Patent Publication Number: US-2018044167-A1

Title: Mems device and process

Description:
This invention relates to micro-electro-mechanical system (MEMS) devices and processes, and in particular to MEMS transducers and methods of fabrication, especially a MEMS capacitive microphone. 
     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. 
     Some transducers formed using MEMS fabrication processes comprise a flexible membrane with one electrode for read-out/drive deposited on the membrane and another electrode supported in spaced relation to the membrane electrode in a transverse direction, typically supported on a structural layer referred to as a back-plate. In the case of MEMS pressure sensors and microphones, read out is usually accomplished by measuring the capacitance between the electrodes. The membrane is moved by air pressure differences which causes the flexible membrane to move relative to the fixed back-plate and thus vary the capacitance. 
       FIGS. 1 a , and 1 b    show a cross section and perspective view, respectively, of a known capacitive MEMS microphone device  100  and  FIG. 1 c    shows a plan view of the membrane and membrane electrode thereof. The capacitive MEMS 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 membrane electrode  102  is mechanically coupled to the flexible membrane  101  to form a first capacitive plate of the capacitive microphone device. A back-plate electrode  103 , which is mechanically coupled to a generally rigid structural layer referred to as a back-plate layer  104 , forms a second capacitive plate of the capacitive microphone device. In the example shown in  FIG. 1 a    the back-plate electrode  103  is embedded within the back-plate structure  104  although other arrangements are possible, for instance where the back-plate electrode  103  is mechanically attached to a surface of the back-plate structure  104  in a traverse direction relative to the membrane electrode  102 . 
     In some applications, the microphone  100  may be arranged in use such that incident sound is received via the back-plate  104 . In such embodiments a plurality of holes  112 , hereinafter referred to as acoustic holes, are arranged in the back-plate  104  so as to allow free movement of air molecules, such that the sound waves can enter a cavity  110  between the membrane  101  and back-plate  104 . On the other side of the membrane a further cavity  109 , between the membrane and substrate, in association with a substrate cavity  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 acoustic holes to allow air to freely move between the second cavity and a further volume above the back-plate. In this case the substrate cavity serves as a front volume and the further volume serves as a back volume. 
     In use, in response to a pressure wave incident on the microphone  100 , the membrane  101  is deflected from its equilibrium or quiescent position. The distance between the membrane electrode  102  and the back-plate electrode  103  is correspondingly altered, giving rise to a change in capacitance between the two electrodes  102  and  103  that is subsequently detected by electronic circuitry (not shown). Bleed holes  111  through the membrane  101  allow the pressure in the cavities  108 / 109  and  110  to equalise over 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. 
     In some MEMS microphones the flexible membrane layer  101  is a thin layer of crystalline or polycrystalline material, although it may be formed in practice by several layers of material which are deposited in successive steps. The flexible membrane  101  may, for example, be formed from silicon nitride Si 3 N 4  or polysilicon. Crystalline and polycrystalline materials have high strength and low plastic deformation, both of which are highly desirable in the construction of a membrane. The membrane electrode  102  in such microphones is typically a thin layer of metal, e.g. aluminium, which, as illustrated in  FIG. 1 c   , is located in the centre of the membrane  101 , i.e. that part of the membrane which displaces the most. It will be appreciated by those skilled in the art that the membrane electrode may be formed by an alloy such as aluminium-silicon for example. The membrane electrode may, for example, cover approximately 40% of the central area of the membrane. One problem with such a material system however is that the metal of the membrane electrode  102  may undergo some plastic deformation with relatively high or repeated displacement, i.e. the metal of the membrane electrode may be deformed so it will not return to exactly the same starting position. Since the flexible membrane  101  and the membrane electrode  102  are mechanically coupled to one another this can lead to an overall change in the quiescent position of the flexible membrane  101  and/or a change in the stress properties of the overall membrane  101 / 102 . 
     Typically the membrane layer  101  and membrane electrode  102  may be fabricated so as to be substantially planar in the quiescent position, i.e. with no pressure differential across the membrane, as illustrated in  FIG. 1 a   . The membrane layer may be formed so as to be substantially parallel to the back-plate layer in this quiescent position, so that the membrane electrode  102  is parallel to the back-plate electrode  103 . As mentioned however due the properties of the metal membrane electrode  102  the overall membrane  101 / 102  may, over time, become permanently deformed from this quiescent configuration. 
       FIG. 2  illustrates the permanent deformation which can occur to the quiescent position, illustrated by the dashed line, of the membrane  101 / 102 . Through use, the metal electrode  102  has been plastically deformed leading to a change in stress distribution and warping of the whole membrane structure  101 / 102 . 
     It can be seen that the membrane electrode  102  in its quiescent position thus has a different spacing from the back-plate electrode  103  from the original quiescent position immediately after manufacture. This can lead to a DC offset in the measurement signal from such a transducer as the capacitance at the quiescent position is not the same. In addition this deformation may lead to change in the stress characteristics of the overall membrane  101 / 102 , which affects how the membrane  101 / 102  reacts to a given signal level which can lead to a change in sensitivity. 
     SUMMARY 
     Embodiments of the present invention relate to MEMS devices and processes that at least mitigate some of the disadvantages mentioned above. 
     Thus according to an aspect of the present invention there is provided a micro-electrical-mechanical system (MEMS) transducer, comprising: a flexible membrane; a membrane electrode coupled to said flexible membrane; and a back-plate, comprising at least one back-plate layer coupled to a back-plate electrode, having a plurality of holes in the back-plate electrode corresponding to a plurality back-plate holes through the back-plate; wherein said membrane electrode comprises at least one opening in the membrane electrode wherein, at least part of the area of said opening corresponds to the area of at least one back-plate hole, in a direction normal to the membrane, and there is no hole in the flexible membrane at said opening in the membrane electrode. 
