Patent Application
20090092273
The invention relates to a sensing element of a silicon condenser microphone and a method for making the same, and in particular, to a silicon microphone structure without a dedicated backplate that employs crossed wire bonding above a diaphragm element to prevent breakage from large diaphragm movements.
In the fast growing consumer electronic product market, there is increasing competition not only in product functionality but also in product reliability performance. For hand held electronic gadgets, the impact proof requirement is becoming more and more stringent. It is not unusual now to require a hand held device like a mobile phone to survive the impact from a 5000 gram weight and/or a free drop from a height of 1.5 meters to a steel plate, a process that can be repeated up to 10 times during a test.
Another electronic device that is also tested under similar conditions, a backplateless silicon microphone, was previously disclosed in a Silicon Matrix Pte Ltd patent application S106-002 and features a movable diaphragm which is supported at its edges, corners, or center by mechanical springs that are anchored to a conductive substrate through rigid pads. In addition, there are stoppers formed above perforated plate extensions of the diaphragm that restrict large movements in a direction perpendicular to an underlying backside hole and thereby minimize breakage. However, the stopper components complicate the fabrication process and there may be a compatibility issue between the stopper and the silicon membrane to which it is attached. Therefore, an improved silicon microphone design is desirable that features a structure to prevent device breakage from strong impact and can be made by a method that does not add complexity to the fabrication process or result in compatibility issues between various components.
One objective of the present invention is to provide a silicon microphone without a dedicated backplate component that has a design feature which prevents a large movement in the suspended diaphragm from breaking the device.
A further objective of the present invention is to provide a silicon microphone design according to the first objective that does not add complexity to the fabrication process.
These objectives are achieved in various embodiments of a silicon microphone design that is comprised of a diaphragm which is suspended over a backside hole formed in a conductive substrate. A plurality of perforated plates is attached to the diaphragm and a spring surrounds the perforated plates and diaphragm. The spring is held to the substrate through a plurality of anchors. Each anchor comprises a rigid pad and an underlying dielectric layer. The shapes of the perforated plates, diaphragm, spring, and rigid pads are all defined by a plurality of slots formed within a membrane layer.
In a first embodiment, the spring and diaphragm are essentially circular in shape, and the spring comprises a circular ring and a plurality of inner beams which is attached to the circular outer edge of the diaphragm. The spring is also comprised of a plurality of outer beams attached to the plurality of rigid pads of the anchors wherein one outer beam is connected to one rigid pad. Such a spring is formed to release in-plane stress and allow more out-plane flexibility. The diaphragm has a diameter slightly larger than the diameter of the underlying backside hole to avoid direct acoustic leakage.
The outer beams of the spring connect to a plurality of anchors which hold the diaphragm, spring, and perforated plates in place but allow movement of the diaphragm, perforated plates, and circular spring in a direction perpendicular to the substrate. Each rigid pad is disposed on a dielectric layer which acts as a spacer to define an air gap between the diaphragm and the substrate. One or more of the rigid pads have an overlying first electrode which is an island of a conductive metal that is connected by wiring to external circuitry. A second electrode of the same material composition is formed on the conductive substrate and is connected to a first electrode to complete a variable capacitor with one pole on the perforated plates and spring, and another pole on the substrate. Preferably, the diaphragm, perforated plates, spring, and rigid pads are coplanar and are made from the same polysilicon membrane layer and the dielectric spacer is a silicon oxide layer. Perforations formed in the perforated plates and in the spring are holes that may be arranged in various designs to allow removal of an underlying dielectric layer during the fabrication process. The holes also allow air ventilation and thus reduce the air damping in the narrow air gap below the diaphragm, spring, and perforated plates during vibrations.
An air gap exists in the dielectric spacer layer between the substrate and the perforated plates, diaphragm, and spring, and a back hole is formed in the substrate below the diaphragm so that a sound signal emanating from beyond the backside of the substrate has a free path to the diaphragm and thereby induces vibrations in the diaphragm. The diaphragm, perforated plates, and perforated spring move up and down (perpendicular to the substrate) in a concerted motion during a vibration. This movement results in a capacitance change between the first and second electrodes which can be converted into an output voltage.
