Three plate silicon-glass-silicon capacitive pressure transducer

A three plate silicon-glass-silicon capacitive pressure transducer includes a conductive silicon diaphragm and substrate relatively spaced by a dielectric body having disposed therein a central electrode positioned between the diaphragm and substrate to form a pressure responsive capacitance with the diaphragm at a value inversely proportional to a pressure signal applied to a pressure sensing surface of the diaphragm.

DESCRIPTION 
1. Technical Field 
This invention relates to capacitive pressure transducers, and more 
particularly to silicon-glass-silicon (SGS) capacitive pressure 
transducers. 
2. Background Art 
Silicon capacitor pressure transducers are known in the art, as described 
in U.S. Pat. No. 3,634,727 to Polye. The Polye pressure transducer 
includes a pair of silicon discs functioning as capacitor plates, each 
having a central aperture. The two discs are vacuum joined along their 
periphery with a silicon-gold eutectic metallic bond to form an internal 
vacuum chamber. The discs, when exposed to an applied pressure, flex 
relative to each other to change the capacitance value between them. This 
change in the device output capacitance is the manifestation of sensed 
pressure. Since the transducer has a quiescent output capacitance value, 
the sensed pressure accuracy is dependent on the magnitude of the change 
in the pressure responsive capacitance versus the quiescent capacitance. 
The quiescent value includes both the static capacitance value of the 
pressure responsive capacitance plus the value of the nonpressure 
responsive capacitance, i.e. the transducer's parasitic capacitance. 
The Polye device has relatively high parasitic capacitance values. This is 
due to the transducer's architecture where the surface area of the 
peripheral bond between the silicon discs is substantial when compared 
with the cross-sectional area of the flexible (i.e. pressure responsive) 
portion of the discs. In addition, the bonded surfaces of the discs are 
much closer to each other than the surfaces of the deflectable portion so 
that they provide a high capacitance value per unit area. The result is 
significant parasitic capacitance which masks the output capacitance 
change (.DELTA.C.sub.o), resulting in low signal to noise ratio. 
In our two copending applications of common assignee, entitled: 
ELECTROSTATIC BONDED, SILICON CAITIVE PRESSURE TRANSDUCER (U.S. Ser. 
No. 310,597, now U.S. Pat. No. 4,415,948 issued Nov. 15, 1983) and 
SILICON-GLASS-SILICON CAITIVE PRESSURE TRANSDUCER (U.S. Ser. No. 
310,598, now U.S. Pat. No. 4,405,970 issued Sept. 20, 1983), filed 
together on Oct. 13, 1981, we disclosed two plate SGS capacitve pressure 
transducers having structures with high signal-to-noise ratios. The 
parasitic capacitance of the structure was reduced by spacing the pressure 
sensitive silicon discs at distances no greater than that between the 
joining surfaces. This is achieved by selective topographical shaping of 
the interior glass borosilicate spacer to create pedestals. These devices 
provide larger variations in the pressure responsive capacitance for 
relatively modest changes in actual pressure. However, they still exhibit 
a finite parasitic capacitance, which may represent the practically 
achievable minimum for two plate devices. 
DISCLOSURE OF INVENTION 
One object of the present invention is to provide a capacitive pressure 
transducer structure which reduces parasitic capacitance to less than that 
achievable in the the prior art two plate devices. Another object of the 
present invention is to provide a structure which itself facilitates 
elimination of the gain attenuation effects of parasitic capacitance in 
the processing of the transducer signal output. 
According to the present invention, a capacitive pressure transducer 
includes a pressure responsive silicon diaphragm and silicon substrate 
spaced apart by a glass dielectric body having an electrode disposed 
therein between the diaphragm and substrate, the electrode positioned at 
an opposite end of a pressure sensitive interstice from the diaphragm so 
as to form a pressure responsive capacitance in combination with the 
diaphragm, at an instantaneous value dependent on the magnitude of a 
pressure signal applied to the diaphragm, the transducer having major 
nonpressure responsive parasitic capacitances between the electrode and 
substrate and the substrate and diaphragm, such that the transducer's 
quiescent output capacitance includes the series equivalent of the 
parasitic capacitance values in parallel with the pressure responsive 
capacitance value. 
