Microelectromechanical system

A microelectromechanical system includes an enclosure defining a cavity and an opening communicating with the cavity; a membrane mounted at the opening; a cantilever located within the cavity, the at least one cantilever comprising a first end, a second end and a fulcrum located between the first end and the second end; a plunger positioned between the membrane and the cantilever and configured to transfer displacement of the membrane to the first end of the cantilever; and a sensing member connected to the second end of the cantilever. The distance between the first end and the fulcrum is less than that between the second end and the fulcrum. The microelectromechanical system has the advantages of high SNR, small package size and high sensitivity. The membrane has a stiffness order of magnitude higher than a conventional membrane, which avoids mechanical collapse and large DC deformation under 1 atm.

FIELD OF THE INVENTION

The present disclosure relates to the field of electroacoustic transducers, and in particular, to a microelectromechanical system.

BACKGROUND

Conventional microelectromechanical system (MEMS) microphones usually comprise a back volume behind the membrane. The back volume is a semi-sealed volume of air that undergoes compression and expansion when there is an input acoustic wave. For a defined package size, the back volume is necessary to allow the membrane to move under external pressure wave. However, the back volume is currently the largest source of acoustic noise, which reduces the signal noise ratio (SNR) of the microphone. The smaller the back volume, the higher the acoustic noise generated by the back volume. It is therefore very difficult to make a microphone with SNR above approximately 74 dB unless the package size is made very large. However, very large package size is not feasible for mobile electronic devices.

The effective way of achieving very high SNR in a normal or smaller sized package is to make the back volume a vacuum. There is one significant challenge with such a type of vacuum back volume microphone. The pressure difference of 1atm between air and vacuum would collapse a normal membrane. Therefore, a very stiff membrane is needed. However, a very stiff membrane causes very low sensitivity. Conventional sensing designs would not work.

Therefore, it is desired to provide an improved MEMS which can at least partly overcome the above problems.

SUMMARY

In one aspect, the present disclosure provides a microelectromechanical system which comprises an enclosure defining a cavity and an opening communicating with the cavity; a membrane mounted at the opening; at least one cantilever located within the cavity, the at least one cantilever comprising a first end, a second end and a fulcrum located between the first end and the second end; a plunger positioned between the membrane and the at least one cantilever and configured to transfer displacement of the membrane to the first end of the at least one cantilever; and a sensing member connected to the second end of the at least one cantilever. The distance between the first end and the fulcrum is less than that between the second end and the fulcrum.

In some embodiments, the cavity is hermetically sealed, with an inside pressure less than an external atmosphere.

In some embodiments, the cavity is vacuum.

Preferably, the sensing member comprises a stationary part fixed with respect to the enclosure and a moveable part connected to the second end of the at least one cantilever and be moveable relative to the stationary part.

In some embodiments, the moveable part comprises a plurality of electrically conductive moveable fingers with a gap formed between every two adjacent moveable fingers; the stationary part comprises a plurality of electrically conductive stationary fingers with a gap formed between every two adjacent stationary fingers; and the moveable fingers are respectively aligned with the gaps of the stationary part and the stationary fingers are respectively aligned with the gaps of the moveable part.

In some embodiments, the at least one cantilever comprises a plurality of triangular-shaped or sector-shaped cantilevers arranged in a circular array.

In some embodiments, the at least one cantilever comprises a pair of rectangular cantilevers arranged in a linear array.

In some embodiments, the at least one cantilever further comprises a rib disposed at a surface thereof.

In some embodiments, the microelectromechanical system further comprises a support disposed between the fulcrum and the enclosure.

In some embodiments, the opening has a size less than that of the cavity.

In some embodiments, the at least one cantilever comprises multiple stages of cantilevers connected successively.

In some embodiments, the multiple stages of cantilevers comprises a first stage of cantilever, a second stage of cantilever and a third stage of cantilever, a first end of the first stage of cantilever is connected to the plunger, a second end of the first stage of cantilever is connected to a first end of the second stage of cantilever via another plunger, a second end of the second stage of cantilever is connected to a first end of the third stage of cantilever via further another plunger, and the moveable part of the sensing member is connected to a second end of the third stage of cantilever.

In some embodiments, every of the first stage of cantilever, second stage of cantilever and third stage of cantilever comprises a fulcrum located between the first end thereof and the second end thereof, and a support disposed between the fulcrum and the enclosure.

In some embodiments, the first end comprises a bending portion with a reduced thickness or one or more slits.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure will be further illustrated with reference to the accompanying drawings. It shall be noted that the elements of similar structures or functions are represented by like reference numerals throughout the figures. The embodiments described herein are not intended as an exhaustive illustration or description of various other embodiments or as a limitation on the scope of the claims or the scope of some other embodiments that are apparent to one of ordinary skills in the art in view of the embodiments described in the Application. In addition, an illustrated embodiment need not have all the aspects or advantages shown.