The plurality of slots which define the plurality of perforated plates, spring, and plurality of rigid pads are openings that have a size that is sufficiently small enough to prevent particles that could inhibit the motion of the silicon microphone from passing through the opening and entering the air gap below. In the exemplary embodiment, there are four perforated plates each having an arc shape with a first side adjoining the outer edge of the diaphragm and three sides defined by slots. A second side opposite the first side may be slightly curved and concentric with the curved outer edge of the diaphragm. Third and fourth sides are preferably shorter than the second side and each of the third and fourth sides are aligned toward the center of the diaphragm and have an end that overlaps an end of the second side. A second end of the third and fourth sides is proximate to the outer edge of the diaphragm. Thus, a third side in each perforated plate faces an adjacent perforated plate and a fourth side in each perforated plate faces an adjacent perforated plate but not the same perforated plate as the third side. Adjacent perforated plates are separated by the inner beams of the spring.
Another important feature is the formation of a plurality of bonding pads outside the outer edge of the spring that enable bonding wires to cross over the diaphragm from a first bond site to a second bond site in a variety of patterns. Thus, if there are “n” bonding pads arrayed on the membrane layer along the outer edge of the spring, the number of bonding wires crossing the diaphragm is “n/2” and these wires are used advantageously to prevent vibrations in the diaphragm and spring from becoming too large and causing device breakage.
In a second embodiment, the perforated spring has three types of slots that may be referred to as inner slots, middle slots, and an outer continuous slot, and the perforated plates are omitted. Although the diaphragm and spring may have a rectangular, square, or other polygonal shapes, the exemplary embodiment shows a circular diaphragm surrounded by a circular spring. The diaphragm may have ribs radiating from a center point to the outer edge in order to strengthen the diaphragm. The circular spring is essentially comprised of two interconnected ring springs and a plurality of perforated beams connecting the outer edge of the outer ring spring to a plurality of anchors. The inner ring spring is attached to certain portions of the edge of the diaphragm. The outer ring spring is attached via perforated beams to a plurality of rigid pads which are anchored to the conductive substrate through a dielectric layer. Inner and outer ring springs are perforated with holes. Furthermore, there is a plurality of “n” bonding pads outside the outer edge of the perforated spring to allow a plurality of “n/2” bonding wires to cross over the diaphragm or circular spring and thereby restrict the motion of the diaphragm and perforated circular spring in a direction perpendicular to the backside hole.
There is a third embodiment similar to the second embodiment except the shape of the diaphragm and surrounding spring are essentially square. Preferably, there is a plurality of sealing ribs proximate to each side of the diaphragm and the sealing ribs may be formed equidistant from the nearest diaphragm side. An outer slot forms an essentially square shape except for the outer slot sections around the pads and perforated beams. Each of the four inner slots have a linear shape and is formed parallel to a side of the diaphragm and is a first distance from the nearest side of the diaphragm. Middle slots have an “L” shape with a first section formed parallel to a first side of the diaphragm and a second section that is formed parallel to a second side of the diaphragm. The ends of adjacent middle slots are separated by a portion of spring.
In a fourth embodiment, each of the perforated beams in the third embodiment is shifted from a corner of the square spring to a position proximate to a midpoint of a side of the square spring. Likewise, each of the pads is moved and connects with an end of a perforated beam opposite the spring. One or more bonding pads are formed on the membrane layer adjacent to a pad along each side of the spring. The inner slots are formed such that a first section of each inner slot is parallel to a first side of the diaphragm and a second section is formed parallel to a second side of the diaphragm, thus forming an “L” shape. An end of the first section and an end of the second section are formed a first distance from the nearest side of a diaphragm edge. Each of middle slots is formed parallel to a side of diaphragm at a second distance from the diaphragm edge wherein the second distance is greater than the first distance.
