WO2018002566A1

WO2018002566A1 - Mems device and process

Info

Publication number: WO2018002566A1
Application number: PCT/GB2016/051974
Authority: WO
Inventors: Marek Sebastian PIECHOCINSKI; Colin Robert Jenkins; Clive Robert Graham
Original assignee: Cirrus Logic International Semiconductor Limited
Priority date: 2016-06-30
Filing date: 2016-06-30
Publication date: 2018-01-04

Abstract

The application describes MEMS transducer structures- comprising a membrane structure having a flexible membrane layer and at least one electrode layer. The electrode layer is spaced from the flexible membrane layer such that at least one air volume extends between the material of the electrode layer and the membrane layer. The electrode layer is supported relative to the flexible membrane by means of a support structure which extends between the first electrode layer and the flexible membrane layer,.

Description

MEMS Device and Process

This Invention relates to a micro-e!ectro-mechanical system (MEMS) device and process, and in particular to a MEMS device and process relating to a transducer, for example a capaciiive microphone.

Various ME S devices are becoming increasingly popular. MEMS transducers, and especially capaciiive microphones, are increasingl being used in portable electronic devices such as mobile telephones and portable computing devices.

Microphone devices formed using ME S fabrication processes typically comprise one or more membranes with electrodes tor read-out/drive deposited on the membranes and/or a substrate, in the case of MEMS pressur sensors and microphones, the read out is usually accomplished by measuring the capacitance between a pair of electrodes which will vary as the distance between the electrodes changes In response to sound waves incident on the membrane surface. Figures 1a and 1b show a schematic diagram and a perspective view, respectively, of a known capaciiive MEMS microphone device SO. The eapacitive microphone device 100 comprises a membrane layer 101 whic 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 eapacitive plate of the eapacitive microphone device. A second electrode 103 is mechanically coupled to a generally rigid structural layer or back-plate 104, which together form a second eapacitive plate of the capaciiive microphone device. In the example shown in Figure 1 a the second electrode 103 is embedded within the back-plate structure 104. The capaoitive microphone Is formed on a substrate 105, for example a silicon wafer which may have upper and lower oxide layers 108, 10? 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 th substrate 105, The substrate cavit 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 saehfida! material, Formation of the first cavity 109 using a sacrificial layer means that the etching of the substrate cavity 108 does not play any part in defining the diameter of the membrane, nstead, 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 b 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 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 1 0 and the second cavity 11 . As mentioned the membrane may be formed by depositing at teas! one membrane layer 101 over a first sacrificial material. In this way the material of the membrane layer(s) may extend into the sypporting structure, i.e. the side wails, 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 arid therefore more rigid structure. Additionally various other material layers could he used In forming the back-plate 104 to control the properties thereof. The use of a silicon nitride materia! system is advantageous in man ways, although other materials may he used, tor 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 instances a further plurality of holes, hereinafter referred to as acoustic holes 112, are arranged In the: back- piate 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 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 If may be substantially sealed. in othe 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 sti!l provided with a plurality of holes to allow air to freely move between the second cavity and a further volume above the back-plate. It should also be noted that whilst Figure 1 shows the back-plate 104 being supported on the opposite s de of the membrane to the substrate 105, arrangements are known where the back-plate 104 is formed closest to the substrate with the membrane layer 101 supported above it

In use, In response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is delbrmed slightly from its equilibrium or quiescent position. The distance between the membrane electrode 102 and the backpiaie 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 timescale (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 fre uencies,

The flexible membrane layer of a MEMS transducer generally comprises a thin layer of a dielectric material - such as a layer of crystalline or poiycrystalline mate al. The membrane layer may, in practice, be formed by several layers of material which are deposited In successive steps. The flexible membrane 101 may, for example, be formed from silicon nitride S U* or po!ysieon. Crystalline and poiycrystalline materials have high strength and low plastic deformation, both of which are highl desirable in the construction of a membrane. The membrane electrode 102 of a MEMS transducer is typically a thin layer of metal, e.g. aluminium, which is typicall 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 he formed by an alloy such as aluminium-silicon for example. The membrane electrode may typically cover, for example, around 40% of area of the

membrane, usually in the central region of the membrane. Thus, known transducer membrane structures are composed of two layers of different material^' - typically a dielectric layer (e.g. SIN) and a conductive layer (e.g. AISI),

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 acros the membrane, as Illustrated in Figur l 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. However, over time, the membrane structure may become deformed -- e,g, as a consequence of relatively high or repeated displacement ~ so that it will not return to exactly the same starting position.

