MEMS dies and MEMS-based sensors

Abstract

The present invention relates to a MEMS die and a MEMS-based sensor. A micro-electromechanical system (MEMS) die comprises: a piston; an electrode facing the piston, wherein the capacitance between the piston and the electrode changes as the distance between the piston and the electrode changes; and an elastic structure (e.g., a gasket or a corrugated wall) disposed between the piston and the electrode, wherein the elastic structure supports the piston and resists movement of the piston relative to the electrode. A back cavity volume is defined by the piston and the elastic structure, and the elastic structure prevents air from leaving the back cavity volume. The piston may be a rigid body made of a conductive material such as a metal or a doped semiconductor. The MEMS die may further comprise a second elastic structure that provides further support for the piston and is disposed within the back cavity volume.

Description

Technical Field

[0001] The present disclosure relates to microelectromechanical systems (MEMS) dies and MEMS-based sensors. Background Art

[0002] Microphone assemblies that include a microelectromechanical system (MEMS) die convert acoustic energy into an electrical signal. Microphone assemblies can be used in mobile communication devices, laptop computers, and appliances, among other devices and machines. An important parameter of a microphone assembly is the acoustic signal-to-noise ratio (SNR), which compares the expected signal level (e.g., the signal amplitude due to acoustic interference captured by the microphone assembly) to the background noise level. In microphone assemblies that include a MEMS acoustic die, the SNR often limits the minimum size that can be achieved and the overall package size of the microphone assembly. Summary of the Invention

[0003] One aspect of the present invention relates to a micro-electromechanical system (MEMS) die.

[0004] Clause 1. A micro-electromechanical system (MEMS) die, the MEMS die comprising:

[0005] piston;

[0006] an electrode facing the piston, wherein a capacitance between the piston and the electrode changes as a distance between the piston and the electrode changes; and

[0007] a resilient structure disposed between the piston and the electrode, wherein the resilient structure supports the piston, the resilient structure resists movement of the piston relative to the electrode, wherein a back volume is defined by the piston and the resilient structure, and the resilient structure prevents air from leaving the back volume.

[0008] Clause 2. The MEMS die of Clause 1, wherein the back volume is surrounded by the piston, the elastic structure, and the electrode.

[0009] Clause 3. The MEMS die of Clause 1, wherein the piston is a rigid body made of a conductive material.

[0010] Clause 4. The MEMS die of Clause 3, wherein the conductive material is a metal.

[0011] Clause 5. The MEMS die of Clause 3, wherein the conductive material is a doped semiconductor.

[0012] Clause 6. The MEMS die of Clause 1, further comprising a second elastic structure disposed within the back volume, wherein the second elastic structure supports the piston.

[0013] Clause 7. The MEMS die of Clause 1, wherein the piston comprises layers of different conductive materials.

[0014] Clause 8. The MEMS die of Clause 1, further comprising a substrate, wherein the substrate supports the spring structure and the electrode.

[0015] Clause 9. The MEMS die of Clause 8, wherein the substrate comprises a plurality of pillars, and the electrodes are distributed between the plurality of pillars.

[0016] Clause 10. The MEMS die of Clause 8, wherein the substrate comprises an electrically insulating layer, and the electrode is embedded in the electrically insulating layer.

[0017] Clause 11. The MEMS die of clause 8, wherein:

[0018] The substrate has a plurality of channels, and

[0019] The dimensions of the channel are such that any point within the channel is less than the thickness of the thermal boundary layer from the nearest surface.

[0020] Clause 12. The MEMS die of clause 11,

[0021] The thermal boundary layer thickness is defined by the following expression:

[0022]

[0023] Where, κ is the thermal conductivity of the gas in the back cavity volume, ρ0 is the density of the gas, C p is the specific heat of the gas at constant pressure, and ω is the angular frequency.

[0024] Clause 13. The MEMS die of clause 11, wherein:

[0025] The base plate includes a plurality of pillars,

[0026] The spaces between the struts constitute the channels.

[0027] Clause 14. The MEMS die of Clause 11, wherein the channel is formed as a ring.

[0028] Clause 15. The MEMS die of Clause 11, wherein the back volume is enclosed by the piston, the elastic structure, and the substrate.

[0029] Clause 16. The MEMS die of clause 1, wherein:

[0030] The piston has a plurality of channels, and

[0031] The dimensions of the channel are such that any point within the channel is less than the thickness of the thermal boundary layer from the nearest surface.

[0032] Clause 17. The MEMS die of clause 16, wherein:

[0033] The piston includes a plurality of struts,

[0034] The spaces between the struts constitute the channels.

[0035] Clause 18. The MEMS die of Clause 16, wherein the channel is formed as a ring.

[0036] Clause 19. The MEMS die of Clause 1, wherein the resilient structure is a gasket.

[0037] Clause 20. The MEMS die of Clause 1, wherein the resilient structure is a corrugated wall.

[0038] Clause 21. The MEMS die of clause 1, further comprising:

[0039] a plurality of support walls; and

[0040] A plurality of outer conductors, wherein each outer conductor of the plurality of outer conductors is attached to the piston at one end and to a support wall of the plurality of support walls at another end.

[0041] Clause 22. The MEMS die of Clause 1, wherein the MEMS die has a vent configured to allow pressure equalization between the back volume and an area external to the MEMS die at frequencies below an audio band.

[0042] One aspect of the present invention relates to a sensor.

[0043] Clause 23. A sensor comprising:

[0044] A micro-electromechanical system (MEMS) die, the MEMS die comprising:

[0045] piston;

[0046] an electrode, the electrode facing the piston, wherein a capacitance exists between the piston and the electrode; and

[0047] an elastic structure, the elastic structure being arranged between the piston and the electrode,

[0048] in:

[0049] The rear chamber volume is defined by the piston and the elastic structure,

[0050] The resilient structure supports the piston and prevents air from leaving the rear volume, and

[0051] During operation of the sensor, the MEMS die outputs a signal based on a change in capacitance due to a change in distance between the piston and the electrode.

[0052] Clause 24. The sensor of clause 23, wherein the back volume is enclosed by the piston, the resilient structure, and the electrode.

[0053] Clause 25. The sensor of clause 23, further comprising a bias voltage source, wherein the piston is electrically connected to the bias voltage source and the electrode outputs the signal.

[0054] Clause 26. The sensor of clause 23, further comprising a bias voltage source, wherein the electrode is electrically connected to the bias voltage source and the piston outputs the signal.

[0055] Clause 27. The sensor of clause 23, wherein the electrode is a first electrode, the sensor further comprising:

[0056] a bias voltage source; and

[0057] a second electrode facing the piston, wherein the first electrode is electrically connected to the bias voltage source, and the second electrode outputs the signal.

[0058] Clause 28. The sensor of clause 23, further comprising:

[0059] a bias voltage source; and

[0060] An external conductor is electrically connected to the piston and the bias voltage source.

[0061] Clause 29. The sensor of clause 23, further comprising:

[0062] integrated circuits; and

[0063] An external conductor is electrically connected to the piston and the integrated circuit.

[0064] Clause 30. The sensor of clause 23, further comprising:

[0065] base;

[0066] a cover attached to the base, the cover having a port, wherein

[0067] The MEMS die is disposed within the housing, and

[0068] Sound enters the housing and causes the piston to move during operation of the sensor.

[0069] Clause 31. The sensor of Clause 23, wherein the MEMS die has a vent configured to allow pressure equalization between the back volume and an area external to the MEMS die at frequencies below an audio band.

[0070] Clause 32. The sensor of Clause 23, wherein the MEMS die includes a second elastic structure disposed within the back volume, wherein the second elastic structure supports the piston.

[0071] Another aspect of the present invention relates to a micro-electromechanical system (MEMS) die.

[0072] Clause 33. A micro-electromechanical system (MEMS) die, the MEMS die comprising:

[0073] case;

[0074] a diaphragm disposed across the opening of the housing, the diaphragm comprising a first electrode,

[0075] wherein the housing and the diaphragm define a back volume, and each point within the back volume is at a distance from a nearest surface that is less than a thermal boundary layer thickness; and

[0076] A second electrode is disposed outside the back cavity volume and facing the diaphragm.

[0077] Clause 34. The MEMS die of clause 33,

[0078] The thermal boundary layer thickness is defined by the following expression:

[0079]

[0080] Where, κ is the thermal conductivity of the gas in the back cavity volume, ρ0 is the density of the gas, C pis the specific heat of the gas at constant pressure, and ω is the angular frequency.

[0081] Clause 35. The MEMS die of Clause 33, further comprising a backplate, wherein the second electrode is disposed on the backplate.

[0082] Clause 36. The MEMS die of Clause 34, further comprising a second backplate disposed between the diaphragm and the opening.

[0083] Clause 37. The MEMS die of Clause 33, further comprising a second diaphragm facing the second electrode, the second diaphragm comprising a third electrode, wherein the second electrode is disposed between the first diaphragm and the second diaphragm.

[0084] Clause 38. The MEMS die of Clause 37, further comprising a plurality of posts extending through the second electrode and connecting the first diaphragm to the second diaphragm.

[0085] Clause 39. The MEMS die of Clause 37, wherein the first diaphragm and the second diaphragm define a sealing region, the pressure in the sealing region being lower than atmospheric pressure.

[0086] Clause 40. The MEMS die of clause 33, wherein:

[0087] The housing includes a plurality of pillars,

[0088] A plurality of channels are defined between the struts.

[0089] Clause 41. The MEMS die of Clause 33, wherein an annular channel is formed into the housing.

[0090] Another aspect of the present invention relates to a sensor.

[0091] Clause 42. A sensor comprising:

[0092] A micro-electromechanical system (MEMS) die, the MEMS die comprising:

[0093] case;

[0094] a diaphragm disposed across the opening of the housing, the diaphragm comprising a first electrode, wherein the housing and the diaphragm enclose a rear cavity volume;

[0095] a second electrode disposed outside the back cavity volume and facing the diaphragm;

[0096] in:

[0097] Every point within the back cavity volume is less than the thermal boundary layer thickness from the nearest surface, and

[0098] During operation of the sensor, the MEMS die outputs a signal based on a capacitance between the first electrode and the second electrode, the capacitance between the first electrode and the second electrode changing due to a change in a distance between the first electrode and the second electrode.

[0099] Clause 43. The sensor according to clause 42, further comprising a second diaphragm facing the second electrode, the second diaphragm comprising a third electrode, wherein:

[0100] The second electrode is arranged between the first diaphragm and the second diaphragm,

[0101] During operation, the MEMS die outputs a second signal based on a capacitance between the third electrode and the second electrode, the capacitance between the third electrode and the second electrode changing due to a change in a distance between the third electrode and the second electrode.

[0102] Clause 44. The sensor of clause 43, further comprising a plurality of posts extending through the second electrode and connecting the first diaphragm to the second diaphragm.

[0103] Clause 45. The sensor of clause 43, wherein the first diaphragm and the second diaphragm define a sealed region, wherein a pressure in the sealed region is lower than atmospheric pressure.

[0104] Clause 46. The sensor of clause 42, wherein:

[0105] The housing includes a plurality of pillars,

[0106] A plurality of channels are defined between the struts.

[0107] Clause 47. The sensor of clause 42, wherein an annular channel is formed into the housing.

[0108] Clause 48. The sensor of clause 42, further comprising:

[0109] a base having a port;

[0110] A cover is attached to the base, wherein a MEMS die is disposed within the cover, and wherein sound enters through the port and moves the diaphragm during operation of the sensor.

[0111] Yet another aspect of the present invention relates to a micro-electromechanical system (MEMS) die.

