WO1993018381A1

WO1993018381A1 - Pressure sensor and method

Info

Publication number: WO1993018381A1
Application number: PCT/NZ1993/000016
Authority: WO
Inventors: Michael Kenneth Andrews; Gary Chandler Turner; Murray Bruce Moore; Paul David Harris; Ian Murray Harris
Original assignee: Industrial Research Limited
Priority date: 1992-03-12
Filing date: 1993-03-12
Publication date: 1993-09-16
Also published as: AU3770093A; JPH07504980A

Abstract

A micro-mechanic gas pressure and/or viscosity sensor using the squeeze film effect comprises a mechanically fixed member (5), an oscillatory member (1) capable of oscillation relative to the fixed member (5), and a thin planar cavity (4) defined between the fixed plate (5) and the oscillatory plate (1) into which gas may pass and be restrained by the squeeze film effect. Electronic means determines either a) gas pressure by reference to the frequency of resulting oscillation of the oscillatory member which is initiated by a voltage pulse applied across the fixed member and the oscillatory member, or b) gas viscosity by reference to the phase difference between a driving voltage applied across the fixed member and the oscillatory member and resulting oscillation of the oscillatory member. The fixed member may form one capacitor plate, and the oscillatory member another capacitor plate. The frequency and/or phase of motion of the oscillatory member can be determined by reference to capacitance across the fixed and oscillatory members. The sensor structure can be produced by etching a substrate (2) to form the fixed member and oscillatory member.

Description

PRESSURE SENSOR AND METHOD TECHNICAL FIELD

The invention comprises a micro-mechanic or micro¬ electronic pressure sensor structure and a pressure sensor employing such a structure.

BACKGROUND ART

Currently, pressures in the atmospheric or barometric range are measured or sensed by relatively low accuracy Bourdon tubes (though sophisticated quartz glass devices can be made) , or • more commonly by bellows-type anaeroid capsules which operate from approximately 1000 hPa to 0.5 hPa. These devices are relatively large, and may suffer hysteresis and drift effects due to the imperfect elastic properties of the materials from which they are • made. Their response is independent of gas type. The range from 100 Pa to 0.1 Pa is usually covered by heat-transfer sensors such as the Pirani and thermocouple gauges. These devices are strongly gas-species dependant, and generally have good resolution over only a limited portion of their range.

Capacitive manometer gauges which are sophisticated versions of the anaeroid capsule are available, which can determine" gas pressure over a wide range, but they are relatively expensive and can suffer drift, particularly at the low pressure end after cycling to high pressures.

Existing methods for the measurement of gas viscosity involve the flow of gas through capillary tubes, the retardation force on spinning objects, the damping or decaying motion of an

- 1 - oscillating disc or vibrating wire, and the band-width of an oscillating quartz crystal.

SUMMARY OF INVENTION

The present invention comprises a micro-mechanical or micro-electronic sensor structure and pressure sensor. The invention provides the capability of measuring gas pressure with high accuracy over a wide range. In a preferred form the invention may be configured to cover the range from the usual barometric region (approximately 1 - 1000 hPa) to the limit of mechanical-pump vacuum systems, a range usually requiring more than one kind of sensing device.

In broad terms the invention in a first aspect may be said to comprise a micro-mechanic gas pressure sensor structure, comprising

a mechanically fixed member;

an oscillatory member capable of oscillation relative to the fixed member;

a thin, planar cavity defined between the fixed plate and the oscillatory plate into which gas to be pressure sensed may pass and be restrained by squeeze film effect,

electronic means for determining the gas pressure by reference to the frequency of resulting oscillation of the oscillatory member initiated by a voltage applied across the fixed member and the oscillatory member and/or gas viscosity by reference to the phase difference between a driving voltage applied across the fixed member and the oscillatory member and resulting oscillation of said oscillatory member.

