NAT'L INST. OF STAND & TECH

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Ref. 1T509?'^ OCT14,975

f5£l NBS TECHNICAL NOTE 591

A Technique for Measuring

the Surface Temperature of

Transistors by Means of

Fluorescent Phosphor
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NBS TECHNICAL NOTE 591

ISSUED JULY 1971

Nat. Bur. Stand. (U.S.), Tech. Note 591, 49 pages (July 1971)

CODEN: NBTNA

A Technique for Measuring the

Surface Temperature of Transistors

by Means of Fluorescent Phosphor

David J. Brenner

Measurement Engineering Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

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CONTENTS

Page

Preface iv

Introduction 2

Purpose 2

The Need for Temperature Measurement 2

Techniques for Measurement of Surface Temperature ... 3

Discussion of Experiment 8

Experiment Outline 8

Mechanical and Electrical Design 8

Transistors Tested 11

Deposition of the Phosphor Coating 12

Ultraviolet Lamp Characteristics 19

Ultraviolet-Light Monitor 19

Filter Selection 20

Camera Optics 21

Film and Processing 23

Microdensitometer 23

FiLm Image Evaluation 24

Analysis and Evaluation 28

Purpose and Conventions 28

Stability of Transistor Parameters 28

Phosphor Characteristics ' 29

Heat Transfer 31

Temperature Range 32

Limitations of the Film 34

Calibration Negatives 36

"Identical" Negatives 38

Spacial Resolution 4-0 Stray Light 4-1 Vacuum and Air Environments 42

Irreversible Effects 42

Practical Results 43

Summary 45

References 4&

in
PREFACE

This NBS Technical Note is based on the author's Master's Thesis,

"Application of the Phosphor-Ultraviolet Technique to Measurement of

the Surface Temperature of Uncapped Power Transistors", which was submitted to the University of Maryland in 1969. The work was performed

at, and supported in part by, the National Bureau of Standards. The

author wishes to thank Harry A. Schafft of NBS for suggesting the topic,

and for assistance in the preparation of this Technical Note. Thanks

are also due to John R. Maurey, also at NBS, for use of the microdensitometer employed in the work.

IV
A TECHNIQUE FOR MEASURING THE SURFACE TEMPERATURE OF TRANSISTORS

BY MEANS OF FLUORESCENT PHOSPHOR

David J. Brenner

A technique that uses temperature-sensitive phosphor

(TSP) to make quantitative measurements of transistor-chip

surface temperatures over the range of 170 - 210 C is

described. The experimental procedure consisted of depositing a thin coating of phosphor on uncapped power transistors,- and then photographing the visible fluorescence

generated under near-ultraviolet (uv) excitation. The

cooler regions produced a brighter fluorescence than the

hotter areas. The surface temperatures of operating

transistors were determined by comparing, on a point-bypoint basis, the photographic density of operating photographs to the density of a family of calibration photographs. Repeated temperature measurements always fell

within a 4 C overall error range when the phospor surface was scanned with a 25 x 25 (M aperture. The error band

dropped to less than 2 C when a 50 x 50 \m aperture was used.

Coarseness of the phosphor coating seemed to limit spacial

resolution to about 20 \m. Any narrow temperature range

between 20 and 4-00 C can probably be measured by selecting

a suitable level of uv irradiation and/or a suitable phosphor.

Key words: Surface temperature; temperature measurement;

temperature-sensitive phosphor; thermographic phosphor;

transistor-chip temperature.
1. INTRODUCTION

1.1. Purpose

The suitability of temperature-sensitive phosphor (TSP) for making

quantitative temperature measurements has not been reported in the

literature. It is the purpose of this report to provide information on

this subject. In particular, the use of TSP in making quantitative

measurements of the surface temperature of power transistors operating

under steady-state (dc) conditions is described. Emphasis is placed on

describing the experimental techniques, defining the limitations of the

apparatus, and determining the reproducibility of the temperature measurements obtained. Although papers by Blanks [l] and Goryunov, Ovechkin, Tolkacheva, and Feoktistov [2] describe the use of TSP to obtain

temperature measurements on transistor chips, neither paper stated the

measurement accuracy obtained.

1.2. The Need for Temperature Measurement

Most of the heat developed by a transistor is produced in its

collector junction, where the highest temperature is found. In power

transistors it is common for the collector current to be distributed

unevenly over the area of the collector junction [3], thus causing

nonuniform heating of the junction and local hot spots.

Hot-spot temperature is an important parameter because it strongly

influences transistor life expectancy [4]. Measurement of the collectorjunction temperature profile provides information that is very important

for reliability studies, the design of transistor structures, and determining temperatures produced by actual circuit conditions.

Since the collector junction is buried within the silicon chip

that forms the transistor, its temperature cannot be measured directly.

An indirect measurement technique that is frequently employed is the

utilization of a temperature -sensitive electrical parameter of the

transistor as a temperature sensor [5]. This approach is very convenient because the electrical parameters can be measured at the terminals

2
of encapsulated transistor chips. However, the temperature -sensitive

1

parameter technique is not satisfactory if local hot spots are present

because averaging of the parameter over the whole junction area results

in underestimating the hot spot temperature [3]. When non-uniform heating occurs, collector junction temperatures can be measured only on a transistor whose junction is close to the surface of the silicon chip;

it being assumed that the chip surface temperature, which can be measured, will closely approximate the junction temperature. Whether or not this is true is a judgment that must be made individually for each

transistor tested. This report does not attempt to resolve the problem.

1.3. Techniques for Measurement of Surface Temperature

Infrared Microradiometer . Quantitative surface-temperature measurements on transistors can be made with an infrared microradiometer

(IRM). An IBM measures temperature on a point-by-point basis, with a spot size diameter in the order of 25 um. Basically the technique

employs a microscope designed for use in the infrared, and a detector

which measures the infrared power reaching the microscope "eyepiece."

The temperature indication is an electric signal and can be read on a meter, recorder, or other instrument. What the IRM actually measures

2

is the infrared radiance of the surface under test. It is necessary

when operated at collector-to-base voltages above about 20 V, transistors are likely to develop one or more hot spots. A large increase in

the junction-to-case thermal resistance occurs at the hot spot. Thermal

resistance measurements made using the temperature-sensitive parameter

technique will show the increase in thermal resistance qualitatively

and can be used as a test for the onset of hot spots.

2

Radiance describes the strength of radiation emitted from a surface,

and is defined to be the power emitted per unit solid angle per unit

area. For example, radiance can be expressed as (w/sr)/cm , where sr means steradian.
to paint the transistor surface with an opaque coating if accurate

measurements are to be obtained. This is required because silicon and

silicon dioxide transmit infrared and thereby allow radiation (in particular, recombination radiation) that originates within the transistor

to be detected by the IRM [4]. The opaque coating also provides a surface of uniform and reproducible emissivity. N At the present time

the IRM is the only commercially available instrument that can make

quantitative measurements of transistor surface temperature. Although

the IRM can provide good accuracy and spacial resolution, its cost

and complexity place it out of reach of many who have need of its capabilities.

Liquid Crystals . A relatively simple and inexpensive technique

sometimes used for surface temperature measurement employs liquid crystals [6]. These are organic materials which maintain a fair amount of

molecular order when in the liquid state. The ordered molecules of

some liquid crystals can function as an optical grating whose internal

spacing is a function of temperature and can produce temperature-dependent, reversible color changes. The colors are produced by selective reflection of the spectral components of white light. To measure

surface temperature, the surface is coated with a thin layer of a suitable liquid crystal and then illuminated obliquely by a beam of

white light. If the eye of the observer is maintained at an appropriate

position, then the color seen will depend on the temperature of the

liquid crystal. In order to prevent reflection of unwanted light from

the substrate surface, it is usually necessary to blacken the substrate

before it is coated with the liquid crystal. Although liquid crystals

for thermographic work are available for temperatures from C to at

least 230 C, whether they can provide quantitative measurements at the

higher temperatures has not been reported in the literature.

