JP2018098546A - Piezoelectric mems microphone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high sensitivity low noise piezoelectric MEMS microphone suitable for small and thin package mounting, by eliminating the problem that substantial characteristics deteriorate when reducing the capacity of back cavity.SOLUTION: A sense electrode film 22, a sense piezoelectric film 21a, a drive piezoelectric film 21b, a reference electrode film 26, a drive electrode film 23, and a support film 25 are provided as a diaphragm, an amplifier circuit 24 for amplifying a sense signal outputted from the sense electrode film 22 is provided, and vibration (amplitude) of the diaphragm due to acoustic signal pressure is suppressed by feeding a signal, amplified by the amplifier circuit 24, back to the drive electrode film 23 as a drive signal.SELECTED DRAWING: Figure 2a

Description

  The present invention relates to a piezoelectric MEMS microphone, and more particularly to a high sensitivity and low noise piezoelectric MEMS microphone suitable for mounting a small package.

2. Description of the Related Art Conventionally, for example, a smartphone having a great demand has been often used as a micro electro mechanical system (MEMS) microphone that is small and thin and has high-temperature processing resistance during solder flow.
9A and 9B show a schematic configuration of a conventional MEMS microphone. FIG. 9A is a top port type. Reference numeral 1 in this figure denotes a substrate of a package, 2 A lid, 3 is an opening (port) opened in the lid, 4 is a MEMS acoustic transducer, 5 is an ASIC (Special Purpose Semiconductor Integrated Circuit), and 100 is a back cavity.

  FIG. 9B shows a bottom port type. In this case, an opening 6 is provided on the substrate 1 side. In the bottom port type, the opening 6 of the substrate 1 and in the top port type, the opening 3 of the lid 2 serves as an input port for the acoustic signal pressure, and the openings 3 and 6 sandwich the acoustic transducer 4 (vibrating plate portion). The closed space on the opposite side becomes the back cavity. The ASIC 5 includes an analog amplifier circuit, a bias voltage circuit, an analog-digital conversion circuit, or the like, and a general MEMS microphone has a configuration in which the MEMS acoustic transducer 4 and the ASIC 5 are mounted in a small package.

  According to such a microphone, the acoustic signal pressure input from the opening 3 on the lid side in the top port type and the opening 6 on the substrate side in the bottom port type is captured by the acoustic transducer 4. The vibration of the diaphragm is converted into an electrical signal, and then a signal processed by the ASIC 5 is output.

A. Dehe, M. Wurzer, M. Fuldner and U. Krumbein, “The Infineon Silicon MEMS Microphone,” AMA Conferences 2013−SENSOR 2013, OPTO 2013, IRS 2 2013, pp.95−99, 2013. R. Littrell * and K. Grosh, “Noise minimization in micromachined piezoelectric microphones,” 21st Int. Congress on Acoustic (ICA), 2pEAa3, 2013.

  By the way, in the back cavity 100 that is the closed space on the opposite side of the acoustic transducer 4 to which the acoustic signal pressure is input, the trapped air is compressed and expanded according to the vibration of the diaphragm of the acoustic transducer 4. Work as a compliance.

FIG. 9C shows a simplified acoustic equivalent circuit of a microphone.
In the figure, the input acoustic signal pressure Pain is divided by the acoustic compliance Cm of the diaphragm and the acoustic compliance Cbc of the back cavity, and the effective acoustic signal pressure Pam applied to the diaphragm is expressed by the following Equation 1.
Pam = Cbc / (Cm + Cbc) × Pain (1)
In this formula (1), if the acoustic compliance Cm is large, the effective acoustic signal pressure Pam is small, and it is known that the main characteristics of the microphone such as the effective sensitivity and signal-to-noise ratio of the acoustic transducer 4 are deteriorated. (Non-Patent Document 1).

  In particular, since the acoustic compliance of the back cavity 100 is proportional to its volume, it is a limiting factor that greatly affects the characteristics of the top port type [FIG. 9A], which has a smaller back cavity volume than the bottom port type [FIG. 9B]. It has become. In the top port type, since the volume of the back cavity 100 is proportional to the thickness of the MEMS substrate (silicon substrate) 1, it is difficult to make the substrate 1 thinner.

