MEMS device and process

Abstract

The application describes a MEMS transducer in which first and second conductive elements of a capacitor are both provided on the membrane. The membrane is shaped such that the first and second conductive elements are displaced relative to each other when the flexible membrane deflects in response to a pressure differential across the membrane. For example the membrane may be corrugated.

Claims

1. A MEMS transducer comprising: a flexible membrane provided with an electrode pair, the electrode pair comprising first and second conductive elements; the flexible membrane comprising a series of peaks, wherein serially adjacent peaks are separated by a side wall portion, and wherein the first conductive element is provided on a first peak and the second conductive element is provided on a second peak; and wherein the first and second conductive elements are displaced relative to each other when the flexible membrane deflects in response to a pressure differential across the membrane.

2. A MEMS transducer as claimed in claim 1, wherein the membrane is shaped to include a series of alternate ridges and grooves.

3. A MEMS transducer as claimed in claim 1, comprising a plurality of electrode pairs.

4. A MEMS transducer as claimed in claim 3, wherein the plurality of electrode pairs form a single set of electrode pairs and wherein the transducer is arranged/operable to provide a single output signal representing the change in capacitance between first and second conductive elements of the single set of electrode pairs.

5. A MEMS transducer as claimed in claim 3, wherein the plurality of electrode pairs form first and second sets of electrode pairs, and wherein the transducer is arranged/operable to provide corresponding first and second output signals representing the change in capacitance between first and second conductive elements of each respective set.

6. A MEMS transducer as claimed in claim 1, wherein a bias voltage is applied to one or more of the conductive elements of an electrode pair.

7. A MEMS transducer as claimed in claim 1, wherein the flexible membrane is supported in a fixed relation relative to a substrate.

8. A MEMS transducer as claimed in claim 1, wherein the flexible membrane comprises a crystalline or polycrystalline material, such as silicon nitride.

9. A MEMS transducer as claimed in claim 1, wherein said transducer comprises a capacitive sensor such as a microphone or a speaker.

10. A MEMS transducer comprising: a flexible membrane provided with an electrode pair, the electrode pair comprising first and second conductive elements; the flexible membrane comprising a series of peaks, wherein serially adjacent peaks are separated by a side wall portion, and wherein the first conductive element is provided on a first side wall portion and the second conductive element is provided on a second side wall portion; and wherein the first and second conductive elements are displaced relative to each other when the flexible membrane deflects in response to a pressure differential across the membrane.

11. A MEMS transducer as claimed in claim 10, wherein the membrane is shaped to include a series of alternate ridges and grooves.

12. A MEMS transducer as claimed in claim 10, comprising a plurality of electrode pairs.

13. A MEMS transducer as claimed in claim 12, wherein the plurality of electrode pairs form a single set of electrode pairs and wherein the transducer is arranged/operable to provide a single output signal representing the change in capacitance between first and second conductive elements of the single set of electrode pairs.

14. A MEMS transducer as claimed in claim 12, wherein the plurality of electrode pairs form first and second sets of electrode pairs, and wherein the transducer is arranged/operable to provide corresponding first and second output signals representing the change in capacitance between first and second conductive elements of each respective set.

15. A MEMS transducer as claimed in claim 10, wherein a bias voltage is applied to one or more of the conductive elements of an electrode pair.

16. A MEMS transducer as claimed in claim 10, wherein the flexible membrane is supported in a fixed relation relative to a substrate.

17. A MEMS transducer as claimed in claim 10, wherein the flexible membrane comprises a crystalline or polycrystalline material, such as silicon nitride.

18. A MEMS transducer as claimed in claim 10, wherein said transducer comprises a capacitive sensor such as a microphone or a speaker.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding 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:

(2) FIGS. 1a and 1b illustrate known capacitive MEMS transducers in section and cut-away perspective views;

(3) FIG. 2 is a sectional view of a flexible membrane according to one example embodiment;

(4) FIGS. 3a to 3c provide various views of a flexible membrane according to another example embodiment;

(5) FIG. 4a shows a graphical representation of the change in capacitance of the first and second sets of electrode pairs shown in FIGS. 3a to 3c plotted against the displacement of the membrane;

(6) FIG. 4b shows a graphical representation of the differential voltage signal obtained from the two plots shown in FIG. 4a plotted against the displacement of the membrane;

(7) FIGS. 5a and 5b show a cross-section through a membrane according to a further embodiment of the present invention;

(8) FIGS. 6a to 6d illustrate in cross-section a number of membrane designs according to various examples embodying the present invention;

(9) FIGS. 7a and 7b each illustrate a plan view of a membrane design according to examples embodying the present invention; and

(10) FIG. 8 illustrates steps of a process flow for a non-planar membrane according to an embodiment of the present invention.

