U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Semiconductor Meas

Automated Scanning

Energy Electron Probe (ASLEEP)

I or Semiconductor Wafer Diagnostics
NATIONAL BUREAU OF STANDARDS

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Semiconductor Measurement Technology:

Automated Scanning Low-Energy Electron Probe (ASLEEP)

for Semiconductor Wafer Diagnostics

A. Christou

Naval Research Laboratory

Washington, D.C. 20375

This activity was supported by

The Defense Advanced Research Projects Agency

U.S DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1978
Library of Congress Cataloging in Publication Data

Christou, A. Automated scanning low-energy electron probe (ASLEEP) for semiconductor wafer diagnostics. (Semiconductor measurement technology) (NBS special publication ; 400-30)

1. Probes (Electronic instruments). 2. Semiconductor wafers—Testing. 1. Title. II. Series. III. Series: United States. National Bureau of Standards. Special publication 400-30. QC100.U57 no. 400-30 [TK7878.6] 602.1s [621.3815'48] 78-2200

National Bureau of Standards Special Publication 400-30

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-30, 38 pages (Apr. 1978) CODEN; XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

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

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(Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

Section Page

1. INTRODUCTION 1

2. INSTRUMENTATION 4

2.1 Electron Generation 4

2.2 Analog and Interface Electronics 8

2.3 Spatial and Voltage Resolution 10

3. EXPERIMENTAL INVESTIGATIONS 10

3.1 Oxide Uniformity 10

3.2 Silicon 11

3.2.1 Semiconductor Defect Density 11

3.2.2 Semiconductor Resistivity-Impurity Profiling 14

3.2.3 Integrated Circuit Testing 17

3.3 ASLEEP Investigations on III-V Compounds 19

3.3.1 Indium Phosphide 19

3.3.2 Gallium Arsenide 22

4. CONCLUSIONS 28

ACKNOWLEDGMENTS 31

REFERENCES 31

LIST OF ILLUSTRATIONS

Page

Figure 1. The ASLEEP vacuum system showing the high vacuum, oil-free pump

station 2

Figure 2. Photograph of the computer and interface electronics 3

Figure 3. Schematic showing the electron gun assembly for the ASLEEP system ... 5

Figure 4. Schematic diagram showing the electron source connections 5

Figure 5. Definition of surface potential relationships and semilog plot for two

different work functions 7

Figure 6. Block diagram of computer automated scanning low-energy electron

probe 9

iii
LIST OF ILLUSTRATIONS - (Cont'd)

Page

Figure 7. Three-dimensional surface potential map of capacitors 12

Figure 8. ASLEEP micrograph showing dislocation pipes in silicon wafer as denoted by arrows 13

Figure 9. X-ray topograph showing Lomer-Cattrell dislocation networks 15

Figure 10. ASLEEP image of X-ray topograph wafer 15

Figure 11. Energy band diagram for silicon showing the position of the Fermi level,

the intrinsic Fermi level, the band gap, electron affinity, and surface

potential 16

Figure 12. Work functions of ion-implanted silicon

(A) Expected work function values from Fermi level shifts ig

(B) Expected work function values after Allen and Gobeh [5] ig

(C) Measured work function values 18

Figure 13. Optical photomicrograph of integrated circuit utilized in the

ASLEEP experiment 20

Figure 14. Micrographs of integrated circuit showing specific area analyzed

(A) Optical photomicrograph of test circuit 21

(B) ASLEEP image of test circuit 21

Figure 15. ASLEEP image and Auger spectra of indium, phosphide surface after an acetone rinse 23

Figure 16. ASLEEP image and Auger spectra of MB-etched [8] indium

phosphide surface 24

Figure 17. ASLEEP image and Auger electron spectra of indium phosphide

annealed at 330 C for 15 min 25

Figure 18. ASLEEP image of indium phosphide prior to oxidation treatment 26

Figure 19. ASLEEP image of indium phosphide after oxidation 27

Figure 20. A comparison of ASLEEP image, SEM image, and scanning Auger

images of gallium arsenide contamination 29

Figure 21. ASLEEP image of gallium arsenide wafer showing the effect of two

different etch treatments 30

iv
PREFACE

This study was carried out at the Naval Research Laboratory as a part of the Semiconductor Technology Program in the Electronic Technology Division at the National Bureau of

Standards. The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance, interchangeability, and reliability of discrete semiconductor

devices and integrated circuits through improvements in measurement technology for use

in specifying materials and devices in national and international commerce and for use by

industry in controlling device fabrication processes. The work was supported by the

Defense Advanced Research Projects Agency* through the National Bureau of Standards'

Semiconductor Technology Program, NBS Order Nos. 501718 and 711782. The contract was monitored by R. L. Raybold as the Contracting Officer's Technical Representative

(COTR). Drs. W. M. Bullis and K. F. Galloway provided technical review of this report

for the National Bureau of Standards.

Certain commercial equipment, instruments, or materials are identified in this report in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the material or equipment identified is necessarily the best available

for the purpose.

Larger scale drawings of the mechanical parts and detaiiled listings of the computer

programs and subroutines are available on request from the COTR, TECH-A-361, National

Bureau of Standards, Washington, DC 20234.

Through ARPA Order 2397.

V

AUTOMATED SCANNING LOW ENERGY ELECTRON PROBE

(ASLEEP) FOR SEMICONDUCTOR WAFER DIAGNOSTICS

A. Christou

Naval Research Laboratory

Washington, D.C. 20375

This report summarizes the results of a three-year effort in the development of a computer Automated Scanning Low-Energy Electron Probe

(ASLEEP) for semiconductor wafer diagnostics. Experiments designed

to explore the measurement capabilities of ASLEEP for measurements on silicon, gallium arsenide, and indium phosphide are described. Four

areas were emphasized: 1) semiconductor resistivity, 2) semiconductor

defect density, 3) lateral inhomogeneities, and 4) oxide uniformity.

