Source: http://patents.com/us-8247945.html
Timestamp: 2018-12-13 20:16:17
Document Index: 487144264

Matched Legal Cases: ['Application No. 06744966', 'Application No. 200680017137', 'application No. 200680025783', 'application No. 2008', 'application No. 200680017137', 'application No. 200680025783', 'application No. 200680021083', 'application No. 2008', 'Application No. 2008']

US Patent # 8,247,945. Micro-electro-mechanical transducers - Patents.com
United States Patent 8,247,945
Huang August 21, 2012
A micro-electro-mechanical transducer (such as a cMUT) is disclosed. The transducer has a substrate, a top plate, and a resilient structure therebetween. The resilient structure has multiple connectors distributed over the device element area to vertically transport the top player with distributed support. The resilient structure may be cantilevers formed using a middle spring layer covering cavities on the substrate. Connectors define a transducing space below the top plate. The resilient structure enables a vertical displacement of the connectors, which transports the top plate in a piston-like motion to change the transducing space and to effectuate energy transformation. No separate cells are necessary for each addressable transducer element. Multiple device elements can be made on the same substrate.
Appl. No.: 11/914,597
PCT No.: PCT/IB2006/051568
PCT Pub. No.: WO2006/123300
Current International Class: H02N 1/00 (20060101); H01L 21/768 (20060101); B41J 2/045 (20060101); H04R 19/00 (20060101); A61B 8/00 (20060101)
05047916 Feb., 1993 JP
2000508860 Jul., 2000 JP
2002076269 Mar., 2002 JP
2002191180 Jul., 2002 JP
2002250665 Sep., 2002 JP
2003017503 Jan., 2003 JP
2003151978 May., 2003 JP
2003526207 Sep., 2003 JP
2005078068 Mar., 2005 JP
2008541434 Nov., 2008 JP
WO2005120130 Dec., 2005 WO
Extended Europoean Search Report mailed on Feb. 18, 2011 for European Patent Application No. 06744966.0, a counterpart foreign application for U.S Appl. No. 11/914,584. cited by other .
Translated Chinese Office Action mailed Mar. 23, 2011 for Chinese Patent Application No. 200680017137.1, a counterpart foreign application of U.S. Appl. No. 11/914,597. cited by other .
Final Office Action for U.S. Appl. 11/917,666 mailed on Apr. 25, 2011, Yongli Huang, "Micro-Electro Mechanical Transducer Having an Insulation Extension". cited by other .
Non-Final Office Action for U.S. Appl. 11/914,584 mailed on Apr. 25, 2011, Yongli Huang, "Through-Wafer Interconnection". cited by other .
Chinese Office Action mailed Jun. 28, 2011 for Chinese patent application No. 200680025783.2, a counterpart foreign application of U.S. Appl. No. 11/914,584. cited by other .
Translated Japanese Office Action mailed Jul. 8, 2011 for Japanese patent application No. 2008-511848, a counterpart foreign application of U.S. Appl. No. 11/914,597. cited by other .
Office action for U.S. Appl. No. 11/914,608, mailed on Aug. 4, 2011, Huang, "Micro-Electro-Mechanical Transducers", 8 pages. cited by other .
Chinese Office Action mailed Feb. 16, 2012 for Chinese patent application No. 200680017137.1, a counterpart foreign application of U.S. Appl. No. 11/914,597, 7 pages. cited by other .
Translated Chinese Office Action mailed Nov. 14, 2011 for Chinese patent application No. 200680025783.2, a counterpart foreign application of U.S. Appl. No. 11/914,584, 11 pages. cited by other .
Translated Chinese Office Action mailed Dec. 16, 2011 for Chinese patent application No. 200680021083.6, a counterpart foreign application of U.S. Appl. No. 11/917,666, 5 pages. cited by other .
Translated Japanese Office Action mailed Oct. 21, 2011 for Japanese patent application No. 2008-516496, a counterpart foreign application of U.S. Appl. No. 11/917,666, 6 pages. cited by other .
Japanese Office Action mailed May 18, 2012 for Japanese Patent Application No. 2008-511846, a counterpart foreign application of US Patent No. 8,105,941, 8 pages. cited by other.
This application is a national stage application of international patent application PCT/IB2006/051568, claiming priority from U.S. Provisional Application Ser. No. 60/682,619, filed May 18, 2005; Ser. No. 60/692,038, filed Jun. 17, 2005; Ser. No. 60/705,606, filed Aug. 3, 2005; and Ser. No. 60/744,242, filed Apr. 4, 2006, which applications are incorporated herein by reference in their entirety.
1. A micro-electro-mechanical transducer having a movable mechanical part to transform energy, the transducer comprising an addressable device element having a device element periphery defining a device element area, wherein the addressable device element comprises: a substrate; a top plate layer disposed above the substrate; and a resilient structure between the substrate and the top plate layer, wherein: the resilient structure has a lower portion connecting to the substrate and an upper portion connecting to the top plate layer; the upper portion of the resilient structure has a plurality of connectors connecting to the top plate layer; the plurality of connectors are distributed over the device element area and have at least one connector located substantially away from the device element periphery toward an inner region of the device element area; the top plate layer is located over the connectors to define a transducing space below the top plate layer; the addressable device element has at least one transducing member to induce energy transformation; and the resilient structure enables a vertical displacement of the plurality of connectors to transport the top plate layer in a substantially vertical direction, thus changing the transducing space and activating the transducing member, wherein: the transducer comprises a plurality of the addressable device elements; and the plurality of addressable device elements are separated by a separation trench formed into at least part of the top plate layer.
2. The micro-electro-mechanical transducer of claim 1, wherein the resilient structure comprises a middle spring layer placed over the substrate, the middle spring layer and the substrate defining at least one cavity inside the device element area, the cavity dividing the device element area into at least two opposing sides, at least one side having an anchor anchoring the middle spring layer, and wherein the plurality of connectors are each horizontally distanced from the anchor by a sufficient length to define a cantilever between the connector and the anchor.
3. The micro-electro-mechanical transducer of claim 2, wherein the top plate is placed over the plurality of connectors, each connector having a desired height standing from the top of the middle spring layer to separate the top plate from the middle spring layer to define the transducing space below the top plate layer.
4. The micro-electro-mechanical transducer of claim 2 wherein the substrate and the middle spring layer define a plurality of cavities dividing the device element area into segments each substantially smaller than the device element area.
5. The micro-electro-mechanical transducer of claim 4 wherein the plurality of cavities comprises a first plurality of extended cavities dividing the device element area into a plurality of islands, and wherein at least some of the islands anchor the middle spring layer.
6. The micro-electro-mechanical transducer of claim 5 wherein the first plurality of extended cavities comprises straight cavities that are substantially parallel to each other.
7. The micro-electro-mechanical transducer of claim 5 wherein the first plurality of extended cavities comprises circular cavities.
8. The micro-electro-mechanical transducer of claim 5 wherein the first plurality of extended cavities comprise circular cavities of decreasing sizes each containing the circular cavities of smaller sizes.
9. The micro-electro-mechanical transducer of claim 5 wherein the first plurality of extended cavities comprising circular cavities of similar sizes arranged side-by-side.
