AU2005227359A1 - A layered microphone structure - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): HEARWORKS PTY LTD Invention Title: A LAYERED MICROPHONE STRUCTURE The following statement is a full description of this invention, including the best method of performing it known to me/us: PC'I'/AUU I/UUZ4U eceived 19 September 2001 -1 0 A Layered Microphone Structure INDTechnical Field The present invention relates to variable capacitor microphones and to a OC method of fabricating the microphones.

Background of the Invention I A typical capacitive or condenser microphone includes a first electrode formed by a diaphragm and a second electrode formed by a back plate. The diaphragm and back plate are paired together to form a capacitor in which capacitance varies with sound pressure incident on the diaphragm. Miniature microphones used in hearing aids and other applications typically also include an electret material between the electrodes of the capacitor. Such miniaturised microphones are reaching the limits of traditional manufacturing techniques. As the microphone components are reduced in size, it becomes more difficult to maintain the same level of manufacturing precision, resulting in microphones with variability in sensitivity and impedance. Attempts have been made to overcome the difficulties associated with miniaturising traditional microphones by using Semiconductor fabrication techniques to make solid state microphones. This technology can potentially increase the manufacturing precision and reduce the manufacturing cost for large volumes, but restricts the choice of transducer component materials to silicon and associated materials. Consequently, the mechanical characteristics are also restricted. For example, silicon diaphragms have a low compliance, resulting in reduced acoustic sensitivity. Such problems have slowed the commercial introduction of a silicon-based microphone.

-1B- Summary of the Invention C In one aspect the present invention provides a capacitive microphone for generating an output signal comprising a moveable electrode and first and second fixed O electrodes, the moveable electrode is disposed between the first and second fixed D 5 electrodes, the first fixed electrode and moveable electrode form a first capacitor, the second fixed electrode and moveable electrode form a second capacitor, and the first and second capacitors are effectively connected in parallel to generate a single output t¢ signal from the microphone, the signal being output by the moveable electrode of the microphone.

PCT/AU01/00240 i :eived 19 September 2001 -2- The microphone structure can be manufactured to high tolerances, for example using optical lithography techniques, or by optical machining (controlled removal of the O top layer by laser ablation). Furthermore, the separation between the first and second electrodes can be carefully controlled by selecting the appropriate spacer layer thickness and shape.

In Cc Acoustic signals are measured by applying a bias voltage to the electrodes N,1 and measuring the change in capacitance as the moveable electrode moves. The moveable electrode may be secured to one face of a diaphragm such that N,1 10 it deflects with the diaphragm, and the fixed electrode may be secured to a back plate.

The moveable and fixed electrodes may comprise electrically conductive coatings formed on opposing faces of the diaphragm and back plate, respectively.

The spacer layer may be shaped to provide support to at least a peripheral region of the moveable electrode, while defining at least one "active" region of the moveable electrode radially inward of the peripheral region where there is a gap between the electrodes, allowing the moveable electrode to deflect. For example, the peripheral region may be annular, providing an active region which is circular and enclosed by the peripheral region. The peripheral region may extend to the perimeter of the moveable electrode.

The microphone may detect acoustic signals travelling through the fixed or moveable electrodes. For example, the microphone may detect acoustic signals travelling in a direction which is generally perpendicular to the moveable electrodes.

Alternatively, or additionally, the microphone may detect acoustic signals travelling in a direction which is generally parallel to the moveable electrode, through apertures or socalled side vents, in the microphone. The at least one spacer layer may comprise a plurality, of spacer layers. The plurality of spacer layers may be stacked one on top of another such that they either partially or completely overlap. Alternatively, the spacer layers may be arranged such that none of the spacers overlap. A side vent for introducing acoustic signals into the microphone may be formed by providing a separation between neighbouring spacer layers. Acoustic signals may be funnelled into such a side vent via a tube or housing. In one example, the spacer layer is in the form of arc-shaped strips arranged end-to-end around the peripheral region of the diaphragm with a separation between oppositely-facing ends of neighbouring strips such that side AMENDED SHEET

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PCT/AUO 1/00240 S( teceived 19 September 2001 -3vents are formed. A microphone with a side vent may alternatively be formed between non-contacting oppositely-facing end portions of a "C"-shaped one-piece spacer layer.

O The spacer layer may support a central region, while not supporting the ND perimeter of the moveable electrode: Alternatively, the spacer may provide support to the peripheral region of the moveable electrode in addition to a central region, while providing a gap between the electrodes over the remainder of the moveable electrode C area. It is known that when a moveable electrode area becomes too large, forces on the moveable electrode such as electrostatic forces can cause it to collapse and fail. The t spacer layer preferably provides support to the moveable electrode in a way which prevents diaphragm failure. For example, the spacer layer may provide support to areas where deflection of the moveable electrode would be the greatest, such as radiallyinward of the peripheral region.

