High sensitivity variable capacitance transducer

A semiconductor capacitance transducer includes adjoining integrated sensor and reference capacitance transducers formed from silicon wafers. The transducers are parallel plate transducers which are structurally the same except that one plate of the sensor transducer is a thin force sensing diaphragm which deflects in response to selected environmental phenomena while the corresponding plate of the reference transducer is adapted to deform in response to some, but not all, of the selected environmental phenomena. By comparing the capacitance of the transducers, the effects of the phenomena which deform the reference transducer can be distinguished from the effects of the phenomena which do not deform the reference transducer. A particular application of the present invention allows thermal effects on the sensor transducer to be distinguished from the effects of pressure. Also, the sensor and reference transducers are rendered substantially free from thermal stress by constructing their plates of the same semiconductor material and electrically isolating the plates with surface passivation layers.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
This invention relates to variable semiconductor capacitance transducers 
and in particular to capacitance transducers adapted to measure pressure 
variations which include an integrated semiconductor reference capacitance 
transducer. 
2. DESCRIPTION OF PRIOR ART 
Various semiconductor variable capacitance pressure transducers have been 
constructed. Basically, they disclose forming one plate of a parallel 
plate capacitor of a semiconductor material with the other plate being a 
dielectric such as quartz. Generally, oppositely disposed areas on the 
respective plates are metallized to provide the conductive regions of the 
capacitor. Doping the semiconductor plate with a high impurity 
concentration has also been utilized to form a conductive region in the 
semiconductor capacitor plate. Typically, one semiconductor plate includes 
a thin diaphragm portion which deflects in response to a pressure 
differential across it. 
The utility of such semiconductor pressure transducers in the microbar 
pressure range is greatly limited by changes in capacitance due to thermal 
effects. Temperature changes cause variation in plate separation due to 
thermal expansion or contraction of the materials between the plates. 
Plate area is also varied by the thermal expansion or contraction of the 
plates. These variations in plate separation and area are called thermal 
offset. Temperature changes also cause deflection of the diaphragm due to 
stress parallel to the surface of the plates. The stress results from 
differences in the coefficients of thermal expansion of the dielectric 
plate and the semiconductor plate. Such stress is termed thermal stress. 
Thermal stress increases with the thickness of the dielectric above the 
plates and with the area of the surface of the dielectric plate. 
Correction of these thermal effects problems has been left to extensive 
calibration and the selection of dielectrics with coefficients of thermal 
expansion similar to the semiconductor material. 
Such calibration is expensive and time consuming and even if the 
coefficient of thermal expansion of the dielectric is similar to the 
semiconductor material, thermal off-set is still a problem. Additionally, 
when one plate of the capacitor is a dielectric, metallization of the 
dielectric surface requires additional processing in order to provide a 
conductive area thereupon. 
Thus, the sensitivity and utility of capacitive pressure transducers would 
be greatly increased if the thermal effects on a capacitive transducer 
could be distinguished from the effects of pressure thereon. Batch 
processing and the fabrication of integrated circuits which included 
capacitive transducers would be greatly facilitated if both plates of a 
capacitive transducer could be made of the same semiconductor material, 
thus requiring only one processing line. Thermal stress would be reduced 
if thick dielectric materials could be eliminated from the transducer. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide a 
variable capacitive transducer which is readilay adapted to produce an 
output compensated for selected environmental effects exclusive of the 
phenomena to be measured. 
It is a further object of the present invention to provide a semiconductor 
capacitance transducer which can be easily batch processed. 
It is another object of the present invention to provide a capacitance 
transducer which is substantially free from thermal stress. 
It is another object of the present invention to provide a capacitance 
transducer which is sensitive to microbar pressure variations. 
It is a further object of the present invention to provide a capacitance 
transducer which can be easily incorporated in IC circuits. 
These objects, among others, are accomplished by providing two parallel 
plate capacitors (i.e. a sensor capacitor and a reference capacitor) which 
are constructed from semiconductor materials using standard integrated 
circuit processing. The capacitors are identical except that the lower 
plate of the sensor capacitor includes a thin, force sensing diaphragm 
which is adapted to deflect in response to pressure differentials across 
it which lie in a preselected range. The lower plate of the reference 
capacitor is substantially thicker than the diaphragm and is thereby 
adapted to remain substantially inert with respect to the selected 
pressure range. Because the capacitors are otherwise identical, variations 
in the plate separation and area of one capacitor due to forces other than 
those which affect the lower plates of the transducers differently (e.g., 
pressures lying within the preselected pressure range), will be the same 
as variations in the plate separation and area of the other capacitor. 