There is a fifth embodiment in which the slot configuration in the third embodiment has been modified to include a fourth type of slot to give a triple folded spring configuration. In this example, there are inner slots as described previously and the middle slots are now replaced by middle inner slots. There is also a plurality of middle outer slots formed between middle inner slots and the outer slot. In the exemplary embodiment, there are four middle inner slots and four middle outer slots. Each middle outer slot has one section formed parallel to a first side of the diaphragm and a second section formed parallel to a second side of the diaphragm. A middle outer slot has two ends that are formed a third distance from the nearest side of the diaphragm. The third distance is greater than the second distance of the middle inner slots. Thus, a first portion of the spring is between the inner slots and middle inner slots, a second portion is formed between the middle inner slots and middle outer slots, and a third portion is formed between the middle outer slots and continuous outer slot.
a is a top view depicting a backplateless silicon microphone with a circular spring, perforated plates, and diaphragm, and additional bonding pads for attaching bonding wires thereto according to a first embodiment of the present invention.
b is a cross-section along a first plane that bisects the backplateless silicon microphone in
a is a top view similar to
b is a cross-section along the second plane in
a is a top view of a backplateless silicon microphone with a double folded and perforated circular spring, and additional bonding pads for bonded wires according to a second embodiment of the present invention.
b is cross-sectional view of the silicon microphone structure in
a is a top view of the silicon microphone according to the second embodiment that shows a second plane which intersects two bonding pads and a second electrode.
b is a cross-sectional view of the silicon microphone in
The present invention discloses a backplateless silicon microphone design that takes advantage of a folded and perforated spring and crossed bonding wires to improve resistance to breakage from strong impact. The figures are not necessarily drawn to scale and the relative sizes of various elements in the structures may be different than in an actual device. The present invention also encompasses a method of forming a silicon microphone according to an embodiment described herein. The terms “surface microstructure” may be used interchangeably with “silicon microphone”.
Referring to
The diaphragm 11 is made of doped silicon, doped polysilicon, Au, Ni, Cu, or other semiconductor materials or metals and is supported along its outer edge 11a by attachment to portions of the circular spring 12 and portions of perforated plates 19 that are comprised of the same material and have the same thickness as the diaphragm 11. The circular spring 12 has a perimeter that is interrupted at a plurality of locations where a plurality of “m” outer beams 12a are formed and serve as connections to a plurality of “m” pads 13 outside the perimeter of the circular spring where “m” is preferably >3. The pads 13 are also made from the same membrane layer 10 as the diaphragm 11, perforated plates 19, and circular spring 12. Unlike the circular spring 12, perforated plates 19, and diaphragm 11 which have flexibility to vibrate in a direction perpendicular to the underlying backside hole 15, the pads 13 are rigidly held in position by attachment to an underlying dielectric layer (not shown) which in turn is formed on the substrate 8. Each pad 13 and underlying portion of dielectric layer form a rigid structure called an anchor. Outer beams 12a provide torsional stress buffering at the pads 13 which in one embodiment are formed equidistant from the diaphragm center 11c. There is a continuous outer slot 22 that separates the pads 13 and circular spring 12 including outer beams 12a from the membrane layer 10.
One important feature is that the circular spring 12 is comprised of a plurality of slots 14a, 14b, 22 that each represent a narrow gap having a width of about 3 to 10 microns. Thus, the circular spring 12 can release in-plane stress and has more out-plane flexibility. The circular spring is also comprised of a plurality of inner beams 12b connected to the outer edge 11a of the diaphragm 11 and formed between adjacent slots 14a. The size of slots 14a, 14b may be minimized based on processing constraints to prevent particles from passing through the slot into the underlying air gap (not shown) and thereby restricting the motion of the diaphragm 11 and spring 12 in a direction perpendicular to the backside hole 15. In the exemplary embodiment, there are four arc shaped perforated plates 19 arranged around the outer edge 11a of the diaphragm 11. The shape of a perforated plate 19 is defined by a slot 14b opposite a side of the perforated plate that adjoins outer edge 11a and two slots 14a connected to slot 14b.
