Patent ID: 12240748

DETAILED DESCRIPTION

Described herein are various types of micro-electro-mechanical systems (MEMS) dies and sensors that incorporate such MEMS dies.

According to an embodiment, a MEMS die has a back volume in which every point within the back volume is no more than the width of one thermal boundary layer from a solid surface.

In an embodiment, a MEMS die includes a piston; an electrode facing the piston, wherein a 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 pleated wall) disposed between the piston and the electrode, wherein the resilient structure supports the piston and resists the movement of the piston with respect to the electrode. A back volume is bounded by the piston and the resilient structure and the resilient structure blocks air from leaving the back volume. The piston may be a rigid body made of a conductive material, such as metal or a doped semiconductor. The MEMS die may also include a second resilient structure, which provides further support to the piston and is disposed within the back volume. In some embodiments, the piston comprises a distinct layer of conductive material (e.g., an electrode that is distinct and made of a different material than the rest of the piston).

According to an embodiment, the MEMS die further includes a substrate that supports the resilient structure and the electrode. There are many possible configurations for the substrate. Examples include: (a) the substrate has pillars, with the electrode distributed among the pillars, (b) the substrate includes an electrically insulating layer in which the electrode is embedded, (c) the substrate has a plurality of channels (e.g., spaces between pillars in the substrate or a plurality of channels formed into rings) and the dimensions of the channels is such that any point within the channels is less than a thermal boundary layer thickness from a nearest surface.

There are also many possible configurations for the piston. Examples include: (a) the piston has pillars, (b) the piston has a plurality of channels (e.g., spaces between pillars in the substrate or a plurality of channels formed into rings) and the dimensions of the channels is such that any point within the channels is less than a thermal boundary layer thickness from a nearest surface.

In an embodiment, the MEMS die also has support walls and external conductors, in which each conductor is attached to the piston at one end and to a support wall at the other end. The MEMS die may also include a vent (e.g., in the piston or in the resilient structure). The vent is configured to permit pressure equalization between the back volume and a region (e.g., a volume) external to the MEMS die at non-acoustic frequencies.

The MEMS die may be part of a sensor (e.g., an acoustic sensor) in which the MEMS die outputs a signal that is based on a change in a capacitance between the piston and the electrode resulting from a change in the distance between the piston and the electrode. In various embodiments, the sensor includes and 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 the signal. In other embodiments, the electrode is electrically connected to the bias voltage source and the piston outputs the 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 the signal. The sensor may include a base and a can attached to the base, in which the MEMS die is disposed within the can and sound enters the can and causes the piston to move, thereby causing a change in distance between the piston and the electrode.

According to an embodiment, a sensor includes a MEMS die that has an enclosure; a diaphragm disposed across an opening of the enclosure, in which the diaphragm includes a first electrode and the enclosure and diaphragm enclose a back volume; a second electrode disposed outside of the back volume and facing the diaphragm. Every point within the back volume is less than a thermal boundary layer thickness from a nearest surface. During operation of the sensor, the MEMS die outputs a signal that is based on a 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. In some embodiments, the MEMS die also includes a second diaphragm facing the second electrode, in which the second diaphragm has a third electrode and the second electrode is disposed between the first diaphragm and the second diaphragm. During operation, the MEMS die outputs a second signal that is based on a 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. Posts connecting the first diaphragm and the second diaphragm may extend through the second electrode. The sensor may also include a base having a port and a can attached to the base, where the MEMS die is disposed within the can and wherein sound enters through the port and causes the diaphragm to move during operation of the sensor.

In some embodiments, the first diaphragm and the second diaphragm define a sealed region in which the pressure is lower than an atmospheric pressure.

In various embodiments, the enclosure has a plurality of pillars, and there is a plurality of channels defined between the pillars. In other embodiments, ring-shaped channels are formed into the enclosure.

In an embodiment, a MEMS die includes an enclosure; a first diaphragm disposed across an opening of the enclosure, in which the enclosure and the first diaphragm define a back volume and every point within the back volume is less than a thermal boundary layer thickness from a nearest surface; a second diaphragm disposed outside of the back volume and facing the first diaphragm; a solid dielectric disposed between the first diaphragm and the second diaphragm, the solid dielectric having apertures; a first electrode oriented lengthwise along and parallel to an axis, the first electrode having a first end coupled to the first diaphragm and a second end coupled to the second diaphragm, the first electrode extending through an aperture; a second electrode oriented lengthwise along and parallel to the axis, the second electrode having a first end attached to the second diaphragm and a second end disposed within an aperture; and a third electrode oriented lengthwise along and parallel to the axis, the third electrode having a first end attached to the first diaphragm and a second end disposed within an aperture. The second end of the second electrode and the second end of the third electrode may be disposed within the same aperture.

According to an embodiment, (a) the first electrode is one of a first set of electrodes, each of which has a first end coupled to the first diaphragm, a second end coupled to the second diaphragm, and which extends through an aperture of the plurality of apertures, (b) the second electrode is one of a second set of electrodes, each of which has a first end attached to the second diaphragm and a second end disposed within an aperture, and (c) the third electrode is one of a third set of electrodes, each of which has a first end attached to the first diaphragm and a second end disposed within an aperture.

Pressure microphones typically include a diaphragm that responds to the pressure difference on either side of it. Turning toFIG.1, in an omnidirectional microphone10one side of the diaphragm12is coupled to an outside environment14and the pressure on that side of the diaphragm12is the sum of atmospheric pressure (Patm) and the desired acoustic signal (Pac). The pressure on the other side of the diaphragm12is provided by a back volume16which is acoustically isolated from the outside environment14yet maintains atmospheric pressure in it through a small acoustic leak15.

A small-signal lumped element model for the omnidirectional microphone10ofFIG.1is shown inFIG.2. The compliance of the diaphragm12and the back volume16are represented by capacitors CDand CBV, respectively. The resistance of the acoustic leak15is represented by RLeak. The acoustic signal is represented as an AC signal source. The pressure across the diaphragm12, PD, causes the diaphragm12to move. Notice that the atmospheric pressure, which is present on both sides of the diaphragm12, is no factor in the diaphragm motion and is not included in this small signal model. Notice also, that when the back volume compliance (CBV) is large compared to that of the diaphragm (CD), most of the acoustic pressure is present across the diaphragm12. If the back volume compliance (CBV) is small compared to that of the diaphragm (CD), very little of the acoustic pressure is present across the diaphragm12. The acoustic leak resistance (RLeak) acts in conjunction with the parallel combination of the back volume compliance (CBV) and the diaphragm compliance (CD) to form a high pass filter. Thus only acoustic pressure signals above a certain frequency will be present across the diaphragm12.

The acoustic leak, being a real resistance, generates thermal noise. This noise appears as a noise pressure across the diaphragm12. But the parallel combination of the back volume compliance (CBV) and the diaphragm compliance (CD) 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 Boltzmann's constant, T is absolute temperature in Kelvin, and C is the parallel combination of the two compliances (CDand CBV). Thus for a particular low frequency cut-off, the noise due to the acoustic leak generally increases with smaller microphones. The only option to reduce this noise is to lower the cut-off frequency for smaller microphones. Traditional A-Weighting depreciates the significance of the low frequency leak noise even for very small microphones with sufficiently low cut-off frequencies.

This has been the traditional view of microphones above a certain size. However, for small microphones another factor becomes significant. As pointed out by Kuntzman et al. (hereafter “Kuntzman”), “Thermal Boundary Layer Limitations on the Performance of Micromachined Microphones,” J. Acoust. Soc. Am. 144(5), 2018, which is incorporated by reference herein, the thermal boundary layer is that factor. Kuntzman discloses the effects of acoustic compression and expansion of air within the back volume of a microphone assembly as a function of the dimensions of the microphone assembly enclosure (e.g., as a function of the back volume of the microphone assembly). Kuntzman states: “for cases in which the thermal boundary layer becomes sufficiently large relative to the enclosure dimensions, which occurs for small enclosures and at low frequencies, compression and expansion of the air within the enclosure transitions from adiabatic to isothermal and a correction to the adiabatic cavity impedance becomes necessary. Heat transfer at the enclosure walls dissipates energy from the system and results in acoustic damping, which contributes thermal-acoustic noise according to the fluctuation-dissipation theorem.” Kuntzman further states: “the acoustic damping resulting from thermal relaxation losses in the enclosure can be a significant noise contributor, particularly for small enclosure sizes for which the losses are most prominent.” Stated generally, Kuntzman teaches that it is desirable to increase the back volume for a microphone assembly to reduce thermal-acoustic noise.

