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Devaney, John R/Notes on SEM examlna«on QC100 .U57 NO.400-, 35, 1977 C.Z NB&-KUB NBS SPECIAL PUBLICATION 400

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

Semiconductor Measurement Technology:

Notes on SEM Examination of

Microelectronic Devices
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards* was established by an act of Congress March 3, 1901. The Bureau's overall goal is strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To t end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific a technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to p: mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institi for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and I Office of Experimental Technology Incentives Program.

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1077

Semiconductor Measurement Technology:

? Notes on SEM Examination of

T „ . Microelectronic Devices

John R. Devaney

Hi-Rel Laboratories

Monrovia, Ca. 91016

K. O. Leedy and W. J. Keery

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Jointly supported by: The Defense Advanced Research Projects Agency

and The National Bureau of Standards

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

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

\j^<^NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1977
Library of Congress Cataloging in Publication Data

Devaney, John R. Notes on SEM examination of microelectronic devices. (Semiconductor measurement technology) (NBS special publication ; 400-35)

Supt. of Docs, no.: €13.10:400-35.

1. Miniature electronic equipment—Testing. 2. SemiconductorsTesting. 3. Scanning electron microscope—Technique. I. Leedy, K. O., joint author. II. Keery, W. J., joint author. III. Title. IV. Series. V.

Series: United States. National Bureau of Standards. Special

publication ; 400-35. QC100.U57 no. 400-35 [TK7874] 602Ms [621.38r7r028] 77-608011

National Bureau of Standards Special Publication 400-35

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-35, 54 pages (Apr. l977) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $1.10

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Semiconduotor Measurement Technology

:

Notes on SEM Examination of Microelectronic Devices

TABLE OF CONTENTS

PAGE

1. Introduction 1

2. Secondary and Backscattered Electron Images 3

3. Accelerating Voltage 8

4. Beam Current

,

14

5. Specimen Tilt 14

6. Scan Line and Frame Time 18

7. Artifacts Introduced by the Beam 26

8. Conclusion 36

Appendix I - Voltage Contrast 38

Electron-Beam-Induced Current , . . 38

Appendix II - Semiconductor Specimen Preparation 43

References 47

TABLE OF FIGURES

PAGE

1. Primary electron penetration showing the volumes of

secondary and backscattered electron generation 4

2. Emission trajectories of secondary and backscattered

electron signals ..... 4

3. Difference in the apparent illumination of a device with

respect to the imaging electrons 5

4. Backscattered and secondary electron images of a relatively

smooth surface 6

5. Micrograph of a wire bond and probe mark in which the

shadow effect produced by the backscattered electron image

reveals some upturned metal not seen in the secondary image . 9

6. Series of micrographs which illustrates the change in contrast as the accelerating voltage is varied 10

7. A portion of a microcircuit imaged with a beam voltage at

3 kV and 30 kV 13

iii
PAGE

8. A portion of a microcircuit imaged at high and low beam

voltage illustrating an application of the greater beam

penetration at 20 kV 15

9. Compression ball bond intermetallic formation imaged at

3 kV and 20 kV illustrating an application of high

accelerating voltages , . 16

10. Diagram illustrating specimen tilt angle 17

11. Series of micrographs which illustrate the changes in contrast and SNR as specimen tilt angle is increased from 0

to 60 deg 19

12. The effect of specimen tilting when viewing a passivated

microcircuit 22

13. Series of micrographs illustrating the effects of variations

in scan line time, frame time, and the number of lines in

the micrograph 24

14. Two examples of charging 27

15. Micrographs which show the effects of charging of areas outside the field of view 29

16. Additional examples of the effects of charging 31

17. The use of backseat tered electrons rather than secondaries

to form the image to eliminate charging effects 34

18. Contamination due to the polymerization of pump oil 35

19. A secondary electron image of a biased microcircuit showing

voltage contrast 39

20. Micrographs of an interdigitated npn transistor made with

a beam current of 350 pA and a beam voltage of 30 kV 41

21. EBIC images of portions of two similar shift registers with

the arrows indicating the region of difference 42

22. A microcircuit window covered by globs of metal introduced

by poor evaporation techniques 46

iv
Semioonductor Measurement Technology

:

Notes on SEM Examination of Microelectronic Devices*

John R. Devaney

Hi-Rel Laboratories

and

K. 0. Leedy and W. J. Keery

National Bureau of Standards

AbstTact: This report reviews selected scanning electron

microscope (SEM) techniques which are appropriate for the exam- ination of microelectronic devices. Illustrated are the results

of individual variations in SEM operating parameters such as accelerating voltage, specimen tilt, scan line time, and frame

time. Techniques which utilize secondary and backscattered

electron emissions are compared and electron-beam-induced current and voltage contrast modes are discussed. Specimen preparation and beam-induced charging artifacts are also discussed.

This report demonstrates the need for flexibility in selecting

and using SEM parameters and analytic procedures to obtain the

maximum information when examining semiconductor devices.

Key Words: Backscattered emission; EBIC; microelectronic

devices; scanning electron microscope; secondary emission; voltage contrast.

1 . Introduction

The scanning electron microscope (SEM) is an instrument widely

used in the electronic components industry. Its advantages over other

types of microscopes for topographical examination are its greater depth

of focus, spatial resolution, and the ease of specimen preparation. In

addition to secondary and backscattered electron emission techniques,

voltage contrast and electron-beam-induced-current modes can be used to

gain additional information on semiconductor devices.

This report is intended to describe selected techniques for obtaining optimum scanning electron micrographs of semiconductor device specimens. It is assumed that the reader is familiar with basic SEM principles.^"^ The various techniques described here are manipulations of

*This work was conducted as a part of the Semiconductor Technology Program of the National Bureau of Standards and was supported by the Defense Advanced Research Projects Agency (Order No. 2397) and the NBS.

The work at Hi-Rel Laboratories was funded under Contract 4-35897.

1
instrument variables. Unless otherwise indicated, the variable of interest was changed while all others were held constant.

The devices examined were made by standard silicon technology.

All passivating layers were first removed and the devices were then

mounted on specimen stubs. (The preparation of specimens is described

in Appendix II.) Secondary electron imaging is emphasized but the backscattered electron image is also discussed. The voltage contrast and

the electron-beam-induced-current (EBIC) modes are briefly addressed in

Appendix I.