Such a spacer layer allows the acoustic sensitivity of a microphone to be increased by manufacturing the diaphragm from a material, such as a polymer, which has a relatively high mechanical compliance compared to silicon nitride and other silicon derivatives. Although it is known to increase the sensitivity of microphones by using a diaphragm with a high mechanical compliance, the reduced stiffness of such materials conventionally places limits on the area of the diaphragm.

In one example, the spacer layer includes one or more island-like structures which provide support to selected central regions of the moveable electrode and give rise to active regions of the moveable electrode around each island. In another example, the spacer layer includes fingers extending from the peripheral region in a generally radially-inward direction. Areas between fingers give rise to active regions of the moveable electrode. Fingers on opposite sides of the moveable electrode may be interleaved. One or more fingers may extend radially inward from the peripheral region and branch from there into a plurality of further fingers. Combinations of these various types of fingers may also be used. In yet another example, the spacer layer includes one or more strips connecting opposite parts of the peripheral region such that the moveable electrode is divided into two or more isolated active regions, or "cells". The spacer layer, may have a grid-like configuration which divides the moveable electrode into more than two cells. A wide variety of grid-like spacer layer configurations are possible. In a preferred embodiment, the spacer layer comprises a sheet in which a plurality of apertures are formed in the sheet, each aperture defining the shape of a cell AMENDED SHEET

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PCT/AU01/00240 Received 19 September 2001 -4- Sin which the moveable electrode may deflect either independently or in unison with Sother cells. Each aperture may be circular, triangular, square, rectangular, polygonal or O any one of a variety of other shapes. The apertures may be arranged in the spacer layer IO in a variety of different patterns. For example, the apertures could be aligned in straight or curved rows or distributed radially about a central point. The spacer layer may also comprise a combination of a grid-like structure and one or more finger structures.

Cc It is preferable to maximise the active area of the moveable electrode since microphone sensitivity is maximised when the active area is maximised. The spacer layer allows the forces on each cell to be reduced to below that which would collapse the moveable electrode. The sensitivity of the microphone can then be raised by increasing the active area without being limited by collapsing forces. Unusually large microphones can be formed by dividing a large moveable electrode into numerous small-area cells. The moveable electrode, or moveable electrode plus diaphragm, can be made from higher compliance materials than would be possible with a microphone having a large but single active area.

The back plate, spacer layer, or diaphragm may be made from one or more polymer sheet materials, such as polyester, and preferably all of these components are made from a polymer material. A combination of different materials may be used in the microphone. Polymer sheet material has the advantage that it can be cut to high precision using laser cutting techniques. In one particular embodiment, the diaphragm and back plate are formed from sheets of polyester. The moveable and fixed electrodes may comprise conductive gold films coated on faces of the diaphragm and back plate, respectively. The impedance and sensitivity of the microphone can be controlled by changing the shape, size and thickness of the spacer layer.

Vibration of the moveable electrode will be at least partially restricted, and is preferably prevented entirely, wherever the moveable electrode makes contact with the spacer layer. This may-be achieved by attaching the moveable electrode directly to the spacer layer, such as with adhesive or fasteners, in order to limit deflection away from the spacer. Alternatively, the moveable electrode may be held down against the spacer layer by a clamping means disposed on an opposite face of the diaphragm to the spacer layer. Preferably, the clamping means only restricts deflection of the moveable, electrode over areas which are supported by the spacer layer. Clamping the moveable AMENDED SHEEI

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WO 01/67809 PCT/AUOI/00240 electrode between the spacer layer and clamping means allows deflection to be confined more effectively to each cell. A plurality of clamped cells thus effectively operate as small vibrationally-isolated moveable electrodes acting in unison. The clamping means may be held in place by an external pressure applied against the clamping means towards the moveable electrode. Additionally, or alternatively, an adhesive may be used to hold the clamping means in place such that deflection of the moveable electrode is restricted.

The clamping means may be a flat sheet with a shape and size very similar to that of the spacer. In one embodiment, the clamping means and spacer layer are both an 3 annulus having an identical shape, and identical internal and external diameters. In another embodiment, the clamping means and spacer layer both have an identical gridlike structure. In a further embodiment, the spacer layer and clamping means both comprise layers in which rows of hexagonal apertures are formed. When the microphone is assembled, the clamping means and spacer layer are positioned such that the hexagonal apertures are aligned.

