MEMS acoustic sensor and assembly

The disclosure relates to a MEMS sensor and an assembly including the MEMS sensor and an electrical circuit disposed in an assembly housing. The sensor includes a suspended structure (148) having a top diaphragm (118), a central electrode (120) and a bottom diaphragm (122) connected by a pillar portion (134). A peripheral portion of the suspended structure is coupled to a support structure (114), forming a low pressure cavity (130). The MEMS sensor includes a top electrode (136) disposed between the top diaphragm and the central electrode and a bottom electrode (138) disposed between the bottom diaphragm and central electrode each coupled to the support structure, wherein in the event of a sound pressure condition, the suspended structures moves up or down together, while the top electrode and the bottom electrode remain substantially stationary.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to MEMS sensors and more particularly to MEMS sensors assemblies and MEMS dies for such assemblies.

BACKGROUND

MEMS sensors are often deployed in various electronic devices such as cellular phones, mobile devices, headsets, hands free systems, smart televisions, smart speakers, portable computers, etc. Such microphones typically include a transducer disposed within a housing, for converting sound waves into an electrical signal that represents the sound. Generally, temperature changes, radiation and other conditions external to the microphone can cause thermo-acoustic effects (e.g., temperature and air pressure changes) within the housing. In wireless communication devices, for example, some radio frequencies induce thermal-acoustic effects in microphones integrated with the device.

DETAILED DESCRIPTION

The present disclosure describes MEMS sensors and assemblies. The acoustic industry and its engineers are searching for ways and designs to provide enhanced sensitivity and sound quality in a robust structure. MEMS sensors and assemblies generally comprise a transduction element disposed within a housing and configured to generate an electrical signal representative of a sensed condition for output to an integrated circuit of the sensor assembly. MEMS sensors and assemblies can be implemented as an acoustic sensor, a vibration sensor, a pressure sensor, a temperature sensor, or a humidity sensor, among others, and combinations thereof.

MEMS sensors and assemblies are described below but the teachings are applicable to other types of sensors described herein. The MEMS acoustic sensor assembly can include a transduction element, an integrated circuit (IC), and a housing. The housing can enclose the transducer and the integrated circuit. The transducer can convert a sound into a signal electrically representing the sound and provide the electrical signal to the IC. The IC can process (e.g., amplify, buffer, filter, digitize, etc.) the signal and output a processed signal at an external-device (e.g., host) interface of the sensor assembly. The housing generally protects and isolates the transducer and the IC from the effects of RF energy on the performance of the transducer assembly. The IC may output the processed audio signal to an external electrical device or host.

Referring toFIG. 1, an embodiment of an acoustic transducer assembly (ATA)100is shown. In its simplest form, the ATA100includes: a base102(e.g., a substrate, such as a printed circuit board (PCB)) having a host-device interface104with for example a plurality of contacts106; a lid108mounted on the base102to form an assembly housing110, a MEMS sensor112, disposed in the assembly housing and acoustically coupled to a sound port of the assembly housing, a sound port113, IC115disposed in the assembly housing, lead(s)117to contacts106and MEMS sensor112.

The ATA100can be integrated with a cellular phone, mobile device, headset, hearing aid device, smart television, smart speaker, or any other type of host device. In some embodiments, the ATA100can include additional components not shown inFIG. 1.

As should be understood by those skilled in the art, the host-device interface104can be a surface-mount interface or can be in the form of leads for a through-hole interface. The MEMS sensor112can be a capacitive, piezoresistive, resonant, or optical, among others. These and other suitable transduction elements can be fabricated as microelectromechanical systems (MEMS) devices. The sound port113can be located in the base102as shown inFIG. 1, for a bottom-port implementation, or on the lid108for a top-port implementation, for example. The MEMS sensor112, such as in the form of a MEMS die, can be mounted over the sound port113. The integrated circuit or ASIC functions can include: impedance matching (buffering) or charge sampling; filtering; amplification; A/D (for digital mics); output signal protocol formatting (e.g., PDM, Soundwire) and the like, for example. In addition, other microphones, such as “smart mics”, can include a separate DSP for higher order processing like voice recognition, noise suppression and authentication. The geometric shape and size of the ATA100, lid108, housing110and MEMS sensor112, can vary widely based on the use case. By way of example, the ATA100, lid108, housing110and MEMS sensor112can be generally circular, cylindrical, square, octagon, or any other polygon shape or geometry having a use case.