10. The micro-electro-mechanical transducer of claim 5 wherein the plurality of cavities comprises a second plurality of extended cavities, and wherein the second plurality of extended cavities crosses the first plurality of extended cavities to form a grid of cavities defining a plurality of discrete islands, wherein at least some of the plurality of discrete islands anchor the middle spring layer.
11. The micro-electro-mechanical transducer of claim 2 wherein the substrate has a plurality of discrete simple cavities arranged side-by-side.
12. The micro-electro-mechanical transducer of claim 11 wherein each discrete simple cavity divides the substrate into a respective inner island and a common outer portion.
13. The micro-electro-mechanical transducer of claim 2 wherein the at least one cavity and the corresponding anchor(s) together occupy at least half of the device element area.
14. The micro-electro-mechanical transducer of claim 2 wherein in the addressable device element the middle spring layer is a contiguous layer covering the at least one cavity and the corresponding anchor(s).
15. The micro-electro-mechanical transducer of claim 2 wherein the middle spring layer is a contiguous layer covering substantially the entire device element area.
16. The micro-electro-mechanical transducer of claim 2 wherein at least one cavity has an annular shape dividing the device element area into an inner island and an outer region.
17. The micro-electro-mechanical transducer of claim 16 wherein the inner island anchors the middle spring layer.
18. The micro-electro-mechanical transducer of claim 16 wherein both the inner island and the outer region anchor the middle spring layer.
19. The micro-electro-mechanical transducer of claim 2 wherein the two opposing sides of at least one cavity are two opposing anchors each anchoring the middle spring layer.
20. The micro-electro-mechanical transducer of claim 19 wherein at least one connector is located between the two opposing anchors and horizontally distanced from each of the two opposing anchors by a sufficient length to define two cantilevers connected head-to-head.
21. The micro-electro-mechanical transducer of claim 2 wherein the cavity has an annular shape, the middle spring layer covers the entire cavity, and the at least one connector is disposed around the annular shape directly above a middle area of the cavity to define a two-dimensional plane cantilever.
22. The micro-electro-mechanical transducer of claim 1 wherein the plurality of connectors define a connector density profile indicating an average connector density per area location, and the resilient structure has a spring strength corresponding to each connector as an exerting point, wherein the average connector density and the spring strength combinedly define an effective spring intensity profile indicating spring strength per area location.
23. The micro-electro-mechanical transducer of claim 22 wherein the effective spring intensity is substantially uniform over the device element area.
24. The micro-electro-mechanical transducer of claim 22 wherein the effective spring intensity is significantly higher at area locations near a center of the device element area than locations near a perimeter of the device element area.
25. The micro-electro-mechanical transducer of claim 22 wherein the device element area comprises a first region and a second region, the first region having a first thickness of the top plate layer and a first effective spring intensity, and the second region having a second thickness of the top plate layer and a second effective spring intensity, the second thickness being smaller than the first thickness and the second effective spring intensity being less than the first effective spring intensity.
26. The micro-electro-mechanical transducer of claim 22 wherein the device element area comprises a perimeter surrounding a middle region, wherein the top plate layer is flexible near the perimeter, but rigid in the middle region, clamped at a perimeter sidewall and supported by at least one cantilever in the middle region.
27. The micro-electro-mechanical transducer of claim 1 wherein in the addressable device element the top plate layer has a thickness profile having at least two different thicknesses across the device element area.
28. The micro-electro-mechanical transducer of claim 27 wherein the at least two different thicknesses comprise a first thickness corresponding to a normal thickness of the top plate layer and a second thickness corresponding to a bottom thickness of a cavity formed on the top plate layer, the first thickness and the second thickness regularly alternating across the device element area.
29. The micro-electro-mechanical transducer of claim 1 wherein the top plate layer comprises a first material and a second material different from the first material.
30. The micro-electro-mechanical transducer of claim 29 wherein the first material comprises a plurality of segments divided by separation trenches and the second material spans over the trenches to mechanically connect segments and/or at least partially fills the separation trenches.
31. The micro-electro-mechanical transducer of claim 1 which is a capacitance micromachined ultrasonic transducer (cMUT), wherein the addressable device element has a top electrode on the top plate layer and a bottom electrode on either the resilient structure or the substrate.
32. The micro-electro-mechanical transducer of claim 31 wherein resilient structure comprises a middle spring layer, and the bottom electrode is on the middle spring layer.
33. The micro-electro-mechanical transducer of claim 32 wherein the middle spring layer comprises a conductive material to effectuate the bottom electrode.
34. The micro-electro-mechanical transducer of claim 31 wherein the top electrode is a separate conductive layer deposited on the top plate layer.
35. The micro-electro-mechanical transducer of claim 1 which is a pMUT, wherein the transducing member comprises a piezoelectric member disposed on the top plate or the middle spring layer.
36. The micro-electro-mechanical transducer of claim 1 which is a mMUT, wherein the transducing member comprises a magnetic member disposed on the top plate layer.
37. The micro-electro-mechanical transducer of claim 1 wherein the top plate layer comprises holes formed therein.
38. The micro-electro-mechanical transducer of claim 37 wherein the holes have a size and location profile optimized for a desired rigidity/mass ratio of the top plate layer.
39. The micro-electro-mechanical transducer of claim 1 wherein the vertical displacement of the connector is limited to a maximum displacement no greater than the height of the transducing space.
40. The micro-electro-mechanical transducer of claim 39 wherein the maximum displacement of the connector is at least one third smaller than the height of the transducing space.
41. The micro-electro-mechanical transducer of claim 39 wherein the maximum vertical displacement of the connector is limited by a motion stopper disposed in the cavity.
42. The micro-electro-mechanical transducer of claim 1 wherein the top plate is significantly more rigid than the resilient structure and is substantially unbent when transported by the vertical displacement of the connector(s).
43. A micro-electro-mechanical transducer having a movable mechanical part to transform energy, the transducer comprising: a substrate, a middle spring layer and a top plate layer arranged in that order such that a bottom side of the top plate layer faces a top side of the middle spring layer and a bottom side of the middle spring layer faces a front side of the substrate, wherein the transducer comprises at least one addressable device element defining a device element area of the substrate and a corresponding device element area of the top plate layer, each addressable device element having a transducing member and being further characterized in that: the device element area has at least one cavity between the substrate and the middle spring layer, each cavity being sidewalled by at least one anchor anchoring the middle spring layer; the middle spring layer covers at least a part of each cavity; at least one connector of a desired height is disposed above each cavity, the connector standing from the top of the middle spring layer; the top plate is placed over the connector(s) to define a transducing space below the top plate layer, the top plate layer including a first electrode and at least a portion of the middle spring layer serving as at least a portion of a second electrode; each connector is horizontally distanced from one of the at least one anchor by a sufficient length to define a cantilever between the connector and the anchor, wherein the cantilever and the cavity enable a vertical displacement of the connector to transport the top plate layer substantially vertically, thus changing the transducing space and activating the transducing member; and the at least one anchor is conductive to serve as a conductor to access the second electrode.
44. The micro-electro-mechanical transducer of claim 43, wherein at least one cavity is in an inner area of the substrate and divides the substrate into at least two segments.
