Reference book not to be

1

1967NBS TECHNICAL NOTE

278

Scanning Electron Probe

Microanalysis

KURT F. J. HEINRICH

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 278

ISSUED FEBRUARY 3, 1967

Scanning Electron Probe

Microanalysis

Kurt F. J. Heinrich

Spectrochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Research

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402

Price 30 cents.
TABLE OF CONTENTS

PAGE

1. INTRODUCTION . . . 1

2. SCANNING ELECTRON MICROSCOPY 5

3. SCANNING X-RAY IMAGES l4

4. THE PULSE RECORDING X-RAY IMAGE l4

5. THE EXPANDED CONTRAST X-RAY IMAGE . 22

6. CONCENTRATION MAPPING 22

7. QUANTITATIVE AREA SCANNING 27

8. CATHODOLUMINESCENCE 33

9. MULTIPLE SCAN EXPOSURES 35

10. REFERENCES . 36

LIST OF FIGURES

FIGURE NO.

1. Scanning images of gold ore specimen 4

2. Topographic effects on target current

image and backscatter image ......... 7

3. Secondary electron images 9

4. Micro images of recrystallized dental alloy . . 11

5. Secondary electron images of a transistor

under various biasing conditions 13

6. Combined recording of secondary and backscattered electrons from a thin film

magnetic memory plane } with "deflection

modulation display" 5 and secondary electron

recording of the edge of a magnetic track . . 15

7. Schematic diagrams of pulse recordings,

expanded contrast scanning, and concentration mapping 16

8. Effect of number of accumulated x-ray

photons. Tazewell meteorite l8

9. Effect of electron beam energy. Tazewell

meteorite 20

li
LIST OF FIGURES (con't)

FIGURE NO. PAGE

10. Recrystallized dental alloy, unprepared

surface 21

11. Tazewell meteorite: representations of

the Fe Ka signal 23

12. Oscilloscope beam brightness as a function

of x-ray signal intensity: pulse registration, expanded contrast technique, fourlevel concentration map 25

13. Dental alloy, polished surface 28

14. Diffusion in a specimen consisting of niobium

wires imbedded in tin 31

15. Sapphire crystals imbedded in a nickel

base alloy 34

16. Infrared cathodoluminescence scanning image . . 35

in
SCANNING ELECTRON PROBE MICROANALYSIS

Kurt F. J. Heinrich

ABSTRACT

The combination of electron microprobe x-ray emission

spectrometry with the scanning techniques first developed for

the scanning electron microscope permits using the scanning

electron probe as a microscope sensitive to elemental

composition. This technique is particularly useful in the

many applications in which spatial distribution of one or

more elements in a specimen is more important than local

composition. Although oscilloscope representation of probe

scanning is usually obtained by the simple technique of

producing a dot of light for each arriving photon, more

sophisticated scanning techniques such as expanded contrast

registration and concentration mapping can provide more

quantitative information. Signals other than x-rays, such as

target current, electron backscatter, or cathodoluminescence

may be used for image formation. Electron beam scanning can

also be performed in a discontinuous fashion, so that the

electron beam irradiates in succession a number of spots

arranged in a square or rectangular pattern, and the number

of photons registered in each position is retained in the

memory of a multichannel analyzer. The application of these

diverse scanning techniques is illustrated.

Key Words: Electron probe, microanalysis, x-ray spectrometry,

scanning electron microscopy, electron backscatter,

cathodoluminescence

.

IV
SCANNING ELECTRON PROBE MICROANALYSIS*

Kurt F. J. Heinrich

Spectrochemical Analysis Section

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 202^4

1 . INTRODUCTION

The static electron probe microanalyzer, as developed by

Castaing [l] in Guinier's laboratory, can be considered a

highly specialized x-ray spectrograph . The primary purpose

of this Instrument is the spectrochemical analysis of

microscopic regions of a specimen. In accordance with the

observations of Moseley [2], the atomic numbers of the target

components (excepting hydrogen and helium) can be determined

by observing the wavelengths of their characteristic x-ray

emission lines. The intensities of the lines are related to

the local concentrations of the components [3*^*5]. To

limit the dimensions of the volume which emits radiation, a well-focused beam of electrons is used for x-ray excitation.

The depth of penetration of the electrons is controlled by

their energy. As the penetrating electrons spread laterally

as well as in depth, the smallest effective beam size, or

lateral resolution, is also related to the initial beam energy,

i.e., the accelerating voltage.

The focusing of the electron beam is achieved by the

well-known principles of electron optics [6], The amount of

energy that can be compressed into a narrow electron beam is

Presented at the 69th Annual Meeting, ASTM, Atlantic City.

N. J., June 1966; to be published in an ASTM Special

Technical Publication.
limited; hence the intensity of x-rays that can be produced

is also subject to stringent limitations. A relatively high

efficiency of x-ray collection in the spectrometer is

therefore desirable. Such an efficiency, coupled with

wavelength resolution sufficient for spectrochemical

analysis, is achieved by the use of focusing bent and ground

crystals, according to Johannson [73. Bent but not ground

crystals (e,g, mica) can also be used in the arrangement

described by Johann [8,9].

