Silicon diaphragm capacitive pressure transducer

A capacitive pressure transducer utilizing a plate of electrically conductive semi-conductor material sandwiched between a pair of plates of inorganic electrically insulating material and electrostatically bonded thereto. A pair of concentric circular recesses are etched in the plates in such a way as to form cavities on opposing sides of the conductive plate and to define a diaphragm area on that plate. Apertures are drilled through the insulating plates to expose the cavities to the pressures to be measured. The surfaces of the insulating plates in the cavities have deposited on them electrically conductive surfaces which form capacitors with the diaphragm area on the conductive plate. A stress relief area is provided on the high pressure side surrounding the diaphragm area. In that stress relief area, the conductive plate is unrestrained by the adjacent insulating plate. Provision of this stress relief makes it possible to extend the pressure range of the transducer beyond the range which would normally be useful for the dimensions of the particular transducer.

BACKGROUND OF THE INVENTION 
This invention relates to a variable capacitance pressure transducer of the 
type which utilizes a plate of electrically conductive semi-conductor 
material, such as silicon, as a diaphragm material. This conductive plate 
is sandwiched between a pair of plates of inorganic electrically 
insulating material, such as borosilicate glass, with the plates being 
electrostatically bonded together to form the transducer. 
Silicon-diaphragm transducers of this construction typically have a pair 
of concentric circular recesses etched in the opposite faces of the 
silicon plate or, alternatively, in the surfaces of the glass plates so as 
to form cavities on opposing sides of the silicon plate to define a 
diaphragm area on that plate. Circular, thin-film metallic deposits are 
laid on the surface of the glass plates in the cavities so that the 
deposits form capacitors with the diaphragm. 
When the cavities are exposed to different pressures, the diaphragm 
deflects and increases the electrical capacity on one side while 
decreasing the electrical capacity on the other side. Thus, it is possible 
with a measuring circuit responsive to those changes in electrical 
capacity to obtain a pressure difference measurement. This measurement may 
be a measure of gage pressure when one cavity is exposed to the ambient 
pressure or it may be what is termed a pressure difference measurement 
when cavities on opposite sides of the diaphragm are both exposed to 
process pressures. 
Pressure transducers such as described above are shown in U. S. Pat. No. 
4,257,274-Shimada et al, issued on Mar. 24, 1981. A method for producing 
pressure transducers of this type is described in U.S. Pat. No. 4,261,086 
issued to Giachino on Apr. 14, 1981. 
The pressure range of pressure differences which can be measured by devices 
of the type described above is determined by the diameter of the diaphragm 
area and the diaphragm thickness. For a fixed diameter of the diaphragm 
area, the range can be increased by increasing the thickness of the 
diaphragm. As the range increases, the forces acting on the diaphragm 
increase in proportion to the pressure times the diaphragm area. A 
pressure will be reached where those forces exceed the material strength. 
In order to avoid failure, the choice of dimensions for the cavity and the 
diaphragm will normally be made by specifying first a cavity diameter 
small enough to contain the pressure being measured and second, the 
thickness of the diaphragm is chosen to give the desired diaphragm 
deflections for the pressure range being measured. It is, of course, 
desirable to extend the pressure range of these devices as much as 
possible to provide units which are broadly applicable. 
The inventors have noticed that upon exposure of pressure transducers of 
this type to excessive pressure differences, failure of the transducer 
occurs as a result of excessive diaphragm deflections, which causes radial 
cracking of the glass plate on the high pressure side. It is therefore an 
object of the present invention to prevent the formation of such cracks in 
the pressure range in which they normally develop and thereby extend the 
pressure range of this type of transducer. 
SUMMARY OF THE INVENTION 
A capacitive pressure transducer is provided which utilizes a plate of 
electrically conductive semi-conductor material sandwiched between a pair 
of plates of inorganic electrically insulating material and 
electrostatically bonded thereto. A pair of concentric circular recesses 
are etched in the plates in such a way as to form cavities on opposing 
sides of the conductive plate and to define a diaphragm area on that 
plate. Apertures are drilled through the insulating plates to expose the 
cavities to the pressures to be measured. The surfaces of the insulating 
plates in the cavities have deposited on them electrically conductive 
surfaces which form capacitors with the diaphragm area on the conductive 
plate. A stress relief area is provided on the high pressure side 
surrounding the diaphragm area. In that stress relief area, the conductive 
plate is unrestrained by the adjacent insulating plate. Provision of this 
stress relief makes it possible to extend the pressure range of the 
transducer beyond the range which would normally be useful for the 
dimensions of the particular transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the pressure transducer structure shown utilizes a 
plate 10 of electrically conductive semi-conductor material such as single 
crystal silicon, sandwiched between a pair of plates 16 and 16a of 
inorganic electrically insulating material, such as borosilicate glass. 