45. The micro-electro-mechanical transducer of claim 43 wherein at least one cavity has an annular shape dividing the substrate into an inner island and an outer region.
46. The micro-electro-mechanical transducer of claim 43 wherein the cavity is sidewalled by two opposing anchors each anchoring the middle spring layer, and the at least one connector is located between the two opposing anchors and horizontally distanced from each of the two opposing anchors by a sufficient length to define a bridge containing two cantilevers connected head-to-head.
47. The micro-electro-mechanical transducer of claim 43 wherein the cavity has an annular shape, the middle spring layer covers the entire cavity, and the at least one connector is disposed around the annular shape directly above a middle area of the cavity to define a two-dimensional plane cantilever.
48. A micromachined ultrasonic transducer (MUT) structure comprising: a substrate, a middle spring layer and the top plate layer arranged in that order such that a bottom side of the top plate layer faces a top side of the middle spring layer and a bottom side of the middle spring layer faces a front side of the substrate, wherein the MUT structure comprises a plurality of addressable MUT elements each defined by a MUT element area of the substrate and a corresponding MUT element area of the top plate layer, each of the plurality of addressable MUT elements having a transducing member and being further characterized in that: the MUT element area has at least one cavity between the substrate and the middle spring layer, each cavity being sidewalled by at least one anchor anchoring the middle spring layer; the middle spring layer covers at least a part of each cavity; at least one connector of a desired height disposed above each cavity, the connector standing from the top of the middle spring layer; the top plate is placed over the connector(s) to define a transducing space below the top plate layer, the top plate layer including a first electrode; each connector is horizontally distanced from one of the at least one anchor by a sufficient length to define a cantilever between the connector and the anchor, wherein the cantilever and the cavity enable a vertical displacement of the connector to transport the top plate layer substantially vertically with a piston-like motion, thus changing the transducing space and activating the transducing member; and the substrate including at least a portion of a second electrode.
49. The MUT structure of claim 48 wherein the plurality of addressable MUT elements are separated by a separation trench formed through at least part of the top plate layer.
50. The MUT structure of claim 49 wherein the separation trench is sealed using a sealing material.
51. The MUT structure of claim 50 wherein the sealing material is a sealing layer placed over top of the separation trench.
52. The MUT structure of claim 51, wherein the middle spring layer is a silicon layer, and the bottom electrode comprises a highly doped region on the middle spring layer.
53. The MUT structure of claim 48 wherein the top plate layer comprises holes formed therethrough, the holes having a size and location profile that is optimized for a desired rigidity/mass ratio of the top plate layer.
54. The MUT structure of claim 48 further comprising an integrated circuit built in the top plate layer.
55. The micro-electro-mechanical transducer of claim 43, wherein the substrate is conductive.
56. The micro-electro-mechanical transducer of claim 43, wherein at least one of the substrate or the anchor includes a conductive layer.
57. The MUT structure of claim 48, wherein the substrate is conductive.
58. The MUT structure of claim 48, wherein the at least one anchor is conductive.
59. The MUT structure of claim 48, wherein the substrate includes a conductive layer.
60. The MUT structure of claim 48, wherein the anchor includes a conductive layer.
Alternatively the membrane can be actuated and the displacement of the membranes detected using a piezoelectric transducer (pMUT) and a magnetic transducer (mMUT). FIG. 2 shows a pMUT cell 20 which has a similar structure to the cMUT cell 10 except that the capacitor (electrodes 12 and 14) are replaced by a piezoelectric member 24 on the membrane 26. FIG. 3 shows a mMUT cell 30 which has a similar structure to the cMUT cell 10 except that the capacitor (electrodes 12 and 14) are replaced by a magnetic member 34 on the membrane 36.
Methods of fabrication for making a cMUT shown in FIGS. 1A-1C have been developed. Exemplary methods are disclosed in U.S. Pat. Nos. 6,632,178 and 6, 958, 255.
This application discloses a micro-electro-mechanical transducer (such as a cMUT) that has a movable mechanical part to transform energy. The transducer has a substrate, a top plate, and a resilient structure therebetween. The resilient structure has multiple connectors distributed over the device element area to vertically transport the top player with distributed support not limited to edges. The resilient structure may be cantilevers, or cantilever-containing structures such as bridges or crossbars, formed using a middle spring layer covering cavities on the substrate. Connectors define a transducing space below the top plate. The resilient structure such as cantilevers enable a vertical displacement of the connectors, which transports the top plate in a piston-like motion to change the transducing space and to effectuate energy transformation. No separate cells are necessary for each addressable transducer element. Multiple device elements can be made on the same substrate.
According to one aspect of the invention, a micro-electro-mechanical transducer is characterized in that:
(a) the resilient structure has a lower portion connecting to the substrate and an upper portion connecting to the top plate layer;
(b) the upper portion of the resilient structure has a plurality of connectors connecting to the top plate layer;
(c) the plurality of connectors are distributed over the device element area and have at least one connector located substantially away from the device element periphery toward an inner region of the device element area;
(d) a transducing space is defined below the top plate layer;
(e) the addressable device element has at least one transducing member to induce energy transformation; and
(f) the resilient structure enables a vertical displacement of the plurality of connectors to transport the top plate layer in a substantially vertical direction, thus changing the transducing space and activating the transducing member.
In one embodiment, the resilient structure has a middle spring layer placed over the substrate. The middle spring layer and the substrate define at least one cavity inside the device element area. The cavity divides the device element area into at least two opposing sides, at least one side having an anchor anchoring the middle spring layer. The plurality of connectors are each horizontally distanced from the anchor by a sufficient length to define a cantilever or a bridge between the connector and the anchor. In this embodiment, each connector may have a desired height and stand from the top of the middle spring layer to separate the top plate from the middle spring layer to define the transducing space below the top plate layer.
In one preferred embodiment, the substrate and the middle spring layer define a plurality of cavities dividing the device element area into segments each substantially smaller than the device element area. Multiple cantilevers or bridges may be defined over the plurality of cavities to provide a distributed support to the top plate layer.
In one embodiment, the cavities and the corresponding anchors together occupy at least half the device element area. Within the device element area the middle spring layer may be a contiguous layer covering the top surface of the anchor. In one embodiment, the contiguous middle spring layer covers at least half of the device element area, and preferably the contiguous middle spring layer covers substantially the entire device element area.
In one embodiment, the cavity has an annular shape dividing the substrate wafer into an inner island and an outer region, and the inner island constitutes the anchor. The anchor may include a first anchor comprising the inner island and an opposing second anchor comprising the outer region.
In one embodiment the cavity comprises a first plurality of extended cavities forming a pattern of cavities and anchors on the substrate wafer. Various patterns may be formed.
The cantilevers may be of a variety of formations. For example, the cavity may be defined by at least two anchors opposing each other, and the middle spring layer within the device element area may connect to both opposing anchors. The connector may be located between the two opposing anchors and horizontally distanced from each of the two opposing anchors by a sufficient length to define two cantilevers connected head-to-head at a location where the connector is positioned. For another example, the anchor may define a circular sidewall surrounding the cavity, the middle spring layer may cover the entire cavity, and the connector may be at a position directly above a middle area of the cavity to define a two-dimensional plane cantilever.