Temperature-Sensitive Phosphor . Another relatively simple and

inexpensive method of making surface temperature measurements is provided by temperature-sensitive phosphors (TSP). The use of phosphors

in making temperature measurements originated with work done by Urback,

Nail, and Pearlman [7] in the 194-0' s. In the succeeding years, improved

phosphors and a variety of procedures for applying the phosphor to the

4
surface under test have been achieved [$] . This technique employs

phosphors which exhibit a strongly temperature-dependent fluorescence

during irradiation by near-ultraviolet (uv) light. Good temperature

sensitivity in a desired temperature range is achieved by selecting a suitable combination of phosphor and uv intensity. In its simplest

form the technique consists of coating a surface with a thin layer of

phosphor and then observing the visible fluorescent pattern that is

produced under uv irradiation. This arrangement suffices to obtain a qualitative picture of the temperature profile, which is sometimes all

that is needed, as when only location or size of a hot spot is of

interest (see photographs in figure l). If the test conditions are carefully controlled, then the radiance of the phosphor fluorescence

can provide quantitative temperature measurements. The remainder of

this report describes the use of TSP in making quantitative temperature

measurements.
(a) (b) (c)

V = 20 V

CE

I. = 800 mA

o T = 195 °C max

«•*• m

V

0E

=^° V

I c

= 400 mA

T = 210 °C max

V^ - 60 V

I. = 267 mA

o T = 215 °C max

All photographs are 16X enlargements of the same 2N3767 silicon power

transistor dissipating 16 watts. The transistor is mounted on a heatsink held at 40 °C.

Figure 1. TRANSISTOR HOT SPOTS. This family of photographs shows how

hot spots are displayed by a phosphor-coated transistor chip.

The photographs show the fluorescent pattern produced under

uv illumination. The location, size, and relative temperature of the hot spots (dark region on photographs) are obvious. Note that the hot spot temperature increases as the

collector-to-emitter voltage of the transistor is increased,

even tnough the total power dissipation is held constant.

This effect was seen in all transistors tested.
A. vacuum photodiode

B. aluminum housing

C. uv-lamp bulb

D. uv-transmitting filter

E. bakelite thermal insulator

F. ground-glass diffuser

G. plain-glass diagonal

H. light stop

I. TUT

J. heat sink

K. vacuum gauge

L. film

M. 35-mm camera body

N. extension tube

0. focusing lens

P. uv-absorbing filter

Q. uv-light absorber

R. small plain-glass diagonal

S. optical window

T. channel for cooling fluid

U. resistance heater

V. thermocouple

W. vacuum chamber

X. outlet to vacuum pump

drawing not to scale

Figure 2. DIAGRAM OF APPARATUS.

7
2. DISCUSSION OF EXPERIMENT

2.1. Experiment Outline

A brief description of the experiment will provide a framework

for presenting the material in this section. The silicon chip of the

transistor under test (TUT) was coated with a thin layer of phosphor

and then mounted on a temperature-controlled heat sink. Light from a uv lamp was reflected down onto the TUT by a small glass diagonal and

caused the phosphor coating on the TUT to produce a green fluorescence.

The fluorescent light passed upward, through a uv-absorbing filter and

a lens, to a photosensitive film. An 8X enlargement of the phosphor

coating was recorded by the film. After development the film was analyzed with a microdensitometer to extract the temperature data. In

order to obtain quantitative temperature measurements the variables of

the measuring system, such as uv intensity and film development, were defined and maintained constant throughout the course of the experiments,

The reproducibility obtained when measurements were repeated was used

as the criterion for judging performance of the TSP.

2.2. Mechanical and Electrical Design

Figure 2 shows the experimental apparatus used. A heat sink (J),

made from a 3/8" thick copper plate, accommodates a chromel-alumel

thermocouple (V), resistance heaters (U), and channels (T) for a cooling fluid. The thermocouple is soldered to the bottom of a narrow,

1/4" deep hole drilled in the plate, and is located under the geometric

center of the case of the TUT. It should be noted, however, that the

transistor chip is not necessarily located at the center of its case. The heating elements enable the heat sink to serve as a hot plate for

use in calibrating phosphor radiance as a function of temperature.

A cooling fluid, either water or air, is contained by 5-mm outside diameter glass tubes that pass through the heat sink and the wall

of the vacuum chamber (W). These two tubes also provide the mechanical

support for the heat sink. In order to ensure a good thermal path

between the glass tubing and the heat sink, the outside of the tubes

8
was ground to produce a snug fit in the heat sink. A coating of thermally conductive, zinc-oxide loaded, silicone grease was used to further

improve the thermal contact. The steps taken to provide good thermal

contact are important because the heat transfer ordinarily provided by

air is not available when the chamber is evacuated. With a water flow

of 5 cm /s, the thermal resistance between heat sink and coolant was

about 1.7 C/W. This is close to the thermal resistance calculated for

the wall of the glass tube alone and is not affected by vacuum conditions. The glass tubes pass through oversize holes in the wall of the

vacuum chamber, and vacuum sealing is achieved with silicone rubber

to allow movement of the tubes as the heat sink changes dimensions.

Heat-conducting grease was also used between the heat sink and

TUT. It was necessary to remove the excess grease which squeezed out

as the TUT was bolted to the heat sink. Otherwise, volatile components

of the grease were found to evaporate at elevated temperatures and con- dense on the optical window (S) of the vacuum chamber. Power resistors

having axial leads were used for the heaters. (Their ceramic bodies were out-of-round and were not altered. ) Installation consisted of

coating the resistors with a water-base ceramic cement and then, before

the cement dried, inserting them into holes drilled in the heat sink.

This construction did not result in good thermal contact between heater

sink in vacuum; consequently, the heaters ran much hotter than the heat

sink. In fact, if the heaters were maintaining the heat sink at a fixed elevated temperature under vacuum, then quickly releasing the

vacuum would cause the temperature of the heat sink to increase several

degrees.

The vacuum chamber was evacuated by a mechanical vacuum pump. A

pressure of less than 0.1 mm Hg was maintained during the operation of

3The process of grinding to the required diameter also improved the

roundness of the glass tubing. Before grinding, its outer surface was

about 1% out-of-round.
the equipment and was continuously monitored by a thermocouple vacuum

gauge (K). This modest vacuum was sufficient to provide thermal insulation and protect the transistor and heat sink from excessive oxidation at elevated temperatures. The collector-to-base leakage current

of some transistors was observed to permanently increase severalfold

when the silicon chip was heated in air.

The electronic equipment used in the experiment is conventional

in its design. One or more regulated power supplies, which were manually

set to the voltage desired, provided the collector-to-emitter voltage

for the TUT.

The voltage was measured by a potentiometer having an error of

less than ±0.5% and reproducibility to within ±0.2%. The desired collector current was also set manually, and a servo amplifier maintained

this value by adjusting the base drive to the TUT. A null type of

circuit measured the collector current with an error of less than

±0.6% and reproducibility to within ±0.2% o A current-limiting resistor

was placed in series with the collector of the TUT to protect the

equipment in the event that the TUT broke down. Sometimes the TUT

was also saved. The collector-to-emitter voltage was monitored by an oscilloscope to check for oscillation. Some of the transistors tested

did oscillate at about 1 MHz in the test circuit used, but the connection of a 10 nF capacitor between the collector and emitter terminals

stopped the oscillation.

A servo control that governed the power applied to the heaters

maintained the heat sink at any desired temperature between 20 C and

2-46 C. The heat-sink thermocouple, whose reference junction was in an

ice bath, provided the control signal. Heat-sink temperature was known to within ±2 C of its true value, and was believed to be reproducible to within a 0,5 C overall range. The intensity of the uv lamp

(C) was stabilized by a servo control that used a vacuum photodiode

(A) to monitor the uv. Film exposure time was measured by an electric

stopclock that was actuated by the flashbulb contact in the camera body (M).

