Patent Publication Number: US-2018035190-A1

Title: Mems device and process

Description:
REFERENCE TO PREVIOUSLY FILED APPLICATIONS 
     This application is a continuation of U.S. Non-provisional patent application Ser. No. 14/430438, filed Mar. 23, 2015, which is a  371  application of International Application No. PCT/GB2013/052459, filed Sep. 19, 2013, which claims priority to U.S. Provisional Patent Application Ser. No. 61/704876, filed Sep. 24, 2012, and United Kingdom Patent Application No. 1217010.6, filed Sep. 24, 2012, all of which are incorporated by reference herein in their entirety. 
    
    
     FIELD OF DISCLOSURE 
     This invention relates to a micro-electro-mechanical system (MEMS) device and process, and in particular to a MEMS device and process relating to a transducer, for example a capacitive microphone. 
     BACKGROUND 
     Various MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephones and portable computing devices. 
     Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between the electrodes. In the case of output transducers, the membrane is moved by electrostatic forces generated by varying a potential difference applied across the electrodes. 
       FIGS. 1 a  and 1 b    show a cross section and a cut-away perspective view, respectively, of known capacitive MEMS microphone devices  100 . The capacitive microphone device  100  comprises a membrane layer  101  which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode  102  is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode  103  is mechanically coupled to a generally rigid structural layer or back-plate  104 , which together form a second capacitive plate of the capacitive microphone device. In the example shown in Figure la the second electrode  103  is embedded within the back-plate structure  104 . 
     The capacitive microphone is formed on a substrate  105 , for example a silicon wafer which may have upper and lower oxide layers  106 ,  107  formed thereon. A cavity  108  in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is provided below the membrane, and may be formed using a “back-etch” through the substrate  105 . The substrate cavity  108  connects to a first cavity  109  located directly below the membrane. These cavities  108  and  109  may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes  102  and  103  is a second cavity  110 . 
     The first cavity  109  may be formed using a first sacrificial layer during the fabrication process, i.e. using a material to define the first cavity which can subsequently be removed, and depositing the membrane layer  101  over the first sacrificial material. Formation of the first cavity  109  using a sacrificial layer means that subsequent etching of the substrate cavity  108  does not play any part in defining the diameter of the membrane. Instead, the diameter of the membrane is defined by the diameter of the first cavity  109  (which in turn is defined by the diameter of the first sacrificial layer) in combination with the diameter of the second cavity  110  (which in turn may be defined by the diameter of a second sacrificial layer). The diameter of the first cavity  109  formed using the first sacrificial layer can be controlled more accurately than the diameter of a back-etch process performed using a wet-etch or a dry-etch. Etching the substrate cavity  108  will therefore define an opening in the surface of the substrate underlying the membrane  101 . 
     A plurality of holes, hereinafter referred to as bleed holes  111 , connect the first cavity  109  and the second cavity  110 . 
     As mentioned the membrane may be formed by depositing at least one membrane layer  101  over a first sacrificial material. In this way the material of the membrane layer(s) may extend into the supporting structure, i.e. the side walls, supporting the membrane. The membrane and back-plate layer may be formed from substantially the same material as one another, for instance both the membrane and back-plate may be formed by depositing silicon nitride layers. The membrane layer may be dimensioned to have the required flexibility whereas the back-plate may be deposited to be a thicker and therefore more rigid structure. Additionally various other material layers could be used in forming the back-plate  104  to control the properties thereof. The use of a silicon nitride material system is advantageous in many ways, although other materials may be used, for instance MEMS transducers using polysilicon membranes are known. 
     In some applications, the microphone may be arranged in use such that incident sound is received via the back-plate. In such embodiments a further plurality of holes, hereinafter referred to as acoustic holes  112 , are arranged in the back-plate  104  so as to allow free movement of air molecules, such that the sound waves can enter the second cavity  110 . The first and second cavities  109  and  110  in association with the substrate  108  allow the membrane  101  to move in response to the sound waves entering via the acoustic holes  112  in the back-plate  104 . In such instances the substrate cavity  108  is conventionally termed a “back volume”, and it may be substantially sealed. 
     In other applications, the microphone may be arranged so that sound may be received via the substrate cavity  108  in use. In such applications the back-plate  104  is typically still provided with a plurality of holes to allow air to freely move between the second cavity and a further volume above the back-plate. 
     In other applications the microphone may be arranged such that, some sound components may be received via the substrate cavity  108  and other sound components may be received via the back-plate  104 . 
     In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium position. The distance between the lower electrode  102  and the upper electrode  103  is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown). The bleed holes allow the pressure in the first and second cavities to equalise over a relatively long timescales (in acoustic frequency terms) which reduces the effect of low frequency pressure variations, e.g. arising from temperature variations and the like, but without impacting on sensitivity at the desired acoustic frequencies. 