The effects of thermal-acoustic noise are most significant at low frequencies, as indicated by Thompson et al. (hereafter “Thompson”), “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 by reference herein. Thompson states: “the change in microphone sensitivity from thermal effects is caused by the change in the compliance of the [microphone] enclosure at low frequencies . . . the thermal resistance could possibly affect the internal noise of the microphone if the noise from this resistance were comparable to or greater than the other thermal noise sources in the microphone.” The thermal-acoustic noise contribution is expected to be greatest for MEMS microphones with small enclosure volumes and low frequencies, where the distances between solid surfaces are 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 may be determined approximately as

δt=2⁢κω⁢ρ0⁢Cp
where ω is the operating angular frequency of the microphone, and where κ is the thermal conductivity, ρ0is the density, and Cpis the specific heat at constant pressure of the gas inside the microphone assembly (e.g., within the back volume of the microphone assembly). The relationship above confirms the dependency between the thermal boundary layer thickness and the operating frequency of the microphone.

The materials that comprise a microphone, metals and plastics for instance, all have much larger thermal capacities than air. Thus at each surface of the back volume, there is heat exchange with the boundary materials and these surfaces are essentially isothermal. The heat exchange is frequency dependent and contributes to the impedance of the back volume. In essence, when the air in the back volume is compressed, its temperature rises. At a given frequency, the portion of the air within a diffusion length of a boundary gives up this heat to the boundary material. When the air in the back volume rarifies, the temperature of the air drops but the portion of the air within a diffusion length of a boundary gains heat from the boundary material.

FIG.3depicts the thermal boundary layer18for the omnidirectional microphone10ofFIG.1. In this figure, the thermal boundary layer18is shaded to depict how the thickness20of the thermal boundary layer18changes with frequency. Darker shading corresponds to the thickness20at higher frequency. Thus, at high frequencies, the thermal boundary layer18is quite thin while at low frequencies, the thermal boundary layer18is thicker. The impact of the thermal boundary layer18on the model is shown inFIG.4. The compliance of the back volume is now replaced with a complex impedance ZBV. The real part of the complex impedance depends on frequency and microphone size and thus a noise contribution is made to the pressure across the diaphragm. The analysis of this noise effect is complex but addressed in Kuntzman. In essence, as the microphone gets smaller, the thermal boundary layer expands to consume more of the total back volume and when integrated, the total noise effect on the pressure across the diaphragm goes up as the microphone size goes down. This is another expected kT/C effect.

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

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

As used herein, the phrase “enclosed volume” or “enclosed back volume” refers to a volume (such as a back volume) that is substantially enclosed but may not be fully enclosed. For example, the enclosed volume may refer to a volume that is fluidly connected with an environment surrounding the MEMS die via a pierce or opening in a diaphragm, in a piston, or in a resilient structure.

FIG.5andFIG.6show a side cross-sectional view of a MEMS die100for a microphone. The MEMS die100is configured to use a capacitive acoustic transduction method to generate an electrical signal in response to acoustic disturbances incident on the MEMS die100. In other embodiments, the MEMS die100may be use another type of transduction, such as piezoelectric transduction, piezoresistive transduction, or optical transduction. The MEMS die100includes a substrate102, an electrode104, and a movable diaphragm106. The movable diaphragm106may be or may include an electrode and may be referred to as a “first electrode,” while the electrode104may be referred as a “second electrode.” The diaphragm106and the electrode104, along with the gap therebetween (which includes insulative material such as air), form a capacitive element. The electrode104may sometimes be referred to as a counter electrode. The substrate102supports the electrode104and the diaphragm106. As shown inFIG.5, the electrode104is coupled directly to the substrate102along an entire lower surface108of the electrode104. The substrate102is large relative to the diaphragm106(and relative to the electrode104), which ensures that the electrode104is rigidly supported. In particular, a combined thickness109of the substrate102and the electrode104is an order of magnitude greater than a thickness112of the diaphragm106. In other embodiments, the relative thickness between the substrate102and the diaphragm106may be different.

According to an embodiment, the electrode104is deposited directly onto a first surface (e.g., an upper surface) of the substrate102. In some embodiments the electrode104is deposited onto or otherwise connected to an insulator114. The insulator114may be made from silicon nitride or another dielectric material. The electrode104may be made from polycrystalline silicon or another suitable conductor. In the embodiment shown inFIG.5, the electrode104is “sandwiched” or otherwise disposed between the substrate102and the insulator114. The electrode104is at least partially embedded within a lower surface of the insulator114and is directly coupled to the substrate102. In other embodiments, the position of the electrode104may be different (e.g., the electrode104may be embedded within or formed onto an upper surface of the insulator114). In yet other embodiments, the electrode104may extend to an outer perimeter of the volume between the electrode104and the diaphragm106(e.g., the diameter of the electrode104may be approximately the same as the diameter of the diaphragm106.

In an embodiment, the diaphragm106is oriented parallel (or substantially parallel) to the electrode104(or insulator114, whichever is on top) and is spaced apart from the electrode104to form a gap. In various embodiments, the gap represents a height118of a cylindrically-shaped cavity (e.g., a cylindrically-shaped volume between the insulator114and the diaphragm106, or a cylindrically-shaped volume between the electrode104and the diaphragm106in those embodiments where the electrode104is on top of the insulator114). The volume between the electrode104and the diaphragm106forms an entire back volume103for the MEMS die100(and, to the extent that the MEMS die100can be considered to be a microphone, the entire back volume of the microphone) as will be further described. The diaphragm106is spaced apart from the electrode104by at least an intermediate layer120. A first side122of the intermediate layer120is coupled to the insulator114, which, in turn, is coupled to electrode104. A second side124of the intermediate layer120is coupled to the diaphragm106along at least a portion of the perimeter of the diaphragm106. A height126of the intermediate layer120(e.g., an axial height of the intermediate layer120parallel to a central axis128of the MEMS die100), plus a height/thickness of the insulator114between the electrode104and the intermediate layer120, is approximately equal to a distance between the diaphragm106and the electrode104(e.g., the height118). In other embodiments, the distance between the diaphragm106and the electrode104is approximately equal to the height of the intermediate layer120. In various embodiments, the intermediate layer120includes a sacrificial layer (e.g., an oxide layer, a phosphosilicate glass (PSG) layer, a nitride layer, or any other suitable material) that is deposited or otherwise formed onto the electrode104. In some embodiments, the intermediate layer120may be made from silicon oxide or other materials that can be etched without affecting the substrate102, the electrode104, or the diaphragm106.

In an embodiment, the diaphragm106is made from polycrystalline silicon or another conductive material. In other embodiments, the diaphragm106includes both an insulating layer and a conductive layer. As shown inFIG.6, a first side132of the diaphragm106faces the back volume103. A second side134of the diaphragm106, opposing the first side132, faces toward a front volume105for the microphone assembly. Sound energy131(e.g., sound waves, acoustic disturbances, etc.) incident on the second side134diaphragm106from the front volume105causes the diaphragm106to move toward or away from the electrode104. The change in distance between the electrode104and the diaphragm106(e.g., the change in the height118) results in a corresponding change in capacitance. An electrical signal representative of the change in capacitance may be generated and transmitted to portions of a microphone assembly in which the MEMS die100is incorporated, such as to an integrated circuit (not shown), for processing.

According to an embodiment, the electrode104is a solid, unperforated structure, such that the volume between the insulator114and the diaphragm106forms an entire back volume103for the MEMS die100. In contrast, for MEMS dies that include a perforated counter electrode (e.g., a backplate with multiple through-hole openings), the back volume includes both the volume between the structure opposing the diaphragm (the insulator114and, if exposed, the electrode104) and the diaphragm106as well as any additional fluid (e.g., air) volume on an opposing side of the opposing structure to which the space between the electrode104and the diaphragm106is fluidly connected.

