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NBSIR 78-1488

Measurement Techniques for Solar

Cells,

Quarterly Report:

September 15 to December 31, 1977

NBS

Publi

cations

NAT’L INST, OF STAND & TECH

AlllDL

D. E. Sawyer, H. K. Kessler,

and H. A. Schafft

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, D.C. 20234

July 1978

-Tirval

*

QC

-

100

''lared for jartment of Energy

“"shington, D.C. 20545

National Bureau of Standards APR 17 1979

c NBSIR 78-1488

MEASUREMENT TECHNIQUES FOR SOLAR

CELLS, o

QUARTERLY REPORT:

SEPTEMBER 15 TO DECEMBER 31, 1977

D. E. Sawyer, H. K. Kessler,

and H. A. Schafft

Electron Devices Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, D.C. 20234

July 1978

Prepared for Department of Energy

Washington, D.C. 20545

N 6S / i Nil- - I r- - 7

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technoiogy

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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CONTENTS

Page

Preface iv

Executive Summary 1

1 . Introduction 2

2. Proposed Applications of the Laser Flying-Spot Scanner

to Solar Cells 3

2.1 Overview of Some Cell Problems 3

2.2 Spatial Variation of Cell Response 4

2.3 Magnitude and Spatial Variations in Cell Emitter

Sheet Resistance 5 2.4 Variations in Grid-Metallization Ohmic Contact 9

3. Work Performed During Reporting Period 10

3.1 Development of Solar Cell Device and Material Measurement Techniques 10

3.1.1 Laser Flying-Spot Scanner Modifications 10

3.1.2 Equipment for Light-Biasing Cells 12

3.1.3 Analysis 13

3.2 Workshops and Symposia 18

4. References 19

Appendix A 20

Appendix B 23

LIST OF FIGURES

1. Silicon p-n junction diode photoresponse 6

2. Items a through c explain the variation in photoresponse

with increasing frequency and distance from the top electrode depicted in figure 1 8

3. Solar cell clamping and rocking unit attached to the microscope cross-slide stage 11

4. Representation of the two-dimensional parasitic resistancecapacitance nature of a solar cell 14

5. The one-dimensional analog of the cell between a pair of

grids shown in figure 4 16

iii
PREFACE

This work was conducted as a part of the Semiconductor Technology Program of the National Bureau of Standards (NBS) . This program serves to

focus NBS research to enhance the performance, interchangeability, and

reliability of integrated circuits and other semiconductor devices including solar cells 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. This research leads to carefully evaluated and well-documented

test procedures and associated technology. Special emphasis is placed

on the dissemination of the results of the research to the appropriate

technical community. Application of these results by industry will

contribute to higher yields, lower cost, and higher reliability of semiconductor devices. Improved measurement technology also leads to greater

economy in government procurement by providing a common basis for the

purchase specifications of government agencies and, in addition, provides

a basis for controlled improvements in fabrication processes and in essential device characteristics.

The segment of the Semiconductor Technology Program described in this

quarterly report is supported by the Division of Solar Technology of the

Department of Energy (DoE) under DoE Task Order A054-SE of Interagency

Agreement EX-77-A-01-6010 . The contract is monitored by Dr. Donald L.

Feucht, Chief of DoE's Advanced Materials R&D Branch. The NBS point

of contact for information on the various task elements of this project

is Dr. David E. Sawyer of the Electron Devices Division at the National

Bureau of Standards.

Certain commercial equipment, instruments, or materials are identified

in this report in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the purpose .

IV
Measurement Techniques for Solar Cells

I QUARTERLY REPORT

September 15 to December 31, 1977

By

D. E. Sawyer, H. K. Kessler,

and H. A. Schafft

EXECUTIVE SUMMARY

This report covers research performed in the period September

15 to December 31, 1977 on the Program on Solar Cell Measurement Technique Development and Other Services by the Electron

Devices Division of the National Bureau of Standards. The objectives of the program are to assist the DoE thin-film photovoltaic effort by developing solar cell device and material measurement techniques using the NBS-developed laser flying-spot

scanner, and by assisting DoE in organizing and hosting appropriate workshops and symposia and providing general consultation

and liaison services.

In this report several proposed applications of the laser scanner*

for solar cells are described: measuring the spatial variation

in cell response for various levels of bias light to map the behavior of the short-circuit photocurrent and the open-circuit

photovoltage over the cell area, measuring magnitude and spatial

variation in cell emitter sheet resistance, and determining emitter metallization regions making poor ohmic contact. Some additions and slight modifications were made to the scanner this

first quarter to enhance its usefulness for examining solar

cells. These include addition of lower magnification microscope objective lenses with longer working distances that allow the scanning of cells between the metallization stripes.

These lenses also provide space for inserting fiber-optic lightguides for cell light-biasing. In addition, a cell mounting

* The scanner is described in Appendix B in enough detail so that

the research described in this and subsequent reports can be

understood without going outside this series.

1
stage was constructed. This stage uses vacuum to hold the cell

and incorporates micrometers to adjust the plane of the cell

with respect to the scanning beam and the reflected-light optical path.

Means for achieving high-intensity light-biasing were investigated. A high-pressure gas-discharge xenon lamp, similar to

those used in solar simulators, was tried, but it generated an excessive amount of noise amplitude, modulating the lamp output

and producing radio-frequency noise in the system to about 35

MHz. A quartz-halogen incandescent lamp, however, is expected

to be adequately quiet when energized with a stable power supply.

The case of the scanner scanning between parallel "emitter" metallization stripes with a line of light modulated at high frequencies was analyzed. It is shown that the minimum-to-maximum

’signal ratio depends only on the modulating frequency, the

(reverse.-biased) junction capacitance (assumed spatially constant) , and the emitter sheet resistance. The modulating frequency is known and the capacitance can be measured nondestructively. The analysis predicts that the emitter sheet resistance

can be obtained by fitting the analytical value for the minimumto-maximum ratio to the experimentally observed ratio.

The first workshop. Stability of (Thin-Film) Solar Cells and Materials, has been planned. This workshop is scheduled to be held

May 1 to 3, 1978 at the National Bureau of Standards, Gaithersburg,

Maryland.

1 . INTRODUCTION

This report covers work performed in the period September 15 to December

31, 1977 on the Program on Solar Cell Measurement Technique Development

and Other Services by the Electron Devices Division of the National Bureau of Standards under Task Order A054-SE of Interagency Agreement EX-77-

A-01-6010.

The objectives of the program are to assist the DoE Advanced Materials

R&D Branch photovoltaic effort in the following ways:

2
1 . by developing solar cell device and material measurement techniques using the laser flying-spot scanner originally developed

at NBS for use on integrated circuits and discrete transistors,

and

2. by assisting DoE in organizing and hosting appropriate Workshops

and Symposia, and by providing general consultation and liaison

services

Several proposed applications of the laser scanner for solar cells are described in section 2. Activities and accomplishments in both project

areas during this reporting period are described in section 3. The outline of the first workshop is presented in Appendix A. Because this is

the first quarterly report for the program, it is felt that its usefulness would be enhanced by a description of the laser scanning equipment.