Even in the bottom port type, if the package is to be reduced in size and thickness, the volume of the back cavity 100 is reduced, and the characteristics of the microphone are sacrificed.
As described above, the present situation is that the MEMS microphones are limited in size and thickness due to acoustic limitations.
On the other hand, in smartphones, which are the main market for MEMS microphones, the demand for smaller and thinner parts has become stricter year by year. Needless to say, in the wearable terminal market, such as smart watches, which have been attracting attention in recent years, there is a demand for smaller and thinner layers than smartphones.

  The present invention has been made in view of the above problems, and its object is to eliminate the disadvantage that the substantial characteristics of the microphone deteriorate when the volume of the back cavity is reduced, and is suitable for mounting a small and thin package. An object of the present invention is to provide a piezoelectric MEMS microphone with high sensitivity and low noise.

In order to achieve the above object, a piezoelectric MEMS microphone according to the invention of claim 1 is produced by a diaphragm having at least two or more piezoelectric films for converting an acoustic signal pressure into an electric signal by a piezoelectric effect. A MEMS acoustic transducer having a sense electrode for outputting an electrical signal, a drive electrode for applying vibration to the diaphragm by the electrical signal, and an amplifier circuit for amplifying the electrical signal output from the sense electrode The signal amplified by the amplifier circuit is fed back to the drive electrode to suppress vibration of the diaphragm due to acoustic signal pressure.
The MEMS acoustic transducer according to a second aspect of the invention includes a sensing piezoelectric film and a driving piezoelectric film as the diaphragm, and a drive electrode film, the driving piezoelectric film, and a reference electrode for providing a reference potential on the support film. The film, the piezoelectric film for sensing, and the sense electrode film are sequentially stacked.

  The MEMS acoustic transducer according to a third aspect of the invention divides a detection area into a plurality of areas, includes a piezoelectric film for sensing and a piezoelectric film for driving as the diaphragm in each area, and each of the support films in the divided areas. On top of this, a drive electrode film, the drive piezoelectric film, a drive reference electrode film for applying a reference potential, a dielectric film serving as an insulating layer, a sense reference electrode film for series connection, the sense piezoelectric film, and a sense electrode Films are sequentially stacked, and a plurality of divided regions are connected in series by the connection of the sense electrode film and the sensing reference electrode film between the divided regions, and an electric signal superimposed is output.

  According to the above configuration, the vibration of the diaphragm due to the acoustic signal pressure is converted into an electric signal, and this electric signal is output from the sense electrode film. The electric signal is amplified by the amplifier circuit and fed back to the drive electrode film. If it does, the vibration which becomes an antiphase with respect to the vibration by acoustic signal pressure will be added from the piezoelectric film for a drive, and it will become possible to suppress the vibration (amplitude) of a diaphragm effectively.

That is, the acoustic compliance Cm of the diaphragm is expressed as follows, where ΔV is the volume of the back cavity displaced by the diaphragm that has received the acoustic signal pressure Pam.
Cm = ΔV / Pam (2)
It is represented by
As can be seen from Equation (2), if the volume ΔV is reduced by suppressing vibration of the diaphragm, the acoustic compliance Cm of the diaphragm is reduced. In Equation (1), the acoustic compliance Cm of the diaphragm is reduced. The acoustic compliance Cbc of the back cavity can be kept sufficiently small. As a result, the effective acoustic signal pressure Pam expressed by the above equation (1) is made substantially equal to the input acoustic signal pressure Pain regardless of the back cavity volume. It becomes possible.

  According to the present invention, when the volume of the back cavity is reduced, the disadvantage that the substantial characteristics of the microphone are deteriorated can be solved, and a high sensitivity and low noise piezoelectric MEMS microphone suitable for mounting a small and thin package. Can be realized.

