PRESSURE SENSORThis invention relates to pressure sensors, and in particular to devices for differential pressure measurement. The invention also relates to a method of fabricating such a device.
There is an increasing requirement in industrial applications for differential pressure measurement. Typically it is necessary to measure pressure differentials of a few pascals under a background pressure of 1000 atmospheres or more.
Conditions of this nature are experienced when attempting to measure flow rates of fluids, e.g. oil or gas under high pressure or well-head conditions.
Currently available devices capable of performing this function are complex and relatively costly.
The object of the invention is to minimise or to overcome this disadvantage.
According to the invention there is provided a differential pressure sensor, including first and second resilient diaphragm members one of which is provided with a trough within which a strain sensor element is disposed, wherein said diaphragm members are bonded together whereby said trough defines a cavity between the diaphragm members, the arrangement being such that distortion of the diaphragm members in response to a pressure differential apply, a corresponding strain to the sensor element.
According to the invention there is further provided a differential pressure sensor, including first and second resilient diaphragm member one of which is provided with a trough, a resonator element mounted on a taut filament suspension access the trough, the resonant frequency of the element being determined by a tension within the filament suspension, and means for driving and interrogating said resonator element, wherein said diaphragm members are bonded together whereby said trough defines a cavity between the diaphragm members, the arrangement being such that distortion of the diaphragm members in response to a pressure differential applied thereto causes a corresponding change in the tension in said filament suspension thereby causing a change in the resonant frequency of the resonator element indicative of the magnitude of the pressure differential.
The arrangement is such that the pressure sensing element is shielded from direct exposure to external pressures.
An embodiment of the invention will now be described with reference to the corresponding drawings in which:Fig. 1 is a general view of the differential pressure sensor;Figs. 2 and 3 are cross-sectional views of the sensor of Fig. 1.
Fig. 4 illustrates the construction of the sensor diaphragm of the pressure sensor of Fig. 1.
Fig. 5 illustrates the construction of the other diaphragm of the pressure sensor; and Figs. 6 to 8 illustrate process steps in the fabrication of the differential process sensor.
Referring to Figs. 1 to 5, the pressure sensor is formed from two resilient diaphragm members 11, 12 bonded together e.g. by a layer. 13 of an adhesive.
Typically the members 11 and 12 are formed from single crystal silicon. One of the diaphragm members (11) is generally flat whilst the other (12) has a trough or ridge portion 14 which, when the diaphragm members are bonded together, defines a cavity 15 therebetween.
Advantageously, the cavity 15 is evacuated to minimise damping of the resonator and thus enhance the sensitivity of the device.
A resonator sensor element 16 is disposed within the cavity 15 and is integral with the second diaphragm member 12. The resonator element 16 is thus protected from externally applied pressure. Typically the resonator 16 consists of a pair of balanced paddles 161 supported on a taut filament 162 bridging the trough 13. The resonator element 16 may be driven at its resonant frequency by electrostatic coupling to the twopaddle members 161. For this purpose, the first diaphragm member 11 is provided on its inwardly facing surface with a pair of metallised conductor tracks 51 (Fig. 5) each of which terminates at a corresponding electrode 52. The arrangement is such that, when the two diaphragm members are assembled together, each electrode 52 is in register with a corresponding paddle member 161 of the resonator 16.
In a typical sensor construction, the diaphragm member 11 and 12 are mounted between a pair of coaxially arranged tubes 71, 72 (Fig. 8) via which a pressure differential is applied to the diaphragm members.
Flexure of the diaphragm assembly in response to a difference in applied pressure between the outer faces of the diaphragm causes a corresponding change in tension in the filament 162. This results in a corresponding change in the resonator frequency indicative of that pressure difference. The response of the diaphragm assembly to an applied pressure differential is determined by the elastic modulus of the diaphragm material and by the area and thickness of each diaphragm. The manner in which diaphragm displacement may be calculated will be apparent to those skilled in the art.
It will be understood that whilst a resonator element has been described as the displacement sensing device, other means of sensing displacement of the diaphragm assembly may also be employed.
Referring now to Fig. 6 a typical fabrication sequence for the sensor of Figs. 1 to 5 will be described. A wafer 51 (Fig.6) of single crystal silicon is masked and selectively etched to define a trough 52 in one face of the wafer. A patterned region corresponding to the resonator element is provided with a etch stop, e.g. by boron doping. Material is thus etched away to leave the resonator element 16 bridging the trough 52. The silicon may be aetched with a mixture of aqueous potassium hydroxide and isopropanol, a mixture of ethylene diamine and catechol, or hydrazine hydrate. These etches cut anisotropically in a precise direction relative to the crystal planes.In particular, anisotropic etching of a ElOO) single crystal silicon surface proceed at an angle of 350 1 12 12 from the normal to the plane of the surface.
Thus, the depth of a trough formed in such a surface may be accurately determined from the lateral dimensions of the mask from which the trough is defined.
The other face of the wafer 51 is then masked and selectively etched to remove material reducing the thickness of the wafer and to define a ridge 61 in register with the trough 52. The ridge may be formed by anisotropic etching in a similar manner to the formation of the trough. In some applications the ridge and trough may be etched simultaneously.
The etched structure is secured (Fig. 8) with a layer 13 of adhesive, e.g. an epoxy resin, to a second thinned wafer 71. The cavity defined by the trough 51 may be evacuated during this assembly. Finally the diaphragm assembly is mounted between a pair of tubular members 71, 72 to form the finished device. It will be appreciated that in a typical fabrication process a plurality of devices are formed on a common silicon substrate wafer, which is then subdivided after the device fabrication process has been completed.
The differential pressure sensor described above may be employed in a variety of pressure sensing applications. It is however of particular use on a differential pressure sensing element of a fluid flowmeter e.g. for the measurement of gas or oil flow rates under high pressure conditions. In such an instrument an obstruction, typically an orifice plate, is placed in the fluid flow. Flow rate is then determined from the pressure differential between the upstream and downstream sides of the obstruction.