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NATL INST OF STANDARDS & TECH R.I.C.

A11 1009851 87

Koyama, Richard Y/A wafer chuck (or use QC100 .U57 V400-55;1979 C.I NBS-PUB-C 19 NBS SPECIAL PUBLICATION 400-55

Nes

PUBLICATIONS

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

m

Semiconductor Measurement Technology:

A Wafer Chuck

for Use Between -196 and 350°C
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

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and the Institute for Computer Sciences and Technology.

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I————

The National Bureau of Standards was reorganized, effective April 9, 1978.
.wuwnai Bureau o? Standams

MAR 1 5 1979

Semiconductor Measurement Technology:

A Wafer Chuck

for Use Between -196 and 350 °C

R. Y. Koyama and M. G. Buehler

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Sponsored by:

Division of Electric Energy Systems

Department of Energy

Washington, DC 20545

The Defense Advanced Research Projects Agency

1400 Wilson Boulevard

Arlington, VA 22209

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

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued January 1979

and

OF
Library of Congress Catalog Card Number: 78-600155

National Bureau of Standards Special Publication 400-55

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-55, 24 pages (Jan. 1979) CODEN; XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

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

Stock No. 003-003-02017-6 Price $1.20

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

Page

List of Figures iv

Preface vi

Abstract and Key Words 1

1. Introduction 1

2. The Variable-Temperature Wafer Chuck 2

3. Automatic Probing Apparatus 3

4. Thermometry 6

5. Defect Mapping 10

6. Automatic Wafer Probing 12

7 . Summary 12

8. Acknowledgments 12

9. References 13

Appendix A — Mechanical Drawings for Parts 14

Appendix B — The Linearity of AV 17

iii
LIST OF FIGURES

Page

1. Isometric view of the major thermal chuck components ..... 2

2. Inverted view of the chuck body with the four 150-W

heater cartridges exposed 3

3. Photograph of the assembled thermal chuck 3

4. Front view of the box enclosing the automatic wafer

prober 4

5. Back view of the box enclosing the automatic wafer

prober 4

6. Inside view of the box with the top cover removed 4

7. Typical experimental setup with instrumentation 6

8. Typical heating and cooling response curves for the

chuck above room temperature and below room tempera-

-ture 7

9. Typical temperature and heating rate curves for a thermally stimulated measurement with approximately 75 percent of full power applied to the heaters 7

10. Forward voltage drop characteristic for a specific n"^p

gold-diffused diode 8

11, Wafer map of the forward voltage drop of the n p diodes ... 8

12. Curves representing the spread of the forward voltage

drop of a selected group of 170 diodes as a function of

temperature, a linear plot representing the "natural"

spread of the forward voltage drop due to device variations, and the deduced spatial variation of the chuck

temperature as a function of the temperature 9

13. Histograms showing the variation of the temperature at which emission from the gold donor and the gold acceptor

occurs; the nonuniformity of the chuck temperature under

dynamic conditions of approximately 7°C/s is inferred

from this variation 10

14. The thermally stimulated capacitance and current responses

of a gold-diffused r&p diode; the heating rate is about

7°C/s 11

15. Wafer map of the gold donor defect density 11

iv
LIST OF FIGURES (continued)

Page

Al. Mechanical drawings for tLe top plate, wafer index

stop, and the index stop pin 14

A2. Mechanical drawing for the chuck body 15

A3. Mechanical drawings for the cover plate and mounting

ring 16

A4. Mechanical drawings for components of the wafer transport slide mechanism 16

V
PREFACE

This work was conducted as part of the Semiconductor Technology Program at the National Bureau of Standards. Portions of this work were supported by the Division of Electric Energy Systems, Department of

Energy (Order Number A021-EES) , the Defense Advanced Research Projects

Agency (Order Number 2397), and the NBS.

Disclaimer'

Certain commercially available materials or instruments are identified

in this publication for the purpose of providing a complete description of the work performed. The experiments reported do not constitute a complete evaluation of the performance characteristics of the

products so identified. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards

nor does it imply that the items identified are necessarily the best

available for the purpose.

vi
Semiconduatop Measurement Technology

:

A WAFER CHUCK FOR USE BETWEEN -196 and 350°C

R. Y. Koyama and M. G. Buehler

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

ABSTRACT

This report describes the design and characterization of a variabletemperature wafer apparatus for use in the detection of electrically

active defects which produce deep levels in the band gap of silicon.

In its present form, the wafer chuck can heat and cool wafers as large

as 51 mm in diameter over the temperature range from -196 to 350°C; heating rates as high as 7°C/s have been achieved. The uniformity of the

temperature across the chuck under static conditions is estimated to be better than ±0.4°C. Construction details of the chuck are given in

an appendix. The use of this apparatus is illustrated by wafer mapping

the gold defect density in diodes fabricated across a silicon wafer.

Key Words: Deep level measurements; defect mapping; hot/cold wafer

chuck; thermal wafer chuck; thermally stimulated measurements; wafer

chuck, variable temperature.