Although oxide charging problems limit the utility of ASLEEP in its present form for silicon, defects and lateral inhomogeneities could be

readily detected in gallium arsenide and indium phosphide.

Key words: Automated scanning low-energy electron probe; lateral inhomogeneity; oxide uniformity; scanning low-energy electron probe;

semiconductor defect density; semiconductor resistivity; semiconductor

wafer diagnostics.

I. INTRODUCTION

The scanning low-energy electron probe (SLEEP) is an electron beam probing technique whereby an electron beam is first accelerated (to provide beam definition) and then

decelerated by a grid placed in front of the sample to be probed [1] . The sample under

investigation is scanned by the electron beam in the retarding field region. Only those elec- trons whose energies are sufficient to overcome the local sample potential barriers are col- lected. This collected current is measured in the sample-cathode circuit. Similarly, electrons

with insufficient energy to overcome sample surface potential are reflected from the sample

and may be collected to form the mirror mode operation. Thus, either the directly collected current or reflected current provides a surface potential map of the sample. In the work

reported here, only the collected current mode was utilized.

The SLEEP technique is inherently simple. The basic apparatus consists of a low-energy

gun structure (800 to 900 V) and a standard vidicon electromagnetic beam focusing and

deflection system. Depending on the type of data, precision, accuracy and detail required,

control and collection can be carried out by manual techniques or by minicomputer.

Similarly, depending on the sophistication of the measurement required, vacuum equipment

may range from the rudimentary (10~^ torr) to the more advanced oil-free ultra-high vacuum (10~ll torr).

In the present investigation a Data General Nova 800 computer has been interfaced to

the scanning low-energy electron probe to control data collection and signal processing, thus

forming an Automated Scanning Low-Energy Electron Probe (ASLEEP) system. An ultra- high vacuum system was also utilized in order to provide for a contamination free system.

Figure 1 shows the vacuum system for the ASLEEP experiments while figure 2 shows the

computer and interface electronics. The ASLEEP experiments have been designed to explore

1
Figure 1. The ASLEEP vacuum system showing the high vacuum,

oil-free pump station.

2
3
the measurement capabilities of the system and the applicability of ASLEEP to the solution

of problems in the two major semiconductor materials technologies: silicon and III-V compounds (gallium arsenide and indium phosphide). In the silicon technology, ASLEEP

was utilized for wafer resistivity -homogeneity studies and dielectric homogeneity studies. In the III-V compound technology, ASLEEP was utilized to investigate lateral inhomogeneities

resulting from stoichiometry variations and semiconductor defects.

2. INSTRUMENTATION

2.1 Electron Generation

The electron gun design chosen for ASLEEP was one which had been used by other

researchers [1] in work function studies. A schematic of the assembled gun structure is shown in figure 3, while figure 4 shows the interconnections of the electron gun to the rest of the system. The gun structure is similar to that of a commercial vidicon tube and consists

of a drift tube, three grids, and a cathode. The target end of the drift tube is covered with a 1000-mesh nickel screen having a 40% transparency.

The cathode is an iridium ribbon 500 jUm thick and 4 mm wide. A thoria coating is plated onto the ribbon in the region where high electron emission is required. The plating

bath consists of 5 g thorium oxide, 75 mg thorium nitrate, and 100 mL ethyl alcohol. The

iridium was biased as the positive lead at a level of 5 to 8 mA at 200 V during the coating

operation. A total coating thickness of 50 jum was required. Thorium nitrate is added to

increase the conductivity of the bath. On portions of the ribbon where high electron emission is not desired, the thoria may be scraped off with a cleaned razor blade. The cathode is activated by quickly raising the filament temperature above its normal operating value. The

filament current required to obtain a given electron emission will then decline and after

about 30 min the emission will stabilize at a filament current of 4 to 5 A.

Directly heated thoria-coated iridium emission sources axe used for ASLEEP rather than

the standard indirectly heated oxide cathode primarily because of the nature of the application. As a diagnostic tool of general utilizability, it is expected that the gun system will be

exposed to air, and therefore to water vapor, upon each sample insertion. "Oxide" cathodes

degrade rapidly under such conditions while thoria-coated iridium has been found to give

good repeatable emission characteristics [ 1 ]

.

Fundamental to the utilization of this system is the measurement of the retarding field current over the target area. This current is given by

where V is the applied voltage, k is Boltzmann's constant, q the electronic charge and T is the

absolute temperature. The minimum current, If^ , is given by the Richardson equation (2)

where 0^ is the cathode work function and a constant. Equation (1) is shown in a semilog plot in figure 5 for two different values of target work function. The region below

the horizontal line marked /rpj^ is the deep retarding region.

(1)

Im =AoT^ exp

90c

kT (2)

4
Figure 3. Schematic showing the electron gun assembly for the ASLEEP

system.

DIRECTLY

HEATED CATHODE-^

I—GRID I r— GRID 2

t—GRID 3

rDRIFT TUBE

FOCUS COIL

t f

30 V

TARGET

500 V

^6V

ELECTROMETER

+800V

6

Figure 4. Schematic diagram showing the electron source connections.