There are several factors to consider when judging a good micrograph: signal-to-noise ratio (SNR) , point-to-point resolution (PPR)

,

and contrast. The SNR is the ratio between rms amplitude of the desired signal and the rms fluctuation due to noise. The SNR is a measure of the efficiency of the signal generation. The PPR is the quantitative measure of the SEM resolving power which defines the ability

of the SEM to resolve two adjacent points. Resolution is dependent on

the interaction volume of the electron beam in the specimen which is

dependent on the primary electron beam diameter and the atomic number

of the specimen surface. It also depends on the nature of the signal

detection system. The contrast is the degree of difference in tone between the lightest and darkest areas of an SEM image. There are a number of types of contrast: topographical, chemical (atomic number),

voltage, current, crystallographic, and magnetic. These are generated

by the primary electron beam interaction with the differences in topography, chemistry, electrical potential, crystallography, and magnetic

fields of the device.

Artifacts introduced by vacuum evaporated thin films, diffusion

pump oil polymerization in the SEM, and careless sample preparation are

of concern in the interpretation of SEM images . Specimen charging due

to stray electrostatic activity in the SEM specimen chamber can be an- other source of misleading information. Contaminants introduced during

manufacture or failure of semiconductor devices cannot be considered

anomalous, since such contaminants are often the subject of SEM investigation.

The SEM operating parameters that affect micrograph quality for

semiconductor device specimens are the type of image (secondary or backscattered electron), the accelerating voltage of the primary beam, the

current of the primary beam, the specimen tilt, the beam scan rate, and

the frame time. These are each considered in detail in subsequent sections of this paper. The effect of the electron beam on the specimen

and anomalous artifacts introduced by the beam are also considered.

Procedures for specimen coating are addressed in Appendix II.

2
2. Secondary and Backscattered Electron Images

The detection of secondary electron emission is the most often

used mode for operation of an SEM. The secondary electrons are the low

energy electrons emitted from the specimen when it is bombarded by a high energy electron beam. Secondary electrons are arbitrarily defined

as those having kinetic energies less than 50 eV. Secondary electrons

are generated throughout the volume of primary electron penetration,

; but only those emitted within several interatomic distances ('\^10 nm)

from the surface escape, as shown in figure 1. As illustrated in figI ure 2a, the secondary electrons emitted from the specimen are detected

by an electron detector. The secondary electrons are deflected along

curved trajectories toward the positively biased detector regardless

of the initial direction of their emission.

Backscattered electrons are primary beam electrons with energies

|i greater than 50 eV scattered from the specimen. They can come from

depths of up to several micrometers beneath the surface. These backscattered electrons have kinetic energies ranging from 50 eV to near

that of the incident primary electron energy and, unlike secondary eleci trons, these higher energy electrons tend to travel in straight lines.

I As shown in figure 2b, the backscattered electrons are detected only

when intercepted by the solid angle of the detector. The backscattered

electron mode is often referred to as the reflection mode.

The secondary electron image appears to be illuminated from all

! directions and has no definite shadows, while the backscattered electron

image appears as if there were oblique illumination of the specimen from

the direction of the detector. This type of illumination forms images

with very distinct shadows. Figure 3 demonstrates the omnidirectional

illumination of a device in a secondary electron image in (a) and the

shadowy unidirectional illumination of the same device in a backscatterI ed electron image in (b)

.

The secondary electron image (SEX) usually has better point-topoint resolution and depth of focus than the backscattered electron image (BEX). The detector normally employed in commercially available

SEM's is the Everhart-Thornley (E-T) detector which is optimized for

I detection of secondary electrons. Therefore, in the backscattered mode,

(1 large beam currents are required to generate sufficient signal electrons,

and the electrons forming the image are from a larger volume than in the

secondary electron image. The advantage of the BEX is the information

obtained relating to the atomic number of the material and surface topography of the device. The topographical information is characterized by

high contrast due to the highly directional nature of the backscattered

electrons and the highly directional E-T detector normally employed.

I Figure 4 shows three attempts to record a micrograph of a device which

' failed to produce suitable topographical contrast in the secondary electron mode. Three different accelerating voltages were used: (a) 3, (b)

3
Figure 1. Primary electron penetration showing the volumes of secondary

and backscattered electron generation.

BEAM

Figure 2. Emission trajectories of secondary and backscattered electron

signals

.

4
a. Omnidirectional illumination in a secondary electron

image

.

b. Shadowy unidirectional illumination in a backscattered

electron image.

Figure 3. Difference in the apparent illumination of a device with

respect to the imaging electrons.

5
a. Secondary electron image recorded at 3 kV.
7
5, and (c) 10 kV. Figure 4d shows a BEI of the exceptional topographical and chemical contrast of this same region. This particular device

had very low secondary emission characteristics. In figure 5a a BEI of

a probe mark was used to reveal information not seen in (b), an SEI.

3. Accelerating Voltage

The accelerating voltage of the electron beam is a high potential

difference applied between the filament (cathode) and anode of the SEM

electron gun. This voltage accelerates the electrons emitted from the

cathode towards the specimen surface. Commercial scanning electron microscopes are capable of a wide range of accelerating voltages from 1 to

50 kV. The use of either a high or low accelerating voltage is advantageous for different types of specimens. In general, the higher the ac- celerating voltage, the smaller the diameter of the primary beam, resulting in better point-to-point resolution. However, for semiconductors,

the minimum spot size obtained by using a higher accelerating voltage

does not always yield the optimum micrograph.

If the accelerating voltage is reduced, the electron beam penetration into the specimen is reduced. This may result in improved contrast

due to both an increase in secondary electron emission at the surface of a device and a decrease in the background secondary emission generated

by backscattered electrons at considerable distances from the beam impact point. The increased emission improves the SNR, thus the contrast.

To illustrate the effects of accelerating voltage on secondary

electron emission, six different accelerating voltages were used to obtain SEM images of the same semiconductor device; the resulting micrographs are displayed in figure 6. The accelerating voltages chosen represent a typical range available on most commercial scanning electron

microscopes: 2, 3, 5, 10, 20, and 30 kV. The same beam current was

used in each case. The chemical and topographical contrast deteriorates

as accelerating voltage is increased. There is also a progressive increase in the noise, as indicated by the grainy appearance of the micrographs made with higher voltages. The image with the most noise is at

30 kV where the beam penetration is at a maximum so that fewer secondary

electrons are generated within an escape depth of the surface. It can be concluded that lower accelerating voltages are superior to higher ac- celerating voltages for obtaining better contrast and less noise in the

imaging of this semiconductor device.

Figure 7 demonstrates the superiority of lower accelerating voltages for resolving surface detail at low magnification. Figure 7a shows an area of a microcircuit examined using a primary electron beam of 3

kV. Note the resolution of surface structure on the metallization.

Figure 7b shows this same area examined using a 30-kV primary electron

beam, and it is obvious that most of the surface structure is obscured.