Referring toFIG.1andFIG.2, a microelectromechanical system10comprises an enclosure20defining a cavity22and an opening24communicating with the cavity22, a membrane30mounted at the opening24, at least one cantilever40located within the cavity22, and a plunger50positioned between the membrane30and the cantilevers40.

In some embodiments, the cavity22enclosed by the enclosure and the membrane30is sealed to external atmosphere and vacuum. Alternatively, the pressure inside the cavity22can be in a reduced atmosphere, for example, 0.1 atmosphere pressure.

Each cantilever40comprises a first end42, a second end44and a fulcrum46located between the first end42and the second end44. One or more ribs48may be formed at a surface of the cantilever40facing the membrane30or opposite from the membrane30for increasing the stiffness of the cantilever40. A support49is arranged between the fulcrum46and the enclosure20such that the cantilever40is pivotable about the fulcrum46. The distance between the second end44and the fulcrum46is greater than the distance between the first end42and the fulcrum46. In some embodiments, the distance between the second end44and the fulcrum46is equal to or greater than ten times of the distance between the first end42and the fulcrum46. The hinged cantilever40is provided with a bending portion47at the first end42thereof. The bending portion47is created by forming a cutout/recess/slit at the bending portion to reduce the stiffness of that region, which facilitates bending of the cantilever40at the bending portion47.

The first end42is connected to the plunger50. The plunger50is configured to transfer displacement of the membrane30to the first end42of the cantilevers40.

In some embodiments, the enclosure20is cylinder-shaped or polygon-shaped and a plurality of cantilevers40is arranged in a circular array. Each cantilever40has a triangular shape or sector shape. The first ends42are located at the center of the circular array of cantilevers40and the second ends44are located at the periphery of the circular array of cantilevers40. The cantilevers40may be separately formed and arranged in a circular array with a gap formed between every two adjacent cantilevers40. Alternatively, the cantilevers40are integrally formed as a single piece with a gap formed between every two adjacent cantilevers40.

The microelectromechanical system10further comprises a sensing member60. The sensing member60comprises a moveable part62connected to the second end44of the cantilever40and a stationary part64attached to the enclosure20. The moveable part62comprises a plurality of spaced movable fingers622with gaps624formed there between and the stationary part64comprises a plurality of spaced stationary fingers642with gaps644formed there between. The movable fingers622are aligned with the gaps644of the stationary part64respectively and the stationary fingers642are aligned with the gaps624of the movable part62respectively. The fingers622and642may be made of conductive material or comprise conductive elements. Thus, the fingers622and642are electrically conductive and a capacitance is formed between the movable part62and the stationary part64. The capacitance between the moveable part62and the stationary part64is based on the overlap of the moveable fingers622and the stationary fingers624. For zero AC acoustic pressure wave applied to membrane30, there is a fixed defined overlap between moveable finger622and stationary fingers642in the plane perpendicular to that shown inFIG.2. When AC pressure is applied to the membrane30, the moveable fingers622move in the direction perpendicular to the plane shown inFIG.2, changing the amount of overlap with stationary fingers642. The further that the moveable fingers622move away from the stationary fingers642in the direction perpendicular to the plane shown inFIG.2, the lower the overlap with the gaps624, and the lesser the capacitance. Similarly, the closer that the moveable fingers622move towards the stationary fingers642in the direction perpendicular to the plane shown inFIG.2, the greater the capacitance. An electronic signal will be generated and output in response to change of the capacitance.

Referring toFIG.3, alternatively, the cantilevers40are arranged in a linear array and each cantilever has a rectangular shape. For a linear array of rectangular cantilevers40, each pair of cantilevers40can be attached to a single membrane30.

FIG.4andFIG.5illustrate deformation of the membrane30and the cantilevers40when the membrane30receives an AC sound wave entering the microphone.FIG.4illustrates deformation of the membrane30and the cantilevers40when a negative pressure is exerted on the membrane30.FIG.5illustrates deformation of the membrane30and the cantilevers40when a positive pressure is exerted on the membrane30. As shown inFIGS.4andFIG.5, when the membrane30is displaced under a pressure difference between the inner surface and the outer surface of the membrane30, the first ends42of the cantilevers40are pulled upwardly or pushed downwardly by the plunger50, which results in the cantilevers40being pivoted about the fulcrums46and the second ends44of the cantilevers40together with the moveably parts64of the sensing member60being moved downwardly or upwardly to thereby change overlap between the moveable fingers622and the stationary fingers642. As the distance between the second end44and the fulcrum46is much greater than the distance between the first end42and the fulcrum46, the displacement of the membrane30is effectively amplified and the sensitivity of the microelectromechanical system10is increased.

