TECHNICAL FIELD
The present invention generally relates to a pressure sensor and in particular, a micro-electro mechanical (MEMS) pressure sensor.
CO-PENDING APPLICATIONS
Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention simultaneously with the present application:
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| 10/965,922 |
10/965,922 |
10/965,902 |
10/965,903 |
10/965,904 |
| 10/965,927 |
10/965,718 |
10/965,746 |
10/965,747 |
6,968,744 |
| 10/965,899 |
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The disclosures of these co-pending applications are incorporated herein by cross-reference.
CROSS REFERENCES TO RELATED APPLICATIONS
The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference.
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| 10/868,866 |
6716666 |
6949217 |
6,750,083 |
7,014,451 |
| 6,991,207 |
6,777,259 |
6,557,978 |
6,923,524 |
6,766,998 |
| 10/853,270 |
6,759,723 |
6,967,354 |
6,870,259 |
6,925,875 |
| 10/898,214 |
11/242,916 |
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BACKGROUND ART
The invention has wide-ranging application across many fields of industry. It is particularly suited to pressure measurement in harsh or dynamic environments that would preclude many other pressure sensors. These applications include, but are not limited to:
-
- monitoring engine pressure (cars, aircraft, ships, fuel cells)
- sensors for high speed wind tunnels
- sensors to monitor explosions
- sensors for boilers
- sensors for dish-washing machines
- sensors for irons (both domestic and industrial)
- sensors for other steam based machines where overpressure can lead to destruction and loss of life
However, in the interests of brevity, the invention will be described with particular reference to a tire pressure monitor and an associated method of production. It will be appreciated that the Tire Pressure Monitoring System (TPMS) described herein is purely illustrative and the invention has much broader application.
Transportation Recall Enhancement, Accountability and Documentation (TREAD) legislation in the United States seeks to require all U.S. motor vehicles to be fitted with a tire pressure monitoring system (TPMS). This is outlined in U.S. Dept. of Transportation, “Federal Motor Vehicle Safety Standards: Tire Pressure Monitoring Systems; Controls and Displays”, U.S. Federal Register, Vol. 66, No. 144, 2001, pp. 38982-39004. The impetus for this development comes from recent Firestone/Ford Explorer incidents which led to a number of fatal accidents. A careful assessment of tire inflation data found that approximately 35% of in-use tires are under inflated, whilst an assessment of the effect of a TPMS found that between 50 to 80 fatalities, and 6000 to 10,000 non-fatal injuries, per annum could possibly be prevented. This is discussed in U.S. Dept. of Transportation, “Tire Pressure Monitoring System,” FMVSS No. 138,2001. European legislation also appears likely to require the fitting of a TPMS to increase tire life, in an effort to reduce the number of tires in use by 60% in the next 20 years, so as to minimise the environmental impacts.
Two different kinds of TPMS are currently known to be available in the marketplace. One kind of TPMS is based on differences in rotational speed of wheels when a tire is low in pressure. The asynchronicity in rotational speed can be detected using a vehicle's anti-braking system (ABS), if present. The second kind of TPMS measures tire pressure directly and transmits a signal to a central processor. (prior art) illustrates a schematic of a typical pressure measurement based TPMS 10. Sensors 12, provided with a transmitter, measure pressure in tires 13 and transmit a signal 14 to antenna 16. The data can then be relayed to a receiver 15 and processed and displayed to a driver of the vehicle 17 on display 18.
Table 1 lists some presently known TPMS manufacturers/providers. Motorola and Pacific Industries have each developed a TPMS, whilst other companies listed in Table 1 act as suppliers for TPMS manufacturers, including some automobile producers that install their own TPMS.
| TABLE 1 |
|
|
| Pressure sensor manufacturers involved in TPMS. |
| Company |
Supplier to |
Type of Sensor |
|
| Motorola |
Motorola |
Capacitance |
| Pacific Industries |
Pacific Industries |
Piezoresistive |
| SensoNor |
Siemens, TRW, Beru, |
Piezoresistive |
|
Porsche, BMW, Ferrari, |
|
Mercedes, Toyota |
| Siemens |
Goodyear |
Piezoresistive |
| Transense Technologies |
Under development |
Surface Acoustic |
|
|
Wave |
| TRW/Novasensor |
Smartire, Michelin, |
Piezoresistive |
|
Schrader, Cycloid |
|
There are two main types of pressure sensor; resistive or capacitive. Both types of these sensors rely on deflection of a membrane under an applied pressure difference. One side of the membrane is exposed to internal pressure of a tire while the other side of the membrane forms one wall of a sealed cavity filled with gas at a reference pressure.