10
2.3. Transistors Tested

Any power transistor that has the semiconductor surface unobscured

(as by connecting wires, heavy metallization, etc. ) is a satisfactory

test object. Three medium-power transistor types, the 2N3739, 2N3741,

and 2N3767 were chosen for experimentation. The transistors are rated

to dissipate either 20 W or 25 W when their case temperature is 25 C,

and to operate with junction temperatures up to 175 C or 200 C. The

higher ratings apply to the 2N3741. Junction temperatures well above

these values were frequently produced during the experiments and usually

did not noticeably degrade the transistors.

All of the transistors tested came packaged in T0-66 (small diamond

shape) cases. In order to reach the silicon chip, either a lathe or a small saw was used to carefully cut off the cap of the transistor case. The active portion of each transistor was a square silicon chip approximately 1.5 mm on an edge, and whose surface was composed of silicon

dioxide and aluminum. A protective coating of silicone varnish or silicone rubber was frequently found on top of the chip and required

removal before a phosphor layer could be deposited. The aluminum

metallization on the emitter and base fingers of the chip was soft

and was sometimes scratched and damaged during the removal of the

protective coating. Also, the wires that connected the emitter and

base regions to the header terminals were sometimes broken by handling.

These were thin, soft aluminum wires, and the strength of the bonds at

the wire ends was low. A minor problem encountered was that the transistor chips were randomly located on their headers, thus making it

necessary to provide a means of bringing the off-center chips into the

field of view of the camera. The maximum placement difference observed

was approximately 2 mm overall. Adjustment was made by sliding the

vacuum chamber (W) on its upper lid, as required.

Several transistor types were found that were quite rugged mechanically, but these were discovered too late to be employed in this

project. Two such types were the 2N3583 and 2N3878. The main advantage

of these transistors was that the aluminum metallization on the silicon

11
chip was protected from mechanical damage by a hard, glassy layer of

aluminum oxide. Another advantage was that the electrical conductors

joining the chip to the header terminals were sturdy and firmly anchored

at both ends.

2.4. Deposition of the Phosphor Coating

The thickness chosen for the phosphor coating is determined by the

requirements of the temperature measurement. A uniform, single layer

(monolayer) of phosphor grains provides the best measurement of detailed

temperature profiles. Because the transistors under test might develop

very small hot spots, a monolayer coating is desirable. It is difficult

to produce a good monolayer coating, however, and a thin coating is also

somewhat transparent (fig. 3). This transparency causes the brightness

of the coating fluorescence to be influenced not only by temperature,

but also by the reflectivity of the surface coated and by variations

in coating thickness. An advantage of transparency is that the substrate surface detail serves as a map upon which the temperature profile

will be superimposed. Thick coatings, on the other hand, are opaque,

fluoresence more brightly, and may be easier to produce. But the thick

coating is not able to display the finer details of the temperature

profile, if there happen to be any. Coatings 40 urn or more in thickness

can probably be considered opaque.

There are several ways that a thin layer of phosphor can be deposited on a transistor chip. One method is to form a suspension of

phosphor particles in a liquid and let the phosphor settle out onto

the chip surface. The choice of liquid is quite broad: Ethyl alcohol

is frequently used; water has the advantage of holding the phosphor

in suspension for a longer time; lacquer or other adhesive vehicle

can be used if a permanent coating is desired; and Blanks [l] claims

to have obtained near-continuous coatings about 13 urn thick using a silicone varnish. Electrostatic deposition techniques might also be

usable.

12
a) 2N3767 transistor chip. These

transistors had bare aluminum

metallization.

b) Transistor chip, similar

to that at left, but with

thin phosphor coating.

Note that metallization

pattern is visible through

the coating.

c) 2N3878 transistor chip. These

transistors had the aluminum

metallization covered by a layer of aluminum oxide.

d) Transistor chip, similar

to that at left, but with

thin phosphor coating.

Note that metallization

pattern is not easily seen

thru the phosphor coating.

Figure 3. VISIBILITY OF TRANSISTOR SURFACE THRU PHOSPHOR COATING.

The photographs were taken at room temperature with diffuse

white light illumination. Compare with figure 5a and 5b,

which shows the appearance of the phosphor coating under

low-angle, grazing illumination.

13
A floatation technique was used to form the phosphor ' coatings

for this project. A thin, floating phosphor layer was first formed on

water, and then transferred to the transistor chips. The phosphor was supported by the surface tension "skin" of the water. None of the

other liquids that were tried could support the phosphor. Surface

tension effects were also used to compact the phosphor granules into a close-knit layer that resisted break-up and thus was easy to work with.

A sketch of the apparatus used to phosphor-coat the transistor

chip is shown in figure 4. A tall metal tube was placed upright on

the rim of a dish of water. Inside the tube were a phosphor reservoir

and an air jet. Puffs of air from the jet lifted the phosphor particles

out of the reservoir and allowed them to fall onto the water. Although

most of the phosphor sank to the bottom, useless, some did float on

the surface of the water. After a thin layer of phosphor was deposited

over most of the water, the metal tube was removed from the rim of the

dish. Under 45X magnification the coating appeared to consist of a monolayer along with a number of small clusters of phosphor particles.

The excess phosphor that formed the clusters was removed by directing

a gentle stream of air across the surface. When evaporation of the

water was prevented, the floating layer could be stored for many weeks.

The surface-tension force of the water was used to compact the

phosphor particles. This was accomplished by introducing a small

amount of oil at the edge of the water, causing the phosphor particles

to be driven together. Several oils were tried before a satisfactory

Ttadelin Thermographic Phosphor #2090 was used. It is one of a series

of four phosphors, covering the range from room temperature to 400 C,

that is available from the United States Radium Company, Morristown,

New Jersey. These are the only phosphors known to the author which

are manufactured for the express purpose of making temperature measurements.

5 Commercial products are identified in this paper in order to adequately

describe the experiment. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards, nor does it imply that the products identified are necessarily the best

available for the purpose.

14
to 15 psi air supply

ir>

-A

brass cylinder

air jet, 0.35 mm ID

phosphor

floating layer of phosphor

crystallizing dish

7 cm dia x 5 cm

sunken phosphor

Figure 4. APPARATUS USED TO PRODUCE THIN PHOSPHOR COATINGS,

15
one was found. Some, such as general purpose lubricating oils, were too

vigorous and caused the phosphor particles to be driven upon one another,

thereby increasing the coating thickness. While in contrast to this,

the type of oil that is used in mechanical vacuum pumps produced almost

no compaction (this oil spread out very slowly on water). As it turned

out, the residual oils remaining on recently washed hands gave the best

results and produced admirable compaction of the phosphor, although

this may not work for everyone. Simply dipping a finger into the water

for a moment would usually compact the phosphor quite nicely. Prior to

the compaction, however, the water had to be free of oil and other

surface-tension reducing materials; otherwise the surface tension would

be too weak to produce adequate compaction.

The floating phosphor layer could be easily transferred to surfaces

that were easily wet by water. Satisfactory phosphor coatings could not

be deposited on water-repellent surfaces because the water rolled off the

surface, carrying the phosphor with it. Good coatings could sometimes

be obtained on surfaces having intermediate wettability characteristics.

Learning how to make the silicon transistor chips wettable was a major

problem.

Although a few chips were easily wet by water as received, most

were covered by a water-repellent coating, of which several types were encountered. It was found that some of these coatings could be

softened and removed with the aid of solvents such as methylene chloride,

paint remover, or commercial preparations such as those used for chemically removing the enamel insulation from magnet wire. Frequently it was necessary to use mechanical means to remove the softened coating,

sometimes resulting in damage to the transistor. The 2N3739 transistors

were occasionally coated with a blob of white silicone rubber, but

commercial preparations are available which easily dissolve this. Once

the water-repellent coating was removed, final cleaning with detergent

and water was sometimes found to be helpful.