The scanner is described in enough detail in Appendix B so that the re- search described in this report and others to come can be understood

without the reader needing to supplement these reports with other publications .

2. PROPOSED APPLICATIONS OF THE LASER FLYING-SPOT

SCANNER TO SOLAR CELLS

2.1 Overview of Some Cell Problems

The requirements placed on terrestrial solar cells developed for DoE's

Photovoltaic Program are unique in that a combination of adequate efficiency, low cost, and long life is demanded. Satisfying this set of

requirements means that some serious problems have to be faced and overcome. Some of the cells which are leading contenders for wide-spread

deployment may be made from poorly understood materials by fabrication

techniques which history has shown can sometimes lead to inexplicable

periods during which cells of adequate performance cannot be provided.

An alternate approach is to use relatively expensive but well-understood

materials such as single-crystal silicon or gallium arsenide, using only

small amounts , and to rely on low-cost reflectors or Fresnel lenses to

collect and focus the sunlight on the cell with concentration ratios of

up to several thousand. The adequate design of a concentrator solar cell

3
can be a formidable problem. An appropriate analysis would begin with a device model suitable for computer simulation which would be both threedimensional and allow the device parameters to vary in their values over

this space. There are disadvantages to this approach, however. One is

that the comprehensiveness of the model could obscure the cause and effect relationships between device parameter values chosen and overall

cell efficiency. That is, the possible usefulness of the computer results in predicting what should be done to improve the cell may be obscured by the many quantities one could vary; it is not known on an a priori basis which quantities are important and must be specified ac- curately, and which quantities may be neglected for the sake of simplicity, to obtain a solution which may lead to better design insight. A

third important class of cells, nonconcentrator types made from singlecrystal materials, employs fabrication techniques which are quite similar to those developed by the integrated circuit industry. One significant difference between solar cells and integrated circuits is that the

solar cell "emitter" (that portion above the p-n junction) is quite thin

to enhance the cell response in the blue region of the spectrum, typically less than a half a micrometer in the more recent efficient designs.

This means that a uniform and low-resistance contact to the emitter is more difficult to achieve for silicon solar cells without shorting

through than it is for integrated circuits where the emitters may be

several times as thick. The laser flying-spot scanner can help ex- amine several of these problems, and appropriate scanner utilizations

are sketched below.

2.2 Spatial Variation of Cell Response

It would be extremely useful, especially for concentrator cells, if one could measure the spatial uniformity of the cell over its surface.

This could provide a check of the cell's design and fabrication steps.

We will discuss in this first of the laser scanning activity reports

how the scanner can be used in a rather simple manner to do this. It

can reveal those portions of a cell which are not as effective as other

parts in producing load power under illumination, and the scanner can present this information in a quantitative manner so as to allow an ac4
curate assessment of the cell deficiencies. This is a straightforward

scanner application. The technique described can be used for any cell

composition, and it will provide information badly needed for concentrator cell design and assessment. It can also be used to obtain a better understanding of the behavior of nonconcentrator cells, both con- ventional p-n junction single-crystal ones and cells made using structures and materials of a more exploratory nature. The technique itself

is very simple. The cell is spot-scanned while it is uniformly illuminated by a bias light that can be increased step-wise in intensity. Under these conditions , the scanner display screen will present a map of

the small-signal, or incremental, cell response. By scanning with various cell load values between zero ohms (the cell termination for measurement of short-circuit photocurrent) and infinity (the cell termination

for measurement of open-circuit photovoltage) , one could obtain maps

which when compared with one another show the variation of these important parameters as the laser beam scans the photosensitive surface. The

maps can further be interpreted to yield desired information about the

cell's point-to-point operation. Establishing the validity of this measurement technique would be extremely valuable to the solar cell community. One obvious utilization is that it would allow one to compare

solar cell design theory and experiment so that one may determine which

I of the many parameters that would be included in a two- or threedimensional computer analysis are really important to specify and control .

2 . 3 Magnitude and Spatial Variations

in Cell Emitter Sheet Resistance

The photomicrograph in figure la shows the silicon chip used for the

first laser scanning observations of lateral (sheet) resistance. There

are nine diodes, each about 250 ym on a side, formed by planar diffusion

into p-type silicon of about 5 fl*cm resistivity. The diffusion was ntype and the junction depth was about 1 ym. These values of junction

depth and bulk resistivity are close to those used for commercial silicon solar cells. The nine-diode chip was bonded to a header using con5
Figure 1. Silicon p~n junction diode photoresponse. The diode scanned

is the central one in the upper left array of nine. The upper right photograph, figure lb, is the photoresponse to unmodulated 0.633-ym light

and with the diode reverse biased 5 V. The figure Ic photograph is for

the same conditions except that the screen now displays the diode response

to the light component modulated at 500 MHz, and the figure Id photograph

is the 1.0-GHz response.

6
ventional techniques, and a lead connecting one of the two ohmic contacts

on the central diode was run to a header post.

The other photographs in figure 1 are photographs of the scanner display

screen with the central diode scanned using laser light of wavelength

0.633 pm and with a diode reverse bias of 5 V. The presentation mode is

a mixture of the conventional z-axis, i.e., intensity modulation along

with vertical deflection of the horizontal scan lines by the same signal.

The photograph shown as figure lb is the diode's response to unmodulated

light; it is the small-signal dc or "video" diode response. The photograph shown as figure Ic is the response to the 500-MHz modulated component from the same laser and with the same bias conditions, while the

photograph shown as figure Id is the corresponding 1,0-GHz response. The

gain of the display screen was adjusted to maintain the same maximum vertical deflection for all three photographs. This maximum occurs at the

edge of the bonded contact. It is seen that the 500-MHz and 1.0-GHz photoresponses from portions of the diode away from the bonded contact drop

off with increasing frequency and distance.

Figure 2 cam be used to explain the results shown in figures lb through

Id. Figure 2a is a cross section through the diode and figure 2b is

the corresponding one-dimensional electrical representation of the diode.

The diffused emitter region is represented by a distributed resistor and

the junction transition region by a distributed capacitor. One would ex- pect such a network to behave in the manner observed; the signal to the

radio receiver would decrease with increasing signal frequency and distance of the laser excitation from the bonded contact, as sketched in

figure 2c, because the attenuation due to the distributed resistancecapacitance line increases with these quantities. There are three independent variables for a given cell geometry: the signal frequency, the

capacitance per unit area, and the sheet resistance. The sheet resistance is the only unknown; this is the quantity one would like to measure and it should be obtainable by fitting the measured response with

the analytical solution appropriate for the cell geometry. It is noteworthy that this proposed method for sheet resistance measurements on

7
o

b

c

<wy|

EQUIV.

ra

CIRCU

M

IT

SIGNAL

Figure 2. Items a through c explain the variation in photoresponse with

increasing frequency and distance from the top electrode depicted in fig- ure 1: the structure acts as a distributed resistance-capacit£mce transmission line and the response variation can be described in terms of the

incremental resistance and capacitance line parameters.

completed cells is unique in that it requires the use of contacts which

are normally a part of the cell, and that it is completely nondamaging

to the cell.

