Pressure sensor device and method of sensing pressure

Abstract

A pressure sensor device comprises a device package (110) arranged to define a cavity (116) having an opening for fluid communication with an internal volume thereof. The cavity (116) comprises a side wall (114, 115). An elongate pressure sensor element (100) is provided and has a proximal end (120) and a distal end (122). The side wall (114, 115) is arranged to hold fixedly the proximal end (120) of the pressure sensor element (100) therein so that the pressure sensor element (100) is cantilever-suspended from the side wall (114, 115) within the cavity (116).

Claims

1. A pressure sensor device comprising: a device package configured to define a cavity having an opening for fluid communication with an internal volume thereof, the cavity comprising a side wall; an elongate pressure sensor element having a proximal end and a distal end; wherein the side wall is configured to hold fixedly the proximal end of the pressure sensor element therein and wherein the pressure sensor element includes a closed cavity therein, a substrate layer comprising a recessed portion formed therein, the recessed portion defining an open sensor element cavity portion; a membrane layer disposed adjacent the substrate layer, the substrate laver and the membrane layer cooperating to close the open sensor element cavity portion and to define the closed cavity; and the closed cavity is held fixedly by the side wall so that the pressure sensor element is cantilever-suspended from the side wall within the cavity.

2. The device according to claim 1, wherein the side wall substantially surrounds the proximal end of the pressure sensor element.

3. The device according to claim 1, wherein the pressure sensor element is surrounded by a fluid.

4. The device according to claim 3, wherein the fluid is a liquid or a gelatinous material.

5. The device according to claim 1, wherein the pressure sensor element comprises a closed cavity therein.

6. The device according to claim 5, wherein the pressure sensor element comprises: a substrate layer; and a membrane layer disposed adjacent the substrate layer, the substrate layer and the membrane layer cooperating to define the closed cavity.

7. The device according to claim 6, wherein the substrate layer comprises a recessed portion formed therein, the recessed portion defining an open sensor element cavity portion.

8. The device according to claim 7, wherein the membrane layer is arranged to close the open sensor element cavity portion.

9. The device according to claim 1, wherein the device package comprises a mating surface for fixing to a counterpart mating surface when in use, wherein the mating surface defines the opening of the cavity of the device package and is a keying surface.

10. The device according to claim 9, wherein the keying surface is a roughened surface.

11. The device according to claim 1, wherein the pressure sensor element is a Micro Electromechanical Systems element.

12. The device according to claim 1, wherein the pressure sensor element is formed from two wafers.

13. The device according to claim 6, wherein the membrane is configured to separate, when in use, an internal volume of the closed cavity from a medium under test.

14. The device according to claim 5, wherein an internal pressure of the closed cavity defines a reference pressure.

15. An absolute pressure sensor device comprising the pressure sensor according to claim 1.

16. An automotive vehicle comprising the pressure sensor device according to claim 1.

17. A method of sensing pressure, the method comprising: providing a device package defining a cavity having an opening for fluid communication with an internal volume thereof, the cavity having a side wall, the side wall holding an elongate pressure sensor element fixedly at a first end of the pressure sensor element; providing a substrate layer and forming a recessed portion therein to define an open sensor element cavity portion; disposing a membrane layer adjacent the substrate layer, the substrate layer and the membrane layer cooperating to close the open sensor element cavity portion and to define a closed cavity; and the side wall holding fixedly the elongate pressure sensor element comprising the closed cavity so that the pressure sensor element is cantilever-suspending within the cavity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of a pressure sensor element constituting part of an embodiment of the invention;

(3) FIG. 2 is a schematic plan view of the pressure sensor element of FIG. 1;

(4) FIGS. 3 to 5 are schematic diagrams of different stages of formation of the pressure sensor element of FIG. 1;

(5) FIG. 6 is a schematic diagram of a package constituting an embodiment of the invention;

(6) FIG. 7 is a schematic diagram of a pressure sensor device constituting an embodiment of the invention;

(7) FIG. 8 is a schematic diagram of a part-formed package to receive the pressure sensor element constituting another embodiment of the invention;

(8) FIG. 9 is a flow diagram of a method of forming the pressure sensor device of FIG. 7 constituting another embodiment of the invention;

(9) FIG. 10 is a schematic diagram of another pressure sensor device constituting yet another embodiment of the invention.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

(10) Throughout the following description, identical reference numerals will be used to identify like parts.