Shortly after the publication of Castaing's thesis, an

instrument quite similar to Castalng's was described by

Borovskii [10], It is interesting to note that Borovskii 1 s illustration of his instrument shows the electron optical

column mounted on a movable arm of the (fixed) spectrometer.

Such an arrangement does not seem to be advantageous from a design standpoint; it shows, however, that Borovskii considered

the electron optical column as an accessory to his

spectrometer. This concept also determined the type of

investigations performed in the USSR in the earlier years with

this instrument.

An electron optical system similar to that of Castalng's

instrument was employed In von Ardenne ' s scanning electron

microscope, developed in the late thirties [11]. The purpose

of this instrument is, however, quite different: a focused

beam of electrons is made to scan in a raster fashion over

the surface of a specimen. A detector of secondary electrons

receives a signal, the Intensity of which varies during the

scan as a function of the specimen surface topography. This

signal can be used to modulate the brightness of an

oscilloscope beam scanning in synchronism with the electron

beam of the microscope „ Hence, a magnified image of the

scanned specimen area appears on the oscilloscope screen. In

comparison with transmission electron microscopy, this method

offers the advantage that replica techniques or thinning the
specimen to transparency (to electrons) are not required.

The practical limit of resolution is on the order of hundreds

of angstroms.

It is thus considerably poorer than that obtainable with

the transmission method, although better by a factor of about

100 than that of electron probe microanalysis. In spite of

the poorer resolution, this technique was further developed

and found useful for diverse applications [12,13].

Cosslett and Duncumb [14] demonstrated that the basic

features of the scanning electron microscope can be

incorporated into the design of an electron probe microanalyzer

An instrument containing these combined characteristics is

called a scanning electron probe microanalyzer . The additions

to a static analyzer are those of a scanning (beam deflecting)

system and of an oscilloscope for observation and photographic

recording of scan images. These features can be added to the

static microprobe without limiting its normal operating

characteristics in any way. The resulting instrument has all

the capabilities of a static microprobe, and the essential

elements of a scanning electron microscope. Most present-day

microprobes do not have a beam as narrow as that of a typical

scanning electron microscope, and only a few are equipped for

detection of secondary (low energy) electrons. Their

performance as an electron microscope is thus somewhat limited,

particularly with respect to resolution. On the other hand,

the combination of scanning and x-ray detection gives rise to

an entirely new function, namely scanning x-ray microscopy

(figure 1).

The range of techniques feasible with such an instrument

is indeed unusually broad. This versatility cannot be fully

exploited unless the operator keeps in mind that his

instrument is a microscope as well as a spectro chemical

analyzer . To clarify this point it is now necessary to

describe in more detail the scanning electron microscope.
Figure 1. Scanning images of gold ore specimen.

Upper left: target current image. Upper right:

electron backscattering image. Lower left: Au Lou

image. Lower right: Ti Ka image.

Instrumental conditions: 20 keV, specimen current:

5 x 10~° amp.

Scanned area: 240[~i x 240^.

4
2. SCANNING ELECTRON MICROSCOPY

The basic features of the electron optical systems used

in both instruments for demagnification of the cathode spot

image are identical. To obtain, however, electron beams of a diameter smaller than 0. 1m., the two lenses commonly used in

microprobes are insufficient. Since the addition of a third

lens to the system represents a major change, this step has

not been taken as yet in the instruments that are commercially

manufactured. Indeed, as long as electron microscopy is

merely considered subservient to x-ray emission techniques,

it could be argued that it is not particularly advantageous

to push the electron image resolution much beyond that of the

x-ray technique » Yet, considerable effort is made to reduce

the effective volume of x-ray emission by using softer x-ray

lines [15], lower accelerating voltages [16], and thinner

specimens [17]. Backscattered electrons are also used for

quantitative analysis, with somewhat improved resolution [18].

Finally, the analyst who is receptive to the possibilities of

scanning electron microscopy soon realizes the independent

merits of this technique. It must, therefore, be expected

that future scanning electron probes will be provided with

electron optics of higher resolution.

Resolution, however, depends not only upon the

dimensions of the electron beam but also upon the spread of

the electrons within the target and upon the choice of signal

used for image formation. An energy analysis of the electrons

re-emitted from the target [19] reveals the presence of two

distinct fractions. The fraction of the primary electrons

which is re-emitted from the target after one or several

collisions with the target atoms is called backscattered

electrons . Electrons of much lower energy are ejected as a

consequence of diverse processes such as primary and

fluorescent ionization and the Auger phenomenon. [20] These

low energy electrons are called secondary electrons.
Backscattered electrons have energies of the order of

magnitude of those forming the primary beam; hence, they can

be detected with relatively simple devices, especially at

primary energies of 20 keV or higher. However, most of these

electrons have reached considerable depths within the specimen

(up to more than one micron) before re-emerging, so that the

inherent lack of resolution of backscatter images would render

them useless for work with very fine beams. Besides, even at

the relatively high currents employed in electron microprobes

(10**' - 10""' amp for instruments having efficient x-ray

detection systems), the amount of electrons reaching the

typical backscatter detector is not very high, so that grainy

images may be obtained.