[0112] Clause 49. A micro-electromechanical system (MEMS) die, the MEMS die comprising:

[0113] case;

[0114] a first diaphragm disposed across the opening of the housing, wherein the housing and the first diaphragm define a back volume, and each point within the back volume is at a distance from a nearest surface that is less than a thermal boundary layer thickness;

[0115] a second diaphragm disposed outside the rear cavity volume and facing the first diaphragm;

[0116] a solid dielectric disposed between the first diaphragm and the second diaphragm, the solid dielectric having a plurality of openings;

[0117] a first electrode, a first end of the first electrode coupled to the first diaphragm and a second end of the first electrode coupled to the second diaphragm, the first electrode extending through an aperture of the plurality of apertures;

[0118] a second electrode oriented longitudinally and parallel to the first electrode, a first end of the second electrode being attached to the second diaphragm and a second end of the second electrode being disposed within an aperture of the plurality of apertures; and

[0119] A third electrode is longitudinally oriented and parallel to the first electrode and the second electrode, a first end of the third electrode being attached to the first diaphragm and a second end of the third electrode being disposed within an aperture of the plurality of apertures.

[0120] Clause 50. The MEMS die of Clause 49, wherein the second end of the second electrode and the second end of the third electrode are disposed within a same orifice of the plurality of orifices.

[0121] Clause 51. The MEMS die of clause 49, wherein:

[0122] The first electrode is one of a first plurality of electrodes, a first end of each of the first plurality of electrodes is coupled to the first diaphragm, a second end of each of the first plurality of electrodes is coupled to the second diaphragm, and the first electrode extends through an aperture of the plurality of apertures,

[0123] The second electrode is one electrode in a second plurality of electrodes, a first end of each electrode in the second plurality of electrodes being attached to the second diaphragm, and a second end of each electrode in the second plurality of electrodes being disposed within an aperture in the plurality of apertures, and

[0124] The third electrode is one electrode in a third plurality of electrodes, a first end of each electrode in the third plurality of electrodes being attached to the first diaphragm, and a second end of each electrode in the third plurality of electrodes being disposed within an aperture in the plurality of apertures. BRIEF DESCRIPTION OF THE DRAWINGS

[0125] The foregoing and other features of the present disclosure will become more apparent from the following description and appended claims, taken in conjunction with the accompanying drawings, which illustrate only several embodiments according to the present disclosure and, therefore, should not be considered limiting of its scope.

[0126] Figure 1 is a side cross-sectional view of a MEMS microphone according to one embodiment.

[0127] Figure 2 According to one embodiment Figure 1 Lumped element model of a MEMS microphone signal.

[0128] Figure 3 is a side cross-sectional view of a MEMS microphone illustrating a thermal boundary layer within a back volume of the MEMS microphone according to an embodiment.

[0129] Figure 4 According to one embodiment Figure 3 Lumped element model of a MEMS microphone signal.

[0130] Figure 5 is a side cross-sectional view of a MEMS die according to one embodiment.

[0131] Figure 6 yes Figure 5 Partial reproduction showing the area near the back cavity volume of the MEMS die.

[0132] Figure 7 is a side cross-sectional view of a MEMS die having a piezoelectric element according to one embodiment.

[0133] Figure 8 is a side cross-sectional view of a MEMS die having a piezoelectric element according to another embodiment.

[0134] Figure 9 is a graph of acoustic noise as a function of the back volume of a MEMS die, according to an embodiment.

[0135] Figure 10 is a graph showing the variation of thermal boundary layer thickness as a function of sound frequency.

[0136] Figure 11is a graph showing acoustic damping as a function of frequency as a function of gap height within the MEMS die for both a conventional microphone assembly and a microphone assembly configured in accordance with embodiments of the present disclosure.

[0137] Figure 12 is a graph of acoustic SNR as a function of gap height within the MEMS die.

[0138] Figure 13 is a graph of acoustic SNR and sensitivity as a function of gap height within the MEMS die.

[0139] Figure 14 is a side cross-sectional view of a MEMS die according to another embodiment.

[0140] Figure 15 is a graph of acoustic SNR as a function of gap height within a MEMS die configured in accordance with an embodiment, over a range of different diaphragm through-hole diameters of the MEMS die.

[0141] Figure 16 are perspective and cross-sectional views of a MEMS die according to another embodiment.

[0142] Figure 17 yes Figure 16 Side cross-sectional view of the MEMS die.

[0143] Figure 18 are perspective and cross-sectional views of a MEMS die according to another embodiment.

[0144] Figure 19 is a side cross-sectional view of a MEMS die according to another embodiment.

[0145] Figure 20 is a side cross-sectional view of a MEMS die integrally formed on an integrated circuit according to one embodiment.

[0146] Figure 21 is a side cross-sectional view of a microphone assembly according to one embodiment.

[0147] Figure 22 is a side cross-sectional view of a microphone assembly according to another embodiment.

[0148] Figure 23 is a side cross-sectional view of a microphone assembly according to another embodiment.

[0149] Figure 24A is a side cross-sectional view of a MEMS die with a movable piston according to one embodiment.

[0150] Figure 24B As Figure 24AA side cross-sectional view of a MEMS die showing a variation of a MEMS die.

[0151] Figure 24C yes Figure 24A A partial three-dimensional cross-sectional view of a possible implementation of an electrode.

[0152] Figure 24D yes Figure 24A A top view of another possible implementation of an electrode.

[0153] Figure 25A is a side cross-sectional view of a MEMS die with a movable piston according to one embodiment.

[0154] Figure 25B yes Figure 25A Top view of the piston, wall, and outer conductor of a possible implementation of a MEMS die.

[0155] Figure 25C As Figure 25A A side cross-sectional view of a MEMS die showing a variation of a MEMS die.

[0156] Figure 25D As Figure 25A A side cross-sectional view of another variation of the MEMS die.

[0157] Figure 25E yes Figure 25A A partial perspective cross-sectional view of a possible implementation of a substrate for a MEMS die.

[0158] Figure 25F yes Figure 25A FIG. 1 is a top cross-sectional view of another possible implementation of a substrate for a MEMS die.

[0159] Figure 26A is a side cross-sectional view of a MEMS die with a movable piston according to one embodiment.

[0160] Figure 26B yes Figure 26A Partial perspective sectional view of a possible implementation of a base plate of a piston.

[0161] Figure 26C yes Figure 26A Bottom view of a possible implementation of a piston for a MEMS die.

[0162] Figure 26D As Figure 26A A side cross-sectional view of a MEMS die showing a variation of a MEMS die.

[0163] Figure 26E As Figure 26AA side cross-sectional view of another variation of the MEMS die.

[0164] Figure 26F According to another embodiment, Figure 26A A side cross-sectional view of a MEMS die showing yet another variation of the MEMS die.

[0165] Figure 27 is a side cross-sectional view of a MEMS die including a dual-diaphragm assembly according to one embodiment.

[0166] Figure 28A is a side cross-sectional view of a MEMS die including a dual-diaphragm assembly according to another embodiment.

[0167] Figure 28B According to one embodiment, a single diaphragm assembly is included but having Figure 28A A side cross-sectional view of a MEMS die with the same housing and substrate.

[0168] Figure 28C yes Figure 28B Side cross-sectional view of a variation of the MEMS die.

[0169] Figure 29 According to one embodiment, Figure 27 The MEMS die sensor.

[0170] Figure 30 According to one embodiment, Figure 28A The MEMS die sensor.

[0171] Figure 31A and Figure 31B The operation of a dielectric motor MEMS device according to one embodiment is shown.

[0172] Figure 32 The operation of a dielectric motor MEMS device according to another embodiment is shown.

[0173] Figure 33 FIG. 1 is a perspective view of a dual-diaphragm solid dielectric MEMS component according to one embodiment.

[0174] Figure 34 According to one embodiment, Figure 33 A side cross-sectional view of the components of a MEMS device.

[0175] Figure 35 According to one embodiment, Figure 33 A side cross-sectional view of another MEMS device of the assembly. DETAILED DESCRIPTION

[0176] Various types of microelectromechanical systems (MEMS) dies and sensors incorporating such MEMS dies are described herein.

[0177] According to one embodiment, the MEMS die has a back volume, wherein every point within the back volume is no more than a width of a thermal boundary layer away from a solid surface.

[0178] In one embodiment, a MEMS die includes a piston; an electrode facing the piston, wherein the capacitance between the piston and the electrode changes as the distance between the piston and the electrode changes; and a resilient structure (e.g., a gasket or a corrugated wall) disposed between the piston and the electrode, wherein the resilient structure supports the piston and resists movement of the piston relative to the electrode. A back cavity volume is defined by the piston and the resilient structure, and the resilient structure prevents air from leaving the back cavity volume. The piston can be a rigid body made of a conductive material such as a metal or a doped semiconductor. The MEMS die can also include a second resilient structure that provides further support for the piston and is disposed within the back cavity volume. In some embodiments, the piston includes a layer of a different conductive material (e.g., an electrode that is different from and made of a different material than the rest of the piston).

[0179] According to one embodiment, the MEMS die further includes a substrate that supports the elastic structure and the electrodes. There are many possible configurations for the substrate. Examples include: (a) the substrate has pillars with electrodes distributed between the pillars, (b) the substrate includes an electrically insulating layer with electrodes embedded therein, and (c) the substrate has multiple channels (e.g., spaces between the pillars in the substrate or multiple channels formed as a ring), where the channels are sized such that any point within the channel is less than the thickness of the thermal boundary layer from the nearest surface.

[0180] There are many possible configurations for the piston. Examples include: (a) a piston having struts, (b) a piston having multiple channels (e.g., spaces between the struts in the base plate or multiple channels formed as a ring), where the channels are sized such that any point within the channel is less than the thermal boundary layer thickness from the nearest surface.

[0181] In one embodiment, the MEMS die further has a support wall and external conductors, wherein each conductor is attached to the piston at one end and to the support wall at the other end. The MEMS die may also include a vent (e.g., in the piston or in the elastic structure). The vent is configured to allow pressure equalization between the back cavity volume and an area (e.g., volume) external to the MEMS die at non-acoustic frequencies.

[0182] The MEMS die can be part of a sensor (e.g., an acoustic sensor) wherein the MEMS die outputs a signal based on a change in capacitance between the piston and the electrode caused by a change in the distance between the piston and the electrode. In various embodiments, the sensor includes and a bias voltage source. In some embodiments, the piston is electrically connected to the bias voltage source (e.g., via an external conductor) and the electrode outputs a signal. In other embodiments, the electrode is electrically connected to the bias voltage source and the piston outputs a signal (e.g., to an integrated circuit). In some embodiments, there is a second electrode facing the piston and the first electrode is electrically connected to the bias voltage source and the second electrode outputs a signal. The sensor can include a base and a cover attached to the base, wherein the MEMS die is disposed within the cover and sound enters the cover and moves the piston, thereby changing the distance between the piston and the electrode.

[0183] According to one embodiment, a sensor includes a MEMS die having a housing; a diaphragm disposed over an opening in the housing, wherein the diaphragm includes a first electrode, and the housing and the diaphragm surround a back volume; and a second electrode disposed outside the back volume and facing the diaphragm. Each point within the back volume is located at a distance from a nearest surface that is less than a thermal boundary layer thickness. During operation of the sensor, the MEMS die outputs a signal based on a capacitance between the first electrode and the second electrode, the capacitance varying due to a change in the distance between the first and second electrodes. In some embodiments, the MEMS die further includes a second diaphragm facing the second electrode, wherein the second diaphragm includes a third electrode, and the second electrode is disposed between the first and second diaphragms. During operation, the MEMS die outputs a second signal based on a capacitance between the third electrode and the second electrode, the capacitance varying due to a change in the distance between the third electrode and the second electrode. A post connecting the first and second diaphragms may extend through the second electrode. The sensor may also include a base having a port and a cover attached to the base, wherein the MEMS die is disposed within the cover, and wherein during operation of the sensor sound enters through the port and moves the diaphragm.

[0184] In some embodiments, the first diaphragm and the second diaphragm define a sealed region at a pressure lower than atmospheric pressure.

[0185] In various embodiments, the housing has a plurality of struts with a plurality of channels defined therebetween. In other embodiments, an annular channel is formed in the housing.