Preferably the frequency and/or phase of motion of the oscillatory member are determined using capacitance techniques. The fixed and oscillatory members may form two capacitor plates and the frequency and/or phase of oscillation may be determined by reference to capacitance across the fixed and oscillatory members. Associated electronics may provide pressure and/or viscosity signals(s), display, or information in any desired form. Other suitable techniques for detecting the frequency and/or phase of motion of the oscillatory member could be used however, such as by employing strain sensors such as piezo-resistors for example, in or associated with the oscillatory member, for example, and capacitance measurement is described in detail herein as one possible technique.

In broad terms the invention in a third aspect may be said to comprise a method of determining gas pressure comprising passing the gas through a thin planar cavity defined by and between a mechanically fixed member and an oscillatory member capable of oscillation relative to the fixed member, between which members the gas may be restrained by squeeze film effect, applying a driving voltage between the fixed member and oscillatory member to cause the oscillatory member to oscillate, and determining gas pressure by reference to the frequency of oscillation of the oscillatory member. In broad terms in a fourth aspect of the invention may be said to comprise a method of determining gas viscosity comprising passing the gas through a thin planar cavity defined by and between a mechanically fixed member and an oscillatory member capable of oscillation relative to the fixed member, applying a driving voltage between the fixed member and oscillatory member, and determining the gas viscosity by reference to the phase difference between the applied voltage and an oscillatory current passed by the structure.

In the sensor micro-structure of the invention, most preferably the oscillatory member is a diaphragm or diaphragm-like member of semi-conductor material and most preferably crystalline silicon. The sensor micro-structure of the invention may then be fabricated utilising standard semiconductor micro-mechanic or micro-electronic technology. The sensor micro-structure is miniaturised, potentially capable of mass-production, and can be made to incorporate other functions such as temperature measurement or control and signal processing on the same piece of silicon which forms the pressure sensor micro-structure. Such technology is known for producing microdevices such as motors, accelerometers, and the like.

The device exploits both the electrical and elastic properties of silicon, and since electronic silicon is in the form of an ultrapure single crystal, the elastic properties of the device should not show the hysteresis and creep effects exhibited by metal-based elastic devices. Unlike most gauges, the pressure sensor micro-structure of the invention is a resonating device, and a pressure sensor incorporating the micro-structure can have a digital output which can be coupled directly to a computer- controlled system.

In an alternative arrangement the'oscillatory member may be in the form of other than a diaphragm or diaphragm-like structure, but still capable of oscillation relative to the fixed plate of the device, such as a thin member of silicon fixed at only one side or edge for "hinged" oscillation relative to the fixed plate for example.

DESCRIPTION OF DRAWINGS

The sensor micro-structure, pressure and viscosity sensors., and methods of the invention will be further described with reference to the accompanying schematic drawings, which show by way of example two preferred form of sensor micro-structures of the invention, without intending to be limiting. In the drawings:

Fig. 1 is a schematic plan view of one preferred form of the micro-structure (in the direction of arrow A in Fig.2),

Fig. 2 is a schematic cross-sectional plan view of micro- structure of Fig. 1 along line B-B of Fig. 1,

Fig. 3 is a schematic cross-sectional view similar to Fig. 2 but of another preferred form of sensor micro-structure of the invention, Fig. 4 is a schematic plan view of the micro-structure of Fig. 3 in the direction of arrow C in Fig. 3 (note that in Figs 3 and 4 only parts of the micro-structure different from that of Figs 1 and 2 are shown, for clarity) ,

Fig. 5A shows arrangements of electronics for a pressure sensor of the invention, and Fig. 5B shows the operation of the device,

Fig. 6 is a schematic plan view similar to Figs 1 and 4, of a third preferred form of sensor micro-structure of the invention.

Fig. 7 is a schematic cross-sectional view similar to Figs 2 and 3, of the sensor micro-structure of Fig. 7 in the direction of arrow D in Fig. 7,

Fig. 8 shows arrangements of electronics for a viscosity meter of the invention, and

Fig. 9 is a schematic cross-sectional view of a sensor micro-structure similar to that of Fig. 6, configured particularly as a viscosity sensor.

