NBS AUlOO =165357

^ —-Ipos&techwc. publications

NBS SPECIAL PUBLICATION 427
NATIONAL BUREAU OF STANDARDS

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JJATlONAIi BUKiiAO

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OCT i^^^^

^ Secondary Ion Mass Spectrometry

^^^^^ •^n^r-^.^O .^n.knrrJ,.. y^M:^n

Proceedings of a Workshop on ^ ^ Secondary Ion Mass Spectrometry and

Ion Microprobe Mass Analysis

Held at the National Bureau of Standards

Gaithersburg, Md., September 16-18, 1974

Edited by

K. F. J. Heinrich and D, E. Newbury

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director

Issued October 1975
Library of Congress Cataloging in Publication Data

Workshop on Secondary Ion Mass Spectrometry and Ion Micro- probe Mass Analysis, National Bureau of Standards, 1974. Secondary Ion Mass Spectrometry.

(National Bureau of Standards Special Publication; 427) "CODEN: XNBSAV."

Supt. of Docs No.: €13.10:427.

1. Mass Spectrometry—Congresses. 2. Microprobe Analysis

Congresses. I. Heinrich, Kurt F. J. II. Newbury, D. E. III. Title. IV. Series: United States. National Bureau of Standards. Special Publication; 427. QC100.U57 No. 427 [QD96.M3] 389'.08s [545'.33] 75-619223

National Bureau of Standards Special Publication 427

Nat. Bur. Stand. (U.S.), Spec. Publ. 427, 230' pages '(Oct. 1975) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OBTICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:427). Price $3,00 . (Add 25 percent additional for other than U.S. mailing).
ABSTRACT

This book is the formal report of the Workshop on Secondary Ion

Mass Spectrometry (SIMS) and Ion Microprobe Mass Analysis held at the

National Bureau of Standards in September 1974. Invited and contributed

papers cover a range of topics in the SIMS field: design of SIMS

instrumentation; factors affecting secondary ion collection; techniques

of reducing secondary ion mass spectra to yield quantitative

compositional information; comparisons of SIMS with Auger electron

spectroscopy (AES); techniques of obtaining elemental depth profiles and

the instrumental and physical factors affecting such profiles; and

applications of SIMS to the study of geological samples. The papers

include both tutorial reviews and detailed reports on current research

in SIMS. The volume should be of interest to all workers in the SIMS

and surface analysis fields.

Key words: Auger electron spectroscopy; elemental depth profiling; ion

microprobe mass analysis; ion optics; local thermal equilibrium (LTE);

mineral analysis; secondary ion mass spectrometry (SIMS); surface

analysis.

Certain commercial products are

to specify adequately the experimental

identification imply recommendation

Bureau of Standards, nor does it imply

necessarily the best available for the

iii

identified in this paper in order

procedure. In no case does such

or endorsement by the National

that the products identified are

purpose.

FOREWORD

The Analytical Chemistry Division of the National Bureau of

Standards, Institute for Materials Research, is concerned with the

development of new techniques of analysis as well as the improvement of

existing methods of chemical analysis. Since one of its goals is to

improve the dissemination of knowledge, the NBS Analytical Chemistry

Division has sponsored a series of special conferences, symposia and

workshops on various topics in analytical chemistry. The workshops

normally are devoted to detailed discussions of relatively sharply

defined subjects. The objective is to bring together interested

scientists from throughout the world to exchange information and advance

the state-of-the-art with leading workers keynoting the discussions.

Because of the specific nature and recent origin of these fields, there

is. frequently no other satisfactory venue available for detailed

discussions to take place; such discussions are often overwhelmed when

attached to large international meetings which cover the gamut of

analytical chemistry. Past topics of these workshops and the published

proceedings include: Quantitative Electron Probe Microanalysis (NBS

Special Publication 298), Aerosol Measurements (NBS Special Publication

412), and Oil Pollution Monitoring (NBS Special Publication 409). These

proceedings are civailable from the Superintendent of Documents,

Government Printing Office, Washington, D. C. 20402.

This volume contains the proceedings of another workshop in this

series on the topic of Secondary Ion Mass Spectrometry (SIMS). The

three-day meeting attracted seventy-five participants from the U.S.,

Europe and Australia. The workshop format consisted of keynote talks

given by eminent workers in several specialties, followed by extensive

discussion. The papers which make up this volume include papers

reporting the views of the keynote speakers as well as shorter

contributions based on floor discussions.

The large number of attendees to this meeting, considering' the

relative scarcity of SIMS instrumentation (about 50 SIMS instruments

exist worldwide), indicates that this topic is rapidly growing in

interest and importance. This publication should serve to characterize

the current state of SIMS as an emerging measurement technology, and

should be a valuable reference to workers in this field.

John D. Hoffman, Director

Institute for Materials Research

National Bureau of Standards

V

PREFACE

_ Microanalysis by means of secondary ion mass spectrometry is still

in the initial phases of exploration. The observation of elements of

low atomic number, the measurement of isotope ratios, and the shallow

depth of sampling achievable with an ion microprobe or ion microscope

are the outstandingly attractive features of this technique. They have

already produced exciting results in diverse fields of application,

including the study of distribution in depth of dopants and impurities

in semiconductor materials, the characterization of the microscopic

distribution of hydrogen cind boron in alloys, and the analysis of

microscopic particles. On the other hand, the experimental difficulties

and instrumental limitations, the uncertainties involving theoretical

models of ion-target interaction, and the seriou,s problems in obtaining

appropriate standards have vexed the analysts involved in the practice

of ion probe microanalysis.

Since the cost of purchase and maintenance of the instrument is

very high, the prospective user needs all the information he can obtain

in order to assess the potential gains and the limitations of secondary

ion mass spectrometry applied to his specific needs.

The present publication is not intended to answer all important

questions on the subject. Rather, it presents a series of papers on the

principles of instrumentation, theory and applications in selected

fields

There is no other book currently available that is exclusively

dedicated to this subject, and we are confident that the present

publication will be of interest and help to actual or potential users of

secondary ion mass spectrometry.

K. F. J. Heinrich D. E. Newbury

vii

CONTENTS

Page

Abstract ..... iii

Foreword v Preface vii

The Ion Microprobe - Instrumentation and Techniques 1

fl. Liebl

Looking at the Collection Efficiency Problem Through the

Ion Microscope Optics .

_ 33

G. Slodzian

fligh 'lass Resolution Secondary Ion ^ass Spectrometry 63

P, Williams and C.A. Evans, Jr.

A Comparison of Mass Spectra from Three Ion Probes 69

J.G. Bradley, D.Y. Jerome and C.A. Evans, Jr.

A Critical Discussion of the Local Thermal Equilibrium Model

for the Quantitative Correction of Sputtered Ion Intensities . . 79

C.A. Andersen

An Outline of Secondary Ion Emission Models 121

J.M. Schroeer

Empirical Quantitation Procedures in SIMS 129

J. A. Mcllugh

Application of SIMS Microanalysis Techniques to Trace

Element and Isotopic Studies in Geochemistry and Cosmochemistry 135

J.F. Lovering

ix
Factors that Influence an Elemental Denth Concentration Profile • • 179

J. A. McHugh

A Comparison of Auger Electron Spectroscopy (AES) and Secondary

Ion Mass Spectrometry (SIMS) 191

J.M. Morabito

Key Word Index 225

X
NBS Spec. Publ. 427, Workshop on Secondary Ion Moss Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Ga Ithersburg, Maryland, September' 16- 18, 1974 (Issued October, 1975).

THE ION MICROPROBE-INSTRUMENTATION AND TECHNIQUES

H. Liebl

Max-Planck-Institut fiir Plasmaphysik

EURATOM Association

8046 Garching

Munich, Federal Republic of Germany

I . INTRODUCTION

After the success of the electron microprobe, several new methods

for the microanalysis of solids have been tried and developed, and the

evolution is still in progress. Setting aside special methods for

particular applications like autoradiography or field desorption

microscopy, there are in principle two ways of microanalysis, shown in

figure 1: emission microscopy combined with spectroscopy (a), and the

microprobe technique (b). In both cases the sample is induced by some primary radiation to emit some secondary radiation, characteristic of

its composition, which is then analyzed. The desired lateral resolution

is achieved in case (a) by forming a magnified image of the surface with

appropriate optics, using the transmitted component of the secondary

radiation. The image thus shows the topographical distribution of the

atomic or molecular species emitting that transmitted radiation. For

spot analyses the radiation is collected behind a hole in the screen and

the secondary spectrum is scanned. In case (b) the lateral resolution

is achieved by focusing the primary radiation to a microprobe having a diameter which depends on the desired resolution. By an x-y deflection

device any spot within a certain area on the sample can be hit and thus

spot analyses be made. Distribution pictures are obtained by raster

scanning the sample area and using the transmitted secondary signal to

modulate the intensity of an oscilloscope, the x-y deflection of which

is synchronized with the primary beam raster scan.

If the primary radiation can be focused by lenses, but not the

secondary radiation, the microprobe technique is the only possible way.

This is the case with the electron microprobe X-ray analyzer. When the

primary radiation is made up of ions, and sputtered secondary ions are used as secondary radiation, then both avenues are open and have been

taken. Instruments of type (a) are appropriately called "ion

microanalyzers," while instruments of type (b) should be referred as "ion microprobe analyzers" for distinction. Ion microanalyzers were

built by two groups in France. Goutte and coworkers [1,2] used a "Wien

filter" for mass analysis of the secondary ions. The elegant designs of

1
SIMS/IMMA

Castaing and Slodzian [3,4], which to them were, as Castaing put it [5],

"a pleasant exercise in optics," led to the development of the Cameca

Ion Microanalyzer [6].

Figure 1. The two ways of microanalysis: (a) direct imaging combined

with analysis, (b) microprobe method. (A, sampled area; B,

image of A; P, primary source; SO, secondary optics; SF,

secondary filter; D, detector; SP, spectrum; PF, primary

filter; PO, primary optics; R, raster scanning).

It has been my frequently expressed opinion [7] that ion microprobe

analyzers have an intrinsic advantage over ion microanalyzers on the

basis of absolute sensitivity. In an ibn microanalyzer the overall

transmission of secondary ions from sample to detector is linked to the

lateral resolution because the lens aberrations of the imaging optics

impose restrictions on the solid angle and energy bandwidth utilized.

On the other hand, in an ion microprobe analyzer the secondary optics

can be designed to accommodate a much larger solid angle and energy

bandwidth with the same mass resolution. Further, one can try to

improve the lateral resolution by making finer primary beams, without

degrading the transmission for the secondary ions.

Since I am obviously prejudiced in favor of ion microprobe

analyzers, I am going to review only these instruments. Ion

microanalyzers are the topic of a separate paper at this meeting.

2
Ion Microprobe Instrumentation

fOkV

Woiierbehd/le/

Anode

NochbejchleuntgLngj

stvfe

Konohttoht Kohr

KothotienKanol

Ooseinloii

Itntiitr blend*

kuiibrennweittqt

dtktrojlatiter.t

1^-

' i?8

^'"'''inervngitinse

(inblick fjr lichl /

' \

mikrotkop 5eoba:tttung

beitrahllej Objekl

Figure 2. Early ion microprobe (v. Ardenne 1939 [9]).

II. EVOLUTION AND PRESENT INSTRUMENTATION

Ion microprobe analyzers have two groups of ancestors: Ion

microprobes and secondary ion mass spectrometers. Both groups still

exist separately, ion microprobes for microetching, scanning ion

microscopy and ion implantation, and secondary ion mass spectrometers

for bulk and surface analysis. (Not all apes evolved into humans,

3
SIMS/IMMA

either.) Both groups can be traced back about 35 years. Sloane and

Press [8] studied the sputtering of negative ions upon positive ion

bombardment in 1938. The first ion microprobe was published in 1939 by von Ardenne [9]. For history's sake, his setup is shown in figure 2.

Here the beam from a canal -ray tube, having a large energy spread, is

focused by an einzel lens to a probe of about 30 ym diameter. Figure 3,

from the same publication, shows a proposed setup for micromachining,

with two stage demagnifi cation of the ion probe, and in situ

observation of the thin sample by transmission electron microscopy. Ten

i'lektronenUrohf

Rohr

Kondemoripufe

Objekt

Oektroitotische Unit I

flektrostatisthe lime I •i2kV

biM3

Konalttrahlffohr

~-

Zur Aufteilung det itrahlenqangei

I /i'achbesih/eunigongi

' Bogengos

Figure 3. Ion microprobe combined with

microscope (v. Ardenne 1939 [9]).

transmission electron

4
Ion Microprobe Instrumentation

100 KSi

Figure 4. Sputtering ion source with primary beam from canalray tube

(Herzog and Viehbbck 1949 [10]).

years later Herzog and Viehbock [10] published their sputter ion source

(fig. 4). They also used a canal-ray tube to generate an intense and

energetic ion beam. It was guided down a long and narrow tube affording

high flow resistance, so that the sputtering took place in a relatively

good vacuum. In those days canal -ray tubes were the most convenient and

5
SIMS/IMMA

VAf>OR TRAP mCrcurv diffusion pump

Figure 5. Sputtering ion source with focused primary

duoplasmatron (Liebl and Herzog 1964 [12]).

beam from

6
Ion Microprobe Instrumentation

widely used source of ions from gases for double-focusing mass spectrographs. In the mass spectrograms lines from the cathode material

would show up, as a consequence of cathode sputtering, and this fact had

been utilized to obtain mass lines from solids. But this source was the

first to produce secondary ions with a primary ion beam technique. This

thread was picked up again in 1961, when I worked in Herzog's group at

Geophysics Corporation of America (6CA), and we had a contract for a .sputtering ion source [11]. The final version [12] of the resulting

device is shown in figure 5. In 1956, von Ardenne's duoplasmatron ion source had become known [13], which was superior by orders of magnitude

to the old canal -ray tube with respect to gas consumption, energy

spread, and brightness. The version shown in this figure was of my own design, trimmed to yield a very pure ion beam of high brightness. The

use of ceramic permanent magnets instead of a coil as customary, brought

certain advantages. The ion beam extracted from it had only a few electron volts of energy spread and thus could be focused very sharply with

Hp* 02*

Figure 6. Mass spectrum of primary argon beam with impurities [14].

a 1:1 imaging lens onto the sample to a minimum diameter of about 300

ym, corresponding to the size of the exit hole in the anode of the

duoplasmatron. For beam adjustment x-y deflection was provided. The

secondary ions were extracted at a right angle to the primary beam from

the tilted sample surface and focused onto the mass spectrometer

entrance slit. The composition of the primary beam could be simply

7
SIMS/IMMA

checked by using the sample surface as an electrostatic mirror, thereby

deflecting a fraction of the primary beam into the mass spectrometer.

In the course of this work it turned out that, even with all precautions, the primary beam carried varying amounts of impurity ions,

besides the doubly charged and diatomic ions of the gas used [14]. This

is seen in figure 6, which shows the impurities in an argon ion beam on

a logarithmic scale. It also turned out that the secondary ion yield

was extremely matrix-sensitive. Therefore, when I joined Applied

Research Laboratories (ARL) in 1964 and started designing the IMMA (Ion

Microprobe Mass Analyzer), I decided that the primary beam had to be

mass-analyzed [15]. Figure 7 shows the final layout of the IMMA [16],

based on my original design, with later improvements added. The

duoplasmatron is basically similar to that of figure 5, but the hot

filament is replaced by a hollow cathode to allow stable operation with

reactive gases. The intermediate electrode can be offset to allow

extraction of negative ions, which are useful in the analysis of

insulating samples (see below). The extracted ion beam is mass-analyzed

by a 90° magnetic field of the wedge type, formed between plane but

inclined pole faces. In such a field, the ions follow trochodial

Figure 7. Final layout of IMMA (ARL) [15,16].

8
Ion Microprobe Instrumentation

trajectories. The merits of this choice are two-directional focusin^g

and no second order angular aberration with a geometrically simple

setup. The mass-analyzed emerging beam is focused with two stage

demagnif ication by einzel lenses onto the sample. The secondary ions are accelerated into a mass spectrometer, designed to provide

particularly high transmission. It consists of an einzel lens, a 45°

spherical condenser, and a wedge magnet of the same type as for the

primary beam analysis. The detector is an ion-electron converter plus

scintillator and photomultiplier. Positive or negative secondary ions

can be detected with equal sensitivity. Between tbe magnet and the exit

slit two pairs of deflection plates are placed which can be used to

switch in rapid sequence between two neighboring masses. ihis "peak

switch" is most useful for isotopic ratio measurements.

The IMMA prototype was in operation two years after it was started

and I presented the first secondary ion micrographs at the 1967 ASTM

meeting on mass spectrometry in Denver, Colorado [17]. Duning the

assembly of the IMMA, Long of Cambridge University published a thorough

feasibility study of an ion microprobe analyzer [18], shown schematically in figure 8. His intended application was isotopic ratio

To pump

Objective lens

Deflection plates

Detector 1

Figure 8. Proposed ion microprobe analyzer (long 1965 [18]).

measurements on geological samples. A student of his, Drummond, started

construction, and in 1967, they published scanning ion micrographs,

obtained with secondary electrons, showing about 0.5 \im resolution [19].

Figure 9 shows their primary beam column [20] with a duoplasmatron , a symmetric einzel lens as condenser, and an asymmetric einzel lens as

"9
SIMS/IMMA

duopU*natron

lon source

d«ll«ct.on platts lit condenser lens

(paraffin)

differential pumping

aperture

Figure 9. Scanning ion microfjrobe (Drummond and Long 1967 [19]).

objective lens. Such an asymmetric lens has a reduced spherical

aberration compared to symmetric lenses, if the working distance is made

Very short [21]. At this condition, Long and Drummond" reported a^^robe

diameter of only 0.2 ym. Recently, Long and Coles [22] have published

applications with the completed instrument. Essential design features

of figure 9 were incorporated into the primary beam column which was developed subsequently by Associated Electrical Industries, Ltd. as an accessory to their MS7 mass spectrometers [23] (fig. 10). The main

changes are a hollow cathode in the duoplasmatron and the addition of a magnet. The mass spectrometer is of the Mattauch-Herzog type with

optional electric or photoplate detection and a mass resolution up to

10000. The third commercial ion microprobe analyzer on the market was developed by Hitachi, and first announced in 1969 at the Mass Spectrometry Conference in Kyoto [24]. The latest version [25] is shown in

figure 11. A point of interest is the placement of a multiplier close

to the sample, which facilitates scanning ion microscopy by secondary

electrons, with the sample biased negatively. In a recent installation

of this instrument a Wien filter was inserted in the primary column for

mass separation [25].

Besides the commercially available ion microprobe analyzers, there

are now several laboratory instruments in operation. Figure 12 shows

the combined ion and electron microprobe (UMPA), which I started in 1968

at the Max-Planck-Institut fiir Plasmaphysik near Munich [7]. It was conceived as a research instrument, suited not only for bulk analysis

with high lateral resolution, but also for surface research. Therefore,

the apparatus is built in UHV-technology to reach pressures under 10"^

torr. As the primary beam, not only ions can be used but also

10
Ion Microprobe Instrumentation

electrons. The secondary analyzer can be used as an energy spectrometer

or as a mass spectrometer, both with two-directional focusing. For the

primary beam focusing "hybrid lenses" are used, which are made by

fitting a nonmagnetic aperture electrode into the gap of a conventional

magnetic l^ns. Such a lens can act as magnetic lens for electrons, as an einzel lens for ions or electrons, and as a combined lens for ions and

Figure 10. AEI ion microprobe analyzer (drawing from C. A. Evans, Jr.,

Anal. Chem. 44 (1972) 67A)

.

electrons simultaneously. The idea behind the 180° primary ion magnet

is shown in figure 13. Sector magnets as well as the Wien filter

perform in addition to the mass separation an energy dispersion,

resulting in an elongation of the focused spot, especially at lower beam

energies. Here the beam is made to enter the 180° magnet parallel, the mass separating slit is placed in the middle, and the ions leave the

magnet as a parallel beam, only slightly broadened due to their energy

spread. At the condenser lens focus, no lateral energy dispersion is

11
SIMS/IMMA i

!

Figure 11. Hitachi IMA-2 ion microprobe analyzer [25].

left; the focus is round. Another advantage is that the ion acceleration voltage and the magnetic field need not be extremely constant. We are in the process of attaching an energydispersive X-ray spectrometer,

which will facilitate conventional electron analysis.

Another prototype instrument, based on this design, has been built

by Meier at Varian MAT in Bremen [27]. A scheme of it is shown in

figure 14. The primary magnet is turned to the other side to make more

room around the sample stage for various attachments such as a residual

gas analyzer and a backscattered electron detector, or an energy

dispersive X-ray spectrometer. This instrument is also built using UHVtechnology. At the Austrian research site Seibersdorf near Vienna,

Rudenauer and Steiger recently finished assembly of another UHV-ion

microprobe analyzer [28] (fig. 15). The primary column is rather

conventional with duoplasmatron and two asymmetric einzel lenses, but

without mass analysis. The secondary mass spectrometer is rather

12
Ion Microprobe Instrumentation

Figure 12. Combined ion and electron microprobe analyzer (UMPA) [7].

unusual in that it actually consists of two double focusing mass spectrometers in tandem. The first one has an energy-dispersed

intermediate focus. The first mass spectrum is formed at a slit inside

a 180° uniform field magnet, serving as entrahce slit of the second mass

spectrometer. This arrangement is distinguished by great abundance

sensitivity because of the virtual elimination of the scattered ion

background from the major peaks.

Now I am going to talk about some instrumental details worthy of

closer consideration. One crucial design feature is the effective

secondary ion collection from the sample, because it enters directly

into the total transmission and therewith into the absolute sensitivity.

The relation between the consumed sample volume and the number of ions

collected at the detector, which stem from a monoisotopic constituent

present with the atomic concentration C in a matrix of atom density N (atoms/cm^) is given by

13
SIMS/IMMA

n = CN a Az (1)

with a = samples area (cm ) Az = depth interval (cm]

Y

= ionization ratio of detected ionic species

(sputtered ions/sputtered particles)

n = total transmission factor from sample to

detector

.

In order to measure a minimum concentration Cn,,-^ with a precision of

±10%, 100 ions have to be collected. Taking N = 6 x lO^cm"^ as an average figure, we obtain the volume which has to be eroded

- 2

1

a Az - 1.6 X 10 /ync . mm

(2)

For a 1 ym area, the eroded depth interval Az becomes

Az ^ 1.6 X lO'-^^/YnCmm

(3)

\

4h 1

1 4

Figure 13. Mass separation without energy dispersion (primary beam of

UMPA) [7].

14
*Ion Microprobe Instrumentation

Primiirmagnel -

EinzcainMOuopUsmatron

Kondtnsator

I?

«

Sckundirmagnvl

EinztMins*

Mus«ns(Mlt SEVU

- Einl«nkmagnet

Eltklrorwnkanorle

Hybridlinse I

AMenkung

Hybhdiinse n

I - SEV m RiickstreOetektronen

En*rgie-<lisp«rsiv»s Spektrometer

- Hikroskop

Kiihltall*

RMlgasanalysator

Sekundirelvktronen

Target - ^ Strom

- Heizurtg

Figure 14. Layout of Varian MAT prototype ion and electron microprobe

analyzer [27].

Assuming for example an ionization ratio of 10"^ and a transmission of

10%, the depth interval required for C^^-p = 100 ppm comes out to be

160A. This would also be about the depth interval required to make one scanning image of that low concentration constituent. For one scanning

image of the matrix (C = 1), Az comes out to be 1.6 x 10"^ A, which

means that only 0.006 monolayers are required. With a transmission of

0.1% instead of 10%, these erosion figures are 100 times as large, i.e.,

1.6 ym in the one case and 0.6 monolayers in the other. These examples

demonstrate the importance of effective secondary ion collection.

Figure 16 shows the arrangements used with IMMA [16] and UMPA [7]. Both

afford high collection efficiency owing to appropriate shaping of the

acceleration field. In both cases in situ microscopic observation with

a numerical aperture as high as 0.25 is possible over a metal mirror

having a central hole for passage of the primary beam. In case (b) the

objective lens can be placed somewhat closer to the sample, which for a given beam brightness yields a higher current in the spot. On the other

hand, the displacement of the spot due to the secondary acceleration

field is larger than in case (a). Another scheme worth mentioning was

15
SIMS/IMMA

Ti-subl.

Figure 15. Ion microprobe analyzer of RUdenauer and Steiger [28].

proposed by Goutte et al. [29] (fig. 17). Here tne secondary ions were

to be accelerated over a short distance and then deflected out by a little electrostatic prism, the outer plate of which is perforated for

passage of the primary beam. Another point of interest is the secondary

ion analysis by a quadrupole mass filter, which had been done first by

Krohn in SIMS work [30] but has not been used in ion microprobes yet.

16
Ion Microprobe Instrumentation

PRIMARY

Figure 16. Secondary ion acceleration with (a) IMMA (Fig. 7) and (b)

UMPA (Fig. 12).

Projektionlinse

Quadrupolfilter Vervielfocher

yonen-Elektronon

^^^^^^^^^

ignal

Figure 17. Secondary ion acceleration through apertures, deflection of

electrostatic prism, and analysis by quadrupole mass filter

(Goutte et at. [29]).

17
SIMS/IMMA

Outer Electrode of

Electrostatic Lers

lOV

20V

FVimary Ion Beam

' To Mass Spectrometer

Secondary Ion

Beam

Shield

Electrode

WW

Specimen Holder

or

Figure 18. Hitachi IMA-2 sample stage [31].

Figure 19. Secondary ion collection system for ANL 100-inch mass spectrometer (Krohn and Ringo [35]).

18
Ion Microprobe Instrumentation

Figure 18 shows the sample stage arrangement of the Hitachi IMA-2 [31].

A point of interest here is the electron spray gun used to prevent

charge-up of insulating samples. This method had also been used with

the sputter ion source of figure 5 by Herzog and co-workers [32].

Andersen, Roden and Robinson [33] have demonstrated the. usefulness of

using a negative primary beam to alleviate the charge-up problem. The

combined ion and electron microprobes (fig. 12 and 14) offer the

additional possibility of using a positive ion beam plus an electron

beam simultaneously.

III. CURRENT DEVELOPMENT

The mass spectrometers of the ion microprobe analyzer described so far are designed for higher transmission and medium resolution, save the

AEI instrument. Since they are all double-focusing instruments

transmission can be traded for higher resolution. But there is a recognized need for a high transmission (10%) - high resolution (10,000)

instrument, particularly for geochemical [34] and biochemical [35]

applications. One step in that direction has been made by Krohn and

Ringo [35] at ANL. They are developing a microprobe attachment for the

Argonne 100-inch mass spectrometer. Figure 19 shows their secondary ion

collection system, developed to achieve optimal matching with the

acceptance phase space of their mass spectrometer. It is made up of

three einzel lenses, but the crucial feature is the division of the

specimen stage into sections of different potentials.

I have mentioned above that the current into a given spot becomes

larger if the distance from the objective lens to the sample can be made

the shorter. Figure 20 shows calculated diameters of the beam focus for

different beam currents as a function of the convergence angle. A beam

of typical brightness and energy spread from a duoplasmatron is assumed.

The curves are the RMS values of the combined contributions from

Gaussian diameter (dg), spherical aberration (dg)* and chromatic

aberration' (d^) [36,37]', The values of the aberration constants Cg and

Cq used here are typical of the objective lenses used in IMMA [15] or UMPA [7] (3 to 4 cm focal length). The optimum convergence angles, to

be defined by the final aperture, are where these curves have minima.

Figure 21 shows similar plots, but for an objective lens of only 0.5 cm

focal length. It shows that it should be possible to obtain a spot of

less than 2000 A diameter with 10"^^ A beam current. These plots apply

for a 10 keV beam with a 10 eV energy spread. Using a higher beam

energy would increase the brightness and therefore move the dg-lines

down, and it would further decrease the relative energy spread AO/U and

therefore move the dc-lines down. So the minima of the curves would be

lower, too. But higher beam energies should not be used with respect to

undesirable effects upon the samples. Hill [38] has reported a 0.3 urn diameter spot with a 30 keV Ar''"beam of 5 x 10"^° A current, and

Drummond and Long have reported a 0.2 ym diameter spot with a 20 keV

Ar'*"beam [20] (current not reported).

19
SIMS/IMMA

From the above consideration on sample consumption, it is clear

that striving for submicron spot sizes would make sense, even though the

required depth interval will be increased, provided that the secondary

ion transmission can be made high. It is obvious that the secondary

ions cannot be extracted sideways from the sample, if it is very close

to the objective lens. I have figured out a way to overcome this

problem [39]. Figure 22 shows the solution: The secondary ions are extracted back through the objective lens. The same set of electrodes

is used as objective lens for the primary beam and for secondary ion

acceleration. The problem here is that the primary beam has to be

confined by a small aperture to obtain the optimum convergence angle,

but the secondary ion collection should be efficient. This is

accomplished by choosing the potentials in such a way that the primary

beam is focused by the two lens fields onto the sample, while the

secondary beam has a crossover in the aperture fitted inside the middle

e-lectrode, and leaves the assembly as a parallel beam (fig. 23). Figure

24 shows the incorporation of a Schwarzschi Id-type microscopic objective

for viewing the sample in situ. The secondary beam is separated from

t-he primary beam by a deflection condenser having a bore in the outer

CONVERGENCE HALF-ANGLE 6

Figure 20. Calculated diameters of focused ion beam vs. convergence

angle obtainable with einzel lens of 3 cm focal length.

Contributions; dg, Gaussian diameter; dg, spherical

aberration, dc, chromatic aberration.

shell (visible at the upper rim of fig. 24). A complete ion microprobe

analyzer with this lens, called the COALA (Combined Objective and Anode

Lens Analyzer), is presently under assembly.

20
Ion Microprobe Instrumentation

CONVERGENCE HALF -ANGLE 0

Figure 21. Beam diameters like Fig. 20, but with einzel lens of 0.5 cm

focal length.

IV. DEPTH PROFILING

One of the most important applications of ion microprobe analyzers

is the measurement of concentration profiles [40]. While the lateral

resolution is about 1 ym for the present instruments, the depth

resolution can be 100 times better, around 100 A. With an ion

microprobe analyzer, a concentration in-depth profile can be taken from

a very small area. In order to obtain even erosion without sloping

walls, several methods can be used. The simplest way (fig. 25a )^ is to use an unfocused beam hitting the final aperture. Since the current

density across the aperture is then uniform, a flat-bottomed crater with

very steep walls will result. Higher current densities and therefore

erosion rates can be obtained, if such a uniform beam is weakly focused

by the objective lens (fig. 25b). The current density will stay

uniform, as long as the lens aberrations are still negligible. Although

the erosion progresses evenly, in these two cases, there will be a contribution to the secondary ion current from the narrow rim zone,

amounting to perhaps a few percent of the total current.

It depends on the problem at hand whether this is tolerable or not.

Even this small contribution is eliminated with the so-called

"electronic aperture" scheme [16] (fig. 26). The focused primary beam

is raster scanned across the area under investigation. The scaler

counting the mass-analyzed secondary ions is electronically gated by

21
SIMS/IMMA

signals derived from the scan voltages, so that only the ions from the

central portion of the rastered area are counted. Figure 27 shows

optical micrographs of an erosion area obtained by raster scanning [43].

The interferometric micrograph demonstrates how evenly the erosion

progresses.

Figure 22. Combined objective and anode lens of COALA (Combined

Objective and Anode Lens Apparatus). 0, Sample; W, Wehnelt;

A", anode; B, grounded electrode; P] ^2 ^3,4* deflection

plates; L, light cone [39].

Tamura et al. [44] at Hitachi have used the so-called stop-scan

method, where the beam raster scans for erosion and in between is

directed to any desired spot for analysis. They also have used the

energy stop of the mass spectrometer as mechanical aperture to suppress

ions from the rim zone, by making it smaller than the image of the

rastered area.

22
Ion Microprobe Instrumentation

Figure 23. Primary (S-j) and secondary ($2) beams in COALA lens in an off-axis scan position.

V. ION MICROPROBE ANALYSIS BY OTHER TECHNIQUES

The analysis of secondary ions is not the only method for bulk or surface characterization by application of ion beams. Other techniques

are: Low-energy noble ion scattering spectroscopy (ISS), Rutherfordbackscattering of high energy light ions, nuclear reactions induced by

high energy light ions, and ion-induced X-ray emission. Cookson and

Pilling [55] at Harwell have built a magnetic quadrupole quadruplet

capable of focusing light ions up to 3 MeV energy to a spot of 4 ym

diameter, and have reported measurements using these techniques (except

ISS) with a microbeam.

Another effect which has also been utilized for surface

characterization by ion microprobe techniques is the so-called "ionoluminescence." A significant fraction of the sputtered neutrals and

ions come off the surface in excited states, and subsequently relax

23.
SIMS/IMMA

Figure 24. COALA lens with microscopic mirror optics. Separation of

beams by deflection condenser (upper rim of figure).

under photon emission. Whoever has worked with intense heavy ion beams

has seen the glow above a bombarded target. Figure 28 shows an arrangement published by Goutte et al. [29] in 1967. A beam of alkali

ions from a thermionic hairpin ion source is focused by a single einzel

24
Ion Microprobe Instrumentation

lens onto the sample. The emitted light is focused by a concave mirror,

into a photomultiplier. Interference filters were used for analysis.

PRIMARY BEAM

SECONDARY

IONS

SAMPLED AREA

a) COLL/MATED

PRIMARY BEAM

PRIMARY BEAM

SECONDARY

IONS

SAMPLED^ AREA

bj UNDERFOCUSED

PRIMARY BEAM

Figure 25. Even erosion obtained with (a) collimated primary beam (b)

underfocused primary beam.

PRIMARY BEAM

SCAN VOLTAGE

MASS

SPECTROMETER

SECONDARY

IONS

SAMPLED

AREA

Figure 26. Scheme of electronic aperture.

25
SIMS/IMMA

Figure 29 shows scanning micrographs obtained by this technique.

Several groups now have been working to investigate the ionoluminescence [45,46]. It seems that it may find more application in ion

microprobe analysis.

Figure 27. Optical micrographs of raster eroded area. Sample: 2000 A

Al on (111) Si surface. Beam: 10 ym diameter, 10 keV Ar"*". Erosion depth: 2600 A.

VI. ION SOURCES

The duoplasmatron-type source has proven to be a good workhorse ion

source, affording high brightness, low energy spread, and versatility as

a source for a variety of ionic species. After the benefits of using

reactive species instead of the commonly used argon had been

demonstrated by Andersen [47] following a suggestion of mine, the

replacement of the hot filaments by hollow cathodes made stable

operation with reactive gases possible. In the plots of beam diameters

(fig. 20 and 21) the positions of the Gaussian beam size (dg) lines for

different beam currents depend on the brightness of the ion gun. A

26
Ion Microprobe Instrumentation

Canon^^K^

Tube

cat hod que

Ions

Bases de temps

imngps » Lentille

Miroir

Cible

Filtre interferentiel

PM.EMI 6256B

R. = 1M£2

Figure 28. Scanning ion microprobe using iono-luminescence (Goutte et

al. 1967 [29]).

higher brightness value moves the dgline for a given beam current down

toward smaller spots. The only ion source with higher brightness values

than duoplasmatrons are field ionization and field evaporation sources

[48-51]. These might have a future in ion microprobes, especially field

evaporation sources operated with liquid alkali metals, since alkali

ions have been found to have a similar beneficial effect on the

production of negative secondary ions as ions of electronegative gases

on the production of positive secondary ions [30,52]. However, because

27
SIMS/IMMA

of the particular emission characteristics of field, ion sources,

completely different ion optics would have to be used for microprobe

formation [53],

VII. COMPLETE THREE-DIMENSIONAL ANALYSIS

In most ion microprobe analyzers only one of the secondary ionic

species can be detected at a time; the others are wasted. An exception

is the AEI-instrument, if used with photoplate detection, since the

Mattauch-Herzog mass spectrograph produces a simultaneous complete mass

spectrum. But photoplate detection has its drawbacks, especially the

limited dynamic range. One way out could be multiple electric

detection, facilitating on-line data acquisition.

Another possibility to record all masses of an eroded volume would

be to use digital scanning with a pulsed primary beam, to analyze each

burst of secondary ions with a time-of-flight mass spectrometer, which

records the different masses successively, and to store the output

information. This way, a complete mass spectrum from each volume

element of the eroded volume would be on record and could be displayed

in whichever way one wished. A novel type of time-of-f1 ight mass spectrometer which is energy-focusing and, therefore, appears well

suited to accommodate the rather large energy spread of sputtered ions,

has been developed by Poschenrieder [54].

28
Ion Microprobe Instrumentation

It seems that the ion microprobe analyzer has established itself as

a valuable new tool for the analytical laboratory, but that there is room for improvement of its instrumental and analytical capabilities.

VI n. REFERENCES

1. Bernard, R. and Goutte, C. R., Acad. Sci . 246 , 2597-99 (1958).

2. Goutte, R., Guillaud, C, Javelas, R., and M6riaux, J. Microscopie

3, 473-82 (1964).

3. Castaing, R., and Slodzian, G., J. Microscopie 1_, 395 (1962).

4. Castaing, R., Proc. 4th Int. Conf. on X-Ray Optics and

Microanalysis, 48, Hermann, Paris (1966).

5. Castaing, R., Proc. 6th Int. Conf. on X-Ray Optics and

Microanalysis, 399-407, Univ. Tokyo Press (1972).

6. Rouberol, J.-M., Guernet, J., Dechamps, P., Dagnot, J. -P. and Guyon

de la Berge, J.-M., Proc. bth Int. Conf. on X-Ray Optics and

Microanalysis, 311-18, Springer (1969).

7. Liebl., H., Int. J. Mass Spectrom. Ion Phys. 6_, 401-12 (1971).

8. Sloane, R. H., and Press, R., Proc. Roy. Soc. A. 168, 284-301

(1938).

9. Ardenne, M. v., Z. Techn. Phys. 2£, 344-46 (1939).

10. Herzog, R. F. K. and Viehbbck, F. P., Phys. Rev. 76, 855-56 (1949).

11. Liebl, H. J. and Herzog, R. F. K., J. Appl . Phys. 34, 2893-96

(1963).

12. Liebl, H. J. and Herzog, R. F. K., 12th Ann. Conf. Mass Spectrom.

Allied Topics, ASTM E-14, 393-97, Canada (1964).

13. Ardenne, M. v.. Tab. d. Elektronenphysik, lonenphysik und

Ubermikroskopie, Vol. I, p. 544, VEB Deutsch. Verlag d.

Wissensch., Berlin, Germany (1956).

14. Herzog, R. F. K., Liebl, H. J., Poschenrieder, W. P., and

Barrington, A. E., NASA Report N 65-23741.

15. Liebl. H., J. Appl. Phys. 38, 5277-83 (1967).

16. Whatley, T. A., Slack, C. B., and Davidson, E., see Ref. 5, p. 417-

22.

29
SIMS/IMMA

17. Liebl, H., 15th Ann. Conf. Mass. Spectrom. All. Top., ASTM E-14,

Paper 160, Denver, Col. (1967).

18. Long, J. V. P., Brit.. J. Appl . Phys. 16, 1277-84 (1965).

19. Drummond, I. W., Long, J. V. P., Nature 21_5, 950-52 (1967).

20,. Drummond, I. W., and Long, J. V. P., Proc. 1st Int. Conf. on Ion

Sources, 459-68, Saclay, France (1969).

21. Septier, A., CERN Report No. 60-39.

22. Coles J. N., and Long, J. V. P., Phil. Mag. 29. 457-71 (1974).

23. Banner, A. E., Bateman, R. H., Halliday, J. S., and Wildig, E., AEI

New Product Information (1972).

24. Tamura, H., Kondo, T., Doi , H., Omura, I. and Taya, S., in Recent

Developments in Mass Spectroscopy, 205-9, Univ. Tokyo Press (1970).

25. , Tamura, H., fcCondo, T., Hirano, T., see ref. 5, 423-29.

26. Matsumoto, R., Sato, K., and Suzuki, K., 6th Int. Vac. Congr.,

Kyoto (1974).

27. Meier, S., 7. Koll. Arbeitskr. EDO, June 1974, Frankfurt/Main,

Germany.

28. Rudenauer, F. 6., and Steiger, W., 6th Int. Vac. Congr., Kyoto

(1974).

29. Goutte, R., Guillaud, C, Javelas, R., and Meriaux, J. -P., Optik

26, 574-81 (1967).

30. Krohn, V. E., J. Appl. Phys. 33, 3523-25 (1962).

31. Nakamura, K., Aoki , S., Tamura, H., and Doi, H., see ref. 5, 447-

54.

32. Herzog, R. F. K., Poschenrieder, W. P., Rudenauer, F. G., and

Satkiewicz, F. G., see ref. 17, paper 93.

33. Andersen, C. A., Roden, H. J., and Robinson, C. F., J. Appl. Phys.

40, 3419-20 (1969).

34. Lovering, J. F., Comments on Earth Sciences: Geophysics (1973) (in

press)

.

35. Krohn, V. E., and Ringo, G. R., Rev. Sci . Instr. 43, 1771-72

(1972).

30
Ion Microprobe Instrumentation

36. Wilson, R. 6., and Brewer, G. R., Ion Beams with Applications to

Ion Implantation, Wiley, New York (1973).

37. Seliger, R. L., and Fleming, W. P., J. Appl . Phys. 45, 1416-22

(1974).

38. Hill, A. R.. Nature 21^, 202-3 (1968).

39. Liebl, H., Vacuum 22^, 619-21 (1972).

40. Liebl, H., J. Vac. Sci . Technol . (in press).

41. Tamura, H., Kondo, T., and Doi , H., Adv. Mass Spectrom. 5, 441-443

(1971) .

42. Blewer, R. S., and Guthrie, J. W., Surf. Sci. 32, 743-47 (1972).

43. Staudenmaier, G., Liebl., H., and Hofer, W. (unpublished).

44. Tamura, H., Kondo, T., Konomata, I., Nakamura, K., and Nakajima,

J., 6th Int. Vac. Congr., Kyoto (1974).

45. Martel, J. G., and Olson, N. T., Nucl . Instr. Method. 105, 269-75

(1972) .

46. Tolk, N. H., Simms, D. L., Foley, E. B., and White, C. W., Rad.

Eff. 18^, 221-20 (1973) (for more references on iono-luminescence

see ref . 45, 46)

.

47. Andersen, C. A., Int. J. Mass Spectrom. Ion Phys. 2, 61-74 (1969).

48. Muller, E. W., and Tsong, T. T., Field Ion Microscopy, Elsevier,

New York (1969).

49. Mahoney, J. F., Yahiku, A. Y., Daley, H. L., Moore, R. D., and

Perel, J., J. Appl. Phys. 40, 5101 (1969).

50. Evans, C. A., Jr. and Hendricks, C. D., Rev. Sci. Instr. 43, 1527

(1972). ~

51. Krohn, V. E., J. Appl. Phys. 45, 1144 (1974).

52. Andersen, C. A., Int. J. Mass. Spectrom. Ion Phys. 3, 413 (1970).

53. Heil, H., and Guckenberger, R., Proc. Symp. Ion Sources and Form.

Ion Beams, Rep. No. BNL-50310, 183 (1971).

54. Poschenrieder, W. P., Int. J. Mass Spectrom. Ion Phys. 3, 357

(1972) .

55. Cookson, J. A., and Pilling, F. D., Thin Solid Films 19, 381

(1973) . ~"

31

NBS Spec. Publ. 427, Workshop on Secondary Ion Mass Spectrometry (SIMS) and Ion MIcroprobe Mass

Analysis held at NBS, Ga ithers burg, Maryland, September 16-18, 1974 (Issued October, 1975).

