HK1261735B - Mems device and process - Google Patents

Description

MEMS device and method

Technical Field

The present invention relates to micro-electro-mechanical systems (MEMS) devices and methods, and in particular to MEMS devices and methods relating to transducers, such as capacitive microphones.

Background

A variety of MEMS devices are becoming increasingly popular. MEMS transducers, and in particular MEMS condenser microphones, are increasingly used in portable electronic devices, such as mobile phones and portable computing devices.

Microphone devices formed using MEMS fabrication methods typically include one or more membranes, with electrodes for readout/actuation deposited on the membrane and/or substrate. In the case of MEMS pressure sensors and microphones, readout is typically achieved by measuring the capacitance between a pair of electrodes, which will vary as the distance between the electrodes changes in response to acoustic waves incident on the surface of the membrane.

Fig. 1a and 1b show a schematic and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone apparatus 100 includes a membrane layer 101, the membrane layer 101 forming a flexible membrane that is free to move in response to pressure differences generated by sound waves. The first electrode 102 is mechanically coupled to the flexible membrane and together they form a first capacitive plate of the capacitive microphone device. The second electrode 103 is mechanically coupled to a substantially rigid structural layer or back-plate 104, which together form a second capacitive plate of the capacitive microphone device. In the embodiment shown in fig. 1a, the second electrode 103 is embedded in the back-plate structure 104.

The condenser microphone is formed on a substrate 105, the substrate 105 being, for example, a silicon wafer, which may have an upper oxide layer 106 and a lower oxide layer 107 formed thereon. A cavity 108 in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is disposed below the film and may be formed through the substrate 105 using a "back-etch". The substrate chamber 108 is connected to a first chamber 109 positioned directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume, allowing the membrane to move in response to acoustic excitation. Disposed between the first electrode 102 and the second electrode 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 may be subsequently removed, and depositing the film layer 101 over the first sacrificial material. The use of a sacrificial layer to form the first cavity 109 means that the etching of the substrate cavity 108 does not play any role in defining the diameter of the membrane. Instead, the diameter of the membrane is defined by the diameter of the first cavity 109 (which in turn is defined by the diameter of the first sacrificial layer) in combination with the diameter of the second cavity 110 (which in turn is defined by the diameter of the second sacrificial layer). The diameter of the first cavity 109 formed using the first sacrificial layer may be more precisely controlled than the diameter of the first cavity 109 formed using a back etching process performed using wet etching or dry etching. Thus, etching the substrate cavity 108 will define an opening in the surface of the substrate below the membrane 101.

A plurality of holes (hereinafter referred to as discharge holes) 111 connect the first chamber 109 and the second chamber 110.

As mentioned, the film may be formed by depositing at least one film layer 101 over a first sacrificial material. In this way, the material of the film layer(s) may extend into the support structure (i.e., the sidewalls) supporting the film. The membrane and backplate layers may be formed from substantially the same material as each other, for example both the membrane and backplate may be formed by depositing a silicon nitride layer. The film layer may be dimensioned to have the required flexibility, while the back plate may be deposited as a thicker and thus more rigid structure. In addition, various other layers of materials may be used in forming the backplate 104 to control the properties of the backplate. The use of silicon nitride material systems is advantageous in many respects, although other materials may be used, such as MEMS transducers using polysilicon films are known.

In some applications, the microphone may be arranged in use such that incident sound is received via the backplate. In such a case, an additional plurality of holes (hereinafter referred to as sound holes 112) are arranged in the back plate 104 so as to allow air molecules to move freely so that sound waves can enter the second cavity 110. The first cavity 109 and the second cavity 110 associated with the substrate cavity 108 allow the membrane 101 to move in response to sound waves entering through the sound aperture 112 in the backplate 104. In such a case, the substrate chamber 108 is generally referred to as a "back volume" and it may be substantially sealed.

In other applications, the microphone may be arranged such that sound may be received via the substrate cavity 108 when in use. In such applications, the back plate 104 is still typically provided with a plurality of holes to allow air to move freely between the second chamber and another volume above the back plate.

It should also be noted that although fig. 1 shows the backplate 104 supported on the opposite side of the membrane to the substrate 105, arrangements are known in which the backplate 104 is formed closest to the substrate, with the membrane layer 101 supported above the backplate 104.

In use, the membrane deforms slightly from its equilibrium position in response to sound waves corresponding to pressure waves incident on the microphone. The distance between the lower electrode 102 and the upper electrode 103 is correspondingly altered, causing a change in capacitance between the two electrodes, which is subsequently detected by electronic circuitry (not shown). The vent hole allows the pressure in the first and second chambers to be balanced over a relatively long period of time (in terms of acoustic frequency), which reduces the effect of low frequency pressure variations, e.g. caused by temperature variations, etc., but does not affect the sensitivity at the desired acoustic frequency.

The transducer shown in fig. 1 is illustrated as having substantially vertical sidewalls supporting a membrane layer 101 spaced from a backplate 104. This can lead to high stress concentrations at the corners formed in the material layers forming the film, given the nature of the deposition process. Sloped or beveled sidewalls may be used to reduce stress concentrations. Additionally or alternatively, it is known to include a number of support structures, such as posts, to help support the membrane in a manner that reduces stress concentrations. Such pillars are formed by: the first sacrificial material used to define the first cavity 109 is patterned such that the substrate 105 is exposed in many areas prior to deposition of the material forming the layer 101. However, this process may result in dimples in the upper surface of the backplate layer in the region of the posts.

MEMS transducers, such as those shown in fig. 1, may be usefully employed in a range of devices, including portable devices. Particularly when used in portable devices, it is desirable that the MEMS transducer be robust enough to withstand the intended handling and use of the device. Therefore, it is generally desirable to improve the flexibility of MEMS devices.

Thus, for useful use in portable electronic devices, such transducers should be able to withstand the intended handling and use of the portable device, which may include the device being accidentally dropped.