     The transducer shown in  FIG. 1  is illustrated with substantially vertical side walls supporting the membrane layer  101  in spaced relation from the back-plate  104 . Given the nature of the deposition process this can lead to a high stress concentration at the corners formed in the material layer that forms the membrane. Sloped or slanted side walls may be used to reduce the stress concentration. Additionally or alternatively it is known to include a number of support structures such as columns to help support the membrane in a way which reduces stress concentration as illustrated in  FIGS. 2 a , 2 b  and 2 c   .  FIG. 2 a    shows a transducer  200  in plan view, but omitting the uppermost part of the back-plate layer  104  for clarity.  FIGS. 2 b  and 2 c    illustrate the periphery of a MEMS microphone structure (along the line A-A′ shown in  FIG. 2 a   ) in perspective and cross sectional views respectively, where similar components are identified by the same numerals as used in  FIG. 1 . 
     In this example the MEMS device  200  is formed with a plurality of support structures  201 , which in this example are formed as supporting columns, arranged around the periphery of the membrane. The columns are formed by patterning the first sacrificial material used to define the first cavity  109  such that the substrate  105  is exposed in a number of areas before depositing the material forming the membrane layer  101  ( FIG. 2 b    shows one membrane layer being deposited directly on the substrate but it will be appreciated that that there may be various intermediate layers on the substrate and the membrane may be formed by depositing multiple membrane layers). Likewise the second sacrificial material used to define the second cavity  110  is patterned so that membrane layer  101  is exposed in the same areas prior to depositing the material of the back-plate layer. This results in a plurality of columns being formed around the periphery of the membrane, as shown in  FIG. 2 a   , which provide support to the membrane but with a reduced stress concentration compared to the arrangement shown in  FIG. 1 . The columns are preferably formed with a stepped profile and/or slanted side walls to minimise stress. This process can lead to dimples in the upper surface of the back-plate layer in the area of the columns. 
       FIG. 2 a    also shows that a conductive track  204  extends from the membrane electrode  102  across the membrane to a contact point outside the suspended portion of the membrane for electrical connection to a buffer amplifier or other circuitry either co-integrated on the same substrate or via pads and wire-bond or flip-chip connections to circuitry on other substrates in the same or another package. 
     MEMS transducers such as those shown in  FIGS. 1 and 2  may usefully be used in a range of devices, including portable devices. Especially when used for portable devices it is desirable that the MEMS transducers are sufficiently rugged to survive expected handling and use of the device. There is therefore a general desire to improve the resilience of MEMS devices. 
     SUMMARY 
     The present invention is therefore concerned with improving the robustness and/or resilience of MEMS devices. 
     Thus according to an aspect of the present invention there is provided a MEMS transducer comprising:
         at least one membrane layer supported so as to define a flexible membrane; and   a strengthening layer mechanically coupled to said membrane layer;   wherein the strengthening layer is disposed around the majority of a peripheral area of the flexible membrane but does not extend over the whole flexible membrane.       

     The strengthening layer may extend radially from a support structure supporting the membrane layer onto a flexible part of the membrane layer and may form part of a side wall of the support structure. The support structure may comprise a supporting column structure. 
     The membrane layer may be supported relative to a first surface of a substrate, the substrate having at least one substrate cavity therein defining an area of opening in the first surface of the substrate, wherein the perimeter of said area of opening is at least partly within an area underlying said flexible membrane. In such case the strengthening layer may be disposed such that the perimeter of the area of opening at least partly underlies the peripheral area of flexible membrane having the strengthening layer. The strengthening layer may be disposed on the membrane layer in an area of the membrane which may make contact with the perimeter of the opening area if sufficiently deflected. 
     The strengthening layer may be continuous in a radial direction from within a support structure supporting the membrane layer to a point inward on the flexible membrane beyond the perimeter of said opening area. 
     The strengthening layer may be disposed around 75% or more of the perimeter of the membrane or around substantially the entire perimeter of the membrane. 
     Radially inside the strengthening layer may be a circumferential area of flexible membrane, the majority of which is free of the material of the strengthening layer. Around a majority of the perimeter of the flexible membrane, there may be a region radially inside the strengthening layer which is free of the material of the strengthening layer. 
     The strengthening layer may comprises a layer of material which has a greater elasticity, a greater plasticity and/or a greater ductility than the material of the membrane layer and/or which is in greater tensile stress than the material of the membrane layer. 
     The strengthening layer may comprise a layer comprising at least one of the group of: titanium, aluminium, copper and gold or an alloy thereof. 
     The strengthening layer may comprise a plurality of layers of different materials having different materials characteristics. Said material characteristics may comprise at least one of: elasticity, plasticity, ductility, tensile stress, yield stress and Young&#39;s modulus. 
     A first strengthening layer may be coupled to the membrane layer in said peripheral area and the transducer further comprises a second strengthening layer having a different geometry on the membrane layer to the first strengthening layer. 
     The thickness of the strengthening layer may be between about 30 and 100 nm inclusive. 
     The membrane structure may comprise a membrane electrode and the strengthening layer may be formed from the same material as the membrane electrode. The material forming the membrane electrode may be discrete from the strengthening layer disposed in the peripheral area of the membrane. A first area of first material may be disposed so as to form the membrane electrode and a second area of first material disposed to form said strengthening layer wherein said second area substantially surrounds said first area. Between the first and second areas there may be a third area which is mainly devoid of any first material. 