After a satisfactory floating phosphor layer was obtained, the

transistor was lowered into the water at a phosphor-free region.

(Phosphor must not be allowed to adhere to the silicon chip in this

16
step. ) The very thin oil film on the surface of the water did not cause trouble. The transistor was then oriented so that the chip faced upward;

and slowly lifted up through the phosphor layer and out of the water.

As the transistor was withdrawn from the water, it picked up a singlelayer coating of phosphor that adhered tenaciously. But if the rim of

the transistor case happened to trap a pool of water, then the phosphor

layer would float on top. When this happened, the phosphor layer was floated down to the chip by removing the water with absorbent paper.

The transistor was then set aside to dry. Drying time was just a few

minutes if the transistor was heated slightly. The short drying time was one of the advantages of the floatation method. By way of comparison, a coating deposited from a liquid suspension had to be left un- disturbed until the liquid evaporated naturally, usually several hours.

Attempts to hasten the drying, such as by mechanically removing excess water or by heating, usually produced convection currents that ruined

the coating.

The dry phosphor coating withstood careful handling quite well,

but sharp jolts and strong air currents had to be avoided. The phosphor

could be easily removed with a soft brush or a water spray. Because the

phosphor always covered much more than just the silicon chip, it was removed from areas whose temperature was not of interest - - the excess phosphor was a source of potentially interfering light that could

increase the error of the temperature measurements.

The average diameter of the individual phosphor grains is 9um.

When deposited on the silicon dioxide or aluminum oxide regions of a transistor chip, the phosphor grains produced a coating that roughly

approximated a monolayer, but some clumps of phosphor grains were also

present (fig. 5). The height of the clumps was usually less than 30um.

On bare aluminum metallization a much higher proportion of phosphor

clumps were produced and the clump size was larger, ranging up to about

50 urn in diameter.

17
Phosphor-coated transistor

(similar to that shown in

Fig. 3b) illuminated by

low-angle, grazing light

to emphasize the texture

of the phosphor coating.

15X

b) Thin phosphor coating (on

a black anodized aluminum

plate) similar to that in

a., above. Diffuse, white

light illumination used.

40X

c) Thin phosphor coating (on a plain silicon chip)

similar to a, above. This

photograph was made by a scanning electron microscope

and was supplied by the NBS

Electronic Technology Division.

Optical microscopes do not have

sufficient depth-of-field at

high magnification to show in

focus an entire phosphor grain.

205X

Figure 5. TEXTURE OF THIN PHOSPHOR COATING,

temperature.

18

Phosphor is at room
2.5. Ultraviolet Lamp Characteristics

The uv light source was a 100-watt high-pressure mercury arc bulb

(C) having an internal spotlight reflector. This type of bulb emits

light over a wide spectral range which includes the uv, visible, and

infrared regions. Since the uv light was used to generate a weak,

green fluorescence, it was necessary to remove all competing light

from the uv beam. A uv transmitting filter (D), which was supplied as part of the uv lamp, absorbed most of the visible light emitted by the

bulb. Measurement of the lamp's (including its filter) spectral output

revealed very strong radiation of the 366-nm (uv) line of mercury, a small amount of violet light (probably near 405 nm), and a continuum

that began at about 680 nm (deep red) and extended into the infrared.

The continuum was probably ordinary heat radiation from the hot bulb.

The red and the violet radiation was visible to the eye.

The mercury-arc bulb was energized by 60-Hz alternating current.

Because the uv light intensity is able to follow the cyclic variation

of the current, its intensity varied with time like a full-wave rectified 60-Hz sine wave. Whenever this report mentions the intensity of

the uv light, it is the simple average value that is meant unless

specifically stated to the contrary.

Short term drift of the uv intensity, after warmup and with

constant voltage, was roughly 1% per minute. The intensity changes

commonly occurred in jumps of about this magnitude and frequency.

Occasionally, and for no apparent reason, the lamp stability was much

better than this. A lamp operated from a dc power source would

probably have better stability.

2.6. Ultraviolet-Light Monitor

The ground-glass diffuser (F) that the uv light encounters at the

entrance to the test apparatus was empirically found to be necessary to

obtain a meaningful indication from the vacuum photodiode. With the

ground glass removed it was observed that various positions of the uv

19
lamp which produced an identical output current from the photodiode,

did not necessarily produce identical uv intensity at the phosphor surface. The ground glass greatly reduced this ambiguity.

After passing through the ground glass, the uv light came to a glass diagonal (G) that reflected about 10J& of the light up to the

photodiode. In order to prevent the hot heat sink in the vacuum chamber

from affecting the photodiode sensitivity, the photodiode was mounted in

an aluminum housing that was thermally insulated from the supporting

structure. The vacuum photodiode has an S-ll spectral response and

exhibits good temporal stability. The S-ll response has peak sensitivity at about 4-20 nm, and approximately an 80% response at 366 nm. Its

sensitivity to red and infrared light is low.

The photodiode and the servo control associated with the uv lamp

acted to stabilize the average value of uv intensity. It should be

mentioned, however, that the radiance of the phosphor fluorescence is a non-linear function of the incident uv intensity (see Section 3.3), so holding the average value of the uv intensity fixed does not guarantee

that the time-averaged fluorescent brightness remains constant. For

example, the brightness of the phosphor fluorescence would probably be

slightly weaker if the uv lamp were energized by a dc rather than an ac current, both having the same time-averaged (not rms) value.

2.7. Filter Selection

About 10% of the uv light striking the small glass diagonal (R) is

reflected downward to the phosphor-coated surface under test. The re- sulting fluorescent light, along with the portion of the incident light

reflected from the vacuum window (S) and from the phosphor surface, then

passes upward to a uv-absorbing filter (P ) that allows the fluorescent

light to pass on to the film (L), or other fluorescence detector, while

blocking all other light to which the detector is sensitive. In order

to keep the intensity of competing light, in particular the deep-red

portion, acceptably low it was necessary to prevent direct reflection

20
into the camera lens (0) from the main light beam of the uv lamp. These

reflections were kept low by proper orientation and shaping of the sur- faces in the path of the main beam and by coating them with a flat-black

paint.

The effort required to separate the fluorescent light from its com- petition depended upon the detector employed to view the fluorescence.

The simplest situation occurred when the eye was used as the detector,

for even with no filter the eye saw only the desired green fluorescence.

This was a consequence of the spectral response of the eye and the

relative intensities of the competing radiation. In most cases, however, the fluorescence was recorded on film. The film used was sensitive to wavelengths between approximately 360 and 650 nm. Hence the

film was blind to the red and infrared light present, but was sensitive to the violet and uv. Many filters are available which absorb

violet and uv light. A #2A Wratten gelatin filter, 100 urn thick, was

used for the present work. This filter transmits wavelengths longer

than about 4-10 nm [9], In cases where the uv lamp emits too much blue

light, a #8 Wratten filter would be required. This filter transmits

wavelengths longer than 4.70 nm. Because these filters exhibited a weak

fluorescence when placed in the direct beam of the uv lamp, they had to

be shielded from the strong primary beam of the uv light. One of these

filters was also necessary when a photomultiplier tube having an S-ll

spectral response was used to measure the phosphor fluorescence.

Selenium and silicon photo-voltaic cells were found to be too sensitive

to the deep red and infrared transmitted by these filters to be useful.

2.8. Camera Optics

The plain glass diagonal (R) that deflected the uv light onto the

TUT was, unavoidably, in the path of the fluorescent light passing from

the phosphor coating up to the camera lens. In order to minimize distortion, the diagonal was made as small as permissible, its approximate

dimensions being 7.5 mm diameter and 0.1 mm thick. Approximately l/3 of

21
the light from the TUT intercepted the diagonal. A totally reflecting

mirror could have been used if more intense uv illumination of the

phosphor were required.