If a device such as a mobile phone is subjected to a fall, this can result not only in mechanical shock due to the impact, but also in high pressure pulses incident on the MEMS transducer. For example, a mobile phone may have an acoustic/acoustic port for a MEMS microphone on one face of the device. If the face of the device falls to ground, some air may be compressed by the falling device and forced into the sound port. This can result in high pressure pulses being incident on the transducer. It has been found that in conventional MEMS transducers of the form described above, high pressure pulses can potentially cause damage to the transducer.

The sacrificial material used to define the first and second cavities is sized to provide a desired equilibrium spacing between the film 101 and the substrate 105 and between the film 101 and the backplate 104 to provide good sensitivity and dynamic range in use. In normal operation, the membrane may deform within the volume defined by the first cavity and the second cavity without contacting the back plate and/or the substrate 105.

However, in response to a high pressure pulse, the film layer 101 may exhibit a greater amount of deformation than usual. Fig. 2a illustrates a situation where the membrane has been deformed downwards after a high pressure event, and fig. 2b shows a situation where the membrane has been displaced upwards.

Consider the following scenario: where the microphone is arranged to receive incident sound from a sound port arranged above the backplate 104 and the sound port pressure increases suddenly, for example because trapped air is forced into the sound port when the device falls. This may result in the pressure in the second chamber 110 being significantly greater than the pressure in the first chamber 109, thereby displacing the membrane downwards to a greater extent than usual. This may result in relatively large stresses at the point 301 where the film layer 101 forms part of the sidewalls of the support structure 201, and in some cases, may therefore result in delamination of the film layer from the rest of the sidewall structure. Further, if the pressure differential is sufficiently large, the membrane may be in contact with the substrate 105 at the edge of the substrate defined by the sidewall 202 of the opening of the substrate cavity 108. Typically, the edge of the substrate at the location of the opening of the substrate cavity has a relatively sharp angle, so the membrane may deform around this edge, resulting in a large stress concentration at this point 302.

As previously mentioned, the film layer 101 will typically be formed of one or more thin layers of semiconductor material, such as silicon nitride. While such materials may be flexible when subjected to uniform stress, the film material may be relatively brittle if there is significant local out-of-plane stress introduced into the film at point 302, such as by contact with the edge of the opening of the substrate cavity 108. Thus, contact between the membrane and the edge of the opening of the substrate cavity in this way may lead to damage, such as membrane cracking.

The vent discussed above with reference to fig. 1 will provide a flow path between the first and second chambers, and thus the flow of air through the vent will reduce the pressure differential acting on the membrane over time. However, the discharge orifices are typically intentionally arranged to provide a limited amount of flow in order to provide a desired frequency response. Thus, a high pressure differential may be maintained across the membrane for a relatively long period of time before the flow through the vent hole acts to equalize the pressure in the first and second chambers. The time required for equalization through the exhaust orifices may be varied by altering the size and/or number of exhaust orifices, but this may adversely affect transducer performance.

Since the high pressure caused by trapped air may last for a relatively long time, the pressure in the first and second chambers may be equalized by means of the vent holes as discussed. Thus, the pressure in the first chamber and the substrate chamber may increase until the pressure is equalized. However, once air is no longer forced into the sound port, the pressure in the sound port will decrease very quickly, and because the back plate typically has a low acoustic impedance, the pressure in the second chamber will decrease quickly. At this point, the pressure in the first chamber may be significantly greater than the pressure in the second chamber, and thus the membrane may deform upwards, again to a greater extent than usual. Furthermore, this may result in significant stress in the region 301 where the membrane layer 101 meets the sidewalls of the support structure. If the pressure differential is large enough, the membrane may be displaced far enough to contact the backplate 104. This may limit the amount of travel of the membrane compared to the situation shown in fig. 2a, but in addition this may introduce stress into the membrane layer at the point 303 where it contacts the back plate 104. Furthermore, this pressure differential may take some time to decrease by virtue of the flow through the vent hole.

It will be appreciated that the two situations may also occur when sound is received via the substrate cavity 108, but in the reverse order.

Fig. 3a to 3c show a previously proposed MEMS transducer comprising a flexible membrane 101 and a variable vent structure 401 in the form of a movable portion or "flap" 402. The movable flap portion is defined by a thin channel 403, which thin channel 403 extends through the membrane and partially separates the movable flap portion from the rest of the membrane, while remaining attached to the rest of the membrane via a connecting portion 404.

The movable flap part 402 is arranged such that its equilibrium position, i.e. the position it assumes when substantially no pressure difference acts on the movable part, is in the plane of the membrane. In response to a pressure differential across the movable portion of the vent structure, the movable portion is deflected away from the plane of the membrane to expose the aperture in the membrane. In this way, the size of the flow path between the first volume above the membrane and the second volume below the membrane through the vent structure varies in response to a variable pressure differential acting on the moveable portion.

Fig. 3b illustrates a portion of the membrane and the variable vent in a perspective view. In this embodiment, the pressure in the volume below the membrane is sufficiently greater than the pressure in the volume above the membrane that the movable flap portion 402 has deflected upward away from the rest of the membrane surface. This opens a flow channel through the membrane, i.e. effectively opens a hole in the substrate. If the pressure differential increases sufficiently, the movable portion 402 may deflect further, providing a greater amount of opening, i.e., a greater flow path.

Thus, the movable portion may assume a range of positions. These positions depend on the pressure differential acting on the movable portion (or variable vent). The extent to which the moveable portion is deflected also determines the extent to which the moveable portion blocks/exposes an aperture through the membrane, thereby determining the size of the flow path.

The structure shown in fig. 3 has been shown to function so as to reduce the pressure differential acting on the membrane at relatively high pressure differentials. However, the pressure pulse profile due to air being forced into the sound port of the host device, for example, due to an impact, may peak often within a few milliseconds. Thus, unless the vent structure is ideally able to respond quickly in this time frame, it may still be damaged by a high pressure or "overpressure" event.

