PIEZOELECTRIC ACOUSTIC MEMS TRANSDUCER AND FABRICATION METHOD THEREOF
1. A piezoelectric MEMS transducer, comprising:
- a body of semiconductor material having a central axis and a peripheral area, the body including;
a plurality of beams extending transversely to the central axis and each having a first end and a second end, the first ends of the beams being coupled to the peripheral area of the body and the second ends facing the central axis;
a membrane extending transversely to the central axis and underneath the plurality of beams; and
a pillar extending parallel to the central axis and rigid with respect to the second ends of the beams and the membrane; and
a plurality of piezoelectric sensing elements arranged on the plurality of beams.
A piezoelectric MEMS transducer formed in a body of semiconductor material, which has a central axis and a peripheral area and comprises a plurality of beams, transverse to the central axis and having a first end, coupled to the peripheral area of the body, and a second end, facing the central axis; a membrane, transverse to the central axis and arranged underneath the plurality of beams; and a pillar, parallel to the central axis and rigid with the second end of the beams and to the membrane. The MEMS transducer further comprises a plurality of piezoelectric sensing elements arranged on the plurality of beams.
- 1. A piezoelectric MEMS transducer, comprising:
a body of semiconductor material having a central axis and a peripheral area, the body including; a plurality of beams extending transversely to the central axis and each having a first end and a second end, the first ends of the beams being coupled to the peripheral area of the body and the second ends facing the central axis; a membrane extending transversely to the central axis and underneath the plurality of beams; and a pillar extending parallel to the central axis and rigid with respect to the second ends of the beams and the membrane; and a plurality of piezoelectric sensing elements arranged on the plurality of beams.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
- 15. An electronic device comprising:
a piezoelectric MEMS transducer that includes; a body of semiconductor material having a central axis (S) and a peripheral area, the body including; a membrane extending transversely to the central axis and underneath the plurality of beams; and a plurality of piezoelectric sensing elements arranged on the plurality of beams; a signal-processing block coupled to the transducer; a microprocessor, coupled to the signal-processing block; a memory coupled to the microprocessor; and an input/output unit, coupled to the microprocessor.
- View Dependent Claims (16, 17)
- 18. A method for fabricating a MEMS transducer, comprising:
forming, in a body of semiconductor material, a membrane having a first and a second membrane surface; forming a pillar of semiconductor material, projecting transversely to the first membrane surface; forming a plurality of beams, extending in a direction transverse to the pillar and on the membrane, the beams each having a first end and a second end, the first ends of the beams being coupled to a peripheral area of the body and the second ends being fixed with respect to the pillar; and forming a plurality of piezoelectric sensing elements on the plurality of beams.
- View Dependent Claims (19, 20, 21, 22, 23, 24, 25)
The present disclosure relates to a piezoelectric acoustic MEMS (Micro-Electro-Mechanical Systems) transducer and to the fabrication method thereof.
As is known, MEMS techniques of micromachining of semiconductor devices allow forming MEMS structures within semiconductor layers, deposited (for example, a polycrystalline silicon layer) or grown (for example, an epitaxial layer) on sacrificial layers that are at least in part removed through etching.
For instance, electroacoustic MEMS transducers (microphones) comprise a flexible membrane integrated in or on a semiconductor material die, as illustrated in
Membrane bending measurement may be of different types. For instance, bending detection may be of a piezoresistive or piezoelectric type, by integrating piezoresistive or piezoelectric elements in or on the membrane; of a capacitive type, wherein the membrane is capacitively coupled to another conductive region of the die; and of an electromagnetic type, wherein, for example, a coil is coupled to a magnetic region. In all cases, the variation of an electrical signal resulting from membrane deflection is measured.
In particular, capacitive microphones are currently widely used in various types of mobile devices, such as smartphones, PCs, tablets, and the like.
However, microphones of this type are disadvantageous. In fact, capacitive microphones may present reliability problems due to contamination by external particles and/or other contaminants (for example, water, dust, soldering vapors, etc.). In particular, the external particles may be trapped between the electrodes of the capacitor, acting as mechanical blocks for the membrane deflection and generating electrical leakage paths, thus causing malfunctioning and a reduction in performance of the microphone. In addition, the presence of contaminants between the electrodes of the capacitor may cause permanent damage to the microphone. Consequently, it is not possible to use a capacitive MEMS microphone in an environment such as water.
To overcome the above limitations, microphones of a piezoelectric type have recently been proposed, exploiting piezoelectricity, i.e., the capacity of some materials to generate a voltage when subjected to a deformation. In particular, piezoelectric microphones are able to operate even when they are immersed in fluids other than air (for example, water and non-conductive liquids) and are not affected by malfunctioning and/or reduction in performance due to contaminants and external particles as MEMS capacitive microphones.