Thus, for example, variations in plate separation and area due to thermal 
expansion of the semiconductor material of the transducers (i.e., thermal 
offset) will be the same for both the reference and sensor capacitor. 
Thus, if the reference and sensor capacitors are subjected to pressure 
differentials across the lower plates at the same time that they are 
subjected to a temperature change, the difference in plate separation 
between the two capacitors will be due almost exclusively to pressure 
differentials falling within the selected range. Furthermore, the effects 
of thermal stress are minimized by constructing each transducer entirely 
of silicon, except for very thin passivation layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The variable capacitance pressure transducer 10 (see FIG. 3) includes two 
adjacent parallel plate pressure transducers 12 and 14. Transducer 12 is a 
variable sensor transducer and transducer 14 is a reference transducer. 
The lower plates of transducers 12 and 14 are formed in an N-type 
epitaxial layer 16 which is grown on a monocrystalline, silicon substrate 
wafer 18 (i.e., a first semiconductor body) of P-type conductivity having 
an upper flat surface 20 of (100) crystal orientation. Epitaxial layer 16 
includes an upper flat principal surface 22. 
As shown in FIGS. 1 and 3, transducers 12 and 14 are the same except for 
aperture 24 in substrate 18 which underlies epitaxial layer 16 in the area 
of sensor transducer 12. Aperture 24 will generally have tapered sides 26 
and 28 which extend from rim 30 to rim 32. Aperture 24 spans the entire 
depth of substrate 18 and exposes the lower surface 34 of epitaxial layer 
16. 
The lower plates of transducers 12 and 14 are formed by P+ areas 36 and 38 
which extend into epitaxial layer 16 from principal flat surface 22. Areas 
36 and 38 are heavily doped to insure that they have sufficient electrical 
conductivity to function as capacitor plates. Areas 36 and 38 are of the 
same dimensions and are circular. Area 36 of sensor transducer 12 is 
centered within rim 32 above circular aperture 24. 
The area of epitaxial layer 16 within rim 32 forms force sensing diaphragm 
40. Clearly diaphragm 40 can be made of various thicknesses and of various 
materials. However, the thickness of and the material comprising diaphragm 
40 should be chosen so that diaphragm 40 will measurably deflect in 
response to the application thereto of whatever force or forces are of 
interest. The present invention is particularly adapted to measuring 
pressure differentials across diaphragm 40. However, clearly transducer 12 
could be adapted to measure any phenomena (as a function of capacitance) 
which would create a force imbalance across diaphragm 40. An example of a 
sensor transducer 12 which is sensitive to microbar pressure differentials 
across diaphragm 40 is included below. 
Electrical contacts to P+ areas 36 and 38 are provided by way of P+ 
channels 42 and 44 which join the perimeters of P+ areas 36 and 38, 
respectively. Channels 42 and 44 extend into epitaxial layer 16 from 
surface 22 and extend outward from P+ areas 36 and 38 in opposite 
directions parallel to surface 22. 
A first passivation layer 46 of silicon dioxide covers surface 22 of 
epitaxial layer 16. Contact openings 48 and 50 extend through passivation 
layer 46 to provide access to P+ channels 42 and 44. Metallized runs 52 
and 54 run along the upper surface 56 of passivation layer 46 and contact 
P+ channels 42 and 44 through openings 48 and 50, respectively. Runs 52 
and 54 lead to contact pads 58 and 60 respectively, which also lie on 
surface 56. Another contact pad 62 is formed in a third opening 64 through 
passivation layer 46. Contact pad 62 serves as a contact to the most 
positive voltage in the system to insure that N epitaxial layer 16 is at a 
voltage which is at least as high as that of P+ areas 36, 38, 42 and 44. 
This, of course, insures that the P-N junctions in epitaxial layer 16 at 
the interface of the P+ areas 36, 38, 42 and 44 can never be forward 
biased and, therefore, P+ areas 36 and 42 are electrically isolated within 
epitaxial layer 16 from P+ areas 38 and 44. 