Disclosure of Invention

The present invention relates to improving the resilience of MEMS devices to high pressure pulses incident on a MEMS transducer. In particular, the present invention relates to improving the response time of a vent structure disposed on a flexible membrane of a MEMS transducer. The present invention is therefore directed to facilitating equalization of the pressure differential created between the upper and lower surfaces of the membrane.

According to an aspect of the present invention, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one movable portion, wherein the vent structure is configured such that, in response to a pressure differential across the vent structure, the movable portion is rotatable about a first axis of rotation and a second axis of rotation, the axes of rotation extending in the plane of the membrane.

According to another aspect of the present invention, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one movable portion connected to the membrane by a joint structure (joint structure), wherein the vent structure is configured such that, in response to a pressure difference across the vent structure, the movable portion is rotatable about a first axis of rotation extending in a plane of the membrane and a second axis of rotation substantially orthogonal to and extending in the plane of the movable portion, wherein the first and second axes of rotation intersect at the joint structure.

In response to a pressure differential across the movable portion of the vent structure, the movable portion deflects to reveal a variable-sized aperture (aperture) in the flexible membrane. Thus, in response to a pressure differential across the movable portion of the vent structure, the movable portion deflects to provide a flow path through the flexible membrane. The size of the aperture, and hence the size of the flow path, increases as the movable portion deflects. This assists in the equalisation of the pressures acting on the opposite surfaces of the membrane and tends to return the movable portion to its equilibrium position.

The flexible membrane exhibits an equilibrium position that may be considered to correspond to a minimum dimension of a flow path through the flexible membrane. Thus, in the equilibrium position, the pressure difference across the vent structure is not sufficient to cause the movable portion to deflect, and the size of the flow path through the membrane is minimal/negligible.

The movable portion can potentially rotate about two axes of rotation in response to a pressure differential across the vent structure. In equilibrium, these two axes of rotation may be considered to extend substantially in the plane of the membrane. However, once the movable portion has been deflected above or below the plane of the membrane, the second axis of rotation may be considered to extend in the plane of the movable portion. The axes of rotation may or may not be orthogonal to each other.

The axis of rotation may conveniently be defined with respect to a joint structure or "hinge" connecting the/each movable portion to the flexible membrane. The tab structure may be defined by one or more channels disposed within the membrane layer. The joint construction may for example comprise a simple connecting portion, or "neck" of film material, which forms the connection between the movable portion and the rest of the flexible film. Thus, the movable portion may be defined by a slit or channel extending through the membrane material, thereby separating the movable portion from the rest of the membrane and thus defining the perimeter shape of the movable portion. In its simplest form, the joint structure may comprise a connecting portion defined between two terminal ends or two terminal areas of a channel. The joint structure may further comprise a generally elongated beam structure extending adjacent the attachment portion and positioned between the remainder of the membrane and the attachment portion.

Thus, the channel defining the moveable portion may be considered to define a path between two end points of the channel. Each endpoint may be considered to be at one termination region of the channel. The width of the connecting portion defined between the termination regions of the channels may be considered as the distance between a first point on one termination region of the channel and a corresponding point on the other termination region of the channel.

The first axis of rotation may be considered to be substantially coincident with or parallel to the width of the joint construction. Thus, the first axis of rotation may coincide with or be parallel to a width across a connection portion formed between two terminating ends or regions of a channel separating the movable portion from the remainder of the membrane. Alternatively, the first axis of rotation may coincide with or be parallel to a longitudinal section of the substantially elongated beam of the joint construction. Rotation of the moveable portion about this first axis of rotation will cause the moveable portion to deflect above or below the remainder of the surface of the membrane, depending on the resultant force acting on the vent structure.

At equilibrium, the second axis of rotation also extends in the plane of the membrane and has a component substantially perpendicular to the width of the joint structure. Rotation of the movable portion about this second axis of rotation tends to cause one lateral edge/corner of the movable portion (depending on the shape of the movable portion) to deflect upwardly relative to the plane of the membrane surface, while causing the opposite lateral edge/corner to deflect downwardly relative to the plane of the membrane surface.

Thus, the joint arrangement may be considered to comprise a double hinge allowing the or each movable part to rotate about the first and second axes of rotation.

Thus, the movable part can be "tilted" around the joint structure relative to the plane of the membrane by rotating around the second axis of rotation. This tilting about the second axis of rotation, which tends to initially occur after a high pressure event, tends to result in a greater proportion of the apertures initially being exposed than would be revealed if the movable portion were considered to rotate only about the first axis of rotation.

The ability of the movable portion to rotate about the second axis of rotation advantageously enables the vent structure to open more rapidly in response to a pressure differential across the membrane. This advantageously results in a faster equalization of pressure across the membrane after a pressure pulse event.

According to another aspect of the present invention, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one movable portion, wherein the/each movable portion is connected to the membrane by a joint structure having a width, the vent structure being configured such that, in response to a pressure differential across the vent structure, the movable portion is rotatable about an axis of rotation having a component perpendicular to the width of the joint structure.

Thus, the vent structure may be configured such that rotation of the movable portion occurs about an axis of rotation having a component perpendicular to the width direction of the joint structure. This may for example be the result of the shape of the movable parts and/or the way each movable part is connected to the rest of the membrane.

According to another aspect of the present invention, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one movable portion, wherein the movable portion is connected to a remainder of the membrane by a joint structure, and wherein an imaginary line extending from a center of the connecting portion across the movable portion in a direction substantially orthogonal to a width of the connecting portion divides the movable portion into a first segment and a second segment, the first segment having a larger surface area than the second segment.