     The area of each of the holes in the back-plate electrode may be greater than the area of each of said plurality of back-plate holes. 
     The flexible membrane may comprises a crystalline or polycrystalline material, such as silicon nitride. The flexible membrane may have intrinsic stress. The membrane electrode may comprise metal or a metal alloy, such as aluminium, possibly with silicon. 
     There may be a plurality of openings in the membrane electrode where, for each of said plurality of openings in the membrane electrode, at least part of the area of said opening corresponds to the area of at least one back-plate hole, in a direction normal to the membrane, and there is no hole in the flexible membrane at said opening in the membrane electrode. At least some of said plurality of openings in the membrane electrode may comprise a hole in said membrane electrode. At least some of said plurality of openings in the membrane electrode may be substantially the same shape and/or substantially the same size as at least some of said plurality of holes in the back-plate electrode. 
     The area at least some of said plurality of openings in the membrane electrode may align with the area of at least some of said plurality of holes in the back-plate electrode, in a direction normal to the membrane. The area of at least some of said plurality of openings in the membrane electrode may be greater than the area of at least some of said plurality of holes in the back-plate electrode and/or the area of at least some of said plurality of openings in the membrane electrode are smaller than the area of at least some of said plurality of holes in the back-plate electrode. The width of at least some of said plurality of openings in the membrane electrode may differ from the width of at least some of said plurality of holes in the back-plate electrode by up to 2 microns. 
     At least one of said plurality of openings in the membrane electrode may be generally circular in shape and/or at least one of said plurality of openings in the membrane electrode may be non-circular in shape. 
     In some instances at least one of said plurality of openings in the membrane electrode is not completely bounded by electrode material. 
     The distribution of the plurality of openings in the membrane electrode may be configured to provide even stress as the membrane flexes. 
     In some embodiments less than 20% of the stress in the flexible membrane-membrane electrode structure is due to said flexible membrane and/or less than 25% of the surface area of said flexible membrane is covered by electrode material. The outer perimeter of the electrode may enclose an area of membrane greater than 35% of the area of said membrane. 
     The membrane electrode may have a thickness of no greater than 100 nm. 
     The transducer may be a capacitive transducer and/or may be a microphone. The transducer may be packaged within a housing. In one embodiment the package may comprise: a substrate comprising a plurality of layers, with at least one layer comprising a conductive material and at least one layer comprising a non-conductive material wherein the MEMS transducer is electrically coupled to the substrate. There may be a plurality of electrical connections for electrically coupling the MEMS transducer to circuitry external to the package. The package may also have a cover mechanically coupled to the substrate wherein the MEMS transducer is enclosed within the volume created by the substrate and said cover; and a port for enabling signals to stimulate the MEMS transducer. 
     The MEMS transducer may be connected to an integrated circuit for read-out and/or drive of said transducer. 
     The MEMS transducer may be used in an audio device which may be at least one of: a portable device; a communications device; a computing device; a battery powered device; an audio player; a video player; a gaming device; a mobile telephone; a laptop, notebook or tablet computer. 
     The invention also provides a method of fabricating a micro-electrical-mechanical system (MEMS) transducer comprising:
         forming at least one membrane layer coupled to a membrane electrode to provide a flexible membrane in use;   forming at least one back-plate layer coupled to a back-plate electrode to provide a rigid back-plate in use having a plurality of back-plate holes through the at least one back-plate layer and back-plate electrode;   wherein forming at least one membrane layer coupled to a membrane electrode comprises forming a membrane electrode having at least one opening, wherein at least part of the area of said opening corresponds to the area of at least one back-plate hole, in a direction normal to the membrane, and there is no hole in the flexible membrane at said opening in the membrane electrode.       

     Forming the membrane electrode may comprise depositing a layer of membrane electrode material and patterning said layer of membrane electrode to provide said at least one opening. 
     In a further aspect there is provided a micro-electrical-mechanical system (MEMS) transducer, comprising: a flexible membrane; and an electrode coupled to said flexible membrane; wherein said electrode comprises a plurality of openings in the electrode wherein there is no hole in the flexible membrane at said openings in the electrode. 
     A further aspect provides a micro-electrical-mechanical system (MEMS) transducer, comprising: a flexible membrane comprising a first material; and a membrane electrode coupled to said flexible membrane comprising a second material; wherein the area of the first material within the outer boundary of the membrane electrode is greater than the area of the second material within the outer boundary of the membrane electrode. 
     The first material may occupy the whole of the area within the outer boundary of the membrane electrode. 
     In another aspect there is provided a micro-electrical-mechanical system (MEMS) transducer, comprising: a membrane comprising at least one flexible membrane layer and a membrane electrode; and a back-plate having a plurality of acoustic holes therein; wherein said membrane electrode has a plurality of openings that at least partly align with said acoustic holes and which do not form part of a hole through the membrane. 
     In a further aspect there is provided a micro-electrical-mechanical system (MEMS) transducer, comprising: a flexible membrane; a back-plate; comprising at least one acoustic hole, wherein said flexible membrane and said back-plate are spaced in relation to one another in a first direction which is substantially normal to the plane of said flexible membrane; and a membrane electrode coupled to said flexible membrane, wherein there is at least one area of absence of electrode material within said electrode which corresponds to said at least one acoustic hole, in a direction normal to the membrane. 