In the exemplary embodiment, slot 14b is essentially concentric to the nearest section of outer edge 11a and has two ends wherein one end overlaps an end of a slot 14a and a second end overlaps an end of a second slot 14a. Slots 14a are aligned toward the diaphragm center 11c and preferably have a shorter length than slots 14b. A slot 14a in one perforated plate 19 faces a slot 14a in an adjacent perforated plate 19 and the facing slots 14a are separated by an inner beam 12a of circular spring 12. Preferably, all slots 14b are disposed the same distance from the diaphragm center 11c. Alternatively, other designs for the plurality of slots may be used. However, each perforated plate 19 should be defined by at least one slot aligned in a direction that is substantially concentric to the nearest section of outer edge 11a. There is a plurality of perforations 20 or holes arranged in various patterns within each perforated plate 19 to allow air ventilation and reduce the air damping in the narrow air gap (not shown) between the perforated plates and substrate 8 during vibrations.
The circular spring 12 is also comprised of a plurality of perforations 20 that may be formed in a variety of patterns within inner beams 12b, between slots 14b and slot 22, and within the outer beams 12a. The perforations 20 can reduce the air damping in the narrow air gap (not shown) between the circular spring 12 and substrate 8 during vibrations. Perforations 20 in the circular spring 12 and perforated plates 19 are also used to facilitate the removal of portions of an underlying dielectric layer (not shown) during the fabrication process and thereby aid in the formation of a narrow air gap below the diaphragm 11, perforated plates 19, and spring 12. The pads 13 may have a circular shape and are positioned at the end of each outer beam 12a. There is also a plurality of ”n” bonding pads 16 made of Al. Cu, Au, or other composite metal materials formed on the membrane layer 10 outside the slot 22. As shown in
Returning to
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In the exemplary embodiment, there is a first bonding wire 21a that connects a first pair of bonding pads 16a, 16b. In addition, there is a second bonding wire 21b which connects a second pair of bonding pads 16a, 16b, a third bonding wire 21c connecting a third pair of bonding pads 16a, 16b, and a fourth bonding wire 21d connecting a fourth pair of bonding pads 16a, 16b. In this case, all four bonding wires 21a-21d cross above the diaphragm center 11c. Bonding wires 21a-21d may be comprised of Al or Au and may be formed by using conventional wedge bonding or a thermalsonic ball bonding process as known by those skilled in the art. Each bonding wire 21a-21d has a first end and a second end wherein a first end is attached to a first bonding pad 16a and a second end is attached to a second bonding pad 16b.
Referring to
Together, the four bonding wires 21a-21d form a stopper that restricts the motion of the diaphragm 11, perforated plates 19, and spring 12 in a z-direction and thereby prevents device breakage. It should be understood that the configuration where bonding wire 21b crosses over bonding wire 21a is not required. The critical aspect of the bonding scheme is that the bonding wires 21a-21d cross above the diaphragm 11 to limit the loop height h in at least one and preferably for a plurality of bonding wires, and provide an improved restraint for diaphragm movement compared with an edge restraint as in the prior art.
Referring to
Referring to
In a silicon-on-insulator (SOI) application, the dielectric layer 9 may be comprised of silicon oxide and the substrate 8 is made of silicon. Optionally, the dielectric layer 9 may be comprised of other dielectric materials used in the art and may be a composite with a plurality of layers therein.
As mentioned previously, there is a first electrode 17 comprised of a metal or composite such as Cr/Au above one or more pads 13. A first electrode 17 serves as a connecting point to external wiring. Additionally, there are one or more second electrodes (not shown) with the same composition as a first electrode formed on the top surface of substrate 8. It should be understood that the backplateless silicon microphone 1 is also comprised of a voltage bias source (including a bias resistor) and a source follower preamplifier but these components are not shown in order to simplify the drawing. A vibration in the diaphragm 11, perforated plates 19, and circular spring 12 is induced by a sound signal that passes through the backside hole 15 and impinges on the bottom surface of the diaphragm that faces the air gap 7. A vibration will cause a change in capacitance in the variable capacitor circuit that is converted into a low impedance voltage output by the source follower preamplifier as understood by those skilled in the art.