The amount of backscattered electrons emitted in a direction towards the detector depends upon the angles between

the primary beam, the specimen surface, and the direction of

emergence. Consequently the Intensity of the signal varies

with the specimen topography (figure 2). This effect causes

a particularly rich contrast when shadow effects are produced

by cracks, holes, or protruding features or when the detector

is located at an acute angle with respect to the specimen

surface. Backscatterlng is also to a great extent a function

of the atomic number of the target components (figure l).

The fraction of primary electrons re-emitted by a target of

low atomic number, such as beryllium or boron, is close to

zero, while elements of high atomic number backscatter as

many as half of the Impinging electrons [18], This

compositional effect on backscatter can produce useful

contrast In flat surfaces; the dual source of variations in

backscatterlng may also complicate the interpretation of

backscatter Images.

While the amount of backscatter of a given target

material is nearly independent of the acceleration voltage

[21], the relative amount of secondary electron emission (of
Figure 2. Topographic effects on target current image (left)

and backscatter image (right).

Specimen: fractured surface of galena.

Instrumental conditions: 20 key, target current?

5 x 10"*° amp.

Scanned area: 400^1 x 480p.

.
energies below 50 eV) increases rapidly at lower energy levels

of primary acceleration. Furthermore, secondary electrons

emitted through a wide solid angle can be collected by

electrostatic or magnetic devices. Hence, at the low beam

currents employed for high resolution scanning electron

microscopy (10 -10~" amp) the use of secondary electrons

is more favorable from a statistical point of view. Secondary

electrons are produced at the specimen surface, or very close

to it, so that they can form images of high resolution. For

these reasons, secondary electrons are employed extensively

in scanning electron microscopy. Although their detection

requires more sophisticated techniques, the superior quality

of the images they produce will assure their increased use

even where highest resolution is not necessary (figure 3).

The possibility of operating with low beam currents has the

additional advantage that the destruction of delicate specimens

by heat and radiation is minimized. This is particularly

important when organic specimens are investigated.

The intensity of secondary emission is a function of the

specimen composition, although the dependence is more

complicated than in the case of backscattered electrons. Due

to the low energy level of the secondary electrons, their

intensity is particularly sensitive to local charges which

may exist at the specimen surface.

A simple way of obtaining a signal for image formation

is to isolate the specimen from ground and observe the

variations of the current flowing from the target to ground

during the scanning process ( target or specimen current , figures 1,2,4,8,10,11, 13-15) • As this current is of the order

of magnitude of the beam current, the noise and the graininess

of the image are conveniently low. It must be recognized,

however, that the nature of this signal is somewhat complex

[18,19]. At higher beam energies, the predominant factor in

8
200 MICRONS 10 MICRONS

Figure 3. Secondary electron images.

Upper left: machine screw. Upper right: tension

break of a wire. Lower left: Insect. Lower right' diatom.

Courtesy of Applied Research Laboratories, Inc.
the variation of target current is the electron backscattering,

and in favorable cases the measurement of target current can

be used to quantitatively determine the amount of backscattering [18] . As the accelerating voltage is lowered, the effect

of secondary emission becomes more important. Electrons other

than those of the primary beam, emitted by diverse parts of the

instrument, contribute as a rule to the target current. At

low primary energies, the increase of secondary emission and

its variation with small changes of specimen charge may

establish a self-regulating system, so that drainage of the

target current to ground becomes unnecessary. In such a case,

the specimen need not be electrically conductive. The

usefulness of the target current as an image former is therefore

greatest at the relatively high primary energies (5-30 keV)

currently used in electron probe analysis.

Perhaps the most striking quality of scanning electron

micrographs is their "lifelikeness . " The nature of such

images is much closer to that of macroscopic vision of objects

than in the case of transmission methods such as light or

electron transmission microscopy or x-ray radiography.

Another impressive advantage of this method, particularly at

high magnifications, is the great depth of focus (figure 4).

It is also useful in practice that the magnification can be

changed continuously and over a very wide range (by a factor

of 10-3 ). Except where color perception is important, an

efficient scanning system using secondary electron detection

and a wide range of magnification could replace in many

instances the rather imperfect light optics now used in many

microprobes.

The raster movement of the electron beam with respect to

the specimen can be achieved by electrostatic or electromagnetic beam deflection, or by mechanical displacement of

the specimen. While electrostatic systems are frequently

used because of their simplicity, electromagnetic systems are

10
OPTICAL PHOTOMICROGRAPH X250 TARGET CURRENT

Figure 4. Micro images of recrystallized dental alloy.

Photomicrograph (left) and target current

image (right J. _g Instrumental conditions: 20 keV, 5 x 10 amp

Scanned area: 550u x 440u.