Eight solder bump sites 64, 66, 68, 70, 72, 74, 76 and 78 (see FIG. 1) are 
formed on upper surface 56 of passivation layer 46. Four of each of these 
bump sites are spaced around the perimeters of P+ areas 36 and 38 at 90 
degree intervals. The bump sites do not penetrate layer 46. The eight bump 
sites along with contact pads 58, 60 and 62 are all formed of an aluminum 
bump 80 (see FIG. 4). Each bump is covered by a composite bonding layer 82 
of chromium, copper and gold. Aluminum is utilized as the lower bump 80 
since it adheres well to silicon and silicon dioxide whereas the upper 
layer 82 is necessary to insure a strong bond between aluminum bump 80 and 
the solder bumps of the upper plate (see below). Two more metallized runs 
84 and 86 lead from the aluminum bumps 80 of bump sites 66 and 72 
respectively, to a fourth contact pad 88. Runs 84 and 86 overlie 
passivation layer 46. Pad 88 is formed the same as pads 58, 60 and 62. Two 
additional passivation layers, a silicon nitride layer 90 and an upper 
silicon dioxide layer 92 overlie passivation layer 46 and runs 52, 54, 84 
and 86. Eight identical openings 94 (see FIG. 4) are provided above each 
of the eight bump sites. Four identical openings 95 are provided above 
each of the contact pads 58, 60, 62 and 88. 
Transducers 12 and 14 have a common upper plate 96 formed from a 
monocrystalline silicon wafer (i.e., a second semiconductor body, see FIG. 
3). A lower principal flat surface 98 of wafer 96 is covered with a fourth 
passivation layer 100 of silicon dioxide. Upper plate 96 is doped with an 
N+ concentration of impurities to provide sufficient electrical 
conductivity therein for plate 96 to function as one plate of a capacitor. 
Eight polysilicon stops 102, 104, 106, 108, 110, 112, 114 and 116 are 
employed to separate upper plate 98 from epitaxial layer 16 (see FIG. 2). 
The stops are symmetrically disposed in two groups of four around the 
perimeters of P+ areas 36 and 38 with a stop positioned adjacent to a bump 
site at one of eight stop site locations 102A, 104A, 106A, 108A, 110A, 
112A, 114A and 116A (see FIG. 1). The polysilicon stops of FIG. 2 are 
located at the corresponding numerical polysilicon stop sites of FIG. 1, 
for example stop 102 is positioned at stop site 102A. The stops are 
secured to passivation layer 100 by any standard thin film technique such 
as vacuum deposition. Preferably, the stops are all of the same height to 
insure an equal plate separation between the plates of transducers 12 and 
14 which facilitates analysis of changes in capacitance due to variations 
in plate separation. Also, for ease of processing, it is preferred that 
the stops be of the same shape and dimension. 
The polysilicon stops are held firmly between passivation layers 100 and 92 
by eight solder bumps 118, 120, 122, 124 (see FIG. 3) and four bumps not 
shown. The solder bumps are formed at eight locations on upper wafer 96 
designated by eight openings 126, 128, 130, 132, 134, 136, 138 and 140 in 
passivation layer 100 (see FIG. 2). The eight openings in layer 100 for 
the solder bumps are, of course, in registered relationship with the eight 
bump site locations 64 through 78. Each solder bump is composed of a 
solder layer 142. The solder bumps are covered by a composite 
chromium-copper-gold bonding layer 144 and an aluminum layer 146. 
As shown in FIGS. 3 and 4, the aluminum layer 146 fills openings 126 
through 140 and adheres to upper silicon wafer 96. Composite layer 144 
serves to join the solder layer 142 to the aluminum layer 146. FIG. 4 
shows solder bump 118 at opening 130 being positioned above its respective 
bump site 64 prior to the joining of wafers 96 and epitaxial layer 16. 
Bump 118 would be joined to site 64 by solder layer 142 being positioned 
in opening 94 of bump site 64 and placed in firm contact with composite 
layer 82. Solder layer 142 would then be heated and reflowed and a secure 
bond would be formed between layers 142 and 82. Note that preferably each 
bump site opening is initially larger in diameter than the solder bumps, 
so that the solder bump will reflow to fill the bump site. Since all eight 
bumps would be contacted with their respective bump sites at the same time 
and bonded under the same conditions, the upper and lower plates would be 
pressed together by uniform pressure across passivation layers 92 and 100. 