The structures shown in Figs 1 and 3 are the preferred forms for a pressure sensor, while that shown in Fig. 6 is preferred for-a viscosity sensor. DETAILED DESCRIPTION OF PREFERRED FORMS

Referring to Fig. 1, the first preferred form sensor micro-structure shown in Figs 1 and 2 comprises a shaped or "micromachined" silicon chip 1 bonded to a substrate 2 which is insulated from the chip 1 and may be formed of glass, for example, or any other suitable material. The substrate 2 is separated from the chip 1 by spacers 3 which may be formed as part of either the silicon chip 1 or the substrate 2. The spacers 3 are not continuous and are provided at the corners of the chip 1. A thin planar cavity 4 is defined between the silicon chip 1 and the substrate 2 such that gas may pass freely into the thin cavity 4, between the substrate 2 and the silicon chip 1. It is essential that the design of the micro-structure in any case is such that gas pressure inside the cavity 4 equals that outside, when the chip 1 is at rest.

A thin metallic layer 5 is deposited as a continuous film or in a suitable pattern on the surface of the substrate 2, together with an electrical connecting track 5a to a bond-pad 5b outside the cavity 4 as shown (see Fig. 2). An electrical connection between the substrate 2 and the silicon chip 1, in the form shown, is via a second metallisation la path from the chip 1 to the substrate 2 by way of one of the spacers 3 to a similar bond-pad lb. In the form of the device shown the connections to the device are made at the substrate level, but alternatively they could be on the silicon chip 1, and alternatively again they could be oriented in other directions than those shown in Figs 1 and 2. In the preferred form described the device comprises a capacitor, one plate of which is the metallic layer 5 on the substrate 2 and the other is the silicon chip 1. The dielectric is the gas space between the plates 1 and 2. The second plate can be formed by the entire conducting silicon chip, or a diffused or implanted area could be defined as shown and indicated at 6, to either raise the conductivity of the plate or to define and junction-isolate the second plate from the remainder of the silicon, if, for example, it was intended to include other electronic elements on the chip.

Part of the silicon chip 1 in the form shown is made compliant by removing the bulk of the silicon under the cavity 4, so that it forms a diaphragm or a diaphragm-like oscillatory member. A central mass lc is defined in the diaphragm centre to lower the natural resonant frequency of the diaphragm/chip 1, and to keep the silicon chip plate planar and parallel to the upper plate as the diaphragm oscillates in its fundamental mode.

The substrate 2 is mechanically fixed, with the metallic layer 5. The linear dimensions 1 of the diaphragm of the silicon chip 1 greatly exceed the plate separation dimension d (see Fig. 1) across the gas cavity 4.

Figs 3 and 4 show a second preferred form of pressure micro-structure, which is slightly altered over the structure of the first form of Figs 1 and 2. The second form of micro-structure similarly comprises a substrate 2 comprising a metallic layer or capacitor plate 5 spaced from the chip 1 as previously. The micro- mechanic structure generally indicated in Figs 3 and 4 includes an oscillatory cantilever on the second plate of the capacitor. The cantilever is constructed by reiifoving three supporting edges of the diaphragm of the structure of Figs 1 and 2, to leave a member supported along the remaining edge Id. The cantilever dimensions 1 and w are comparable, and greatly exceed the spacing d. Fig. 4 is a plan view of the structure of Fig. 3 in the direction of arrow C in Fig. 3 but showing the outline shape of the chip 1 only. As in the form of Figs 1 and 2, this form of the micro-structure also comprises a mass lc to lower the natural resonant frequency of the chip 1, in this case formed at or towards the free end of the cantilever, away from its hinged mounting at edge Id. Similarly the second plate can be formed by the entire conducting silicon chip or. by a diffused area 6 as before.