One aspect of the invention is a micro-electro-mechanical transducer wherein the middle spring layer has a plurality of connectors connecting to the top plate layer. The plurality of connectors is distributed across the device element area to define a connector density profile having an average connector density per area location. Each cantilever thus formed has a spring strength, which when combined with the respective average connector density defines an effective spring intensity profile in which the effective spring intensity is the spring strength per area location. The effective spring intensity may either be substantially uniform over the device element area or have a nonuniform distribution profile. For example, the effective spring intensity may be significantly higher at area locations near a center of the device element area than locations near a perimeter of the device element area.
In one embodiment having multiple connectors, the device element area comprises two regions having different thicknesses of the top plate layer and different spring intensities. For example, a first region may have a greater thickness of the top plate layer and great effective spring intensity than the second region, or vice versa. In another embodiment the device element area comprises a perimeter surrounding a middle region, wherein the top plate layer is flexible near the parameter but rigid in the middle region, clamped at a perimeter sidewall and supported by at least one cantilever in the middle region.
The micro-electro-mechanical transducer may have additional characteristics. For example, the addressable device element the top plate layer may have a thickness profile having at least two different thicknesses across the device element area. The two different thicknesses may comprise a first thickness corresponding to a normal thickness of the top plate layer and a second thickness corresponding to a bottom thickness of a cavity formed on the top plate layer, the first thickness and the second thickness regularly alternating across the device element area. For another example, the addressable device element the top plate layer may comprise a first material and a second material different from the first material. In one embodiment, the first material has a plurality of segments divided by separation trenches and the second material spans over the segments to connect the segments of the first material. The second material may also at least partially fill the separation trenches.
In one embodiment, the micro-electro-mechanical transducer is a capacitance micromachined ultrasonic transducer (cMUT), wherein each addressable device element has a top electrode on the top plate layer and a bottom electrode on either the middle spring layer or the substrate wafer. Example, the middle spring layer may comprise a conductive material to effectuate as the bottom electrode. Alternatively, the top electrode may be a separate conductive layer deposited on the top plate layer.
In another embodiment, the micro-electro-mechanical transducer has a plurality of addressable device elements, which may be separated by a separation trench formed through at least part of the top plate layer. The substrate wafer may be a conductive wafer, and the addressable device element is insulated by an embedded insulator formed on the substrate wafer. The embedded insulator comprises an insulation cavity having a desired overall thickness and being filled with a dielectric material. Alternatively, the addressable device element is insulated by an embedded insulator formed on the substrate wafer, the embedded insulator comprising a patterned cavity containing solid lines of oxidized native material of the substrate wafer. The embedded insulator may also be formed on the top plate layer.
In one embodiment, the addressable device element is addressed from the back side of the substrate wafer via a through-wafer interconnect formed through the substrate wafer. For example, a conductive substrate wafer may be used, and the through-wafer interconnect may include an annular trench surrounding a through-wafer conductor made of a native material of the conductive wafer. A dielectric material may be used to fill the annular trench to insulate the through-wafer conductor from the rest of the substrate wafer. In one particular configuration, the through-wafer conductor is positioned directly underneath the addressable device element.
The micro-electro-mechanical transducer may also be a pMUT, wherein the transducing member comprises a piezoelectric member disposed on the middle spring layer or the top plate layer. The micro-electro-mechanical transducer may also be a mMUT, wherein the transducing member comprises a magnetic member disposed on the middle spring layer or the top plate layer.
The performance of the micro-electro-mechanical transducer may be enhanced by careful selection of the top plate material or configuration. For example, the top plate layer may have holes formed therethrough. The holes may have a size and location profile that optimizes a rigidity/mass ratio of the top plate layer. Furthermore, in order to avoid electric shorting problem, the maximum vertical displacement of the connector may be limited to be no greater than, preferably less than, the height of the transducing space between the top plate layer and the top surface of the anchor including any intervening layer. In one embodiment, the maximum vertical displacement of the connector is limited by a motion stopper disposed in the cavity.
According to another aspect of the invention, a micromachined ultrasonic transducer (MUT) structure has a substrate wafer, a middle spring layer and the top plate layer arranged in that order such that a bottom side of the top plate layer faces a top side of the middle spring layer and a bottom side of the middle spring layer faces a front side of the substrate wafer. The MUT structure comprises a plurality of addressable MUT elements each defined by a MUT element area of the substrate wafer and a corresponding MUT element area of the top plate layer. Each of the plurality of addressable MUT element has a transducing member and is further characterized in that:
(a) the device element area has at least one cavity between the substrate and the middle spring layer, each cavity being sidewalled by at least one anchor anchoring the middle spring layer;
(b) the middle spring layer covers at least a part of each cavity;
(c) at least one connector of a desired height disposed above each cavity, the connector standing from the top of the middle spring layer;
(d) the top plate is placed over the connector(s) to define a transducing space below the top plate layer;
(e) each connector is horizontally distanced from one of the at least one anchor by a sufficient length to define a cantilever between the connector and the anchor, wherein the cantilever and the cavity enable a vertical displacement of the connector to transport the top plate layer substantially vertically with a piston-like motion, thus changing the transducing space and activating the transducing member; and
(f) the at least one cavity and the corresponding anchor(s) occupy at least half of the MUT element area.
The MUT structure may have additional features as summarized in above.
FIG. 5A shows a cross-section view of a cMUT structure in accordance with the present invention showing a complete cMUT element and parts of neighboring cMUT elements.
FIG. 5B shows a schematic top view of the cMUT element shown in FIG. 5A without showing the top plate.
FIG. 5C shows an enlarged view of a selected cMUT portion of the cMUT structure of FIG. 5A.
FIG. 6 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a first example of an addressable cMUT element in accordance with the present invention.
FIG. 7 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a second example of an addressable cMUT element in accordance with the present invention.
FIG. 8 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a third example of an addressable cMUT element in accordance with the present invention.
FIG. 9 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fourth example of an addressable cMUT element in accordance with the present invention.
FIG. 10 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fifth example of an addressable cMUT element in accordance with the present invention.
FIG. 11 shows a cross-section view of another cMUT structure of the present invention showing a complete cMUT element and parts of neighboring cMUT elements.
FIGS. 12A-12J show examples of spring (cantilever) distribution profiles.
The present invention is envisioned to do away with the cMUT cell requirement. As will be shown next, although the present invention may still be used to fabricate cMUT elements each having multiple cells, it does not have to be. In fact, due to the inherent characteristics of the cMUT design in accordance with the present invention, it may be preferable from a fabrication point of view to make each addressable cMUT element without any internal cell boundaries. In addition, even when an addressable cMUT element in accordance with the present invention is made of multiple smaller segments (such as areas each having its own top plate layer segment and middle spring layer segment separated from that of others), these smaller segments do not need to have a clamped perimeter or a peripheral support wall, and further do not need to be identical to each other in size or shape.