It is a figure which shows the basic principle of the piezoelectric type MEMS microphone of this invention. It is a figure which shows the specific structure (feedback circuit) of the piezoelectric MEMS microphone of 1st Example (the acoustic transducer part is what hatched partially in AA sectional drawing of FIG. 2b). It is a top view of the acoustic transducer part of 1st Example. It is explanatory drawing which shows the operation | movement principle of 1st Example. It is a figure (what a part of section was hatched) which shows the concrete composition (feedback circuit) of the piezoelectric type MEMS microphone of the 2nd example. It is a top view of the acoustic transducer part of 2nd Example. It is a graph which shows the piezoelectric film thickness dependence of the diaphragm acoustic compliance in 1st Example. It is a graph which shows the back cavity volume dependence of the signal noise ratio of 1st Example (when aluminum nitride is used as a piezoelectric film). It is a graph which shows the back cavity volume dependence of the signal noise ratio of 1st Example (when scandium aluminum nitride is used as a piezoelectric film). It is a graph which shows the amplifier circuit gain width dependence of the signal noise ratio in 1st Example. It is a figure which shows the mounting form [Figure (A), (B)] of the conventional MEMS microphone, and the simplified acoustic equivalent circuit [Figure (C)].

FIG. 1 shows the basic principle of the piezoelectric MEMS microphone of the present invention. In the figure, reference numeral 11 is a diaphragm having a plurality of piezoelectric films, 12 is a sense electrode, 13 is a drive electrode, and 14 is an amplifier circuit. is there.
The vibration plate 11 is vibrated and displaced by, for example, an acoustic signal pressure input from the sense electrode side, and the mechanical vibration is converted into an electric signal (sense signal) by the sense electrode 12. The part that converts the acoustic signal into an electrical signal is a part called an acoustic transducer, and is produced on, for example, a silicon substrate using a MEMS manufacturing technique.

  The sense signal output from the sense electrode 12 is amplified by the amplifier circuit 14. In general, since an electrical signal converted by an acoustic transducer has a high equivalent output impedance and a low signal strength, in the present invention, impedance conversion is performed by the amplifier circuit 14 and an amplified drive signal is generated and fed back to the drive electrode 13. Let When this drive signal is applied to the drive electrode 13, a mechanical drag having a phase opposite to that of the acoustic signal pressure, that is, a force for deforming the diaphragm 11 by the acoustic signal pressure is applied to the diaphragm 11 by inverse conversion of acoustic (mechanical) electrical conversion. The power to cancel is given. In FIG. 1, the polarity and the amplification gain are selected so that the amplifier circuit 14 gives a force to cancel the deformation of the diaphragm 11.

  In this way, the so-called negative feedback principle suppresses the sense signal to approximately 1 / L of the loop gain (considering not only the gain of the amplifier circuit 14 but also the acoustoelectric conversion gain). As a result, the vibration of the diaphragm 11 is suppressed, and the acoustic compliance (Cm) of the diaphragm 11 defined by Equation (2) can be kept sufficiently small relative to the acoustic compliance (Cbc) of the back cavity. Therefore, even if the volume of the back cavity is reduced, the basic characteristics of the microphone such as sensitivity and signal / noise ratio are not deteriorated, and the microphone can be mounted in a small and thin package. The output from the amplifying circuit 14 (drive voltage signal) is used as the output of the MEMS microphone. At this time, a buffer circuit for taking out to the outside and an analog-digital conversion circuit for taking out as a digital signal are added. May be.

FIG. 2a shows a specific configuration of the first embodiment (acoustic transducer portion is a cross-sectional view taken along line AA in FIG. 2b), and FIG. 2b shows a configuration of the acoustic transducer portion (viewed from the top surface). Table 1 shows the characteristics and performance of various piezoelectric film materials.
In FIG. 2a, reference numeral 21a is a sense piezoelectric film (thin film), 21b is a drive piezoelectric film (thin film), 22 is a sense electrode film (thin film), 23 is a drive electrode film (thin film), and 25 is a support film (dielectric). Reference numeral (thin film) 26 is a reference electrode film (thin film) connected to a reference potential (ground 50). The sense electrode film 22, the sense piezoelectric film 21a, the reference electrode film 26, the drive piezoelectric film 21b, and the drive The electrode film 23 and the support film 25 constitute a diaphragm. Reference numeral 27 denotes a silicon (Si) substrate. This acoustic transducer is manufactured using the MEMS manufacturing technique by sequentially laminating the above-described members on the silicon substrate 27.

  As shown in Table 1 below, for example, an aluminum nitride (AIN) film is used for the piezoelectric films 21a and 21b, and a molybdenum (Mo) film is used for the electrode films 22 and 23, for example. The crystal orientation (piezoelectric polarity) of the sensing piezoelectric film 21a and the driving piezoelectric film 21b made of aluminum nitride is the same. As the support film 25, a dielectric thin film such as a silicon nitride film or a silicon oxide film, or aluminum nitride similar to the piezoelectric film is used. The support film 25 may be a silicon thin film. In that case, the support film 25 can also be doped with impurities to be used as a drive electrode film.