An omnidirectional microphone may be formed by introducing acoustic signals into the microphone from one face of the moveable electrode only, such as the face of the moveable electrode facing away from the spacer layer. A directional microphone may be formed by introducing acoustic signals into the microphone from 0 both sides of the moveable electrode. Such a microphone may include an acoustic inlet in the back plate and fixed electrode, and/or a side vent formed by a gap or channel in the spacer layer, for the passage of acoustic signals. The microphone may further include an acoustic delay element in communication with the acoustic inlet. An acoustic signal reaching the moveable electrode via the acoustic delay element will be time-delayed with respect to an acoustic signal reaching the opposite side of the moveable electrode. The microphone is thus made sensitive to the spatial direction-ofarrival of acoustic signals. As the time delay of the acoustic delay element approaches infinity, the microphone becomes an omnidirectional device. In embodiments in which the moveable electrode is divided into a plurality of cells, the back plate and fixed electrode preferably includes a separate acoustic inlet beneath each cell. The acoustic delay element may communicate with the plurality of cells via the plurality of acoustic inlets.

PCT/AUO1/00240 i; -ceived 19 September 2001 -6- "1 Known acoustic delay elements may be used with the microphone. Preferably, Othe acoustic delay element is formed by the combination of an acoustic resistance 0 element in conjunction with a rear chamber (also known as cavity) that forms an acoustic compliance. The resistance and the compliance act to create a time delay. The rear chamber communicates with each respective diaphragm cell via each respective acoustic inlet in the back plate and fixed electrode. The chamber may terminate with a rear plate which hasan acoustic resistance formed by a porous passage. Alternatively, I the rear plate may be sealed off and not have a porous passage. In this case, the rear port effectively creates an infinite delay, resulting in an omnidirectional microphone.

10 The above microphones may be enhanced further by including a second fixed electrode on an opposite side of the moveable electrode to the first-mentioned fixed electrode. The moveable electrode is thus disposed between two fixed electrodes, effectively forming two capacitors in parallel. If the moveable electrode moves towards the first fixed electrode, the first capacitance will increase while the second capacitance will decrease, and vice versa. The first and second capacitances are preferably combined together..

The second fixed electrode may be secured to a "front plate". Preferably, the second fixed electrode comprises a conductive coating formed on the front plate, and the moveable electrode comprises conductive coatings formed on opposite faces of a diaphragm, preferably with an electrical connection between the two coatings. Diaphragm deflection can then be detected from a second capacitance @(between the second fixed electrode and moveable electrode) in addition to the first-mentioned capacitance. The second fixed electrode may be separated from the moveable electrode by a second spacer layer identical to any one of the spacer layers described above. Preferably, the second spacer layer between the second electrode and moveable electrode is aligned with an identically-shaped first spacer layer between the moveable electrode and first electrode. It is preferable that the first and second spacers substantially overlap. A clamping means may be provided to hold the front plate in place.

Any one of the above microphones may be constructed as either a purely capacitive microphone, or optionally, as a capacitive microphone with an electret layer between each pair of fixed and moveable electrodes to create a baseline electric field.

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PCT/AUO 1/00240 .'eceived 19 September 2001 -7- SThe baseline electric field can be enhanced with an externally-applied electric field.

The electret is preferably attached to a fixed electrode in order to avoid reducing the O compliance of the diaphragm.

INO

N, A second aspect of the present invention provides a stacked microphone formed from a plurality of sub-microphones, each sub-microphone being in accordance with any one of the microphones described above, wherein the sub-microphones are e3stacked one on top of the other such that all moveable and fixed electrodes are substantially parallel. The second aspect allows the first fixed electrode of a first sub- 10 microphone to also function as a fixed electrode of a second sub-microphone. Each stacked microphone may have a front plate and/or a back plate at opposite ends.

Acoustic signals may be introduced through side vents formed by one or more spacer layers as described above.

A third aspect of the invention provides a method of separating a fixed electrode from a moveable electrode in a capacitive microphone, the method comprising a step of mounting at least one layer between the fixed electrode and moveable electrode, the at least one layer including a spacer layer formed from one or more sheets of material. The at least one spacer layer may be in accordance with any one of the spacer layers described above.

The components of any one of the embodiments described above can be manufactured from a variety of materials using a variety of different techniques. For example, the spacer layer, back plate, front plate, and clamping means can be made from silicon or associated materials using known lithography and etching techniques.

SAlternatively, laser patterned ablation of multiple layers may be used to form the diaphragm, spacer layer, back plate, front plate, and clamping means from sheets of material. Many layers can be machined simultaneously (bulk machining) in this way using light masks with multiple patterns of the same part.