LOOKING AT THE COLLECTION EFFICIENCY PROBLEM

THROUGH THE ION MICROSCOPE OPTICS

6. Slodzian

University of Paris

Orsay, France

I. INTRODUCTION

Secondary ion emission is used in various instruments to provide

for elemental and isotopic analysis of solid samples over small areas or

in thin surface layers. In addition, many people have been studying the

processes of ion emission. However, it seems that not enough attention

has been paid to the data collecting procedure, that is, to the energy

and angular discriminations introduced by the collecting optics coupled

with' the energy and mass spectrometers. This paper is an attempt to

cast some light on these problems. Although it specifically deals with

the ion microscope optics, the principles here invoked are thought to be

general enough to provide us with conclusions which will hold in other

situations, possibly with minor alterations.

When a primary ion beam is directed against the target surface,

each "point" of the surface becomes a source of sputtered secondary

ions. The question is how to use this localized emission so as to

obtain a localized analysis. One way is to look for a focusing system

which would make the beam emerging from each point source converge on a corresponding image point. Two preliminary and obvious remarks can be

made:

- The secondary ions exhibit an energy distribution with a maximum of

several eV and a tail extending to several hundred eV, In order to re- dace the energy dispersion and to handle the secondary beam more easily,

it appears convenient to accelerate the ions as they leave the target.

- Magnetic^ lenses are too weak to focus ions and, therefore, we have to use electrostatic lenses. It is known that ions accelerated by the same voltage will follow trajectories which are independent of the ratio e/m

(e and m are the charge and the mass of the ions respectively) (fig. 1).

Thus, we see that the focusing system will necessarily comprise two

stages: an acceleration stage and an electrostatic focusing stage. In addition, since the magnification of the ion image will be the same regardless of the particular ion species used, from a geometrical point

of view, the focusing system acts like a device which would reconstruct

a kind of expanded thin layer of the target made of the material

sputtered from the target. If the laws of secondary emission were

simple,^ the ion image would faithfully reproduce the composition of the

sample, except for aberrations which might give some blurring of the

image.,

33
SIMS/IMMA

Figure 1 Schematic view of the general actio'n of the objective lens

in an ion microscope. A thin layer CqBq in diameter is "reconstructed" in the

C'B'

.

image plane into^an enlarged ion inlage

Once the enlarged image is obtained, it has to be split into as many elemental images as there are different elements (or isotopes).

This can be done by a combination of sector fields^ similar to those

used in mass spectrometry which allow an ion image made up of a given

§pecies to be filtered out. Then the filtered ion image is converted

into an equivalent electron image. This latter image is either observed

on a fluorescent screen or directly recorded on a photographic film, in

addition, direct measurements can be performed on a specified limited

area selected by aperturing at the electron image.

First, let us look at the properties of the focusing system (often

called an emission or immersion lens) in some detail.

II. EMISSION LENS

(1) Secondary ion characterization

The secondary ions released at the surface are defined both by

their kinetic energy c|)q (in electron volts) and their velocity direction

(otg, i/^o) where ao and }pQ are the emergence angle' (colatitude) and .the

azimuthal angle (fig. 2) respectively. When a layer of a thickness dz over an area doo has been sputtered away, d^N-j ions of a given species

(M^, for instance)^ are ejected with their kinetic energy (\)q in the

range d(j)Q, and their direction (aQ, ^q) within the solid angle (IQq:

34
Ion Collection Efficiency

where fo(<|)o» ^o) is a function which depends on emission parameters

(nature of the target, angle, energy and nature of the primary ions,

relative orientation of the primary beam with respect to the crystalline

directions of the target, surface reactions); for the sake of simplicity

we assume the sample to be homogeneous.

4>o /

Ao

dz

Figure 2 Characterization of the secondary ions as they are emitted:

energy (j)o expressed in electron volts, direction of their

velocity (ao, ^Pq) » and geometrical parameters (df2o solid

angle, dao emitting area)..

(2) Acceleration space

The ions are accelerated by the uniform electrostatic field Eq set

between the target M (at the voltage V) and the electrode N (at ground

potential). In the electrode N, there is a round hole through which the

ions leave the acceleration space. Near the hole, the field is no longer uniform and the equi potential surfaces are curved, having

cylindrical symmetry around the Z- axis of the hole (fig. 3). This

curvature of the equi potential surfaces induces focusing effects. The

combination of acceleration and passage through the hole can be

approximated in the following way:

- Up to the plane of the electrode N, the field is assumed to be uniform

so that the trajectories between "M" and "N" are segments of parabolas.

- At the plane N there is an abrupt change in the electrostatic field

which goes from the value Eq to zero. Since the flux is conserved, an infinitely strong radial field must appear. This radial field has a divergent effect but it is acting only over an infinitesimal distance.

35
SIMS/IMMA

Figure 3 Acceleration space; section of the curved equi potential

surfaces.

It can be shown [1] that, under these conditions, the passage through

the hole does not change the distance r from the axis z'z at which a trajectory pierces the plane of N, but that it changes the meridian

component, r' = dr/dz, of the trajectory direction:

A^' = J • -TTT-^r^^^-^ = ^ . -—r-^ (2) ^ * V + *o cos^ %

~ 4D \ *o— 2 cos a V o

where D is the distance between M and N. Thus, the hole is acting as if

it were a divergent lens placed in the plane N with its optical axis

coinciding with z'z and with a focal length 4D (1 + ((|)q/V) cos^aQ). The

term containing (jjQ describes the chromatic effect of such a lens, and

if 4)q/V«1 , it can be neglected in a first approximation. It is easy

to show that, just before entering the divergent lens, the trajectories

which are followed by ion§ which left point Aq look as if they were coming from a virtual point A situated at a j^i stance D behind M (fig.

4). Of course, the relation between A and A or B and 6 is the same for

any point of the surface.

The effect of the uniform field alone is to produce a virtual image

at a magnification of one. Further, an ion (4>o, ol^» i>o) leaving a point

Aq situated on z'z for instance, will look as if it came from A with the

energy V + c()q = V and its velocity will have the direction (a, ^Iq) where

S. is given by

sin = ^y-+ <t>^ sin a, or a ^^¥^rv~ sin (3)

35
Ion Collection Efficiency

Y M N

«l>o'°<o)

0

D

Figure 4 Approximate optics of collecting system: AAq - D/3,

Again, this kind of relation holds for any point on the surface.

Thus, the image looks as if it received ions from an illumin ating

aperture situated at infinity with an apparent angular diameter 2v%7V.

The divergent lens will act to bring the illuminating aperture from the

infinity to its focal point 0 situated at a distance 4D behind N, and it

will give an image A with a magnification 2/3 situated at a distance

40/ 3 behind N.

Finally, an ion {(t>Q^ olq » \1)q) leaving A(j_will look as if it came

from a point A, with a velocity direction (a, %): a= 3/2/<))p/V sin oq.

in addition, its trajectory will l ook as if it had passed through point

P in the focal plane, OP = 40/(|)o/V sin aoJ it can also be shown that

ions ((t)p, ap, ipq) leaving any point of the surface look as if they had

passed tnrougn point P. Since point P is characterized by the energy

and the initial angles of ejection, we will find the angular ana energy distribution in the focal plane of the divergent lens (fig. 5).

This situation is very similar to that encountered in electron

microscopy where the diffraction pattern is located in the focal plane

of the objective lens: here the diffraction pattern is replaced by an emission pattern .

CAq ^ 3D, AAq 0.

37
SIMS/IMMA

Figure 5 Three-dimensional views of ion trajectories, with the same energy and ejection direction. In the illuminating aperture

plane. OP = p, PP^= dp. dz = pdpd^i^oAs the image [A] and the illuminating aperture [0] (which is often

termed "cross-over") are virtual, it is necessary to add a lens or a combination of lenses (transfer optics) in order to obtain real images

of the target surface and the cross-over. These real images can be

apertured hy metal diaphragms which will limit the viewing field to the

diameter CB and the illuminating aperture to the diameter JK = 2a (fig.

6). The limitation applied on the cross-over diameter implies that, for

each energy (^q, the ions emitted with an angle ao greater than olq^^ are not collected:

a =^ 40/4.0/ 1 V+(})o) sin a^^ (4)

In other words, all th e ions emitted with an energy lower than (})om are collected: a =^ 4Dv^^jjj7V'.

The radius of the aberration spot around A is about

^a - 4>o/V)'^ sin ao(l - cos Uq). Because of the limitations on the

cross-over, will reach a maximum for (^q = ({jq,^ and = -n/Z. Thus

38
Ion Collection Efficiency

the resolving spatial limit is of the order of = (y <^om/^o) where y is

a dimension less coefficient between 1 arid 2 (1),

L

Figure 6 Objective lens; collecting optics coupled with a transfer

optics reduced to one electrostatic lens.

(3) Illumination of the virtual object

The virtual image [A] plays the part of a virtual object for the

transfer optics which will produce the real images actually observed.

It is therefore necessary to correlate the illumination of this virtual

object and the emission function fg ((|)q, aQ, \1)q).

Let us take a small area da surrounding the point A, and a solid

angle dJ2 around the direction (a, ^) (fig. 7). The number of ions

emitted with the initial energy (J)q in the range d^Qt as a thickness dz

is sputtered away, is given by:

= £((|)Q,a,\p) dJ2dad<t)^dz (5)

From the Lagrange-Helmhol tz relation (or the equivalent Abbe's

aplanatic sines relation) we know that:

sin a /da = +V sin a /3a (6)

39
SIMS/IMMA

and by differentiation against and rearranging the expressions we

get:

d) sin a cos a da da = fd) +V) sin a cos a dada ^O O 0 0 0 o

^ (7)

In addition, we know that^

Thus:

dfi da cosa

0 0 0

= fd) +V) dJ^da cos a

o ^^o ^ (8)

Quantities like dJ^dacos a measure what may be termed the

"geometrical extent" of the beam emerging from da in the solid angle! dn

around a direction making an angle a with respect to the normal to da.

dOo > COLLECTING M

da

OPTICS 1 0

Figure 7 Schematic diagram of the effects of the collecting optics.

Let us start with d^N^- ions emitted by dog in the solid angle dJ^Q

and in the energy range d(t>o (fig. 7). When these ions leave the

acceleration space, they look as if they were , emerging from a surface

area da, in a solid angle dfi and in an energy range d<j)Q. Therefore, we may deduce that:

£

o £

(9)

cosa^

o o

40
Ion Collection Efficiency

Now, we wish to introduce some averaging because the variations of

fo ('J'o' "^Q* ^o) with and may be very complicated.^ Let us take a function Fq {<Pq, Uq) such that (fig. 7):

Tr/2

I f i<^^yOL^,\l)^ sin a da = y F (d) ) (10) o

If the emission of secondary ions followed Lambert's law for a fixed value of ^q, fg would be equal to Fq cos ap. Although we know

that it is not in general the case, we shall make the approximation by

which Fq cos aQ replaces fQ; this approximation is valid as long as we

are only looking for integral contribution to ion currents. Along these

lines, we can make one step further and introduce a function Fq (4)q)

defined by:

O ^^0*^0 o ^^o-' (11)

Here again, we will make the approximation that Fq = T^,

This approximation and the former one are justified as long -as we are interested in the total illumination of the area doQ and not in the

details of the angular distribution of the ions.

In summary, the number d^N^- ions is given by the expressions:

d^N. = T^C<f)o) sin a^ cos a^ da^ d^p^ da^ d<p^ dz (12)

and

d^N. = ) sin a cos a da d^p da dcf) dz (13)

The expression for T {(^q) can be deduced in the following way:

f(4>o, aA) = -x-^ • ° ° ° ° COS a (14)

^o cosa

o V + 4 = -T-^ Fo^*o''''o^ (15) o

41
SIMS/IMMA

Thus, it is possible to put down:

^(<t>Q*OL,\p) = F((()^,i|^) COS a (16)

Then, by averaging according to the same procedure as previously:

27r F((J,^) =

2TT

FCflJo,^) (17)

It should be noted that

V + (J)

- Because of the limitation of the illuminating aperture at a diameter

2a, the angle cannot be greater than

*o ^ *om ^^^^ /t'o/CV+cl)^) sin a^^^ = a/4D (19)

If «(>o < 'f'om' collected.

(4) Illumination of the illuminating aperture

The illuminating aperture, located in the plane [0], contains much

"imental information

emitted by an area

pIp. ^q) (fig. 5), where

experimental information on ion emission. Indeed, all the ions (<i)Q, a^,

emitted by an area Oq look as if they were emerging from a ^ point

p = OP = 4D A^/(Y+(t>^) • sin (20)

Near point P, over an area dz = pdpdij^Q, the ion density is given by

the expression:

9Z
Ion Collection Efficiency

Figure 8 The continuous line qives a polar graph of the emission

function OR = fo(<(5o» ^o) given % and d)n ; the broken

line gives a polar graph of the function OR' = Fo(<|3o» ^o)

cos Oq.

If, with proper transfer optics, we produce a real image of the

illuminating aperture, it becomes possible to have direct access to

''o^'i'o' "O' ^o^» ^^^^ ^° emission pattern of the sample.

(5) Transfer Optics

The purpose of the transfer optics is to allow a better fit between

the collecting optics and the mass spectrometer in order to meet

specific requirements. For instance, after the spectrometer has been

set to give a higher mass resolution, the transfer optics can be

adjusted so as to maintain the same useful yield but this can only be

achieved over a smaller analyzed field. At this stage, the size of the

primary beam has no importance at all since:

43
SIMS/IMMA

- the spatial resolving limit is determined by the objective aberrations.

- the analyzed field is apertured by diaphragm T'y^.

- the useful yield is determined by the magnification of the

illuminating pupil at the plane T'^.

From the illuminating aperture on, the target and the collecting

system will be replaced by the virtual object [A] and its illuminating

aperture [0]. The transfer optics has to transform the virtual objects

[A] and [0] into real images [A'] and [0']. It may consist of a single

electrostatic lens as shown in fig. 6,^ or be composed of several

lenses

.

TRANSFER

OPTICS

Figure 9 Schematic diagram of the general effects of the transfer

optics.

Let us examine the illumination of [A'] (fig. 9). Again, from the

Lagrange-Helmholtz (or aplanatic) relation, we may deduce that:

V sin a cos a da da = V sin a' cos a' da' da' (22)

but here, V = V since before and after the transfer optics, the

particles are moving in regions which are at the same potential.

Therefore, the quantity, sinacosadada is invariant, no matter how complicated the transfer optics is.

The geometrical extent of the beam

dU = sin a cos a dadij^da (23)

is also invariant since ^ = Tp'

.

In order to average the emission pattern, it is then possible to

define functions like f U , a', i/^'), F' ((j>., ip') and F' {<t>J) . And it

IS not difficult to show that:

44
Ion Collection Efficiency

V + (j)

If the transfer optics is reduced to one lens, the system formed by

the collecting and the transfer optics is called an immersion lens (or

emission lens) and it plays the part of an objective lens in a microscope.

III. INFLUENCE OF THE FILTERING OPTICS

We do not intend to enter into details; we just need to

characterize the filtering optics by features pertinent to our problem:

- The filtering optics will accept an ion beam with a given geometrical

extent AUg and let through ions in a given energy band pass A<^q.

- Since we are dealing with images, we choose a filtering optics which

has a cylindrical symmetry. Nevertheless, this point is not essential;

it would be possible to come to similar conclusions with any kind of

filtering optics.

Figure 10 Characterization of the geometrical parameters of the spectrometer .

45
SIMS/IMMA

^ The accepted beam is physically defined by two diaphragms and

J l\ aligned on the axis of the transfer optics and separated bf a distance hiz; hi and hz are the radii of T'^ and T'a respectively.

Usually, hi land hz are much smaller than hiz, so that « 1 (fig. 10).

At [A'], the number of ions arriving on da', in a s^olid angle dfi',

is:

d^N. ^ ^(<^J cos a' dfi' da' d* dz 10 (25)

^0

cosa' dfi' da' is the geometrical extent of the beam and (cIjq) dc^Q dz

is equivalent to brightness in light optics. With simple geometry, it

is easy to show that any plane perpendicular to the axis will have the same brightness F' (4)^) dcfjQ dz. Therefore, if [0'] and [A'] are inot,

for instance, in coincidence with T'^ and T^y^, the total number of

accepted ions will not be reduced. However, once more, using simple

geometrical arguments, one can see that there are some inconveniences to

a situation where [0'] and [A'] do not coincide with T'c and T^^. On

figure 11 which represents one among many possible situations,

essentially two remarks are to be made:

IMAGE PLANE

Figure 11 "Shadow effects." In the image plane, the hatched lines

show the "half-light" region, the thick continuous line

shows the "full-shade" region, the light continuous line

shows the "full -light" region. In the illuminating aperture

plane, the thick continuous line gives the part of the

illuminating aperture which contributes to the illumination

of a given point in the "full-light" region.

46
Ion Collection Efficiency

- Part of the analyzed area will be in "full light," another part will

be in "half light" (hatched regions on the figure); the illumination of

this latter region continuously decreases towards the edges. Thus,

localized analysis should be restricted to the fully illuminated region.

- Different points of the analyzed field in the fully illuminated region

will receive ions which are not coming from exactly the same location

inside the illuminating aperture. Therefore, the analysis of each

micro-area is not made with exactly the same type of ions.

Henceforth, we shall assume that the transfer optics is adjusted in

such a way that [0'] and [A'] coincide with T'^ and T'/\ respectively.^

Let us compute the number d^N^ of ions accepted by the spectrometer

in the energy range d(j)Q, Since a' is small the total geometrical extent

of the beam accepted by the spectrometer is 2 2 AU = .2 (26)

s , 2

d^N- = P" Ccl)^) ^"s- ^^^^

IV. THE USEFUL ViELD

(1) Definition

The destructive nature of the analysis implies the necessity to

sputter a given volume of material in order to get (M+) ions of type

w to the detector. During the sputtering process, a volume v has been

removed, which contained Nq atoms among which one could find

N(M) = C(M) atoms of the element M, where C(M) is the atomic

concentration.

There is a close connection between the geometrical localization

(the volume being analyzed) and the sensitivity limit^ (being defined as the smallest concentration detectable with a given precision in a given

volume). Let us define a "useful yield":

Ni cm") (28)

T = ^ N (M)

To achieve a precision of p/10^ in a measurement, it is necessary

to produce a number N-j ions at least equal to 10'*/p^ and therefore to

sputter away a volume v = (1/nC) (10'*/p^Ty) where n stands for the number

of atoms per unit volume.

It should be noted that the shape of the volume is left unspecified

so that the arguments given here are valid for micro-areas and for indepth analysis as well

.

47
Ion Collection Efficiency

It is clear that there is an imperative need for getting the best

yield possible. This yield will depend upon basic ionization and

sputtering processes which control the emission function fp ((j)Q, aQ, %)

of M"*" ions, but will also depend on instrumental parameters.

(2) Influence of the transfer optics on the useful yield

Because of the coincidence between T^c, T';^ and [0'], [A'], the

illuminating aperture [0] and the analyzed field area of the target will

be respectively restricted to diameters 2a and dg {oq = 7TdQ/4). The

number of ions accepted by the spectrometer in the range dcf)Q is:

2 V .

(^5'

Further, from the Lagrange-Helmholtz relation integrated over the

geometrical extent of the beam AUg, we may deduce:

TT (j) sin^a a = (Y + (b )AU V AU (30) 0 cm o ^ ^Q-^ s s

We have to examine two cases:

2 ^ ^om ; *om - V, a - 4D /TTY sin a^^ (31)

16

AU3 =

TT<|) V

o . 2 — sm a a om o

2

ira a

16 D

(32)

48
SIMS/IMMA

1

TTQ^ sin^a F (<b )dd) dz = —

, 2 (33)

160"^ 0 0 0 o

4)

TT • aJ^(<^J d(j,^ dz

O O^'^O'^ ^o (34)

(35)

d^N. = 7ra^ F~ ((|)^) d<\,^ dz

1 0 0 ^^o ^o (36)

Thus, in principle, it is possible to compute dN^-:

dN. = TTO dz

o

om

o^^o"^ ^o om

(i) +A(t) ^om

om

o ^^o^

d<|) (37)

Since the volume QqCIz contains n.C(M) Oq dz atoms M, we get the

following expression:

T = K " nC

(38)

49
Ion Collection Efficiency

where:

r^om

K„ = TT om

F (d) )d(|) +

6 +A(i) o ^^o^

d({)

(39)

om

For a given sample (where n and C are known), the useful yield can

be obtained experimentally by measuring the number of ions M"*" produced

by the sputtering of a layer of thickness z. The expressions of xy and

dN^- are useful if one wishes to evaluate how these quantities are evolving when -instrumental parameters are changed as we shall see a little further.

in other respects, it should be noticed that the diaphragms T'£ and

T'y^ do not exactly play the same role:

- Ty does not change if the diameter of T'/^ is reduced (that is, if the

analyzed field is reduced) since N-j and Nq are reduced in the same proportion.

- T^J decreases if the diameter of T'^ is reduced (that is, if the

diameter of the illuminating aperture is reduced). But we have to keep

in mind that if some emission anisotropy occurs, the illumination of the

diaphragm T'e will not be uniform and consequently the number of ions of

a given species accepted by the spectrometer may not be proportional to

the open area of T'^.

For a given experimental setup* as far as emission anisotropy can

be neglected, is a well defined number essential to determine what

performance one should expect. Let us tafte an example and put in some

numbers.

A tantalum oxide film formed by anodic oxidation in an aqueous

solution of ammonium citrate is bombarded with an argon beam. The

measured useful yield of Ta"*" ions is about 2 • 10"^ The area analyzed

with this particular instrument [2] is 1 .25x10^"!^ • Since n is known,

it is easy to compute that for a precision of ± 5%, a layer of at least

100 A thick must be sputtered away in order to detect an impurity

which would have the same x^, as tantalum and be present in the

concentration range of 0.1 ppm. But, as already mentioned, besides the

instrumental factors, the ionization processes play an important role in

the values obtained for Xl,. For instance, x^,(Ta+) is 10^ times smaller

when pure tantalum is bombarded with argon ions, than when TaaOs is

bombarded under the same conditions.

50
SIMS/IMMA

Let Us make some additional remarks:

- We have not taken into account the chromatic aberrations given by the

transfer optics which might slightly change the former results.

- Some care . should be taken to make sure that no diaphragms other than

T'e and T'/\ are stopping the ions supposed to be accepted by the

spectrometer (electrode N for instance).

- Ty is dependent on the "brightness" {y/<PoWo{<t>o) seen by the

spectrometer, on AU^ and on (A(})q/V). FqC^q) is ' related to the

ionization processes so that when everything possible has been done to

increase the ionization efficiency (bombardment with reactive primaries,

surface reactions, . . .), the only thing which remains to be done is to

increase the accelerating voltage V. We already know that, at a given

voltage V, all ions of which the energies are (j)^ < <^Q^t are collected;

consequently a further increase of V will not change the collection

efficiency for these ions s_ince it is already 100%; it will just

increase the brightness (V/4)q)Fq((J)q) . This increase in brightness is

not followed by an increase in ion current because the ions <^q ^ <^Qm are now filling a beam which has geometrical extent smaller than AU^ allowed

by the spectrometer. This is another way of saying that all these ions are already completely collected, which incidently shows that if for

some reasons we wish to discard the ions (})o^4>om' it would be possible to

improve the mass resolution without reducing the sensitivity obtained

with the ions (|)p < <^q^. Furthermore, any increase of V will also

increase (})om. so that ions with higher energies will now be collected

with 100% efficiency. For those ions which are only partially collected

because their energies are still higher than the new setting of (|)om>

' an increase of V has the effect of increasing the solid angle of emission

in which they are collected: Their total contribution to the secondary

ion current, provided < A^q, is increased. From the "spectrometer

point of view," this increase is due to a higher brightness combined

with a complete filling of the diaphragms T'e and T';\; there is also, to

a lesser degree, some contribution of the band pass since our assumption

is that the ratio A(|)q/V only is fixed.

It is worthwhile to note that the contribution of ions like ()>o >

cf)r)ni in a range 6<t)Q is proportional to the voltage V, but this is a direct consequence of using a Lambert's law to average the emission

function. Later on, we will come back to these points.

V. SOME CONSEQUENCES

A. Bombarding Modes

Until now, none of our arguments about and the image

illumination included the bombarding mode of the sample. Thence, it is

obvious that the results are valid for both a broad uniform primary beam

and a localized spot of primary ions (probe) which may eventually be

scanned over the field to be analyzed.

51
Ion Collection Efficiency

However, between the two bombarding modes, a fundamental difference

appears when one asks for local analysis on the micrometer scale:

- With a broad primary beam, an adequately apertured objective lens is

necessary in order to obtain images with the required resolving limit.

- With a narrow primary beam, the objective lens plays the part of a collecting system only, and the resolving limit is entirely depending on

the size of the primary spot. ^

r »

(1) Broad primary beam

The resolving limit is on the order of Z = ((j)om/V) D- From the

.expression for Xy it is easy to deduce that any improvement of the

spatial resolution will cost a decrease of xy. Moreover, the relative

band pass A(|)o/V of the spectrometer cannot be too large because of the

chromatic aberrations of the objective lens.

Let us look for the relationships existing between I and Xy. We

already know that I and Xy depend on cfj^ and that, through T'e^, <Pq^

itself depends on the radius "a" of the illuminating aperture [0]. Let

(xy)^ be the useful yield for a given "a".

nC V 16D

Thus:

(Tu)^ = — .K^ = • H (41) ^ ^ nCD lenCD'^

When "a" is changed into "a"', I and (xy)^ will be changed into I' and (xy)^' respectively, and it is easy to see that:

(42)

If we just need a rule of thumb, we may assume to a first (and very

crude) approximation that does not depend on (j)om'. (Ty)ji -i^/i') {tu)p,' Therefore, with only one experimental measuremejit

of (xy)^' we are able to get an estimate of (xy)j^ for any value of a.^°

52
SIMS/IMMA

Now, let l^z be the minimum volume that must be sputtered in order

to obtain a measurement with a given statistical fluctuation ± p/100:

nC p^(T^) P VK^

If we make use of the former approximation and choose I' = lym,

(tu)^' = (tu)i; then:

nC (Tj^

where all the lengths are in micrometers. According to this expression,

we can estimate the minimum thickness required to make an analysis over

an area with a given precision. We can also estimate the "ultimate

resolving limit"^^ which will be reached when z equals I:

1 10

7

(45)

It is remarkable that i depends on (xy): (measured when the optics

is adjusted so as to give il = 1 ym) according to a power (-1/4) only.

It suggests 'that even wide changes in the useful yield from one element

to another should not affect too much the ultimate resolving limit.

(2) Localized bombardment

Localized analysis can also be obtained by reducing the size of the

bombarding beam to a small spot. For instance, let us imagine that a probe of one micrometer diameter strikes the sample and that the optics

is adjusted in such a way that the emission lens gives an image with a resolving limit of one micrometer. As the spatial resolving limit does

not depend on the chromatic aberrations of the objective lens, it is

possible to take a larger band pass (A4)q)s. The increase of then

comes from the increase of since the total current of ions NT*" is:

^^i = *OHiK£^o (46)

It is well known that FQ{(j)p) presents a maximum around several

electron-volts and depends on elements and ionization processes. Let us take an example which is known to favor the emission of high energy

versus low energy ions: a polycrystalline nickel sample bombarded by

53
Ion Collection Efficiency

argon ions. The angle of incidence is about 50°, the energy of the

primaries when they strike the target is 6.2 keV, the secondary Ni"*" ions are accelerated at 3.8 kV and the objective lens is set to collect all

the ions up to (pQm approximately equal to 0.7 eV. From the energy

distribution curve (3) it is easy to deduce the following table of the

increments of 0/-n){(t>om ^a) where the arbitrary value 100 has been given

to i^/n){<^om h) fo"" A*o = 200 eV:

*0 , 10 20 40 60 100 200

A

IT 50 15 11.5 10 5 4.2 4.3

From the above table one can see that an energy band A())q as small as 10 eV would contribute 65 percent to the total current that

could be obtained with a band pass of 200 eV. Of course, if for some

reason or another the low energy ions are discriminated, the relative

contribution of high energy ions is more important and the total current

is more sensitive to the width of the band pass.

Attention should be paid to the fact that, in general, the previous

numbers are only rough estimates because Fo(io) depend on many

factors. But, as far as ionization processes are concerned, we think

that when "chemical emission"^^ is involved, these numbers underestimate

the contribution of low energy ions.

VI. PLAYING WITH THE TRANSFER OPTICS

(1) Increasing

Let us start with an accelerating voltage V and a spectrometer

characterized by A^iq/V and AU^, which lead to a given mass resolving

power M/AM. For given adjustments of the transfer optics, let us assume

that we have a, £, M/AM and do fixed.

Now, just by changing the adjustments of the transfer optics, we

may obtain (since AUg = TTa^ao/64 D^):

a' = k a, £' = £,

^ , d' = ^ , = (47)

' * AM ' k ' ^ om om

where k is a dimensionless factor.

We have to find out what is the value of (K^)' in order to estimate

the increase of xy. Again, this estimate can only be obtained through a numerical integration of Fo(<t>o) and Fo((|)o)/<|>o- But we can understand

54 •
SIMS/IMMA

the physical reason which leads to an increase of 1^: the secondary

ions are collected in a greater emission solid angle. As k increases,

(j)'Qm increases and consequently the energy range in which ions are collected with 100 percent efficiency becomes larger. A further

increase of k will widen the energy band of ions collected with 100

percent efficiency and increase the emission solid angle of ions which

are only partly collected.

Let us try to make an estimate on the example already given . The transfer optics is now set to allow ions with <^qj^ =10 eV

to be completely collected. (With that specific instrument used, it

would lead to il' = 10 ym and do - 65 ym if we had started with

^om " ^-^ ^ " Vim and dp - 250 ym.) From tne energy distribution

curve corrected for Lambert's law, the following table can be drawn up

(with the same convention as previously) . ^

*0 10 20 40 60 100 200

A

7T 41 21 16 6.5 7 8.5

The ratio between (^^^ and (j)om is close to 8 (botfi measured in

the range A(()o = 200 eV) . Thence, t^y - 8 when k^ - 14.? , which gives

a ratio dN'i/dNi - 0.56 because the analyzed field area is reduced by a factor l/k^.^** But again, this is just an example to show how the

things work and from one situation to another the results may be

somewhat different.

As we are seeking higher useful yields ^ on one hand the size of the

analyzed field is reduced (d'=dQ/k) and on the other hand the spatial

resolving limit of the objective lens is deterioratea (r=k^l). If a resolving limit lower than 1' is needed, t'len we must reduce the size of

the primary beam and match it (as far as possible) to the localization

we wish to get. But it should be noted that, because the size of the

analyzed field is now reduced, an image over a greater area can only be

obtained if the sample is mechanically scanned or if the primary beam is

rastered over the sample and the ion image scanned synchronously in

front of the diaphragm T'/\, The objective lens will ttien essentially

play the part of a collecting system. If we keep increasing k, 1^ will

keep increasing but it will be less and less rewarding because ^o{(Pq)

decreases with d'g and because the chromatic aberrations will lower the

expected gain. The limit will be reached as 1' becomes of the order of

d*.

However, before we run into such difficulties, there is room for

improvement, and for this purpose the ion microscope optics coupled with

a transfer optics is a tool which seems very adequate. The adequacy

partly is due to the fact that it should not be difficult to switch from

an ion image with a spatial resolving limit better than one micrometer,

55
Ion Collection Efficiency

and well adapted for surveying the sample, to the measurement of

secondary ions coming from a selected area, with the best possible

adjustment of the transfer optics giving the highest consist nt with

the specific analytical problem treated. Of course, depending on the

size of the analyzed field area, the reduction of the spatial resolving

power^^ may or may not require a localized primary beam (the positionof

the probe could be adjusted and observed directly on the ion image for

instance). But in all these questions, qne should keep in mind that

there is a close connection between spatial localization, sensitivity

and concentration.

Another point has to be considered. For a given spectrometer and

collecting geometry, there must be a given analyzed field area Oq.^^ As

a layer of thickness z is sputtered, this area will produce a given

number N-j of ions K*" (at a fixed voltage V). We. have shown that an increase of Ty leads to a decrease (in the example previously cited) or

let us say at a stationary value of N-j. However, since low energy and

higher energy ions are differently affected, it might happen that the

ratio of the emission currents of two different elements would

significantly depend on the setting of the transfer optics which

controls (})om- But at the present stage, such guesses (although not

completely irrelevant) need more experimental support. However, if we

are in. a case where emission anisotropy is significant, it looks

worthwhile to collect the emission 1n as large a solid angle as possible

in order to improve the reproducibility of the measurements.

(2) Improving mass resolution over small areas

To improve the mass resolution, we have to reduce the entrance and

the aperture diaphragm T'^ and T'a. This will reduce AUs, the

geometrical extent of the beam accepted by the spectrometer.^®

To improve the mass resolution by a factor of X, we must divide hj^

bv X and ha by /k (because the angular aberrations are depending on hl).^^ The geometrical extent becomes AUg = (1/X^)(AU5). Now, let us adjust the transfer optics in order to keep the same value for "a," that

is the same xu- Since AUg = (7ra^aA/64 D^), this implies that the

new diameter of the field is d' = Aq/Xx^^^).

Thus, any improvement of the mass resolution which maintains

unchanged xn, will reduce the analyzed field. Inside this field, the

resolving limit will also remain constant. However, the total signal

will be reduced by a factor X^ since a' = Qq/X^.

Here, we reach the interesting conclusion: wherever a small area

has to be analyzed with a high mass resolution, the transfer optics may

be adjusted to fulfill this requirement with no loss of signal provided

a' is greater than the analyzed area.

Of course, it is also possible to adjust the transfer optics in

order to combine the effects described in (1) and (2) this section and

obtain a higher Tu and a better mass resolving power over a reduced

56
SIMS/IMMA

area. But in this case, according to the analytical requirements

(localization, concentration, sensitivity . . .), the use of a probe

(coupled with an ion microscope type optics) may or may not ultimately

become desirable.

In order to make an estimate, let us start with the conditions

already mentioned, il = 1 ym, do = 270 ym, M/AM - 330 and t^. If we choose X = 9 and if we make the proper adjustments, we can get d' ~ 10

ym, (M/AM)' ^ 3000 and leave £ and unchanged. Therefore, if on a grain of 10 ym, we had to perform either trace analysis or isotopic

ratio measurements and if on both we had to avoid mass interferences, it

would be possible to do it without reducing Xy, that is vithout losing

sensitivity (on this specific grain), and still keep a spatial resolving

limit of the order of 1 ym. Thus, generally, the transfer optics allows

one to maintain the same collection efficiency at high mass resolution

as that obtained at lower mass resolution, but at the cost of a reduction in the field of view.

Furthermore, as already mentioned, .if we accept to reduce the

analyzed field, we may try to increase xy. For instance if we play with

the factor k and take k' = k*^, then d" ^ 7 ym, a" - 2 ym, xiJ - 2tu'> (M/AM)' stays unchanged. Taking into account the aberrations of tne

objective lens, we may then expect that the ions emitted from an area of

5 ym in diameter are collected with a 100 percent efficiency up to 2 eV

(the contribution of -ions over 2 eV being dimmed by a factor 2/(pQ

instead of l/<J)o) and the mass resolving power is still of the order of

3000.

Once more, these numbers are given here just to show how the things

work and to show with practical examples that what is really needed, in

local analysis, is to have an instrument that one could adjust to his

specific problem.

VII. CONCLUSION

Looking at the collection efficiency problem from a very general

point of v'ew, we have discussed the importance of the transfer optics

and of the different modes of bombardment.

However, one point has been left to discussion: the information

that might be present in an ion microscope. Let us imagine that we are looking at viewing field of 250 ym in diameter with a spatial resolution

of one micrometer. The image contains 5.10** points of information of a square micrometer. To record such an image on a photographic plate with

an optical density d =0.7 will take p time t (seconds) and will require

from each square micrometer 1000^° (which already has ti statistical

fluctuation^ of ±3 percent). In order to get 5.10'* points of information

with the same statistics using a one micrometer probe with the same primary density as the former bombardment, would require a time tj

= 5.10^ t [xu/(xu)sl. If, as we have already discussed^ the ratio of the

useful yields in both situations is about 3/5, then tg - 3.10"* t. Even

if something was wrong by a factor of ten, in our estimates, tg would

$7
Ion Collection Efficiency

only be 3.10^ time^ higher than t. Therefore, one second exposure time

with an ion microscope would have to be compared to nearly one hour with

a scanning probe. Moreover, with "channel plates," it will probably be

possible to gain at least a factor of 10 on the exposure times. Then,

each square micrometer will have to release 40 ions to produce the same

optical density (thus the statistical fluctuation limit will soon be

reached).

This example was given to show why a microscope is so useful: it

allows a very easy and fast survey work on a sample. This way, many

unexpected features can be observed, saving a lot of time and preparing

the path for more precise measurements if necessary. That means that

when the precision of densitometry measurements on the ion micrographs

is not suitable for a given problem, any specific well located and

limited area can be apertured at the ion image (or at the converted

electron image) and the ion currents emitted^ by this area can be

directly recorded. Particularly, from what has been set forth

throughout this paper, no reason appears that would prevent'- the

measurements made with the collecting optics of an ion microscope to be

less suitable for subsequent operation to render the analysis

quantitative. On the contrary, having a better (although approximate)

knowledge of the categories of collected ions, it should be possible to

investigate more deeply the influence that the instrumental factors

undoubtedly have on the quantitation tests.

Moreover, we have shown how the use of a probe could benefit from

the optics of the ion microscope. So , we don't see any basic

contradiction between "ion microscopy" and "ion probing," provided each

one is used properly in order to meet the particular analytical

requirements. In that respect, the transfer optics plays a key role and

it should bring a great flexibility into the analytical practical

operations, and allow more sophisticated analysis, such as for instance

trace analysis, avoiding interference problems on small grains, or maybe

precise measurements of isotopic ratios on small grains.

VIII. REFERENCES

[1] Zworykin, V.K., Morton, G. A., Ramberg, E. 6., Hillier, J., and A.

W. Vance, Electron Optics and the Electron Microscope, Wiley, New

York (1949).

Grivet, P., Electron Optics, Pergamon Press (Oxford 1965).

Magnan, C, Traite de Microscopie Electronique (Vol. I) Hermann

Paris (1961).

Slodzian, G., Ann. Phys. (Paris) 9 (1964) 591.

[2] Hernandez, R., Lanusse, P., Slodzian, G. and Vidal, R., Recherche

Aerospatiale (Paris) 6 (1972) 313.

[3] Blaise, G. and Slodzian, G.. Rev. Phys. Appl . (Paris) 8 (1973) 247.

58
SIMS/IMMA

Footnotes

1. The emission processes prevent this from being true. These

processes will not be discussed in this paper.

2. The optics of filtering systems is too involved to be discussed

here.

3. It could be M" or any other kind of ion.

4. Of course, the higher the accelerating voltage V the better the

approximation. But it should be mentioned that some workers are using a field-free region followed by an acceleration space. In this case, if V and D are still the acceleration voltage and

distance; the change in slope Ar' is of the order of

(r/4D)(V/(|)QCos^ao) as the particle enters the acceleration space.

Here, it is a convergent effect which might introduce some energy

discrimination essentially at low energies.

5. This statement is obvious for meridian trajectories like, for

instance, trajectories leaving point A. For ions coming from

points such as B and following skew trajectories (fig. 5), the

statement remains true provided the azimuthal angle is measured in

a frame of reference where the z'z axis is now placed along 08.

Anyway, whatever the emission point, all the ions an, ^q) are coming from the point P of the illumination aperture (fig. 5)

(aberrations being neglected).

6. The complications come from emission anisotropy: sometimes surface

relief effects could also play some part.

7. Figure 6 is included just as an example; particularly the relative

position of [0'] and [A'] could be inverted if necessary.

8. It should be noted that the relative positions of [0] and [A']

could as well be inverted.

9. The term "sensitivity limit" here defined is an ultimate sensitivity which could only be achieved if one had a perfect

detector and if there were no "background ions" or other spurious

effects of that kind. What is termed "detectability" is the

practical sensitivity actually achieved with a given instrument on

a given sample for a given element or isotope.

10. This approximation is acceptable if (j>gn, « A4)q, provided that the

contribution of FqUq) to the integrals is small at very low

energies and thence if we are looking for situations where a < a'.

11. It was already mentioned that the spatial resolving limit Jig is not

equal to I but the coefficient u is difficult to estimate because

it depends partly on Fo((j)o). Moreover, it should not be forgotten

59
SIMS/IMMA

that we are here dealing with approximate trajectories. Anyway,

introducing £a, the expressions here computed would become:

and:

.4 . . £

4

1 • . ,

2 ^2 ^ a (49)

Even if y were of the order of 2, the "ultimate spatial revolving

limit" would only be multiplied by a factor of y/2: £3 l^/2.

12. "Chemical emission": emission process occurring either when the

sample is bombarded with reactive primary ions, when a reactive gas

is blown on the surface during the bombardment, or when an oxide, a halide or any other ionic compound is sputtered.

13. The experimental procedure may introduce some systematic errors.

Indeed, what is measured is the contribution of (^q energy ions in

the range 6(^q emitted in a^ solid angle 2 tt(1 - cos aom) so that the

averaging which leads to Fq((J)o) actually takes into account the

values of fo(4)o» ^o* '^o) ^0 included in the range (0, aom)

instead of (0, 7r/2).

14. The ratio dNj/dNi is only relevant for thin-film analysis. Of

course, for an area < a', the sputtering of a given thickness z would lead to a ratio H\/H^ = {T^^)*/T^^.

15. It could also very well be that, for a specific problem, what is

rather needed is a better resolving power whatever value takes

and this could also be achieved with such an experimental setup.

16. Which can eventually be extended by proper scanning methods as we

have already mentioned.

17. It can be seen from the two tables given in this paper.

18. Eventually, A<^q also would have to be slightly reduced. But we

shall not examine here which effect it may have on t^. It will be

a small effect anyway.

19. In fact, the real situation is more complicated because the

complete configuration of second order aberrations of the

spectrometer has to be taken into account. Particularly, since the

filtering optics is not Gaussian (although it may be stigmatic) the

aberrations do not have cylindrical symmetry. Therefore, the shape

of the diaphragms (round hole versus rectangular slits) have to be

adjusted properly. Only a detailed study of these aberrations can 60
Ion Collection Efficiency

help to determine the best aperturing which will lead to a given

improvement in mass resolution and to the best useful yield. The

estimates made here show that such a goal is attainable.

These experimental numbers are obtained in the following way: The

ion currents emitted from an area of 60 ym in diameter are measured

directly after a proper aperturing at the ion image. Then,

micrographs are taken with- different exposure times. From optical

density measurements on each micrograph it is easy to draw the

calibration curve—optical density versus number of ions emitted

per square micrometer. It is worthwhile to note that if we had to

get an image with an optical density d » 0.7 from a one square micrometer area containing an element at the atomic concentration of

1 percent and presenting a useful yield = 10"^, it would require

the removal of 10® atoms. Such a number would correspond to the

sputtering of a layer thickness of approximately 20A. If the primary beam density was set to give a sputtering rate of loVsec, it

should take 2 seconds to record the image of the given area and to

get, with no extra effort, the images of 5 x 10** adjacent areas of

one micrometer square. Moreover, from this example it can be seen

that the only influence of the sputtering rate is to determine the

exposure time.

61

NBS Spec. Publ. 427, Workshop on Secondary Ion Mass Spectrometry (SIMS) and Ion Microprobe

Analysis held at NBS, Go Ithersburg, Maryland, September 16-18, 1974 (Issued October, 1975).

HIGH MASS RESOLUTION SECONDARY ION MASS SPECTROMETRY

Peter Williams and C. A. Evans, Jr.

Materials Research Laboratory

University of Illinois

Urbana, IL 61801 , U.S.A.