INTRODUCTION

The detection of electrically active defects

which produce deep levels in the band gap of

semiconductors is an important aspect of material and device characterization in the

semiconductor industry. Techniques such as transient capacitance and current measurements [1], thermally stimulated current and

capacitance measurements (TSM) [2], and deep

level transient spectroscopy (DLTS) [3] are extremely sensitive and useful for establishing energy levels, emission rates, and densities of deep level defects. These techniques depend on the manipulation of the

specimen temperature during some portion of

the measurement; typically, temperatures in

the range from -196 to as high as 300°C are required, depending on the semiconductor band

gap energy.

Traditionally, deep level measurements have

been performed in cryostats on test devices

which have been scribed from wafers and individually packaged [2]. Great advantage

could be derived by the ability to perform

these deep level measurements on devices

which can be probed in wafer form. This

would eliminate operations such as scribing,

dicing, die bonding, wire bonding, and pack1
age sealing in the processing, and shorten

the time from wafer completion to deep level

test measurements. Because the wafer stays

intact, variations of parameters such as defect density and electrical device characteristics across the wafer could easily be determined. In addition to being a useful re- search tool in the laboratory, the wafer

handling capability would allow these measurements to be used as a routine production

diagnostic tool. The use of these techniques at intermediate points in the processing of wafers to determine the effect of particular processing steps on the activation

or introduction of defect centers has particular application in the processing of power

devices [4] and solar cell devices in which

the lifetime of minority carriers is an es- pecially important concern.

In this paper, a variable-temperature wafer

apparatus capable of performing deep level*

measurements on wafers is described. By us- ing liquid nitrogen as the coolant , the apparatus covers the temperature range between

-196 to over 350°C with heating rates as high

as 7°C/s. Sensitivity for electrical measurements is sufficient to allow current measurements as low as 0.2 pA or capacitance changes

(1 MHz) as small as 5 fF.

THE VARIABLE-TEMPERATURE WAFER CHUCK

The thermal and electrical requirements of

deep level measurements led to the design

and construction of the thermal chuck shown

in figure 1. It shows an isometric view of

Figure 1. Isometric view of the major thermal chuck components

.

the component parts of the chuck with the

various parts identified. A nickel-plated

copper top plate (3) has vacuum grooves for

holding the test wafer in place, a hole for

thermocouple insertion, and mounting holes

for a wafer index-stop (2) . This top plate

is isolated from the chuck body (5) by an insulator plate (4) which must provide good

electrical isolation as well as good thermal

conduction; a circular sapphire plate (0.25 mm thick by 67 mm in diameter) has been

found to perform satisfactorily. The heart

of the assembly is the chuck body (5) which

is also fabricated from copper and plated

with nickel. The chuck body consists of an integral cooling cavity and holes for four

heater cartridges (10) . An inverted view

of this part with the heaters exposed is

shown in figure 2. A stainless steel cover plate (6) encloses the cooling cavity and

attaches to the manifold (9) for the coolant fluid. The 150-W heater cartridges (10) are silver soldered into the chuck body (5)

which is subsequently heli-arc welded to the cover plate (6) and the stainless steel

mounting ring (7) . Since the mounting ring

(7) is attached to supporting hardware, it

has been designed with a thin rib (0.25 mm

thick) on the circumference to minimize heat

" This apparatus would also be useful and

has been used for bias-temperature stress

(BTS) measurements on metal-insulatorsemiconductor (MIS) structures. BTS mea- surements with thermal stress of 300°C

have been utilized to detect mobile ion

contamination in silicon dioxide MIS ca- pacitors .
transfer to the support; this is necessary

in order to minimize mechanical motion of

the chuck due to differential thermal expansion and contraction of the support hardware.

The power leads for the heater cartridges

are shielded along their entire length with

either solid (11) or flexible metal shields

(not shown) . The top surface of the chuck

body (5) is identical (including vacuum

grooves) to the top plate (3) ; for measurements which do not require electrical isolation, this feature allows faster thermal re- sponse by mounting the wafer directly on the

chuck body. Figure 3 shows a photograph of

the assembled thermal chuck. The numbers

are keyed to those in figure 1; in addition,

the two thermocouples (12) , the two chuck vacuum supply pipes (13) for wafer hold-down.

Figure 2 . Inverted view of the chuck body

with the four 150-W heater cartridges ex- posed (see fig. 1 for number key to components )

.

and one pair of three sets of locking and

leveling screws (lA) are indicated. Each of

the two type K [5] thermocouples has dual

elements and is mounted to the chuck body

and top plate by inserting it into holes

drilled in the chuck body (5) or the top

plate (3) . To eliminate possible electrical

interference, the thermocouple in the top

plate is an isolated type.

Figure 3. Photograph of the assembled thermal chuck (see fig. 1 and text for number

key to components )

.

AUTOMATIC PROBING APPARATUS

To build in the capability for automation of

measurements, the thermal chuck assembly was adapted to a modified automatic wafer prober

(Teledyne - TAG, PR-100) . This prober has a probe-ring assembly which can accommodate as many as 60 individually adjustable probes.

Once mounted and adjusted, the probes remain

fixed. Automatic probing is accomplished by

moving the chuck-mounted wafer to the appropriate position relative to the probes.