11
less susceptible to accidental misalignment. Furthermore,

their insertion in the electron beam column presents fewer

design problems. Mechanical displacement of the specimen was

used in von Ardenne's microscope [11]. This idea was also

applied to the scanning version of the French CAMECA microprobe [22], in order to avoid defocusing of the x-ray optics

due to beam displacement

.

It greatly aids the observation of the specimen if the

scanning operation can be performed at such a speed as to

obtain a continuous image on the oscilloscope screen. Such a system has been introduced recently in several commercial

instruments. It requires detectors and electronic systems

capable of rapid changes, and intense signals are needed to

reduce noise effects at these high scanning rates. This

renders such a system ineffective for high resolution scanning.

It should be possible instead to use a memory system in which

the information is scanned at relatively low speed, stored

during operation, and displayed at a high speed such as

employed in television systems.

The use of secondary electron or target current scans

offers several interesting applications. Since the emission

of secondary electrons varies greatly with minor variations

of local electrical potential, secondary electron images of

semiconductor devices can be used to map out the voltage

distribution on the surface of such devices (figure 5),

[12,23]. Lander et al. [24] described the observation of

imperfections or dislocations by a target current technique.

Dorsey [25] showed that the deflection of low energy electrons

by magnetic fields can be used to register two-dimensional

images of recordings on magnetic tape, using a differential

secondary electron detector. Among other means of

representation he uses the "deflection modulation display"

previously employed by Everhart (see [25]). This technique

of display was also applied to the combined recording of

12
ON V

OFF?

IK

IOK b^c

AAAr

E

6V

2N2I02

(A) SCHEMATIC (B) ALL TERMINALS GROUNDED

(C) TRANSISTOR OFF (D) TRANSISTOR ON

Figure 5. Secondary electron images of a transistor under

various biasing conditions.

15
backscatter and secondary electrons, yielding information

concerning the composition of the specimen surface (figure 6).

3. SCANNING X-RAY IMAGES

The combination of the area scanning technique with x-ray

detection produces x-ray area scan images. It is this type of

representation that motivated the development of the scanning

electron probe microanalyzer, and that is most frequently

obtained with such an instrument. The speed at which this

technique provides information concerning the distribution of

an element over a microscopic area is of great practical

importance. While x-ray area scans may be preliminary to the

quantitative analysis of selected spots, in many cases they

yield all of the information that is needed. A metallurgist

may, for instance, request the identification of a precipitate,

an inclusion, or an oxide phase formed within a crack. He is

interested in such cases in local variations rather than in

exact values of elemental concentration; thus, his requirements

are best met by using the microanalyzer as a scanning x-ray

microscope . Failure to take full advantage of this technique

will result in a considerable waste of time. A demonstration

of the scanning technique to the person requesting the analysis,

and insistence that the microprobe operator, rather than the

requester, determine the technique to be applied to the solution

of a particular problem, will be of great aid in establishing

an efficient service program.

4. THE PULSE RECORDING X-RAY IMAGE

The most common x-ray area scanning technique is shown

schematically in figure 7(A). A focusing x-ray spectrometer

is tuned to a characteristic line of the element under

investigation. The pulses produced in the detector are

amplified and used to modulate the beam brightness of an

oscilloscope scanning in synchronism with the microprobe.

Thus, each detected photon produces a dot of light on the

14
Figure 6. Combined recording of secondary and backscattered

electrons from a thin film magnetic memory plane,

with "deflection modulation display" (top), and

secondary electron recording of the edge of a magnetic track (bottom).

Acceleration voltage: 20 keV.

Area scanned: 1mm x 1mm.

Courtesy of J. R. Dorsey.

15
(A)

PROBE

X-RAY DETECTOR

I

OSCILLOSCOPE

HORIZONTAL DRIVE VERTICAL DRIVE

I Z

AC AMPLIFIER

a GUN BIAS

(B)

PROBE

I

HORIZONTAL DRIVE VERTICAL DRIVE

OSCILLOSCOPE

X — RAY DETECTOR W AC AMPLIFIER

GUN BIAS

DC AMPLIFIER J> iratemete'r! J^>

[bTasI

PROBE SCOPE E SCOPE I

HORIZONTAL DRIVE VERTICAL DRIVE

X-RAY DETECTOR

GUN

BIAS

(C) 1

AC AMPLIFIER

RATEMETER KH9 DC AMPLIFIER

MASK

PHOTOMULTIPLIER

Figure 7. Schematic diagrams of (A), pulse recordings (B),

expanded contrast scanning and (C), concentration

mapping.

16
oscilloscope screen. A photographic camera records these

dots of light, and the sum of the dots recorded during the

exposure forms the scanning image. We will call this method

the pulse recording method (figures 1,8,9,10,11,13).