Ion bombardment of a solid surface produces a variety of secondary

ionic species. In addition to the analytically important singly charged

monatomic ions, polyatomic and multiply charged ions are produced which

can complicate the qualitative and quantitative interpretation of

secondary ion mass spectra. The polyatomic ions can result from the

ionization of hydrocarbons or other residual gases, either in or on the

sample, from clusters of the matrix atoms, and from clusters formed by

the bombarding ion with the matrix atoms. Such ions can occur at the same nominal mass as the ions of analytical interest, causing a mass

spectral interference.

A variety of techniques has been used to circumvent the mass

spectral interference problem. Good vacuum and sample handling

techniques can reduce the hydrocarbon levels. Discrimination against

ions of low initial kinetic energy can improve atomic to molecular ion

ratios, although this technique reduces the intensity of both [1].

Other techniques to reduce or eliminate cluster formation at the mass of

interest include changing the mass and chemical nature of the primary

ion or the polarity of the secondary ions. Alternatively, the mass spectrum can be treated as the superimposition of spectra from a multi component mixture. If the individual spectrum of each component is

known and the combined spectrum contains at least N peaks for an Ncomponent mixture, the contribution of the major components may be

obtained by standard mathematical techniques,. A limitation of this last

technique for low abundance components is the statistical noise on the

multi component peaks.

The one common feature of all the above techniques is that they

leave a degree of ambiguity in the analysis. Thus none of these

techniques allow the analyst to state with certainty the identity of an

ion. He can only state that the contributions of possible interferences

have been reduced.

High mass resolution techniques have been used in combination with

both large-beam [2,3] and micro-beam [4] ion probes to improve the

analytical capabilities of SIMS. Two significant benefits result from

the use of high mass resolution. The first is the obvious separation.
SIMS/IMMA

in a number of cases, of the analytical ion from the interfering species

permitting accurate and unambiguous intensity measurement. Second, and more important, high mass resolution mass spectrometers permit precise

mass determination of the secondary ions and thus unequivocal

qualitative identification of the ion species. The value of this aspect

of high resolution cannot be overemphasized since there are generally

several possible atomic compositions for ions at a given mass number,

and precise mass determination is the only technique capable of making

an unambiguous identification.

Resolving Power (10% valley)

Fig. 1 Variation of transmission with resolving power on AEI-IM20 ion

microprobe.

64
High Mass Resolution SIMS

To date only modified spark-source mass spectrometers have been

used for high resolution SIMS [2-4]. Such instruments have smaller

angular (a) acceptance than the specially designed, low resolution

microprobe [5] and have no z-focusing (where z is the long axis of the

entrance slit) in the mass spectrometer. However, the use of high

secondary ion accelerating potentials and the proper phasematching

optics in the ion source makes the transmission of these instruments,

used at low resolution, comparable in theory and practice with the high

transmission, low resolution microprobe. Transmission decreases, of

course, as resolution is increased -- approximately linearly for a wellfocused mass spectrometer. The transmission-resolution behavior of the

AEI-IM20 ion microprobe at the University of Illinois is shown in figure

1.

High Mass Resolution Spectrum

of Lobradorite

Moss (omul

Fig. 2 Labradorite sample -- Potassium region.

At the lowest resolving power attainable with this instrument, the

sensitivity for ^^Fe from an iron sample is 8 x 10^ counts/sec/nA of

primary beam. This corresponds to a measured transmission (resolved ion

current/secondary ions produced) of 1 percent. The loss of transmission

at high resolution is offset to some degree by the higher abundance

sensitivity (fewer ions scattered from neighboring peaks into the

analytical peak) of a mass spectrometer operated at high resolution.

65
SIMS/IMMA

Mass 55

Mass 56

LABRADORITE (IRON REGION)

Fe = .12 at%

Res. Power 3000 (10% Valley)

Mass 57

Fig. 3 Labradorite sample -- Iron region. Mass assignments of the

polymer species to within ±1 millimass unit (±18 ppm) were

made from direct measurement of peak position from the chart.

Exact masses of monomer species and hydrocarbons were used for

calibration.

Two modes of usage of high mass resolution ion microprobe mass spectrometry are shown in figure 2 and 3. These show spectra taken from

Lake County Oregon Labradorite using the AEI-IM20 ion microprobe. High mass resolution may be used simply to check on the "purity" of specific

peaks --to check, for example, whether the vacuum techniques used have

adequately suppressed hydrocarbon interferences, or whether peaks due to

66
High Mass Resolution SIMS

monoisotopic species are interference-free. (In^terferences in the

peaks of multi-isotopic species can frequently be detected through

anomalous isotope ratios.) Figure 2 shows this mode of usage. Here the

problem was to determine whether the ^^K"^ peak was significantly

contaminated with ^^Na^^O"*". Since little ^^Na^^O"*" was present, low

resolution measurements would suffice to determine ^^K"*" abundance.

A more complex situation is shown in figure 3. Here the ions of

interest are iron and manganese, and it may be seen that not only the

manganese peak, but also all the iron isotope peaks are masked by

interferences. A resolving power (R.P.) of 3000 (10 percent valley)

permits the separation, qualitative identification (via precise mass

(a) (b)

RP 7740 RP 4350

X30

CaO

56,Fe

High Resolution SIMS

Labradorite (Fe = .12at%)

Fig. 4 High resolution performance of AEI-IM20 microprobe.

67
SIMS/IMMA

determination) and quantitative determination of Fe and Mn in the

presence of interfering polymeric species. Simple measurement directly

from the chart recording is adequate to determine masses to within about

1 millimass unit in a multiplet. The assignment of precise masses then

allows the identification of the ion species. Although they act as interferences at low resolution, the hydrocarbon peaks become invaluable

mass markers at high resolution. Judicious use of isotopic abundance

data can aid identification. Thus ^^Si2 and '^''Ca^^O would not be

resolved, but it may be estimated that ^^SU is less than about 10

percent of the CaO signal from the absence of ^^Si^^Si at mass 57 where

it would be almost resolved from CaOH. ^^Mn is estimated to be about 40

ppma (by comparison with ^^Fe). Note that ^^Fe (20 ppma) is clearly

resolved and detectable (at about 20 counts/second). A detection limit

for iron in this sample is estimated to be 10 ppma. This limit is

dictated by the large CaO interference and the magnitude of the

secondary signal

.

That sensitivity is still respectable at the highest resolving

power of the mass spectrometer is demonstrated by the mass spectrum of

figure 4a, taken on the same sample. ^^Fe (1000 ppma) is still

detectable {^20 counts/second) at 7740 R.P. (10 percent valley). If the

collector slit width is increased, without increasing the source slit

width, essentially flat-topped peaks are obtained at 4300 R.P.

facilitating ion counting on the top of a peak for accurate quantitative

measurements or isotope ratio determinations (fig. 4b).

REFERENCES

1. Herzog, R. F. K., Poschenrieder, V. P., and Satkiewicz, F. G., GCA

"Corp., Bedford, Mass., for NASA, NASA-Cr89326 (N67-39205) (1967).

2. Hernandez, R., Lanusse, P., Slodzian, G., and Vidal, G., La

Recherche Aerospatiale No. 1972-6, 313 (1972).

3. Blattner, R. ^1., Baker, J. A., and Evans, C. A., Jr., Anal. Chem.

46, 2171 (1974).

4. Bakale, D. K. , Colby, B. N., and Evans, C. A., Jr., "High Mass

Resolution Ion Microprobe Mass Spectrometry of Complex Matrices,"

Anal. Chem. (accepted for publication).

5. Liebl, H. J., J. Appl . Phys. 38, 5277 (1967).

This research was supported in part by the National Science Foundation

under Grants DMR-7203026 and GP-43220X.

68
NBS Spec. Publ. 427, Workshop on Secondary Ion Mass Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Ga ithers burg, Maryland, September 16-18, 1974 (Issued October, 1975).

A COMPARISON OF MASS SPECTRA FROM

THREE ION PROBES

James G. Bradley*., Dominique Y. Jerome**,

and Charles A. Evans***

*Charles Arms Laboratory

Division of Geologic and Planetary Sciences

California Institute of Technology

Pasadena, California 91125

**Laboratoire de Mineralogie et de Cristal lographie

Associg au CNRS

University de Paris VI 4 Place Jussieu

75230 Paris-Cedix 05, FRANCE

***Materials Research Laboratory

University of Illinois

Urbana, IL 61801 , U.S.A.

An intercomparison of the major element mass spectra from an Applied Research Laboratories' Ion Microprobe Mass Analyzer (ARL), the

Cameca Ion Analyzer IMS-300 (Cameca) and the AEI Scientific Apparatus

MS-702 Ion Probe (AEI) has been made using a natural mineral [1]. The

comparisons are on the basis of the relative detected intensities of

"Na"^, 2^A1 + , ^^Si+ and '*°Ca+ secondary ions (Si's). The work was done with the intent of determining the feasibility of doing quantitative analyses of mineral samples for the major and minor cations on the three machines and to determine the extent to which the three

instruments provide a common result that can be used to advance the

theory of quantitative secondary ion mass spectroscopy (SIMS).

The intercomparison was done on samples of Lake County Oregon

Labradorite (LC) [2,3]. Lake County is a crystalline feldspar of low

potassium content so that it is a plagioclase feldspar, and its sodium

and calcium content is such that it is classified as labradorite. Its

major element composition as determined by electron probe analysis at

NASA-JSC is given in Table 1 [4]. This analysis is in good agreement

with the wet chemical analysis found in reference 2. Lake County has

proven to be a particularly useful microanalytical standard for SIMS

comparisons because it possesses four major cations with no

69
SIMS/IMMA

significant interferences for secondary ion mass spectrometry, it is

quite homogeneous from sample to sample and within a given sample

there are very large volumes without foreign inclusions. This mineral

has also been used as a trace element standard [5]. (Warning: no particular sample of Lake County or other mineral should be assumed to

be chemically homogeneous or the same as any other sample without

detailed checks made by accurate and precise analytical techniques.)

Table 1: Composition of Lake County Labradorite

Element Atomic percent concentrations

Si 18.1

Al 12.7

Ca 5.0

Na 2.5

K 0.062

Fe 0.12

0 61 .6

All the data presented for the ARL and AEI instruments and some preliminary Cameca data (not presented here) were obtained from

analysis of a single sample of Lake County Labradorite. This is the same sample whose analysis is given in Table 1. The sample was extensively checked for inclusions and , sodium-calcium zoning with the

electron probe and was found to be homogeneous within counting

statistics. The data presented for the Cameca instrument were

obtained from analysis of a separate sample. An extensive check of

this sample with an electron probe found it to be free of inclusions

and zoning and homogeneous in major elements within counting

statistics. The major element compositions of the two samples agree

within a fraction of a percent.

The differences in the mass spectra of the instruments are illustrated in figure 1. In section A are plotted the true abundances

of silicon, aluminum, calcium, and sodium in the sample, assuming

61.5% oxygen. In sections B through E the data represent the relative

intensities of the detected calcium, silicon, aluminum, and sodium

secondary ion beams with the sum of intensities adjusted to give a total of 38.5%. For example the data for silicon as plotted is the

result of the calculation

[2^Si'']/0.9921

^^~['*°Ca'']/.9697 + [ ^ ^si-"] /Q . 9221 + [^'Al""] + [^^Na""]

38,. 5

70
Spectra. Comparison

where ["M"'"] is the actual detected secondary ion intensity in

counts/second or -amperes. The factor 38.5% reflects the fact that the

non-oxygen elements of the true bulk composition represent about 38.5%

of all the sample atoms. The introduction of this factor makes easier

the comparison of true versus ion probe compositions although there is no reason to believe that any ion probe raw analysis should agree with

the true composition. No predictive data reduction scheme (such as CARISMA) has been used on any of the data presented here.

5." Apparent Composition of Lake County Labrcdorite

20 r

15-

a.Eo

OJCO

CD Si*

c

<uo

Eo<

Al*

10-

j- Ca*

0^

No*

A

ARL Relative Ion Intensities

Beta Aperture Size

B

£ 2 5

Q. a>

.<« m

5 E E

_i Q. a

q:

UJ<

-oCa

oNo

^Al

oSi

0 0.1 0.2 0.3 0.4 Beta Aperture Size (Inches)

Figure 1. True bulk composition of Lake County Labradorite (section

A) and the composition as indicated by relative ion

intensities (sums adjusted to 38.5%) are displayed as measured by ARL, Cameca and AEI ion probes (sections B-E).

Section B represents the relative secondary ion intensities

in the ARL instrument as the beta aperture diameter was

varied. The analyzed spot was kept in its nominal on-axis

design standard position. "Normal" operation is at a beta

aperture diameter of 0.31 inches. The alpha aperture was

kept at its normal diameter of 0.25,inches.

71
SIMS/IMMA

Section C represents the relative secondary ion intensities

in the ARL instrument with the analyzed sample spot moved

about 300 microns toward the secondary ion pickup electrode

relative to its standard on-axis position. The alpha aperture diameter was 0.25 inches. The beta aperture diameter

was 0.31 inches.

Sections D and E represent the relative secondary ion

intensities as measured on the Cameca and AEI ion probes

under their "normal" operating conditions.

The ARL data were obtained witn a pulse counting detector and so have been corrected for dead time. The Cameca and AEI data were

obtained as analog signals from the output of a scintillationphotomultiplier detector and an electron multiplier detector

respectively. In all cases the data from measurement of the ^^Si"*" and

'*°Ca"'' intensities have been corrected for their isotopic abundances to

give total silicon and calcium signals. The secondary mass spectrometers and detection systems of all three instruments may

introduce a mass dependent discrimination for certain ions; no effort

has been made to correct for this discrimination. It should be noted

that analog detection systems typically show more mass discrimination

than pulse counting systems.

Section B of figure 1 shows the variation in the relative ion

intensities as measured in the ARL instrument as the beta aperture

diameter was^ varied. The beta aperture is between the electric and

magnetic sectors and limits the angular spread and^ energy spread of

the ions transmitted to the magnetic sector. The alpha aperture was

set at 0.25 inches for these measurements. In the ARL instrument, the

alpha aperture is an entrance aperture limiting the maximum angular

divergence of the secondary ion beam that can be transmitted to the

electric and magnetic sectors. Before measuring the ion intensities

at each aperture setting, the sample bias voltage was adjusted in

accordance with the ARL recommended procedure. That is, the sample

voltage was increased somewhat beyond that required for maximum

secondary ion transmission in order to qualitatively flatten the top

of a peak displayed on the console oscilloscope (in this case '*°Ca''"). It is likely that this procedure serves to position the secondary ion

beam so that the highest energy ions do not pass through the beta

aperture.

The trends in relative secondary ion intensities (section B, fig.

1) should be considered rather subtle, as the average secondary ion

intensity increased a factor of 30 in going from the narrowest to the

widest aperture. The repeatability at any one aperture is rather poor

because of the qualitative tuning procedure. For example, the details

of the ^^Na"*" trend are not repeatable and probably meaningless.

72
Spectra Comparison

Section C of figure 1 shows the relative secondary ion

intensities that were measured with the primary ion beam deflected to move its impact point on the sample approximately 300 micrometers

closer to the secondary ion pickup electrode than it would be in the

"design standard configuration." The sensitivity for aluminum (counts

per second per nanoampere of primary ion beam) increased approximately

a factor of two with deflection of the primary beam. Deflecting the

primary ion beam apparently caused the secondary mass spectrometer to

transmit more of the same "kind" of ions that increasing the beta

aperture diameter caused it to transmit.

Sections D and E display the relative secondary ion intensities

detected in running LC in the Cameca and AEI instruments. The

similarity of Cameca and AEI relative intensities is quite striking.

The relative ion intensities detected by the ARL under normal

operating conditions (fig. 1, section B; alpha aperture 0.25 inches

diameter, beta aperture 0.31 inches diameter and primary beam on axis)

are strikingly different from those of AEI and Cameca. The

similarities and dissimilarities between the spectra produced by the

three instruments may reflect some "universal" genuine property of

Si's emitted from an 0" bombarded LC sample or the relationships may

be entirely due to combinations of the widely different parameters and

characteristics of the three instruments.

The 0" primary ions in the Cameca strike the sample with an energy of about 14.5 keV at an angle of about 45° to the sample

surface; the Si's are drawn off perpendicular to the sample surface

and sent through a single focusing mass spectrometer at an energy of

4.5 keV. The Si's in the Cameca are detected by a scintillationphotomultiplier detector with the ion to electron conversion occuring

at an impact energy of about 55 keV.

The 0" primary ions in the AEI instrument strike the sample with

an energy of 35 keV at an angle of 73° to the sample surface; the Si's

are drawn off the sample surface, by an electrode arrangement that

bends them through an angle of about 60° with respect to a normal to

the sample surface and sends them through a double focusing secondary

mass spectrometer at an energy of 10 keV. The SI 's are detected by an electron multiplier with the impact energy on the conversion dynode of

about 13 keV.

In the ARL instrument, the 0" primary ions strike the sample

normal to the surface with an energy of 21.5 keV; the Si's are drawn

off the sample surface by an electrode arrangement that bends them

through an angle of 45° and sends them through a double focusing

secondary spectrometer at an energy of 1.5 keV. The ions are detected

by a scintillation-photomultiplier detector with ion to electron

conversion occurring at an energy of 16.5 keV.

The dissimilarity of the ARL compared to the Cameca and AEI

spectra cannot be due only to different primary ion impact energy

73
SIMS/IMMA

since its 21.5 keV impact energy is between that of the Cameca (15

keV) and AEI (35 keV). It cannot be due to some simple property of

the ARL secondary ion pickup geometry or to its double focusing

secondary spectrometer, since the AEI instrument is similarly configured. The normal incidence of the primary ions in the ARL is a possible explanation but seems unlikely because the primary beam

impact in the AEI is only 17° away from the normal. Mass

discrimination can be ruled out since even a strong discrimination

(inversely proportional to the square root of ion mass) in any

instrument is qualitatively of the wrong nature as well as of

insufficient magnitude to bring about better agreement between the ARL

spectrum and the others.

It seems, likely that the source of the disagreement of the ARL

spectrum with the others lies in which ions are being transmitted.

The secondary ion pickup electrode arrangement in each instrument is

capable of discriminating against ions of different initial energies

and angles of emission.

The rather simple Cameca pickup geometry permits calculation of.

its energy discrimination properties. A normally equipped Cameca

instrument will not transmit Si's with more than about 0.5 eV of

velocity parallel to the sample surface. Hence the more energetic

Si's will be strongly discriminated against although those few ions of

higher energy, say more than about 5 eV above sample voltage, which

are directed almost normal to the sample surface will still enter the mass spectrometer. The mass spectrometer further discriminates

against high energy ions because the electrostatic mirror will not

reflect tons of energy more than about (in the case of these

experiments) 20 eV above sample voltage. Further, since the mass spectrometer is single focusing the mass peak shape for a particular

ion is distorted according to the proportion of ion energies present.

A typical peak shape (at mass resolution approximately 300) is shown

in figure 2. The occurrence of the maximum on the low mass side of

the^eak represents a high production of low energy ions and/or high

transmission for low energy ions. The presence of a low tail on the

high mass side represents a reduced secondary ion production and/or

low transmission for energetic ions. Hence the ratio of the low

energy peak to the high energy tail at the 20 eV mirror cutoff is some crude measure of the degree to which high energy ions are produced at

the sample surface. This peak to tail ratio was measured for each of

the four ions dealt with here; the results are listed in Table 2.

Unfortunately the peak to tail ratio was not measured for molecular

ions. The points to note are that there are rather few high energy

sodium and calcium ions, rather many higher energy aluminum and

silicon ions and that the Cameca peak heights as reported here

represent essentially the intensities of the lower energy ions.

The secondary ion pickup geometry of the AEI instrument can also

discriminate against certain ion energies. The bending of the

secondary ions through an angle of 60° causes a spatial separation of

the trajectories of ions of different energies. Hence only a finite

74
Spectra Comparison

Typical Cameca

Peak Shape

Figure 2. The typical shape of a single-mass peak (at a mass resolution of about 300) from the singlefocusing Cameca ion

probe as displayed by magnetic scanning.

Table 2: Relative low energy peak to

high energy tail ratios as measured on the Cameca ion

probe.

Ion

^

Ratio

^oca"*" 73

^ssi"" 7.4

27^"^ 15

2 3Na* 240

75
SIMS/IMMA

energy spread can be transmitted through the apertures of the pickup

electrode. There is a secondary ion steering repel ler over the sample

region that allows maximizing of the detected ion intensities. If the

greatest production of secondary ions occurs at low energies then

maximizing of intensities will occur with low energy ions transmitted

preferentially and only some fraction of the higher energy ions

transmitted. • If only the low energy ions are transmitted or if an insignificant number of high energy ions are produced, then the

relative ion intensities determined by the AEI peak heights will be

the same as that produced by the Cameca instrument despite the double

focusing character of the AEI spectrometer.

The ARL instrument can discriminate against ions of certain

energies for the same reason as the AEI, but the ARL pickup system

does not include a secondary ion steering repel ler to permit

adjustment of which ion energies are transmitted through the pickup

electrode apertures. The trend of ARL relative ion intensities with

iiacreasing beta aperture is consistent with an increasing transmission

of low energy ions (those represented in the Cameca and AEI peak

heights) due to a sample voltage adjustment procedure which always

sets approximately the same upper limit on the energy of transmitted

ions and hence decreases the low energy limit as the beta aperture

size increases.

The increase in aluminum (and other ion) sensitivity and

alteration of relative -ion intensities with displaced primary ion

bombardment position in the ARL (fig. 1, section C) are consistent

with improved transmission of the more abundant low energy ions, since

the alteration of the relative intensities is such that the improved

transmission is for the same type ion re^presented in the Cameca (and

AEI) peak heights. The greater magnitude of relative intensity

changes with spot position change (versus beta aperture changes)

indicates that most of the discrimination is occurring in the pickup

electrode.

If for the moment the spectral differences can be ruled out as due to differences in sputtering conditions, the data seem to indicate

that the ARL machine discriminates against the transmission of low

energy ions and that the low energy ions are dominant in the sputtered

spectrum. Also the data are consistent with the Cameca and AEI

discriminating against higher energy ions, the AEI only if the low

energy ions are dominant in the secondary ion energy spectra.

It is felt that the results presented here truly represent

instrumental characteristics as determined under the specified

operating conditions and with the instruments otherwise tuned by

experienced operators, but the measurements should be confirmed in

other laboratories. The systematic effects of varying sputtering and

tuning conditions need to be studied within each instrument to

determine the extent that each can yield altered relative ion intensities. Measurements on other multiple element standards would be

re
Spectra. Comparison

helpful in revealing the systematics of an elemental discrimination

based on sputtered ion energy differences.

The similarities and dissimilarities of the relative secondary

ion intensities as measured by various instruments must be understood,

particularly ' in so far as they reflect only instrumental differences,

before the data from any instrument can be used to establish the

validity of any theory for the quantification of secondary ion mass spectroscopy.

Acknowledgements

Bradley was supported by a National Research Council Research

Associateship during the work at NASA-JSC. His initial work in France

was generously made possible by a travel grant from Centre National de

la Recherche Scientifique of France.

Bradley and Jerome acknowledge valuable and stimulating

discussions with Professor Georges Slodzian in whose laboratory the

Cameca measurements were done.

The work at the University of Illinois was supported by the

National Science Foundation Grants GP-42330X and DMR-7203026.

References

1. The measurements on the ARL instrument were done on the

instrument of the Geochemistry Branch, Planetary and Earth

Sciences Division, National Aeronautics and Space. Administration,

Lyndon B. Johnson Space Center, Houston, Texas.

The measurements on the AEI instrument were done on the

instrument of the Materials Research Laboratory, University of

Illinois, Urbana, Illinois.

The Cameca measurements were made on the instrument of the

Laboratoire de Physique des Sol ides, Universite Paris-Sud, Orsay,

France.

2. Stewart, D. B., Walker, G. W., Wright, T. L., and Fahey, J. J.,

Physical properties of calcic labradorite from Lake County,

Oregon, Amer. Mineral

51 . 177-197 (1966).

3. Limited quantities of this mineral can be obtained by writing to

the National Museum of Natural History, Smithsonian Institution,

Washington, D. C. 20560 and referring to the sample number

115900.

4. Analyst Roy Brown, Lockheed Electronics Corporation.

5. Meyer, C, Jr., Anderson, D. H., Bradley, J. G., Proc. Fifth

Lunar Sci . Conf., Geochim. Cosmochim. Acta, Pergamon Press.

77
(

I

I
NBS Spec. Publ. 427, Workshop on Secondary Ion Mass Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Go ithersburg, Maryland, September 16-18, 1974 (Issued October, 1975).

A CRITICAL DISCUSSION OF THE LOCAL THERMAL

EQUILIBRIUM MODEL FOR .THE QUANTITATIVE CORRECTION

OF SPUTTERED ION INTENSITIES

C. A. Andersen

Hasler Research Center

Applied Research Laboratories

95 La Patera Lane

Go! eta, California 93017

I. INTRODUCTION

Over the past few years we have developed an analytical method for

the quantitative interpretation of sputtered ion intensities. On the

basis of experimental observations, it has been postulated that the

production of sputtered ions is a function of the electronic properties

of the surface of the sample which, in turn, are related to the

surface chemistry. We. have learned that we can control the electronic

properties of the sample surface and enhance and stabilize the yield

of secondary sample ions through control of the chemistry of the surface

by the proper selection of the primary bombarding ion beam. The

emission of positive ions is enhanced by the increased electronic work

function cheated by bombarding with an electronegative gas such as oxygen [1], while the emission of negative ions is enhanced by the

decreased work function created by bombarding with an electropositive

gas such as cesium [2].

The quantitative model that has been developed [3] to convert the

enhanced and stabilized ion yields to atomic concentrations is based on

a local thermal equilibrium (LTE) model. This working hypothesis states

that the majority of sputtered ions, atoms, molecules, and electrons are

in thermal equilibrium with each other and that these equilibrium

concentrations can be predicted through the use of the proper Saha

equations [4].

The ionization process postulated in this model can be thought of

as an equilibrium dissociation reaction such as that given in equation

1.

M^^M* + e" (1)

79
SIMS/IMMA

The equilibrium constant for this reaction is simply written as the

concentrations of the products over the concentrations of the reactants

as given in equation 2.

'

+ n .n M

At thermal equilibrium the equilibrium constant can be calculated

according to the Saha-Eggert ionization equation [5], as given in

equation 3:

/ZTTkT . "^e--"^Mn V^e- ^. E/kT

(3)

MO ' ^y[0

Where h is Planck's constant, k is Boltzmann's constant, T is the absolute temperature in °K, m is the mass, B the internal partition function, and E the dissociation energy of the species in question (in this case it is the first ionization potential of the atom). A

practical logarithmic form of equation 3 is given in equation 4 and

shows that the ratio of positive ions to neutral atoms of an element is

a function of the temperature and electron density of the assemblage:

logCxij^^/rij^o) = 15.38 + log 2(Bj^/Bj^) + 1.5 log T ^

(4)

5040CIp-AE)

T log Nowhere nf/|"'"and nw^are the numbers of atoms of element M in two adjacent

charge states, B|v|'''and B^^ are the internal partition functions of tnese

charge states, 2 is the partition function of a free electron, Ip is the

ionization potential depression due to Coulomb interactions of the

charged particles, and ne" is the number of electrons per cm^.

It can be seen from equation 4 that, given the partition functions

and the ionization potential, the ratio of the number of singly charged

ions to neutral atoms of an element is predicted by the temperature and

electron density of the assemblage. This is the basis for a quantitative method since the singly charged ions are directly observed

in the mass spectrum. Therefore, from the predicted ratio of singly

charged to neutral atoms, the total number of atoms of an element

present in the sputtering assemblage can be calculated. If all the

elements observed in the mass spectrum are treated in this way, the

atomic composition of the sample is derived.

80
LTE Model

The sputtering assemblage is assumed to be basically a dense plasma

of coexisting positive and negative atomic and molecular ions,

electrons, and neutral atoms and molecules. Under the present

experimental ion-bombardment conditions, most of the particles are

neutral atoms and singly charged positive ions. There are, however, for

elements of high electron affinity and high oxygen affinity, significant

numbers of negative ions and oxide molecules in equilibrium with the

positive ions and neutral atoms. The equilibrium concentrations of

these species can be calculated with the appropriate Saha equations, as will be shown shortly, and the intensities of the positive ions

corrected for their abundance. All the observed intensities of singly

charged positive ions in the mass spectrum are corrected in this way to

give the total atomic composition of the sputtering plasma, which is

.directly related to that of the solid. We have previously demonstrated

[4] that such a correction procedure generally works well.

A substantial degree of success using this model has also been

reported: by R. Shimizu et al. [6] using an Ar"*" beam and a Hitachi IMA2 to analyze a steel standard; by F. G. Rudenauer and W. Steiger [7]

using an Ar^ beam and a specially designed ion microprobe with tandem

^nass spectrometers to analyze a steel sample; by A. Lodding et al. [8]

fusing an 0" beam and a CAMECA IMS 300 ion analyzer to analyze known

apatite samples; by C. Meyer et al. [9] using an 0" beam and an ARL IMMA

to analyze plagioclase standards; by J. Schroeer [10] using an Ot beam

and an AEI ion microprobe attachment to the MS 702 to analyze several

NBS steel standards; by D. Simons [11] using the same equipment and an 0" beam to analyze a plagioclase standard (Lake County); and by J.

Lovering [12] using an 0" beam and an ARL IMMA to analyze many

different mineral standards. J. F. Bradley [13] has also reported

element intensity ratios for Lake County plagioclase obtained with a iCAMECA IMS 300 and an 0" bombarding ion beam that appear to be identical

to those reported by Simons [11]. In addition Z. Jurela [14] has

proposed, and demonstrated with a good deal of success, a similar

thermodynamic model which differs from that discussed here in that it

stresses a nonequilibrium state. An independent evaluation [10] of

Jurela's nonequilibrium model using several NBS steel standards,

however, indicated that it did not account for the observed ion

intensities with as good accuracy as did the local thermal equilibrium

model

.

Based on the apparent successful application of the method by

several investigators using different instruments to many different

types of samples, it is appropriate to critically discuss the LTE model

in terms of the confirming and negating evidence and also in terms of a possible physical basis.

II. CONFIRMING EVIDENCE FOR LTE

There are six main observational indications that local thermal

equilibrium exists between the electrons, ions, atoms, and molecules of

the sputtering assemblage. These are:

81
SIMS/IMMA

1. Positive Ions in Equilibrium with Neutral Atoms

Equation 4 was developed to describe thermal ionization and is

taken as one of the validity criteria for the establishment of local

thermal equilibrium [15]. An example of the application of this

equAtion to sputtered positive ion data obtained from a wellcharacterized mineral standard is, given in figure 1. In this case, the

effective temperature and electron density of the sputtering region were obtained by solving the Saha-Eggert ionization equations for the best

fit of the observed singly charged ion intensities to 'the known concentrations of the elements in the sample. The symbols II and I refer to

the singly charged ion and neutral atom, respectively. AE in this tase

SPUTTERED POSITIVE ION'S

CLINOPYROXENE (AC-362-C1)

1.0 T

1 1 1 1 r 1 1 1 1 1 1

CO

n

CN 0

t a

Z z

O)

-1.0

Fe\si

t = ll,000°K ^^-^"^S.

Ne= 1.17x 10^9 \

1 1 1 1 1 1 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Ip(eV)

FIGURE 1: Saha-Eggert ionization equation applied to sputtered positive ion data from a mineral standard bombarded with ^^0"

at 16 keV.

82
LTE Model

is 0.66 eV. Equation 4 is basically a linear equation in two unknowns,

T and ng". The values for two observed singly-charged-to-neutral atom

ratios, therefore, permit a solution for the temperature and electron

density of the sputterea region. in practice, the- neutral atom

intensities are not observed but the composition of the sample is known

and the ratio of the corrected ion intensities thereby established. The

fit of the data to the Saha-Eggert ionization equation indicates that

the singly charged ions in the sputtering -assemblage are in thermal

equilibrium with the neutral atoms and that the equation can be used to

predict this ratio.

2. Negative Ions in Equilibrium with Neutral Atoms

The electron attachment process under equilibrium conditions can be

written as: M"^M° + e' (5)

The dissociation constant of this reaction can be written in a form

analogous to equation 2 and evaluated according to a Saha equation

analogous to equation 3. A practical logarithmic form of the latter

equation is given in equation 6:

log(nj^./nj^o) = -15.38 + log (gj,^- /2gj^ ) -1.5 log T +

5040E^ (6) + log n

where Eg is the electron affinity of the neutral atom of element M, and

g represents the statistical ground state of the negative ion or the

neutral atom.

Experimental confirmation that sputtered negative ion yields fit

such a relationship at elevated temperatures is given in figure 2. Here

the ratios of the number of negative ions to neutral atoms for three

elements from a mineral standard are plotted versus the electron

affinity of the atom. The statistical weights of the ions and atoms

have not been included because they are unknown. This simplification is

discussed in another paper [4]. The effective temperature and electron

density were obtained from the experimental data using equation 6,

setting the statistical weights equal to one, and the known atomic

concentrations of the elements in the sample. Again, there seems to be

evidence of local thermal equilibrium between negative ions and neutral

atoms

In this example the temperature is noticeably lower than that

observed for the positive ion data of figure 1. The significance of

this difference is not fully appreciated at this time because of a 83
SIMS/IMMA

2

c o

-1.0

-2.0

-3.0

r— T"

SPUTTERED NEGATIVE IONS

.BERYL: Be^ Si^ O^g.

-1.0

^^O", 11 kV

Ng= 2.35 X 10

1.0

(eV)

2.0

FIGURE 2: Saha equation applied to sputtered negative ion data from a 'mineral standard bombarded with ^^0~ at 11 keV.

general paucity of negative ion data. It is thought, however, that the

relatively high electron density at this lower temperature is

significant. More work with negative ions is earnestly needed.

3. Molecular Ions in Equilibrium with Atomic Ions

The molecular and atomic species under conditions of local thermal

equilibrium are related through dissociation reactions such as:

MO' M° + 0° (7)

84
LTE Model

where the superscript refers to neutral particles. Similar equations

could be written for the dissociation of charged molecules to charged

atoms. The dissociation constant of this reaction can be written in a form analogous to equation 2 and evaluated according to a Guldberg-Waage

equation that is analogous to Saha equation 3. Using an approximation

for the partition function of the diatomic molecule [16], the latter

equation is given in equation 8.

1.0 ~I I I I I i I

1

1

I PLAGIOCLASE (AC -362)

'O', 16 kV _ T = 11,500°, Ng = 3.16x 10

0.10

/M0\

\ M /obs.

FIGURE 3: Observed positive atomic and monoxide ion intensities

(corrected for degree of ionization) as a function of the

calculated dissociation constant and the average concentration of free oxygen. Data from a mineral standard bombarded

with ^^0", at 16 keV.

85
SIMS/IMMA

where is the rotational constant for the electronic state referred to

a vibrationless condition, to° is the vibrational constant, g° is the

statistical weight of the electronic ground state, and Vd° is the

dissociation energy of the neutral molecule. Equation 8 can be used to

determine the neutral molecular monoxide-to-metal atom ratio if the

concentration of neutral free oxygen is known [4].

The accuracy of this approach is illustrated in figure 3 where data

obtained from a silicate mineral standard bombarded with a pure beam of

^®0" are shown. The observed positive atomic , and molecular ion

intensities of both M^^O and M^^O have been corrected for their degree

of ionization and plotted as a function of the calculated neutral

molecular dissociation constant and the average concentration of neutral

free ^^0 or in the sputtering region. The average concentration of

neutral free oxygen, Nq, was determined from the experimental data to be

the average value calculated using the observed MO'^/M"*' intensity ratios

(corrected for their degree of ionization) of each of the molecule and

atom pairs illustrated in figure 3. The fit of the data demonstrates

that the molecular ions observed in the mass spectrum are the products

of equilibrium reactions and that their relative concentrations can be

calculated with equation 8.

Another indication of the equilibrium between molecules and atoms

that is implied in the data discussed above can be obtained by comparing

the temperatures derived from the molecules to that derived from the

atoms. As explained in subsection 1 above, a temperature is derived

from the sputtered atomic ion data by using the LTE model (CARISMA)

[3,4] to match the observed sputtered ion intensities to the known

composition of the standard sample. This temperature is known as the

CARISMA temperature and relates the ratio of ions to atoms of a specific

element at a given electron density. It is also possible to derive a temperature from the molecular ion spectra, without knowing the

composition of the sample, by forming the ratio of molecular ion

intensities (corrected for their degree of ionization) of a pair of

molecules, MO and M'O, and using equation 9. Equation 9 is simply a ratio formed from equation 8 for two oxide molecules with different

cations.

B.M

MM MOMO M

3/2

(9)

1-10 exp

1-10 exp

^-0. 6250)

^

„ /-5040 .,, V 0 exp (^-^j^CV^.V^

86
LTE Model

ANDESINE O^O"): T(°K)

kV -r / 5' A' \ T (CAR ISMA)

18 10200 9700

10075

10440

11225

12150

11480

13560

14600

14280

16740

16 10550

14 10850

11 11375

8 1^050

TABLE I: LTE (CARI^MA) temperatures compared to temperatures derived

from molecular oxide data from a silicate bombarded at

different energies.

Table I gives the results of such a comparison based on data

derived from a mineral standard bombarded with ^^0" at several different

accelerating potentials. Two sets of Si and Al monoxide molecular ion

data, 'one incorporating natural ^^0 from the sample and the other

incorporating implanted ^^0 from the bombarding beam, were compared to

the CARISMA temperatures.

It can be seen that, although the temperatures don't match

perfectly, they are of the same order and trend in the same direction as the accelerating potential is changed. (This important inverse

relationship will be returned to later.) The temperature differences

noted may be real or may be related to errors in the physical constants

required to solve the equations. In any case the data again support

local thermal equilibrium between atoms and molecules.

4. Perfect Mixing of Reaction Particles

The internal consistency of the data for molecules formed from

metal atoms of the sample with both natural ^^0 from the sample and with

bombardment-introduced ^^0, shown in both the above data and those of

figure 3, demonstrates that (under these experimental conditions) the

molecules observed in the mass spectrum are not directly related to the

molecular structure of the solid but are the reaction products of a collection of implanted atoms and sample atoms interacting at an elevated temperature. The perfect mixing implied is an indication of

equilibrium among the particles.

A further indication of the perfect mixing of atoms, observed in

the sputtered ion data discussed above, is obtained by determining the

87
SIMS/IMMA

ratio of the implanted to .natural oxygen for the different pairs of

atoms and molecules,. The atom ratio of ^^O/^^O can be determined: (1)

directly from the sputtered ion intensity ratio of these two isotopes;

(2) from the ratio of Si^^O/Si^^O according to equation lOa-f; and (3)

from the ratio of ^^02/^^62 according to equation lla-f. The equations

are self-explanatory, being analogous to equations 2 and 3, . and the

resijlts of the analysis are given in Table II.

ANDES1Ne(^^0"): n,8^/n„^

kV (n.8o/n,6o) ("360/320 X 0.838

V/2

("283,18^023^^ 0.896)

18.

16.

14.

11.

8.

2.68

2.78

2.62

2.65

2.54

2.64

2.50

2.21

2.52

2.70

3.78

2.87

2.58

2.83

TABLE II: The ratio of implanted to sample ^^0 -gas derived

from atomic and molecular oxide data from a silicate

bombarded at different keV.

n.MO nMO

(10a)

(b)

Cc)

^ _.. /27TkTZT "»M-M^/^ Vo "V^T

88
LTE Model

(e)

(f)

Clla)

Kn,16Q n

16 n

0-

16

0-

1/2 (c)

n

n

18

16

0

0

T 1/2

n

16

r

0

i^'l^o

^2 ^2J

n

n

18

16

0

0

in^g^A3/2 1/2

(e)

n

n

18 0

16

n. 16o \"18

0-

0,

X 0 . 838)

\l/2

89
SIMS/IMMA

FIGURE 4: Ratio of implanted to sample gas as a function of the

calculated sputtered atom yield for silicates bombarded with

^H-, ^^0-, and ^^Cl" at 16 keV.

It can be seen in Table II that the ^^O/^^O ratios obtained from

all three combinations of atoms and molecules are essentially equivalent, indicating that the ^^0 and ^^0 atoms are equally distributed

among the oxygen and silicon monoxide molecules. The differences noted

are probably the results of errors in the data.

It is also interesting to note that the ratio of ^^O/^^O is

approximately constant with accelerating potential and that it has a value of about 2.6. This indicates that there are about 2.6 times as many implanted atoms in the sputtering assemblage as there were ^^0

atoms originally in the solid sample. Since the mineral was originally

90
LTE Model

about 62 percent oxygen, the new oxygen content of . the sputtering

assemblage must be about 85 percent. This would appear to be a large

amount of entrapped gas for the solid to contain.

Figure 4 shows the fairly consistent ^^O/^^O ratio measured in five

different mineral types at an accelerating potential of 16 kV plotted as

a function of the calculated [2] sputtered atom yield (in relative

units). The sputtered atom yield was chosen because it had previously

been shown [17] that the amount of inert gas trapped in a metal was inversely proportional to this function. Also shown on the plot are the

ratios of implanted to sample gas obtained when mineral specimens are bombarded with and ^^Cl beams. The inverse relationship appears to

hold and it can be seen that as the sputtering yield drops the amount of

entrapped bombarding gas increases greatly with the ratio of bombarding

gas to sample gas approaching 100 in the case of H bombardment.

5. Electrons in Equilibrium with the Sample Surface

We have recorded the temperatures and electron densities that have

been derived from the analysis of several hundred standard samples using

the described [4] LTE model and have noted a definite relationship

between these two parameters [18]. This functional relationship may be

interpreted by returning to the dissociation reaction given in equation

2 where it is noted that the equilibrium constant of the reaction is

given in terms of the concentrations of the ions, atoms and electrons.

It is important to understand, however, that it is not necessary to assume that in this equation the electrons come only from the ionization

of the atoms [19]. Some, or the majority, of the electrons, required to

explain the degree of ionization observed, could arise from other

processes. A possible source, for example, could be thermally emitted

electrons coming from a hot spot on the surface of the bombarded sample

such as might be associated with the postulated local plasma.

Electron emission from a hot surface can generally be described by

Richardson's equation [20].

2 -*o/^T

(12)

where I is the electron yield of the surface in A/cm^, is

Richardson's constant, T is the absolute temperature of the surface, k

is Boltzmann's constant, and <^q is the work function of the surface at

absolute zero temperature. This formulation gives the electron emission

in terms of the electronic work function and the temperature of the

surface. N^, the electron concentration per cubic centimeter, required

for the dissociation equation can be obtained from I, the electron

emission per square centimeter, by using the relationship given in

equation 13 [19]:

91
SIMS/IMMA

N = 4.034 X 10"^^ • T"^/^I 03)

Richardson's equation can thus be tested to see if it gives the

dependence noted in the observed relationship between temperature and

electron density. A satisfactory result will be considered to be a straight line fit of the temperature and electron density data on a Richardson plot and a reasonable electronic work function for the

materials derived from the slope of that line.

This test has been made in the next two figures where the LTEderived electron densities have been plotted against the LTE-derived

temperatures according to Richardson's equation. Figure 5 represents

163 silicate analyses [4]. It should be noted that these are the

analyses of known standard reference materials and that the temperatures

and electron densities plotted are obtained independently of each other

by simply allowing a computer using an LTE model (CARISMA) to pick the

temperatures and electron densities that best explain the observed

sputtered ion intensities with respect to the known sample compositions.

The heavy line through the points is a least squares fit that gives

better than 95 percent correlation. Richardson's relation on the basis

of these data would predict an average electronic work function for the

emitting surfaces of 11.18 volts, defined at absolute zero temperature.

This value may be slightly higher than expected but is not entirely

inconsistent with known band gaps of insulators [21]. The temperature

coefficient of the work function is negative and about 4x10""* eV/°K,

which is also of about the right magnitude [22]. The light dashed line

on the plot is related to our experience with metals analysis which is

given in figure 6.