A 50-mm focal length lens, taken from a "35-mm" camera, was used

for the focusing lens (0). A 36-cm long extension tube (N) between the

lens and camera body (M) provided SX magnification of the TUT. In order

to obtain sufficient working distance between lens and TUT it was necessary to mount the lens so the end that faces the film when used for

ordinary picture-taking, is facing the TUT instead. The distance

between the TUT and the nearest lens surface was about 50 mm. An un- desired consequence of using the lens backward is that its speed

becomes approximately f/3.5, although it is rated as f/l.4 when used

normally.

A 35-mm single-lens reflex camera body (M) was used to view the

TUT and to hold the film. In a single-lens reflex camera the same lens

is used to form the image for both the film and the view finder. A

hinged, movable mirror determines in which place the image is formed.

The advantage of this arrangement is that the image seen in the view

finder is always the same as the image recorded by the film.

The viewing screen used in the view finder had a ground glass surface, except for a central circular area that was clear. A fairly

sharp and bright image was visible in the clear region, but accurate

focusing on the TUT was difficult to achieve. The image seen on the

ground glass was of low brightness and never appeared sharply focused,

although focusing accuracy seemed about as good as for the clear region.

As long as the phosphor coating on the TUT fluoresced brightly enough

to produce adequate film exposure with the 35 seconds exposure time used,

it was bright enough to be seen in the clear region of the viewing

screen.Many viewing screens intended for taking ordinary snapshots are not

satisfactory for photomicroscopy work. In particular, viewing screens depending on either a split-image rangefinder or a Fresnel viewing

region for focusing are not acceptable. The problem arises because the

light reaching the viewing screen from the TUT has a narrow angular

22
spread. The small angular spread renders the split-image range finder

inoperative. It also accentuates the coarse texture of the Fresnel

pattern, and again prevents satisfactory focusing.

2.9. Film and Processing

A fast (ASA speed of 4-00), panchromatic black-and-white film '

was employed. An exposure time of 35 seconds was used in photographing

the phosphor fluorescence. For this exposure time a light intensity of

approximately 0.0001 meter-candles at the film is required to produce

the minimum usuable film exposure [10], This minimum usuable exposure

limited the highest temperature measurable on the TUT to approximately

210°C for the combination of uv intensity, film, camera optics, and

exposure time selected. The resolution of the film, for a high contrast

subject, was estimated to be 30 line-pairs per millimeter.

2.10. Microdensitometer

The microdensitometer employed automatically made a straight-line

scan across the film, producing an ink-on-paper recording of film

density versus position. For the purpose at hand the microdensitometer

was set up to produce recordings having a calibration factor of 0.15

density units per centimeter in the vertical direction, and a 10-mm

displacement in the horizontal direction for a 1-mm displacement at the

film.

Photographic density is defined as the logarithm (to the base ten)

of the ratio of incident light flux to the light flux transmitted by

the film. Thus if the film transmits 50% of the incident light, its

Kodak Tri-X Pan Film, TX 135-20.

7Film was developed in full-strength Kodak D-76 developer at 24°C.

Development time was 10 min, with agitation for about 4- seconds at halfminute intervals.

23
density is 0.30 du (du = density unit). After making a number of

density measurements on the film negatives, it was noticed that the

graph of density versus exposure had about twice the slope that was shown by the published film data [10]. The discrepancy was probably

due to the fact that the measured value of density is dependent upon

the details of the optical system used to make the measurement. It

turned out that it is standard film-industry practice to specify the

diffuse density of films. The term diffuse density means that when

measuring the transmitted light flux, all transmitted light is included.

However, the instrument used by the author did not measure the total

transmitted flux, but only that portion which passed straight through

the film and could thus be collected by the microscope objective of the

microdensitometer; light scattered by the film was not detected. The

diffuse density of any scattering medium is always less than the density

indicated by the author's instrument. When quantitative values of film

density are given in this report, the values are the net density above

the fog and base density of the film, as measured with the microdensitometer used by the author.

A square scanning aperture, either 200 or 4-00 (im on an edge with

respect to the film, was used when making the microdensitometer recording. Since the film image was an 8X enlargement, the scanning aperture

with respect to the TUT was either 25 or 50 urn on an edge.

2.11. Film Image Evaluation

The raw data produced by each experiment was a filmstrip of

negative transparencies of the fluorescent surface of the TUT. From

these negatives the temperature profile of the transistor could be

obtained. The method used to obtain temperature data from the

negatives is described in this section.

It is convenient to divide the negatives into two categories;

calibration negatives and operating negatives. A calibration negative

is simply a picture of the phosphor surface taken when no power was

24
applied to the transistor, so that the entire surface was at the temperature of the heat sink. (The resistance heaters mounted in the heat

sink brought it to the desired test temperature. ) Therefore the negative is a record of the radiance of the phosphor fluorescence for this

temperature. A set of calibration negatives is shown in figure 6a.

Note that although all parts of the transistor were at the same temperature, the density of each negative varies from point to point. For

this reason intercomparison of negatives had to be made on a point-bypoint basis when temperature data was sought.

An operating negative is a picture taken when the TUT was dissipating electric power. Figure 6b shows a typical operating negative. Although the location and approximate temperature of the hot spot can be seen at a glance, quantitative temperature measurements can be obtained

only by correlating an operating negative with the calibration negatives.

This correlation was accomplished by using a microdensitometer to generate a family of density (temperature )-versus-position traces from the

calibration negatives, upon which a corresponding trace from an operating negative was then superimposed (fig. 6c). Each trace covered

exactly the same path across the transistor surface, so a vertical line

drawn on this graph corresponds to a particular point along the path.

Wherever the trace of the operating negative intersects a calibration

trace, its temperature is known. Hence these intersections establish

the temperature profile along the line scanned.

The temperature profile of the entire surface of the TUT can be

built up from the temperature profiles of individual lines, but this is

a very slow process unless the procedure is automated. Most of the

negatives evaluated in this project were scanned along only one path,

chosen to include the hot spot of the TUT. Blanks [l] used two projectors, a single reference negative, and a differential light meter

to measure film density, a procedure that is faster but less accurate

than that used here.

Since all negatives had to be scanned along the same path on the

transistor image, reference markers were placed on two corners of the

transistor chip before photographs were taken. This was done by removing

25
' m til

iiiiiriiTrni

200°C 190°C 180°C

(a) Calibration negatives.

15 watts

(b) Operating

negative

.

Figure 6.

Millimeters

(c) Microdensitometer

traces of negatives

along indicated

path.

(d ) Temperatures

plotted on an

enlargement of

the operating

negative

.

ANALYSIS OF FILM NEGATIVES. The negatives in (a) and (b) are about the size of those examined by the microdensitometer.

The emitter lead is at the bottom of the chip, the base lead

at the top. Contrary to the case shown here, all other transistors tested developed their hot spot near the emitter lead.

26
the phosphor from the corners and placing a few grains of another phosphor on the cleared area. One grain at each corner was arbitrarily

chosen for use as the position reference. The marker phosphor maintained a reasonably strong fluorescence throughout the temperature

range used for the calibration negatives. Grains of marker phosphor

are seen in the lower two corners of the chip in figure 6d. The dark

spots in figure 6a were caused by misplaced marker phosphor.

27
3. ANALYSIS AND EVALUATION

3.1. Purpose and Conventions

Many of the characteristics and limitations of temperature-sensitive phosphors were revealed by correlating microdensitometer recordings

of the film negatives with the test conditions under which the exposures

were made. The purpose of this section is to present the relevant ex- perimental data and from this establish the characteristics of the phosphor. In order to simplify the presentation it is desirable to adopt

the convention that length dimensions on the film will be referenced to

the transistor surface. Or if the film image is not that of a transistor, then film dimensions will be referenced back to an imaginary

transistor. If, for example, the microdensitometer aperture views a 400 x 400 \±m area of the film, the text will state that the aperture is

50 x 50 |j.m, it being understood that the dimensions at the film have

been divided by 8 in order to reference the measurement to the transistor.