Upper wafer 96 would then be positioned over substrate 18 as shown by 
dashed line 147 in FIG. 1. 
It is preferable that the polysilicon stops and the solder bumps be located 
beyond the perimeters of P+ areas 36 and 38 of the transducers 12 and 14. 
This simplifies the parallel plate structure of transducers 12 and 14 and 
insures a uniform vertical electric field between the plates of the 
transducers. It also allows the separation of the transducer plates to 
vary directly in response to forces applied thereto over a large pressure 
range because the plates are, in effect, clamped only at the edges 
thereof. 
The structure of the integrated capacitance transducer is completed by 
securing a glass tube 148 (shown in cross section in FIG. 3) to lower rim 
30 of aperture 24. Note that in FIG. 3, four flattened edges 106b, 108b, 
114b and 116b of stops 106, 108, 114 and 116 respectively, are presented 
in the cross-sectional view. 
One method of making an integrated capacitance transducer according to the 
present invention is shown in FIGS. 5A, 5B, 6A, 6B and 6C. The various 
photomasks referred to in processing the upper and lower plates of device 
10 are not shown. All impurity dopings can be performed by ion 
implantation or diffusion. FIGS. 5A and 5B show the results of various 
processing steps on the upper wafer 96 of device 10. With respect to FIG. 
5A, a silicon upper wafer 96 with flat lower surface 98 is provided. Wafer 
96 is doped with an N+ concentration. S.sub.i O.sub.2 passivation layer 
100 is then grown by standard oxidizing techniques on lower surface 98. 
Photolithography is then employed to mask layer 100 to expose the solder 
bump openings (only openings 130, 132, 138 and 140 are shown in FIG. 5A). 
The eight solder bump openings are etched through layer 100. Standard thin 
film deposition is then utilized to form the eight polysilicon stops (FIG. 
5A shows stops 106, 108, 114 and 116). 
Referring to FIG. 5B, standard thin film deposition is again employed to 
form the triple layered metallization at each of the bump openings. 
Metallized layers 142, 144 and 146 are deposited in turn through the same 
thin film mask to form the solder bumps (FIG. 5B shows only bumps 118, 
120, 122 and 124). 
FIGS. 6A, 6B and 6C show the results of processing steps on the substrate 
18 to form the lower plates. First, FIG. 6A shows that silicon substrate 
18 is a P-type starting material. Epitaxial layer 16 is then grown on 
substrate 18 and is doped to an N concentration. A first photomask (not 
shown) is applied to the upper surface 22 of epitaxial layer 16. The first 
photomask is exposed and developed to expose areas on surface 22 
immediately above areas 36 and 38, and channels 42 and 44. Areas 36 and 
38, and channels 42 and 44 are doped with a P+ concentration to a depth 
less than the thickness of epitaxial layer 16. Since this doping step is 
relatively precise, ion implantation is the preferable means for forming 
areas 36, 38, 42 and 44. 
The first photomask is removed and silicon dioxide layer 46 is then grown 
on upper surface 22 (see FIG. 6B). A second photomask (not shown) is 
employed to allow openings 48 and 50 to be etched above channels 42 and 
44, respectively. 
Continuing with FIG. 6B, the second photomask is removed and metallized 
runs 52 and 54 are provided by standard thin film deposition techniques, 
followed by further masking and the deposition of metallized layers 80 and 
82 at each of the eight bump sites as well as at the various contact pad 
sites (again by standard thin film techniques). Passivation layers 90 and 
92 are then deposited over passivation layer 46, the metallized runs, bump 
sites and contact pads. Standard photolithographic techniques are then 
employed to provide openings in each layer 90 and 92 at each of the eight 
bump sites (i.e. openings 94) and the four contact pads (i.e., openings 
95). 
Turning to FIG. 6C, the lower surface 150 of substrate 18 is etched to form 
aperture 24 to the depth of epitaxial layer 16, thereby forming diaphragm 
40. As is well known, the etching rate of the N epitaxial layer is much 
slower than the P-substrate 18. This difference in etch rates thus 
facilitates the etching of aperture 24 to the depth of lower surface 34 of 
layer 16 by allowing a larger tolerance in timing the etching than if only 
a P-substrate were employed. 
Finally, upper wafer 96 with the solder bumps attached is pressed against 
substrate 18 with the solder bumps in registered relationship with their 
corresponding bump sites, and the entire structure is heated to a 
temperature sufficient to allow the solder to reflow and adhere to layer 
82 at each of the bump sites. Glass tube 148 is secured to lower surface 
150 to seal the periphery of rim 30. 