The loss of visible surface structure is the result of excessive penetration and diminished surface electron emission.

8
a. Backscattered electron image with the arrow

showing the upturned metal.

b. Secondary electron image in which flat illumination

fails to reveal metal detail.

Figure 5. Micrograph of a wire bond and probe mark in which the shadow

effect produced by the backscattered electron image reveals

some upturned metal not seen in the secondary image.

9
a. Micrograph recorded at 2 kV xn which the chemical

and topographical contrast is outstanding.

b. Micrograph recorded at 3 kV in which the chemical

and topographical contrast is still very good.

Figure 6. Series of micrographs which illustrates the change in contrast as the accelerating voltage is varied.

10
c. Micrograph recorded at 5 kV.

d. Micrograph recorded at 10 kV showing decreased

contrast

.

11
e. Micrograph recorded at 20 kV showing further

decreased contrast.

I

The greater beam penetration depth at a high accelerating voltage

can be advantageously used even at low magnification to provide subsurface information. Figure 8a shows the surface of a device covered with

contamination viewed at 3 kV while figure 8b shows the same area viewed

at 20 kV. The original contaminated surface is not seen at the higher

beam energy and the device surface itself can be studied.

The advantage of using high accelerating voltage for high magnification applications is illustrated in figure 9 which shows micrographs

of the intermetallic region between a compression ball bond and a pad

at a magnification of 10,000x. The upper micrograph was recorded with

an accelerating voltage of 3 kV and the lower with an accelerating voltage of 10 kV. The lower accelerating voltage produces electrons of lower velocity, resulting in an increased width of the primary electron

beam and poorer point-to-point resolution in the image, thus the fuzziness in figure 9a. The higher accelerating voltage used for figure 9b

produces a much sharper primary beam, and the PPR and general resolution are improved.

4. Beam Current

The micrographs illustrating this report were taken using a tungsten hairpin filament which is the most common electron source used in

commercial instruments. The use of a brighter source would permit higher beam current density, thus better SNR for the same beam diameter, or,

conversely, would yield the same beam current and SNR using a finer

probe. Two brighter electron sources are becoming available for use on

an SEM. The LaB5 gun^^ is approximately ten times brighter than the

tungsten filament. This source usually adds somewhat to instrument complexity. A field emission source is the brightest source presently

available. It is about 1000 times brighter than the tungsten filament

but requires a correspondingly better vacuum for operation. At the present level of development, it has stability and lifetime problems.

5. Specimen Tilt

Semiconductor devices to be examined in an SEM specimen chamber

must first be mechanically and electrically secured to a holder designed

to be mechanically attached to the SEM stage. The ability of a stage to

incline the specimen surface toward the electron detector by means of an external control is termed specimen tilt. The angle of inclination (9)

formed by specimen tilting is defined as the angle between the plane

normal to the path of the primary electron beam and the specimen surface,

as shown in figure 10. The primary function of tilt is to obtain a favorable position for viewing the specimen. Advantages obtained by proper adjustment of specimen tilt include increased electron emission, improved contrast, and reduced electrostatic charging.

14
a. With a beain voltage of 3 kV, there is clearly visible

a layer of contamination on the device surface.

With a beam voltage of 20 kV, details of the device

surface under the contamination become visible.

Figure 8. A portion of a microcircuit imaged at high and low beam

voltage illustrating an application of the greater beam

penetration at 20 kV.

15
b.

Figure 9. Compression ball bond intermetallic formation imaged at 3 kV

(a) and 20 kV (b) illustrating an application of high accelerating voltages.

16
PRIMARY BEAM

Figure 10. Diagram illustrating specimen tilt angle.

17
Improvement in both secondary and backscattered electron emission

occurs because the primary electron beam penetration is reduced as the

tilt angle is increased from 0 to 90 deg. For specimens at a high tilt,

more of the primary electron penetration volume is within an escape

depth of the surface, thus more electrons are emitted. Increasing the

number of emitted electrons improves the SNR, ^which in turn improves

contrast. Both a high tilt angle and a low accelerating voltage limit

primary beam penetration.

Another reason that a tilt ang=le of 45 deg or greater improves the

SNR and contrast is that the SEM goniometer stage is designed to tilt

the specimen surface toward the E-T detector which is usually optimized

for a tilt angle of 45°. Therefore, tilting causes an increase in the

number of signal electrons that can reach the electron detector.

These effects are demonstrated in figure 11. These micrographs

were made with an accelerating voltage of 5 kV and a constant beam current. In figure 11a where 0 is 0 deg, there is little topographical or chemical contrast. Figures lib and 11c demonstrate the advantage of

tilt in revealing the character of the topography more clearly. In the

latter micrograph, a distinct improvement in SNR and topographical contrast can be noted when compared to figure 11a. An interesting development in chemical contrast is seen in figure lid. Such intense contrast

of this quality can only be attained using an accelerating voltage less

than approximately 7 kV with a tilt angle of about 45 deg or greater.

As 6 increases to 60 deg, enhancement of chemical contrast and SNR continues as shown in figure lie. Examination at high angles provides an increased capability of interpreting the quality of the metallization,

bonding, and oxide steps which is valuable to SEM semiconductor analysis. This is especially true for regions of the device where tools

have been used to trim, bond, and cut portions of the device. These are areas where heavy mechanical damage often occurs.

Another very useful consequence of tilting a semiconductor device

can be seen in figure 12. Figure 12a shows a passivated microcircuit

examined at a 9 of 0 -deg without any prior preparation to prevent charging. Charging is manifested by the very bright regions with the very

dark adjacent areas which results in the obscuring of detail in both.

In figure 12b the same circuit is examined at a 6 of 60 deg. The high

tilt angle has increased the number of backscattered and secondary electrons which leave the surface, thus reducing the net charge in the insulating layer. The resolution is not good in this image, but it demonstrates that specimen tilt is a technique which can be used to reduce

specimen charging, as well as improve contrast. Further discussion on specimen charging will be found in Section 7.

6 . Scan Line and Frame Time

In an SEM the same sweep or scan generator is used to synchronously

deflect both the primary electron beam in the column and the electron

beam in the cathode ray tube (CRT) used for imaging.

18
a. Tilt angle is 0 deg.

b. Tilt angle is 20 deg.

Figure 11. Series of micrographs which illustrate the changes in

contrast and SNR as specimen tilt angle is increased

from 0 to 60 deg.

19
c. Tilt angle is 40 deg.

d. Tilt angle is 50 deg.