     The strengthening layer may be disposed on the opposite side of the membrane layer to a substrate. The transducer may also comprise a back-plate structure wherein the membrane layer is supported to be flexible with respect to said back-plate structure. 
     The transducer may be a capacitive sensor such as a microphone. The transducer may comprise readout circuitry. The transducer may be located within a package having a sound port. The transducer may be implemented in an electronic device which may be at least one of: a portable device; a battery powered device; an audio device; a computing device; a communications device; a personal media player; a mobile telephone; a games device; and a voice controlled device. 
     In a further aspect the invention provides a method of fabricating a MEMS transducer having a flexible membrane, the method comprising: 
     forming a membrane layer to form at least part of said flexible membrane; and 
     forming a strengthening layer mechanically coupled to an area of said membrane layer that corresponds to a peripheral area of the flexible membrane; 
     wherein the strengthening layer does not extend over the whole of the flexible membrane. 
     The method may be used to form transducer according to any of the embodiments discussed above. In particular the strengthening layer may extend radially from an area on the membrane layer that corresponds to the membrane support onto an area that corresponds to the flexible part of the membrane. The method may involve forming at least one side wall structure to support the flexible membrane wherein part of the strengthening layer forms part of the at least one side wall structure. The method may involve forming the membrane layer so as to be suspended over a first surface of a substrate and forming at least one cavity in the substrate to define an area of opening in the first surface of the substrate, wherein the perimeter of the opening area underlies the area of strengthening layer. 
     Forming the strengthening layer may comprise depositing a layer of first material onto the membrane layer and patterning the layer of first material to form the strengthening layer. The method may also involve forming a patterned layer of first sacrificial material on a substrate; forming the membrane layer over the first sacrificial material and the substrate; and forming the strengthening layer on the membrane layer in an area corresponding to the peripheral area of the first sacrificial material and area where the membrane layer contacts the substrate. 
     The method may involve forming a membrane electrode mechanically coupled to the membrane layer, wherein the material of the strengthening layer is the same material as the membrane electrode. Forming the strengthening layer may involve forming a layer of first material on the membrane layer and patterning the layer of first material to form the membrane electrode and the strengthening layer. 
     In a further aspect the invention provides a MEMS transducer comprising: 
     a flexible membrane suspended over a first surface of a substrate, 
     the substrate having one or more substrate cavities, the one or more substrate cavities defining an opening area in the first surface; 
     wherein a peripheral area of the suspended membrane carries a strengthening layer, the strengthening layer overlying the perimeter of the opening area; and 
     a non-peripheral area of the surface of the membrane is devoid of the material of the strengthening layer. 
     In a yet further aspect there is provided a MEMS transducer comprising: 
     a membrane structure comprising: 
     a flexible membrane; 
     a first area of first material disposed so as to form a membrane electrode; and 
     a second area of first material disposed to form a strengthening layer for said flexible membrane; 
     wherein the second area substantially surrounds the first area and between the first and second areas there is a third area, the majority of which is devoid of any first material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example only, with reference to the accompanying drawings, of which: 
         FIGS. 1 a  and 1 b    illustrate known capacitive MEMS transducers in section and cut-away perspective views; 
         FIGS. 2 a , 2 b  and 2 c    illustrate plan, sectional and perspective views of another known capacitive MEMS transducer; 
         FIGS. 3 a  and 3 b    illustrate how a high pressure event may affect the membrane; 
         FIGS. 4 a  and 4 b    illustrate an embodiment of a MEMS transducer according to the present invention; 
         FIG. 5  illustrates the effect of a high pressure impulse on an embodiment of the present invention; 
         FIG. 6  illustrates a cross section of further embodiment of the present invention; 
         FIG. 7  illustrates a plan view of a yet further aspect of the invention; and 
         FIGS. 8 a - f    illustrate one suitable process for forming a MEMS transducer according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As described above MEMS transducers such as shown in  FIGS. 1 and 2  may be usefully employed in a variety of different devices and increasingly are becoming popular for use in portable electronic devices such as mobile telephones, mobile computing devices and/or personal media players and the like. 
     To be useful for use in portable electronic devices such transducers should be able to survive the expected handling and use of the portable device, which may include the device being accidentally dropped. 
     If a device such as a mobile telephone is subject to a fall, this can result not only in a mechanical shock due to impact but also a high pressure impulse incident on a MEMS transducer. For example, a mobile telephone may have a sound port for a MEMS microphone on one face of the device. If the device falls onto that face, some air may be compressed by the falling device and forced into the sound port. This may result in a high pressure impulse incident on the transducer. It has been found that in conventional MEMS transducers of the form described above high pressure impulses can potentially lead to damage of the transducer. 
     Referring again  FIGS. 2 a -2 c   , as previously described, a MEMS transducer  200  may have a membrane layer  101  and a back-plate layer  104  formed so that a membrane is suspended above a surface of a substrate  105  to define a first cavity  109  and the back-plate  104  is suspended above the membrane to form a second cavity  110 . Note as used herein the term substrate will be used to refer to the one or more layers of material above which the membrane is suspended. This may typically comprise a silicon wafer and may also include one or more deposited layers, possibly including layers of the same material used to form the membrane layer. 
