WO2009116957A2

WO2009116957A2 - Water resistant ultra-low pressure sensor

Info

Publication number: WO2009116957A2
Application number: PCT/SG2009/000099
Authority: WO
Inventors: Kit-Wai Kok; Kok Meng Ong; Karthikeyan Kumaraswamy; Bryan Keith Patmon; Kathirgamasundaram Sooriakumar; Angeline Tee Lay Har
Original assignee: Sensfab Pte Ltd
Priority date: 2008-03-19
Filing date: 2009-03-19
Publication date: 2009-09-24
Also published as: SG155795A1; WO2009116957A3

Abstract

A device (100) for sensing an acoustic signal comprises a backplate of electrically conductive material, with the backplate defining at least one channel, and a diaphragm (21 ) of electrically conductive material that is connected to, and insulated from the backplate. The diaphragm defines a flexible member having a top side (98) and a bottom side (99) and the flexible member cooperates with the backplate to define an air gap. A first bond pad is formed on the diaphragm and a second bond is formed on the backplate. The bottom side of the flexible layer is provided with a water resistant layer (50), and the backplate, diaphragm, first bond pad and second bond pad combine to form a sensor for sensing the acoustic signal.

Description

WATER RESISTANT ULTRA-LOW PRESSURE SENSOR

PRIORITY STATEMENT

[0001] This application claims priority to Singapore patent application SG 200802224-6, filed on March 19, 2008.

FIELD OF THE INVENTION

[0002] The present invention relates to a sensor, particularly a water resistant ultra- low pressure sensor and method for the fabrication of same. In particular, the invention relates to a water resistant ultra-low pressure sensor for acoustic applications, for example in the form of a silicon microphone, and a method for the fabrication of such a sensor.

BACKGROUND OF THE INVENTION

[0003] A capacitive microphone device typically includes a diaphragm having an electrode attached to a flexible member and a backplate parallel to the flexible member attached to another electrode. The backplate is relatively rigid and typically includes a plurality of holes to allow air to move between the backplate and the flexible member. The backplate and flexible member form the parallel plates of a capacitor. Acoustic pressure on the diaphragm causes it to deflect which in turn changes the capacitance of the capacitor. The change in capacitance is processed by electronic circuitry to provide an electrical signal that corresponds to the change. [0004] Microelectro-mechanical systems (MEMS), including miniature microphones, are fabricated with techniques commonly used for making integrated circuits. Potential uses for MEMS microphones include microphones for hearing aids and mobile telephones, and pressure sensors for vehicles.

[0005] Many available MEMS microphones involve a complex fabrication process that includes numerous masking and etching steps. As the complexity of the fabrication process increases there is a greater risk of the devices failing the testing process and being unusable.

[0006] Applicant has developed a number of improvements in pressure sensors, such as silicon microphones. For example, International Publication WO 2004/105428 describes a silicon microphone of the above type that includes a flexible diaphragm that extends over an aperture. A backplate is also provided that combines with the flexible diaphragm to form the parallel plates of a capacitor for the microphone. However, this and many known examples are so-called "top-side" application sensors. That is, in use the sensor is packaged in a device, for example a mobile telephone, such that an acoustic signal travels through a hole in the device and is indirectly received by the sensor.

[0007] More recently, a sensor has been developed for a "bottom-side" application, the sensor having a high back volume for acoustic applications. In this case, the sensor is formed such that a backplate of the sensor has channels extending therethrough, and is located above a flexible member defined by a thin section of a diaphragm of the sensor. In bottom-side applications, the sensor is mounted on a printed circuit board (PCB) such that the sensor straddles an aperture in the PCB. Any signal passing through the aperture is in direct communication with the flexible member defined by the thin section of the diaphragm of the sensor.

[0008] More recently, there has been an emerging need for sensors having the advantages of so-called "bottom-side" application sensors, but also having water proof or water resistant capability. Such sensors would have application in, for example, water resistant watches and water resistant mobile phones which require water resistant microphones.

