A11103 fl7^b7

QC100 ^FECIAL PUBLICATION 400-27

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

Semiconductor Measurement lechnotogy:

Laser Scanning of Active

Semiconductor Devices

Videotape Script
NATIONAL BUREAU OF STANDARDS

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Semiconductor Measurement Technology:

Laser Scanning of Active

Semiconductor Devices-Videotape Script

David E. Sawyer and

David W. Berning

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

APR 2 0 197$ CPC/00

This activity was supported by

The Defense Advanced Research Projects Agency and The National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE

James A. Baker, III, Under Secretary

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

U 5 NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director Ml Issued February 1976
Library of Congress Cataloging in Publication Data

Sawyer, David E. Laser scanning of active semiconductor devices—videotape

script. (Semiconductor measurement technology) (NBS special publi- cation ; 400-27) "This activity was supported by the Defense Advanced Research Projects Agency and the National Bureau of Standards."

Supt. of Docs. No.: C 13.10:400-27

1. Semiconductors—Testing. 2. Optical scanners. 3. Lasers. L Berning, David W., joint author. IL Title. III. Series. IV. Series: United States. National Bureau of Standards. Special publication ; 400-27. QC100.U57 no. 400-27 [TK7871.85] 602Ms.

[621.3815'2'028] 75-619427

National Bureau of Standards Special Publication 400-27

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-27, 26 pages (Feb. 1976) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For sale by the Superinlendenl of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SO Catalog No. C13.10:400-27). Price $1.20 (Add 25 percent additional for other than U.S. mailing).
PREFACE

This is the script for the second in a series of videotape presentations being

distributed on loan without charge in order to disseminate more effectively to the

semiconductor community the m.easurement technology improvements developed under the

Semiconductor Technology Program in the Electronic Technology Division of the National

Bureau of Standards (NBS).

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. Its major thrusts are the

development of carefully evaluated and well documented test procedures and associated

technology and the dissemination of such information to the electronics community.

Application of the output by industry will contribute to higher yields, lower cost,

and higher reliability of semiconductor devices. The output provides a common basis

for the purchase specifications of government agencies which will lead to greater

economy in government procurement. In addition, improved measurement technology

will provide a basis for controlled improvements in fabrication processes and in es- sential device characteristics.

The Program receives direct financial support principally from three major

sponsors: The Defense Advanced Research Projects Agency (ARPA),* the Defense Nuclear

Agency (DNA),"'" and the National Bureau of Standards.^ The ARPA-supported portion of

the Program, Advancement of Reliability, Processing, and Automation for Integrated

Circuits with the National Bureau of Standards (ARPA/IC/NBS) , addresses critical

Defense Department problems in the yield, reliability, and availability of integrated

circuits. The DNA-supported portion of the Program emphasizes aspects of the work

which relate to radiation response of electron devices for use in m.ilitary systems.

There is considerable overlap between the interests of DNA and ARPA. Measurement

oriented activity appropriate to the mission of NBS is a critical element in the

achievement of the objectives of both other agencies.

Essential assistance to the Program is also received from the semiconductor industry through cooperative experiments and technical exchanges. NBS interacts with

industrial users and suppliers of semiconductor devices through participation in

standardizing organizations; through direct consultations with device and material

suppliers, government agencies, and other users; and through periodically scheduled

symposia and workshops. In addition, progress reports are regularly prepared for

issuance in the NBS Special Publication 400 sub-series. More detailed reports such

as state-of-the-art reviews, literature compilations, and summaries of technical efforts conducted within the Program are issued as these activities are completed.

Reports of this type which are published by NBS also appear in the Special Publication 400 sub-series. Announcements of availability of all publications in this subseries are sent by the Government Printing Office to those who have requested this

service. A request form for this purpose may be found at the end of this report.

Through ARPA Order 2397, Program Code 6D10 (NBS Cost Center 4257555).

''Through Inter-Agency Cost Reimbursement Order 76-806 (NBS Cost Center 4259522).

'^Through Scientific and Technical Research Services Cost Center ^4257100

.

iii

Semioonduator Measurement Technology

Laser Scanning of Active Semiconductor Devices

Videotape Script

by

David E. Sawyer

and

David W. Berning

This is the script of a videotape presentation which describes new

and powerful applications for laser scanning in semiconductor device

design and reliability work. The design of the scanner is described in

detail and many of its applications are displayed and discussed. The optical scanner can, in a completely nondestructive way, reveal the inner

workings of semiconductor devices. For example, it is shown that the scanner can (1) map dc and high-frequency gains in transistors, (2) reveal

areas of the device operating in a nonlinear manner, (3) electronically

map temperature in the transistor, and (4) detect the location of hot

spots that can develop for certain operating conditions. The vehicle used

to show these capabilities of the scanner is a bipolar interdigitated UHF

transistor. A dual input NAND gate is used to demonstrate the use of the scanner to determine internal logic states and otherwise observe internal

operation of the circuit. To show the ability of the scanner to examine

MOS devices without detectable degradation, a MOS shift register is used.

The location and progress of internal logic in the register is clearly

shown by the scanner. Not only can internal logic be mapped and marginallyoperating logic cells detected, but individual logic states can be changed

by the scanner without affecting other elements.

Key Words: Failure analysis; hot spots; integrated circuits; laser scanner; measurement method; nondestructive test; nonlinear operation; optical

scanner; reliability; transistors.