20
Tilt angle is 60 deg.

21
a. With a tilt angle of 0 deg, a large amount of charging is apparent

due to trapping of primary electrons in the glass layer.

b. With a tilt angle of 60 deg, the charging has been greatly reduced

because fewer electrons are captured in the glass.

Figure 12. The effect of specimen tilting when viewing a passivated

microcircuit

.

22
The line scan time is the amount of time taken for a single line

to be scanned from left to right across the CRT display screen. The

frame time is the amount of time taken to complete a compilation of single lines sweeping the display screen from top to bottom. These two parameters determine the number of lines , the number of picture points

,

and the beam dwell time for each picture point in the SEM image. The

amount of time the electron beam rests on a point of the specimen surface determines the number of electrons emitted from that point. Therefore for a given beam current and detector gain, it is the number of picture points and dwell time per point which ultimately determine the SNR.

These principles apply whether the scan generator is a digital or analog

type. The range of scan line and frame times on commercial SEM's is

large. The factor to be considered when selecting the line scan and

frame time for SEM photomicrography is the selection of a suitable number of scan lines per frame so that none are visible within the resolution limits of the eye. Another consideration is that the number of scan lines used should not be so high that the scan time per point is

reduced causing the SNR to suffer significantly. In addition, the frame

time should be short enough to allow the economical recording of micrographs .

In order to illustrate the proper selection of line scan and frame

time for SEM photomicrography, micrographs of the same semiconductor device recorded under different raster time conditions are compared. The

values of accelerating voltage, beam current, and tilt are held constant.

If too few scan lines are used, the individual lines can be differentiated in the micrograph. The result is poor PPR caused by the picture

voids between the lines. This effect can be seen with about 500 scan

lines per frame or less. (Due to the loss of resolution in printing,

this example is not shown.) In order to fill these picture voids and

improve the PPR, an image of the device was recorded using a frame time

of 10 s and scan line time of 0.005 s. The resulting micrograph, shown

in figure 13a, has a total of 2000 lines; as a result, there are no picture line voids. However, there is an increase in noise due to the reduced scan time per point producing a lower SNR. It is apparent that

for a frame time of 10 s, neither 500 nor 2000 scan lines are suitable

for photomicrography.

Figure 13b was recorded using a 10 s frame and a 0.01 s line scan

time, producing a total of 1000 lines. It shows an improvement in PPR

and SNR. However, there is still considerable noise in the image. To

decrease this noise, the line scan time was increased to 0.1 s, and the

frame time increased to 100 s, keeping a total of 1000 scan lines. The

result shown in figure 13c demonstrates excellent PPR, SNR, and contrast.

However, the economics of the SEM operation might dictate that micrographs be made in less than 100 s. Figure 13d shows a compromise between SEM resolution and efficiency. This micrograph was recorded with

a 40 s frame time, and a scan line time of 0.04 s, keeping the 1000 scan

line total. These final values are generally very effective for good

micrograph quality. Of course, should a better SNR be required, one method of improvement would be to increase the frame time.

23
a. Scan time 0.005 sec/line and 2000 lines per frame. There is increased noise due to the short dwell time per picture point.

b. Scan time 0.01 sec/line and 1000 lines per frame. The image shows

better PPR and SNR as compared to (a)

.

Figure 13. Series of micrographs illustrating the effects of variations

in scan line time, frame time, and the number of lines in

the micrograph.

24
c. Scan time 0.1 sec /line and 1000 lines per frame. Contrast, PPR,

and SNR are excellent.

d. Scan time 0.04 sec/line and 1000 lines per frame. This represents

a compromise giving good contrast, PPR, and SNR while limiting total

exposure to 40 seconds for efficient use of time.

25
In setting up the SEM, focusing and stigmator adjustment are most

conveniently performed using a frame time of 1 to 5 s. An expedient

technique is to reduce the display screen raster size to 3 by 3 cm or 4

by 4 cm instead of the standard 10 by 10 cm to eliminate slow scan

flicker and aid focus and stigmator adjustment by providing a more continuous image.

A separate monitor which scans at a television rate is a versatile

means for quick examination of semiconductor devices. However, the 500-

line picture is scanned in 1/30 s giving a very short scan time per picture point and reducing the SNR greatly. To increase the SNR for adequate viewing, it is necessary to increase the beam current and consequently beam size which in turn reduces the PPR. The resulting picture

is useful as a visual display for rapid search and for the study of dynamic effects but seldom is suitable for good photomicrographs.

7 . Artifacts Introduced by the Beam

In scanning electron microscopy, three of the most common artifacts introduced by the primary electron beam are electrostatic charging, vacuum pump oil poljnnerization, and electron beam damage. These

artifacts present special problems for examining semiconductor devices

because of the materials used in their construction and the fact that

active electronic components are susceptible to radiation damage.

Electrostatic charging is due to the presence of a negative potential on an insulating portion or point of the specimen surface. Charging is caused by the collection of electrons on the surface of the insulator. The accumulation of electrons builds up a space charge which

can deflect the incident beam, causing severe image distortion. The

presence of the surface charge will also change secondary emission

greatly. A charged area can cause the darkening of the image of a nearby uncharged region by deflecting the emitted secondaries away from the

detector. The charging effect is time dependent since a charged region

may discharge by a breakdown to ground and then the region can recharge.

Portions of semiconductor devices charge readily, because of the

insulating materials used in their fabrication. Glass, alumina, and

silicon dioxide are examples of insulating materials used as substrates,

packages, or passivating layers in semiconductor fabrication. These materials, along with nonconductive contaminant particles, are possible

sites for charging.

In order to recognize how charging manifests itself, some examples

are given. One example has already been seen in figure 12a in the discussion of tilt angle. Figure 14a shows glass passivation which has become charged over a metallization pattern. A single nonconducting particle whose image has become extremely bright is shown in figure 14b.

As a result of the high negative potential at its surface, the secondary

electron emission has been reduced in the vicinity of the particle.

26
A glass passivation layer which has become

charged by the electron beam.
It is not necessary for an area of the specimen to be in the field

of view for it to become charged. There can be a charge build-up where

scattered electrons strike a surface. Figure 15a shows a device which

had been previously viewed with an accelerating voltage of 10 kV at a higher magnification. The raster area during that exposure is shown by

the darkened region near the top of the device. The electrons scattered

from this area formed a charge build-up on the ceramic package in the

area that appears bright (indicated by the arrow) . The darkened area

around this bright spot indicates the suppression of secondary electron

emission in the vicinity of the charged region. The effect of this suppression can be seen more clearly in the next two figures which show

two versions of the high magnification view. Figure 15b is a micrograph

made before the charge build-up while figure 15c was made afterward.