In particular, in piezoelectric microphones, sensitive regions of piezoelectric material, such as aluminum nitride (AlN) and PZT (lead zirconate titanate) are formed on the membrane, in proximity of areas with a maximum stress. In presence of sound waves that cause deflection of the membrane, the sensitive regions, which deflect together with the membrane, give rise to a voltage variation correlated to the intensity of the detected sound wave. An interface circuit connected to the MEMS microphone amplifies and processes the electrical signal generated by the latter and outputs an analog or digital signal that can then be processed by a microcontroller of an associated electronic device.
An example of piezoelectric MEMS microphone is described in U.S. Pat. No. 8,896,184 and is illustrated in
However, the above known solution has some disadvantages.
In fact, the size of the ventilation hole 9 of the MEMS microphone 5 depends upon the gradient of stress on the stack of layers (and, in particular, in the piezoelectric material layer or layers) of each beam 8A, 8B, for example due to the residual stress, which, even at rest, causes an undesirable deflection of the beams, thus varying the size of the ventilation hole 9. The size variation of the ventilation hole 9 entails a less precise control of the position of the roll-off-frequency point (which determines the low-frequency behavior of the MEMS microphone 5). This is undesirable, since the position of the roll-off-frequency point can vary up to ±50 Hz, being incompatible with current market requirements where, in many cases, it is desired to have a maximum variation of roll-off frequency of ±10 Hz.
Furthermore, the MEMS microphones of a piezoelectric type currently on the market have a low sensitivity and, thus, a low SNR (Signal-to-Noise Ratio, in particular due to the noise intrinsic in the MEMS microphone and caused by the material and by the viscous resistances generated by the movement of the air of the microphone).
At least one embodiment of the present disclosure provides a MEMS transducer of a piezoelectric type that overcomes drawbacks of the prior art.
According to the present disclosure, a MEMS transducer and a fabrication method thereof are provided.
For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:
With reference to
The substrate 22 is traversed, from the second surface 22B, by a through cavity 23 having, for example, in top plan view, a circular shape with diameter d1, laterally delimited by a wall 23A.
A first dielectric layer 25 extends on the first surface 22A and is, for example, of USG (Undoped Silicate Glass), thermal silicon oxide (SiO2) or TEOS (TetraEthyl OrthoSilicate).
The sensitive region 36 extends on the first dielectric layer 25 and comprises a frame portion 30A extending peripherally, and a mobile portion 30B, carried and surrounded by the frame portion 30A. The frame portion 30A surrounds and supports the mobile portion 30B, anchoring it to the substrate 22.
In particular, the mobile portion 30B is formed by a membrane 32 suspended over the through cavity 23; a pillar 34 monolithic with the membrane 32; and a plurality of beams 39 rigid with the frame portion 30A and connected to the membrane 32 by the pillar 34, monolithically with the latter. The membrane 32, the pillar 34, and the beams 39 are of semiconductor material, for example polysilicon.
With reference to
In the embodiment of
The frame portion 30A of the sensitive region 36 comprises a first structural layer 37, overlying the first dielectric layer 25; a second dielectric layer 33, overlying the structural layer 37; a second structural layer 38, overlying the second dielectric layer 33; and a plurality of first anchorage elements 31 and second anchorage elements 35.
In detail, the first structural layer 37 is, for example, of polysilicon and has the same thickness and characteristics as the membrane 32, being formed simultaneously therewith, as described hereinafter with reference to
The second dielectric layer 33 is, for example made of USG, silicon oxide, or TEOS and defines the distance between the membrane 32 and the plurality of beams 39 and, thus, the height (measured along axis Z of reference system XYZ) of the pillar 34.
The second structural layer 38 is, for example, of polysilicon or silicon nitride (Si3N4) and has the same thickness and characteristics as the beams 39, being formed simultaneously therewith, as described hereinafter with reference to
The first anchorage elements 31 extend through the first dielectric layer 25 between the substrate 22 and the first structural layer 37. The first anchorage elements 31 thus have the same thickness as the first dielectric layer 25 (measured along axis Z of the reference system XYZ), and are monolithic with the membrane 32, being formed in the same manufacturing step, as described hereinafter with reference to
In the illustrated embodiment, a recess 40 extends between the first structural layer 37, the first anchorages 31, and the substrate 22 is recessed with respect to the wall 23A of the through cavity 23 and is, in top plan view (
The second anchorage elements 35 extend through the second dielectric layer 33, between the first and second structural layers 37, 38, and thus have the same thickness as the second dielectric layer 33.
The sensitive region 36 further comprises a plurality of sensing elements 50, each arranged at the first end 41 of a respective beam 39 and precisely straddling the first end 41 and the frame portion 30A.
