In recent years, microchip technology has been used to make sensors to measure an increasing number of physical phenomena. Previously proposed pressure sensors using micro-chip technology for measuring the differential pressure of two fluids have usually involved a delicate sensing-element. This has the disadvantage that in some applications the sensors are easily damaged. This often results in high costs due to having to replace damaged sensors.

It is the aim of the present invention to provide a robust and low cost capacitive differential pressure sensor which overcomes the aforementioned problems.

Accordingly, the present invention is directed to a capacitive differential pressure sensor in which the capacitive element comprises two layers, at least one of which layers is made of silicon, a cavity between the two layers, electrical output means which allow the capacitance of the sensor to be measured, in which the sensor further comprises a hole formed between the outside of one of the layers and the cavity, the outsides of the respective layers are isolated by one or more flexible diaphragms, transmission fluids are contained between the one or more

diaphragms and the respective layer, such that the cavity is exposed to a first pressure, and the outside of the other layer is exposed to a second pressure. The capacitance generated by the variation in the distance between the layers can be measured and is directly related to the pressure difference.

Preferably both layers are made of silicon.

In a preferred embodiment the layer containing the hole has a metal film deposited on it inside the cavity.

Advantageously, the layer bearing the metal is glass.

Preferably, the metal layer is a gold layer.

In a preferred embodiment, the one or more diaphragms are made of an elastomeric or elastic material, preferably a rubber material or flexible metal such as stainless steel.

Preferably, both sides of the sensor are evacuated before the trapped fluid is added. This provides the advantage that the system is clear of any unwanted fluid which might adversely affect the readings of the sensor.

This provides the further advantage that the whole of the sensor is filled with the trapped fluid.

Advantageously, the fluids contained within the one or more diaphragms are electrically inert fluids which are impermeable to the one or more diaphragms. Preferably the fluids are silicone oils or perfluorinated hydrocarbons.

Preferably, the output means comprise contact pads which are attached to each layer of the sensor to enable

leads to be connected.

The present invention provides the advantages of being able to accurately measure differential pressure in a wide range of liquids and gases regardless of their conductivity, cleanliness or corrosiveness. The microchips for the capacitive differential pressure sensors can be easily produced on a large scale by normal microchip manufacturing processes. The sensor has a good mechanical robustness. The sensor can withstand a pressure which may be many times greater than the measured differential pressure, for example, when one side of the pressure sensor is disconnected from the system being measured.

An example of a capacitive differential pressure sensor made in accordance with the present invention will be described hereinbelow with reference to the attached drawings in which: Figure 1 shows a cross-section of the microchip of the sensor; Figure 2 shows a cross-section of the sensor as a whole; Figure 3 shows a cross-section of a modified version of the sensor in Figure 2; and Figure 4 shows a cross-section through an enlarged part of the sensor shown in Figure 2.

In Figure 1, there is a base layer 10, which in this case is made of glass. The base layer 10 has attached to it a silicon layer 12. There is a cavity 14 between the

two layers 10 and 12. On top of the base layer 10 in the cavity 14 is a metal film 16. The metal film 16 is preferably made of gold. A hole 18 is formed between the outside of base layer 10 and the cavity 14. Attached to the outside of the silicon layer 12 is a bond pad 20, which is attached to a lead 22. Another bond pad 24 is connected to a lead 26 on the inside of the base layer 10. The outside of the base layer 10 is exposed to a pressure P2 and the outside of the silicon layer 12 is exposed to a pressure P1.

The sensor is typically 3mm square. The cavity 14 is also preferably circular with a radius of approximately lmm and a depth of approximately lym. The metal film 16 on the base layer 10 forms one plate of a capacitor and the silicon layer 12 above the metal film 16 forms the other plate of the capacitor. The capacitance of the sensor is defined by the standard equation: C = E,E3l/d where E = permittivity of free space, Er = relative permittivity of the material in the cavity, A = area of the capacitance plate, and d = gap between the capacitance plates.

The capacitive differential pressure sensor works by differential pressure being applied across the silicon layer 12 which acts as a diaphragm. This causes the cavity gap to change and hence produces a change in the

capacitance of the sensor. The hole 18 allows the pressure P2 to be applied to the cavity side of the silicon layer 12. Therefore if pressure P2 > P1, then the capacitance will decrease, whereas if P2 < P1, then the capacitance will increase. The capacitive differential pressure sensor can be tuned to respond to different full scale pressure differentials by altering the dimensions of the silicon.

The silicon and glass layers 10 and 12 are joined via an anodic bonding process which forms a hermetic seal. In practice, the metallisation of the metal film 16 on the base layer 10 also contains a hermetically sealed, horizontal leadthrough 25, which extends to the bond pad 24 at the edge of the sensor. The silicon surface is oxidised in this area thus preventing the shorting of the leadthrough to the silicon. Silicon is removed from the silicon layer 12, in the position above the bond pad 24 on the glass layer 10 to enable electrical contact to be made to the metal film 16 on the glass layer 10. The other contact is made to the silicon layer 12 which is processed in a standard manner to provide the ohmic contact bond pad 20.

A modification of this design would be to replace the glass layer 10 by a second silicon layer. In this case the horizontal leadthrough would not be required and the electrical contact can be made to the bottom surface.