According to another aspect of the present invention, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one movable portion, wherein the movable portion is connected to a remainder of the membrane by a connecting portion having a width, and wherein the movable portion is asymmetric about an imaginary line extending across the movable portion from a center of the connecting portion in a direction substantially orthogonal to the width of the connecting portion.

Due to the pressure difference across the vent structure, a resultant force acts on the moveable portion, causing the moveable portion to deflect. Thus, in some cases, such as in an impact event that results in a difference between the pressure acting on one planar surface of the movable portion (i.e., force/unit area) and the pressure acting on the opposite planar surface of the movable portion, a resultant force acts on the surface of the movable portion in the direction of the greater pressure.

Due to the pressure difference across the vent (e.g. in the direction from the upper surface to the lower surface of the movable part or vice versa) the total force acting on the larger surface area of one section of the movable part will be larger than the total force acting on the smaller surface area of said section. This causes the movable part to rotate about the second axis of rotation.

According to another aspect, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one movable portion and a coupling structure disposed on a coupling edge of the movable portion that couples the movable portion to the flexible membrane, wherein the coupling structure is disposed at an off-center location on the coupling edge.

According to another aspect of the present invention, there is provided a MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one moveable portion, wherein the moveable portion is connected to the remainder of the membrane by a joint structure, and wherein an imaginary line extending substantially orthogonally across the moveable portion from the center of the joint structure divides the moveable portion into a first section and a second section such that, in response to a pressure differential across the membrane, a resultant force occurring on the first portion causes a moment about the imaginary line that is greater than a moment caused by a resultant force occurring on the second portion.

According to another aspect of the invention there is provided a MEMS transducer comprising a vent structure comprising a movable portion which is inclined in response to a pressure differential across the vent structure such that one edge of the movable portion is deflected below the plane of the membrane and an opposite edge of the movable portion is deflected above the plane of the membrane.

In general, a MEMS transducer is provided that includes at least one vent structure disposed in a flexible membrane of the transducer. The MEMS transducer may be a capacitive microphone. The flexible membrane may be supported between a first volume and a second volume, and a flow path may be provided between the first volume and the second volume through the vent. The vent structure may include a movable portion that is movable to open an aperture extending from the first volume to the second volume. The movable portion may occupy at least some, and possibly most, of the area of the aperture at rest, but is movable in response to a local pressure differential across the aperture so as to vary the size of the aperture which opens to provide a flow path. In other words, the movable portion may effectively close at least a portion of the aperture at equilibrium, but is movable so as to vary the extent to which the aperture is closed. The movable portion is preferably arranged to keep the aperture (i.e. aperture) closed under normal operating pressure differences, but larger at higher pressure differences that could potentially cause damage to the membrane to increase the size of the flow path, e.g. to make the aperture less closed. Thus, the vent may be considered a variable aperture.

Thus, the vent structure acts as a pressure relief valve to reduce the pressure differential acting on the membrane. However, unlike a vent hole in the membrane (if present) that has a fixed area and thus a fixed sized flow path, the variable vent has a flow path size, i.e., porosity, that varies in response to the pressure differential. The degree to which the vent allows venting is therefore dependent on the pressure difference acting across the vent, which is significantly dependent on the pressure of at least one of the first and second volumes. Thus, the vent structure provides a variable acoustic impedance.

The transducer may comprise a back plate structure, wherein the flexible membrane layer is supported relative to the back plate structure. The back plate structure may comprise a plurality of holes through the back plate structure. When at least one vent structure is formed in the flexible film layer, at least one of the holes through the back plate structure may comprise a vent hole at a location corresponding to the location of the vent structure in the flexible film layer. The area of the vent hole in the back plate may extend laterally away from the open area of the vent hole in the flexible membrane at a location when the variable vent hole in the flexible membrane is first opened. When at least one vent structure is formed in the flexible membrane layer and includes a moveable portion connected to the remainder of the membrane via a beam structure and the moveable portion and beam structure are defined by a channel extending through the flexible membrane; the position of the channel in the membrane which in use does not form part of the variable flow path through the membrane may then be arranged not to substantially overlap with the position of any of the plurality of apertures in the backplate structure.

The transducer may be a capacitive sensor, such as a microphone. The transducer may include readout circuitry (analog and/or digital). The transducer and circuitry may be provided together on a single semiconductor chip-e.g., an integrated microphone. Alternatively, the transducer may be on one chip and the circuitry may be provided on a second chip. The transducer may be positioned within an enclosure having an acoustic port (i.e., an acoustic port). The transducer may be implemented in an electronic device, which may be at least one of: a portable device; a battery powered device; an audio device; a computing device; a communication device; a personal media player; a mobile phone; a tablet device; a game device; and a voice control device.

Features of any given aspect may be combined with features of any other aspect, and various features described herein may be implemented in any combination in a given embodiment.

An associated method of manufacturing a MEMS transducer is provided for each of the above aspects.

Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIGS. 1a and 1b illustrate a known capacitive MEMS transducer in a cross-sectional view and a cut-away perspective view;

figures 2a and 2b illustrate how a high pressure event may affect the membrane;

3 a-3 c illustrate known variable vent configurations;

FIG. 4 illustrates a flexible membrane having a vent structure according to a first embodiment, wherein the vent structure is in an equilibrium position;

FIG. 5 illustrates a flexible membrane having a vent structure according to a first embodiment with a first pressure differential across the vent;

FIG. 6 illustrates the flexible membrane with a vent structure according to the first embodiment with a second pressure differential across the vent;

FIG. 7 illustrates many other vent configurations that exemplify the invention; and

fig. 8a to 8h illustrate various MEMS transducer packages.

Detailed Description

Embodiments of the present invention relate to a MEMS transducer comprising a transducer structure comprising a flexible membrane supported between a first volume and a second volume. The first volume may for example comprise a first cavity 109 between the membrane and the substrate and/or a volume formed in the substrate 108. The second volume may include the second cavity 110 between the membrane and the back plate and/or any volume in fluid communication with the second cavity (e.g., an acoustic port in a top port embodiment).