In further accord with the present invention all three plates, the 
diaphragm, substrate and electrode are electrically conductive, the 
diaphragm and substrate being conductive silicon and the electrode being 
preferably metal, to allow direct connection of electronic signal 
conditioning apparatus to all three plates to facilitate the elimination, 
by signal conditioning, of the electrode to substrate capacitance, thereby 
eliminating parasitic capacitance signal attenuation of the transducer 
output. In still further accord with the present invention the central 
electrode includes a central plateau portion with major surfaces parallel 
to major surfaces on each of the diaphragm and substrate plates, the 
diameter of the central plateau being less than that of both the diaphragm 
and substrate. 
The capacitive pressure transducer of the present invention is a three 
plate device, by addition of a central electrode positioned between the 
diaphragm and substrate. As a result, spacing between the pressure 
responsive capacitance plates is reduced, and the parasitic capacitance is 
divided into segments so as to facilitate cancellation with associated 
signal conditioning apparatus. The parasitic capacitance between electrode 
and substrate is in series with that between the substrate and diaphragm 
which, together, are in parallel with the pressure responsive capacitance 
between the electrode to diaphragm junction of the transducer. As a result 
the absolute value of the parasitic capacitance is reduced. 
Furthermore, by providing separate signals to the electrode to diaphragm 
junction, the electrode to substrate junction, and the substrate to 
diaphragm junctions of the transducer, the pressure responsive capacitance 
is effectively isolated from the attenuating effects of the parasitic 
capacitance, as described in detail in a copending application of the same 
assignee, entitled: CAITIVE PRESSURE TRANSDUCER SIGNAL CONDITIONING 
CIRCUIT, U.S. Ser. No. 527,530, filed on even date herewith by Barry Male. 
As a result, the present capacitive transducer structure is well suited 
for low pressure applications due to the high gain sensitivity achievable 
by the transducer's output capacitance. 
These and other objects, features, and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a best mode embodiment thereof, as illustrated in the accompanying 
drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 is an elevated cross section view, not to scale, of the present 
(SGS) capacitive pressure transducer 10. A silicon (Si) substrate 12 
having a metal electrode 13 on a base side is bonded on a mounting side to 
a glass dielectric laminate having four layers 14-17 of borosilicate 
glass, e.g. Corning 7070 Glass, or "Pyrex". The glass dielectric layers 
support a center electrode 18 in position between the substrate and a 
pressure responsive silicon diaphragm 20. A chamber, or interstice 22 is 
formed in the glass dielectric concentric with a central plateau portion 
24 of the central electrode. The chamber allows diaphragm flexure in 
response to pressure signals applied to sensing surface 26. Diaphragm 
flexure changes the value of the interstitial capacitance between 
diaphragm and electrode, which transduces the applied pressure signal to a 
measurable electronic signal. For absolute pressure measurements, the 
outer surface 28 of the substrate electrode is exposed to a vacuum. In the 
case of .DELTA.P measurements, surfaces 26, 28 (diaphragm and substrate 
electrode) are exposed to the different pressure signals between which the 
difference pressure is to be measured. 
Referring to FIG. 2, fabrication of the three plate transducer begins with 
preparing a silicon base substrate 12 to a selected thickness. The actual 
thickness depends on the transducer's sensed pressure range. For a 0-50 
PSI device the substrate would be on the order of 800 Microns (.mu.M). A 
first layer 14 of borosilicate glass dielectric, typically Corning 7070 
Glass, or "Pyrex", if rf sputtered on the substrate surface 30. The 
sputtering is done with 25% oxygen. To ensure that the glass is 
sufficiently oxidized to take full advantage of the borosilicate 
properties, e.g. dielectric and expansion coefficients, the deposited 
layer is annealed by exposure to steam at elevated temperatures, on the 
order of 555.degree. C., for one hour. Annealing saturates the glass to 
provide a "wet" glass layer. This "relaxes" the glass and promotes a 
better field assisted bond of the glass to the silicon substrate. 
Following annealing, a thin borosilicate glass layer 15 is rf sputtered 
over the "wet" glass to a thickness of approximately 0.5 .mu.M. This 
second layer, which is not annealed ("dry glass"), seals the wet glass to 
allow metallization of the center metal electrode 18. 
The metal electrode 18 (FIG. 3) is formed on the exposed surface of layer 
15 by deposition of electrically conductive material, including metals, 
such as chromium, aluminum, or copper, or semiconductors, such as silicon. 
The electrode is deposited to a thickness on the order of 0.5 .mu.M using 
known techniques, such as rf sputtering, vacuum evaporation, or chemical 
vapor deposition. The electrode layer is geometrically patterned using 
standard photolithography and etching procedures to form a plateau 24 at 
one end. The plateau, forms a capacitor plate which in combination with 
the diaphragm provides the pressure sensitive capacitor element. 
