Inorganic bonding materials minimize outgassing and improve electrical and mechanical properties of semiconductor strain-gage transducers in high-vacuum and high-temperature operations. The two basic methods described are ceramic-glass-bonding and metallic bond formation between the strain gage and the substrate

Woodruff, N. L.

English

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NASA TECH BRIEF 
fl
July 1970	 Brief 70-10215 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Inorganic Bonding of Semiconductor Strain Gages 
An improved method of bonding semiconductor 
strain gages to metallic force collectors of transducers 
has been developed and evaluated. This innovation 
involves the use of inorganic bonding materials in-
stead of the conventional organic adhesives normally 
employed for this purpose. Organic epoxies have 
excellent mechanical properties when they are used 
within their design limits. When they are exposed to 
ultra-high vacuum, however, outgassing occurs, 
and when their operating temperatures rise above 
350° F, hysteresis develops and the transducer be-
comes unstable. Consequently, the replacement of 
the organic adhesives with inorganic bonding ma-
terials appeared to be a way to minimize the out-
gassing and to improve the electrical and mechanical 
properties of the semiconductor strain-gage trans-
ducers in high-vacuum and high-temperature opera-
tions. Two basic methods were examined: (1) the use 
of an inorganic substance, such as glass, as the 
bonding material; and (2) the forming of a metallic 
bond between the strain gage and the substrate. 
In developing the first method, a ceramic-type 
glass with a relatively low melting point was found 
to be the best material because its firing-temperature 
range of 425° to 450°C is considerably lower than 
that of vitreous glasses. Further, it is readily adapt-
able to conventional methods of attaching leads to 
silicon. Because the devitrifying ceramic glass, after 
it has been cooled, can be reheated to relatively high 
temperatures without remelting, the sensing element 
of the transducer has much higher temperature poten-
tial than one with which organic adhesives have been 
used. When this sensing element is used in conjunc-
tion with an all-welded transducer housing and high-
temperature auxiliary hardware, for example, it is 
possible to extend the operating temperature range 
of the silicon strain-gage transducer to 800°F. The 
development of a piezoresistive transducer of this 
type for high-temperature operation is very desirable. 
The second method investigated was the metallic 
or eutectic bond. Establishing such a bond between 
the silicon strain gage and the force-collecting sub-
strate involves the use of a p-n junction to isolate the 
diffused strain gage from the silicon' substrate, thus 
forming the strain-sensing portion of the silicon de-
vice. The substrate, in turn, is eutectically bonded to 
the metallic force collector. Both the metallic and 
silicon substrates are coated with gold; and the sensor 
and the collector are heated to a temperature above 
370°C, which causes the interface to melt. The dis-
solved silicon redeposits on the single crystal sub-
strate during subsequent cooling, and an intermetallic 
bond results. This method of bonding is difficult to 
accomplish, because a uniform and complete bond 
over the entire area of the strain gage is necessary for 
the effective transmission of strain to the diffused 
sensing elements, and because the relatively thin 
sensor is very fragile. 
The ceramic-glass-bonding technique has been very 
successful, and has made practical the construction 
of unique force collectors with many desirable fea-
tures. The deleterious outgassing that is characterized 
with organic bonds is minimized, hysteresis is reduced, 
and repeatability is improved. In addition, the 
inorganic glass is not as susceptible as epoxies to 
degradation by intense radiation. The metallic bonding 
technique is difficult, but is worthy of further in-
vestigation. Its advantages are essentially those de-
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19700000207 2020-03-17T00:12:11+00:00Z
scribed for the glass-bonding method, and, in addi-
tion, it is likely that even less outgassing will occur 
in ultra-high vacuum environments. 
Notes: 
1. The bonding methods described in this document 
are applicable for use in hard vacuums, such as 
those of high-temperature, RF vacuum tubes. 
2. The following documentation may be obtained 
from:
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Norton L. Woodruff of

Kulite Semiconductor Products, Incorporated
undercontract to

Goddard Space Flight Center
(GSC- 10833) 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.65) 
Reference: NASA CR-100015 (N69-19138), 
The Development and Evaluation of an 
Inorganic Bonding Method for Semi-
conductor Strain Gages for Space and 
Ultra High Vacuum Applications
S 
S 
Brief 70-10215	 Category 08


Inorganic bonding of semiconductor strain gages

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