     As mentioned above a sacrificial material may be used to define the dimensions of the first cavity and hence the dimensions of the membrane. As discussed sacrificial material may be deposited and patterned relatively accurately to provide good control over the membrane dimensions. A substrate cavity is also provided in the substrate  105 , typically by means of a back etch. To ensure that it is the dimensions of the first cavity  109  which determine the membrane dimensions, the substrate cavity is arranged to have a smaller diameter than the first cavity at the point  202  where the substrate cavity and first cavity meet, in other words the opening of the substrate cavity at the surface of the substrate has a smaller diameter than the first cavity. This means that in such a structure the membrane is suspended above a section of the substrate, indicated by arrow  203 , before reaching the opening of the substrate cavity, i.e. the opening of the substrate cavity  108  in the surface of the substrate is within the area of the flexible membrane. 
     The sacrificial material used to define the first and second cavities is dimensioned so as to provide a desired equilibrium separation between the membrane layer  101  and the substrate  105  and also between the membrane layer  101  and the back-plate  104  so as to provide good sensitivity and dynamic range in use. In normal operation the membrane may deform within the volume defined by the first and second cavities without contacting the back-plate and/or substrate  105 . 
     In response to a high pressure impulse however the membrane layer  101  may exhibit a greater amount of deformation than usual.  FIG. 3 a    illustrates the situation where the membrane has been deformed downwards following a high pressure event and  FIG. 3 b    shows the situation where the membrane has been displaced upwards. 
     Consider the situation where the microphone is arranged to receive incident sound from a sound port arranged above the back-plate  104  and the sound port pressure suddenly increases, for instance as a result of air trapped when the device falls being forced into the sound port. This may result the pressure in the second cavity  110  being significantly greater than the pressure in the first cavity  109 , displacing the membrane downwards to greater extent than is usual. This may result in a relatively large stress at point  301  where membrane layer  101  forms part of the sidewall of supporting structure  201  and, in some instances, delamination of the membrane layer from the rest of the sidewall structure. Further, if the pressure difference is great enough the membrane may make contact with the substrate  105  at the edge of the substrate defined by the side wall  202  of the opening of substrate cavity  108 . Typically the edge of the substrate at the location of the opening of the substrate cavity has a relatively sharp angle and thus the membrane may be deformed round this edge, leading to a large stress concentration at this point  302 . 
     As mentioned previously the membrane layer  101  will typically be formed from one or more thin layers of semiconductor material, such as silicon nitride. Whilst such a material can be flexible when subject to even stresses, if there is a significant localised out-of-plane stress such as may be introduced into the membrane at point  302  by contact with the edge of the opening of the substrate cavity  108 , the membrane material can be relatively brittle. Thus contact between the membrane and the edge of the opening of the substrate cavity in this way can lead to damage such as cracking of the membrane. 
     As the high pressure caused by trapped air may persist for a relatively long time, the pressure in the first and second cavities may equalise by virtue of the bleed holes (not shown in  FIG. 2 or 3 ) discussed above with relation to  FIG. 1 . Thus the pressure in the first cavity, and substrate cavity, may increase until the pressures are equalized. However once air is no longer being forced into the sound port the pressure in the sound port will reduce quite quickly and, as typically the back-plate has a low acoustic impedance, the pressure in the second cavity will quickly reduce. At this point the pressure in the first cavity may be significantly greater than the pressure in the second cavity and thus the membrane may be deformed upwards, again to a greater extent than may usually be in the case. Again this may lead to a significant stress in region  301  where the membrane layer  101  meets the sidewall of the supporting structure. If the pressure difference is large enough the membrane may be displaced far enough to contact the back-plate  104 . This may limit the amount of travel of the membrane as compared with the situation shown in  FIG. 3 a    but again this may introduce stress into the membrane layer at the point  303  where it contacts the back-plate  104 . 
     It should be appreciated that both of these situations can also occur when sound is received via the substrate cavity  108  but in the opposite order. Whilst both situations may lead to damage of the membrane it is believed that the situation shown in  FIG. 3 a    is more likely to lead to damage. 
     To reduce the likelihood of damage in such high pressure situations embodiments of the present invention provide MEMS transducers having at least one membrane layer supported so as to define a flexible membrane and a strengthening layer mechanically coupled to the membrane layer. The strengthening layer is disposed around the majority of a peripheral area of the flexible membrane but does not extend over the whole flexible membrane. 
     The use of a strengthening layer mechanically coupled to peripheral area of the membrane layer helps avoid damage to the membrane. The strengthening layer will act to reinforce the peripheral area of the membrane and can aid in stiffening the membrane at this point. The strengthening layer also helps distribute any stress in the membrane layer. The strengthening layer can thus ensure that the peak stress experienced by the membrane in the peripheral area for a given acoustic overload is reduced and is thus less likely to exceed the yield stress of the membrane in this area, with thus less likelihood of damage. 
     The strengthening layer may be coupled to the membrane layer in any convenient arrangement, i.e. it may be disposed on top of, or underneath, a membrane layer, or sandwiched between two membrane layers. There may be a plurality of strengthening layers, some or all disposed on top of each other in some areas of a membrane. 
