CLAIM OF PRIORITY

The application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 62/376,658 filed August 18, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND

Micro-electro-mechanical systems (MEMS) technology has enabled the development of acoustic transducers such as microphones using silicon-wafer deposition techniques.

Microphones fabricated this way are commonly referred to as MEMS microphones and can be made in various forms such as capacitive microphones or piezoelectric microphones. MEMS capacitive microphones and electret condenser microphones (ECMs) are used in consumer electronics and have an advantage over typical piezoelectric MEMS microphones in that they have historically had greater sensitivity and lower noise floors. However, each of these more ubiquitous technologies has its own disadvantages. For standard ECMs, they cannot be mounted to a printed circuit board using the typical lead-free solder processing commonly used to attach microchips to the board. MEMS capacitive microphones, which are often used in cell phones, have a backplate that is a source of noise in the microphones. MEMS capacitive microphones also have a smaller dynamic range than typical piezoelectric MEMS microphones.

SUMMARY

In some examples, a piezoelectric, acoustic device comprises substrate; a plurality of plates, each plate comprising a first electrode layer, a piezoelectric layer and a second electrode layer, wherein the piezoelectric layer is in between the first electrode layer and the second electrode layer, and wherein, for each plate, a base of the plate is affixed to the substrate and a remaining portion of the plate is unaffixed to the substrate; and a plate coupling structure affixed to a first one of the plates and to a second one of the plates, wherein the plate coupling structure at least partly covers a gap between a first edge of the first one of the plates and a second edge of the second one of the plates, wherein the first edge faces the second edge; wherein the plate coupling structure comprises a mismatch reduction element and a stopping element, wherein the mismatch reduction element is configured to reduce an amount of mismatch in deflection of the first one of the plates and the second one of the plates, and wherein the stopping element is configured to enable the mismatch reduction element to expand only up to a specified amount.

This aspect includes one or more of the following features, taken in any combination. The mismatch reduction element comprises a spring. The piezoelectric, acoustic device comprises a microphone. The piezoelectric, acoustic device comprises a

Microelectromechanical systems (MEMS) microphone. A plate comprises a cantilevered beam. The stopping element comprises a plurality of interdigitated fingers. A first portion of the interdigitated fingers are part of the first one of the plates and extend into the second one of the plates. The mismatch reduction element extends from a tip of an interdigitated finger in the first portion to a tip of another one interdigitated finger in the first portion. A second portion of the interdigitated fingers are part of the second one of the plates, and wherein the mismatch reduction element is unconnected to one or more interdigitated fingers included in the second portion. The mismatch reduction element is affixed to an end portion of at least one of the interdigitated fingers. The first one of the plates is adjacent to the second one of the plates. The actions include a vertical gap between at least one of the interdigitated fingers and the mismatch reduction element. The vertical gap is approximately in a range of 0.1 μπι to 1.0 μπι. The piezoelectric, acoustic device is a transducer. The plurality of plates comprise tapered, transducer beams, with a tapered, transducer beam having a beam base, a beam end, and a beam body, with the beam body tapered from the beam base to the beam end and disposed between the beam base and the beam end, the tapered, transducer beams connected in a cantilever arrangement over the substrate by having beam bases attached to the substrate, beam ends converging towards a common each, and with each beam body free from the substrate and with each beam end free and unattached. The stopping element is comprised of aluminum nitride. The actions include overlapping portions affixed to respective plates, wherein each of the plates comprises openings sized to fit respective stopping elements, and wherein an element of an overlapping portion is affixed to a plate at least partly between two openings, wherein the overlapping portion comprises a strip portion and wherein the strip portion is affixed to the plate between two elements of the overlapping portion and at least partly covers an opening sized to a fit stopping element. DESCRIPTION OF DRAWINGS

FIG. 1 is diagram of a plate.

FIG. 2 is a diagram of plates arranged in a gap-controlling geometry.

FIG. 3 is a diagram of modeled deflection.

FIG. 4 is a diagram of modeled deflection along a gap of two adjacent plates in a gap controlling geometry.

FIG. 5 is a diagram of a MEMS die and a magnified view of a center of the MEMS die. FIG. 6 is a diagram of a fabrication process.

FIGS. 7, 8 and 10 are each a diagram of a view of a center of a piezoelectric sensor plate with plate coupling structures.

FIG. 9A is a diagram of a transducing element with plate coupling structures.

FIG. 9B is a diagram of one of the plate coupling structures shown in FIG. 9A.

FIG. 11 is a diagram of a portion of a plate coupling structure that includes fingers and overlapping portions.