InFIG. 2, a MEMS die or sensor112, in the form of an acoustic sensor in a vacuum is shown. The MEMS sensor112includes: a housing114(also referred to as a “support structure” to differentiate from the assembly housing); a transducer116disposed in the support structure114, the transducer116including a top diaphragm118, a central electrode120and a bottom diaphragm122; the top diaphragm118and the bottom diaphragm122including a middle portion124and a peripheral portion126extending to and coupled with a wall128of the support structure114, forming a low pressure cavity130; the central electrode120extends along and between the middle portions124of the top diaphragm118and the bottom diaphragm122and includes a termination132point free from contacting the wall128; the central electrode120is connected to the top diaphragm118and the bottom diaphragm122by a pillar structure134, the transducer116further including a top electrode136disposed between the top diaphragm118and the central electrode120and a bottom electrode138disposed between the bottom diaphragm122and central electrode120each coupled to the wall128of the support structure114, wherein in the event of a positive acoustic pressure condition or a negative acoustic pressure condition, the top diaphragm118, the bottom diaphragm122and central electrode120move up or down together, while the top electrode136and the bottom electrode138remain substantially stationary. Advantageously, this construction can provide enhanced sensitivity and sound quality, a desired 3D footprint and a robust structure.

Also advantageously, in one embodiment, the top diaphragm118, the bottom diaphragm122and central electrode120can move up/down or float, to provide improved sensitivity of the transducer116and are constructed to allow movement of part of the transducer116when exposed to sound pressures, which can provide improved sound quality.

InFIG. 2, the peripheral portions126of the top diaphragm118and the bottom diaphragm122include an adjacent corrugation region140. In one embodiment, the corrugation region140allows part of the transducer to float during a positive or negative acoustic pressure condition, defining an independent suspension system, while another part remains stationary. The corrugation region140can be defined as an independent suspension system, which helps to provide compliance sufficiently high to provide high sensitivity and/or movement of a diaphragm in situations in which an acoustic transducer is exposed to sound pressures. The corrugation region should also be constructed in a smooth-shaped manner to reduce stress concentrations, to increase diaphragm robustness and reduce premature failure. In one embodiment, it should be noted, that the top and bottom electrodes136and138are fixed or coupled to the wall128and fail to have such a corrugation region (or independent suspension system) and thus act differently and independently from the top118and bottom diaphragms122. Stated differently, the corrugation region140, as detailed herein, can provide a compliant linkage that includes at least some of its movability and mobility being flexible while also allowing a predetermined deflection.

InFIG. 2, the pillar structure134includes a plurality of pillars142and the central electrode120has holes144configured and aligned to allow the plurality of pillars142to extend therethrough and connects (at connection(s)146) to the central electrode120. Advantageously, this structure allows and enables the tandem movement of the top diaphragm118, the bottom diaphragm122and the central electrode120, when exposed to sound pressures. In one embodiment, the pillars142comprise a non-conductive material.

In more detail, the central electrode120is constructed to move with the top diaphragm118and the bottom diaphragm122. Advantageously, in one embodiment, this structure is constructed and adapted to allow the three components (the central electrode120, the top diaphragm118and the bottom diaphragm122, hereafter referred to as suspended structure148) to move in tandem with each other when exposed to sound pressures, independent of the stationary structure150(top and bottom electrodes136and138), which remain stationary. Stated differently, the suspended structure148can dynamically move up and down like in a piston-like arrangement with respect to a stationary structure.

InFIG. 2-4, the transducer116is shown at an atmospheric pressure condition with arrow152, a positive pressure condition with arrow154and a negative pressure condition with arrow156.

As shown inFIG. 2, during an atmospheric pressure condition (or at rest), shown as arrow152, the transducer116includes a first gap158that is about half or less of a second gap160, wherein the first gap158is defined as a distance from the bottom electrode138to the bottom diaphragm122and the second gap160is defined as a distance between the bottom electrode138and the central electrode120. Continuing, a third gap162is about half or less of a fourth gap164, wherein the third gap162is defined as a distance from the top electrode136to the top diaphragm118and the fourth gap164is defined as a distance between the top electrode136and the central electrode120.

InFIGS. 2-4, the top diaphragm118includes a first, second, third and fourth lower section172,174,176and178, respectively, shown in dashed line, between a left wall128and pillar142, pillar to adjacent pillar, pillar to next adjacent pillar, and pillar142to a right wall128, moving from left to right. These lower sections172,174,176and178, in dashed line, are examples of localized deformations at an instant in time. They are localized and curve inwardly, due to the vacuum or low-pressure cavity130. Likewise, the bottom diaphragm122includes a first, second, third and fourth upper sections180,182,184and186, respectively, located between a left wall128and pillar142, pillar to adjacent pillar, pillar to adjacent next pillar, and pillar142to a right wall128, moving from left to right. These upper sections180,182,184and186, in dashed line, are examples of localized curved inwardly deformations at an instant in time. They are localized and curve inwardly, due to the vacuum or low-pressure cavity130. In practice, these deformations are substantially consistent (or permanent), if there are no pressure leaks, and can also vary due to changes and fluctuations in ambient pressure and sound pressure. InFIGS. 2-4, in practice, the first and third gaps158and162, will change due to such localized deformations, as shown by second section174of the top diaphragm118and the second section182of the bottom diaphragm122.