The general function of device 10 allows for extremely accurate and 
sensitive determinations of pressure differentials. As shown in FIG. 3, 
the pressure of the environment to be measured, i.e. P.sub.E, is applied 
to the lower surface of diaphragm 40 by way of tube 148. A reference 
pressure P.sub.Ref, is provided between the upper and lower plates of both 
transducers 12 and 14. P.sub.Ref can be supplied by enclosing device 10 in 
a common hermetically sealed transducer container with a portal for 
supplying the reference pressure. When P.sub.E differs from P.sub.Ref, 
diaphragm 40 will deflect and the separation between area 36 and upper 
plate 96 will vary correspondingly. Since lower plate 38 of transducer 14 
is not exposed to the environmental pressure, no variation of plate 
separation of transducer 14 will occur due to a difference in pressure 
between the reference pressure and environment pressure. 
However, forces other than the pressure differential between P.sub.Ref and 
P.sub.E which affect the plate separation and plate area of transducer 12, 
can equally affect the plate separation and plate area of transducer 14. 
In particular, when the temperature that device 10 is subjected to 
changes, the various layers of materials within device 10 will expand or 
contract depending on the sign and magnitude of the coefficient of thermal 
expansion of the materials. Variations in plate separation due to the 
thermal expansion or contraction of materials between layer 16 and wafer 
96, and variations in plate area due to the thermal expansion or 
contraction of wafers 18 and 96 is termed thermal off-set. Stress between 
adjacent, joined layers parallel to the surface of the layers due to 
differences in the coefficients of thermal expansion of the layers is 
called thermal stress. 
In a device according to the present invention, variation in the plate 
separation and plate area of the sensor and reference transducers 12 and 
14 will be the same because the vertical structure of each transducer is 
the same except for aperture 24 and because wafers 18 and 96 are both 
silicon. Expansion or contraction in the vertical direction due to a 
temperature change will affect the vertical position of epitaxial layer 16 
(and therefore the vertical position of P+ areas 36 and 38) equally for 
both transducers 12 and 14 despite aperture 24. Areas 36 and 38 are the 
same and each will expand or contract equally because they are both formed 
in epitaxial layer 16. Thus, thermal off-set will be the same for both 
transducers 12 and 14. Therefore, since a pressure differential across 
diaphragm 40 will affect sensor transducer 12 but not reference transducer 
14, variations in plate separation of sensor 12 due to the pressure 
differential can be distinguished from variations due to thermal off-set 
by subtracting the variation in capacitance of transducer 14 (or a 
corresponding electrical signal such as voltage) from the variation in 
capacitance of sensor transducer 12. 
Furthermore, variations in plate separation of the transducers due to 
thermal stress are virtually eliminated. Since the transducers 12 and 14 
are composed entirely of silicon other than passivation layers 46, 90, 92 
and 100 and the metallization, the coefficient of thermal expansion is the 
same for both the upper and lower plates of each transducer. Thus, no 
thermal stress appears due to wafer 96, substrate 18, epitaxial layer 16 
or the polysilicon stops. The metallized areas, i.e. the solder bumps and 
the contact pads, are very malleable with small areas of contact to wafers 
18 and 96, thus their contribution to thermal stress is very small. 
Similarly, the passivation layers 46, 90, 92 and 100 are very thin and 
therefore shear forces applied to adjoining layers and silicon wafers 18 
and 96 due to the passivation layers are also very small. 
Nevertheless, because force sensing diaphragm 40 is much thinner than 
substrate 18 which underlies P+ area 38, any small but non-negligible 
thermal stress which might be present in device 10 due to dissimilar 
material resting on the silicon wafer surfaces, may deform diaphragm 40 
more than principal surface 22 above P+ area 38. Thus, for an even more 
precise transducer, an aperture identical to aperture 24 could be formed 
underneath P+ area 38 of reference transducer 14, thereby making reference 
transducer 14 strictly equivalent to sensor transducer 12. This would 
insure that all variations of plate separation between transducers 12 and 
14 due to thermal effects (as well as any effect to which both transducers 
were subjected) would be the same. If this alternative embodiment were 
employed and pressure differentials were to be measured, tube 148 (or its 
equivalent) would be a requirement to insure that the lower plate of the 
two transducers were not subjected to the same pressure differential. It 
also entails, of course, the obvious additional processing of forming two 
apertures instead of one. 