     The strengthening layer is disposed around a peripheral area of the membrane, i.e. towards the outer edge of the flexible membrane. Thus the strengthening layer is provided in the vicinity of at least some of the possible points of high stress identified above, i.e. those points of the membrane surface which may experience a high stress concentration when the membrane is subject to a large deformation. To ensure that the strengthening layer provides sufficient strengthening to the membrane the strengthening layer is provided around a majority of the periphery of the membrane. By majority is meant more than  50 % of the perimeter of the membrane. 
     The strengthening layer is not provided over the whole of the membrane however, which thus means the strengthening layer does not have any significant negative impact on the performance of the transducer. For example disposing the strengthening layer over a large portion of a microphone membrane might adversely affect its flexibility, and hence reduce the acoustic sensitivity of the microphone. Embodiments of the present invention thus provide strengthening of the membrane in areas where it is advantageous, i.e. locations on the membrane that may be subject to particularly high stress in response to a large pressure difference acting on the membrane. 
       FIGS. 4 a  and 4 b    illustrates plan and section views of one embodiment of a MEMS transducer  400  according to an embodiment of the invention. The transducer  400  has a similar structure to that described above with reference to  FIG. 2  and similar features are identified using the same reference numerals. 
     In the embodiment shown in  FIG. 4  there is an strengthening layer  401  which is disposed on top of the membrane layer  101 . 
     In this embodiment the strengthening layer is disposed on top of the membrane layer  101  as a continuous layer around the entire periphery of the membrane. The strengthening layer  401  is disposed on the part of the membrane layer  101  which forms part of the support columns  201  and extends inward on the flexible membrane beyond the edge  202  of the substrate cavity—or more particularly beyond the point at which the membrane layer  101  may make contact with the edge  202  of the opening of the substrate cavity in a high pressure situation. Thus the perimeter of the opening of the substrate cavity in the substrate surface underlies the strengthening layer. In other words the strengthening layer partly overhangs the substrate cavity. 
     The material of the strengthening layer advantageously has a greater elasticity than the material of the membrane layer and/or may be in greater tensile stress than the material of the membrane layer. Such a strengthening layer can help re-distribute the stress experienced by the membrane layer, especially in a membrane layer formed from material such as silicon nitride. The strengthening layer may additionally or alternatively comprise a material which has a greater plasticity than the material of the membrane layer, for example the strengthening layer may comprise a material with a greater ductility than the material of the membrane. The strengthening layer may thus help the membrane layer to flex without damage and can help reduce the stress within the membrane layer when subject to high pressures/large membrane deflection. The strengthening layer may also reduce the chance of catastrophic failure. 
       FIG. 5  illustrates a similar situation to that shown in  FIG. 3 a   , where the membrane is deflected downwards by a relatively large pressure difference, to the extent that the membrane layer  101  contacts the substrate  105  at the edge  202  of the substrate cavity. In this instance however the presence of strengthening layer  401  at point  501  on the membrane layer, where the membrane layer becomes part of the side wall of the support structure  201 , helps reduce the stress experienced by the membrane layer at this part. The stress can be distributed throughout the strengthening layer  401  and, as the strengthening layer  401  also extends into the side wall structure, the stress can also be distributed to the support structure  201 . 
     The presence of strengthening layer  401  at point  502  on the membrane layer, where the membrane layer impacts on the edge  202  of the opening of the substrate cavity also helps strengthen the membrane layer and distribute stress at this point. This may reduce the amount of deflection of the membrane layer at this point in response to a larger than usual pressure difference acting on the membrane. The strengthening layer will also help provide resilience to mechanical shock. 
     The strengthening layer may also add resilience in the situation shown in  FIG. 3 b    where the membrane is deflected so as to make contact with the back-plate. At a certain pressure level the middle of the membrane (or the membrane electrode) may just about make contact with the back-plate. At this point the stress in the membrane may be relatively evenly distributed—apart from at the periphery of the membrane as discussed earlier. Thus the strengthening layer may help distribute stress experienced at the edge of the membrane. 
     In the embodiment shown in  FIGS. 4 a  and 4 b    the strengthening layer  401  is disposed to be continuous in a radial direction throughout the whole of the support structure and to extend inward on the membrane to a point beyond the limit of the opening of the substrate cavity, i.e. the first material will, in equilibrium position, overhang the opening of the substrate cavity. Conveniently the strengthening layer  401  overhangs the substrate cavity by a small amount, such that, as shown in  FIG. 5 , if the membrane is deflected to such an extent to contact the substrate, the point of contact corresponds to a location which is coupled to the strengthening layer. In some embodiments the strengthening layer may extend inward into the flexible membrane to a point further then the expected point of contact with the substrate cavity, for instance the strengthening layer may extend for of the order of 50 μm or so inwards past the likely point of contact. Extending the strengthening layer radially inward beyond the likely point of contact can provide advantageous in terms of bend radius of the membrane. 
