Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Physical deformation
Reexamination Certificate
1997-09-25
2004-03-02
Coleman, W. David (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Physical deformation
Reexamination Certificate
active
06700174
ABSTRACT:
FIELD OF USE
The present invention relates to a pressure sensor which undergoes physical movement in response to an applied external force. This invention also relates to techniques for fabricating such a pressure sensor.
BACKGROUND ART
Conventional silicon micromachined pressure sensors typically use either piezo-resistive or capacitive elements to sense the deflection of a thin silicon diaphragm. Piezo-resistive elements are much more common than capacitive elements because the piezo-resistive elements have a lower cost, as well as greater product familiarity and acceptance.
FIG. 1A
is a top view of a conventional silicon micromachined piezo-resistive pressure sensor
1
. Pressure sensor
1
is fabricated on a silicon substrate
2
having an area of 2 mm by 2 mm and a thickness on the order of 500 &mgr;m. To increase the sensitivity of pressure sensor
1
, substrate
2
is fabricated to include a frame
2
a
, an annular diaphragm
2
b
and a circular platform
2
c
. Diaphragm
2
b
is etched to have a thickness on the order of 10 &mgr;m, while frame
2
a
, and platform
2
c
remain at a thickness of approximately 500 &mgr;m. As a result, the deformation of substrate
2
will be concentrated within the annular diaphragm
2
b
, thereby increasing the sensitivity of pressure sensor
1
.
Four Wheatstone bridge circuits
3
a
,
3
b
,
3
c
and
3
d
are formed on substrate
2
. Each of these Wheatstone bridge circuits includes a plurality of contact pads
4
, a plurality of piezo-resistive elements
5
, and conductive traces for connecting the pads
4
and piezo-resistive elements
5
. Piezo-resistive elements
5
are formed by ion implanting impurity regions into the annular diaphragm
2
b
. The resistances of piezo-resistive elements
5
change in response to mechanical stresses applied to the crystalline substrate
2
. More specifically, the resistances of piezo-resistive elements
5
change in response to compression and dilation of diaphragm
2
b
. This annular diaphragm
2
b
and the position of piezo-resistive elements
5
provides a 25 to 50 times increase in the gauge factor, such that pressure sensor
1
can provide an output voltage on the order or 2 to 3 mV/V when designed for full range of differential pressure on the order of a 4 inch water column (WC).
In the past, pressure sensor
1
has typically been used for high pressure range sensing applications in the automobile world. Such applications include, for example, measurements of manifold absolute pressure, transmission fluid pressure, coolant and power steering pressure and tire pressure.
The effectiveness of pressure sensor
1
is determined by a combination of two physical effects, which can be explained in terms of a mechanical amplifier cascaded with an electrical amplifier. The mechanical amplifier is diaphragm
2
b
which converts pressure into displacement. The electrical amplifier is the combination of piezo-resistive elements
5
and Wheatstone bridge circuits
3
a
-
3
d
, which convert displacement into output voltage.
There are a number of inherent disadvantages associated with pressure sensor
1
. First, platform
2
c
acts as a seismic mass which causes an excessive amount of dynamic deflection in response to shock and vibration (i.e., noise). Platform
2
c
can further cause an excessive amount of static deflection in response to gravity, thereby making the sensor highly sensitive to mounting positions). As a result, the operation of pressure sensor
1
can be affected by the position and environment in which pressure sensor
1
is mounted.
In addition, piezo-resistive elements
5
act as pyro-resistors, thereby making pressure sensor
1
extremely sensitive to temperature changes. As a result, sophisticated temperature compensation schemes must typically be used with pressure sensor
1
. It is typical that even after such temperature compensation is provided, the temperature effects are on the order of 1 to 2 percent of full range.
Furthermore, annular diaphragm
2
b
is typically very fragile, thereby rendering pressure sensor
1
prone to damage during transportation, handling and assembly. Also, while the annular diaphragm
2
b
increases the sensitivity of the mechanical amplifier portion of pressure sensor
1
, the shape of annular diaphragm
2
b
limits the linear elastic range the diaphragm
2
b
. As a result, the performance of pressure sensor
1
can be nonlinear if the deformation of diaphragm
2
b
exceeds the linear elastic range of the silicon diaphragm.
Moreover, because of the inherent stiffness of silicon substrate
2
, pressure sensor
1
is better suited for high pressure applications (i.e., measuring pressures greater than 1 psi), rather than low pressure applications (i.e., measuring pressures less than 1 psi).
FIG. 1B
is a cross sectional view of a conventional capacitive differential pressure sensor
20
which is used to measure pressure. Pressure sensor
20
is formed by sandwiching an etched silicon diaphragm
29
(which is etched from a silicon substrate
28
) between an upper glass plate
30
and a lower glass plate
27
. Pressure ports
25
and
26
are formed through the upper and lower glass plates
30
and
27
, respectively, to vent silicon diaphragm
29
. Aluminum is sputtered to the inner surfaces of the upper and lower glass plates to form fixed capacitor plates
23
and
24
. Connectors
21
and
22
extend from plates
23
and
24
, respectively, along the walls of pressure ports
25
and
26
, to the outer surfaces of the upper and lower glass plates
30
and
27
. The silicon diaphragm
29
forms a movable center capacitive plate of the sensor
20
in a configuration similar to a capacitive potentiometer. A positive pressure applied to pressure port
25
causes the silicon diaphragm
29
to deflect toward the lower glass plate
27
, thereby increasing the capacitance between diaphragm
29
and plate
24
, while decreasing the capacitance between diaphragm
29
and plate
23
. The imbalance, which is directly proportional to pressure, is detected by an electronic circuit.
Pressure sensor
20
to exhibits the following disadvantages. First, silicon diaphragm
29
, being relatively thick (i.e., having a thickness of at least about 5 microns), can experience an excessive amount of dynamic deflection in response to shock and vibration. Furthermore, as silicon diaphragm
29
is made thinner for low pressure applications (i.e., a thickness of approximately 5 microns) it is difficult to fabricate a substantially planar diaphragm. A non-planar diaphragm can result in erroneous capacitance measurements. Moreover, as silicon diaphragm
29
is made thinner for low pressure applications, the diaphragm becomes very fragile, thereby rendering pressure sensor
20
prone to damage during transportation, handling and assembly.
It would therefore be desirable to have a low-cost, reliable pressure sensor which is relatively insensitive to temperature, dynamic shock and gravitational forces. It would also be desirable if such pressure sensor is relatively sturdy and has a wide linear elastic range. It would further be desirable if such pressure sensor were well suited for low pressure applications.
SUMMARY
Accordingly, the present invention provides a sensitive pressure sensor which includes a flexible membrane, such as low-stress silicon nitride, which is supported by a semiconductor frame. The flexible membrane extends over the frame, and an inherent tensile stress is present in the membrane. A thin film strain gage material, such as nickel-chrome, is deposited over the flexible membrane to form one or more variable resistance resistors over the flexible membrane.
When an external pressure, such as a dynamic pressure drop due to an air flow, is applied to the membrane, the membrane is deformed out of plane. When the membrane is deformed out of plane, the variable resistance resistors increase in length, and thereby increase in resistance. The increase in resistance is monitored by an electronic circuit, such as a Wheatstone bridge circuit. The sensor circuit generates an outp
Miu Denny K.
Tang Weilong
Bever Hoffman & Harms LLP
Coleman W. David
Dutton Brian
Hoffman E. Eric
Integrated Micromachines, Inc.
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