The resistive-type sensors typically employ silicon-based micro-machining to form a Wheatstone bridge with four piezoresistors on one face of the membrane. The sensor responds to stress induced in the membrane. For capacitive-type sensors, the membrane forms one plate of a capacitor. In this case, the sensor responds to deflection induced in the membrane. Preferably, the responses should be linear with pressure, for predicability, up to at least a critical point.
Transense Technologies, listed in Table 1, have developed a different type of sensor, based on surface acoustic wave detection. This sensor relies on interferometric measurement of the stress-induced deflection of a reflective membrane. A fibre-optic cable both transmits and receives laser light, with one end of the fibre-optic cable being inserted into the interferometer. This system is discussed in Tran, T. A. Miller III, W. V., Murphy, K. A., Vengsarkar, A. M. and Claus, R. O., “Stablized Extrinsic Fiber Optic Fabry-Perot Sensor for Surface Acoustic Wave Detection”, Proc. Fiber Optic and Laser Sensors IX, SPIE vol. 1584, pp 178-186, 1991.
Presently, there are also a variety of different kinds of deployment means for sensors in a TPMS, including valve cap and valve stem based systems, systems with the sensor mounted on the wheel rim or wheel hub, and also a tire-wheel system developed by an alliance of several tire manufacturers which has a sensor embedded in the wheel frame itself. These different kinds of deployment in TPMS are listed in Table 2.
| TABLE 2 |
|
|
| Specifications of TPMS in production. |
|
|
|
Warning |
|
|
| Company/ |
Type of |
|
Level |
Accuracy |
| Group |
System |
Fitted to |
(psi) |
(psi) |
Sampling |
|
| Beru |
Wheel Rim |
Audi, BMW, |
user set |
1 |
every 3 sec, |
|
|
Mercedes |
|
|
transmitted |
|
|
|
|
|
every 54 sec |
| Cycloid |
Wheel Cap |
Ford, |
18 |
1 |
30 sec/10 min |
|
(pump) |
Goodyear |
| Fleet |
Valve Cap |
heavy |
20 |
1 |
3.5 sec |
|
|
vehicles |
| Johnson |
Valve Stem |
AM |
19.9 |
1 |
15 min |
| Michelin/ |
PAX |
Renault, |
? |
? |
? |
| Goodyear/Pirelli/ |
System |
Caddillac |
| Dunlop |
| Motorola |
Wheel Rim |
AM |
? |
? |
6 sec |
| Omron |
Valve Stem |
AM |
? |
? |
? |
| Pacific Industries |
Valve Stem |
AM |
20.3/user |
1.8 |
15 sec/10 min |
|
|
|
set |
| Schrader |
Valve Stem |
Corvette, |
22 |
2% |
? |
|
|
Peugeot, |
|
|
Cadillac |
| Smartire |
Wheel Rim |
Aston |
? |
1.5 |
6 sec |
|
|
Martin, |
|
|
Lincoln, AM |
|
| AM = products fitted to a vehicle after vehicle purchase (After Market). |
To increase battery life, most TPMS are in stand-by mode for the majority of time, only operating at set intervals. The U.S. legislation requires the system to alert the driver within a set time of detecting significant tire under-inflation conditions. It also requires a warning light to signal when the tire is either 20% or 25% under-inflated. Most of the devices presently available in the market are accurate to within ±1 psi, which represents ±3 % for a tire pressure of 30 psi. More generally, the sensor should perform in a harsh environment, with temperatures up to 130° C. and accelerations of 1000 g or more. Tire pressure increases and decreases in response to corresponding changes in temperature. Most systems presently available include a sensor to account for thermally induced changes in tire pressure sensor sensitivity (Menini, Ph., Blasquez, G., Pons, P., Douziech, C. Favaro, P. and Dondon, Ph., “Optimization of a BiCMOS Integratetd Transducer for Self-Compensated Capacitive Pressure Sensor,” Proc. 6th IEEE Int. Conf Electronics, Circuits and Systems, Vol 2, pp.1059-1063, 1999).