Sawyer

* Hello. My name is David Sawyer, and seated next to me is David Berning. We (000) are staff members of the Electronic Technology Division at ihe National Bureau of

Standards, Washington, D. C, and we would like to tell you about our work with an optical scanner which we developed. This scanner has been used to reveal, in a com- pletely non-destructive way, the inner workings of semiconductor devices by taking

advantage of the photosensitivity of semiconductor materials. We have looked both

at discrete devices and at integrated circuits (IC's) with it.

The effect of scanning a device is to create current-carriers inside to excite

it electrically, and this allows us to map the operation of the device over its active

*Tape counter setting.

1
area. In nan> cases, this can be done while the device is functioning in its usual (018) manner in a circuit. Ve will demonstrate this with photographs showing the results

of scennine bipolar and >iO£ IC's to rc^veal internal logic states, and we will show

tiiat the scanner may also be used to change the state of an embedded logic cell re- versibly. Then a discrete device, a UKF transistor, will be used to demonstrate

sciiie of our newer scanner applications by revealing the location of hot-spots, and

Trapping regions where the transistor is operating in a nonlinear fashion. Some of our latest work with the sane transistor type suggests that the scanner can be used

also to map electronically the temperature variations within functioning devices.

During the presentation, we will be making sug.gestions as to how scanner setups such

as this m.ight be used in device design and also used to assist in evaluating device

reliability. But first, we would like to tell you briefly hov7 the scanner works and

list som.e of its attributes. David?

Berning

In the design of the scanner we have incorporated ideas from Sawyer's previous

efforts [1], as well as ideas obtained from other sources [2]. The scanner is quite

simple in principle. Let me show you how it works. [Graphic 1] On the upper right,

optical radiation from a low-power cw helium-neon laser is reflected in sequence

from, four mirrors. The first two fold the optical path to make the scanner more

compact and rugged. The second two, labeled V and K, are vertical and horizontal

deflection m.irrors, respectively, which oscillate in orthogonal directions. The

laser beam is directed through the cam- era tube of a standard microscope onto

the specimien. The beam is focussed toa small spot on the specimen and is deflected across it in a raster pattern.

The same electrical sources that drive

the deflection m.irrors also deflect the

spot on a cathode ray display screen in

synchronism with the laser scan. The

display screen is on the lower left of

the chart. By electrically connecting

the scanned specimen to modulate the

spot on the display screen, a map of

the device's photoresponse is obtained. Let's go back to the mirror scanning

DISPLAY

m^

MIXER

REFLECTED LIGHT

Graphic 1

[1] Potter, C. N. , and Sawyer, D. E., A Flying-Spot Scanner, Rev. Sci. Instrum. 39 , 180-183 (1968).

[2] McKahon, R. E. , Laser Tests IC's with Light Touch, Electronics 44, 92-95 , (April 12, 1971).

2
systein. The lens L-, between the vertical and horizontal deflection mirrors refocus- (080) ses the vertical deflection, fron; V , onto the horizontal pirror, H [3]. The bearr diverges from the horizontal deflection rriirror, H, to form the scanning raster.

>';irror 11 is located at the exit-pupil point of the microscope camera tube. The scan raster typically covers all of the field of view that can be seen with the eye when

the microscope is used in its usual manner. For the microscope which we have, it is not possible to scan the specimen and observe tb.e specimen with the eye at the same

time. Switching between these two functions is performed by moving a prism within

the microscope. Perhaps this is a good thing, for even though the lasers we use are low-powered there always is the possibility that under certain conditions the laser

beam, might be reflected directly into the eye and cause som.e discomfort. But it would be useful if we could always know which portion of a specimen is being scanned,

and this is one of the main functions of the ref lected-light circuit. The reflectedlight circuit makes use of a half-silvered mirror in the microscope, lers 1^2, and a photocell. The mirror com;es as an integral part of the m.icroscope vertical illumii- nator. Laser radiation reflected from the specim.en is directed by the half-silvered

mirror onto the lens and photocell. The photocell signal ir;odulates the display

screen to present a picture of the device surface topography. Used this way, we

could call the apparatus a "flying-spot microscope". It is useful in its own right,

and of course no electrical connections to the specimens are required. But the main

purpose of the flying-spot scanner is to learn the specific details of device operation by monitoring the electrical effects of electron-hole pairs photogenerated

within the device, and so the primary purpose of the reflected-light circuit is to pinpoint the device response with relation to surface features such as metallization

areas. This is accomplished simply by mixing together the signals from the scanned

specim;en and the photocell.

Sawyer

We extended the capability of our scanner by adding a second dedicated laser.

[Graphic 2] We can change between then by sliding a mirror — which carries the

second laser beam. from, the left — in or out of the path of the first laser beam. I will tell you the reason for having two lasers in a moment. But let us first finish

the description of the entire scanning system. [Graphic 3] This is the schematic

of the complete system and shows a few items which did not appear before. The laser

on the left operates at 0.633 ym, in the visible red, and the right one at 1.15 pm,

in the near infrared. They both are cw helium-neon lasers, one is equipped with

mirrors to permit it to oscillate at 0.633 um, and the other with mirrors for 1.15 ym. operation. The system also has a rotatable analyzer in the 0.633-ym laser path on

[3] Beiser, L. , Laser Beam Scan Enhancement through Periodic Aperture Transfer,

Applied Optics 7_, 647-650 (1968).

3
the left, a half-wave plate in the 1.15-pm path, on the upper ripht, and a radio- (15 frequency receiver in the speciir.en siena] path, just to the left of the nicrcscope

specinien stage.