To reduce the likelihood of damage in high pressure situations, the transducer arrangement includes at least one vent arrangement in communication with at least one of the first and second volumes. The vent structure includes at least one movable portion that is movable in response to a pressure differential across the vent structure.

Fig. 4, 5 and 6 illustrate a flexible membrane 501 according to a first embodiment.

Referring to the embodiment illustrated in fig. 4, 5 and 6, the flexible membrane 501 includes a vent structure having three movable portions 502a, 502b, 502 c. In this embodiment, each of the movable portions is an irregular polygon and is connected to the remainder of the membrane by a joint structure. In this embodiment, the joint structure includes a connecting portion 505 and a beam structure 503.

Each movable part is formed by a channel 403, the channel 403 extending from the upper surface of the membrane to the lower surface of the channel. Each movable portion may be partially separated from the rest of the membrane 501 by a channel 403 etched through the membrane. Each beam structure is formed by an auxiliary channel 504, which auxiliary channel 504 extends through the membrane and may be formed by etching through the membrane.

The connecting portion comprises a portion or "neck" of the membrane material defined by a terminating end or region of the channel 403. For example, referring back to fig. 3a, it will be appreciated that a connecting portion comprising a very shallow neck of membrane material may be formed directly between the terminating ends of the channel 403. The distance between the terminating ends represents the width w of the connecting portion. Alternatively, as illustrated by the tab 502b of fig. 4a, the terminating region of the channel 403 extends inwardly towards the central region of the movable portion, thereby defining a more pronounced neck of film material having a width w. The width of the connecting portion defined between the termination regions of the channels may be considered as the distance between a first point on one termination region of the channel and a corresponding point on the other termination region of the channel.

As another example, and as illustrated by the flap 502C of fig. 4a, the termination region of the channel may define a C-shaped or U-shaped path such that a line drawn between the termination ends will intersect the channel. In this case, it will be understood that the connection portion is not provided directly at the termination of the channel 403, althoughTerminal endBetweenBut the connecting portion is still a neck of membrane material defined between the termination regions of the channels. Each movable portion is connected to the rest of the membrane along a connecting edge substantially coinciding with the cooperating edge of the membrane. The connecting edge of the movable part is separated from the rest of the membrane by a channel 403, except at the joint structure. In this embodiment, the joint structure is provided at an eccentric position on the connecting edge of the movable part. It will be understood that the "connecting edge" of the movable portion is defined as the edge that includes the joint and extends between points X and Y shown on figure 4 a.

It will of course be understood that the channel 403 does represent a path for air to flow through the membrane, however the channel 403 may be formed to have a very narrow width so that when the movable flap portions are in an equilibrium position whereby the movable portions engage to substantially close the aperture, the flow of air through the channel will be minimal or negligible.

The width of the channel 403 may be limited by: the photolithographic process constraints on the minimum etchable gap, or the need for some mechanical gap for the movable element to bend and flex but still pass over the rest of the structure. Furthermore, a narrow gap will tend to have a large proportion of manufacturing tolerances, resulting in a large variation in acoustic impedance when closed and thus a wide variation in low frequency roll-off of, for example, a microphone.

A typical width may be 1 μm relative to a typical vent structure ranging from 20 μm to 50 μm. However, the width may be one tenth or ten times greater depending on acoustic specifications or manufacturing process capabilities. As mentioned, the line width of the channels defining the moveable vent portion may affect factors such as low frequency roll off. In selecting the appropriate line widths, the effects of different widths may be simulated and/or different designs may be manufactured and tested.

Fig. 4a, 4b and 4c show vent structures in a substantially closed or "balanced" state when the pressure differential across the membrane is at or near zero. Fig. 5a, 5b and 5c show the position of the movable part of the vent arrangement at a first relatively low pressure differential. Fig. 6a, 6b and 6c show the position of the movable part of the vent arrangement at a second relatively high pressure differential.

Fig. 4a, 4b and 4c show a plan view, a side view and a front view, respectively, of the flexible membrane 501 when the pressure difference across the membrane is zero or close to zero and the movable parts 502a, 502b and 502c are thus substantially in plane or "flush" with the planar surface of the membrane. In this case, the flow path through the membrane is substantially closed (with any minimal air flow through the membrane depending on the size of the channel 403).

Referring to fig. 4a, which shows a plan view from the upper surface of the flexure, it can be seen that the moveable portion 502 exhibits an irregular polygonal shape. Each movable portion may be considered to extend on either side of an imaginary "centre line" C extending from the centre of the joint structure across the movable portion in a direction substantially orthogonal to a line coincident with or parallel to the width w of the connecting portion. The centerline C divides the movable portion into a first segment a1 and a second segment a 2. The movable portion 502 is asymmetrical about the imaginary centre line C and it can be seen that the first section a1 exhibits a larger surface area than the second section a 2.

The movable portion is deflected out of the plane of the membrane after an event that causes a pressure differential between the pressures experienced on the upper and lower surfaces of the membrane. Fig. 5a, 5b and 5c show a plan view, a side view and a front view, respectively, of the flexible membrane 501 in case of a first pressure difference across the vent. As can be seen most clearly from fig. 5b and 5c, each of the movable portions 502 has been rotated about a first axis of rotation R1 and a second axis of rotation R2 (shown on 5 a) so as to deflect the movable portion away from the remainder of the film so as to reveal an aperture a in the film. The pores a provide a flow path through the membrane.

The first axis of rotation R1 of each movable portion is substantially coincident with or parallel to the elongated portion of the beam 503. The aperture a may be considered to be substantially circular (although it will be appreciated that in this embodiment the outer edge of the aperture is formed from a straight edge). Thus, the first axis of rotation R1 may be considered to have a tangential component t — that is, a component that may break off tangentially with respect to the vent structure or aperture. In this embodiment, the first axis of rotation is substantially coincident with the tangential component.