Movement in the x-y plane is accomplished by

a table on which the chuck is mounted; after

reaching the desired position, a small vertical motion (z) raises the chuck to make contact between the wafer and the measurement

probes. After initial alignment of first the

wafer to the prober axes and then the measurement probes to the contact pad geometry, the

prober can automatically index the wafer from

die to die at a preset index interval. In addition, a computer interface is utilized to control and execute a variety of electrical

parameter measurements of devices fabricated

on the wafer.

The low temperature requirements of the

deep level measurements dictate the use of a cryogenic fluid for the thermal chuck

coolant. Since it is important to maintain

a low relative humidity to minimize or eliminate the condensation of water vapor on the

wafer at low temperatures , the thermal chuck

and the wafer prober were enclosed in a sealed box. The enclosure is continuously

purged ('^^2.5 L/min) with dry nitrogen.

The main features of the box are shown in

figures 4, 5, 6, and 7. (Refer to table 1

3
Figure 4. Front view of the box enclosing Figure 5, Back view of the box enclosing

the automatic wafer prober (see table 1 for the automatic wafer prober (see table 1 for

number key to components ) . number key to components )

.

Figure 6. Inside view of the box with the top cover removed (see table 1 for number key to components and figs. 1-3 for details of the thermal chuck).

4
Table 1. List of the Major Components.

A. Thermal chuck components

1. 51-mm diameter wafer

2. Wafer index stop (stainless steel)

3. Top plate (nickel-plated copper)

4. Insulator plate (0.25-mm by 67-mm

diameter sapphire)

5. Chuck body (nickel-plated copper)

6. Cover plate (stainless steel)

7. Mounting ring (stainless steel)

8. Base plate (stainless steel)

B. Prober and enclosure components

15. Probe ring height manipulator

16. Stereo microscope

17. Prober control panel

18. Top viewport

19. Wafer transfer probe

20. Wafer 6-alignment manipulator

21. Microscope mounting post

22. Wafer transfer slide

23. Front viewport

24. Prober control connector

25. Liquid nitrogen f eed-throughs (2 eacl

C. Instrumentation

35. Digital voltmeter

36. Voltage source

9. Coolant manifold (2 each, stainless

steel)

10. Heater cartridge (4 each)

11. Electrostatic shields (4 each, stainless steel)

12. Thermocouple (2 each)

13. Chuck vacuum supply (2 each)

14. Leveling and locking screws (3 pairs)

26. Dry nitrogen f eed-throughs (2 each)

27. Heater power, thermocouples, etc.

28. Prober bridge assembly

29. Probe ring height micrometer

30. Probe ring support

31. Multiprobe assembly

32. Probe ring

33. Probe and manipulator (6 each)

34. Telescoping tubes/stainless steel

bellows (1 pair)

37. Capacitance meter

38. Electrometer

for the numerical listing of the components.) All sealing surfaces utilize gaskets or "0" rings. Utilities are interfaced at the back of the box; these include

the prober control (24), liquid nitrogen

f eed-throughs (25), dry nitrogen feedthroughs (26), heater power, and thermocouples (27) . Figure 6 shows an inside

view of the prober box. The thermal chuck

assembly is seen in the foreground. In operation, a wafer is loaded into the box

by the wafer transfer slide (22); once inside the box, the wafer is picked up with

the vacuum-operated wafer transfer probe

(19) and placed on the surface of the thermal chuck. Specimen wafers can be changed

in a matter of seconds without having to open the box; the wafer slide assembly (22)

which is sealed to the box in both the fully

open and fully closed positions, minimizes

penetration of room air into the box. (Additional details of the chuck and the wafer

slide mechanism are given in Appendix A.)

Wafer alignment to the prober axis is ac- complished by rotating the chuck assembly

about an axis perpendicular to its center

with the e-manipulator (20) ; this is done

while viewing the wafer with the microscope

(16) through the top viewport (18) . The

bridge structure (28), which supports the

probe ring (32), is shown displaced toward

the rear for clarity; the probe ring is nor- mally in the region above the thermal chuck.

Mounted on the probe ring (32) are six individually adjustable probe manipulators

(33), and a 6 by 6 multiprobe array (31).

The multiprobe is a fixed array of 36 individually biasable probes which can be used

for simultaneous probing of 36 devices

spaced at the appropriate interval on the

wafer; this multiprobe is usable to at least

300°C.* Neither the individual probes (33) nor the multiprobe array (31) is externally

adjustable or accessible after the box is

* The multiprobe array was designed to allow BTS on 36 devices simultaneously during one thermal cycle of the chuck.

5
closed. However, fine adjustment on the

probe contact pressure is available with the

height micrometer (29) which is accessible

externally through the probe ring height

manipulator (15) . Under measurement conditions, the top and front viewports (18, 23) are covered to prevent light from entering

the box. The x, y, and z positions of the

thermal chuck are controlled externally from

the control panel (17) . Figure 7 shows a view of the prober box along with its associated complement of commercially available

instrumentation for the TSM technique.

