A novel method of mounting thin samples for electrical measurements is described. A vacuum chuck holds a mounting plate, which, in turn, holds the sample. Contacts on the mounting plate establish electrical connection to the sample. The attachment of wires directly to the samples is unnecessary. Measurements can be made at temperatures from 77 to 1000 K. As an application of the apparatus, resistivity and Hall measurements of a thin silicon carbide sample are presented

Matus, L. G.

Summers, R. L.

English

NASA Technical Reports Server

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NASA Technical Memorandum 87256 
• NASA-TM-87256 19860015488 
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Apparatus for Electrical Measurements 
of Thin Films from 77 to 1000 K 
Lawrence G. Matus and Robert L. Summers 
Lewis Research Center 
Cleveland, Ohio 
March 1986 
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J.:lINGLEY RESEARCH CENTER 
LIBRARY, NASA 
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https://ntrs.nasa.gov/search.jsp?R=19860015488 2020-03-20T15:31:58+00:00Z
APPARATUS FOR ELECTRICAL MEASUREMENTS OF THIN FILMS FROM 77 TO 1000 K 
Lawrence G. Matus and Robert L. Summers 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135 
\1 
SUMMARY 
A novel method of mounting thin samples for electrical measurements is 
described. A vacuum chuck holds a mounting plate, which, in turn, holds the 
sample. Contacts on the mounting plate establish electrical connection to the 
sample. The attachment of wires directly to the samples is unnecessary . 
. Measurements can be made at temperatures from 77 to 1000 K. As an application 
of the apparatus, resistivity and Hall measurements of a thin silicon carbide 
sample are presented. 
INTRODUCTION 
The NASA Lewis Research Center, as a part of its effort in high tempera-
ture electronics, currently has a research program aimed at developing silicon 
carbide (SiC). SiC is of interest because of its potential as a semiconductor 
material for high temperature applications. During this research, electrical 
measurements are required for the routine characterization of the SiC material. 
From these measurements, basic electrical parameters such as carrier concen-
tration, resistivity, and Hall mobility are derived. If these measurements 
are made as a function of temperature, additional information such as the 
impurity doping concentration, impurity ionization energy, and the band gap 
energy can also be obtained. 
The combination cryostat-furnace apparatus described herein has been built 
for the purpose of making resistivity and Hall measurements as a function of 
temperature (77 to 1000 K) on thin samples (the order of 10 pm) of SiC. The 
measurements were made using the van der Pauw technique (ref. 1). For the Hall 
measurements, an electromagnet capable of generating 1 T (10 kG) was utilized. 
In designing this apparatus, the ease of sample mounting was a primary 
concern. Direct electrical connection to the sample by wire bonding or similar 
techniques had been considered but rejected. Bonding to these fragile samples 
is time consuming and frequently results in destruction of the substrates. 
Instead, a vacuum chuck concept was adopted which simultaneously holds the 
sample in place and establishes electrical contact. With this mounting method, 
measurements on many samples can be made quickly. For measurements as a func-
tion of temperature, either cooled or heated nitrogen gas was c1rculated over 
the sample and the mount1ng hardware. 
DESCRIPTION OF THE APPARATUS 
The photograph in figure 1 shows three elements that make up the cryostat/ 
furnace apparatus. These elements are (1) the tubular vacuum jacket, (2) the 
sample mounting assembly, and (3) the heating/cooling assembly. Figure 2 
illustrates the three elements in their assembled configuration and gives rele-
vant dimensions of the apparatus. These dimensions were forced by the dimen-
sions of the electromagnet and the SiC samples. The vacuum jacket must fit 
between the magnet pole pieces (51 mm air gap) and the sample mounting plate 
must' receive the SiC sample. A typical SiC sample is a 5 mm square platelet. 