Now let us show you what the scanner actually looks like. [Graphic 4] This is

a photograph of the scanner in our laboratory. A frar.ie holding all of the optics

for the systen is rigidly attached tc the microscope stand. One of the lasers is easily identified in the view. It is ir.ounted on the right h.and side of the frame.

An 8 X 10 display screen, on the left, provides a large display that can easily be seen by several persons. Tlie signal generators that provide the vertical and horizontal deflection signals for the scanning mirrors and tiie display can be seen above

the display screen. A preamplifier

Graphic 2

A/2 PLATE

» ^

-0 ^

R.RCVR

Graphic 3

and mixing amplifier are located

above the generators. The shelf above

the scanner holds some supporting

electronics for the system.. These

include the radio receiver, scanning

m.irror power amplifiers, laser pov/er

supplies, a dc to TOO khz laser modulator, and seme other power supplies

used for specimen excitation.

Ise added the second laser to the scanner to give us greater measurement flexability. This flexability

comes fromi the difference in penetration depths of the radiation from the two lasers. Light and near-infrared

radiation incident on silicon creates electron-hole pairs with a generation

rate which exponentially trails off

with distance into the material. The

penetration depends on the wavelength

of the incident radiation. For each

wavelength we can associate a characteristic penetration length as that

distance required to decrease the

pair generation rate to 36.8% of its

value at the surface. This 36.8%

value is simply the reciprocal of e, the base of the natural logarithm

system.. The 0.633-Mm visible light

from the first laser mentioned has a
Graphic 4

characteristic penetration depth in silicon of about 3 micrometers [4]. Because (198) most modern silicon devices have their active regions within a few micrometers of

the surface, the 0.633-ijm laser is quite effective in mapping active regions of op- erating devices. For this reason, this laser has been our "work horse" for device

scanning. The light from the laser is al.ready polarized, and so tlie rotatable analyzer I mentioned earlier provides a convenient way to adjust the illumination intensity. The intensity at the specimen can be varied to produce junction photocurrents over the range from about 10 picoamperes to about one-tenth of a milliampere.

Silicon at room temperature is almost transparent to 1.15 ym infrared radiation

from our second laser. The characteristic penetration depth of this radiation is about one centimeter [4]. Because of this weak absorption we need to utilize as much of the laser intensity as we can. There are several air-glass interfaces within the microscope which can attenuate the infrared radiation selectively, depending

on the specific angle of polarization at each interface, and so it is helpful to be

able to rotate the plane of polarization of the laser to find the optim.un incident

polarization angle through the system. This is the purpose of the half-wave plate

I mentioned earlier . The half-wave plate is rotated to yield a maximum specimen

signal.

We use the infrared laser for two classes of measurements, both of which make

use of the penetrating nature of the radiation. In tlie first, we can use the re- flected light circuit to look through the silicon wafer and observe irregularities

at the silicon-header interface. This application uses the scanner in the

[4] Dash, V/. C. , and Newman, R. , Intrinsic Optical Absorption in Single-Crystal

Germanium and Silicon at 77° and 300°K, Phys. Rev. 99_, 1151-1155 (1955).

5
"flving-spot microscope" mode v^e mentioned earlier. The second application makes use (238) of the temperature sensitivity of the silicon absorption. It produces a larger signal on the display screen for those device portions v/hich are warmer than others.

Utilizing this sensitivity, we have an electronic technique for thermal mapping of devices which appears to have a number of advantages over the more traditional methods. Vie will show you an example of this application later on.

Since the scanner's optical system, uses refractive optical elements, one has to refocus the scanning raster with the use of the reflected light circuit when one changes from one vjavelength to the other. But this is not too bothersome, although

a system, using only reflecting optics would obviate this minor Inconvenience.

During norm>al operation, the lasers produce optical radiation not at just one discrete wavelength but at a series of discrete wavelengths centered about the nominal wavelengtli. These wavelengths correspond to the individual allowed axial modes

of the laser. Mixing of these wavelengths has thic effect of modulating the light ^

simultanecvisly at several frequencies. The modulation frequencies are multiples of

a fundamental one, and for our visible laser, the light is self-modulated at 500 KHz

and 1.0 Gttz to a degree adequate for determining the response of devices to light

modulated at these frequencies. The corresponding frequencies for our infrared laser

are 385 and 770 MKz . We can determiine the response of a specim.en to optical radiation miodulated at these frequencies. Looking again at the diagram, of the scanning

system, [Graphic 5] we determine this response by inserting a radio receiver between

the specimen on the m;icroscope stage and the m.ixing amplifier and tuning the receiver

to the desired modulation frequency. This technique has been used to map the

frequency dependence of device ch.aracter istics and also R-C effects in distributed

passive structures.

To complete the coverage of the important attributes of the scanner we need to describe briefly the deflection

circuits, mixing amplifier, and display

screen. And, we should m.ake some state- ments about the optical resolution of

the system. David, if you will summarize these item^s, then we will move

along to show some results we have obtained using the scanner on operating

devices

MIXER

REFLECTED LIGHT

Graphic 5

6
Berning

The keynote of our scanner design was flexibility. IJe did not want the range (287) of applications to be limited by the equipment, so we designed the scanner to be as versatile as possible. For example, the rate of the horizontal or vertical mirror

deflection sweeps, either or both may be varied between 3 mHz and 1,1 kHz. The up- per frequency is established by mechanical resonance of the mirrors. The lowfrequency range allows us to observe phenomena which are intrinsically slow, carrier

trapping for example. But this is not a consideration in much of our work. We generally use a 1 kHz horizontal sweep frequency and a 3 Hz vertical sweep. This gives

us a 330 line scan which yields adequate definition.

