Method of manufacturing a pressure sensor

Abstract

A method of manufacturing an overheat or fire alarm detection system, comprises the steps of micromachining a pressure sensor and securing a sensor tube in fluid communication with the pressure sensor. The sensor tube may comprise a hollow tube containing a material that evolves gas upon heating. The micromachining step may comprise doping at least a portion of a first layer, forming a cavity at least partially within the doped portion and forming a deformable diaphragm over the cavity.

Claims

1. A method of manufacturing an overheat or fire alarm detection system, comprising the steps of: micromachining a pressure sensor, and securing a sensor tube in fluid communication with said pressure sensor; wherein said micromachining step comprises: forming one or more layers including a first layer, doping at least a portion of a first layer to form a doped portion; to form an electrical terminal, forming a cavity at least partially within said doped portion; and forming a deformable diaphragm over said cavity, wherein said micromachining step comprises forming a recess in an upper surface of said diaphragm, said upper surface facing away from said first layer and being at least partially aligned with said cavity, and wherein said micromachining step comprises forming at least two recesses having different depths.

2. The method of claim 1, wherein said sensor tube comprises a hollow tube containing a material that evolves gas upon heating.

3. The method of claim 1, wherein said micromachining step comprises: forming a plurality of doped portions; forming a cavity partially within each doped portion; and forming a plurality of recesses in said upper surface, each recess being at least partially aligned with a cavity.

4. The method of claim 1, wherein said step of forming one or more layers comprises forming a flexible, electrically conductive layer between said deformable diaphragm and said first layer.

5. A pneumatic pressure sensor comprising: a first layer having a plurality of cavities in a first surface; and a deformable diaphragm having: a first surface facing and covering said cavities, and a second surface facing away from said first layer and having a plurality of recesses, wherein each recess is at least partially aligned with one of said cavities and at least a first recess has a greater depth than a second recess; wherein said first layer comprises a semiconductor wafer having a plurality of doped portions and one or more of said cavities is at least partially within one of said doped portions; and wherein each doped portion provides an electrical terminal that is closable by the movement of a portion of said diaphragm into one of the plurality of cavities.

6. The pneumatic pressure sensor of claim 5, wherein said first layer and said diaphragm each have a thickness of 100 μm or less.

7. A method of manufacturing the pneumatic pressure sensor of claim 5, comprising the steps of: micromachining a first layer having a plurality of cavities in a first surface; and micromachining a deformable diaphragm that covers said cavities and has a plurality of recesses of different depths in a second surface of said diaphragm that faces away from said first layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Some exemplary embodiments of the present disclosure will now be described by way of example only and with reference to FIGS. 1 to 2, of which:

(2) FIGS. 1a to 1c are various views of a pressure sensor according to an exemplary embodiment of the present disclosure; and

(3) FIG. 2 shows a schematic cross-sectional view of an overheat or fire alarm system according to an exemplary embodiment of the present disclosure.

(4) The figures are not to scale.

DETAILED DESCRIPTION

(5) FIG. 1a shows a perspective view of an exemplary micromachined pressure sensor 10. The sensor 10 comprises a substrate 12 (‘first layer’) and a deformable diaphragm 14.

(6) The diaphragm 14 may be formed of a ceramic material such as silicon nitride (Si3N4). The substrate 12 may be formed of a semiconductor such as silicon.

(7) The diaphragm 14 comprises three recesses 16a, 16b, 16c in an upper (‘second’) surface 14b. The recesses 16a, 16b, 16c are circular and equally spaced.

(8) Located between diaphragm 14 and substrate 12 is an intervening electrically conductive, flexible metal layer 18. The metal layer 18 contacts the upper (‘first’) surface 12a of the substrate 12 and a lower (‘first’) surface 14a of diaphragm 14. The diaphragm 14, the substrate 12 and the intervening metal layer 18 are all circular and substantially the same size.

(9) The diaphragm 14 and the intervening metal layer 18 may be formed via deposition. Features of the layers, such as recesses 16a, 16b, 16c and cavities 19 may be formed by etching the respective layer.

(10) FIG. 1b shows an overhead plan view of the pressure sensor 10. The recesses have a diameter D2 of 10 microns and the diaphragm 14 has a diameter D1 of 30 microns. The diameter of the diaphragm 14 represents the greatest overall dimension of the sensor 10. Other dimensions will be suitable.

