Pressure transducer

A pressure transducer has a first electrode fabricated with a predetermined shape upon a substrate surface. A first insulator encloses that portion of the first electrode not enclosed by the substrate; a second insulator forms a wall to a predetermined height above the substrate surface, and outwardly adjacent to the entire periphery of the first electrode. The wall has a base in pressure-tight connection to the substrate and a top opposite to the base. A deflectable second electrode is fabricated in pressure-tight connection across the wall top to enclose, at a reference pressure, a cavity between the first and second electrodes and the wall; the height of the wall, the shape of the first and second electrodes and the deflection characteristics of said second electrode, with respect to pressure, are all predeterminately selected to provide a desired relationship of the capacitance between the first and second electrodes and the pressure incident upon that surface of the second electrode opposite to the cavity.

BACKGROUND OF THE INVENTION 
The present application relates to pressure transducers and, more 
particularly, to a novel pressure transducer of the deflecting membrane 
type, having increased dynamic range and substantially uniform relative 
sensitivity over an essentially arbitrary monotonic curve of capacitance 
versus pressure. 
It is well known that there are many actual and potential uses for pressure 
sensors in today's high-technology environment. For example, in the 
transportation industries, almost all engines have at least one pressure 
transducer; in fact, an aircraft jet engine has several pressure 
transducers which must have a relative accuracy of about 1 percent of 
reading over a range spanning several decades of pressure. Because the 
tolerable error is extremely small at the lower-pressure end of the 
pressure range (when error is expressed as a fraction of full-scale range) 
these pressure transducers are relatively expensive. It is therefore 
highly desirable to provide a pressure transducer which not only has a 
wide dynamic range, but which also has a predetermined monotonic curve 
relating the input pressure to the magnitude of a predetermined electrical 
output parameter. That is, it is desirable to provide a response curve 
which has a well-defined input/output relationship; for example, an 
essentially logarithmic relationship of the output parameter, e.g. an 
electrical capacitance C, with changes in input pressure P (or C=k log P, 
where k is a scale factor). It is also desirable that the pressure 
transducer have a predetermined sensitivity characteristic over its entire 
range, e.g. response that is accurate to within a constant fraction of the 
current reading, and the like. It is further desirable to provide a 
pressure transducer which is rugged and compatible with semiconductor 
technology, so that the transducer can be integrated upon a semiconductor 
integrated circuit chip. 
PRIOR ART 
Pressure transducers, based upon a change in the electrical capacitance 
between a deflecting conductive membrane electrode and an adjacent fixed 
electrode, are well known in the art. Pressure transducer designers know 
that a larger area capacitive transducer has a greater low-pressure 
sensitivity than a smaller area transducer, which latter type, conversely, 
has a greater high-pressure sensitivity. The state-of-the-art at the 
present time suggests that a combination of separate transducers, one 
having superior low-pressure sensitivity and the other having superior 
high-pressure sensitivity (with perhaps additional transducers having 
optimum sensitivity at intermediate pressures therebetween) may be more 
cost effective than utilizing a single transducer with very high pressure 
sensitivity. This is so because the average pressure transducer, 
particularly of the capacitive type, is sensitive over a smaller range of 
pressure than that pressure range required in large dynamic range 
applications, such as the aircraft engine control application previously 
mentioned hereinabove. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the invention, a pressure transducer comprises: a first 
electrode fabricated with a predetermined shape upon a substrate surface; 
a first insulator enclosing that portion of the first electrode not 
enclosed by the substrate; a second insulator forming a wall to a 
predetermined height above the substrate surface and outwardly adjacent to 
the entire periphery of the first electrode, said wall having a base in 
pressure-tight connection to the substrate and a top opposite to said 
base; and a deflectable second electrode fabricated in pressure-tight 
connection across the wall top to enclose, at a reference pressure, a 
cavity between said first and second electrodes and said wall; the height 
of said wall, the shape of said first and second electrodes and the 
deflection characteristics of said second electrode with respect to 
pressure, being predeterminately selected to provide a desired 
relationship of the capacitance between said first and second electrodes 
and the pressure incident upon that surface of said second electrode 
opposite to said cavity. 