3.2. Stability of Transistor Parameters

The transistors were subjected to some vigorous treatment while

they were test objects. Each transistor had its protective cap re- moved, and the transistor chip was likely to be subjected to various

organic solvents, water, detergent, mechanical rubbing, weeks of ex- posure to phosphor and air, and high-temperature operation in vacuum. Surprisingly, the effect of these operations upon electrical parameters

of the transistors appeared to be negligible.

The common-emitter current gain ( P) and the collector-to-base leakage current (l p^) of two or three samples of each of the three tranGJjL)

sistor types investigated were measured before their cases were opened.

The 3 tests were made with the transistors dissipating 2 W and at

medium to high collector currents. The I measurements were made at

several voltages ranging from about 20% to nearly the maximum of the

28
published collector junction breakdown voltage. Measurements of both

3 and IpTM were first made with the heat sink at 25°C, then repeated

at 125°C, and finally rerun at 25°C. Changes in p of ±2% between the

two 25 °C tests were common, but never exceeded ±4%. A similar comparison for I pian produced changes as large as -65% and +100%. Large

changes in I nT3n were common, although one of the transistors changed

only about 12%. These parameters were also measured on three of the

transistors at the end of their use as temperature measurement subjects,

For all three the changes that occurred before their cases were opened

were larger than those occurring afterward.

The above measurements of and I. were made with the exposed

silicon chip in darkness to avoid a photo-current of approximately

100 |iA that would be produced by irradiation from the uv lamp. The

good stability of the electrical parameters of the exposed chips may

have been due to the vacuum environment that was provided when the

chips were operated at high temperature.

3.3. Phosphor Characteristics

The dependence of the phosphor's fluorescent radiance upon temperature and uv irradiation was measured and is shown in figure 7. The

fluorescence was measured by replacing the camera and lens assembly

with a photomultiplier tube having an S-ll spectral response.

Important characteristics of the phosphor that were revealed are:

1

)

For a given intensity of uv irradiation there is a span

of approximately 50°C over which the temperature co- efficient of fluorescent brightness is essentially con- stant and near its maximum value of 6.1%/°C.

2) Increasing the intensity of the uv irradiation by a factor of three causes the entire temperature span of

maximum sensitivity to shift approximately 20°C upward.

29
ra •H§

to

ca ¥>•H

ca

<uos

•Hn

s

CDO

CO

<DU

§

1000

500

200

100

50

20

10

0.5

80 120 160 200 240 230

Temperature, °C

Figure 7. FLUORESCENT RADIANCE OF #2090 PHOSPHOR AS A FUNCTION OF

TEMPERATURE AND UV EXCITATION. The phosphor is a thin, some- what open coating on an aluminum plate. The uv excitation

for each curve is identified by, and is approximately proportional to, the cathode current developed by the uv monitoring vacuum photodiode. Dashed lines show experimental

data whose accuracy is not assured.

30
3) If the phosphor is held at some fixed temperature within the span of high sensitivity, then the fluorescent

radiance is proportional to the 1.9 power of the uv intensity. In other words a 10$ change in uv irradiation

causes 19$ change in the radiance of the fluorescence.

This large sensitivity of radiance to the uv intensity

is a consequence of the fact that the temperature at

which the brightness begins to drop sharply, changes

with the uv irradiation.

The following empirical equation is useful for interpolating the

data given in figure 7:

(T -T ) •R2

= (I

2/I)1,9R (1.061) X 2

when (l)

R = radiance of fluorescence in arbitrary units,

I = uv irradiation incident on the phosphor in arbitrary units,

T = temperature of phosphor in degrees Celsius.

Subscripts 1 & 2 identify initial and final states, respectively. This

equation is valid only if the temperature coefficient of brightness is near its maximum value of 6.1$/°C at both points. Figure 7 can be used

to determine whether this requirement is satisfied.

3.4. Heat Transfer

For the calibration negatives the temperature of the surface of

the transistor chip was very close to the temperature of the heat sink.

To demonstrate that this was the case, the maximum possible temperature

difference was calculated as follows: The total radiated power from a 1.5 x 1.5 mm black-body surface at 200°C, calculated from the StefanBoltzmann 4-th-power radiation law, is less than 30 mW. And based on

the heater power required to maintain the heat sink at 200°C, heat loss

from the heat sink surface to the residual air around it was less than

2

5 mW/mm , making the conductive heat loss from the surface of the silicon chip less than 10 mW. The thermal resistance from collector

31
junction to heat sink was less than 10°C/W, so the temperature difference between collector junction and heat sink was less than 0.4°C even

when the most pessimistic assumptions were made.

No independent measurement of transistor surface temperature was made while transistors were dissipating electric power. Independent

measurement would have required the use of an infrared microradiometer

and was beyond the scope of this project.

The mechanism of heat transfer from the transistor surface to the

phosphor was the same for both the calibration and the operating

negatives. In both cases heat was conducted from the transistor sur- face into the phosphor layer. Temperature differences between transistor and phosphor, if any, were probably the same for both the calibration and the operating negatives, and were therefore accounted for

by the calibration negatives.

3.5. Temperature Range

The temperature range over which useful measurements could be made

was influenced by many parameters, some of which could be selected to

suit the requirements of the user. For any set of parameters chosen,

the useful temperature range was easily found by examining the filmdensity versus phosphor-temperature characteristic obtained. Figure 8

shows this characteristic for the parameters used in the present investigation. At temperatures between 180 and 200°C the relation of

film density to phosphor temperature remained fairly constant at approximately 0.05 density units (du) per Celsius degree. At temperatures

below 180°C the sensitivity fell off because the phosphor fluorescence

became less temperature dependent. Above 200°C the sensitivity dropped

sharply because the fluorescence became too weak to produce an image on

the film.

32
to

CM

>»

-P rH

•H

CO

cu Q

o

o

-m4 Um— scanning aperture (= 50 urn)

0.5 0.9 1.3

Distance Along Transistor, mm

Figure 8. FAMILY OF CALIBRATION NEGATIVE TRACES.

33
3.6. Limitations of the Film

Ideally, the photographic density of the film should have been determined only by the brightness pattern of the phosphor fluorescence.

In fact, the film itself had small inherent density variations. Although these variations warranted consideration, they were not a major

source of error in the temperature measurements.

The microdensitometer recordings in figure 9 display the density

variations inherent in the film itself. Each exposure was made by re- moving the camera lens and uniformly illuminating the film, so local

variations of the film density could not be attributed to exposure

variations. Since temperature measurements were made by matching the

densities of two negatives at the same point along a scanning path,

only frame-to-frame variations at a particular point are significant.

The largest variations observed were 0.05 and 0.03 du for 25 x 25 um and

50 x 50 um scanning apertures, respectively. In other words, the variations caused no more than 1°C error. These values applied for any

pair of negatives on the same roll of film when the density was about

1.0 du. The density variations of unexposed frames were always smaller

than for frames exposed to light, so dust or dirt on the negatives was

not a factor here

.

The boundaries around highly localized hot spots were regions across which the radiance of the phosphor fluorescence usually changed very

rapidly. This resulted in steep exposure gradients on the camera film,

a condition that could conceivably have modified the expected film density. Although these exposure gradients probably did not introduce

significant errors, as a precautionary measure the phenomenon will be

explained more fully.

The nature of the effect can be demonstrated by making a microdensitometer trace across the boundary between two regions that have re- ceived different exposures. Ideally, the recording should show a simple

step change in the film density. But in practice a slight overshoot us- ually occurs on both sides of the transition [11], as seen at the lefthand side of the traces in figure 9. This is caused by the action of

34
cm

• H

(u.