For the most part, the apparatus is constructed of nonmagnetic stainless steel. 
The sample mounting assembly and the heating/cooling assembly slide into 
opposite ends of the vacuum jacket such that the cooled or heated nitrogen gas 
is injected in close proximity to the sample and the sample mounting hardware. 
The vacuum jacket is mounted between the pole pieces of the magnet and connec-
ted to a mechanical vacuum pump. The use of the vacuum jacket minimizes the 
heat conduction to and from the ambient environment. To further reduce the 
heat loss from the sample mounting volume, the void spaces in the sample mount-
ing assembly and the heating/cooling assembly are packed with a ceramic fiber 
insulation. 
Low temperature measurements approaching 77 K were obtained by flowing 
cryogenic nitrogen while electrical resistance heating of dry nitrogen was used 
to obtain measurements at temperatures approaching 1000 K. By balancing the 
flows of these two nitrogen gas sources, measurements at any temperature from 
77 to 1000 K could be made. 
To monitor the temperature, two thermocouples were initially utilized. 
One thermocouple was connected to the bowl of the vacuum chuck while the second 
sensed gas temperature near the sample. The original reasoning was that when 
the temperature was being changed, the chuck temperature would lag behind the 
gas ambient temperature. In practice, no significant temperature difference 
was noted. Since the bowl thermocouple did, on occasion, interfere with the 
electrical measurement leads, the bowl thermocouple was eliminated. All tem-
perature measurements are gas temperatures near the sample. 
A unique feature of this apparatus is the method of mounting and maintain-
ing electrical contact to the sample under evaluation. The mounting method 
had to retain the sample and sustain electrical contact over a wide temperature 
range and not introduce distortions into the magnetic field during the Hall 
measurements. Various methods of clamping were considered and rejected. 
Instead, the mechanical mounting force is provided by the vacuum chuck. The 
mounting stage is shown in figure 3 as well as the "exploded view" in figure 
2. With this mounting method, the vacuum force acts upon raised contacts on 
both the sample and the mounting plate to provide the necessary electrical 
connections. 
The mounting plate is made of a machinable glass-ceramic insulating 
material. Both surfaces of the plate, as well as the surface of the vacuum 
chuck are polished to enhance the vacuum seal between the various elements. 
The vacuum source for the chuck is a commercial venturi type pump. Holes 
through the mounting plate communicate the vacuum to the interface between the 
plate and the sample. Atmospheric pressure, acting upon the sample then pro-
vides the mechanical pressure to hold the sample in place. 
In this application, the vacuum holes through the mounting plate are con-
centric with the various electrical contacts so that the contacts on the mount-
ing plate are in the form of conducting annuli. Each sample has a pattern of 
2 
solid contacts similar to that of the mounting plate. The sample is positioned 
on the plate so that the contacts are aligned and then the vacuum is applied. 
The contacts on both the samples and the mounting plate are sputter-
deposited through metal masks. The deposited pattern on the mounting plate 
includes conductors from the conducting annuli to a second set of contacts near 
the periphery of the plate. These peripheral contacts are used to connect to 
the lead wires which, in turn, are connected to the appropriate measurement 
devices. 
For this application, gold contacts were used. In addition, gold wire was 
used for electrical leads to minimize any thermally induced contact voltages. 
The van der Pauw measurement requires a pattern of four contacts. For this 
work, the contact size was 0.5 mm2, and they were arranged in a square pat-
tern with 2.5 mm center-to-center spacing. 
In applications of this technique to other measurements, one must consider 
the problems of contact material, size, and spacing as well as the effect of 