5
The electrons from the cathode are accelerated by the grids and directed onto the end

cap of the drift tube. In the drift tube, standard vidicon magnetic deflection and focus

techniques image the defining aperture on the target under study. The 700 V applied to the drift tube and nickel mesh screen creates a field of 4000 V/cm in front of the target

which decelerates the electrons. A tantalum aperture defines the electron beam size. The

target specimen is connected to ground by a current amplifier with the result that the target

is virtually at ground potential. Any electrons collected by the target are amplified by the

current amplifier and fed to the electronic control circuitry. To make a surface potential

measurement, the cathode potential is swept with a saw tooth which starts increasing the

beam energy from the deep retarding region. When the beam energy is high enough, such

that a predetermined threshold current is collected by the target, the sawtooth is turned off. The magnitude of the sawtooth voltage at turn off is a direct measurement of surface

potential at a particular beam position for a constant (known) cathode work function.

The application of the electron gun to the complicated semiconductor diagnostic problems has revealed two important weaknesses in the gun design which must be corrected

before ASLEEP can become a routine diagnostic tool. These weaknesses are: a) energy fil- tering of the electron beam due to the magnetic field of the directly heated cathode, and

b) limited electron emission density from the cathode.

a) Energy Filtering of the Electron Beam: Energy filtering of the electron beam arises

in the following way. A directly heated thoriated iridium cathode was chosen for the

electron gun because, as stated above, it is not poisoned by exposure to water vapor. However, the thoriated cathode must operate at a higher temperature than a barium oxide

cathode and, consequently, it is directly rather than indirectly heated. A direct current of

4.5 A is required to attain emission temperature. This current results in a large magnetic

field which deflects the emitted electrons sharply away from the centerline of the electron

gun. External alignment coils are used to compensate for this field, but since the magnetic

field from the cathode varies inversely £is distance from the filament and the field from the

alignment coils is uniform, the compensation is not complete. The uncompensated field

gives all of the electrons a radial component of velocity which directs them into the walls of

the electron gun. This effect is strongest on the low-energy electrons and they tend to be

filtered out before the electron beam reaches the target. If the log of the target current is plotted as a function of bias voltage between the cathode and the target, a plot is obtained

which contains two regions. At high bias voltages the current is independent of voltage.

This is the saturation region. At lower bias voltages the log of current is proportional to

bias voltage. This is the retarding field region. In the case with no electron filtering the

slope in the retarding field region is the temperature of the cathode. The removal of the lowenergy electrons reduces the bea m current undesirably and makes the cathode appear hotter.

This apparent increase in temperature manifests itself as a reduction in the slope of the

retarding field plots which results in a reduction in the accuracy of the surface potential

measurement as the surface potential is defined by the point where the saturation region and

the retarding field region intersect on the plot.

The magnetic field could be reduced by redesigning the filament holder so that the cur- rent followed a straight bifilar loop, which provides self-canceling of the magnetic field. Indirectly heated cathodes would also be a desirable solution to this problem as the heater

current required is much smaller and the heating windings may be arranged to minimize the

magnetic field. Field emission sources could also be used as described in reference [2]

.

b) Limited Electron Emission Density: The ASLEEP system requires a small monoenergetic beam of electrons. It is impossible to extract electrons from a large area of the

6
Figure 5. Definition of surface potential relationships and semilog plot

for two different work functions.

7
cathode and focus them into a tight spot and still meet this requirement because the focusing

action would give the electrons a radial component of velocity w^hich spreads their energy

distribution. Consequently, ASLEEP uses a pinhole aperture and deflection and focus techniques similar to a vidicon tube. Thus, the electrons which participate in the surface potential

measurement originate from a region of the cathode approximately the size of the pinhole

aperture. Due to the focusing technique used, the electron spot size at the target is also

approximately the same as the pinhole aperture. Thus, reducing the aperture size to increase

resolution results in reduced beam current and reduced signal to noise ratio. Thus, it is essential that the current density emitted by the cathode be as large as possible.

2.2 Analog and Interface Electronics

A complete block diagram of the ASLEEP system is shown in figure 6. The

electron beam is deflected by signals provided to the deflection amplifiers. The x and y

deflection channels are identical; thus the discussion is confined to the x axis. There are two possible sources of x-cixis deflection information: the computer, or a 12-bit word

set by front-panel-mounted octal-thumbwheel switches (x-axis manual). The computer

communicates through a series of interfaces which were purchased from the computer supplier. These interfaces provide a 16-bit word, of which 12 bits are fed to the x-axis input

select board which provides solid-state switching on a front panel input to determine which

source of data will be used. The x-axis D/A converter converts the digital signal to an analog

voltage which drives the x-axis deflection amplifier and an oscilloscope. The deflection amplifier drives the deflection yoke. The deflection amplifier has a step response of 1.9 jus/A and a small signal bandwddth of 1.5 MHz. It is a direct-coupled feedback amplifier which operates

as a voltage-to-current converter. The x-axis signal is also fed to the x axis of an oscilloscope

which provides an analog presentation of the surface potential.

The cathode input provides a selectable voltage to the cathode of the electron gun.

There are two possible sources of cathode voltage data: the computer, or a 12-bit word set by front-panel-mounted octal-thumbwheel switches (cathode manual). The cathode input

select board provides solid state switching on a front panel input to determine which source

of data will be used. The cathode D/A converter converts the digital signal to an analog

voltage which is applied to the cathode. When in the computer mode of operation, it is possible to apply a voltage ramp or any other function to the cathode via software. The sys- tem also has a 4-digit LED display which can display singly the quantities: x-axis beam

coordinate, y-axis beam coordinate, and cathode voltage. In order to reduce cabling, the

data are multiplexed and the multiplexed data are sent to the display demultiplexer, timing,

and display.