DETAILED DESCRIPTION

When fabricating a microphone with gap controlling geometry (such as that described in U.S. Patent No. 9,055,372, the entire contents of which are incorporated herein by reference), sensor yield is reduced due to manufacturing non-idealities relative to sensor yield without these non-idealities. For example, if the in-plane residual stress is not the same in two orthogonal in- plane directions, and this difference varies through the thickness of the film stack, then the tips of the plates can have different amounts of vertical deflection. They could also have different amounts of vertical deflection if they were slightly different lengths. This different deflection of adjacent plates is undesirable because it increases the gap between plates and reduces the acoustic resistance through the sensor as described in U.S. Patent No. 9,055,372. For example, a MEMS microphone transducer design has two 0.5 μπι thick layers of aluminum nitride (A1N) stacked on top of each other. The residual film stress in the X-direction (oxx res) for the bottom layer is 400 MPa and that in the Y direction (oyy res) is 435 MPa. The residual film stress in both the X-direction and Y-direction is 400 MPa for the top layer. This difference in X versus Y stress will cause a plate deflection of approximately 15 μπι for 380 μπι long plates.

For example, referring to FIG. 1, plate 10 is shown. In this example, plate 10 is one of four plates in a gap controlling geometry. Plate 10 includes edges 10a, 10b and base 10c. Base 10c is fixed to a substrate (not shown) and the remaining structure is free to move, creating a fixed-free-free triangular cantilever plate. For this example, the

length of plate 10 is 380 μπι from base to tip and the thickness is 1 μπι thick A1N. This includes two 500 nm thick layers which add up to a total of 1 μπι.

Referring to FIG. 2, acoustic device 20 includes four plates 22, 24, 26, 28 arranged in a gap-controlling geometry. In this example, a gap exists between edges of the plates that face each other. For example, gap 30 exists between edge 34 of plate 22 and edge 32 of plate 28. In this example, each triangular plate is a cantilever plate. This is a gap

controlling geometry because as the plates 22, 24, 26, 28 bend up or down due to

variations in residual stress, the gaps remain relatively small compared to gaps in two facing rectangular cantilevers for example. However, mismatches in stress in the X and Y directions can still cause these plates 22, 24, 26, 28 to deflect differently and have gaps that are much larger than those created when the stress in X and Y directions is the same.

Referring to FIG. 3, diagram 40 shows modeled deflection of the four plates when the bottom layer stress is 400 MPa in the X-direction and 435 MPa in the Y-direction and the top layer stress is 400 MPa in both the X and Y directions. The two opposing pairs of plates have matched deflection but the difference in X and Y stress causes the adjacent plates to have different vertical deflections, enlarging the gap between plates.

Referring to FIG. 4, diagram 50 shows modeled deflection along the gap of two adjacent plates in the gap controlling geometry. The conditions are the same as those used for FIG 3. The gap between plates is largest at the tip, where it is about 15 μπι.

In addition to deflection mismatch of adjacent plates, another limitation of cantilever- plate based piezoelectric MEMS microphones is that they do not have a backplate to stop over excursion, as capacitive MEMS microphones do. In some cases, very high pressure levels or very high acceleration levels can cause the piezoelectric microphone diaphragm or plates to bend enough to cause breakage. A capacitive MEMS microphone prevents diaphragms from breaking off the microphone by using a very stiff backplate, which prevents the microphone diaphragm from deflecting too far, thereby limiting the maximum stress in the diaphragm. A capacitive MEMS microphone necessarily includes a backplate to form the capacitor with a diaphragm (e.g., the front plate). A piezoelectric MEMS microphone typically does not have a backplate, which would generate noise in the microphone.

In order to both reduce mismatch between adjacent plates and prevent breakage due to excessive bending, a plate coupling structure is described here. This structure consists of an interdigitated finger element and a mismatch reduction element. This structure has a non-linear behavior such that it provides two regimes of operation. The first operating regime is called "normal operation." In this normal operation, an applied acoustic pressure produces a large change in piezoelectric material stress relative to the second regime. The second operating regime is called "overload protection." In overload protection, an applied pressure or acceleration produces a relatively small change in piezoelectric material stress relative to normal operation.

Referring to FIG. 5, diagram 59 illustrates a MEMS microphone die 60 (e.g., a MEMS transducer) that includes four triangular plates 64, 66, 68 and 70 meeting at a center location 61. In this example, each base of plates 64, 66, 68 and 70 is affixed to substrate 62. Diagram 59 also includes magnified view 63 of the center of the MEMS die 60 showing plate coupling structures 71a and 70b-70d connecting adjacent plates. Magnified view 63 illustrates portion 64a of plate 64, portion 66a of plate 66, portion 68a of plate 68, and portion 70a of plate 70. In this example, plate coupling structure 71a is affixed to portions 66a, 70a and at least partially covers a gap between an edge of portion 70a and an edge of portion 66a.