The mixing amplifier was designed by us, and it is flat from dc to about 2 MHz. Filters are built-in whiich provide low- frequency roll-off when desired. The dc capability is a necessity when slow sweeps are employed. Its use is generally desirable at other times as well, since it yields a more accurate display; however, drifting and noisy specimens necessitate using the filters sometimes. The mixing amplifier contains three channels, which permit us to perform algebraic operations on the

amplifier inputs. For example, we can obtain the specimen output for the unmodulated

component of the laser radiation. U'e call this the specimen's video response.

Simultaneously we can pick off the specimen output at the laser R.F. frequency com.ponent

with the R.F. receiver, and can subtract the two to present on the screen only the

difference to shou the change in device behavior with frequency. At the same time, we may add the reflected light signal so that the net change in electrical response

is superimposed on a picture of the surface of the device. The display mode also

may be varied. One mode modulates the intensity of the electron beam, on the 8 X 10 inch screen in the conventional manner while the other m^ode injects the signal into

the vertical deflection amplifier. This latter, "Y-axis", modulation mode produces

pictures that appear to have a three-dimensional quality. One of the advantages of

the vertical-deflection mode is that it allows the specim.en response to be quantified, since the screen vertical deflection can be calibrated in terms of specimen

signal current or voltage. The next chart [Graphic 6] demonstrates a mixture of

both the intensity m.odiilation and the vertical deflection display modes.

This is a photograph of the display screen where the specimen scanned is a chrome-on-glass pattern test target placed over a photodiode which generates the

electrical signal monitored. You see only a very small portion of the test pattern.

The display was generated using the visible laser. The finest pattern of lines

shown here is on the upper right. This pattern consists of Z-ym wide bars spaced

2 vim apart which are clearly resolved. So we say that our optical resolution is

somewhat better than 2 pm. The same measurements made using the infrared laser re- sults in a slight degradation of detail. Ve. associate with the infrared laser usage

a resolution of better than 3 ym,, referred to the specimen.

7
One further point before we move on to devices, all the device results which (347) we wil]. show are qualitative. But we believe the scanner also can provide quantitative infornation if we deternine, with a calibrated photodiode, for example, the

photon flux at the scanned specimen and if we calibrate the display screen in terms

of photovoltage or photocurrent from the specimen. I7e could then attacbi numbers to the input-output , interactions and in this way quantify specimen behavior.

A

b'

Sawyer

Now that we have described the scanner to you in sone detail, let us move on to show you some of the ways th.e scanner can be used. A good example of the capabilities of the scanner is shown by the results we obtained som.e time ago with a type

7438 dual-input NAND gate scanned with visible light. In this chart [Graphic 7] we have a schematic of the 7438. It is a bipolar IC with the output transistor norm.ally in its high or nonconducting state.

To sv/itch the output transistor to its

low state, that is into saturation, re- quires that both inputs be in their high

state. The inputs are labeled A and B and the output is labeled Y. In the

truth table on the lower right, the designations 1 and 0 tell, respectively,

whether the specified circuit state is high or low. The circuit state is specified by a trio of numbers representing

A, B, and Y, in that order. For examiple,

the case in which both A and B are high

to cause th.e output to go low is listed

in the truth table as 110.

The electrical response of the circuit to the scanning light spot is re- vealed in the following manner. The effect of the light spot is to generate

current carriers in the silicon. V.tien these current carriers are generated

near active elements they m.ay alter the current through the supply bus, depending on the operating condition of the

element scanned. This change in current

is detected as a ch.ange in the voltage

drop across a resistor inserted in the

supply bus, which is shown in the upper

Graphic 6

1/4

7438

V cc TO VIDEO

AMP

FOR DISPLAY

A B Y

0 0 1 0 1 1

1 0 1

1 1 0

iGlMD

Graphic 7

8
right. The voltaee signal serves as tlie input to the nixing an.plifier and display (390) screen. For example, a transistor operating so that it has significant sir.all-signel current gain will show a large photoresponse to tlie scanning light spot and conse- quently produce a bright image on the display screen. On the other hand, a transistor operating with little small-signal current gain, such as when it is in saturation,

will produce no image. Thus, from the photoresponse of the elements in the circuit,

the different logic states can be deduced. T^aturally, changing the state of the MAND

circuit yields a new photoresponse display of the active elements.

The next graphic [Graphic 8] shows photographs of the display screen for the

four different states. The photograph on the upper left shows the image on the display screen for the 001 condition. The large rectangular pattern of horizontal

stripes in the upper right is the output transistor's base-collector region scanned

by the laser. The transistor is in its high state and, as expected, an appreciable

photoresponse is obtained because the photocurrent is am.plified by the small-signal

current gain of the transistor. In order for the output transistor to be in its

high state, the truth table predicts that the input transistor have one or both

emitter inputs in their low state. The input transistor is located directly below

the output transistor, and as can be seen, no photoresponse is obtained in this area.

This is because no amplification of the photogenerated current in the input transistor can occur in the circuit unless both inputs are in their high state.