An exemplary process sequence for fabricating the backplateless silicon microphone 1 comprises forming a dielectric layer 9 such as silicon oxide by a conventional oxidation or deposition methods on the substrate 8 which may be doped silicon that is polished on both of its front and back sides. A membrane layer 10 is deposited on the dielectric layer 9 and will be subsequently patterned to form diaphragm 11, circular spring 12, pads 13, and perforated plates 19. Those skilled in the art will appreciate that the membrane layer 10 and dielectric layer 9 could also be formed directly by a well known wafer bonding process. In an SOI approach where the dielectric layer 9 is silicon oxide and the membrane layer 10 is doped silicon, substrate 8 and the membrane layer are provided with a resistivity of <0.02 ohm-cm.
Next, a hardmask comprised of one or more layers that will subsequently be used for fabricating a backside hole is formed on the back side 8a of substrate 8. In one embodiment, the hard mask is comprised of a thermal oxide layer 3 grown by a well known LPCVD method on the substrate 8 and a silicon nitride layer 4 deposited by an LPCVD method on thermal oxide layer 3. Note that the hard mask is simultaneously grown on the membrane film on the opposite side of the substrate 8 but is subsequently removed by well known wet chemical or dry etching methods.
One or more via openings (not shown) are formed in the dielectric layer 9 and membrane layer 10 to expose certain portions of the substrate 8. Then a conductive layer that will be used to form first electrodes, second electrodes, and bonding pads is formed on the membrane layer 10 and in the via openings by using a conventional physical vapor deposition (PVD) method. A photomask (not shown) is employed to selectively etch portions of the conductive layer to define one or more first electrodes 17 and bonding pads 16 on the membrane layer 10, and one or more second electrodes 18 within the via openings.
Next, the membrane layer 10 is selectively etched with a second photomask (not shown) to form slots 14a, 14b, 22. Perforations 20 are also formed by etching the patterned second photomask layer but are not shown in
Conventional processing then follows in which the substrate 8 is diced to physically separate silicon microphone devices from each other. There is a final release step in which a portion of the dielectric layer 9 is removed to form the air gap 7. The perforations 20 may facilitate the removal of selected portions of the dielectric layer 9 during this step. In an SOI embodiment, a dielectric layer 9 made of oxide is removed to form an air gap 7 by a timed etch involving a buffered HF solution, for example. The dielectric layer 9 is removed with proper control so that portions of the dielectric layer below the pads 13 can be kept intact.
Referring to
An important feature is that the circular spring 33 has a plurality of middle slots 34a and plurality of inner slots 34b formed therein and each slot represents a narrow gap which is typically 3 to 10 microns wide along a diameter of the circular spring. Moreover, there is a continuous outer slot 34c that surrounds the spring 33, beams 33a, and pads 32 and separates the aforementioned elements from the surrounding membrane layer 30. The size of the gap in the inner slots 34b, middle slots 34a, and outer slot 34c is minimized based on processing constraints to prevent particles from entering the air gap (not shown) below the diaphragm 31. The middle slots 34a and inner slots 34b are patterned in such a way that the separation between any two inner slots 34b is aligned to a central portion of the nearest middle slot 34a. The circular spring 33 is essentially comprised of two interconnected rings, an inner ring formed between the outer edge 31a and the middle slots 34a, and an outer ring formed between the middle slots 34a and the outer slot 34c. As a result, the two interconnected rings within circular spring 33 enable a release of in-plane stress and allow more out-plane flexibility.
In the exemplary embodiment, there is a first set of three inner slots 34b arranged around the outer edge 31a of the diaphragm 31. Each inner slot 34b has a lengthwise direction that forms a curved (arc) shape which is concentric to the curved outer edge 31a, and is formed a first distance from the outer edge 31a. Each inner slot 34b has two ends and the distance between the two ends is defined as the length of an inner slot 34b which is preferably equivalent for all inner slots 34b. Preferably, the distance between a middle slot 34a and the nearest point on an adjacent inner slot 34b is less than the length of an inner slot 34b. Likewise, there is a second set of three middle slots 34a arranged in a circular pattern between the inner slots 34b and the outer slot 34c. Each middle slot 34a is formed a second distance from the diaphragm center 31c and the second distance is greater than the first distance. The arc length of each middle slot 34a may be the same as or larger than the length of an inner slot 34b. Each middle slot 34a has two ends and an arc shape that is concentric to the curved outer edge 31a and the distance between an end of one middle slot 34a and the nearest point on an adjacent inner slot 34b is preferably less than the length of a middle slot 34a.