(11) Referring to FIG. 1, a pressure sensing element 100, for example a MEMS pressure sensor element, of a pressure sensor device comprises a substrate layer 102 having a cavity 104 formed therein and a surface membrane layer 106 sealingly disposed adjacent the substrate 102. The substrate 100 can be formed from any suitable material, for example silicon or glass. For some applications, use of a silicon substrate is advantageous, because silicon has desirable mechanical properties and/or permits integration of piezo resistors with the silicon substrate will allowing isolation thereof within the crystalline structure of the silicon. The surface membrane 106 can also be formed from any suitable material, for example silicon. Turning to FIG. 2, the pressure sensor element 100 is elongate and, in this example, of a generally rectangular shape. However, the skilled person will appreciate that other shapes can be employed.

(12) Referring to FIGS. 3 to 9, a wafer constituting the substrate 102 is obtained (Step 200) from silicon grown using any suitable growth technique (FIG. 3). A recessed position or recess 108 defining an open sensor element cavity position is then formed (Step 202) in the substrate 102 (FIG. 4) using, in this example, a surface micromachining technique. However, the skilled person will appreciate that other semiconductor processing techniques can be employed to form the recess 108, for example using any suitable etching technique, such as a chemical etching technique. It should also be appreciated that although in this example a single recess is described as being formed in the substrate 102, more than one recess can be formed in the substrate 102.

(13) Thereafter, the membrane 106, which in this example is ground and polished from a thicker initial silicon wafer separately, is then disposed (Step 204) fixedly adjacent the substrate 102 (FIG. 5), for example by way of bonding using any suitable technique, for example a MEMS wafer bonding technique, such as fusion bonding, anodic bonding, glass frit bonding, or eutectic bonding, thereby closing the recess 108 and hence forming the closed cavity 104. The membrane 106 is applied to the substrate 102 so that the cavity 104 formed comprises a vacuum therein, for example by applying the membrane 106 to the substrate 102 in a vacuum.

(14) In another embodiment, the pressure sensor element 100 can be formed by providing an initial silicon wafer having a first side and an opposite second side. The silicon wafer is etched from the first side to form an open cavity therein. The silicon wafer is etched sufficiently deeply to leave only a thin layer of a region of the wall of the silicon substrate at the second side of the silicon wafer. This thinned backside wall constitutes an integrally formed membrane. A glass wafer is then bonded to the open first side of the etched silicon wafer to close the open cavity, for example using an anodic bonding technique. During fabrication, piezo-resistors can be defined on the second side prior to etching so that the piezo-resistors are disposed on the exterior of the pressure sensor element 100 on the thin layer of the region of the wall of the silicon substrate.

(15) Referring to FIG. 6, the pressure sensor element 100 is a first part of the pressure sensor device. In this regard, a second part is a device package 110. In this example, the package 110 comprises a base 112 and side walls 114. The side walls 114 extend away from the base 112 and are, in this example, substantially perpendicular to the base 112. However, the skilled person should appreciate that the side walls 114 need not extend away from the base 112 substantially perpendicularly, but can extend away at other angles, for example between about 70° and about 90°. The base 112 and the side walls 114 are arranged so that the package 110 comprises an open cavity 116 therein for allowing fluid communication with an internal volume of the open cavity 116. In this example, the open cavity 116 is larger in area than an adjacent surface of the pressure sensor element 100, for example an outer surface 107 of the membrane 106. The area of the open cavity 116 can therefore be, for example, up to about 50 times larger than the area of the adjacent surface of the portion of the pressure sensor element 100 residing in the open cavity 116, such as up to about 10 times larger. The depth of the open cavity 116 can be between about two and five times the thickness of the pressure sensor element 100. The volume of the open cavity 116 is not significantly larger than the volume of the portion of the pressure sensor element 100 residing in the open cavity 116 in order to limit the pressure applied to the base 112 and/or side walls 114 of the package 110. In this regard, the volume of the open cavity 116 can be, for example, up to 250 times larger than the volume of the portion of the pressure sensor element 100 residing in the open cavity 116, for example 20 times greater. The force applied to the body of the package 110, for example the base 112 and the side walls 114, is therefore limited so as to avoid damage to the package 100. One of the side walls 114 comprises, when completed, a concave portion, for example a side recess 118, formed therein. The side recess 118 is, in this example, substantially hyperrectangular in form and arranged to receive a part of the pressure sensor element 100 snugly therein.