SUMMARY OF THE INVENTION

[0009] In accordance with a first aspect, a device for sensing an acoustic signal comprises a backplate of electrically conductive material, with the backplate defining at least one channel, and a diaphragm of electrically conductive material that is connected to, and insulated from the backplate. The diaphragm defines a flexible member having a top side and a bottom side, and the flexible member cooperates with the backplate to define an air gap. A first bond pad is formed on the diaphragm, and a second bond pad is formed on the backplate. The bottom side of the flexible layer is provided with a water resistant layer, and the backplate, diaphragm, first bond pad and second bond pad combine to form a sensor for sensing the acoustic signal. [0010] From the foregoing disclosure and the following more detailed description of various embodiments it will be apparent to those skilled in the art that the present invention provides a significant advance in the technology of acoustic sensors. Particularly significant in this regard is the potential the invention affords for providing a water resistant sensor which can also withstand low pressure. Additional features and advantages of various preferred embodiments will be better understood in view of the detailed description provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1 shows a cross section view of a first wafer and a second wafer.

[0012] Fig. 2 is a cross section view of the first wafer and the second wafer after oxide layer deposition.

[0013] Fig. 3 is a cross section view of the first wafer following patterning and etching of a cavity.

[0014] Fig. 4 is a cross section view of the first wafer following patterning and etching of at least one contact cavity.

[0015] Fig. 5 is a cross section view of the first wafer following patterning and etching of at least one bond pad cavity. [0016] Fig. 6 is a cross section view of the first wafer and second wafer bonded together.

[0017] Fig. 7 is a cross section view of the first wafer bonded to the second wafer following patterning and etching to form a thin section of a diaphragm.

[0018] Fig. 8 is a cross section of an alternate embodiment with a handle wafer.

[0019] Fig. 9 is a cross section of an alternate embodiment with a glass wafer.

[0020] Fig. 10 is a cross section of an alternate embodiment with a hydrophobic layer deposited on the thin section of the diaphragm.

[0021] Fig. 11 is a cross section of an alternate embodiment (flipped 180°) with the oxide layer removed, leaving a ground surface.

[0022] Fig. 12 is a cross section of an alternate embodiment similar to Fig. 11 , but with a second wafer cavity added.

[0023] Fig. 13 a cross section view of another embodiment similar to Fig. 11 , wherein holes and a small cavity are formed in the second wafer.

[0024] Fig. 14 a cross section view of another embodiment similar to Fig. 13, wherein holes are extended through to an air gap to form channels. [0025] Fig. 15 shows a cross section view of the embodiment of Fig. 14, with bond pads.

[0026] Fig. 16 shows a cross section view of an embodiment of a sensor in accordance with one embodiment.

[0027] Fig. 17 is a cross section view of a device using the sensor of Fig. 16 where the sensor is mounted over an aperture.

[0028] Fig. 18 is a cross section view of a device in accordance with another embodiment where the device is provided with an equalisation hole.

[0029] Fig. 19 is a cross section view of a device in accordance with another embodiment where the device is provided with a waterproof coating.

[0030] Fig. 20 is a cross section view of a device in accordance with another embodiment where the device is provided with a waterproof coating and an equalisation hole.

[0031] Fig. 21 is a cross section view of a device in accordance with another embodiment where the device is provided with an equalisation chip.

[0032] It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the sensor as disclosed here, including, for example, the specific dimensions of the layers of the wafers, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to help provide clear understanding. In particular, thin features may be thickened, for example, for clarity of illustration. All references to direction and position, unless otherwise indicated, refer to the orientation illustrated in the drawings.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OFTHE INVENTION

[0033] It will be apparent to those skilled in the art, that is, to those who have knowledge or experience in this area of technology, that many uses and design variations are possible for the sensor disclosed here. The following detailed discussion of various alternative and preferred features and embodiments will illustrate the general principles of the invention with reference to a sensor suitable for use in mobile telephones, hearing aids and pressure sensors for vehicles. Other embodiments suitable for other applications will be apparent to those skilled in the art given the benefit of this disclosure.