Embodiments of the present disclosure may also include other types of MEMS dies. For example, the MEMS die may be piezoelectric, piezoresistive, or optically transductive.FIG.7shows an embodiment of a piezoelectric MEMS die175. The piezoelectric MEMS die175includes a substrate177and a diaphragm179coupled to the substrate177and spaced apart from the substrate177. The piezoelectric MEMS die175also includes a piezoelectric layer181connected to the diaphragm179. As shown inFIG.7, the piezoelectric layer181may be connected (e.g., deposited onto or otherwise coupled) to a lower surface183of the diaphragm179. In other embodiments, as shown inFIG.8, the piezoelectric layer181may be connected to an upper surface185of the diaphragm179. In either case, the volume between the substrate177and the diaphragm179forms an entire back volume187for the piezoelectric MEMS die175.

FIG.9shows a plot of the A-weighted acoustic noise200in the audio frequency band (e.g., range) of 20 Hz to 20 kHz (hereafter “acoustic noise”) of a MEMS die as a function of the size of the back volume of the MEMS die. In particular,FIG.9shows the simulated relationship between the acoustic noise200and the back volume for a MEMS die with a counter electrode and diaphragm of fixed size (e.g., for a diaphragm with fixed diameter), in which the MEMS die is being used as a transducer. In the simulation, the back volume was varied within a range between approximately 0.0006 mm3and 10 mm3by changing the size of the gap (e.g., height118ofFIG.5) between 0.5 um and 8 mm. As shown inFIG.9, the acoustic noise200increases with decreasing back volume (e.g., decrease in the height118ofFIG.5) within a range between approximately 9 mm3and 0.1 mm3. The trend in acoustic noise200between approximately 9 mm3and 0.1 mm3is consistent with the discussion provided in both Kuntzman and Thompson, which teach that the acoustic noise increases as the size of the back volume decreases. Surprisingly, a reversal in the trend is observed (for the simulated diaphragm diameter) below a back volume of approximately 0.1 mm3(in the size range of the MEMS die). As shown inFIG.9, at a back volume of approximately 0.0006 mm3, the acoustic noise200has returned to levels that are approximately equal to those achieved at 4 mm3(e.g., a reduction in total back volume by a factor of approximately 7500).

FIG.10shows a plot of the relationship between the thermal boundary layer thickness300and the operating frequency of a MEMS die being used as a transducer (e.g., the MEMS transducer modeled inFIG.9, and assuming air is provided within the volume between the counter electrode and the diaphragm). The thermal boundary layer thickness300is shown to decrease with increasing operating frequency. This dependency is shown graphically inFIG.10over a range of operating frequencies within the audio frequency band of the MEMS die (e.g., within a human audible frequency range between approximately 20 Hz to 20 kHz).

As shown inFIG.10, when the size of the gap (e.g., the height) between the counter electrode and the diaphragm is large (e.g., when the gap is greater than 500 μm), the thermal boundary layer thickness300is less than the size of the gap over a majority of the audio frequency band of the MEMS die. As the gap decreases, the thermal boundary layer thickness300becomes equal to or greater than the size of the gap over a larger proportion of the audio frequency band. It is within this range of gap sizes that the thermal-acoustic noise contribution is greatest and the overall SNR of the MEMS die is reduced (e.g., the MEMS die being used as a transducer).

The approximate range of gap sizes that correspond with improved SNR performance (e.g., corresponding with back volumes fromFIG.9for which the reversal in the trend of acoustic noise is observed) is identified by horizontal lines302toward the bottom ofFIG.10. As shown, the size of the gap (e.g., height118shown inFIG.5) is less than approximately two times the boundary layer thickness300within the back volume103over a majority of the audio frequency band of the MEMS die100(e.g., between 20 Hz and 20 kHz). In other words, the back volume103is dimensioned such that the distance between any point or location within the back volume103and the nearest solid surface contacting the back volume103is less than a single thermal boundary layer thickness300. For example, as shown inFIG.6, a point119approximately half way in between the diaphragm106and the insulator114is spaced less than one thermal boundary layer thickness300from a back volume facing surface of both the diaphragm106and the insulator114(the solid surfaces of the back volume that are closest to point119).

Based on this data (and data fromFIG.9), two different thermal regimes and mechanisms appear to exist depending on whether the size of the gap (e.g., the height118) is 1) greater than two times the thermal boundary layer thickness over the majority of the audio frequency band or 2) less than two times the thermal boundary layer thickness over the majority of the audio frequency band. The fact that acoustic noise decreases at very small gap heights (less than two orders of magnitude less than most existing microphone assemblies) is an unforeseen benefit that has not been previously identified.

FIG.11shows the back volume damping (hereafter “damping”) as a function of frequency of a MEMS die operating as a transducer within these two different thermal regimes. The upper set of curves400show the damping for MEMS dies having a gap size that is greater than the thermal boundary layer thickness. The direction of decreasing gap size for the curves400is indicated by dashed arrow402. As shown inFIG.11, as the size of the gap decreases, the damping (and related thermal noise) increases (e.g., the total noise over the audio frequency band of the MEMS die increases). The lower set of curves404show the damping response for MEMS dies where the size of the gap is less than the thermal boundary layer thickness (e.g., less than two times the thermal boundary layer thickness, similar to the MEMS die100ofFIG.5andFIG.6). The direction of decreasing gap size for the curves404inFIG.11is indicated by dashed arrow406. The damping (and related thermal noise) is shown to decrease as the size of the gap decreases. Additionally, unlike the trend exhibited by the upper set of curves400, the lower set of curves404exhibits an approximately flat damping response as a function of frequency. Such properties may be particularly advantageous for applications such as beam forming for signal processing, and other applications where the sensitivity of the MEMS die is reduced at low frequencies.

FIG.12shows the acoustic SNR as a function of the gap size for three different values of the surface area of the diaphragm (e.g., the diameter of the diaphragm, and correspondingly, the diameter of the back volume) for a microphone assembly. Curves of acoustic SNR are provided over a range of different surface areas for 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 in SNR with surface area is opposite to the trend in SNR with the size of the gap (e.g., the height between the counter electrode and the diaphragm), the effect of the gap has been observed to dominate.

The results shown inFIGS.9-12were simulated assuming piston-like diaphragm displacement (e.g., assuming that the diaphragm does not curve or bow, and that all points along the surface of the diaphragm move by an equal amount). In reality, the diaphragm106(seeFIG.5) will not displace uniformly in a piston-like motion but will instead bow or curve under the bias voltage applied to the MEMS die100(and further as a result of sound pressure incident on the diaphragm106). The movement of the diaphragm106will therefore move the air within the gap in both an axial direction (e.g., vertically up and down as shown inFIG.5) and a radial direction (e.g., horizontally left and right as shown inFIG.5). The radial velocity component of air within the back volume103will result in viscous losses, which will increase acoustic noise for the MEMS die above the values shown inFIG.12.

FIG.13shows a plot of the acoustic SNR as a function of the size of the gap between the counter electrode and the diaphragm (the vertical spacing between the counter electrode and the diaphragm). Curve500shows the acoustic SNR for a MEMS die that is modeled assuming a piston-like diaphragm motion. Curve502shows the acoustic SNR for a MEMS die that is modeled assuming that the diaphragm bends (e.g., curves) with the application of a bias voltage to the MEMS die. As shown inFIG.13, the effect of actual diaphragm bending and movement is most prominent at small gap sizes (e.g., below 5 μm in this case). At gap sizes between 5 μm and 11 μm, viscous effects associated with diaphragm movement are significantly reduced. One way to counteract the effects of diaphragm displacement/movement, as shown inFIG.13, is to constrain the size of the gap to within a range between approximately 5 μm and 12 μm, or another suitable range depending on the geometry of the back volume. Alternatively, or in combination, the bias voltage applied to the MEMS die may be adjusted (e.g., increased) to increase the sensitivity of the microphone assembly to at least partially offset the effects of the additional acoustic noise resulting from viscous losses.