The two photographs on the upper right and on the lower left are for the 101

and Oil states, respectively. For neither of these two states are both emitters of

the input transistor in their high state, and so amplification occurs. The last

photograph, on the lower right, is for the 110 state. The output is in saturation

and so no photoresponse is seen in this area. The tvjo L-shaped lighted areas on the

lower right quadrant are the base-collector regions around the two emdtters of the

input transistor. The illum.inated

region to the left is tb.e interstage transistor, and the horizontal bar below this transistor is

the load resistor for the input transistor. This diffused re- sistor shows up on the display

screen because the resistor m.ate- rial, silicon, is photoconductive

.

In order for this photoconductivity to be observed, a current must

flow through the resistor, and be

amiplified by the interstage tran- sistor which occurs only in the

110 state. Graphic 8
It is sometimes useful to see the surface cf the circuit to identify various

areas on such displays. In the next graphic [Graphic 9] we have three photographs

of the sair.e circuit but now with the reflected light node added. On the top, is a display generated solely frorr reflected light. In the other two, the electrica],

signal from the supply bus h.as been added to the display. The photo' on the lovrer left is for the 001 state, and the one on the right is for the 110 state.

The fact that a diffused resistor could be detected by its photoconductivity

led us to believe that we r.ight be able to tell wh;ich transistors in an MOS array

were ON by observing channel photoconductivity. \-e were successful in doing this

and David Berning vzill describe the results.

Eerning

The scanner was used with visible light to observe logic information passing

through a MOS shift register. The shift register was a static dual 128-bit P-1jOS

ion-inplanted device and the operation of the circuit was easily observed without

alteriijg the device characteristics. In order to be able to observe the logic flow

in the device with the laser scanner, the package leads are connected as appropriate

for normal circuit operation. The information which describes the circuit operation

is extracted by monitoring variations in power supply current to the device, as was explained for the bipolar NAND gate.

To prepare you for viewing the shift register scanned in real tim.e, I would

like to first show you some pliotographs of the display screen when tbie scanner

was used to view the shift register. [Graphic lOA] The upper left photograph shows

about one third of one of the 128 bit registers and some associated input/output

circuitry on the far right of the photograph. In the m.ain part cf the register a

Graphic 9
random pattern of logical I's and O's can be seen which appear as arrangerents of

dark and light areas along four columns, starting from the left. The logic flows

in the register along a snake-like path which begins in the left hand column at the

bottom. It flows up this first column, then do\<m the second column, up the third,

and down the fourth to where it switches the output circuitry at the bottoi:. of the

fourth column. The logical I's and O's are easily identified in tliis particular

shift register: if we look along the direction of motion, and tliis v;ould be the

vertical direction for the first column, the I's are those elements bounded on the

right by two parallel light bars, and the O's are the elemients bounded on the left

by two parallel light bars. So, for example, in the first colunn on the left and

starting from, the bottom, the first two cells are I's and the next six are O's.

The upper right photograph shows the same register after a single clock pulse

has been injected through the clock input and a logical 1 injected through the data

input. To help you see how the logical I's and O's have shifted, Xv-e have labeled

them in the first column of the two upper photographs. [Graphic lOE] The output

has also switched states.

Demonstrated liere is the operatioir of a good device. Our laser scanning technique should be able to detect sections of the device which are marginal. VJhen this

circuit was operated with reduced power supply voltage, it was observed that some

logic cells did not always function properly. In looking at a defective circuit, v;e have been able to detect improper internal operation.

It was also found while scanning this MOS device that internal logic states can

be changed non-destructively by increasing the laser intensity with th.e polarizeranalyzer. In fact, the laser has to be attenuated before stable logic states can

Graphic lOA

11
exist in the register. Logical I's can be selectively changed to O's and vice ver- (487)

sa. V;e do this by first decreasing the laser scan raster to a region on the logic

cell where a change is desired. The scan raster is decreased l;y electronically decreasing the excursion of the laser deflection mirrors. The laser intensity is then

increased to change the state of the cell, after which the beam is returned to its

previous intensity. The laser raster scan then can be restored to its original

size. This operation changes the state only of the desired cell and does not affect

the state of any other cells in the register. In this case aiming the laser to one side of the logic cell changes a 0 to a 1, and aiir.ing to the other side changes a 1 to 0. Aiming the laser is accomplished by moving the microscope stage on which the

device rests. The bottom, photograph shows the register as it appeared in the upper

right except that the laser was used to selectively change one of the logic cells.

The cell changed is the center one in a group of five logical I's located in the

third colum.n. The group is enclosed V7ith a white line to help you see them.

[Graphic IOC] You may be able to note that no other logic cell has been changed and

no input pulses were injected into the device. The ability to change internal logic states non-destructively may have far reaching benefits in the testing of LSI devices

which have internal processing blocks which are not accessible externally.

Now that you have had a chance to become acquainted with what the shift register looks like as revealed by the scanner, let's look at a videotape we made in our laboratory of the progression of logic through the f'OS shift register. [Videotape

begins] This was taped in real tim^e from, the scanner's display screen. You see here as you saw in the previous photographs a random pattern of logical I's and O's. The device can easily be moved on the scanner to see the areas of interest. All 128

storage units in this register can easily be located.

Graphic lOB

12
Graphic IOC

We are now going to enter a series of logical I's into the register by using (525) the clock and data inputs on the device. These can be observed moving 'Into the register at the bottom of the first column. As each logical 1 is injected all of the

other logical states move along the register as they should — up the first column,

down the second, up the third, and down the fourth to where the output stages re- ceive either a 1 or a 0, and switch as appropriate.