contact potentials or resistance. The use of a conducting annulus as a contact 
does limit the size of the contact. In some applications, gold as a deposited 
contact may not be appropriate. The influence of contact size and geometry 
upon the van der Pauw measurement is discussed in reference 1. 
APPLICATION 
The apparatus described has been used to obtain electrical measurements 
upon SiC samples. These samples are epitaxially grown upon commercial silicon 
wafers (ref. 2). With care, thicker (>10 ~m) SiC layers can be removed from 
the silicon and handled without breakage. For thin layers, the measurements 
must be made with the silicon in place. The resistivity of the silicon 
decreases rapidly with increasing temperature and, in turn, introduces large 
errors into any measurement. Obviously, at higher temperatures, meaningful 
-measurements cannot be made with the silicon layer present. The higher tem-
perature measurements can be made only on the thicker samples. 
Figure 4 presents typical measurements of carrier concentration, resis-
tivity, and Hall mobility as a function of temperature. These data are from a 
cubic SiC sample having a thickness of 17 ~m. 
The apparatus described in this report was constructed for the specific 
purpose of SiC characterization. It is expected however, that the vacuum chuck 
concept would be useful in a variety of measurement applications. 
REFERENCES 
1. van der Pauw, L.J.: A Method of Measuring Specific Resistivity and Hall 
Effect of Discs of Arbitrary Shape. Philips Res. Rep., vol. 13, no. 1, 
Feb. 1958, pp. 1-9. 
2. Nishino, S.; Powell, J.A.; and Will, H.A.: Production of Large-Area 
Single-Crystal Wafers of Cubic SiC for Semiconductor Devices. Appl. 
Phys. Lett., vol. 42, no. 5, Mar. 1, 1983, pp. 460-462. 
3 
CRYOGENIC 
GAS JACKET 
VACUUM 
Figure 1. - Elements of cryostat/furnace apparatus. 
51 mm OD 
MOUNTING PLATE 
25 mill d ia, x 3 m m J 
EXPLODED VIEW 
I THERMOCOUPLE LEADS CHUCK VACUUM MEASUREMENT LEADS 
\- CHUCK VACUUM 
\ 
\ 
-~ 
VACUUM CHUCK 
13 mm dia. 
Figure 2. - Assembled configuration of cryostat/furnace apparatus. Exploded view details the sample mounting hardware, 
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Figure 3. - Sample mounting hardware. 
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Figure 4. - Electrical measurements for 17-~m-thick cubic SiC sample. 
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1. Report No. 2. Government Accession No. 
NASA TH-87256 
4. Title and Subtitle 
Apparatus for Electr1cal Measurements of 
Thin Films from 77 to 1000 K 
7. Author(s) 
Lawrence G. Matus and Robert L. Summers 
9. Performing Organization Name and Address 
Nat10nal Aeronaut1cs and Space Admin1strat10n 
Lewis Research Center 
Cleveland, Ohio 44135 
12. Sponsoring Agency Name and Address 
National Aeronaut1cs and Space Adm1n1strat1on 
Wash1ngton, D.C. 20546 
15. Supplementary Notes 
16. Abstract 
3. Recipient's Catalog No. 
5. Report Date 
March 1986 
6. Performing Organization Code 
505-62-01 
8. Performing Organization Report No. 
E-2839-1 
10. Work Unit No. 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
A novel method of mounting thin samples for electrical measurements 1s descr1bed. 
A vacuum chuck holds a mount1ng plate, wh1ch, 1n turn, holds the sample. Con-
tacts on the mount1ng plate estab11sh electr1cal connect10n to the sample. The 
attachment of w1res d1rectly to the samples is unnecessary. Measurements can be 
made at temperatures from 77 to 1000 K. As an app11cat1on of the apparatus, 
res1st1vity and Hall measurements of a th1n s111con carbide sample are presented. 
17. Key Words (Suggested by Author(s)) 
Apparatus; Electrical measurements 
Sample mount1ng; Thin f11ms 
18. Distribution Statement 
Unclassif1ed - un11mited 
STAR Category 35 
19. Security Classlf. (of this report) 
Unc1ass1f1ed 
20. Security Class If. (of this page) 
Unc1ass1fied 
21. No. of pages 
"For sale by the National Technical Information Service, Springfield, Virginia 22161 
22. Price" 
.. 
End of Document 


Apparatus for electrical measurements of thin films from 77 to 1000 K

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