Tire pressure sensors operate in a harsh environment. They are subjected to substantial shock loading and large temperature variations. For longevity, the sensor components need to be robust. However, the sensor should be kept as small as possible to minimize power consumption and allow it to be installed in restrictive spaces such as the valve stem.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge.
SUMMARY OF THE INVENTION
Accordingly the present invention provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects from any pressure difference between the reference pressure and the fluid pressure, the membrane being at least partially formed from conductive material; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein,
the flexible membrane is less than 3 microns thick.
The operational range of the pressure sensor requires the membrane to have a certain deflection. For a given material, the deflection of the membrane will depend on, inter alia, its area and its thickness. Minimizing the thickness of the membrane allows the use of.a high yield strength membrane material. A thinner membrane also allows the area of the membrane to be reduced. Reducing the area of the membrane reduces the power consumption and the overall size of the sensor. A high yield strength material is better able to withstand the extreme conditions within the tire and a compact design can be installed in restricted spaces such as the valve stem.
A first related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects from any pressure difference between the reference pressure and the fluid pressure, the membrane being at least partially formed from conductive material; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein,
the chamber, the flexible membrane and the associated circuitry are formed on and through a wafer substrate using lithographically masked etching and deposition techniques.
The lithographically masked etching and deposition techniques used in the semiconductor chip manufacturing industry can produce many separate devices from a single wafer with high yields and low defect rates. Applying these fabrication techniques to MEMS pressure sensors and associated CMOS circuitry allows high volumes and high yields that dramatically reduce the unit cost of individual sensors.
A second related aspect provides a pressure. sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects due to pressure differentials between the reference pressure and the fluid pressure; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein,
the membrane is less than 0.1 grams.
Designing and fabricating the sensor to minimize the mass of the membrane decreases the effects of acceleration on the membrane deflection. At the same time, a low mass has no effect on the membrane deflection from the pressure differential between the reference fluid and the air pressure.
A third related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
first and second chambers having first and second flexible membranes respectively, the first and second flexible membranes configured to deflect in response to pressure differences within the first and second chambers respectively, the first membrane arranged for exposure to the fluid pressure and the second membrane sealed from the fluid pressure; and,
associated circuitry for converting the deflection of the first flexible membrane into an output signal related to the fluid pressure, and converting the deflection of the second membrane into an adjustment of the output signal to compensate for the temperature of the sensor.
By sealing the second chamber from the tire pressure, the deflection of the second membrane can be determined as a function of temperature. This can be used to calibrate the output signal from the first chamber to remove the effects of temperature variation.
A fourth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects due to pressure differentials between the reference pressure and the fluid pressure; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein, the membrane is at least partially formed from a conductive ceramic material.
Conductive ceramics, such as metal ceramics, have previously been used to coat tool steels because of its corrosion and wear resistance. Surprisingly, it can be deposited as a thin membrane with sufficient flexibility for sensing pressure while retaining its corrosion and wear resistance. Furthermore, these materials are generally well suited to micro fabrication processes and electrically conductive so they can be used in capacitative and resistive type pressure sensors.
A fifth related-aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a first wafer substrate with a front side and an opposing back side, a chamber-partially defined by a flexible membrane formed on the front side and at least one hole etched from the back side to the chamber;
a second wafer on the back side of the first wafer to seal the at least one hole; wherein,
the chamber contains a fluid at a reference pressure, such that the flexible membrane deflects due to pressure differentials between the reference pressure and the fluid pressure; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein, the second wafer is wafer bonded to the first wafer substrate.
Wafer bonding offers an effective non-adhesive solution. It provides a hermetic seal with only minor changes to the fabrication procedure. Skilled workers in this field will readily understand that the most prevalent forms of wafer bonding are:
direct wafer, or silicon fusion, bonding;
anodic, or electrostatic Mallory process bonding; and, intermediate layer bonding.
These forms of wafer bonding are discussed in detail below, and they all avoid the unacceptable air permeability associated with adhesive and polymer coatings.