Accordingly, a fourth aspect of the invention provides a method of fabricating components of a capacitive microphone formed from layers comprising a back plate, a diaphragm, and a spacer layer for separating the back plate from the diaphragm, and a AMENDED SHEEl

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WO 01/67809 PCT/AUOI/00240 -8- O clamping layer for clamping the diaphragm against the spacer layer, the method S comprising the steps of: O providing a pattern for the layers; transferring the pattern for each layer onto at least one lithographic mask; i and passing laser light through each lithographic mask such that unmasked CC light removes unwanted material from each layer.

C1 The capacitive microphone may be in accordance with any one of the t embodiments described above. In an embodiment of this method, the pattern for each D layer is transferred onto a chrome-on-quartz lithographic mask using electron beam lithography. The laser light typically has a wavelength of 193 nm light and is produced by an excimer laser. A number of layers may be formed simultaneously in this way (i.e.

"bulk machining"), which lowers the production cost of a microphone.

The diaphragm, spacer layer, back plate, front plate, and clamping means may also be manufactured using the "LIGA" process, or a LIGA-like process. LIGA is a three-stage process which can be used for the manufacture of high aspect ratio, 3-D microstructures in a wide variety of materials metals, polymers, ceramics and glasses). The name is derived from the German acronym Lithographie, Galvanoformung und Abformung, i.e. lithography, electroplating and moulding.

The diaphragm, spacer layer, back plate, front plate, and clamping means may also be mechanically stamped out using a process similar to that used to form compact discs. All of these techniques have the advantage that the components can be manufactured to a high tolerance, such as to within microns or sub-microns, while allowing simultaneous production of many parts. Any one of a variety of known cutting or forming techniques may be used to shape the layers in the microphone.

A fifth aspect of the invention provides a method of fabricating components of a capacitive microphone formed from layers comprising a back plate, a diaphragm and a spacer layer for separating the back plate from the diaphragm, and a clamping layer for clamping the diaphragm bracket to the spacer layer, the method comprising the steps of: providing a pattern for each of the layers; transferring the pattern onto each of the layers using a laser beam.

WO 01/67809 PCT/AU01/00240 -9- The step of transferring the pattern -onto each layer using a laser beam may 4_ involve relative movement of the laser with respect to each layer, such as by 0 direct-writing with a laser beam onto the layer.

INO Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to 0 imply the inclusion of a stated element or integer or group of elements or integers but e¢ not the exclusion of any other element or integer or group of elements or integers.

CIn order that the present invention may be more clearly understood, tt n embodiments of the invention will now be described, by way of example, with reference 0 to the accompanying drawings.

Brief Description of the Drawings Figure 1 shows a top view of a first embodiment of a microphone.

Figure 2 shows. an exploded cross-sectional view through section I-I of Figure 1.

Figure 3 shows plan views of each of the microphone layers shown in Figure 2.

Figure 4 shows a top view of a second embodiment of a microphone.

Figure 5 shows an exploded cross-sectional view through section II-II of Figure 4.

Figure 6 shows plan views of each of the microphone layers shown in Figure Figure 7 shows plan views of seven alternative embodiments of spacer layers.

Figure 8 shows a top view of a third embodiment of a microphone.

Figure 9 shows an exploded cross-sectional view through section llI-III of Figure 8.

Figure 10 shows a top view of a fourth embodiment of a microphone.

Figure 11 shows an exploded cross-sectional view through section IV-IV of Figure Figure 12 shows an exploded cross-sectional view of a fifth embodiment of a microphone.

Figure 13 shows a plan view of a C-shaped spacer layer shown in Figure 12.

Figure 14 shows an exploded cross-sectional view of a sixth embodiment of a microphone.

Figure 15 shows an exploded cross-sectional view of a seventh embodiment of a microphone.

Figure 16 shows an exploded cross-sectional view of an eighth embodiment of a microphone as seen though section V-V of Figure 17.

Figure 17 shows a pictorial view of a spacer layer with channels shown in Figure 16.

PCT/AU0 1/00240 S( :ceived 19 September 2001 Detailed Description of the Drawings A first embodiment of a microphone 2 will now be described with reference to O Figures 1 to 3. The microphone is composed of successive parallel layers, namely a clamping layer 4, an elastically resilient diaphragm 6, a moveable electrode 8 attached to one side of the diaphragm 6, a spacer layer 10, an electret layer 11, a fixed electrode S12 attached to a back plate 14, rear cavity walls 16, and a rear plate 18. Plan Views of tt% each of the layers are shown in Figure 3. The clamping layer 4 and spacer layer 10 are c" both sheets of material which have been cut into the shape of an annulus sized to Ssupport the periphery of the diaphragm 6. The clamping layer 4 is also attached to the 10 diaphragm 6 with an adhesive. The spacer layer forms a thin cavity 19 between the moveable and fixed electrodes. The electrodes 8, 12 are formed by coating two faces of the diaphragm 6 and back plate 14, respectively, with a conductive material. An acoustic signal incident.on the.diaphragm 6 causes diaphragm deflection, as indicated by arrows 6a, which in turn causes a change in capacitance between electrodes 8, 12.