     The invention further provides a micro-electrical-mechanical system (MEMS) transducer, comprising: a flexible membrane; a membrane electrode coupled to said flexible membrane, wherein said membrane electrode has a plurality of membrane electrode openings; and a back-plate in spaced relation to said flexible membrane; said back-plate having a plurality of back-plate holes; wherein said plurality of membrane electrode openings and said plurality of back-plate holes are located in substantially the same place and are substantially the same size. 
     In a further aspect there is provided a micro-electrical-mechanical system (MEMS) transducer, comprising: a flexible membrane; and an electrode coupled to said flexible membrane; wherein said electrode is patterned with a plurality of openings. 
     The electrode may be patterned with said openings to reduce creep and/or stress relaxation of the electrode. 
    
    
     
       DRAWINGS 
       The invention will now be described by way of example only, with reference to the accompanying drawings, of which: 
         FIGS. 1 a , 1 b  and 1 c    illustrate known MEMS transducers in device section, plan views and membrane plan views; 
         FIG. 2  illustrates how a membrane may be plastically deformed; 
         FIG. 3  illustrates openings in a membrane electrode according to an embodiment of the invention; 
         FIGS. 4 a  and 4 b    illustrate openings in a membrane electrode according to further embodiments of the invention; 
         FIG. 5  illustrates the openings in the electrode membrane being in an offset configuration according to an embodiment of the invention; 
         FIG. 6  illustrates a plan view of a membrane electrode according to an embodiment of the invention showing the arrangement of the openings in the membrane electrode; 
         FIG. 7  illustrates another arrangement of openings in the membrane; 
         FIGS. 8 a , 8 b  and 8 c    illustrate further arrangements of openings in the membrane; 
         FIG. 9  illustrates an array of MEMS transducers situated in the same substrate; 
         FIG. 10  illustrates a MEMS package; and 
         FIGS. 11 a  and 11 b    illustrate further MEMS packages. 
     
    
    
     DETAILED DESCRIPTION 
     This invention will be described in relation to a MEMS transducer in the form of a MEMS capacitive microphone. It will be appreciated, however, that the invention is equally applicable to other types of MEMS transducers including capacitive-type transducers. 
     As mentioned above for MEMS sensors having a metal membrane electrode provided on a flexible membrane layer, especially a membrane layer which is a crystalline material, plastic deformation of the metal in use may mean that the quiescent position of the membrane and/or stress characteristics can change overtime with use. This can result in an unwanted DC offset and/or a change in sensitivity of the sensor and the subsequent quality of the acoustic signal being reproduced may be significantly degraded. 
     Typically the membrane electrode is relatively thin, for example of the order of about 60 nm or so, and using a thinner membrane electrode layer may not be possible or practical for conventional MEMS processes. Using a smaller diameter electrode on the membrane would reduce the amount of metal to undergo plastic deformation but would also result in a loss of sensitivity—as capacitance is linked to electrode area. This is undesirable, especially as the capacitance of a MEMS sensor is relatively low anyway. 
     Embodiments of the present invention relate to micro-electrical-mechanical system (MEMS) transducers comprising a membrane electrode coupled to a flexible membrane, and a back-plate comprising at least one back-plate layer coupled to a back-plate electrode, where the back-plate has a plurality of back-plate holes through the back-plate. The flexible membrane and the back-plate may be supported in spaced relation to one another so as to be substantially parallel (at the quiescent position). In embodiments of the present invention the membrane electrode comprises at least opening wherein, at least part of the area of the opening corresponds to the area of a back-plate hole in a direction normal to the membrane. In other words the area of at least part of the opening in the membrane electrode aligns (in a direction normal to the membrane) with at least part of the area of a back-plate hole. In embodiments of the invention there is no hole in the flexible membrane at said opening in the membrane electrode, i.e. the flexible membrane may be continuous in the area of the opening in the membrane electrode and the area of the membrane electrode is not part of a hole through the entire membrane. In some embodiments there may be a plurality of such openings in the membrane electrode. 
     The openings in the membrane electrode are areas devoid of substantially any material of the membrane electrode, and thus may form cavities, apertures or holes in the membrane electrode material, e.g. the metal. By providing such openings in the membrane electrode, the total amount of metal forming the membrane electrode can be reduced compared to a membrane electrode having a similar size diameter but without any such openings, i.e. the membrane electrode having the openings provides less coverage of the flexible membrane. 
     As previously stated the openings in the membrane electrode do not necessarily correspond to holes in the membrane and thus the openings can be seen as an area of absence of electrode material (but at least partly bounded by electrode material), where there is still continuous material of the flexible membrane, i.e. there is a hole in the membrane electrode material only and not the flexible membrane. 
     The openings in the membrane electrode may preferably be arranged so that these openings, i.e. the areas of absence of membrane electrode material, are at least partly aligned with the holes in the back-plate, e.g. acoustic holes. As the acoustic holes are present throughout the whole back-plate, at least some of the acoustic holes in the back-plate correspond, whether in whole or in part, to holes in the back-plate electrode, i.e. areas of absence of back-plate electrode. The openings in the membrane electrode and the holes in the back-plate electrode are aligned, partially or wholly, in a traverse direction, i.e. a direction normal to the membrane. As used herein the term normal to the membrane shall mean a direction which is substantially normal to the plane defined by the bound edges of the membrane. Obviously in use the membrane may deflect and the direction of the local normal to part of the membrane may vary, but the direction normal to the whole membrane can still be seen as the direction normal to the plane of including the fixed edges of the membrane. 