Those electrons which have enough energy to overcome the surface potential barrier of

the target constitute a current which is amplified either by a current-to-voltage converter or by a nanovoltmeter amplifier operating as a shunt picoammeter. The output of the current

amplifier is fed to the threshold detector. When the target current reaches a preselected

value, a selected bit on a specified interface is set. Computer software tests this bit and when

it is set, stops the cathode voltage ramp. The value of the ramp at that instant is the relative

surface potential of that point on the target. The output of the current amplifier is also fed

to the z axis (intensity control) of an oscilloscope to provide a TV-type picture of variation

in surface potential.

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The programming language chosen for ASLEEP was BASIC since it is widely used and

programs may be interactively written and debugged at the computer terminal. BASIC also

supports calls to user-written assembly language subroutines. This feature is essential to the

NRL implementation of ASLEEP, because it is through the assembly language subroutines

that the computer controls the electron beam position, cathode voltage, and data output.

2.3 Spatial and Voltage Resolution

Utilizing an interdigitated test structure of aluminum on silicon dioxide and consisting

of 6-/im-wide lines on 12-nm centers, the minimum spatial resolution attained was 6 jum.

To minimize charging effects, a special test structure with minimum exposed oxide was constructed for voltage resolution measurements. A gold-metallized alumina substrate was

divided into two large pads separated by an 80-/im gap. An adjustable voltage was applied

between the two pads and its effect on the video signal was studied. A single line was

scanned across the pads and the target current was displayed on the oscilloscope. The

potential on each pad was adjusted so that the currents on each were equal and less than

1 nA. Then the potential of one pad was adjusted until the target current was displaced by

the width of the noise in the scope. A 300-mV change was adequate to do this with a target

current amplifier bandwidth of 20 kHz. The persistence of the phosphor gives a filtering

effect which allows a greater resolution in the TV scans. At a target current of less than

1 nA and even with careful shielding and grounding, the bias box which applies the voltage

between the pads acts as a source of 60-Hz pickup. In addition, it adds noise by increasing

the shunt capacitance on the input of the target current amplifier. A 20-Hz low-pass filter was inserted in the video circuit to remove these noise sources and the voltage resolution

experiment repeated. In this case, the voltage resolution measured was 25 mV.

The computer was used to measure voltage resolution by applying the cathode ramp,

threshold detector, and signal averaging. Measurements made over a period of less than a minute were compared and showed standard deviations of 1 mV, but long term drifts

increased the standard deviation to 10 mV. The long term drifts were due to the beam

wandering around on the target surface and variations in heater current.

3. EXPERIMENTAL INVESTIGATIONS

The results, to be more fully described below, indicate that oxide charging problems

limit the utility of ASLEEP, in its present form, for silicon. However, in the case of gallium

arsenide and indium phosphide, by utihzing in situ oxide desorbing anneals, native oxides

may be removed, thus allowing ASLEEP to study a wide range of problems. The problems

studied by ASLEEP include surface contamination effects and lateral inhomogeneities due

to stoichiometry variations. In addition, defect and fault detection can be readily attained on both the gallium arsenide and indium phosphide vdthout the obscuring effects of oxide

charging.

3.1 Oxide Uniformity

Oxide uniformity was studied by utilizing MOS capacitors. Large scale effects such as capacitor shorting to the substrate by defects in the oxide were readily identified by

10
ASLEEP. Figure 7 shows a three-dimensional surface potential map of 0.7-mm diameter

aluminum capacitor dots on 0.89-mm centers on silicon dioxide. Increasing surface potential

is directed downward. The oxide and some of the dots are charged to -10 V. The dots

which are raised are over regions of thinner oxide where the capacitance is large enough that

the signal is capacitively coupled to the target current amplifier.

However, charging of the capacitors by the electron beam made it difficult to extract

quantitative information. During the mapping operation, to determine the coordinates of

each capacitor, the capacitors were scanned and charged. The oxide of the samples weis of

high enough quality that over a week was required for them to discharge. This problem

could be avoided by using a sample which has a reference mark on it to which the capacitor

coordinate could be related. Then only the reference mark would be scanned to determine

the coordinates of all of the capacitors. The procedure for obtaining a quantitative measure- ment is outlined as follows:

a. The uncharged capacitor is probed with a small current to measure the surface po

tential. It is important in the case of high resistivity oxide capacitors to use a current small

enough that the charge deposited during the surface potential measurement is negligible

compared to the charge deposited in step b.

b. Then the sample is charged by scanning with a few volts applied to the cathode. The

charge placed on the sample is Q = jAt where / = charging current, A = capacitor area, and

t = charging time.

c. After charging, the sample is probed and the surface potential is measured again. The

capacitance may be calculated from C = Q/AV, where AFis the change in surface potential.

d. If the thickness, h, of the oxide dielectric in the capacitor is known and assumed to be constant, the dielectric constant may be determined from e = Ch/A. By scanning a number of capacitors the variation in e will determine the homogeneity of the oxide.

3.2 Silicon

3.2.1. Semiconductor Defect Density

Edge dislocation pipes and oxidation stacking faults were investigated by ASLEEP.

Edge dislocations were induced by oxidizing a (100) oriented silicon slice at 950°C for 1 h

in order to induce dislocation climb to the oxide-silicon interface. The oxide was stripped

from the sample before analysis. Typical edge dislocation pipes are shown in figure 8 as indicated by the arrow. Also shown are the electrically active region of the dislocation pipe

and the recombination contrast of the radial defect structure which surrounds the core of

the dislocation pipe. The broad contrast zone has been attributed to correspond to an

increased minority carrier flow surrounding the dislocation core [3] . This effect results from the annular depletion region predicted from electron-beam-induced-current (EBIC)

images [4] . At the center of each defect a surface micropit was observed by scanning elec- tron microscopy (SEM) analysis. The above analysis illustrates the applicability of ASLEEP

to defect density studies if the defects to be analyzed are larger than the 6-/im spatial

resolution limit of the instrument.