A number of problems are associated with the previously considered transducer designs. In particular both the membrane and the membrane electrode will suffer intrinsic mechanical stress after manufacture, for instance due to being deposited at relatively high temperatures of a few hundred degrees Celsius and desiring en return to room temperature to contract by different amounts due to greatly different thermal coefficients of expansion yet being intimately

mechanically coupled together. Not being abl to Immediately dissipate the stored energy due to the stress, i.e. not able to fully release the stress by independent mechanical contraction, the composite structure of electrode and membrane will tend to deform,, simila to the well-known operation of bi-meial!lc strip, thermostat sensors. Over a long time,, especially when subject to repeated mechanical exercising as typical of a microphone membrane in use, the metal electrode layer in particular may foe subject to creep or plastic deformation as it anneals to reduce its stored stress energy - being unable to release it in any other way. Thus, the equilibrium or quiescent position of th membrane structure comprising the membrane and the membrane electrode is sensitive to manufacturing conditions from day one and can also change over time.

Figure 2 illustrates the permanent deformation which can occur to the quiescent position of the membrane 101/102, it can be seen that the quiescent position of the membrane, and thus the spacing between the back-plate electrode 103 and the membrane electrode 102. therefore changes from its position immediately after manufacture - shown by the dashed line - to the deformed quiescent position. 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, More importantly, for a.c, audio signals, the change in

capacitance leads to a variation in the signal charge fo a given acoustic stimulus, i.e. the acousto-elecf rical sensitivity of the microphone..

In addition, the elasticity of the composite electrode-membrane structure 101/102 Is sensitive to the mechanical stress of the electrode and membrane layers. Any variatio In manufacturing conditions and the subsequent stress release via metal creep or suchlike will affect, the values af the stress of these layers. The deformation due to the stress mismatch will also directly affect the values of quiescent stress.

Thus, it can be appreciated that the membrane structure and associated transducer may suffer an increased manufacturing variation In initial sensitivity and furthermore experience a change - or drift - In sensitivity over tim meaning that the transducer performance cannot be kept constant.

Furthermore, the metal of the membrane eiectrode may undergo some plastic deformation as a consequence of relatively high or repeated displacement from the quiescent/equilibrium position- Thus, the metal of the membrane electrode may be deformed so It will not return to its original position, Since the flexible membrane 101 and the membrane electrode 102 are mechanically coupled to one another this can also lead to an overall change In the quiescent position of the flexible membrane 101 and/or a change in the stress properties and thus the elasticity of the overall membrane structure.

Embodiments of the present invention relate to MEMS transducers and processes which seek to alleviate some of the aforementioned disadvantages, in particular by providing a transducer which exhibits an improved consistency In sensitivity or performance initially and over time.

According to a first aspect of the present invention there is provided a MEVIS transducer comprising a membrane structure, the membrane structure comprising a flexible membrane layer and a first electrode layer, the first electrode layer being supported relative to the flexible membrane laye so as to be spaced f om the flexible membrane layer.

The separation of the electrode layer from the snembrane layer advantageously serves to reduce coupling of initial post-manufactur or time-dependent stress differences between electrode layer and membrane layer and thus

advantageously alleviate problems associated with the warping of the membrane structure and elasticity variation due to coupled stress, both initially and over time, initial manufacturing variation and drift overtime of transducer sensitivity may also be improved. The supporting of the electrode relative to the flexible membrane layer allows the electrode layer to still substantiall mechanically follow the movement of the membrane layer to deliver an output signal despite the separation. The first electrode ayer is preferably supported relative to the flexible

membrane b means of a support structure which extends between the first electrode layer and the flexible membrane layer, According to. embodiments of the invention, at least one air volume extends between the material of the first electrode layer and the membrane layer.

The membrane structure may comprise a second electrode layer, said second electrode layer being disposed on the opposite side of the flexible membrane to the first electrode. Preferably, the second electrode layer Is supported relative to the flexible membrane layer so as to be spaced from the flexible membrane layer.

Preferably, the second electrode layer is supported relative to the flexible membrane by moans of a support structure which extends between the second electrode layer and the flexible membrane layer- According to embodiments of the present invention, the first electrode layer and/or the second electrode layer may comprise a -continuous sheet of material. Alternatively, the fi st electrode layer and/or the second electrode layer comprises one or more openings. The openings in the may be generally circular in shape and/or the openings may be nomcireuiar in shape. Thus, the first electrode layer and/or the second electrode layer ma be formed in a lattice shape. The lattice shape may comprise a plurality of ; polygons which define openings In the electrode layer. Thus, the first and/or second electrode layer can foe considered to comprise a lattice of connectivity which is suspended relative to the membrane layer b means of a support structure.

The electrode laye may have a thickness of around iOOnm. The electrode layer may be spaced from the membrane layer by a separation of around lOOnm. The support structure which serves to facilitate the suspension of the/each electrode layer above/below the membrane layer may comprise a plurality of support elements. The support elements comprise dielectric or conductive materia!. The support elements may be disposed at or near the periphery of the membrane structure. Alternatively the support elements may be distributed over the surface of the membrane.

Alternatively, the support structure which serves to facilitate the suspension of the/each electrode layer above/below the membrane layer, may comprise a conductive layer having a plurality of openings. Thus, the support structure may comprise a lattice structure.