[0034] Turning now to the drawings, Fig. 1 shows a cross section view of a first wafer 10 and a second wafer 11 to be used to fabricate a sensor. The first wafer 10 includes a first layer 12 of highly doped silicon, a second layer 13 of silicon substrate and an intermediate oxide layer 14. The first layer 12 may include p⁺⁺ doped silicon and the second layer 13 may include an n-type substrate. Alternatively, the first layer 12 may include an n⁺⁺ doped silicon and the second layer 13 may include a p-type substrate. The first layer 12 is of the order of 4 microns thick and the oxide layer 14 is of the order of 2 microns thick. The thickness of these layers will generally depend on the characteristics required for the sensor. The second layer 13 may be larger than the first layer 12 and the oxide layer 14. For example, the second layer 13 may be in the order of 400 to 600 microns thick.

[0035] The second wafer 11 is formed from silicon. The second wafer 11 is heavily doped and may be either p-type or n-type silicon. In certain embodiments, the second wafer 11 is formed from silicon. It will be readily apparent to those skilled in the art, given the benefit of this disclosure, that different silicon surfaces or structures may be used. The first wafer 10 has a first major surface 15 formed from the heavily doped silicon of the first layer 12 and a second major surface 16 formed from the silicon of the second layer 13. Likewise, the second wafer 11 includes a first major surface 17 and a second major surface 18 formed from the heavily doped silicon of the second wafer 11.

[0036] In fabricating the sensor, the first wafer 10 and the second wafer 11 can be initially processed separately before being bonded together and further processed. Fig. 2 shows the first wafer 10 and second wafer 11 after oxide layers 19 have been formed on the major surfaces of the wafers 10 and 11. An oxide layer 19 is typically formed on the major surfaces of the wafers 10 and 11 through thermal growth or a deposition process. Forming oxide layers 19 on both major surfaces 15-16 and 17- 18 of the first wafer 10 and second wafer 11 respectively reduces the risk of distorting the wafer that may occur if oxide were only formed on one major surface on each wafer. That being said, it is an alternative embodiment to that illustrated in Fig. 2, that an oxide layer 19 is only formed on the first major surface 15 of the first wafer 10 and the first major surface 17 of the second wafer 11. The thickness of the oxide layers 19 is less than the thickness of the first and second wafers 10 and 11.

[0037] Fig. 3 illustrates the first wafer 10 in which a cavity 20 has been patterned and etched. In particular, the cavity 20 has been patterned and etched through the oxide layer 19 on the first major surface 15 of the first layer 12 of the first wafer 10, and into the first layer 12 of the first wafer 10. In this step, a portion of the heavily doped silicon forming the first layer 12 is etched away to produce a thin section 21 of the heavily doped silicon of the first layer 12. The thickness of the thin section 21 will determine the properties of the sensor eventually fabricated as this thin section 21 of highly doped silicon will form the flexible member of the diaphragm of the sensor. A wet or dry silicon etch may be used here. In one embodiment a reactive ion etch (RIE) is used to form the cavity 20. Generally, the etch is a time etch. Therefore, the final thickness of the thin section or flexible member 21 of the diaphragm 41 is dependent on the etching time. Further, the desired shape of the cavity 20 will generally be dictated by the desired properties of the sensor.

[0038] Following etching of the cavity 20 into the first layer 12 of the first wafer 10, contact cavities 22, illustrated in Fig. 4, are patterned and etched into the first layer 12 of the first wafer 10 through the oxide layer 19. These cavities 22 extend through the first layer 12 to the oxide layer 14 of the first wafer 10. Again, any suitable etching process may be used to form the contact cavities 22. The contact cavities 22 electrically isolate the diaphragm from the backplate. Although there is a layer of oxide between the diaphragm and backplate, this oxide layer has the potential to form a parasitic capacitance. The contact cavities reduce the amount of parasitic capacitance, leading to better sensitivity and lower noise.

[0039] Referring to Fig. 5, at this stage a bond pad cavity 23 may optionally be formed by patterning and etching the oxide layer 19 formed on the first major surface 15 of the first layer 12 of the first wafer 10. This may again be achieved through any suitable etching process.