Charging of the package has suppressed a wide range of signal electrons

and reduced the contrast even though the charged area is not located

in the area of the micrograph.

Charging is often seen as a distortion of the image in the photomicrograph. Figure 16a shows the characteristic bright areas of charging,

but in addition there are bright lines breaking the continuity of the

image at the top and center. These are caused by unsynchronized discharges of the charged specimen surface during scanning. Figure 16b is

an example of a particularly deceptive charging syroptom. There is poor

PPR which no amount of manipulation of the SEM controls could significantly improve. In this case, there is a charge build-up in the residual glass left behind by poor etching. As a result, the primary beam

cross-section is distorted from its normal circular shape to that of an ellipse which results in a loss of PPR. This distortion is very similar

to the uncorrectable instrumentation astigmatism caused by dirty apertures in the electron beam column. Figure 16c shows severe image distortion of a metallization window. The entire raster has been slowly

deflected by the charged residual glass during the exposure of the micrograph. This effect is best seen in the long blurred streaks in the upper half of the image.

It is not difficult to see that charging can render SEM images useless. Reducing the electron beam current will reduce but not eliminate

charging. Two techniques for reducing the effects of charging are illustrated in figure 16d. The penetration of primary electrons into an insulating surface, and their storage on and beneath the surface is the cause of charging. In figure 16d, a microcircuit similar to the microcircuit in figure 15 is shown. However, the high magnification raster

at the top of the die in figure 16d was irradiated using a high tilt

angle and a 3-kV accelerating voltage instead of 10 kV as in figure 15c.

The energy of the backscattered electrons is 3 keV which is too low for

these electrons to penetrate and charge the top of the die well, unlike

the 10-keV backscattered electrons which caused the charging seen in

figure 15a. In the section on specimen tilt, figure 12 demonstrates

that specimen tilt could be used as a method to reduce electron penetration, and, therefore, charging. Using low accelerating voltage and

specimen tilt is the most expedient means of preventing charging. It

28
a. A low magnification photo of a microcircuit which has just been ex amined at a higher magnification. The dark area of the circuit

shows the area scanned. Backscattered electrons have caused charg

ing of the ceramic package which has caused a darkening. (See arrows .

)

b. A circuit similar to that shown in (a). This micrograph was made

shortly after the beam was turned on and shows adequate contrast

and topographic detail.

Figure 15. Micrographs which show the effects of charging of areas outside the field of view.

29
The same area as shown in (b) after a long exposure to the electron

beam. There is a reduction in the number of secondary electrons

detected which results in a reduction in contrast. This is due to

charging which is occurring outside the field of view.

30
A sample showing typical bright areas of charging and horizontal

streaks caused by unsynchronized discharges.

Poor PPR which cannot be corrected by any adjustment of time SEM

controls

.

Figure 16. Additional examples of the effects of charging.

31
Severe image distortion caused by charging of residual glass not

removed by the etching procedure.
can be seen in figure 16d that another by-product of these techniques

is better contrast. The contrast in the dark square area in figure 16d

is rich in comparison to that in figure 15a.

Figure 17a shows a secondary electron image of a device with a charged passivation layer. Figure 17b shows a backscattered electron

image of the same area. The characteristic dark areas adjacent to regions of charging is a result of the distortion in the paths of secondary electrons while higher energy backscattered electrons are not affected. Therefore, using backscattered electrons will eliminate contrast distortion due to charging. A treatment of electron beam interactions with metals and insulators and effects of specimen tilt and accelerating voltage has been given by Oatley.^^

One technique commonly used to prevent specimen charging, applying

a conductive coating over the sample, has not been covered. Because of

the nature of semiconductor devices, such a coating should be a last resort and undertaken only after careful consideration of all phases of

the examination to which the device will be subjected. Once coated, a device can no longer be operated nor will use of voltage contrast and

EBIC modes of SEM examination be possible. X-ray analysis can become

more difficult after application of any coating but carbon. See Appendix II for further discussion of specimen preparation.

Scanning a semiconductor surface at high magnification can cross- polymerize hydrocarbon monolayers deposited by an oil-pumped vacuum system. This effect appears as a very dark square area as in figure 18.

Such contamination is an unwanted artifact, and can be avoided by not

making prolonged high magnification scans on a localized area. All focusing should be done outside, but adjacent to, the surface area to be

photomicrographed . Alternatively, the hydrocarbon diffusion pump oil

can be replaced with an oil that will not be polymerized by the primary

beam. A good alternative is found in perfluorinated polyether oil.

This oil fractures into molecules of low molecular weight which are

swept away by the vacuum system. Another method is to employ a cold

finger or trap to help reduce specimen contamination by capturing the

oil contaminates on a cold surface. Pump oil contamination is not a problem with an SEM equipped with a dry-pumped vacuum system.

The SEM primary electron beam damages the semiconductor device.

Electron beam penetration into the device results in ionization processes which change the surface oxide properties, and thus degrade the

electrical parameters. Digital bipolar devices are very resistant to

damage due to electron beam irradiation. Linear bipolar devices are susceptible to parameter changes during SEM examination; however, it is

unlikely that it would cause them to cease functioning. MOS devices

are readily damaged by exposure to an electron beam. Even a very short

exposure can make them inoperative. ^

For devices for which optimum electrical performance is required

after examination, the only way to eliminate electron beam damage is to

33
Secondary electron image of circuit with charged glass passivation.

Backscattered electron image of the area shown in (a) . The high

energy reflected electrons are not substantially perturbed by any

charge in the glass.

ure 17. The use of backscattered electrons rather than secondaries

to form the image to eliminate charging effects.

34
igure 18. Contamination due to the polymerization

of pump oil.

35
not examine the device with an SEM. However, if an SEM examination

must be performed, an accelerating voltage as low as possible considering the resolution which is required should be used. Also, the lowest

practical beam current should be used and the specimen should be irradiated only as long as absolutely necessary. Appropriate alignment techniques are required so that time is not lost searching for the area of

interest. If the examination does not include voltage contrast or EBIC

modes, a bias should not be applied to the device junctions. It should

be noted in subsequent use of the device that the electron beam. could

have made some irreversible changes in the device.

8 , Conclusion

SEM operation procedures must not become stereotyped for all semiconductor device applications if optimum images are to be obtained.

This report demonstrates the need for flexibility in selecting and using

SEM parameters and analytic procedures to yield the maximum information

from semiconductor devices. Table 1 summarizes the results of varying

significant SEM operating parameters.

The primary consideration in topographical analysis of semiconduc--

tor devices in the SEM is the reduction of primary electron beam penetration. The methods described for accomplishing this are low accelerating voltage and high specimen tilt. These techniques result in improved SNR and contrast, less charging, and, in general, good resolution.