     In a MEMS capacitive sensor the capacitance developed between the back-plate electrode and membrane electrode will depend on the separation between the electrodes and also the area of the electrodes. It will be appreciated that the presence of the acoustic holes in the back-plate clearly means that there are a plurality of holes in the back-plate electrode. Thus, for a continuous membrane electrode such as illustrated in  FIGS. 1 a  and 1 c   , there will be areas of membrane electrode which are located, in a direction normal to the membrane, in the location of an acoustic hole in the back-plate. 
     As one skilled in the art will appreciate in a parallel plate capacitor which is charged/biased there will be an electrostatic field component running from one plate to the other in a direction perpendicular to the plates. However in the areas of the acoustic holes there is no back-plate electrode and thus no perpendicular field component. Embodiments of the present invention therefore provide openings in the membrane electrode in locations that generally correspond to the location of the holes in the back-plate and hence the back-plate electrode, in other words in areas such that the area of a back-plate hole overlaps, at least partly, with the area of an opening in the back-plate electrode. In such areas there would be no perpendicular field component, even if there were membrane electrode material present, because of the absence of back-plate electrode. Providing openings in the membrane electrode in such areas may therefore have a limited impact on the resulting capacitance of the sensor and hence on sensitivity of the sensor. 
     The advantage of having such openings in the membrane electrode is that less membrane electrode material can be used than otherwise would be the case, but without any significant detriment to the resulting capacitance. The result of less membrane electrode material, e.g. metal, being formed on the flexible membrane results in the amount of material that is susceptible to plastic deformation being reduced and the proportion of membrane material to electrode material being increased. Thus the resulting complete membrane structure is less likely to be deformed plastically with the attendant disadvantages, but without sacrificing sensitivity or signal to noise ratio. 
     In addition the openings in the membrane electrode can effectively break up the overall area of the membrane electrode into smaller areas. In other words the maximum continuous extent of membrane electrode in any direction may be relatively low because of the openings (and reduced compared to a membrane electrode without openings). For example consider that the openings comprise an array of holes in the membrane electrode. Between the holes may be narrow sections of membrane electrode. Narrower sections of metal may provide a better response to elongations, in terms of being less likely to deform the underlying membrane, than wider sections. Thus patterning the membrane electrode with openings can be advantageous not just in reducing the overall amount of membrane electrode but also providing a membrane electrode pattern that is less likely to result in deformation to the flexible membrane. Patterning of the membrane electrode with openings can therefore to reduce stress relaxation/creep, i.e. reduce the impact of metal creep. 
     This is particularly advantageous where the flexible membrane comprises a crystalline or polycrystalline material, such as one or more layers of silicon-nitride Si 3 N 4  and the membrane electrode comprises a metal, such as aluminium, titanium, gold or copper, or alloys thereof. In such material systems the flexible membrane layer may be formed so as to have a desired intrinsic stress and any plastic deformation of the membrane electrode may alter the response of the sensor. 
     A more exact explanation and analysis would include the effect of non-perpendicular fringing electrostatic fields that extend from the edge of a hole in the back-plate electrode to any membrane electrode area that corresponds to the area of the acoustic hole. However such fringing fields may only extend laterally for about 1-2 μm and the area of the acoustic holes may be much larger, for example of the order of about 10 μm or so in diameter, so this effect does not materially affect the argument. 
     It will therefore be appreciated that in embodiments of the invention the acoustic holes in the back-plate and the back-plate electrode correspond to the openings in the membrane electrode. By correspond it is meant that the position of the openings in the membrane will be substantially the same as the position of the holes in the back-plate electrode as viewed from a direction normal to the membrane (or back-plate electrode). Therefore were the areas of the acoustic holes to be projected onto the membrane in such a direction, the areas of the projected holes would at least partly map on to the area of the openings in the membrane electrode. 
     The sizes of the openings in the membrane electrode and acoustic holes in the back-plate, or more particularly the holes in the back-plate electrode, could, in some embodiments, be substantially the same size and possibly also the same shape. Thus the openings in the membrane electrode could correspond in size and location to the holes in the back-plate (or holes in the back-plate electrode). However the sizes and/or shape of the membrane openings and back-plate holes do not need to be same (and one could be several factors larger than the other), however, they still could correspond in terms of location, and in this sense, for example, several openings could underlie one hole. 
     Embodiments of the invention thus provide a plurality of openings in the membrane electrode which, at least partly, align with holes in the back-plate and hence holes in the back-plate electrode and which do not form part of a hole in the membrane. It will be appreciated however that there may, in addition, be other openings in the membrane electrode which do not align with any back-plate hole and/or which do form part of a hole through the membrane. For instance there may be one or more bleed holes, or other pressure relief hole, through the membrane and some may be provided in the area of the membrane electrode. 
       FIG. 3  illustrates one embodiment of the present invention.  FIG. 3  shows a membrane electrode  302  formed on a flexible membrane  301 . The membrane electrode  302  has a plurality of openings  313  in the electrode material  302  where there is no coverage of the membrane  301 . These openings (or areas of absence)  313  reduce the amount of electrode material  302  which is deposited on the membrane  301  (for a given diameter of electrode) and therefore increases proportion of membrane material to electrode material compared to the electrode without such openings. This in turn will lead to a membrane structure  301 / 302  which has reduced plastic deformation. In use this structure will  301 / 302  deform less and this improve the operation of the MEMS transducer  100  compared to a membrane electrode without openings. 