To observe the logic levels in this delicate device, the laser beam had to be

attenuated with the polarizer-analyzer. If the laser intensity is increased by a

sm.all am.ount, one can observe the logic elements switching randomly. As was stated

before, if the scanning raster is reduced to a point and aimed at any individual

cell, the cell's state can be changed predictably.

If the laser intensity is increased still further, the logic cells appear to saturate

I'H^en the laser intensity is restored to its original level, once again stable

states form in the shift register, and these states can be moved through the circuit

as before. [Videotape ends]

Sawyer

We would like to show you next how the scanner can be used to map, alm.ost simultaneously, low-frequency electrical characteristics, the high-frequency electrical characteristics, and the thermal behavior of a device, and also tell about the

uniformity of operation over the device area when it is working within its normal

frequency range. We will dem.onstrate this with a type 2N4431 UHF transistor. This

widely available transistor type is designed to furnish 5 watts at UHF frequencies
up to 1,000 MHz. The surface topology for the transistors we have worked with is (571) shown next by the photomicrographs in the next graphic. [Graphic 11] There are

four in-line cells electrically connected in parallel, and these are shown in the

left photograph. The total active area of the transistor is a rectangle 1.2 mrp. long

and 0.15 mm wide. The photograph on the right is an enlargement. You may be able

to see that the emitter and base fingers are interdigitated with the em.ltter fingers

coming in from the right, and the base fingers coming in from, the left. The finger

metallization is 2 pro in width, and the stripe separation is 8 pm. The metal stripe

separation allows the active device regions to be accessible to laser irradiation.

For the results that David Berning will show, the devices were scanned while they were connected and biased in the commion-em.itter configuration. The signal for the

display screen image was taken from a 60-^2 resistor which served as the collector

load. For several of the transistors, we found that regions of high tem.perature

,

so called "hot-spots", would form within the bias range listed in the manufacturer's

data sheet. We used this observation to aid in understanding the m.any things the scanner can tell us about the way a device really w'orks.

Berning

This [Graphic 12] is an array of observations on a 2N4431 transistor. The up- per left image is the infrared response for a collector-emitter voltage of 26 V and a collector current of 250 niA. The transistor is operating just outside of the

hot-spot regime. Not much of the incident optical energy is absorbed in the device

active regions and so the display screen signal is weak. The infrared response was actually quite uniform over the device, and the apparent nonuniformity captured in

Graphic 11

14
Graphic 12

this photograph was due to systeni noise which had the effect of modulating the pre- (600) sentation of the photoresponse . The lower left photograph v;as made for the sai!;c scanning conditions but V7ith the bias adjusted sligl:tly to put the transistor into

hot-spot operation. The region of enhanced photoresponse, the v/hite area, is tlie hot-spot region as confirmed with the use of a passive infrared microscope. 1\\e

photoresponse is proportional to the number of electron-hole pairs photogenerated

in the device active region. For radiation from the infrared laser, which is lightly

absorbed at room tem.perature , the num.ber of the pairs photcgeneiated increases with

tem.perature because the optical absorption coefficient increases with tem.perature.

This is due primarily to the v?ell-known change in the silicon bandgap with tem.pera- ture [5].

The upper right photograph is for the sam.e hot-spot conditions, but it was m.ade using the video response to the visible laser instead of to the infrared laser. The

optical absorption for the visible wavelength is large enough so that essentially

all of the incident lig'^'t is absorbed in the active regions at rccmi temperature and

no enhancement in the hot-spot portion is observed. The lower right photograph is

for the sam.e bias and optical wavelength e3;cept that now we have used the radio re- ceiver, and the image produced is the. response of the transistor to the component of

the laser light modulated at SCO MI'.z . We interpret th.is image as the spatial m.ap of

the 500 MHz gain of the transistor. The hot-spot area is revealed quite dram.atically, and we see that there are actually two hot spots symjiietric about a eel] boundary.

[5] Smith, R. A., Semioonductort- (Camhrldge University Press, 196]) 204-207.

i 15
Savtyer [At Blackboard]

Altb.ounh tlie resiilts shov.'n here, both for tlie therma]. piappinp and tbie mapping (623) of transistor f;ain at 500 IIH:-; , Xvere obtained vith static biasing , siiri.lar ireasiirep.ents witbi the transistor operating in its own RF circuit should be feasible.

>'ow \.'e uould libe to shov; tliat the scanner car be used to detect regions \vhere

nonlinear behavior is occurring '.ithln functioning semiconductor devices. As far as ve knov.-j this has no\'er beer dene before.

Tl-;e deiponstration vehicle is the same C^O"'!' '^'f ?.VA^31 transistors that v.-as used

for the jow- and h i gb-f re cuen cy wapplng, and for the electronic therr.'al mapping I'e- scribed. But, Iiefd'e we shov? the results, we first sbiould describe how tbie r.'.ethod ror n:appinj; nonlinear be?;avior works. Th.is iCrjipliic 13 — drawn at hlackboa rci j is a general representation of an ami^l if}ing element, such as a transistor or an entire integrated circuit. For siPDlicity, let's sliow the eler.ent amplifying at tne single frequency f-, , althougii other frequencies mav also be present, such as side-bands, if f, were modulated. Put generality is not lost by considering f^ alone. ^o describe our tecb.nique, let us assume that a portion of th.e specimen

is operating in a linear manner: that is, the local minority carrier concentration

is proportional to the input current, and the input-output characteristic is shovjn

by this sclid-line. [Graphic 13, right half] Othier portions of the device may behave in a nonlinear manner as shown by this dashed line. The overall behavior will

lie sorewhere hetv;een these tvv'o curves. Ey m.onitoring the electrical output and

changing, the operating conditions, we will know that nonlinearity is occurring but

we will not know whether it is occurring uniformily or only in certain, as yet un- specified, portions of the specimen.