Figure 2 shows the capacitive differential pressure sensor 48 of Figure 1 prepared for use with a two-fluid

system. It is important that the measuring surfaces of the capacitive differential pressure sensor remain clean and the fluid in the gap is not contaminated, hence ensuring that the permittivity remains constant.

The sensor 48 is mounted on a printed circuit board 72. This is clamped between two lower parts 62, 54 and a cover 98, by clamping means, in this instance, bolts 52 from above and below. Each bolt 52 screws into a threaded section in lower part 62. In addition, an inner casing 74 is sealed to the printed circuit board to provide a pressure tight chamber 95. The bottom lower part 54 has two openings 56 and 57 which lead through to enlarged areas 58 and 59. The openings 56 and 57 are equipped with screw threads 60 to enable the attachment of pipes from the system to be measured, such that the pipe connected to the opening 56 has a pressure P1 and the other pipe attached to the opening 57 has a pressure P2. The upper lower part 62 is fitted above the bottom lower part 54. A diaphragm 64 is held between the bottom lower part 54 and the upper lower part 62. The main criteria for the selection of material for the diaphragm 64 is that it is impermeable to the fluid of the system to be measured and that it is further impermeable to whatever the fluid is contained between the diaphragm 64 and the respective layers 10 and 12. The diaphragm 64 may be made of an elastomeric or elastic material, preferably rubber. However the diaphragm may be made of a flexible metal such as stainless steel.

In the upper lower part 62 are two openings 66 and 67 which are enlarged at the bottom of the middle part 62 to match the enlarged areas 58 and 59 of the openings 56 and 57 in the bottom lower part 54 through the diaphragm 64. The openings 66 and 67 then narrow to top portions 68 and 69, which communicate with two openings 78 and 80 in the printed circuit board 72. The printed circuit board 72 is fitted above the upper lower part 62 and is pressure tightly sealed by sealing means 73 around the openings 78 and 80. The printed circuit board 72 is wholly covered by the cover 98 which is sealed to its edge by adhesive sealant or gasket 70. The printed circuit board 72 is partially covered by the inner casing 74, which is held on the board 72 by adhesive sealant or gasket 77 and packing 71 situated between its top side and the cover 98 obviously the inner casing could be secured by means other than packing 71. In the printed circuit board 72, the opening 78 communicates with the opening 67 of the middle part 62 which is at pressure P2 and the opening 80 communicates with the other opening 66 in the upper lower part 62 which is at pressure P1. The opening 80 then leads through the printed circuit board 72 to the chamber 95 which is bordered on one side by the printed circuit board 72, which has the sensor 48 mounted on top of it, and on the other sides by the inner casing 74. The opening 78 goes through the printed circuit board 72 and enters the cavity of the sensor 48 through its underside.

During the construction of the sensor all fluid is evacuated from the openings 66 and 67, holes 70 and 80 and chamber 83 and the cavity 16 in the sensor 48 before they are filled with the fluid. This is facilitated by the fact that cover 98 lower parts 70 and 62 can be held together by upper bolts 52 which screw into lower part 62. Once the sub-assembly has been evacuated it is then filled with the fluid and the diaphragm added and the lower parts of the sensor then added and held on by the lower bolts 52. The use of upper and lower bolts thus ensures ease of assembly whilst allowing evacuation of all unwanted fluids.

Figure 3 shows a modified version of the sensor shown in Figure 2. As can be seen the printed circuit board 72 is completely contained within the cover 98. This has advantages in assembly and also not exposing the edges of the printed circuit board 72 to outside abrasion. In this instance the cover 98 seals via adhesive sealant or gaskit 70 onto the lower part 62. Furthermore extra sealing means 73 are not required at the far ends of the printed circuit board 72. The seal between the lower part 54 and the diaphragm 64 is improved by the addition of sealing means 100 which can be 0 rings.

The sensor 48 is held on the printed circuit board 72 by a mounting 88 which is more clearly shown in Figure 4.

The mounting 88 has two prongs 90 which pass through the printed circuit board 72 and is held on the printed circuit board 72 by adhesive sealant. The sensor 48 is held on the

mounting 88 by die attach sealant 92. The sensor 48 has gold wires 94 which attach to the prongs 90 which allow the output of the sensor 48 to be measured through tracks (not shown) on the printed circuit board 72. The printed circuit board 72 is connected by leads 85 to an electrical connection 86 for external readings to be taken. The pressure P1 applied to the opening 56 by the system to be measured is transmitted through the diaphragm 64 to the fluid contained in the opening 66, hole 80 and chamber 83, and thence to the top of the sensor 48. The pressure P2 applied to the opening 56 by the system to be measured is similarly transmitted through the diaphragm 64 to the fluid contained in the opening 67, hole 78 and the cavity in the sensor 48.

The printed circuit board 72 may additionally incorporate support circuitry to control, adjust, condition, compensate, amplify 6r convert the electrical output of the sensor 48. This enables the output of the sensor as a whole to be a simple nature not requiring large amounts of external circuitry.

However, the choice of fluids to fill the sensor is dependent upon the material from which the diaphragm 64 is made. Preferably such fluids are silicone oils or perfluorinated hydrocarbons.

It will be readily apparent to a reader of ordinary skill in the art that whilst such a capacitive differential pressure sensor, as described above, is ideally suited for measuring differential pressures in water, such a sensor could easily be used for measuring other fluid pressures.