Figure 6 is similar to figure 5 except here the heavy line is a least squares fit through 60 metal alloy analyses we have made [4], The

points were derived from the LTE model using CARISMA in exactly the same way as those shown in figure 5. The average work function of the

electron emitting surfaces using these data is now predicted by

Richardson's equation to be 8.54 volts at absolute zero. The

correlation is in excess of 90 percent and the temperature coefficient

of the work function is nearly identical to that observed for the

silicates. The work function again might seem a little high but

probably is not excessive for an oxygen-saturated metal surface [19].

The lower dashed line is that derived from the silicate analyses of

figure 5. The upper dashed line is given for comparison and represents

the actual electron emission with temperature of a heated tungsten

filament [23]. It is interesting to note here the temperature and

electron density observed by Shimizu et al. [6] using an LTE model to

interpret the results of their analysis using an argon beam. The

temperature reported (6000°-6500°K) is very similar to those we have

noted on similar materials but their electron density (Ne= 5.6-

5.2xl0^®e"/cm^) is much larger indicating, according to the Richardson

equation, a considerably lower electronic work function of the bombarded

surface. This is exactly the result to be expected when bombarding a 92^
LTE Model
94
LTE Model

metal with an inert gas such as argon as opposed to an electronegative

reactive gas- such as oxygen [1]. The increased work function created

when bombarding with the reactive gas decreases the electron emission

and consequently the electron density of the local plasma which

increases the absolute sputtered ion yield, see equation 4. This same effect, of course, can be realized by analyzing a sample or a point on the sample that naturally has a higher work function. This phenomenon

is often observed in sputtered ion micrographs of polycrystal 1 ine

samples where both bright and dark grains of a material with a constant

composition are seen. Prolonged bombardment with an oxygen beam will

eventually overcome the natural work function differences and produce an image of equal intensity for grains of equivalent composition.

The good fit of the LTE-derived data to Richardson's equation,

coupled with the derivation of a reasonable electronic work function for

•the surfaces, indicates that the electrons are in equilibrium with the

surface which is at an elevated temperature. This further suggests, by

inference, that all the sputtered particles are in equilibrium with a hot spot on the surface of the sample.

6. Equivalence of Temperatures Derived from Optical Emission

and Sputtered Ion Data

The emission of ultraviolet and visible radiation from samples

subjected to energetic (few keV) ion bombardment has generally been

observed by many investigators [24-30]. The behavior they have noted

concerning the relative changes in intensity of the radiation with

respect to oxygen coverage of the sample and with respect to insulator

versus metal is exactly analogous to the behavior noted previously for

the intensity of sputtered ions [2,3]. This indicates that the

processes governing the emission of radiating excited particles are the same as those that govern sputtered ion emission.

Recently, Shimizu and his colleagues [31] have spectrally analyzed

the optical radiation generated from aluminum and aluminum oxide samples

bombarded with argon ions at 14 keV. Using Saha's relationships,

according to the LTE model, they have found the radiation from the

sputtered excited neutral atoms to yield temperatures of several

thousand degrees Kelvin ('v5000°K) . They found similar results when

analyzing the spectral data of Kerkow [32]. The temperatures of the

insulator (AI2O3) were generally higher than those of the metal (Al),

again paralleling sputtered ion relationships [4]. These temperatures,

they emphasized, are similar to the temperatures they had previously

derived, using an LTE model, in their analysis of sputtered ion data

obtained from iron samples bombarded under similar experimental

conditions (6). They were also able to dramatically change the shape of

the optically emitting region above the surface of an Ar-bombarded

tantalum sample by simply applying a weak magnetic field to the sample

[33].

These results are consistent with an LTE model and indicate that,

the sputtered neutral atoms exhibit degrees of excitation equivalent to

95
SIMS/IMMA

the high temperatures derived from the sputtered ion data. The results

also indicate that the radiating excited neutral atoms interact and

exchange charges with the sputtered ions above the surface of the

sample, as demonstrated by the effect of the superposed magnetic field.

III. ACCURACY OF THEORETICAL CALCULATION METHOD

Before discussing the negating evidence for the LTE model , it would

be well to analyze the inherent accuracy of the equations and constants

being used in our calculations (CARISMA) to test 'the model. Table III lists the physical constants required to solve the LTE equations

according to our partitular approach (CARISMA), their source in the

literature [4], and an estimate of the accuracy with which each constant

is known.

CONSTANTS FOR ANALYTICAL CALCULATIONS

PARAMETER COMMENTS SOURCES

ISOTOPIC COMPOSITION ACCURACY < 1 - 5% F. A. WHITE (1968) IONIZATION POTENTIAL (IP) • ELEMENT

• OXIDE

• DIOXIDE

ACCURACY < 1 - 15%

FOR 25 ELEMENTS ONLY

FOR 13 ELEMENTS ONLY

SAMSONOV (1968) VEDENEYEV (1966) AND.OTHERS

PARTITION FUNCTION (B)

• NEUTRAL ATOM

AND

POSITIVE ION

• MONOXIDE

FOR 72 ELEMENTS, <7000''K FOR 25 ELEMENTS, >7000<'K

(Fe, >7000''K) ASSUMED EQUAL TO ATOM VALUE

DeGALftN (1958) DRAWIN AND FELENBOK (1965) SPARKS AND FISCHEL (1971)

MONOXIDE

• ROTATIONAL CONSTANT (B|) FOR 78 ELEMENTS ROSEN (1971) and EXTRAPOLATION

i • VIBRATIONAL CONSTANT (cj°)

• STATISTICAL WT. GROUND STATE (g") • DISSOCIATION ENERGY (V^) " (ACCURACY 5-30%) ROSEN (1971) AND OTHERS

DIOXIDE DISSOCIATION ENERGY ACCURACY 10-30% VEDENEYEV (1966) ELECTRON AFFINITY (EA)

1 ACCURACY ~1 - 20% VEDENEYEV (1966), SAMSONOV (1968), PAGE AND GOODE (1969), GRAY (1965), AND OTHERS

STATISTICAL WT. ATOM GROUND STATE NOT PRESENTLY USED

TABLE III: Physical constants used in CARISMA, their limitations and sources.

The most important part of the correction applied to the positive

ion intensities of most elements is that given by equation 4 which

predicts the ratio of the number of singly charged positive ions to

neutral atoms of an element. In fact for the purposes of illustration

in figure 1, the positive ion intensities corrected for the atomic

concentration of the elements have been plotted as a function of

equation 4 only, and the molecular oxide ^nd negative ion contributions

have been neglected. Generally, the molecular oxide and negative ion

96
LTE Model

corrections become significant only at lower temperatures and at higher

values of the electron density respectively. Equation 4 depends on the

ionization potential of the neutral atom and on the internal partition

functions of the singly charged positive ion and of the neutral atom.

In this respect, the limitations imposed by the available physical

constants set the maximum upper limit of our program (CARISMA) at

17000°K, mainly because of a lack of the partition functions of the

neutral atoms of the elements at higher temperatures.

P.F.

100 =

"I r

100

10

1 I r MANGANESE

J L J L

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 1000/T

STEI6ER AND RUDENAUER (1971)

FIGURE 7: Partition functions of the neutral atoms of iron and man- ganese as a function of temperature (ref. 34).

97
SIMS/IMMA

The second most important part of the correction to be applied to

test the LTE model involves the calculation of the number of metal atoms

that appear as molecules (generally as oxides when bombarding with

oxygen) rather than as atoms. The remaining parts of the correction,

i.e. for negative ions and for dioxide molecules, are usually not of

significant proportions and are discussed in more detail in reference 4.

An illustration of this problem is given in figure 7, taken from

Steiger and Riidenauer [34], where the partition functions of the neutral

atoms of Fe and Mn are shown as functions of temperature. The partition

functions vary only slowly in the low temperature range (<5000°K) but

change rapidly in the higher temperature range where we generally work.

We use partition functions that have been calculated as functions of AE

by Drawin and Felebok [35], wherever possible, and extrapolate their

data to 17000°K, if their data are not given in this temperature range.

For most of the other elements we use partition functions from De Gal an et al. [36]. Their data stop at 7000°K and we use the ratio of the

partition functions of the singly charged to neutral atom at this

temperature for all higher temperatures. For some elements such as U and Th, we have to be satisfied with a single value for this ratio

established at 506o°K [37]. The effect of these limitations on the

accuracy of the computed results is unknown because the inaccuracies in

the partition functions are unknown. The effect, however, scales

directly in magnitude and is probably considerable for some of the

elements that are in the group known only in the low temperature region.

In addition to the internal partition functions the other important

constant required in the calculation of equation 4 is the ionization

potential of the neutral atom. An example of the variability that can

be found in the literature for what might be thought to be a well

established physical parameter is given in Table IV. The reported

ionization potential of U is seen to have varied by about 25 percent in

the last ten years. Errors in the ionization potential scale

exponentially (see equation 3) and can lead to very large errors in the

calculation of the positive-ion to atom ratio (10 percent error can result in a change by a factor of two or more). We (CARISMA) have been

using the 1968 Samsonov [38] value in our calculations. The appearance

of the value by Avni and Klein [39] questions the validity of that

choice and possibly bears on some of the errors we have experienced with

our U and Th analysis, as will be discussed shortly.

As stated above, the molecular oxide terms are only important at

lower temperatures and high concentrations of oxygen. This correction

is mainly influenced by the exponential dissociation energy term of

equation 8. The dissociation energies and molecular constants of the

neutral monoxide molecules of most of the elements can be found in the

literature. The accuracies of these data, however, are quite variable

and lead to variable accuracies in the final calculations. The

dissociation energies and the vibrational, rotational and electronic

constants used for the neutral monoxide molecules are generally taken

from a compilation on diatomic molecules by Rosen [40] supplemented

mainly by data from Gaydon [41]. In general, many of the ionization

.98
LTE Model

THORIUM AND URANIUM IONIZATION POTENTIAL

SOURCE Th U

*Avni and Klein (1973) 7.5 + 0.3 eV 6.3 + 0.3 eV

Mann (1964)

^ 1 1 _i_ A AC 6.11 + O.Uo

Bakulina and lonov (1959) 6.08 + 0.08

DeMaria (1950) 5.4 + 0.3

Gingerich and Lee (1954) 4.9

Samsonov (1958) 6.95 5.65

Spectrochimica Acta. 28

. p. 331.

TABLE IV: Variation in the ionization potentials of Th and U found in

the literature.

potentials of the neutral monoxide molecules are missing. To overcome

this situation the final correction equation in CARISMA [4] has been

written so as to be independent of the ionization potential of the

molecule. In order to accomplish this, however, the rather drastic

approximation is made that the dissociation constants of the positive

and negative molecular ions are equal to those of the neutral molecules.

The errors generated by these assumptions are discussed more fully in

reference 4 but generally lead to errors of unknown magnitude in the

final correction. The errors could be large, however, because of their

exponential dependence. With these inadequacies in the test procedure

in mind we can proceed to examine the negating evidence for LTE.

,IV. NEGATING EVIDENCE FOR LTE

There are three lines of observational data which indicate that the

sputtering assemblage of electrons, ions, atoms, and molecules are not

in local thermal equilibrium or at least not in exact LTE. These are: 99
SIMS/IMMA

1. Apparent Non-Equilibrium of Heavy Molecular Oxide Components

As was discussed in reference 4, the group of elements that have

the largest molecular oxide components tend to show significant errors

in their calculated concentration values when corrected according to our

LTE (CARISr^) model. This is shown in figure 8 where CARISMA-corrected

ion microprobe results for several elements of this group are shown

compared to the "true" values established by several other analytical

techniques [4]. The heavy lines represent a perfect one-to-one

correspondence and each line has its own concentration scale as indicated. The samples analyzed are coded in the legend. As

illustrated, the general result is that the ion microprobe results are

too small compared to the true values, i.e. at the temperature required

by the lighter atomic ion data CARISMA predicts the ratio of monoxide

molecular components to elemental components for the elements

illustrated in figure 8 that are much smaller than what are actua]^ly

ION MICROPROBE MASS ANALYZER

FIGURE 8: Comparison of the analyses using the LTE model (CARISMA)

with those of other analytical techniques for Zr, Mo, Nb,

La, Ce and P in metal and silicate matrices.

100
LTE Model

observed in the mass spectra. The observed intensities of the molecular

oxide ions of the heavy atomic weight elements of high oxygen affinity,

in fact, often exceed the intensities of their component metallic atomic

ions.

Our preliminary results [4] indicated that the errors, although of

unknown origin, were constant in many different samples as shown by the

dashed line in figure 8. Recent work, however, indicates that this

simple conclusion is probably not valid. It now appears that the errors

found for a particular element can be quite variable. This is best

demonstrated by looking at an extreme case. The errors of largest

0 2 4 6 8 10 12 'JO/U orTho/Th

FIGURE 9: Error factors by which the LTE model (CARISMA) derived U and

Th concentration values must be multiplied to _obtain the

true values as a function of the observed liO^/U'^ and

ThO+ZTh"*" ratios.

101
SIMS/IMMA

magnitude encountered using our LTE (CARISMA) model are found for U and

Th. Figure 9 shows the errors recently determined for these elements in

a set of zirconolite mineral samples [42] and in the NBS glass standard

SRM 612. The factors (AU or ATh) by which the CARISMA calculated

concentration value must be multiplied to obtain the true concentration,

are plotted versus the observed UO/U or ThO/Th ion ratios respectively.

The molecular oxide to atomic ion ratio, MO/M, is chosen because it

represents an internal thermometer that can be used to judge the

temperature of the sputtering assemblage [4]. These results indicate

that as the temperature decreases (increasing MO/M ratio), the error

increases. The generally good correspondence of the errors with 'the

MO/M ratio, which is directly observed in the mass spectrum, leads to an extremely simple empirical correction that has been effectively used to

quantitatively analyze uranium in radiometric age dating work [43]. It

is apparent that the errors can be very large; much larger, in fact,

than can be generated by possible errors in the ionization potential

such as discussed in Table IV (i.e.j changing the uranium ionization

potential from 5.65 to 6.3 decreases the error factor by only about 20

percent in a typical case). The effects of errors in the oiher

constants discussed in Table III are probably equally small, but the

effect of the assumption as to the equivalence of the positive and

negative ion and neutral molecular oxide dissociation constants is

unknown and may be significant.

The errors, however, are not instrument dependent (assuming the

instrument is operated in a standard analytical mode) as is illustrated

in figure 10. Figure 10 shows the CARISMA error factors determined for

the rare earth elements by two different operators [44] using the same

set of standards [45] and two different ion probes (IMMA) of the $ame

manufacture. It is apparent that the error factors are essentially the same in each laboratory for these elements as indicated by the solid

one-to-one correspondence line.

As mentioned earlier, the molecular oxide ions are expected to be

most important at low temperatures, assuming a constant concentration of

oxygen. The large oxide components observed in the mass spectrum,

therefore, lead to the conclusion that the heavy molecular oxides are at

a lower temperature than are the lighter atoms and electrons of the

sputtering assemblage. It is possible to calculate this apparent lower

temperature by assuming that the total observed error is in the

temperature dependence of the dissociation constant of the heavy

molecule (see equation 8). This was done for the set of zirconolite

mineral and NBS glass data discussed above and the results are given in

Table V.

In Table V, Tp represents the CARISMA temperature determined from

the ion intensity data of the major elements, AU and ATh are the errors

in the U and Th analyses at that temperature, and Tg is the temperature

required for the molecule in order to obtain the true U or Th

concentration. AT is the apparent temperature drop of the heavy

molecule. It can be seen that the relative temperature decrease (AT/Tg)

is fairly constant in these samples, showing less than a 30 percent
LTE Model

FIGURE 10: Error factors for the rare-earth elements derived by two

different ion microprobe laboratories (ref. 44).

total variation (standard deviation of 10 percent or less) although the error factors, themselves, range over factors of ten.

At present, it is not known if this improvement is fortuitous or actually indicates a lower temperature for the heavy molecular oxides.

The errors seem to be related only to the heavy atoms that have large

oxide molecules such as U and Th and those elements mentioned in

reference 4. Pb which has nearly the same mass as U has only a small

molecular oxide and a very much smaller error. The error also generally

decreases with atomic weight. Significant errors only begin to appear

10^3
SIMS/IMMA

TEMPERATURE DIFFERENCE OF ELECTRONS AND MOLECULES

SAMPLE Te AU aT/Te aTh aT/Tf

ZlKLUNULl 1 tCK) 1 1 ncn iiybu cc o bb.o flirt .540 87.8 .507

1 77on

.519 27.1 .461

1 /lonn 14zUU on o .500 17.5 .444

1 /icnn "Zl It 55,

r"i ^ .517 31.3 .476

II fZh\ 1 oncn izybU 28.9 .452

1 1 con

27.0 .427

II fC'D\ Cod; 1 "znnn

.511 34.9 .496

II f'7IK\ C/A) 111 cn 11160 /in /

.453 35.1 .418

(7C) 12180 36.1 ,474 27.5 .450

(8A) 10000 27.7 .380

NBS (GCE) 13640 8.7 .447 5.00 .380

(H) 15^180 10.9 .470 6.48 .580

0.492*. 032 0.437±.048

TABLE V: Error factors for Th and U in several LTE-corrected mineral

analyses listed with their CARISMA temperatures (Tg) and

the possible reduction in the temperature (AT) required to

obtain the true analysis.
LTE Model

around atomic number 40 and above. The lighter molecular oxides

generally have the temperature (Te) established by CARISMA for the atoms

and ions of the sample, see figure 3.

Finally, it is possible that the observed errors are related to the

abnormally large energy spreads (up to several hundred electron volts)

observed [46] to be associated with the metallic atomic ion component of

these molecular oxides. The large energy spreads may result in a physical discrimination by the ion probe itself against the total

integrated intensity of the atomic ion species, or may indicate a second

ion generating process that is not controlled by LTE. More work is in

progress on this subject.

2. Apparent Non-Equilibrium of Multiply Charged Ions

The LTE model appears to fail in its ability to predict the

observed ratio of multiply charged ions to singly charged ions and,

possibly, also in its ability to predict the correct ratio of singly

charged ions to neutral atoms for those elements which have a large

first ionization potential, e.g.^ fluorine (Ip=17.4 eV).

An illustration of this behavior for multiply charged ions is given

in figure 11 where data collected from several analyses of three similar

silicate minerals are shown. This graph is similar to that given in

figure 1 and is actually a plot of the Saha-Eggert ionization equation.

The ratios of the singly charged ions to neutral atoms modified by their

partition functions are plotted versus the first ionization potential of

the element (AE = 0.55 eV) using the method described in the discussion

of figure 1. The doubly charged to singly charged ion ratios (++) and

the triply to doubly charged ion ratios (+++) are those actually

observed in the mass spectra. They are modified by their partition

functions and are plotted versus the second and third ionization

potentials of the element (AE = 1.10 and 1.65 eV, respectively).

The slope of the line on such a plot gives the temperature of the

assemblage and the absolute position of the line gives the electron

density. It can be seen that the slopes of the two lines drawn through

the singly and doubly charged ratio data are identical , within

experimental uncertainty, but that the doubly charged data lie above the

singly charged data. This indicates that, although the doubly charged

ions are at essentially the same temperature as the singly charged ions

and neutral atoms, they exist in a region of approximately 50 times

lower electron density. This result was noted earlier [4] and leads to

a problem in directly using the observed ratios of doubly to singly

charged ions of a pair of elements for obtaining the electron density

appropriate to the sputtered singly charged ions and neutral atoms. The

electron density is too low by a rather large, but seemingly constant,

factor. As can be seen in figure 11, the triply charged data appear to

be at even a lower electron density.

The reason for this apparent violation of LTE is not understood at

present. Possibly it is related to the relative velocities of the

105
SIMS/IMMA

different chairged species and the probability for an electronic

transition between the ion and the surface. In the mode of analysis

used to acquire the data of figure 11, a positive bias of about 1500

•6.0

-7.0

•8.0

SILICATE

"o", 17 K«v

T 10200' K

e

Si' t

9

O**

10 16 20 26 30 36 40 46 60

Z. - Uv)

FIGURE 11: Saha-Eggert ionization equation applied to singly, doubly.

and triply charged positive ions sputtered from silicate

standard with ^^O" at 17 keV.

106
LTE Model

volts is applied to the surface of the sample. According to the

discussion of van der Weg and Rol [47] this could lead to a greater

probability of escape without an electronic transition to a given

distance from the surface of the sample for the multiply charged ions of

an element because of their larger velocities in the applied field. The

observed data would then indicate that an electron density gradient

exists which decreases away from the positively charged surface of the

sample.

It is also quite possible that some completely different mechanism

controls the production of the majority of the multiply charged ions.

This possibility is consistent with the observations of Joyes [48] who

notes a strong kinetic character in the emission of Al and Al"'""'"^ but

only a very small kinetic character in the emission of the low energy

Al"*" ions which, of course, represent the great majority of the sputtered

Al ions.

Figure 11 also shows that the data for doubly-charged ions of very

high ionization potential appear to be at even lower electron densities.

It may also be true that singly-charged ions of high ionization

potential, such as fluorine, do not exhibit the same electron density as that shown by the elements of lower ionization potential. Fluorine

generally has ion abundances that are too large, sometimes apparently by

as much as an order of magnitude or more, with respect to the ion

abundances of the lower ionization potential elements in the same

sample. The reason for this is not understood and more work needs to be

done.

We are presently performing our quantitative analysis of the high

ionization potential elements by using working curves constructed from

many different samples of the same general chemistry [49]. This

approach must be carefully checked when a substantially different sample

composition is analyzed. For example, Hinthorne [50] showed that the

slope of the fluorine working curve changed by 30-40 percent between

silicates and phosphates. The working curves are also very instrumentdependent and extreme care should be taken when attempting to transfer

so-called sensitivity factors between instruments of different design.

For example, the slope of the F working curve (fVP"^ x P% versus F%) in

a phosphate matrix was found to be 6.5 in an Applied Research

Laboratories Ion Microprobe Mass Analyser (IMMA) [50] and 0.136 in a CAMECA Ion Microscope (IMS) [8].

3. Non-Maxwellian Energy Distribution of the Sputtered Ions

The LTE model also appears to fail when the energy distributions of

sputtered particles are considered. The particles of a thermal

equilibrium assemblage are expected to exhibit a Maxwellian energy

distribution characterized by a common temperature. An example of the

observed energy distributions of singly charged Al and Si ions sputtered

from a glass sample is given in figure 12 [51]. The long, high energy

tails observed do not appear to be Maxwellian in character and the

energy distributions are found to have different shapes for two

107
SIMS/IMMA

different elements from the same sample. The peaks of the energy

distributions are also found to be at fairly high values ('^'6 eV) com- pared to the temperatures calculated for these ions using the LTE model.

In general, the lower sputtered-ion energies (<20 eV), which

contain the great majority of the sputtered ions, can be fit to a Maxwellian energy distribution. The temperature coefficients of these

distributions are, however, very large ('\^7xlO'*°K in the present

example). In fact, Maxwellian energy distributions for low-energy

sputtered ions have been reported by Stanton [52] and by Coswell and

Smith [53]. Secondary electrons generated under ion bombardment also

display energy distributions that contain the majority (>95%) of the

electrons in the low energy region (<20 eV) [54].

The higher-energy ions, however, are observed to deviate from the

Maxwellian distributions by a generally continuously increasing factor

as energy increases. Sometimes small bumps are noticed in the very high

energy regions of the distributions [55]. The reasons for these

deviations are not understood at present, but have been ascribed by

Stanton [52] to a changing amplitude function in the Maxwellian

distribution and used by Jurela [55] as evidence for a second mechanism

for the generation of sputtered ions.

108
LTE Model

It is obvious that much more work needs to be done in this area for

a proper understanding of the meaning of the energy distributions. It

should be remembered that these charged particles exist in the presence

of a static electric field and its effect on the overall shape of the

energy distribution curves must be taken into account [56]. Another

important modifying influence on the shape of the energy distribution

curve may be the potential barrier at the surface of the sample, or the

binding energy of the atom to the surface of the sample, which must be overcome in order for the atom to be sputtered [57,58].

V. POSSIBLE PHYSICAL MECHANISM

An obvious question to be asked at this point is whether or not

there is a physical mechanism that can possibly explain the apparent LTE

conditions postulated in this work. The most likely candidate for this

purpose would appear to be the thermal spike model originated by Seitz

and Koehler [59] and discussed by Thompson and Nelson [60] and Vinegard

[61]. A thermal spike is pictured as a local volume of a target crystal

wherein the majority of the atoms are temporarily in motion as the

result of an atomic collision cascade initiated by the process of high

energy ion bombardment. To satisfy our data requirements we would

imagine this collision cascade as intersecting the surface of the

sample. The phenomenon of thermal spikes has been used to interpret

sputtering data [62,63] and has also been proposed by Jurela [14] to be

the possible source of the high temperatures encountered in his model of

secondary sputtered ion emission.

An exhaustive review of thermal spike theory will not be made here,

but rather the predictions of a recent preliminary theoretical study by

Sigmund [64] will be used to examine its possible application to our

data. Sigmund 's study predicts the time constant of the spike, the

lifetime, of the cascade (the slowing down time of the projectile atom),

and the effective maximum energy density (energy per atom) within the

spike as functions of the energy and mass of the bombarding ion beam and

the mass of the target sample. An important conclusion drawn by Sigmund

is that a tendency towards local thermal equilibrium exists in the spike

which is dependent on how much the time constant of the spike (x)

exceeds the lifetime of the cascade (tq). These times and the effective

energy per atom have been calculated according to Sigmund 's model [64]

and the results are given in Table VI.

Table VI lists experimental data obtained from silicate standard

samples (Z ^ 10) bombarded with ^H", ^^0", and ^^Cl" and from a heavy

metal standard (Waspalloy, Z ^ 29) bombarded with ^^0", all at 15 keV.

The LTE temperatures obtained from these data according to CARISMA are listed for comparison with effective energies per atom and the times

calculated according to Sigmund's model. It is obvious that the

calculated spike energy approaches that predicted by the LTE model only

in the case of the heavy matrix. The very light bombarding ion (H)

would appear to result in only a very small value for the effective

energy per atom compared to that apparently predicted by the LTE model

.

109
SIMS/IMMA

THERi'lAL SPIKE PARAMETERS'

15 KEV TIME CONSTANT CASCADE LIFETIME

MAX. ENERGY DENSITYTEMP. SPIKETEMP.

CARIS.'IA

ION/SAMPLE T (sec) To (sec) T/To E (eV) T(°K) T(°K)

VSI LI GATE lA X 10"^ 3.7 X 10"" 6.5 X 10^ .002 23 14150

"O/SILICATE 5.7 X 10"^^ 1.2 X 10"" 4.8 X 10^ .05 580 14500

"Cl/SILICATE 2.3 X 10"^^ 1.7 X 10"" 1.4 X 10^ .10 1160 14300

"O/WASPALLOY 1.0 X 10"^^ 1.2 X 10~" 8.3 X 10^ .52 5990 8500

*P. SIGMUND, APPL. RHYS. LEH 25. 169. 1974. (USING M

= 1/3).

TABLE VI: Comparison of LTE (CARISMA) temperatures with the effective

maximum energy density per atom, the atomic cascade

lifetime, and the thermal spike time constant derived from

Sigmund's model (ref. 64).

These calculations were made using m = 1/3 in the elastic scattering

power potential [64]. The calculation of the effective energy per atom

is very sensitive to the choice of this exponent and although it is

probably a good choice for low keV oxygen bombardment [65], it may not

apply as well to bombarding ions or targets of greatly different mass.

The time constant of the spike according to this model is generally

on the order of 10"^^ seconds and exceeds the lifetime of the cascade by

about two orders of magnitude. This is about 100 periods of atomic

vibration and would appear to be within the range where concepts of

thermal equilibrium have some validity [66]. The calculated time

constant for the H gas bombardment experiment is exceedingly long which

may be related to the problem of the choice of the power potential

exponent discussed above.

Another important conclusion from Sigmund's model is that, the

effective energy per atom in the spike should decrease in a simple

linear relationship with increasing bombarding ion energy. This

unsuspected inverse relationship was observed earlier in the data given

in Table I. Experiments performed by bombarding a silicate (andesine)

and a heavy matrix (Waspalloy) with ^^0" ion beams of different energy

had previously yielded data which the LTE model interpreted to be

no
LTE Model .

decreasing temperatures with increasing bombardment energy. These data

are given in Table VII as a function of the true bombarding beam potential (the primary accelerating potential plus 1.5' kV of additional

extraction potential at the sample).

ANDESINE "^CARISMA WASPALLOY "^CARISMA

keV eV/A(Z=0) _K eV/A(Z=0) J<

9.5 35.2 16740 36.5 14940

12.5 31.6 14280 33.8 12360

15.5 28.1 14640 31.2 9260

17.5 25.7 13560 29.4 7760

19.5 23.4 11480

TABLE VII: Comparison of LTE (CARISMA) temperatures with the energy

lost in the surface of samples bombarded at different keV

using the data of figures 14 and 15 (ref. 68).

The data are plotted in figure 13 as an inverse function of the

bombarding ion energy according to the model of Sigmund [64], The

general correlation is apparent.

Earlier, T. Ishitani and his colleagues used their Monte Carlo

simulation program [67] to calculate the energy loss of the bombarding

oxygen beam with depth in the sample for andesine and Waspalloy as a function of energy [68]. These simulations are shown in figures 14 and

'15. It is instructive to note that the relative difference of the

deposited energy at some depth in the sample (such as, for example, that

associated with the mean of the energy loss distribution with energy)

does not reflect the observed CARISMA temperature differences. This is

especially obvious in the Waspalloy data where the two sets of data

actually trend in the opposite manner. It is the energy losses in the

first collisions at the very surface that reflect the trend of the

CARISMA temperatures. This again indicates that the hot zone we are sampling involves only the uppermost surface layers of the sample. In order to illustrate this, the energy losses at pZ equal to zero depth

werg taken from the figures and are listed in Table VII (the units of

eV/A were chosen for convenience). The data are also plotted in figure

16. The correlation of the CARISMA temperatures with the energy

deposited in the surface layer according to a Monte Carlo simulation is

apparent.

.111
SIMS/IMMA

(E)"^ (keV)

FIGURE 13: LTE (CARISMA) derived temperatures for a silicate (andesine)

and heavy metal alloy (Waspalloy) bombarded with ^^0" at

different energies as a function of the inverse of the

bombarding beam accelerating potential.

It would appear that there is some hope of interpreting the

apparent high temperatures indicated by the LTE model through either the

thermal spike model directly or a computer simulation' of the energy loss

of the bombarding ion beam and the dissipation of this energy among the

affected atoms.

VI. CONCLUSION

In conclusion it can be said that there is a good deal of

experimental evidence that a simple model based on the concept of local

thermal equilibrium of the sputtering assemblage can be used to

interpret sputtered ion yields with good accuracy. The practicality of

the model for quantitative elemental analysis of a wide variety of

112
LTE Model

FIGURE 14: Monte Carlo simulation of the energy loss of a bombarding

^^0' beam as a function of depth in andesine for different energies (ref. 68).

samples with many different instrument designs is generally confirmed.

The physical basis of the model, however, is very much an area of

speculation with the particular aspects of the very high temperatures

predicted, possibly exceeding the critical temperature [69] of the

sample [14b], and of the establishment of equilibrium between the atoms

and the electrons being questions of particular concern.

113
SIMS/IMMA

"0 Waspalloy

6

DEPTH 3Z (jjg/cm2)

Ishitani (1973)

FIGURE 15: Monte Carlo simulation of the energy loss of a bombarding

^®0" beam as a function of depth in Waspalloy for different energies (ref. 68).

114
LTE Model

o

o

o

20

19

18

17

16 -

15 -

14

13

12

1

10 -

9

8

7

1—

1—I—I—I—I—

"1—I—I—I—I—

J LJ I I I 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 {®VA)z=0

FIGURE 16: Energy loss (eV/A) at the surface (pz=0) of samples using

data given in figure 14 and 15 as functions of the LTE

(CARISMA) derived temperatures.

VII. REFERENCES

1. Andersen, C. A., Third Natl. Electron Microprobe Conf. Chicago, IL,

1968; Int. J. Mass Spectrom. Ion Phys. 2, 61 (1969).

2. Andersen, C. A., Fourth Natl. Electron Microprobe Conf. Pasadena,

CA, 1969; Int. J. Mass Spectrom. Ion Phys. 3, 413 (1970).

3. Andersen, C. A., Fifth, Sixth, and Seventh Natl. Electron

Microprobe Conf. New York, 1970; Pittsburgh, 1971; San Francisco,

1972.

4. Andersen, C. A. and Hinthorne, J. R., Analytical Chem. , 45, 1421

(1973).

"

5. Saha, M. N., Phil. Mag., 40, 472 (1920); Z. Phys., 6, 40 (1921; and

Eggert, J., Z. Phys., 20,"570 (1919).

6. Shimizu, R., Ishitani, T. and Ueshima, Y., Jap J. Appl . Phys., 13,

250 (1974).

115
SIMS/IMMA

7. RLidenauer, F. 6. and Steiger, W., Proc. "Sth Intl. Vacuum Cong.,

Kyoto, Japan, 1974.

8. Lodding, A., Gourgout, J. M., Petersson, L. G. and Pros tell, G., Z. Naturforsch., 29a, 897 (1974).

9. Meyer, C, Jr., Anderson, D. H., and Bradley, J. G., Proceedings

5th Lunar Science Conf., Supplement 5, Geochim. et Cosmochim. Acta,

1, 685 (197 ).

10. Schroeer, J., Workshop on Secondary Ion Mass Spectrometry, National

Bureau of Standards, Gaithersburg, 1974, paper 6.

11. Simons, D., University of Illinois (Urbana), private communication.

12. Levering, J. F., Workshop on Secondary Ion Mass Spectrometry,

National Bureau of Standards, Gaithersburg, 1974, paper 8.

13. Bradley, J. G., Workshop on Secondary Ion Mass Spectrometry,

National Bureau of Standards, Gaithersburg, 1974, paper 4.

14. Jurela, Z., (a) Radiation Effects, 13, 167 (1972) and (b) Int. J.

Mass Spectrom. Ion Phys., 12_, 33 (19737.

15. Boumans, P. W. J. M., Theory of S-peotvoohemioal Exaitation^

Plenum Press, N. Y., 1966, p. 80.

16. Herzberg, G., Moleoular Spectra and Molecular Structure^ J. Spectra

of Diatomic Molecules3 D. van Nostrand, Princeton, N. J., 1950,

pp. 123, 125, 467.

17. Almen, 0., and Bruce, G., Nucl . Instr. Methods, 21. 257 (1961).

18. Andersen, C. A., Hinthorne, J. R., Eighth Natl. Electron Microprobe

Conf. , New Orleans, 1973.

19. Langmuir, I., and Kingdon, K. H., Proc. Royal Soc. , A107 , 61 (1925).

20. Dekker, A. J., Solid State Physics^ Prentice-Hall, Englewood

Cliffs, N. J., 1962, p. 220.

21. Shankland, T. J., Science, July, 51 (1968).

22. Haas, G. A., and Thomas, R. E., Measurement of Physical Properties

y

Ed. E. Passaglia, Interscience Publ . , N. Y., 1972, p. 91.

23. Hermann, G., and Wagener, S., The Oxide Coated Cathode^ Chapman

and Hall, London, 1951, Vol. 2, p. 74.

24. Snoek, C, van der Weg, W. F., and Rol . P. K., Physica, 30, 341

(1964). ~"

116
LTE Model

25. Tsong, I. S. T., Phys. Stat. Sol. (a). 7, 451 (1971).

26. White, C. W., Simms, D. L., and Tolk, N. H., Science, 177 , 481

(1972).

27. Tolk, N. H., Simms, D. L., Foley, E. B., and White, C. W.

Radiation Effects, 18, 221 (1973).

28. Kerkdijk, C, and Thomas, E. W., Physica, 63, 577 (1973).

29. Thomas, G. E., and de Kluizenaar, E. E., Le Vide, 167, 190 (1974).

30. Tsong, I. S. T., and McLaren, A. C, Nature, 248, 43 (1974).

31. Kato, M., Shimizu, R., and Ishitani, T., Techn. Reports of Osaka

Univ., 24, No. 1199, 451 (1974).

32. Kerkow, H., Phys. Stat. Sol. (a), 10, 501 (1971).

33. Shimizu, R. , Kato, M., and Ishitani, T., Jap. J. Appl . Phys., V3.

1477 (1974).

34. Steiger, W., and Rlidenauer, F. G., J. Mass Spectrom. Ion Phys., 1_3,

411 (1974).

35. Drawin, H. W., Felenbok, P., Data for Plasmas in Loaal

Thermodynamic Equilibrium^ Gautheir-Vil lars , Paris, 1965.

36. De Gal an, L., Smith, R., and Winefordner, J. D., Spectrochim.

Acta, 23B, 521 (1968).

37. Corlin, C. H., J. Astrophys. 136^, 916 (1962).

38. Samsonov, 6. V., Handbook of the Physiaoahemiaal Properties of the

Elements, IFI/Plenum, N. Y., 1968, p. 20.

39. Avni, R., and Klein, F. S., Spectrochim. Acta, 28B^, 331 (1973).

40. Rosen, B., Speatrosaopia Data Relative to Diatomic Molecules

Pergamon Press, N. Y., 1970.

41 . Gaydon , A . G . , Dissociation Energies and Spectra of Diatomic

Molecules, 3rd ed.. Chapman and Hall, London, 1968.

42. Lovering, J. F., Dept. Geology, Univ. Melbourne, Australia.

Personal communication.

43. Hinthorne, J. R., Andersen, C. A., and Conrad, R. L., to be

submitted to Geochimica et Cosmochimica Acta (1975).

44. Lovering, J. F., In press, Coiments on Earth Sciences: Geophysics

(1975).

117
SIMS/IMMA

•45. Duke, M. J.j Center for Volcanology, Univ. Oregon, Eugene, Oregon.

Personal communication.

46. Fralick, R. D., and Roden, H. J., Applied Research Laboratories,

Goleta, CA, personal communication.

47. van der Weg, W. F., and Rol , P. K., Nucl . Instr. Methods, 38, 274

(1965): —

48. Joyes, P., Radiation Effects, 19^, 235 (1973).

49. Hinthorne, J. R., and Andersen, C. A., Am.. Mineral . , 60, 143

(1975). —

50. Hinthorne, J. R., 26th Pittsb. Conf. on Analytical Chem. and Appl

.

Spect., Cleveland, 1975.

51. Roden, H. J., Fralick, R. D., 26th Pittsb. Conf. on Analytical

Chem. and Appl. Spect., Cleveland, 1975.

52. Stanton, H. E., J. Appl. Phys., 31_, 678 (1960).

53. Cowell, G. K. , and Smith, H. P., Jr., J. Appl. Phys. 43, 412

(1972) .

54. Lonchet, F., Viel , L., Benazeth, C, Fagot, B., and Colombie, N.,

Radiation Effects, 14, 123 (1972).

55. Jurela, Z., Ion Surface Intevaotion^ Sputtering and Related

Phenomena^ Eds. R. Behrisch, W. Herland, W. Poschenrieder , P. Staib and H. Verbeek, Gordon and Breach Science Publ . , London,

1973, p. 161.

56. Blaise, G., and Slodzian, 6., Revue de Physique Appliqu^e, 8, 105

(1973) .

57. Miyagawa, S., J. Appl. Phys., 44, 5617 (1973).

58. Harrison, D. E., Jr., Moore, W. L., Jr., and Hoi combe, H. T.,

Radiation Effects, 17_, 167 (1973).

59. Seitz, F., and Koehler, J. S., Solid State Physios^ Eds. F. Seitz

and D. Turnbull, Academic Press, N. Y., 1956, Vol. 2, p. 305.

60. Thompson, M. W., and Nelson, R. S., Phil. Mag., 7_, 2015 (1962).

61. Vineyard, 6. H., Faraday Soc. London, Discussion, 31_, 7 (1961).

62. Anderson, H. H., and Bay, H. L., Radiation Effects, 19, 139 (1973).

63. Kelly, R., and Lam, N. Q., Radiation Effects, 19. 39 (1973).

118
LTE Model

64. Sigmund, P., Appl . Phys. Letters, 25^, 169 (1974).

65. Sigmund, P., H. C. Orsted Institute, Copenhagen, personal

communication.

66. Dienes, G. J., and Vineyard, G. H., Radiation Effeois in Solids^

Interscience Publ . , N. Y., 1957, p. 36.

.67. Ishitani, T., Shimizu, R., and Murata, K., Jap. J. Appl. Phys.,

Jl, 125 (1972); and Phys. Stat. Sol.(b), 50, 681 (1972).

68. Ishitani, T., and Shimizu, R., Dept. of Appl. Phys., Osaka Univ.,

Osaka, Japan, personal communication.

69. Gates, D. S., and Thodos, G., A. I. Ch. E. Journal, 6, 50 (1960)

and D. A. Young and B. J. Alder, Phys. Rev. A, 3, 364 (T971 )

.

119

NBS Spec. Publ. 427, Workshop on Secondary Ion Moss Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Ga ithersburg, Maryland, September 16-18, 1974 (Issued October, 1975).

AN OUTLINE OF SECONDARY ION EMISSION MODELS

Juergen M. Schroeer

Department of Physics

Illinois State University

Normal , 111 inois 61761

I . Types of Models

The models for the emission of secondary ions can be classified as follows:

A. Emission of Atomic Ions:

1. Thermodynamic Models

a) Saha-Langmuir

b) Andersen-Hinthorne

c) Jurela

2. Quantum-mechanical Models

a) Schroeer

b) Sroubek

c) Van der Weg and Bierman, Benninghoven

d) Joyes (Paris group)

e) Rudenauer et al.

f) MacDonald

B. Emission of Molecular Ions:

a) Benmngnoven (dissociation) and Slodzian (chemical

ionization)

b) Valence model of Plog, Wiedman, and Benninghoven

c) Fingerprint model of Werner

II. Definitions

Total sputtering yield:

S =

(f^"^) + (M*) + (MO) + CM") + molecules (i)

(A+)

where +, -» and o refer to the charge of the sputtered particle, * is

excited neutral, M is the sputtered metal atom. A"*" is the bombarding

ion, and (M"^) is the number of sputtered positive metal atoms, etc.

Secondary ion yield:

3^ = ^

(2)

(A*)

121
SIMS/IMMA

Probability of ionization:

(3)

(M+) + (M*) + (MO) + S

If we make the simplifying assumption that we are dealing with a matrix m of a single element, in which various impurities i_with concentrations

£i are dissolved, then the apparent secondary ion yield for the

iffipurities is

In (4) we made the further assumption, that after equilibrium has been

reached, the total sputtering yield for the impurities is the same as for the matrix atoms, i.e.j Si=Sm. Also,

since Cm 1 . So,

C-i =

1 m rel m

The spectroscopist measures the Y's, and wants the c's. He needs to

know Rr^el'

III. Langmuir-Saha Equation [1]

The Langmuir-Saha equation

where I_ is the ionization energy and the work function, holds if we have thermal equilibrium between a surface at temperature T and the

evaporated atoms, ions and electrons. Equation (7) is applicable to

thermal desorption, but does not work for sputtering.

122
Ion Emission Models

IV. Model of Andersen and Hinthorne [2]

The model assumes a plasma "at the surface" where there is local

thermal equilibrium between atoms, ions and electrons, such as M°^M+ + 6". (8)

The equilibrium constant is described by the Saha-Eggert equation

i = (ZTM k

1)3/2 2 B (T) e \

(M°) ® B°(T) \ kT /

where T is the plasma temperature, B the partition functions (tabulated

in the literature), AE the reduction in the ionization energy due to the

plasma, Ng the electron concentration, and mg the electron mass. This

model works very well for the bombardment of alloys and minerals with

oxygen or other reactive gases at energies around 10 keV. T'vSOOO to

15,000 K. T and Hq are fitting parameters. [3] R'*"(rel) can be

calculated [2,4] from (9). If (M+)«(M°), then

+ B. (T)

R (rel) = constant exp (- — I . (10)

B.°(T)

V. Model of Jurela [5]

Jurela uses the Dobretsov equation which deals with surface

ionization through non-equilibrium thermodynamics:

^ " B°(T) — '

' ^-''•^

where T is the local surface temperature, and Iq=1-M. AI is the image

potential energy at a critical distance from the surface, which is

approximately equal to one-half of a lattice constant. The model was tested by bombarding many pure elements with 8 and 40 keV argon ions.