The geometry of the counter electrode may also be adjusted to reduce the radial velocity component of air within the back volume resulting from non-piston-like diaphragm movement. For example,FIG.14shows a MEMS die600that includes a curved electrode604. In particular, an upper surface632(e.g., first surface, back volume facing surface, etc.) of the insulator614is shaped to approximately match the curvature of the diaphragm606under application of a bias voltage such that, during operation, the distance between the diaphragm606and the electrode604(which is also curved in this embodiment) is approximately equal throughout the back volume611(e.g., in a lateral direction, away from a central axis of the MEMS die). To achieve this, the electrode604and the diaphragm606are not parallel in a resting situation (e.g., when the bias voltage is removed). As shown inFIG.14, the electrode604is deposited or otherwise formed onto a recessed portion636of a substrate602of the MEMS die600. The curvature of the electrode604is a function of the bias voltage applied to the MEMS die600, the dimensions of the back volume611, and the thickness of the diaphragm606.

Returning toFIG.6, the MEMS die100is shown to include an opening or pierce138that extends through the diaphragm106(e.g., from the first side132of the diaphragm106to the second side134of the diaphragm106). The pierce138is disposed at a central position on the diaphragm106in coaxial arrangement relative to the central axis128of the MEMS die100. The pierce138is a circular through-hole in the diaphragm106. In other embodiments, the size, shape, location, and or number of openings in the diaphragm106may be different.

FIG.15shows the acoustic SNR of a MEMS die (configured like the MEMS die100) as a function of the size of the gap for a range of different diameters for the pierce (e.g., the pierce138). As shown inFIG.15, the pierce introduces acoustic noise into the MEMS die100(see alsoFIG.5), particularly at small gap sizes (e.g., below 5 μm). The rate of change (e.g., increase) of the acoustic noise also increases with the diameter of the pierce. In the MEMS die100ofFIG.5, the diameter140of the pierce138is within a range between approximately 0.25 μm and 2 μm to minimize the effects of the pierce138on the overall acoustic SNR. It should be appreciated that the optimal range of pierce138diameters will vary depending on the thickness of the diaphragm106and the geometry of the back volume103.

The sensitivity of the MEMS die100may also be improved by increasing the compliance of air in the back volume103(e.g., by reducing the stiffness of the air contained within the back volume). In an embodiment, this is accomplished by providing channels in the MEMS substrate, such that every point within the channels is no further away from a solid surface than the thickness of a single thermal boundary layer. Turning toFIG.16andFIG.17, a MEMS die700configured accordingly is shown. The MEMS die700includes an electrode704and a substrate702that includes a plurality of channels742formed into the electrode704and substrate702. More specifically, the MEMS die700is structured with the channels742formed with dimensions such that any point within the channels742is less than a single thermal boundary layer thickness from a nearest boundary surface. In the embodiment ofFIG.16, each one of the plurality of channels742extends away from the diaphragm706in a substantially perpendicular orientation relative to the diaphragm706(e.g., parallel to a central axis of the MEMS die700). The channels742extend through the electrode704. Among other benefits, the channels742increase the overall compliance of air within the MEMS die700(e.g., by adding air volume away from the space between the electrode704and the diaphragm706) without fully penetrating through the substrate702.

The channels742in the substrate702are sized to reduce thermal-acoustic noise within the MEMS die700. Specifically, a width744(e.g., diameter) of each one of the plurality of channels742is less than two times the thermal boundary layer thickness within the back volume over a majority of an audio frequency band of the MEMS die700, such that the distance between any point or location within the back volume is within a single thermal boundary layer thickness from a nearest solid surface of the substrate or the diaphragm over a majority of the audio frequency band. The depth745of each of the channels742is approximately equal to the size of the gap, shown as height718(e.g., the distance between the electrode704and the diaphragm706). It will be appreciated that the geometry of the channels742may be different in various embodiments. For instance, in other embodiments the depth745may be different from the size of the gap.

In an embodiment, channels in a MEMS substrate may be defined by pillars. An example of such a MEMS die is shown inFIG.18. The MEMS die, generally labelled750, includes an electrode754and a substrate752forming a cavity756(e.g., back volume) in which a plurality of pillars758are disposed. The pillars758are cylinders that extend upwardly from a lower surface of the cavity756in a substantially perpendicular orientation relative to the lower surface (the pillars758extend toward the diaphragm706). In other embodiments, the shape of the pillars758may be different. The pillars758may be formed into a substrate752for the MEMS die750. A conductive layer715is deposited onto or otherwise connected to an upper surface of each one of the pillars758. Together, the conductive layers715form a single electrode. A lateral distance between adjacent pillars758(e.g., a radial distance relative to a central axis of each of the pillars758) is less than two times the thermal boundary layer thickness over a majority of an audio frequency band of the MEMS die750. In other embodiments, the geometry of the channels (FIG.16andFIG.17) and pillars (FIG.18) may be different. In some embodiments, a porous silicon substrate may be used in lieu of channels or pillars. Among other benefits, using a porous silicon substrate increases the effective compliance of the air within the back volume, without requiring additional manufacturing operations to form channels, pillars, or other geometry into the substrate.

Turning toFIG.19, an example of a MEMS die that uses porous silicon in accordance with an embodiment will now be described. The MEMS die, generally labeled770, includes a substrate772. Being formed of silicon, the substrate772can be doped to make it conductive so that the surface of a porous region774is effectively the counter electrode for a capacitive transducer. The size of the pores776is much less than a single thermal boundary layer thickness and yet allows air flow in all directions. The percentage of open volume in the porous region774can be controlled by well-known electrochemical processes and can be made fairly large. The gap size, shown as height778, between the upper surface of the porous region774(e.g., the counter electrode) and the diaphragm780still must be less than two thermal boundary layer thicknesses, but in this embodiment the gap size does not dominate the size of the back volume782and thus the sensitivity of the MEMS die770. Alternatively, a sintered material could be used instead of porous silicon with characteristic pore sizes as described above.

Among other benefits, a reduction in the required back volume of a MEMS die as set forth in the above-discussed embodiments allows the overall footprint (e.g., package size, etc.) of a microphone assembly that uses the MEMS die to be substantially reduced. Moreover, because the counter electrode is a solid, unperforated structure, the MEMS die may be integrated with other components of the microphone assembly to further reduce the package size of the microphone assembly. For example,FIG.20shows monolithic integration of a MEMS die800with an integrated circuit (IC)802. The IC802may be an application specific integrated circuit (ASIC). Alternatively, the IC802may include another type of semiconductor die integrating various analog, analog-to-digital, and/or digital circuits. As shown inFIG.20, the IC802forms a substrate for the MEMS die800. The MEMS die800is integrally formed on the IC802as a single unitary structure. An electrode804of the MEMS die800is directly coupled to IC802along an entire lower surface808of the electrode804.

According to an embodiment, the geometry of the electrode804may be the same or similar to the geometry of the electrode104described with reference toFIG.5. As shown inFIG.20, the electrode804is directly coupled to the IC802(e.g., formed onto an upper surface of the IC802). The IC802includes a substrate810and an upper portion812coupled to a first surface (e.g., an upper surface, etc.) of the substrate810. The IC802additionally includes a plurality of transistors813embedded in the upper surface of the substrate810, between the substrate810and the upper portion812. The upper portion812is structured to electrically couple (e.g., connect, etc.) the electrode804to the IC802and/or to other parts of the microphone assembly (not shown). In particular, the upper portion812includes a plurality of metal layers814embedded within the upper portion812. The metal layers814electrically connect the electrode804to a contact disposed at an outer surface of the upper portion812(e.g., to an outer surface of the combined MEMS die800and IC802).

According to an embodiment, as shown inFIG.21, the combination of the MEMS die800and the IC802die is configured to fit within a microphone assembly, shown as assembly900. As shown inFIG.21, the assembly900includes a housing including a base902, a cover904(e.g., a housing lid), and a sound port906. In some embodiments, the base902is a printed circuit board. The cover904is coupled to the base902(e.g., the cover904may be mounted onto a peripheral edge of the base902). Together, the cover904and the base902form an enclosed volume for the assembly900(e.g., a front volume910of the MEMS die800). As shown inFIG.21, the sound port906is disposed on the cover904and is structured to convey sound waves to the MEMS die800located within the enclosed volume. Alternatively, the sound port906may be disposed on the base902. The sound waves (e.g., sound pressure, etc.) move the diaphragm806of the MEMS die800, which changes the size of the gap (e.g., the height818) between the diaphragm806and the electrode804. The volume between the electrode804and the diaphragm806forms an entire back volume911for the MEMS die800, which, advantageously, reduces the overall footprint of the microphone assembly900, without limiting the acoustic SNR that can be achieved.