Alternatively, other designs for the slots 34a, 34b and pads 32 may be used. For example, the number of slots within each set of middle slots 34a or inner slots 34b may be greater than three and the number of perforated beams 33a and pads 32 may be larger than three.
The circular spring 33 is also comprised of a plurality of holes or perforations 40 that may be formed in a variety of patterns between the diaphragm 31 and outer slot 34c, and within the beams 33a. The perforations 40 are needed to allow air ventilation and thus reduce the air damping in the narrow air gap (not shown) between the circular spring 33 and substrate 8 during vibrations. The pads 32 may have a circular shape and are positioned at the end of each perforated beam 33a. There is also a plurality of ribs 39 formed within diaphragm 31 to strengthen that element. Each rib 39 may extend from the diaphragm center 31c to the outer edge 31a and may gradually become wider as the distance from the diaphragm center increases.
Another important feature is a plurality of bonding pads 36 that are arrayed outside the outer slot 34c. There is at least one bonding pad 36 formed between two adjacent pads 32. The bonding pads 36 may be comprised of the same metal as in a first electrode 37 or in a second electrode 38 and are formed on the membrane layer 30 at locations outside the circular spring 33. In one embodiment, the bonding pads 36 are equidistant from the diaphragm center 31c. In the exemplary embodiment, there are four bonding pads 36 formed between each pair of adjacent pads 32. However, the present invention also encompasses an embodiment where there are an unequal number of bonding pads between adjacent pads 32. For example, there may be three bonding pads between a first pad 32 and a second pad 32 and four bonding pads between the second pad and a third pad 32.
One or more of the pads 32 may have a first electrode 37 formed thereon. A first electrode 37 may be comprised of a metal layer such as Cr/Au that serves as a connecting point to external wiring. Additionally, there are one or more second electrodes 38 with the same composition as a first electrode 37. The second electrodes 38 may be formed on the substrate 28 and may be formed at a greater distance from the diaphragm center 31c than a bonding pad 36 or first electrode 37. A first electrode 37 and second electrode 38 may have a circular shape and are connected by wiring (not shown) to form a variable capacitor with one pole on the perforated spring 33 and another pole on the substrate 28. From a top view, a first electrode 37 has a smaller diameter than that of a pad 32 to allow for some overlay error and undercut release during fabrication. Optionally, the first and second electrodes 37, 38 may be a single or composite layer comprised of Al, Ti, Ta, Ni, Cu, Au or other metals.
Referring to
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Bonding pads 36a-36d serve as termination points for a plurality of bonding wires that cross over the circular spring 33, and in some cases the diaphragm 31, and thereby function as stoppers to prevent large vibrations or strong impact in the aforementioned moveable elements from breaking the device. First bonding pads 36a, 36c differ only in that a first bonding pad 36a is formed between a pad 32 and an adjacent second bonding pad 36d while a first bonding pad 36c is formed between a second bonding pad 36b and a second bonding pad 36d. Note that a second bonding pad 36b is formed between a pad 32 and a first bonding pad 36c while a second bonding pad 36d is formed between a first bonding pad 36a and a first bonding pad 36c. There is a first bonding pad 36a opposite every second bonding pad 36b and a first bonding pad 36c opposite each second bonding pad 36d. In this embodiment, first bonding pads (36a or 36c) and second bonding pads (36b or 36d) are formed in an alternating fashion along the outer slot 34c. Optionally, when n=2, there is only one bonding wire (not shown) connecting a first bonding pad 36a and a second bonding pad 36b and the bonding wire preferably crosses over the center of the diaphragm 31.