Solving for the temperature, he found values between 2000 and

123
SIMS/IMMA

5000 K, which he feels are more reasonable than the temperatures in the

model of Andersen and Hinthorne.

He assumes that the sputtered atoms leave the surface in a neutral,

unexcited state and are subsequently ionized by a quantum-mechanical

transition of the atoms' valence electrons to the top of the conduction

band of the metal. The ionization probability is calculated using the

adiabatic approximation, giving

where A is the surface binding energy of the sputtered atom, ti is

Planck's constant divided by 2it, v^ its velocity, n_ a fitting parameter

('\^2.5), a^ is about one-half the surface layer thickness and is used as a fitting parameter i"^] .5 A). The model has been tested with reasonable

success [4,6,7] by bombarding pure metals with 12 keV argon ions.

Limited success has been obtained for the analysis of alloys. [8]

Sroubek's basic approach is the same as Schroeer's, but he makes

use of molecular orbital calculations to evaluate the quantum-mechanical

transition probabilities. Depending on the mathematical approximations

made, he obtains

VI . Model of Schroeer

(12)

VII. Model of Sroubek [9]

or

3

When he considers the possibility of level crossings between the

electron levels of the solid and those of the sputtered atom, then

124
Ion Emission Models

T,+ V 1 1/2 (-15) ^ "

[

a (I - cDj J

Experimental, tests are very limited so far. [9]

VIII. Model of van der Weg and Bierman, [10] and Benninghoven [11],

They assume that the sputtered particle leaves 'the surface as an ion and is subsequently neutralized by electrons from the. sol id through

resonance tunneling and Auger transitions,

R % exp (- —j, ^

^

witha=^2A, 0=^4x10^** sec"^. This was tested for the excitation of

Cu under 80 keV argon bombardment. [10]

IX. Model of Joyes et al. [12]

They assume that the atoms are sputtered as neutrals, but with an inner electron excited to an outer electron shell. Upon de-excitation,

the available energy is given to a valence electron which is ejected

(this is called auto-ionization) . The mathematics is very complex;

tests have been limited so far to the bombardment of a few transition

elements by argon. [13]

X. Model of Rudenauer et al. [4,14]

They take Schroeer's ionization model, combine it with Sigmund's

sputtering theory, and apply it to the secondary ion emission from

alloys.

XI. Model of MacDonald [15]

He finds an empirical expression for R"''(v) by measuring S"''(v) and

S(v) separately:

R'^Cv) = S''(.v)/S(v), (17)

where v_ is the velocity of the sputtered atom. He feels that the

experimental results are consistent with the assumption that we have

auto-ionization from an excited state, at least, for the sputtering of Cu

by 10 keV argon.

125
SIMS/IMMA

XII. Molecular Ions*

Atomic and molecular ions can be produced through the breaking of

bonds of chemical compounds on the surface: dissociative [11] or chemical [16] ionization. The secondary ion spectrum depends on the

atomic and molecular species present on the surface, and on the

condition of the surface, such as its state of oxidation.

The empirical valence model of Plog et al. [17] quite successfully

accounts for the relative intensities of ions in series such as M"*", MO"*", MOz"^, etc., emitted from oxidized metal surfaces and from oxides.

The fingerprint model of Werner [18] tries to explain the secondary

ion spectrum of an unknown surface (such as oxidized Cr) as a superposition of the secondary ion spectra characteristic of known

compounds (such as CraO, Cr202j CraO, etc.).

XIII. Acknowl edgements

.Financial support through a Faculty Research Award from Illinois

State University is gratefully acknow'aifdged. A sajjbatical leave at the

Physikal isches Institut der Universitat Munster made me aware of some

very recent work, which I could therefore include in this written

summary.

[1

[2

[3

[4

[5

[6

[7

[8

[9

XIV. References

Dresser, M. S., J. Appl . Phys. 39 (1968) 338, Zemel; J., J. Chem.

Phys. 28 (1958) 410.

Andersen, C. A. and Hinthorne, J. R., Anal. Chem. 45_ (1973) 1421.

Shimizu, R., Ishitani, T. and Ueshima, Y., Jap. J. Appl. Phys. 1_3 (1974) '250.

RUdenauer, F. G., Steiger, W. and Portenschlag, R., Mtkrochimica

Acta (Wien). Suppl . 5 (1974) 421.

Jurela, Z., Int. J. Mass Spectr. and Ion Phys. 12 (1973) 33.

Schroeer, J., Rhodin, T. and Bradley, R., Surface Science 34_ (173)

571.

Wittmaak, K.,- "Energy Dependence of the Secondary Ion Yield of

Metals and Semiconductors," to be published.

Schroeer, J., Vacuum 22_ (1972) 603.

Sroubek, Z., Surface Science 44 (1974) 47.

126
Ion Emission Models

[10] Van der Weg, W. F. and Bierman, D. J., Physica 44 (1969) 177 and

206.

[11] Benninghoven, A., Z. Physik 220 (1969) 419.

[12] Joyes, P., J. Physique 29 (1968) 774, and 30 (1969) 243 and 365,

Castaing,' R. and Hennequin, J.-F. , Adv. in Mass Spectrometry 5 (1971) 419, and Joyes, P., Radiation Effects 19 (1973) 235.

[13] Blaise, G. and Slodzian, G., J. Physique 31_ (1970) 93.

[14] Gries, W. and Riidenauer, F. G., "A Quantitative Model for the

Interpretation of Secondary Ion Mass Spectra of Dilute Alloys," to

be published in Int. J. Mass Spectrometry and Ion Physics.

-[15] MacDonald, R. J., Surface Science 43^ (1974) 653.

[16] Castaing, R. and Hennequin, J.-F., Adv. in Mass Spectrometry 5^ (1971) 419 and Slodzian, G. and Hennequin, J.-F., Compte rendus

heb.des stances Acad. Sci . (Paris) Serie B 263^ (1966) 1246.

[17] Plog, C, Wiedmann, L. and Benninghoven, A., "Empirical Model for

the Calculation of Secondary Ion Intensities from Oxidized Metal

Surfaces and Metal Oxides," to be published in Surface Science.

[18] Werner, H. W., -de Grefte, H. A. M. and van den Berg, J., Adv. in

Mass Spectrometry 6 (1974) 673.

127

NBS Spec. Publ. 427, Workshop on Secondary Ion Moss Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Gclthersburg, Maryland, September 16-18, 1974 (Issued October, 1975).

EMPIRICAL QUANTITATION PROCEDURES IN SIMS

J. A. McHugh

General Electric Company

Knolls Atomic Power Laboratory

Schenectady, New York 12301

The secondary ion yield is very sensitive to the state of the

surface, the matrix, and the effects induced by the primary ion beam.

Therefore, the ratio of secondary ion intensities of an element from

various points on a surface is not always a measure of relative element

concentration. Caution must be exercised especially when interpreting

ion images of the surface. Such a case is illustrated in the ion

micrographs of a nickel base alloy [1] shown in figure 1. The Ni, Fe,

and Cr content of each grain is the same, yet the absolute ion yield of

an element shows considerable variation (as much as a factor of 3).

The variation is brought about by effects dependent on the chemical

nature and the crystal structure of the material, and the relative

orientation of the grains in the plane of the surface; such effects are primary ion channeling, radiation-induced recrystal 1 ization , differences

in the concentration of implanted oxygen and differences in the angular

distributipn of secondary ions ejected from the various grain

orientations. The relative changes in ion intensity between grains are approximately equal for all elements, and if the intensities are referenced to the ion intensity of a major constituent at each point,

the differences between grains are reduced or eliminated. One concludes

from observations of this type that any quantitation procedure is better

based on relative ion yields from a point of analysis rather than on an absolute yield.

The precision or reproducibility of relative ion yield measurements

by SIMS is quite satisfactory {<_ 5 percent, under controlled

conditions). It would not be worthwhile to attempt quantitation of SIMS

data if reasonable precision did not exist. Considering the complexity

of secondary ion emission and the basic differences in SIMS instruments,

it is difficult to envision a purely theoretical model applicable to all

SIMS instruments, samples and analysis conditions. For example, SIMS

instruments do not collect all the secondary ions produced at the

sample, nor do they transmit the same representative fraction of ions.

This results from inherent differences in transmission efficiency as a function of secondary ion initial energy. Therefore, any practical

quantitation approach must reduce variables to a minimum and be

adaptable to any SIMS instrument. The most straightforward approach is an empirical method.

129
SIMS/IMMA

Figure 1 Ion micrographs from a grain boundary region of

an Inconel sample that previously had been bombarded with a large-diameter 18.5 keV '°0" beam

(periphery of the bombarded region is shown by

the dotted line in the figure, along with the

position of the grain boundary -- the solid line,

and the Ti-rich inclusions the small triangles). Enhanced secondary ion emission is

noted from grain 1 compared to grain 2. (From

McHugh [1])

The basic ingredients of a useful empirical method are:

1. A set of homogeneous standards representing a wide variety of

matrix types,

2. Standardized instrument operating conditions.

3. Relative elemental sensitivity factors (5/\) derived from

standards, and

4. Methods to extend to other matrices.

130
Empirical Quantitation

The foundation of any empirical method is a set of good standards

which represent a variety of matrix types. The standards must be

homogeneous on a micro-scale and should not be susceptible to surface

and bulk element fractionation caused by sputtering and ion implantation

effects. To achieve success with a quantitation method based on calibration with standards, it is important to standardize instrument

operating parameters — primary ion species, energy and current density,

sample environment, detector efficiency, and the energy band-pass of the

secondary ion analyzer. Once one fixes these conditions, meaningful

sample analyses are possible with relative elemental sensitivity factors

derived from standards of the same or similar composition to the sample.

The relative elemental sensitivity factor, Sy^, is defined by the

following equation:

s, = - (1)

iVe C ref ref

where i« = secondary ion signal for element "A" (sum of

all isotopic components)

= atomic concentration of element "A"

Sa = relative elemental sensitivity factor for ele-

" ment "A."

The relative ion yields from a standard are normalized to a common

reference element. The secondary ion signals of an element "A" and the

reference element are divided by their respective atomic concentrations

in the standard to provide a common concentration base for comparison.

Once a set of relative elemental sensitivity factors, 5/\, has been

generated, the measured secondary ion signals of the unknown sample of

similar composition to the standards can be reduced to relative atomic

concentration by employing the following relationship.

The absolute ion signals or relative ion signals, when divided by S^,

form a set of values proportional to the atomic concentration of the

•elGment in the sample. The set of relative atomic concentrations can

ba normalized to 100 percent and the result will be the composition of

the matrix in atomic percent provided the SIMS instrument is operating

131
SIMS/IMMA

at a sensitivity sufficient to detect all major constituents of the

matrix. In a well-behaved matrix, quantitative accuracies of 10

percent can be achieved with this approach. However, such an approach

places great demands on the standards and on sample homogeneity.

A standard for every matrix that one could encounter is an unreasonable requirement. Therefore, empirical methods are needed to

extend and adjust sensitivity factors from a few known standards to any

desired matrix. The sensitivity factor will show a dependency on a parameter (let us call it es) that characterizes the electronic

properties of the secondary-ion emitting surface. The general behavior

of Sf^ as a function of^e will follow the pattern shown in figure 2.

The type of information given in this figure is derived from standards

that differ widely in matrix composition, but possess a common reference

element. The simplest approach for defining e is to choose two

elements that have matrix-sensitive sensitivity factors {e.g., 5iand

from fig. 2), and let eg = kSi/S? where k is an arbitrary constant.

Other types of information present in the secondary ion spectri that

could characterize ai^e the secondary ion ratios M'''^/M''", MO^M and

MN"''/M'''. The ratios M''"''"/M''' and under the proper circumstances MO /M"*" and

W^/W^ are to a first approximation independent of concentration, and

therefore, are the most direct path for establishing eg

unknown matrix or surface condition. The practical value of these

ratios in defining eg has not been demonstrated as yet.

Once we obtain the function 5/\(es) for all elements we can attempt

the quantitation of secondary ion signals from an unknown matrix

provided our detection sensitivity is sufficiently high to detect all

the major constituents. Our first task is to establish eg for the

matrix or sample being analyzed. This can be done in one of three ways:

one method is to use information present in the secondary ion

spectra—^information such as M'^"*"/M"'", MOVm'^j MnVm"*" or any other

secondary ion ratio sensitive to eg. Such a method is the most

preferrable because no knowledge about sample composition is required;

however, at present we lack the needed information to use this method

effectively. Another method depends on establishing eg from the

relative ion signals and the known concentration of two or more constituents in the matrix that possess a matrix-sensitive S^^/Sq ratio.

This method is quite useful for trace element analyses in a wellcharacterized matrix. The final method is to choose eg solely from

analytical experience and judgment. Such an approach is not as undesirable as one might expect. At least the extremes are defined, and

a better, estimate of 5/\ can be 'obtained (see fig. 2).

The analysis of solids with an oxygen primary ion beam and the

methods outlined above have given favorable results. In fact, a single

set of sensitivity factors gives accuracies for elemental concentrations

much better than a factor of 2 for most elements in a number of quite

different matrices—AI2O3 and Zr, Fe and Ni base alloys. Therefore, any

model that applies a matrix-sensitive correction to relative sensitivity

factors could only improve the results. The attractive features of the

132
Empirical Quantitation

approach described above are: the model is simple, should be applicable

to any instrument, is not dependent on theoretical or physical

constants, and is based solely on standards and measurements within the

instrument.

Figure 2 The dependence of the relative elemental sensitivity factor on the character of the sample

surface. Information of the type illustrated

in this figure can be derived for any SIMS instrument from a set of standard samples. Establishing for a given matrix determined the Sp^

values required for quantitating relative secondary ion yield data, e.g.^ compare the set of

Sa values for elements 1 through 7 in matrices

A and B.

The accuracy of any quantitation procedure not involving a direct

sample-standard comparison depends on the susceptibility of the matrix

to surface [2-5] and bulk element fractionation caused by sputtering and

ion implantation effects. Element fractionation in the primary ion

implant zone depends on the characteristics of the matrix, the

particular constituent and its mobility, the chemical character of the

133
SIMS/IMMA

implanted pr4fnary ion, and the electric field gradients established in

the sample from surface charging. SIMS results for many different

samples suggest that • this problem is not a major threat to analytical

accuracy in most situations; nevertheless, there are situations where

anomalous effects are observed and fractionation in the implant zone is

a reasonable explanation for the effect.

REFERENCES

1. McHugh, J. A. 10th National Meeting Society for Applied

Spectroscopy, St. Louis, Mo., Oct. 18-22, 1971 (unpublished).

2. Honig, R. E. in Advances in Mass Spectrometry , West, A. R. Editor,

Vol. 6, 337, Elsevier Applied Sciences Publishers, London (1974).

3. Tarng, M. L. and Wehner, G. K. J. Appl . Phys. 43, 2268 (1972).

4. Dahlgren, S. D. and McClanahan, E. D. J. Appl. Phys. 43, 1514

(1972). ~

5. Feng, H. C. and Chen, J. M. J. Phys. C: Solid St. Physics 7_, L75

(1974).

134
NBS Spec. Publ. 427, Workshop on Secondary Ion Mass Spectrometry (SIMS) and Ion Microprobe Moss

Analysis held at NBS, Gaithersborg, Maryland, September 16-18, 1974 (Issued October, 1975).

APPLICATION OF SIMS MICROANALYSIS TECHNIQUES

TO TRACE ELEMENT AND ISOTOPIC STUDIES

IN GEOCHEMISTRY AND COSMOCHEMISTRY

J. F. Levering

Department of Geology

University of Melbourne

Parkville, Victoria, Australia 3052

I. INTRODUCTION

The development of the electron microprobe during the late 1950's

provided a powerful tool for rapid, accurate, nondestructive analysis of

micrometer-size areas in a wide variety of samples. These unique

capabilities of the electron microprobe were soon realized by the

chemical geologist and the last decade has seen this instrument

revolutionize the fields of analytical petrology and experimental

petrology by providing major element analyses of co-existing micrometer

diameter phases in geological materials, meteorites and in experimental

charges. Perhaps the best demonstration of the microanalytical power of

the electron microprobe has been its major contribution to the

unprecedented rapid accumulation of basic chemical data on the mineral,

rock and glass samples from the lunar surface returned during the Apollo

missions. It is fair to say that our current detailed knowledge of the

chemical evolution of the lunar volcanic rocks, breccias and soils would

simply not have existed if Castaing had not developed the concept of the

electron microprobe when he did in 1951.

However the analytical capabilities of the electron microprobe have

certain limitations which restrict its ability to contribute basic

analytical data to support what many consider to be the next major

advances in geochemical and cosmochemical research. These advances

promise to develop from trace element partitioning studies between co- existing phases in rocks, meteorites and experimental products.

Clearly, more detailed and rigorous understanding of such fundamental

petrol ogical problems as the nature and degree of melting involved in

magma production, as well as the subsequent differentiation of magmas as they undergo partial crystallization during their ascent, intrusion and

storage in magma chambers and final eruption, will depend on an accurate

and precise knowledge of the effects of temperature, pressure and

composition on the partitioning of trace elements in the ppm and ppb

concentration ranges between minerals and their parental melts.

Similarly, an accurate knowledge of trace element partitioning and dif135.
SIMS/IMMA

fusion gradients in minerals in metamorphic rocks is needed to quantify

the role of temperature, pressure, fluid pressure and composition in

metamorphic processes. Furthermore, trace element partitioning data

between the constituent phases of chondrites are needed to provide more

precise models for chpndrite origins as a function of condensation

processes in the primitive solar nebula and subsequent metamorphic

•processes within parent meteorite bodies.

The problem with the electron microprobe is that analytical

sensitivity is limited to around 10'^^-10"^^g by the relatively high Xray continuum background generated along with characteristic X-ray

spectra of the constituent elements in the sample. This places a detection limit of around 50 ppm for relatively heavy elements (e.g-.j

Fe) and about 1000 ppm for the lighter elements {e.g.j F) in micrometer

diameter analysis volumes with the electron microprobe [1]. It should

be emphasized that these are simply detection limits, and realistic

limits for meaningful trace element determinations by the electron

microprobe may be from 2 to 10 times higher.

The electron microprobe is also severely limited in the depth

resolution of analysis by the significant penetration of the electron

beam into the sample to depths of several micrometers for silicate

matrices. This limitation means that the electron microprobe is

unsuitable for providing data for another potentially important field of

future geochemical research—the nature and composition of ultrathin

(ca. 1000a) intergranular material in rocks and elemental enrichments on crystal surfaces of phases in rocks. Recent work has shown that the

migration of hydrothermal fluids exsolved from magmas and also derived

from wall rocks around a cooling intrusive probably plays an important

role in the two-way exchange of dissolved elements between wall rocks

and intrusive with implications both to chemical fractionations during

late-stage igneous alteration and the formation of hydrothermal ore fluids. The most reactive sites for the solution of these dissolved

elements would be at intergranular surfaces in rocks, and virtually

nothing is known at present of the chemical peculiarities of these

sites.

A third limitation of the electron microprobe is its inability to

detect H and Li and its low sensitivity to the light elements with

atomic number below ten. Consequently little is known of the behavior

of the light elements during igneous, metamorphic and other geologically

important processes. As a particular example of the importance of light

element studies in geology, there is a pressing need for an accurate

method for the microanalysis of trace amounts of hydrogen in the lattice

of minerals usually considered to be anhydrous, since experiments . have

shown that the deformational properties of these phases can be

drastically altered by trace hydrogen in their lattices [2].

In summary, some of the most important advances in chemical geology

to be made in the near future require the development of instruments

capable of in situ microanalysis of micrometer-size phases at elemental

abundance levels in the ppm and ppb range, with adequate sensitivity to

136
Geochemical Applications

cover the analysis of light elements (including hydrogen), and with a depth resolution during analysis of the order of a few hundred angstroms

so that it is possible to study the composition of intercrystal 1 ine

material and the surface chemistry of phases in rocks. All these

capabilities are inherent in the analytical technique known generally as secondary ion mass spectrometry (SIMS) which forms the basis of the

family of analytical instruments such as the ion microscope of Castaing

and Slodzian [3] and the ion microprobe mass analyzer (IMMA) of Liebl

[4]. The SIMS technique has the added capability of in situ isotopic

analysis of micrometer-size phases, opening up the possibility of Rb/Sr

and Pb/Pb dating of individual grains of suitable phases in rocks.

Clearly the elegant capabilities of the SIMS microanalytical technique,

when fully developed, should provide the chemical geologist with a single instrument .which approaches the concept of an "ultimate weapon"

as far as in situ microanalytical capability is concerned. The purpose

of this work is to examine the progress to date of the development of

SIMS analytical methods for geological materials to see how near the

chemical geologist is to realizing his "ultimate weapon."

II. SIMS INSTRUMENTATION AND TECHNIQUES

A detailed history of the development of SIMS instrumentation and a discussion of the pattern of future development has been given by Liebl

[5], and Robinson [6] has also discussed the development of ion

microprobe instrumentation. The first SIMS instrument with microanalytical capability was developed by Castaing and Slodzian [3] as a secondary ion emission microscope. In their original instrument,

secondary ions sputtered from a sample surface by bombardment with a 0.5 mm diameter beam of argon ions are mass analyzed, and particular ionic

species are selected to form an enlarged image of the sample showing the

distribution of the selected ionic species within the irradiated area

with a spatial resolution of a few microns. Further developments

allowed sputtered ion intensities from selected areas in the sample with

dimensions of around 2 micrometers to be measured so that quantitative

SIMS microanalysis of these areas can be made. This improved instrument

is manufactured commercially by CAMECA (France) as the Ion Analyzer IMS

300.

A different design approach was used by Liebl [4] in his

development of the ion microprobe mass analyzer (IMMA) as an analogue of

the electron microprobe X-ray analyzer in which the electron microprobe

was replaced by an ion microprobe and the X-ray analyzer by a secondary

ion mass spectrometer. Essentially IMMA involves the production of a focused (minimum 2 micrometers diameter), high-energy beam of ions of a single species to bombard a sample surface selected for microanalysis by means of an optical microscope. The secondary ions sputtered from the

sample are extracted and analyzed according to their mass-to-charge

ratio in a secondary ion mass spectrometer. A commercial IMMA (fig. 1)

based on Liebl's design is manufactured by Applied Research Laboratories

(U.S.A.). A similar IMMA-type design approach has been used by Hitachi

(Japan) in their IMA-2 instrument and by A.E.I. (United Kingdom) in

137
SIMS/IMMA

their ion microprobe attachment (IM 20) for the MS 702R mass spectrometer.

ANODE

TARGET

Figure 1. Schematic diagram of the Applied Research Laboratories ion

microprobe mass analyzer (IMMA). Reproduced by permission.

The ,use of a focused primary ion beam provides IMMA with an analytical spatial resolution comparable to that of the electron

microprobe. However, the extremely low background noise level in the

secondary ion detection system used means that elemental abundances can

be measured for trace elements as well as for major elements.

Furthermore, the very light elements (including hydrogen) can be

analyzed and isotopic ratios determined. In addition the IMMA is

capable of isotopically analyzing successive atomic layers of the sample

surface as they are eroded away by the impinging ion beam.

Although the secondary ion emission microscope of Castaing and

Slodzian (as manufactured by CAMECA) is capable of microanalysis with

trace element sensitivity, it is fair to say that most quantitative

analytical data published to date on geological samples have been

determined on IMMA instruments as manufactured by Applied Research

Laboratories (A.R.L.). Furthermore my own ion microprobe analysis

experience was gained in the A.R.L.-IMMA laboratory jointly operated by

the National Bureau of Standards and the Goddard Space Flight Center

138
Geochemical Applications

under the direction of K. F. J. Heinrich and situated in the National

Bureau of Standards. Consequently much of the following discussion of

the methods of SIMS microanalysis of geological samples will be based on the use of A.R.L.-IMMA instruments but the general principles should be

applicable to other IMMA-type instruments and to secondary ion emission

microscopes

,

III. IMMA ANALYSIS OF GEOLOGICAL MATERIALS

1 . General

a. Specimen preparation and handling

As with electron microprobe analysis, geological specimens are prepared with highly polished surfaces both as thick samples (for opaque

phases) or as thin sections (for transparent phases) with standard techniques [1]. The standard A.R.L.-IMMA instrument has facilities for

optical viewing of the sample during analysis (at magnifications up to

300X) in reflected light only, but D. H. Anderson (personal communication) has developed a simple modification to provide viewing in transmitted light for polished thin sections.

b. Surface contamination

Since IMMA analyses are essentially surface analyses and can be

restricted to a few tens of angstroms depth, great care must be taken to

minimize surface contamination of the sample during its preparation and

analysis.

Other possible sources of surface contamination during specimen

preparation are the polishing compounds (e.^.j, alumina, silicon

carbide, etc.) and the material in polishing laps {e.g.^ lead) used in

the polishing process. Where phases of very different hardness co-exist

in the sample it is possible to find the softer material smearing over

the harder one. A good example is the smearing of metal phase over adjacent silicate phases in the preparation of polished surfaces of

chondritic meteorites.

Possible sources of surface contamination during analysis arise

from condensibles on the sample surface both from impurities in the vacuum environment of the instrument and also from material transferred

by sputtering from a previous analysis site on the sample. In the

A.R.L.-IMMA, ion and sorption pumps in combination provide a rigorously

oil -free vacuum system to minimize silicone and hydrocarbon

contamination. A "cold dome" at liquid nitrogen temperatures placed

immediately above the sample is essential during quantitative analysis

to minimize other environmental condensibles, in particular hydrogen

from residual water vapour. The control of material deposited from

previously sputtered analysis sites is more difficult although the "cold

dome" helps to trap some.

139
SIMS/IMMA

The best way to overcome the problem of possible surface

contamination is to "preclean" the analysis surface by rastering the

primary ion beam over an area somewhat larger than the analysis spot for

sufficient time to sputter away several hundred Angstroms from the

surface. Of course, this "preclean" treatment is not possible if

surfa'ce analysis studies are being undertaken and under those

circumstances the only recourse is to recognize possible surface

contamination sources and minimize their effects as far as possible.

c. Analysis of nonconducting samples

The IMMA analysis of conducting samples provides no special

problems provided the sample is in electrical contact with the sample

holder. Conducting paints are completely suitable for making such

contacts.

With nonconducting samples, problems arise during analysis because

of two separate effects:

1) electrical charge build-up on the sample surface resulting from

the effects of the charged primary ion beam used for

sputtering,

2) the electric equi potentials on the sample may be severely

distorted by variations in dielectric constant of the saqiple

from point to point.

In the first case, electrical charge buildup can cause the point of

impact of the primary. ion beam to become unstable so that control of the

analysis site is lost. This instability can also lead to drastic

changes in the collection efficiency of the selected sputtered ions onto

the detector since the bombarded area is stigmatically focused directly

onto the resolving slit of the secondary ion mass spectrometer. Chargeup can also lead to significant deceleration of the impinging ion beam.

In the Hitachi IMA-2 instrument, surface charge-up of insulators is

controlled by flooding the impact site with low energy electrons from a thermal electron emitter [7], However, Andersen et al. [8] have shown

that electrical charge-up on insulators is prevented if negatively

charged primary ion beams are used instead of positive beams. The

mechanism seems to be that a departing secondary electron takes away the

negative charge brought in by a negative ion. Consequently IMMA

analyses of most geological samples are carrieid out with negative

primary ion beams.

In the second case, residual surface charge arising from

triboelectric, piezoelectric or thermoelectric effects may cause the

sample surface potential to vary from the sample level by hundreds of

volts [6]. In the IMMA design, the sputtered secondary ions are extracted into the secondary mass spectrometer by raising the sample

surface potential to a controlled value. If the electrical equipotentials are severely distorted by variations in the dielectric

constant of the sample from point to point .(as would occur with a 140
Geochemical Applications

polyphase rock sample) then the efficiency of extraction of the

sputtered ions into the secondary mass spectrometer fluctuates

drastically as different points on the sample are bombarded. To overcome this problem and fix the surface potential of the sample at a controlled value, insulators {e.g., rock samples) are vacuum coated with

a 100 to 200 A thick layer of carbon (or conducting metals) before

analysis, by the same techniques as for electron microprobe analysis of

insulators [1]. However it shou-ld be emphasized that the effect of the

conducting coating in IMMA analysis of insulators is different from that

in the electron microprobe in which it acts to carry away the charge

delivered by the electron beam.

In practice, before ion intensity data are measured in IMMA on a coated sample, the conducting coat immediately under the point of beam

impact must first be removed by erosion of the primary ion beam so that

secondary ion spectra are not contaminated by the coating material. For

normal primary ion beam conditions (see below) , and with standard

thickness carbon coatings, a period of about 10 minutes of erosion by

the beam scanning in a raster mode with dimensions which are slightly

larger than the analysis area is usually sufficient. For oxygen-free

samples, this preliminary bombardment also serves to establish the

conditioning of the chemical nature of the sample surface by the implantation of the oxygen from the primary ion beam normally employed in

IMMA analysis.

Very thin samples (up to several hundred angstroms) or small

particles (a few microns in diameter) on conductive mounts need no special treatment as they cannot support a large enough potential

difference from_ the mount to disturb either the primary or secondary ion

beams significantly [6], providing negative primary ion beams are used.

d. Effect of ion bombardment on geological samples

Under normal conditions used for IMMA analysis, the site of impact

of the stationary primary beam is normally marked by a conical crater

which may be up to a micrometer or more deep after a lengthy trace

element analysis has been completed. For most matrices no other visible

phenomena are apparent at the point of impact of the primary beam.

It is also important to prevent significant heating of the sample;

otherwise volatile elements may be preferentially lost from around the

bombarded area and pre-existing elemental diffusion gradients could be

modified. However, the use of focussed primary ion beams means that

even with specific power density inputs as high as lOOw/cm^, a beam of 2 micrometers diameter delivers only a total of 12.4 yW [6] to the sample.

This low total power input would result in a much lower general

temperature rise than with the same power density over a larger beam

diameter, although the thermal conductivity of the sample is also

important in assessing the temperature rise in the sample during

sputtering.

Nevertheless some matrices are apparently unstable under ion

bombardment and varying sputtered ion intensities may result in these

141
SIMS/IMMA

cases. For example, the U.S. National Bureau of Standards SRM 610-617

series of standard trace element glasses are generally unsatisfactory as ion microprobe standards since they contain 14% NaaO and such Na-rich

glasses are highly unstable with respect to sputtered Na ion intensities

even under micrometer-diameter primary ion beams [9]. Similarly

instabilities are found in the analysis of Na-rich glasses in the

electron microprobe and it is probable that similar hypotheses may

explain both phenomena [1].

2. Analytical Conditions

a. Primary ion beam

A fundamental requirement for the development of quantitative IMMA

analysis is that there be a constant, high level of intensity of sput-,

tered secondary ions, constant with respect to time, during the primary

ion bombardment. Most workers have used inert gas {e.g.j argon) primary

ion beams for SIMS analysis, but Andersen [10,11] has shown that when an aluminum metal sample is bombarded with '*°Ar''" ions, the sputtered ^'Al"*" ion yield falls exponentially with time as the ability to extract these

ions from the sample is progressively destroyed by continued bombardment

(fig. 2). Andersen postulated that the diminishing production of Al"^

Figure 2. Variation with time of the sputtered ^'Al ion intensity from

aluminum metal using primary ion beams of an inert gas

{'*°Ar+) and an electro-negative gas (^^O"^) [10].

ions was due to the removal of a previously formed thin oxidized film on the sample during the bombardment, and he found that if a primary ion

beam of a reactive, electronegative gas (such as oxygen) was used, the

yield of Al"*" ions rose rapidly to a maximum stable output (fig. 2).

M2
Geochemical Applications

Presumably the electronegative ion beam implanted into tne sample

surface forms surface compounds (in this case oxides) which produce

stable, high intensity yields of sputtered positive ions. Andersen

concluded that the emission of positive ions is enhanced and stabilized

by the increased electronic work function of the sample surface created

by bombardment with an electronegative gas, such as oxygen. The same situation applies to the production of sputtered negative ions if the

work function of the sample surface is decreased by bombarding with ions

of an electropositive gas, such as cesium.

Since most elements of general interest to geochemists and

cosmochemists have their highest yields in the positive secondary ion

spectrum [11], an electronegative gas, normally oxygen, is commonly used

in the IMMA analysis of geological specimens. To reduce the possible

charge-up of nonconducting samples (see above), negative primary ion

beams {i.e.^ ^^0~) are used. The acceleration potential for the ^^0"

ions normally used in IMMA analysis is ^20 keV, and Andersen and

Hinthorne [12] have shown that essentially identical analytical results

are obtained for a number of accelerating potentials within the range 11 to 18 keV.

M/e Mte

Figure 3. Secondary ion mass spectra from IMMA study of terrestrial

zirconolites (K = Kola Peninsula, U.S.S.R.) and lunar

zirconolite (Apollo 11 basalt 10047).

143
SIMS/IMMA

Experience to date suggests that an optimum v-alue fbr prirtiary beam

currents for useful elemental analytical sensitivities is around 1-10 x 10"^ amperes. Minimum diameter of the primary beam corresponding to a 10 X 10"^ amp beam current is about 20 ym. Any significant drift in the

primary beam current should be monitored and the raw secondary ion

intensities should be normalized to a standard value of primary beam

current. However, experience with at least the A.R.L.-IMMA instrument

has shown that, provided the duoplasmatron ion source is clean and has

been operating for around two hours (together with the ion optics

electronics), the drift in beam current over an analysis is usually less

than 2 percent.

b. Ion counting of sputtered secondary ions.

Raw sputtered secondary ion intensities must also be corrected for

detector background and detector dead-time. In the A.R.L.-IMMA, the

sputtered secondary ion beams are detected by a 'high gain device of the

Daly type that permits single ion counting [13]. Experience has sljiown

that these detectors have detector backgrounds which are essentially

constant over the 0-300 mass range at <3 counts/second. Detector and

counting electronics dead-times in this instrument are typically around

30 nanoseconds. Significant corrections are required to ion intensities

in the 10^ - 10^ counts/second range which are typical for major element

secondary ion intensities from geological samples. Consequently an accurate determination of the dead-time for each IMMA ion counting

system is extremely important.

IV. QUANTITATIVE IMMA ANALYSIS OF GEOLOGICAL MATERIALS

1 . Nature of Secondary Ion Mass Spectra

A detailed model of the mechanisms of secondary ion production is

not yet available but a mass spectrum of ionic species sputtered from

even the simplest mineral phases shows complexities which highlight the

difficulty of quantitative analysis with IMMA [14]. An extreme example

is shown in figure 3 in which secondary ion mass spectra of a terrestrial and a lunar example of the complex mineral zirconolite are illustrated. Previous electron microprobe studies of zirconolites have

shown that this mineral has an extremely complex composition in which more than 25 elements occur with abundance levels >0.1% by weight [36].

The secondary ion mass spectra are highly complex with peaks occurring

at virtually every mass position in the mass range 0-282. Not all peaks

present in the spectra have been identified in figure 3 but sufficient

peaks have been labeled to show that the positive secondary ion spectra

recorded contain all singly charged ions of the 25 or more elements

present, together with XO""", X02'^ and other multiple atom clusters as well as multiply charged metal ions. All this complexity exists despite

the fact that the use of chemically reactive primary ion beams {e.g.^

^^0") greatly reduces the number of multiple atom clusters present [6].

Given the complexity of secondary ion mass spectra for minerals, serious

problems arise with the possibility of overlap^ of contaminating ionic

144
Geochemical Applications

species with the ion peak to be determined. These problems are particularly serious with those instruments {e.g.^ CAMECA, A.R.L.,

Hitachi) with secondary ion mass spectrometers of relatively low

resolution (^^^500). Part of the problem can be minimized by carefully

controlling the abundance of possible contaminants in the sputtering

source. Experience with IMMA analysis of geological materials shows

that, provided only cryogenic and ion pumping vacuum systems are used,

hydrocarbon contamination in the secondary mass spectrum is not a significant problem. Another source of possible contamination is the

production of a variety of atomic and molecular ions in the

duoplasmatron ion source from impurities in the source gas and from the

materials used to construct the ion source. These contaminating species

are removed in the A.R.L.-IMMA by means of a primary mass analyzer which

allows selection of only one ionic species to form the primary ion beam.

A further source of secondary ion contamination is from complex hydride

ions produced in the sputtering source from residual water in the vacuum. These hydride ions can be significantly depleted if a "cold

dome" at liquid nitrogen temperature is placed just above the sample.

However, the presence of the "cold dome" does not prevent the formation

of hydride ions in the sputtering source if the phase analyzed contains

significant amounts of OH in its lattice. For example, IMMA analyses of

OH-bearing terrestrial zirconolites (Table 5) posed a problem with

an originally unrecognized ^^(CaOH)"'" contamination of the ^^Fe"*" peak

measured to determine the iron content of this phase.

Undoubtedly many peaks are seriously contaminated by multiply

charged, polyatomic and complex molecular ions which are produced in the

sputtering source and for which correlation procedures may, or may not,

be available. Experience with a wide variety of sample matrices soon

builds up an appreciation of significant peak contamination problems but

the IMMA analyst must be extremely thorough in searching for all

possible peak contaminants. Computer print-outs of selected peak

overlaps are available (D. H. Anderson - personal communication) but it

is clear that a basic problem with IMMA analysis using currently

available instruments is the low resolution of the secondary ion mass spectrometers, which were generally designed for high transmissivities

and wide energy bandpass.

However, even with the existing low resolution instruments, it is

often possible to correct analytical peaks for significant interfering

peaks. For example [15] in an IMMA analysis of a clinopyroxene, peaks

with mass/charge (M/e) ratios of 40 and 41 may contain the following

species:

M/e Species Normalized peak intensities

39 39k 5.15 X 10^

40 '*°Ca"' + 24j^gi6Q+ 4.49 X 10^

41 6.96 X 10^

.145
SIMS/IMMA

In order to calculate the '*''Ca''" intensity for the analysis, we need to

subtract the'MgO"^ component from the 40 peak.

First of all the '*^K'*" component in the 41 peak can be calculated

from the "clean" ^^K"*" as follows knowing that in natural potassium:

' = 6.88

^'K 93.1

Hence:

fij^H- ^ 39j^+ ^ 6.88 = 5.15 X 10^ X 6.88 = 3.81 x 10^

93 1 5371

Then the ^^Mg ^^O"*" component in the 41 peak is given by:

2 5Mg ^^0-" = 41 peak - '^^K''

= 6.96 X 10^ - 3.81 X 10^ = 6.58 x 10^

knowing 2'^Mg^/2 5Mg+ = then

2.Mg+ = 25^g 160+ X' = 6.58 X 10^ X 7.90 =

5.20 X 10"*

so that the corrected '*''Ca'^ = 4.47 x 10^ - 5.20 x 10** =

4.42 X 10^

Computer programs have now been developed by Colby [16] and

Andersen and Hinthorne [17] to carry out spectrum stripping of

contaminated peaks provided the interfering ion species are identifiable. In the general case one extra peak must be measured over

the total number of ion species present. A specific example is given

below on computer spectrum stripping of interferences on the lead

isotopes. Corrections made in this way are not serious provided the

species to be corrected for interference is a major contributor to the

peak measured; however, if the species sought is a trace element and the

interfering species is a major contributor, then serious errors can arise. An example would be trace element determinations of Ca in an Mgrich olivine where the '*°Ca''" peak is interfere^ with by a significant

^^Mg ^^0+ and the '*'*Ca+ peak by ^^si ^^o^.

146
Geochemical Applications

2. Quantitative IMMA Analysis by Comparison with Standard Samples

Some of the earliest attempts at quantitative IMMA analysis [10]

followed the familiar "working curve" approach or comparison of absolute

secondary ion intensities from unknown samples with a suite of standard

samples with known elemental compositions. However, it is now realized

that such an approach has severe limitations since absolute sputtered

ion intensities depend critical ly. on the work function of the surface

from which the ions are produced. For a particular element at a given

abundance level, the intensity of the sputtered ions may change by more

than an order of magnitude when the work function of the surface changes

by a few tenths of a volt [11]. Such local variations are common within

a particular sample and between two separate samples, so that in many

cases simple comparison of absolute sputtered intensities from standards

and samples becomes meaningless.

Bence [18] has also shown that the particular crystal lographic

orientation of the mineral surface being sputtered can seriously affect

the absolute intensities of secondary ions and again lead to problems of

absolute intensity comparisons between standards and samples.

A partial solution to the problems of IMMA analysis using

sample/standard comparisons follows from observations that, despite

local variations in work function values and crystal lographic

orientations, the ratio of different sputtered ion species from a given

mineral matrix is approximately constant for that matrix [6,18]. An

example of this approach is the IMMA analysis of F (in humites,

amphiboles) and H (in micas, zeolites, epidotes, amphiboles, humites) by

Hinthorne and Andersen [19]. They constructed working curves based on

H"'"/Si''' and F''"/Si'^ ion intensity ratios (corrected for natural isotopic

abundance) f^om a series of standard silicate samples (normalized for

the different silicon abundances) with known fluorine, hydrogen, and

silicon abundances.' Although it is well known that even ion intensity

ratios may change with matrix changes [19], apparently the silicates

used in constructing these working curves are sufficiently similar for

the ionization efficiency ratios of fluorine, hydrogen, and silicon to

remain constant to a first approximation. But Andersen and Hinthorne

[19] give a strong warning that it would be unwise to extend the use of

these working curves to very different silicate matrices, to oxides or metals without first carrying out a very thorough investigation.

Nevertheless, J. R. Hinthorne (personal communication) has recently

found that the fluorine working curves can also be used for fluorine

analysis of sphenes.

3. Quantitative IMMA Analysis by CARISMA - Local Thermal

Equilibrium Approach by C. A. Andersen

The major limitations of the "working curve" approach to IMMA

analysis, arising from uncertainty of matrix affects on the relative

ionization of different elements and the difficulty of obtaining

suitably homogeneous standards for trace elements in a wide variety of

matrices, led Andersen [11] and later Andersen and Hinthorne [12,13] to

147
SIMS/IMMA

develop a model of secondary ion generation of theoretically universal

applicability for quantitative IMMA analysis. A more detailed

discussion of their approach is given elsewhere in this volume by C. A.

Andersen but, in general terms, their method has been initially based on the use of ^^0" primary ion beams and assumes that the sputtering source

resembles a dense "plasma" of positive and negative ions, neutral atoms

and of oxide molecules in local thermodynamic equilibrium. Provided the

temperature (T), electron density (Ng) and concentration of free oxygen

are known in the "plasma," then, Andersen and Hinthorne conclude, it is

possible to calculate the atomic composition of the sputtering source

(and of the sputtered sample) from measurement of the singly charged

positive ion intensities in the sputtered mass spectrum.

The complex calculations involved are carried out by means of a computer program entitled CARISMA (Corrections to Applied Research

Laboratories Ion Sputtering Mass Analysers) which includes several

options for establishing the temperature and electron density in the

sputtering source. The best tested option (option 3) follows an "internal standard" approach and depends on an independent knowledge of

the atomic concentration of two or more elements in the sample. Singly

charged positive ion intensities for these internal standard elements

are measured, along with related metal —metal oxide ion intensities from

one of the combinations O-O2, Si-SiO, Al-AlO, Ca-CaO to determine the

free oxygen concentration in the source. These data are then used as input to CARISMA which searches the T and Ng space to find a set of

values for these two parameters consistent with the observed internal

standard ion intensities and their known elemental compositions in the

sample. The observed ion intensities for all other elements analyzed in

the sample are then corrected for these same T and Ng parameters to give

the atomic composition of the sample.

