Capacitive pressure detector independent of temperature

A capacitive pressure detector, which comprises a support plate (5, 6; 19), a stationary capacitor plate (4; 7) disposed on the support plate, (5, 6; 19), and a silicon plate (2; 13) disposed on the support plate (5, 6; 19) so that it surrounds the stationary capacitor plate (4; 7), the middle portion of the silicon plate having been made thinner so that it forms membrane-like structure (3; 16) acting as a mobile capacitor plate. The support plate consists of a silicon layer (6; 21) and of a glass layer (5; 20) attached onto said silicon layer and placed against the silicon plate (2; 13), said glass layer being essentially thinner than the silicon layer. Hence, in the combination plate (5, 6; 20, 21) in this way obtained, owing to the elasticity coefficients and thermal expansion coefficients of the different layers (5 and 6; 20 and 21), the difference in thermal expansion between the combination plate (5, 6; 20, 21) and the silicon membrane (3; 16) is essentially reduced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is concerned with a capacitive pressure detector. 
2. Description of the Related Art 
In respect of the prior-art technology, reference should be made to the 
following publications: 
[1] U.S. Pat. No. 4,386,453 (Gianchino et al.) 
[2] U.S. Pat. No. 4,257,274 (Shimada et al.) 
[3] U.S. Pat. No. 4,332,000 (Petersen) 
[4] U.S. Pat. No. 4,390,925 (Freud) 
[5] U.S. Pat. No. 3,397,278 (Pomerantz) 
[6] K. E. Bean, "Anisotropic Etching of Silicon", IEEE Transactions on 
Electron Devices, Vol. ED-25 (1978) No. 10, pp. 1185-93. 
It is well-known that miniaturized capacitive pressure detectors can be 
made of silicon and glass (cited papers [1] to [4]). Silicon can be 
processed by means of chemigraphic etching (cited paper [6]), patterned by 
microlithographic means, and the silicon and glass parts can be joined 
together by means of an electrostatic method (cited paper [5]). 
SUMMARY OF THE INVENTION 
Suitable for use in detectors are, e.g., Corning Glass, type 7740, "Pyrex", 
or the "Tempax" glass of Schott. They contain ions of alkali metals, which 
is favourable for the formation of an electrostatic joint. The thermal 
expansion of these glasses is also of the same order of magnitude as 
compared with silicon. At room temperature, the thermal expansion 
coefficient of silicon is 2.5 ppm/.degree.C., and that of the noted 
glasses about 3.2 ppm/.degree.C. At higher temperatures, silicon expands 
non-linearly and exceeds the corresponding coefficient of glass. 
The difference in the thermal expansion coefficients, about 0.7 
ppm/.degree.C., is the most important factor affecting the dependence on 
temperature of capacitive pressure detectors of silicon-glass construction 
.

FIG. 1 is a schematical illustration of the basis for the dependence on 
temperature in a capacitive pressure detector. A silicon piece 2 provided 
with a thin portion 3 deflected by the effect of pressure is attached to a 
glass plate 1 by means of an electrostatic method (cited paper [5]). The 
difference in pressure effective across the portion 3 deflects said 
portion 3 and changes the distance between it and a stationary capacitor 
plate 4 placed on the glass plate 1, and the capacitance between them. 
If the thermal expansion coefficients of silicon and glass are of different 
magnitude, when the temperature rises, a horizontal force F arises in the 
portion 3 sensitive to pressure. If the glass expands more than silicon, 
the force F attempts to reduce the deflection caused by the pressure P in 
the portion 3. If the sensitivity to pressure of the deflection without 
the force F is S.sub.o, with the effective force F it is 
##EQU1## 
wherein a is the side of the membrane 3 (if the membrane is square) or the 
diameter of the membrane 3 (if the membrane is circular), the coefficient 
K is 0.27 in the case of a square membrane and 0.2 in the case of a 
circular membrane, and .epsilon..sub.Si is the deformation (elongation) 
caused by the force F in the silicon membrane 3. 