The image depicted in figure 8 was formed by initially collecting the electron current

and then differentiating it. Differentiation of the signal de-emphasized low frequency

11
Figure 7. Three-dimensional surface potential map of capacitors.

12
Figure 8. ASLEEP micrograph showing dislocation pipes in silicon wafer

as denoted by arrows. The dimensions of area examined are 2 mm by 2 mm.

13
variations in the target signal due to decreasing electron beam intensity off axis of the elec- tron gun. Differentiation also removed the dc component of the target current and emphasized small variations superimposed on the dc level. The mosaic (6 mm X 7.5 mm) was

assembled by scanning a region of the target and then repositioning the target to bring • another area into view of the electron gun. Experiments on sputtering to remove native

oxides which have been attempted on other Scimples indicated that the fine lines in the

mosaic are produced by variations in the native oxide and by surface contamination.

The second silicon defect density experiment conducted was the analysis of a (100)

wafer which contained process-induced defects. The wafer had been examined by X-ray

topography and showed the characteristic Lomer-Cottrell dislocation networks (figure 9).

Examination of the sample by ASLEEP indicated the presence of dislocation lines (figure 10)

with the expected 60-deg angle of intersection, but it was impossible to establish a one-to- one correlation with the features in the X-ray topograph. Further examinations of other

regions of the same wafer did not reveal additional instances of these dislocations in regions

where the X-ray topograph indicated they would be found. These results can be explained

as follows: (1) if the surface defects have an associated surface potential variation of less than ±10 mV and/or a lateral extent of less than 6 jum, ASLEEP cannot resolve the defect; and

(2) if the oxide/contamination effects described above dominate surface potential variations

associated with the dislocations, the defect structure cannot be correlated with the X-ray

topographs. These results do not rule out this application for ASLEEP, but further support

the need for highly controlled in situ surface cleaning techniques. The general applicability

of ASLEEP for examination of silicon is dependent upon the development of such processes.

3.2.2. Semiconductor Resistivity-Impurity Profiling

Changes in semiconductor surface potential resulting from a variation in impurity concentration were investigated using ASLEEP imaging. Figure 11 shows a hypothetical energy

band diagram showing the position of the Fermi level, Ep, the intrinsic Fermi level, E-, the

band gap, Eg, the electron affinity x, the surface potential 0„ in the n-type region and the

surface potential 0^ of the p-type region. From figure 11,

<t>n=X^^ -(Ep-Ei) (3)

and

E

<^P=X + -^ +iEi-Ep). (4)

Substituting for the electron and hole concentrations yields the final two expressions for

surface potential:

<t>n=X^^ -kTU

f_

(5)

•^P =X + -| -kTSln ^. (6) ^ Pi

14
Figure 9. X-ray topograph showing Lomer-Cattrell dislocation networks.
VACUUM LEVEL

Er,/2 Jef-Ei

Eg/2

P- TYPE

EfEv N-TYPE

Figure 11. Energy band diagram for silicon showing the position of the

Fermi level, the intrinsic Fermi level, the band gap, electron

affinity, and surface potential.

16
The difference in surface potential between two n-type regions of doping densities n-^ and

is given by

E„ /t. Eg n„

A(t)„=x + ~- -kT^n — - x--^ +kTZn — = kT in — . (7) " 2 n. 2 n. n.

I II

It should be emphasized that this simple analysis does not take into account the effect of

surface states on the surface potential measurements discussed below.

Semiconductor resistivity was investigated by utilizing ion-implanted calibration

samples. A p on p+ wafer of silicon was implanted with phosphorus to create regions with

donor densities of 1 X 1020 cm'^ , 1 X IQl^ cm-3, 1 X lO^S cm-3 and 1 X lO^^ cm'^. The

p-type material was doped to 2 X 10^^ cm"^. After the sample was implanted and annealed

at 1000° C for 30 min, the oxide which grew during the anneal was stripped and the resistivity

of the sample was measured by the four-probe method to verify that the expected carrier

concentrations were present. The sample was then given an HF dip and placed in the

ASLEEP system. The UHV system was pumped until a pressure of 3 X 10"^ torr was

reached, the electron gun system was operated for 2 h until the emission stabilized, and then

surface potential measurements were made.

Figure 12a shows the surface potential variations expected for the implanted impurity

concentrations as based on a simple one-donor level model. Figure 12b shows the surface

potential variation following the work of Allen and Gobeli [5] for a clean silicon surface

dominated by surface states. It may be seen that in either case the surface potential variation, over the concentration range chosen, is well within the resolution of ASLEEP. Figure

12c shows typical experimental observations in the implanted samples. It was postulated

that oxide/contamination effects were dominating doping effects since the potential shifts are not consistent with figures 12a or 12b and in the area of the junction between the 10^^

cm~^ and 10^'^ cm~^ doped n-type regions and the p-type region all traces of doping fade

out and the three regions became indistinguishable.

The sample was then sputtered with 2-kV argon ions to remove the surface contamination layer and native oxide. Work function measurements were made after sputtering, but

the resiilts were similar to those shown in figure 12c. The sample was then annealed in a vacuum of 2 X 10"*^ torr for 2 h at 600° C. Work function measurements were made again

and the results were similar to those shovm in figure 12c. These results lead to the conclusion that native oxide and/or vacuum-processing-related contamination dominated

the surface potential variation. The NRL ASLEEP system has a demonstrated resolution of

10 mV. If the silicon surface were clean either prior to sputter/anneal or subsequent to

sputter/anneal, the work of Allen and Gobeli [5] indicates that the surface potential variations should have been easily resolved. It was suspected, then, that the vacuum system

sputter/anneal processing was inadequate to completely remove native oxides and/or con- tamination layers in situ. This limitation was verified by subsequent anneal/sputter experiments utilizing low-energy electron diffraction (LEED) and Auger electron spectroscopy.