In the exemplary embodiment, there are three bonding wires 41 wherein each bonding wire 41 connects a first bonding pad 36a and a second bonding pad 36b and crosses over the circular spring 33 and diaphragm 31. Furthermore, there are three bonding wires 42 wherein each bonding wire 42 connects a first bonding pad 36c and a second bonding pad 36d and crosses over a circular spring 33 but not over the diaphragm 31. One or more bonding wires 41 may cross over a bonding wire 42 and one or more bonding wires 42 may cross over a bonding wire 41 to provide a high degree of restraint in limiting the upward motion (out of the plane of the paper) of the diaphragm 31 and circular spring 33 during a large vibration or strong impact. Thus, the bonding wires 41, 42 are advantageously used as a stopper to prevent the moveable elements from moving too far away from the substrate 28 and thereby prevent device breakage. Bonding wires 41, 42 may be made of Al or Au and may be formed by employing a well known thermalsonic gold wire bonding process or with a conventional wedge bonding process.
Alternatively, other wire bonding designs may be used to restrain the movement of diaphragm 31 and circular spring 33. Preferably, each bonding scheme comprises a plurality of bonding wires in which one or more bonding wires cross over the diaphragm 31 to provide maximum restraint during vibrations.
Referring to
In addition, there are three sets of slots. The outer slot 34c forms an essentially square shape except for the outer slot sections around the pads 32 and perforated beams 33a. Each of the four inner slots 34b has a linear shape and is formed parallel to a side of the diaphragm 31 and is a first distance from the nearest side of the diaphragm. The middle slots 34a each have an “L” shape and a first section formed parallel to a first side of the diaphragm 31 and a second section that is formed parallel to a second side of the diaphragm. An end on first section and an end on second section are formed a second distance from a nearest side of the diaphragm 31 wherein the second distance is greater than the first distance. The ends of adjacent middle slots 34a are separated by a portion of spring 33. Preferably, there is an inner slot 34b formed between the diaphragm 31 and an end of a middle slot 34a.
The spring 33 in the third embodiment is considered to have a double folded spring configuration wherein an inner folded spring portion is formed between the inner slots 34b and the middle slots 34a and an outer folded spring portion is formed between the middle slots and the outer slot 34c.
Other aspects of the second embodiment are carried forth in the third embodiment such as a plurality of “n” bonding pads 36 formed outside the outer slot 34c on membrane layer 30 and preferably between adjacent pads 32. There is a first electrode 37 formed on one or more pads 32 and one or more second electrodes 38 formed on substrate 28. From a top view, the sides (outer edges) of the diaphragm 31 and sealing ribs 31r are a greater distance (x, y direction) from the diaphragm center 31c than the backside hole 35 which may have a square shape. The third embodiment also encompasses a bonding wire protection scheme in which “n/2” bonding wires (not shown) are used to connect the “n” bonding pads 32 as described in previous embodiments.
Referring to
A bonding wire protection scheme is employed similar to that described in the first two embodiments. In particular, a plurality of “n/2” bonding wires (not shown) connect a plurality of “n” bonding pads 36 and thereby restrict the upward motion of the diaphragm 31 away from the backside hole 35 during large vibrations caused by a strong impact or a large sound signal. Preferably, each of the “n/2” bonding wires cross over at least a portion of the diaphragm 31 or spring 33.
Referring to
As in the previous embodiments, a bonding wire protection scheme comprised of “n/2” bonding wires (not shown) connecting “n” bonding pads 36 is advantageously employed to restrict the upward motion of the diaphragm 31 away from the backside hole 35 and thereby imparts impact proof resistance to the silicon microphone 60. Preferably, each of the “n/2” bonding wires cross over at least a portion of the diaphragm 31 or spring 33. The bonding pads 36 may be formed equidistant from the outer slot 34c. Preferably, there are one or more bonding pads 36 between adjacent pads 32.
All of the embodiments provide an advantage over prior art in that the backplateless silicon microphone disclosed herein has the improved impact proof capability because the bonding wires provide restraint over the entire surface of the diaphragm and circular spring compared with only an edge restraint in the prior art. Moreover, the bonding wires can be fabricated during the same process that forms wire connections between first electrodes and second electrodes and thus do not add any complexity to the production process. In addition, the unique slot design allows in-plane stress to be released and permits more out-plane flexibility to prevent device breakage.
While this invention has been particularly shown and described with reference to, the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.
RELATED PATENT APPLICATION This application is related to the following: Docket # S106-002, Ser. No. 11/500114, filing date Aug. 7, 2006; assigned to a common assignee and herein incorporated by reference in its entirety.