If the glass part 1 is much thicker than the silicon part 2, the following 
equation applied approximately: 
EQU .epsilon..sub.Si =.DELTA..alpha..DELTA.t, (2) 
wherein .DELTA..alpha. is the difference between the thermal expansion 
coefficients of silicon and glass and .DELTA.t is the change in 
temperature. 
S.sub.o and .epsilon..sub.Si are dependent on temperature. The dependence 
on temperature of S.sub.o results from the dependence on temperature of 
the coefficients of elasticity of silicon. The temperature coefficient of 
the sensitivity S is 
##EQU2## 
With silicon at (100)-level (1/S.sub.o)(.delta.S.sub.o 
/.delta.T).apprxeq.70 ppm/.degree.C. If the value a/h=20 is chosen, 
(1/S)(.delta.S/.delta.T) .beta. is obtained, i.e., the phenomena cancel 
each other. However, the ratio a/h=20 is suitable only for detectors 
measuring rather high pressures (about 50 bars). 
If the pressure to be measured is lower than 1 bar, an appropriate ratio is 
a/h.gtorsim.80. That results in a temperature coefficient .gtorsim.1000 
ppm/.degree.C. 
Thus, it should be possible to reduce the difference in the thermal 
expansion coefficients .DELTA..alpha. in order that detectors intended for 
low pressure ranges should be stable relative the temperature. A suitable 
glass quality, whose thermal expansion coefficient were closer to that of 
silicon, is, however, not commercially available. The thermal expansion of 
the support plate 1 can, however, be brought to the desired level by means 
of the construction shown in FIG. 2. 
It is an object of the present invention to eliminate the above drawback 
and to bring the thermal expansion of the support plate in the pressure 
detector to the desired level. 
The invention is based on the following ideas: 
The temperature coefficient of the sensitivity of a capacitive pressure 
detector can be minimized when the difference in the thermal expansion of 
the insulating support plate and of the pressure-sensitive silicon 
membrane is adjusted to such a level that it overrules the 
temperature-dependence of the elastic properties of silicon. 
The insulating support plate consists of glass and silicon plates joined 
together. 
The thermal expansion coefficient of the support plate of sandwich 
structure can be brought to the desired level by selecting the thicknesses 
of the silicon and glass layers appropriately. 
In the sandwich structure, silicon and glass are joined together by means 
of an electrostatic method (cited paper [5]). 
The thickness of the glass layer is from 50 .mu.m to 1 mm. 
More specifically, the pressure detector in accordance with the invention 
includes a support plate which consists of a silicon layer and of a glass 
layer attached onto the silicon layer and placed against the silicon 
plate, with the glass layer being essentially thinner than the silicon 
layer so that in the combined support plate, owing to the elasticity 
coefficients and thermal expansion coefficients of the different layers, 
the difference in thermal expansion between the combined support plate and 
the silicon membrane is essentially reduced. 
By means of the invention, remarkable advantages are obtained. Thus, by 
means of the invention, it is possible to regulate the temperature 
coefficient of the insulating support plate in the capacitive pressure 
detector in the desired way. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The invention will be examined in more detail in the following with the aid 
of the exemplifying embodiments shown in FIGS. 2 to 4. 
FIG. 2 is a sectional side view of one detector in accordance with the 
invention. 
FIG. 3 is a sectional side view of a second detector in accordance with the 
invention. 
FIG. 4 shows a section along plane A--A in FIG. 3. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 2, the thick glass part 1 shown in FIG. 1 has been replaced by a 
thin glass 5 and a thicker silicon plate 6 joined together. The parts 5 
and 6 can be joined together best by means of the electrostatic method of 
cited paper [5]. 
When the silicon plate 6 is thick as compared with the glass plate, the 
deflection can be overlooked, and the thermal expansion coefficient of the 
sandwich structure can be calculated as follows: 
##EQU3## 
wherein .alpha..sub.2 is the thermal expansion coefficient of silicon and 
.alpha..sub.1 that of glass, m is the ratio of the thicknesses of the 
glass and the silicon, and n is the ratio of the coefficients of 
elasticity of glass and silicon. 