3.2.3. Integrated Circuit Testing

The final application of ASLEEP to silicon was in the area of contactless integrated

circuit testing. The ASLEEP system can be used to analyze integrated circuits provided

access tabs are made available to critical portions of the circuit. The low-energy beam

17
N REGION P REGION

O CT> 00 o 'o 'o ><

CM

> >

o o CD CD

cn 00

> >>

-800 MVo o o CO CO

00 <D O

'o "o ^

> > o

5 2 s

o o >— 00

A. EXPECTED WORK FUNCTION VALUES DUE

TO FERMI LEVEL SHIFT

B. EXPECTED WORK FUNCTION VALUES

AFTER ALLEN AND GOBELI (REF. 5) C. MEASURED WORK FUNCTION VALUES

Figure 12. Work functions of ion-implanted silicon. (A) Expected work function values from Fermi level shifts. (B) Expected work function values after Allen and Gobeli [5]

.

(C) Measured work function values.

18
provides a nondestructive electrical contact to the device and can be programmed to provide

acceptance testing of electrical performance on an on-line application.

Three integrated circuit chips were pulled from the production line before the

glass overcoat was applied; hence, all of the metalhzation was exposed (fig. 13). At first examination of the circuit, 1.5-V electrons possessed adequate energy to cross the surface

potential barrier of the metal interconnects (fig. 14b shows the ASLEEP image). However,

as scanning continued, the oxide visible between the interconnects charged to the extent

that tangential fields created at the sample surface prevented any electrons from being col- lected by the metallization. Increasing the electron energy brings the image back until the

oxide charges further. Once the saimple has charged, irradiation with high-energy electrons

or ultraviolet light was required to discharge the sample. This problem makes it impossible

to view a surface potential image of the integrated circuit for more than a few minutes. In an ASLEEP system for production line testing of integrated circuits, it would be possible

to mechanically index the circuits so they could be precisely positioned in front of the

electron gun. The computer could then steer the beam so that only the metal interconnects

would be proved with low-energy electrons.

3.3 ASLEEP Investigations on III-V Compounds

Annealing III-V compounds in an ultrahigh-vacuum chamber has been shown to produce a perfectly clean and well ordered surface [6, 7] . Thus, annealing compounds such as gallium arsenide (GaAs) and indium phosphide (InP) in the ASLEEP vacuum surface should

eliminate the oxide charging problems encountered with the silicon experiments.

3.3.1. Indium Phosphide

ASLEEP experiments were conducted as a function of anneal temperature on (100)

n-type indium phosphide grown on semi-insulating (SI) surfaces. Areas on the indium

phosphide surface which exhibited variations in surface potential were analyzed by

Auger electron spectroscopy capable of 100-nm resolution. The indium phosphide

surface was initially rinsed in acetone and polished by a 1% (by volume) solution of bromine

in methanol for 5 min. The same surface was then etched in a dilute solution by hydrofluoric acid for 1 min, rinsed in de-ionized water, and then etched with the MB-etching

solution which consists of 10 mL of hydrochloric acid, 10 mL of hydrofluoric acid and

5 drops of hydrogen peroxide in 40 mL of water [8] for 10 min (the entire procedure is denoted as the MB etch). For purposes of comparison, two indium phosphide samples were mounted side by side for ASLEEP examination. One sample had the acetone rinse with no

further etching, while the second sample had the MB etch described above. Prior to the

ASLEEP investigation, an in situ Auger-anneal experiment indicated that desorption of

hydrocarbons from indium phosphide surfaces takes place at 250°C. Further annealing

above 250°C results in the desorption of phosphorus oxide at. 275°C leaving behind an

oxidized indium-rich layer which is further desorbed in the vicinity of 310°C to 330°C.

Decomposition of indium phosphide was found to occur at 420° C.

I The ASLEEP images of the acetone-cleaned and MB-etched indium phosphide sur- faces are shown in figures 15 and 16. Typical regions analyzed with Auger electron

spectroscopy (AES) are also indicated on the figures. Regions which showed variations in

j surface potential were invariably identified with a high oxygen content as indicated by the

oxygen transition at 510 eV. The phosphorus L2,3 VV transition [9] at 120 eV was re- duced in magnitude from the expected phosphorus LVV peak height and was also characj terized by new peaks at 100 eV and 110 eV. These peaks are phosphorus oxide peaks denoted as PL2,30L2,30L2,3 at 110 eV and PL2,30Li V at 100 eV (not shown on photo19
Figure 13. Optical photomicrograph of integrated circuit utilized in the ASLEEP experiment. The dimensions of the integrated circuit are 120 mm by 120 mm.

20

I
21
graph). The indium transition at 404 eV was also reduced in magnitude from the expected

indium peak height. The extra peaks and above characteristics were only observed in areas A and B and are associated with the desorption of phosphorus compounds. The above characteristics were not apparent in other regions analyzed and are not consistent with transitions

involving indium and indium compounds. A small dip in the spectra at 84 eV was also observed, which indicates the presence of the PL2 3OL1OL1 transition. The MB-etched sample

had a limited number of similar regions as did the acetone-cleaned sample as shown in figure

16. The Auger spectrum of each region analyzed on the MB-etched sample was also characterized by the large oxygen peak at 510 eV and phosphorus oxide peaks at 110 eV and 84 eV.