(11) FIG. 1c shows a cross-sectional view of the sensor 10 taken along line A-A in FIG. 1b. As discussed above, the sensor 10 comprises a three layer structure, namely the substrate 12, the metal layer 18 and the diaphragm 14. The substrate 12 comprises doped portion 13 and un-doped portions. A single recess 16a can be seen. Below recess 16a, is a cavity 19 formed in doped portion 13. The cavity 19 is defined between the upper surface 13a of doped portion 13, the walls 12c of the un-doped portion 12 and the lower surface 14a of diaphragm. The cavity 19 is substantially aligned with recess 16a. Similar doped portions and cavities are formed below the two other recesses 16b, 16c.

(12) The thickness of the diaphragm T1 is 1.0 μm and the depth T2 of the cavity 19 in the doped portion is 0.5 μm, although other dimensions will be suitable.

(13) The portion 14c of the diaphragm below the recess 16a, the doped portion 13, the cavity 19 and the intervening metal layer 18 form a pneumatic pressure switch. When subjected to sufficient pressure on its upper surface 14b, the portion 14c will deform into the cavity 19 (carrying with it a portion of metal layer 18). When the metal layer 18 contacts doped portion 13 an electrical circuit (not shown) is completed. This triggers an alarm to indicate that a certain temperature threshold has been detected. The sensor 10 shown, therefore has three distinct switches.

(14) The recesses 16a, 16b, 16c may each have a different depths d1, d2 and d3. This provides a different pressure set point, as the thickness of portion 14c below a recess will be inversely proportional to the amount of pressure needed to deform that portion into a cavity 19. The pressure set point will also depend on the depth of cavity 19 as the portion 14c will have further to deform until it makes contact (via metal layer 18) with doped portion 13.

(15) In use, all three switches may be used to monitor different temperature conditions, such as overheat, fire and integrity. Alternatively, the user may only connect to a switch having a desired pressure set point.

(16) FIG. 2 shows a cross-sectional view of an exemplary overheat or fire alarm system 30 comprising the sensor 10 (of FIGS. 1a to 1c) secured to a sensor tube 20. The sensor 10 is shown enlarged for illustrative purposes only. A thermal insulation block 26, made for example of ceramic, is attached around the sensor 10. A sleeve 28 is wrapped around block 26 and a part of the sensor tube 20 to ensure a hermetic seal between sensor 10 and tube 20.

(17) The sensor tube 20 comprises an interior space 21 and a solid core 22. The interior space 21 is filled with an inert gas such as helium. The solid core 22 is formed of a material that evolves a gas, such as hydrogen, upon heating. The material may be titanium hydride. The tube 20 comprises a metallic casing 24, made for example of an Inconel alloy. A shallow gap or plenum 23 is formed between sensor 10 (and specifically the upper surface 14b of the diaphragm 14) and a first end 20a of the sensor tube 20. The recesses 16a, 16b, 16c are in fluid communication with the interior 21 of the sensor tube 20.

(18) The sensor tube 20 has an outer diameter D3 of 1.6 mm and an inner diameter D4 of 0.9 mm. The solid core 22 has a diameter D5 of 0.66 mm. Other dimensions will be suitable.

(19) Heating the sensor tube 20 first causes the helium gas to expand. This applies a pressure on the recesses 16a, 16b, 16c. Depending on the pressure set point of each switch, one or more switches may be activated. Further heating of the sensor tube 20 causes the core 22 to evolve hydrogen gas. This causes one or more switches to activate.

(20) One of the pressure switches may provide an integrity alarm if the pressure drops below a certain threshold. The threshold could be set as the normal operating pressure of the helium gas fill. If the pressure drops below this threshold, then it may be indicative of a leak in the sensor tube (or between the tube 20 and the sensor 10). The integrity switch may be normally closed and only open when the pressure drops below the threshold. The opening of the normally closed switch (i.e. the opening of the electric circuit between the metal layer 18 and the doped portion) may trigger an alarm.

(21) The exemplary overheat or fire alarm system 30 can therefore be used to provide a number of different alarm signals indicative of different temperatures or conditions.

(22) The foregoing description is only exemplary of the principles of the invention. Many modifications and variations are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than using the example embodiments which have been specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.