For real cells, the analytic complexities are greater than that for the

single one-dimensional line. The excitation is better represented by a volume with radial symmetry than by a mathematical line , but these realities should not introduce insurmountable analysis problems. For example,

solar cell grids are at least a few millimeters apart. For light modulated at frequencies which are in the range of 500 MHz, the complex diffusion length in the semiconductor, e.g., silicon, shrinks to the order

of a few micrometers, a distance somewhat larger than the one-half micrometer radius of the light spot in the NBS scanning system, but tiny

in comparison with the grid spacing. Thus, for any realistic analysis,

the spot can be considered a point source, and this simplifies the re- quired analysis.

8
The experimental technique described above should also reveal variations

in the sheet resistance of the (top) diffused layer. These variations

can be a problem and may significantly affect solar cell output power,

particularly for cells made from the more exploratory semiconductor materials in which inhomogeneities may be expected. Variations in sheet re- sistance due to any cause would be revealed by variations in the parameter chosen to fit the experimental data (the cell response) over the ac- tive area. After the analysis for the uniform sheet resistance case has

been completed, an analysis may then be conducted to establish the range

of sheet resistance variations which one may expect to reasonably detect.

The capacitance of solar cells made from the more exploratory materials

may also vary over the cell area. Analyses of resistance-capacitance

transmission lines such as those representing the distributed nature of

solar cells usually produce answers in terms of resistance-capacitance

.

products, and so one might conclude that the method then would not yield

unambiguous values for sheet resistance variations. However, spatial

variations in junction capacitance may be controlled independently by

varying the cell bias voltage. The incremental sheet resistance is not

a strong function of this voltage, but the capacitance per unit area is,

and it probably may be made to vary in a predictable manner to allow the

spatial variations in resistance and capacitance to be obtained independently .

2.4 Variations in Grid-Metallization Ohmic Contact

The preliminary results shown in figure 1 suggest that the laser scanner

can be used to detect locations where the grid metallization forms a poor ohmic contact with the diffused layer. For uniform ohmic contacts

the photoresponse along the grids will be uniform; but in regions of poor

ohmic contact, the photoresponse will be less than that observed near

good contacts

.

9
3. WORK PERFORMED DURING REPORTING PERIOD

3.1 Development of Solar Cell Device and Material

Measurement Techniques

3.1.1 Laser Flying-Spot Scanner Modifications

Some modifications were made to the laser flying-spot scanner to enhance

its usefulness for scanning solar cells. The modifications were made in

the vicinity of the specimen stage. The microscope objective lens was changed to one having both a lower magnification, 2.5X as contrasted with

the 8X one most frequently used for scanning integrated circuits. The

lower magnification provides a longer working distance (45 mm) and allows

most solar cells to be scanned from one grid-line to another without moving the cell. The longer working distance provides the space between the

lens and the scanned specimen needed when fiber optic bundles are used to

bias the cell with steady-state light while scanning. (The light-biasing

work is discussed in section 3.1.2.)

Most of the devices previously scanned were ICs and discrete transistors.

In general, they were easy to mount for scanning since most of these devices came equipped with pins or wire leads which could be inserted in

appropriate sockets clamped onto the microscope specimen stage. The

plane of the scanned device could be easily adjusted by bending the device leads so that the light reflected from surface features such as a metallization is directed toward the germanium photodiode used in the

reflected-light circuit. However, neither the same holding arrangement

nor the same method for aligning the reflected-light path is workable

with the usual solar cell. The usual cell is flat, and if leads are an integral part, then they are not as a rule adequate for holding the cell.

To solve this problem, the clcimping and rocking unit shown in figure 3 was designed and constructed to fit between the solar cell and the microscope x-y cross-slide stage; this latter retains its original function

of placing the scanned specimen into the horizontal plane. The upper

part of the unit is a vacuum chuck having replaceable top plates to ac- commodate cells of various sizes. The rocking feature is controlled by

the two micrometer heads which can be seen in the photograph. These

10
Figure 3. Solar cell clamping and rocking unit attached to the microscope cross-slide stage. Cell clamping is performed with a vacuum holddown chuck magnetically held to the top plate of the rocking mechanism.

Rocking (the orientation of the vertical axis) is controlled by micrometers. This photograph was made with the scanner in operation, and the

pattern between grids approximately in the middle of the scanned cell is

the raster of scanning light.

11
permit precise rotation of the vacuum chuck around the x- and y-axes to

direct the reflected light to the germanium photodiode, and the chuck is

spring loaded against the micrometer tips to maintain the settings of

the micrometer heads.

3.1.2 Equipment for Light-Biasing Cells

One of the technical activities in the Program is to determine the smallsignal photoresponse of various portions of cells while they are subjected to background light levels which are uniform in space and time.

One should be able to interpret the small-signal scanned-cell photoresponse and determine the adequacy of the cell design and fabrication,

tell a good deal about uniformity of the cell materials, and even localize cell defects. One interim specification for the illuminator is

that it should produce, for nonconcentrator cells, a short-circuit cur- rent response equivalent to exposing the cell to an insolation of 20 suns at the cell-scanned area. Noise or modulation of the light, by

e.g., an inadequately filtered lamp power supply, should be a minimum;

otherwise the scanner signal-amplifiers will process this extraneous optical signal along with the cell response to the laser scanning spot and

obscure the display information.

The light sources considered first were high-pressure gas discharge lamps

such as are used in solar simulators. One such lamp was set up in a test

bench and powered by a filtered dc source. Monitoring the light output

with a photodiode revealed spikes and noise bursts which at times ex- ceeded ten percent of the average light output and which had frequency

components measurable to about 35 MHz. A dual-trace oscilloscope monitoring simultaneously the light output and the lamp voltage showed that

the extraneous light signal originated within the lamp; undoubtedly this

is associated with instabilities inherent in the gas discharge. Our con- clusion is that, while such sources may be "quiet" enough for use in

solar simulators where one is concerned with the average value of the

light, they are not suitable for our needs.

Quartz-halogen sources were next investigated. A new model quartzhalogen high-intensity illuminator designed to be used with the same

12
model microscope as the one incorporated in the scanner was tried, but

the results were disappointing; the short-circuit cell current for a small area (2 by 2 cm) cell was equal to only one-sun insolation. An

examination of the optical path between the lamp and the specimen stage

showed that although the illuminator was more than adequate for its intended usage, it was not designed for efficient utilization of the lamp's

output, and the optics between the illuminator and the specimen constituted a high f-number system.

It is apparent that a light-bias system has to be tailored for the task.

The approach, which will be taken is to try a quartz-halogen incandescent

lamp operating from a stable and low-ripple power supply, using flexible

fiber-optic light pipes to direct the light to the cell area being

scanned and a low f-number lens system to couple the lamp to the light

pipes

3.1.3 Analysis

This section continues the subject introduced in section 2.3: determination of the magnitude of and spatial variations in the cell emitter sheet

resistance by scanning with laser light modulated at high frequencies.