The samples shown in figures 15 and 16 were subsequently annealed at 330° C for 15

min. The ASLEEP images of figure 17 showed a perfectly uniform surface potential map

for the MB-etched surface and a relatively well behaved surface potential map for the ace- tone-cleaned surface. The Auger spectra of regions shown indicate that oxygen has been

desorbed probably in the form of the phosphorus and indium oxides. The 510-eV oxygen

transition has disappeared while the indium and phosphorus transitions have recovered the

peak-to-peak height expected for uncontaminated indium phosphide. All of the observed

features of the Auger spectra may be attributed to indium and phosphorus, together with

their characteristic energy loss peaks. Immediately after attaining the ASLEEP images,

LEED experiments were conducted on the annealed surfaces. It is noted that prior to annealing, the LEED pattern was obscured completely. Upon heating to 330°C, the LEED pattern

appeared at 35 eV and consisted of two sets of 4 X 1 patterns rotated 90 deg. This pattern

is in general agreement with the reconstructed indium phosphide surface observed by McRae

[7] . From this experiment we have concluded that ASLEEP can successfully map surface

potential variations which arise from local changes in material stoichiometry.

In order to further test the previous observations that surface stoichiometry variations

in indium phosphide resulted in surface potential differences, controlled oxidation experiments were conducted on the originally desorbed indium phosphide surface. The ASLEEP

image prior to oxidation is shown in figure 18. Random AES profiles also shown in

figure 18 indicate features attributed to indium and phosphorus. The indium: phosphorus

peak ratio is approximately 0.33. Oxygen was allowed to leak into the UHV ASLEEP

system until the pressure approached 1 X 10"^ torr, while the substrate was maintained at 125°C. After an exposure of 10 min at 10"^ torr, the system was evacuated to the 10"^ ton- range. The ASLEEP image after oxidation (fig. 19) shows irregular regions with one well

defined flat region. The Auger spectrum is typical of contaminated indium phosphide with

carbon and oxygen present; the peak-to-peak height of the phosphorus transition at 120 eV

is drastically reduced from that expected for uncontaminated indium phosphide. The

irregular regions, in addition to the phosphorus L2 3 VV transition at 120 eV, also showed

the previously discussed phosphorus oxide peaks at 110 and 100 eV. . Exposure at 10"^ torr

also decreased the phosphorus: indium peak ratio which is indicative of oxygen penetration

into the bulk. Therefore, from the oxidation experiment, localized surface potentiad variations can be attained which are attributed to indium, phosphorus, and oxygen variations.

3.3.2. Gallium Arsenide

ASLEEP imaging was applied to the study of gallium arsenide (1) surface contamination and (2) surface etchants. State-of-the-art etchants for gallium arsenide utilize halogens

such as chlorine and bromine which segregate at select surface sites resulting in surface

potential variations. Figure 20 shows a comparison between the ASLEEP image, SEM

22
23
24
25
26
27
image and scanning spot Auger oxygen and chlorine images on n-type gallium arsenide taken

after a standard etch treatment. The wafer was processed as follows : 2 min in MB etching solution followed by a dip in 10 mL of hydrofluoric acid for 10 s and a cascade rinse. The round

features of the ASLEEP image are shown to consist of adsorbed oxygen and chlorine,

while the elliptical feature is probably due predominantly to adsorbed chlorides. In this case the correlation of ASLEEP with Auger and SEM imaging has clearly explained the

features observed in the ASLEEP image.

In a second experiment an n-type gallium arsenide epitaxial layer grown on an n^-type

gallium arsenide substrate was utilized for comparison of two candidate etchants. The gallium

arsenide sample was divided in half and both halves were degreased, rinsed in methyl alcohol,

and silver indium germanium contacts were evaporated on the back. The small half was process

as follows: dipped in 10-mL hydrofluoric acid for 2 s, followed by a deionized water rinse;

etched in 10-mL hydrochloric acid for 2 min; dipped in 10-mL solution of hydrofluoric acid foi 10 s, followed by a cascade rinse. The second half was processed as follows: dipped in 10-mL

hydrofluoric acid solution for 2 s; rinsed in deionized water; dipped in 10-mL solution of hydro

chloric acid for 30 s; rinsed in deionized water; dipped in 10-mL hydrofluoric acid solution for

10 s, and etched in MB-etching solution for 10 min, followed by a cascade rinse. The samples were dried in nitrogen and immediately mounted in the ASLEEP system. The MB-etched half

as shovm in figure 21 has a lower density of topographical features and a lower overall surface

potential than the routinely treated half. Statistical data were taken in order to determine the

surface potential difference between the MB-etched side and the standard side. The surface potential difference is 1.18 V, and it is clear that the distribution function for the MB-etch is tighter than for the standard etch. An optical inspection of the sample was made and it be- came obvious that most of the features seen in the ASLEEP image were not present in the

optical images. In particular, the swirl which appears on the MB-etched side was not observed by optical microscopy nor by the scanning electron microscope. These experiments were performed in order to demonstrate the utilizability of ASLEEP for optimizing semiconductor processing.

4. CONCLUSIONS

This investigation has revealed the following points in connection with the application

of ASLEEP to the silicon investigations.

1. ASLEEP has the capability to measure quantitatively the surface work function

variations associated with the doping level normally occurring in the silicon device technology. This is so, even taking into account the effect of surface states on compressing the

Fermi level shift. As shovm in the work of Gobeli and Allen [5] ,.for an n-tyoe material the

work function variation is from approximately 4.82 to 4.70 eV (A0 — 0.12 eV) for doping

density changes from approximately 10^^ to 10^0 cm"^. A similar variation occurs for

p-type material up to 2 X lO^^ cm-3. The resolution of ASLEEP is 10 mV and thus this

remains an important application of the ASLEEP technology. However, the present investi- gation including utilization of LEED and Auger analysis has shown that native oxides and

processing related contamination factors must be controlled in order to allow meaningful

measurement of the surface potential.