Liquid nitrogen is circulated to the movable

chuck from the f eed-throughs (25) at the

back of the box through a pair of stainless

steel welded bellows. Except for extension

and compression, these bellows are confined

within a pair of telescoping tubes (34) to minimize vibrational motion of the bellows

caused by expansion of the liquid nitrogen.

These tubes also serve to insulate the metal bellows from the box ambient. Excessive

cooling of the box ambient, and eventual

cooling of the prober mechanism under prolonged liquid nitrogen use was found to cause mechanical malfunction of the x-y

translation mechanism.

Figure 7. Typical experimental setup with

instrumentation (see table 1 for number key

to components )

THERMOMETRY

In order to realize useful information from

any variable-temperature measurement system,

appropriate attention must be given to the

thermometry. In this case, there are two aspects: (1) measurement of the actual wafer or device temperature and (2) the uniformity of the chuck temperature. Each of

these aspects needs to be considered under

both static (isothermal) and dynamic conditions of the chuck. In addition, for dynamic temperature measurements, the heating

rate must be established. The importance of

each particular aspect depends on the measurement that is being made. For example,

in order to measure the energy level of an unknown deep level defect, it is necessary

to know the specimen temperature accurately

when using the DLTS [3] technique; for TSM

[2], one needs to know both the temperature

and the heating rate. On the other hand,

when the objective is to simply measure the

density of a deep level defect by the thermally stimulated capacitance technique, an accurate knowledge of the temperature or heating rate is not required.

The temperature of the top plate is monitored by an isolated thermocouple which is

inserted into a hole from the side. The

temperature-sensitive junction is positioned

at the center. The thermocouple itself was checked for absolute calibration at -195.66°C

[6], and a measurement of the thermocouple

output voltage against temperature over the

range from -196 to 20°C confirmed that its

characteristics conformed to published thermocouple calibration charts [5].

Figure 8 shows a set of typical heating and

cooling cycles for two different measurement

sequences (the temperatures were measured

with the thermocouple in the top plate). The

upper curve represents heating from room temperature to over 300°C and back to room tem6
perature.* Heating to 300°C requires about

70 s and cooling back to room temperature re- quires less than 2 min. The lower curve represents a temperature cycle required for TSM.

Cooling to liquid nitrogen temperature is ac- complished in less than 4 min. The increase

in cooling rate just prior to reaching -196''C

is due to the complete filling of the chuck

with liquid nitrogen. Heating back to room

temperature requires about 45 s . (Although

TSM of mid-gap defects in silicon are com- pleted at room temperature, higher band gap

materials may require heating above room temperature.) Figure 9 shows a detailed heating curve and a heating rate curve from liquid nitrogen temperature. This typical heating curve was obtained with approximately 75 percent of full power applied to the chuck

heaters . Although high heating rates are re- quired for good signal-to-noise ratios in TSM,

the nonlinearity of the heating rate is of no consequence [2] as long as the rate at the

emission temperature is known. With this apparatus, heating rates as high as 7°C/s can be obtained.

0 100 200 300

TIME (s| Figure 8. Typical heating and cooling re- sponse curves for the chuck: a) above room

temperature, and b) below room temperature

(t5^ical for TSM).

Under dynamic conditions as illustrated in

figures 8 and 9, the indicated thermocouple

temperature and the actual chuck/wafer temperature can be significantly different. Be- cause the thermocouple junction is isolated

from the chuck top plate, it has a thermal

time constant approaching several hundred

milliseconds. During a typical TSM scan

(heating from -196 to 23°C) the device temperature is higher than the theirmocouple indicates. Temperature calibration for a TSM

scan is accomplished by calibrating the forward voltage drop of a junction diode (fabricated on the wafer) against the thermocouple temperature under static isothermal con- ditions (assuming that there is no temperature gradient between the fabricated junction

and the thermocouple) . Under dynamic conditions, the diode response is much faster than

the thermocouple; thus, the measured forward

voltage drop of the diode gives the actual

device temperature.

The nonuniformity of the chuck temperature

under "isothermal" conditions was determined

by measuring the variation of the forward

.voltage drop, Vp, of an array of golddiffused r&p diodes spaced at 2.54-mm intervals on a 51-mm diameter wafer [7]. Static

temperature conditions were maintained at the

center of the chuck by a temperature control

0

'

1 ' 1

1 1 1 1

1

1

-50

-100 / HEATING RATE

-150 TEMPERATUBE

-200

1 1 1 1 . 1 . 1

6.0

4 0

2.0

TIME |sl Figure 9 . Typical temperature and heating

rate curves for a thermally stimulated measurement with approximately 75 percent of

full power applied to the heaters

.

" This cycle would be typical of that used

for a BTS test, except that the stress

period at 300° C was omitted. It should

also be noted that the actual temperature

during BTS is an important parameter and

should be accurately known.

7
system which monitored the thermocouple output. At a given temperature set point, the

control system provides enough heating power

to balance the cooling input from the liquid

nitrogen. Under these conditions, the uncertainty of the set point temperature was about

±0.1°C. The forward voltage drop of the diode array (at a fixed forward current) was recorded as a function of wafer position for

a sequence of temperatures between liquid

nitrogen temperature and SO'C. The measured

variation of the voltage drop with position

is due to both the inherent differences of

the diodes as a result of their fabrication

and the possible nonuniformity of the chuck

temperature.