(16) Of course, the skilled person will appreciate that whilst a substantially hyperrectangular open cavity structure has been described above, the open cavity 116 can possess a different form, for example the open cavity 116 can be a blind bore.

(17) Referring to FIG. 7, the pressure sensor element 100 of a pressure sensor device 124 has a proximal end 120 and a distal end 122 with respect to the side wall comprising the side recess 118 in which the pressure sensor element 100 is ultimately disposed. The proximal end 120 of the pressure sensor element 100 is located in the side recess 118 of the package 110 so that the pressure sensor element 100 is cantilever-suspended from the side wall 114 within the open cavity 116. As such, in this example, the side wall 114 in which the side recess 118 is disposed substantially surrounds the proximal end 120 of the pressure sensor element 100. In this example, the side wall 114 in which the proximal end 120 of the pressure sensor element 100 is disposed, and thus the side recess 118, are formed in two stages. In this regard, one technique to form the pressure sensor device 124 comprises part-forming (Step 206) the device package 110 (FIG. 8) comprising a lead frame (not shown) using any suitable moulding technique, forming the side walls 114 of the device package 110 such that a side wall 121 of the side walls 114 comprises a shoulder 123 therein to receive the pressure sensor element 100. In another example, the side walls 114 can be completely moulded and the shoulder 123 can be formed using any suitable erosion technique, for example drilling. In yet another example, the package 110 can be mounded to comprise a relatively shallow recess therein, and a part of the package 110 can then be eroded further, for example by drilling, to form the side walls 114 and complete the definition of the shoulder 123, by leaving a portion of the shallow recess intact. The proximal end 120 of the pressure sensor element 100 is then disposed (Step 208) on the shoulder 123 of the side wall 121 so that the pressure sensor element 100 extends into the open cavity 116. At this stage, wire bonding is performed (Step 210) to connect electrically the pressure sensor element 100 to, for example, the lead frame (not shown) of the device package 110 and/or any other semiconductor device or integrated circuit to be packaged with the pressure sensor element 100. The side recess 118 and the side wall 121 are then completed (Step 212) by applying an encapsulant, for example a glob top, over the proximal end 120 of the pressure sensor element 100. It should be appreciated that this is just one technique for forming the device package 100 with the pressure sensor element 100 therein and other manufacturing techniques can be employed.

(18) In operation, the open cavity 116 enables the pressure sensor element 100 to be exposed to a fluid, for example a medium of which pressure is to be sensed. The pressure sensor element 100 is thus surrounded by a fluid, which in use can be a liquid or gelatinous material. The membrane 106, of course, separates the internal volume of the closed cavity 104 from the fluid under test. In this regard, the internal pressure of the closed cavity 104 constitutes a reference pressure for the pressure senor element 100.

(19) The pressure sensor device 124 is, in this example, an absolute pressure sensor device, which can be employed in relation to various fields of endeavour, for example in an automotive vehicle, such as an automobile.

(20) Turning to FIG. 10, the package 110 comprises a mating surface 126 for fixing to a counterpart mating surface 128 of an element or apparatus to which the pressure sensor device 124 is to be connected or coupled. The mating surface 126 defines the opening of the open cavity 116 of the package 110 and is, in this example, a keying surface. The keying surface can be formed, for example, by roughening the mating surface 126.

(21) The skilled person should appreciate that the above-described implementations are merely examples of the various implementations that are conceivable within the scope of the appended claims. Indeed, it should be appreciated that the pressure sensor element 100 can be formed by techniques other than micromachining techniques. Similarly, the pressure sensor device can be used for applications other sensing absolute pressure.