In one aspect of the present invention, a resilient structure (such as the middle spring layer 520 in the example shown in FIG. 5A) having multiple connectors distributed over the device element area is provided to vertically transport the top player. The multiple connectors facilitate distributed support not limited to edges (peripheral areas) only. In the exemplary structure shown in FIG. 5A, for example, the top plate 540 is connected to the middle spring layer 520 through multiple plate-spring connectors 530, 530a and 530b. Although connectors 530b are located near the device element peripheral defined by separation trenches 515, other connectors 530 and 530a are substantially away from the device element peripheral toward the center of the device element area to provide a more flexible, more efficient, and more distributed support to the top plate layer 540. This distributed support may help build a cMUT element of a larger size.
FIG. 5B is a schematic top view of the cMUT element 500 without showing the top plate 540. In cross-sectional view of the cMUT element 500 taken along broken line indicated in FIG. 5B is shown in FIG. 5A. In this exemplary configuration, the cMUT element 500 is patterned with concentric squares. In the center is a simple discrete connector 530 positioned on top of the cavity 502 of a simple shape (as shown in the cross-sectional view in FIG. 5A the connector 530 is placed on the middle layer 520 which covers the cavity 502), which is surrounded by a circular sidewall anchor 503, which in turn is surrounded by a circular cavity 502a. Another larger circular connector 530a is positioned on top of the circular cavity 502a, which is surrounded by a circular sidewall anchor 503a of the next level. As will be shown in FIGS. 6-10, however, a variety of different configurations are possible with the present invention.
FIG. 5C is an enlarged view of a selected cMUT portion 510, which is a part of the complete cMUT element 500. The selected cMUT portion 510 is a basic unit, of the complete cMUT element 500 (and other cMUT elements 500A and 500B). The structure of the selected cMUT portion 510 provides a basis to understand the complete cMUT element 500.
As shown in FIG. 5C, the selected cMUT portion 510 includes two halves centered on an imaginary vertical line (not shown) going through the structure from the top to the bottom. As mentioned, for certain application such as a CMUT with a high operation frequency, a full CMUT element or device may use only one cMUT portion 510. For other applications, it may be preferred to use a combination of multiple basic unit is shown in FIG. 5C and FIG. 5D.
The basic structure unit of the cMUT element is built on a substrate wafer 501 on top of which there is a standing feature (referred to as "sidewall anchor" hereinafter) 503 having two sidewalls on two opposing sides bordering cavities 502 and 502a, respectively. The standing feature (sidewall anchor) 503 may be an integrated part of the substrate 501 formed as a result of forming the cavities 502 and 502a, but may also be an additional structure added onto a separate substrate. In one embodiment, for example, the sidewall anchor 503 is part of the middle spring layer 520. The substrate of 501 may either be made of a nonconductive material or a conductive material such as silicon or polysilicon. In a configuration where the sidewall anchor 503 is a separate structure, conductivity of the sidewall anchor 503 may be the same as or different from that of the substrate 501. For example, the substrate 501 may be made of a nonconductive material while the sidewall anchor 503 a conductive material such as silicon or polysilicon.
The cMUT structure shown also has the second cavity 502a long the other side of sidewall anchor 503. Depending on how and where the cMUT portion 510 is taken from the cMUT element 500, the second cavity 502a may either belong to a different and separate cavity, or just another portion of a same circular or extended cavity as the cavity 502. The selected cMUT portion 510 also has a second connector 530a in the other half. Again, depending on how and where the cMUT portion 510 is taken from the cMUT element 500, the second connector 530a may either be a part of a different and separate connector, or just another portion of a same circular or extended connector as the connector 530.
The cMUT structure portion 510 further has these components: a middle spring layer 520 which is preferably an elastic membrane, a bottom electrode 525 placed on the middle spring layer 520, a connector 530 which stands on top of the middle spring layer 520, an insulation layer 535 sitting over the connector 530, a top plate 540 connected to the connector 530 through an intervening insulation layer 535, and a top electrode 550.
The connector 530 stands out from the middle spring layer 520 to define a transducing space 560 below the top plate 540. The transducing space 560 is generally defined between the top plate layer 540 and the top surface of the middle spring layer 520 or the top surface of the sidewall anchor 503, whichever is higher. Where there is an intervening layer between the top plate layer 540 and the top surface of the middle spring layer 520 or the top surface of the sidewall anchor 503, the available transducing space may be reduced. For example, if another layer is deposited over the sidewall anchor 503, and yet another layer is deposited on the bottom of the top plate 540, the transducing space would be defined between these two intervening layers. In the exemplary configuration shown in FIG. 5C, the actual height of the available transducing space 560 may be reduced by the thicknesses of the insulation layer 535, the bottom electrode 525 and the middle spring layer 520. It should be noted that in some embodiments it is possible to have the entire height between the top plate layer 540 and the top surface of the sidewall anchor 503 available for the transducing space 560. For example, a conductive substrate wafer may itself be used to effectuate a bottom electrode on the substrate (e.g., on the sidewall anchor 503) without requiring a separate electrode layer; and cantilevers may be made with segments of middle spring layers connected to the sidewall anchor 503 at sides flush with or lower than the top surface of the sidewall anchor 503, instead of using a continuous middle spring layer placed on top of the sidewall anchor 503.
The connector 530 is horizontally distanced from the sidewall anchor 503 by a sufficient length to define a cantilever anchored at the sidewall anchor 503 with an exerting end 522 on the connector 530. The cantilever and the cavity 502 enable a vertical displacement of the connector 530, which transports the top plate 540 substantially vertically with a piston-like motion, thus changing the transducing space 560. When the both halves of the cMUT structure 510 move in the same phase, the vertical piston-like motion is further assured.
In this particular example shown, top surface of the sidewall anchor 503 is covered by the middle spring layer 520, which in turn is covered by the bottom electrode 525. Furthermore, the top plate 540 and the connector 530 do not connect with each other directly but are intervened by the insulation layer 535 therebetween. The transducing space 560 between the top plate 540 and the top surface of the sidewall anchor 503 is therefore partially occupied by the middle spring layer 520, the bottom electrode 525 and the insulation layer 535. It is noted that the part of the middle spring layer 520 covering the top surface of the sidewall anchor 503, the bottom electrode 525 and the insulation layer 535 are optional. In any event, in order to achieve the intended energy transformation, the transducing space 560 should not be entirely occupied by these extra layers if they are included in the structure.
The connector 530 (which is also shown in FIG. 5C) stands on the middle spring layer, and is horizontally distanced from the sidewalls of both the sidewall anchor 503 and the sidewall anchor 503a. The middle spring layer 520 between the sidewall anchor 503 and the sidewall anchor 503a defines a double-cantilever anchored at the sidewall anchor 503 and the sidewall anchor 503a. The double-cantilever is connected head-to-head at location 522 where the connector 530 is located to form a bridge.
The top plate 540 is placed over the connector 530, which separates the top plate 540 from the middle spring layer 520 to define a transducing space below the top plate. The double-cantilever and the cavity 502 enable a vertical displacement of the connector 530, which transports the top plate 540 substantially vertically, thus changing the transducing space and activating a transducing member in the transducer for energy transformation.