[0186] In one embodiment, a MEMS die includes: a housing; a first diaphragm disposed on an opening of the housing, wherein the housing and the first diaphragm define a back volume and each point within the back volume is at a distance from a nearest surface less than a thermal boundary layer thickness; a second diaphragm disposed outside the back volume and facing the first diaphragm; a solid dielectric disposed between the first and second diaphragms, the solid dielectric having an orifice; a first electrode oriented longitudinally along an axis and parallel to the axis, a first end of the first electrode coupled to the first diaphragm and a second end of the first electrode coupled to the second diaphragm, the first electrode extending through the orifice; a second electrode oriented longitudinally along the axis and parallel to the axis, a first end of the second electrode attached to the second diaphragm and a second end disposed within the orifice; and a third electrode oriented longitudinally along the axis and parallel to the axis, a first end of the third electrode attached to the first diaphragm and a second end disposed within the orifice. The second end of the second electrode and the second end of the third electrode may be disposed within the same orifice.

[0187] According to one embodiment, (a) the first electrode is one of a first set of electrodes, the first end of each electrode in the first set of electrodes is connected to the first diaphragm and the second end of each electrode is connected to the second diaphragm, and the first electrode extends through an orifice of a plurality of orifices, (b) the second electrode is one of a second set of electrodes, the first end of each electrode in the second set of electrodes is attached to the second diaphragm and the second end of each electrode is disposed within the orifice, and (c) the third electrode is one of a third set of electrodes, the first end of each electrode in the third set of electrodes is attached to the first diaphragm and the second end of each electrode is disposed within the orifice.

[0188] A pressure microphone typically includes a diaphragm that responds to pressure differences on either side. Figure 1 In the omnidirectional microphone 10, one side of the diaphragm 12 is coupled to the external environment 14, and the pressure on this side of the diaphragm 12 is the atmospheric pressure (P atm ) and the desired acoustic signal (P ac The pressure on the other side of the diaphragm 12 is provided by a back volume 16 which is acoustically isolated from the external environment 14 but maintains atmospheric pressure therein through a small acoustic leak 15.

[0189] Figure 2 Shown Figure 1 The small signal lumped element model of the omnidirectional microphone 10 is shown in FIG. The compliance of the diaphragm 12 and the back cavity volume 16 are respectively represented by the capacitor CD and C BV The impedance of the acoustic leakage portion 15 is represented by R Leak The acoustic signal is represented by an AC signal source. The pressure P on the diaphragm 12 is D The diaphragm 12 is moved. Note that the atmospheric pressure present on both sides of the diaphragm 12 is not a factor in the diaphragm motion and is not included in this small signal model. Also note that when the compliance of the diaphragm (C D ) compared to the posterior cavity volume compliance (C BV ) is large, most of the sound pressure exists on the diaphragm 12. If the compliance of the diaphragm (C D ) compared to the posterior cavity volume compliance (C BV ) is small, very little sound pressure exists on the diaphragm 12. Acoustic leakage impedance (R Leak ) combined with the posterior cavity volume compliance (C BV ) and diaphragm compliance (C D ) in parallel combination to form a high-pass filter. Therefore, only sound pressure signals above a certain frequency will appear on the diaphragm 12.

[0190] The acoustic leakage part as the actual impedance generates thermal noise. This noise appears as noise pressure on the diaphragm 12. However, the back cavity volume compliance (C BV ) and diaphragm compliance (C D ) limits the noise to low frequencies so that when the noise is integrated over the audio frequency range (the noise is band-limited, so this is equivalent to integrating from zero to infinity), the result is the well-known quantity kT / C, where k is the Boltzmann constant, T is the absolute temperature in Kelvin, and C is the ratio of the two compliances (C D and C BV ). Therefore, for a given low-frequency cutoff, the noise caused by acoustic leakage generally increases with smaller microphones. The only option to reduce this noise is to lower the cutoff frequency of the smaller microphone. Even for very small microphones with a sufficiently low cutoff frequency, conventional A-weighting reduces the importance of low-frequency leakage noise.

[0191] This is the conventional view of microphones above a certain size. However, for small microphones, another factor becomes important. As noted by Kuntzman et al. (hereinafter “Kuntzman”) in “Thermal Boundary Layer Limitations on the Performance of Micromachined Microphones,” J. Acoust. Soc. Am. 144(5), 2018, which is incorporated herein by reference, the thermal boundary layer is that factor. Kuntzman discloses the effects of acoustic compression and expansion of the air within the back volume of the microphone assembly as a function of the size of the microphone assembly housing (e.g., as a function of the back volume of the microphone assembly). Kuntzman states: “For the case where the thermal boundary layer becomes large enough relative to the housing size, which occurs for small housings and low frequencies, the compression and expansion of the air within the housing transitions from adiabatic to isothermal, and a correction for the adiabatic chamber impedance is required. Heat transfer at the housing walls dissipates energy from the system and results in acoustic damping, which contributes to thermoacoustic noise according to the wave dissipation theorem.” Kuntzman also states that “acoustic damping due to thermal relaxation losses in the enclosure can be a significant noise contributor, particularly for small enclosure sizes where the losses are most significant.” In general, Kuntzman teaches that it is desirable to increase the back volume of the microphone assembly to reduce thermoacoustic noise.

[0192] The effects of thermoacoustic noise are most significant at low frequencies, as noted by Thompson et al. (hereinafter “Thompson”), in “Thermal Boundary Layer Effects on the Acoustical Impedance of Enclosures and Consequences for Acoustical Sensing Devices,” J. Acoust. Soc. Am. 123(3), 2008, which is incorporated herein by reference. Thompson states: “Thermal-induced changes in microphone sensitivity are caused by changes in the compliance of the [microphone] enclosure at low frequencies…If the noise from the thermal impedance is comparable to or greater than other thermal noise sources in the microphone, then this impedance may affect the internal noise of the microphone.” The thermoacoustic noise contribution is expected to be greatest for MEMS microphones with small enclosure volumes and low frequencies, where the distance between solid surfaces is on the order of the thickness of the thermal boundary layer within the back volume (which increases with decreasing operating frequency). The thermal boundary layer thickness can be roughly determined as

[0193]

[0194] where ω is the operating angular frequency of the microphone, and where κ is the thermal conductivity, ρ0 is the density, and C p is the specific heat of the gas inside the microphone assembly (eg, within the back volume of the microphone assembly) at constant pressure. The above relationship demonstrates the correlation between the thermal boundary layer thickness and the operating frequency of the microphone.

[0195] For example, materials including microphones, metals, and plastics all have a much greater heat capacity than air. Therefore, at every surface of the back volume, there is heat exchange with the boundary material, and these surfaces are essentially isothermal. This heat exchange is frequency-dependent and contributes to the impedance of the back volume. Essentially, when the air in the back volume is compressed, its temperature rises. At a given frequency, the portion of air within the boundary diffusion length releases this heat to the boundary material. As the air in the back volume thins out, the air temperature drops, but the portion of air within the boundary diffusion length gains heat from the boundary material.

[0196] Figure 3 Shown Figure 1 Figure 1 shows the thermal boundary layer 18 of an omnidirectional microphone 10. In this figure, the thermal boundary layer 18 is shaded to illustrate how the thickness 20 of the thermal boundary layer 18 changes with frequency. Darker shading corresponds to the thickness 20 at higher frequencies. Thus, at high frequencies, the thermal boundary layer 18 is quite thin, while at low frequencies, the thermal boundary layer 18 is thicker. Figure 4 The influence of the thermal boundary layer 18 on the model is shown in FIG. Now, using the complex impedance Z BV The real part of the complex impedance depends on frequency and microphone size, so the noise contributes to the pressure on the diaphragm. The analysis of this noise effect is complex but is addressed in Kuntzman. Essentially, as the microphone becomes smaller, the thermal boundary layer expands to consume more of the total back cavity volume, and when integrated, the total noise contribution to the pressure on the diaphragm increases as the microphone size decreases. This is another expected kT / C effect.

[0197] According to one embodiment, there is a size region that contradicts this conventional wisdom. At very small sizes, where the thermal boundary layer consumes the entire back cavity volume, and particularly for frequencies below audio (<20kHz), where a significant portion of the thermal boundary layer volume exists within the total back cavity volume, the trend of increasing noise reverses. If the noise is integrated from zero to infinite frequency, kT / C still increases with smaller sizes, as expected. However, if the integration is performed only over the audio frequency band, the result is a value less than kT / C (integrated from zero to infinity). The audio band noise power portion of kT / C decreases as the back cavity volume size decreases.

[0198] In general, systems and devices for providing high acoustic signal-to-noise ratio (SNR) performance for a MEMS die in a microphone (e.g., used as an acoustic transducer) are disclosed herein. Also disclosed herein is a MEMS die wherein, for a MEMS transducer employing such a die, the distance between any point within the back volume and the nearest solid surface to that point is less than the thickness of a single thermal boundary layer at the upper limit of the audio frequency band. Because the thermal boundary layer thickness increases with decreasing frequency (as described above), this constraint ensures that the distance between any point within the back volume and the nearest solid surface is less than the thickness of a single thermal boundary layer over most of the audio frequency band of the MEMS die. As used below, the upper limit is the upper frequency limit of the audio frequency band of interest detected by the microphone. For example, the upper limit can be the upper limit range of the frequency band for which the integrated circuit is monitoring audio signals (e.g., 20 kHz). Note that the MEMS die can react to audio signals above the upper limit, and the microphone can detect audio signals above the upper limit. However, the design of the microphone will be optimized for a given upper limit.

[0199] As used herein, the phrases "enclosed volume" or "enclosed back volume" refer to a volume (e.g., a back volume) that is substantially enclosed but may not be completely enclosed. For example, an enclosed volume may refer to a volume that is fluidically connected to the environment surrounding the MEMS die via perforations or openings in a diaphragm, piston, or elastic structure.

[0200] Figure 5 and Figure 6 A side cross-sectional view of a MEMS die 100 of a microphone is shown. The MEMS die 100 is configured to generate an electrical signal in response to an acoustic disturbance incident on the MEMS die 100 using a capacitive acoustic transduction method. In other embodiments, the MEMS die 100 may use another type of conversion, such as piezoelectric conversion, piezoresistive conversion, or optical conversion. The MEMS die 100 includes a substrate 102, an electrode 104, and a movable diaphragm 106. The movable diaphragm 106 may be or may include an electrode and may be referred to as a "first electrode", while the electrode 104 may be referred to as a "second electrode". The diaphragm 106 and the electrode 104 and the gap therebetween (including an insulating material such as air) form a capacitive element. The electrode 104 may sometimes be referred to as a counter electrode. The substrate 102 supports the electrode 104 and the diaphragm 106. As Figure 5 As shown, the electrode 104 is directly coupled to the substrate 102 along the entire lower surface 108 of the electrode 104. The substrate 102 is relatively large relative to the diaphragm 106 (and relative to the electrode 104), which ensures that the electrode 104 is rigidly supported. In particular, the combined thickness 109 of the substrate 102 and the electrode 104 is an order of magnitude greater than the thickness 112 of the diaphragm 106. In other embodiments, the relative thickness between the substrate 102 and the diaphragm 106 can be different.

[0201] According to one embodiment, the electrode 104 is deposited directly on the first surface (e.g., the upper surface) of the substrate 102. In some embodiments, the electrode 104 is deposited on or otherwise connected to the insulator 114. The insulator 114 can be made of silicon nitride or another dielectric material. The electrode 104 can be made of polysilicon or other suitable conductors. Figure 5 In the illustrated embodiment, the electrode 104 is "sandwiched" or otherwise disposed between the substrate 102 and the insulator 114. The electrode 104 is at least partially embedded within the lower surface of the insulator 114 and directly coupled to the substrate 102. In other embodiments, the location of the electrode 104 may be different (e.g., the electrode 104 may be embedded within or formed on the upper surface of the insulator 114). In other embodiments, the electrode 104 may extend to the outer perimeter of the volume between the electrode 104 and the diaphragm 106 (e.g., the diameter of the electrode 104 may be approximately the same as the diameter of the diaphragm 106).