Generally however, because the passivation layers (46, 90, 92 and 100) are 
so thin and the solder bumps so malleable, the structure of the present 
invention is sufficient to negate thermal stress effects even in the 
microbar pressure range without the additional aperture. In prior 
semiconductor transducers, thermal stress was almost totally due to the 
use of an upper plate of dielectric material which had a different 
coefficient of thermal expansion from that of the lower plate which was 
formed from a semiconductor material. Thermal stress increases with 
increased thickness of the dielectric plate and increased area of the 
upper dielectirc plate. The present invention provides electrical 
isolation of the upper and lower plates by use of standard semiconductor 
surface passivation layers. Even totaled, the thickness of the passivation 
layers secured to a single wafer is generally at most several thousand 
angstroms (it is preferable to keep the passivation layer thickness to 
less than one micron). For microbar pressure variations, the contribution 
to thermal stress of these passivation layers to diaphragm deflection is 
of little consequence and is vastly less than the thermal stress due to an 
upper plate of a dielectric material. Further, even if the polysilicon 
stops are made of a material which is different from the upper or lower 
plates, if the height of the stops and the area of contact between these 
stops and the plates are small, the contribution to thermal stress by the 
stops will be minimal. Whatever the particular dimensions of transducers 
12 or 14, thermal stress is greatly reduced from configurations where one 
plate is a dielectric material by replacing that dielectric material with 
a semiconductor plate. And, thermal stress is virtually eliminated by 
relying solely on standard passivation layers as the means to electrically 
isolate the upper and lower plates. Electrically isolating plates by means 
of passivation layers is, of course, a method of increasing the 
sensitivity of a semiconductor transducer whether that transducer is in an 
integrated circuit or not. 
In the first embodiment, if the thickness of substrate 18 is sufficient, 
the entire lower surface 150 of substrate 18 could be subjected to the 
environmental pressure P.sub.E, without employing restrictive tube 148 
immediately around rim 30. This is because the thicker lower plate of 
reference transducer 14 would not be sensitive to the pressure range of 
interest and thus would remain unaffected by the difference between 
P.sub.E and P.sub.Ref. 
Thus, formation of the reference transducer 14 of the same materials and 
structure as the sensor transducer 12, except for aperture 24, allows the 
capacitances of transducer 12 to vary in response to forces due to a 
selected first phenomenon (e.g., pressure, a pressure differential or a 
range of pressure differentials to which diaphragm 40 will measurably 
deflect) and provides an integrated semiconductor capacitance transducer 
which can readily be adapted to produce an output which is compensated for 
the effects of a second phenomenon (e.g., temperature, a temperature 
change or a range of temperature changes). It is preferable to have the 
reference transducer 14 adjacent to the sensor transducer 12 so that each 
transducer will be subjected to virtually the identical temperature. 
Of course, transducers 12 and 14 should remain far enough apart so that the 
electric field of one transducer does not substantially affect the 
electric field of the other transducer. In a parallel plate configuration, 
for example, the fringing of the electrical field at the ends of the 
plates drops off rapidly and normally its effect can be ignored beyond a 
distance on the order of the plate separation. Standard computations of 
electric field configuration and intensity can be employed if necessary to 
determine the distance between the reference and sensor transducer to 
insure independence of the capacitances of the transducers. Transducers 12 
and 14 should also remain far enough apart so that the deflection of 
diaphragm 40 will not affect the area of principal surface 22 within P+ 
area 38. This distance will depend on the rigidity of the material 
employed, as is well known, and generally several mils is sufficient. 
It has been found that in the linear range the deflection of diaphragm 40 
can be obtained for pressures applied to diaphragm 40 by: 
##EQU1## 
where y=diaphragm deflection 
P=Pressure to be measured 
a=diaphragm radius 
t=Diaphragm thickness 
E=Young's Modulus of silicon, i.e., 1.51.times.10.sup.12 dynes/cm 
.nu.=Poissons ratio for silicon diaphragm, i.e., 0.18. 