2. The question of the utilizability of ASLEEP for defect and fault detection in silicon was resolved during the contract period. Large defect structures such as dislocation

pipes were observed and correlated with SEM and optical microscopy. In the case of oxidation-induced stacking faults which are below the spatial and surface potential resolution

I

28
SEN IMAGE 120X ASLEEP IMAGE 90X

0 IMAGE 120X Cl IMAGE 120X

Figure 20. A comparison of ASLEEP image, SEM image,

and scanning Auger images of gallium arsenide

contamination.

29
Figure 21. ASLEEP image of gallium arsenide wafer showing the effects of two

different etch treatments.

30
limits of the system, ASLEEP is clearly not applicable. In the case of certain surface and

sub-surface faults, it is believed that the surface contamination problem described above is dominant and must be solved before these defects can be observed.

3. Studies of the uniformity of oxide thickness and dielectric constant proved to be

impractical since provision was not made for routine discharging of the oxide surface after

initial charging by the scanning electron beam. However, discharging could be accomplished

by pulse sampling techniques.

4. In order to avoid unwanted variations in electron energy, the retarding field must be

uniform, which means that the target must be flat.

5. Improved gun design is required to reduce beam size, increase current density, and

reduce the loss of low-energy electrons from the beam.

6. Oxide charging, spatial resolution limitations, and deflection amplifier drifts make

examination of state-of-the-art integrated circuits marginal ^Adth the present equipment.

However, further development effort could solve these problems.

The application of ASLEEP to indium phosphide and gallium arsenide proved successful since oxides aind contaminants were successfully desorbed by in situ annealing. ASLEEP

was therefore successfully utilized to study the following problems to provide data which

could be used to optimize indium phosphide and gallium arsenide processing.

1. Surface potential variations in indium phosphide and gallium arsenide due to

lateral inhomogeneities.

2. Surface potential variations in indium phosphide and gallium arsenide due to adsorbed contaminants from processing.

3. Effects of various processing steps on surface potential variations.

ACKNOWLEDGMENTS

The author acknowledges contributions from Mr. W. Jenkins and Mr. W. Wilkins who

constructed the system and carried out the first experiments on silicon, Mr. George Nelson

who was responsible for the computer interfacing, and Dr. J. E. Davey who supplied many

of the original ideas and experiments.

REFERENCES

1. Haas, G. A., and Thomas, R. E., Electron Beam Scanning Technique for Measuring

Surface Work Function Variations, Surface Science 4, 64-75 (1966).

2. Strayer, W., Mackie, W., and Swanson, L. W., Work Function Measurements by the Field

Emission Retarding Potential Method, Surface Science 34, 225-235 (1973),

3. Ravi, K. V., and Varker, C. J., Edge Dislocations in Silicon, J. Appl. Phys., 45, 263-271

(1974).

4. Ravi, K. V., Dislocation Pipes in Oxidized SiUcon, J. Appl. Phys. 43, 1785-1791 (1972).

31
5. Allen, F. G., and Gobeli, G. W., Work Function, Photoelectric Threshold, and Surface

States of Atomically Clean Silicon, Phys. Rev. 127, 150-159 (1962).

6. Chang, K. H., and Meijer, P. H., Surface Properties of Indium Phosphide, J. Vac. Sci. Technol. 14, 789-796 (1977).

7. "MacRae, A. U., LEED Examination of Indium Phosphide and Gallium Arsenide, J. Vac.

Sci. Technol. 4, 247-255 (1966).

8. Bark, Marvin, NRL, private communication.

9. Siegbahn, K., "ESCA-Atomic, Molecular and Solid State Structure Studied by Means of

Electron Spectroscopy," in Nova ACTA REGIAE Soc, Ser. 4, Vol. 20, North Holland

Publishing Co.. Amsterdam-London (1967).

I. S. GOVERNMENT PRINTING OFFICE : 1978—261-238/50

32
NBS-114A (REV. 7-731

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Acces.sion BIBLIOGRAPHIC DATA No. SHEET NBS Spec. Publ. 400-30

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Semioonduotov Measurement Teohnology : Automated Scanning LowEnergy Electron Probe (ASLEEP) For

Seirn conductor Wafer Diagnostics

5. F'ubl ication Date April 1978

6. Performing Organization Code

7. AUTHOR(S)

A. Christou

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Naval Research Laboratory

Washington, DC 20375

10. Project/Task/Work Unit No.

11. Contra'-f / Granl Hf>^ - NfeS Orders 50T718

and 711782

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

ARPA

1400 Wilson Boulevard

Arlington, VA 22209

13. Type of Report & Period Covered Final - 8/74 - 9/77

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 78-2200

16i ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This report summarizes the results of a three-year effort in the development of a computer Automated Scanning Low-Energy Electron Probe (ASLEEP) for semiconductor wafer

diagnostics. Experiments designed to explore the measurement capabilities of ASLEEP

for measurements on s''''icon, gallium arsenide, and indium phosphide are described.

Four areas were emphai. zed: 1) semiconductor resistivity, 2) semiconductor defect

density, 3) lateral ini;omogeneities , and 4) oxide uniformity. Although oxide charging

problems limit the utility of ASLEEP in its present form for silicon, defects and

lateral inhomogeneities could be readily detected in gallium arsenide and indium

phosphide.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

inhomogeneity; oxide uniformity; scanning low-energy electron probe; semiconductor

defect density; semiconductor resistivity; semiconductor wafer diagnostics.

18. AVAILABILITY |^ Unlimited

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