Each sensing element 50 (
In use, an acoustic pressure wave acts on the membrane 32, exerting thereon a force that deflects the membrane 32 without deforming it; the force exerted by the acoustic pressure wave on the membrane 32 is then transmitted, concentrated, by the pillar 34 to the second end 42 of the plurality of beams 39, maximizing displacement (and thus deflection) of the plurality of beams 39 at the second end 42. In other words, the mobile portion 30B of the sensitive region 36 moves according to a piston-like movement because of displacement of the membrane 32 and of the pillar 34, so that the plurality of beams 39 moves according to a lever-arm effect as a result of the force exerted by the external acoustic pressure.
Deflection of the sensitive region 36 consequently generates a stress accumulation and, thus, a maximum strain (in absolute value) at the first and second ends 41, 42 of the beams 39; the accumulation of stress is then detected, on the first end 41, by the sensing elements 50, the layers of piezoelectric material 56 whereof are deformed by the stress. Consequently, according to the known inverse piezoelectric effect, deformation of the piezoelectric material layer 56 causes a charge accumulation between the first and second electrodes 55, 57, which, in turn, generates a corresponding electrical signal; the electrical signal is then transmitted to external processing circuits and devices through the metallization layer 65 (electrical lines 43).
In the MEMS transducer of
Moreover, the first sacrificial layer 125 has already been defined according to known masking and definition techniques so as to form a first plurality of anchorage openings 131′.
Next, the first transduction layer 132 is defined according to known photolithographic techniques so as to form the ventilation opening 45.
Next, the second sacrificial layer 133 is defined using photolithographic techniques known per se so as to form a second plurality of anchorage openings 133′ (arranged in a peripheral area of the second sacrificial layer 133 which is designed to form the frame portion 30A of the sensitive region 36) and a pillar opening 133″, arranged in a central portion of the second sacrificial layer 133, where the pillar 34 is to be formed.
In the embodiment of
Next, the sensing elements 50 are formed in the area that is designed to form the frame portion 30A of the sensitive region 36. In particular, the first electrode 55, the piezoelectric material layer 56, and the second electrode 57 are deposited in sequence. Next, the stack thus formed is defined in a per se known manner (for example, using photolithographic techniques). Then, the passivation layer 59 is deposited and defined on the sensing elements 50, in a per se known manner, to form the plurality of contact openings 60. Next, the metallization layer 65 is deposited and defined on the passivation layer 59 so as to form the electrical lines 43.
Then, the wafer 100 is diced, thus obtaining one or more MEMS transducers 20 that have the basic structure represented in
In greater detail, in the MEMS microphone 220 of
The reinforcement structure 270 has, for example, a cobweb shape, as visible in the top plan view of
Consequently, transmission of the stress, caused by the acoustic pressure, to the first and second ends 41, 42 of the beams 39 is optimized as compared to the MEMS microphone 20.
Next, fabrication steps are carried out similar to those described with reference to
With reference to
In particular, the top plan view of
In particular, here the membrane 532, the recess (not illustrated), and the ventilation opening 545 have a polygonal (for example, octagonal) shape. In addition, each beam 539 carries further sensing elements 550′, arranged in proximity of the pillar 534. In detail, each further sensing element 550′ is arranged on a corresponding beam 539 at the respective second end 542.
This embodiment may advantageously be used in applications where it is desirable for the MEMS microphone 520 to have a higher sensitivity. In fact, in use, each further sensing element 550′, in a way similar to the sensing elements 550, is subjected to a high stress, due to deformation of the respective beam 539, and generates a corresponding electrical signal.
With reference to
This embodiment enables detection of the sound at larger sound intensity ranges, without any loss of sensitivity. In fact, this embodiment allows stiffer beams to be obtained as compared to the embodiment described with reference to
With reference to
This embodiment has the advantage of optimizing the electrical capacitance of the piezoelectric actuator 750, the voltage of the signal generated by the MEMS microphone 720 is to be read. In fact, in the present embodiment, optimization of the electrical capacitance makes it possible to have an improvement of the value of SNR of the MEMS microphone 720.
With reference to
In particular, in
This embodiment may advantageously be used in applications where it is desirable for the MEMS microphone 820 to have a higher sensitivity.
With reference to
With reference to
In detail, here, the MEMS transducer (also referred to hereinafter as “MEMS microphone 1220”) has a general structure similar to the MEMS microphone 20 of
In detail, the membrane 1232, of a generally circular shape in top plan view (
In practice, with this configuration, the membrane 1232 projects on the outside of the cavity 1223 and faces the substrate 1222 with the projections 1300. In this way, the vertical movement (parallel to the axis Z) of the membrane 1232 is limited by the interference between the projections 1300 and the substrate 1222, protecting the membrane 1232 from external mechanical impact that might break it and thus jeopardize operation of the MEMS transducer 1220, without, on the other hand, modifying substantially air flow through the ventilation opening 1245.