The forward voltage characteristic for a single device near the center of the wafer

is shown in figure 10. The measurements

were made with stationary temperatures at a forward current of 10 uA. There are two well-defined linear regions of the characteristic described by

V^, = 0.415 - 2.77 X 10-3t (1)

-196°C <_ T < -40°C

= 0.432 - 2.12 x lO'^T (2)

-WC < T £ 35°C

where T is the temperature in °C; Pearson's

correlation coefficients for V^i and Vp2 ^re

0.99996 and 0.99991, respectively. As ex- plained by Sclar and Pollack [8], an extrapolation of Vpi to absolute zero (-273°C) gives

Vpi (-273''C) = 1.171 V which corresponds to the intrinsic band gap of silicon at -273°C.

If all devices on the wafer had identical

forward voltage characteristics, temperature

profiling would simply be accomplished by

measuring the voltage drop as a function of

wafer position. Unfortunately, due to the

nonuniformity of the gold diffusion process

in the fabrication of the wafer, each device

has a unique slope; however, all characteristics intersect Vp = 1.171 V at -273°C. Figure 11 is a map of the forward voltage drop

at 24''C of approximately 300 devices spaced

at 2.54-mm intervals on the wafer. The total

variation from light regions to dark regions

is 0.3664 to 0.4091 V, respectively. Rotation of the wafer on the chuck causes rotatation of the pattern indicating that the image is due to the wafer and not the chuck.

In fact, at 24*'C, it is fair to assume that

the chuck is in equilibrium with its sur- roundings (no heater power or cooling applied) and thus that a true isothermal condition exists.

For the purpose of this temperature nonuniformity determination, high and low outliers were removed from the array (not measured)

and only a select group consisting of 170 devices with fairly uniform characteristics

was measured. At each temperature, "isothermal" conditions were established by the con- troller and the 170 devices were probed to measure the forward voltage drop. The data

points (a) of figure 12 show the measured

difference

0.8

ST 0.6

0.4

01- J__r 200 100 SO

TEMPERATURE {°Z\

Figure 10. Forward voltage drop characteristic for a specific n+p gold-diffused diode.

Figure 11. Wafer map of the forward voltage

drop of the n+p diodes. Approximately 300

data points are plotted with the dark-tolight shading representing a forward voltage

variation from 0.4091 to 0.3664 V, respectively. (A quadrant of four squares represents one data point . ) 8
^\ ^Fmax ^Fmin *

where Vfj^iax ^Fmin respectively, the

largest and smallest forward voltage drop of

the measured group at each temperature. The

spread in the measured voltage drops ranges

from 2.5 mV at liquid nitrogen temperature

(-196°C) to 11.5 mV at 58°C. The vertical

height of each data point represents the

measurement uncertainty as determined from

several measurements at each temperature.

The measured spread, AVj,j, is due to two con- tributions: (1) the "natural" spread of

voltages (AVd) as a result of nonuniformity

of device characteristics, and (2) the nonuniformity of the local wafer temperature (AT),

resulting in a deviation of AVj^ from AVq.

The line labeled (b) in figure 12 represents

an estimate of the temperature dependence of

the natural spread, AVq. It was established

by drawing a straight line through the room temperature data point (24°C) and the point

AV = 0 at -273°C. The reasonable assumption

that the chuck is isothermal at 2^°C means

that the measured spread is due only to the

natural variations in the individual devices

In addition, since all devices have an ex- trapolated forward voltage drop of 1.171 V

at absolute zero (-273°C) [8], then the natural spread, AVj) (-273°C) = 0. Hence, the two points on the line AVj) are fixed; the as sumption of linearity is justified in Appendix B.

In the region above -20''C, the data points

of the measured spread essentially follow

the straight line of the natural spread;

this suggests that there is no contribution

to the measured spread from nonuniformities

of the chuck temperature. The largest devia

tions between the measured spread and the

natural spread occur between -160 and -SO^C.

Assuming that this deviation is due only to variations in the chuck temperature , and

that the measured spread is the sum of the two independent contributions (i.e., AVj^ =

1 '

1 ' 1

1 1 1 1 1 1

1

1

(b) AV.^X I

I I

I I I (a) AVm

. . - ^ - .

1

A ^ ^

1 1 1 1

(c) AT

1 1 1 -250 -200 -150 -100 -50 0 50

TEMPERATURE ("C)

1.0

u

oh

<

o

Figure 12. Curves representing: a) the spread of the forward voltage drop of a selected

group of 170 diodes as a function of temperature, b) a linear plot representing the "natural

spread of the forward voltage drop due to device variations, and c) the deduced spatial vari

ation of the chuck temperature as a function of the temperature.
AVj) + AVj, where AV'j is the contribution due

to variations in the chuck temperature uniformity) , then the nonuniformity of the chuck

temperature can be calculated. This is

shown plotted in figure 12 as curve (c) and was determined from the difference between

AV^ and AVq, divided by the slope of a typical diode characteristic given in eq (1)

.