Alternatively, the support structure may comprise (only) a first partial lattice structure, and the electrode layer may comprise only a second partial lattice structure, the first and second lattice structures cooperating to provide electrical connection to all desired elements of the electrod layer.

It will he appreciated that the first and/or second electrode layer may be

fabricated - e.g. by deposition - as a distinct processing step. Alternatively, the first and/or second electrode layer may be fabricated during deposition of a single layer of material which is subsequently processed to form the support structure and the electrode layer. Thus, embodiments of the present invention are envisaged in whic the support structure is formed of the same material as the electrode layer. Preferably,: however, the bulk of the conductive material forms the electrode layer which is suspended relative to the flexible membrane with the support structure layer. Alternatively, embodiments of the present invention are envisaged in which the support structure Is formed of the same material as the membrane layer. According to embodiments of the present invention, the openings of the support structure form a plurality of air volumes which extend between the materia! of the first electrode layer and the flexible membrane layer. The openings of the support structure may increase in size from a region towards the centre of the membrane structure to a region at or near the periphery of the membrane structure. Alternatively, the openings of the support structure ma be

substantially of uniform size.

In embodiments wherein the electrode layer and the support layer each comprise a plurality of openings, the openings of the electrode layer may be laterally offset from the openings of the support layer.

The first electrode layer (as a substantially continuous sheet or lattice pattern or partial lattice pattern) may overlie substantially ail of the membrane or there may be a region of the membran where the. electrode layer or pattern is absent.. This reaion may comprise at least a central region of the membrane.

The flexible membrane may comprise a crystalline or poiycrysta!line material, Preferably the flexible membrane layer comprises silicon nitride. The first andfor second electrode layer may comprise metal or a metal alloy. Preferably, the electrode comprises aluminium., silicon, doped silicon or polysilicGn.

Embodiments of the present Invention advantageously demonstrate a reduction In the degree of deformation of the quiescent or equilibrium position of the membrane structure over time. The separation of the first electrode layer from the membrane layer serves to at least partially decouple the electrode material from the membrane material in one or more regions thus mitigating the mismatch between the mechanical properties of the membrane and the conductive electrode. Thus embodiments of the present invention advantageously reduce the area of interface betwee the membrane material and the metal electrode, thereby serving to reduce the mechanical influence of the metal electrode layer on the membrane layer. According to preferred embodiments the time- dependent drift of the M EMS transducer is alleviated. Furthermore,

embodiments of the present invention may demonstrate an enhancement in the capacitance, since the overall working area of the electrode layer can foe advantageously increased within existing electrode boundaries. As a result the sensitivity of the transducer may be enhanced .

Some of the invention relate to structures of the electrode layer or support structures which allow relaxation of stress of the electrode layer, thus

advantageously reducing any Impact on the stress or deformation of the membrane, and so reducing any consequent sensitivity variation, initially or over time. The transducer may: comprise a back-plate structure wherein the flexible membrane layer Is supported with respect to said back-plate structure. The back-plate structure ma comprises a plurality of holes through the back-plate structure, The transducer may be a capactive sensor such as a microp! one. The transducer may comprise readout,_, i.e. amplification, circuitry. The transducer may be located within a package having a sound orts ie, an acoustic port. The transducer may he Implemented in an electronic device which may he 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 tablet device; a games device; and a voice controlled device.

Features of any given aspect may be combined with the features of any other aspect and the various: features described herein may be Implemented in any combination In a given embodiment. Associated .methods of fabricating a MEMS transducer arB provided for each of the above aspects,

Fo r a better ynderstanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which Figures la and 1b illustrate known capacitive Ei tS transducers in section and perspective views;

Figures 2 illustrates how a membrane may be deformed; Figures 3 illustrates a previously considered membrane structure;

Figures 4a to 4s show cross-secilonai views through various membrane structures according to embodiments of the present invention; Figures 5a to Se show a membrane structure according to a further embodiment of the present invention;

Figure 6a to 6d show further membrane structures according to embodiments of the present invention;

Figures 7a to ?g show cross-sectional views tlirough various membrane structures according to further embodiments of the present invention; and

Figures 8 to Sh show. a series of drawings to illustrate a process for making a membrane structure embodying the present Invention.

Throughout this description any features which are similar to features In other figures have been given the same reference numerals. igure 3 shows a top view of a previously considered membrane structure comprising a planar membrane layer 101 and an electrode 102. The electrode - which is typically formed of metal or metal allo ~ is patterned to incorporate a plurality of openings 113. In this specific example the openings are generally hexagonal In shape.

It will be appreciated that microphone sensitivity is a function of capacitance which is directly proportional to the area of the conductive electrode. Membrane structures which incorporate a patterned electrode layer, or lattice, as shown In Figure 3 may therefore potentially demonstrate a deterioration in the sensitivit of the transducer as compared to sheet electrode designs, albeit mitigated somewhat by the effect of electrostatic fringing fields.