(11) Throughout this description any features which are similar to features in other figures have been given the same reference numerals.

DESCRIPTION

(12) Embodiments of the present invention relate to MEMS transducers comprising a non-planar flexible membrane.

(13) FIG. 2 illustrates a cross-section through a flexible membrane 215 according to a first example embodiment. The membrane comprises a groove 214 formed of a pair of sidewalls 204a and 204b which extend from the surface of the membrane to a peak of the groove in the form of planar surface 205. In this particular example, the groove projects below the main planar surface of the membrane. However, it will be appreciated that the FIG. 2 embodiment could similarly comprise a ridge which projects above the main planar surface of the membrane. Thus, the membrane can be considered to exhibit a change or discontinuity in the z-dimension (i.e. orthogonal to the plane of the membrane defined in the x and y dimensions) as illustrated in FIG. 2. The membrane 215 is provided with first and second conductive elements 202a and 202b which are respectively provided on the first and second sidewalls 204a and 204b of the groove.

(14) In use the electrodes are connected to circuitry (not shown) which allows the capacitance to be measured.

(15) When a pressure wave due to acoustic noise is incident on the flexible membrane 215, the membrane is displaced from its equilibrium position. A displacement of the membrane results in a corresponding displacement in the distance between the conductive elements 202a and 202b, which is measurable as a change of capacitance. For example, an upward displacement of the flexible membrane 215 will cause a separation of the two conductive elements, resulting in a measurable reduction in the capacitance arising between the conductive elements. The measured change in capacitance allows the change in distance between the elements, and thus the amount of displacement of the membrane to be determined.

(16) The FIG. 2 design represents an alternative membrane structure wherein a pair of capacitive platesor conductive elementsare provided on, or associated with, the membrane. This facilitates the generation of measurable capacitance laterally across the membrane as first and second conductive elements are provided a different lateral positions with respect to the notional plane of the membrane.

(17) According to this embodiment, the need for a fixed electrode positioned above the membranee.g. as part of a backplate structureis mitigated. This enables the possibility of a transducer configuration according to which the usual backplate structure, or other support structure for the fixed electrode, may be omitted. Such a transducer configuration will benefit from a significant reduction in acoustic noise.

(18) FIG. 3a illustrates a cross-section through a membrane substantially in the equilibrium position according to a further embodiment of the present invention. The membrane 314 comprises a plurality of alternate ridges and grooves which project above and below a notional main plane 316 of the membrane. Each of the ridges and grooves comprises a pair of sloped sidewalls. The membrane exhibits a generally square-wave shape. The peak of each ridge/groove comprises a substantially planar surface 317 which extends longitudinally across the membrane. This can be seen more clearly from FIG. 3c which is a perspective view of a part of the membrane 314.

(19) The membrane 314 comprises a plurality of electrode pairs 312each comprising first and second conductive elements 302a and 302bwhich are indicated in FIG. 3 by the dashed lines. It will be appreciated that the conductive elements may belong to only one electrode pair, as shown in FIG. 3a or, alternatively, one or more of the conductive elements may belong to more than one electrode pairi.e. a conductive element may be shared by two or more electrode pairs. This will be determined by the associated circuitry and the manner in which the conductive elements are connected for readout. The electrode pairs form two sets of electrode pairs 312a and 312b.

(20) In this embodiment, first and second conductive elements forming an electrode pair are located on the sidewalls of the ridges/grooves so as to be substantially facing one another. When the membrane is considered at the equilibrium position, as shown in FIG. 3a, the conductive elements of each conductive pair in a given set are provided at substantially the same offset from the notional main plane of the membrane 316. Thus, the first set of electrode pairs 312a is offset in the +z direction and the second set of electrode pairs 312b are offset by the same distance in the z direction.

(21) In use, a bias voltage is applied to one or both of the first and second conductive elements. The potential difference applied across each of the electrode pairs may be the same or different.

(22) Again, when a pressure wave due to acoustic noise is incident on the flexible membrane 315, the membrane is displaced from its equilibrium position. FIG. 3b shows the membrane 315 when it has been displaced upwardly from equilibrium in the +z direction. Following displacement of the membrane as shown in FIG. 3b, the notional main plane of the membrane 316 can be considered to be distorted into an arc which extends between the peripheral supporting edge of the membrane. The conductive elements forming the first set of electrode pairs 312a have been displaced such that the distance between respective first and second conductive elements of an electrode pair has increased. Conversely, the conductive elements forming the second set of electrode pairs 312b have been displaced such that the distance between the respective first and second conductive elements of an electrode pair has decreased.