In a presently preferred embodiment, the transducer is fabricated by: 
fabricating the first electrode and then covering all of the first 
electrode and at least an adjacent portion of the substrate surface with a 
layer of a first insulative material; fabricating a layer of a second 
insulative material, different from the first layer material, to cover all 
of the first layer to at least beyond an imaginary line defining the 
periphery of the cavity to be formed, and to a predetermined depth; 
fabricating a thin film of a conductive second electrode material upon the 
second insulator layer free surface opposite to the substrate surface; 
forming an array of a plurality of apertures through the second electrode 
film; introducing, through the apertures, an etchant which does not 
appreciable affect the first insulator and second electrode materials and 
which dissolves away second insulator layer material between the first 
layer and second electrode in an amount sufficient to form the cavity of a 
desired shape and with a wall of second insulator material extending in 
pressure-tight manner between the substrate surface and second electrode; 
and then applying sufficient amounts of second electrode material to seal 
closed all of the apertures and the cavity. 
Accordingly, it is an object of the present invention to provide a novel 
pressure transducer having a relatively large dynamic range and uniform 
relative sensitivity, while being capable of fabrication utilizing 
semiconductor integrated circuit technology. 
This and other objects of the present invention will become apparent upon a 
reading of the following detailed description, when considered in 
conjunction with the drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring initially to FIGS. 1, 1' and 1a-1c, a presently preferred 
embodiment of my novel pressure transducer 10 is fabricated upon a surface 
10-1a of a substrate 10-1 of a non-conductive material, such as a ceramic 
or semiconductor substrate of a hybrid or integrated electronic circuit, 
and the like (see FIG. 1a). A first conductive electrode 11 is fabricated 
upon substrate surface 10-1a; first electrode 11 can be fabricated of any 
conductive material compatible with the material of the underlying 
substrate. A first insulative material of a first insulative layer 12 is 
fabricated to cover the first electrode, at least to the extend that the 
electrode is located within the transducer area. That is, first electrode 
11 has a first end 11a (see FIG. 1) and a second end 11b each of which 
either extend past or define opposite sides of the transducer area along a 
major axis x; the active area of transducer 10 is present between x-axis 
locations X1 and X2. A first transducer lead 10a is the continuation of 
the conductive first electrode, as from the narrow second end 11b thereof. 
Transducer 10 includes an insulative supporting wall 14 fabricated upon 
substrate surface 10-1a beyond the periphery of first electrode 11 and its 
associated insulative coating 12; wall 14 preferably has a substantially 
constant height H (see FIG. 2a) and has a predetermined shape which 
continuously encloses the entire transducer area. Wall 14 includes (FIG. 
1) a first end wall portion 14a, with its interior edge outwardly adjacent 
to line X1, and a second end wall portion 14b, with its interior edge 
outwardly adjacent to line X2; end wall portions 14a and 14b are connected 
by continuous, opposed side wall portions 14c. The dimensions of first and 
second end support wall portions 14a and 14b and the shape of curved 
support side wall portions 14c are determined by a generally curved, but 
monotonic, equation y=f(x). The insulative support wall 14 has a cross 
section with a substantially flat top portion 14d (emphasized by stippling 
in the plan view of FIG. 1). A metallization layer 16 (see FIG. 2a) is 
preferably fabricated upon the support wall top surface 14d, so that a 
thin film membrane second electrode 18 of an elastically-deflecting 
conductive material can be fabricated in attachment to the continuous 
support wall top portions 14d so as to complete the pressure-tight 
enclosure of a cavity 10c formed between second conductor 18, the interior 
surface of support wall 14, the top surface of insulative layer 12, and 
any portion of substrate top surface 10-1a between support wall 14 and 
insulator 12. 