  As shown in FIG. 2b, this acoustic transducer has a through-hole (lower space) serving as a back cavity or an input port for acoustic signal pressure at the center of the substrate 27 below the diaphragm (21a, 21b, 25). 200 is formed, and an inter-beam gap (cut portion) G is formed in the center of the diaphragm, so that a cantilever structure in which one side at the left and right ends is supported (three sides are open) is obtained. In addition, although the diaphragm of an Example is comprised from the two rectangular bodies which oppose, the several wedge shape which has several triangles, polygons, or circular support fixed outer periphery by which one side was supported and fixed. You may be comprised from.

  Further, an amplifier circuit 24 for inputting the sense signal of the sense electrode film 22 is disposed, and an output signal of the amplifier circuit 24 is fed back to the drive electrode film 23 as a drive signal. The amplifier circuit 24 is incorporated in an ASIC (special purpose silicon semiconductor integrated circuit), and the ASIC and the acoustic transducer are mounted together in a small package as shown in FIGS.

  According to such a 1st Example, a sense signal is output from the sense electrode film 22 by a piezoelectric effect because a diaphragm vibrates with an acoustic signal pressure. This sense signal is amplified by the amplifier circuit 24 and fed back to the drive electrode film 23 as a drive signal, whereby a mechanical force is applied to the diaphragm by the reverse piezoelectric effect through the drive piezoelectric film 21b. Vibration (amplitude) is suppressed.

The operation principle of the embodiment will be described with reference to FIG.
Consider a case where an acoustic signal pressure is applied from above in FIG. 3 and the diaphragms (piezoelectric films 21a and 21b and support film 25) having a cantilever structure are curved downward (convex upward). As shown in the part E1 in the figure, tensile stress is applied to the upper half of the diaphragm (beam) (the part including the piezoelectric films 21a and 21b), and compressive stress is applied to the lower half (support film 25). In the vicinity of the center of the plate, there is a surface to which no stress is applied (assuming that it is a core surface s). The two layers of piezoelectric films 21a and 21b are formed above the core surface s. Further, although the position in the height direction of the core surface s differs depending on the material mechanical constant such as Young's modulus of the material constituting the diaphragm and the film thickness, the support film 25 is made of the same aluminum nitride, and the electrode films 22, In the simple case where the film thicknesses 23 and 26 are ignored, it becomes the center plane of the entire thickness.

As described above, tensile stress is generated in the two layers of aluminum nitride piezoelectric films 21a and 21b, and an aluminum nitride thin film is generated so that a negative voltage is generated in the sense electrode film 22 due to the piezoelectric effect at this tensile stress. The crystal orientation (piezoelectric characteristics) is selected (the c-axis is perpendicular to the substrate and upward).
When the generated sense voltage is inverted and amplified by the amplifier circuit 24 and fed back to the drive electrode film 23, the drive piezoelectric film 21b has an action of contracting due to the reverse piezoelectric effect, and the diaphragm is projected upward (projected downward). ) To bend.

  Since the drive piezoelectric film 21b is sandwiched between the sense piezoelectric film 21a and the support film 25, as shown by E2 in FIG. 3, the drive piezoelectric film 21b has tensile stress, the sense piezoelectric film 21a and the sense piezoelectric film 21a. A compressive stress is generated as a reaction in the support film 25. In other words, the feedback drive voltage causes a compressive stress opposite to the tensile stress due to the acoustic signal pressure to occur in the sense piezoelectric film 21a, and if the gain of the amplifier circuit 24 is kept sufficiently large, no sense signal is generated ( Both stresses cancel each other as an average value of the sense electrode region. At the same time, a rotational moment acting to bend (upward) in a direction opposite to the bending (downward) due to the acoustic signal pressure acts on the diaphragm, so that the displacement of the diaphragm is suppressed and the acoustic compliance (Cm) is reduced. However, in order to sufficiently reduce the acoustic compliance, the extension length from the support end of the sense electrode film 22 (αL with respect to the beam length L), the film thickness of the drive piezoelectric film 21b (the total thickness of the diaphragm 2H) [Beta] H) and the film thickness ([gamma] H) of the sensing piezoelectric film 21a must be carefully selected.