(23) Depending on the specific design e.g. dimensions relative to the notional plane, it will be noted that for small upwards displacements, the conductive elements forming the second set of electrode pairs may initially be displaced towards one another, giving an increase in capacitance, before continuing to be displaced away from one another, giving a decrease in capacitance, as the upward displacement of the membrane increases. The opposite will be true for downward deflection of the membrane in the z direction. FIG. 4a shows a graphical representation of the change in capacitance of each of the first and second sets of electrode pairs vs. the displacement of the membrane at the centre of the membrane. The solid line represents the change in capacitance between the electrode pairs of the first set of electrode pairs 312a. The upper, broken line, represents the change in capacitance between conductive elements forming electrode pairs of the second set of electrode pairs 312b. Thus, from this it can be seen that the capacitance of the second set of electrode pairs initially increases and then starts to decrease.

(24) A particular advantage of MEMS transducers incorporating a membrane as illustrated in this embodiment is that two outputs may be generatedone output which represents the change in capacitance between conductive elements of the first set of electrode pairs, and one output which represents the change in capacitance between conductive elements of the second set of electrode pairs. Thus, embodiments which incorporate a membrane provided with first and second sets of electrode pairs, for example as shown in FIG. 3, may be advantageously employed as a differential system. As will be appreciated, differential systems are advantageous in that they allow electrical noise generated from sources such as the transducer power supply or noise picked up from external electrical interference to be removed by e.g. common mode rejection whereby the two output signals generated from the respective changes in capacitance are applied to both inputs of differential amplifier and subtracted.

(25) In addition, and as discussed with respect to the FIG. 2 embodiment, the need for a fixed second electrode, positioned relative to the membrane e.g. as part of a backplate, is mitigated as the change in capacitance can be obtained from the electrode pairs provided on the membrane. Thus, the FIG. 3a embodiment facilitates the generation of a differential signaland thus the rejection of electrical noise present in both output signalsas well as a reduction in the acoustic noise that would otherwise be introduced by the provision of a structure such as a backplate which supports the fixed electrode. This is particularly beneficial when compared with previously considered transducer designs which facilitate the generation of a differential signal. Such prior designs typically utilise a second backplate structure to support a third fixed electrode, such that displacement of the membrane electrode is measured with respect to the second fixed electrode provided in the first backplate structure (first output signal) and with respect to the third fixed electrode provided in the second backplate structure (second output signal) However, although the prior design achieves a doubling of the obtained output signal, and thus may improve the signal to noise ratio (SNR), this improvement may be undermined by the increase in noise that is a consequence of the additional backplate structure.

(26) The difference between the output signals generated by the first and second sets of electrode pairs can thus be processed to obtain a differential signal. FIG. 4b shows a graphical representation of the differential voltage signal obtained from the two plots shown in FIG. 4a plotted against the displacement of the membrane centre. It is noted that the difference between the capacitances of the first and second sets of electrode pairs will continue to grow even when the second set of electrode pairs are being initially displaced towards each other and then subsequently being displaced apart. It can be seen from FIGS. 4a and 4b that although the distances between the conductive elements of the first and second sets of electrode pairs does not vary linearly with membrane displacement, the difference between them does.

(27) The properties of the membrane will affect the performance e.g. sensitivity of the transducer device. For example, the depth and/or width of the ridges/grooves, the stiffness and/or thickness of the membrane material, and the overall shape of the membrane area all parameters which may affect the distance that the first and second conductive elements may be. This, in turn will affect the sensitivity of the transducer device. Embodiments of the present invention are envisaged in which the various membrane properties are selected for a particular sensitivity of the required transducer device and/or for a particular application of the required transducer device.

(28) FIGS. 5a and 5b show a cross-section through a membrane according to a further embodiment of the present invention. The membrane is corrugated in form and thus comprises a plurality of alternate ridges and grooves which project above and below a notional main plane 316 of the membrane. Each of the ridges and grooves comprises a pair of sidewalls. The membrane exhibits a substantially square-wave shape. The peak of each ridge/groove comprises a substantially planar surface which extends longitudinally across the membrane.

(29) In this example the membrane comprises first and second sets of electrode pairs 312a and 312b. However, in this example the conductive elements of the electrode pairs are provided on the top and bottom peak planes of the ridges and grooves. Such an arrangement is relatively easy to fabricate since the metal or conductive material forming the conductive elements may be readily deposited on the peak planes.