Referring now particularly to FIGS. 1' and 1a-1c, transducer 10 with 
hermetically-sealed cavity 10c is fabricated by the following process: 
first, as shown in FIG. 1a, the metallization of first electrode 11 on 
substrate top surface 10-1a is carried out and first electrode is covered 
with the first insulative layer 12. Next, as shown in FIG. 1b, a layer 14 
of a second insulative material is formed atop substrate top surface 
10-1a, enclosing all of first insulator 12; layer 14 is formed to the 
desired height H (on the order of 100 microns). The edge metallization 
layer 16 is then fabricated upon the second insulator layer top surface 
14-1. Layer 16 may typically be formed with substantially constant 
thickness and substantially constant width and with a shape defining the 
edges of the transducer top electrode. A layer of the conductive material 
of second electrode 18 is then formed to cover all of the wall top surface 
14-1, between the opposed portions of metallization 16. The upper surface 
18a of the second electrode metallization layer is covered with a suitable 
resist material (not shown) and an array of holes are formed therethrough; 
an etchant is introduced to form an array of apertures 18b through the 
second electrode thin film layer (see FIGS. 1 and 1'). Thereafter, a 
second etchant is applied, through the second electrode apertures 18b; 
this second etchant does not appreciably affect the materials of 
metallization 16 (if used), thin film membrane layer 18 or first 
insulative material layer 12, but only the underlying second insulative 
material of layer 14. If desired, a suitable protective layer of the 
first, or another, insulative material can be applied to the exterior 
elevations 14' of the support wall, to prevent etching thereof. The second 
etchant removes (FIG. 1c) the underlying portions of second insulative 
material layer 14, as schematically shown by a removed portion 18b-1b, 
substantially symmetrically about the etching axis 18b-1a associated with 
a second electrode layer first aperture 18b-1. Similarly, other portions 
of layer 14 will be etched away, with the left-ward extent of the etched 
portion from each of apertures 18b-2, 18b-3, 18b-4, . . . , corresponding 
to broken curves 18b-2b, 18b-3b, 18b-4b, . . . Thus, the etched cavity 10c 
has an upper boundary formed by the second, or top, electrode 18, a lower 
boundary formed by the first insulative layer 12 and such portions 10-1a' 
of the substrate upper surface as are exposed (the second etchant also 
being selected to not substantially affect the material of substrate 
10-1). The lateral boundaries of cavity 10c are formed by the interior 
surface 14i of the enclosing wall, which surface 14i will typically have a 
curvature in the vertical direction. 
The interior cavity 10c is sealed, after being filled with a chosen gas at 
a chosen pressure, by directing additional second electrode material at 
the second electrode top surface 18a, to attach to the non-apertured 
portions 18c thereof and form an overfill 18d which adheres thereto to 
completely close all of apertures 18b in pressure-tight manner. As a final 
step, a second transducer conductive lead 20 (FIGS. 1 and 2b) is 
fabricated upon a portion of the substrate surface exterior of support 
wall 14, and has a portion 20a which integrally connects the lead 20 with 
the second conductive electrode 18; a second transducer lead 10b is a 
continuation of the conductive second electrode 20. The capacitance C 
between first electrode 11 and second electrode 18 can now be measured by 
electronic circuitry connected to leads 10a and 10b. It will be understood 
that this circuitry can be integrated into substrate 10-1. It will be 
further understood that, when apertures 18 are sealed, cavity 10c contains 
a reference volume of a reference atmosphere at a reference pressure; the 
exact chemical composition and physical parameters (pressure, temperature 
and the like) of the cavity-filling gas can be selected to provide the 
completed pressure transducer 10 with desired physical characteristics, 
such as for compensating temperature variations to which the transducer is 
subject and the like. 
Referring now to FIGS. 1, 2a and 2b, in operation, when the external 
pressure P is equal to the internal pressure of the gas in cavity 10c, 
upper electrode 18 is substantially undeflected and has a reference 
capacitance with respect to first electrode 11. Although the transducer 10 
will function for a range of external pressure P less than the reference 
pressure of the gas in cavity 10c, it is contemplated that typical 
operation will be with external pressure P greater than the reference 
cavity pressure. As the external pressure P is increased above the 
reference cavity pressure, the second electrode 18 is deflected downwardly 
towards first electrode 11. At low pressures, portions of second electrode 
18 adjacent to the wider cavity end (adjacent to line X1) will deflect 
very easily, while at higher pressures, portions of electrode 18 nearer to 
the narrow end 11b (adjacent to line X2) will begin to deflect. At higher 
pressures, the easily-deflected, high-compliance region at the wider 
cavity end will "bottom out", but is prevented from shorting to first 
electrode 11 by the presence of insulative layer 12. The maximum 
deflection distance D is selected to prevent deflecting electrode 18 from 
exceeding its elastic limit. To provide substantially constant relative 
pressure accuracy, wherein each increment of transducer terminal 
capacitance is responsive to an increment of pressure which is 
proportional to the current pressure, a "horn" shape of wall 14 may be 
designed, with the location and size of the upper electrode region then 
undergoing increased deflection at any particular pressure, being 
determined by the use of an appropriate mechanical design tool, such as a 
finite element computer-aided design program and the like. 