Although increasing the electron beam current is effective in improving

SNR, low accelerating voltage is superior. Both techniques reduce PPR,

but high beam current does not provide the striking contrast which low

voltage does. Also, high beam currents may result in specimen charging,

increased hydrocarbon cross-poljmierization, and radiation damage. High

accelerating voltage is to be used both where maximum PPR must be obtained and to yield subsurface information such as beneath contamination

or glassivation layers. These techniques were discussed chiefly in

terms of secondary electron emission. However, suggestions were given

for using the backscattered electron signal to prevent image distortion

due to charging and to gain detail in topography, for using voltage contrast for detecting specimen voltages, and for using EBIC to delineate

pn junctions.

The principles established for selecting a line and frame rate

should be applied no matter what set of signals is used. A 40 s frame

rate and a 0.04 s line rate (1000 scan lines) are frequently applicable.

In any case, specimen coating is to be considered as a last resort to

improve SNR or reduce charging.

36
* c 0) 0) OJ <u cn (u to m to to lA C E (O fO rO <o ta

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37
Appendix I

Voltage Contrast

A voltage contrast image is obtained in the normal secondary electron mode where a potential difference is applied on the specimen surface. This is due to the fact that a secondary electron image is formed

by the collection of low energy electrons (<_ 50 eV) . Surface potentials

and electric field gradients near the specimen surface easily affect the

trajectories of these low energy electrons and modify their detection.

Fewer electrons are collected from areas of positive potential and more

electrons are collected from areas of negative potential. In the sec- ondary electron image, positive areas appear dark and negative areas appear bright. This voltage contrast effect can be easily observed for

potential differences greater than 0.5 V.

Shown in figure 19 is a voltage contrast image of a section of a quad two-input NAND gate with +5 V applied to both inputs (marked A and

B) to force the output (marked Y) low. The voltage contrast technique

is useful for detecting potential differences such as at discontinuities

in metallization, gradients across resistor stripes, or at p-n junctions.

It is a valuable tool for semiconductor analysis.

To examine a device in the voltage contrast mode, it is necessary

to be careful about charging since charging adds a stray electric field

that also influences the paths of the secondary electrons. It is difficult to obtain good voltage contrast from devices mounted in recessed

ceramic packages. Glassivation should be removed, and stray nonconducting particles should be blown off the surface. An accelerating voltage

between 2 and 10 kV is recommended and the specimen should be tilted toward the secondary electron detector. Further details on the detection

of voltage contrast will be found in references by Holt, et at. and

Wells. 1^

Voltage contrast has been used dynamically to check the operation

of semiconductor devices. This is usually done using stroboscopic techniques and TV scan rates. ^^"-^^

Electron-Beam-Induced Current

Electron-beam-induced current (EBIC) in semiconductor devices produced by the electron beam of an SEM can be used to image subsurface device features and to measure certain material parameters. The energy

deposited by the SEM electron beam can be considered primarily as a mechanism for electron-hole pair production in the device materials.

For a simple pn junction, the pairs created in the junction depletion

region are immediately separated by the built-in electric field and can be detected in an external circuit. Electron-hole pairs created in either the n- or p-type regions which diffuse to the junction will also be

38
ure 19. A secondary electron image of a biased

microcircuit showing voltage contrast.

39
separated and collected. EBIC can be used for SEM imaging by modulating

the intensity of a synchronously scanned cathode-ray tube.

EBIC has been used to delineate junctions, locate inversion layers,

measure diffusion lengths and carrier lifetimes, detect crystal imperfections, and measure surface recombination. ' ^

Photomicrographs of an interdigitated npn transistor are shown in

figure 20. Figure 20a is a routine emissive micrograph of the transistor. Figure 20b is an EBIC image when the emitter is the input and the

base and collector are grounded. The accelerating voltage was 30 kV

with a beam current of 350 pA for both photographs.

For an integrated circuit, EBIC imaging can result in a very com- plex display. However, EBIC analysis has been successfully applied to

integrated circuits in a comparative mode. This has been used primarily

as a technique for failure analysis. In circuits of a repetitive nature

such as multiple gate devices, multiple bit shift register, flip-flops,

or any circuit where a circuit function is duplicated, it is often possible to compare "good" and "bad" regions on the same chip. Where a duplication of function on a single chip is not available, the EBIC display

from a defective circuit can be compared with a good circuit. Obviously,

the SEM operating conditions must be held constant from specimen to specimen for meaningful comparisons.

As an example of the comparative technique, a defective low-power

four -bit shift register was compared with a functional device. The defective device failed to shift the input data. The images were obtained

by taking the induced current signal from the substrate with the power

input lead grounded and all other leads floating. The accelerating potential was 20 keV and the beam current was approximately 1 nA. Figure

21a is the EBIC image of a portion of the defective device. For comparison, the same region of a functioning device is shown in figure 21b.

The region of difference is indicated by the arrows. In the image of

the good device, a junction associated with the parallel enable input is

visible, but it is not seen in the defective device. Further examination of this area using other failure analysis techniques confirmed that

this junction was the source of malfunction. In this instance, EBIC

analysis did not identify the exact cause of failure, but it rapidly

identified a small area for further investigation.

When performing an EBIC examination, due to the low signal levels

involved, shielded coaxial cables must be used for all interconnections

and a low-noise, high-gain video amplifier is required. The amplifier

gain is adjusted to obtain the desired contrast. The beam accelerating

voltage is adjusted to control the beam penetration to allow signal collection from the desired depth. A higher than normal beam current might

be necessary to provide the sufficient induced current. The specimen is

usually not tilted and charging is usually not a problem.

40
a. Secondary electron image.

b. EBIC image of the emitter with the base and collector grounded.

Figure 20. Micrographs of an interdigitated npn transistor made with a beam current of 350 pA and a beam voltage of 30 kV.

41
a. Defective shift register.

42
Appendix II

Semiconductor Specimen Preparation

One of the primary advantages of scanning electron microscopy over optical and transmission electron microscopy is the minimum amount of

specimen preparation necessary. However, some preparation is required

and if the study is a failure analysis, the microscopist must take care that the preparations do not obscure the cause of failure being studied.

An examination with an optical microscope should be made of a semiconduc

tor device immediately after delidding the package. The analyst should

observe any contamination that may be present and which appears to be

the result of device failure or poor fabrication. If any solder balls,

fragments of broken passivation, or smeared metallization are noted, the

sample surface should not be disturbed or valuable information could be

lost. Photo documentation of the optical examination is desirable.