A sixth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects due to pressure differentials between the reference pressure and the fluid pressure; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein, the membrane is non-planar.
It is possible to extend the linear range of the pressure-deflection response with a non-planar membrane. Corrugations, a series of raised annuli or other surface features are an added complexity in the fabrication process but can extend the linear range of the sensor by 1 MPa.
A seventh related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the flexible membrane at least partially formed from conductive material, and the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects from any pressure difference between the reference pressure and the fluid pressure;
a conductive layer within the chamber spaced from the flexible membrane; and, associated circuitry incorporating the flexible membrane and the conductive layer; such that,
the conductive layer and the flexible membrane form capacitor electrodes and the deflection of the flexible membrane changes the capacitance which the associated circuitry converts into an output signal indicative of the fluid pressure; wherein,
the conductive layer is arranged such that deflection of the membrane towards the conductive layer can displace the fluid from between the membrane and the conductive layer.
Venting the fluid between the electrodes to the other side of the fixed electrode, while keeping the chamber sealed, avoids the extreme fluid pressure that cause the squeeze film damping.
An eighth related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the flexible membrane is a laminate having at least two layers wherein at least one of the layers is at least partially formed from conductive material, and the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects from any pressure difference between the reference pressure and the fluid pressure; and,
associated circuitry for converting deflection of the flexible membrane into an output signal indicative of the fluid pressure.
Forming the membrane from a number of separately deposited layers alleviates internal stress in the membrane. The layers can be different materials specifically selected to withstand harsh environments.
A ninth related aspect provides a method of fabricating pressure sensor for sensing a fluid pressure, the method of fabrication comprising:
etching a recess in a wafer substrate;
depositing a flexible membrane to cover the recess and define a chamber such that during use the chamber contains a fluid at a reference pressure and the flexible membrane deflects from a pressure difference between the reference pressure and the fluid pressure;
depositing associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; and,
depositing an apertured guard over the membrane.
An aspect closely related to the ninth aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a wafer substrate with a recess;
a flexible membrane covering the recess to define a chamber containing a fluid at a reference pressure, such that the flexible membrane deflects due to pressure differentials between the reference pressure and the fluid pressure;
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; and,
an apertured guard over the membrane formed using lithographically masked etching and deposition techniques.
By depositing material over the membrane to form the guard offers greater time efficiency and accuracy than producing a guard separately and securing it over the membrane. Semiconductor etching and deposition techniques allow highly intricate surface details. The apertures in the guard can be made smaller to exclude more particles from contacting the membrane. The fine tolerances of lithographic deposition permit the guard to be positioned close to the membrane for a more compact overall design.
A tenth related aspect provides a pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects due to pressure differentials between the reference pressure and the fluid pressure, the membrane being at least partially formed from conductive material;
a conductive layer within the chamber spaced from the flexible membrane such that they form opposing electrodes of a capacitor; and,
associated circuitry for converting the deflection of the flexible membrane into. an output signal indicative of the fluid pressure; wherein, the conductive layer is less than 50 microns from the membrane in its undeflected state.
A capacitative pressure sensor with closely spaced electrodes can have small surface area electrodes while maintaining enough capacitance for the required operating range. However, small electrodes reduce the power consumption of the sensor which in turn reduces the battery size needed for the operational life of the sensor.
An eleventh related aspect provides a pressure sensor for sensing a fluid pressure, the pressure sensor comprising:
a chamber partially defined by a flexible membrane, the chamber containing a fluid at a reference pressure, such that the flexible membrane deflects from any pressure difference between the reference pressure and the fluid pressure, the membrane being at least partially formed from conductive material; and,
associated circuitry for converting the deflection of the flexible membrane into an output signal indicative of the fluid pressure; wherein,
the associated circuitry is adapted to be powered by electromagnetic radiation transmitted from a point remote from the sensor.
Beaming energy to the sensor removes the need for long-life batteries, or can be used to supplement or charge the batteries. In either case, the sensor avoids the need for large batteries and is therefore small enough for installation in the valve stem or valve itself.
Optional and Preferred Features
Preferable and optional features of the various broad aspects of the invention are set out below. The skilled worker in the field will understand that while some of the features described below are optional for some of the above broad aspects of the invention, they are essential to other broad aspects.