InFIG. 3, a positive (upward) acoustic pressure condition, shown by arrow154, is illustrated. The suspended structure148inFIG. 3is moved or deflected upwardly, with the bottom diaphragm122curved and moved upwardly, causing the first gap158to decrease and the second gap160to increase. The top diaphragm118is also shown curved and moved upwardly, while the central electrode120remains substantially planar. Note inFIGS. 3 and 4, the suspended structure148is constructed to allow movement, deflecting and/or floating when exposed to sound pressures, while the stationary structure150remains independent and stationary.

In more detail, inFIG. 3, the suspended structure148(the bottom diaphragm122, the central electrode120and the top diaphragm118) moves up, causing the first158and fourth gaps164to decrease and the second160and third gaps162to increase.

In further detail inFIG. 3, due to the bottom diaphragm122moving up, a first capacitance (C1)166between the top electrode136and central electrode120increases and a second capacitance (C2)168between the bottom electrode138and the central electrode120decreases. Note, the capacitance change is inversely proportional to gap change. In the instance of a positive acoustic pressure condition, the first capacitance C1166(0 or 1) and the second capacitance C2168(0 or 1) can be fed to an IC115to process sound. The capacitance signal out of MEMS sensor112is analog and substantially continuous. The IC115can convert the capacitance change, thus voltage change to digital if desired.

In the event of an excessive (high) positive pressure condition inFIG. 3, the first gap158approaches and can become zero, wherein a middle portion of the bottom diaphragm122and the bottom electrode138can make physical contact, which prevents the bottom electrode138, top electrode136and the central electrode120from making contact, due to the fourth gap164being greater than the first gap158.

Now moving toFIG. 4, a negative (downward) acoustic pressure condition (shown as arrow156) is illustrated. The suspended structure148inFIG. 4is moved or deflected downwardly, with the bottom diaphragm122curved and moved downwardly. Since the bottom diaphragm122moves down, the first gap158is increased and the second gap160is decreased. The top diaphragm118is also shown curved and moved downwardly, while the central electrode120remains substantially planar. Note inFIGS. 3 and 4, the suspended structure148is capable of moving, deflecting or floating when exposed to sound pressures, while the stationary structure150remains independent and stationary.

In more detail, when a negative acoustic pressure condition occurs, the suspended structure148moves down causing the first158and fourth gaps164to increase and the second160and third gaps162to decrease.

In further detail inFIG. 4, due to the top and bottom diaphragm118and122movement down, the first capacitance (C1)166between the top electrode136and central electrode120decreases and the second capacitance (C2)168between the bottom electrode138and the central electrode120increases. The capacitance change is inversely proportional to gap change. In the instance of a negative acoustic pressure condition, the first and second capacitance166and168values can be fed to the IC115, to process sound.

In the event of an excessive negative pressure condition inFIG. 4, in the event the third gap162goes to zero, a middle portion of the top diaphragm118and the top electrode136make physical contact, which prevents the bottom electrode138and the central electrode120from making contact, due to the third gap162being greater than the second gap160. Advantageously, this construction can prevent an undesirable “pull in” from occurring. InFIG. 4, an example of a gap approaching zero, is illustrated by the lower section174in dashed-line of the top diaphragm118being spaced immediately adjacent to the top electrode136.

In one embodiment, the transducer includes a suspended structure148defined by the top diaphragm118, the central electrode120and the bottom diaphragm122being suspended about a stationary structure defined as the top and bottom electrodes136and138, at ambient pressure and when exposed to sound pressures. This construction can provide enhanced sensitivity and sound quality in a dynamic environment and a robust structure.

As should be understood by those skilled in the art, various modifications can be made to address certain use cases in connection with this disclosure. For example, various illustrative embodiments can include; (i) The support structure114being generally tubular with the diaphragms and central electrode being complementarily configured to fit and move up or down therein, and the diaphragms and central electrode can be generally disc shaped (or circular) and in a parallel arrangement, to allow diaphragm movement in a piston-like arrangement with respect to the tubular support structure. (ii) The top and bottom diaphragms118and120can be disc shaped and have pillars that have local deformations between adjacent pillars and between pillars and a wall, in one use case. (iii) The top and bottom diaphragms118and120can be disc shaped with a corrugation region140around a periphery. (iv) The central electrode120can include a substrate with a top and bottom electrode on either side, in one embodiment. (v) The top diaphragm118and bottom diaphragm122can include a conductive layer on one or both sides, in another use case. (vii) The number of pillars can vary widely depending on use case.

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. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.).