[0202] In one embodiment, the diaphragm 106 is oriented parallel to (or substantially parallel to) the electrode 104 (or insulator 114, whichever is on top) and is spaced apart from the electrode 104 to form a gap. In various embodiments, the gap represents the height 118 of the cylindrical chamber (e.g., the cylindrical volume between the insulator 114 and the diaphragm 106, or the cylindrical volume between the electrode 104 and the diaphragm 106 in those embodiments where the electrode 104 is located on top of the insulator 114). The volume between the electrode 104 and the diaphragm 106 forms the entire back volume 103 of the MEMS die 100 (and, to the extent that the MEMS die 100 can be considered a microphone, the entire back volume of the microphone), as will be described further. The diaphragm 106 is spaced apart from the electrode 104 by at least one intermediate layer 120. A first side 122 of the intermediate layer 120 is coupled to the insulator 114, which is in turn coupled to the electrode 104. A second side 124 of the intermediate layer 120 is coupled to the diaphragm 106 along at least a portion of the perimeter of the diaphragm 106. The height 126 of the intermediate layer 120 (e.g., the axial height of the intermediate layer 120 parallel to the central axis 128 of the MEMS die 100) plus the height / thickness of the insulator 114 between the electrode 104 and the intermediate layer 120 is approximately equal to the distance between the diaphragm 106 and the electrode 104 (e.g., height 118). In other embodiments, the distance between the diaphragm 106 and the electrode 104 is approximately equal to the height of the intermediate layer 120. In various embodiments, the intermediate layer 120 includes a sacrificial layer (e.g., an oxide layer, a phosphosilicate glass (PSG) layer, a nitride layer, or any other suitable material) deposited or otherwise formed on the electrode 104. In some embodiments, the intermediate layer 120 can be made of silicon oxide or other materials that can be etched without affecting the substrate 102, the electrode 104, or the diaphragm 106.

[0203] In one embodiment, the diaphragm 106 is made of polysilicon or another conductive material. In other embodiments, the diaphragm 106 includes both an insulating layer and a conductive layer. Figure 6 As shown, a first side 132 of the diaphragm 106 faces the rear cavity volume 103. A second side 134 of the diaphragm 106, opposite the first side 132, faces the front cavity volume 105 of the microphone assembly. Acoustic energy 131 (e.g., sound waves, acoustic disturbances, etc.) incident on the second side 134 of the diaphragm 106 from the front cavity volume 105 causes the diaphragm 106 to move toward or away from the electrode 104. The change in distance between the electrode 104 and the diaphragm 106 (e.g., a change in height 118) results in a corresponding change in capacitance. An electrical signal representing the change in capacitance can be generated and transmitted to a portion of the microphone assembly incorporating the MEMS die 100, such as an integrated circuit (not shown), for processing.

[0204] According to one embodiment, the electrode 104 is a solid, non-porous structure such that the volume between the insulator 114 and the diaphragm 106 forms the entire back cavity volume 103 of the MEMS die 100. In contrast, for a MEMS die that includes a perforated counter electrode (e.g., a backplate with a plurality of through-hole openings), the back cavity volume includes the volume between the structure opposite the diaphragm (the insulator 114 and the electrode 104 (if exposed)) and the diaphragm 106, as well as any additional fluid (e.g., air) volume on the opposite side of the opposing structure to which the space between the electrode 104 and the diaphragm 106 is fluidly connected.

[0205] Embodiments of the present disclosure may also include other types of MEMS dies. For example, the MEMS die may be piezoelectric, piezoresistive, or photoconductive. Figure 7 1 shows an embodiment of a piezoelectric MEMS die 175. The piezoelectric MEMS die 175 includes a substrate 177 and a diaphragm 179 coupled to and spaced apart from the substrate 177. The piezoelectric MEMS die 175 also includes a piezoelectric layer 181 connected to the diaphragm 179. Figure 7 As shown, the piezoelectric layer 181 can be connected (e.g., deposited or otherwise coupled) to the lower surface 183 of the diaphragm 179. In other embodiments, as shown in FIG. Figure 8 As shown, the piezoelectric layer 181 can be connected to the upper surface 185 of the diaphragm 179. In either case, the volume between the substrate 177 and the diaphragm 179 forms the entire back volume 187 of the piezoelectric MEMS die 175.

[0206] Figure 9 A graph of A-weighted acoustic noise 200 (hereinafter referred to as "acoustic noise") in the 20 Hz to 20 kHz audio frequency band (eg, range) of a MEMS die is shown as a function of the size of the back volume of the MEMS die. In particular, Figure 9 The simulated relationship between acoustic noise 200 and back cavity volume of a MEMS die having a counter electrode and a diaphragm of fixed size (e.g., a diaphragm with a fixed diameter) is shown, wherein the MEMS die is used as a transducer. In the simulation, the acoustic noise 200 is simulated by varying the size of the gap between 0.5 μm and 8 mm (e.g., Figure 5 The height 118), the volume of the back cavity is about 0.0006mm 3 Up to 10mm 3 The range varies between . Figure 9 As shown, at approximately 9mm 3 to 0.1mm 3 In the range between , the acoustic noise 200 decreases with the decrease of the back cavity volume (for example, Figure 5 The height 118 decreases and increases. At about 9mm 3 to 0.1mm 3The trend in acoustic noise 200 between 2 and 3 is consistent with the discussion provided in Kuntzman and Thompson, who teach that acoustic noise increases as the size of the back cavity volume decreases. Surprisingly, below about 0.1 mm 3 A reversal of the trend (for the simulated diaphragm diameter) is observed for the back cavity volume (within the MEMS die size range). Figure 9 As shown, at approximately 0.0006mm 3 At the back cavity volume, the acoustic noise 200 has returned to the same level as at 4mm 3 (e.g., the total back cavity volume is reduced by a factor of approximately 7500) to approximately the same level as achieved.

[0207] Figure 10 A graph showing the relationship between thermal boundary layer thickness 300 and the operating frequency of a MEMS die used as a transducer (eg, Figure 9 , and assuming that air is provided in the volume between the counter electrode and the diaphragm). It is shown that the thermal boundary layer thickness 300 decreases with increasing operating frequency. Figure 10 This dependence is graphically shown in FIG. 5 over a range of operating frequencies within the audio band of the MEMS die (eg, within the human audible frequency range between approximately 20 Hz and 20 kHz).

[0208] like Figure 10 As shown, when the gap size (e.g., height) between the counter electrode and the diaphragm is large (e.g., when the gap is greater than 500 μm), the thermal boundary layer thickness 300 is less than the gap size over most of the audio frequency band of the MEMS die. As the gap decreases, the thermal boundary layer thickness 300 becomes equal to or greater than the gap size over a larger portion of the audio frequency band. In this gap size range, the contribution of thermal acoustic noise is maximized, and the overall SNR of the MEMS die decreases (e.g., when the MEMS die is used as a transducer).

[0209] corresponding to improved SNR performance (e.g., corresponding to Figure 9 The approximate range of the gap size of the back cavity volume where the reversal of the acoustic noise trend is observed is given by Figure 10 The bottom horizontal line 302 indicates that the size of the gap (e.g., Figure 5 The height 118 shown is less than approximately two times the thickness of the boundary layer 300 within the back volume 103 over most of the audio frequency band (e.g., between 20 Hz and 20 kHz) of the MEMS die 100. In other words, the back volume 103 is sized such that the distance between any point or location within the back volume 103 and the nearest solid surface contacting the back volume 103 is less than the thickness of the single thermal boundary layer 300. For example, Figure 6As shown, a point 119 approximately halfway between the diaphragm 106 and the insulator 114 is spaced less than one thermal boundary layer thickness 300 from the surfaces of both the diaphragm 106 and the insulator 114 that face the back cavity volume (the solid surface of the back cavity volume closest to the point 119 ).

[0210] Based on this data (and from Figure 9 Based on the data from the present invention, two different thermal regimes and mechanisms appear to exist depending on whether the gap size (e.g., height 118) is 1) greater than twice the thickness of the thermal boundary layer over a large portion of the audio frequency band or 2) less than twice the thickness of the thermal boundary layer over a large portion of the audio frequency band. The fact that acoustic noise is reduced at very small gap heights (two orders of magnitude smaller than most existing microphone assemblies) is a previously unidentified and unforeseen benefit.

[0211] Figure 11 The back volume damping (hereinafter referred to as "damping") is shown as a function of frequency for a MEMS die operating as a transducer in two different thermal states. The upper set of curves 400 shows the damping for a MEMS die with a gap size greater than the thickness of the thermal boundary layer. The direction of decreasing gap size of the curve 400 is indicated by the dashed arrow 402. Figure 11 As shown, as the gap size decreases, the damping (and associated thermal noise) increases (e.g., the total noise over the audio band of the MEMS die increases). The lower set of curves 404 shows the damping response of the MEMS die where the gap size is less than the thermal boundary layer thickness (e.g., less than twice the thermal boundary layer thickness, similar to Figure 5 and Figure 6 MEMS die 100). Figure 11 The direction of decreasing gap size for the middle curve 404 is indicated by dashed arrow 406. Damping (and the associated thermal noise) is shown to decrease as the gap size decreases. Additionally, unlike the trend exhibited by the upper set of curves 400, the lower set of curves 404 exhibits a nearly flat damping response as a function of frequency. Such characteristics may be particularly advantageous for applications such as beamforming for signal processing and other applications where the sensitivity of the MEMS die decreases at low frequencies.

[0212] Figure 12The acoustic SNR as a function of gap size is shown for three different values ​​of the surface area of ​​the diaphragm of the microphone assembly (e.g., the diameter of the diaphragm and, correspondingly, the diameter of the back cavity volume). Acoustic SNR curves are provided over a range of different surface areas of the counter electrode and the diaphragm. The acoustic SNR is shown to increase with decreasing gap. The acoustic SNR is shown to decrease with decreasing surface area. Although the trend of the SNR with respect to surface area is opposite to the trend of the SNR with respect to gap size (e.g., the height between the counter electrode and the diaphragm), the effect of the gap has been observed to dominate.

[0213] Figures 9 to 12 The results shown in FIG are simulated assuming piston-like diaphragm displacement (eg, assuming the diaphragm does not bend or bow, and that all points along the diaphragm surface move an equal amount). In practice, the diaphragm 106 (see FIG. Figure 5 ) will not be displaced uniformly in a piston-like motion, but will instead become arched or bent under the bias voltage applied to the MEMS die 100 (and further due to the acoustic pressure incident on the diaphragm 106). Thus, the movement of the diaphragm 106 will be in an axial direction (e.g., as Figure 5 vertically upward and downward as shown) and radially (e.g., as shown Figure 5 The radial velocity component of the air in the back volume 103 will cause viscous losses, which will increase the acoustic noise of the MEMS die to Figure 12 Above the value shown.

[0214] Figure 13 Graphs of acoustic SNR as a function of the gap size between the counter electrode and the diaphragm (the vertical spacing between the counter electrode and the diaphragm) are shown. Curve 500 shows the acoustic SNR for a MEMS die modeled as assuming piston-like diaphragm motion. Curve 502 shows the acoustic SNR for a MEMS die modeled as assuming the diaphragm flexes (e.g., bends) as a bias voltage is applied to the MEMS die. Figure 13 As shown, the effects of actual diaphragm buckling and movement are most pronounced at small gap sizes (e.g., below 5 μm in this case). At gap sizes between 5 μm and 11 μm, the viscous effects associated with diaphragm movement are significantly reduced. Figure 13 As shown, one way to offset the effects of diaphragm displacement / movement is to limit the size of the gap to a range between approximately 5 μm and 12 μm, or to another suitable range depending on the geometry of the back cavity volume. Alternatively or in combination, the bias voltage applied to the MEMS die can be adjusted (e.g., increased) to increase the sensitivity of the microphone assembly, thereby at least partially offsetting the effects of the additional acoustic noise caused by viscous losses.