If the optical examination did not reveal any obvious device problems, the glass passivation layer should be removed from the semiconductor surface. Typically, the glass is removed by a wet acid or gas plasma etch. This is necessary because the glass does not repliccite the underlying metallization stripes or oxide steps. Inadequate removal of

the glass will leave a residue of glass on the metallization which would

make topographical interpretation difficult. The residue also charges,

which will normally manifest itself in an inability to focus as illustrated in figure 16c. The specimen should be firmly fixed to the specimen holder using an electrically conductive adhesive or its equivalent

and the holder should be firmly mounted in the stage and make electrical

contact to it.

Coating the specimen with a thin (5- to 20-nm thick) layer of car- bon or metal may be used to enhance secondary emission and reduce charging. However, this deposited coating has potential drawbacks: its application is time consuming, there is die surface detail which is covered by the thin film coating, and the device is rendered inoperable.

Despite these drawbacks, specimen coating is the method of suppressing

charging and increasing the electrical emissivity of a specimen most often utilized. When applied with the specimen surface parallel to the

rays of evaporating coating material, subtle changes in surface topography can be accentuated by shadowing.

In order to reduce the amount of data which is obscured by specimen

coating, certain guidelines should be observed. The selection of the

evaporant material is the first consideration. Criteria for selection

of a material include: electron emissivity, replication ability, oxidation rate, and melting point. Gold has excellent electron emissivity,

but, at high magnifications, has an agglomerate appearance. Silver has

the best replicating ability, but, in a few days, it tarnishes and the

specimen cannot be viewed. Gold-palladium alloy is a compromise between

high electron emissivity and replicating ability, but the heat from its

43
high melting point may alter the die surface. Aluminum is a good alternative, having good replicating ability, a low melting point, and adequate electron emissivity. On a very irregular specimen surface, carbon

is often used on a first layer to establish surface conductivity because

its atoms deflect from surrounding surfaces and reapproach the semiconductor surface from many different angles. A second, very thin film of a noble metal is then applied to improve electron emissivity. For x-ray

microanalysis, carbon can be used exclusively to establish conduction,

while avoiding extraneous x-ray information.

The film should be the thinnest possible coating that will suppress

charging. The best procedure is to deposit a very thin film of a noble

metal (<10 nm thick) and then view the specimen in the SEM to see if the

charging has been suppressed. This procedure can be repeated as often

as necessary to satisfactorily suppress charging. Although very time

consuming, this procedure will limit the amount of die surface data lost.

An evaporated thin film will not be uniform on a semiconductor die

surface because of the irregular metallization and oxide steps. A gimbal mechanism may be used to spin the specimen through a complex movement exposing each die surface step for an equal amount of time to multiple evaporate sources, but it is still doubtful that the thin film will

be truly uniform. There are two reasons for this. First, there are variations in the specimen source distance while spinning and the thickness of an evaporated thin film depends on the inverse square law. Also,

there are changes during evaporation in the nominal angle of deposition.

To prevent the evaporant source heat from altering die surface topography during evaporation, three precautions should be taken. (1) The

specimen should be kept at least 3 to 4 cm from the evaporant source(s).

(2) A pre-evaporation shield should be kept between the evaporant source

and specimen until the coating material has begun to evaporate. (3) A

coating material with a low melting point, such as aluminum, should be

used.

An alternative to vacuum evaporation as a specimen coating technique is sputter coating. In the evaporation technique, the coating

material is heated to a sufficiently high temperature so that it evaporates, depositing a thin film on the die surface. In the sputter coating technique, atoms from the coating material are ejected when bombarded by relatively high energy particles. Some of the ejected atoms

will land on the die surface to be coated. The particles must be of

sufficiently high energy in order to overcome the binding energy of the

atoms at the surface of the target. The fast heavy particles used to

erode the target are usually derived from an ionized inert gas, and are produced by three main processes: ion-beam sputtering, radio-frequency

sputtering, and direct current sputtering.

The main advantage of sputter coating is that a continuous thin

film is deposited even on parts of the die surface which are complex,

and those surfaces which are not directly facing the rays of sputtered

4A
atoms. This occurs due to the fact that, under the high pressures used,

the coating material atoms experience many collisions and approach the

die surface from all directions. This effect is achieved without using

a gimbal movement. Therefore, the sputter coating techniqtie has good

surface replication ability, and there is a reduced chance of surface

damage by heating. Noble metals should be used because of their emissivity. Aluminum is not adaptable to this technique because its surface

oxide prevents sputtering.

The techniques advised for specimen coating should be rigorously

followed. Thin film evaporations which are performed improperly will

ruin the possibility for SEM examination. Figure 22 pictures a microcircuit window which has been buried under large globs of aluminum from a poor evaporation process. This specimen is a total loss for examination

purposes because of the excessive artifacts introduced.

45
Figure 22. A microcircuit window covered by globs

of metal introduced by poor evaporation

techniques

.

46
References

1. Goldstein, J. I., and Yakowitz, H. , eds . , VvaQtioal Seanning Eleatvon Microscope and Electron Prohe Microanalysis (Plenum Press

,

New York, 1975)

.

2. Hearle, J. W. S., Sparrow, J. T. , and Cross, P. M., The Use of the

Scanning Electron Microscope (Pergamon Press, Oxford, 1972).

3. Holt, D. B., Muir, M. D., Grant, P. R. , and Boswawa, I. M. , eds..

Quantitative Scanning Electron Microscopy (Academic Press, London,

1974).

4. Johari, 0., ed., Scanning Electron Microscopy/1976/I (Part IV) (IIT

Research Institute, Chicago, 1976).

5. Leedy, K. 0. , A Bibliography on Electron Beam Induced Current Analysis of Semiconductor Devices, Solid State Technology 19_, No. 2,

45-48 (1977).

6. Pease, R. F. W. , Fundamentals of Scanning Electron Microscopy,

Scanning Electron Microscopy/1971 (Part JJ , pp. 9-16 (IIT Research

Institute, Chicago, 1971).

7. Thornton, P. R. , Scanning Electron Microscopy - Applications to Materials and Device Science (Barnes and Noble, Inc., New York, 1968).

8. Wells, 0. C, Scanning Electron Microscopy (McGraw Hill, New York,

1974)

.

9. Wittry, D. B., Electron Beam Interactions in Solids, Scanning Electron Microscopy/1970 , pp. 409-416 (IIT Research Institute, Chicago,

1970).

10. Everhart, T. E., and Thornley, R. F. M. , Wide-band detector for

micro-microampere low-energy electron currents, J. Sci. Instrim. 37 , 246-248 (1960).