The effect on the scanning image of the statistics of

random photon arrival has already been mentioned. The

statistical "noise" superposed upon the information may

adversely affect the image quality by producing granular

texture. This impairs the comparison of shades (i.e., signal

intensity). Furthermore, it also reduces the definition of

contours (figure 8). It is good practice to control the

statistical factor by using a scaler to count the photons

collected for the scanj several frames are thus recorded

until the desired number of photons is registered. This is

also helpful in controlling the total brightness of scanning

pictures, provided that some control Is exerted upon the

brightness of a single light dot. It is, however, impossible

to set a fixed rule concerning the minimum number of photons

to be accumulated. The author uses, under average conditions,

200,000 counts per exposure. This number can be reduced when

the element under investigation is present in small localized

areas, and should be increased if it extends over most of the

surface, if a complex structure or small concentration changes

must be illustrated, or when the signal to be observed is

close to the background level. In order to avoid overexposure

of the photograph, the brightness of the individual photon

signal must be varied inversely to the number of photons per

area unit at the brightest level.

When low concentrations are registered, the existence of

background, chiefly due to continuous radiation, is quite

bothersome. This is particularly true if the specimen contains

areas of varying mean atomic number, and if the wavelength of

the observed line is short (< 1 a). As the intensity of

continuous radiation varies markedly with the mean atomic

17
Figure 8. Effect of number of accumulated x-ray photons.

Tazewell meteorite. Upper left: current image

„

Upper right: Ni Ka, 20,000 cts. Lower left:

Ni Ka 56,000 cts. Lower right: Ni Ka, 560,000 cts c Scanned area 7°I-L x 55u-.

18
number of the target, variations in background intensity maythen simulate or mask low concentrations of the element to be

investigated. In quantitative analysis this background

contribution is determined separately and subtracted from the

observed signal intensity. An analogous correction procedure

cannot be applied to the x-ray pulse recording method; hence

a high line-to-background ratio is very desirable when this

scanning technique is performed. The use of narrow detector

slits, and of pulse height selection, and the avoidance of

analytical lines of short wavelength, help to minimize the

background effects.

The magnification of the image obtained in the scanning

procedure is determined by the ratio of the deflections on

the oscilloscope screen to those on the specimen; this ratio

can be made as "high as wanted. The useful magnification is,

however, determined by the spatial resolution of the technique.

The effective excited volume of the specimen under typical

conditions has a diameter of 2-3 microns. To obtain images

of reasonable sharpness, this distance should be represented

by no more than 1-2 mm on the images. Therefore, magnifications of more than 500-1000 are hardly justified. This

corresponds, for the conventional 8-cm oscilloscope screen,

to a minimum scanning area of 80-160 microns diameter.

The excited volume diminishes when the operating voltage

is reduced [16] (figure 9). The minimum excitation potential

of the characteristic line sets a limit to this approach;

improved resolution can sometimes be achieved by switching to

lines of longer wavelength, which can be excited at lower

potentials [15].

If quantitative analysis is to be performed, it is

important to prepare the surface to be analyzed to a high

degree of flatness. For qualitative purposes, however, this

precaution is frequently unnecessary, as illustrated in

figure 10. The inspection of specimens as received permits

19
_____ .

'".

'

!

-

X1 • , 1

I

-

1

i

.^pB

Figure 9. Effect of electron beam energy „ Tazewell

meteorite. Ni Ka (left) and Fe Ka (right) area

scanning images.

Acceleration voltage: 20 keV (above) and 40 keV

(below)

.

Scanned area: 192m. x 192 h.

20
TARGET CURRENT SnLa

Figure 10. Recrystallized dental alloys unprepared surface.

Same focus and conditions as figure 4. Upper left:

Ag Lou x-ray scan. Upper right? Hg Ma x-ray scan.

Lower left: target current scan. Lower right: Sn

Leu x-ray scan.

Are"a scanned: 275m x 220m.

21
the identification of features located at the specimen

surfaces which could be destroyed by the conventional techniques

of specimen preparation.

5. THE EXPANDED CONTRAST X-RAY IMAGE

The contrast produced by a signal -which is only slightly

higher than the background can be enhanced so that only signal

levels above background are recorded. Melford [26] proposed

and demonstrated such an "expanded contrast method" in 'which

brightness modulation by the ratemeter signal has been employed

for image formation (figure 7B). With this arrangement, an

arbitrary voltage level (bias) can be subtracted from the

ratemeter output so that the onset of intensity registration

at the oscilloscope screen can be shifted above the background

level. This method can also be used to enhance differences at

high concentration levels (figure 11). A difference of 5$ in

the concentration of an element at the 50$ level is difficult

to detect on a pulse registration image. If, instead,

Melford l s technique is used, and the bias is set just below

the signal level for 50$, the difference Is easily noticed,

provided that sufficient counts are collected to overcome the

effects of statistical fluctuations.

The finite time constant of the ratemeter makes

relatively slow scanning speeds necessary. Therefore the scan

is performed in a single frame sweep. The sweeping speed,

the number of lines per frame, and the ratemeter time constant

must be Judiciously chosen in order to avoid statistical

jitter of the image brightness as well as loss of spatial

resolution due to the inertia of the ratemeter output at high

time constants.

6. CONCENTRATION MAPPING

Although the expanded contrast method permits emphasizing

small changes at a high concentration level, the information

obtained is comparative rather than quantitative. A more

quantitative approach can be taken by using a device producing

22
Figure 11. Tazewell meteorite: representations of the Fe Ka

signal.