[0215] The geometry of the counter electrode can also be adjusted to reduce the radial velocity component of the air in the back volume caused by the non-piston-like diaphragm movement. For example, Figure 14 A MEMS die 600 is shown that includes a curved electrode 604. In particular, an upper surface 632 (e.g., a first surface, a surface facing the back cavity volume, etc.) of the insulator 614 is shaped to approximately match the curvature of the diaphragm 606 when a bias voltage is applied, such that during operation, the distance between the diaphragm 606 and the electrode 604 (which is also curved in this embodiment) is approximately equal throughout the back cavity volume 611 (e.g., in a lateral direction, away from the central axis of the MEMS die). To achieve this, the electrode 604 and the diaphragm 606 are not parallel when at rest (e.g., when the bias voltage is removed). As shown Figure 14 As shown, electrode 604 is deposited or otherwise formed on recessed portion 636 of substrate 602 of MEMS die 600. The curvature of electrode 604 is a function of the bias voltage applied to MEMS die 600, the size of back volume 611, and the thickness of diaphragm 606.

[0216] Back to Figure 6 , the MEMS die 100 is shown as including an opening or perforation 138 extending through the diaphragm 106 (e.g., from a first side 132 of the diaphragm 106 to a second side 134 of the diaphragm 106). The perforation 138 is centrally located on the diaphragm 106 in a coaxial arrangement relative to the central axis 128 of the MEMS die 100. The perforation 138 is a circular through-hole in the diaphragm 106. In other embodiments, the size, shape, location, and / or number of the openings in the diaphragm 106 may vary.

[0217] Figure 15 The acoustic SNR of a MEMS die (configured similarly to MEMS die 100) as a function of gap size is shown for a range of different diameters of through-holes (e.g., through-hole 138). Figure 15 As shown, the perforations introduce acoustic noise into the MEMS die 100 (see also Figure 5 ), especially at small gap sizes (e.g., less than 5 μm). The rate of change (e.g., increase) of acoustic noise also increases with the diameter of the perforation. Figure 5 In the MEMS die 100, the diameter 140 of the through-hole 138 is in the range of about 0.25 μm to 2 μm to minimize the impact of the through-hole 138 on the overall acoustic SNR. It should be understood that the optimal range of the diameter of the through-hole 138 will vary depending on the thickness of the diaphragm 106 and the geometry of the back volume 103.

[0218] The sensitivity of the MEMS die 100 can also be improved by increasing the compliance of the air in the back cavity volume 103 (e.g., by reducing the stiffness of the air contained within the back cavity volume). In one embodiment, this is achieved by providing a channel in the MEMS substrate such that every point in the channel is no more than the thickness of a single thermal boundary layer from a solid surface. Figure 16 and Figure 17 , a correspondingly configured MEMS die 700 is shown. The MEMS die 700 includes an electrode 704 and a substrate 702, the substrate 702 including a plurality of channels 742 formed therein. More specifically, the MEMS die 700 is configured with the channels 742 sized such that any point within the channels 742 is less than a single thermal boundary layer thickness from the nearest boundary surface. Figure 16 In an embodiment, each of the plurality of channels 742 extends away from the diaphragm 706 in a substantially perpendicular orientation relative to the diaphragm 706 (e.g., parallel to the central axis of the MEMS die 700). The channels 742 extend through the electrode 704. Among other benefits, the channels 742 increase the overall compliance of the air within the MEMS die 700 (e.g., by increasing the volume of air away from the space between the electrode 704 and the diaphragm 706) without completely penetrating the substrate 702.

[0219] The channels 742 in the substrate 702 are sized to reduce thermoacoustic noise within the MEMS die 700. Specifically, the width 744 (e.g., diameter) of each of the plurality of channels 742 is less than twice the thickness of the thermal boundary layer within the back cavity volume of the MEMS die 700 over most of the audio frequency band, such that over most of the audio frequency band, any point or location within the back cavity volume is within a single thermal boundary layer thickness from the nearest solid surface of the substrate or diaphragm. The depth 745 of each channel 742 is approximately equal to the size of the gap, as shown by the height 718 (e.g., the distance between the electrode 704 and the diaphragm 706). It should be understood that the geometry of the channels 742 may vary in various embodiments. For example, in other embodiments, the depth 745 may be different from the size of the gap.

[0220] In one embodiment, channels in a MEMS substrate may be defined by pillars. Figure 18An example of such a MEMS die is shown in FIG. The MEMS die, generally designated 750, includes an electrode 754 and a substrate 752 that form a chamber 756 (e.g., a back volume) in which a plurality of pillars 758 are disposed. The pillars 758 are cylinders that extend upward from the lower surface of the chamber 756 in a substantially perpendicular orientation relative to the lower surface (the pillars 758 extend toward the diaphragm 706). In other embodiments, the shape of the pillars 758 may be different. The pillars 758 may be formed in the substrate 752 of the MEMS die 750. A conductive layer 715 is deposited on or otherwise connected to the upper surface of each of the pillars 758. The conductive layers 715 together form a single electrode. Over most of the audio frequency band of the MEMS die 750, the lateral distance between adjacent pillars 758 (e.g., the radial distance relative to the central axis of each pillar 758) is less than twice the thickness of the thermal boundary layer. In other embodiments, the channel ( Figure 16 and Figure 17 ) and pillars ( Figure 18 ) can be different. In some embodiments, a porous silicon substrate can be used in place of channels or pillars. Among other benefits, using a porous silicon substrate increases the effective compliance of the air within the back cavity volume without requiring additional manufacturing operations to form channels, pillars, or other geometries in the substrate.

[0221] Go to Figure 19 , an example of a MEMS die using porous silicon according to one embodiment will now be described. The MEMS die, generally designated 770, includes a substrate 772. In the case of being formed from silicon, the substrate 772 can be doped to make it conductive so that the surface of the porous region 774 is actually the counter electrode of the capacitive transducer. The size of the pores 776 is much smaller than the thickness of a single thermal boundary layer and also allows air to flow in all directions. The percentage of open volume in the porous region 774 can be controlled by well-known electrochemical processes and can be made quite large. The gap size (shown as height 778) between the upper surface of the porous region 774 (e.g., the counter electrode) and the diaphragm 780 must still be less than two thermal boundary layer thicknesses, but in this embodiment, the gap size does not dominate the size of the back cavity volume 782 and therefore does not dominate the sensitivity of the MEMS die 770. Alternatively, a sintered material can be used instead of porous silicon having the above-mentioned characteristic pore size.

[0222] Among other benefits, the reduction in the required back volume of the MEMS die as described in the embodiments discussed above allows the overall footprint (e.g., package size, etc.) of a microphone assembly using the MEMS die to be substantially reduced. Furthermore, because the counter electrode is a solid, non-perforated structure, the MEMS die can be integrated with other components of the microphone assembly to further reduce the package size of the microphone assembly. For example, Figure 20 The monolithic integration of a MEMS die 800 and an integrated circuit (IC) 802 is shown. IC 802 may be an application specific integrated circuit (ASIC). Alternatively, IC 802 may include another type of semiconductor die that integrates various analog, analog-digital, and / or digital circuits. Figure 20 As shown, IC 802 forms a substrate for MEMS die 800. MEMS die 800 is integrally formed as a single, unitary structure on IC 802. Electrodes 804 of MEMS die 800 are directly coupled to IC 802 along the entire lower surface 808 of electrodes 804.

[0223] According to one embodiment, the geometry of the electrode 804 may be similar to that of the reference electrode 804. Figure 5 The electrodes 104 are of the same or similar geometry as described above. Figure 20 As shown, the electrode 804 is directly coupled to the IC 802 (e.g., formed on the upper surface of the IC 802). The IC 802 includes a substrate 810 and an upper portion 812 coupled to a first surface (e.g., upper surface, etc.) of the substrate 810. The IC 802 also includes a plurality of transistors 813 embedded in the upper surface of the substrate 810, between the substrate 810 and the upper portion 812. The upper portion 812 is configured to electrically couple (e.g., connect, etc.) the electrode 804 to the IC 802 and / or other components of the microphone assembly (not shown). In particular, the upper portion 812 includes a plurality of metal layers 814 embedded within the upper portion 812. The metal layers 814 electrically connect the electrode 804 to contacts disposed on an outer surface of the upper portion 812 (e.g., to an outer surface of the combined MEMS die 800 and IC 802).

[0224] According to one embodiment, Figure 21 As shown, the combination of the MEMS die 800 and the IC die 802 is configured to be assembled within a microphone assembly, as shown in assembly 900. Figure 21As shown, assembly 900 includes a housing including a base 902, a cover 904 (e.g., a housing cover), and a sound port 906. In some embodiments, base 902 is a printed circuit board. Cover 904 is coupled to base 902 (e.g., cover 904 can be mounted on a peripheral edge of base 902). Cover 904 and base 902 together form an enclosed volume of assembly 900 (e.g., front cavity volume 910 of MEMS die 800). Figure 21 As shown, a sound port 906 is disposed on the cover 904 and is configured to transmit sound waves to the MEMS die 800 located within the enclosed volume. Alternatively, the sound port 906 can be disposed on the base 902. Sound waves (e.g., sound pressure, etc.) move the diaphragm 806 of the MEMS die 800, which changes the gap size (e.g., height 818) between the diaphragm 806 and the electrode 804. The volume between the electrode 804 and the diaphragm 806 forms the entire back volume 911 of the MEMS die 800, which advantageously reduces the overall footprint of the microphone assembly 900 without limiting the acoustic SNR that can be achieved.

[0225] like Figure 21 As shown, substrate 810 is coupled to a first surface of base 902 within enclosed volume 908. In some embodiments, the assembly may form part of a compact computing device (e.g., a portable communication device, a smartphone, a smart speaker, an Internet of Things (IoT) device, etc.), wherein one, two, three, or more components may be integrated for picking up and processing various types of acoustic signals, such as speech and music.

[0226] exist Figure 21 In an embodiment, the MEMS die 800 is configured to generate an electrical signal (eg, a voltage) at the output in response to acoustic activity incident on the sound port 906. Figure 21 As shown, the output portion includes pads or terminals of the MEMS die 800 that are electrically connected to the circuit via one or more bond wires 912. The assembly 900 may also include electrical contacts disposed on a surface of the base 902 external to the cover 904. The contacts may be electrically coupled to the circuit (e.g., via bond wires or electrical traces embedded within the base 902) and may be configured to electrically connect the microphone assembly 900 to one of a variety of host devices.

[0227] Figure 21 The arrangement of the components of the microphone assembly embodiment should not be considered limiting. Many alternatives are possible without departing from the inventive concepts disclosed herein. For example, Figure 22Another embodiment of a microphone assembly 1000 is shown that includes a MEMS die 1100 that is a flip chip bonded to a base 1002 of the microphone assembly 1000. The MEMS die 1100 is separated from (and electrically connected to) the base 1002 by solder balls 1003. The MEMS die 1100 is arranged to receive acoustic energy through an acoustic port 1006 centrally located within the base 1002. The MEMS die 1100 is suspended within a cavity formed between the base 1002 and the cover 1004 of the microphone assembly 1000.

[0228] Figure 23 Shows something like Figure 22 1000, but in which the lid has been replaced by an encapsulant 1201 that surrounds a MEMS die 1300. Among other benefits, the encapsulant 1201 insulates the MEMS die 1300 and helps support the MEMS die 1300 in position above a base 1202 of the microphone assembly 1200. The encapsulant may include a curable epoxy or any other suitable material.

[0229] One potential problem with using a diaphragm in a capacitive MEMS sensor is that the dynamic movement of the diaphragm itself results in lateral velocity gradients and viscous losses. Any initial diaphragm deflection due to, for example, an applied bias voltage exacerbates viscous losses. According to an embodiment of the present disclosure, a capacitive MEMS sensor uses a piston (e.g., a rigid silicon wafer) instead of a diaphragm. In one embodiment, the piston is supported by an elastic structure (e.g., a soft rubber seal, a gasket (e.g., made of PDMS), or a corrugated wall).