As shown inFIG.21, the substrate810is coupled to a first surface of the base902within the enclosed volume908. 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.), where one, two, three or more assemblies may be integrated for picking-up and processing various types of acoustic signals such as speech and music.

In the embodiment ofFIG.21, the MEMS die800is configured to generate an electrical signal (e.g., a voltage) at an output in response to acoustic activity incident on the sound port906. As shown inFIG.21, the output includes a pad or terminal of the MEMS die800that is electrically connected to the electrical circuit via one or more bonding wires912. The assembly900may further include electrical contacts disposed on a surface of the base902outside of the cover904. The contacts may be electrically coupled to the electrical circuit (e.g. via bonding wires or electrical traces embedded within the base902) and may be configured to electrically connect the microphone assembly900to one of a variety of host devices.

The arrangement of components for the microphone assembly embodiment ofFIG.21should not be considered limiting. Many alternatives are possible without departing from the inventive concepts disclosed herein. For example,FIG.22shows another embodiment of a microphone assembly1000that includes a MEMS die1100that is flip-chip bonded to a base1002of the microphone assembly1000. The MEMS die1100is separated from the base1002(and electrically connected to the base1002) by balls of solder1003. The MEMS die1100is arranged to receive sound energy through a sound port1006disposed centrally within the base1002. The MEMS die1100is suspended within a cavity formed between the base1002and a cover1004of the microphone assembly1000.

FIG.23shows another embodiment of a microphone assembly1200that is similar to the microphone assembly1000ofFIG.22, but where the cover has been replaced by an encapsulant1201that surrounds the MEMS die1300. Among other benefits, the encapsulant1201insulates the MEMS die1300and helps to support the MEMS die1300in position above the base1202of the microphone assembly1200. The encapsulant may include a curable epoxy or any other suitable material.

One potential problem with using a diaphragm in a capacitive MEMS sensor is that the dynamic movement of the diaphragm itself results in a lateral velocity gradient and viscosity-induced losses. Any initial diaphragm deflection due to, for example, applied bias voltage accentuates the viscous losses. According to an embodiment of the disclosure, a capacitive MEMS sensor uses a piston (e.g., a rigid piece of silicon) in place of a diaphragm. In an embodiment, the piston is supported by a resilient structure (e.g., a soft rubber seal, a gasket (made, for example, of PDMS), or a bellowed wall).

In an embodiment, the resilient structure provides a seal that prevents the lateral movement of air, which reduces noise.

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

Turning toFIG.24A, a MEMS die that uses a piston instead of a diaphragm according to an embodiment will now be described. The MEMS die2400includes a piston2402that 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 electrode2404facing the piston2402, and a resilient structure2406that supports the piston2402on the electrode2404. A back volume2408is bounded on top by the piston2402, bounded on the bottom by the electrode2404, and bounded on all sides by the resilient structure2406. Together, the piston2402, electrode2404, and the resilient structure2406enclose a back volume2408. In some embodiments, the electrode2404is part of a substrate that also includes insulative material that supports the electrode2404. The resilient structure2406prevents air or other gas from leaving the back volume2408(e.g., blocks the air or other gas from travelling in direction radially outward from a central portion of the back volume2408). Put another way, the resilient structure2406seals the back volume2408to prevent lateral velocity gradients in the captured air volume. Possible implementations of the resilient structure2406include a gasket made of polydimethylsiloxane (PDMS) or rubber. Another possible implementation of the resilient structure2406is a pleated wall (made of, for example, PDMS, rubber, or made of more rigid materials such as silicon, silicon nitride, or aluminum). An example of a suitable structure having a pleated wall is a flexible bellow seal. Yet another possible implementation of the resilient structure2406is a thin diaphragm at the perimeter.

According to an embodiment, the piston2402has a vent2411that allows pressure between the back volume2408and the area external to the MEMS die2400to be equalized when the piston2402is at rest or is moving up and down relative to the electrode2404at non-acoustic frequencies. The vent2411is a hole that is configured to permit pressure equalization between the back volume and a region (e.g., a volume) external to the MEMS die at frequencies below the audio band. For example, the vent2411may have a geometry and dimensions that result in an impedance that the vent2411has no impact on the flow of air at non-audio frequencies but which blocks flow of air at acoustic frequencies. The vent2411need not be located in the piston2402, but may be anywhere (e.g., in the resilient structure2406) to facilitate pressure equalization.

In the embodiment depicted inFIG.24A, the piston2402and the electrode2404form a capacitive element, with the piston2402serving as a first electrode of the capacitive element, the electrode2404serving as a second electrode of the capacitive element, and the air or other gas in the back volume2408acting as a dielectric. During operation, the piston2402and the resilient structure2406behave like a classic spring-mass system. The capacitance between the piston2402and the electrode2404changes as the distance between them changes (i.e., as distance increases or decreases). The resilient structure2406resists the movement of the piston2402towards the electrode (e.g., in response to incoming pressure waves, such as those produced by sound). In particular, as the piston2402moves towards the electrode2404, the resilient structure2406compresses, builds up potential energy, and (when the external force pushing the piston2402decreases sufficiently) pushes the piston2402away from electrode2404. Further MEMS die embodiments with a piston-spring structure will be described with respect toFIG.24B,FIG.25A,FIG.25C,FIG.25D,FIG.26A,FIG.26D,FIG.26E, andFIG.26F. It is to be understood, however, that the principles and variations described in conjunction withFIG.24A(including the inclusion of a vent) may also be applied to these further MEMS die embodiments.

In an embodiment, when the MEMS die2400is integrated into a microphone, the piston2402is electrically connected to a bias voltage source2409and the electrode2404is electrically connected to an integrated circuit2410(e.g., to an amplifier input thereof). Alternatively, the piston2402may be electrically connected to the integrated circuit2410and the electrode2404may be electrically connected to the bias voltage source2409.

Referring toFIG.24B, a variant of the MEMS die2400configured according to an embodiment is shown. The MEMS die2450includes a piston2403that has an insulative portion2403aand an electrode2403b. In this embodiment, it is the electrode2403bthat functions as the first electrode in the capacitive element (with the electrode2404acting as the second electrode).

In an embodiment, channels2412are formed into the electrode2404ofFIG.24AandFIG.24B. In an embodiment, the dimensions of the channels2412are such that any point within the channels2412is less than a single thermal boundary layer thickness from a nearest surface. Each of the channels2412extends away from the piston2402in a substantially perpendicular orientation relative to the piston2402(e.g., parallel to a central axis of the MEMS die). Among other benefits, the channels2412increase the overall compliance of air within the MEMS die (e.g., by adding air volume away from the space between the electrode and the piston) without fully penetrating through electrode2404. The space within the channels2412is part of the back volume.

Turning toFIG.24C, a perspective view of the electrode2404ofFIG.24A(after the MEMS die2400is cross sectioned along line A-A′) according to an embodiment is shown. In this embodiment, pillars2414are formed into the electrode2404and the spaces between the pillars2414constitute the channels2412. The channels2412and the pillars2414may be of any shape as long as any point within the channel is less than a thermal boundary layer thickness from a surface of a pillar2414at the upper limit of audio frequency.

Turning toFIG.24D, a top view of the electrode2404ofFIG.24A(after the MEMS die2400is cross sectioned along line A-A′) according to another embodiment is shown. In this embodiment, the channels2412are formed in the electrode2404in concentric rings.