K=a geometrical factor which is equal to 1 for the geometry of the 
preferred embodiment 
Similarly, the maximum pressure to retain linearity between pressure and 
deflection is given by: 
##EQU2## 
(Equations 1 and 2 are provided in R. J. Roark, "Formulas for Stress and 
Strain", Table X, Second Edition, McGraw-Hill, New York and London (1943) 
and S. Timoshenko and S. Woinowsky-Krieger, "Theory of Plates and Shells," 
McGraw-Hill, New York (1959), the contents of which are herein 
incorporated by reference). 
Some typical dimensions of a device which could be adapted to be sensitive 
to microbar pressure differentials are as follows: diaphragm 40 a diameter 
of 60 mils; epitaxial layer 16 a thickness of 26 microns; P+ active areas 
36 and 38 a diameter of 54 mils; a 6 mil gap between the diaphragm 
perimeter 32 and the nearest point of the polysilicon stops (i.e. distance 
"d" in FIG. 1); a 1,000 angstrom thickness for silicon dioxide layer 46; a 
5,000 angstrom thickness for silicon nitride layer 90 and silicon dioxide 
layer 92 each; a 0.5 micron height for aluminum bump 80; a 1.3 micron 
height for composite layer 82 with the chromium being 1,500 angstroms, the 
copper 10,000 angstroms and the gold 1,300 angstroms; a 5 micron height 
for the polysilicon stops; a 1,000 angstrom thickness for silicon dioxide 
layer 100; a 15 mil thickness for substrate 18; a 25 mil separation 
between P+ areas 36 and 38; an approximately 130 mil.times.260 mil mil 
substrate 18; and an approximately 88 mil.times.176 mil upper wafer 96. 
Utilizing these dimensions, the plate separation between P+ areas 36 and 
38 and the lower surface 98 of upper wafer 96 is approximately 6.2 microns 
(i.e. 5 micron polysilicon stop plus two 0.1 micron silicon dioxide layers 
46 and 100 plus a 0.5 micron silicon dioxide layer 92 plus a 0.5 micron 
silicon nitride layer 90). To reduce the total thickness of passivation 
layers 46, 90 and 92 to less than one micron, in the event that the above 
dimensions contribute excessively to thermal stress, it is preferable to 
reduce the thickness of layers 90 and 92 to 4,000 angstroms each. The only 
limit with regard to the thickness of the passivation layers (or layer if 
only one is used to isolate wafers 18 and 96) is that they be thick 
enough to substantially electrically isolate wafers 18 and 96. 
Isolation of P+ areas 36 and 38 by reverse biasing the P-N junctions 
between those areas and epitaxial area 16 introduces a junction 
capacitance between the epitaxial area 16 and each P+ area. These junction 
capacitances could limit the sensitivity of the output since the junction 
capacitances are in series with their respective transducer capacitances. 
This problem could be alleviated, for example, by designing the circuitry 
employing device 10 so that transducers 12 and 14 are in opposite arms of 
a bridge circuit wherein each arm of the bridge circuit contains an 
impedance formed by a capacitance equal to the capacitance of transducers 
12 and 14 individually, including their respective junction capacitances. 
For simplicity, transducers 12 and 14, should preferably under initial 
conditions have the same capacitance and the same junction capacitances. 
As is well known, in such an impedance bridge configuration, the 
individual magnitudes of the capacitances will not be important and 
variations in capacitances 12 and 14 can be readily determined to a high 
degree of sensitivity. Further, the use of metal plates on dielectric 
wafers would avoid this junction capacitance problem and maintain 
capacitor isolation. 
If the junction capacitances discussed above are sufficiently small so that 
pressure differentials across diaphragm 40 can be measured to a desired 
degree of accuracy, the operational amplifier circuit of FIG. 7 can be 
used in conjunction with device 10. The operational amplifier circuit of 
FIG. 7 is a typical circuit which employs eight MOSFETS. The input to the 
gate of MOSFET 152 is controlled by a voltage divider comprised of sensor 
capacitor 12 and capacitor 154. Similarly, the input to the gate of MOSFET 
156 is controlled by a second voltage divider comprised of reference 
capacitor 14 and capacitor 158. MOSFETS 160 and 162 serve as loads to the 
drains of MOSFETS 152 and 156, respectively. MOSFETS 164, 166 and 168 
serve as a current mirror. Nodal analysis requires that the source 
currents of MOSFETS 152 and 156 (i.e., IB and IC) always equal IA, which 
is constant. Preferably, MOSFETS 152 and 156 are identical as are MOSFETS 
160 and 162, and capacitors 154 and 158. 