FIG.6andFIG.7illustrate the working principle of a hinged cantilever. Each hinged cantilever40made of a thin silicon plate has a bending point47created by slits formed therein, and a fulcrum46which is also created by slits in the same plate. The fulcrum46is next to the rigid ribs48which extend all the way out to the sensing area. As the cavity22is in a reduced atmosphere or vacuum and there is limited or no acoustics in the cavity, these ribs48are designed to be strong but light, with low inertia. When the membrane30is displaced by z1, the second end of the cantilever40together with the moveable part62of the sensing member is displaced in the opposite direction by z2where z2is greater than z1, therefore increasing the mechanical sensitivity of the microelectromechanical system10that applies the cantilever40.

In the above-described embodiments, the size of the opening24is less than that of the cavity22. Thus, the small diameter, thick membrane30which avoids mechanical collapse and very large DC deformation under 1atm, has a stiffness order of magnitude higher than a conventional membrane. The microelectromechanical system10applies the cantilevers40to amplify the displacement of the membrane30by at least a factor of approximately 10 or more, which is ideally suited to a perimeter based comb drive which can achieve enhanced electrostatic sensitivity. The hinged cantilevers40may be arranged in either a circular array in which case each cantilever40is triangular or sector or a linear array in which case each cantilever is rectangular. The first end42of each cantilever40is attached to the membrane30and the second end44is connected with the moveable part62of the sensing member60. In general, there can be an array of membrane30all connected to a central plunger50which drives the array of hinged cantilevers40. The cantilevers40could be attached to various electrostatic sensing members but a comb drive with spaced fingers is preferred due to high sensitivity and large allowed range of motion in the z direction for the moveable part62of the comb drive type sensing member60.

To further increase mechanical sensitivity of the microelectromechanical system10, a chain of cantilevers could be used with one stage of hinged cantilevers driving the next stage, resulting in higher amplification. This chain of cantilevers could be optimised to achieve a non-linear displacement amplification of the membrane such that the initial large DC displacement due to atmosphere is not amplified as much as displacements when the DC pressure is between 0.5-1 atm. This would be a type of passive DC position control.

As shown inFIGS.8-9, the microelectromechanical system10comprises a chain of cantilevers40connected successively. In this embodiment, the cantilevers40comprise first stage of cantilevers40a, second stage of cantilevers40b, and third stage of cantilevers40c. The first ends42aof the first stage of cantilevers40aare connected to the first plunger50a. The second ends44aof the first stage of cantilevers40aare connected to the first ends42bof the second stage of cantilevers40bvia second plungers50band the second ends44bof the second stage of cantilevers40bare connected to the first ends42cof the third stage of cantilevers40cvia third plungers50c. The second ends44cof the third stage of cantilevers40care connected to the moveable part62of the sensing member60. The fulcrums46a,46b,46care respectively disposed between the first ends and the second ends of the cantilevers40. The distance between the first ends42a,42b,42cand the fulcrums46a,46b,46care respectively less than that between the second ends44a,44b,44cand the fulcrums46. Assume the distance between the second ends44aand the fulcrums46ais N1 times of the distance between the first ends42aand the fulcrums46a, the distance between the second ends44band the fulcrums46bis N2 times of the distance between the first ends42band the fulcrums46b, and the distance between the second ends44cand the fulcrums46cis N3 times of the distance between the first ends42cand the fulcrums46c, when the membrane30is displaced under a pressure difference between the inner surface and the outer surface of the membrane30, the moveable part62of the sensing member60will achieve a displacement which is substantially N1*N2*N3 times of the displacement of the membrane30. The displacement of the membrane30is greatly amplified.

The microelectromechanical system10according to the above disclosure has the advantage of high SNR, small package size and normal or high sensitivity. By also increasing mechanical sensitivity, a standard (−38 dB V/Pa) or higher total transducer sensitivity needed for a vacuum back volume design can be more easily reached. By making the cavity/back volume a vacuum, it does not need to be bigger than a conventional front volume and can be much smaller than that. This raises the possibility of a higher SNR microphone in a smaller package size than conventionally used, which is very attractive for all microphone applications, particularly mobile applications.

Although the invention is described with reference to one or more embodiments, the above description of the embodiments is used only to enable people skilled in the art to practice or use the invention. It should be appreciated by those skilled in the art that various modifications are possible without departing from the spirit or scope of the present invention. The embodiments illustrated above should not be interpreted as limits to the present invention, and the scope of the invention is to be determined by reference to the claims that follow.