The above novel cMUT design has essentially done away with the conventional concept of the cell insulation wall which divides a cMUT element into cells and is required to support and clamp the membrane at the perimeter of each cMUT cell. The conventional cMUT designs shown in FIG. 1, including the cMUT design as disclosed in the U.S. Pat. No. 7,030,536, all require a cell insulation wall (e.g., cell insulation wall 18 in FIGS. 1A-1C, or the compliant cell insulation wall 48 in FIGS. 4A-4B) at the perimeter of each cMUT cell to support and clamp the perimeter of the membrane. The cell insulation wall defines the prior art cMUT cells. Within the perimeter defined by the insulation wall, the bottom electrode in the prior art cMUT structure is either directly deposited on the substrate 11 (FIG. 1) or deposited on the top of a pedestal 43 (FIG. 4A), which is separated from the cell insulation wall. Because the majority of the membrane (16 in FIG. 1 or 46 in FIG. 4A) is unsupported, the size of each cell and the selection of the materials for the membrane is limited.
In contrast, with the cMUT design as shown in FIG. 5A and FIG. 5B, the cMUT element is no longer required to be divided into cells and thus has no need of an insulation wall to define cell perimeters. The top plate layer 540 and the top electrode 550 are supported by a resilient structure (multiple cantilevers in the embodiment shown) through multiple connectors that can be freely positioned and distributed based on need, thus effectively distributing the entire load of the top plate layer 540 over multiple springs (cantilevers). This solves the problem of the cell limitation inherent to the prior art designs. The middle spring layer 520 and a bottom electrode 525 are supported by multiple sidewall anchors 503 which can also be distributed across the entire substrate wafer 501 based on need, rather than being limited to peripheral areas. There is no central pedestal and a separate peripheral compliant support structure. Only multiple sidewall anchors 503 distributed across the entire device element area are required to support the middle spring layer 520 and the bottom electrode 525. Each anchor 503 serves both as a support to the bottom electrode and as an anchor to a corresponding cantilever (or two cantilevers in the configuration shown in FIG. 5A and FIG. 5B, one on each side of the sidewall anchor 503).
With this design, a cMUT element with a very large active area may be formed. The operating frequency or frequency response of the cMUT element may be adjusted not only by selection of the materials for the top plate 540 but also by the configuration of the multiple cantilevers, including the spring strength of individual cantilevers and the cantilever distribution density profile across the area of the cMUT element.
The entire top plate 540 formed above the substrate wafer 501 may be movable without any clamped or fastened area. If desired, multiple cMUT elements can be formed by forming separation trenches 515 through the top plate 540 and the middle spring layer 520. In principle, however, the entire cMUT structure with a very large active area may be used as a single cMUT element.
Furthermore, with the cMUT structure design in FIG. 5A and FIG. 5B, the top plate 540 may be separated into a plurality of smaller top plates of identical or different sizes and shapes. Each smaller top plate may be addressed as a single cMUT element; alternatively multiple smaller top plates may be combined together and addressed as a single cMUT element.
Furthermore, unlike the flexible membrane clamped on its edges (or posts) in the conventional cMUTs, the top plate 540 shown in FIG. 5A and FIG. 5B can be designed to be either flexible or rigid. With a rigid top plate, the whole surface of the cMUT, which may include any number of separate smaller top plates 540, may be movable with a very uniform displacement profile.
In the exemplary cMUT structure 500 shown in FIGS. 5A-5B, the middle spring layer 520 is a continuous layer placed over the top of the multiple sidewall anchors 503 and a multiple cavities 502. The middle spring layer 520 thus occupies the entire device element area longitudinally (i.e., in the lateral or surface dimension). This configuration allows the entire top plate 540 to be supported by cantilevers through connectors 530 that can be placed or distributed all over the entire device element area without being limited to places near the edge of the element. It is appreciated, however, that the middle spring layer 520 may comprise multiple small segments that are either connected to each other at certain points or completely separated from each other. Particularly, segments of middle spring layer 520 may be used in a way where each segment is connected or anchored to a respective sidewall anchor 503 and covers at least part of a respective cavity 502 to allow cantilevers to be formed, but leaves part of the top surface of the sidewall anchors 503 uncovered. Such a configuration may allow the flexibility of using different materials and different thicknesses for the middle spring layer 520, but may complicate the fabrication process.
Regardless of the configurations of the middle spring layer, it is envisioned that in some preferred embodiments, the cantilever-forming anchor surface(s) (the top surface of the sidewall anchors 503) and the associated cantilever areas of cMUT element 500 together cover at least half of the device element area in order to achieve an effectively distributed cantilever-support of the top plate 540 and greater effective active areas. The cantilever areas may be defined as the total area of the cavities, or alternatively as the total area of the middle spring layer 520 covering the cavities. Preferably the cantilever-forming anchor areas and the cantilever areas are at least 80% of the device element area, and further preferably near 100% of the device element area, as is the case showing in FIG. 5A. Pedestals that do not serve to anchor a cantilever is not required, and preferably not formed at all within the device element area of the substrate wafer 501 in order to maximize the cantilever-forming anchor areas as described above. In some embodiment, the cantilever areas and the associated connector areas are minimized while the cantilever-forming anchor areas are maximized for higher effective electrical field area for energy transformation. For a given number of the cantilevers, the cantilever area may be minimized by using a thinner middle spring layer with shorter cantilever dimensions while still achieving desired spring strength. These configurations provide a potential advantage over the prior art designs that have peripheral support for clamped membranes.
(9) The present invention provides more design flexibility for the MUTs than those of the prior arts. For example, the top plate 540 may be designed to be of different shapes and configurations; the embedded springs (e.g. cantilevers made from the middle spring layer as shown in FIGS. 5A and 5B) may have different shapes and different spring constants by selecting the proper membrane dimensions. Moreover, the embedded springs can attach to the different locations on the top plate 540.
FIGS. 6-11 show five exemplary designs of the spring (cantilever) distribution in accordance with the present invention. FIG. 6 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a first example of an addressable cMUT element in accordance with the present invention. The addressable cMUT element 600 has an overall square shape, and includes:
(1) a peripheral cavity 602a with an annular shape defining the outer rim of the cMUT element 600;
(2) a circular connector 630a placed over the peripheral cavity 602a;
(3) an outer anchor 603a surrounded by the peripheral cavity 602a, the outer anchor 603a being contiguous throughout the general device element area of the substrate wafer except for the inner cut-out areas as shown;
(4) four identical inner cavities 602b each having an annular shape, each cavity 602b dividing the substrate wafer into an inner island (respective anchor 603b) and an outer region (the common outer anchor 603a in this particular example);
(5) four identical inner circular connectors 630b each placed over a respective inner cavity 602b; and
(6) four identical inner anchors 603b surrounded by a respective inner cavity 602b.
In this example, the outfit anchor 603a is connected to the circular connector 630a through a middle spring layer (not shown). The middle spring layer may be a single continuous layer covering the entire device element area, or separate areas connected to the anchors at selected positions. Although the middle spring layer is thin in its vertical dimension, it may be of any desired span or size on its surface (longitudinal or lateral dimension) up to covering the entire device element area. Preferably, the middle spring layer is contiguous over large areas to make plane cantilevers instead of separate narrow strip cantilevers. In the example shown in FIG. 6, because the circular connector 630a is a contiguous closed circle, a single plane cantilever may be formed over the peripheral cavity 602a, anchored on the circular rim of the outer anchor 603a. However, it is appreciated that the connector 630a (and other connectors herein) may be of any pattern, including separate segments. Similar formation possibilities apply to the four connectors 630b and the cantilevers formed over the four cavities 602b. It is appreciated that the shapes and relative sizes of the cavities 602b, connectors 630b and the anchors 603b can be varied rather freely without departing from the general concept of the present invention. In particular, the term "circular" used in this description refers to a formation that has a substantially close-ended configuration, and does not suggest in any way the item or formation preferred to is round-shaped or in any other particular shape.