One typical design example is as follows. However, it is assumed that the support film 25 is made of the same aluminum nitride as the piezoelectric films 21a and 21b, and the approximate analysis is performed ignoring the film thicknesses of the electrode films 22 and 23.
For example, in a cantilever-structured diaphragm, the length from the support end (stretched length L) is 350 microns and the width is 1400 microns facing each other, and the diaphragm thickness (2H) is 1 .68 microns (H = 0.84 microns) and its resonant frequency is set to about 20 kHz. The extension length (αL) of the sense electrode film 22 is fixed at 154 microns (α = 0.44) from the viewpoint of optimizing the signal to noise ratio.

  FIG. 5 shows calculated values of the acoustic compliance change of the diaphragm when the thickness of the sense piezoelectric film 21a is changed using the film thickness of the drive piezoelectric film 21b as a parameter. For example, if the drive piezoelectric film thickness is 0.33 micron (β = 0.4), and the sense piezoelectric film thickness is also 0.33 micron (γ = 0.4), the acoustic compliance Can be made substantially zero.

FIG. 6 shows the calculation of the dependency of the signal-to-noise ratio on the back cavity volume between the conventional structure and the structure of the embodiment, with the gain of the amplifier circuit 24 being infinite under the above conditions. In the conventional structure to be compared, aluminum nitride is used as a cantilever having the same size as that of the embodiment, a reference electrode is provided on the diaphragm thickness center plane (core surface s), and the uppermost and lowermost surfaces of the piezoelectric film are provided. Each of the sense electrodes was arranged with the above-mentioned optimum extension length (the film thickness was 0.33 microns).
As shown in FIG. 6, in the conventional structure, the signal-to-noise ratio rapidly deteriorates as the back cavity volume is reduced. In contrast, in the embodiment structure, the signal-to-noise ratio can be kept substantially constant even when the back cavity volume is reduced. In the conventional structure, since the sense electrode is doubled, the signal-to-noise ratio is about 3 dB larger than that of the structure in the case where the back cavity volume is large, and the standard back cavity volume of the bottom port type is 3 mm. ^In the case of ³ , the conventional structure is slightly better.

In addition, in the top port type in which both characteristic lines cross at 2.5 mm ³ and the opening of the MEMS transducer becomes the back cavity, the back cavity volume (silicon substrate thickness≈0.6 mm) is about 0.6 mm ³ . The structure was superior to about 5.6 dB. When the layer thickness of the silicon substrate 27 is further reduced, the superiority or inferiority of both becomes more remarkable, and it can be seen that the structure of the embodiment is a structure suitable for promoting the miniaturization and thinning of the MEMS microphone. As a matter of course, the characteristics of the piezoelectric MEMS microphone differ depending on the characteristics of the piezoelectric material.

Table 1 shows typical piezoelectric materials such as aluminum nitride (AlN), scandium aluminum nitride (AI _x-1 Sc _x N), zinc oxide (ZnO), and lead zirconate titanate (PZT) that affect the microphone characteristics. Comparison of material constants such as Young's modulus and transverse piezoelectric strain coefficient giving

As shown in Table 1, the corresponding figure of merit in the signal-noise ratio (FOM) is represented by the ratio of the coupling coefficient (k _{31 ²⁾} and the loss angle (tan [delta), the greater the performance index, substantially to The signal-to-noise ratio can be improved in a proportional manner. Compared to zinc oxide and lead zirconate titanate, aluminum nitride has a 6 to 40 times higher performance index and is a suitable material for piezoelectric MEMS microphones. Further, it is known that scandium aluminum nitride (AI _x-1 Sc _x N) obtained by adding scandium to aluminum nitride has a higher transverse piezoelectric strain coefficient than aluminum nitride. For example, when the ratio of scandium is 35%, it can be expected that the figure of merit is improved by about 7 times.