The largest deviation is about 0.8°C near

-65**C, resulting in an uncertainty of ±O.A°C

at a given temperature set point. Where this

uncertainty is critical, it would be necessary to make a specific calibration at the

location of interest rather than rely on the

indicated thermocouple temperature.

A curious consequence of these results is

the fact that the measured spread, AVj^, can be smaller than the natural spread, AVd.

This is explained by the location of particular devices. In general, the shading of

figure 11 reveals that the low voltage devices are near the center and the high voltage ones are near the edge of the wafer.

Thus, a situation where the edge region is warmer than the center region would cause a decrease in the measured voltage spread. In- tuition might suggest that the central re- gion would be warmer than the edge due to the presence of the heaters. However, a study of the forward voltage wafer maps

failed to reveal the "image" of the heaters

and corroborates the interpretation given

above

.

A direct measure of the uniformity of the

chuck temperature under dynamic conditions

was not made. However, an indirect measure

was obtained from the analysis of a series

of TSM scans which were made as a function

of wafer position under the same dynamic

heating conditions. The location of the measured emission current peak or the depletion capacitance change in a junction diode

caused by the discharge of a specific deep

level defect is a function of both the heating rate and the temperature [2]. With the

heating rate fixed at approximately 7°C/s

by maintaining a constant heating power, any

variations in the temperature at which the

emission occurs can be interpreted as being

due to variations in the chuck temperature

with position. This analysis was performed

at two different temperatures utilizing the

gold donor level (which discharges at about

-135°C) [9] and the gold acceptor level

(which discharges at about -45°C) [9]. The

results are represented in the two histograms of figure 13 which plot the number of

devices on the wafer versus the temperature

at which the emission occurs ; the temperature was measured with the top-plate thermocouple and corrected for the heating rate

lag. These results suggest that the variability of the chuck temperature with position under dynamic conditions is on the order of tS^C. Where these variations are important to a measurement, it would be necessary to correct or account for them.

(f) UJ 16

U

>

[jj Q 12

LLO

q: 8

LU

T» -135°C

TSS -45°C

5 8°C 5 4OC

Figure 13. Histograms showing the variation

of the temperature at which emission from

the gold donor (-135°C) and the gold acceptor (-45°C) occurs; the nonuniformity of the

chuck temperature under dynamic conditions

of approximately 7°C/s is inferred from this

variation

.

DEFECT MAPPING

As an example of the use of this apparatus

,

a wafer map of the defect distribution across

a gold-diffused silicon wafer was made. Thermally stimulated measurements were made on an array of n^p gated diodes fabricated by a 2-Mm phosphorus diffusion into <111>, 5 to 10 n«cm, p-type silicon wafers. The gold defect center was introduced by evaporating

gold on the back surface of the wafer and

diffusing at 825 °C for 24 h. Figure 14 shows

the thermally stimulated response from a typical diode on the processed wafer. The upper

curve is the capacitance response and the

lower is the current response. In each case.
the response was measured by first cooling

the device to near liquid nitrogen temperature. Zero bias was applied to the diode to charge all defects with majority carriers

(holes); a reverse bias of 15 V was then applied to form a depletion region. The cur- rent or capacitance was measured with the depletion bias maintained while the wafer was heated with a heating rate of about 7°C/s.

At the appropriate temperature, the gold defect emits its trapped hole causing a measurable current, a slight collapse of the depletion region, and a measurable capacitance

increase. In this example, the emission is

due to the gold donor level located at 0.35

eV above the valence band [9]. The system

noise in the x-y recorder tracings was approximately 2 fF and 0.1 pA for the capacitance and current, respectively.

The gold donor density was determined by

measuring the thermally stimulated capacitance response as a function of wafer position. Following the work of Buehler [2],

the defect density is given by

N 2(C^ - C^)

^A-^D

(3)

Nj-/(N^ - Nj)) is the ratio of the defect density to the net background acceptor density;

Cf and C-j^ are given in figure 14a. This ex- pression is valid for the case when only one

9 9 charge carrier is emitted, and C-^ » Cj^^

where C^, is the zero bias diode capacitance.

These conditions are satisfied here. Note

3.24

1

1 1 1

'

1

=: 3.20

:| 0 002 pF

3.16

- la) / CAPACITANCE

——r ; 1

\ ^OlpA

\^(b) CURRENT

-163 -143 -123 -103 TEMPERATURE I°C1

1.5

1.0

0.5

0

Figure 14-. The thermally stimulated a) ca- pacitance and b) current responses of a golddiffused n^-p diode; the heating rate is about

7°C/s.

that in this measurement, only the total

charge flow as evidenced by the change in

capacitance is important; the result is independent of the heating rate and also does

not depend on an accurate knowledge of the

temperature. An independent measure of the

average acceptor density of the depletion

region is required and was determined from

a measurement of the capacitance-voltage

characteristic of the sam.e junction used for

the TSM. The net acceptor density, - Np, was calculated from the standard Schottky

relation:

2Ci2c22(V2 - Vi)

q e (Ci2 - C22)

s

(4)

Ci, Vj and C2, V2 are the capacitancevoltage pairs taken from the diode C-V characteristic; q is the electronic charge, and

Eg is the dielectric constant for silicon.