Figure 4 is the representation of a solar cell showing the passive cell

elements. Grids, shown by shading, are used to reduce the lateral resistance of the top, thin diffused region above the p-n junction, the so- called "emitter" region. These grid fingers are connected together and

to a common top electrode which makes up one of the two electrical contacts to the solar cell, the other being an ohmic contact to the back

surface of the cell wafer.

It is well known that the energy conversion efficiency of a cell may be

severely reduced if the lateral resistance of the "emitter" is excessive,

since all the photocurrent is constrained to flow laterally along the

"emitter," and so almost all junction solar cells employ gridding to reduce this resistance [1] . Several gridding schemes may be employed. For

example, the grids may be in the form of radial "spokes," or it may be in

a crosshatch pattern, but the most common pattern is the "finger" configuration shown. This is the configuration which will be assumed in the

13
Figure 4. Representation of the two-dimensional parasitic resistancecapacitance nature of a solar cell.

14
analysis which follows. The purpose of this analysis is to determine the

conditions under which the laser flying-spot scanner, using light modulated at high frequencies, can be used to determine in a nondamaging manner

the sheet resistance of the "emitter." With this known through measurements made on the cells themselves, one can attempt to control its value

on later cells and optimize cell performance . After techniques have been

developed for in situ emitter sheet resistance measurements, one can then

work toward developing techniques for measuring variations in this resistance . The emitter sheet-resistance measurement technique which will be

described uses only the contacts to the cell (the electrode terminals)

which are always an integral part of the completed cell.

The cell is assumed reverse biased in figure 4. (If it were not so biased, the equivalent circuit would include additional passive elements

representing the junction diffusion admittance.) The state of affairs

depicted corresponds to an instant in time when the laser spot is approximately midway between the two grid stripes. The device equivalent

circuit is that of a distributed resistance-capacitance network due to

the "emitter" sheet resistance and the transition-region capacitance of

the reverse-biased p-n junction. The net result of carrier photogeneration due to the laser spot is to place a current generator across the

transition-region capacitance just below the light spot [2,3,4], as shown

in the figure. If the light is modulated, then the equivalent current

generator will have both ac and dc components. (There will always be a dc component, because it is not possible for light to have an average

value of zero I) Intuitively, one would expect that the ac voltage measured between the top and back electrodes to vary with the location of

the light spot, reading a maximum just at the edge of a grid, and a common trough-shaped minimum midway between a pair of grids . One would ex- pect this because of the symmetry of the grid pattern and because of

greater attenuation of the signal current by the two-dimensional

resistance-capacitance network as the distance between the light spot

and its nearest grid line is increased.

For the geometry shown, the analysis is simplified if a line of laser

light parallel to the grids is used, rather than a spot, and the scan15
0 ^

LOAD Z

X

I

Figure 5. The one-dimensional analog of the cell between a pair of grids

shown in figure 4. The current generator Iq represents the equivalent

current generator associated with the modulated laser light.

ning is performed by moving the light line parallel to itself. If the

line is much longer than the separation of the grids, the problem can be treated as one dimensional, and figure 5 is the equivalent electrical

circuit of the portion of the solar cell between two grids separated,

edge to edge , a distance I . Scanning with a line of light will probably

not be performed in the program. However, it is useful to analyze this

one -dimensional case because the solution can be obtained in a closedform expression which may be examined to anticipate the important results for the more realistic two-dimensional case. In figure 5 the

scanning line is at a distance p from the edge of the left grid. It is

clear that all such cell portions are equivalent. It is only necessary

to analyze one, eind the (as yet unspecified in value) impedance of the

rest of the solar cell may be lumped with the cell terminating impedemce

(also unspecified) yielding a resulting load impedance

The equations relating the line current i (x) which flows in the plane

of the "emitter," the voltage v(x) across the line, the capacitance per

unit length c of the line, the "emitter" lateral resistance r per unit

length, the angular modulating frequency w, and the coordinate distance

X are easily derived and are

( 1 )

( 2 )

16
where j = (-1) 1/2

Differentiating eq (2) and substituting into eq (1)

,

- B^v = 0

where

B = (jco/wo) ,

(3)

(4)

and WQ is the angular frequency

ojQ = (rc)~^ . (5)

The general solution of eq (3) is

v(x) = Ae^^ + Be"^^ . (6)

The constants A and B are chosen to satisfy the boundary conditions.

From eq (2) , the line current is ... $ -Bx 3 - Bx i(x) = — Be - — Ae . (7) r r

Two sets of equations similar to eqs (6) and (7) are required; one to

represent the line to the left of p, and the other for the line to the

right of p. Designating the quantities to the left by the subscript I,

and to the right by the subscript II, and with io representing the magnitude of the current generator due to the modulated laser light, the

two sets of equations are solved subject to the boundary conditions

ill(p) - ij(p) = io

Vi(p) = Vji(P)

( 8 )

(9)

v^(0) = = Z^[i^^(i) - i^(0)] (10)

The voltage across the load Z^, i.e., the output voltage the

scanning light line at the arbitrary position 0 ^ p ^ il is

= ioZ, out ) {sinh[B(il - p) ] + sinh(Bp)}

2BZ„ ( 11 ) {sinh(BH) + [cosh (BS-) - 1] }

17
As the laser light line is swept from one grid stripe to another,

undergoes a maximiam-minimum-maximum excursion having the ratio

V ^ (min) V ^ . ... . , out _ out/p - i,/2 _ 2

sinh (Bi^/2)

V .(max) V . , - ~ sinh(6J.) out out/p =0

( 12 )

It is noteworthy that this ratio does not depend on the effective load

impedance Z^. All the quantities in eq (12), with the exception of r, are known, or can be found readily for the cell using nondamaging methods, and so selecting a value of r which brings eq (12) into agreement

with the measured ratio v ^(min)/v ^ (max) yields the emitter sheet

out out

resistance

.

3.2 Workshops and Symposia

An important part of this program is to set up workshops and symposia

for DoE Headquarters on topics identified as important to the R&D New

Materials Branch objectives.

The first workshop will be on the Stability of (Thin Film) Solar Cells

and Materials and is scheduled for May 1 to 3, 1978 at the National Bu- reau of Standards in Gaithersburg, Maryland. One of its main purposes

is to identify and discuss problems and obstacles to achieving 20-year

life for terrestrial solar cells and to plan how these problems may be

overcome by the use of test and measurement procedures designed to en- hance the prediction and control of material and device stability.

This is a timely topic because DoE by 1985 will have committed itself

to a restricted choice for thin-film materials and configurations [5]

.

An outline of the workshop, as it was structured during the reporting

period, follows. The workshop was originally conceived by NBS to be a one-day affair, but as the needs of the thin-film solar cell community

were examined more carefully it became obvious that one day would not

be sufficient. After consultation with DoE Headquarters, it was decided that a more intensive workshop program was required to meet the

needs of the solar community. As a consequence, it was decided to

shift program priorities to emphasize the stability workshop activities.

By the end of this reporting period, the workshop was planned to occupy

two and one-half days.