       FIG. 3  also shows the back-plate  304  and back-plate electrode  303  which have acoustic holes  312  through them. These acoustic holes  312  allow acoustic communication between the cavity between the membrane and back-plate and a volume on the other side of the membrane (which could be a sound port or a back-volume). The acoustic holes  312  extend through both the back-plate  304  and the back-plate electrode  303 , and thus there are holes through the entire back-plate structure  303 / 304 . It should be noted the holes  314  in back-plate electrode  303  may have the same dimensions as the acoustic hole  312  to which they correspond but this is not always the case and the holes  314  in the back-plate electrode  303  could be larger as illustrated, for instance so that no back-plate electrode material  303  is exposed in the side wall of the acoustic holes  312 . 
     In the embodiment shown in  FIG. 3  the holes  314  in the back-plate electrode  303  directly overlay the openings  313  in the membrane electrode  301 . In other words the size and location of the openings  313  in the membrane electrode  302  substantially correspond (at the quiescent position) to the size and location of the holes  314  in the back-plate electrode  303 . This means that (at quiescent position) for substantially any area where there is any membrane electrode  302  there is also overlying back-plate electrode  303  (assuming the overall size and location of the perimeter of the back-plate electrode  303  at least matches that of the membrane electrode  302 ). However for any area within an opening  313  in the membrane electrode  302  there may be no overlying back-plate electrode  303 . 
     In use, an electric field is generated between the back-plate electrode  303  and the membrane electrode  302 . This field may comprise a linear field component  324  running from one electrode to another in a direction perpendicular to the electrodes (note only one section of linear field component  324  is shown in  FIG. 3  for clarity). In addition fringe field components (not illustrated) may also occur, bowing into the space between the acoustic holes  312  and the openings  313  in the membrane electrode  302 . 
     In the case of a parallel plate capacitor, the linear field component  324  will extend from one plate to another in a direction perpendicular to the plates, forming equipotential surfaces. This produces a uniform electric field which capacitively couples the two plates  302 ,  303  to one another. The arrangement shown in  FIG. 3  ensures that membrane electrode material  302  is generally provided in areas where there could be a linear field component  324  but is generally omitted in areas where there may be no linear field component  324 . 
     Such an arrangement can significantly reduce the amount of membrane electrode material  302  provided on the flexible membrane layer  301  but without a significant decrease in capacitance. 
     It will of course be appreciated that a membrane electrode  302  will normally be deposited in the central part of the membrane  301  only. Thus there may be acoustic holes  312  in the back-plate  303 / 304  which would not, in any case, overlap any membrane electrode  302 . If the back-plate electrode  303  is larger than the membrane electrode  302  there may also be holes  313  in the back-plate electrode which would not, in any case, overlap any membrane electrode  302 . It will be understood that the openings  313  in the membrane electrode  302  are areas that are at least partly bounded by membrane electrode material  302 . 
     In one embodiment openings  313  may be provided in the membrane electrode  302  such that the majority of holes  312  in the back-plate electrode  303 / 304  that lie within a central area corresponding to the outer perimeter of the membrane electrode  302  have an associated opening  313  in membrane electrode  302 . In other words there may only be a relatively small proportion of holes in the back-plate electrode  312  which correspond to a location on the membrane electrode  302  where there is no opening. As will be described below this can significantly reduce the extent of coverage of the membrane electrode  302  compared to an electrode without any openings. 
     It should be noted that the embodiment shown in  FIG. 3  illustrates that the flexible membrane  301  does not contain any holes corresponding to the openings  313  in the membrane electrode  302 , i.e. the material, silicon nitride for example, of the flexible membrane  301  is continuous in the area of the openings  313  in the membrane electrode  302 . In some MEMS sensors there may be holes through the membrane structure. As discussed above in respect of  FIG. 1 , bleed holes  111  may be provided for low-frequency pressure equalisation and whilst such bleed holes  111  are typically located outside the area of membrane  301  coupled to the membrane electrode  302  this is not necessarily the case and there could be bleed holes  111 , or other holes through the membrane electrode  302  and flexible membrane layer  301 . Such holes through the entire membrane structure  302 / 303  will thus provide openings in the membrane electrode  302  but only in combination with a hole in the membrane layer  301 . Further any such holes through the membrane  302 / 303  may not be specifically aligned with back-plate holes  312 . Any holes through the membrane electrode corresponding to bleed holes, or other holes, through the entire membrane structure will thus be in addition the openings in the membrane electrode described above. 
     It should also be noted that even though there is no material of the membrane electrode  302 , e.g., metal or alloy, within the openings  313  such openings do not have to be void of any material. For example there could be an additional membrane layer on top of the membrane layer  301  which at least partly fills the opening  313  in the membrane electrode  302 . Additionally, or alternatively, the openings  313  could be at least partly filled with a dielectric material. 
       FIGS. 4 a  and 4 b    illustrate two other possible embodiments in which the openings  313  of the membrane electrode  301  still correspond to the acoustic holes  312  through the back-plate  304  and the second electrode  303 , however the pattern of acoustic holes  312  in the back-plate electrode  303  does not exactly map on to the pattern of openings  313  in the membrane electrode  302 . 
     In both the embodiments shown in  FIGS. 4 a  and 4 b    there is a degree of overlap between the membrane electrode  302  and back-plate electrode  303  providing electrode areas that will, in use, have a linear field  324  between them. In the embodiments of  FIGS. 4 a  and 4 b    there is at least some area of membrane electrode  302  that is not overlapped by back-plate electrode  303  or vice versa. These ‘non-overlapped’ areas of either electrode  302 / 303  can still contribute to the capacitance of the device through fringe fields which will extend from the edge of one electrode to at least some of the non-overlapped portion of the other electrode. 