Figs 7 and 8 show a third preferred form of micro- structure similar to that of Figs 1 and 2. The third form of micro-structure similarly comprises a substrate 2 comprising a metallic layer or capacitor plate 5 spaced from the chip 1 as previously. Section le have been removed from the thinned diaphragm region of the chip 1 to form cut outs le in turn defining a central section of the chip If. Central section If is thus supported by connecting beams lg. Fig. 7 shows the micro-structure in the direction of arrow D in Fig. 6, in schematic cross-section.

The micro-structure in either case shown or in any other possible configuration of the invention preferably includes a port through which gas enters the cavity 4 and which incorporates a filter to prevent small particles which may be in the gas from entering the cavity.

In each case, the micro-structure described constitutes a mechanically resonant system the natural frequency and damping coefficient of which are strongly affected by the gas pressure between the rigid substrate 2 and the oscillatory member or chip 1. In the preferred forms of microstructure shown in the drawings the capacitor configuration allows the mechanical oscillations of the chip 1 to be transduced into electrical signals and, when an associated electronic circuit is added to the micro-structure to form a pressure sensor unit, can in turn be used to both detect and maintain the oscillation by applying a feedback signal to the mechanical system by way of the force exerted between capacitor plates when a voltage is applied across them. As stated, instead of providing the metallic layer 5 or other conductive layer or equivalent and configuring the micro-structure as a capacitor other techniques could be used to- detect the motion of the oscillatory member. For example, piezo-resistors could be provided in or be associated with the chip 1 and with associated electronics motion of the oscillating member detected by reference to changes in resistance of the piezo-resistors.

A feature of the micro-structure is that there is never any net force from the ambient gas applied across the oscillatory member, so that cycling between high and low pressures is not accompanied by hysteresis in the micro-mechanic structure. Central to the operation of the micro-mechanic structure of the invention is the relative trapping of a sample of gas between the fixed and oscillatory members. The efficiency of the trapping depends on the geometry of the cavity 4, the gas pressure and viscosity, and the frequency of the oscillation of the plates. Since the ratio 1/d of the planar dimensions of the chip 1 to the depth of the cavity 4 is large, the gas forms a film. The dynamics of such films, called "squeeze films" have been studied in connection with air bearings, and the motion expected of the system can be characterised by a dimensionless number, called the squeeze number. At a high pressure or squeeze number the gas in the cavity 4 is trapped because for example, as the oscillatory member oscillates, the gas cannot flow out in the time available to maintain a constant pressure since the gas flow is impeded by viscous effects. The resonant frequency of the structure is relatively high, because the gas in the cavity acts as a dissipative spring, adding to the spring constant of the silicon structure. The dimensions of the device can be chosen such that at higher pressures, the system natural frequency is dominated by the trapped gas.

At a low gas pressure or squeeze number the resonant frequency of the structure is lower. The gas present is too dilute to have a significant spring constant in comparison with that of the diaphragm, and the oscillation frequency of the combined gas- microstructure system approaches that of the silicon/insulator device alone. At lower pressures the damping of the device falls dramatically with pressure, and approaches the very low values associated with resonant structures made of single-crystal silicon. Further quantitative information on the gas pressure may therefore be derived from a measurement of the quality factor Q of the electronically-maintained oscillator.

When the micro-structure is driven at a frequency corresponding to a low squeeze number, gas is not trapped but is pumped dissipatively in to and out of the cavity without compression. The amount of dissipation depends on the gas viscosity. In this regime the micro-structure acts as a damped spring, and the gas viscosity can be inferred from its drive characteristics.

Fig. 5B schematically illustrates the principle of operation of the device as a gas pressure sensor. If the silicon chip is caused to oscillate, for example by an initial voltage pulse applied across the structure of the device, at higher gas pressures in the cavity 4 the chip will oscillate at a relatively high resonant frequency and will be relatively rapidly damped back to zero while at lower gas pressures the chip 4 will oscillate at a relatively low frequency with greater amplitude, and damping to zero will be much slower.