[0040] In Fig. 6, the first and second wafers 10 and 11 are bonded together. The major surfaces bonded together, via respective oxide layers 19, are the first major surface 15 of the first wafer 10 and the first major surface 17 of the second wafer 11. In one embodiment the wafers 10 and 11 are bonded together through their respective oxide layers 19 using fusion bonding. In bonding the wafers 10 and 11 together, an air gap 24 is formed between the wafers 10 and 11 corresponding with the cavity 20 formed in a previous etching step.

[0041] At Fig. 7, following bonding of the two wafers 10 and 11 , a cavity 25 is patterned and etched through the oxide layer 19 formed on the second major surface 16 of the first wafer 10, through the silicon of the second layer 13 of the first wafer 10 and through the intermediate oxide layer 14 of the first wafer 10. The cavity is formed in a position corresponding to the position of the air gap 24. Thus, the thin section 21 previously formed is exposed to the cavity 25.

[0042] If a support member, such as a glass wafer support, is desired, this may be applied as illustrated in Figs. 8 and 9. In this embodiment, the oxide layer 19 formed on the second major surface 16 of the first wafer 10 and a portion of the second major surface 16 are subjected to a grinding operation to thin the second layer 13 of the first wafer 10. This produces a ground surface, which is a handle wafer 26, on the first wafer 10. It should, however, be understood that any other suitable method for removal of the oxide layer 19 and thinning of the second layer 13 may be employed. After thinning of the second layer 13, a glass wafer 27 that has been previously prepared is bonded to the handle wafer 26 of the second layer 13. The glass wafer 27 includes a central aperture 28 that cooperates with the previously formed cavity 25. This ensures that the sensor will function correctly when fabrication is completed. If the glass wafer 27 is not provided with an aperture, one may be formed in the glass wafer 27. For example, if the glass wafer 27 is solid, this may itself be patterned and etched to provide the aperture 28. In such a case, a masking layer of chrome and gold may be deposited onto the glass wafer 27 and the aperture 28 may be formed by wet or dry etching, for example using HF. The glass wafer 27 may be formed from Borofloat™ glass manufactured by Schott, or a borosilicate glass such as Pyrex™ manufactured by Corning.

[0043] According to one embodiment of the invention, a water resistant layer is formed on the thin section 21 , more particularly on the bottom side of the thin section 21. With reference to Fig. 10, a thin layer 50 of a hydrophobic material is deposited on a bottom side 99 of the thin section 21 of the diaphragm by molecular vapor deposition. The thin section 21 of the diaphragm has a top side 98 opposite the bottom side 99 and in contact with the air gap 24. The thin layer 50 is a self assembled monolayer coating that provides a hydrophobic surface exhibiting a water contact angle of greater than 100 degrees and may be in the range of from 100 to 120 degrees, from 100 to 115 degrees, or from 100 to 112 degrees, 100 to 110 degrees, or from 100 to 105 degrees. As such, water will not wet the surface of the water resistant layer 50, but will roll off easily when the surface is tilted. The layer 50 in this particular embodiment is less than 20 Angstroms and most preferably less than 5 Angstroms or only a few Angstroms thick. As such, the layer 50 does not affect the performance or properties of the sensor.

[0044] The hydrophobic material may be a self-assembling monolayer (SAM). A specific example of the hydrophobic material is a SAM which is deposited by wet or vapor deposition techniques. More particularly, the water resistant layer may comprise one or more layers formed by Molecular Vapor Deposition ™ (MVD). In a particular embodiment, the water resistant layer may be a few monolayers formed by Molecular Vapor Deposition ™ (MVD). In particular, the water resistant layer may comprise two, three or four monolayers. In a particular embodiment, the water resistant layer may be formed from self assembled monolayers with low work of adhesion. A range of values for work of adhesion may be from 0.1 to 40 mJ/m². Examples of such self assembled monolayer coatings formed by MVD include but are not limited to dimethyldichlorosilane (DDMS), perfluorodecyltrichlorosilane (FDTS) and tridecafluoro-1 ,1 ,2,2-tetrahydrooctyltrichlorosilane (FOTS). In specific examples, the work of adhesion value for DDMS may be 3 mJ/m², 5mJ/m² for FOTS, and 36mJ/m² for FDTS coatings. Other suitable materials for the hydrophobic layer will be readily apparent to those skilled in the art given the benefit of this disclosure.