But the scanner can tell us \^'hich portions are behaving nonlinearly. We optically scan th.e specimen with the laser output modulated at the frequency f ^

, and

with the electrical input at f^ as before. For the portions tliat are behaving linearly, the electrical output will contain the two input frequencies f^ and f^ only. One can always represent a curved line, such as the dashed line, by a power series;

and, the output for th.e portion specified by tliis curve will consist of frequency

Graphic 13

16
coruponcnts. other than just and f^. The first term after the linear term in the (663) pcv;er series is the square term, and this will produce an output at both the sum

frequency, f^ plus f ^ , and the difference frequency between f-^^ and f^. If we now

connect a radio receiver to the output of the scanned specimen and tune to either

the sun? or difference frequency, we will get an output froni the receiver only when

the laser is incident on a portion of the speciner that has some nonlinearity in its

electrical characteristics; and, if we connect th.e receiver to the display screen,

then we will present only those nonlinear portions on the screen.

David Berning will show you a set of pliotographs , showing nonlinearities in a 2;tAA31 revealed in this m.anner after some introductory rem.arks

.

We used a 2N4'^31 which could be put into hot-spot operation, as revealed both

b}' the use of a passive IP scanner and by the electronic techniques previously described. The transistor V7as connected [Graphic 14 — drawn at blackboard] in a simple comjiicn- emitter circuit with a 60-r; collector load resistor and a variable dc

base current supply. A UIIF signal generator was used to furnish R.F. base drive,

and a 30 i^Ttlz I.F. araplifier and detector was connected across the 60-fi collector

load. The detector output was fed into the display screen's mixer-amplifier. The

signal generator was set at 470 }V.z , which is within the operating frequency range

of the transistor. If nonlinearities are present v;ithin the transistor, then_ the

470 KHz electrical input and th.e 500 MHz optical modulation of the visible laser

will irdy. to produce an output on the display screen from^ the 30 MJiz I.F. am.plifier.

We have a plot of the low- f requen.cy input-output characteristics for the 2N4431

scanned. [Graphic 15] The curve v.'as obtained from a recorder trace of the dc

Berning [z\t Blacl<board]

470 MHz

rt7

Graphic 14

17
collector-current, base-current characteristics. The base current range is from 0 (696) to 13 iriA, and the collector range is fron 0 to 520 m^.. The deviation from linearity

in the center is associated with hot-spotting, as we establishied by thermally m.ap- ping the device. Had the device been linear, its characteristics would have followed

the dashed ].ine, which we have sketched in. If the transistor's R.F. characteristic

is similar to the static characteristic shown here, then we should obtain a signal

on the displ_ay screen wh.en the transistor is biased near cut off, in the hot-spot

region, and near saturation; and, we sliould see relatively little when the transistor

is biased at the various linear regions in between.

The photograph, on the left-hand side shows the topology of the transistor, and

this photograph was made using only the reflected light circuit of the scanner. The

active region appears dark in this photograph and it is bounded by the reflective

base and emitter contacts at the ends of the interdigita ted fingers.

In the next series of graphics we will show the results of using the I.F. output of t!ie scanner for various operating points on the input-output characteristic,

going from cut off to saturation. So, the central region in the photograph that now

appears dark will be light when there is significant I.F. output which indicates

nonlinear operation. VJith that introduction let's look at the next graphic [Graphic

16] which is a combination of the display screen photograph for the transistor near

cut off and the collector-current base-current graph v;ith the dc operating point indicated by the arrow. IJe obtain a strong I.F. output because of the strong electrical nonlinearity . In the next graphic [Graphic 17] we have moved into the first

linear region, and the signal has decreased. t'oving on, [Graphic 18] the hot-spot

region has been entered. It is interesting to observe that hot-spotting can influence electrical linearity in cells other than lust the one or ones with a hot spot.

Graphic 15

18
As you can see, the ncnlinearity is fairly iinifornOy distributed ever the entire (723) active area of the four cells. Perhaps tliis is due to coupling in the coranon collector substrate. As we move up the cb.aracteristic , [Graphic 191 the behavior is still quite nonlinear. Here, [Graphic 20] even though the device is still operating

in the hot-spot regime, thie recorder trace shews that the input-output characteristic is fairly linear, and so not muchi f requenc^^-mixing occurs. Now, [Graphic 21] we

are approaching the higb.-current boundary for hot-spotting, and the nonlinearity in- creases. Here, [Graphic 22] the operating point is outside of the hot-spot region,

and the cliaracteristic again is fairly linear and so little mixing occurs. Finally,

for the last condition, [Graphic 23] the device is near saturation, and again non- linearity is observed on the display screen. You may be able to see that tlie nonlinearity is somewhat more pronounced along the right-hand emitter side. VJe suspect that this is so because the base contact is on the left, and a small amou.nt of

debiasing of the right-hand emitter side is occurring due to the dc voltage drop

along the base fingers.