18
The workshop outline detailed in Appendix A states its purpose and scope

as well as providing an overview of the specific discussion topics. It was mailed to members of the Workshop Steering Committee and discussed

at a meeting of the Committee at DoE Headquarters on December 12, 1977.

4 . REFERENCES

1. Hovel, H. J., Solar Cells, Semiconductors and Semimetals, Vol. 11,

R. K. Willardson and A. C. Beers, Eds, (Academic Press, New York,

1975)

2. Gardner, W. W., Depletion-Layer Photoeffects in Semiconductors, Phys

Rev. 116 , 84-87 (1959)

3. Anderson, L. K. , Detection of Microwave Modulated Light, 1964 International Solid-State Circuits Conference Digest of Technical Papers,

pp. 60-61.

4. Sawyer, D. E., A Study on p-n Junction Photodetectors Utilizing Internal Parametric Amplification, Sperry Rand Research Report SRRC-PR

65-34, issued April 1965, Final Report of Work Performed 1 April

1964 - 31 March 1965. Prepared for the Department of the Navy, Bu- reau of Naval Weapons, Research Division (RRRE-3) , Washington, DC.

Contract No. N600 (19) 61770.

5. U.S. Department of Energy National Photovoltaic Program Plan, February 3, 1978.

19
APPENDIX A

Outline of the Workshop

on Stability of (Thin-Film) Solar Cells and Materials

I . Purpose of the workshop

The purpose is to identify and discuss the problems and obstacles

to achieving 20-year reliability for thin-film solar cells and to plan

how these problems and obstacles may be overcome. The participants will

also be asked if, in their opinion, cell stability testing should be an automatic part of the cell development work for cells which use exploratory materials or configurations (in some cases, this might be done by

the same organization charged with developing the cell, but in most

cases would be done by some other)

.

A. Specific and desired outcomes from the workshop

1. Definition of the problem areas,

2. Determining how to structure research leading to better

cell and material stability,

3. Development of "standard," i.e., important, tests that

should be performed, and

4. Guidelines for the design of stable cells.

B . A number of benefits to the Advanced Materials R&D photovoltaic community should accrue in the process of achieving the

above goals, including

1. Identification of expertise in

a. stability considerations in general and

b. thin-film device stability,

2. Informing them on degradation phenomena,

3. Increasing their awareness of the need to "build in" cell

reliability, and

4. Providing them with some of the resources needed to achieve

built-in reliability and freedom from instabilities.

II . Scope of the workshop

The concern is with phenomena intimately related to the thin-film

cell proper, i.e., occurring in the volume bounded by the semiconductor

surfaces and the surfaces of the cell metallization. Analyses of general problems external to the active device, such as delamination of en- capsulants and changes in optical transmission of window materials, will

not be included. (The topics excluded should be covered adequately elsewhere — in Doe's single-crystal silicon and gallium arsenide array efforts, for example.) However, reliability and degradation problems

20
which are found for thin-film solar cells will not be excluded simply

because they may also be found in devices which are other than those of

concern to the Advanced Materials R&D Branch. This means, for example,

that the widely observed degradation of solar cells due to metallization

corrosion is a proper subject here, if the context is thin-film cells.

Ill . Workshop structure

A. Overview

The workshop will take the better part of two days and with a sequence which will begin with the experiences of other technical com- munities in engineering stability and freedom from the effects of degradation mechanisms and will move on to stability considerations appropriate to the thin-film solar cell community. This progression is chosen

because it should provide the participants with the background and perspective needed to appreciate the \abiquitous and complex nature of the

problem. It should also, by exposing the workshop participants to similar problems in these other fields, suggest means whereby solutions may

be found. Most of the workshop talks will be invited, but participation

of the audience will be encouraged.

B . Papers

The papers are expected to fall into the following three areas;

1. Papers which provide a perspective on the problem areas from

the vantage point of work performed on more mature devices by

larger technical communities (by the integrated circuit industry, and in the p-n j\inction single-crystal silicon solar

cell effort, for example);

2. Papers which address degradation and instability mechanisms

appropriate to thin-film photovoltaic devices; and

3. Papers which recommend appropriate tests and screens.

C. Some suggested topics for the morning of the first day (the

theme is "What we can learn from others")

1. Device stability, reliability and degradation-mode data ac- cumulated for the DoD community: what this should teach the

photovoltaic commxinity, and how to make use of this accumulated experience. The talk would be given by a representative of the Rome Air Development Command or the Reliability

Analysis Center.

2. The experiences of JPL, NASA/LeRC, Sandia in areas of general concern such as mechanical integrity, delamination of

cell constituents, changes in optical properties of window

materials, corrosion of cell metallization, etc.

3. Special concerns in concentrator cells.

21
4. Newer results suggesting that significant bulk/surface

changes may occur in some silicon p-n junction singlecrystal cells.

D. Some suggested topics for the afternoon of the first day

(the theme is "What are (or should be) the particular con- cerns of the thin-film community")

1. Growth with time of oxides beyond initial/ optimum thickness, in MOS "Schottky" cells,

2. Nature of hydrogen bonding may determine stability of

amorphous silicon cells,

3. Field-aided copper migration, and cadmium diffusion,

may degrade CdS/Cu2 S, or CdS/copper-temary cells,

4. General thin-film phenomena; for example, the consequences of grain boundaries such as enhanced diffusion,

and

5. Accelerated testing of thin-film materials and structures .

E. The morning of the second day , working groups will develop

position documents on, for example:

1. what are the degradation phenomena which one needs to test

for,

2. recommended test conditions, i.e., what are the tests which

are appropriate to carry out,

3. appropriateness of accelerated life testing, and

4. the use of special test vehicles (test patterns) to magnify

chcinges which would be occurring in the "real" cell, and so allow short-duration tests to predict the long-term cell

changes

.

F. The summary session would be held in the early afternoon of

the second day. This would:

1. summarize what has been learned, and

2. present recommended test and measurement procedures designed

to enhance the ability of one to predict material and device

stability.

22
Laser Scanning of

Active Integrated Circuits and

Discrete Semiconductor Devices*

D. E. Sawyer, D. W. Berning, and D. C. Lewis**

Institute for Applied Technology

National Bureau of Standards

Washington, D.C.

The device laser scanning work conducted in the Electronic Technology Division of the

National Bureau of Standards (NBS) is described. The scanner constructed at NBS is sketched briefly; this is followed by illustrations demonstrating its usefulness for de- termining on a point-by-point basis the inner workings of active semiconductor devices. The scanner is non-damaging to all devices tried and it has been used to map d.c. and

high-frequency gain variations in transistors, reveal areas of the device operating in a nonlinear manner, electronically map temperatures within devices, determine internal logic states in IC’s and selectively change these states at will. It has also been used to perform

hitherto impossible measurements on flip-chip bonded devices, that is, seeing the circuit

electrical operation and the metallization pattern through the back side of the chip. Applications to other strucures and devices including solar cells are suggesed.