[0045] As used herein, the term "water resistant" includes within its scope resistance to permeation by water, for example through a layer. The term also includes substantial resistance to permeation of water, including water proofing, whereby permeation of water is prevented or substantially prevented. The resistance to permeation may be assisted with a mechanical barrier, and/or by a chemical barrier, for example using a hydrophobic material.

[0046] Fig. 11 shows the subassembly flipped so that top side 98 is now on the top of the paper. As illustrated in Fig. 11 , following etching of the cavity 25 in the second layer 13 of the first wafer 10, and optionally after bonding of the glass wafer 27 to the second layer 13, the second major surface 18 of the second wafer 11 and the oxide layer 19 formed on it are subjected to grinding. This leaves a ground surface 29 of the second wafer 11 exposed. Optionally a cavity 30 may be formed in the second wafer 11 by patterning and etching the ground surface 29 of the second wafer 11 as shown in Fig. 12. It will be appreciated that grinding of the second major surface 18 of the second wafer 11 and the oxide layer 19 may be conducted prior to etching of the cavity 25.

[0047] As shown in Fig. 13, a plurality of holes 31 can be patterned and etched into the highly doped silicon of the second wafer 11 in a region associated with the air gap 24 and, therefore, the thin section 21. A further small cavity 32 is also etched into the second wafer 11. This cavity 32 is associated with an air gap 33 formed by the bond pad cavity 23 when the first and second wafers 10 and 11 are bonded together. When the holes 31 and small cavity 32 are formed, a global etch is conducted such that the holes 31 extend through to the air gap 24 and the small cavity 32 extends through to the air gap 33. In effect, channels 34 are formed that extend through the second wafer 11 to the air gap 24, and a deeper cavity 35 is formed.

[0048] Referring to Figs. 14-15, following formation of the channels 34 by global etching, a shadow mask 36 is put in place over the second wafer 11 and bond pads 37 and 38 are deposited, for example by deposition of aluminium. A first bond pad 37 is deposited on an area of the first wafer 10 exposed through the cavity 35, while a second bond pad 38 is deposited on an area of the second wafer 11.

[0049] When fabrication is complete, a sensor 40 in accordance with one embodiments is provided as illustrated in Fig. 16. This includes a backplate 39 formed from the second wafer 11 that includes a plurality of channels 34. The plurality of channels 34 extend to an air gap 24 defined by the first wafer 10. More particularly, the air gap is defined by the thin section 21 of the diaphragm 41 and the backplate, and is seen to extend from the top side 98. The thin section 21 associated with the air gap 24 defines a flexible member of the diaphragm 41. Diaphragm 41 has the bottom side 99 and the top side 98 at thin section 21. A pair of bond pads 37 and 38 are associated with the first wafer 10 and second wafer 11 respectively. The bond pad 37 is formed on the diaphragm 41 and the bond pad 38 is formed on the backplate 39. It will be appreciated that the sensor is formed such that the backplate 39 and therefore the channels 34 extending through the backplate 39 are located above the flexible member defined by the thin section 21 in the orientation shown in Fig. 16. This advantageously creates the so-called "bottom side" application. [0050] Fig. 17 shows a device 100 having the sensor 40 mounted on a PCB 42 such that the sensor 40 straddles an aperture 43 in the PCB 42. That is, as shown in Fig. 17, the thin section 21 with the water resistant layer 50 is positioned directly over the aperture 43. As such, any signal passing through the aperture 43 is in direct communication with the flexible member defined by the thin section 21 of the diaphragm 41 of the sensor 40. The hydrophobic layer is attached to the bottom side 99 of the thin section 21 of diaphragm 41. The bond pads 37 and 38 are associated with wires 44 that may be connected with other components 45 of the device 100. A cap 46 of the device cooperates with the PCB 42 to define a back volume 47, and the sensor 40 is positioned in the back volume. The sensor 40 is advantageously mounted over the aperture 43. Therefore, the signal, designated by the arrow, can travel directly to the sensor 40 and in particular the flexible member of the sensor 40.