In this example, plate coupling structure 71a includes mismatch reduction elements 72, 74 and stopping elements 76a-76e that are configured to prevent a mismatch reduction elements from stretching too far. In this example, each of stopping elements 76a-76e is an interdigitated finger or interdigitated element. In this case, the interdigitated fingers are an extension of the plates. These interdigitated fingers extend into the adjacent plates. For example, interdigitated element 76b is an extension of portion 70a of plate 70 and extends into portion 66a of plate 66, which is adjacent to plate 70. A mismatch reduction element (e.g., one of mismatch reduction elements 72, 74) then extends from the tip of one finger to that of the next finger on the same plate, skipping over the finger of the adjacent plate. For example, mismatch reduction element 74 extends from tip 77a of finger 76a to tip 77b of finger 76c to tip 77c of finger 76e, skipping over fingers 76b, 76d of adjacent plate 66. In this example, fingers 76a, 76c, 76e all are on the same plate - plate 70. In operation, the mismatch reduction element may touch the finger of the neighboring plate, keeping the plates from having excessive mismatch. When the microphone is exposed to high pressure or acceleration, however, the plates deflect significantly and the mismatch reduction elements of adjacent plates eventually come into contact with each other. When the mismatch reduction elements come into contact, the boundary conditions of the plates change and the operating regime changes from "normal operation" to "overload protection." This change in boundary conditions causes the deflection shape to change and reduces stress in the piezoelectric material caused by an applied pressure or acceleration relative to the stress in the piezoelectric material of a plate without the plate coupling structure.

Referring to FIG. 6, fabrication process 80 is shown for building the plate coupling structure. In this example, top view 80a and cross-section view 80b illustrate a step in which the plates and interdigitated fingers are defined by a first etch of the plate material or materials. As shown in top view 80c and cross-section view 80d, a sacrificial layer is deposited and vias are etched into this sacrificial layer. These vias will become the points where the mismatch reduction element attaches to the tips of the interdigitated fingers. As shown in top view 80e and cross-section view 80f, the mismatch reduction element layer is deposited and patterned. The mismatch reduction element material fills the vias, connecting to the fingers of one plate while overlapping but not connecting to the fingers of the adjacent plate. In the end, the device is released by removing the sacrificial material under the mismatch reduction element as well as the sacrificial material under the piezoelectric plates.

Some design parameters of this design are the finger length and the nominal vertical gap between the finger and the mismatch reduction element. The length of the fingers will determine the transition point from normal operation to overload protection. The longer the fingers, the larger the range of normal operation. In this embodiment, the finger length increases as the fingers get closer to the plate tip. The length increases such that many of the mismatch reduction elements come into contact at the same time to reduce the peak stress in the mismatch reduction elements. The nominal vertical gap between the finger and the mismatch reduction element is also critical. If this gap is small, fingers on both adjacent plates can come into contact with the mismatch reduction elements before the mismatch reduction elements contact each other. This creates three regimes, normal operation (e.g., defined as a change in boundary condition as described above), a first overload protection regime in which mismatch reduction elements are not contacting each other, and a second overload protection regime in which mismatch reduction elements are in contact with each other. If this first overload protection regime is undesirable, then the vertical gap between the finger and the mismatch reduction element must be adjusted accordingly. In practice, a gap between 0.1 μπι and 1.0 μπι is reasonable but it will depend on the geometry of the interdigitated fingers and the desired range of normal operation.

Referring to FIG. 7, view 90 of a center of a piezoelectric plate sensor 92 is shown. In this example, piezoelectric plate sensor 92 includes plate coupling structure 94 with trapezoidal interdigitated fingers 91, 96, 98, 100, 102 and mismatch reduction elements 93, 95, 97, 99, 101. In this example, each of mismatch reduction elements 93, 95, 97, 99, 101 is connected to a root (i.e., end portion) of a finger instead of a tip. By using trapezoidal interdigitated fingers, the majority of the stress induced by contact is in the plate material instead of the mismatch reduction elements 93, 95, 97, 99, 101. This is advantageous in cases where the plate material is stronger than that of the mismatch reduction elements 93, 95, 97, 99, 101. Similar to the foregoing embodiments above, the geometry of the trapezoidal fingers can be changed from the base to the tip of the plate in order to allow many of the plates to come into contact at approximately the same level of deflection. This allows the stress to be spread out over many fingers instead of being concentrated on a single finger.