O

Lm

Density 6

0.

•H O

-»J U*—scanning aperture (= 25 urn)

0.4 0.3

Distance, mm

1.2 1.6

(a)

CM

-P

•H

COC

CD O

•H

d

D

IN

1

H h— scann ing ape:rture (= 50 ujn)

0.4 0.8

Distance, mm

1.2 1.6

(b)

Figure 9. DENSITY VARIATION INHERENT IN THE PIIM. The density structure of negatives that were exposed by a uniform blanket of

light is shown. Each graph shows the results for two frames

on the same filmstrip. The same negatives and scanning path

were used for both (a) and (b).

35
the film developer. The developing solution becomes somewhat depleted

in the area that received the greater exposure. Some of this depleted

developer diffuses into the area of lesser exposure, depressing the

amount of development in that region. At the same time, strong

developer from the area of lesser exposure also diffuses across the

boundary, causing increased development of the heavily exposed image.

3.7. Calibration Negatives

Examining a single calibration photograph reveals several important characteristics of the TSP. Microdensitometer scans across a typical calibration negative are shown in figure 10. The negative is a photograph of the fluorescence of a phosphor-coated transistor at 190°C.

Keeping in mind that the entire phosphor-coated surface was at the same,

uniform temperature, the most striking feature of the recording is its

jagged structure. Ideally, the radiance of the phosphor fluorescence

would have been the same everywhere. The density variations seen in

this recording were caused primarily by unevenness of the phosphor coating. Clumps of phosphor grains, which fluoresced brightly, produced

the density peaks, while the valleys usually correspond to thinner and more uniform regions of the coating. The deep valley about 0.4 mm along

the scanning path, however, was caused by a void in the phosphor layer.

One of the limitations of the TSP was caused by these variations

in the phosphor coating. Notice that they produced density gradients

that are typically 7 du/mm. The film could be positioned in the microdensitometer within an overall error span of 15 urn, with respect to the

surface of the actual transistor. It was unusual, however, for this

span to exceed 7 urn. This means that when the calibration negative was temporarily removed, and then returned again to the microdensitometer,

its new position usually differed from the original position by less

than 7 urn. If a positioning error of 7 urn did occur, then density readings on the new scan might change (0.007 mm) x (7 du/mm), or 0.05 du.

In practice, discrepancies were usually somewhat less than this, even

36
-P

•H2

Q

•H

0.5 0.9 1.3

Distance Along Transistor, mm

(a)

-p•HW

•H

1-1

• r-l H K— scanning apei•ture (= 50 urn)

tOO

ITN O

0.1 0.5 0.9 1.3 1.7

Distance Along Transistor, mm

(b)

Figure 10. DENSITY STRUCTURE OF A CALIBRATION NEGATIVE. The two curves show the smoothing that occurs when a larger scanning aperture is used. Both scans were made along the same path on

the same 190°C calibration negative.

37
when several months elapsed between examinations and the negative re- ceived moderate handling.

Figure 10b shows the recording obtained when the scanning path of

figure 10a was retraced, but with a 50 x 50 urn aperture. The larger

aperture is seen to substantially reduce the slope of the curve over most of the recording. Density peaks that appear in this recording

were caused primarily by the thicker phosphor coating that deposited on

the transistor metallization. A minor benefit of the larger aperture

was that the peaks and valleys that appeared in the microdensitometer

recording corresponded to the brightness variations seen by direct

visual observation of the phosphor. When the small aperture was used,

the local detail that was obtained tended to obscure the nature of the

overall picture.

3.8. "Identical" Negatives

The preceding section discussed the reproducibility that was obtained when duplicate microdensitometer recordings were made of the

same negative. The present section will discuss the reproducibility

obtained by "identical" negative pairs. Or in other words, did identical exposures produce identical negatives?

Calibration negatives, all of the same transistor held at the same temperature, were used to evaluate photographic reproducibility. The

test conditions for each exposure were as identical as the experimental

equipment could make them. To this end, only negatives from the same'

filmstrip were compared, and all exposures were made during the same run in order to minimize possible shifting of the phosphor. It was found to be immaterial whether the exposures were adjacent frames or were at opposite ends of the filmstrip. Applying power to the transistor between calibration exposures also had no effect on the results.

The microdensitometer recordings in figure 11 show the reproducibility that was typically achieved between two "identical" negatives.

The largest discrepancy observed between such negative pairs at any one point was 0.1 du for the 25 x 25 \xm aperture, and 0.05 du for the

38
0.5 0.9 1.3

Distance Along Transistor, mm

(a)

-p•HQ

a

•H

H K— scann Lng aper ture (= 50 \im)

[

0.1 0.5 0.9 1.3

Distance Along Transistor, mm

1.7

(b)

Figure 11. DENSITY VARIATION BETWEEN "IDENTICAL" CALIBRATION NEGATIVES.

Figure a shows microdensitometer traces for two "identical"

190°C calibration negatives, each scanned along the same

path. Dots are used to distinguish between the two negatives. Figure b repeats the traces using a larger aperture.

39
50 x 50 |im aperture. Such frame -to -frame differences were the predominant source of error in the temperature measurements and seemed to be

caused by spontaneous changes in the position of individual phosphor

particles. These density variations were around three times larger

than the inherent variation in the film itself, and about twice as great as the density discrepancies observed between repeated scans of

the same negative.

3.9. Spacial Resolution

As used in this section, spacial resolution refers to the ability

of a phosphor coating to resolve fine patterns of surface temperature.

Because it is difficult to produce temperature variations having high

spacial frequency and at the same time accurately known temperatures

within the temperature pattern, only qualitative measurements of

spacial resolution were obtained.

Visualize a surface divided into two isothermal regions with an infinitely sharp gradient at their common boundary. The spacial resolution of a temperature-measuring system can be evaluated by noting the

distance required for its temperature reading to go from one temperature to the other as the boundary is traversed. The transition

distance cannot be smaller than the scanning aperture used. In addition

to aperture several other factors can also limit resolution. Among

these are the diffusion of heat once it passes into the phosphor coating,

and the possibility that the photographic optics may not form a sharp

image on the film. Or, fluorescent light from the cool (brighter) side

of the transition might illuminate the phosphor on the hot side of the

boundary. The steepest gradient actually found was a temperature

change of 40°C that occurred over a distance of 0.09 mm, or 450°C/mm.

It is not known whether this was the gradient actually existing on the

transistor, or if the observed gradient represented a limitation of the

TSP technique.

Since the use of the larger, 50 x 50 urn scanning aperture improved

40
the repeatability of the temperature measurement, it became desirable to

know whether or not spaoial detail of the temperature profile was sacrificed for this benefit. A test for this was made by scanning a negative

first with the 25 x 25 urni aperture and then with the 50 x 50 u.m aperture.

The operating negative that showed the narrowest hot spot was selected

for this test. The small aperture characterized the hot spot as having

a peak temperature of 197°C, and a width of 140 urn between the points

that were at 190° C. The large aperture characterized the hot spot as having essentially the same peak temperature, and a width of 160 |_im

between the 190°C points. Thus for all temperature profiles encountered,

the larger scanning aperture appeared to provide adequate spacial resolution.

3.10. Stray Light

Since temperatures could vary widely on the surface of an operating transistor, brightness of the phosphor fluorescence could also vary

greatly from point to point. When a wide brightness range existed,

there was the possibility that light from bright areas would illuminate

the darker areas. Such illumination would degrade measurements in the

darker (hotter) areas because the spurious light was not present when

the high-temperature calibration photographs were taken. This characteristic is an inherent weakness of the TSP technique. Means by

which spurious illumination was kept to a minimum were

:

1) Limiting the phosphor-coated area to only the regions

whose temperature was to be determined.