Preferably the sensor is powered by radio waves transmitted from a remote source. Preferably the sensor is a capacitative pressure sensor with a conductive layer within the chamber spaced from the flexible membrane such that they form opposing electrodes of a capacitor. In a further preferred form the conductive layer is less than 50 microns from the membrane in its undeflected state.
Preferably, the membrane is circular with a diameter less than 500 microns. In a further preferred form the membrane is less than 300 microns and in specific embodiments the diameter is 100 microns.
In some preferred embodiments the membrane is approximately 0.5 μm thick. In further embodiments, the membrane is a 100-micron diameter circular film. Preferably the metal ceramic is a metal nitride. In specific embodiments, the membrane is titanium nitride, tantalum nitride, and vanadium nitride. The membrane may also be form from mixed metal nitrides. The mixed metal nitrides may be titanium silicon nitride, tantalum silicon nitride, vanadium silicon nitride, titanium aluminium silicon nitride, tantalum aluminium silicon nitride and so on.
Preferably, the flexible membrane is a laminate having at least two layers wherein at least one of the layers is at least partially formed from conductive material. The layers within the laminate may be formed from the deposition of different metal ceramics. Preferably, the metal ceramics are metal nitrides or mixed metal nitrides, such as titanium nitride, titanium aluminium nitride, tantalum silicon nitride, titanium aluminium silicon nitride, tantalum aluminium silicon nitride and so on. Layers of the laminate may also be metal such as titanium or vanadium.
In a particularly preferred form, the sensor further comprises a second chamber with a second membrane, the second chamber being sealed from the fluid pressure and the second membrane deflecting from a predetermined pressure difference in the second chamber; wherein,
the associated circuitry converts the deflection of the second membrane into an adjustment of the output signal to compensate for the temperature of the sensor.
In some embodiments, the sensor is formed on and through a silicon wafer using lithographically masked etching and deposition techniques. In a further preferred form, the sensor is a capacitative sensor, wherein a conductive layer is deposited in each of the first and second chambers and the first and second flexible membranes are conductive, such that, the conductive layer in the first chamber and the first flexible membrane form capacitor electrodes wherein the deflection of the first flexible membrane changes the capacitance which the associated circuitry converts to the output signal.
Preferably, there is provided a CMOS layer disposed between the metallic layer and the substrate. In some embodiments, the sensor is additionally provided a passivation layer at least partially deposited over the metallic layer.
Optionally, the pressure sensor is adapted to sense the air pressure within a pneumatic tire.
Conveniently, the wafer is a first wafer substrate with a front side and an opposing back side, a recess etched into the front side and at least one hole etched from the back side to the recess; and the sensor further comprises:
a second wafer on the back side of the first wafer to seal the at least one hole; wherein,
the second is wafer bonded to the wafer substrate.
Optionally, the wafer bonding is direct wafer bonding wherein the contacting surfaces of the first and second wafers are ultra clean, and activated by making them hydrophilic or hydrophobic prior to bonding, and then brought into contact at high temperature, preferably around 1000° C. Anodic bonding offers another option wherein the contacting surfaces of the first and second wafers have a large voltage applied across them. The wafers may be in a vacuum, air or an inert gas when the bond is formed. Intermediate layer bonding is a third option wherein a layer of low melting point material is applied to one or both of the contacting surfaces of the first and second wafers so that heat and pressure forms the wafer bond. Preferably the low melting point material is silicon nitride or titanium nitride. This option avoids the high surface cleanliness required by direct silicon bonding and the high voltages required by anodic bonding.
In some embodiments, the membrane is non-planar and preferably corrugated. In a further preferred form, the flexible corrugated membrane has a corrugation factor of 8. In a particularly preferred form, the sensor has a linear response up to about 1 MPa. In specific embodiments, the membrane corrugations have a period of between 5 microns and 15 microns, preferably about 10 microns. In some preferred embodiments, the membrane is substantially circular and the corrugations are-annular. In a particularly preferred form, the corrugations have a substantially square-shaped cross-sectional profile.
Preferably the sensor further comprises an apertured guard over the membrane formed using lithographically masked etching and deposition techniques. In these embodiments, the guard is laminate having at least two layers. In a particularly preferred form the layers are different materials such as silicon nitride and silicon dioxide.
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