The electret layer 11 creates an electric field between the two electrodes 8, 12, and is bonded to the back plate 14. The electret in this embodiment is formed from amorphous teflon, but it will be understood that alternative electret materials may be substituted.

The conductive material is a 100 nanometre vacuum-deposited gold film. However, it is understood that conductive coatings can be made from a variety of other known materials, including aluminium, titanium, chromium, copper, or conductive oxides such as indium tin oxide ("ITffO"). Alternatively, if the diaphragfn and back plate are themselves formed from conductive materials, conductive coatings are not required.

Microphone output is measured via electrical leads 21 connected to the electrodes 8, 12.

When the microphone 2 is assembled, two electrodes 8, 12 are separated by the spacer layer 10. The diaphragm 6 is clamped together with the moveable electrode 6 against the spacer layer by the clamping layer 4. The clamping layer 4, diaphragm 6, spacer layer 10, and back plate 14 may all be cut out of sheet material. In this embodiment, the sheet material is polyester, but it will be appreciated by those skilled in the art that many other types of sheet material may also be suitable, including polycarbonate silicon, silicon nitride, and silicon dioxide.

An acoustic delay element 30 is attached to the back plate 14. The acoustic delay element 30 comprises a chamber 32 formed between cylindrical walls'16 extending downwardly from the back plate 14, and a porous passage 38 in the rear plate AMENDED SHEET

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PCT/AUO 1/00240 I -ived 19 September 2001 -11- 18. The porous passage 38 functions as an acoustic resistive element. The chamber 32 Sis in communication with the cavity 19 beneath the diaphragm via an inlet 34 in the O back plate 14 and fixed electrode 12, and effectively time-delays acoustic signals NO travelling to the diaphragm 6 via the back plate 14.

In a particular example, typical dimensions of the microphone are as follows: active electrode surface area: 2 mm 2 C* diaphragm layer thickness 6 9m; t-i spacer layer thickness: 6 20 pm; 0 electret (teflon) layer thickness 50 [Lm; c 10 0 rear cavity height: 1 mm; 0 electrode layer thickness 100 nm.

These dimensions are intended as an example only, and the microphone may be made larger or smaller than this example, depending on requirements. It will be understood by a person skilled, in the art that dimensions of at least some of the microphone components will have limits on size. For example, if the diaphragm diameter made too large the electrostatic forces acting on it will force it to collapse onto the back plate. An advantage of this particular embodiment is that the active microphone diaphragm area is divided into smaller, independently-supported cells. The effective microphone surface area, and hence acoustic sensitivity, can be increased without concern for diaphragm collapse compared to a single cell diaphragm of equivalent active area.

Any one of a variety of known cutting or forming techniques may be used to shape the layers in the microphone. For example, the clamping layer 4, diaphragm 6, spacer layer 10, back plate 14, and rear plate 18 could each be cut by a laser or mechanically stamped out. The process of cutting out any one of these layers using a laser involves the following steps. A sheet of suitable material for example polyester, with the appropriate thickness for one or more of the layers of the microphone is placed on a moveable table. The table can be moved in two horizontal directions and Y) relative to a laser positioned above the material to be cut. The movement of the table is controlled by a Computer Numerical Control (CNC) program. The CNC program controls the positioning of the table and light fluence from the laser. Using the CNC each layer of the microphone can be cut to any two-dimensional pattern. Each layer can be further patterned in the vertical dimension by limiting the laser radiation fluence.

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PCT/AUO 1/0U240 eceived 19 September 2001 -12- A LIGA or LIGA-like process, or a process akin to the known process of stamping CDs could also be used to form any one of the layers.

O A second embodiment of the microphone 50 will now be described with \O reference to Figures 4, 5 and 6. The same reference numerals will be used where the features are the same as in the previous embodiment. The microphone again consists of successive parallel layers but differs in that seven hexagonal apertures 52a are formed in Cc the spacer layer 54, and seven hexagonal apertures 52b are formed in the clamping layer N, 56 to define seven hexagonal active regions 58 of the diaphragm 6, referred to as cells, V)between support portions 60 which support central regions of the diaphragm. A cavity 59 is provided beneath each cell 58 to enable diaphragm deflection to take place in the cell 58. Plan views of each of the microphone layers are shown in Figure 6. An electret layer is not shown in this embodiment, but it is understood that an electret layer could be sandwiched between the spacer layer 54 and fixed electrode 62 as with the first embodiment. Such an electret would have the same plan view pattern as the fixed electrode 62 and back plate 64 shown in Figure 6. The hexagonal apertures 52a in the clamping layer 56 are identical to, and aligned above, the apertures 52b in the spacer layer 54. The microphone also includes an acoustic delay element 30 which is identical to the acoustic delay element 30 described above with respect to Figure 2. The chamber 32 is in communication with each of the diaphragm cavities 59 by a plurality of acoustic inlets 66 in the back plate 64 and fixed electrode 62, each inlet 66 being located beneath a diaphragm cell 58 andcavity 59.