[0051] Bondpads 37, 38 are electrically connected to the diaphragm and backplate respectively. The diaphragm and backplate form a capacitor. When a pressure is sensed, the diaphragm will deflect and there will be a change in capacitance with respect to the backplate. This change in capacitance (an electrical signal) will be transmitted via the bondpads to a preamplifier. Wiring 44 connects the microphone sensor to the preamplifier whereby the signal will be converted, amplified or further processed. Wiring 44 is preferably formed of a conductive material such as aluminium or gold.

[0052] The materials forming the backplate 39 and diaphragm 41 may be any highly doped material, for example any p+ or n+ material. Preferably, the backplate is formed from a silicon wafer including an oxide layer on at least one side thereof, and the diaphragm is formed from a silicon-on-insulator (SOI) wafer including a layer of heavily doped silicon, a layer of silicon and an intermediate oxide layer. Alternatively, the diaphragm may be formed from doped polysilicon.

[0053] Fig. 18 shows an alternate embodiment of a device 110 wherein a sealing material 51 is provided between the glass wafer 27 of the sensor 140 and the PCB 42. In turn, the PCB 42 may then be mounted on a customer PCB 52 using a sealing material 51. Preferably the sealing material 51 is water resistant, so that in this configuration, the package will have water resistant capabilities with the sensor 140 being located within a water resistant or substantially waterproof space. Further protection may be had by extending hydrophobic layer 150 beyond the thin section 21 and down the glass wafer 27 to the sealing material 51 as shown so that the sealing material, cap 46 and the water resistant layer 150 cooperate to define a substantially waterproof space. To allow for some small amount of air to pass through to account for pressure changes and/or temperature changes, a small equalisation hole 96 may be provided in cap 46. Fig. 19 shows another embodiment of a device 120 having a sensor 240 where an equalisation hole is not used, and a hydrophobic layer 250 extends from the thin section 21 of the diaphragm 41 to the glass wafer 27, and from there around PCB 42 and cap 250, effectively entirely surrounding the sensor 240 with a hydrophobic layer.

[0054] Fig. 20 is another embodiment of a device 130 having a sensor 240 similar to Fig. 19, with the hydrophobic layer on the PCB 42 and cap 46, but with the addition of the equalisation hole 96. Fig. 21 is another embodiment of a device 160 similar to Fig. 18, but the equalisation hole in the cap is replaced with an equalisation chip 77 having a pathway 78, preferably a serpentine (non-straight) pathway. The chip 77 is mounted on the PCB 42 and the PCB has an equalisation access 79 aligned with the pathway 78. This design advantageous allows for the low airflow through the pathway but effective prevention of moisture entering the back volume 47.

[0055] The sensor and arrangements according to the invention may provide a number of advantages. For example, the positioning of the sensor on a PCB as described above may by design advantageously alleviate problems associated with moisture entering the package. The inclusion of a water resistant layer as described provides more specific water resistance to the arrangements. The sensor allows for arrangement having a large back volume. With regard to acoustic applications, back volume is important to the acoustic performance of a device as it affects sensitivity. The bottom side application method simply allows the total volume enclosed to be the back volume, greatly improving sensitivity. Also, with bottom side application, a hole can be punched in a front of the device, for example the front keypad area of a mobile phone, and with a hole drilled in the PCB sound can travel directly to the sensor. This shorter path of travel enables a lower device profile since no air channel is needed below the hole. Such features, together with water resistant properties, are considered to be most desirous.

[0056] From the foregoing disclosure and detailed description of certain preferred embodiments, it will be apparent that various modifications, additions and other alternative embodiments are possible without departing from the true scope and spirit of the invention. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to use the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

What is claimed is:

1. A device comprising, in combination: a backplate of electrically conductive material, the backplate defining at least one channel; a diaphragm of electrically conductive material that is insulated from the backplate, the diaphragm defining a flexible member having a top side and a bottom side, and the flexible member cooperating with the backplate to define an air gap; a first bond pad formed on the diaphragm; and a second bond pad formed on the backplate; wherein the bottom side of the flexible layer is provided with a water resistant layer, and the backplate, diaphragm, first bond pad and second bond pad combine to form a sensor for sensing an acoustic signal.