With added electronics appropriately matched and calibrated the pressure sensor micro-structure of the invention may determine gas pressure from the resonant frequency of the structure. It has been found that the square of the resonant frequency is approximately linearly related to the gas pressure, since the spring constant of a trapped element of gas is proportional to its pressure, while in the vacuum region the effect of the silicon spring constant is dominant and yields a resonant frequency which varies approximately linearly with pressure.

Fig. 5A shows, by way of example and without intending to be limited to any particular electronics configuration or mode of operation, one arrangement of electronics for use with the micro-structure as a gas pressure sensor using capacitance techniques to detect motion of the oscillatory member. In Fig. 5A MS indicates the physical micro-structure, such as that shown in Figs 1 and 2 or Figs 3 and 4, or Figs 6 and 7, for example. MS is charged, to a voltage V by the function generator 8, except for a brief period when a voltage pulse different from V is applied. This sets the moveable plate of MS in motion, and generates a damped oscillatory current at the input of amplifier 9. This signal is amplified and passed to a frequency detector 11, which locks the frequency and phase of the voltage-controlled oscillator 10 to the decaying signal from the micro-structure. The voltage controlled oscillator provides a pressure signal output, for example to a display, control microprocessor or the like in a control system, memory or whatever. After a suitable number of oscillation cycles, the function generator 8 produces another voltage pulse which is applied to MS to maintain its oscillation and its resonant frequency. It should be emphasised that the particular electronic configuration described is given as an example and other configurations are possible. For example, it is possible that instead of applying a series of triggering pulses to the chip to maintain oscillation, a continuous frequency could be applied to the chip.

With added electronics appropriately matched and calibrated the sensor micro-structure may determine gas viscosity. It has been found that provided the driving force applied to the micro-structure is at a frequency well below the resonant frequency of the structure, there is a simple relationship between the phase ø of the motion of the chip with respect to the driving force, and the driving frequency. Further, the slope of a plot of tan ø versus the frequency f of the driving force is directly proportional to the gas viscosity.

Fig. 8 shows, by way of example and without intending to be limiting, one arrangement of electronics for use with the micro-structure as a gas viscosity sensor using capacitance techniques to detect motion of the oscillatory member. In Fig. 8 MS indicates the physical micro-structure, such as that shown in Figs 1 and 2 or Figs 3 and 4 or Figs 6 and 7 for example. Oscillator 12 applies a small sine wave voltage of frequency f superimposed on a DC voltage across the micro-structure. This results in the chip 1 oscillating with a strong component of motion at the drive frequency f. Applying the driving voltage as an alternating voltage superimposed on a DC voltage is desirable because it results in the chip oscillating at frequency f whereas without the DC component the chip moves at a frequency 2f.

A capacitance metering circuit 13 operating at a frequency much higher than f measures the plate capacitance from which may be determined the plate position. The phase comparator 14 compares the phases of the drive signal and the capacitance signal and, provided that the system is not overdriven, this phase is found to be independent of amplitude. The gas viscosity has been found to be proportional to this phase difference. Accurate measures of amplitude or capacitance are not required; the only measurements required are drive frequency and phase difference.

The micro-structure may be immersed in the gas, the viscosity of which is being sensed, or if the micro-structure is of the form shown in Fig. 6, in which gas can enter the cavity from the silicon side via the cut outs, i.e. a stream of gas may be directed to the sensing device as schematically shown in Fig. 9. A cover 15 extends over the chip as shown, and gas passes into the sensor to envelop the chip and the thin space between the chip and the substrate through inlet port 16 and passes out through outlet port 17. Other equivalent arrangements are possible.

The sensor micro-structure of the invention may be employed in any suitable application, particularly where measurement over a wide gas pressure with accuracy is required. The sensor may comprise part of a micro-electronic device in a control system or in numerous applications requiring pressure and/or viscosity measurement.