T

he effect of directing light of energy somewhat

greater than that of the band gap on a semiconductor is to create electron-hole pairs within the material which can be collected as a photocurrent. If the light is focused to a spot and moved over exposed semiconductor portions of a device, it is possible to learn a

great deal about the internal operation of the device by interpreting the photoresponse. The interpretation is facilitated if the photoresponse is presented on the screen

of a cathode ray tube whose electron beam is deflected

in synchronism with the moving light spot since a “picture” of the photoresponse of the device is presented.

Interpretation of the display can yield, on a point-bypoint basis, information about the device which can be

used to determine internal characteristics. This probing

technique has applications in device testing, device design, and reliability. Provided that the photon energy of

the light is less than a few times the band gap energy of

the semiconductor one would not expect either short or

long term device degradation to occur due to the light

(provided of course that the optical power density at the specimen is not excessive). No device damage due to the

flying-spot scanner has been observed.

Scanner Description

The scanner to be described here is based on previous

work''* ® and several innovations. A schematic outline of the scanner is shown in Fig. 1; a photograph of the apparatus is shown in Fig. 2.

*This work was conducted as part of the National Bureau of Standards program on Semiconductor Measurement Technology with principal funding from the Defense Advanced

Research Projects Agency through ARPA Order 2397. ••Present address: Office of Naval Research, Washington, DC

22217.

Light for the laser scanner is provided by either of two low power He-Ne cw lasers. The wavelength of one laser is 1.15 ixm; the wavelength of the second laser

is 0.633 jam.

Mirrors M, and M, are used to fold the output from

the 1.15-;xm beam so that the scanner can be made

more compact and rugged; mirror M4 is used for the same purpose with the 0.633-/im beam. Mirror M3 is used to select which laser output is to be used with the scanner; it can be used to block the 1.15-^m beam and

simultaneously insert the 0.633-,/im beam into the scanner, or it can be used to deflect the 0.633-/xm beam out

of the optical circuit of the scanner. The analyzer shown at the output of the 0.633-/im

laser in Fig. 1 is used to control the intensity of the

radiation delivered to the device under test. Since the output of the laser is plane polarized, rotation of the analyzer can vary the transmissivity from 0 to 1. There

is no intensity control on the 1.15-/im laser.

DISPLAY SCREEN Fig. I —The light and signal paths of the dual laser scanner.

SOLID STATE TECHNOLOGY/June 1977

23
Fig. 2—Photograph of the laser scanner.

Fig. 3—Optical photograph of the surface topology for a microwave transistor consisting of four cells.

Mirrors V and H are electrically driven to provide

orthogonal deflections of the light beam. Light from mirror H passes through a microscope and is focused to a spot on the device to be examined. The same electrical signals that drive mirrors V and H also deflect a spot on

a cathode ray display screen in synchronism with the

laser scan. The display screen is on the lower left of Fig. 1. The lens Li between the vertical and horizontal deflection mirrors refocuses the vertical deflection from V

onto the horizontal mirror, H. The beam diverges from

the horizontal deflection mirror, H, to form the scanning

raster. The scan raster typically covers the same field of view that can be seen with the eye when the microscope

is used in its customary manner. Light reflected from the specimen is used to identify the portion of the specimen

being scanned. The reflected-light circuit uses a half- silvered mirror in the microscope which is an integral

part of the microscope’s vertical illuminator, a lens L,, and a photocell. Laser radiation reflected from any point on the specimen is directed by the half-silvered mirror onto the lens and is focused to a fixed point on the

photocell. TLe photocell signal modulates the display screen to present a picture of the device surface topography. Used this way, one could call the apparatus a

“flying-spot microscope.” The primary purpose of the

reflected-light circuit is to permit correlation of the device response with surface features such as metallization areas. This is accomplished simply by mixing to- gether the signals from the scanned specimen and the

photocell. Alternatively, a color display screen has been

used with the photocell signal fed into one color channel and the electrical signal (s) from the scanned speci- men into the other(s). A recent publication'* describes

in detail the construction and operation of the entire scanner. Two lasers are used for a greater measurement flexibility. Visible or near-infrared radiation incident on sili- con 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. The visible light from the 0.633-/tm

laser has a characteristic penetration depth of about 3

i(xm in silicon.® Because most modern silicon devices have their active regions within a few micrometers of

the surface, the 0.633-/im laser is quite effective in exciting active regions of silicon operating devices. The

intensity at the specimen can be varied to produce junction photocurrents over the range from about 10 pA to about 0.1 mA.

Silicon at room temperature is almost transparent to the 1.15-/xm infrared radiation from the second laser; the characteristic penetration depth of this radiation

is about 1 cm.® The infrared laser is used for three

classes of measurements: 1) examination of the silicon- header interface; 2) device temperature profiling; and

3) examination of the device through the backside of the silicon chip. Each of these applications makes use of the penetrating nature of the radiation. In the first application, the reflected-light circuit is used to look through the silicon wafer and observe irregularities at the silicon-header interface in the “flying-spot microscope” mode. The second application makes use of the temperature sensitivity of the silicon absorption; a larger

signal is produced on the display screen for those device

portions which are warmer than others. Utilizing this sensitivity, one has an electronic technique for thermal mapping of devices which appears to have a number of advantages over the more traditional methods.® The

third application uses the penetrating infrared radiation

to photogenerate carriers deep within silicon devices;

this capability allows the operation of devices that are bonded face down (e.g., beam lead devices) to be ex- amined.

Mapping of Device Electrical and Thermal Characteristics Figure 3 shows photographs of the metallization pattern of a type 2N4431 UHF transistor. This widely

available type is designed to furnish 5 W at frequencies

up to 1 GHz. There are four in-line cells electrically connected in parallel; these are shown in the left photograph. The total active area of the transistor is a rec- tangle 1.2 mm long and 0.15 mm wide. The emitter

24

June 1977/SOLID STATE TECHNOLOGY
%

and base fingers are interdigitated with the emitter fing- ers coming in from the right, and the base fingers coming in from the left. The finger metallization is 2 /jn in width, and the stripe separation is 8 /im. The metal

stripe separation allows the active device regions to be

accessible to laser irradiation. For the results that will be shown, the devices were scanned while they were

connected and biased in the common-emitter configuration. The signal for the display screen was taken from a 60 ohm resistor which served as the collector load. For

several of the transistors, it was found that regions of high temperature, so-called “hot-spots,” would form

within the acceptable operating range listed in the manufacturer’s data sheet. This observation was used to aid

in understanding the many things the scanner can tell us about the way a device really works. Figure 4 shows

photographs of the display screen. The upper left image

is the infrared response for a collector-emitter voltage

of 26 V and a collector current of 250 mA. The transis- tor is operating just outside of the hot-spot region. Little of the incident optical energy is absorbed in the device

active regions so the display screen signal is weak. The

infrared response was actually quite uniform over the

device, and the apparent nonuniformity captured in this photograph was due to system noise which had the ef- fect of modulating the presentation of the photoresponse.