FIG. 7 shows a calculation result of the back cavity volume dependency of the signal-to-noise ratio when scandium aluminum nitride (AI _x-1 Sc _x N: x = 0.35) is used as the piezoelectric films (21a, 21b). The shape corresponding to 6) is shown. The plane dimensions are the same as the values in FIG. 6, but the total film thickness of scandium aluminum nitride is increased to 1.86 microns in order to set the resonance frequency of the diaphragm to 20 kHz, and the parameters of α, β, and γ are as described above. The value was the same.
As shown in FIG. 7, in this case, the back cavity dependency is qualitatively the same as in FIG. 6, but the absolute value of the signal-to-noise ratio is improved by about 8 dB.

FIG. 8 shows the result of calculating the gain dependency of the amplifier circuit 24 of the signal-to-noise ratio when the above scandium aluminum nitride is used. This is the case where the back cavity volume is 0.6 mm ³ described above. And two examples in the case of 0.1 mm ³ in which the substrate 27 is thinned to 100 microns or less. From FIG. 8, it can be seen that the deterioration of the signal-to-noise ratio can be suppressed to 1 dB or less if the gain is 50 dB or more when the former is 0.6 mm ³ and the gain is 67 dB or more even when the latter is 0.1 mm ³ .

  The drive voltage in the first embodiment is, for example, 44 mV / Pa (−27 dBV / Pa). By the way, a high frequency region proportional to the reciprocal of a time constant defined by the product of an equivalent drive voltage output resistance (output impedance of drive voltage, resistance of drive electrode film 23 and resistance of reference electrode film 26) and drive electrode capacitance. It is necessary to make the cutoff frequency (drive voltage cutoff frequency) larger than the band required for the microphone. For example, when the microphone band is 10 kHz and the drive voltage cutoff frequency is 20 kHz, the equivalent drive voltage output resistance needs to be smaller than 45 kΩ. The power consumption required for driving in this case is as small as 17 μW at the required acoustic signal pressure of 120 dBSPL, which is not a problem in practice.

  FIG. 4a shows the configuration of the acoustic transducer of the second embodiment (cross-sectional view of the left side portion of the diaphragm of FIG. 4b), and FIG. 4b shows the configuration of the acoustic transducer portion (plan view). In the second embodiment, rectangular diaphragms (cantilever structure) similar to those in the first embodiment are arranged to face each other, but as shown in FIG. 4b, the upper senses of the left and right diaphragms are sensed. The area is divided into areas I to IV, a total of eight areas are provided, and a sense electrode film and a reference electrode film are connected in series between adjacent sense areas so that the sense voltage is superimposed.

  In FIG. 4a, 32 from the upper side is a sense electrode film, 31a is a sensing piezoelectric film, 36a is a sensing reference electrode film to which a reference potential is applied, 38 is an insulating film (dielectric thin film), and 36b is a reference potential (ground). 50) is connected to the drive reference electrode film, 31b is the drive piezoelectric film, 33 is the drive electrode film, and 35 is the support film. These are formed on the silicon substrate as a vibration plate as in FIG. 2a. Laminated in sequence and manufactured using MEMS manufacturing technology. The sense electrode film 32, the sense piezoelectric film 31a, and the sense reference electrode film 36a serve as a sense region, and the drive electrode film 33, the drive piezoelectric film 31b, and the drive reference electrode film 36b serve as a drive region.

  In FIG. 4b, 37 is a silicon substrate, 200 is a through-hole (lower space) formed in the silicon substrate 37, 40a is a sense signal output pad, 40b is a sense reference pad, and 41a to 41e are wirings. In the second embodiment, the sense area described with reference to FIG. 4a is divided into eight, and the sense electrode film 32 on the left side of the area I is connected to the sense signal output pad 40a via the wiring 41a. The sense electrode film 32 in the region II is connected to the reference electrode film 36a by the wiring 41b, so that the sensing reference electrode film 36a and the sense electrode film 32 between the eight regions are connected in series by the wirings 41b to 41d in order. A sense reference pad is connected to the sense reference electrode film 36a on the right side of 1 through a wiring 41e, and finally the amplifier circuit (24) is connected to the diaphragm through the sense signal output pad 40a and the sense reference pad 40b. Connect to. Note that the drive region composed of the drive electrode film 33, the drive piezoelectric film 31b, and the drive reference electrode film 36b does not need to be divided and is the same as in the first embodiment.