Furthermore, as visible in
The MEMS transducer 1220 includes plural anchorage elements 1235 spaced apart from each other by portions of a second dielectric layer 1233. The MEMS transducer 1220 may be manufactured according to any one of the fabrication processes described with reference to
Alternatively, in a further embodiment, the projections 1300 may be missing, and the membrane 1232 may have, for example, a circular shape with a greater diameter than the cavity 1223. In this case, the peripheral portion of the membrane overlies and faces the substrate 1222 to be limited in its movement along axis Z along its entire circumference. Also in this case, the arrest element of
In the present MEMS transducer, manufactured according to any of the embodiments described with reference to
The first electrode 1501, the first piezoelectric material layer 1502, and the second electrode 1503 of each bimorphous cell 1500 form a first capacitor, having a first electrical capacitance Cp1. Likewise, the second electrode 1503, the second piezoelectric material layer 1504, and the third electrode 1505 of each bimorphous cell 1500 form a second capacitor, having a second electrical capacitance Cp2. In the embodiments illustrated by way of example in
The bimorphous cells 1500 of
With reference to
The above configuration is advantageously used in applications where it is desirable to maximize the output voltage Vout and minimize the equivalent capacitance between the cells C1, Cn. In addition, in different embodiments, each beam of the present MEMS transducer may comprise a number of bimorphous cells, connected together according to any of the configurations discussed with reference to
The MEMS transducer 2020 is substantially the same as the MEMS transducer 220 of
Thus, the ventilation opening 2045 is a hole defined by larger facing surfaces, that is the lateral wall 2031A of the anchorage element 2031 and a lateral wall 2080A of the membrane protrusion 2080, compared to the facing surfaces of the ventilation opening of the MEMS transducer 220 of
The MEMS microphone illustrated in
The electronic device 1100 is, for example, a portable mobile communication device, such as a mobile phone, a PDA (Personal Digital Assistant), a notebook, but also a voice recorder, an audio-file player with voice-recording capacity, etc. Alternatively, the electronic device 1100 may be an acoustic apparatus, such as a head-set system, a hydrophone, that is able to work under water, or else a hearing-aid device.
The electronic device 1100 of
The advantages of the present piezoelectric electroacoustic MEMS transducer clearly emerge from the foregoing description.
In particular, thanks to the structure of the mobile portion and positioning of the sensing elements on the ends of the beams the MEMS transducer has high sensitivity and low noise and, thus, high SNR; in fact, in use, the mobile portion performs a piston-like movement and the present configuration allows the lever-arm effect to be exploited, since the stress accumulated at the end of each beam is efficiently detected by the sensing elements.
Moreover, the sensitivity and flexibility of the mobile portion can be adjusted by appropriately configuring the beams (as illustrated, for example, in
In addition, the presence of a reinforcement structure on the membrane enables an increase in the pressure transmission efficiency; in fact, the reinforcement structure enables stiffening of the membrane without adding any significant contribution of mass, rendering it less subject to undesirable deflections. Consequently, in use, transmission of the stress to the ends of each beam is more efficient and contributes to rendering the MEMS microphone more sensitive.
Moreover, by positioning the sensing elements at the ends of each beam it is possible to reduce considerably the dependence of the sensitivity of the MEMS transducer upon the residual stress of the used piezoelectric materials. In fact, in this case, the sensing elements are arranged where the stress is maximum (in absolute value), and are thus able to detect the acoustic pressure in a precise way, with a higher sensitivity and with a lower contribution of noise.
In addition, the width of the ventilation opening is here precisely defined and according to known photolithographic techniques; this allows to precisely set, at a design level, the roll-off point, and, thus, the low-frequency behavior of the MEMS transducer. The roll-off point is consequently independent from possible residual stresses in the piezoelectric materials of the piezoelectric actuator.
Finally, the present MEMS transducer is resistant to water and liquid/solid contaminants/particulate coming from the outside since the sensing structure is of a piezoelectric type and does not have any parts where the contaminant can penetrate and obstruct the operating movement of the MEMS microphone.
Finally, it is clear that modifications and variations may be made to the MEMS transducer described and illustrated herein, without thereby departing from the scope of the present disclosure. For instance, the various embodiments described may be combined so as to provide further solutions. In particular, the beams, the membrane, the recess, and the ventilation opening may have different shapes. Moreover, all the membranes may have the reinforcement structure in all the embodiments.
Furthermore, the present MEMS microphone may have arrest elements in all presented embodiments, and the shapes may vary with respect to the ones illustrated.
In addition, a membrane protrusion like the protrusion 2080 of
The reinforcement structures of the membrane may be internal, i.e., facing the transduction frame.
The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.