The lower left photograph was made for the same scanning conditions but with the bias adjusted slightly to put

the transistor into hot-spot operation.® The region of enhanced photoresponse, the white area, is the hot-spot

region as confirmed with the use of a passive infrared microscope. The 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 temperature, the number of pairs photogenerated increases with temperature

because the optical absorption coefficient increases with

temperature. This is due primarily to the well-known

change in the silicon bandgap with temperature and an

increase in the phonon population. It is possible to as- sign temperature values to this thermal-enhancement of

the 1.15-,/im response, and a paper describing the details

is in preparation.^

The upper right photograph is for the same hot-spot

conditions, but it was made using the response to the

visible laser. Essentially all of the incident light is ab- sorbed in the active regions at room temperature and so no enhancement in the hot-spot is observed. During normal operation, each laser produces optical radiation at a series of discrete wavelengths centered about the nominal wavelength corresponding to the in- dividual allowed axial modes of the laser. Self-mixing

of these wavelengths modulates the light simultaneously

at several frequencies. The modulation frequencies are

multiples of a fundamental one, and for the particular

visible laser used, the light is self-modulated at 500 MHz

and 1.0 GHz to a degree adequate for determining the response of devices to light modulated at these frequencies. One inserts a radio receiver between the specimen

Fig. 4—Electrical responses of microwave transistor. Top

left: infrared photoresponse outside the hot-spot regime;

bottom left: infrared photoresponse in the hot-spot regime;

top right: visible photoresponse in the hot-spot regime;

bottom right: visible photoresponse with the modulated

light at 500 MHz.

and the display electronics and tunes the receiver to the selected frequency.

The lower right photograph in Fig. 4 is for the same

bias and optical wavelength as for the upper right photograph. In this case the radio receiver is used and the image produced is the response of the transistor to the component of the laser light modulated at 500 MHz.

This image is interpreted as the spatial map of the 500-

MHz gain of the transistor. It is well known that the hot- spot formation can influence high-frequency device

gain.® The hot-spot area is revealed quite dramatically,

and it can be seen that there are actually two hot spots symmetric about a cell boundary.

By scanning a device with an electrical signal applied

to its usual input port, one can map on the display screen the regions of the device which are operating nonlinearly at the signal frequency. All that is required is that an i.f. amplifier and detector, tuned to the differ- ence frequency of the electrical signal and a laser mode

beat, be inserted between the device output and the dis- play screen.® Nonlinearities can indicate stress points

and so this mapping may be useful in predicting certain types of device failures. It may also be useful in selecting

linear devices for applications in which many devices are used cascaded throughout a communications system;

component nonlinearity in such .applications is to be

avoided since it may produce cross-talk between channels. For these reasons, this unique ability to map device

nonlinearities should make the laser scanner invaluable

for device design, testing, and reliability.

Internal Operation of Integrated Circuits The results of scanning a type 7438 dual-input NAND

gate IC with 0.633-/im light are shown in Fig. 5. The

schematic of the bipolar IC appears in the upper right

quadrant along with the truth table. The output transistor (the output is labeled Y) is normally in its high, or

SOLID STATE TECHNOLOGY/June 1977

25
Fig. 5—Type 7438 dual-input NAND gate. Top right: sche- matic and truth table; top left: reflected-light image; bottom

left: photoresponse superimposed on reflected-light image

with output in high state; bottom right: photoresponse

superimposed on reflected-light image with output in low

state.

Fig. 6—Photograph of the display when scanning through

the back side of a silicon-on-sapphire C-MOS device.

The photograph in the lower left quadrant of Fig. 5

is the superposition of the scanned IC’s electrical re- sponse and the reflected-light metallization signal for the 001, 101, and Oil states; all give the same results. The

output transistor is in its high state, and an appreciable

photoresponse is obtained from portions of the basecollector junction not covered by metallization. The input

transistor is located directly below the output transistor, and very little photoresponse is observed from it because no appreciable amplification of the photocurrent generated in the input transistor can occur unless the transistor is out of saturation, i.e., unless both inputs are in their high states. For the photograph in the lower right quadrant of

Fig. 5, the circuit state is 110, and the output transistor

is in saturation with little photoresponse obtained from

it. The input transistor is out of saturation, and the two L-shaped areas, one under another, below the out- put transistor are the unobstructed base-collector junction regions around the emitters. TTie illuminated re- gion to the left is the inter-stage transistor, and the hori- zontal bar below this is the 4K ohm load for the input

transistor. MOS IC’s also have been inspected with the laser scanner. In addition to observing the states of all the

logic elements, and the states of the input-output cir- cuits in a shift register, it has been possible to change

the state of a desired logic element from 0 to a 1 or vice versa nondestructively with the laser spot.'® This ability could have far-reaching implications for the testing of LSI circuits, particularly for testing those portions that are embedded, i.e., with no direct connections to external leads.

nonconducting state, and to switch it into its low state,

i.e., into saturation, requires that both inputs A and B be

in their high states. The overall NAND gate operation

is specified by the trio of numbers representing A, B, and Y, in that order. As an example, the case in which

A and B are high to cause the output to go low is listed in the table as 110. The top left photograph was generated solely by the scanner’s reflected light circuit and

it shows the metallization. The output transistor occupies the upper right quadrant. To reveal the NAND

gate’s internal operation,- it was biased for normal ser- vice with a 330 ohm resistor inserted in the supply bus,

and the voltage variation across this resistor during laser scanning modulated the display screen. A transistor operating with significant small-signal current gain will show a large phbtoresponse and consequently 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 al- most no image. Thus, from the photoresponse of the elements in the circuit, the different logic states can be

deduced. Naturally, changing the state of the NAND

circuit yields a new photoresponse display of the active elements.

Examining IC’s Through the Wafer

In the work described so far, the scanning light beam

was incident on the side of the chip containing the active circuit. But, it is possible to scan a device using

penetrating light such that the light enters the back

surface and passes almost entirely through the chip before being partially absorbed in the device active region or reflected by the metallization. The 1.15-/un He-Ne

laser wavelength is nearly optimum for accomplishing

this in silicon devices. Because of the penetration, photocurrents from deep within the chip may contribute to the overall response. Light scattering due to back surface irregularities must be minimized in order to maintain resolution, and

this requires either that the back side be smooth, or that a drop of liquid be placed on the surface having an

index of refraction approximately equal to that of the semiconductor.

Back surface scanning was successfully tried on three

different types of devices: C-MOS on sapphire, bulk C- MOS, and a bulk bipolar circuit. The C-MOS on sapphire circuit, a type 4007, was

scanned from the back side using visible light (X =

0.633 >un) because sapphire is optically transparent. A

drop of light oil was deposited on the back of the de26

June 1977/SOLID STATE TECHNOLOGY
vice substrate to provide an optically smooth surface. The photograph of the scanner display shown in Fig. 6 was obtained using the reflected light signal mixed with

the photoresponse of the inverter. Of the six transistors shown in this figure, only the lower right hand one was

electrically connected. The five brightly lit stripes are

the gate area of the electrically “on” device. The bulk silicon C-MOS integrated circuit was also

a 4007; however, since the substrate was silicon, it was

necessary to use the 1.15-/un wavelength. Figure 7 shows the reflected light image. No oil was needed as

the back side was a polished surface. The third device was a beam-leaded bulk silicon bi- polar 709 operational amplifier. In this case oil was

deposited on the back of the de.vice die to provide an

optically smooth surface. Figure 8a shows the 1.15-/un

scanning image obtained when the amplifier was connected to provide a gain of 10 and with 0 V input. The

reflected-light signal is superimposed on the photoresponse. Figure 8b shows the device for the same conditions except that the input voltage is now —1.5V, a value sufficiently large to saturate the 709. These three examples demonstrate a capability of the scanner which can be very useful when metallization on the top of the chip obscures the underlying device

active regions but these regions are accessible from the back, or when it is desired to inspect devices bonded

into frames or packages “face down” with direct access

to the active surfaces blocked by mounting materials.