The second axis of rotation R2 extends across the vent structure in the plane of the membrane. Thus, R2 has a component Rc that extends from the tab structure toward the center of the vent structure in the plane of the membrane. The second axis of rotation R2 may be substantially orthogonal to the first axis of rotation R1.

Considering the axis of rotation taken in relation to the first axis of rotation R1, it can be seen that a first pressure difference acting on the movable part has caused a rotation about R1 in order to deflect the movable part upwards out of the plane of the membrane.

As can be seen from fig. 5b and 5c, the rotational movement about the second axis of rotation R2 tends to cause the moveable portion to "tilt" due to the first pressure differential acting across the vent structure. Thus, the lateral edge of section a1 of the movable portion has been deflected upwards with respect to the plane of the membrane, while the lateral edge of section a2 of the movable portion has been deflected downwards with respect to the plane of the membrane. Since the amount of rotation occurring about the first axis of rotation R1 is relatively small, the lateral edge of the segment a2 of the movable portion that has deflected downwardly about R2 actually protrudes slightly below the surface of the membrane. This can be seen in fig. 5 b.

At this relatively low pressure differential, the pressure differential across the vent tends to cause greater rotation about R2 than about R1 as the vent structure is closer to the equilibrium position. Continued deflection of the movable portion about the second axis of rotation tends to result in a greater proportion of apertures (and thus a greater flow path) to be revealed than would be the case due to rotation about R1. This enables the vent structure to open more quickly from an equilibrium position in response to a high pressure event, thus revealing a larger aperture in response to a relatively low pressure differential. This advantageously allows the relative pressures above and below the membrane to equalize more quickly, thereby protecting the transducer from potential damage.

Fig. 6a, 6b and 6c show a plan view, a side view and a front view, respectively, of the flexible membrane 501 in case of a second pressure difference across the vent, which is higher than the first pressure difference discussed above with reference to fig. 5. In this case, the movable portion has experienced a large deflection due to the pressure difference across the vent structure. In particular, rotation about the first axis R1 provided by twisting or twisting of the beam 503 tends to cause the movable portion 502 to deflect upwardly toward a plane orthogonal to the plane of the remainder of the membrane. At this relatively high pressure differential, continued deflection of the movable portion tends to occur about R1.

The/each moveable portion of the vent structure may rotate about either or both of the axes of rotation in response to a pressure differential across the membrane. For example, consider a pressure curve that occurs after an event in which trapped air is forced into the host device's sound port, for example, due to a host device falling and hitting a surface. The pressure at the sound port will rise over time and will reach a peak pressure differential, and then decrease as the vent structure opens to equalize the pressure across the vent. As the air pressure increases, the/each movable part may initially rotate about an axis having a component perpendicular to the hinge in the horizontal plane ("second axis of rotation"). The/each movable part may also rotate simultaneously about the first axis as the pressure continues to rise. Then, under higher pressure, the/each movable part can rotate about an axis having a component substantially parallel to the joint construction in the horizontal plane (the "first" axis of rotation).

The vent structure shown in fig. 4, 5 and 6 includes three movable portions arranged around the perimeter of the membrane aperture a. It will be appreciated, however, that this design can be generalized to incorporate any number of moveable portions of any shape (including a spherical rod/clover), which has been found to be particularly beneficial as it balances the conflicting goals of: the speed of response, which is proportional to the number of movable portions, and the "leakage" (i.e., the flow of air through the membrane that occurs when the vent is in the equilibrium position) associated with placing more channels in the membrane to define the movable portions.

Fig. 7a and 7b show example alternative vent configurations.

Fig. 7a shows a flexible membrane 601 for a MEMS transducer, the flexible membrane 601 comprising a single movable part 602. The movable portion 602 extends on either side of an imaginary "centerline" C that extends from the center of the joint structure 604 across the movable portion in a direction generally orthogonal to a line drawn in the width direction across the attachment portion or across the width of the attachment portion. Thus, the centerline C divides the movable portion into a first section a1 and a second section a 2. The movable portion 602 is asymmetrical about the imaginary centre line C and it can be seen that the first section a1 exhibits a larger surface area than the second section a 2. Starting from point P on the centre line C, the distance from point P to a first line d1, drawn substantially orthogonally to the lateral edge of the first segment a1 of the movable part, is greater than the distance from point P to a second line, drawn substantially orthogonally to the lateral edge of the second segment a 2.

The movable portion is connected to the rest of the membrane along the connecting edge 603 at a position offset from the center of the connecting edge. The movable portion may rotate about a first axis of rotation R1 and a second axis of rotation R2 in response to a pressure differential across the membrane 601.

The vent may act as a non-linear vent, which is a vent in which the size of the flow path is not fixed and in which the degree to which the vent opens and the rate of flow through the vent varies in a non-linear manner with the pressure differential.

Fig. 7b shows a flexible membrane 601 for a MEMS transducer, which flexible membrane 601 comprises two movable parts 702a and 702b, each movable part being connected to the rest of the membrane by a connecting part.

Accordingly, embodiments of the present invention generally relate to a MEMS transducer including a transducer structure having a flexible membrane supported between a first volume and a second volume and at least one vent structure. The vent structure has at least one movable portion that is movable in response to a high pressure differential across the movable portion to provide a flow path for an exhaust fluid (e.g., gas from at least one of the first and second volumes).

Embodiments have been described in terms of venting air from a volume. The same principle applies to other gases and indeed to other fluids, possibly including liquids. In some embodiments, the transducer may be arranged in a sealed environment filled with a fluid other than air, the sealed environment being arranged to allow transmission of pressure waves to or from outside the sealed environment. There may still be large pressure differences that may be generated in the sealed environment, and the use of a variable vent may be beneficial in such embodiments.