Figure 7 shows further examples of flat sheets 70 which could be used as either a clamping layer or a spacer layer in any of the embodiments described above or below.

It can. be-seen..that .the two dimensional configuration of the spacer/clamping layer can be varied in many different ways. The examples shown in Figure 7 are: interleaved fingers 72; inwardly-directed radial fingers 74; concentrically-distributed circular apertures 76 for defining nine circular cells 78 in a diaphragm; a combination of square apertures 80 and triangular apertures 82; two crossed strips 84 for defining four quarter-circle cells 86 in a diaphragm; four circular apertures 86 for defining four circular cells 88 in a diaphragm; and two crossed strips 84 with four fingers which branch out from the centre.

A third embodiment of the microphone 100 will now be described with reference to Figures 8 and 9. The same reference numerals will be used where the AMENDED SHEET

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PCT/AUO 1/00240 Seceived 19 September 2001 -13- Sfeatures are the same as in the previous embodiments. This embodiment includes a first fixed electrode 102 and a second fixed electrode 104 on opposite sides of a diaphragm O 6. The first and second fixed electrodes 102, 104 are attached to a back plate 106 and a ",front plate 108, respectively, such that both electrodes face the diaphragm The diaphragm 6 is separated from the first fixed electrode 102 by a first spacer layer 112, and from the second fixed electrode 104 by a second spacer layer 110. Both sides of the Cc diaphragm are coated with a conductive material to form a pair of parallel moveable electrodes 114 which are electrically connected together. However, it will be Sunderstood that there may be situations in which it is preferable to electrically isolate the two moveable electrodes 114. The first and second fixed electrodes 102, 104 comprise electrically conductive coatings formed on the back plate 106 and front plate 106, respectively. Any one of the conductive coatings described in the first embodiment, such as a 100 nanometre gold film, can be used to form the fixed and moveable electrodes 102, 104, 114. The microphone 100 effectively forms two capacitors, the first capacitor consisting of the first fixed electrode 102 in conjunction with the moveable electrode 114, and the second capacitor being formed by the second fixed electrode 104 in conjunction with the moveable electrode 114. Electrical leads 116 attached to each of the electrodes enable electrical output of the microphone to be measured. A first acoustic inlet 118 is formed through the back plate 106 and first fixed electrode 102, and a second acoustic inlet 120 is formed through the front plate 108 and second fixed electrode 104. The rear plate 122 in this embodiment is a solid plate without a porous section, creating a very high infinite acoustic resistance. Each of the layers of this microphone can be made in the same way as the equivalent layers described in the first embodiment.

A fourth embodiment of the microphone 130 is shown in Figures 10 and 11.

As with the third embodiment, this microphone includes a first fixed electrode 132 and a second fixed electrode 134 on opposite sides of a diaphragm 6. The first and second fixed electrodes 132, 134 are attached to a back plate 136 and a front plate 138, respectively, such that both electrodes 132, 134 face the diaphragm 6. However, unlike the third embodiment, this microphone is configured as a "multi-cell" microphone, that is, the first spacer layer 142, second spacer layer 140 and clamping layer 144 divide the diaphragm 6 into a plurality of cells. In this case, each spacer and clamping layer 140, 142, 144 includes four circular apertures 146 arranged in the same spacer layer pattern AMENDED SHEET

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PCT/AUO1/00240 I eived 19 September 2001 -14which is shown in Figure and therefore defines four circular cells. The first and.

Ssecond spacer layers also define a cylindrical cavity 148, 150 on opposite sides of each O diaphragm cell. Acoustic signals enter the microphone 130 via four front acoustic inlets IO152 in the front plate 138 and the second fixed electrode 134. Each front acoustic inlet 152 is aligned with the centre of a cavity 150. The chamber 32 of the acoustic delay element 30 is in communication with four respective diaphragm cavities 148 (formed by C" the back spacer layer 142) via four respective acoustic inlets 154 in the back plate 136 and first fixed electrode 132, each inlet 154 being located beneath a cavity 148.