     Having the strengthening layer extend continuously radially from within the support structure to overhang the substrate cavity (and thus be present at the point of possible contact between the membrane layer  101  and edge  202  of the substrate cavity) is particularly advantageous as it allows stress experienced at point  502  to be distributed within the support structure. 
     In some embodiments however it may be sufficient to dispose the strengthening layer at a first location of the membrane layer, where contact with another part of the transducer structure is possible, but where such strengthening layer does not extend as far as the support structure for the membrane. For example, as illustrated in  FIG. 6  there may be a strengthening layer  401   a  disposed in the area of the membrane which may contact the edge  202  of the substrate cavity in response to a high pressure differential. This strengthening layer  401   a  does not extend as far as the support structure  201 . It should be noted that this strengthening layer  401   a  is still disposed toward the outer part of the flexible membrane and thus is disposed around the periphery of the membrane. 
     As mentioned previously in embodiments such as shown in  FIG. 4 , where there is a substrate cavity defining an opening in the substrate that is within the area of the flexible membrane, a downward deflection of the membrane such that the membrane contacts the edge of the opening in the substrate surface may be a particular source of failure for conventional MEMS transducers. Providing strengthening layer  401   a  even just in this location may therefore offer significant improvements in the resilience and robustness of MEMS transducers. Thus embodiments of the present invention may relate to MEMS transducers comprising an opening in the surface of substrate for a substrate cavity that is within the area of the flexible membrane, wherein the membrane comprises a strengthening layer coupled to only part of the membrane in the location where the membrane may, in use, contact the edge of the opening. 
     Alternatively there may be a strengthening layer  401   b  which extends from within the side wall of a support structure inwards onto at least part of the flexible membrane but which may not extend as far as the substrate cavity (if present). In some embodiments there may be two discrete strengthening layers, i.e. strengthening layers  401   a  and  401   b  may be both present as discrete strengthening layers. In this instance the strengthening layers  401   a  and  401   b  may have the same material characteristics but in some embodiments different materials and/or different thicknesses could be used for the different strengthening layers. 
     It will be noted that the embodiment shown in  FIG. 4  includes a membrane support in the form of pillars or columns as described above in relation to  FIG. 2 . The same principles apply to embodiments of MEMS transducers which simply have a continuous sidewall from which the membrane layer extends to form the flexible membrane, for example similar to sidewall arrangement shown in  FIG. 1 . For example in some embodiments the structure shown to the left of line  402  may not be present (in other words consider the structure to the right of line  402  being present at any cross-section through the edge of the transducer and not just in certain locations). It can clearly be seen that the same principles apply and the layer  402  may extend from within the continuous side wall inwards onto the membrane layer  101 . It will also be clearly appreciated that the membrane support may take other forms, e.g. vertical side walls etc. In general however all such support structures will involve a membrane layer extending from some sort of side wall structure and the point at which the membrane layer emerges from the side wall may experience a greater stress than other parts of the membrane when a large pressure differential causes a large deflection of the membrane. Thus one aspect of embodiments of the present invention is the use of a strengthening layer which is coupled to a membrane layer so that the strengthening layer extends from within a side wall structure to a flexible part of the membrane—but which is not disposed over the entire membrane. 
     Referring back to  FIG. 4 a    the strengthening layer  401  may be continuous around the entire periphery of the membrane. As shown this may result in a strengthening layer which, for a generally circular membrane, has an annular shape (for membranes of different shapes the layer  401  may generally correspond to the shape of the perimeter of the membrane). 
     In some embodiments however the strengthening layer may be divided into one or more sections around the periphery of the periphery of the membrane, for instance there may be a plurality of sections  401   c  separated by relatively small gaps as indicated by the dotted lines. 
     As discussed above the strengthening layer is provided around the majority of the periphery of the membrane, i.e. more than 50% of the perimeter. Advantageously the strengthening layer  401  may extend for more than 50% of the perimeter of the membrane. In some embodiments the strengthening layer may be provided around 75% or more of the perimeter of the membrane and possible around at least 90% of the perimeter of the membrane or substantially the entire perimeter of the membrane. Conveniently, if the strengthening layer is divided into different sections these may be relatively evenly spread around the perimeter of the membrane so as to provide strengthening evenly around the whole membrane. 
     In the embodiment shown in  FIG. 4 a    the material of the conductive track  204  for connecting the membrane electrode  102  to the contact pad  205  on the substrate is formed on top of the strengthening layer  401 . As one skilled in the art will appreciate this will result from the strengthening layer  401  being formed prior to deposition and patterning of the material of the conductive track  204 —which usually is a metal material and is usually part of the same material used to form the electrode  102 . Depositing the strengthening layer  401  prior to forming the conductive track ensures that the strengthening layer contacts the membrane layer even in the vicinity of the conductive track. In other embodiments however the material forming the strengthening layer may be deposited after the material used to form the conductive track  204  and thus may be deposited on top of conductive track  204 . 