[0230] In one embodiment, the resilient structure provides a seal that prevents lateral movement of air, which reduces noise.

[0231] According to one embodiment, the MEMS die includes a vent that allows the pressure in the back volume to equalize with ambient pressure (but only at non-acoustic frequencies - it is sealed at acoustic frequencies).

[0232] Go to Figure 24AA MEMS die using a piston instead of a diaphragm, according to one embodiment, will now be described. MEMS die 2400 includes a piston 2402 that is rigid (e.g., made of relatively thick silicon) and conductive (e.g., made of a conductive material such as a metal or a doped semiconductor such as crystalline silicon), an electrode 2404 facing piston 2402, and a resilient structure 2406 that supports piston 2402 on electrode 2404. A back cavity volume 2408 is bounded at the top by piston 2402, at the bottom by electrode 2404, and on all sides by resilient structure 2406. Piston 2402, electrode 2404, and resilient structure 2406 together enclose back cavity volume 2408. In some embodiments, electrode 2404 is part of a substrate that also includes an insulating material supporting electrode 2404. Resilient structure 2406 prevents air or other gas from leaving back cavity volume 2408 (e.g., prevents air or other gas from traveling in a direction radially outward from a central portion of back cavity volume 2408). In other words, the elastic structure 2406 seals the back cavity volume 2408 to prevent lateral velocity gradients in the trapped air volume. Possible implementations of the elastic structure 2406 include gaskets made of polydimethylsiloxane (PDMS) or rubber. Another possible implementation of the elastic structure 2406 is a corrugated wall (e.g., made of PDMS, rubber, or a more rigid material such as silicon, silicon nitride, or aluminum). An example of a suitable structure with a corrugated wall is a flexible bellows seal. Another possible implementation of the elastic structure 2406 is a thin diaphragm at the periphery.

[0233] According to one embodiment, the piston 2402 has a vent 2411 that allows pressure equalization between the back cavity volume 2408 and an area outside the MEMS die 2400 when the piston 2402 is at rest or moving up and down relative to the electrode 2404 at non-acoustic frequencies. The vent 2411 is a hole configured to allow pressure equalization between the back cavity volume and an area (e.g., volume) outside the MEMS die at frequencies below the audio band. For example, the vent 2411 can have a geometry and size that results in impedance such that the vent 2411 has no effect on air flow at non-audio frequencies but blocks air flow at audio frequencies. The vent 2411 does not have to be located in the piston 2402, but can be anywhere (e.g., in the resilient structure 2406) to facilitate pressure equalization.

[0234] exist Figure 24AIn the illustrated embodiment, the piston 2402 and the electrode 2404 form a capacitive element, with the piston 2402 acting as a first electrode of the capacitive element, the electrode 2404 acting as a second electrode of the capacitive element, and the air or other gas in the back cavity volume 2408 acting as a dielectric. During operation, the piston 2402 and the elastic structure 2406 behave like a classic spring-mass system. The capacitance between the piston 2402 and the electrode 2404 changes as the distance between them changes (i.e., increases or decreases as the distance increases). The elastic structure 2406 resists movement of the piston 2402 toward the electrode (e.g., in response to an incoming pressure wave, such as a pressure wave generated by sound). In particular, as the piston 2402 moves toward the electrode 2404, the elastic structure 2406 compresses, accumulates potential energy, and (when the external force pushing the piston 2402 is sufficiently reduced) pushes the piston 2402 away from the electrode 2404. Reference will be made to Figure 24B 、 Figure 25A 、 Figure 25C 、 Figure 25D 、 Figure 26A 、 Figure 26D 、 Figure 26E and Figure 26F Other MEMS die embodiments are described with piston-spring structures. However, it should be understood that in combination Figure 24A The principles and variations described (including the inclusion of vents) can also be applied to these other MEMS die embodiments.

[0235] In one embodiment, when the MEMS die 2400 is integrated into a microphone, the piston 2402 is electrically connected to a bias voltage source 2409, and the electrode 2404 is electrically connected to an integrated circuit 2410 (e.g., an amplifier input thereof). Alternatively, the piston 2402 can be electrically connected to the integrated circuit 2410, and the electrode 2404 can be electrically connected to the bias voltage source 2409.

[0236] refer to Figure 24B , shows a variation of a MEMS die 2400 configured according to one embodiment. MEMS die 2450 includes a piston 2403 having an insulating portion 2403a and an electrode 2403b. In this embodiment, electrode 2403b serves as the first electrode in the capacitive element (electrode 2404 serves as the second electrode).

[0237] In one embodiment, the channel 2412 is formed in Figure 24A and Figure 24B2404. In one embodiment, the dimensions of the channel 2412 are such that any point within the channel 2412 is less than a single thermal boundary layer thickness from the nearest surface. Each of the channels 2412 extends away from the piston 2402 in a substantially perpendicular orientation relative to the piston 2402 (e.g., parallel to the central axis of the MEMS die). Among other benefits, the channel 2412 increases the overall compliance of the air within the MEMS die (e.g., by increasing the volume of air away from the space between the electrode and the piston) without completely penetrating the electrode 2404. The space within the channel 2412 is part of the back cavity volume.

[0238] Go to Figure 24C , showing an embodiment according to Figure 24A FIG24 is a perspective view of electrode 2404 (after sectioning MEMS die 2400 along line AA'). In this embodiment, pillars 2414 are formed in electrode 2404, with the spaces between pillars 2414 forming channels 2412. Channel 2412 and pillars 2414 can be of any shape, as long as any point within the channel is less than the thermal boundary layer thickness at the upper audio frequency limit from the surface of pillars 2414.

[0239] Go to Figure 24D , showing a method according to another embodiment Figure 24A 24. A top view of the electrode 2404 of FIG. 24 (after the MEMS die 2400 is sectioned along line AA′). In this embodiment, channels 2412 are formed in the electrode 2404 in the form of concentric rings.

[0240] In various embodiments, the electrode facing the piston (on the opposite side of the gap / dielectric) is part of a layered structure generally referred to as a substrate, which may include, for example, a polysilicon layer and an insulator layer, wherein the electrode is embedded in the substrate (e.g., within the insulating material) or disposed on the surface of the substrate (e.g., on top of the insulating material). Figure 25A , shows a MEMS die configured according to this embodiment. MEMS die 2500 includes a piston 2502 that is configured similar to Figure 24A The piston 2402 has the same possible variations, including different electrodes and insulating materials. The MEMS die 2500 also includes a substrate 2504 and an elastic structure 2506 that supports the piston 2502 on the substrate 2504. Possible implementations of the piston 2502 and the elastic structure 2506 include combining Figure 24A and Figure 24B25. The piston and resilient structure of FIG. 25 are those discussed above. The back cavity volume 2508 is bounded at the top by the piston 2502, at the bottom by the substrate 2504, and on all sides by the resilient structure 2506. The piston 2502, the substrate 2504, and the resilient structure 2506 surround the back cavity volume 2508. The resilient structure 2506 prevents air from leaving the back cavity volume 2508, and in particular, blocks air from traveling in a direction radially outward from a central portion of the back cavity volume 2508. The substrate 2504 includes an insulating layer 2510 (e.g., made of silicon dioxide or silicon nitride) and an electrode 2512 embedded in the insulating layer 2510. The electrode 2512 faces the piston 2502 such that a capacitance exists between the piston 2502 and the electrode 2512 (the piston 2502 acts as a first electrode of the capacitor, the electrode 2512 acts as a second electrode of the capacitor, and the air or other gas in the back cavity volume 2508 acts as a dielectric). However, in some embodiments, piston 2502 includes an insulating material and a conductive portion or layer, wherein the conductive portion or layer functions as a first electrode of a capacitor. In one embodiment, when MEMS die 2500 is integrated into a microphone, piston 2502 is electrically connected to bias voltage source 2514, and electrode 2512 is electrically connected to integrated circuit 2516 (e.g., an amplifier input thereof). Alternatively, piston 2502 can be electrically connected to integrated circuit 2516, and electrode 2512 can be electrically connected to bias voltage source 2514.

[0241] In one embodiment, the channel 2518 is formed in the substrate 2504, and the electrode 2512 spans the channel 2518. In one embodiment, the dimensions of the channel 2518 are such that any point within the channel 2518 is less than a single thermal boundary layer thickness from the nearest surface. Figure 25A In an embodiment, each of the channels 2518 extends away from the piston 2502 in a substantially perpendicular orientation relative to the piston 2502 (e.g., parallel to the central axis of the MEMS die 2500). The channels 2518 extend through the electrode 2512. Among other benefits, the channels 2516 increase the overall compliance of the air within the MEMS die 2500 (e.g., by increasing the volume of air away from the space between the second electrode 2512 and the piston 2502) without completely penetrating the substrate 2504. The space within the channels 2518 is part of the back cavity volume 2508.

[0242] According to one embodiment, the MEMS die 2500 includes a wall 2520 and one or more external conductors 2522. Each external conductor 2522 is electrically connected to the piston 2502 at one end (or to an electrode on the piston 2502 if the piston includes an insulating material) and is electrically connected to the wall 2520 at the other end. In one embodiment, each of the one or more external conductors 2522 is a resilient member, such as a metal spring. The wall 2520 is connected to the bias voltage source 2514 through a contact 2524 (e.g., a eutectic metal contact) and a conductive path 2526 (with through-silicon vias (TSVs) as appropriate) within the insulating layer 2510 of the substrate 2504. Figure 25B The wall 2520 and outer conductor 2522 are shown viewed from above the piston 2502.

[0243] In combination Figure 25A In a variation of the described embodiment, the function of electrode 2512 may be Figure 25C 25. The embodiment of the present invention is performed by two electrodes shown in the MEMS die 2550. In this embodiment, the function of electrode 2512 is performed by electrodes 2512a and 2512b, both of which are shown embedded in substrate 2504 (in insulating layer 2510). In this embodiment, electrode 2512a is electrically connected to bias voltage source 2514 via conductive path 2526a, and electrode 2512b is electrically connected to integrated circuit 2516 (e.g., an amplifier input thereof) via conductive path 2526b.

[0244] In one embodiment, the piston is connected to an electrical potential through a very large resistor. In one embodiment, the electrical potential can be electrical ground. The impedance of the resistor should be large enough to set the electrical corner frequency to be lower than the desired low acoustic corner frequency (e.g., 20 Hz). In one embodiment, the impedance can be 10^12 ohms. In other embodiments, the impedance can be less than or greater than 10^12 ohms. In one embodiment, the resistor is formed by the leakage conductance of the elastic structure 2506b and the insulating layer 2510. In operation, the piston is connected to an electrical potential in DC sensing, but is electrically isolated in AC sensing. In such an embodiment, movement of the piston toward and away from the electrode facing the piston causes the capacitance between the piston and the electrode to change and induces a signal in one of the electrodes (the other electrode is supplied with a DC voltage). An example of this is shown in FIG. Figure 25D Shown in (which is combined Figure 25C A variation of the embodiment described. Figure 25D, MEMS die 2560 has a piston 2502 configured to electrically float. In this embodiment, electrode 2512a is connected to bias voltage source 2414 via conductive path 2526a, and electrode 2412b is connected to integrated circuit 2416 (e.g., an amplifier input thereof) via conductive path 2526b.

[0245] Go to Figure 25E , showing a method according to one embodiment along ( Figure 25A 25. In this embodiment, pillars 2530 are formed in substrate 2504, and the spaces between pillars 2530 constitute channels 2518. At least some of the pillars (in some embodiments, all of the pillars) include a conductive layer 2532 connected (e.g., to a TSV running down the long axis of the pillar) to a single common conductor, such that all of the conductive layers are part of electrode 2512. This substrate configuration can also be used Figure 25C and Figure 25D In an embodiment of the present invention, but with two electrically separated electrodes (e.g., with two separated conductive ribs).