The plateau surface area, which may be circular, is essentially concentric 
with substrate surface 30. An alternative, less desirable method of 
depositing and patterning the electrode involves the use of a metal mask. 
This eliminates the photo resistant etching procedures, but is less 
accurate. Following complete electrode formation, a third layer 16 of 
borosilicate glass is deposited over the electrode and the exposed surface 
area of layer 15. This third glass layer is similarly deposited by rf 
sputtering to a thickness on the order of 2 .mu.M. 
FIG. 4 illustrates the results of the next steps of forming an aperture, or 
well, 22 in layer 16, concentric with the plateau 24 surface of the 
electrode, and sealing this aperture with glass layer 17. The aperture is 
formed by known photolithographic techniques and use of a selective 
etching compound, such as hydroflouric acid, which attacks the glass 
dielectric layer but not the electrode. The well diameter at the exposed 
surface of layer 16 is greater than that of the plateau to allow for that 
portion of the diaphragm which cannot flex; the part immediately adjacent 
to the upper periphery of the well, which is constrained. Extending the 
plateau surface area into this "fringe" area of the diaphragm would only 
add to the parasitic capacitance. The exposed metal of the electrode 
itself acts as an etch stop, for control of the critical dimension of the 
vacuum cavity. Over etching the glass in a small annular area surrounding 
the electrode does not significantly affect the device properties. 
The layer 17 of borosilicate glass is then sputtered over the etched 
surface, e.g. the remaining surface area of layer 16, the exposed surface 
of the electrode plateau 24 at the bottom of the well, and any exposed 
surfaces of layer 15. Although other techniques may be used for depositing 
all of the glass layers, sputtering allows for exacting dimensional 
control. This thickness of layer 17 is similarly on the order of 0.5 
.mu.M. This seals the electrode surface so as to prevent arcing during the 
later field assisted bonding step, and also prevents electrical shorting 
of the diaphragm (20, FIG. 1) interior surface to the electrode under 
severe deflection. 
FIGS. 5, 6 illustrate the remaining steps in fabrication. The silicon 
diaphragm 20 is processed to a selected thickness. For the 0-50 PSI sensed 
pressure range the diaphgram thickness is selected to provide 2.0 .mu.M 
deflection. The diaphragm is then field-assist bonded to the structure by 
placing the diaphragm in close proximity to the exposed peripheral surface 
34 of the borosilicate layer 17. The diaphragm and structure are then 
heated to temperatures in the range of 500.degree. C., in a vacuum chamber 
at approximately 10.sup.-6 Torr pressure, with a DC voltage between 75-125 
VDC applied from the diaphragm 20 (+) to the glass layer 17 (-) for five 
to ten minutes. The resulting electrostatic field causes the diaphragm and 
glass layer 17 to attract each other as the current flow through the 
silicon glass interface provides a seal around the surface 34 to transform 
the well 22 into a vacuum chamber. 
The substrate electrode 13 is formed on the completed structure by rf 
sputtering conductive metalization on the exposed outside substrate 
surface. The metalization may be applied in two layers, a first layer of 
nickel on the order of 500 angstroms, deposited directly on the silicon 
substrate surface, and a second layer of gold, approximately 5,000 
angstroms, deposited on the nickel to allow wire bonding for electrical 
connection. It should be understood, however, that other methods may be 
used, such as alloying gold without recourse to the nickel layer. All of 
which is known in the art. 
FIG. 6 illustrates the last step of etching the transducer structure from 
the silicon diaphragm 20 down through glass layers 16, 17, to expose 
contact surface 36 of the electrode 18. A conductive metal layer such as 
nickel, gold, may then be deposited to again provide electrical connection 
to the center electrode. The nickel-gold cannot be deposited prior to 
electrostatic bonding since the bonding temperatures are above the 
gold-silicon eutectic temperature, and the nickel layer does not isolate 
them. 