Figures 4a to e each show a cross-sectional view through various membrane structures comprising a flexible membrane 101 and a first electrode.

Figure 4a shows a cross section through the line A~A of the membrane structure shown in Figure 3. The metal forming the electrode 102 can h seen to he directly In contact with the flexible membrane layer 101. Thus any stress In the electrode is directly coupled into the membrane, and the electrode can substantially only attempt to relax Its stress by affecting the membrane stress.

Figure 4b shows a cross-sectional view of a membrane structure for a IvTEIVIS transduce according to a first embodiment of the present invention. The membrane structure comprises a first electrode layer 102 comprising a sheet electrode- hich is supported, by means of a support structure comprising a plurality of support: elements 114, in a spaced relationship m the membrane layer 101 . A volume of air 118 extends between the electrode layer and the flexible membrane. The support elements 11 may comprise a plurality of vertical spacers, or isolated mounts, which allow the first electrode layer to be suspended above the membrane layer. The membrane structure may be provided with such support elements provided around some or all of the periphery of the membrane, near to parts of the periphery at which the membrane Is anGhored by means not illustrated in this figure t the surrounding silicon substrate. The elements serve to establish a spacing of the electrode layer in the vertical direction - l.e, orthogonally to the plane of the membrane - without significantly coupling the electrode layer in the plane of the membrane. The support elements may be formed of conductive or dielectric materia!.

Alternatively both the membrane and the electrode may be anchored

independently, for instance at the sidewall of the baekplate, with the electrode anchored at a higher part of the sidewall than the electrode such that the sidewall provides the support structure as Illustrated in Figure X, in the structure shown on the left,. Thus in some embodiments the support structure may comprise a continuous structure, for example a continuous ring around a circular periphery.

The volume of air substantially trapped between the electrode and the membrane may be substantially trapped, except maybe for bleed holes in the membrane structure or electrode layers, or with limited access to other volumes of the structure via the periphery of the membrane, The volume of air may thus serve as an air cushion to couple any acoustically stimulated movement of the membrane to the electrode. Thus, according to this embodiment, it will be appreciated that the electrode layer is physically separated froni the membrane (although it will also be appreciated that the electrode layer is indirectly connected to the membrane layer via the support elements 114). As such, since the electrode layer is not in direct contact with the membrane layer except a or near the periphery., the mechanical influence of the electrode layer i 02 on the membrane layer is reduced and the membrane structure may advantageously demonstrate an improvement in time-dependent warping of the membrane. Moreover, in comparison to the membrane structure shown in Figures 3 and 4a, the area of the metal electrode is increased and, thus, the transducer sensitivity is enhanced.

Figi re 4c shows a cross-sectional view of a membrane structure for a MEMS transducer according to a second embodiment of the present invention. The membrane structure comprises a first electrode layer 102 and a support structure 114. The first electrode layer 102 comprises a continuous sheet of electrode material, The support structure 114 comprise a patterned layer of conductive material ~ similar to the single electrode layer shown in Figure 4a ~ comprising a plurality of openings. The openings define a plurality of air volumes 115 - which extend between the first electrode layer 102 and the membrane layer 101 , In this example the material of the first electrode layer, the metal elements of the lattice structure forming the support structure 114 and the membrane effectively form a plurality of closed air volumes within the

membrane structure. . Thus, as a consequence of the electrode layer not being connected to the membrane layer at each of the air volumes, a significant proportion of the electrode material of the first electrode layer Is mechanically separate from the membrane layer, thereby allowing at least some stress release by local vertical thinning of the electrode material_* Although illustrated as approximately equal, preferably the support structure Is significantly less wide than the air volume width, to allow increased stress relaxation.

Figure 4d shows a cross sectional view of a membrane structure for a MEMS transducer according to a third embodiment of the present Invention, The first electrode layer 102 comprises a layer of electrode material that Is patterned to comprise a plurality of openings 116 within the plane of the first electrode layer, Although not apparent from this cross-sectional view, the electrode layer 102 Is formed in a lattice shape - e.g. by a process of patterning or etching of the electrode material Th lattice shaped electrode can be considered to comprise a plurality of interconnected conductive elements. The first electrode layer 102 is supported so as to foe spaced from the membrane layer by means of one or more support elements (not shown) which may be formed of the same material as the electrode or of an insulating material.