r A fifth embodiment of the microphone 160 is shown in Figures 12 and 13. As with the third and fourth embodiments, this microphone is equivalent to two parallelplate capacitors connected in parallel. The microphone 160 includes a diaphragm 6 with 100 nanometre-thick gold film electrodes 114 formed on opposite faces, as well as a first fixed electrode 162 and a second. fixed electrode 164. Each fixed electrode 162, 164 is formed from an electrically conductive material which is thick enough and strong enough to be self-supporting and perform the dual functions of an electrode and a front plate or back plate. Thus, this embodiment does not include any front plate or back plate. It will be understood that such self-supporting electrodes 162, 164 may be used in any of the other embodiments described herein as an alternative to a conductive coating formed on a front plate or back plate. The diaphragm 6 is separated from the first fixed electrode 162 by an annular first spacer layer 166, and is separated from the second fixed electrode 164 by a second spacer layer in the form of three component spacer layers 168, 170 stacked one on top of the other. The component spacer layers 168, 170 comprise two annular layers 168 and a C-shaped spacer layer 170 sandwiched therebetween. The C-shaped.component layer 170 forms a side vent 172 for receiving acoustic signals, while the annular component spacer layers 168 provide support around the entire peripheral region 174 of the diaphragm 6. If required, the stacked component spacer layers 168, 170 could be used in any of the other microphone embodiments described herein. Unlike embodiments described above, acoustic signals enter this microphone through the side vents 172. Thus, the first and second fixed electrodes 162, 164 do not include any acoustic inlets for the passage of acoustic signals. A pipe or funnel may be attached to the side vent 172 to detect sound from a particular location.

A sixth embodiment of the microphone 180 will now be described with reference to Figure 14. This microphone 180 is formed by stacking two of the AMENDED SHEEI

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PCT/AUO1/00240 eceived 19 September 2001 microphones 160 shown in the fifth embodiment (Figure 12) one on top of the other.

SThe resultant microphone structure is equivalent to four parallel-plate capacitors O connected in parallel.

The microphone 180 comprises a lower sub-microphone 182 and an upper submicrophone 184. The lower sub-microphone 182 has a first fixed electrode 186, and the upper sub-microphone 184 has a second fixed electrode 188, and both sub-microphones Sshare a centrally-located third fixed electrode 190. All other features of the submicrophones are the same as those shown in Figures 12 and 13. As with the fifth embodiment, side vents 172 are formed by C-shaped component spacer layers 170 sandwiched between annular component spacer layers 168. An acoustic inlet tube 190 divides into two branch tubes 192 which feed acoustic signals into the two respective side vents 172. Since acoustic signals may only enter the microphone via a single inlet port 194, the microphone is omnidirectional.

A seventh embodiment of the microphone 200 is shown in Figure 15. This embodiment is similar to the stacked microphone shown in Figure 14. Again, the microphone 200 is formed from an upper sub-microphone 202 and a lower submicrophone 204, with each sub-microphone having a first spacer layer 206, 208 and a second spacer layer 168, 170. The second spacer layer 168, 170 of each submicrophone 204 forms first side vents 172 with a C-shaped component spacer layer 170 sandwiched between two component annular spacer layers 168. This microphone differs from the sixth embodiment in that the first spacer layer 206, 208 of each submicrophone also comprises a C-shaped component spacer layer 206 sandwiched between two annular second spacer layers 208. Each second C-shaped spacer layer 206 forms a second side vent 210 located on an opposite side of the microphone to the.first sidevents 172. A second inlet tube 212 divides into two branch tubes 214 for feeding acoustic signals into the second side vents 210. A resistance element 216 is provided in the second inlet tube 212 which together with the cavity enclosed by the spacer introduces an acoustic delay with respect to the first inlet tube 190, and thus make the microphone 200 directional.

An eighth embodiment of the microphone 220 will now be described with reference to Figures 16 and 17. This embodiment also has similarities to the stacked microphone shown in Figure 14. Again, the microphone 220 is formed from an upper sub-microphone 222 and a lower sub-microphone 224, with each sub-microphone AMENDED SHEE"

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PCT/AUO1/00240 eceived 19 September 2001 -16having a first spacer layer 230 and a back spacer layer 226, 228. Each first spacer layer S230 is cylindrical and does not include any side vents. Each second spacer layer 226, O 228 comprises a double layer formed by a first annular component layer 226 which is ,O cylindrical, and a second annular component layer 228 which is cylindrical and includes three radial channels 231 in an upper edge 232 of the cylinder walls 234. The first annular component layer 226 is stacked on top of the second annular component layer 228 and forms a bridge over the three channels 231. Each channel 231 in the second annular component layer 228 provides the function of a side vent for receiving acoustic t signals. This example of a spacer layer 226, 228 may also be used in any of the other embodiments described herein where one or more side vents are required. It will be understood that the number of channels 231 in the second annular component layer 226 can be varied, depending on the number of side vents required. The entire microphone 220 is disposed within a cylindrical housing 238 which is closed off at one end 240.