The difference between the temperature coefficients of silicon and the 
sandwich structure (5, 6) is 
##EQU4## 
In the case of glass and silicon, n=0.36. If we choose m=0.5, we obtain 
.DELTA..alpha..apprxeq.0.15 (.alpha..sub.1 -.alpha..sub.2). Thus, it is 
possible to reduce the mismatching between the thermal expansion 
coefficients of silicon and glass essentially. By selecting the ratio m of 
the thicknesses of the glass and silicon plates appropriately, it is 
possible to reach a situation in which the temperature dependencies of the 
difference in thermal expansions and of the elasticity coefficients 
overrule each other. 
The invention can also be applied, e.g., to the construction of a detector 
for absolute pressure in accordance with FIGS. 3 and 4. Into a silicon 
piece 13, a cavity 14 has been machined for a vacuum capsule as well as a 
recess 8 for the gap between the plates of a pressure-sensitive capacitor. 
The vacuum capsule is closed by a plate 15, which consists of a thinner 
glass layer 17 and of a thicker silicon plate 18. The plates 17 and 18 
have first been joined together by means of an electrostatic method (cited 
paper [5]). Thereupon the sandwich structure 15 has been attached in a 
vacuum to the silicon piece 13 by means of the same method. 
As part of the silicon piece 13, a thin silicon membrane 16 is disposed 
between the vacuum capsule 14 and the gap 8 between the capacitor plates. 
The silicon membrane 16 is deflected towards the vacuum capsule 14 by the 
effect of external pressure. The silicon piece 13 is attached to a support 
piece 19. The face of the support piece 19 is made of an insulating 
material, and onto it a thin metal film has been deposited into which a 
stationary capacitor disc 7, a conductor 12, and terminal areas 10 and 11 
have been patterned. A capacitor sensitive to pressure is formed between 
the flexible silicon membrane 16 and the deposited metal pattern 7. The 
capacitance of the detector can be measured across the terminal areas 10 
and 11. The terminal area 11 makes an electric contact with the silicon 
piece 13, and the terminal area 10 is in conductive contact with the metal 
pattern 7 via the conductor 12, which runs along a tunnel 9. The pressure 
to be measured can also act upon the silicon membrane 16 via the tunnel 9. 
The support piece 19 consists of a thin glass layer 20 and of a silicon 
plate 21, which have been attached to each other by means of an 
electrostatic method (cited paper [5]). The silicon piece 13 is attached 
to the support plate 19 by the same method. 
The temperature dependence of the pressure detector construction described 
herein is essentially lower than that of a construction in which the plate 
15 closing the vacuum capsule 14 and the support plate 19 are made 
exclusively of glass. Since the pressure detector construction is 
symmetrical, the torque dependent on the temperature and derived from the 
sandwich structure of the plates 15 and 19 overrule each other. 
Within the scope of the invention, it is also possible to conceive 
solutions different from the exemplifying embodiments described above. 
Thus, as the support plate, it is also possible to use a silicon plate 
onto which an insulating film has been deposited. However, the film must 
be thick in order to avoid stray capacitances, and the depositing of a 
thick film is slow and costly. The joining of a polished glass plate by 
means of the electrostatic method (cited paper [5]) is a rapid and 
inexpensive step of work. 
The dimensioning of the embodiment according to FIGS. 3 and 4 is as follows 
(typical ranges in parenthese): 
______________________________________ 
Glass plates 17 and 20 
width: 4 mm (2 to 6 mm) 
thickness: 100 .mu.m (10 to 200 .mu.m) 
Silicon plates 18 and 21 
width: 4 mm (2 to 6 mm) 
thickness: 1 mm (0.5 to 1.5 mm) 
Silicon plate 13 
thickness of plate: 
0.4 mm (0.2 to 0.5 mm) 
thickness of membrane: 5 to 200 .mu.m (depending 
on pressure) 
Stationary capacitor plate 7 and terminals 10 and 11 
thickness: 0.2 .mu.m (0.1 to 1.0 .mu.m) 
Capacitor gap 8 
width: 4 .mu.m (1 to 10 .mu.m). 
______________________________________ 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.