Pressure-to-capacitance transducer 10 has a maximum sensitivity at low 
pressure and has a decreasing sensitivity with increasing pressure and 
upper electrode deflection. In the specific embodiment illustrated herein, 
with the deflecting thin film electrode geometry being elongated along a 
major axis x in the rest plane of the electrode, and with variable 
distance between the supporting sidewalls 14c along a minor axis y, 
deflection at the wider end of electrode 18 will be greatest as pressure 
is increased from the rest pressure. Thus, the top electrode is deflected 
continuously downward; at a first pressure P.sub.1, greater than the 
reference pressure P.sub.0, the top electrode has a shape as shown by 
electrode 18-1, near the wider transducer end, in FIG. 2a. At the same 
time, the deflection of top electrode 18 at the narrow end (as shown in 
FIG. 2b) will be substantially negligible. For a somewhat greater pressure 
P.sub.2 &gt;P.sub.1, the wide-end deflection will be even greater, as shown 
by increasingly-downwardly-deflected electrode 18-2. The pressure can be 
further increased until the top electrode, near its wide end, "bottoms 
out". For even greater pressure P.sub.3 &gt;P.sub.2, electrode 18-3 bottoms 
out to an increasingly greater extent against the top surface of first 
insulative layer 12; capacitance increases until the downward deflection 
ceases at any particular Y line (perpendicular to the X direction along 
the major sensor axis). Even then, a region closer to the transducer 
narrower end will still continue to display deflection and the device will 
still display sensitivity. Thus, the deflection of upper electrode 18 at 
the narrow end will increase for even greater pressures, until, at some 
pressure P.sub.4 &gt;P.sub.3, the downwardly deflected upper electrode 18-4 
will contact the first insulative layer. Thereafter, a maximum pressure 
P.sub.m is reached at which a maximum amount of the upper electrode 18, 
adjacent the narrower end of the transducer, has "bottomed out" and no 
further deformation occurs; the maximum capacitance is exhibited. 
The pressure sensitivity is a function of geometry at a particular location 
along the elongated transducer dimension (X). It is possible to tailor the 
sensitivity at a particular pressure by tailoring the slope of the 
deflecting electrode boundary. Because of the concentration of stress at 
the edge of the transducer cavity 10c when the elastic film upper 
electrode 18 deforms under the influence of external pressure P, a fairly 
sharp angle may develop at the high pressure limit, at the interior upper 
edge of wall 14/metallizing layer 16, if used. The film of layer 18 is 
therefore subjected to very large tension at its outer surface and very 
large compression at the surface in contact with the edge of the cavity at 
wall 14 (and/or layer 16, if used). It is contemplated that the strain 
will not exceed a few tenths of one percent of the elastic limit of the 
film material. For example, if film 18 is tungsten of about 1 micron 
thickness, then less than 1% of the elastic limit is reached, at a maximum 
external pressure P of 50 atmospheres, for a circular cavity of about 
10-20 microns diameter. An identical geometry can be maintained at the 
edges of an arbitrarily large-diameter cavity, if there is an insulating 
support about 0.1 microns beneath the elastic film. Thus, the width of the 
transducer at narrow end X2 is on the order of 15 microns wide for the 
deflecting thin film electrode 18 to just reach the insulating support 
layer 12, near the narrow end of the transducer, with an external pressure 
P of 50 atmospheres. Even though the film in the wider portions of the 
transducer will have bottomed out at somewhat lower pressures, the edges 
will have the same deflection geometry because the support will hold the 
deflection at 0.1 microns and provide a zero slope boundary condition at 
the same distance from the edges as in the narrower region of the 
transducer. 
The hermetic deflecting thin-film member, which desirably is extremely 
elastic (with as little sensitivity to vibration or creep as possible), 
can be a ductile metal, or even more attractively, a refractory metal. 
Advantageously, the cavity is fabricated by utilizing silicon dioxide for 
the second insulative material and tungsten as the material for electrode 
18. Because tungsten can be selectively deposited, i.e. deposited only 
where previous tungsten exists, the selective deposition of additional 
tungsten for portions 18d, sealing apertures 18b, is attractive. The use 
of tungsten and silicon dioxide allows the use of hydrofluoric acid for 
the second insulative material etchant. 
While one presently preferred embodiment of my novel pressure transducer is 
described in detail herein, many modifications and variations will now 
become apparent to those skilled in the art. It is my intent, therefore, 
to be limited only by the scope of the appending claims and not by the 
specific details and instrumentalities presented by way of explanation 
herein.