The first electrode layer 102 can be seen to define a plurality of interconnected electrode elements which extend in the line of the cross sectional view. Each of the electrode elements can be considered to form a conductive bridge with th space underneath the bhdge defining an air volume 115 as indicated by the dashed line. Each arm or bridge of the lattice is free to expand or contract in width or height proving degrees of freedom to allow any metal stress to he substantially accommodated leaving only relatively little residua! stress requiring to be released via transmission to the membrane via the support elements,

Figur© 4@ shows a cross sectional view of a membrane structure for a MEMS transducer according to a fourth embodiment of the present invention. In this example the first electrode layer 102 again comprises a layer of electrode material that is patterned to comprise a plurality of openings 118 within the plane of the first electrode layer. Again, as in Figure 46, the electrode layer 102 is formed in a lattice shape and can he considered to comprise a plurality of interconnected conductive elements. The electrode layer is supported in a spaced relation relative to the membrane layer by means of a support structure. The support structure 114 is patterned to incorporate a plurality of openings. The electrode layers are fabricated such that the openings of the electrode layer are laterally offset from the openings of the support structure. This figure portrays an illustrative case in which the bridges and support structures are at least somewhat aligned along the chosen cross-sectional plane: for e,g. a hexagonal lattice there will he no such plane, but this figure illustrates the general concept of the bridges linked by a series of conductive support structures.

Figures Sa to S© show perspective views of a membrane structure according to a fifth embodiment of the present Invention, In this example the electrode layer 102 is patterned to incorporate a plurality of openings, Specifically, the electrode layer Is spaced from the membrane layer by means of a support structure 1 14, The support structure 1 14 In this embodiment comprises the same conductive material that forms the electrode layer 102, The support structure 1 14 thus extends between the electrode layer 102 and the membrane 101 ,

As illustrated most clearly In the plan view of Figure 6e, the support structure may comprise a first partial lattice structure comprising elements 182, and the electrode layer may comprise second partial lattice structure comprising elements 180, laid out superimposed on a hexagonal grid pattern 184, The individual separate elements of the first partial lattice structure serve as

conductive bridges between adjacent elements of the second partial lattice structure, which in turn provide conductive paths between the adjacent elements of the first partial lattice structure. Thus the first and second lattice structures cooperate to provide -electrical connection to all desired elements of the electrode layer, Some or all elements at or near the periphery of the membrane may he coupled to readout circuitry, thus coupling the whole electrode structure to the readout circuitry.

The elements of the first partial lattice structure are isolated and small so accumulate little stress and are free to release this stress in several directions. The elements of the second partial lattice structure are isolated and small so accumulate little stress, so also have little effect on the membrane. In further variants, the size of the elements of the second partial lattice structure may he reduced further, for instance by spanning only a part of each edge of the polygon rather than the whole side, with the adjacent electrode element expanded to maintain connectivity, In variants illustrated in perspective in Figures 5A and SB, elements of the first partial lattice structure are extended laterally in a cantilever fashion to increase their area to increase the sensing capacitance..

As illustrated in Figures 6a and 6b, which also further Illustrate cantilever structures, a membrane structure embodying the present invention In which the electrode comprises material that is spaced from the flexible membrane layer and thus incorporates one or more air volumes which extend between the spaced material and the membrane layer, may he fabricated from further processing to the electrode structure shown In Figure 3. Thus, the material that will form the first electrode layer can be deposited onto a patterned electrode layer as shown In Figure 3 (which ultimately forms the support structure for the first electrode layer) so that. the material of the first electrode layer extends partially or fully between opposite edges defining the openings of the support layer. As shown In Figures 8a and 6b the material of the first electrode layer 102 is therefore spaced from the membrane layer by an air volume and thus forms a cantilever o "partial cross-over" with respect to the plane of the support structure 114, As shown in Fig e 6c and 6d, the material of the first electrode layer 114 forms a "full cross-over" with respect to the plane of the, support structure 114. The resultant membrane structure will benefit from increased capacitance due to the increased conductive material, whilst the openings of the support structure provide an improvement in the stresses that arise betwee the electrode material and the membrane as comparted to previously considered designs incorporating a single electrode layer comprising a continuous sheet of metal in direct contact with the membrane. According to one or more embodiments of the present Invention, the

mechanical decoupling of the electrode layer by the presenc of one or more air volumes which extend between the first electrode layer and the membrane layer CQykS be beneficially used to alleviate so-called diaphragm edge cud. In view of the problem of the membrane curling at the edge due to the stress mis-match that arises between the two layers, it is typical for the conductive metal electrode material to be deposited in region towards the centre of the

membrane and not close to the edge of the membrane. However, this prior solution represents a reduction in the metal electrode area and thus undermines the sensitivit of the transducer,. Embodiments of the present invention enable the electrode to extend over a greater proportion of the membrane including the region at or near the periphery of the membrane.

According to preferred embodiments which utilise a support structure

comprising a lattice-like structure In conjunction with an electrode layer also comprising a lattice-like structure. It is proposed to vary the area of the openings provided in electrode layer and/or the area of the openings in the support structure so as to control or modulate membrane edge curl, Edge curl leads to air leakage around the membrane, i.e. an acoustic bypass path which particularly impairs low-frequency sensitivity and so controlling the edge curl allows for more effective control of the leakage and. thus, improved control of microphone low-frequency roll-off, figures 7a to ?g each show a cross-sectional view through various membrane structure designs which comprise a flexible membrane layer 101. and at least first electrode 102 supported In a spaced relation from the membrane by means of a support structure 114.