The silicon plate is electrostatically bonded between the borosilicate 
glass plates along the entire interface. In accordance with the 
electrostatic bonding procedure set forth in U.S. Pat. No. 3,397,278, 
issued to Pomerantz on Aug. 13, 1968, this bonding process provides a bond 
between the glass plates and the silicon plate which approaches the 
strength of those materials themselves. 
The silicon plate has circular recesses 12 and 12a etched into its opposite 
faces to form a diaphragm area in the silicon plate. This area is the area 
of reduced cross-section and is that portion of the silicon plate which 
will be deflected when exposed to a pressure difference. 
Each of the glass plates 16 and 16a has a corresponding circular thin metal 
film 18 and 18a deposited on that surface of the glass plate facing the 
recesses 12 and 12a in the silicon plate. Each of these thin film deposits 
18 and 18a is connected to one of the thick film deposits 20 and 20a, 
respectively. An interconnecting thin film deposit 22 and 22a is laid on 
the surface of the respective apertures 24 and 24a in the glass plates. 
It will be evident that each of the deposits 18 and 18a forms a plate of a 
capacitor, the other plate of which is the diaphragm area of the silicon 
plate 10. Thus, when a high pressure is introduced into the centrally 
located aperture 24 and a lower pressure is introduced into the other 
centrally located aperture 24a, the diaphragm area of the silicon plate 10 
will deflect in a downward direction. This changes the capacity between 
the deposit 18 and the plate 10 as well as between deposit 18a and the 
plate 10. These capacity changes are in opposite direction and may be 
utilized in a bridge circuit, for example, to measure the pressure 
difference across the diaphragm. For this purpose, separate electrical 
connections can be made to the deposit 20 as well as the deposit 20a and 
the plate 10. 
When the high pressure introduced through aperture 24 exceeds the low 
pressure introduced through aperture 24a by an amount which is excessive 
for the dimensions and material of the transducer, the deflections of the 
diaphragm area of the silicon plate 10 in combination with the strong 
bonding along the interface as provided by the electrostatic bonding 
procedure, causes the glass plate 16 on the high pressure side of the 
transducer to fail in tension as by the formation of radial cracks 26 in 
the glass plate. This cracking starts near the silicon-glass interface at 
the outside edge of the etched recess in the silicon and separates 
radially with increasing pressure. The crack is the result of the tensile 
reaction stresses developed in the glass by the flexing of the silicon 
diaphragm. The crack growth stops when sufficient tensile stresses in the 
glass plate on the high pressure side have been converted into compressive 
reaction stresses in the glass plate on the low pressure side. Since glass 
is much stronger in compression than in tension, the device can continue 
to function without further failure at a reduced full scale design 
pressure. 
It is, of course, desirable to design the transducer for a particular full 
scale pressure, and therefore it is beneficial to be able to utilize the 
transducer in pressure ranges which would normally cause the above 
mentioned cracks to be produced without, however, having to accommodate 
the decrease in design pressure resulting from these cracks. To this end 
there is provided by this invention a stress relief area surrounding the 
diaphragm area so there will be no restraint on deflection of the silicon 
plate by that portion of the glass plate on the high pressure side which 
is just adjacent to the recess, or, in other words, in that area 
surrounding the diaphragm area where cracking would be expected. 
One form of the present invention which provides the necessary stress 
relief is shown in FIG. 2 wherein the etched recess on the high pressure 
side of the silicon plate 10 is increased in diameter. This increase in 
diameter may, for example, be of a magnitude such that the diameter of the 
recess 12 on the high pressure side approximates 1.3 to 1.5 times the 
diameter of recess 12a on the low pressure side. It has been found that 
with such an increase in diameter on the high pressure side, the 
transducer will not fail by fractures in the glass but instead is limited 
by the strength of the silicon diaphragm. 
Another form of the present invention is shown in FIG. 3 wherein the stress 
relief is provided by etching a recess in the glass plate on the high 
pressure side of the transducer while omitting any etched recesses in the 
silicon plate on that side. These recesses should, as mentioned with 
regard to FIG. 2, be on the order of 1.3 to 1.5 times the diameter of the 
recesses on the low pressure side to avoid cracking in the glass plate. 
It will, of course, be evident to those skilled in the art that the recess 
on the low pressure side of the diaphragm may be etched into the glass 
rather than into the silicon plate. Thus, both recesses may be etched 
either into the silicon plate or into the respective glass plates or, 
alternatively, one of the recesses may be etched into the glass and one 
into the silicon. Still another variation of the present invention is 
provided by preventing the electrostatic bonding over that area of 
interface between the plate 16 and the plate 10 on the periphery of the 
recess 12 where the cracking would normally occur. By preventing the 
bonding from occurring in that area cracking is avoided, for that area of 
the silicon plate will be unrestrained by the glass plate 16. Thus, there 
is provided the necessary stress relief in avoidance of cracking upon the 
application of overpressure conditions.