       FIG. 4 a    illustrates an embodiment wherein the width of the openings  313  in the membrane electrode  302  is smaller than the width of the openings  314  through the back-plate electrode  303 , in other words the openings  313  in the membrane electrode  302  are smaller than the opening  312  in the back-plate  304  and the opening  314  in the back-plate electrode  303 . In this embodiment, in addition to the linear field component  324  between the back-plate electrode  303  and the membrane electrode  302 , in use there may also be fringing field components (not illustrated) between the edge of the back-plate electrode  303  and at least some parts of the membrane electrode  302  within the area of the holes  312 / 314  in the back-plate electrode  303 . Typically such fringing fields may extend in practice for at about 1-2 microns in a lateral direction. Thus the openings  313  in the membrane electrode  302  may have a diameter which is smaller than that of the holes  314  in the back-plate electrode  303  by an amount in the region of up to 2 microns, say 1-2 microns. In such an embodiment there may be potentially no drop in sensitivity compared to an electrode having no openings as the total area of membrane electrode which contributes to the capacitance may be substantially the same in each case. In other words, any electrode material  303  disposed within an area inside about 2 microns from the edge of the hole  314  in the back-plate electrode  303  may not contribute significantly to the capacitance in any case. 
       FIG. 4 b    shows an alternative embodiment where the width of the openings in the membrane electrode  313  is greater than the width of the holes  314  through the back-plate electrode  303 . This embodiment therefore provides larger openings than that shown in  FIG. 3  (for a given size of holes in the back-plate electrode) and thus this embodiment provides a membrane electrode  302  with even less membrane electrode material. This will further decrease the likelihood that the overall membrane would be plastically deformed or the extent of any plastic deformation. The difference between the width of the openings in the membrane electrode  302  and holes  314  in the back-plate electrode  303  may again be of the order of 1-2 microns, and similarly to the embodiment in  FIG. 4 a   , fringe fields (not illustrated) will extend from the non-overlapped back-plate electrode  303  to the membrane electrode  302 . 
     In the embodiments described with reference to  FIGS. 3 and 4   a  and  4   b  the openings  313  in the membrane electrode  302  are substantially centred on the centre of the holes  314  in the back-plate electrode  303 . In some embodiments however the openings  313  in the membrane electrode  302  may be offset with respect to the holes  314  in the back-plate electrode  303  (whether or not the openings  313  in the membrane electrode  302  are the same or a different size to the holes  314  in the back-plate electrode  303 ) as illustrated in  FIG. 5 . Of course other arrangements are possible. In some embodiments it may be desired to pattern the voids  313  in the membrane electrode  301  in a particular way, for instance to achieve a desired stress distribution, which results in an offset between the openings  313  in the membrane electrode  302  and the holes  314  in the back-plate electrode  303  and/or there may be a desire to offset the openings from the holes in the quiescent position so that there is a preferred alignment when the membrane  301 / 302  is deflected by an incident pressure wave. 
     It should be noted that the size of the openings and/or the alignment of the openings with respect to the holes in the back-plate electrode may vary across the membrane electrode. Thus in one part of the membrane electrode one particular size/alignment may be used, e.g. that shown in  FIG. 3 , but in another part of the membrane electrode a different size/alignment may be used, e.g. that shown in  FIG. 4 a    or  4   b  or  FIG. 5 . 
       FIG. 6  illustrates a plan view of the membrane  301  and membrane electrode  302  of one embodiment of a MEMS transducer. As described previously the membrane electrode  302  may be deposited over the central part of the membrane  301  and the outer boundary of the membrane electrode  302  may encompass an area of the central part of the membrane which is of the order of 40% or so of the membrane area.  FIG. 6  shows the plan view of the openings  313  that are formed in the membrane electrode  302 . The conductive track  615  is also shown in  FIG. 6 , which connects the membrane electrode  302  to the associated circuitry (not shown). The conductive track  615  is typically formed of the same material as the membrane electrode  302 . The openings  313  shown in  FIG. 6  form an array in a hexagonal pattern (illustrated by the dotted hexagon) within the outer boundary of the membrane electrode  302 . In this embodiment there are also openings formed in the periphery, i.e. outer boundary, of the membrane electrode. These openings are not completely enclosed by membrane electrode material and, as the corresponding hole in the back-plate electrode may not fully fall within the outer boundary of the membrane electrode, the openings can vary in size depending on where they are positioned in the array. 
     Note as used herein the outer boundary of the membrane electrode refers to the general perimeter that the electrode there would have if there were no openings in the membrane, i.e. in this instance the outer boundary would be a generally circular perimeter. In general terms the outer boundary of the membrane electrode can be thought of as a perimeter which completely surrounds the membrane electrode and which has substantially the smallest possible length. Thus in the embodiment shown in  FIG. 6  the outer boundary would follow the circular perimeter of the electrode until the mouth of an opening was reached—the outer boundary would then be a substantially straight line across the mouth of the opening, where it would then again follow the circular perimeter of the electrode. The conductive track  615  will not generally be considered part of the membrane electrode but in some instances the end of the conductive track and start of the electrode may be not be clearly defined. 
     It will be appreciated that the presence of openings in the membrane electrode within the outer boundary of the membrane electrode, where there are no holes in the flexible membrane, thus means that the area of flexible membrane material that lies within the outer boundary of the membrane electrode is greater than the area of material forming the membrane electrode. In some embodiments the material of the flexible membrane occupies the whole of the area within the outer boundary of the membrane electrode. 