The foregoing describes the sensor micro-structure, pressure and/or viscosity sensors, and methods of the invention including preferred forms thereof. From the foregoing description it will be apparent that the principle of the sensors or methods of the invention may be embodied in other micro-structure designs besides the specific forms shown in the drawings and described, and such alterations and modifications as will be apparent to those skilled in the art are intended to be incorporated within the scope hereof, as defined in the following claims.

Claims

1. A micro-mechanic gas pressure and/or viscosity sensor, comprising: a mechanically fixed member; an oscillatory member capable of oscillation relative to the fixed member; a thin planar cavity defined by the fixed plate and the oscillatory plate between the fixed plate and oscillatory plate, into which gas may pass and be restrained by the squeeze film effect, electronic means for determining the gas pressure by reference to the frequency of resulting oscillation of the oscillatory member initiated by a voltage applied across the fixed member and the oscillatory member and/or gas viscosity by reference to the phase difference between a driving voltage applied across the fixed member and the oscillatory member and resulting oscillation of said oscillatory member.

2. A gas pressure and/or viscosity sensor as claimed in claim 1, wherein the fixed member forms one capacitor plate, the oscillatory member forms another capacitor plate, and said electronic means comprises means to determine the frequency and/or phase of motion of the oscillatory member by reference to capacitance across the fixed and oscillatory members.

3. A gas pressure and/or viscosity sensor as claimed in either one of claims 1 and 2, produced by etching of a substrate to form the fixed member and oscillatory member.

4. A gas pressure and/or viscosity sensor as claimed in any one of claims 1 to 3, wherein the oscillatory member is formed from crystalline silicon.

5. A gas pressure and/or viscosity sensor as claimed in any one of claims 1 to 4, wherein the electronic means for determining gas pressure comprises a voltage generator to provide the applied voltage and a frequency detector to detect the oscillatory current passed by the structure.

6. A gas pressure and/or viscosity sensor as claimed in any one of claims 1 to 4, wherein the electronic means for determining gas viscosity comprises a voltage generator to provide the applied voltage and a phase detector and comparator means to detect the phase of the oscillatory current passed by the structure and compare same to the phase of the driving voltage.

7. A method of determining gas pressure comprising passing the gas through a thin planar cavity defined by and between a mechanically fixed member and an oscillatory member capable of oscillation relative to the fixed member, between which members the gas may be restrained by squeeze film effect, applying a driving voltage between the fixed member and oscillatory member to cause the oscillatory member to oscillate, and determining gas pressure by reference to the frequency of oscillation of the oscillatory member.

8. A method as claimed in claim 7 wherein the fixed member forms one capacitor plate and the oscillatory member forms another capacitor plate and the frequency of motion of the oscillatory member is determined by reference to capacitance across the fixed and oscillatory members.

9. A method of determining gas viscosity comprising passing the gas through a thin planar cavity defined by and between a mechanically fixed member and an oscillatory member capable of oscillation relative to the fixed member, applying a driving voltage between the fixed member and oscillatory member, and determining the gas viscosity by reference to the phase difference between the applied voltage and an oscillatory current passed by the structure.

10. A micro-mechanic gas pressure sensor, comprising: a mechanically fixed member; an oscillatory member capable of oscillation relative to the fixed member; a thin planar cavity defined by the fixed plate and the oscillatory plate between the fixed plate and oscillatory plate into which gas may pass, and electronic means for determining the gas pressure by reference to the frequency of resulting oscillation of the oscillatory member initiated by a voltage applied across the fixed member and the oscillatory member.

11. A gas pressure sensor as claimed in claim 10, wherein the fixed member forms one capacitor plate, the oscillatory member forms another capacitor plate, and said electronic means comprises to determine the frequency of motion of the oscillatory member by reference to capacitance across the fixed and oscillatory members.