2. The device according to claim 1 further comprising a printed circuit board defining an aperture, wherein the diaphragm is mounted over the aperture.

3. The device according to claim 2 further comprising a cap attached to the printed circuit board, and cooperating with the printed circuit board to define a back volume, wherein the sensor is positioned in the back volume.

4. The device of claim 3 wherein the cap has an equalisation hole on the cap.

5. The device of claim 4 further comprising a wafer attached to the PCB with a sealing material, wherein the water resistant layer extends along the wafer to the sealing material.

6. The device of claim 5 wherein the water resistant layer further extends around the printed circuit board and the cap.

7. The device of claim 3 further comprising a wafer attached to the printed circuit board with a sealing material, wherein the water resistant layer extends along the wafer to the sealing material and further extends around the printed circuit board and the cap.

8. The device of claim 3 further comprising an equalisation access formed in the printed circuit board, and an equalisation chip mounted on the printed circuit board defining an equalisation pathway aligned with the equalisation access.

6. The device of claim 1 wherein the water resistant layer is a self assembling monolayer (SAM) having a thickness of less than 10 Angstroms.

7. The device of claim 1 wherein the diaphragm is insulated from the backplate by an oxide layer.

8. The device of claim 1 wherein the backplate is formed from a silicon wafer and further comprises an oxide layer.

12. The device of claim 1 , wherein the diaphragm is formed from an silicon-on- insulator wafer having one of a layer of heavily doped silicon, a layer of silicon and an intermediate oxide layer, and a layer comprising a doped polysilicon wafer.

13. The device of claim 1 further comprising a glass wafer bonded with the printed circuit board.

14. The device of claim 1 wherein the backplate includes a cavity extending above the plurality of backplate holes.

15. A method of manufacturing a sensor including: providing a first wafer including a layer of heavily doped silicon, a layer of silicon and an intermediate oxide layer, the layer of heavily doped silicon defining a first major surface of the first wafer and the layer of silicon defining a second major surface of the first wafer; providing a second wafer of heavily doped silicon having a first major surface and a second major surface; forming a layer of oxide on at least the first major surface of the first wafer; forming a layer of oxide on at least the first major surface of the second wafer; patterning and etching a cavity through the oxide layer on the first major surface of the first wafer and into the layer of heavily doped silicon of the first wafer; patterning and etching contact cavities through the oxide layer on the first major surface of the first wafer and through the layer of heavily doped silicon of the first wafer; bonding the first major surface of the first wafer to the first major surface of the second wafer such that the cavity formed in the first major surface of the first wafer defines an air gap between the first wafer and the second wafer; patterning and etching a cavity into the layer of silicon defining the second major surface of the first wafer thereby forming a flexible member from the layer of heavily doped silicon of the first wafer, the flexible member being associated with the air gap formed between the first wafer and the second wafer and having an enclosed top side and an exposed bottom side; forming a water resistant layer on the exposed bottom side of the flexible member; thinning the second wafer at its second major surface; patterning and etching a plurality of holes in the second major surface of the second wafer, the plurality of holes being associated with the air gap formed between the first wafer and the second wafer; and forming at least one bond pad on the layer of heavily doped silicon of the first wafer and at least one bond pad on the second wafer.

16. A method of manufacturing a sensor according to claim 15, wherein the water resistant layer is a monolayer formed by molecular vapor deposition.

17. A method of manufacturing a sensor according to claim 15, including bonding a support member to the second major surface of the first wafer at any stage after patterning and etching of the cavity into the layer of silicon defining the second major surface of the first wafer.

18. A method of manufacturing a sensor according to claim 15, including patterning and etching a cavity in the second major surface of the second wafer prior to the step of patterning and etching the plurality of holes in the second major surface of the second wafer.