Referring to FIG. 8, view 120 of a center of piezoelectric sensor plate 122 is shown. In this example, piezoelectric sensor plate 124 includes plate coupling structure 124 with T-shaped interdigitated fingers 128. In plate coupling structure 124, spring 126 is a mismatch reduction element. Similar to the trapezoidal finger, a T-shaped finger directs the maximum stress in the plate material. Here, the T-shaped interdigitated fingers 128 also prevent the spring 126 from overextending and breaking. Unlike the foregoing embodiments, this structure has a mismatch reduction element (i.e., spring 126) that is nominally in a same vertical plane as the piezoelectric sensor plate 122 and T-shaped interdigitated fingers 128. This means that this plate coupling structure 124 is more susceptible to failure due to an inability of the spring 126 to hold both plates in the same plane. The previous two embodiments can only fail if some part of the plate or plate coupling structure is physically broken. Because all elements in this embodiment of FIG. 8 are in the same vertical plane, however, the fabrication is simpler. There is no need for vias and an additional layer of material. Referring to FIG. 9A, transducing element 150 includes plates 151, 152, 154, 156, 158, 160 and plate coupling structures 162, 164, 166, 168, 170, 172. Referring to FIG. 9B, a close-up view of plate coupling structure 162 is shown. In this example, plate coupling structure 162 is a same plate coupling structure as each of plate coupling structures 164, 166, 168, 170, 172. In this example, plate coupling structure 164 includes spring 164a, fingers 164b-164r, 165a-165b and 165c-165p and overlapping portions 166, 168. In this example, each of overlapping portions is represented by the shaded black regions in FIG. 9B that extend horizontally along a length of the plate coupling structure. In this example, an overlapping portion is a piece of structural material that is affixed or otherwise connected to a plate. In this example, spring 164a acts as the mismatch reduction element and overlapping portions prevent the failure described for FIG 8.

In some example, one or more of overlapping portions 166, 168 stick to one or more of fingers 164b-164r, 165a-165b and 165c-165p when they touch. Therefore, the microphone sensitivity degrades if one or more of fingers 164b-164r, 165a-165b and 165c-165p touches one or more of overlapping portions 166, 168. As such, spring 164a prevents one or more of fingers 164b-164r, 165a-165b and 165c-165p from touching one or more of overlapping portions 166, 168, e.g., during normal operation. In this example, overlapping portions 166, 168 are used to prevent plates 151, 152, 154, 156, 158, 160 (FIG. 9A) from getting out of plane and coming apart, breaking spring 164a.

Referring to FIG. 10, diagram 200 shows a close-up view of a center of a transducing element that includes plates 202, 204, 206, 208. In this example, plate coupling structure 210 is attached to plates 202, 206. In this example, plate coupling structure 210 includes spring 212, fingers 214, 216, 218, 220 and overlapping portions 222, 224.

Referring to FIG. 11, diagram 250 illustrates a zoomed-in view of fingers and

overlapping portions of a plate coupling structure. In this example, a plate coupling structure (attached to plates 252, 254) includes fingers 256, 258, 260, 262, 264, 266, 268. In this example, fingers 262, 264, 266, 268 are part of plate 254 (i.e., are extensions of plate 254) and plate 252 includes openings sized to fit each of fingers 262, 264, 266, 268. Similarly, each of fingers 256, 258, 260 is part of plate 252 and plate 254 includes openings sized to fit each of each of fingers 256, 258, 260. In this example, overlapping portion 270 includes elements 270a-270c and is affixed to plate 252 along an edge of a gap (not shown) between plate 252 and plate 254. Each of elements 270a-270c is sized to fit between portions of plate 252 that include the openings for fingers. Overlapping portion 270 also includes strip portion 270d that connects elements 270a, 270b and lies over the opening in plate 252 that is sized for finger 264. In this example, overlapping portion 272 is affixed to plate 254 and includes elements 272a-272b and includes strip portion 272c.

Other embodiments are within the scope and spirit of the description and the claims. The use of the term "a" herein and throughout the application is not used in a limiting manner and therefore is not meant to exclude a multiple meaning or a "one or more" meaning for the term "a." Additionally, to the extent priority is claimed to a provisional patent application, it should be understood that the provisional patent application is not limiting but includes examples of how the techniques described herein may be implemented.

A number of embodiments of the invention have been described. Nevertheless, it will be understood by one of ordinary skill in the art that various modifications may be made without departing from the spirit and scope of the claims and the techniques described herein.