The values of depletion capacitance were measured with = 5 V and V2 = 15 V. These

data were used to calculate the defect density from eq (3) . Figure 15 displays the

gold donor defect density as a function of

position on the wafer. The darker areas rep- resent regions of higher density, and the

variation is from 2.34 to 3.61 x 10

13 cm

-3

The system noise (2 fF) suggests that donor

defect densities in the range of 1 x 10^^

cm~^ would be detectable in this wafer.

Figure 15. Wafer map of the gold donor defect density. The variation from light to

dark regions is 2.3 to 3.6 x 10^3 cm" 3. (Approximately 70 data points are plotted on a grid corresponding to the 5.08-mm spacing of

the devices on the wafer. Shading at intermediate points is derived by interpolation.)

11
AUTOMATIC WAFER PROBING

A further aspect of this system which has

not been discussed in detail is its utility

as an automated wafer probing system. This

feature in itself is similar to many commercial wafer probing systems; the unique feature of this system is that measurements un- der static temperature conditions can be

made at any temperature in the range of -196

to 300 "C* For example, the wafer maps of

forward voltage drop (e.g., fig. 11) for the

temperature uniformity measurements were

made under computer control at a series of

temperatures from -196 to +58°C. The measurement of the data for the map at each

temperature requires about 6 min. In addition, such device parameters as diode re- verse leakage current [10], reverse recovery lifetime, and open circuit voltage decay

lifetime have been mapped and have been seen

to correlate with the measured defect density. This instrument can utilize the fact

that most semiconductor properties are temperature dependent and provides a new dimension to measurements for diagnostics of fabricated wafers.

SUMMARY

A variable-temperature wafer chuck capable

of excursions from -196 to 350°C has been

designed, constructed, and evaluated. The

chuck is part of an automatic wafer probing

apparatus which is housed in a sealed enclosure to provide a dry environment. The apparatus can be used for a variety of measurement functions but was primarily designed for

detection of deep levels in processed silicon

wafers. As an example, the gold donor defect

density in p-type silicon was mapped as a function of position on the wafer and graphically revealed the nonuniform defect distribution. The use of such apparatus as a diagnostic tool for monitoring defects during wafer fabrication should greatly enhance the

process engineer's ability to control his

process

.

ACKNOWLEDGMENTS

The authors gratefully acknowledge many helpful discussions with colleagues in the Electron Devices Division. In particular, special thanks go to W. M. Bullis for his perceptive reading of the manuscript, to L. M.

Smith, G. G. Morgan, and W. A. Cullins , who

were primarily responsible for the fabrication of the apparatus, and to Y. M. Liu, who

fabricated the gold-diffused wafer for the

defect density wafer map.

* The automatic probing feature is employed

only for measurements at static temperatures. Measurements requiring dynamic temperature excursions are performed manually

on individual devices because small mo- tions of the chuck (x, y, and particularly

z) due to thermal expansion and contraction cause automatic probing to be unre- liable .

12
REFERENCES

1. Sah, C. T., Bulk and Interface Imperfections in Semiconductors, Sotid-State

Electvonios 19, 975-990 (1976).

2. Buehler, M. G., Impurity Centers in p-n

Junctions Determined from Shifts in the

Thermally Stimulated Current and Capacitance Response with Heating Rate, SotidState Eleotronics 15, 69-79 (1972).

3. Lang, D. V., Deep Level Transient Spectroscopy: A New Method to Characterize

Traps in Semiconductors, J. Appl. Phys.

45, 3023-3032 (1974).

4. Measurement Techniques for High Power

Semiconductor Materials and Devices:

Annual Report, January 1 to December 31,

1977, NBSIR 78-1474, F. F. Gettinger,

Ed, pp. 30-38.

5. Powell, R. L., Hall, W. J., and Hyink,

C. H. , Jr., Thermocouple Reference Data

Expressed on the Kelvin Scale, NBS Report 10,724, p. 81 (November 1971).

6. Calibration was performed by the Heat

Division of the National Bureau of Standards .

7. The wafer was a "Compac-P" device wafer

obtained from Texas Instruments, Inc.,

Dallas , Texas

.

8. Sclar, N. , and Pollock, D. B., On Diode

Thermometers, Solid-State Electronias

15, 473-480 (1972).

9. Sah, C. T., Forbes, L., Rosier, L. I.,

Tasch, A. F., Jr., and Tole, A. B.

,

Thermal Emission Rates of Carriers at Gold Centers in Silicon, Appl. Phys,

Letters 15, 145-148 (1969).

10. Koyama, R. Y., Wafer Mapping of Electrically Active Defects, Semiaonduator

Charaaterization Techniques^ Proceedings Volume 78-3, P. A. Barnes and G. A. Rozgonyi, Eds, pp. 220-228 (ECS,

Princeton, 1978).

13
APPENDIX A

Mechanical Drawings for Parts

Given in figures Al through A4 are the mechanical drawings for the basic components of the

variable-temperature chuck and the wafer transport slide mechanism.