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 layer of polycrystalline silicon and a layer of insulator, with the electrode being embedded in the substrate (e.g., within the insulative material) or disposed on the surface of the substrate (e.g., on top of the insulative material). Turning toFIG.25A, a MEMS die configured according to such an embodiment is shown. The MEMS die2500includes a piston2502, which is configured like the piston2402ofFIG.24Aand has the same possible variations, including a distinct electrode and insulative material. The MEMS die2500further includes a substrate2504, and a resilient structure2506that supports the piston2502on the substrate2504. Possible implementations of the piston2502and the resilient structure2506include those discussed in conjunction with piston and resilient structures ofFIG.24AandFIG.24B. A back volume2508is bounded on top by the piston2502, bounded on the bottom by the substrate2504, and bounded on all sides by the resilient structure2506. The piston2502, substrate2504, and the resilient structure2506enclose a back volume2508. The resilient structure2506prevents air from leaving the back volume2508, and particularly blocks air from travelling in a direction radially outward from a central portion of the back volume2508. The substrate2504includes an insulative layer2510(made, for example, of silicon dioxide or silicon nitride) and an electrode2512that is embedded in the insulative layer2510. The electrode2512faces the piston2502such that a capacitance exists between the piston2502and the electrode2512(with the piston2502acting as a first electrode of a capacitor, the electrode2512acting as a second electrode of the capacitor, and the air or other gas in the back volume2508acting as the dielectric). However, in some embodiments, the piston2502includes both insulative material and a conductive portion or layer, in which the conductive portion or layer acts as the first electrode of the capacitor. In an embodiment, when the MEMS die2500is integrated into a microphone, the piston2502is electrically connected to a bias voltage source2514and the electrode2512is electrically connected to an integrated circuit2516(e.g., to an amplifier input thereof). Alternatively, the piston2502may be electrically connected to the integrated circuit2516and the electrode2512may be electrically connected to the bias voltage source2514.

In an embodiment, channels2518are formed into the substrate2504, and the electrode2512spans the channels2518. In an embodiment, the dimensions of the channels2518are such that any point within the channels2518is less than a single thermal boundary layer thickness from a nearest surface. In the embodiment ofFIG.25A, each of the channels2518extends away from the piston2502in a substantially perpendicular orientation relative to the piston2502(e.g., parallel to a central axis of the MEMS die2500). The channels2518extend through the electrode2512. Among other benefits, the channels2516increase the overall compliance of air within the MEMS die2500(e.g., by adding air volume away from the space between the second electrode2512and the piston2502) without fully penetrating through the substrate2504. The space within the channels2518is part of the back volume2508.

According to an embodiment, the MEMS die2500includes walls2520and one or more external conductors2522. Each external conductor2522is electrically connected to the piston2502(or to an electrode on the piston2502if the piston includes insulative material) at one end and to a wall2520at the other end. In an embodiment, each of the one or more external conductors2522is a resilient member, such as a metallic spring. The walls2520are connected to the bias voltage source2514by way of a contact2524(e.g., a eutectic metal contact) and a conductive path2526within the insulative layer2510of the substrate2504(with through-silicon vias (TSVs) as appropriate).FIG.25Bdepicts the walls2520and the external conductors2522as seen from above the piston2502.

In a variation on the embodiment described in conjunction withFIG.25A, the function of the electrode2512may be carried out by two electrodes, shown in the MEMS die2550ofFIG.25C. In this embodiment, the function of the electrode2512is carried out by an electrode2512aand an electrode2512b, both of which are depicted as being embedded in the substrate2504(in the insulative layer2510). In this embodiment, the electrode2512ais electrically connected to the bias voltage source2514via a conductive path2526aand the electrode2512bis electrically connected to the integrated circuit2516(e.g., to an amplifier input thereof) via a conductive path2526b.

In an embodiment, the piston is connected to an electrical potential through a very large resistor. In an embodiment, the electrical potential may be electrical ground. The resistance of the resistor should be large enough to set the electrical corner frequency below that of the desired low acoustic corner frequency (e.g. 20 Hz). In an embodiment, the resistance may be 10{circumflex over ( )}12 ohm. In other embodiments, the resistance may be less or more than 10{circumflex over ( )}12 ohm. In an embodiment, the resistor is formed by the electrical leakage conductance of resilient structure2506band insulative layer2510. In operation, the piston is connected to an electrical potential in a DC sense, but is electrically isolated in the AC sense. In such an embodiment, the movement of the piston towards and away from the electrodes facing the piston results in a change in capacitance between the piston and the electrodes and induces a signal in one of the electrodes (the other electrode being supplied with a DC voltage). An example of this is shown inFIG.25D(which is a variation on the embodiment described in conjunction withFIG.25C). InFIG.25D, a MEMS die2560has the piston2502configured to be electrically floating. In this embodiment, the electrode2512ais connected to the bias voltage source2414via a conductive path2526aand the electrode2412bis connected to the integrated circuit2416(e.g., to an amplifier input thereof) via a conductive path2526b.

Turning toFIG.25E, a partial perspective view of the substrate2504cross sectioned along line B-B′ (ofFIG.25A) according to an embodiment is shown. In this embodiment, pillars2530are formed into the substrate2504and the spaces between the pillars2530constitute the channels2518. At least some of the pillars (all of them in some embodiments) include an electrically conductive layer2532that is connected (e.g., with a TSV down the long axis of the pillar) to a single, common conductor so that all of the electrically conductive layers are part of the electrode2512. This substrate configuration may also be used in the embodiments ofFIG.25CandFIG.25D, but with two electrically separate conductive layers instead of the single electrically conductive layer2522that is shown inFIG.25E.

Turning toFIG.25F, a top view of the substrate2504cross sectioned along line C-C′ (ofFIG.25A) according to another embodiment is shown. In this embodiment, the channels2518are formed into the substrate2504in rings, such as the concentric rings shown inFIG.25Fbetween parts of the electrode2512, which is also formed in concentric rings2536. In this embodiment, a single conductive rib2534electrically connects the rings2536of the electrode2512. This substrate configuration may also be used in the embodiments ofFIG.25CandFIG.25D, but with two separate electrodes (e.g., with two separate conductive ribs).

According to various embodiments, channels are provided in the piston to increase the compliance of the air in the back volume. Turning toFIG.26A, a MEMS die configured according to such an embodiment is shown. The MEMS die2600includes a piston2602that is rigid (e.g., made of relatively thick silicon) and that is conductive (e.g., a semiconductor like crystalline silicon), a substrate2604, and a resilient structure2606that supports the piston2602on the substrate2604. Possible implementations of the piston2602and the resilient structure2606include those discussed in conjunction with the piston2402and the resilient structure2406ofFIG.24A. The piston2602, substrate2604, and the resilient structure2606enclose a back volume2608. The resilient structure2606blocks air from leaving the back volume2608, and particularly blocks air from travelling in a direction radially outward from a central portion of the back volume2608. The substrate2604includes an insulative layer2610(made, for example, of silicon dioxide) and an electrode2612that is embedded in the insulative layer2610.

Referring still toFIG.26A, the electrode2612faces the piston2602such that a capacitance exists between the piston2602and the electrode2612(with the piston2602acting as a first electrode of a capacitor, the electrode2612acting as a second electrode of the capacitor, and the air or other gas in the back volume2608acting as the dielectric). However, in some embodiments, the piston2602includes both insulative material and a conductive portion or layer, in which the conductive portion or layer acts as the first electrode of the capacitor. In an embodiment, when the MEMS die2600is integrated into a microphone, the piston2602is electrically connected to a bias voltage source2614(e.g., via external conductors2622) and the electrode2612is electrically connected to an integrated circuit2616(e.g., to an amplifier input thereof) (e.g., via a conductive path2622). Alternatively, the piston2602may be electrically connected to the integrated circuit2616and the electrode2612may be electrically connected to the bias voltage source2614.

According to an embodiment, the piston2602includes channels2618. The space within the channels2618is part of the back volume2608. The dimensions of the channels2618are such that any point within the channels2618is less than a single thermal boundary layer thickness from a nearest surface. In the embodiment ofFIG.26A, each of the channels2618extends away from the substrate2604in a substantially perpendicular orientation relative to the substrate2604(e.g., parallel to a central axis of the MEMS die2600). Among other benefits, the channels2618increase the overall compliance of air within the MEMS die2600(e.g., by adding air volume away from the space between the substrate2604) without fully penetrating through the piston2602.

According to an embodiment, the MEMS die2600includes walls2620and one or more external conductors2622. Each external conductor2622is configured as described previously with respect to the external conductor2522ofFIG.25A(including the possible implementations) and their function and relationship to the walls2620is also as described previously with respect the walls2520ofFIG.25AandFIG.25B.