[0246] Go to Figure 25F , showing a method according to another embodiment along ( Figure 25A In this embodiment, the channel 2518 is formed in the substrate 2504 in the form of a ring, such as Figure 25F The concentric rings shown between portions of the electrode 2512 are also formed in the form of concentric rings 2536. In this embodiment, a single conductive rib 2534 electrically connects the rings 2536 of the electrode 2512. This substrate configuration can also be used Figure 25C and Figure 25D In an embodiment of the present invention, but with two electrically separated electrodes (e.g., with two separated conductive ribs).

[0247] According to various embodiments, a channel is provided in the piston to increase the compliance of the air in the rear chamber volume. Figure 26A , shows a MEMS die configured according to this embodiment. MEMS die 2600 includes a piston 2602 that is rigid (e.g., made of relatively thick silicon) and conductive (e.g., a semiconductor like crystalline silicon), a substrate 2604, and a resilient structure 2606 that supports the piston 2602 on the substrate 2604. Possible implementations of the piston 2602 and the resilient structure 2606 include combining Figure 24A24. The piston 2602 and the resilient structure 2406 of FIG. 24 are similar to those discussed above. The piston 2602, the substrate 2604, and the resilient structure 2606 surround a back cavity volume 2608. The resilient structure 2606 prevents air from leaving the back cavity volume 2608, and in particular, prevents air from traveling in a direction radially outward from a central portion of the back cavity volume 2608. The substrate 2604 includes an insulating layer 2610 (e.g., made of silicon dioxide) and an electrode 2612 embedded in the insulating layer 2610.

[0248] Still refer to Figure 26A , electrode 2612 faces piston 2602 such that there is capacitance between piston 2602 and electrode 2612 (piston 2602 acts as a first electrode of a capacitor, electrode 2612 acts as a second electrode of the capacitor, and the air or other gas in back volume 2608 acts as a dielectric). However, in some embodiments, piston 2602 includes an insulating material and a conductive portion or layer, wherein the conductive portion or layer acts as the first electrode of the capacitor. In one embodiment, when the MEMS die 2600 is integrated into a microphone, piston 2602 is electrically connected to bias voltage source 2614 (e.g., via external conductor 2622), and electrode 2612 is electrically connected to integrated circuit 2616 (e.g., its amplifier input) (e.g., via conductor path 2622). Alternatively, piston 2602 can be electrically connected to integrated circuit 2616, and electrode 2612 can be electrically connected to bias voltage source 2614.

[0249] According to one embodiment, the piston 2602 includes a channel 2618. The space within the channel 2618 is part of the back cavity volume 2608. The dimensions of the channel 2618 are such that any point within the channel 2618 is less than a single thermal boundary layer thickness from the nearest surface. Figure 26A In an embodiment, each of the channels 2618 extends away from the substrate 2604 in a substantially perpendicular orientation relative to the substrate 2604 (e.g., parallel to the central axis of the MEMS die 2600). Among other benefits, the channels 2618 increase the overall compliance of the air within the MEMS die 2600 (e.g., by increasing the volume of air in the space away from the substrate 2604) without completely penetrating the piston 2602.

[0250] According to one embodiment, the MEMS die 2600 includes a wall 2620 and one or more outer conductors 2622. Each outer conductor 2622 is configured as previously described with respect to Figure 25A The outer conductors 2522 (including possible implementations) are described above, and their function and relationship to the wall 2620 are also as previously described with respect to Figure 25A and Figure 25B The wall 2520 is described.

[0251] Figure 26B A partial perspective view of a cross section of the piston 2602 along line DD′ is shown. In this embodiment, struts 2624 are formed in the piston 2602 , and the spaces between the struts 2624 constitute the channels 2618 .

[0252] Go to Figure 26C , shows a bottom view of the piston 2602 taken along line DD' according to another embodiment. In this embodiment, the channels 2618 are formed into the piston 2602 in the form of concentric rings.

[0253] In combination Figure 26A In a variation of the described embodiment, the function of electrode 2612 may be Figure 26D 26. The embodiment of the present invention is performed by two electrodes shown in the MEMS die 2650. In this embodiment, the function of electrode 2612 is performed by electrode 2612a and electrode 2612b. In this embodiment, electrode 2612a is connected to a bias voltage source 2614, and electrode 2612b is connected to an integrated circuit 2616 (e.g., an amplifier input thereof) via respective conductive paths 2622a and 2622b.

[0254] In combination Figure 26D In a variation of the described embodiment, piston 2602 may be electrically floated, such as Figure 26E 26. In this embodiment, electrode 2612a is connected to bias voltage source 2614 via conductive path 2622a, and electrode 2612b is connected to integrated circuit 2616 (eg, an amplifier input thereof) via conductive path 2622b.

[0255] In various MEMS die embodiments described herein (e.g., Figure 24B 、 Figure 25A 、 Figure 25C 、 Figure 25D 、 Figure 26A 、 Figure 26D and Figure 26E In a MEMS die, the MEMS die may further include an elastic structure inside the back volume (e.g., near the central axis of the MEMS die). This elastic structure is in addition to the elastic structure at the periphery of the piston. An example of this embodiment is Figure 26F , where the MEMS die 2670 is configured similar to Figure 26A MEMS die 2600, except that MEMS die 2670 includes a second spring structure 2607 inside back volume 2608. One possible advantage of second spring structure 2607 is that it can increase the resonant frequency of MEMS die 2670 to limit adverse effects due to having resonances in or near the audio frequency band.

[0256] In the various embodiments of the MEMS die described above, certain innovations allow the back volume to be configured so that no point within the back volume is greater than the thickness of a single thermal boundary layer from a solid surface. The following is a description of other embodiments of a MEMS die in combination with a diaphragm assembly having such a back volume configuration.

[0257] For example, go to Figure 27 , the MEMS die 2700 includes a dual-diaphragm diaphragm assembly 2701. The dual-diaphragm assembly 2701 includes a first diaphragm 2702 including a first electrode 2704, a second diaphragm 2706 including a second electrode 2708, and a backplate 2710 including a third electrode 2712. The first diaphragm 2702 and the second diaphragm 2706 are oriented so that they face each other, and the backplate 2710 is disposed between and faces both the first diaphragm 2702 and the second diaphragm 2706. The first diaphragm 2702 and the second diaphragm 2706 are connected to each other by a post 2713 extending through the backplate 2710. The MEMS die 2700 also includes a spacer 2714 sandwiched between and connected to the first diaphragm 2702 and the backplate 2710, and a spacer 2716 sandwiched between and connected to the second diaphragm 2706 and the backplate 2710. The area 2716 between the first diaphragm 2702 and the second diaphragm 2706 is sealed and at a pressure lower than standard atmospheric pressure (e.g., 50% of atmospheric pressure) and can be at or near a vacuum. The MEMS die 2700 also includes a housing 2718 formed of and serving as a substrate. The second diaphragm 2706 is attached to the housing 2718 via a spacer 2720.

[0258] In one embodiment, the channel 2722 is formed into the housing 2718. In one embodiment, the dimensions of the channel 2722 are such that any point within the channel 2722 is less than the thickness of a single thermal boundary layer from the nearest surface. The channel 2722 increases the overall compliance of the air within the MEMS die 2700. The space within the channel 2722 is part of the back cavity volume. Possible configurations of the topography of the substrate 2718 include incorporating a substrate having pillars or rings, respectively. Figure 24C and Figure 24D Those configurations discussed.

[0259] Variants of the MEMS die 2700 are Figure 28A The MEMS die 2800 includes Figure 27The dual diaphragm assembly 2701 is similar to the dual diaphragm assembly 2701, but with a substrate labeled 2802. A housing 2806 is attached to the first diaphragm 2702 (via spacer 2805). A channel 2810 is formed into the housing 2806. In one embodiment, the dimensions of the channel 2810 are such that any point within the channel 2810 is less than a single thermal boundary layer thickness from the nearest surface. The space within the channel 2810 is part of the back cavity volume. Possible configurations of the topography of the housing 2806 include incorporating a housing having struts or rings, respectively. Figure 24C and Figure 24D The substrate 2802 has a hole 2820 through which sound passes. The second diaphragm 2706 is attached to the substrate 2802 via a spacer 2822.

[0260] Variants of the MEMS die 2800 are Figure 28A 28. MEMS die 2850 includes a housing 2806 and a substrate 2802. Additionally, MEMS die 2850 has a single diaphragm 2852 and a backplate 2854 facing diaphragm 2852. Diaphragm 2852 has a first electrode 2856, and backplate 2854 has a second electrode 2858. Diaphragm 2852 is attached to housing 2806 via spacer 2860. Backplate 2854 is attached to diaphragm 2852 via spacer 2862, and to substrate 2802 via spacer 2864. During operation, sound enters hole 2820, passes through backplate 2854, and strikes diaphragm 2852.

[0261] Another variation of the MEMS die 2800 is Figure 28C The MEMS die 2870 is similar to Figure 28B The MEMS die 2850 is different in that the positions of the backplate 2854 and the diaphragm 2852 are reversed, and there is a second backplate 2862 with an electrode 2864.

[0262] Go to Figure 29 , described in the context of sensors Figure 27In an example use of a MEMS die 2700, the sensor is specifically an acoustic sensor, referred to as microphone 2900. Microphone 2900 has a housing 2902, which includes a cover 2904 and a base 2906. Cover 2904 is attached to base 2906. A pressure wave (e.g., a sound wave) enters a port 2908 on cover 2904 and strikes the first diaphragm 2702, causing it to bend and inducing a corresponding bend in the second diaphragm 2706 (via struts 2713). As a result, the distance between the first electrode 2704 and the third electrode 2712 changes the capacitance between the first electrode 2704 and the third electrode 2712. Similarly, the distance between the second electrode 2708 and the third electrode 2712 changes, thereby changing the capacitance between the second electrode 2708 and the third electrode 2712. These capacitance changes are read by circuitry (e.g., an integrated circuit) 2910, which is attached to the MEMS die 2700 via one or more signal paths (e.g., wires) 2912. The circuit 2910 then interprets the signal representing the change in capacitance as, for example, sound.The circuit then provides a further signal representing the interpretation to an external device via one or more additional signal paths (eg, wires).

[0263] Go to Figure 30 , an example use of MEMS die 2800 is described in the context of a sensor, specifically, an acoustic sensor referred to as microphone 3000. Microphone 3000 has a housing 3002, which includes a cover 3004 and a base 3006. Cover 3004 is attached to base 3006. A pressure wave (e.g., a sound wave) enters port 3008 on base 3006, passes through hole 2820 in substrate 2802, and strikes second diaphragm 2806, causing it to bend and inducing a corresponding bend in first diaphragm 2802 (via struts 2813). As a result, the distance between first electrode 2804 and third electrode 2812 changes the capacitance between first electrode 2804 and third electrode 2812. Similarly, the distance between second electrode 2808 and third electrode 2812 changes, thereby changing the capacitance between second electrode 2808 and third electrode 2812. These changes in capacitance are read by a circuit (e.g., an integrated circuit) 3010, which is attached to the MEMS die 2800 via one or more signal paths (e.g., wires) 3012. The circuit 3010 then interprets the signal representing the change in capacitance as, for example, sound. The circuit then provides another signal representing the interpretation to an external device via one or more additional signal paths (e.g., wires).

[0264] Other types of dual diaphragm assemblies can also be used instead Figure 27 、 Figure 28A 、 Figure 29 and Figure 30The dual diaphragm assembly 2701. For example, a dielectric motor MEMS assembly can be used. Figure 31A 、 Figure 31B and Figure 32 The principle of such an assembly according to one embodiment is described.