     It will be seen that the strengthening layer  401  is only provided in the peripheral area of the flexible membrane, extending from the support structure to overhang the opening of the substrate cavity. The strengthening layer is not provided over the whole of the membrane layer and thus has a limited impact on transducer performance. The strengthening layer is thus disposed such that radially inside the strengthening layer is a circumferential area of flexible membrane, the majority of which is free of the material of the strengthening layer. In other words, around a majority of the perimeter of the flexible membrane, there is a region radially inside the strengthening layer which is free of the material of the strengthening layer. The strengthening layer is thus provided in an outer peripheral region of the membrane and a substantial area within the membrane is devoid of any strengthening layer. 
     The material(s) used to form the strengthening layer may be compatible with standard device fabrication processes and thus may advantageously be material(s) compatible with standard CMOS processing. As mentioned the material of the strengthening layer may be chosen to have a greater plasticity, e.g. ductility, than the material of the membrane layer and/or be in greater tensile stress. Suitable materials include various CMOS compatible metals such as titanium, aluminium, copper and gold as well as alloys of such materials, e.g. such as aluminium nitride. The strengthening layer may comprise layers of one or more of these metals, e.g. for assisting adhesion to other structural layers. The strengthening layer may comprise a plurality of layers of different material, for example layers with different material characteristics. The different layers may comprise materials with deliberately different physical properties such as elasticity, plasticity, ductility, tensile stress, Young&#39;s modulus, and/or fracture stress point to attempt to optimise the properties of the resulting laminate sandwich to provide better ruggedness. 
     In some embodiments, there may be more than one strengthening layer provided at a given point on the membrane layer, i.e. there may be a layer of first material and also a layer of second material in the same location, the layers of first and second material together providing strengthening to the membrane layer. The first and second strengthening layers may be coterminous or there may be some areas where there is only first or second material. In other words there may be first and second strengthening layers having different geometries, i.e. having different patterns on the membrane layer. 
     The strengthening layer may be relatively thin. For instance, when using a metal as the strengthening layer a thickness of around 30-100 nm, for example about 60 nm may be sufficient. With a MEMS transducer using silicon nitride as the membrane layer the membrane layer may have a thickness of around 0.4 μm. In such an embodiment a metal strengthening layer of the order of 60 nm thick may be disposed as shown in  FIG. 4 b    to provide increased robustness of the transducer with no significant detrimental impact on performance 
     It may be possible in some fabrication processes to use thinner metal layers, for instance of the order of 30-40 nm or less. However in some fabrication processes the deposition process may not reliably result in a continuous metal layer at such relatively low thicknesses, possibly producing discrete islands of metal—reducing the effectiveness of the strengthening layer. 
     Conveniently the strengthening layer  401  may have a relatively uniform layer thickness, at least in the part disposed on the flexible membrane, such that a single deposition step may be used for the material of the strengthening layer. However in some embodiments it may be beneficial for different parts of the strengthening layer  401  to have different thicknesses. 
     In one embodiment the material used to form the strengthening layer may conveniently be the same material as used to form the membrane electrode. In this embodiment the strengthening layer may be formed using the same general process steps as used to create the membrane electrode. In this embodiment no additional process steps may be required compared with the conventional fabrication process but the step of patterning the metal layer used to form the membrane electrode additionally comprises patterning the metal layer to form the strengthening layer as illustrated in  FIG. 7 . 
       FIG. 7  shows a plan view of a transducer (omitting the back-plate for clarity) which shows membrane electrode  102 —together with conductive track  204 , formed on the electrode as described previously. In this embodiment however the metal used to form the electrode  102  has also been patterned to form strengthening layer  701  around substantially the whole of the periphery of the membrane as discussed above. 
     In the embodiment shown in  FIG. 7  the metal layer is patterned so that the material of strengthening layer  701  is discrete from the electrode  102  and conductive track  204 , in other words the strengthening layer  701  is substantially isolated from the conductive track  204 . However this is not essential and in other embodiments the material could be continuous to form conductive track  204  and the material of the strengthening layer  701 . As the strengthening layer  701  is only provided around the periphery of the membrane it would not significantly impact on device performance. 
     It will of course be appreciated that the strengthening metal layer  701  will form part of the column support structures. This detail is omitted from  FIG. 7  for clarity. 
     In the embodiment shown in  FIG. 7  the material that forms the strengthening layer  701  thus also forms the membrane electrode and thus some parts of the membrane within the outer peripheral area are also provided with the same material as the membrane layer (i.e. electrode area  102 ). However it will be appreciated that there is a circumferential area radially inside the strengthening layer (i.e. that area between the strengthening layer  107  and electrode  102 ) the majority of which is free of material of the strengthening layer. Only a small part of the membrane in this area is covered in material used to form the strengthening layer, i.e. that section of the membrane which bears the conductive track  204 . 
     Embodiments of the invention therefore also provide a MEMS transducer having a membrane comprising a first area of first material disposed so as to form a membrane electrode and a second area of first material disposed to form a strengthening layer. The second area may substantially surround the first area and may for instance by the outer part of the membrane, but be largely separate therefore. In other words between the first and second areas there may be a third area which is mainly or substantially devoid of any first material. The first material in the second area may extend into the side walls support structure for the membrane. 
     As mentioned the use of the same material layer to form the strengthening layer and the membrane electrode is a particular advantage as it allows embodiments of the invention to be implemented using existing process flows with only a change to one mask layout.  FIGS. 8 a -8 f    illustrate one example of fabrication of a transducer such as shown in  FIG. 7 . 