Application to Photodetectors and Solar Cells The laser scanner is an obvious candidate for analyzing photodetector operation. Devices made from a number of materials, including silicon and indium arsenide,

and designed for optical communication, have been

scanned with unmodulated light, and light modulated at frequencies up to 1.2 GHz, to study gain and spatial response variations. These devices have been both those

that utilized internal (avalanche) gain, as well as those which utilized simple photodetection. Even vacuum devices have been scanned; measurements have been made

of the uniformity of cathode response for a 931 -A photomultiplier to 0.633-jum light modulated at 120 MHz.

Experimental solar cells designed for space applications

have been laser scanned and the results compared with

those obtained with a scanning electron microscope in the electron-beam-induced-current (EBIC) mode. The

defects located with the SEM were also found with the

laser scanner, in these preliminary experiments. The

scanner described in this paper should also be useful in observing variations in solar cell response with light

level, and this could be invaluable for cell design. This

could be done by scanning the cell with a “small sig- nal” laser while at the same time exposing the cell uni- formly to an auxiliary source which would be varied

step-wise in intensity.”

Summary

The scanner work described makes use of the fact

Fig. 7—Photograph of the display when scanning through

the back side of a bulk silicon C-MOS device with the infra- red laser.

Fig. 8—Images obtained when scanning a commercial flip- chip operational amplifier with the infrared laser, (a) In- put at 0 V. (b) Input at —1.5 V.

that semiconducting materials are .photosensitive as part

of their basic nature. Thus, semiconductor devices such

as diodes, transistors, and entire integrated circuits made

from these materials can be studied using optical radiation. The effect of the optical radiation is to generate

electron-hole pairs in a non-damaging way within the specimen. These current-carriers can stimulate device behavior by taking the place of signal current-carriers which are normally supplied by leads fixed to the device. In contrast to signals applied via the leads, which are

fixed in position, the optical excitation can be moved

SOLID STATE TECHNOLOGY/June 1977

27
Laser Scanning

^ver the surface and within the bulk, and the response

of the structures can be studied on a point-by-point

basis to learn the inner workings of the device. In several cases, the scanner has been used to observe

device phenomena which one has not been able to study

before. The scanner can map the flow of logical information in MOS and bipolar IC’s. It can map the operation

of transistors at ultrahigh frequencies; it can electronically map the temperature distribution; it can pinpoint

the portions of operating devices which are operating in a non-linear manner; and it has the unique ability to look through the back side of silicon chips to observe

circuit operation.

References

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

2. McMahon, R. E., “Laser Tests IC’s with Light Touch,”

Electronics, vol. 44, 92-95, (April 12, 1971).

3. Beiser, L., ‘Xaser Beam Scan Enhancement through Peri- odic Aperture Transfer,” Applied Optics, vol. 7, 647-650

(1968).

4. Sawyer, D. E., and Berning, D. W., “A Laser Scanner for Semiconductor Devices,” NBS Special Publication 400-24, February 1977. Available from the Superintendent of Documents, U.S. Govt. Printing Office, SD Cat. No. C13.10:

400-24.

5. Dash, W. C., and Newman, R., "Intrinsic Optical Absorption in Single-Crystal Germanium and Silicon at 77° and

300°K," Phys. Rev., vol. 99. 1151-1155 (1955).

6. Sawyer, D. E., and Berning, D. W., ‘Thermal Mapping of

Transistors with a Laser Scanner," Proc. IEEE, vol. 64, 1634-1635 (1976).

7. Sawyer, D. E., and Lanyon, H. P. D., "Determining Sili- con Device Temperatures By Measuring the Thermally-En- hanced 1.15 Aim Photoresponse,” to be published.

8. Oettinger, F. F., and Rubin, S., “The Use of Current Gain

as an Indicator for the Formation of Hot Spots Due to Current Crowding in Power Transistors,” IEEE 1 0th Annual Proceedings, Reliability Physics, 12-18 (1972).

9. Sawyer, D. E., and Berning, D. W., “Mapping Nonlinearities Over the Active Regions of Semiconductor Devices,”

Proc. IEEE, vol. 64, 1635-1637 (1976).

10. Sawyer, D. E., and Berning, D. W., "Laser Scanning of MOS IC’s Reveals Internal Logic States Non-destructively,” Proc. IEEE, vol. 64, 393-394 (1976).

11. Lanyon, H. P. D., Worcester Polytechnic Institute, Worcester, MA, private communication.

June 1977/SOLID STATE TECHNOLOGY

28
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC ATION OR R IMPORT NO. NBSIR 78-1488

2. Gov’t Acccs.sion No.3. Recipient ’.s Acce.s.sion No.

4. ITTLL AND SUBTITLE

Measurement Technique for Solar Cells: Quarterly Report

September 15 to December 31, 1977

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

D. E. Sawyer, H. K. Kessler, and H. A. Schafft

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

Task Order A054-SE

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

Advanced Materials R&D Branch

Division of Solar Technology

Department of Energy

20 Massachusetts Avenue, N.W. Washington, D. C. 20545

13. Type of Report & Period Interim

Sept. 1 5-Dec. 31 , 1 977

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) This is the quarterly report of the work performed in the three month period

September 15 - December 31, 1977. The objectives of the program are to assist the DOE

thin-film photovoltaic effort by developing solar cell device and material measurement

techniques using the NBS-developed laser flying-spot scanner, and by assisting DOE in

organizing and hosting appropriate workshops and symposia and providing general consultation and liaison services.

Several possible applications of the laser flying-spot scanner to solar cells are discussed including measurement of the spatial variation in cell response versus bias

light to map cell behavior under various insolation conditions, measuring magnitude

and spatial variation in cell emitter sheet resistance, and determining emitter metallization regions making poor ohmic contact. Additions and modifications to the optical

and mechanical portions of the scanner made to enhance its usefulness for solar cells

are described. An analysis is presented for scanning, with light modulated at high

frequencies, the most common solar cell geometry. The results predict that the cell

emitter sheet resistance can be obtained by analyzing the laser display screen presentation. Scanner background information is included, in an appendix.

The outline of the Workshop on the Stability of (Thin Film) Solar Cells and Materials

is presented. This workshop is scheduled to be held May 1-3, 1978 at NBS, Gaithersburg

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)

Device measurements; laser scanning; light-biasing; metallization; ohmic contacts;

reliability; semiconductor measurements; sheet resistance; solar cells; solar cell

stabi 1 ity.

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