  The sense piezoelectric film 31a and the drive piezoelectric film 31b are made of, for example, an aluminum nitride film or a scandium aluminum nitride film, and the sense electrode film 32 and the drive electrode film 33 are made of molybdenum. When an aluminum nitride film is used, the crystal orientation (piezoelectric polarity) of both piezoelectric films 31a and 31b is the same. As in the first embodiment, as the support film 35 and the insulating film 38, a dielectric film such as a silicon nitride film or a silicon oxide film or aluminum nitride is used, and when a silicon thin film is used as the support film 35, impurities are doped. Thus, it can also be used as the drive electrode film 31b for conductivity.

  Also in the acoustic transducer of the second embodiment, the through-hole 200 is formed in the central portion of the substrate 37, and the gap G between the beams is provided in the central portion of the diaphragm, so that the cantilever held on one side of the left and right ends. It becomes a beam structure. The diaphragm may be composed of a plurality of triangles, polygons having one side supported and fixed, or a plurality of wedges having a circular support fixed outer periphery.

According to the second embodiment, the sense voltage can be superimposed and added to increase it by 8 (18 dB), and the requirements for the input conversion noise and gain of the amplifier circuit (24) can be relaxed. For example, the gain required for the amplifier circuit shown in FIG. 8 can be reduced by 18 dB.
Similarly to the first embodiment, if the stretch length of the sense electrode film 32, the sense piezoelectric film thickness, and the drive piezoelectric film thickness are carefully selected, the acoustic compliance (Cm) of the diaphragm is set to the back cavity (for example, 200). Compared to the acoustic compliance (Cbc), the back cavity volume can be reduced without impairing the signal-to-noise ratio and sensitivity.

In the above embodiment, an example of a diaphragm having a cantilever structure is shown. However, a diaphragm having a both-end support structure in which both ends are supported and fixed to a substrate and a disk-shaped vibration in which an outer periphery is supported and fixed to a substrate. By applying the above embodiment to the structure of the plate, a high-sensitivity and low-noise piezoelectric MEMS microphone suitable for mounting a small and thin package can be produced.
In the embodiment, the sense piezoelectric film 31a and the drive piezoelectric film 31b are arranged one by one. However, a plurality of piezoelectric films may be provided in the vertical direction.

5. ASIC (Special Purpose Semiconductor Integrated Circuit),
11 ... diaphragm, 12 ... sense electrode,
13 ... Drive electrode 14,24 ... Amplifier circuit,
21a, 31a ... Sense piezoelectric film,
21b, 31b ... Drive piezoelectric film,
22, 32 ... sense electrode film,
23, 33 ... drive electrode film,
25, 35 ... support film, 26 ... reference electrode film,
27, 37 ... silicon substrate,
36a ... Sense reference electrode film,
36b ... Reference electrode film for drive,
38. Insulating film (dielectric film),
40a: sense signal output pad,
40b ... Sense reference pad,
41a-41e ... wiring,
200 ... through hole.

Claims (3)

A diaphragm having at least two piezoelectric films that convert an acoustic signal pressure into an electric signal by a piezoelectric effect, a sense electrode for outputting an electric signal generated by the diaphragm, and applying vibration to the diaphragm by the electric signal A MEMS acoustic transducer having a drive electrode for;
An amplification circuit that amplifies the electrical signal output from the sense electrode,
A piezoelectric MEMS microphone that suppresses vibration of the diaphragm due to acoustic signal pressure by feeding back the signal amplified by the amplifier circuit to the drive electrode. The MEMS acoustic transducer includes a sensing piezoelectric film and a driving piezoelectric film as the diaphragm,
2. The piezoelectric element according to claim 1, wherein a drive electrode film, the drive piezoelectric film, a reference electrode film for applying a reference potential, the sense piezoelectric film, and the sense electrode film are sequentially stacked on the support film. Type MEMS microphone. The MEMS acoustic transducer divides a detection area into a plurality of areas, and includes a sensing piezoelectric film and a driving piezoelectric film as the diaphragm in each area,
On each support film in the divided region, a drive electrode film, the above drive piezoelectric film, a drive reference electrode film for applying a reference potential, a dielectric film serving as an insulating layer, a sense reference electrode film for series connection, and the above A piezoelectric film for sensing and a sense electrode film are sequentially stacked,
2. The piezoelectric MEMS according to claim 1, wherein a plurality of divided regions are connected in series to output an electric signal superimposed by connecting the sense electrode film and the sense reference electrode film between the divided regions. microphone.

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