     In the embodiment illustrated in  FIG. 6  there is an opening in the membrane electrode associated with substantially all the acoustic holes in the back-plate that fall, at least partly within the outer boundary of the membrane electrode. In one embodiment the diameter of the openings may be of the order of about 10 μm with the shortest distance between adjacent openings being of the order of about 5 μm. 
     This density of openings can significantly reduce the amount of material forming the membrane electrode. Typically the outer perimeter of a membrane electrode may encompass an area of the membrane of at least 35%. For example consider a membrane electrode of a first diameter such that the outer boundary of the membrane electrode encompasses an area of about 40% of the centre of the membrane. Without any openings in the membrane electrode the coverage of the membrane by the electrode is thus about 40%. With openings of the order of about 10 μm diameter separated by about 5 μm the extent of coverage of the membrane drops to about 25% or less. 
     This changes the relative contributions of the membrane layer  301  and electrode  302  to the total stress of the membrane structure. In the conventional transducer having a metal, e.g. aluminium, or an alloy, e.g. Aluminium-Silicon, electrode covering about 40% of a crystalline, i.e. silicon nitride, or polycrystalline membrane, the membrane electrode  302  may contribute about one third of the total stress. By reducing the coverage of electrode material  302 , in this particular case from about 40% to less than 25% for example, the stress contribution due to the membrane electrode  302  is reduced. If the intrinsic stress of the membrane  301 , e.g. silicon nitride, is adjusted to compensate to provide the same overall stress the membrane electrode  302  will contribute less than 20% to the total stress. 
     It should be noted that  FIG. 6  shows the openings  313  are discrete openings. In some embodiments however at least some openings could be linked to one another by narrow channels (not illustrated) in the membrane electrode  302 . 
       FIG. 7  shows an alternative pattern of openings  313 , which form a rectangular pattern (again bounded by the circular membrane electrode  302 ). This embodiment does not have openings  313  in the periphery of the membrane electrode  302  as illustrated in  FIG. 6 . In other words openings  313  are only provided in the membrane electrode  302  that correspond to holes in the back-plate electrode which fall fully within the outer boundary of the membrane electrode. 
       FIG. 8 a    shows a further alternative array in openings  313 , wherein the openings  313  are square in shape and are arranged in a rectangular pattern bounded by the circular membrane electrode boundary. The openings in the membrane electrode could have any desired shape as can those holes (acoustic and electrode) associated with the back-plate. 
     As discussed the particular arrangement of the openings  313  in the membrane electrode  302  may be chosen so at to provide a desired stress characteristic for the membrane  302 / 303 . For instance the distribution of the plurality of openings in the membrane electrode may be configured to provide even stress as the membrane flexes. In some embodiments the arrangement of the acoustic holes may therefore be designed to match a desired pattern of openings in the membrane electrode. 
       FIGS. 8 b  and 8 c    shows further examples of openings in the membrane electrode.  FIG. 8 b    shows a plan view of flexible membrane  301  and the electrode  302  and lead track  615 . In this example there are a series of arcuate openings  313  in the membrane electrode. The arcuate openings in effect divide the membrane electrode into a series of concentric annuluses and a central circular portion. As clearly the various parts of the membrane electrode must be electrically connected the arcuate opening  313  do not completely separate the membrane electrode sections and a path  801  connects all the parts of the membrane electrode. In some embodiments it may be desired that the pattern of the membrane electrode is generally symmetric (rotationally and or about an axis) and thus other paths  803  could also comprise material of the membrane electrode. 
     The position and size of the arcuate openings is arranged with respect to the holes  314  in the back-plate electrode so that the openings align with a number of the holes  314 . The position of only a few holes  314  in the back-plate electrode are shown for clarity. In this embodiment the outer boundary of the membrane electrode will be the perimeter of the outer annular section. 
       FIG. 8 c    shows a further example in which the membrane electrode  302  has a single continuous opening  313  that spirals into the centre of the membrane electrode. Thus the resulting membrane electrode pattern resembles a spiral on the flexible membrane  301 . Again the path and size of the spiral opening  313  is arranged to coincide with the area of a number of the holes  314  in the back-plate electrode. This membrane electrode is therefore relatively narrow at all points with the advantages of reduced metal creep. In this embodiment the outer boundary of the membrane electrode would run across the mouth of the spiral opening (as indicated by the dotted line) and then extend along the periphery of the electrode. 
     The embodiments discussed above have been described with reference to MEMS transducers where the back-plate, which supports the back-plate electrode in spaced relation to the membrane and membrane electrode, has a plurality of acoustic holes running through the back-plate. As described above the openings in the membrane electrode can be aligned with the holes in the back-plate electrode so as to reduce the amount of metal on the membrane electrode without a significant impact on capacitance. 
     Embodiments of the present invention also apply to transducers such as pressure sensors where the back-plate (which may form part of a substrate) is not provided with a plurality of acoustic holes. In this case there may still be a plurality of openings in the membrane electrode in embodiments of the invention. As discussed above providing openings in the membrane electrode can be advantageous in reducing the total amount of metal required as compared to a membrane electrode without openings. If the openings are relatively small, for instance of the order of a diameter of about 1 to 4 microns, there may not be a significant drop in capacitance as fringing fields from the edges of the openings in the membrane electrode will extend to the areas of the back-plate electrode where there is no linear field component. Thus the overall sensitivity of the sensor may not be significantly affected but the amount of metal and hence the extent of plastic deformation, may be decreased. Large openings could be used if desired, possibly with a small drop in sensitivity but less chance of DC offset or sensitivity changes in use. 