Furthermore, the cantilevers formed over cavities 602b may be anchored either on the outer anchor 603a or on the inner anchor 603b, or both (to form a bridge-like double cantilever connected head to head).
FIG. 7 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a second example of an addressable cMUT element in accordance with the present invention. The addressable cMUT element 700 has an overall square shape, and includes:
(1) a plurality of circular cavities 702a, 702b, 702c, and 702d of decreasing sizes each containing the trenches of smaller sizes (if there is any);
(2) a plurality of circular connectors 730a, 730b, 730c and 730d placed over the cavities 702a, 702b, 702c, and 702d, respectively; and
(3) a plurality of anchors 703a, 703b, 703c and 703d surrounded by the cavities 702a, 702b, 702c, and 702d, respectively.
FIG. 8 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a third example of an addressable cMUT element in accordance with the present invention. The addressable cMUT element 800 has an overall square shape, and includes:
(1) an outer circular cavity 802a surrounding an outer circular connector 830a;
(2) a plurality of extended straight cavities 802b substantially parallel to each other;
(3) a plurality of extended straight connectors 830b substantially parallel to each other and placed over the respective cavity 802b; and
(4) a plurality of extended straight anchors 803b parallel to each other and alternating with cavities 802b therebetween.
FIG. 9 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fourth example of an addressable cMUT element in accordance with the present invention. The addressable cMUT element 900 has an overall square shape, and includes:
(1) an outer circular cavity 902a surrounding an outer circular connector 930a;
(2) a first plurality of extended straight cavities 902b substantially parallel to each other;
(3) a first plurality of extended straight connectors 930b substantially parallel to each other and placed over the respective cavity 902b;
(4) a second plurality of extended straight cavities 902c substantially parallel to each other; and
(3) a second plurality of extended straight connectors 930c substantially parallel to each other and placed over the respective cavity 902c;
The second plurality of extended straight cavities 902c intersects with the first plurality of extended straight cavities 902b to form a grid of cavities defining an array of discrete islands (anchors) 903. The resultant cavity formation is a multiple small square-shaped circular cavities.
FIG. 10 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fifth example of an addressable cMUT element in accordance with the present invention. The addressable cMUT element 1000 has an overall square shape, and includes:
(1) an outer circular cavity 1002a surrounding an outer circular connector 1030a and a contiguous anchor 1003;
(2) a plurality of simple cavities 1002b separated from each other and arranged side-by-side; and
(3) a plurality of discrete connectors 1030b placed over a respective simple cavity 1002.
In this configuration, plurality of simple and discrete cavities 1002b and plurality of discrete connectors 1030b share a common contiguous anchor 1003.
It should be noted that although the exemplary formations in FIGS. 6-10 all have an outer circular cavity defining the boundary or perimeter of the addressable cMUT element, such an outer circular cavity is not required. This may be illustrated using the next example with reference to FIG. 11.
FIG. 11 is a cross-section view of a cMUT structure showing a complete cMUT element 1100 and parts of neighboring cMUT elements 1100A and 1100B, one on each side. The cMUT element 1100 is very similar to the cMUT element 500 in FIGS. 5A and 5B, except that the separations that form the cMUT elements (1100, 1100A and 1100B) are at different locations on the substrate than that in FIGS. 5A and 5B. Like the cMUT structure in FIGS. 5A and 5B, the cMUT structure in FIG. 11 is built on a substrate wafer 1101 and also has a middle spring layer 1120 and a top plate 1140. The cMUT elements 1100, 1102 and 1104 are separated by separation trenches 1115 formed through the top plate 1140 and the middle spring layer 1120. Because the separation trenches 1115 are cut through transducing spaces 1160 instead of connectors 1130 (as is the case in FIGS. 5A and 5B), the boundary of the cMUT element 1100 at the substrate wafer level is half of the anchors 1103 instead of the cavities as in the cMUT element structure 500.
Furthermore, the exemplary formations in FIGS. 6-10 all have an inclusive circular connector surrounding the bulk of the cMUT element. Such a circular connector at an outer peripheral of the cMUT element may be used to seal the cMUT element during fabrication. Sealing may be necessary for applications in a liquid medium but may not be necessary for applications in air or vacuum. In any event, an inclusive circular connector at an outer peripheral is not required.
As shown in the above examples, wide varieties of formations are available within the spirit of the present invention. Islands may be formed on the substrate wafer by forming cavities. Although in principle not all islands must serve as an anchor to form a cantilever, it is preferred that as many as islands are effectively utilized as anchors to optimize the performance and design freedom.
As shown in the above examples, multiple cantilevers may be formed in the cantilever-forming area. The middle spring layer may have a plurality of connectors connecting to the top plate layer. The plurality of connectors may be distributed across the device element area to define a connector density profile indicating an average connector density per area at each location on the middle spring layer. Each connector and its corresponding cantilever may be considered as a spring having a spring strength, which when combined with the respective average connector density at the location can define an effective spring intensity profile. For example, the effective spring intensity may be defined as spring strength per unit area at a location, which would be determined by combining the spring strength of individual cantilevers and the number of cantilevers in a unit area (spring density).
As shown in the examples described herein, the anchors (e.g., sidewall anchors 503) and their corresponding connectors may be distributed across the substrate wafer in any pattern to achieve desired support characteristics such as device frequency responses and mode shapes.
FIGS. 12A-12J show examples of spring (cantilever) distribution profiles. These examples also combine spring distribution profiles with special shapes and patterns of the top plate layer.
FIG. 12A shows a cMUT element 1200A having a plurality of identical cantilevers each represented by a spring symbol 1270 evenly distributed over the device element area. The resultant effective spring intensity is substantially uniform over the device element area.
FIG. 12B shows another cMUT element 1200B having two types of cantilevers represented by springs 1270a and springs 1270b. Springs 1270b use thicker spring symbols to represent stronger spring strength. In this example, the effective spring intensity is significantly higher at area locations near a center of the device element area than locations near a perimeter of the device element area.
FIG. 12C shows another cMUT element 1200C having two types of cantilevers represented by springs 1270a and stronger springs 1270b. In this example, the effective spring intensity is lower at area locations near a center of the device element area than locations near a perimeter of the device element area. In addition, the springs 1270a bar non-uniformly distributed in the device element area.
FIG. 12D shows another cMUT element 1200D having two types of cantilevers represented by springs 1270a and stronger springs 1270b. In this example, the device element area comprises a first region 1210a and a second region 1210b. The thickness of the top plate layer 1240D is greater in the first region 1210a than in the second region 1210b. The thickness in the first region 1210a may correspond to a normal thickness of the top plate layer 1240D and the smaller thickness in the second region 1210b may correspond to a bottom thickness of a cavity formed on the top plate layer 1240D.