Figure 7a shows membrane structure comprising a first electrode layer 102a and a support layer 114. Both the first electrode layer and the support layer are patterned to incorporate a plurality of openings. The openings formed In the support layer form a plurality of air volumes 115 which extend between tbe first electrode layer and the membrane layer, in this example the size of the openings across the membrane layer is substantially constant, f igure 7b shows a membrane structure comprising a first electrode layer 102a and a second electrode layer 102b. The second electrode layer 102h is formed on the opposite side of the flexible membrane layer 101 to the first electrode 102a. Each of the first and second electrode layers 1G2a, 102b ss supported in fixed relation relative to the flexible membrane by a support layer 114a, 114b such that the material forming the respective electrode layer is supported in spaced relation from the membrane and is thus separated from the membrane layer by a plurality of air volumes 115 formed by openings 115 in the respective support layer, in this example the size of the openings 1 6 across the

membrane layer is substantially constant.

A membrane structure according to Figure ?b may be usefully employed in conjunction with a MEMS transducer which utilises first and second baokp!ates (each Incorporating a backplate electrode) which are respectively positioned above and below the membrane structure..

Rgyre 7c shows a membrane structure that is similar to the membrane

structure shown In Figure 7b. However, in this example the air volumes 116 of the second electrode are laterally offset witb respect to tbe openings of the first electrode. This arrangement can beneficially serve to mitigate the occurrence of stress concentrations arising within the membrane structure since the stress arising between the membrane and the first electrode may tend to cancel or mitigate the stress maxima and minima arising between the membrane and the second electrode. Furthermore, a membrane structure embodying this design may experience a..rippling effect which therefore provides degree of freedom to the membrane and, thus, alleviates stress. Again, In this example the size of the openings across the membrane layer Is substantially constant.

Figure 74 shows a membrane structure comprising a first electrode layer 102a and a support layer 114, Both the first electrode layer and the support layer are patterned to Incorporate a plurality of openings. The openings formed in the support layer form a plurality of air volumes 115 which extend between the first electrode layer and the membrane layer. In this example the size of the openings in both the first electrode layer and the support layer vary from a region at the centre ef the membrane towards an outer or peripheral region of the membrane. Thus, the openings which form the air volumes 115 similarly vary in size, Specifically, the area of the air volumes 115_. formed by the openings in the support layer increase in size from a region towards the centre of the membrane to a region at or near the periphery of the membrane,. This arrangement - specifically the increase in the sfee of the openings towards the periphery of the membrane structure ~ advantageously serves to mitigate membrane edg curl.

Figure 7e shows a membrane structure comprising a first electrode layer 102a and a second electrode layer 102b. The second electrode layer 102b Is formed on the opposite side of t e flexible membrane layer 101 to the first electrode 102a< Each of the first and second electrode layers 102a, 1 2b Is supported in fixed relation relative to the flexible membrane by a support layer 114a, 114 such that the material forming the respective electrode layer is supported in spaced relation from the membrane and is thus separated from the membrane layer by a plurality of air volumes 1 5 formed by openings 115 in the respective: support layer. As in the example shown in Figure 7ύ, th size of the openings in both the electrode layer and the support layer vary from a region at the centre of the membrane towards an outer or peripheral region of the membrane. Thus, the openings which form the air volumes 115 similarly vary in size, Specifically, the area of the air volumes 115 formed by the openings in the support layer increase in size from a region towards the centre of the membrane to a region at or near the periphery of the membrane. The increase in the size of the openings towards the periphery of the membrane structure advantageously serves to mitigate membrane edge curl since it serves to reduce local f ractional metal area coverage to thereby reduce stress in the peripheral region.

Furthermore, the trade off in terms of a reduced capacitance is less critical since the membrane experiences less displacement than the central region of the mem bra n structure .

A membrane structure according to Figure 7e may he usefully employed in conjunction with a MEMS transducer which utilises first and second backplates (each incorporating a baclpiafe electrode) which are respectively positioned above and below the membrane structure.

Figure 7f shows a membrane structure comprising an electrode layer 102 and a support layer 1 14. According to this example, the electrode layer 102 does not extend over a central region of the flexible membrane layer. Thus, within the central region of the membrane structure, the first electrode is formed only of a single support layer which Is provided so as to directly overlay the membrane layer 101. Furthermore, the size of the openings in support layer 1 4 Increase from a region at or near the boundary of the central region to a region at or near the periphery of the membrane In a manner similar to the embodiment shown in Figure 7e, In the regio laterally outside the central region of the membrane i.e. the region where the electrode layer is spaced above the membrane layer by means of the underlying support layer 114, the openings in the support layer form air volumes 115 which extend between the electrode layer and the membrane. Transducers incorporating membrane structures embodying the Figure 7f example are advantageous in that the electrode layer 102 need be provided on just a fraction of the membrane structure ~ e,g> on around 10% of the area of the membrane—in the peripheral region of the membrane. Thus, the presence of decoupling air-bridges, or air volumes, are provided at the edge of the membrane structure which may beneficially mitigate membrane edge-cud.