Graphic 16

Graphic 18

Graphic 20

Graphic 22

19
Sawyer

Although the observations were made on a transistor operating in the UMF region, (738) there should not be any probleir. using this technique to detect operating nonlinearities in either h.ighor or lower frequency ranges, or even at av:dio frequencies; it

sbtould only be necessary to choose appropriate laser modulation and I.F. frequencies,

and to modulate the 1 ^ser at the required frequency.

One of the requirements, of course, for applying any of the scanning ideas that we have demonstrated is that one has to get light into the device. ^^ut , the trend

in bipolar device design appears to be toward interdigit ation, r.ow even for pov;cr supply transistors, and this al lov.'s optical access. Optical access to IC's is facilitated when pclvsiliccn is used in place of metal, and polysilicon is widely used

now. For th.ese two reasons, it is generally easier to obtain scanning results from

devices now than it vras just a fev.' years ago.

'est of our scanning v:ork has been on devices for whiich the chips have been

mounted on a header. But 5oii;e of th.ese can be scanned at the wafer level; that is,

before the chip is mounted. This is most fea.sible for low power devices which do not require optinur,! heat-sinking during testing.

A].l of the work shewn today was performed on siliccn devices. However, \jc also

have scanned devices made frop other sem.iconducting materials, gern-aniinn and indium,

arsenide, for example.

Tc sum up the T'or!>- which ve have shown today , v.-e maVcG use of the fact that

semiconducting r'aterials are photosensitive d.n an electricciJ sense as part of their

basic nature. Thi's , semiconductor devices such as diodes, transistors, and entire

integrated circuits made from, these m.aterials can be studied using optical radiation.

The effect of t^ie optical radiation is to rcnerate electron-hole pairs in a nondai-iaging way within the SDecir;en. These current-carriers can stimailate device beiTavicr by taking the place of signal current-carriers u-hich. are norm.ally supplied by

leads iixed to tb.e device. In contrast tc signals applied via the leads, Xv'hich are rixed in position, the optical excitation can be moved over t'-.e surface and within

the bulk., and the response of the structures can be studied on a point-l'V-pc int

basis tc learn the inner ^.'orkings of the device.

\-!e have developed a versatile optical scanner th.at can be applied tc a variety

oi device problems. In several cases we have observed device phenomena which, one

has net been able to study before. The scanner can m:ap the flou of logical information in ;',0S and bipolar IC's, it can map the operation of transistors at ultrahigh

frequencies, it can electronically map the tem.peraturc distribution, and it can pinpoint the portions of operating devices v.'hich are operating in a nonlinear manner. These .ire just some e:-;an-:plps of the applications of the scanner we have developed

i.'hich we felt would be of greatest interest tc you.

2G
If you have questions about the suitability of loser scsnning for these or any (777) other app].ications, to shed light on YOUR problems, or if you would like to explore

with us the possibility of assenbling such a scanner at your facilities, please

telephone or v/rite us. Contact either David Perning or me, David Sav.-ycr. Our address is Electronic Technology Division, National Bureau of Standards, Washington,

D. C. 20234.

CREDITS

Jointly Supported by: Defense Advanced research Projects Agency and

National Bureau of Standards

Technical Adviser: Karry A. Schafft

Craphics: R.andy Myones , Rayii;ond V. Gates, and Marilyn B. Ugiansky

Light Show: David W. Berning

Associate Producer: Lynn '-yvill

Producer; Ron Meininger

Recorded at: HQ USAF TV Studio, Pentagon, October 1975.

21
FORM NB5-114A (1-711 U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Acces.sion BIBLIOGRAPHIC DATA No. SHEET :;CS Spec. Publ . 40Q-:^7

3. Recipient's .^ccessicin .No.

4. TITLE AND SUBTITLE

Semiconductor Measurement TeohnoloQi/ : Laser Sconninn of

Active Semiconductor Devices — Videotape Script

5. Publication Date February 1776

6. Performing Organization Code

7. AUTHOR! S) David E. Savjyer and David U. Eernina

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11, Oontriict/ Grant No. ARPA Order 2397; Procran

CndP fimn

12. Sponsoring Organization Name and Address

fIBS — Washington, D. C. 2023^

ARPA — 1400 Wi Ison Boulevard, Arlington, VA 22209

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 75-619427

16. 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 is the script of a videotape presentation v;hich describes new and powerful

applications for laser scanning in semiconductor device design and reliability v/ork.

The design of the scanner is described in detail and many of its applications are displayed and discussed. The optical scanner can, in a completely nondestructive way,

reveal the inner workings of semiconductor devices. For example, it is shown that the scanner can (1) nap dc and high-frequency gains in transistors, (2) reveal areas of the

device operating in a nonlinear manner, (3) electronically map temperature in the

transistor, and (4) detect the location of hot spots that can develop for certain

operating conditions. The vehicle used to show these capabilities of the scanner is a bipolar interdigitated UHF transistor. A dual input NAMD gate is used to demonstrate

the use of the scanner to determine internal logic states and otherwise observe internal

operation of the circuit. To show the ability of the scanner to examine MOS devices

without detectable degradation, a MOS shift register is used. The location and progress

of internal logic in the register is clearly shown by the scanner, flot only can internal logic be mapped and marginally-operating logic cells detected, but individual

logic states can be changed by the scanner without affecting other elements.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Failure analysis; hot spots; integrated circuits; laser scanner; measurement method;

nondestructive test; nonlinear operation; optical scanner; reliability; transistors.

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