FIG.26Bshows a partial perspective view of a cross section of the piston2602along line D-D′. In this embodiment, pillars2624are formed into the piston2602and the spaces between the pillars2624constitute the channels2618.

Turning toFIG.26C, a bottom view of the piston2602cross sectioned along line D-D′ according to another embodiment is shown. In this embodiment, the channels2618are formed into the piston2602in concentric rings.

In a variation on the embodiment described in conjunction withFIG.26A, the function of the electrode2612may be carried out by two electrodes, as shown in the MEMS die2650ofFIG.26D. In this embodiment, the functions of the electrode2612are carried out by an electrode2612aand an electrode2612b. In this embodiment, the electrode2612ais connected to the bias voltage source2614and the electrode2612bis connected to the integrated circuit2616(e.g., to an amplifier input thereof) via respective conductive pathways2622aand2622b.

In a variation on the embodiment described in conjunction withFIG.26D, the piston2602may be electrically floating, as shown in the MEMS die2660ofFIG.26E. In this embodiment, the electrode2612ais connected to the bias voltage source2614via a conductive path2622aand the electrode2612bis connected to the integrated circuit2616(e.g., to an amplifier input thereof) via a conductive path2622b.

In the various MEMS die embodiments described herein (e.g., the MEMS dies ofFIG.24B,FIG.25A,FIG.25C,FIG.25D,FIG.26A,FIG.26D, andFIG.26E), the MEMS die may also include a resilient structure inside the back volume (e.g., near a central axis of the MEMS die). Such a resilient structure is in addition to a resilient structure at the periphery of the piston. An example of such an embodiment is shown inFIG.26F, in which a MEMS die2670is configured like the MEMS die2600ofFIG.26Aexcept that the MEMS die2670includes a second resilient structure2607inside the back volume2608. One possible advantage to the second resilient structure2607is that it may increase the resonance frequency for the MEMS die2670to limit adverse effects that would result from having the resonance in or near the audio band.

In various embodiments of MEMS dies described above, certain innovations allow for a back volume configured such that every point within the back volume is no more than a single thermal boundary layer thickness from a solid surface. What follows is a description of further embodiments in which MEMS dies with such back volume configurations are combined with diaphragm assemblies.

For example, turning toFIG.27, a MEMS die2700includes a dual-diaphragm diaphragm assembly2701. The dual-diaphragm assembly2701includes a first diaphragm2702comprising a first electrode2704, a second diaphragm2706comprising a second electrode2708, and a backplate2710comprising a third electrode2712. The first diaphragm2702and the second diaphragm2706are oriented so that they face one another, and the backplate2710is disposed in between the first diaphragm2702and the second diaphragm2706and facing both the first diaphragm2702and the second diaphragm2706. The first diaphragm2702and the second diaphragm2706are connected to one another by pillars2713, which extend through the backplate2710. The MEMS die2700further includes a spacer2714that is sandwiched between and connected to the first diaphragm2702and the backplate2710, and a spacer2716that is sandwiched between and connected to the second diaphragm2706and the backplate2710. A region2716between the first diaphragm2702and the second diaphragm2706is sealed off and is at a lower pressure than standard atmospheric pressure (e.g., 50% atmospheric pressure) and may be at or near a vacuum. The MEMS die2700further includes an enclosure2718that is formed from and acts as a substrate. The second diaphragm2706is attached to the enclosure2718via a spacer2720.

In an embodiment, channels2722are formed into the enclosure2718. In an embodiment, the dimensions of the channels2722are such that any point within the channels2722is less than a single thermal boundary layer thickness from a nearest surface. The channels2722increase the overall compliance of air within the MEMS die2700. The space within the channels2722is part of the back volume. Possible configurations for the topography of the substrate2718include those discussed in conjunction withFIG.24CandFIG.24Dwith pillars or rings, respectively.

A variation of the MEMS die2700is shown inFIG.28A. The MEMS die2800includes the dial-diaphragm assembly2701ofFIG.27, but with a substrate, labeled2802. Attached to the first diaphragm2702(via a spacer2805) is an enclosure2806. Channels2810are formed into the enclosure2806. In an embodiment, the dimensions of the channels2810are such that any point within the channels2810is less than a single thermal boundary layer thickness from a nearest surface. The space within the channels2810is part of the back volume. Possible configurations for the topography of the enclosure2806include those discussed in conjunction withFIG.26BandFIG.26Cwith pillars or rings, respectively. The substrate2802has a hole2820through which sound passes. The second diaphragm2706is attached to the substrate2802via a spacer2822.

A variation of the MEMS die2800is shown inFIG.28B. A MEMS die2850includes the enclosure2806and the substrate2802. Additionally, the MEMS die2850has a single diaphragm2852and a backplate2854facing the diaphragm2852. The diaphragm2852has a first electrode2856and the backplate2854has a second electrode2858. The diaphragm2852is attached to the enclosure2806via a spacer2860. The backplate2854is attached to the diaphragm2852via a spacer2862and to the substrate2802via a spacer2864. During operation, sound enters the hole2820, passes through the backplate2854and strikes the diaphragm2852.

Another variation of the MEMS die2800is shown inFIG.28C. The MEMS die2870is similar to the MEMS die2850ofFIG.28Bexcept that the positions of the backplate2854and the diaphragm2852are reversed and there is a second backplate2862that has an electrode2864.

Turning toFIG.29, an example use of the MEMS die2700ofFIG.27is depicted in the context of a sensor, specifically an acoustic sensor referred to as a microphone2900. The microphone2900has a housing2902that includes a can2904and a base2906. The can2904is attached to the base2906. Pressure waves (e.g., sound waves) enter a port2908on the can2904and strike the first diaphragm2702, cause it to flex and to induce corresponding flexion in the second diaphragm2706(via the pillars2713). Consequently, the distance between the first electrode2704and the third electrode2712, altering the capacitance between the first electrode2704and third electrode2712. Likewise, the distance between the second electrode2708and the third electrode2712changes, altering the capacitance between the second electrode2708and the third electrode2712. These capacitance changes are read by an electrical circuit (e.g., integrated circuit)2910, which is attached to MEMS die2700via one or more signal pathways (e.g., wires)2912. The electrical circuit2910then interprets the signals representing the capacitance changes and interprets the signals as, for example, sound. The electrical circuit then provides further signals representing the interpretation via one or more additional signal pathways (e.g., wires) to external devices.

Turning toFIG.30, an example use of the MEMS die2800is depicted in the context of a sensor, specifically an acoustic sensor referred to as a microphone3000. The microphone3000has a housing3002that includes a can3004and a base3006. The can3004is attached to the base3006. Pressure waves (e.g., sound waves) enter a port3008on the base3006, pass through the hole2820in the substrate2802and strike the second diaphragm2806, cause it to flex and to induce corresponding flexion in the first diaphragm2802(via the pillars2813). Consequently, the distance between the first electrode2804and the third electrode2812, altering the capacitance between the first electrode2804and third electrode2812. Likewise, the distance between the second electrode2808and the third electrode2812changes, altering the capacitance between the second electrode2808and the third electrode2812. These capacitance changes are read by an electrical circuit (e.g., integrated circuit)3010, which is attached to MEMS die2800via one or more signal pathways (e.g., wires)3012. The electrical circuit3010then interprets the signals representing the capacitance changes and interprets the signals as, for example, sound. The electrical circuit then provides further signals representing the interpretation via one or more additional signal pathways (e.g., wires) to external devices.

Other types of dual-diaphragm assemblies may also be used in place of the dual-diaphragm assembly2701ofFIG.27,FIG.28A,FIG.29, andFIG.30. For example, a dielectric motor MEMS assembly may be used. The principles of such an assembly according to an embodiment will now be described with reference toFIG.31A,FIG.31B, andFIG.32.