FIG. 12E shows another cMUT element 1200E having a plurality of cantilevers represented by springs 1270. In this example, the top plate layer 1240E has a thickness profile having two different thicknesses 1212a and 1212b alternating across the device element area. The greater thickness may correspond to a normal thickness of the top plate layer 1240E while the smaller thickness may correspond to a bottom thickness of a cavity formed on the top plate layer 1240E. The two thicknesses 1212a and 1212b may regularly alternate across the device element area. This configuration may help to improve the rigid/mass ratio of the top plate.
FIG. 12F shows another cMUT element 1200F having a plurality of cantilevers represented by springs 1270. In this example, the top plate layer 1240F is made of two different materials. The first material may make up a plurality of segments 1241 divided by separation trenches 1245. The second material 1242 may be provided as connectors which connect the segments 1241 by spanning over the trenches 1245. The second material 1242 may be a sealing material to fill or partially fill the trenches 1245.
FIG. 12G shows another cMUT element 1200G having a plurality of cantilevers represented by springs 1270 which are uniformly distributed across the cMUT area. In this example, the top plate layer 1240G has a thickness profile in which the top plate layer 1240G is thicker in a central region than in a region close to the perimeter of the cMUT element 1200G.
FIG. 12H shows another cMUT element 1200H which is similar to the cMUT element 1200G except that the cMUT element 1200H has two types of cantilevers represented by springs 1270a and stronger springs 1270b. The stronger springs 1270b are deployed at edges (perimeter) of the cMUT element 1200H.
FIG. 12I shows another cMUT element 1200I having a plurality of cantilevers represented by springs 1270 which are uniformly distributed in a middle region of the cMUT area. In this example, the top plate layer 1240I has a thickness profile in which the top plate layer 1240G is significantly thinner in the perimeter of the cMUT element then in the middle region. In one embodiment, the top plate layer 1240I is very thin and flexible near the parameter but rigid in the middle region. The top plate layer 1240I may even be clamped (or fixed) or nearly clamped at perimeter sidewalls 1281 without the support of a cantilever (spring). Even in this special case, however, there are no clamped (or fixed) areas within the surface of cMUT elements except for the edges, and the cMUT element 1200I still has at least a central portion of the top plate 1249I supported by the embedded springs located underneath to move in the piston-like motion. Therefore, the cMUT in such special configurations would still benefit from the potential advantages of the present invention over the conventional cMUTs.
FIG. 12J shows another cMUT element 1200J which is similar to the cMUT element 1200I except that the springs supporting the middle region of the top plate layer 1240J has two types of different springs strengths and distributed with a desired distribution pattern.
In the cMUT structure of the present invention, the top electrode can be located in the any position as long as it moves with the top plate. But the top electrode desirably should be designed to maximize the electric field between two electrodes without electric shorting. For example, if the top plate is conductive (e.g. highly doped silicon), the top electrode (usually a metal layer) may be place on the top of the top plate. If the top plate is not conductive (e.g., using silicon nitride, LTO, or PMMA), the top electrode may be placed at the bottom side of the top plate layer, and preferably on the top of an insulation layer (e.g., insulation layer 535 in FIG. 5B) which is placed over the plate-spring connectors. This helps to obtain a stronger electrical field between two electrodes.
One highly effective method for forming or introducing top plate in a cMUT structure of the present invention is to use wafer-bonding technology in which a prime silicon wafer or an SOI wafer having a desired silicon layer on an insulation layer is bonded to the substrate and the middle spring layer. In that case, the resultant top plate would include a silicon layer or polysilicon layer. Such a layer is relatively conductive for the purpose of the applications envisioned in the present invention. Correspondingly, measures to reduce the chance for shorting between the top electrode and the bottom electrode may be used.
An insulation layer such as the insulation layer 535 in FIG. 5B is used to prevent shorting between two electrodes in case the two electrodes contact each other during the operation. The insulation layer can be located on either the top or the bottom side of the transducing space defined between two electrodes. The insulation layer 535 in FIG. 5B is an example of the former configuration. Usually it is preferable to have the insulation layer placed on the side of the top plate instead of having it on the middle spring layer. This is because the properties of the middle spring layer needs to be well controlled, and having an insulation layer thereon may impose an unfavorable restriction for such controlling of the properties and behaviors. The insulation layer may either be a contiguous single layer or be patterned. If the middle spring layer itself is made of a dialectical material (e.g., silicon nitride), the insulation layer may be optional.
In one design, the depth (vertical height) of the cavity in the substrate wafer is designed to be no greater than, preferably smaller than, the depth (vertical height) of the available transducing space between the top plate and the middle spring layer. In this design, the middle spring layer would be stopped by the bottom of the cavity in the substrate wafer before the conductive top plate and the top electrode hit the bottom electrode to cause shorting between two electrodes. This would effectively prevent the top plate layer from touching the bottom electrode to cause a short, and thus eliminate the need for an insulation layer under the top plate layer.
FIG. 14 shows an enlarged view of a selected portion of a cMUT element having a motion stopper to limit the maximum vertical displacement of the top plate. The selected cMUT portion 1410 is similar to the selected cMUT portion 510 in FIG. 5 but has motion stoppers 1490 to prevent shorting. The selected cMUT portion 1410 has two halves centered on an imaginary vertical line (not shown) going through the structure from the top to the bottom. The basic structure of the cMUT element is built on a substrate wafer 1401 and has these components: a cavity 1402 defined by a sidewall anchor 1403, a middle spring layer 1420 which is preferably an elastic membrane, a bottom electrode 1425 placed on the middle spring layer 1400, a connector 1430 which stands on top of the middle spring layer 1420, a top plate 1440, and a top electrode 1450. Unlike the cMUT structure in FIGS. 5A-5B, there is no intervening insulation layer between the top plate layer 1440 and the connector 1430.
Ideally, the top plate should be light (i.e., with less mass) and rigid. An acoustic ratio of 1st resonant frequency over the total mass, defined as a rigidity/mass ratio, of the top plate may be used to evaluate the material selection and structure design of the top plate. A higher value of the rigidity/mass ratio is generally preferred for the top plate. Several different designs considerations are described below.
(2) If a certain material is already selected for the top plate, some structure designs may be used to further improve the rigidity/mass ratio. For example, making a material porous usually increases the rigidity/mass ratio defined above. Porous silicon can be formed by electrochemical etching of silicon in an HF rich electrolyte. For another example, the top plate can be micromachined to have a well-designed hollow structure to achieve a larger ratio of 1st resonant frequency over the total mass. The micromachining may be done by etching with a desired mask pattern.
FIGS. 15A-15D illustrate examples of holed or hallowed structures applied to the top plate to optimize the properties such as the rigidity/mass ratio of the top plate.
In particular, the micro-electro-mechanical transducer in accordance with the present invention may be fabricated using the novel fabrication methods disclosed in international patent applications PCT/IB2006/051566, entitled THROUGH-WAFER INTERCONNECTION; PCT/IB2006/051567, entitled METHODS FOR FABRICATING MICRO-ELECTRO-MECHANICAL DEVICES; and PCT/IB2006/051569, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS.
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