(30) FIG. 6 illustrates in cross-section a number of membrane designs according to various examples embodying the present invention. The membranes are supported in fixed relation to a transducer substrate 620.

(31) FIG. 6a shows a membrane 615 comprising a plurality of arcuate ridges. First and second conductive elements 302a and 302b forming an electrode pair are provided on the sidewalls of a given ridge. This example facilitates the generation of a single output signal.

(32) FIG. 6b shows a membrane 615 comprising a series of alternate ridges and grooves. The membrane is shaped to exhibit a substantially sinuous form. The membrane is provided with first and second sets of electrode pairs 312a and 312b. The conductive elements of the electrode pairs are provided on the peaks of the membrane. The conductive elements forming the second set of electrode pairs 312b are thus provided on the negative peak, or trough, that is below the notional main plan of the membrane. This example facilitates the generation of two output signalsone from each of the sets of electrode pairs 312a and 312band thus the generation of a differential signal that advantageously allows electrical noise present in the output signal to be eliminated by common mode rejection.

(33) FIG. 6c shows a membrane comprising a plurality of rectangular shaped ridges. First and second conductive elements 302a and 302b forming an electrode are provided on the peak surfaces of adjacent ridges.

(34) FIG. 6d shows a membrane shaped to comprise a plurality of alternate ridges and grooves. The membrane exhibits a truncated triangular form. As in FIG. 6b, the membrane is provided with first and second sets of electrode pairs 312a and 312b. The conductive elements of the electrode pairs are provided on the peaks of the membrane. In this example, two of the conductive elements of each set of electrode pairs are shared by two, adjacent, electrode pairs. This example facilitates the generation of two output signalsone from each of the sets of electrode pairs 312a and 312band thus the generation of a differential signal that advantageously allows electrical noise present in the output signal to be eliminated by common mode rejection.

(35) FIGS. 7a and 7b each illustrate a plan view of a membrane design according to examples embodying the present invention. The lines drawn on FIGS. 7a and 7b represent the peaks of the groove and/or ridges of the membrane. Although drawn as a line, it will be appreciated that the peaks may actually comprise a planar surface such as that shown in FIG. 3c, when the membrane is shaped to provide a plurality of ridges and/or grooves having a generally square wave profile or having a truncated triangular wave profile.

(36) As shown in FIG. 7a, the peaks of the plurality of ridges extend linearly across the membrane which will be anchored with respect to the underlying membrane (not shown). The ridges can be considered to define a longitudinal axis, wherein the axes of the ridges are substantially parallel and generally aligned in a first direction. It is envisaged that the plurality of ridges may alternatively define a plurality of curved lines.

(37) As shown in FIG. 7b the peaks of each of the plurality of ridges may define a closed circle or polygonin this case a rectanglewhich are arranged radially with respect to the membrane centre. The size of each of the rectangles defined by the peaks increases from a region at or near the centre of the membrane towards the periphery of the membrane. Thus, the FIG. 7b embodiment can be considered to comprise a plurality of concentric corrugations. The peaks may be symmetrically arranged about a common axis of symmetry, with the centre of the membrane or alternatively the membrane may be shaped so that the peaks follow the shape of the perimeter of the membrane.

(38) Embodiments of the present invention facilitate the generation of measurable capacitance laterally across the membrane. Thus, first and second conductive elements are provided at different lateral positions on the membrane.