Upper left: x-ray line scan of Fe Ka, (Fe), and

zero intensity level (0), superposed upon a target

current area scan. The line (L) marks the locus

of the line scan, with respect to the target

current area scan. Upper right: pulse registration

x-ray area scan. Lower left: expanded contrast

image. Lower right: four-level concentration map.

Scanned area: 138m- x 110n.

23
abrupt changes of the oscilloscope beam brightness at predetermined ratemeter signal levels. The principle of this

technique, called concentration mapping , is illustrated on the

graph in figure 12, in which the relationship of scope beam

brightness to x-ray signal intensity is schematically plotted

for the pulse registration image, for the enhanced contrast

x-ray image, and for a concentration map. It can be

appreciated that in principle this method shows the location

of quantitatively defined concentration rangesj to obtain

this information, all concentration differences within each

range must be ignored.

The first concentration maps to be obtained [27] used an

auxiliary oscilloscope (not employed for image visualization)

as a ratemeter output analyzer. This optical approach to

concentration mapping, which is the most flexible and easiest

to improvise, will illustrate the procedure (figure JC) . The

ratemeter output deflects the beam of the auxiliary oscilloscope.

Let us assume that this deflection is along the vertical screen

axis. In the simplest variation of the technique, deflection

along the other axis is not used. The oscilloscope beam will

thus display vertically the ratemeter output voltage. The

oscilloscope screen is now covered by a mask cut so that only

a limited sector of the beam path remains visible to an

observer in front of the oscilloscope. This observer would

' see the light only when the ratemeter output causes a beam

deflection between two limits corresponding to well-defined

concentrations of the element being investigated. Hence the

appearance of light indicates that the concentration of this

element at the point of impact of the electron beam is within

the preselected concentration range. The observer is now

replaced by a phototube, the output of which is made to

modulate the signal brightness of the main oscilloscope. The

beam of this main oscilloscope scans in synchronism with the

electron beam on the specimen, as in the scanning techniques

24
WHITE

IMAGE

BRIGHTNESS

LEVEL

BLACK

S I yS^

/ I X

/r ~/^

v/ c I /

/\ r-/- J

I /

I

. J

X-RAY INTENSITY

Figure 12. Oscilloscope beam brightness as a function of

x-ray signal intensity? a. pulse registration,

b. expanded contrast technique, c. four-level

concentration map.

25
described previously. The result of the operation is an

image on "which areas containing the element of interest within

the preset concentration range are marked by a characteristic

level of brightness.

A variation of this procedure consists in replacing the

signal cutout of the mask by a series of slits. In this case,

several equiconcentration lines will be produced in one

operation. This is useful for specimens, such as diffusion

couples, in which the lines corresponding to different levels

cannot be misinterpreted.

In another variant, two partially overlapping ranges are

represented by adjacent cut-outs. By means of a horizontal

square-wave deflection, the beam of the auxiliary oscilloscope

is made to switch periodically from one cut-out to the other.

The range common to both cut-outs appears white on the

concentration map, while areas cover by one cut-out only appear

grey, or dotted if the square-wave-frequency is slow [27].

Interesting combinations of boundary conditions can be

obtained by applying one ratemeter output to the horizontal

axis of the auxiliary oscilloscope, and another ratemeter

output — corresponding to another element — to the vertical

axis. Each point on the auxiliary screen corresponds thus to

a coincidence of defined concentration levels of the two

elements (including absence of one or both).

The optical concentration mapping device can be replaced

by an electronic equivalent [28], The electronic concentration

mapper provides an output voltage or current which is constant

within several adjustable input voltage ranges, changing

abruptly in passing from one range to another. The output of

this device is used to modulate the brightness of the imageforming oscilloscope. The illustrations reproduced here were

made using a device, constructed at the National Bureau of

Standards, that can employ up to four ranges, so that images

in black, white, and two grey shades can be produced.

26
The application of this technique is not restricted to

x-ray signals. With instruments capable of analysis by

target current measurement [18], target current concentration

maps [29] can be produced in appropriate systems, usually in

much shorter time than the corresponding x-ray concentration

maps (figure 13). These types of images can be used with

advantage for quantitative metallography [26,30,31].

7. QUANTITATIVE AREA SCANNING

A scanning technique capable of yielding quantitative

information is of great practical interest. Its application

would be of benefit in the study of subjects such as homogeneity, diffusion of elements, etc. It must be recognized,

however, that a scanning procedure has limitations which

affect the accuracy of the data obtained by it. The most

important factors to be considered are:

1. counting statistics

2. defocusing of the x-ray optics

3. instability of excitation

4. problems of calibration

These potential sources of error must be considered

before a quantitative evaluation of area scans is attempted.

The importance of the statistical limitations of accuracy

can be illustrated by considering the scanning image as

composed of a number of discrete image elements. In order to

obtain acceptable quantitative information, the effects of

counting statistics must be kept within tolerable bounds for

each of these elements.