Embodiments of the present invention also relate to a MEMS transducer comprising a flexible membrane supported between a first volume and a second volume and a vent structure connecting the first volume and the second volume. The vent provides a flow path that varies in size with the pressure differential across the membrane.

Embodiments of the present invention also relate to a MEMS transducer having a membrane supported between a first volume and a second volume, wherein an acoustic impedance between the first volume and the second volume is variable with a pressure differential between the volumes.

Although various embodiments describe MEMS capacitive microphones, the invention is also applicable to any form of MEMS transducer other than a microphone, such as a pressure sensor or an ultrasonic transmitter/receiver.

Embodiments of the present invention may be usefully implemented in a range of different material systems, however, the embodiments described herein are particularly advantageous for MEMS transducers having a membrane layer comprising silicon nitride.

It should be noted that the embodiments described above may be used in a range of devices, including but not limited to: an analog microphone, a digital microphone, a pressure sensor, or an ultrasound transducer. The present invention may also be used in a number of applications, including but not limited to: consumer applications, medical applications, industrial applications, and automation applications. For example, typical consumer applications include: portable audio players, laptops, mobile phones, PDAs, and personal computers. Embodiments may also be used in voice activated devices or voice controlled devices. Typical medical applications include hearing aids. Typical industrial applications include active noise cancellation. Typical automated applications include hands-free settings, acoustic impact sensors, and active noise cancellation.

One or more transducers according to any of the above-described embodiments may be incorporated into a package. Fig. 8a to 8g illustrate a number of different packaging arrangements. Each of fig. 8 a-8 g shows the transducer elements positioned in the package, but it is understood that in some embodiments, there may be more than one transducer (e.g., an array of transducers), and that multiple transducers may be formed on the same transducer substrate (i.e., a monolithic transducer substrate), or may be formed as separate transducers with separate transducer substrates, each of which is bonded to the package substrate.

Fig. 8a shows a first arrangement in which the transducer 1100 is positioned within a cover 1101 on a package substrate 1102, the cover 1101 forming at least part of the housing. In this embodiment, the cover may be a metal housing bonded to the base. The package substrate may include at least one insulating layer. The package substrate may further include at least one conductive layer. The package substrate may be a semiconductor material, or may be formed of a material such as a PCB, ceramic, or the like. Where the cover 1101 is metallic or the cover 1101 itself includes a conductive layer, the cover may be electrically coupled to the conductive layer of the substrate, e.g., such that the housing provides shielding against electromagnetic interference (EMI). Bond wires 1103 may connect the transducer to bond pads on the package substrate. In some embodiments, readout circuitry (e.g., amplifier circuitry) may be positioned within a housing formed in or connected to the package substrate. Vias (not shown) through the package substrate may connect to contacts (i.e., solder pads) 1104 for electrically connecting external circuitry (not shown) to the package to allow transmission of electrical signals to/from the transducer 1100. In the embodiment shown in fig. 8a, there is one sound port or acoustic port in the lid 1101 to allow sound to enter the enclosure, and the transducer is arranged in a top port arrangement.

Fig. 8b illustrates an alternative arrangement in which the sound port is provided in the package substrate 1102 and may be sealed in use. The ring 1105 may be a sealing ring or a solder pad ring (for use in forming the solder ring), and the ring 1105 may be disposed around the perimeter of the sound port on the outside of the package to allow the sound path to the sound port to be sealed when, in use, connecting the package to another PCB, for example. In this embodiment, the transducer is arranged in a bottom port arrangement, and the volume defined by the housing 1101 forms part of the back volume of the transducer.

Fig. 8c illustrates an embodiment in which the transducer structure is inverted and flip-chip bonded to the package substrate via connectors 1106 instead of bond wires connecting the transducer to the package substrate. In this embodiment, the acoustic port is in the enclosure substrate such that the enclosure is arranged in a bottom port arrangement.

Figure 8d illustrates an alternative embodiment to that of figure 8b, in which housing 1107 is formed from a multiple material panel (e.g., PCB, etc.). In this case, the housing 1107 may include one or more conductive layers and/or one or more insulating layers. Fig. 8d shows a sound port in the package substrate. Figure 8e shows an alternative arrangement to that of figure 8b, in which the housing 1107 is formed from multiple material panels (e.g., a PCB as described with respect to figure 8d, etc.). Fig. 8f shows another embodiment, where the transducer structure is joined via connection 1106 to an upper layer of the housing, which may be, for example, a PCB or a layered conductive/insulating material. However, in this embodiment, the electrical connection to the package is still via contacts (solder pads) 1404 on the package substrate, such as vias (not shown) in the package substrate and conductive traces (trace) to the transducer on the inside of the housing. Figure 8g illustrates an alternative embodiment to that of figure 8c, in which the transducer is flip-chip bonded to an enclosure base in a housing 1107, the housing 1107 being formed from a panel of material (e.g., a PCB as described with respect to figure 8d, etc.).

In general, as illustrated in fig. 8h, one or more transducers may be positioned in one package, which is then operatively interconnected to another substrate, such as a motherboard, as is known in the art.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed as limiting their scope.

Claims (32)

1. A MEMS transducer comprising: a flexible membrane having a vent structure comprising at least one moveable portion, wherein the moveable portion is connected to the remainder of the membrane by a single connecting portion having a width, and wherein the moveable portion is asymmetric about an imaginary line extending across the moveable portion from the center of the connecting portion in a direction substantially orthogonal to the width of the connecting portion.

2. A MEMS transducer comprising:

a flexible membrane having a vent structure comprising at least one movable portion connected to the membrane by a single joint structure, the vent structure being configured such that, in response to a pressure differential across the vent structure, the movable portion is rotatable about a first axis of rotation and a second axis of rotation, the axes of rotation extending in the plane of the membrane when the vent structure is in an equilibrium position, wherein the first axis of rotation and the second axis of rotation intersect at the joint structure.