(39) FIG. 8 illustrates a sequence of stepsnumbered 1 to 27of a process flow for fabricating a non-planar membrane having a single corrugation, or groove, according to an embodiment of the present invention. For clarity, only a single corrugation is illustrated in this figure. Step 1: Bare silicon substrate is provided. Step 2: A Resist (not shown) is laid down on one sidetermed the front side for the purposes of this explanationof the substrate and is patterned. A timed etch is performed into the Si substrate. The depth of this etch should be deeper than the proposed corrugation. Step 3: A thin layer of polyimide (PI) is spun onto the wafer. This step plays a role in controlling the depth of corrugation and subsequently acts as an Etch Stop (ESPI) in a subsequent back etch through the substrate (step 23). Step 4: A resist layer is laid down prior to planarization. Step 5: Resist layer is patterned. Step 6: Front side is planarized by Chemical-Mechanical Planarization (CMP) and then the resist is removed. Step 7: A corrugation stop layer is laid down. The material used for the corrugation stop should ideally have high selectivity to both PI and Si etch chemistries. For example, aluminium/titanium may be used. This material may also eventually become an electrode on the lower side of the corrugation. The thickness of the corrugation stop will affect the final depth of the corrugation. Step 8: A further resist layer is deposited. Step 9: The resist is patterned. Step 10: The corrugation stop is etched. Step 11: The resist is removed. Step 12: A further PI layer is spun over the corrugation stop. Step 13: A further CMP planarization is performed. Step 14: A further resist layer is deposited. Step 15: The resist is patterned. Step 16: The PI layer is etched down onto the corrugation stop. The corrugation stop serves to define the end point of the etch. Step 17: The resist is removed. Step 18: A layer of Silicon Nitride is deposited on the front surface creating a membrane layer having a corrugation. Step 19: Metal to form one or more conductive element(s) is deposited on top surface. Step 20: A resist layer is spun and patterned. Step 21: The top metal layer is etched to define two conductive elements. (again, important to link terminologyconductive elementsto this process flow). Step 22: A resist mask is deposited on the other side of the substratereferred to as the back side for the purposes of this explanation. Step 23: Back side resist is patterned. Step 24: Back side etch to polyimide which acts as an end point for this etch. Step 25: Remove Back side resist. Step 26: Further backside etch terminating at the corrugation stop. Step 27: Residual polyimide is etched away to fully release the membrane.

(40) Thus according to embodiments of the present invention, a method of fabricating a non-planar flexible membrane comprising at least one conductive element, comprises: forming a first cavity in the upper surface of a substrate (steps 1-2: which may e.g. be referred to as a front-side etch process); providing a first sacrificial layer of material within the cavity (steps 3-6); providing a first patterned layer of conductive/metal material on top of the sacrificial layer within the cavity(steps 7-11); providing a second sacrificial layer of material within the cavity (steps 12-13) and removing the second sacrificial layer in regions overlying the first patterned layer of conductive/metal material to define a second cavity (within the first cavity) above the patterned layer of conductive/metal material (steps 12-16); and depositing a layer of membrane material (e.g. SiN) over the first cavity including over the second cavity (steps 17-18).

(41) Thus, steps 1 to 18 result in the formation of a non-planar membrane layer having a corrugation (a corrugated membrane layer), wherein a conductive element is provided on the underside of the corrugation. Thus, the first layer of conductive metal material is for forming at least one conductive elements on the underside of the membrane layer.

(42) Subsequently, according to a preferred embodiment of the present invention, the method may comprise: providing a second layer of conductive/metal material on top of the corrugated membrane layer (step 19); patterning the second layer of conductive/metal material to define at least one further conductive element.

(43) Thus, the additional steps 19 and 20 result in the formation of a non-planar membrane layer having at least one conductive element provided on both the upper and lower surfaces of the corrugated membrane layer.

(44) Subsequently, according to a preferred embodiment of the present invention, the method further comprises: forming a cavity in the lower surface of the substrate (steps 22-24 which may be referred to as e.g. a back-side etch process); removing the first and second sacrificial layers to release the corrugated layer of membrane material (steps 25 to 27).

(45) It will be appreciated that the number of corrugations (ridges and/or grooves) as well as the number of conductive elements provided on each side of the membrane layer will depend on the intended design of the membrane layer. For example, the method may be readily adapted to provide at least one electrode pair on just one side of the membrane without providing any conductive elements on the other side. First and second conductive elements forming an electrode pair will preferably be located with respect to the corrugation(s) such that the first and second conductive elements are displaced relative to each other when the flexible membrane deflects in response to a pressure differential across the membrane.

(46) It should be understood that the directions provided should not be in any way construed as limiting to any particular orientation of the transducer during any fabrication step and/or its orientation in any package, or indeed the orientation of the package in any apparatus. The relative terms upper, lower, above, below, underside, underneath etc. shall be construed accordingly.

(47) Embodiments of the present invention are particularly applicable to MEMS sensor transducers, especially capacitive transducers such as MEMS microphones and MEMS speakers. It will also be appreciated that other types of MEMS capacitive sensors could be implemented, for example accelerometers, pressure sensors, proximity sensors or flow meters.

(48) Embodiments may be implemented in a host device, especially a portable and/or battery powered host device such as a mobile telephone, and audio player, a video player, a PDA, a mobile computing platform such as a laptop computer or tablet and/or a games device for example or in an accessory device, such a headset, earbud (possibly noise-cancelling), or microphone assembly, designed for wired, or wireless connection with such host devices, possibly via multi-wire cables, multi-pole jacks, or optical fibres and connectors.

(49) 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 numerals or labels in the claims shall not be construed so as to limit their scope.