As an example, we shall assume a specimen composed of

three elements, all present at fairly high concentrations.

The counting rates obtained on the specimen will be assumed

to be on the order of 5000 counts/sec, and the background

counting rates to be negligible. To obtain the counting rates

at one point of the specimen with a coefficient of variation

V = J2L = 1/t/N = 2x10*°

N

27
Figure 13. Dental alloy, polished surface.

Upper left: target current image. Upper right

:

Sn La-, x-ray scan. Lower left: three-level

concenbration map using target current signal.

Lower right: same, with a different sequence of

brightness values. Acceleration voltage: 30 keV,

Scanned area: 132n x 132(i.

28
where N is the average number of counts collected In one operation, we must accumulate a minimum of 2.5 x 10 counts/sec, for

each element [32]. At the counting rate of 5000 counts/sec,

50 seconds of counting time are thus required for each point.

Assuming that the same time is spent for the observation of

the standard, three elements can be measured with one scaler

and spectrometer in 300 sec. This can be reduced to 100 sec

if three spectrometers and scalers are available. Six numbers

must be read from the scalers and recorded. The conventional

correction procedures must then be applied, using slide rule,

desk calculator, or electronic computation.

Let us now apply the same technique to a scanning raster

of 100 lines per frame on the same specimen. For a square

scan, it is reasonable to consider each line as composed of

as many image elements as there are lines in the frame.

Consequently, our area scan is equivalent to performing the

analysis, as described above, of 10,000 points. This would

require a counting time of 10 sec (278 hrs) for each element.

A correction calculation would have to be performed with each

of the 10,000 pairs of numbers generated in the operation.

Due to the sheer volume of this information, it would be

impossible to use it for any practical purpose without further

graphical or statistical procedures.

There are several ways to abbreviate this tedious

operation while preserving the part of the information which

is considered significant. In the first place, the scanning

procedure eliminates the need for manual translation of the

beam position. The next consideration concerns the number of

counts required for each point. When a group of measurements

is interpreted, less statistical precision is in general

needed for the individual measurement. If in our example a coefficient of variation of 10 were to suffice, the time for

one counting operation could be reduced from 50 sec to 2 sec.

With so short a counting period, it would be unnecessary to

29
repeat the standard measurements more than occasionally; we

will assume that the time for standard reading can be neglected.

With these assumptions the total time requirement for the

complete scan is reduced to 5.6 hrs.

It is even more advantageous to increase the counting

rates. This abbreviates the duration of the analysis without

loss of statistical precision. If counting rates can be

increased to 20,000 count/sec, the operating time is further

reduced to 1.4 hrs. Finally, the duration of the analysis

can be shortened by reducing the number of points. A raster

of 40 x 40 points instead of 100 x 100 could be analyzed with

a coefficient of variation of 0.01, at counting rates of

20,000 counts/sec, in 13.3 minutes. This is quite a reasonable

duration for such an operation, particularly when compared to

the first figure of 278 hours.

At a rate of two image elements per second, manual data

recording must be replaced either by graphical procedures

such as the concentration mapping, or by print-out. The first

alternative is preferred when a great number of points is used

and the interest lies primarily in the distribution of a few

concentration levels. The second procedure, in which the

digital nature of the information is conserved, is particularly

useful when further mathematical treatment of the data is

planned. Here it is convenient to store the data obtained on

the image elements in a memory device such as the data storage

section of a multi-channel pulse height analyzer. The decisions

concerning further processing of the data can then be postponed

until all data have been gathered and inspected (figure 14).

In the cylindrically focusing x-ray spectrometers

generally employed in electron probes, the point of x-ray

excitation must lie on the line along which focusing occurs

for the wavelength in question. This line crosses the specimen

surface in vertically placed spectrometers, and Intersects it

in the case of horizontally arranged spectrometers.

30
Figure ik. Diffusion in a specimen consisting of niobium wires

imbedded in tin. The phases that can be observed

are niobium (wire cross-sections) , Nb^Sn, and tin.

Upper left: target current image. Upper right:

Nb L emission, non-dispersively detected by a Ne

detector and a 40 x 40 channel memory core. Both

vertical position and brightness are a function of

signal intensity. Lower left: Nb L radiation

(non-dispersively), three position concentration

map; brightness increases with the concentration of

niobium. Lower right: two-level concentration map

of the same signal, showing in white the distribution of the phase Nb^Sn. Acceleration voltage:

20 keV. Area scanned: kk0\± x 440|i.

31
Consequently, the observed x-ray intensities decrease if the

beam is moved away from the focal position by horizontal

displacement; in vertically arranged spectrometers, the x-ray

intensities also depend critically upon the specimen position

along the vertical axis.

This defocusing effect limits seriously the extent to

to "which x-ray scanning operations by beam displacement can

be interpreted quantitatively [28]. In scans covering several

hundred microns, even the qualitative interpretation may be

affected by serious loss of signal intensity at the edges of

the image. To avoid errors due to this effect, the following

alternatives are suggested:

1. To restrict the width of scanning (to a width of 50m.

or less, depending upon the spectrometer characteristics), for

scanning operations which are to be quantitatively interpreted.