Figure 7g shows an example similar to the Figure 7f example however, the membrane structure comprising a first electrode layer 102a and a second electrode layer 102b. The second electrode layer 02b is formed on the

opposite side of the flexible membrane layer 101 to the first electrode 102a.

Figures Sa to 8b illustrate the steps involved in a possible method of fabricating a membrane^" structure according to one embodiment of the present invention.

As shown in Figure 8a, the mlcrofabtlcation process starts with the deposition of silicon nitride (SI3N onto a planar silicon substrate wafer using known techniques such as a PECVD (plasma enhanced chemical vapour deposition) method. in Figure 8 b, a sacrificial resist layer Is deposited on top of the silicon nitride and this is then patterned (exposure and development) as shown in Figure 8c, A metal electrode is deposited by conformal coating using e.g, a sputtering technique as shown in Figure Sd. Then, a second layer of resist is deposited on top of the metal layer - as shown in Figure 8© ~ which is then patterned as shown in figure 8f. Reactive ion etch is applied to create metal perforation as shown in Figure 8g. Both resist layers are stripped in the last fabrication step, shown in FIg< 8b to create a layer of metal footing the electrode layer with air volumes underneath in spaces previously filed by the sacrificial resist layer, and the support layer comprising the sldewa!ls of the oonformal metal coating as well as the metal portions directly contacting the substrate. Portions of the underlying substrate may be etched from belo in a later step of the process to release the nitride membrane layer. The support metal may extend laterally to electrically connect the electrode structure to associated bias or amplifier

-circuitry whic may either be co-Integrated on the same substrate or ma be integrated on a separate silicon substrate and coupled via bond pads or contact pads.

A MEMS transducer according to the embodiments described here may comprise a capacifive sensor, for example a microphone,

A MEMS transducer according to the embodiments described here may further comprise readout circuitry; for example wherein the readout circuitry may comprise analogue and/or digital circuitry such as a low-noise amplifier, voltage reference and charge pump for providing higher-voltage bias, analogue~to- digiial conversion or output digital interface or more complex analogue or digital signal processing. There may: thus be provided an Integrated circuit comprising a MEMS transducer as described in any of the embodiments herein. One or more MEMS transducers according to the embodiments described here may be located within a package, This package may have one or more sound ports. A MEMS transducer according to the embodiments described here may be located within a package together with a separate Integrated circuit comprising readout circuitry which may comprise analogue and/or digital circuitry such as a low-noise amplifier; voltage reference and charge pump for providing higher^» voltage bias, analogue-to-digltal conversion or output digital interface or more complex analogue or digital signal processing.

A MEMS transducer according to the embodiments described here may be located within a package having a sound port, According to another aspect, there Is provided an electronic device comprising a MEMS transducer according to an of the embodiments described herein. An electronic device may comprise, for example, 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.

According to another aspect, there is provided a method of fabricating a MEMS transduce as described in any of the embodiments herein.

Although the various embodiments describe a MEMS eapaeiiive 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 EMS transducers having membrane layers comprising silicon nitride.

In the embodiments described above it is noted that references to a transducer element may comprise various forms of transducer element For example, a transducer element may comprise a single membrane and back-plate

combination. In another example a transducer element comprises a plurality of individual transducers, for example multiple membrane/back-plate

combinations, The individual transducers of a transducer element may be similar, or configured differently such .that they respond to acoustic signals differently, e.g. the elements may have different sensitivities, A transducer element ma also comprises different Individual transducers positioned to receive acoustic signals- from different acoustic channels. It is noted that in the embodiments described herein a transducer element may compose, for example, a microphone device comprising one or more

membranes with electrodes for read --out/d ive deposited on the membranes and/or a substrate or back-plate, in the case of MEMS pressure sensors and microphones, the electrical output signal may be obtained by measuring a signal related to the capacitance between the electrodes. However, ft is noted that the embodiments are also intended to embrace the output signal being derived by monitoring piezo-reslstive or piazo-alectric elements or indeed a light source. The embodiments are also Intended embrace a transducer element being a capaeitlve output transducer, wherein a membrane is moved by electrostatic forces generated by varying a potential difference applied across the electrodes, including examples of output transducers where piezoelectric elements are manufactured using MEMS techniques and stimulated to cause motion in flexible members .

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, wearable devices, laptops, mobile phones, PDAs and personal computers. Embodiments may also be used In voice activated or voice controlled devices, Typicai medical applications include hearing aids. Typical Industrial applications include active noise cancellation. Typicai automotive applications include hands-free sets, acoustic crash sensors and active noise cancellation.. ίί 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 ^*ar 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.