11. Broers, A. N., High-Resolution Scanning Electron Microscopy of Surfaces, Microprohe Analysis^ C. A. Anderson, ed., pp. 83-121 (Wiley,

New York, 1973)

.

12. Oatley, C. W., The Scanning Electron Microscope y pp. 148-167 (Cambridge University Press, 1972).

13. Galloway, K. F., Keery, W. J., and Leedy, K. 0., Integrated Circuit Damage Resulting from SEM Examination, Proc. 25th Annual Electronics Components Conference, Washington, D.C., May 12-14, 1975,

pp. 263-266.

47
14. Keery, W. J., Leedy, K. 0., and Galloway, K. F., Electron Beam

Effects on Microelectronic Devices, Soanning Electron Miaroscopy/

1976/1 (Part IV), pp. 507-514 (IIT Research Institute, Chicago,

1976)

.

15. Holt, D. B., Muir, M. D. , Grant, P. R. , and Boswawa, I. M. , eds .

,

Quantitative Scanning Electron Microscopy y pp. 117-123 (Academic

Press, London, 1974).

16. Wells, 0. C., Sca'rining Electron Microscopy ^ pp. 180-202 (McGraw

Hill, New York, 1974)

.

17. Plows, G. S., and Nixon, W. C., Stroboscopic Scanning Electron Microscopy, J. Sci. Instnm. (J, Phys. E.) Series 2, Vol. 1, 1968,

pp. 595-600.

18. Plows, G. S., and Nixon, W. C., Operational Testing of L.S.I. Arrays by Stroboscopic Scanning Electron Microscopyj Microelectronics

and Reliability (GB) 10, 317-323 (1971).

19. Gopinath, A., and Hill, M. S., A Technique for the Study of Gunn

Devices at 9.1 GHz Using a Scanning Electron Microscope, IEEE Trans.

Electron devices ED-20 , 610-612 (1973).

20. Holt, D. B., Muir, M. D. , Grant, P. R. , and Boswawa, I. M. , eds.,

Quantitative Scanning Electron Microscopy , pp. 213-286 (Academic

Press, London, 1974).

21. Thornton, P. R. , Scanning Electron Microscopy - Applications to

Materials and Tievice Science, pp. 217-243 (Barnes and Noble, Inc.,

New York, 1968)

.

22. Galloway, K. F. , Leedy, K. 0., and Keery, W. J., Electron-BeamInduced Currents in Simple Device Structures, IEEE Trans. Parts

y

Hybrids^ and Packaging PHP-12 , 231-236 (1976)

.

48
NBS-114A (REV 7-73)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No. SHEET NBS Spec. Publ. 400-35

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Semiaonduator Measurement Technology: Notes on SEM

Examination of Microelectronic Devices

5. Publication Date April 1977

6. Performing Organization Code

7. AUTHOR(S)

John R. Devaney, K. 0. Leedy, and W. J. Keery

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Hi-Rel Laboratories

911 S. Mountain Avenue,

Monrovia, CA 91016

10. Project/Task/Work Unit No.

11. Contract/Grant No. 4-35897

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

iNdLiunai Dureau ot oTanaaras, wasmngton, UL ^:U<io4 ARPA, 1400 Wilson Boulevard, Arlington, VA 22209

13. Type of Report & Period Covered Summary Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

This work was funded by the Defense Advanced Research Projects Agency under

ARPA Order 2397, Program Code 4D10. Library of Congress Catalog Card Number: 77-608011

16. ABSTRACT (A 200-word or less factual summary of most sigriificant information. If document includes a significant hiblinSraohv or literature survey, mention it here.)

This report reviews selected scanning electron microscope (SEM) techniques which are appropriate for the examination of microelectronic devices. Illustrated are the re- sults of individual variations in SEM operating parameters such as accelerating voltage, specimen tilt, scan line time, and frame time. Techniques which utilize secondary and backscattered electron emissions are compared and electron-beam-induced current and voltage contrast modes are discussed. Specimen preparation and beam-induced

charging artifacts are also discussed. This report demonstrates the need for flexibility in selecting and using SEM parameters and analytic procedures to obtain the

maximum information when examining semiconductor devices.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Backs cattered emission; EBIC; microelectronic devices;

scanning electron microscope; secondary emission; voltage contrast.

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Studies and compilations designed mainly for the mathsmatician and theoretical physicist. Topics in mathematcal statistics, theory of experiment design, numerical

liralysis, theoretical physics and chemistry, logical design and programming of computers and computer sys-

:ems. Short numerical tables. Issued quarterly. Annual

Subscription: Domestic, $9.00; Foreign, $11.25.

i

j DIMENSIONS/NBS (formerly Technical News Bulle-

'tin) —This monthly magazine is published to inform

entists, engineers, businessmen, industry, teachers,

Si idents, and consumers of the latest advances in

ijscience and technology, with primary emphasis on the

' "A' irk at NBS. The magazine highlights and reviews

h issues as energy research, fire protection, building

ctchnology, metric conversion, pollution abatement,

[health and safety, and consumer product performance.

In addition, it reports the results of Bureau programs

I in measurement standards and techniques, properties of

: matter and materials, engineering standards and services, instrumentation, and automatic data processing,

i Annual subscription: Domestic, $12.50; Foreign, $15.65.

{ NONPERIODICALS

I Monographs—Major contributions to the technical liter-

{ ature on various subjects related to the Bureau's scienjtific and technical activities. Handbooks—Recommended codes of engineering and

(i industrial practice (including safety codes) developed

in cooperation with interested industries, professional

I I organizations, and regulatory bodies.

iSpecial Publications—Include proceedings of conferences

j sponsored by NBS, NBS annual reports, and other

ii ' special publications appropriate to this grouping such

i| as wall charts, pocket cards, and bibliographies.

!| Applied Mathematics Series—Mathematical tables, manI uals, and studies of special interest to physicists, engii'j neers, chemists, biologists, mathematicians, comI ' puter programmers, and others engaged in scientific

;

i and technical work.

I National Standard Reference Data Series—Provides

[ quantitative data on the physical and chemical proper-

' ties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

jj program coordinated by NBS. Program under authority

' of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for

these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements available from ACS, 1155 Sixteenth

St. N.W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental

functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under proce- dures published by the Department of Commerce in Part

10, Title 15, of the Code of Federal Regulations. The

purpose of the standards is to establish nationally rec- ognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series—Practical information,

based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent

of Documents, Government Printing Office, Washington,

D.C. 20402. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Springfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for

outside sponsors (both government and non-government). In general, initial distribution is handled by the

sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
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