2) Designing the test chamber to prevent reflection of

fluorescent light back to the phosphor coating.

3) Selecting the uv intensity so as to limit the brightness ratio over the phosphor surface. This requirement was met by using the lowest uv intensity that

allowed measurements to be made over the temperature

range required.

41
A certain amount of the light passing from the phosphor surface to

the film was probably scattered by items in its path. The scattering

mechanisms would include dust particles and multiple reflections from

the surfaces of optical components. The overall effect of such

scattered light would be to cause a slight, uniform exposure over the

entire film, thus tending to fill in the areas of minimum exposure.

For the equipment used, stray light was believed to be only a minor

problem, because when sufficiently high hot-spot temperatures were produced, there was no measurable film exposure in the hot spot region,

even though the rest of the phosphor fluoresced brightly.

3.11. Vacuum and Air Environments

Comparison tests were made to determine if a temperature change

resulted when the transistor environment was changed from a pressure of

about 10 atmosphere to almost full atmospheric pressure, all other

parameters, including heat sink temperature, remaining constant. The

hot-spot temperature was about 200°C during these tests. Nearly

identical operating negatives resulted, well within the normal frameto-frame variation discussed in 3.8. This test was not done for calibration negatives because the copper heat sink would have been subject

to excessive oxidation if exposed to air at elevated temperatures.

3.12. Irreversible Effects

An irreversible change in the phosphor fluorescence was observed

with two transistors. In both cases the transistors had been operated

under conditions that caused the hot-spot temperature to become un- usually high. This caused a portion of the phosphor to develop a permanent increase in the brightness of its fluorescence for all temperatures. The temperature required to produce the irreversible effect was estimated to be in the vicinity of 270°C. The transistor used as the

subject for the microdensitometer recordings shown in this chapter had

42
such a temperature-modified region of phosphor. This region is identified by its abnormally high photographic density. Note, for example,

the portions of the recordings in figure 8 that fall between 1.3 and

1.7 mm along the scanning path.

Temperatures that caused the irreversible change in fluorescence

also caused some of the phosphor to permanently bond to the aluminum

metallization on the transistor chip. The lowest temperature required

to produce bonding was not determined.

3.13. Practical Results

Figure 12 shows the kind of single-path temperature profile that was obtained from the filmstrips when evaluated as described in Section

2.11. The profile path was usually chosen to pass through the point of

highest temperature. Such a path was easily found because the location

of the highest hot-spot temperature could be readily identified by

direct visual examination of the operating negatives.

In principal the temperature profile of a complete surface could

have been built up from a large number of single-line profiles. But as

a practical matter, production of a complete profile was found to be

too time consuming and was not attempted. It was estimated that approximately five hours would have been required to produce a single, rather

coarse, overall temperature profile composed of six lines having five

temperature calibration points each. However, the time would probably

be reduced by half if somewhat reduced accuracy were acceptable.

43
210

o

190

170

1

DISTANCE in mm

Figure 12. TEMPERATURE PROFILE ALONG PATH THROUGH A HOT SPOT

The profile is based on figures 6o and 6d.

44
4. SUMMARY

Temperature-sensitive phosphor (TSP) provides a very easy means

for obtaining a visual, qualitative indication of surface temperature.

This report has shown that TSP can also provide quantitative temperature measurements. With the apparatus used, and over the range of

170-210°C, measurements were always reproducible to within an overall

range of <4°C when the phosphor surface was scanned with a 25 x 25 urn aperture, and to within 2°C when scanned with a 50 x 50 urn aperture.

The predominant cause of measurement error seemed to be spontaneous

changes in the position of individual phosphor particles. Unevenness

of the phosphor coating limited the smallest useful scanning aperture

to about 20 x 20 um. Surfaces having only moderate temperature variations could be easily calibrated to make the measurement accuracy

about the same as its reproducibility. The accuracy obtained for surfaces having wide temperature variations appeared to be almost as good,

but a thorough investigation of this case was not made.

The handling and processing received by the transistor chips did

not significantly affect their electrical parameters. The phosphor

coatings were deposited without using a binder and were easily removed

from the chips at the end of the tests, providing that temperatures had

not greatly exceeded 210°C. At higher temperatures some of the phosphor

permanently bonded to the bare aluminum metallization.

45
5. REFERENCES

[1] Blanks, H. S. , Thermographic analysis of electronic components

(Proc. of the Inst, of Radio and Electronics Engineers

Australia) 29_, 337-342 (Oct. 1968).

[2] Goryunov, N. N. , Ovechkin, Yu. A., Tolkacheva, Ya. A., and

Feoktistov, Yu. F. , Observation of thermal fields in semiconductor devices, Instruments and Experimental Techniques (USSR),

439-443 (Mar.- Apr. 1966).

[31 Young, M. R. P., and Peterman, D. A., Reliability Engineering,

Microelectronics and Reliability, 7, 91-103 (1968).

[4] Peterman, David, and Workman, Wilton, Infra-Red radiometry of semiconductor devices, Microelectronics and Reliability, 6, 307-315

(1967).

[5] Reich, Bernard, and Hakin, Edward B. , An appraisal of transistor

thermal resistance measurement techniques, SCP and Solid State

Technology, 8, 21-29 (Apr, 1965).

[6] Fergason, J. L. , Liquid crystals, Scientific American, 211 , 76-85,

(Aug. 1964).

[7] Urbach, F ., Nail, N. R. , and Pearlman, D. , The observation of temperature distributions and of thermal radiation by means of non- linear phosphors, J. Opt. Soc. Am., %£, 1011-1019 (Dec. 1949).

[8] Byler, W. H. , and Hays, F. R. , Fluorescence thermography, Nondestructive Testing, 19., 177-181 (May-June 1961 ).

[9] Kodak Publication No. B-3, Kodak Wratten Filters, 20th Edition

(1960).

[10] Kodak Photo Information Book AF-13, Kodak Black-and-White Films in

Rolls, 3rd Edition (1967).

[11] Kodak Scientific and Technical Data Book P-9, Kodak Photographic

Films and Plates for Scientific and Technical Use, 8th Edition

(1960).

46
-orm NBS-114A (1-71)

1. PUBLICATION OR REPORT NO.

NBS-TN-591

2. Gov't Accession No.U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A Technique for Measuring the Surface Temperature of

Transistors by Means of Fluorescent Phosphor.

5. Publication Date July 1971

6. Performing Organization Code

7. AUTHOR(S)

David Brenner

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Task/ Work Unit No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address 13. Type of Report & Period Covered

Final 1967 - 1969

14. Sponsoring Agency. Code

IS. SUPPLEMENTARY NOTES

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.) A technique that uses temperature-sensitive phosphor (TSP) to make quantitative

measurements of transistor-chip surface temperatures over the range of 170 - 210°C

is described. The experimental procedure consisted of depositing a thin coating

of phosphor on uncapped power transistors, and then photographing the visible

fluorescence generated under near-ultraviolet (uv) excitation. The cooler regions

produced a brighter fluorescence than the hotter areas. The surface temperatures

of operating transistors were determined by comparing, on a point-by-point basis,

the photographic density of operating photographs to the density of a family of

calibration photographs. Repeated temperature measurements always fell within a 4°C overall error range when the phosphor surface was scanned with a 25 x 25 urn aperture. The error band dropped to less than 2°C when a 50 x 50 um aperture was

used. Coarseness of the phosphor coating seemed to limit spacial resolution to

about 20 um. Any narrow temperature range between 20 and 400° C can probably be

measured by selecting a suitable level of uv irradiation and/or a suitable phosphor.

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

Surface temperature; temperature measurement; temperature -sensitive phosphor;

thermographic phosphor; transistor-chip temperature.

18. AVAILABILITY STATEMENT

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I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

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UNCLASSIFIED

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21. NO. OF PAGES

49

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