Claims

1. A MEMS transducer comprising a membrane structure, the membrane structure comprising a flexible membrane layer and a first electrode layer, the

S first electrode layer being supported relative to the flexible membrane layer so as to be spaced from the flexible membrane layer.

2. A MEMS transducer as claimed in any preceding claim, wherein at least one air volume extends between the material of the first electrode layer and the 0 membrane layer.

3. A MEMS transducer as claimed in claim 1 or 2, wherein the membrane structure comprises a second electrode layer, said second electrode layer being disposed on the opposite side of th flexible membrane to the first electrode,5

4. A MEMS transducer as claimed in claim 3, wherein the second electrode layer Is supported relative to the flexible membrane layer so as to be spaced from the flexible membrane layer. 0 6. A MEMS- transducer as claimed in any preceding claim, wherein the first electrode layer and/or the second electrode layer comprises a continuous sheet of material.

6. A MEMS transducer as claimed in any preceding claim wherein the first6 electrode layer and/or the second electrode layer com rises one or more

openings.

7. MEMS transducer as claimed in any preceding claim wherein the first electrode layer and/or the second electrode layer comprises a lattice structure. 8, A MEMS transducer as claimed In any preceding claim, wherein the first electrode layer is supported relative to the flexible membrane by means of a support structure which extends between the first electrod layer and the flexible membrane layer,

9, A MEMS transducer as claimed In any preceding claim, wherein the second electrode layer is supported relative to the flexible membrane by means of a support structure which extends between the second electrode layer and the flexible membrane layer,

10, A MEMS transducer as claimed in claim 8 or 9, wherein the support structure comprises a plurality of support elements.

11, A MEMS transducer as claimed in any preceding claim, wherein the support elements comprise dielectric or conductive material,

12, A MEMS transducer as claimed In claim 10 or 11 , wherein the support: elements are disposed at or near the periphery of the membrane structure.

13, A MEMS transducer as claimed In claim 8 or 9, wherein the support structure comprises a conductive layer having a plurality of openings,

14, A MEMS transducer as claimed in claim 13, wherein the support structure comprises a lattice structure.

15, A E S transducer as claimed in claim 13 or 14 wherein the openings of the support structure form a plurality of air volumes which extend between the material of the first electrode layer and the flexibl membrane layer.

16. A MEMS transducer as claimed In claim 13, 14 or 15, wherein the openings of the support structure increase in. size from a region towards the centre of the membrane structure to a region at or near the periphery of the membrane structure,

17. A MEMS transducer as claimed In claim 10 or 1 , wherein the openings of the support structure are substantially of uniform size.

18. A MEMS transducer as claimed in any one of claims 13 to 16 when appended directly or indirectly to any one of claims 8 or wherein the openings of the first and/or second electrode layer are laterally offset from the openings of the support structure.

19. A MEMS transducer as claimed In any preceding claim, wherein the first electrode layer comprises a hole which overlies at least a central region of membrane layer.

.20. A MEMS transducer comprising a membrane structure, the membrane structure comprising a membrane and an electrode, wherein the material forming the electrode Is separated from the membrane by at least one air volume.

21. A MEMS transducer as claimed in any preceding claim, wherein the flexible membrane comprises a crystalline of polycrystalllne material

22. A MEMS transducer as claimed in claim 21 , wherein the flexible membrane layer comprises silicon nitride.

23. A MEMS transducer as claim in any preceding claim, wherein the first and/or second electrode layer comprises metal, a metal alloy or a metallic compound.

24. A MEMS transducer as claimed in claim 23, wherein the electrode comprises aluminium, aluminium-silicon alloy or titanium nitride,

25. A MEMS transducer as claimed in any preceding claim comprising a back- plate structure wherein the flexible membrane is supported with respect to said back-plate stajcture,

26. A MEMS transducer as claimed in claim 25 wherein said back-plate structure comprises a plurality of holes through the back-plate structure,

27. A M MS transducer as claimed in any preceding claim wherein said transducer comprises a capacfiive sensor

28. A MEMS transducer as claimed m any preceding claim wherein said transducer comprises a microphone,

29. A MEMS transducer as claimed in claim 2? or 28 further composing readout circuitry.

30. A MEMS transducer as claimed in claim 29., wherein the readout circuitry may comprise analogue and/or digital circuitry,

31. A MEMS transducer as claimed in any preceding claim wherein the transducer is located within a package having a sound port.

32. An electronic device comprising a MEMS transducer as claimed in any preceding claim.

33. An electronic device as claimed in claim 32 wherein said device is 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.

34. An integrated circuit comprising a MEMS transducer as claimed in any preced i ng clal m and readout ci reustry.

35. A method of fabricating a MEMS transducer as claimed in any preceding claim.