[0265] Go to Figure 31A In one embodiment, a dielectric motor MEMS device 3100 includes a first electrode 3110 oriented longitudinally along and parallel to an axis 3160. The first electrode 3110 has a first end 3111 and a second end 3112. The MEMS device 3100 also includes a second electrode 3120 oriented longitudinally along and parallel to the axis 3160. The second electrode 3120 has a first end 3121 and a second end 3122. The MEMS device 3100 also includes a third electrode 3130 oriented longitudinally along and parallel to the axis 3160. The third electrode 3130 has a first end 3131 and a second end 3132. The electrodes 3110, 3120, and 3130 can be cylindrical, plate, cube, prism, polyhedron, or other shaped electrodes. The length of the first electrode 3110 is longer than the length of the second electrode 3120 and longer than the length of the third electrode 3130. MEMS device 3100 has a solid dielectric 3150 interspersed between electrodes. Dielectric 3150 can be made of, for example, silicon nitride. Electrodes 3110, 3120, and 3130 are made of conductors or semiconductors, such as electroplated metal or polysilicon.

[0266] In one embodiment, the first electrode 3110 is a conductive pin among a plurality of first conductive pins electrically connected to each other, the second electrode 3120 is a conductive pin among a plurality of second conductive pins electrically connected to each other, and the third electrode 3130 is a conductive pin among a plurality of third conductive pins electrically connected to each other.

[0267] Go to Figure 31B In one embodiment, the dielectric 3150 has a plurality of apertures 3156 extending through the dielectric 3150 in a direction parallel to the axis 3160. The apertures 3156 are shown as cylindrical, but may be of any suitable shape. The first electrode, the second electrode, and the third electrode may each be at least partially located within one of the plurality of apertures 3156. In other words, a portion of the electrode may be located within the aperture and / or the electrode may be located within a portion of the aperture. According to a possible embodiment, at least the second electrode and the third electrode may be only partially located within the aperture 3156, with the ends being located outside the dielectric 3150. For example, the first electrode 3110 may be at least partially located within a first aperture of the plurality of apertures 3156. The second end 3122 of the second electrode 3120 may be located within a second aperture of the plurality of apertures 3156. The first end 3131 of the third electrode 3130 may be located within a third aperture of the plurality of apertures 3156.

[0268] refer to Figure 31B , the dielectric 3150 has a first surface 3151 and a second surface 3152. The first surface 3151 and the second surface 3152 are parallel to a plane perpendicular to the axis 3160. Figure 31B In the configuration shown, the first end 3111 of the first electrode 3110 and the first end 3121 of the second electrode 3120 extend beyond the first surface 3151 , and the corresponding second end 3112 of the first electrode 3110 and the second end 3132 of the third electrode 3130 extend beyond the second surface 3152 .

[0269] According to one embodiment, the electrodes of device 3100 are substantially fixed relative to each other. For example, due to bending and other forces, the electrodes may allow some minor relative movement relative to each other. The dielectric 3150 and the electrodes may also be free to move relative to each other. For example, the dielectric 3150 may move relative to the first, second, and third electrodes parallel to axis 3160. Additionally or alternatively, the first, second, and third electrodes may move relative to the dielectric 3150. The dielectric 3150 may also take other forms, such as one or more segments or members.

[0270] According to possible embodiments, the dielectric 3150 can fill at least 50% of the distance between the first electrode 3120 and the second electrode 3120 (e.g., the distance perpendicular to the first length of the first electrode 3110). For example, the dielectric 3150 can fill at least 75% of the distance, at least 80% of the distance, or between at least 80% and 90% of the distance. However, the dielectric 3150 can fill any amount from 1% to 99% of the distance. The more the dielectric fills the gap between the electrodes, the greater the capacitance change per unit displacement, and thus the greater the force generated for a given bias between the electrodes. Some minimum gap should be maintained between the dielectric and the electrodes, which is subject to manufacturing constraints so that the dielectric and electrodes remain movable relative to each other.

[0271] According to one embodiment, a first capacitance exists between the first electrode 3110 and the second electrode 3120, and a second capacitance exists between the first electrode 3110 and the third electrode 3130. The capacitance between the electrodes is a function of their position relative to the dielectric 3150. For example, when the dielectric 3150 moves relative to the electrodes in a direction parallel to the axis 3160, the values ​​of the first capacitance and the second capacitance can change in opposite directions. Thus, for example, when the dielectric 3150 moves and causes the first capacitance to increase, the second capacitance can simultaneously decrease. However, the electrostatic force on the dielectric 3150 relative to the electrodes 3110, 3120, and 3130 can remain substantially unchanged relative to the displacement.

[0272] According to one embodiment, during operation of the MEMS device 3100, a voltage from a first voltage source V1 is applied between the first electrode 3110 and the second electrode 3120, thereby generating a relatively constant force F1. A voltage from a second voltage source V2 is applied between the first electrode 3110 and the third electrode 3130, thereby generating a relatively constant force F2. Forces F1 and F2 are opposite. If the structure is relatively symmetrical and the magnitudes of the voltage sources V1 and V2 are equal, the forces F1 and F2 are equal, thereby exerting a net zero force between the dielectric and the electrodes. The magnitudes of the voltages from the voltage sources V1 and V2 can be unequal to compensate for structural asymmetry or to intentionally generate a non-zero net force between the dielectric and the electrodes.

[0273] Go to Figure 32 , a dielectric motor MEMS device 3200 according to another embodiment will now be described. In this embodiment, the first electrode 3110 (from Figure 3 ) is performed by a first set of electrically interconnected electrodes 3210-x, the function of the second electrodes 3120 is performed by a second set of electrically interconnected electrodes 3220-x, and the function of the third electrodes 3130 is performed by a third set of electrically interconnected electrodes 3230-x. The second and third electrode sets 3220-x, 3230-x can be staggered on either side of the first electrode set 3210-x. This allows for greater capacitance and an increase in capacitance variation with displacement. Other configurations of conductive and dielectric elements, such as strips or rings, can also be used.

[0274] Go to Figure 33 , showing the combination Figure 32 The dual diaphragm assembly of the MEMS device 3200 is shown. The dual diaphragm assembly 3300 includes a diaphragm 3370 coupled to a first electrode set 3210-x and a third electrode set 3230-x. The diaphragm 3370 has a surface 3372 that is perpendicular to the axis 3360. The surface 3372 is substantially planar (e.g., planar, but with defects on the surface 3372, or slightly curved or uneven, while still allowing the surface 3372 to operate in a manner useful for the diaphragm). The diaphragm 3370 can be made of a sandwich. The dual diaphragm assembly 3300 also includes a second diaphragm 3374 located on a side of the dielectric 3350 opposite the first diaphragm 3370. The second diaphragm 3374 is coupled to the first electrode set 3210-x and the second electrode set 3220-x. The second diaphragm 3374 has a substantially flat surface 3376 and is oriented perpendicular to the axis 3360. The first diaphragm 3370 and the second diaphragm 3374 are spaced apart from the dielectric 3350 to allow relative movement between the electrodes connected to the first diaphragm and the second diaphragm and the dielectric 3350 .

[0275] In one embodiment, a sealed low-pressure region is defined between the diaphragms 3370 and 3374. This low-pressure region serves to reduce noise and damping of the assembly 3300. The first electrode set 3210-x (which is connected to both the first diaphragm 3370 and the second diaphragm 3374) helps prevent the diaphragms from collapsing onto the dielectric 3350. The low-pressure region can be substantially a vacuum (e.g., having a pressure of less than 1 Torr, less than 300 mTorr, or less than 100 mTorr). According to one embodiment, the dielectric 3350 is relatively thick and hard compared to the diaphragms 3370 and 3374, and remains relatively stationary when the diaphragms 3370 and 3374 deflect. The deflection of the diaphragms 3370 and 3374 causes the electrodes 3210-x, 3220-x, and 3230-x to move relative to the dielectric 3350.

[0276] Diaphragms 3370 and 3374 may be made of a dielectric material such as silicon nitride. However, other materials may be used. For example, one or more of diaphragm 3370, diaphragm 3374, and dielectric 3150 may be polyimide.

[0277] In one embodiment, if used as an acoustic sensor, the MEMS die 3300 operates as follows. Pressure (e.g., from sound waves) reaches at least one diaphragm, causing it to flex and move the electrodes attached to the diaphragm relative to the dielectric, thereby changing the corresponding capacitance between the various elements. This change in capacitance manifests as a change in one or more signals output from the MEMS die 3300. The one or more signals are read and interpreted by an IC and passed on to other external components.

[0278] Figure 34 Shown include from Figure 27 MEMS device 3400 has all the elements of MEMS device 2700, but the dual-diaphragm assembly 2701 is replaced by the dual-diaphragm dielectric motor assembly 3300. Other functions of MEMS device 3400 are similar to those of MEMS device 2700.

[0279] Figure 35 The MEMS device 3500 is shown to include Figure 28A The MEMS device 3500 is an embodiment of the present invention that has all the elements of the MEMS device 2800 except that the dual-diaphragm assembly 2701 is replaced by the dual-diaphragm dielectric motor assembly 3300. The other functions of the MEMS device 3500 are similar to those of the MEMS device 2800.

[0280] For the use of plural and / or singular terms in this article, those skilled in the art can convert from plural to singular and / or from singular to plural when appropriate according to the context and / or application. For clarity, various singular / plural permutations can be explicitly set forth in this article.

[0281] Unless otherwise indicated, use of the words "approximately," "about," "approximately," "substantially," etc., refers to plus or minus ten percent.

[0282] The foregoing description of exemplary embodiments has been presented for purposes of illustration and description. This description is not intended to be exhaustive or limited to the precise forms disclosed, but rather may be modified and varied in light of the above teachings or acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

1. A micro-electromechanical system (MEMS) die for a microphone, wherein the MEMS die is used as an acoustic transducer, the MEMS die comprising: piston; an electrode, the electrode facing the piston, wherein the capacitance between the piston and the electrode changes as the distance between the piston and the electrode changes; a resilient structure disposed between the piston and the electrode, wherein the resilient structure supports the piston, the resilient structure resists movement of the piston relative to the electrode, wherein a back volume is defined by the piston and the resilient structure, and the resilient structure prevents air from escaping the back volume; and A second elastic structure is disposed within the rear cavity volume, wherein the second elastic structure supports the piston.

2. The MEMS die of claim 1, wherein: The back volume is surrounded by the piston, the elastic structure and the electrode.

3. The MEMS die of claim 1 , wherein: The piston is a rigid body made of conductive material.

4. The MEMS die of claim 3, wherein: The conductive material is metal.

5. The MEMS die of claim 3, wherein: The conductive material is a doped semiconductor.

6. A sensor for a microphone, the sensor comprising: A micro-electromechanical system (MEMS) die is used as an acoustic transducer, and the MEMS die comprises: piston; an electrode, the electrode facing the piston, wherein a capacitance exists between the piston and the electrode; an elastic structure disposed between the piston and the electrode; and a second elastic structure, the second elastic structure being arranged in the rear cavity volume, in: The rear chamber volume is defined by the piston and the elastic structure, The resilient structure supports the piston and prevents air from leaving the rear volume, The second elastic structure supports the piston, and During operation of the sensor, the MEMS die outputs a signal based on a change in capacitance due to a change in distance between the piston and the electrode.

7. A micro-electromechanical system (MEMS) die, comprising: case; a first diaphragm disposed across the opening of the housing, wherein the housing and the first diaphragm define a back volume, and each point within the back volume is at a distance from a nearest surface that is less than a thermal boundary layer thickness; a second diaphragm disposed outside the rear cavity volume and facing the first diaphragm; a solid dielectric, the solid dielectric being disposed between the first diaphragm and the second diaphragm, the solid dielectric having a plurality of openings; a first electrode, a first end of the first electrode coupled to the first diaphragm and a second end of the first electrode coupled to the second diaphragm, the first electrode extending through an aperture of the plurality of apertures; a second electrode oriented longitudinally and parallel to the first electrode, a first end of the second electrode being attached to the second diaphragm and a second end of the second electrode being disposed within an aperture of the plurality of apertures; and A third electrode is longitudinally oriented and parallel to the first electrode and the second electrode, a first end of the third electrode being attached to the first diaphragm and a second end of the third electrode being disposed within an aperture of the plurality of apertures.

Images