3. A MEMS transducer as claimed in claim 2 wherein the joint structure comprises a connection portion having a width and wherein the first axis of rotation has a component in the plane of the membrane that is substantially coincident with or parallel to the width of the connection portion.

4. A MEMS transducer as claimed in claim 2 or 3 wherein the second axis of rotation has a component substantially perpendicular to the first axis of rotation.

5. A MEMS transducer as claimed in claim 2 or 3 wherein rotation about the first axis of rotation tends to cause the movable portion to deflect above or below the plane of the membrane.

6. A MEMS transducer as claimed in claim 2 or 3 wherein rotation about the second axis of rotation tends to cause the respective movable portion to tilt relative to the plane of the membrane.

7. A MEMS transducer as claimed in claim 2 or 3 wherein the first and second axes of rotation are mutually orthogonal.

8. A MEMS transducer comprising a flexible membrane having a vent structure comprising at least one movable portion, wherein the/each movable portion is connected to the membrane by a single joint structure having a connecting portion, the vent structure being configured such that, in response to a pressure differential across the vent structure, the movable portion is rotatable about an axis of rotation having a component perpendicular to the width of the connecting portion.

9. A MEMS transducer comprising:

a flexible membrane having a vent structure comprising at least one movable portion, wherein the movable portion is connected to the remainder of the membrane by a single connecting portion having a width, and wherein an imaginary line divides the movable portion into a first segment and a second segment, the first segment having a larger surface area than the second segment, the imaginary line extending across the movable portion from the center of the connecting portion in a direction substantially orthogonal to the width of the connecting portion.

10. A MEMS transducer as claimed in any of claims 1, 8 or 9 wherein the moveable portion is rotatable about an axis of rotation having a component in the plane of the moveable portion perpendicular to the width of the connecting portion in response to a pressure differential across the vent structure.

11. A MEMS transducer as claimed in any of claims 1, 2, 8 or 9 wherein the movable portion rotates to expose an aperture in the membrane to provide a flow path through the membrane.

12. A MEMS transducer as claimed in claim 11 wherein the vent structure comprises three movable portions arranged around the outer periphery of the aperture in the membrane.

13. A MEMS transducer as claimed in any of claims 1, 2, 8 or 9 wherein the or each moveable portion has an equilibrium position in which the pressure differential across the membrane is negligible and in which the flow path through the vent structure is at a minimum.

14. A MEMS transducer as claimed in any of claims 1, 2, 8 or 9 wherein the flexible membrane is supported between a first volume and a second volume and wherein a flow path is between the first volume and the second volume.

15. A MEMS transducer as claimed in any of claims 1, 2, 8 or 9 wherein the at least one moveable portion of the flexible membrane is defined by one or more channels running through the flexible membrane.

16. A MEMS transducer as claimed in any of claims 1, 2, 8 or 9 wherein the at least one movable portion is an irregular polygon.

17. A MEMS transducer as claimed in the preceding claim wherein the moveable portion is connected to the remainder of the flexible membrane via a beam structure.

18. A MEMS transducer as claimed in any of claims 1, 2, 8 or 9 comprising a backplate structure, wherein the flexible membrane is supported relative to the backplate structure.

19. A MEMS transducer as claimed in claim 18 wherein the back plate structure comprises a plurality of holes through the back plate structure.

20. A MEMS transducer comprising a flexible membrane having a vent structure comprising a movable portion connected to the membrane by a joint structure, wherein the movable portion is laterally inclined relative to the joint structure in response to a pressure differential across the vent structure such that one lateral edge of the movable portion deflects below the plane of the membrane and an opposite lateral edge of the movable portion deflects above the plane of the membrane.

21. A MEMS transducer as claimed in any of claims 1, 2, 8, 9 or 20 wherein the transducer comprises a capacitive sensor.

22. A MEMS transducer as claimed in any of claims 1, 2, 8, 9 or 20 wherein the transducer comprises a microphone.

23. A MEMS transducer as claimed in claim 21 further comprising readout circuitry.

24. A MEMS transducer as claimed in claim 23 wherein the readout circuitry can comprise analogue circuitry and/or digital circuitry.

25. A MEMS transducer as claimed in any of claims 1, 2, 8, 9 or 20 wherein the transducer is located within a package having an acoustic port.

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

27. The electronic device of claim 26, wherein the device is at least one of: a portable device; a battery powered device; an audio device; a computing device; a communication device; a personal media player; a mobile phone; a game device; and a voice control device.

28. An integrated circuit comprising readout circuitry and a MEMS transducer according to any of claims 1-25.

29. A method of fabricating a MEMS transducer having a flexible membrane, the method comprising:

forming a structure having a flexible membrane supported between a first volume and a second volume; and

forming at least one vent structure in communication with at least one of the first volume and the second volume, the at least one vent structure comprising at least one movable portion connected to the flexible membrane by a joint structure, wherein, in response to a pressure differential across the vent structure, the movable portion is rotatable about an axis of rotation having a component perpendicular to a width of the joint structure in a plane of the membrane.

30. A method of fabricating a MEMS transducer having a flexible membrane, the method comprising:

forming a structure having a flexible membrane supported between a first volume and a second volume; and

forming at least one vent structure in communication with at least one of the first volume and the second volume, the at least one vent structure including at least one movable portion; and

forming a single joint structure on a connecting edge of the movable portion that connects the movable portion to the flexible membrane, wherein the joint structure is formed at an off-center location on the connecting edge.

31. The method of claim 29 or 30, comprising forming a film layer to form at least a portion of the flexible film and forming at least one of the vent structures in the film layer.

32. The method of claim 31, wherein forming the vent structure comprises forming one or more channels through the film layer to enable deflection of a portion of the flexible film away from a surface of a remainder of the flexible film in response to a pressure differential.