0-80 TAP 2 HOLES

Figure Al. Mechanical drawings for the top plate, wafer index stop, and the index stop pin.

14
Figure A2. Mechanical drawing for the chuck body.

15
Figure A3. Mechanical drawings for the cover plate and mounting ring.

2.019 X.0I8 DEEP 4-40 TAP 4 HOLES

WAFER TRANSPORT SLIDE MECHANISM T

"y MAKE ONE

C.R. STEEL

8-32 TAP 2 HOLES —J .785 k- 449*1 !«-/062-Jk

^

! -Ul k-.i2e —J 1^.336

8-32 TAP 4 HOLES

157 DRILL a C'BORE

/ 2 HOLES

^MAKE ONE SET

I 2024 ALUM.

4-40 TAP

MAKE 2 a 2024 ALUM.

CVJ DRILL a C'BORE FOR 4-40 FIL HD.

2 HOLES

I -^1*^062 "0"R J.550»--ti •-.250 ^

r i

I

IN6 GROOVE

,189 DRILL / 4 HOLES

MAKE ONE

2024 ALUM

j-*|600|«- k-l 234-»

-1875

25 I^I.O—^ |*-l.l09-i|*^^^ j*-774» "0"RIN6 GROOVE

Figure A4. Mechanical drawings for components of the wafer transport slide mechanism.

16
APPENDIX B

The Linearity of AV

In the region below ACC, assume a generalized form for eq (1)

:

V„ = A + BT

F

(Bl)

where A and B are constants, and T is the tem- perature in "C, Two specific equations which

represent the highest and lowest voltage devices of the group can, respectively, be written as

:

V = A + B T FH H H

FL

(B2)

(B3)

Hence, an equation for the "natural" spread

of the forward voltages of this group of devices can be written as:

AV (T) = V - V

FH FL

(BA)

Therefore, the assumed linear form for eqs

(B2) and (B3) result in a linear equation for

AVqCT), and hence the assumption made earlier

is justified.

The two boundary conditions required to evaluate eq (B4) are : 1) the observed spread at liquid nitrogen temperature, AV])(-196), and

2) the fact that the intercept at -273°C for

both Vpjj and VpL are identical [10], V-pyi Vpj, or AVj^(-273) = 0. Hence,

AV^(-196)

AVj^(T) = D

77 [273 + T]

(B5)

-196°C < T < -40°C .

In the temperature range for Vp2 [eq (2)],

similar reasoning and the application of boundary conditions at 24''C and -273°C yield the

following equation:

AV (2A)

AVjj(T) = [273 + T]

-10°C < T < 35°C

(B6)

From figure 12, it can be seen that AVj)(-196)

= 2.5 mV and AVd(24) = 10.0 mV; the slopes

calculated from eqs (B5) and (B6) then give

3.25 mV/°C and 3.37 mV/°C, respectively. A

straight line through the liquid nitrogen

data point (-196°C) with a slope of 3.25

mV/°C would be a reasonable representation

for AVq in the range -196 £ T <_ -40°C; the same can be said for a line of slope 3.37

mV/^C through the data point at 24°C. Although coincidental for this case, the slopes

of eqs (B5) and (B6) are virtually identical,

and hence, either line is a good description

for AVq over the entire temperature range of

interest.

17
NBS-114A (REV. 9-76)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP 400-55

2.Gov't. Accession No. 3. Reeipiefit's A.iiicb.Mon No.

4. TITLE AND SUBTITLE

Semiconductor' Measurement Technology

A Wafer Chuck for Use Between -196 and 350 °C

5. Publication Date January 1979

6. Performing Organization Code

7. AUTHOR{S) R. Y. Koyama and M. G. Buehler

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/Work Unit No. 7216441

11. Contract/Grant No. DOE: A021-EES

ARPA: 2397

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreef, City, state. ziP) DOE - Division of Electric Energy Systems, Washington, DC

20545; ARPA - 1400 Wilson Boulevard, Arlington, VA 22209

13. Type of Report & Period Covered Interim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 78-600155

1^ Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual surr^mary of most s i gni fi cant information, ff document includes a significant bibliography or

literature survey, mention it here.) This report describes the design and characterization of a variable-temperature

wafer apparatus for use in the detection of electrically active defects which produce deep levels in the band gap of silicon. In its present form, the wafer chuck

can heat and cool wafers as large as 51 mm in diameter over the temperature range

from j-196 to 350°C; heating rates as high as 7°C/s have been achieved. The uniformity of the temperature across the chuck under static conditions is estimated to be

better than ±0.4°C. Construction details of the chuck are given in an appendix.

The use of this apparatus is illustrated by wafer mapping the gold defect density

in diodes fabricated across a silicon wafer.

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

separated by semicolons) Deep level measurements; defect mapping; hot/cold wafer chuck; thermal wafer chuck;

thermally stimulated measurements; wafer chuck, variable temperature.

18. AVAILABILITY g Unlimited

I I For Official Distribution. Do Not Release to NTIS

I 1 Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003- 0201 7 -6

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