     On substrate  105  first sacrificial material  801  may be deposited and shaped to define the first cavity. The sacrificial material may also be patterned to expose the substrate in certain areas which will form part of the membrane support. The sacrificial material may also be treated to ensure that the sides of the sacrificial material are sloped as will be understood by one skilled in the art. A membrane layer  101  is then deposited on the substrate so as to cover the first sacrificial material as shown in  FIG. 8 a   . Bleed holes in the may be formed in the membrane layer within the area of the membrane at this point and filled with sacrificial material. A metal layer  803  may then be deposited over the whole of the membrane layer as shown in  FIG. 8 b   . This metal layer may then be patterned, as shown in  FIG. 8 c   , by using standard etching techniques, to define a membrane electrode  102  and strengthening layer  701  (as well as a conductive track  204 —not shown in  FIG. 8 ). 
     If required the metal layer may also be patterned so as to avoid the bleed holes  802  provided in the membrane to reduce the effects of low frequency pressure changes (by allowing the pressure in the first and second cavities to equalize over time). However in some embodiments the bleed hole may lie outside the area covered by the strengthening layer and/or electrode. 
     There may be some additional small channels (not shown) provided to aid removal of the sacrificial material. At least some of the these channels may be provided in the outer part of the membrane layer, in which case the metal layer  701  may be patterned to exclude metal from the location of such channels 
     A second sacrificial layer  804  is then deposited and patterned on top of the membrane layer to define the second cavity as shown in  FIG. 8 d   . The back plate structure  104  may then be formed by depositing a first back-plate layer, depositing and patterning a metal layer to form a back-plate electrode  103  and depositing a further back-plate layer as shown in  FIG. 8 e   . 
     Finally, a substrate cavity may be etched through substrate  105  to the sacrificial material and the sacrificial material removed from the first cavity. Further, acoustic holes (not shown) would be etched into the back-plate layer and the sacrificial material may be removed via the acoustic holes, from the second cavity to leave a transducer structure as illustrated in  FIG. 8 f   . This provides a transducer structure with strengthening layer  701  without requiring any additional process steps and without impacting on transducer performance. 
     The foregoing description has focussed mainly on embodiments including a substrate cavity having an opening in the substrate which is at least partly within the area of the membrane. As discussed aspects of the present invention may be particularly beneficial for such embodiments due to the high stress can be encountered if the membrane is deflected enough to contact the substrate cavity. 
     However the principles of the present invention are applicable to other arrangements as well. For instance MEMS transducers where the substrate cavity defines the first cavity, or where the spacing between the membrane and substrate is sufficient that the membrane would never contact the substrate in use may still experience high stress at the point where the membrane layer emerges from the side wall support to form the flexible membrane and may benefit from a strengthening layer which extends from the support structure at least partly onto the part of the membrane layer which forms the flexible membrane. 
     Also it has been proposed, for embodiments having a substrate cavity with an opening which is smaller than the first cavity under the membrane, that various structures could be used to extend over at least part of the area of the substrate cavity and provide a limit to the amount of membrane deflection in response to a large pressure differential. In other words structures may be deliberately introduced for the membrane to contact when subject to the large pressure difference but which do not present to sharp corner that the edge of a substrate cavity presents. In such arrangements the membrane will make contact with some part of such structure(s) and thus may benefit from a layer of first material extending over the membrane to the point of contact. 
     In some embodiments there may therefore be more than one opening in the surface of the substrate leading to one or more substrate cavities. In some embodiments there may be separate substrate cavities which collectively form an acoustic volume for the transducer. The one or more cavities will therefore have an area of opening in the surface of the substrate, i.e. area defined by the outermost parts of the perimeters of the outermost cavity openings. If the perimeter of such area of opening is within the area of the flexible membrane then it is possible, on a large deflection, that the membrane makes contact with the edge of an opening, or part of the structure between or within the substrate cavities. Thus in some embodiments the strengthening layer is disposed on the membrane layer such that the perimeter of said area of opening at least partly underlies the peripheral area of flexible membrane having said strengthening layer. 
     Although the various embodiments describe a MEMS capacitive microphone, the invention is also applicable to any form of MEMS transducers other than microphones, for example pressure sensors or ultrasonic transmitters/receivers. 
     Embodiments of the invention may be usefully implemented in a range of different material systems, however the embodiments described herein are particularly advantageous for MEMS transducers having membrane layers comprising silicon nitride. 
     It is noted that the embodiments described above may be used in a range of devices, including, but not limited to: analogue microphones, digital microphones, pressure sensor or ultrasonic transducers. The invention may also be used in a number of applications, including, but not limited to, consumer applications, medical applications, industrial applications and automotive applications. For example, typical consumer applications include portable audio players, laptops, mobile phones, PDAs and personal computers. The invention may be used in voice controlled devices and may be implemented in home network controllers for audio or other domestic apparatus. Typical medical applications include hearing aids. Typical industrial applications include active noise cancellation. Typical automotive applications include hands-free sets, acoustic crash sensors and active noise cancellation. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.