The housing 238 serves to guide acoustic signals into the microphone via the channels 230.

The present invention also includes within its scope a spacer layer which provides one or more acoustic inlets (eg. acoustic inlet 34 shown in Fig. 2) in addition to side vents (eg. vent 172 in Fig. 13 or channel 231 in Fig. 17). Such a spacer layer may be combined with any one of the microphone embodiments described above.

It will be appreciated by a person skilled in the art that the numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive.

AMENDED SHEET

IPENAU

Claims (17)

1. 2. The microphone according to claim 1, wherein either of the first and second fixed electrodes are separated from the moveable electrode by at least one insulating spacer layer
2. 3. The microphone according to claim 2 wherein the spacer layer is shaped to provide support to the moveable electrode to provide at least one active region of the moveable electrode located inwardly of the peripheral region in which there is a gap between the moveable and a fixed electrode for deflection of the moveable electrode.
3. 4. The microphone according to claim 2, wherein the at least one spacer layer is shaped to provide at least one side vent for facilitating transfer of acoustic signals into and out of the microphone. The microphone according to claim 2, wherein the spacer layer comprises fingers extending in a generally radially-inward direction from a perimeter of the moveable electrode for providing support to the at least one inwardly located region of the moveable electrode.
4. 6. The microphone according to claim 2, wherein the spacer layer includes a plurality of apertures which define a plurality of isolated active regions of the moveable electrode in which there is a gap between the moveable and a fixed electrodes for deflection of the moveable electrode.
5. 7. The microphone according to claim 6, wherein the plurality of apertures are arranged in a grid-like configuration.
6. 8. The microphone according to claim 6, wherein the apertures are hexagonal.
7. 9. The microphone according to claim 6, wherein the plurality of apertures are circular. The microphone according to claim 1, wherein the moveable electrode is O secured to a face of a diaphragm such that it can deflect with the diaphragm. N 11. The microphone according to claim 10, wherein the moveable electrode o comprises an electrically-conductive coating. O I12. The microphone according to claim 1, wherein each fixed electrode is secured to a back plate.
8. 13. The microphone according to claim 12, wherein the fixed electrode comprises an electrically-conductive coating.
9. 14. The microphone according to claim 2, further comprising clamping means for In clamping selected areas of at least one of the moveable electrodes against the spacer layer. The microphone according to claim 1, wherein the microphone includes an acoustic delay element for time-delaying acoustic signals directed to a first -face of the moveable electrode with respect to acoustic signals directed to a second opposite face of the moveable electrode.
10. 16. The microphone according to claim 1, further comprising a back plate, wherein the back plate includes at least one acoustic inlet for introducing acoustic signals into the microphone through the back plate.
11. 17. A stacked microphone formed from a plurality of sub-microphones, each sub- microphone being in accordance with claim 1, wherein the sub-microphones are stacked together such that all moveable and fixed electrodes are substantially parallel.
12. 18. A method of fabricating components of a capacitive microphone formed from layers, the method including the step of fabricating at least one layer of the microphone using a LIGA process.
13. 19. A method of fabricating components of a capacitive microphone formed from layers comprising a back plate, a diaphragm, and a spacer layer for separating the back plate from the diaphragm, and a clamping layer for clamping the diaphragm against the spacer layer, the method comprising the steps of: providing a pattern for the layers; transferring the pattern for each layer onto at least one lithographic mask; and passing laser light through each lithographic mask such that unmasked light removes unwanted material from each layer. A method of fabricating components of a capacitive microphone formed from O layers comprising a back plate, 'a diaphragm, and a spacer layer for separating the back plate from the diaphragm, and a clamping layer for clamping the 0 O diaphragm against the spacer layer, the method comprising the steps of: IND providing a pattern for each of the layers; transferring the pattern onto each layer using a beam of laser light.
14. 21. A method according to claim 20, wherein the step of transferring the pattern t involves relative movement of the laser beam with respect to each layer. S22. A method according to claim 20 or claim 21, wherein the step of transferring the pattern comprises directly writing the pattern onto each layer with the laser beam.
15. 23. A method according to any one of claims 20 to 22, wherein the diaphragm and back plate each include an electrically conductive coating which functions as a moveable electrode and fixed electrode, respectively.
16. 24. A method according to any one of claims 20 to 23, wherein each of the layers comprises a layer of a polymer material. A method according to claim 24, wherein the polymer material is polyester.
17. 26. A method according to, any one of claims 20 to 25, wherein the laser light has a wavelength of substantially 193 nanometres.