       FIG. 9  illustrates an embodiment wherein two transducers are formed within the same substrate  905 . Two membranes ( 901   a  and  901   b ) are shown, each with a respective membrane electrode ( 902   a  and  902   b ), back-plate electrode ( 903   a  and  903   b ) and back-plate ( 904   a  and  904   b ). The back-volume  908  is therefore shared by the two transducers. These transducers could, in some embodiments, be tuned to work at different frequencies to one another. Different transducers may have different membrane and electrode properties such as size, relative areas, spacing etc. to “tune” the transducers. It is also possible that the one transducer may be an output transducer (i.e. a transmitter) and the other may be an input transducer (i.e. a receiver).  FIG. 9  only shows a cross section of a possible embodiment of the present invention, in fact there could be any number of transducers in an array across the surface of the substrate. In other embodiments some or all of multiple transducers formed within the same substrate may have separate back volumes, or subsets of the transducers may each share respective back volumes. 2D array. 
       FIGS. 10, 11   a  and  11   b  illustrate embodiments of the present invention wherein the transducer is packaged, e.g. within a housing to form a MEMS package.  FIG. 10  shows the transducer mounted on a (package) substrate  1018  with a protective cover or lid  1019  coupled to the substrate  1018 . The cover or lid  1019  may shield the transducer, for example providing protection from electromagnetic interference an/d or dust/moisture protection. In some embodiments the cover  1019  may have an acoustic port, i.e. sound hole  1017 , to let pressure waves access the transducer, in which case the package substrate  1018  may be solid, thus covering the back volume formed in the substrate of the transducer (although at least part of the substrate underlying the transducer could have a cavity to increase the area of back-volume). Alternatively the transducer could be arranged with a sound port  1020  through the substrate  1018 , in which case the cover  1019  may be sealed. To provide connections to the transducer and any read-out circuitry the package may have contacts  1024  connected via suitable tracks such as vias. 
     As shown in  FIG. 11 a    associated integrated circuitry  1125  can also be housed within the MEMS package. The associated integrated circuitry  1125  may be directly electrically connected to the both electrodes ( 902  and  903 ) and may bias the MEMS transducer. Furthermore the associated integrated circuitry may amplify the signal produced by the incident pressure waves which cause the flexible membrane  901  to move. In the embodiment illustrated in  FIG. 11 a    the package may be formed from a laminate type structure comprising a substrate  1118 , and a cover or lid  1119  which is separated from the substrate  1118  by spacer side walls. Again the sound port could be provided in the substrate or in the cover. 
       FIG. 11 b    illustrates a further embodiment wherein the MEMS transducer is mounted on the top cover  1119  and may underlie an acoustic port  1117 . In this configuration the volume between the acoustic port and the flexible membrane forms the necessary back-volume. Alternatively an acoustic port  1120  could be formed in the substrate  1118 . In this configuration the substrate contacts  1124  are on the opposite substrate to the MEMS transducer, and appropriate electrical connections may need to run along the side walls of the package. 
     It may also be possible to mount the MEMS transducer to either side of the packaging ( 1018 ,  1019 ,  1118 , or  1119 ) with the back-plate being the side which was physically attached, instead of the substrate  905 . 
     The protective elements ( 1018 ,  1019 ,  1118 , and  1119 ) serve to protect the MEMS transducer from external sources of damage. In some embodiments the package may also provide electromagnetic shielding The package, e.g. the substrate and/or cover, may be partially or fully made from a conductive material or metal. By having at least some metal (or conductive material) in the protective element a Faraday cage will be formed around the MEMS transducer keeping any external EM radiation away from the MEMS transducer. Printed circuit board material or ceramics may also be used in the fabrication of the protective elements. 
     Embodiments of the invention may be produced using standard MEMS processing techniques. For instance, referring back to  FIG. 1 a    a capacitive microphone may be 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  101 , 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 membrane electrode and back-plate electrode  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 . 
     As mentioned the membrane  101  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  101  may be formed by depositing one or more silicon nitride layers. The membrane layer may be dimensioned to have the required flexibility. The membrane electrode may then be deposited on the membrane layer and patterned to have a plurality of opening therein as discussed above. The membrane electrode could be patterned by depositing a metal layer in the general area required and then etching openings into the membrane or alternatively by patterning the membrane prior to deposition of the electrode material. 
     Collectively the metal of the membrane electrode and silicon nitride of the membrane layer provide a desired stress characteristic of the overall membrane structure. In embodiments of the present invention, where less metal is used compared to a conventional transducer, the stress of the flexible membrane layer may be greater to compensate. One skilled in the art will be aware that the stress can be controlled in various ways, such as varying the temperature of deposition. 
     The back-plate layer may be formed by depositing a further layer of sacrificial material to define a cavity between the membrane and back-plate and then depositing the back-plate layer(s) and back-plates electrode. The back-plate may also be formed from silicon nitride but may be deposited to be a thicker and therefore more rigid structure than the membrane. 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. 
     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 . 
     The transducer shown in  FIGS. 1 a  and  b    is illustrated with sloped side walls supporting the membrane layer  101  in spaced relation from the back-plate  104 . Sloped or slanted side walls may be used to reduce the stress concentration. Vertical walls may be used but 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. 
     Other types of MEMS devices may make use of the embodiments described above, these include, but are not limited to pressure sensors, ultrasonic transducers, acceleration monitoring and signal generation transducers. 
     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. Embodiments may also be used in voice activated or voice controlled devices. 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.