Turning toFIG.31A, in an embodiment, a dielectric motor MEMS device3100includes a first electrode3110oriented lengthwise along and parallel to an axis3160. The first electrode3110has a first end3111and a second end3112. The MEMS device3100also includes a second electrode3120oriented lengthwise along and parallel to the axis3160. The second electrode3120has a first end3121and a second end3122. The MEMS device3100further includes a third electrode3130oriented lengthwise along and parallel to the axis3160. The third electrode3130has a first end3131and a second end3132. The electrodes3110,3120, and3130may be cylinders, plates, cuboids, prisms, polyhedrons, or other shapes of electrodes. The length of the first electrode3110is longer than the length of the second electrode3120and longer than the length of the third electrode3130. The MEMS device3100has a solid dielectric3150interspersed among the electrodes. The dielectric3150may be made, for example, of silicon nitride. The electrodes3110,3120, and3130are made of a conductor or a semiconductor, such as plated metals or poly silicon.

In an embodiment, the first electrode3110is an electrically conductive pin of a plurality of first electrically conductive pins electrically connected to each other, the second electrode3120is an electrically conductive pin of a plurality of second electrically conductive pins electrically connected to each other, and the third electrode3130is an electrically conductive pin of a plurality of third electrically conductive pins electrically connected to each other.

Turning toFIG.31B, in an embodiment, the dielectric3150has a plurality of apertures3156that penetrate through the dielectric3150in a direction parallel to an axis3160. The apertures3156are depicted as cylindrical but may be any suitable shape. The first, second, and third electrodes can each be located at least partially within an aperture of the plurality of apertures3156. In other words, part of an electrode may be located within an aperture and/or the electrode may be located in part of the aperture. According to a possible embodiment, at least the second and third electrodes are located only partially within the apertures3156for the ends to be located outside the dielectric3150. For example, the first electrode3110may be disposed at least partially within a first aperture of the plurality of apertures3156. The second end3122of the second electrode3120may be disposed within a second aperture of the plurality of apertures3156. The first end3131of the third electrode3130may be disposed within a third aperture of the plurality of apertures3156.

Referring toFIG.31B, the dielectric3150has a first surface3151and a second surface3152. The first surface3151and the second surface3152are parallel to a plane perpendicular to the axis3160. In the configuration depicted inFIG.31B, the first end3111of the first electrode3110and the first end3121of the second electrode3120extend beyond the first surface3151, and the second ends3112and3132of the respective first and third electrodes3110and3130extend beyond the second surface3152.

According to an embodiment, the electrodes of the device3100are substantially fixed relative to each other. For example, the electrodes may allow for some minor relative movement with respect to one another due to flexion and other forces. The dielectric3150and electrodes may also be free to move relative to each other. For example, the dielectric3150may be movable parallel to the axis3160relative to the first, second, and third electrodes. Additionally or alternatively, the first, second, and third electrodes may be movable relative to the dielectric3150. The dielectric3150may also take other forms such as one or more segments or members.

According to a possible embodiment, the dielectric3150can fill at least 50% of a distance (e.g., a distance perpendicular to the first length of the first electrode3110) between the first and second electrode3120. For example, the dielectric3150may fill at least 75% of the distance, at least 80% of the distance, or at least between 80-90% of the distance. However, the dielectric3150may fill any amount from 1% to 99% of the distance. The more the dielectric fills the gap between the electrodes, the more the capacitance changes per unit of displacement and thus the higher the force generated for a given voltage bias between electrodes. Some minimal gap should remain between the dielectric and the electrodes, subject to fabrication constraints, so that the dielectric and electrodes remain moveable relative to each other.

According to an embodiment, a first capacitance exists between the first electrode3110and the second electrode3120and a second capacitance exists between the first electrode3110and the third electrode3130. The capacitance between electrodes is a function of their positions relative to the dielectric3150. For example, values of the first and second capacitance can change in opposite directions when the dielectric3150moves relative to the electrodes in a direction parallel to the axis3160. Thus, for example, when the dielectric3150moves and causes the first capacitance to increase, the second capacitance may simultaneously decrease. An electrostatic force on the dielectric3150relative to the electrodes3110,3120, and3130may, however, be substantially unchanged relative to the displacement.

According to an embodiment, during operation of the MEMS device3100, a voltage from a first voltage source V1is applied between the first electrode3110and the second electrode3120producing a relatively constant force F1. A voltage from a second voltage source V2is applied between the first electrode3110and the third electrode3130to produce a relatively constant force F2. Forces F1and F2are in opposition. If the structure is relatively symmetric and the magnitude of the voltage sources V1and V2are equal, forces F1and F2are equal and thus a net zero force is exerted between the dielectric and the electrodes. The magnitude of the voltages from the voltage sources V1and V2can be unequal to compensate for asymmetries in the structure or to intentionally create a non-zero net force between the dielectric and the electrodes.

Turning toFIG.32, a dielectric motor MEMS device3200according to another embodiment will now be described. In this embodiment, the function of the first electrode3110(fromFIG.3AandFIG.3B) is carried out by a first set of electrically interconnected electrodes3210-x, the function of the second electrode3120is carried out by a second set of electrically interconnected electrodes3220-x, and the function of the third electrode3130is carried out by a third set of electrically interconnected electrodes3230-x. The second and third sets of electrodes3220-xand3230-xcan be staggered on either side of the first set of electrodes3210-x. This allows for more capacitance and an increase in the change of capacitance with displacement. Other configurations of conductive and dielectric elements, such as bars or rings, may also be used.

Turning toFIG.33, a dual-diaphragm assembly that incorporates the MEMS device3200ofFIG.32is shown. The dual-diaphragm assembly3300includes a diaphragm3370coupled to the first set of electrodes3210-xand the third set of electrodes3230-x. The diaphragm3370has a surface3372perpendicular to the axis3360. The surface3372is substantially planar (e.g., planar but with imperfections on the surface3372or slightly curved or uneven while still allowing the surface3372to operate in a manner useful for a diaphragm). The diaphragm3370may be made of sandwiched layers. The dual-diaphragm assembly3300also includes a second diaphragm3374located on an opposite side of the dielectric3350from the first diaphragm3370. The second diaphragm3374is coupled to the first set of electrodes3210-xand the second set of electrodes3220-x. The second diaphragm3374has substantially planar surface3376and is oriented perpendicular to the axis3360. The first diaphragm3370and the second diaphragm3374are spaced from the dielectric3350to permit relative movement between the electrodes connected to the first and second diaphragms and the dielectric3350.

In an embodiment, a sealed low-pressure region is defined between the diaphragms3370and3374. This low-pressure region serves to reduce noise and damping of the assembly3300. The first set of electrodes3210-x(which are connected to both the first diaphragm3370and the second diaphragm3374) help prevent the diaphragms from collapsing onto the dielectric3350. This low-pressure region may be substantially a vacuum (e.g., with a pressure less than 1 Torr, less than 300 mTorr, or less than 100 mTorr). According to an embodiment, the dielectric3350is relatively thick and stiff compared to the diaphragms3370and3374and remains relatively motionless when the diaphragms3370and3374are deflected. Deflection of the diaphragms3370and3374moves the electrodes3210-x,3220-x, and3230-xrelative to the dielectric3350

The diaphragms3370and3374may be made of a dielectric material, such as silicon nitride. However, other materials can be used. For example, one or more of the diaphragm3370, the diaphragm3374, and the dielectric3150can be polyimide.

In an embodiment, if employed as an acoustic sensor, the MEMS die3300operates as follows. Pressure (e.g., from sound waves) reaches at least one of the diaphragms, causing it to flex and to move the electrodes attached thereto relative to the dielectric, thereby causing respective capacitances between the various elements to change. This change in capacitance manifests as a change in one or more signals being output from the MEMS die3300. The one or more signals are read and interpreted by an IC and passed on to other external components.

FIG.34depicts a MEMS device3400that includes all of the elements of the MEMS device2700fromFIG.27but with the dual-diaphragm assembly2701replaced by the dual-diaphragm dielectric motor assembly3300. The MEMS device3400otherwise functions similarly to the MEMS device2700.

FIG.35depicts an embodiment in which a MEMS device3500includes all of the elements of the MEMS device2800fromFIG.28Abut with the dual diaphragm-assembly2701replaced by the dual-diaphragm dielectric motor assembly3300. The MEMS device3500otherwise functions similarly to the MEMS device2800.

With respect to the use of plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

Unless otherwise noted, the use of the words “approximate,” “about,” “around,” “substantially,” etc., mean plus or minus ten percent.

The foregoing description of illustrative embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be 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.