2. To use scanning by mechanical displacement rather than

beam displacement [22]. This, however, can only be done at

relatively slow speeds. It is advantageous to have capabilities

for qualitative high-speed beam scan and for slow mechanical

specimen scan in the case of quantitative applications.

3. To use semifocusing or non-dispersive [28] x-ray

spectrometers for extensive area scans to be evaluated

quantitatively.

4. To employ signals other than x-rays (e.g. target

current )

.

The variation of x-ray signals as a function of specimen

elevation can seriously affect automatic scanning procedures

with mechanical specimen translation. The control of the

specimen surface elevation is, in fact, one of the most

important functions of the optical microscope mounted within

the electron probe.

Scanning procedures, both by specimen and by beam

translation, preclude in general a frequent check of intensities

by means of standard reference materials. It is therefore

32
important to control the Instrument stability for the duration

of the scan. Some instruments permit the monitoring of a current proportional to the beam current. This permits the

observation of beam fluctuations. It has also been proposed

that long term beam stability can be achieved by a feedback

system monitoring such a current. When stepwise scanning is

performed, a direct correction for beam fluctuation can also

be achieved by integrating or counting the detector pulses at

each point of the specimen for fixed amounts of beam (or

monitor) current rather than for fixed time. It would be

inappropriate to use the target (specimen) current for this

purpose, in view of its variation as a function of specimen

composition and surface topography. The analyst should also

consider that signal fluctuations may arise as well due to

changes of detector efficiency with changes of temperature or

pressure (in the case of flow detectors), or of the crystal

spacing of monochromators due to thermal expansion.

8 . CATHODOLUMINESCENCE

Besides the x-ray signals and those derived from

backscattered and secondary electrons, the emission of light

in the visible and infrared ranges can also be used to obtain

information concerning the target bombarded by electrons. The

intensity and spectral distribution of the cathodoluminescent

emission can be affected by trace amounts of some elements.

The relations between composition and emission in the visible

range are, however, much more complex than those between

composition and x-ray production, so that the use of

cathodoluminescence for analytical purposes is limited to

special cases which have been empirically studied, mainly in

the fields of minerals and ceramics [33].

Cathodoluminescent emission can be recorded by color

photography through the light microscope incorporated in most

microprobes. It is sometimes more advantageous to apply a scanning technique similar to that used for x-ray signals or

33
backscattered electrons [34], In this case colors are not

retained, although colored filters can be used to advantage.

The spatial resolution obtained is, however, excellent, as

the visible radiation is generated very close to the specimen

surface (figure 15).

^ -4sfe'- * W " J& ^MS <

* ^W <^EL gg & ST m

* ^

;v &

..jsP • £^

!r

^

.*•-* A

t

r

Figure 15. Sapphire crystals imbedded in a nickel base alloy,

Left: target current image. Right: cathodoluminescence scanning image.

Acceleration voltage: 20 keV.

Scanned area: 55u x 70u. o

Kyser and Wittry studied the emission of infrared

cathodoluminescent emission from gallium arsenide [35,36] and

showed that local intensity variations can be attributed to

variations in the concentration of impurities or crystal

imperfections (figure 16).

34
INFRARED CATHODOLUMINESCEIMCE

ral

«!;;: :: .... # "" '? *! ' " ; iiiin

Figure 16. Infrared cathodo luminescence scanning image.

The specimen is a tellurium-doped n-type gallium

arsenide, diffused with zinc from the vapor phase

to form a p-n junction. The dark region is p-type

Area scanned: 350H x 250n.

Courtesy of D B. Wittry.

9o MULTIPLE SCAN EXPOSURES

Line and area scans of different types can be superposed

for better presentation [37]. A line scan can frequently be

understood better if accompanied by a target current area

scan marking the beam trajectory of the line scan (figure 11)

Similarly, the combination of a concentration map with a target current area scan is frequently useful.

The use of multiple exposures in different colors [38]

offers further possibilities to combine several signals in a single representation.

35
10 . REFERENCES

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Beams to Metallographical Analysis" (French),

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[9] Ogilvie, R. E., "X-Ray Optics in Electron Microanalysis,"

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[11] von Ardenne, M., "The Scanning Electron Microscope.

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36
[12] Smith, K. C. A. et al., "The Scanning Electron

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(1956).

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r ^ it L17J Theisen, R. and Lemaitre, J., High Precision Microprobe

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Books, Washington, D. C, 1963, p. 391.

[18] Heinrich, K. F. J., "interrelationships of Sample

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[20] Burhop, E. H. S., The Auger Effect and other Radiation- less Transitions , Cambridge University Press,

Cambridge, England, 1952.

[21] Palluel, P., "The Backscattered Component of Secondary

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37
[23] Nealey, C. C, Laakso, C. W e , and Hagen, P. J., "Planar

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38
[33] Long, J. V. P., "The Application of the Electron-Probe

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