In using CARISMA option 3 it is normally advantageous to use as many internal standard elements as possible (up to a maximum of 6

elements) to give the best estimate of the T and Ng parameters for the

analysis. Andersen and Hinthorne [12] state that for most geological

samples, typical values of T range from 5000° to 15,000°K with the Ng

values ranging from '\'l .5 x

10^** cm"* (at lower T) to around 10^° - lO^^cm-^ (at higher T).

If internal standard information is not available for a sample,

CARISMA can be used in option 1 mode when estimated T and N^ values for

the particular sample matrix are used in the input. Experience of IMMA

analysis of a variety of standard samples with widely different matrices

has shown that, if standard operating conditions are used, then

reasonably constant T and Ng parameters are found for any matrix type

[12]. But clearly IMMA analyses calculated by the option 1 mode will

generally be less reliable than those derived using option 3.

In practice, raw secondary ion intensity data arising from an IMMA

analysis must first be corrected for detector background, detector dead-

.148
Geochemical Applications

time, primary beam current drift and overlap of analysis peaks in the

secondary ion mass spectrum by contaminating species as outlined

previously. These corrected ion intensity data are then used as input

to CARISMA, under one of the options described here, and Andersen and

Hinthorne [12] have shown that this approach can provide "generally

acceptable" major and trace element abundances for a. large number of

elements in a wide variety of geological materials {e.g.^ silicates,

oxides, rock glasses) and man-made products {e.g.^ metals and alloys)

whose compositions are "known" from a number of independent analytical

techniques.

However, serious problems arise in attempting a detailed assessment

of the accuracy of these CARISMA corrected analyses of geological

materials. For example, virtually all of the trace element abundances

reported using other analytical techniques are analyses of bulk samples

in which the possibility of contamination by other phases is universally

a problem. Even without such contamination, inhomogenei ties of elemental

distributions within the phase are always possible and could drastically

effect IMMA analyses carried out with finely focussed primary beams.

Even with the major elements, problems arise in comparing IMMA analyses

with other methods of analysis since most of the IMMA analyses reported

by Andersen and Hinthorne [12] were calculated from CARISMA option 3 in

which many of these major elements were themselves used as internal

standards. Given these uncertainties, the accuracy of their CARISMAcorrected IMMA analyses can only be assessed in a general way after

looking at the comparison trends for the various elements in similar

matrices and such an assessment is summarized in Table 1.

4. Independent Assessment of CARISMA-reduced IMMA Analyses

of Geological Materials

The apparent general success of the CARISMA-reduction of IMMA

analytical data, as reported by Andersen and Hinthorne [20], prompted me

to independently investigate the method using the A.R.L.-IMMA instrument

in the Analytical Chemistry Division of the U.S. National Bureau of

Standards. Silicates (plagioclase, clinopyroxene, tranquil 1 ityite)

,

silicate glasses, oxides (chromites, zirconol ites) , and metals from

terrestrial, lunar, meteoritic and man-made materials were analyzed by

IMMA and the CARISMA-reduced data compared with independently derived

data from electron microprobe and other methods. The results are given

in Tables 2 to 8 and standard IMMA analytical conditions described above were used in all cases.

a. Intra-laboratory analytical reproducibility

Little information has been given previously on the reproducibility

of repeated IMMA analysis on a single sample within a single laboratory.

Duplicate analyses from a number of phases are given in Tables 2 to 8

and, for most cases, reproducibility is satisfactory particularly when

the likelihood of inhomogenei ties in the trace element distributions on the micrometer scale is considered.

•149
SiMS/IMMA

IB

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150
Geochemical Applications

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151 •
SIMS/IMMA

Table 3. IMMA analyses of chromite from the

Moama eucrite

Chromite

Electron

microprobe

IMMA Grain 4 Grain 5 atomic per

cent

Mg 1.79^ 2 .18 2.46

Al 4.59^ 6 .59 6.99

Ti 2.04^ 2 . 10 2.04

Cr 20.0^ 17 . 3 16 .9 Fe 13.8^ 13; 9 13.8

ppm atomic

Li 5 . 3 6.0 Na 4 . 8 29

Si 1830 562

4.2 3.3

V 2700 1840 1870

Mn 3500 1940 2110

Ni 124 124

Rb <1 <1 Sr <1 <1

Zr 7.0 5.2

Nb 30 15 Cs 0.6 . <C. 5 Ba 2.2 <2

CARISMA

parameters

Option 3 3 T (°K) 2 ,800*^ , 3,400^^

Ne (cm ^) ob

0.123 X 10

10"

0.690 X 10

a internal standards used in CARISMA option 3 calculations

b values seem unusually low (see text)

rT52
Geochemical Applications

Table 4. IMMA analyses of fission track

c standard glasses.

Glass TT42 Grant glass Saratoga glass

Element imJ \^ ^ W X. \_/ 1 1 Microprobe

IMMA Electron^

Microprobe

IMMA

Microorobe

IMMA

atomic

Mg

^

1 • O D 1 . D 4 U . 1 Z / u . xu

Si ^ 23.91 23.60 25.71 25.90 27.16 27.60

K 0.22 0.224 0.022 0.027 6.90 6.4 3

ca

^

1. 79 1. 75 1.38 1.38 2.68 2.. 72

atomic

Li 119 5.4 8.8

Be < 0.1 <0.1 <0.1

B 5310 108 61.2

Cd <0. 3 <0. 3 <0.2

CARISMA

parameters

Option 3 3 3

T ( °K) 13,860 U ,460 13,800

Ne (cm ^)

^^0 V^^o^^

2 0 1.157x10

22,27

1 . oqg X 10^°

24.22

1 . 044 X 10^0

23.61

a. internal standards used in CARISMA option 3 calculations.

b. analyst : J. Nelen, Mineral Sciences, Smithsonian Institution.

c. samples from B.S. Carpenter (National Bureau of Standards).

153
' SIMS/IMMA

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154
Geochemical Applications

Table 6. IMMA analysis of tranquillityite (grain 20A-27)

in Apollo 11 basalt 10047

Tranquillityite

Electron

Ml T* y o

W

b

IMMA

atomic

per cent

Si 1 . 14'3 7.16

Ti 7.76*^ 7 .75

Fe la. 8^ 18.8

4 . 32 2.15

Al u . / / Ca U • D O Mn U • J. D Y n 7 n Nb

ppm atomic

Li 3.3

B - 25

Na 23

Mg J.1 d4 HO P 105

K '\>200 709

Ba 11

'^'300 34

CARISMA

parameters

Option 3 T (OK)

Ne (cm"-^)

8,960

1 7 0.985x10

a internal standards used in CARISMA option 3 calculations

b

Hf, Pb, Tb, U all <10 ppma

155
SIMS/IMMA

b. Inter-laboratory analytical reproducibility

No data have been published previously on the reproducibility of

CARISMA-reduced analyses on the same samples determined in separate IMMA

laboratories. Table 5 includes IMMA analyses of identical samples of

zirconolite from two separate laboratories and, for most elements, the

reproducibility is satisfactory, particularly when known sample

inhomogenei ties are considered. However, in a few cases, there does seem to be certain systematic differences between the laboratories. For

example, zirconium and uranium are consistently higher in the IMMA Lab 2 analyses while Pb is consistently higher in the IM^IA Lab. 1 analyses.

The explanation of these differences is not clear at present.

c. Assessment of analytical accuracy

Following the discussion above concerning* the problem of comparing

IMMA analyses with independently determined analyses, it is unprofitable

to attempt to discuss in detail the relative accuracy of thei IMMA

analytical data in Tables 2 to 8. A more realistic approach would be

for each individual to make his own assessment of these data on the

basis of his own experience, of the relevance of the independently

derived abundance data available for comparison in the Tables. For

example, in assessing the IMMA analyses of the NBS standard steel sample

(Table 7) it must be remembered that samples SRM661 , 662, 663, 664 and,

to a lesser extent 665, contain significant numbers of inclusions of

various types into which a variety of elements {e.g.^ B, C, Ti , Nb,

Zr, Tb, Si , Mn, S, Se, Ce, Mg, Al , Mo, Fe, Ag, V, etc.) have been

segregated (R. L. Myklebust, personal communication). Consequently the

standard NBS average analyses for these elements as reported in Table 7 can only be compared in a general way with IMMA data measured on highly

localized areas.

The IMMA analyses of the ultrahigh purity iron (FER BNMZ F 1)

sample (Table 8) illustrates other problems which may arise.

Independent studies (I. L. Barnes - personal communication) have

indicated that C, Mn and Cu all occur in this sample at abundance levels

<_ 1 weight ppm so that the higher levels in the IMMA analyses must

reflect analytical problems. It seems that the C may result from hydrocarbon contamination and the Cu could have been smeared on the sample

from the brass sample holder in which it had been polished. The

relatively high Mn abundance reported could have resulted from FeH''' contamination on the ^^Mn"*" peak measured. However, the IMMA abundance

levels for the other elements analyzed are in general agreement with the

ultrahigh purity of this material.

On the overall assessment of the data in Tables 2 to 8, it may be

concluded that the results of these new IMMA analyses of geological

materials confirms the previous conclusions of Andersen and Hinthorne

[20] as summarized in Table 1. Where suitably homogeneous and well

characterized samples have been analyzed by IMMA using CARISMA data

reduction, the results show that relatively accurate determinations of

156
Geochemical Applications

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157
SIMS/IMMA

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158
Geochemical Applications

elemental abundances in geological materials are possible for those

elements in Table 1 (other than those described in "poor correlations")

if option 3 CARISMA reduction of ion intensity data is used. 11}!,

greatest source of error lies in the possibility that incorrect ion

intensities are used as input to CARISMA for one or more internal

standards in option 3. Such errors arise if an unsuspected

contamination exists in the internal standard ion peak measured and

these incorrect data will seriously perturb the "best fit" in T - Ne

space from which the sample composition is calculated.

IMMA Lab. 2 - REE error factor

Figure 4. Error factors for REE analyses using CARISMA-reduced data

from two IMMA laboratories (see Table 5).

159-
SIMS/IMMA

d. Failure of CARISMA analyses

The IMMA analyses of Andersen and Hinthorne [12], and those reported in the present work, found that CARISMA consistently predicted values

for zirconium, niobium, molybdenum, lanthanum, rare earth elements

(REE), hafnium, tungsten, thorium, and uranium abundances which were

much less than the known abundances in the samples analyzed. All these

elements are characterized by large molecular oxide ion components in

the secondary ion spectrum and, in many cases, the metal monoxide ion

peak is actually greater than the atomic ion peak {e.g.^ see fig. 3).

Preliminary work (Lovering, Andersen and Hinthorne, unpublished report)

on IMMA analyses of REE and lanthanum standard glasses in two separate

A.R.L.-IMMA laboratories has indicated variable error factors for these

elements which are relatively reproducible between the two laboratories

(fig. 4). These data suggest that the error factors are not dependent

on instrumental effects but are more likely to be due to inadequacies in

the procedure in CARISMA used to calculate the contribution of REE

molecular oxide species in the sputtering source. Further work is at

present underway to establish the error factors for the elements Zr, Nb,

Mo, Hf, W, Th, U so that the accuracy of IMMA analyses of these

geochemical ly important elements can be improved.

V. APPLICATIONS OF SIMS MICROANALYSIS IN- GEOCHEMISTRY AND

COSMOCHEMISTRY

1 . General

Commercially manufactured SIMS instruments have only been generally

available since about 1970, and quantitative IMMA analysis methods using

the CARISMA approach have only been generally available since 1972 [13].

Consequently the applications of SIMS microanalysis to the study of geochemical and cosmochemical problems as published up to the present are rather limited in number. It must also be remembered that the bulk of

the quantitative IMMA analyses published to date have been produced in a single IMMA laboratory (at Hasler Research Center, Applied Research

Laboratories) by two analysts (C. A. Andersen and J. R. Hinthorne).

Nevertheless the limited applications that have been published serve to

clearly illustrate the potential power of SIMS microanalysis to geology.

2. Trace Element Geochemistry

a. Terrestrial materials

Relatively few data have been published on trace element

distributions in terrestrial rocks and minerals using IMMA analysis.

Andersen and Hinthorne [9] have reported IMMA-derived abundances for a wide range of trace elements in pyroxenes, amphi boles, olivines,

garnets, scapolites, K-feldspars and plagioclase feldspars. They have

also reported determinations of trace elements in glasses produced from

the US6S series of standard rocks {i.e., G-2, AGV-2, GSP-1) and have

160.
Geochemical Applications

thus shown how IMMA can be used to measure trace element abundances in

rocks

.

Andersen [20], and Stout and Hinthorne [21] have reported the first

in situ determinations, using IMMA, of trace element distribution

coefficients between co-existing hornblende and cummingtonite in

cunmingtonite-gedrite hornblende and cumningtonite-garnet-hornblende

assemblages in sillimanite grade metamorphic rocks from the Keen

Dome, New Hampshire. Average distribution coefficients, Kq

(hornblende/cummingtonite) were determined as follows: Li = 3.0, P = 3.0, V = 6.6, Cr = 0.44, Co = 5.0, Sr = 2.5, Y = 10.0. High

concentrations of Li in gedrite and of B, Cs and Ce in hornblende were

also observed together with marked fractionation of Cr, Zr, Pb into

cummingtonite relative to coexisting Fe-Mg [i.e.^ garnet) phase.

Similar partitionings have also been observed (J. R. Hinthorne personal

communication) between the identical co-existing phases in sillimanite

grade rocks of the same bulk composition from Norway.

Trace-element partitioning between co-existing phases in igneous

rocks has yet to be studied in detail using in situ IMMA analysis, but

Andersen and Hinthrone [22] have reported IMMA analyses for a variety of

co-existing silicate and oxide phases in an oceanite, trachyte, and

nephelinite from Hawaii, a tholeiitic basalt and basanite from the

Mojave Desert, a high-alumina basalt from the Mid-Atlantic Ridge, a metallic iron-bearing basalt from Disko Island, and a spinel peridotite

from St. Paul 's Rocks

.

Havette-Ledebt [23] has used ion microscope pictures and

semiquantitative analyses to study the behavior of Ni , Ca and other

elements during the serpentinisation of peridotites from Bagilet Auliac

(France) and the relationships and origin of glass components of

palaeozoic ignimbrites from Jersey, Alegon, Cbevrons, Bas Bocage

Vend^en, Genis and Montagne Noire (France). She also presented ion

microscope studies of the distribution of fission product elements

within the natural atomic pile which had formed spontaneously in 20-30%

U lenses within the uranium ore deposit at Oklo, Gabon.

Hinthorne and Ribbe [24] reported analyses of trace elements (i.e.,

Al, P, Be, Li, Na, K, Ti , V, Mn, Sr, Y, Zr, Cr, Ce) in a number of

samples of chondrodites as part of a study of the B geochemistry of this

phase. Lovering (this work) has carried out IMMA analyses of

zirconolites from several localities in the U.S.S.R. (Table 5) which

confirm the extraordinary chemical complexity of this phase.

b. Meteorites

Jerome and Slodzian [25] have described a semiquantitive ion

microscope study of trace element distributions in the Juvinas eucrite

(basaltic achondrite). They found that two pyroxenes co-exist in

Juvinas with one pyroxene (Caand Ti-rich but relatively poor in Mg, Si,

Fe, Mn) existing as lamellae in another pyroxene (Ca- and Ti-poor and

relatively enriched in Mg, Si, Fe, Mn). Both pyroxenes were Al-poor and

161
SIMS/IMMA

inclusion of a Ti-rich phase (rutile?) occur in the host Ca-poor

pyroxene.

IMMA analyses have been reported to illustrate trace element

fractionations between co-existing silicate-oxide phases in the Bereba

[26], Cachari [22] and Juvinas [22] eucrites (basaltic achondrites) and

between silicate-oxidemetal phases in the new Moama eucrite

(Lovering—See Tables 2, 3, 8 in this work). Similar studies have been

made of trace element distributions between co-existing plagioclaseclinopyroxene-olivine-chromite in the unique Chassigny olivine-rich

achondrite, co-existing cl inopyroxene-ol ivine in the Nakhla diopsideolivine achondrite and in olivine fragments and plagioclase-rich

inclusion in the Allende carbonaceous chondrite [27].

Andersen and Hinthorne* have also listed IMMA analyses of augites

[22] and a REE-rich phosphate [27] in the Angra dos Reis augite

achondrite. The phosphate REE fractionation pattern is essentially

horizontal for the light REE's from Ce to Eu, and^no Eu anomaly of any

kind is observed. In the lunar rocks, phosphate phases usually show

similarly horizontal patterns, Ce to Sm, but all show prominent negatiive

Eu anomalies which are characteristically larger than those in the hbst

rocks [26]. On this basis it can be concluded that the Angra dos Reis

achondrite itself does not have a strong negative Eu anomaly, although

Schnetzler and Phil potts [28] have reported a small negative Eu anomaly

in a total sample. On the other hand, ^^O/^^O ratios indicate a close

relationship between lunar rocks and chondrites, enstatite achondrites

and the Angra dos Reis achondrite (Group II materials), while basaltic

achondrites, hypersthene achondrites and mesosiderites form the distinct

and separate Group I materials [29]. The REE data discussed here would

imply that, within Group II extra-terrestrial materials, the Angra dos

Reis is more closely allied to the chondrites (and enstatite

achondrites) than to the lunar rocks.

c. Lunar rocks and minerals

Since the advent of SIMS instrumentation and the development of

analytical methods for these instruments coincided with the return of

lunar samples during the Apollo lunar landings from 1969 to 1972, it is

not surprising that much of the IMMA analytical data published to date

has been concerned with the analysis of these unique materials.

Andersen et al. [30] reported IMMA analyses of trace elements {e.g.t B,

Be, P, Si , Al , Li , V, Mg, Ti , Cr, Fe, Mn, Hf, Zr, Y, Na, Ca, Sr, K, Ba,

Pb) in glass, clinopyroxenes, pyroxenoids (=pyroxferroite) , plagioclases

and ilmenites in Apollo 11 samples. Fredriksson et al. [31] carried out

IMMA analyses of trace elements {e.g.y Li, B, Na, P, K, Rb, Sr, Y, Zr,

Ba, La, Ce) in low-alkali "normal" glasses, KREEP glasses and

groundmas? -welded glass in breccias from the Apollo IE site. Prinz et

al. [32] used IMMA analysis to characterize various basaltic glass fragments in an Apollo 15 microbreccia on the basis of Li, B, V, Sr, Y, Zr,

Ba, Rb, Na, K, P, Ti , Cr, Mn, La, Ce, Pr, Nd abundances. Similar trace

element suites have been measured by Andersen and Hinthorne in glasses

162
Geochemical Applications

from Apollo 14 [22], Apollo 15 [26], Luna 16 [22], and Luna 20 [27]

sites.

Trace element fractionations have been measured between co-existing

silicate (olivine, clinopyroxene, orthopyroxene , pyroxmangi te,

plagioclase, cristobalite) and oxide (spinel, ilmenite) phases, as well

as in single mineral grains, in a wide range of lunar basalts, norites,

anorthosites and breccias from Apollo 11, 12, 14 [22], Apollo 15

[26,27], Apollo 16 [27], Luna 16 [22] and Luna 20 [27] sites. These

studies have produced an immense quantity of basic trace element

abundance data which have not yet been adequately considered in the

light of current models for the evolution of the lunar rocks.

Meyer et at. [14] have discussed in some detail the problems of

interference {e.g., with Al2'^, CaO+, CaOH"*" and Rb"^ with CaSiO+ and

CaAlO"*") on IMMA analyses of trace elements in lunar plagioclases . Data

are presented for Li, Mg, Ti , Sr and Ba abundances .determined after making corrections for specific interferences and further empirical

corrections to CARISMA-reduced values on the basis of comparisons with

isotope dilution analyses determined on a bulk sample of a single

terrestrial plagioclase. On the basis of their trace element data, they

conclude that considerable variation in plagioclase compositions

observed indicates a wide variety of rock types in the lunar crust

before it was brecciated and sintered to its present state.

Bence and Autier [33] used a CAMECA ion microscope to compare ion

intensities for Li, Na, Mg, Al , K, Ca, Ti , Mn, and Ba along traverses

across a pigeonite-augi te junction in Apollo 15 basalt 15499 and between

pigeonite, augite, Fe-augite and plagioclase in Apollo 12 basalt 12033.

Andersen and Hinthorne [22] have reported IMMA analyses of a zoned

clinopyroxene grr.in from the Apollo 12 fines sample 12032 which has a core of pigeonite surrounded by subcalcic augite and finally riiraned by a pyroxene of intermediate composition. Virtually all the trace elements

analyzed i^'.e., Li, B, P, K, Na, V, Ti , Cr, Ba, Y, Zr, Al , Ca) are highly fractionated into the augite phase and only Sr shows relative

enrichment in the pigeonite.

Scanning ion micrographs of an Apollo 11 lunar basalt 10085 were

used by Andersen and Hinthorne [34,35] to illustrate the fractionation

of U, Th, Pb and REE into the accessory minerals apatite, whitlockite,

zircon, baddeleyite, zirkelite ( = zirconolite [36]) and

tranquillityite. K, Ba, Rb and Sr are localized in K-rich, U- and Thpoor glasses comnonly associated with these accessory phases. Although

absolute determination of REE abundances by CARISMA-reduced IMMA

analyses awaits final quantification, they have calculated REE

fractionation patterns for these phases in arbitrary units with respect

to the known abundances in chondritic meteorites [17, 21, 26, 27, 34,

35]. The various REE's within a single phase are in the correct

relationship to each other and the final calibration will only change

the absolute position of the fractionation curves for each mineral with

respect to each other [27]. A selection of their REE fractionation

curves for a number of lunar phases is shown in figure 5. In general,

163
SIMS/IMMA

Figure 5. Relative REE fractionation patterns (chondrite normalized) of

individual accessory phases in lunar rocks determined by

IMMA analysis [35].

apatites and whitlockites show rather flat fractionation patterns for

the light REE's (La to Gd) with a strong negative Eu anomaly, while Zrrich phases (i.e., baddeleyite, zirkelite = zirconolite,

tranquil! ityite) tend to show patterns with the light REE's which rise

164
Geochemical Applications

with increasing atomic number except for a strong negative Eu anomaly.

One exception to this rule is in the Apollo 11 mare basalt 10047 where

the apatites show rising patterns similar to the patterns normally seen

for Zr-rich phases [26]. Lovering and Wark [37] have shown

independently with electron microprobe analyses that the apatites in

10047 do exhibit this unusual rising REE fractionation pattern.

Andersen and Hinthorne have also reported that Y-bearing armalcolites in

Apollo 16 spinel troctolite 65785 show falling patterns from La to Sm, a negative Eu anomaly and rise to Gd [26] while plagioclase from Apollo 16

anorthosite 60025 shows a positive Eu anomaly in agreement with

independently published bulk- analyses [26]. They have commented that

all accessory phases analyzed by them generally show larger Eu anomalies

than do the bulk rock samples [26].

Lovering (this work—Table 6) has reported an IMMA analysis of

trace element abundances in the same tranquil lityite grain (20A-27) in

Apollo 11 basalt 10047 which had also been analyzed for REE's [26] and

lead isotopic composition (Table 9).

3, v Light Element Geochemistry

Hinthorne and Ribbe [24] have reported IMMA analyses of B in a number of chondrodites and were able to show that B can substitute for

as much as 6% of the Si atoms in this phase. However, in other humite

minerals, the B substitution does not exceed 0.7% of the Si atoms in the

structure.

The geochemical importance of trace determinations of H and F in

minerals have been discussed by Hinthorne and Andersen [19]. They have

developed a technique for H (detection limit 'vlOO weight ppm) and F (detection limit ^-20 weight ppm) analysis by IMMA which depends on the

comparison of H"'"/Si'*" and F"'"/Si''' intensity ratios with working curves which have been constructed using similar data from standard materials

of knowfi compositions. This particular approach to the analytical

problem was required because experience has shown that CARISMA, as it is

presently constituted, is not complete enough to use reliably on elements with high ionization potentials such as F (17.42 eV) and H (13.60 eV).

Lovering (this work) has reported quantitative Li and B analyses of

three glasses used as fission track geochronological standards (Table

4).

4. Isotope Ratio Analysis

a. General

Isotope ratio measurements by SIMS can be carried out either by an automatic, rapid peak-switching system [13] or by high speed mass scanning into a synchronized multichannel scaler [38].

Andersen and Hinthorne [38] have used the multichannel scaler

approach to measure the 2°^Pb/^°^Pb composition of the C.I.T. reference

165
SIMS/IMMA

Table 9. IMMA analyses of lead isotopic composition and Pb/ Pb ages

of uranium- rich phases in lunar rocks and fines.

Lunar localityRock

type (NASA catalogue

number)

Phase Th/U

(atomic)

208p^/206pj, Age"

(xlO' years) Reference

Apollo 11 Anorthositicfragment in fines

(10085)

Ti, Zr silicate (tranquillityite?) 2.16 0.59 0.40 3.90

[34] Baddeleyite

(ZrO^) 0.49 0.133 0 .455 4.11

REE phosphate 5.54 1.78 0 . li 4.79

Mare Basalt

(10047)

Zirconoli te (20A- 1) 1.92 0.502 0.3617±0.0090 3.7510.05

[26] Tranqui 1 li ty ite (20A-27) 0.60 0. 139 0.365910.0228 3.7610.10

Apollo 12 Dark/ Light K-rich

rock

(12013)

Dark Region Zircon 0. 48 0. 144 0.483 4.18

[34]

Wh itlockite

Light Region

Zr-phase

in K-rich vein 1.22 0.381 0.414 3.96 Fines

(12032)

Zircon 0 . 79 0.294 0 .408 3.94

Apollo 14 2-4 mm Fines

(142S7)

Zirconoli te 1.55 0. 383 0.412 3.96

Zirconolite 1.73 0.374 0.425 4.00

<1 mm Fines

(14163) Baddeleyite -- 0.34 0.406 3.93

Basalt

(14310) Y-REE oxide(?) 1.76 0.5003 0.396810.0081 3.9610.03

[40]

Apollo IS Basalt

(15555)

Y- Zr oxide (?) 7. 7 1.506 0.273210.0105 3.36*0.06

Y-Zr oxide(?) 0.8295 0.288810.0153 3.4610.09

Basalt

(15016) Phosphate

Breccia

(1S20S) Zircon 2.0 1. 108 0.411110.0291 4.0110. 11

Apollo 16 Anorthositicgabbro

(68415)

REE phosphate 1.72 0.3778 0.400610.0465 3.9610. 18

Y- Zrphase 1.45 0.3982 0.400710.0740 3.9610.28

Troctolitic

anorthosite

(65785)

Armalcol ite

(16) 1.67 0.388 0.422910.0526 3.9910.19

Armalcolite [26] (13) 1.20 0.316 0.448410.0383 4. 0710. 13

Armalcolite

(IS) 1.08 0.308 0.40910. 1584 3 94 -0.79

Luna 20 Micro- brecciafragments in (22003)

REE phosphate 1.71 0. 5046 0.439710.0122 4. 1210.04

(40)

Y- Zr phase 1. 34 0.3728 0.541810.0433 4.4210.11

±2 standard deviations. ^^^Pb/^^^Pb ages calculated using constants given in text (±2 standard deviations).

166
Geochemical Applications

lead sample and have concluded that IMMA instrumental factors will not

lead to errors of precision or accuracy in Pb isotopic ratios much in excess of 1 to 2 percent (one standard deviation). It should be

remembered that these conclusions refer to measuremients on a pure lead

nitrate sample and IMMA measurements of the isotopic composition of lead

in geological materials may be complicated by interferences with other

ionic species on the lead peaks in the secondary ion spectrum (see

below)

.

b. Molecular ion interference corrections in lead isotope studies

Andersen and Hinthorne [17] have developed a correction program for

molecular ions in the mass range of 202-210 for zircon which calculates

corrected 2°'*Pb, ^^^Pb, ^°^Pb and ^°^Pb relative ion intensities on the

basis of the following interferences:

202 ZrzO, ZrzDH, HfSi

203 ZrzO, ZrzOH, HfSi

204 ZrzO, ZrzOH, HfSi, Pb

205 ZrzOH, HfSi

206 ZrzO, HfSi, HfOz. Pb

207 ZrzOH, HfSi, HfOzH, Pb

208 ZrzO, HfSi, HfOz, Pb

209 HfSi, Hf02, HfOzH, ZrzOH

210 HfSi , HfOz, HfOzH

In their method [17] the ion intensities at each mass unit peak in

range 202-210 are measured and, since there are nine components and nine

peaks, the interferences on each of the lead peaks may be calculated by

properly ordering the corrections in a computer program. The largest

corrections on the Pb isotopes is for ZraO and HfOa while the HfSi is

small and ZraOH, HfOaH are either minor or negligible.

Corrections for nonradiogenic Pb in minerals are normally based on ^"^Pb abundances but this is usually small in Zr-rich phases and the

observed 2°'*Pb"'" ion intensity is usually less than the statistical

variation on the total 204 peak measured. Andersen and Hinthorne [17]

have also included in their lead isotope correction computer jjrogram a common lead correction based on the radiogenic 2°^Pb/^°^Pb predicted

from the measured Th/U ratio in the mineral. This assumes no subsequent

movement of Th relative to U, or of ^°^Pb relative to ^°^Pb.

The following decay constants and 2 38|j/2 3 5u ^.^tios, recently

recommended by Tatsumoto et al. [39], have been used in all data

reported here: >

X235 = 9.8485x10"^° yr-i , X^^e = 1.5525x10-^° yr'^

X232 = 0.49475x10-^° yr"^ 233^/235^ = 137.88.

T67
SIMS/IMMA

c. 207pjj/20 6p^ Geochronology

Andersen and Hinthorne [17] have reported IMMA-derived ^°^Pb/^°^Pb

ages on zircons from two localities (Cardiff Mine, Ontario and

Stillwater Complex) that have been previously analyzed and documented by

conventional techniques. Their data, indicating ages of 1059±39 m.y.

for the Cardiff Mine zircons and 2838±134 m.y. for the Stillwater

Complex samples, are in excellent agreement with independently derived

data published previously. This result is strong evidence in support of

their conclusion that IMMA-derived lead isotopic compositions, when

corrected by the procedure described above, are capable of producing

accurate ^°"^Pb/^°^Pb ages of U-enriched minerals within polished

sections of rocks.

They have also examined a number of U-rich minerals in a variety of

lunar rocks and soils ("fines") [34, 35, 38, 40] and a summary of their

207pb/206pb^ 208p5/206pb^ jh/U ratios and calculated 207pb/206pb gggs

for these phases is given in Table 9. All Pb isotopic ratios have been

corrected for ZraO*, HF02"'", YbOa"^, LuOz"*" and CaO.BaO"*" interferences

wherever applicable to the particular phase analyzed. However, for most

phases, these corrections are usually <0.1%. No common Pb corrections

were applied because ^°'*Pb''" could not be detected. The Th/U atomic

ratios were deterrfiined from observed ThO'''/UO''' intensity ratios and are said by them to have an accuracy of about 10% [38] and are generally

compatible with ^°^Pb/^°^Pb ratios.'

They have concluded that model 2°^Pb/^°^Pb ages calculated from

their IMMA analyses are generally consistent with ages' determined by

other workers using conventional Rb/Sr, K/Ar and Ar/Ar dating techniques

(fig. 6). Their data also indicate a younger age for the "intrusive,"

light (K-rich) material relative to the dark matrix breccia of Apollo 12

rock 12013 which is generally consistent with a two-stage evolution of

Pb in this rock proposed by Tatsumoto [41] on the basis of U-Th-Pb

systematics. /

Studies of the U/Pb systematics in zircon have shown that this

mineral has the ability to retain some age evidence of aji evejit which

preceded the last significant metamorphic event in an area (see summary

by Catanzaro [42]). Up to the present U/Pb dating has been confined to

bulk samples of zircons separated from rocks, but IMMA has the

capability of providing data on the ^°^Ph/^^^U and ^°''Ph/^^^\J ratios

(from which U-Pb dates are calculated) on micrometer areas of individual

grains of zircons within polished thin sections of rocks.

Andersen and Hinthorne [17] have proposed a method for converting

observed U (and Th) and Pb ion intensities measured by IMMA to absolute

U and Pb atomic abundances on the basis of sputtering source

temperatures estimated from observed UO"'"/U"'' (and ThO^Th"*") ratios.

Correction factors calculated from the measured UOVU"*" ratio for each

d. L)/Pb dating of zircons

168-
Geochemical Applications

<rtooC\J OCVJ

CLOct: Q-ocro

4 4

by.

4.2

4.0

3 8

3 6

- 3.4

3.2 -

"I

1 1 r 1 1 1 rX

235= 9 8485x|0-'^yr-l_

A"^= l.5525xlO-'Oyr-'

3.0 3 2 3.4 3.6 3.8 4.0 4 2 4 4 b.y. OTHER LABORATORY RESULTS ( Rb/Sr, K/Ar, Ar/Ar)

Figure 6, 20 7p|j/2oep|j crystallization ages of individual

accessory phases in lunar rocks from the Apollo 14

(68415 sites calculated from IMMA

correlation line) with independe

by «^Rb/^'Sr,'*°K/'*°Ar and/or

IMMA errors shown are ±1 deviation and other method errors include total

ages published including standard deviations quoted.

15 (15555) and 16

and compared (1 :1 ages determined

techniques [17].

U-rich

(14310),

analyses

nt rock

°Ar/'^Ar

standard

range of

analysis are applied to the 232, 238 and 206 sputtered ion intensities

(after correcting the 206 peak for interferences and common Pt

contribution as described above) to derive the ^°^Pb/^^®U atomic ratios.

The ^°^Pb/^^^U ratio is calculated directly from the measured radiogenic

20 7p|3/20 6pb i^^Q^n 23 8U/2 3 5U ^^t^-Q^ j^g 2 06p5/2 3 8u

20 7p|jy23 5y ratios are then plotted on the "concordia" diagram and ages

interpreted on the basis of presently accepted hypotheses for radiogenic

lead evolution in zircons (see summary in 142]).

Using this approach, Andersen and Hinthorne [17] have plotted IMMA

data for '^°^Ph/^^^\i and ^°^Pb/^^^U measured on a number of individual

169
SIMS/IMMA

grains of a zircon sample, from the Cardiff Mine, Ontario onto the

"concordi a" diagram and have shown that they fall on a 1040 m.y.

isochron, in good agreement with independent ages measured by U/Pb, K/Ar

and Rb/Sr techniques.

They have also reported U/Pb studies of U-enriched phases' in the Kri9h light areas which apparently penetrate the dark colored breccia

matrix of'unique Apollo 12 rock 12013 [17]. A single grain of Zr-Nb-Y-

(Ti?) oxide, probably zirconolite, occurring in the light area ,was analyzed by IMMA and a discordant age of 3865 m.y. calculated from the

"concordia" diagram. Similarly a zircon from the dark breccia matrix

was studied by them and discordant age of about 44-70 m.y. calculated.

Subsequent work (J. R. Hinthorne—personal communication) on other Urich phases in both light and dark regions have confirmed these ages.

The apparent older age of the dark matrix relative to the "intrusive"

light material is consistent both with ^°^Pb/^°^Pb ages discussed above

(see Table 9) and also generally with a two-stage model for Pb evolution

on bulk samples proposed by Tatsumoto [41].

e. Pb isotope Cosmochemistry

On the basis of IMMA studies, Hinthorne and Andersen [43] have

reported that Pb (2-35 weight ppm ^°^Pb), associated with geothite

alteration rims around troilite in the Apollo 16 "rusty rock" 66095, has

20 7p|3/206p|3 2 0 8p|3/206ptj ,,^atios which are unsupported by either Th

and/or U. Similarly Meyer and McKay [44] have studied the isotopic

composition of Pb occurring as a thin coating (100-1000 A) on the orange

glass sphenes occurring in the Apollo 17 "orange soil" (74220). Their

data indicate a primitive isotopic composition for this volatile Pb

indicating a source from a U-poor, volatile element rich environment

inside the moon and hot from meteoritic contamination.

f . ®^Rb/^^Sr Geochronology

The use of relatively low resolution secondary ion mass spectrometers in most SIMS instruments poses problems in the ^^Rb/^^Sr

dating of many silicate minerals {e.g.^ plagioclase feldspars) due to

the following selected spectral overlaps:

84 CaSiO, Sis, Sr

85 CaSiO, Si 3, Rb

86 AI2O2, CaAlO, Si 3, Sr

87 CaAlO, SiAlOz. SU» Rb, Sr

88 SiAlOa, SiaOa, Si 3, Sr

Meyer et al. [14] have concluded that, although not all these

interferences are significant {e.g.^ ^®Si2^^02"'' interference on ^^Sr"*" is <3% at 200 weight ppm Sr in plagioclase), the overall uncertainties

of all possible molecular ion interferences means that present

instruments are unsuitable for Rb/Sr dating. On the other hand it is

highly probable that correction procedures can be developed for specific

mineral matrices using the approach used by Andersen and Hinthorne [17]

170
Geochemical Applications

and described above in correcting molecular ion interferences on lead

peaks. In some cases molecular ion interferences will be negligible and

Andersen and Hinthorne [13] have reported good agreement between the

Rb/Sr isochron defined by a suite of K-feldspafs on the basis of

previous isotope dilution studies, and IMMA data for ^^Rb/®^Sr and

°^Sr/®^Sr isotope ratios on the same samples. Their results suggest

that, in these old Rb-enriched phases, KSiO'*' interferences on the ion

peaks measured at mass 85, 86 and 87 are negligible.

g. ^Li/^Li and ^^B/^°B Ratio Measurements in Cosmochemistry

Balsiger et al. [45] have briefly discussed current nucleosynthesis

models for the origin of the light nuclei D, Li, Be and B which either

assume their formation in the solar system or else in earlier galactic

processes. The solar system model alone should lead to local variations

in the ^Li/^Li and ^^B/^°B ratios if the primitive solar system material

was not subsequently thoroughly mixed. Consequently considerable effort

has gone into attempting to document variations in these ratios within

chondri ti c meteori tes

.

An early SIMS study by Poschenrieder et al. [46] suggested local

variations of ^Li/^Li ratios in the Holbrook chondrite from 9.5 to 27.9.

However later ^Li/^Li ratio measuremen-ts by SIMS in a number of

chondrites [47, 48, 49, 50] showed values varying by less than ±10%.

Preliminary ^Li/^Li ratios have been measured by J. R. Hinthorne

(personal communication) in W-1 rock glass, a South African zircon,

glass in Apollo 15 sample 15017 and in various silicate components of

the Allende carbonaceous chondrite using the A.R.L.-IMMA. His data

indicate all_ ^Li/^Li ratios measured lie in the range 11.9 to 12.3 and

are indistinguishable from each other and consistent with the results of

bulk ^Li/^Li ratios measured in a number of separated components from a variety of meteorites by Balsiger et al. [46].

5. Surface Analysis

Hofmann et al. [51] have used the ability of IMMA to measure the

isotopic composition of a solid as a function of depth in the solid with

a depth resolution of tens of angstroms to study the self-diffusion of

potassium in biotite. They heated a biotite sample in an alkali choride

solution enriched with "^^K for 41 days at 650°C and 2000 bars and, on IMMA analysis for ^^K/'*^K ratios, found a decrease with depth indicating

potassium diffusivity in the a- and b- directions to be greater than the

c- direction by 2 to 4 orders of magnitude. In the c- direction, D = 1x10"^'' cm^ ' sec"^ with an estimated error of a factor of 10 which was completely consistent with values calculated from independent methods

previously.

Clearly the surface analysis capability of IMMA provides an important tool for diffusion and surface composition studies in mineral

grains and integranular thin film analysis in rocks.

171
SIMS/IMMA

VI. FUTURE DEVELOPMENTS

1. SIMS Instrumentation

The major need is for the development of SIMS instruments with

secondary ion mass spectrometers which have greatly improved resolution

(at least to 3000 [18] and preferably higher) coupled with high

transmissivities so that present analytical sensitivities can be

retained or even improved further without surrendering the micrometerdiameter spatial resolution of the analysis area. These requirements

pose severe design criteria and it may yet prove impossible to meet them

all using current concepts of mass spectrometer design. An alternative,

though possibly less satisfying, approach is to correct for molecular

ion interference by spectrum stripping using computers. Colby [16] and

Andersen and Hinthorne [17] have shown that this approach can successfully overcome the limited resolution of most present day SIMS

instruments in many analytical situations.

There is also a need to improve spatial resolution of the SIMS

analysis region by decreasing primary beam diameters without seriously

sacrificing analytical sensitivity. Current estimates suggest that 1000

A will be the useful limit to ion beam diameters from duoplasmatron ion sources [5].

The need for independently measured abundances of internal standard

elements, for use in CARISMA reduction of IMMA analytical data with the

option 3 mode, has enhanced the relevance of combined electron

microprobe/IMMA instrument designed by Liebl [52]. This instrument

OUOPUkSMtfRON

ION SOURCE

EW2EL LENS.

ACAMPLFER

AND ItTLTLrU RECTIFIER

PHOTO- MULTIPUER

CHART RECORDER

13cp« CHOfTOJ

O .SkrV

TD PUMPING SYSTEM

MULTIPLE TARGET HOLDER

Figure 7. Schematic diagram of ion beam spectrochemical analyzer

(IBSCA) developed for quantitative analysis of geological

materials [54].

172
Geochemical Applications

would allow elemental abundance determinations to b6 made at an identical sample site first by electron microprobe and then by IMMA.

2. Quantitative IMMA Analysis

Considerable modification of the CARISMA approach to quantitative

IMMA analysis is required in the use of those elements with high

ionization potential {e.g.^ F, H) and those which have high oxide ion

intensities in the secondary mass spectrum ( e."^.^ Zr, Hf, Ta, REE,

etc.). But there is still a major need to unequivocally demonstrate

(and further improve where necessary) the accuracy of the CARISMAreduction of IMMA data by the preparation of critically homogeneous

trace element standard samples with geologically relevant matrices. The

need for such standards is critical if the geochemical community is to

generally accept the accuracy of CARISMA-reduced IMMA analysis for trace

element abundances in geological materials.

3. IBSCA Analysis

Tsong and McLaren [53] have shown that the bombardment of a solid

sample with an energetic ion beam causes the spontaneous emission of

ultraviolet and visible radiation as well as the emission of sputtered

secondary ions. This emitted radiation has been shown by them to

contain the characteristic lines of the constituent elements in the

sample and an instrument known as the ion beam spectrochemical analyzer

(IBSCA) has been constructed to analyze the radiation (fig. 7). The

potential of the IBSCA technique as a quantitative analytical tool has

been empirically demonstrated by comparison of IBSCA analyses of a suite

of plagioclase samples with analyses independently determined by

electron microprobe. Further developments suggest trace element

sensitivities .for the IBSCA technique are possible [54].

The general theory of quantitative IBSCA analysis has been

discussed by Tsong and McLaren [54] who have shown that the intensity of

a spectral line corresponding to a particular element in the sample is

proportional to the product of the rate of mass loss in sputtering and

the weight concentration of that element in the sample. Consequently,

if the mass loss during sputtering can be measured, it is possible to

calculate the concentration of the element in the sample by direct

comparison of intensities with a standard sample.

Clearly the limitation of the method at present is the difficulty

in accurately measuring the mass loss of the sample during sputtering.

However the relative simplicity and cheapness [53] of the apparatus

required for IBSCA analysis will encourage the further development of

the technique in the near future.

VII. ACKNOWLEDGMENTS

The IMMA analyses were carried out during the tenure of a N.A.S.A.

Senior Research Associateship at Goddard Space Flight Center. The

173
SIMS/IMMA

electron microprobe data were largely determined by D. A. Wark under a grant to me from the Australian Research Grants Committee. My own experience with IMMA quantitative analysis has drawn heavily on the

pioneering work of C. A. Andersen, and J. R. Hinthorne of the Hasler

Research Center (Applied Research Laboratories) and -I gratefully

acknowledge their ever-ready assistance, and the co-operation in data

reduction and in providing independent IMMA analyses of some phases in

Tables 2 to 8. I must also acknowledge the considerable support given

to me by my colleagues K. F. J. Heinrich and R. J. Myklebust (National

Bureau of Standards) and L. S. Walter and H. Thomas (N.A.S.A. Goddard

Space Flight Center)

.

VIII. REFERENCES

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in the Earth Sciences, Advances in Geophysics 16 , 57-154 (1973).

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Geophys. J. Roy. Astron. Soc. 14, 19-31 (1967).

• ^ 3. Castaing, R. and Slodzian, G., Microanalyse par Emission lonique

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4. Liebl, H., Ion Microprobe Mass Analyzer, J . Appl . Phys . 38 , 5277-

5283 (1967).

5. Liebl, H., Ion Microprobe Analysers: History and Outlook, Anal.

Chem. 46, 22A-30A (1974).

6. Robinson, C. F., Ion Microprobe Instrumentation in Microprobe

Analysis , Andersen, C. A., ed., pp. 507-530, Wiley-Interscience,

New York (1973).

7. Nakamura, K., Aoki, S., Tamura, H. and Doi , H, A New Technique for

Analyzing Insulating Materials by means of the Ion Microanalyzer in

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and Microanalysis , Shinoda, G,, Kohra, K. and Ichinol<awa, T. , eds.

,

pp. 447-454, Univ. of Tokyo Press, Japan (1972).

8. Andersen, C. A., Roden, H. J. and Robinson, C. F., Negative Ion

Bombardment of Insulators to Alleviate Surface Charge-Up, vh_ Appl

.

Phys. 40, 3419-3420 (1969).

9. Lovering, J. F., Ion Microprobe Mass Analyser (IMMA): Ultimate

Weapon for the Geochemist (?), Comments on^ Earth Science:

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10. Andersen, C. A., Progress in Analytic Methods for the Ion

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(1969).

174
Geochemical Applications

11. Andersen C. A., Analytic Methods for the Ion Microprobe Mass

Analyzer. Part II. Int. J. Mass Spectrom. Ion Phys. 3_, 413-428

(1970).

12. Andersen, C. A. and Hinthorne, J. R., Thermodynamic Approach to the

Quantitative Interpretation of Sputtered Ion Mass Spectra, Anal

Chem. 45,' 1421-1438 (1973).

13. Andersen, C. A. and Hinthorne, J. R., Ion Microprobe Mass Analyzer,

Science 175 , 853-860 (1972).

14. Meyer, C, Jr., Anderson, D. H. and Bradley, J. G., Ion Microprobe

Mass Analysis of Plagioclase from "Non-Mare" Lunar Samples in Proc.

Fifth Lunar Sci . Conf., Geochim. Cosmochim. Acta , Suppl . 5 j_» 685-

. 706.'Pergamon.

15. Hinthorne, J. R. and Andersen, C. A., Ion Microprobe Mass Analyzer

Operation Manual, Sec. 3, pp. 47-48, Applied Research Laboratories,

Sunland, California (1972).

16. Colby, J. W., Ion Microprobe Mass Analysis in Scanning Electron

Microscopy and Electron Probe Microanalysis , Goldstein, J. and

Colby, J. W . , eds. , Plenum P ress. New York ( 1975 )

17. Andersen, C. A. and Hinthorne, J. R., Ion Microprobe Determinations

of Ages of Individual Minerals and the Distribution of U, Th, Pb

and the KREEP Elements in Returned Lunar Material, Progress Report

Lunar Sample Research Program NASA Contract No. NAS 9-13299 (1974).

18. Bence, A. E., Electron and Ion Probes in Mineralogy (abstract),

Geo! . Soc. Amer. Abstr. Programs 5, 546-547 (1973).

19. Hinthorne, J. R. and Andersen, C. A., Microanalysis of Fluorine and

Hydrogen in Silicates with the Ion Microprobe Mass Analyser, Amer.

Mineral. 60, 143-147, (1975),

20. Andersen, C. A., Analytic Methods and Applications of the Ion

Microprobe Mass Analyzer in Microprobe Analysis , Andersen, C. A.,

ed., pp. 531-553, Wiley-Interscience, New York (1973).

21. Stout, J. H. and Hinthorne, J. R., Comparative Ion Microprobe and

Electron Microprobe Analyses of Coexisting Amphi boles and Garnet

(abstract). Geological Soc. Amer., Annual Meeting (Washington, D. C), Abstracts Program pp. 723-724 (1971).

22. Andersen, C. A. and Hinthorne, J. R., Ion Microprobe Mass Analysis

of Lunar Samples, Progress Report Lunar Sample Research Program

NASA Contract No. 9-11566 (1971).

23. Havette-Ledebt, A., Examen de Certains Probl^mes M^thodologiques

Pos^s par 1' utilisation de 1 'Analyseur lonique en G^ologie.

T75
SIMS/IMMA

Application 1 'Etude de Quelques Sujets Patrol ogiques, Thesis,

Universite de Paris-Sud, Centre d'Orsay, p. 1-149 (1974).

24. Hinthorne, J. R. and Ribbe, P. H., Determination of Boron in

Chondrodite by Ion Microprobe Mass Analysis, Amer. Mineral

.

59

, 1123-1126 (1974).

25. Jerome, D. Y. and Slodzian, G., Utilisation du Microanalyseur

lonique on G^ochimie Application i la m^t^orite de Juvjnas, Biill

.

Soc. Francaise Miner. Cristallog. 94, 538-548 (1971).

26. Andersen, C. A. and Hinthorne, J. R., Ion Microprobe Mass Analysis

of Lunar Samples, Progi»ess Report Lunar Sample Research Program

NASA Contract No. NAS 9-13299 (1973).

27. Andersen, C. A. and Hinthorne, J. R., Ion Microprobe Mass Analysis

of Lunar Samples, Progress Report Lunar Sample Research Program

NASA Contract No. NAS 9-11566 (1972).

28. Schnetzler, C. C. and Philpotts, J. A., Genesis of the Calcium-Rich

Achondrites in the Light of Rare Earth and Barium Concentrations in

Meteorite Research , Millman, P. M., ed., Reidel , Dordrecht

(Holland), pp. 206-216 (1969).

29. Taylor, H. P., Duke, M. B., Silver, L. T. and Epstein, S., Oxygen

Isotope Studies of Minerals in Stony Meteorites, Geochim.

Cosmochim. Acta 29, 489-512 (1965).

30. Andersen, C. A., Hinthorne, J. R. and Fredriksson, K.,' Ion

Microprobe Analysis of Lunar Material from Apollo 11, Proc. Apollo

11 Lunar Sci . Conf., Geochim. Cosmochim. Acta. Suppl . 1 , Vol . 1

,

T^9-TF7Tl97Dyr ~

31. Fredrikson, K., Nelen J., Noonan A., Andersen, C. A. and Hinthorne,

J. R., Glasses and Sialic Components in Mare Procellarum Soil,

Proc. Second Lunar Sci . Conf

.

, Geochim. Cosmochim. Acta Suppl. 2,

Vol. 1 , 727-735Tr97iy:

32. Prinz, M., Dowty, E., Keil, K., Andersen, C. A. and Hinthorne, J.

R., Ion Microprobe Study of High-Alumina Basaltic Glasses in Lunar

Science IV^, Chamberlain, J. W. and Watkins, C, eds., pp. 603-605,

Lunar Science Institute, Houston, Texas (1973)..

33. Bence, A. E. and Autier, B., Secondary Ion Analysis of Pyroxenes

from Two Porphyritic Lunar Basalts in The Apollo 15 Lunar Samples , Chamberlain, J. W. and Watkins, C, eds., pp. 191-194, Lunar

Science Institute, Houston, Texas (1972).

34. Andersen, C. A. and Hinthorne, J. R., U, Th, Pb and REE Abundances

and 2°^Pb/2°^Pb Ages of Individual Minerals in Returned Lunar

Material by Ion Microprobe Mass Analysis, Earth Planet. Sci.

Letters 14, 195-200 (1972).

176
Geochemical Applications

35. Andersen, C. A. and Hinthorne, J. R., U, Th, Pb and REE Abundances

and Pb 207/206 Ages of Individual Minerals in Returned Lunar

Material by Ion Microprobe Mass Analysis in Lunar Science III , Watkins, C, ed., pp. 21-23, Lunar Science Institute, Houston,

Texas (1972).

36. Wark, D. A., Reid, A. F., Lovering, J. F. and El Goresy, A.,

Zirconolite (versus Zirkelite) in Lunar Rocks in Lunar Science IV , Chamberlain, J. W. and Watkins, C, eds. pp. 764-766 (1973)

37. Lovering, J. F. and Wark, D. A., Rare Earth Element Fractionation

in Phases Crystallizing from Lunar Late-Stage Magnetic Liquids, in

Lunar Science V, pp. 463-465, Lunar Science Institute, Houston,

Texas (1974).

38. Andersen, C. A. and Hinthorne, J. R., 207pt,/2 0 6ptj ^g^^

Individual Mineral Phases in Lunar 20 Material by Ion Microprobe

Mass Analysis, Geochim. Cosmochim. Acta. 37, 745-754 (1973).

39. Tatsumoto, M., Knight, R. J. and Allegre, C. J., Time Differences

in the Formation of Meteorites as Determined from the Ratio of

Lead-207 to Lead-206, Science 180 , 1279-1283 (1973).

40. Andersen, C. A. and Hinthorne, J. R. , ^o^pb/aoep^ ;^ges and REE

Abundances in Returned Lunar Material by Ion Microprobe Mass

Analysis in Lunar Sciences IV , Chamberlain, J. W. and Watkins, C,

eds., pp. 37-39, Lunar Science Institute, Houston, Texas (1973).

41. Tatsumoto, M., U-Th-Pb Age of Apollo 12 Rock 12013, Earth Planet.

Sci. Letters 9, 193-200 (1970).

42. Cataniaro, E. J., The Interpretation of Zircon Ages in Radiometric

Dating for Geologists , eds. Hamilton, E. J. and FarguhaFii W. MT7

InterscTence, New York, pp. 225-258 (1968).

43. Hinthorne, J. R. and Andersen, C. A., Uranium-Lead and Lead-Lead

Ratios in Lunar Samples 66095 and 12013 by Ion Microprobe Mass

Analysis in Lunar Science V, pp. 337-339, Lunar Science Institute,

Houston, Texar~(T57^T^

44. Meyer, C, Jr. and McKay, D. S., Ion Microprobe Analyses of Lead

and Other Volatiles in Lunar Sample 74220 (Abstract), Los Angeles

(California) Meeting of the Meteoritical Society (1974).

45. Balsiger, H., Geiss, J., Groegler, N. and Wyttenbach, A.,

Distribution and Isotopic Abundance of Lithium in Stone Meteorites,

Earth Planet Sci. Letters 5, 17-22 (1968).

46. Poschenrieder, W. P., Herzog, R. F., and Barrington, A. E., The

Relative Abundance of the Lithium Isotopes in the Hoi brook

Meteorite, Geochim. Cosmochim. Acta 29. 1193-1195 (1965).

f77
SIMS/IMMA

47. Gradsztajn, E., Salome, n., Yaniv, A. and Bernas, R., Isotopic

Analysis of Lithium in the Hoi brook Meteorite and in Terrestrial

Samples with a Sputtering Ion Source Mass Spectrometer, Earth

Planet. Sci . Lett. 3, 387-393 (1967).

48. Bernas, R., Gradsztajn, E. and Yaniv, A., Isotopic Composition of

Lithium in Some Meteorites and the Role of Neutrons in the

Nucleosynthesis of the Light Elements in the Solar System in

Meteorite Research , Mi 11 man, P. M., ed., Reidel, Dordrecht

(Holland), pp. 123-131 (1969).

49. Nishimura, H. and Okano, J., Preliminary Analysis of Meteorites

with an Ion Probe Mass Analyzer (in Japanese), Mass Spectros. Japan

18, 894-904 (1970).

50. Nishimura, H., Fujiwara, T. and Okano, J, Elements and Isotope

Analysis with the Ion Probe Mass Spectrometer in Proceedings of the

Sixth International Conference on X-Ray Optics and Microanalysis , Shinoda, G., Kohra, K. and Ichino1<awa, T. , eds., pp. 431-437, Univ.

of Tokyo Press, Japan (1972).

51. Hofman, A. W., Giletti, B. J., Hinthorne, J. R., Andersen, C. A.

and Comaford, D., Ion Microprobe Analysis of a Potassium SelfDiffusion Experiment in Biotite, Earth Planet. Sci. Letters 24 , •48-52 (1974).

52. Liebl, H., Design of a Combined Ion and Electron Microprobe

Apparatus, Int. J. Mass Spectrom. Ion Phys. 6, 401-412 (1971).

53. Tsong, I. S. T. and McLaren, A. C, Quantitative Spectrochemical

Analyses of Feldspars by Ion Bombardment, Nature 248 , 43-45 (1974).

54. Tsong, I. S. T. and McLaren, A. C, An Ion Beam Spectrochemical

Analyzer - With Application to the Analysis of Rock Forming

Silicates, Spectrochim. Acta (In press, 1975).

178
NBS Spec. Publ. 427, Workshop on Secondary Ion Moss Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Ga Ithersburg, Maryland, September 16-18, 1974 (Issued October, 1975).

FACTORS THAT INFLUENCE AN ELEMENTAL DEPTH CONCENTRATION PROFILE

J. A. McHugh

General Electric Company

Knolls Atomic Power Laboratory

Schenectady, New York 12301

One of the attractive capabilities of secondary ion mass spectrometry (SIMS) is concentration profiling with high depth

resolution for both major and minor constituents. The general procedure

for producing a SIMS depth profile is to monitor the secondary ion

signal of an element of interest as a function of sputter time. In a uniform matrix, time can be translated to depth through suitable

calibration experiments (known film thickness, crater depth measurements, sputter yield, etc.).

Figures 1 and 2 demonstrate the depth profiling capability of SIMS.

Figure 1 is a phosphorus profile from a TaaOs film havinq a ^^P-rich

layer located 230 A below the surface. The true width (full width at

half maximum) of the layer lies somewhere in the range of 50-70 A. The

anodically produced amorphous TaaOsfilm sputters uniformly and thus

forms a good test matrix. The profile was obtained by monitoring the

3^p+ secondary ion current as a function of sputtering time (which is

proportional to the amount of film removed). The time scale was converted to Angstrom units (A) by measuring the time to sputter through

a known thickness of TazOs. The sputter ion beam was 02"'' at 3.5 keV. A

number of effects must be considered and kept under control to produce a profile that approaches the true concentration gradient (for example

fig. 1). These factors will be discussed shortly.

Figure 2 is also a profile produced from a TazOs film; however,

this time the interest is directed at a particular isotope of an element

rather than the element itself. It demonstrates the value of the

isotopic analysis capability of SIMS which is especially valuable in

diffusion and ion implantation studies.

The film was anodically produced in three steps. The second

anodization was carried out in an electrolyte enriched in H2^'0. Each

layer is about 1000 A thick. This particular profile was produced from

a 50 X 50 vim area on the sample. Ta^^0+ was monitored as the measure of

the 0 level. The Ta^^O+ signal from layer 1 and 3 represent the ^^0

contribution from normal oxygen {^.2 percent). Approximately 2 percent

of the oxygen atoms were ^^0 in the enriched layer. The leading and

179
SIMS/IMMA

trailing edges of the profile can give information on the resolution of

the profiling method and information on the transport of film

constituents during film formation.

Figure 1. Phosphorus depth profile generated by 1.75 keV ^^0

bombardment (Oz"^ at 3.5 keV}. The FWHM of the distribution is 78 X. A 1050 A TazOs film with a ^^Prich layer located 230 A below the surface was profiled with an ARL ion microprobe.

The factors that influence an elemental concentration profile

(those factors which produce a distorted representation of the true

concentration gradient) can be divided into two classes: Instrumental

Effects and Ion-Matrix Effects . For the instrumental effects^ one Ra?

some control over reducing or eliminating them. One has very little or

no control over ion-matrix effects.

180
Depth Profiles

Figure 2. The depth concentration profile of 0 in a TaaOs

sample containing a layer enriched in 0. The

profile was performed with 8.5 keV NI on an ARL ion

microprobe and the monitor of the ^^0 concentration

was Ta ^°0+. The ^^0 is ^2 percent of the oxygen in

the enriched layer. The Ta ^^^0+ signal preceding

and following the enriched layer peak is due to the

^^0 level in normal oxygen.

I. INSTRUMENTAL EFFECTS

The instrumental factors that must be considered are: 1)

uniformity of the primary ion current density, 2) constancy of the

primary ion current, 3) redeposition, 4) memory, 5) primary ion beam

tailing and the nonfocused component, 6) chemical purity of the primary

ion beam and 7) residual gas impurities.

It is virtually impossible to extract meaningful depth profile

information from SIMS data unless one maintains constant primary ion

intensity and uniform primary ion current density over the secondary ion

extraction area of the sample. Uniform primary ion current density is

produced usually by defocusing a static or stationary beam, or by

sweeping a beam in raster fashion over the sample surface to produce a region of uniform current density. Constant primary ion current is

desired so that the penetration rate remains constant; therefore, time

is directly proportional to depth in a uniform matrix. Redeposition

[1,2] and memory will also distort a profile. The influence that these

first four points have on a profile is illustrated in figure 3.

The secondary ion intensity of the layer species is measured as a function of time. The ideal profile is indicated in the tail region by

181
'SIMS/IMMA

the dashed curve. For a stationary primary ion beam with nonuniform

current density and a secondary ion analyzer whose information zone extends over the complete beam diameter, a distorted profile (as indicated in fig. 3) is produced because of contributions from the crater

edge.

T

1 r 1 1 r

I 1 I I I —I 1 ' ' 0 . 10 20 30

TIME OR DEPTH (orbitrory units)

Figure 3. Illustration of a concentration profile of a subsurface layer and various instrumental factors that

produce a distorted profile of the true distribution.

We can restrict the information zone to the area of uniform

penetration and remove most of the tail. This can be accomplished by 1)

having the uniform region much larger than the acceptance area of the

secondary ion analyzer, 2) in the direct-imaging analyzer by selective

aperturing, and 3) in the scanning ion probe by gating the ion detector

ojx when the rastered primary ion beam is in .the region of uniform

penetration. Even with the information zone restricted to the uniform

penetration region, a distortion of the ideal profile can result from

res puttering of redeposited material or from contamination of the

surface by layer species that have a source elsewhere in the instrument — "memory."

The tail and the nonfocused component of the primary ion beam are important factors to be considered in scanning ion probe type

instruments—both for depth profiling of trace constituents and trace

element analysis in general. Figure 4 schematically depicts this

situation. The nonfocused component can be as high as 0.1 to 1 percent
Depth Profiles

ION SOURCE

TOY AND

rUM AN/

[ ENERGY AND/OR I

i MOMENTUM ANALYSER I

Figure 4. Schematic representation

of the scattered ion and

neutral component of a focused ion beam.

SCATTERED K)NS AND ENERGETIC NEUTRALS

-SAMPLE

of the total number of beam particles, and it depends on lens design,

residual gas pressure in the primary ion column, location and size of

apertures and the geometrical arrangement of the column elements. A

significant energetic particle component falling outside the area of

interest, and over a large area of the sample, can create serious

problems in trace analysis and depth profiling. The problem is most severe when the elements of the primary column are on a common axis, and

this axis intersects the ion optic axis of the secondary ion analyzer.

Reducing the size of the secondary analyzer acceptance area (at the

sample surface) and deflecting the primary ion beam so that the neutral

and scattered-ion impact region is removed from the focused beam impact

point is one solution to this problem.

A partial solution to the above problem is illustrated in figure 5.

The secondary ion emission induced by the nonfocused component

illuminating the contaminant layer can be suppressed by overcoating the

sample with a substance that is of no consequence in an analysis or

depth profile. Carbon is a very good choice for a number of reasons:

1) it gives a very pure film, 2) it is conducting, and 3) the secondary

ion yields are low from carbon surfaces. This technique is very

beneficial for depth profiles of elements that are major surface

contaminants and for the general trace analysis of solids for

constituents which are also major surface contaminants. It is also

useful in micro-area analyses of O2, N2 and H2 in reactive metals.

183
SIMS/IMMA

The chemical purity of the primary beam is important to prevent

implantation of the sample with elements of interest in an analysis.

For example, a 1 ppm impurity in the primary ion beam would appear as

0.1 ppm atomic concentration in the solid (assuming that sputtering has

proceeded to the point of exposing the primary ion implant region).

Especially important are solids analyses for the elements O2, N2 and H2

Mass analysis of the primary ion beam eliminates this problem.

PRIMARY ION BEAM

Figure 5. An illustration of the effect that a carbon over- coating has on suppressing the secondary ion emission from points other than the analysis point.

It reduces the problems introduced by the tail of

the focused primary ion beam and the nonfocused

component of the beam.

Recontami nation of the surface by residual gases complicates solids

analyses directed at determining the bulk concentration of these

elements. This problem is brought out in the microarea analysis of N2

in Nb. A calibration curve is shown in figure 6. The measure of the

nitrogen level is the NbNVNb''" ratio. Four standards were analyzed:

83, 521, 618 and 1055 ppm N2 by weight. The uncorrected experimental

data and the background corrected calibration curves are given. At the

recontami nation levels present in the instrument, the analysis of N2 at

levels below a few hundred ppm is virtually impossible (background in

this specific instance is ^^SOO ppm). The cross hatched region shows the

typical variation of the uncorrected experimental curve for different

instrument and sample conditions. Higher current density and improved

primary ion beam focusing reduces this effect. However, one cannot

escape this problem without some means to limit the information zone to

the high current density, region of the primary ion beam. It is clear

from figure 6 that depth profiles covering a wide concentration range

can encounter problems. Similar effects result from memory and

redeposition phenomena.

184
Depth Profiles

NITROGEN CONCENTRATION (BY WEIGHT)

Figure 6. Quantitative working curve for the determination

of nitrogen in Nb using an ARL ion microprobe.

The NbNVNb''" secondary ion ratio is used as the measure of nitrogen in the Nb standards. The

departure of the experimental curve from the

corrected curve is due to a background MbN"*" signal

from N2 recontamination of the surface. Higher

current density and optimized current density

distribution in the focused primary ion beam can improve the above results; however, it can not

completely eliminate the effect.

II. ION MATRIX EFFECTS

The major factors that affect a depth concentration profile which

can be classified as ion matrix effects are: 1) the mean escape depth

of secondary ions, 2) recoil implantation, 3) molecular ion

interferences, 4) primary ion beam induced diffusion of matrix species,

5) nonuniform sputter removal of matrix layers, and 6) implanted primary

ion chemical and lattice damage effects.

185
SIMS/IMMA

The escape depths for sputtered particles cover a range from the

outer surface layer to depths >20 A. The median escape depths are generally <10 A. Decreasing the primary ion energy shifts the depth-oforigin distribution for sputtered particles (secondary ions) nearer the

surface and thus reduces contributions from deep layers. The escape

de]}th does not play a significant role in depth resolution or profile

distortion because other factors have a much greater effect.

The effect of primary ion energy on recoil implantation of layer

species or mixing in subsurface layers [3] is demonstrated in figur'e 7.

This is the same ^^P-TaaOs film sample discussed previously. It shows

the profiles for 1.75, 7.75 and 18.5 keV ^^0 bombardment. A dramatic

effect with significant 'distortion of the true shape is evident.

200 200 400 200 400 600

DEPTH (A)

Figure 7. Phosphorus depth profiles of a TaaOs film

containing a ^^P-rich subsurface layer. The

profiles were obtained using normal incident

^^0 sputtering beams of 1.75, 7.75 and 18.6

keV. These results illustrate the effect of

incident ion energy on a measured profile.

(From McHugh [3]).

However, at 18.5 keV some characteristics of the true profile remain.

This is because the mean penetration depth of ^^0 is initially beyond

the layer, and it indicates for certain situations that high energy

beams can provide useful information [3]. In addition, profiling thin

surface films {<100 A) or regions near the surface with a high energy,

mass analyzed, low— Z ion beam is useful because an approximate depth

calibration or sputter rate can be established from the implanted

primary ion profile, and the calculated location and shape of the

primary ion implant profile.

186
Depth Profiles

The remaining factors affect a profile in the following way. A

given sample matrix produces a characteristic group of molecular ion

interferences which can interfere with certain element

analyses—especially at trace levels in low mass resolution instruments

[4-7]. Surface charges can build up on thin insulating films causing

migration of ionic species in the film (e.^. , alkali and halogen

constituents) [8,9]. A uniform primary ion current density does not in

itself imply uniform etching of the surface. This characteristic is

controlled by surface topography, cleanliness, and sample uniformity

which depends on composition, physical state and crystal lographic

orientation.

Figure 8.

The time or depth dependence

of the ^^Fe+ secondary ion

intensity from an iron surface

for normal incidence ^^0* and

t°Ar+ ion bombardment. The

^^Fe"*" peak heights and positions

corresponding to the surface

oxide film were normalized in

the '*°Ar+ and ^^0- cases for

ease of comparison. Rp - 140 A

for 16.5 keV ^^0 ions incident

on an Fe target. Rp is the projected range.

DEPTH (i) 100 200 300

T

=-1 1 1 1 1

Since depth profiles usually extend to depths greater than a few

hundred angstroms, the chemical effects of the implanted primary ion are usually evident in a profile. The chemical effect is demonstrated in

figure 8 where the surface of an iron sample is sputtered with oxygen

and argon ions, and the ^^Fe"*" is measured as a function of time. The

initial peak is due to enhanced emission from a surface oxide film.

Then the growth in intensity is due to reaching the implant zone. The

plateau is generally jreached at depth equal to 2 to 3 times the mean projected range of the primary ion in the solid. The profile with Ar"'" shows just the surface film enhancement. The implantation of the inert

species Ar produces no enhancement. If the sputter etching is ac- complished with a primary particle energy of MOO eV/amu, the changing

matrix condition caused by ion implantation is confined to within 100 A

of the surface; beyond 100 A the implant ion concentration remains

constant and a stable matrix condition is attained. In many cases, the

187
SIMS/IMMA

effects resulting from this type of chemical enhancement can be con- trolled by introducing oxygen gas in the vicinity of the sample.

Adsorption of O2 on surfaces generally enhances and stabilizes the

secondary ion yield [10,11]. The result is that element sensitivity is

improved and certain matrix effects are reduced or eliminated which

gives more meaning to the secondary ion signal as a direct measure of

element concentration. Also, the variation of secondary ion yield with

crystal orientation is eliminated in a number of cases by O2 adsorption

on a surface [12,13]. The usefulness of this method in an analytical

application has been demonstrated [1,14].

From the above example it is evident that the variation of

secondary ion intensity is not always a measure of a relative difference

in element concentration; therefore, care must be exercised in the

interpretation of depth profiles — especially profiles near the

surface or of films composed of dissimilar layered phases, or matrices

with nonhomogenous distribution of trace elements that have a major

influence on the secondary ion emission characteristics of the matrix.

To reduce these effects so that meaningful results are obtained, one of

the following methods should be used 1) apply quantitation procedures to

the data, 2) reference the profiled element intensity to the secondary

ion intensity of one or more uniformly distributed constituents of the

film, or 3) change the character of the secondary ion emitting surface

by performing the profile under conditions of high reactivity gas {e.g.

O2) pressure in vicinity of the sample surface.

III. REFERENCES

[1] Blanchard, B., Hilleret, N., and Quoirin, J. B., J. Radioanal.

Chem. 12, 85 (1972).

[2] Croset, M., J. Radioanal. Chem. . 12, '69 (1972).

[3] McHugh, J. A., Rad. Effects 21, 209 (1974).

[4] Hernandez, R., Lanusse, P., and Slodzian, G., C. R. Acad. Sci. , Ser. B, 271, 1033 (1970).

[5] Hernandez, R., Lanusse, P., Slodzian, G., and Vidal, G., Method.

Phys. Anal. 6, 411 (1970).

[6] Colby, B. N. and Evans, C. A., Jr., Appl . Spect. 27, 274 (1973).

[7] Bakale, D. K. , Colby, B. N., Evans, C. A., Jr., and Woodhouse, J. B., in Proceedings of the Eighth National Conf. of the Electron

Probe Analysis Soc. of America, . p . 7A , New Orl eans , La . , Augus

t

Uy/3). 7 '

[8] Brown, F. and Mackintosh, W. D., J. Electrochem. Soc. 120, 1096

(1973).

1^8
Depth Profiles

[9] Hughes, H. L., Baxter, R. D. , Phillips, B., IEEE Trans. Nucl . Sci

.

NS-19 , 256 (1972).

[10] Slodzian, G., Ann. Phys. 9^591 (1964).

[11] Slodzian, G. and Hennequin, J. F., C. R. Acad. Sci., Ser. B, 263,

1246 (1966).

[12] Bernheim, M., Rad. Effects, 18^ 231 (1973).

[13] Bernheim, M. and Slodzian, G., Int. J. Mass Spectrom. Ion Phys. ,12,

93 (1973).

[14] Blanchard, B., Hilleret, N. and Monnier, J. , Mat. Res. Bull.,

6, 1283 (1971).

189

NBS Spec. Publ. 427, Workshop on Secondary Ion Mass Spectrometry (SIMS) and Ion Microprobe Mass

Analysis held at NBS, Gaithersburg, Maryland, September 16-18, 1974 (Issued October, 1975).

A COMPARISON OF AUGER ELECTRON SPECTROSCOPY

(AES) AND SECONDARY ION MASS SPECTROMETRY (SIMS)

J. M. Morabito

Bell Telephone Laboratories, Inc.

Allentown, Pennsylvania

ABSTRACT

This paper compares the analytical capabilities and limitations of

Secondary Ion Mass Spectrometry (SIMS) and Auger Electron Spectroscopy

(AES) beginning with the modes of operation available: 1) SIMS and AES

spectra, 2) ion and Auger imaging, 3) AES and SIMS depth profiling.

Vacuum requirements, the volume of sample consumed, and the status of

quantitative analyses by SIMS and AES are also discussed; followed by a comparison of SIMS and AES bulk, in depth, and surface detectability

limits. It becomes clear from the data presented that the limitations

and capabilities of both SIMS and AES dictate the use of both, when

possible, in materials analysis. SIMS does have, for many elements, a sensitivity advantage over AES but AES is preferred for the surface an- alyses of small areas {<50 ym^) at concentrations >1 at. %, particularly

when sample removal by sputtering is undesirable.

I.- INTRODUCTION

Fundamentals of the sputtering process as related to Secondary Ion

Emission Mass Spectrometry (SIMS), possible ionization mechanisms,

instrumentation, modes of operation, and methods of SIMS quantitative

analysis have been discussed in previous papers of this monograph. This

paper will discuss the analytical capabilities and limitations of SIMS

in comparison to those of Auger Electron Spectroscopy (AES). Both SIMS

and AES are capable of localized, i.e.^ small selected volume, analysis

from the surface into the bulk and can provide three-dimensional (x, y,

z) microchemical analysis by means of depth profiling (z) and "ion" or

"Auger" imaging (x, y). The limitations of both techniques are inherent

in the instrumentation used, the extent to which the physics of AES and

SIMS is understood, and the availability of accurate data on parameters

such as Auger yields and escape depths, sputtering and secondary ion

yields, atomic and molecular partition functions, etc. The limitations

and capabilities of both SIMS and AES dictate the use of both, when

possible, in materials analysis.

191
SIMS/IMMA

II. MODES OF OPERATION AVAILABLE

The modes of operation available in SIMS and AES are illustrated in

figure 1

.

1 . Spectra

A, SIMS Spectra

Secondary ion mass spectra can be used to. identify all elements

(with the possible exception of neon and helium) and isotopes. Both

positive and negative secondary ion mass spectra can be obtained.

However, the yield (number of secondary ions)/(number of sputtered

atoms) of positive and negative secondary ions can vary [1] by several

orders of magnitude over the periodic table and is a function of parameters such as primary ion energy [2], nature of the primary ion [3]

{e.g.^ argon or oxygen (I6O2 +5 ^^0")), chemistry [4] orientation

[5], and other parameters discussed in

spectrometric detection results in very

hence signal-to-background ratios of

absence of mass interferences.

the literature. The use of mass

low instrumental background, and

10^ - 10^ are achievable in the

ENERGY (eV)

MASS AND AUGER SPECTRA ION AND AUGER IMAGES

DEPTH (h

ANALYSIS IN DEPTH

Figure 1 - Modes of operation available in AES and SIMS.

192
SIMS/AES Comparison

The possibility of mass interference makes ,good mass resolution,

(M/Am), a necessity in SIMS, since mass interferences can limit

detection sensitivity and complicate depth profiles, surface analysis,

and isotope abundance measurements. The mass resolution needed to

resolve elements in the mass range of 27 to 238 from hydrocarbon

interferences is shown in table 1 along with the resolution required to

separate multiply charged and polyatomic species in the mass range of 14

to 58. Mass resolution of 1,000 to 3,000 is not possible with state-ofthe-art quadrupole mass spectrometers or with double focussing

spectrometers of the type described by Liebl [6] without the use of some

type of secondary ion energy discrimination. The technique of lowenergy discrimination (LED) has been described in detail [7,8] and has

been demonstrated to reduce the intensity of polyatomic ion species

without significantly reducing the intensity of the singly charged

species. This type of discrimination is possible since the kinetic

energy distribution of polyatomic and multiply charged ions is lower

than that of monoatomic ions [9]. High-energy filtering is also

desirable since a high energy limit minimizes chromatic aberration [11].

The resolution obtainable with the Castaing-Slodzian [10] mass spectrometer design in combination with a 90° spherical electrostatic

Table 1. Resolution (10% valley) of elements from

hydrocarbon and polyatomic species.

Hydrocarbons

Mass Doublet M/Am

27 C2H3 - ^'Al 642

63 C5H3 - ^^Cu 670

138 C10H18 - ^^^Ba 570

208 C15H28 - ^°'Pb 866

238 Cl7H3^ - ^''U 1100

600 - 1000 is required

Polyatomic

Mass Doublet M/Am

14 Si"*""*" - N"^ 960

28 Si"*" - Fe"^ 2,980

56 Sit - Fe"*" 2,895

58 ^^Si^^Si"^ - Ni"^ 3,800

193
SIMS/IMMA

analyzer is shown in figure 2 [11]. The separation of ^^Fe"*" from ^''AT'',

shown in figure 2, implies a mass resolution (M/Am) of 3,500. Higher

mass resolution in SIMS requires the use of a Mattauch-Herzog type

spectrometer as described by Hernandez et al. [12].

M

AM 3500

Figure 2 - Mass resolving power of IMS 300 with electrostatic analyzer (Reference 11).

The SIMS spectrum also contains information on the chemistry of the

sample (e.^., the presence of oxides, carbides, intermetal 1 ic compound

formation, etc.). Methods have been developed to unravel the chemical

information contained in SIMS spectra by means of standard "fingerprint"

spectra [13] {e.g., from a series of metal oxides such as CrOq, CrO, Cr203, etc.). However, the limitations of "fingerprint" mass spectrometry for information on sample chemistry have not vet bepn systematically investigated.

B. Auger Spectra

Auger spectra can be used to detect and identify all elements with

the exception of hydrogen and helium, but AES spectra provide no information on the isotopes of elements. The variation in Auger yield

114
SIMS/AES Comparison

is less than a factor ten over most of the periodic table for a fixed

primary electron energy. The Auger yield depends primarily on the cross section for ionization (t> , the backscattering factor r, and the escape

depth, d . All of these factors are a function of primary energy. The

backscattiring factor r is due to primary electrons backscattered with

sufficient energy to produce additional Auger electrons. The signal-tobackground, of differentiated Auger spectra is on the order of 100:1.

dN

dE

Ti

(UNDOPED)

Ti 0

a)

•-TiN b)

0 (37.0 at. %N)

To

Ta. N

J L 0 100 200 300 400 500 600 700

ELECTRON ENERGY (eV)

Figure 3 - Spectral interferences of nitrogen and titanium

in AES.

Spectral interferences in AES are less severe than mass interferences since most elements have several (3-5) Auger transitions

over a wide energy range, but there are several combinations of elements

which result in serious interference effects {e.g.^ nitrogen and

titanium, argon and boron). The inherent energy spread of Auger

electrons which determines the width of the Auger peak, and hence,

spectral interferences, is several electron volts. Therefore, Auger

transitions with energies within several eV of each other are difficult

to resolve with electrostatic spectrometers including the higher

resolution double-pass type recently described by Palmberg [14].

195
SIMS/IMMA

However, the double pass spectrometer is also suitable for photoelectron

spectroscopy—more commonly known as ESCA—and many Auger instruments

are being modified to include the ESCA capability. The combination of

ESCA and AES in tne same instrument should reduce spectral interference

difficulties considerably. However, it should be noted that the spatial

resolution of ESCA is two to three orders of magnitude poorer than

that of AES.

Figure 3 shows the Auger spectra of Ti(a) and TiN(b). The 418 eV

titanium transition is more intense than the 387 eV transition in the case of pure titanium, but there is an interference between the titanium

387 eV and the nitrogen 380 eV transitions (shown for TaN(c)). This

interference makes the 387 eV Ti peak more intense than the 418 eV peak

for nitrogen doped Ti films. Unfortunately, oxidation of titanium also

results in an increase in the 387 eV peak which further complicates the

identification of nitrogen in titanium. Figure 4 shows the

photoelectron spectra of TiN and demonstrates the clear separation

between the nitrogen and titanium photoelectron transitions. For a brief comparison of Auger and photoelectron spectroscopy—ESCA—see reference 15.

0 Is

Ti 2s

Ti 2p

>-

l1

z

N Is C s

900 800 700 600 500 400 300 200 100 0

BINDING ENERGY (eV)

Figure 4 - ESCA Spectrum of TiN.

1'96
SIMS/AES Comparison

Auger spectra also contain information on the chemistry of the

sample, but interpretation is difficult since the Auger process can involve as many as three atomic energy levels. The Auger spectra

therefore, like the SIMS spectra, can only provide a fingerprint of the

chemical environment. Changes in the Auger spectra which are used to

provide information on chemical environment include: 1) energy shifts

in the Auger peaks, 2) changes in the peak shape of valence Auger

electrons, 3) additional "fine structure" on the low energy side of the

Auger peak.

2. Ion and Auger Images

Both ion and Auger images can provide a two-dimensional map of the

distribution of surface impurities. Ion imaging requires the

volatilization of material by sputtering; Auger imaging does not. The

lateral resolution of ion images depends on the method used to obtain

the image. The two methods available are by scanning a focussed ion

beam [6] and intensity-modulating the z-axis of a cathode ray tube (CRT) or by focussing the ions by means of an ion-optical system [10]^ Image

resolution by the scanning method is limited by the ion beam diameter

\,2. yjTi. The resolution possible by the ion-focussing approach is limited

only by the chromatic aberration present in the ion optics and is independent of ion beam size. Ion image resolution by this method on the

order of .5 ym has been obtained [16].

The Auger image can only be obtained by the scanning intensity

modulation method and is therefore theoretically limited by the electron

beam size consistent with practical recording times {^20Q - 400 s).

Brandis [17] has obtained an Auger image resolution of .5 ym for a 1,500

mesh silver grid with a beam current of 4 x 10"' A and a recording time

of 200 sec. Auger imaging is limited to concentrations in the range of

1 at. %, but ion images at concentrations below 1 at. % can be obtained

for elements with high secondary ion yield such as Na, Al , Mg. Ion and

Auger images from small (5 - 10 ym^) areas are usually not jtaken due to

poor image resolution, but ion images from precipitates as slnall as 50 A

have been recorded [18]. Table 2 summarizes the salient differences

between Ion and Auger imaging.

3. AES and SIMS Depth Profiling

In-depth profile analysis by AES and SIMS is accomplished by

monitoring a particular or series of Auger and/or secondary ion peaks

while sputtering through the sample. The depth scale (A) is obtained by

calibrating for the sputtering rate (A/s or A/min) by measuring the

sputtering time and the crater depth formed by the sputtering. Crater

depth measurements are typically obtained with interferometers [19] or

by a Talystep [20] which are only accurate for flat bottom craters.

Sputtering rates calculated from crater depth measurements assume a constant sputtering rate during the measurement. Unfortunately, the

sputtering rate (z) can change during the analysis of multicomponent

197
SIMS/IMMA

Table 2. A comparison of Auger and ion 'imaging.

1. Lateral resolution of 'v^.Bym has

been demonstrated for' both.

2. Auger imaging does not require

sample consumption, but is limited

to at % concentration.

3. .Ion imaging requires sample

consumption, but is not limited

to concentrations as high as

1' at % for elements with high

secondary yields.

4. Auger or ion imaging of small areas U - lOym^) is not practical.

films, especially when interdiffusion has occurred. However, Dahlgren

and MacClanahan [21] have shown that the sputtering rate for a twocomponent system under equilibrium conditions is often limited by and

equal to that of the element with the lower sputtering yield. Figure 5a

and figure 5b show the Auger depth profiles of Ti-Pd-Au films (Sample 1 and 2) after interdiffusion has occurred. A thickness monitor was used

to deposit films of different Ti and Pd thickness. The Ti thickness

ratio was 1 .73 and the palladium thickness ratio was The gold

thickness was 3,600 A for both films. Therefore, by measuring the

sputtering rate of gold (zi) and defining the Pd thickness to be from

the point where the gold profile intersects the palladium profile (A) to

the point where the Ti profile begins (B) (as shown in figures 5a and

5b), and using the ratio of the sputtering efficiency of palladium to

gold (.87) to calculate the sputtering rate of. palladium (^2)>« the

thickness of the Pd region of Sample 1 calculates to be 964A and

that of Sample 2 calculates to be 482 A, a ratio of 2. From Point B (figs. 5a and 5b) a titanium rich ternary alloy is present and it is

reasonable to assume that the sputtering rate (23) of this alloy is

limited by and equal to the sputtering rate of titanium which has a lower sputtering yield than the gold or palladium constituents of the

alloy. The thickness of the titanium rich region of Sample 1 is

calculated under this assumption to be 504 ^ and that of Sample 2 to be

892 A» a ratio of 1.77. This example is cited to demonstrate that

accurate depth scales can be obtained for the case of a varying

sputtering rate. The same method could be used for the SIMS profiles

provided chemical effects do not distort the SIMS profiles

significantly.

Other practical problems inherent in the sputtering process used in

SIMS and AES profiling include the possibility of differential

'198
SIMSyAES Comparison

sputtering in the case of alloys which can change sample composition at

the surface, recoil or "knock on" whereby lattice atoms are pusheddeeper into the solid [22], primary ion implantation effects [23,24],

redeposition [25] and crater wall effects [26]. Numerous instrumental

techniques have been developed in both AES and SIMS to avoid crater wall

effects. Both sputtering and instrumental artifacts decrease the depth

resolution possible by AES or SIMS profiling. The magnitude of the

degradation depends on sputtering conditions used and the nature of the

sample.

— — — ^

I

1

I ' I ill I I l!

' 1 \

0 10 ?0 30 40 50 0 10 20 30 40 50 T'ME (MIN) TIME (MIN )

Figure 5 - AES profiles of Ti-Pd-Au films with different

Ti and Pd thicknesses.

As discussed by Evans [27], "depth resolution" can be defined in

several ways, the most common being the ability to distinguish between

two subsurface features. With this definition and in the absence of

sputtering artifacts, the depth resolution possible in AES and SIMS

profiling has been shown [27,281 to be 5 - 10 percent of the thickness

removed by sputtering, e.g.^ 100 A for a 1,000 A film. Depth resolution

can also be evaluated by the distortion imposed on a step function

change in concentration. Figure 6 shows the AES profile obtained on a Pt-Al multilayer film [29] on a silicon substrate. The profile clearly

shows an aluminum-rich zone of 47 A in width {&). The measured

aluminum profile has a Gaussian dlstribjiition , but the actual aluminum

199
SIMS/IMMA

1000

Figure 6 - AES depth profiles for platinum, aluminum, and

silicon in thin films sputtered onto silicon.

profile was most probably a step function (i.e.^ c{x) = c for 0 < x < 6;

c(x) = 0 elsewhere). This distortion is due in part to the sputtering

process, but also to the response of the AES or SIMS techniques to this

change in concentration. The response of AES and SIMS is determined by

the escape depth of the Auger and secondary ions which are in the range

of 5 - 20 A in AES and believed to be in the range of 25 - 50 A in SIMS.

Impurity regions of thickness less than or comparable to the escape

depth of Auger or secondary ions will exhibit an AES or SIMS profile

different in shape than the actual impurity profile even in the absence

of sputtering effects such as "knock on" as recently discussed by Chou

et al. [30]. The lower escape depths of Auger electrons and the lower

primary ion energies (<2 keV compared to >2 keV in SIMS) used in AES

profiling makes the distortion of sharp or step-function like

concentration gradients less of a problem in AES than SIMS.

200
SIMS/AES Comparison

DEPTH —

Figure 7 - SIMS depth profiles for platinum, aluminum, and

silicon in thin films deposited onto silicon.

Figures 7 and 8 are the SIMS profiles obtained on the same Pt-Al film

analyzed by AES (fig. 6). The data shown in figure 7 were obtained

using positive primary argon ions at 5 keV. The secondary ion yield of

^^^Pt"*" is enhanced on the surface and at the Pt-Al interface due to the

presence of impurities such as oxygen, aluminum, and magnesium. This

enhancement is evidence for alloying between Pt and these elements which

could have taken place during deposition of the films or promoted by the

collision cascades produced by the high energy primary argon ions. The

latter possibility has not yet been thoroughly investigated. It is

possible, however, by SIMS to monitor cluster ions such as ^^^Pt^^Al"*",

^^Al^^i"*^. The presence of these species in the SIMS spectra suggest

that alloying has occurred either during film deposition or by the

primary ion impact-collision cascade process. The width of the

aluminum-rich region in the SIMS profile of figures 7 and 8 is similar

to that found during AES profiling (fig. 6). The profiles in figure 8 were obtained using primary Oa"*" ions at 2.5 keV. The same enhancement

effects are observed for Pt and Si using primary oxygen ions (see fig.

7). Note that the 2 ss^-h- profile is similar in shape to that of

^^Al^ssi"*", suggesting that alloying also occurs between AT and Si.

There was nothing in the AES spectra or in the AES profile which

indicated alloying between Pt, Al , and Si. The indication of alloying

from the SIMS data could be an artifact of the sputtering process, but

could also be real and, therefore, an important capability of the SIMS

technique. If it was not an artifact, SIMS could be very useful in the

study of the stability of interfacial phases in thin films [31] which

are not in thermal equilibrium.

201
SIMS/IMMA

III. VACUUM REQUIREMENTS

The mechanism of secondary ion emission is such that vacuum

conditions are perhaps of more concern in SIMS than in AES since the

presence of contaminants such as H2O, CO, CO2, and hydrocarbons not only

complicate analysis for these species, but also reduce the sputtering

rate, contribute to molecular ion formation, and complicate the

ionization mechanism. In addition, the presence of numerous molecular

ions such as M«Uy in the SIMS spectra increases the probability of mass interference effects particularly at the high mass end of the spectra.

Ultrahigh vacuum conditions (10"^° torr) are particularly important for

surface^analysis with SIMS since the extremely low primary ion currents

(10"^ A/cm^) used for surface analysis are considerably below those

required to maintain a contamination-free surface at lU"' or 10-8 torr.

It is for this reason that tne SIMS instrumentation used for surface

studies are designed with ultrahigh vacuum capabilities similar to those

of AES instrumentation, i.e., 10"io torr or better. The possible

presence of contaminants in the sample chamber during AES or SIMS

profiling also means that a minimum sputtering rate must be maintained

during AES or SIMS profiling of chemically active samples such as

I 50 A

DEPTH

Figure 8 - SIMS depth profiles for platinum, aluminum, and

silicon in thin films sputtered onto silicon.

202
SIMS/AES Comparison

metals The portions of the sample sputtered below this minimum rate

should 'not be allowed to contribute to the Auger or secondary ion

spectra since these portions are often contaminated by residuals in the

sample chamber. Sputtering rates in the range of 10 A/min. are usually

sufficient to prevent contamination of active surfaces surrounded by

residuals (H2O, XO, CO^) that ha've a partial pressure in the range of

10" 8 torr.

IV. VOLUME CONSUMED IN SIMS AND AES ANALYSIS

A. SIMS

Since only a small fraction of the sputtered particles are ionized,

a finite volume (6^) of the sample must be volatilized to detect a specified concentration (C) with a precision (p) on the measurement

[32J. This minimum volume can be estimated, by the following equation

[33]

^ 10^ 100

3 _ (CNK)'-' ~^ ^ CD

^ = P Nav/M

where

6^ - Volume (cm^)

C - Concentration (atom fraction)

N - Collection efficiency x transmission of the mass spectrometer

K - Secondary ion yield (ions/sputtered neutral)

p - Precision {%)

a - ISotopic abundance

p - density

M - atomic weight

•^AV

" Arogadro's number

This minimum volume can be expressed in terms of the area analyzed (A)

and the depth U) removed to obtain this required volume. Table 3 shows

the results obtained for the common dopants (B, P, and As) in silicon.

The secondary ion yields used for these calculations were K (boron) = 5.2 X 10"°, K (phosphorus) = 3.5 x lO'^ and K (arsenic) = 5.2 x 10-7.

These yields were estimated from measurements of the number of counts

obtained from a known volume of ion implanted silicon samples and

assuming the transmission collection efficiency factor to be 10 percent.

The precision used was 3 percent. The yields used are conservative, but

it is doubtful that the secondary ion yields of these elements are more

than two_orders of magnitude better, i.e.^ K (boron) is 5.2 x 10 > K > 5.6 X 10"^ under typical instrument operation. The analyzed area was varied from 20 to S x 10^ ym^ and the minimum depth required to

detect concentrations of I0-2, 10"^ and 10"^ (atom fraction) of boron,

phosphorus, and arsenic are listed in table 3. These minimum depths

203
SIMS/IMMA

will increase for elements with lower secondary ion yields {e.g.j Pd,

Ag, Zn) and will decrease for elements of higher secondary ion yield

{e.g., Li, Al , Si, Na).

Table 3. The minimum sputtered depth versus concentration for boron, phosphorus,

and arsenic in silicon.

Area

Element Concentration 20ym2 10 W 9 X lO'^ym^

Depth Depth Depth

10-2 250^

0 5A .55A

10-*^ 2.5ym 500A 55. 5A

10-^ 250ym 5 X 10'*A 5.5 X lO'A

31p+ 10-2 370A 7.4A .82X

31p+ 10-- 3.7ym 740A 82K31p+ 10-^ 370ym 7.4 X lO'^A 8.2 X lO^A

^5As+ 10-2 2500A 50A 5.5A

lO-** 25ym 5000A 555A

^5As+ 10-^ k:500um 5 X lO^A 5.5 X 10"*^

The results clearly show that; 1) the depth resolution possible for

a given concentration can be dependent on the area analyzed depending on

the secondary ion yield, 2) depth profiles of elements with low

secondary ion yields at concentrations below .1 at. % with good depth

resolution—5 to 10 percent of the thickness removed by sputtering—are difficult to obtain with good precision (p) when the analyzed area is

less than 10 ym^, 3) surface analysis (< 20 A depth) with SIMS for

concentrations below 1 at. % of low secondary ion yield elements

requires a large area of analysis. The results shown in table 3 can only serve as an estimate of the required depth, since Equation (1) does

not correct for background and mass interference effects which can only

serve to increase the minimum volume, requirement consistent with a specified precision. Calculations with Equation (1) are, therefore, not

often realistic, but are helpful to establish a "modus operandi" for the

objectives of the experiment.

The requirement of a minimum volatilized volume has important

consequences when the objective is the analysis of small areas (< 20

ym^) with small diffusion depths (< 1 ym) , e.g., a transistor on a 204
SIMS/AES Comparison

1—I—I—I—I—I—I—I—I—I—I

I r -CALCULATED ARSENIC PROFILE

MEASURED ARSENIC PROFILE FOR BORONPLUS-ARSENIC SAMPLE

\-«— EXPONENTIAL TAIL \

\

\

\

-CALCULATED Xje

\

-CALCULATED BORON PROFILE -MEASURED BORON

PROFILE FOR BORON- ^ PLUS-ARSENIC \V^^ SAMPLE

I '! S^^^ I I I I 4 .6 .8 1.0 SILICON DEPTH (/im)

L2 1.4 1.6

Figure 9 - SIMS profiles (measured and calculated) of

arsenic and boron in silicon ion implanted with

both boron and arsenic.

typical silicon integrated circuit. Figure 9 shows SIMS profiles of

arsenic and boron in silicon ion-implanted with both boron and arsenic.

The area of analysis was 9 x lu"^ ym^, but the depth of analysis was limited to 1 ym. Within this volume {'^ 10^ ym^), the entire boron

profile down to 10^^ atoms/cm^ could be detected. The redistribution in

the boron profile compared to the calculated [34] profile (also shown in

fig. 9) has also been observed by Bonis et at. [36] using a nuclear

activation technique. The redistribution of boron is believed to be due

to the arsenic implantation and subsequent anneal. A possible mechanism

has been discussed by Ziegler [36].

It was not possible, within this volume, to detect the entire

arsenic profile. The tail in the arsenic profile is an artifact in the

SIMS technique due, perhaps, to mass interference [37] or redeposition

effects [25]. If real , the' arsenic concentration for all depths

would be greater than the boron concentration. This is not possible

since there is an emitter-base junction (Xj^) at .41 ym. The tail in

the arsenic profile begins at 5 x 10^ ^ atoms/cm^ and occurs at a depth

of 5,000 A. Very little of the boron and arsenic profile could be

measured with a precision of 3 percent if the area of analysis was

205
SIMS/IMMA

decreased to 20 ym^, which would be the area of an actual transistor

fabricated by arsenic (emitter) and boron (base) doping in the range of

10^^ - 10^5 atoms/cm^ by chemical or ion implantation methods. However^

the profiling of elements with high ion yields down to 10^^

atoms/cm^ within a volume of 20 ym^ would be possible. Methods to

increase the ion yield of the common dopants in silicon to the range of

10-1 "1 would be of considerable practical importance in the

silicon integrated circuit technology.

Table 4. The minimum time constant, min required for

Auger surface analysis as a function of beam

size (ym^) for a constant current density (j).

j = 10'^ A/ym^

A A A A

lOVm^ low lOym^ lym"

S/N C{at%) ^min ^min ^min ""min

10/1 10 1 X 10"2s .1 s .1 s 10 s 100/i 1 .1 s 1 s 10 s 100 s

j = 10"^ A/ym^

lO^m^ lO^ym^ lOym^ lym^

S/N C{at%) ^min ^min ^min ^min

10/1 10 1 X lO'^s .01 s .1 s 1 s 100/1 1 .01 s .1 s 1 s 10 s

min. j^OR

B. AES

Chemical analysis by Auger electron detection does not require

material consumption. Removal of material is only used to bring regions

of interest to the surface where they can be analyzed. Reducing the

electron beam size, hence area of analysis, for a constant primary

current density (j) does not increase the depth of sample analyzed but

does increase the time constant necessary to achieve a required signalto-noise ratio on the lock-in amplifier used in the AES detection

scheme. The depth of sample analyzed in AES is determined only by the

escape depth of the Auger electrons (dg). Table 4 shows the minimum

time constant, Am-jp, required to detect concentrations of 10 and 1 at. %

(corresponding to S/N ratios of 10/1 and 100/1) as a function of

206
SIMS/AES Comparison

electron beam size (ym^) for current densities of 10"'^ A and 10"^ A.

A practical limit on the time constant is 10 seconds which is reached at

an area of 1 um^ and concentration of 10 at. % when j is 10"'^ A and at

an area of 1 ym^ and concentration of 1 at. % when j ;= 10~^ A_.„ . It

would be difficult for SIMS (see table 3) to detect elements of low

secondary ion yield down to concentrations of 1 at. % when the area of

analysis is limited to 1 ym^ and the depth to 10 A, but the surface

analysis of these elements down to 1 at. % from small areas {'^ 1 ym^)i

and depths {"^ 10 A) is possible with AES as shown in table 4. From the,

data of Table IV we also see that \^\^ is proportional to 1/jA.

AES with a small focused electron beam is therefore well suited to

the "analysis of actual integrated circuit devices provided current

densities in the range of 10"'' A to 10"^ A can be obtained. Current

densities in this range are achievable, as shown in figure 10, for tungsten and lanthanum hexaboride (LaBe) sources [17]. There is, however,

the possibility of beam damage, electron induced desorption [38], sur- face heating, etc. at these current densities—especially for oxides.

Figure 10 - Primary electron beam diameter versus beam

current.

V. STATUS OF QUANTITATIVE AES AND SIMS ANALYSIS

1 . SIMS

A. Models

Quantitative SIMS analysis by the use of standards, and by the^use

of a local thermal equilibrium (LTE) model and a quantum mechanical

model have been described in this monograph and the literature [33,

39, 40, 41].

207
SIMS/IMMA

Despite the apparent success of the LTE model [40 j, the assumptions

upon which it is based have been questioned [42,43]. In addition, the

often unpredictable effects of active elements on secondary ion yieldschemical effects, matrix and orientational effects on ion yields, and mass interference effects caution against the routine use of any model

[44]. The LTE model, like the BET model [45] used to calculate surface

areas, might only provide a working formalism with the "temperature" and

electron density of the plasma serving as adjustable parameters.

Experiments to prove or disprove the validity of the LTE model are in

progress in several laboratories [46]. These experiments, together with

a more detailed analysis of the energy absorbed and subsequently

dissipated by the lattice atom-primary ion collision process, should

provide some insight into the conditions necessary for the formalism to

apply to ion emission.

B. Standards

The formalism used for quantitative SIMS analysis by the use of

standards has also been described in several publications [33,39]. It

has become common practice to reference or normalize the secondary ion

current from the impurity dopant (i) to the secondary ion signal from

the matrix (m). This normalization procedure corrects for any changes

in the sputtering rate {1) or surface atom density (a) during the

measurement. In-depth profiling and ion imaging can be used to

establish that the dopant is homogeneously distributed throughout the

sample(s) to be used as a standard. The methods which have been used to

prepare standards for quantitative SIMS analysis include ion

implantation [37,47,48], reactive sputtering [49J, bulk doped samples

[50,51], thin film deposition- techniques , etc. The work with standards

has confirmed that SIMS measurements are quantitative particularly when

the dopants are homogeneous at low concentrations, e.g.j, < 10 at.%

total, and there are no chemical and matrix effects. Quantitative SIMS

analysis by the use of standards is preferred over the various model

formalisms provided standards are available and the unknown is similar

in chemistry, density, and crystallinity to the standard. A SIMS

calibration curve for nitrogen in gallium phosphide, using the ion

implantation method of standard preparation, is shown in figure 11 [48].

A. Standards

Quantitative analysis by AES depends, at the present time, on the

use of standards and on an equation of the form [52]

2. AES

-1

C. =

1 rel

X 100 (2)

rel I.

203
SIMS/AES Comparison

at.%, A^^i is the relative Auger yield where C^- is the concentration in

of the impurity (i) referenced to the matrix, I^- is the measured peak

height(s) of the dopant{s), and j is a running index over all the

elements present in the sample. Equation (2) assumes a homogeneous

distribution of dopant and the complete absence of matrix effects since

Are^ "lust often be measured by the use of "pure" elements; e.g.,

the relative Auger yields of iron and copper for a Fe-Cu-Au alloy would

5XI0V

TDTAL COUNT AFTER CORRECTION

Figure 11 - SIMS calibration curve for nitrogen in GaP.

be measured by measuring Auger peak heights from "pure" iron, copper,

and gold samples and referencing these peak heights to gold whose A^.^^ is defined as 1. The Ay.g-|'s measured in this way are not always in

agreement with those measured from a series of Fe-Cu-Au alloys of known

and varying Fe, Cu, and Au composition. The use of Equation (2) . also

assumes that Ay.gi is independent of concentration. In addition,

quantitative analysis by AES requires the use of sputtering to remove

oxide and carbon layers which can attenuate the Auger peak of interest.

There is a surface compositional change due to sputtering and, therefore, the Auger measurements can only provide a relative measure [53J of

the actual concentration in the bulk. It is possible, however, to

calculate the concentration in the bulk from the surface concentration

20T
SIMS/IMMA

measured by AES and the sputtering yields [53]. Unfortunately, the

sputtering yields are not often accurately known. , In contrast, SIMS measures not what remains on the surface but that which leaves and can therefore provide, in principle, the actual concentration in the bulk of

the sample.

.002 .005 .01 .02 .05 0.1 02 05 10

BORON OR PHOSPHORUS/SILICON {1619 eV) PEAK TO PEAK RATIO

Figure 12 - Auger calibration curves for B and P in silicon.

Figure 12 shows the calibration curves obtained for the Si-B and

Si-P systems using bulk grown doped silicon standards [54]. Calibration

curves of this type can be routinely used to provide quantitative AES

analysis and establish AES detectability limits. For a two-component

system, Equation (2) reduces [53] to

^ % (3)

m rel

where is the concentration of the matrix. A plot of C^/C^ vs.

I-j/Ifj, can then be used to calculate Ay,gi of the dopant which can then be

compared to the A^g] calculated by tne use of pure elements, e.g.^ pure

silicon and pure boron.

210
SIMS/AES Comparison

B. • First Order Approximation to Quantitative Auger Analysis

with the CMA Analysis

Quantitative AES analysis by models analogous to those used by the

electron microprobe [55] are not yet available. The reasons for the

present lack, of a model include: *

1) The detection scheme used does not measure the absolute Auger

current (i^^) directly.

2) Auger escape depths (dg) for many elements jiave not yet been

accurately measured or tabulated for routine quantitative

analysis

.

3) Backscattering correction factors (r) are not available for

most elements or easily measured.

4) A tabulation of ionization cross sections is not available.

5) The effect of surface roughness on the Auger signal intensity

has not been sufficiently studied with regard to quantitative

analysis

.

The information available on 1

J to 4) has shown that:

1) The Auger peak-to-peak height from the differentiated energy

distribution is proportional L56J to the Auger current (i/^)

provided the peak shape in the N(E) function does not change,

and the integral of the N(E) peak is a more accurate relative measure of 'the Auger current (E is the kinetic energy of the

Auger electrons) [57].

2) The escape depth of Auger electrons is determined by the

valence band of the element, and in the energy range of 100 eV

to 1 ,500 eV is proportional [58] to yE.

3) Backscattered primary electrons contribute no more than 20-30

percent to the Aujger yield [59].

4) The ionization cross sections are approximately proportional to

1/E^ [60] for K and perhaps L shell ionizations provided the

primary energy (Eq) is a factor 2 or 3 greater than the binding

energy (Eg) of the ionized level.

Therefore, for a given matrix {e.g.^, Si), primary electron current and

energy (where Eq >> Eg), concentration C, and angle of incidence (a) the

Auger yield_ from a smooth surface can be defined as the product,

i|;(i)dpn(E)r. The product of these parameters should be proportional

to T/vC since ri(E), the transmission of the CMA spectrometer, is

approximately proportional to E up to 1 ,uOO eV. However, this linear

dependence of the CMA transmission with energy is only valid when the

acceptance window of the spectrometer is small compared to the natural

211
SIMS/IMMA

line width of the Auger peak. The acceptance window of the spectrometer

is the product of spectrometer energy resolution {^^ .5 percent) and the

kinetic energy of the Auger transition of interest, e.g.^ it is 5 eV at

a kinetic energy of 1000 eV. The transmission of the retarding

field type analyzer is constant with energy and therefore _the

suggested l/ZE dependence would not apply to data obtained with this

type of analyzer.

The Auger transition probability, 4^, is assumed to be close to

unity for the major, more intense, Auger transitions. There are, of

course, several transitions (peak splitting) possible from a single

ionization (KLiLi, KL1L2, etc.) and each has its own transition

probability. For most elements, however, one of the possible

transitions associated with a K, L, M, etc. ionization will predominate

and it is these transitions which are most often . used in qualitative

Auger analysis.

The ratio of the differentiated or integrated Auger signal (I) of

element A normalized to the matrix (l'^) signal to that of element B normalized to the same matrix at a constant concentration C can be

expressed by

E

B

E

The Auger signals (I

ABM

,1 > I ) should be corrected for any variation in

the electron multiplier gain (G) of the CMA with E. The gain of the

electron multiplier in the Physical Electronics CMA has been found to be

essentially constant above 150 eV [61]. The sputtering yield ratio of

elements A and B, Sg/S;^, should approach 1 at low concentrations (< 19

at. %). This sputtering yield ratio is a necessary correction since

most Auger data for quantitative analysis are taken during sputtering

[49,53]. The backscattering ratio, r^/rg, should also be close to 1 at

low concentrations since it is primarily determined by the matrix. For

the same matrix and for major transitions of the same type (KLL, LMN,

etc.) and shape, ^j^/^q'^ 1 and Equation (4) then reduces to:

212
SIMS/AES Comparison

.008

.007

.006

.005

.004

72X1020A/cm'

.003

.002

.001 r

1000 2000 3000 4000

DEPTH A

Figure 13 - Auger profile of oxygen and nitrogen ion implanted into silicon.

The validity of Equation (4) has been tested by the author at low

concentrations {< 10 at. %) by ion-implanting nitrogen and oxygen into

silicon and at higher concentrations (>10 at. %) by preparing nitrogen,

oxygen, and carbon doped tantalum thin films using reactive sputtering.

The concentration of the ion implanted samples was calculated from the

dose (at./cm^) implanted into the sample and the LSS theory [34]. The

measured nitrogen and oxygen profiles, obtained by combining Auger

analysis with argon ion sputtering, have a Gaussian distribution in good

agreement with LSS theory_ [34]. The measured profiles are shown in

figure 13. The implanted concentration (at./cm^) versus the normalized

Auger peak heights over the concentration range where the nitrogen and

oxygen data overlap is shown in figure 14. The use of ion implantation

to calibrate Auger and SIMS data for quantitative analysis has been

discussed in several publications [37,47,48]. The ratio of the nitrogen

and oxygen normalized peak heights, / 1° at the same concentration,

213
SIMS/IMMA

e.g.^ 2, 3, etc. x lO^o at./cm^ (fig. 14) measures to be lJ__witl]L_an

estimated error of <20% at these low concentrations. This ratio

compares very well with the ratio of the square root of the Auger

energies of oxygen (510 eV) and nitrogen (380 eV) which is 1.15 and

suggests that the sputtering yield and backscattering ratio can be

approximated by unity in this low concentration range and the Auger

yields of nitrogen and oxygen are about the same.

V,Si jSij

NORMALIZED AUGER PEAK HEIGHTS (IQ-^

)

Figure 14 - Concentration (At./cm^) of ion implanted nitrogen and oxygen vs. normalized auger peak heights

The validity of Equation (4) at higher concentrations (>10 at. %) was also studied. Figure 15 is a plot of the concentration of nitrogen,

oxygen, and carbon in reactively sputtered tantalum films. The

concentration of these elements was determined by the electron

microprobe [53]. Nitrogen and oxygen which have similar Auger peak

shapes were found to have, within experimental error, the same calibration curve but carbon, with a different Auger peak shape, falls slightly

below both oxygen and nitrogen. The calibration curves for (104 eV) and

(179 eV) should then be more similar to the carbon calibration curve. 214
SIMS/AES Comparison

.025 .05 .1 .2 .3 .5 1.0

X (NORMALIZED AUGER PEAK HEIGHTS)

Figure 15 - Log C (at. %) determined by the electron microprobe vs. log of normalized N. 0. C auger peak

height (X) in reactively sputtered tantalum thin

films

The peak shapes- for nitrogen, oxygen, carbon, beryllium and boron are

. J" V^^^^ Similarity in peak shape would not be required

If the integral rather than the differentiated peak-to-peak height was

used. '

M n J^oi ^S^^i^jH?" Eguation (4) to data obtained on vacuum- cleaved

MgO [52], CM [62 , CdS [61], GaP [61], and KCl [61] is summarized in table 5. The s listed in Table V were calculated assuming that the

for MgO^"^

backscattering ratios are approximately one, e.g..

rel = 1.5

(6)

215
SIMS/IMMA

Aril " ^ since Mg is the matrix. These calculated A^g-|^s (table 5) were

then used together with Equation (2) to calculate the composition from

the measured differentiated peak-to-peak heights (I) which are shown for

MgO, CU2O and CdS in figure 16. The peak heights for GaP and KCl were

taken from Reference 61. The running index j of Equation (2) includes

all the elements present in the sample.

Table 5. Calculated compositions of compound standards.

bample \el Calculated Composition^

MgO 1.50 56 at. 5

lo 0 44 at. % Mg

CU2O 1.34 35 at. 5k 0 65 at. % Cu

CdS 1.57 48.5 at. 5

lo S 51 .5 at. % Cd

GaP 2.99 47 at.

'% p 53 at. % Ga

KCl 1.18 47.4 at.

'% CI 52.6 at. % K

^All dat$ (except MgO) were taken while sputtering.

The results suggest that a sensitivity—relative Auger yield

(Ay,g,) —factor can be estimated without standards by Equation (4)

provided the transitions have similar Auger peak shapes and transition

probabilities in the energy range 100 eV to 1 ,000 eV. The use of

Equation (4) is restricted to data obtained with the CMA spectrometer

and is presented as a semi -empirical attempt to convert measured peakto-peak heights into quantitative atomic concentrations. The

limitations of the suggested 1//E dependence are increased by our present lack of information on transition probabilities for specific

transitions, backscattering correction factors and the corrections

necessary due to sputtering during Auger analysis. In the absence of

sputtering artifacts, the largest error should occur when the transitions chosen are of different type (K, L), shape and natural line width.

Errors due to the above are minimized by integrating the differentiated

Auger peaks. It is highly improbable that the sputtering yield (S) and

backscattering corrections (r) are independent of concentration.

Measurements on alloys indicate that S does change with concentration

[21]. The effect of dopant concentration on the backscattering

correction remains to be established.

215
SIMS/AES Comparison

400 800 1200 ELECTRON ENERGY (eV)

Figure 16 - Auger spectra of vacuum cleaved MgO, single

crystal CU2O, and CdS.

VI. COMPARISON OF SIMS AND AES BULK, IN-DEPTH,

AND SURFACE DETECTABILITY LIMITS

Bulk detectability limits for SIMS refer to operation without

restrictions on the area and depth of analysis. In AES, this implies an electron beam diameter of 100 ym, high primary currents {e.g.^ 20 to

50 yA) and a long time constant on the lock-in amplifier—on the order

of several seconds. Table 6 lists the AES and SIMS bulk detectability

limits [8] for the common dopants (B, P, As) in silicon and for light

elements In tantalum. The bulk detectability of AES is limited to

^10'^'^ atoms/cm^ or .1 at. %. SIMS is clearly a far more sensitive

technique than AES for bulk analysis (see table 6), and thus

particularly useful for the precharacterization of samples prior to

electron microprobe or Auger analysis. Precharacterization with SIMS

can reduce the time necessary for comprehensive AES or electron

microprobe analysis.

217
SIMS/riMA

The detectability limits of SIMS for in-depth analysis depend to a great extent on the depth over which the impurity is diffused, on the

magnitude of the secondary ion yield, on the required precision (p). and

depth resolution needed in the profile. Over a depth of 1,000 A, a precision of 3%, and a depth resolution of 50 to 100 A, the in-depth

detectability of SIMS for elements with ion yields in the 10"^ range

could be as low as 10^^ atoms/cm^, assuming no limitation on the area of

analysis and the absence of mass interference effects. For most

elements, the above conditions would result in an in-depth detectability

limit of from lO^'' atoms/cm^ to 5 x 10^^ atoms/cm^. The in-depth

detectability limits of AES under the same conditions has been measured

[47] to be 10^0 atoms/cm^ or from two to three orders of magnitude

less sensitive than SIMS. The in-depth detectability limits of AES under- the same conditions has been measured [47] to be 10^° atoms/cm^ or from two to three orders of magnitude less sensitive than SIMS. The indepth sensitivities of AES and SIMS become more jimilar as the depth of

the diffused region decreases below 100 A and the area of analysis

is restricted to 10 ym^ or less.

Table 6. Comparison of bulK sensitivity limits tor

P, B, and As in silicon and for N, 0, C in

tantalum using AES and SlMS.

Element Technique Detectabi lity Limit

N, 0, C AES 10

Table 6. Comparison of bulk sensitivity limits for

P, B, and As in silicon and for N, 0, C in

tantalum using AES and SIMS.

Element Technique Detectability Limit

N, 0, C AES - 10'^ atoms/cm^

N SIMS 10'^ atoms/cm'

C, 0 SIMS 10^^ atoms/cm^

P, B . AES 10^^ atoms/cm^

As AES 10^° atoms/cm^

B SIMS lU^" atoms/cm^

P SIMS 10^^ atoms/cm'

As i>IMS lO^^ atoms/cm^

218
SIMS/AES Comparison

The surface detectability ' limit of AES is the same as its bulk

detectability limit, .1 at. %, since the Auger electrons are coming

from the same depth whether the surface or bulk (after material removed

by ion sputtering) analysis is performed. Surface analysis by SIMS

requires the use of small current densities (10"^ A/cm^) to limit

material removal to fractions of a monolayer, and large areas of

analysis .1 cm^) to remove a sufficient volume (6^) of sample.

Surface analysis by SIMS can be complicated by hydrocarbon mass interference effects [12,33], secondary ion enhancement due to the

presence of chemically active elements on the surface [33], arid primary

ion implantation effects [23]. High mass resolution and the

simultaneous detection of a wide mass range by means 'of a photoplate

[12] are particularly useful when performing surface analysis with SIMS.

Although the interpretation of SIMS surface analysis can be difficult,

the surface detectability limit of SIMS is several orders of magnitude

more sensitive than AES. However, surface analysis is more routinely

performed by AES which is the preferred technique when surface analysis

from selected areas <50 ym^ is required, especially when material

removal is undesirable.

VII. SUMMARY AND CONCLUSIONS

Both SIMS and AES have unique analytical capabilities for materials

characterization from the surface into the bulk. They are the only

analytical techniques capable of three-dimensional microchemical

analysis with good lateral and depth resolution. Lateral resolution <1 ym and depth resolution from 5 to 10% of the thickness removed by sputtering can be obtained.

AES surface and bulk analysis is limited to ^ .1 at. % . In-depth

profiling by AES and Auger imaging are limited to ^ 1 at. % (depending

on the material, current density, analysis time, etc.). Quantitative

bulk and in-depth analysis based on standards or by a first order

approximation method are accurate to within 20-30 percent, but a generalized model for quantitative analysis without standards is not yet

available. For a model to be acceptable, it would also have to include

sputtering parameters such as sputtering yields (S) since ion sputtering

is routinely used in AES bulk and indepth profile analysis.

Sputtering is not often used in AES surface analysis, since it

would remove elements of interest from the surface. It is, however,

often necessary to remove contaminants such as carbon, sulfides,

etc. from the surface since they can attenuate or completely mask other

surface impurities. Attenuation effects and the often nonhomogeneous

distribution of impurities within the plane of the surface and within

the escape depth region makes quantitative AES surface analysis of real

surfaces very difficult. Quantitative AES surface analysis is possible,

however, from surfaces prepared under ultrahigh vacuum conditions with

varying amounts of contaminants or cleaved in ultrahigh vacuum.

~ The

analysis of surfaces prepared under such ideal conditions should eventually provide the information on escape depths, backscattering
SIMS/riMA

corrections, ionization cross sections, etc. necessary for routine AES

quantitative analysis without standards.

For optimum detection limits, SIMS surface, in-depth profile, and

bulk analysis should be performed using a large bombarded .area (10"2 - 10"^ cm^) with low current densities (10"^ A/cm^) for surface analysis

and higher current densities (10"^ - 10"^ A/cm^) for in-depth profile

and bulk analysis. Bulk detection limits for many (but not all)

elements, in the absence of mass interferences, can be in the PPM range

for elements with ion yields of 10"^ or better. Surface and in-depth

profile detection limits depend on the requirements of the measurement,

but should be from one to two orders of magnitude less than bulk

detection limits since a reduced sample volume is available for

volatilization. Mass interferences should be reduced in SIMS analysis

which is possible by using high resolution mass spectrometry [12] or some type of energy filtering [7,8,11]. The technique of introducing a controlled oxygen jet [63] on the sample during primary ion bombardment

can be useful in minimizing chemical , matrix and orientation effects

during SIMS in-depth profiling provided the oxide formed is amorphous.

As understanding and experience with SIMS progresses, chemical, matrix,

and orientation effects on secondary ion yields will also be used to

advantage in materials characterization. The ion image is, in fact, a two-dimensional representation of the chemical, matrix, and orientation

effects in SIMS.

Ion images are often required to interpret anomalous structure in

SIMS depth profiles which can be due to the presence of precipitates in

grain boundaries, porosity between thin film layers, changes in ion

yield due to orientation effects, etc. The ion image mode is of

particular value in identifying different phase regions in alloys as a function of heat treatment. Detection limits with the ion imaging mode

are, however, limited since it is often necessary to have an image

within 200 to 500 A of a specified depth in the sample. The

concentration or secondary ion yield of the element must therefore be

high to obtain a sufficient signal from a volume element whose depth is

limited to 200-500 A.

The sensitivity of secondary ion yields to subtle changes in sample

chemistry, crystallography, etc. serves only to complicate quantitative

analysis, but quantitative analysis has been demonstrated in several

laboratories by the use of well -characterized standards. The

quantitative models available in SIMS suggest that standardless analysis

is possible in the absence of chemical and other effects on ion yield,

but there are difficulties with the available models. The major

objection to the LTE model proposed by Andersen [40] is that thermal

equilibrium cannot exist in the target as discussed by Carter [43] and

Werner [42]. It is also not clear if the LTE model is useful for low

primary ion current densities which are most often used in the analysis

of surfaces and thin films.

From results obtained with both AES and SIMS on the same samples

[8], it is quite clear that the techniques are very complementary. AES

(which is less sensitive to the chemical state of the element) is

22D
SIMS/AES Comparison

preferred for routine surface analysis of small (<50 ym^) selected

areas, and for the detection of impurities whose concentration is

greater than 1 at. %. SIMS can provide elemental detection and

identification of most elements below 1 at. %, information on isotopic

abundance, and the chemical state of the impurities {i.e., oxides,

precipitates, etc.). The sensitivity of SIMS makes it ideal for trace

analysis and for the profiling of common dopants used in the silicon

integrated circuit technology. Both techniques are being used in a wide

variety of areas previously inaccessible to comprehensive analytical

investigation.

ACKNOWLEDGMENTS

P. W. Palmberg of Physical Electronics Industries obtained the ESCA

data shown in figure 4. R. K. Lewis of Cameca Instruments performed the

SIMS measurements shown in figures 7 and 8.

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SIMS/PflA

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27. Evans, C. A., 21st National Symposium of the Amer. Vac. Soc,

Anaheim, California, October 1974. Paper to be published in J. of

Vac. Sci. Tech. (Special Issue).

28. Palmberg, P. W., J. Vac. Sci. Tech. 10, 274 (1973).

29. Andrews, J. M., and Morabito, J. M., to be published.

30. Chou, N. J., VanderMeulen, Y. J., Hammer, R., and Cahill, J., Appl.

Phys. Lett. 24, 200 (1974).

31. Narusawa, R., Komiya, S., and Hiraki, A., Appl. Phys. Lett. 22, 389

(1973). ~

222
SIMS/AES Comparison

32. Slodzian, G., Rev. Phys. Appl . , 3(4), 360 (1968).

33. Morabito, J. M., and Lewis, R. K., Anal. Chem. 45, 869 (1973).

34. Lindhard, J., Scharff, M., Schiott, A.. Mat. Fys. Medd., Dan Vid

Selsk, 33, 1 (1973).

35. Bonis , M. , Blanchard, B., DeBrebisson, M., Hilleret, N., and

Monnier, J., Electrochem. Soc. Meeting, Miami, Florida, Abstract

266, p. 658.

36. Ziegler, J. P., Cole, G. W., and Baglin, J. E. E., IBM Research

Report, RC 3643, December 7, 1971.

37. Tsai , J. C. C, Morabito, J. M. , and Lewis, R. K. , lo^n Implantation

in Semiconductors and Other Materials edited by B. L.-Crowder,

Flenum Tress, New York, New York, 89 (1973).

38. Newsham, I. G., Hogue, J. V., and Sands trom, D. R., J. Vac. Sci

Tech., 9, 597 (1972).

39. Werner, H. W., Advances in Applied Spectroscopy , Plenum Press, New

York, 7a, 239 (19697:

40. Andersen, C. A., and Hinthorne, J. R., Anal. Chem. 45^, 1421 (1973).

41. Schroer, J. M., Sur. Sci. 34, 571 (1973).

42. Werner, H. W., Theoretical and Experimental Aspects of Secondary

Ion Mass Spectrometry, Vacuum - in press (1974).

43. Carter, G., Enrico Fermi Summer School Paper, 1973, in print.

44. Morabito. J. M., J. Vac. Sci. Technol . 21, 297 (1974).

45. Brunauer, S., Emmett, P. H., and Teller, E., J. Am. Chem. Soc. 60,

309 (1938). ~

46. Werner, H. W., Philips Research Laboratories, Eindhoven, The

Netherlands. Private communication.

47. Morabito, J. M., and Tsai, J. C. C, Surf. Sci. 33, 422 (1972).

48. Tsai, J. C. C, and Morabito, J. M. , Proc. of IVth International

Conf. on Ion Implantation in Semiconductors and Other Materials,

Japan, 1974 (to be published).

49. Morabito, J. M., Anal. Chem. 46, 189 (1974).

50. Croset, M., International Meeting of Chemical Analysis by Charged

Particle Bombardment, Namur, September 5-8, 1971.

223
siMS/rriA

51. Heinrich, K. F. J., and Myklebust, R. L., Proc. of 8th National

Conference on Electron Probe Analysis, New Orleans, Louisiana,

1973, 3A.

52. Palmberg, P. W. , Anal. Chem. 45^, 549 (1973).

53. Shimizu, H., Ono, M., and Nakayama, K., Surface Sci . 36 17 (1973).

54. Thomas, J. H., and Morabito, J. M., Sur. Sci. 41, 629 (1974).

55. Castaing, R, These de Doctorat, Universite de Paris (1951),

Publication Onera (1955).

56. Weber, R. E., and Johnson, A. L., J. Appl . Phys. 40, 314 (1969).

57. Grant, J. T., and Haas, T. W. , Sur. Sci. 44, 617 (1974).

58. From a plot of escape depth values prepared literature by J. C. Tracy, General Motors Research Laboratories (1973).

59. Meyer, F., and Vrakking, J. J., Sur. Sci. 33, 221 (1972).

60. Rudge, M. R. H., and Schwartz, S. B., Proc. Phys. Soc. (London) 88,

563 (1966).

61. Palmberg, P. W., Riach, G. E., Weber, R. E., and MacDonald, N. C,

Handbook of Auger Electron Spectroscopy , Physical Electronics

Industries, Minn., 3 (1972^

62. The CU2O single crystal was supplied by H. Bennett, National Bureau

of Standards, Washington, D. C.

63. Blanchard, G., Hilleret, N., and Monnier, J., Material Res. Bull

7

6, 1283 (1971).

224
KEY WORD INDEX

Analytical accuracy 96,156

Applications

Cosmochemistry 135-178

Depth profiling 21 .179-.190. 191-224

Geochemistry 135-178

Geochemical surface analysis 171

Geochronology 167-171

Isotope ratio analysis 165

Light element geochemistry 165,

Surface analysis 171,191-224

Trace element geochemistry 160-165

Auger Electron Spectroscopy 191-224

Auger images 197

CARISMA computer program 79-120

Chemical effect 187

Cosmochemistry 135-178

Comparison of ion spectra 69-78

Depth profiling 179-190,191-224

Instrumental effects 181

Ion-matrix effects 185

Detectability limits 217-219

Duoplasmatron 6

Empirical quantitation 129-134

Geochemical applications 135-178

Geochemical surface analysis 171

Geochronology 167-171

High Mass Resolution SIMS 63-68

Interference correction 145,193

Interlaboratory analysis comparison 103,156

Ion beam parameters 20,21,51-54

Ion collection efficiency 33-62

225
Ion emission models, secondary 12T

Ion energy loss with depth 113-115

Ion images 130,197

Ion luminescence 28

Ion microscope 2,33-62

Objective lens 34,39

Emission lens 34

Transfer optics 44

Ion microprobe 1-32

AEI 11,63-68

ARL If 'IMA 8

COALA 22,23,24

Drummond and Long 10

Goutte 17

Herzog and Viehbock 5 Hitachi IMA 12,18

Krohn and Ringo 18

Liebl and Herzog 6

Long 9

Riidenauer and Steiger 28

UMPA 13,13,17

VARIAN 15

Ion Sources 26

Ion Spectra

Argon source 7

Labradorite 65,66,67

Shape in ion microscope 75

Zirconolite 143

Isotope studies 135,165

Knock-on 186

Local analysis 20,53

Local Thermal Equilibrium 79-120

Minimum analyzed volume 14,60,203

Molecular ions 126

Non-conducting samples 140

Optical emission 95

225
Recoil implantation 186

Relative elemental sensitivity factors 131

Resolution, mass 63

Richardson's equation 91

Saha - Eggert ionization equation 80,123

Secondary ion emission models 121-127

Andersen and Hinthorne 79-120,123

Benninghoven 125

Joyes 125

Jurela 123

MacDonald 125

Plog 126

Rudenauer 125

Schroeer 124

Sroubek 124

van der Weg 125

Werner 126

Secondary ion energy distribution 108

Secondary ion properties 34,43,54,55,82-86,108,142

Sensitivity factors 131

Surface analysis 171,191-224

Temperature - electron density 91-98

Thermal equilibrium 79

Thermal spike 109

Trace elements 160

Transmission 54,64

Useful ion yield 14,60,23

Vacuum requirements 202

Volume of analysis 14,60,203

227
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP-427

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Secondary Ion Mass Spectrometry,

Proceedings of a Workshop on Secondary Ion Mass

Spectrometry and Ion Microprobe Mass Analysis

5. Publication Date October 1975

6. Performing Organization Code

7. AUTHOR(S)

K. F. J. Heinrich and D. E. Newburv

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Worlc Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as No. 9.

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 75- 619223

16. ABSTRACT (A 200-word or less factual summary ol most sigriificant intormation. II document includes a significant

bibliography or literature survey, mention it here.)

This book is the formal report of the Workshop on

Secondary Ion Mass Spectrometry (SIMS) and Ion Microprobe

Mass Analysis held at the National Bureau of Standards in

September 1974. Invited and contributed papers cover a range of topics in the SIMS field: design of SIMS instrumentation; factors affecting secondary ion collection;

techniques of reducing secondary ion mass spectra to yield quantitative compositional information; comparisons of

SIMS with Auger electron spectroscopy (AES) ; techniques

of obtaining elemental depth profiles and the instrumental

and pnysical factors affecting such profiles; and applications

of SIMS to the study of geological samples. The papers

include both tutorial reviews and detailed reports on current

research in SIMS. The volume should be of interest to all

workers in the SIMS and surface analysis fields.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) j^^^^^ eloctron spoc t r OS copy ; elemental depth pro- filing; ion microprobe mass analysis; ion optics; local thermal

equilibrium (LTE) ; mineral analysis; secondary ion mass spectrometry

(SIMS) ; surface analysis.

18. AVAILABILITY Unlimited

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