As is well known, different input voltages to the gates of MOSFETS 152 and 
156 result in a variation in the voltage at node 170. The voltage at node 
170 is passed through a second stage amplifier comprised of MOSFETS 164 
and 172 and capacitor 174. The final output is taken at V.sub.out. The 
circuit of FIG. 7 can be incorporated in an IC with device 10. Generally, 
DC biasing components would be added to the circuit of FIG. 7 to set 
V.sub.Ref and to initially set the input voltages. 
In operation, pressure differentials across diaphragm 40 which deflect 
diaphragm 40 but not the lower plate of transducer 14 will provide 
different input voltages to MOSFETS 152 and 156. The voltage at node 170 
will vary and this variation will be amplified and the result obtained at 
Vout. From the values of the components in the circuit of FIG. 7, the 
difference in variation in capacitance of transducers 12 and 14 can be 
readily determined from which the variation in plate separation of 
transducer 12 due solely to pressure can be sound. Then the pressure 
differential across diaphragm 40 is easily calculated using Equation 1. 
Device 10 has herein been described in terms of its use as a pressure 
transducer, however device 10 could be adapted to measure acceleration or 
any other force which would deflect diaphragm 40 substantially greater 
than the lower plate of transducer 14. If acceleration were of interest, 
diaphragm 40 could be positioned perpendicular to the direction of 
acceleration and P+ area 38 could be positioned along the direction of 
acceleration. Temperature changes would then effect transducers 12 and 14 
substantially the same while the acceleration being measured would effect 
only the sensor transducer 12. Device 10 could be adapted to serve as a 
transducer with a compensated output for any force to which diaphragm 40 
would be measurably deformed whereas the lower plate of transducer 14 
would not. Device 10 is, of course, equally capable of serving as a 
transducer for steady state forces applied to the lower plates of 
transducers 12 and 14 as well as variational forces. 
Many variations on the preferred embodiment within the scope of the claimed 
invention are also possible. For example, the polysilicon stops could be 
deleted and plate separation provided solely by the solder bumps. If a 
constant plate separation is employed, care must be taken in the formation 
of the bumps. Transducer 12 and 14 need not be the same or even similar. 
One could be a parallel plate capacitor and one of another configuration. 
Calibration of each transducers output with regard to the range of 
interest of pressure and temperature, or compensating circuitry, would 
then generally be needed to distinguish the effects of the various 
phenomena on the transducers. However, the geometries of the transducers 
could be chosen to provide a fixed relationship between the capacitances 
of the transducers. Further, if one desires to not integrate transducers 
12 and 14 in the same wafers, the upper plates of the transducers can be 
on different wafers, and the lower plates of the transducers can be on 
different wafers (i.e., four separate semiconductor bodies would be 
involved). In that event, the charge holding regions in the plates of the 
respective transducers would generally be oppositely disposed in 
overlapping relationship, as is the case of the integrated circuit 
described above. 
Metal plates on dielectric layers which overlie wafers 18 and 96 can be 
employed instead of doped conductive areas within the wafers. If metal 
plates are used, wafers 18 and 96 could both be dielectrics but at a loss 
of ease of integration. If only one wafer of either wafers 18 or 96 were a 
dielectric, integration could still be accomplished but more than one 
processing line would then be required. Dielectric layers 90, 92 and 100 
(and even layer 46 other than under the metallized runs and contact pads 
on wafer 18) could be eliminated, but at increased risk of deterioration 
of the exposed surfaces. The location of the metallized runs and contact 
pads can clearly be varied greatly. Wafer 96 can use two separate doped 
areas for the upper plates of the transducer. The conductivity types of 
the P and N areas could be reversed. The shape of the "plate" sensitive to 
the phenomena of interest need not be a thin diaphragm. Any shape which 
will measurably deform when exposed to the phenomena of interest is 
sufficient as a charge holding member of the sensor transducer. In the 
processing, the stops could be etched from upper plate 96 and thereby be 
formed of monocrystalline silicon (see FIG. 8). This list of variations is 
given only by way of example. It is not nor is it intended to be 
exhaustive of the embodiments of the device disclosed herein, those 
embodiments being solely defined by the claimed invention. 
It can be seen from the above description that a device constructed in 
accordance with the description will accomplish at least all of the stated 
objectives.