In the present three plate SGS capacitive pressure transducer, the pressure 
responsive capacitor plates, i.e. the electrode plateau 24 and diaphragm 
20 (FIG. 1), are in close proximity; closer than the diaphragm to 
substrate or electrode to substrate spacing to provide a higher capacity 
per unit area for the pressure responsive capacitor than the parasitic 
capacitors. For the 0-50 PSI transducer the plateau cross-sectional area 
and that of the mating surface of the diaphragm is on the order of 0.114 
cm.sup.2. The surface area of the diaphragm and substrate not masked by 
the electrode plateau surface, i.e. the "fringe area", is approximately 
twice that of the plateau; on the order of 0.25 cm.sup.2. The plate 
spacing (d), however, differs by nearly 450 to 1. The spacing between the 
plateau and diaphragm is approximately 2.0 microns at P=0 PSI; that 
between the diaphragm and substrate is on the order 8.0 microns. Of course 
the diaphragm to substrate cpaacitance has a borosilicate glass dielectric 
with a higher dielectric constant (4.25) than the pressure responsive 
capacitance which is in a vacuum (K=1.0). 
Appendix A illustrates the distributed capacitances of the transducer in a 
simplified schematic illustration (A). The pressure responsive capacitance 
C.sub.p is across the electrode (E) to diaphragm (D) junction. The 
electrode (E) to substrate (S) junction includes parasitic capacitances 
C.sub.1 (that occurring between the glass dielectric layer 17 and the 
surface of the electrode plateau 24) and C.sub.2 (that between the 
electrode plateau and substrate). Parasitic capacitances C.sub.3 -C.sub.5 
exist between the diaphragm (D) and substrate (S). Capacitance C.sub.3 is 
in the vacuum of chamber 22 (FIG. 1), between the diaphragm and the bottom 
of the chamber outside of the plateau; the dielectric constant is 1.0. 
Capacitance C.sub.4 is within the dielectric between the chamber and the 
substrate. Finally, capacitance C.sub.5 is between the unmasked surfaces 
of the diaphragm (D) and substrate (S). 
Appendix A lists typical values for the capacitor's dielectric constant 
(K), plate area (A) and plate saving (d) for a 0-50 PSI transducer. The 
parasitic capacitances are resolved into two major values; 
##EQU1## 
as illustrated in drawing (B). The two values are C.sub.x 
=66.times.10.sup.-12 f, and C.sub.Y =124.13.times.10.sup.-12 f. The two 
parasitic capacitances are in electrical series and sum together as the 
ratio of the product to the sum, or C'=43.times.10.sup.-12 f. 
The zero PSI value for capacitance C.sub.p =50.44.times.10.sup.-12 f and at 
50 PSI C.sub.p =140.44.times.10.sup.-12 f. The change in output 
capacitance (.DELTA.C.sub.o) from zero to 50 PSI is on the order of twice 
the zero PSI quiescent value. 
Waveform 38 of FIG. 7 illustrates the change in the C.sub.p capacitance 
C.sub.p as a function of the sensed pressure signal magnitude. The 
waveform is nonlinear, but continuous up to a maximum P.sub.MAX value 40 
at which point the curve becomes less sensitive due to physical contact of 
the diaphragm with the electrode. 
The three plate capacitive pressure transducer architecture permits the 
plates of the sensing capacitance to be in closer proximity than the 
nonpressure responsive capacitances. This allows for a relatively high 
capacitance per unit area which helps equalize the sensing capacitor 
quiescent value with that of the parasitic capacitance. In addition, with 
all three plates being conductive, i.e. diaphragm and substrate being 
conductive silicon (specified N doping levels to allow direct electrical 
connection) and the electrode being either conductive silicon or metal, 
the signal conditioning circuitry of the before referenced copending 
application may be directly connected. As described therein, as is 
illustrated simply in sketch (C) of Appendix A, the conditioning circuitry 
provides the electrode (E) and substrate (S) with equal phase and 
magnitude current signals (I.sub.O, I.sub.Z) resulting in zero current 
flow through the parasitic capacitance C.sub.X. All of the sensing current 
(I.sub.O) flows through the pressure responsive capacitance C.sub.p, 
effectively neutralizing (and making an equivalent zero) the parasitic 
capacitance (C.sub.x). The result is a quiescent capacitance equal to the 
pressure responsive capacitance. At zero PSI, C.sub.O =C.sub.p 
=183.5.times.10.sup.-12. A gain of 1.8 compared with a gain of 
approximately 0.95 without the signal conditioning. As such, the three 
plate transducer may be used in extremely low sensed pressure applications 
in which the highest gain sensitivity is required. 
Similarly, although the invention has been shown and described with respect 
to a best mode embodiment thereof, it should be understood by those 
skilled in the art that the foregoing and various other changes and 
omissions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention.