JP2018137297A - Piezoelectric element - Google Patents

Piezoelectric element Download PDF

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JP2018137297A
JP2018137297A JP2017029541A JP2017029541A JP2018137297A JP 2018137297 A JP2018137297 A JP 2018137297A JP 2017029541 A JP2017029541 A JP 2017029541A JP 2017029541 A JP2017029541 A JP 2017029541A JP 2018137297 A JP2018137297 A JP 2018137297A
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diaphragm
region
thin film
diaphragms
piezoelectric
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JP6894719B2 (en
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王義 山崎
Kimiyoshi Yamazaki
王義 山崎
尚己 桝本
Naomi Masumoto
尚己 桝本
博行 口地
Hiroyuki Kouchi
博行 口地
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New Japan Radio Co Ltd
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Abstract

PROBLEM TO BE SOLVED: To provide a high-sensitive piezoelectric element which has an improved signal-to-noise ratio and is suitable for mass-production by preventing, in a case where cantilever beam-structured diaphragms are adopted, degradation in characteristics caused by a gap between the diaphragms or between the diaphragms and a substrate.SOLUTION: A cantilever beam-type piezoelectric MEMS microphone comprises diaphragms (12 and 14) each including: piezoelectric thin films 7a and 7b; and an electrode thin film 8. The microphone is configured such that one end of each of the diaphragms is supported by a substrate 1. An open end-side region II on the diaphragm is made thicker than a supported end-side region I so that an elastic modulus of the open end-side region II on the diaphragm may be greater than that of the supported end-side region I. In addition, the elastic modulus of the open end side of the diaphragm can be made greater by providing an opening in the region I or by making an elastic modulus of a material for the region II on the diaphragm greater than that of a material for the region I.SELECTED DRAWING: Figure 3

Description

本発明は圧電素子、特に高感度・低雑音となる圧電型ＭＥＭＳマイクロフォン等の圧電素子の構成に関する。   The present invention relates to a configuration of a piezoelectric element, particularly a piezoelectric element such as a piezoelectric MEMS microphone having high sensitivity and low noise.

近年、巨大な需要のあるスマートフォンには、小型で薄くかつハンダフロー時の高温処理耐性を有するＭＥＭＳ（Micro Electro Mechanical System）が多く使われている。その大部分は、音響圧力による振動板の振動変位を対向する固定板との間の容量の変化を電圧変化に変換して出力する容量型ＭＥＭＳマイクロフォン（音響トランスデューサ）である。しかし、この容量型は、振動板と固定板との隙間の空気の流動で生じる音響的抵抗のために、信号雑音比を改善することが困難になりつつある。そこで注目されているのが、圧電薄膜が用いられ基本的に単一となる振動板の歪により、音響振動を電圧変化として取り出す圧電型ＭＥＭＳマイクロフォンである。   2. Description of the Related Art In recent years, MEMS (Micro Electro Mechanical System) that is small and thin and has high-temperature processing resistance during solder flow is often used for smartphones that are in great demand. Most of them are capacitive MEMS microphones (acoustic transducers) that convert a change in capacitance between a diaphragm and an opposing fixed plate due to acoustic pressure into a voltage change and output the change. However, in this capacitive type, it is becoming difficult to improve the signal-to-noise ratio due to acoustic resistance generated by the air flow in the gap between the diaphragm and the fixed plate. Therefore, attention is focused on a piezoelectric MEMS microphone that uses a piezoelectric thin film and extracts acoustic vibration as a voltage change due to distortion of a single diaphragm.

この圧電型ＭＥＭＳマイクロフォンとしては、例えば図８，図９に示されるような片持ち梁構造のものがあり、図８は四角形振動板を持つマイクロフォン、図９は三角形振動板を持つマイクロフォンである。
図８において、符号２は基板に一端が支持され圧電薄膜を有する振動板、３は基板の中央部に開けられた空孔であり、この振動板２は、その一端が基板の空孔３の図の左右端に支持される形で、２つ設けられる。従って、このマイクロフォンの２つの振動板２は、四角形の３辺において開放され、１辺が基板に支持された状態の片持ち梁構造となる。 This piezoelectric MEMS microphone has a cantilever structure as shown in FIGS. 8 and 9, for example. FIG. 8 shows a microphone having a square diaphragm, and FIG. 9 shows a microphone having a triangular diaphragm.
In FIG. 8, reference numeral 2 denotes a diaphragm having one end supported by a substrate and having a piezoelectric thin film, 3 is a hole opened in the center of the substrate, and this diaphragm 2 has one end of the hole 3 of the substrate. Two are provided so as to be supported at the left and right ends of the figure. Accordingly, the two diaphragms 2 of the microphone have a cantilever structure in which the three sides of the quadrangle are opened and one side is supported by the substrate.

図９において、符号４は、三角形の振動板であり、この振動板４は、その一端が基板の空孔３の左右端及び上下端に支持される形で、４つ設けられる。従って、このマイクロフォンの４つの振動板４は、三角形の２辺において開放され、１辺がシリコン基板に支持された状態の片持ち梁構造となる。   In FIG. 9, reference numeral 4 denotes a triangular diaphragm, and four diaphragms 4 are provided such that one end thereof is supported by the left and right ends and the upper and lower ends of the hole 3 of the substrate. Accordingly, the four diaphragms 4 of the microphone have a cantilever structure in which the two sides of the triangle are open and one side is supported by the silicon substrate.

特許第５７０７３２３号公報Japanese Patent No. 5707323 特許第５９３６１５４号公報Japanese Patent No. 5936154

しかしながら、上記圧電型ＭＥＭＳマイクロフォンでは、片持ち梁構造を採用することによって、容量型ＭＥＭＳマイクロフォンに相当する感度や信号雑音比が得られているが、それ以上の特性向上を図るまでには至っていない。
上記片持ち梁構造のマイクロフォンでは、図８において、２つの振動板２の間と振動板−基板間にギャップ（ｇ）があり、図９において、４つの振動板４のそれぞれの間にギャップ（ｇ）があり、これらのギャップを介して音響圧力信号が漏れるために、振動板２，４に対する音響インピーダンスと並列に入るギャップ部分の音響抵抗が小さくなる。このギャップ音響抵抗の低下は、低周波ロールオフ周波数の増大、つまり低周波数での感度劣化をもたらすという不都合があり、更には、信号雑音比を劣化させる要因ともなっている。 However, in the piezoelectric MEMS microphone, the sensitivity and signal-to-noise ratio corresponding to the capacitive MEMS microphone can be obtained by adopting the cantilever structure, but no further improvement in characteristics has been achieved. .
In the cantilever microphone, the gap (g) is present between the two diaphragms 2 and between the diaphragm and the substrate in FIG. 8, and the gap (g) between each of the four diaphragms 4 is illustrated in FIG. g), and since the acoustic pressure signal leaks through these gaps, the acoustic resistance of the gap portion in parallel with the acoustic impedance to the diaphragms 2 and 4 is reduced. This decrease in the gap acoustic resistance has the disadvantage of increasing the low-frequency roll-off frequency, that is, degrading the sensitivity at low frequencies, and is also a factor that degrades the signal-to-noise ratio.

図９の三角形の振動板４では、４つのそれぞれの先端が一箇所に集まるような構造を採用することにより、ギャップに起因する問題がある程度改善されるが、十分ではない。
また、このギャップ（ｇ）の幅は、製作時の加工精度と振動板２，４を構成する薄膜の残留応力の面内均一性によって決まり、例えばギャップを挟んで相対する開放端が高さ方向に変形してギャップの幅が広がること等が生じ、量産レベルでマイクロフォンの特性に影響のない範囲に抑えることは容易ではない。
上記のような問題点は、圧電型マイクロフォン以外でも圧力、振動、変位等を扱うデバイスに使用される他の圧電素子において同様に生じる。 In the triangular diaphragm 4 of FIG. 9, the problem caused by the gap is improved to some extent by adopting a structure in which the four tips are gathered in one place, but it is not sufficient.
The width of the gap (g) is determined by the processing accuracy at the time of manufacture and the in-plane uniformity of the residual stress of the thin film constituting the diaphragms 2 and 4, and for example, the open ends facing each other across the gap are in the height direction. Therefore, it is not easy to reduce the gap to a range that does not affect the characteristics of the microphone at the mass production level.
The above-mentioned problems similarly occur in other piezoelectric elements used in devices that handle pressure, vibration, displacement, etc., other than piezoelectric microphones.

本発明は上記問題点に鑑みてなされたものであり、その目的は、片持ち梁構造の振動板を採用する場合に、各振動板間又は振動板−基板間のギャップによって生じる特性の劣化を解消し、信号雑音比を改善した高感度の量産性に適した圧電素子を提供することにある。   The present invention has been made in view of the above problems, and its object is to reduce the deterioration of characteristics caused by a gap between each diaphragm or between a diaphragm and a substrate when a cantilever structure diaphragm is employed. An object of the present invention is to provide a piezoelectric element suitable for high-sensitivity mass-productivity that has been solved and improved in signal-to-noise ratio.

上記目的を達成するために、請求項１の発明は、圧電薄膜及びこの圧電薄膜を挟んで配置された一対の電極を含む振動板を備え、この振動板の一端が基板に支持されてなる圧電素子において、上記振動板の開放端側の弾性係数を、該振動板の支持端側の弾性係数より大きくしたことを特徴とする。
請求項２の発明は、上記振動板の開放端側を上記支持端側より厚くすることにより、上記振動板の開放端側の弾性係数を大きくしたことを特徴とする。
請求項３の発明は、上記振動板の支持端側に開孔を設けることにより、上記振動板の開放端側の弾性係数を大きくしたことを特徴とする。
請求項４の発明は、上記振動板の開放端側を構成する材料の弾性係数を、上記支持端側を構成する材料の弾性係数より大きくすることにより、上記振動板の開放端側の弾性係数を大きくしたことを特徴とする。
請求項５の発明の上記振動板は、音響圧力によって振動する振動板であることを特徴とする。 In order to achieve the above object, a first aspect of the present invention is a piezoelectric device comprising a diaphragm including a piezoelectric thin film and a pair of electrodes arranged with the piezoelectric thin film interposed therebetween, and one end of the diaphragm supported by a substrate. In the element, the elastic coefficient on the open end side of the diaphragm is made larger than the elastic coefficient on the support end side of the diaphragm.
The invention according to claim 2 is characterized in that the elastic coefficient on the open end side of the diaphragm is increased by making the open end side of the diaphragm thicker than the support end side.
The invention according to claim 3 is characterized in that an elastic coefficient on the open end side of the diaphragm is increased by providing an opening on the support end side of the diaphragm.
According to a fourth aspect of the present invention, the elastic coefficient of the material constituting the open end side of the diaphragm is made larger than the elastic coefficient of the material constituting the support end side, so that the elastic coefficient of the open end side of the diaphragm is increased. It is characterized by having increased.
The diaphragm according to a fifth aspect of the invention is a diaphragm that vibrates by acoustic pressure.

以上の構成によれば、振動板の一端が基板に支持された片持ち梁構造において、開放端側の振動板を支持端側より厚くすること等により、振動板の支持端側の弾性係数に対して開放端側の弾性係数を大きくしたので、ギャップによる音響抵抗の低下が小さくなる。
特に、製造における残留応力による変形も問題であり、隣接する振動板同士又は振動板と基板のギャップの幅は、支持固定端から離れる程、つまり振動板の開放端へ行く程大きくなるが、本発明では、この開放端側の弾性係数を大きくするので、残留応力に起因して変形の大きくなる開放端間のギャップの広がりやバラツキが効果的に抑制される。 According to the above configuration, in the cantilever structure in which one end of the diaphragm is supported by the substrate, the elastic coefficient on the support end side of the diaphragm is increased by making the diaphragm on the open end side thicker than the support end side. On the other hand, since the elastic coefficient on the open end side is increased, the decrease in acoustic resistance due to the gap is reduced.
In particular, deformation due to residual stress in manufacturing is also a problem, and the width of the gap between adjacent diaphragms or between the diaphragm and the substrate increases as they move away from the support fixing end, that is, toward the open end of the diaphragm. In the invention, since the elastic coefficient on the open end side is increased, the spread and variation of the gap between the open ends, which are greatly deformed due to the residual stress, are effectively suppressed.

本発明によれば、片持ち構造の振動板を採用する場合に、各振動板間又は振動板−基板間のギャップによって生じる音響抵抗の低下を抑えることができ、これによって制限される低周波ロールオフ周波数の増大、つまり低周波数での感度劣化を効果的に抑制することが可能となる。また、ギャップ音響抵抗の低下による信号雑音比の劣化が抑制され、高感度の量産性に適した圧電素子、圧電型ＭＥＭＳマイクロフォンが得られるという効果がある。
更に、量産製造時の残留応力によるギャップ幅の広がりに伴う特性劣化が解消されると共に、加工精度と振動板を構成する薄膜の残留応力の面内均一性に対する要求条件が緩和されることにより、製造歩留まりの向上も期待できるという利点がある。 According to the present invention, when a cantilever diaphragm is employed, a decrease in acoustic resistance caused by a gap between the diaphragms or between the diaphragm and the substrate can be suppressed, and the low-frequency roll limited thereby An increase in off frequency, that is, sensitivity deterioration at a low frequency can be effectively suppressed. In addition, the deterioration of the signal-to-noise ratio due to the decrease in the gap acoustic resistance is suppressed, and there is an effect that a piezoelectric element and a piezoelectric MEMS microphone suitable for mass production with high sensitivity can be obtained.
Furthermore, the characteristic deterioration due to the widening of the gap width due to the residual stress at the time of mass production is eliminated, and the requirements for the processing accuracy and the in-plane uniformity of the residual stress of the thin film constituting the diaphragm are relaxed, There is an advantage that an improvement in manufacturing yield can be expected.

本発明の実施例に係る圧電型マイクロフォンの四角形振動板タイプの概略構成を示す平面図である。It is a top view which shows schematic structure of the square diaphragm type of the piezoelectric microphone which concerns on the Example of this invention. 実施例に係る圧電型マイクロフォンの三角形振動板タイプの概略構成を示す平面図である。It is a top view which shows schematic structure of the triangular diaphragm type of the piezoelectric microphone which concerns on an Example. 第１実施例の圧電型マイクロフォンの構成を示す断面図である。It is sectional drawing which shows the structure of the piezoelectric microphone of 1st Example. 第２実施例の圧電型マイクロフォンの構成を示す平面図である。It is a top view which shows the structure of the piezoelectric microphone of 2nd Example. 第３実施例の圧電型マイクロフォンの構成を示す断面図である。It is sectional drawing which shows the structure of the piezoelectric microphone of 3rd Example. 実施例における最大応力等のシミュレーション値を従来と比較して示したグラフ図である。It is the graph which showed the simulation value, such as the maximum stress in an Example, compared with the past. 実施例における規格化されたギャップ音響抵抗軸を示すグラフ図である。It is a graph which shows the gap acoustic resistance axis normalized in the Example. 従来の圧電型マイクロフォンの四角形振動板タイプの概略構成を示す平面図である。It is a top view which shows schematic structure of the square diaphragm type of the conventional piezoelectric microphone. 従来の圧電型マイクロフォンの三角形振動板タイプの概略構成を示す平面図である。It is a top view which shows schematic structure of the triangular diaphragm type of the conventional piezoelectric microphone.

図１に、実施例の圧電型ＭＥＭＳマイクロフォンの四角形振動板タイプ、図２に、三角形振動板タイプの概略が示されており、符号１２は圧電薄膜を有し中央で２つに分割された振動板、３は基板に開けられた空孔である。この図１のマイクロフォンの２つの振動板１２は、四角形の３辺において開放され、１辺（支持端）が基板に支持された状態の片持ち梁構造である。
また、図２の符号１４は、四角形を対角線で分離した三角形の振動板であり、この振動板１４は、その一端が基板の空孔３の左右端及び上下端に支持される形で、４つ設けられており、このマイクロフォンの４つの振動板１４は、三角形の２辺において開放され、１辺（支持端）が基板に支持された状態の片持ち梁構造である。 FIG. 1 shows a rectangular diaphragm type of the piezoelectric MEMS microphone of the embodiment, and FIG. 2 shows an outline of a triangular diaphragm type. Reference numeral 12 denotes a vibration having a piezoelectric thin film and divided into two at the center. Plates 3 are holes formed in the substrate. The two diaphragms 12 of the microphone of FIG. 1 have a cantilever structure in a state in which one side (support end) is supported by a substrate with three sides of a square open.
Further, reference numeral 14 in FIG. 2 is a triangular diaphragm obtained by separating a quadrilateral with a diagonal line. The diaphragm 14 is supported by the left and right ends and upper and lower ends of the holes 3 of the substrate. The four diaphragms 14 of this microphone have a cantilever structure in which two sides of a triangle are open and one side (support end) is supported by a substrate.

図３に、第１実施例の圧電型ＭＥＭＳマイクロフォンの断面の構成（この図３は概念図であり、厚さ方向の縮尺と横方向の縮尺は異なっている）が示されている。符号1はシリコン（Ｓｉ）からなる基板であり、上述した振動板１２，１４は、シリコン酸化膜（ＳｉＯ２）からなる絶縁膜６を挟んで基板１上に作製される。図３において、符号７ａは、下層の圧電薄膜、７ｂは上層の圧電薄膜、８は電極薄膜、９は配線電極、１５は付加薄膜であり、これらによって振動板１２，１４が構成される。上記圧電薄膜７ａ，７ｂは、窒化アルミニウム等、電極薄膜８はモリブデン（Ｍｏ）等からなり、圧電薄膜７ａ，７ｂは３層の電極薄膜８で上下に挟まれる。なお、各電極薄膜８は、各層の電極薄膜８毎に図示しない接続手段により、それぞれ接続されている。 FIG. 3 shows a cross-sectional configuration of the piezoelectric MEMS microphone of the first embodiment (this FIG. 3 is a conceptual diagram, and the scale in the thickness direction is different from the scale in the horizontal direction). Reference numeral 1 denotes a substrate made of silicon (Si), and the above-described diaphragms 12 and 14 are formed on the substrate 1 with an insulating film 6 made of a silicon oxide film (SiO 2 ) interposed therebetween. In FIG. 3, reference numeral 7 a is a lower piezoelectric thin film, 7 b is an upper piezoelectric thin film, 8 is an electrode thin film, 9 is a wiring electrode, and 15 is an additional thin film, and these constitute diaphragms 12 and 14. The piezoelectric thin films 7a and 7b are made of aluminum nitride or the like, the electrode thin film 8 is made of molybdenum (Mo) or the like, and the piezoelectric thin films 7a and 7b are sandwiched between three layers of the electrode thin films 8. Each electrode thin film 8 is connected to each layer of electrode thin film 8 by connection means (not shown).

ここで、圧電型マイクロフォンの基本的な動作原理を説明する。
例えば、図３（或いは図５）の下方の基板１に開けられた空孔３から音響圧力（説明のため準直流的圧力を考える）が加わったときには、圧電薄膜７ａ，７ｂを含む振動板１２，１４は上方に湾曲変位する。その結果、下層の圧電薄膜７ａには引張応力が、上層の圧電薄膜７ｂには圧縮応力が発生する。２層の圧電薄膜の圧電性を示す結晶配向を薄膜の延伸方向に垂直で同一向き（例えば図５で上向き）とすると、横圧電効果によって、下層の圧電薄膜７ａと上層の圧電薄膜７ｂに極性の異なる電圧が発生し、３層の電極薄膜８とそれに接続した配線電極９によって外部に取り出すことができる。そして、音響圧力によって発生する応力は、振動板（又は梁）１２，１４の支持端に近づく程大きく、その先端では、開放端であるため応力は開放されて０となる。従って、電極薄膜８は応力の大きい振動板１２，１４の支持端近傍にのみ支持端から延伸させ、その延伸長を、音響圧力によって電極薄膜８間に発生させる信号エネルギー（電圧の自乗×静電容量／２）を最大化するように設定することで、マイクロフォンとしての信号雑音比を最大化することができる。 Here, the basic operation principle of the piezoelectric microphone will be described.
For example, when an acoustic pressure (considering a quasi-direct current pressure for the sake of explanation) is applied from the hole 3 formed in the substrate 1 below in FIG. 3 (or FIG. 5), the diaphragm 12 including the piezoelectric thin films 7a and 7b. , 14 are curvedly displaced upward. As a result, tensile stress is generated in the lower piezoelectric thin film 7a, and compressive stress is generated in the upper piezoelectric thin film 7b. When the crystal orientation indicating the piezoelectricity of the two layers of piezoelectric thin film is the same as the direction perpendicular to the stretching direction of the thin film (for example, upward in FIG. 5), the lateral piezoelectric effect causes the lower piezoelectric thin film 7a and the upper piezoelectric thin film 7b to Are generated and can be taken out by the three-layered electrode thin film 8 and the wiring electrode 9 connected thereto. The stress generated by the acoustic pressure increases as it approaches the support ends of the diaphragms (or beams) 12 and 14, and the stress is released and becomes zero at the tip because it is an open end. Accordingly, the electrode thin film 8 is stretched from the support end only in the vicinity of the support ends of the diaphragms 12 and 14 having high stress, and the signal energy (voltage square × electrostatic force) generated between the electrode thin films 8 by the acoustic pressure is extended from the support end. By setting so as to maximize the capacity / 2), the signal-to-noise ratio as a microphone can be maximized.

音響圧力以外に、振動板（梁）１２，１４を構成する圧電薄膜等の薄膜の残留応力の製造上の意図的或いは意図しないアンバランスによっても振動板（梁）１２，１４は変形する。例えば、上層の圧電薄膜７ｂに対して下層の圧電薄膜７ａの圧縮応力がより大きい場合には、振動板１２，１４は上向きに変形する。そのため、振動板−支持基板間のギャップ（ｇ）は実質的に大きくなる。また、残留応力の製造上の意図しない不均一性によって、隣接振動板間の変形の大きさは異なり、その結果として隣接振動板間のギャップ（ｇ）も実効的に大きくなる。当然のことながら、振動板１２，１４の先端程、残留応力による変形は大きい。ギャップ音響抵抗はギャップの幅の３乗に反比例して低下することが知られている。低周波感度の劣化を示す低周波ロールオフ周波数はギャップ抵抗に反比例して大きく、つまりギャップの３乗に比例して大きくなる。   In addition to the acoustic pressure, the diaphragms (beams) 12 and 14 are also deformed by an intentional or unintentional imbalance in manufacturing residual stresses of thin films such as piezoelectric thin films constituting the diaphragms (beams) 12 and 14. For example, when the compressive stress of the lower piezoelectric thin film 7a is larger than that of the upper piezoelectric thin film 7b, the diaphragms 12 and 14 are deformed upward. Therefore, the gap (g) between the diaphragm and the support substrate is substantially increased. Further, due to unintended non-uniformity in manufacturing residual stress, the magnitude of deformation between adjacent diaphragms differs, and as a result, the gap (g) between adjacent diaphragms also effectively increases. As a matter of course, the deformation due to the residual stress is larger at the tips of the diaphragms 12 and 14. It is known that the gap acoustic resistance decreases in inverse proportion to the cube of the gap width. The low-frequency roll-off frequency indicating the deterioration of the low-frequency sensitivity increases in inverse proportion to the gap resistance, that is, increases in proportion to the cube of the gap.

そこで、振動板１２，１４において信号を取り出す領域と変形の大きな領域が異なることを利用し、実施例では、図１，図２に示されるように、片持ち梁構造の支持端側の領域Ｉと支持端の対極にある開放端（先端）側の領域IIを設定し、支持端側の領域Ｉに対し変形の大きい領域IIの弾性係数（弾性率）を実質的に異なる値にすることによって、信号エネルギーとギャップを最適化するようにしている。後述の実施例に示されるように、信号を取り出す領域Ｉは相対的に曲がり易く（実効的弾性率を相対的に小さく）して音響信号による応力を大きく保ち、ギャップに悪影響を与える領域IIは湾曲しづらく（実効的弾性係数を相対的に大きく〕することにより、両者の最適化を図ることが可能となる。   Therefore, by utilizing the fact that the signal extraction region and the large deformation region are different in the diaphragms 12 and 14, in the embodiment, as shown in FIGS. 1 and 2, the region I on the support end side of the cantilever structure is used. By setting the region II on the open end (tip) side at the counter electrode and the support end, and making the elastic modulus (elastic modulus) of the region II with large deformation with respect to the region I on the support end side substantially different , Trying to optimize the signal energy and gap. As shown in the examples described later, the region I from which the signal is extracted is relatively easy to bend (the effective elastic modulus is relatively small) to keep the stress due to the acoustic signal large, and the region II that adversely affects the gap is By making it difficult to bend (relatively increasing the effective elastic modulus), it is possible to optimize both.

まず、図３の第１実施例では、振動板１２，１４に上述のように付加薄膜１５を形成し、領域IIの膜厚を領域Ｉの膜厚より厚くすることにより、領域IIの実効的な弾性係数を増大させる。湾曲のバネ定数は、振動板１２，１４を構成する薄膜の２次モーメント、つまり膜厚の３乗に比例して増大する。仮に、領域Ｉと領域IIの膜厚が等しいとすると、弾性係数が膜厚比の３乗（同じ材料を仮定した場合）に比例して増大したことと等価である。第１実施例では、領域IIの上層圧電薄膜７ｂの上に、付加薄膜１５を堆積加工することにより、上記弾性係数の増大を実現している。このようにして、製造時の残留応力によるギャップ（ｇ）の広がりをなくすと共に、ギャップの音響抵抗を大きくすることができる。   First, in the first embodiment of FIG. 3, the additional thin film 15 is formed on the diaphragms 12 and 14 as described above, and the film thickness of the area II is made larger than the film thickness of the area I. Increase the elastic modulus. The spring constant of the curve increases in proportion to the second moment of the thin film constituting the diaphragms 12 and 14, that is, the cube of the film thickness. If the film thicknesses of the region I and the region II are equal, it is equivalent to the elastic modulus increasing in proportion to the cube of the film thickness ratio (assuming the same material). In the first embodiment, the elastic modulus is increased by depositing the additional thin film 15 on the upper piezoelectric thin film 7b of the region II. In this way, the gap (g) can be prevented from expanding due to residual stress during manufacturing, and the acoustic resistance of the gap can be increased.

図６には、ギャップ音響抵抗の増大を試算した結果が示されており、これは、図２に示すような三角形の片持ち梁構造の振動板１４について、振動板１４の長さに対する厚膜の領域IIの長さを変数にして、領域IIがない場合を基準（値１．０）とし、最大応力、共振周波数及び残留応力による変形がどのように変化するかについて、３次元シミュレーター等を使って計算したものである。なお、振動板１４を構成する材料は同一とし、領域IIの膜厚を領域Ｉの膜厚の２倍の厚さにしている。領域IIの長さが長くなるにつれて期待通り残留応力による変形は減少する結果となった。但し、振動板１４の支持端付近の最大応力は漸減し、共振周波数は最初は付加薄膜１５の質量効果によって下がるが、領域IIの長さが長くなると厚膜化によって全体のバネ定数が大きくなり共振周波数が逆に高くなるとの特性を示す。   FIG. 6 shows a result of a trial calculation of an increase in the gap acoustic resistance, which is a thick film with respect to the length of the diaphragm 14 for the diaphragm 14 having a triangular cantilever structure as shown in FIG. Using the length of region II as a variable, the case where region II does not exist is set as a reference (value 1.0), and how the deformation due to the maximum stress, resonance frequency and residual stress changes is determined using a 3D simulator, etc. It is calculated by using. The material constituting the diaphragm 14 is the same, and the thickness of the region II is set to twice the thickness of the region I. As expected, as the length of region II increased, the deformation due to residual stress decreased as expected. However, the maximum stress near the support end of the diaphragm 14 gradually decreases, and the resonance frequency initially decreases due to the mass effect of the additional thin film 15. However, as the length of the region II increases, the overall spring constant increases due to the increase in thickness. In contrast, the resonance frequency is increased.

ところで、領域Ｉの膜厚を一定に保った状態で、出力電圧に相当する最大応力やギャップ音響抵抗に相当するギャップの変化を比較することは必ずしも適当でない。実用上は、共振周波数を一定に保ち（音声の場合は２０ｋＨｚ程度）、かつ信号雑音比を表す電極薄膜間の信号エネルギーとギャップ音響抵抗に相当するギャップの幅の３乗を規格化して比較するのが好ましい。共振数周波数は、領域Ｉと領域IIの膜厚比を一定に保ったまま、領域Ｉの膜厚を調整することによって領域IIの長さの比率に関わらず一定に保つことが可能である。その上で、ギャップ音響抵抗と信号雑音比の積を性能指数（規格化された音響抵抗）として計算し、領域IIの長さの比率に対してプロットしたものが図７に示されている。図６と同様に、領域IIがない場合を基準（値１）とした。この性能指数は領域IIの長さの割合が０．５を超えた辺りから増加し始め、０．７で略３となる。ギャップ音響抵抗をいたずらに大きくする必要はないので、実用上は領域IIの長さの比率を０．６〜０．８程度とすればよい。   By the way, it is not always appropriate to compare the maximum stress corresponding to the output voltage and the change in the gap corresponding to the gap acoustic resistance in a state where the film thickness of the region I is kept constant. In practice, the resonance frequency is kept constant (about 20 kHz in the case of speech), and the signal energy between the electrode thin films representing the signal-to-noise ratio and the third power of the gap width corresponding to the gap acoustic resistance are normalized and compared. Is preferred. The resonance frequency can be kept constant regardless of the ratio of the length of region II by adjusting the film thickness of region I while keeping the film thickness ratio of region I and region II constant. Then, the product of the gap acoustic resistance and the signal-to-noise ratio is calculated as a figure of merit (standardized acoustic resistance) and plotted against the ratio of the length of region II is shown in FIG. As in FIG. 6, the case where there was no region II was set as a reference (value 1). This figure of merit starts to increase when the ratio of the length of the region II exceeds 0.5, and becomes approximately 3 at 0.7. Since it is not necessary to unnecessarily increase the gap acoustic resistance, the length ratio of the region II may be practically set to about 0.6 to 0.8.

以上のように、第１実施例では、領域IIの膜厚を厚くすることによって、実効的な弾性係数を大きくし、残留応力によるギャップの広がり、ギャップ音響抵抗の低下を抑制することができ、低周波数でのロールオフ周波数の増大が抑えられ、信号雑音比を高く保つと共に、量産時のバラツキを抑制することも可能となる。   As described above, in the first embodiment, by increasing the thickness of the region II, the effective elastic modulus can be increased, and the spread of the gap due to the residual stress and the decrease in the gap acoustic resistance can be suppressed. An increase in the roll-off frequency at a low frequency can be suppressed, the signal to noise ratio can be kept high, and variations during mass production can be suppressed.

以下に、図２の三角形振動板タイプの設計例を示す。各支持端の長さは９００μｍで、圧電薄膜７ａ、７ｂとしてそれぞれ０．６μｍで２層の窒化アルミニウム薄膜を用い、電極薄膜８にはそれぞれ厚さ５０ｎｍのモリブデン薄膜を用いる。付加薄膜１５としては、膜厚１．２μｍの窒化アルミニウムを用い、領域IIの長さをそれぞれ３６０μｍとする。薄膜は全てスパッタリング法にて堆積させる。また、ギャップ（ｇ）のパターン幅（残留応力がない場合のギャップの幅）を０．６μｍとする。これにより、共振周波数は約２０ｋＨｚ、ロールオフ周波数は１００Ｈｚ以下にすることができる。但し、残留応力の隣接振動板間のバラツキは２０ＭＰａ程度に抑える必要があるが、これは現状の製造技術で十分達成可能である。なお、圧電薄膜７ａ，７ｂとして、窒化アルミニウムに代えてスカンジウムを添加した横圧電歪係数の大きな窒化スカンジウムアルミニウム（Ａｌ１−ｘＳｃｘＮ）を用いれば、性能はさらに向上する。なお、電極薄膜８の材料は、モリブデン薄膜以外に、プラチナ（Ｐｔ）薄膜、チタン（Ｔｉ）膜膜、イリジウム（Ｉｒ）薄膜、ルテニウム（Ｒｕ）薄膜を用いることもできる。 The design example of the triangular diaphragm type shown in FIG. 2 is shown below. The length of each support end is 900 μm, each of the piezoelectric thin films 7a and 7b is 0.6 μm, two aluminum nitride thin films are used, and each electrode thin film 8 is a 50 nm thick molybdenum thin film. As the additional thin film 15, aluminum nitride having a film thickness of 1.2 μm is used, and the length of each region II is set to 360 μm. All thin films are deposited by sputtering. The pattern width of the gap (g) (the width of the gap when there is no residual stress) is 0.6 μm. Thereby, the resonance frequency can be about 20 kHz, and the roll-off frequency can be 100 Hz or less. However, the variation in residual stress between adjacent diaphragms needs to be suppressed to about 20 MPa, which can be sufficiently achieved by the current manufacturing technology. Note that the performance is further improved by using scandium aluminum nitride (Al 1-x Sc x N) having a large transverse piezoelectric strain coefficient in which scandium is added instead of aluminum nitride as the piezoelectric thin films 7a and 7b. The material of the electrode thin film 8 may be a platinum (Pt) thin film, a titanium (Ti) film, an iridium (Ir) thin film, or a ruthenium (Ru) thin film in addition to the molybdenum thin film.

図４に、第２実施例の構成が示されており、この第２実施例は、領域Ｉの振動板１２に開孔を設けたものである。図４において、領域Ｉと領域IIを持つ振動板１２は、付加薄膜１５が設けられない図３の構成と同様であり、この振動板１２の領域Ｉに空孔３まで貫通する多くの開孔１６、或いは圧電薄膜や電極薄膜の一部を薄膜化した貫通しない孔を設けることによって、領域Ｉの弾性係数を領域IIよりも下げ、この領域IIの実効弾性係数を領域Ｉより高くする。なお、開孔１６の直径を０．５μｍ程度と小さくすることにより、開孔１６を介したギャップ音響抵抗の低下を軽微に抑えることができる。また、第１実施例の構成と併用することも可能であり、その場合には、領域Ｉと領域IIの実効的弾性係数の比をより大きくすることが可能となる。なお、この第２実施例の構成は、三角形振動板(１４)の場合でも同様となる。   FIG. 4 shows the configuration of the second embodiment. In the second embodiment, the diaphragm 12 in the region I is provided with an opening. In FIG. 4, the diaphragm 12 having the region I and the region II is the same as the structure of FIG. 3 in which the additional thin film 15 is not provided, and many openings that penetrate to the hole 3 in the region I of the diaphragm 12. 16 or by providing a non-penetrating hole in which a part of the piezoelectric thin film or electrode thin film is thinned, the elastic modulus of region I is lowered from region II, and the effective elastic modulus of region II is made higher than region I. Note that, by reducing the diameter of the opening 16 to about 0.5 μm, the decrease in the gap acoustic resistance through the opening 16 can be suppressed to a slight level. Further, it can be used in combination with the configuration of the first embodiment, and in that case, the ratio of the effective elastic modulus between the region I and the region II can be increased. The configuration of the second embodiment is the same even in the case of the triangular diaphragm (14).

図５に、第３実施例の構成（この図５も概念図であり、厚さ方向の縮尺と横方向の縮尺は異なっている）が示されており、この第３実施例は、振動板の領域IIを高弾性係数の材料で構成したものである。図５において、振動板１２，１４の領域Ｉの構成は第１実施例と同様であるが、この領域Ｉの圧電薄膜７ａ，７ｂに例えば酸化亜鉛（ＺｎＯ）を用い、領域IIの振動板部分の材料として窒化シリコン、窒化アルミニウム或いは金属のモリブデンやタングステン等、酸化亜鉛に比べて２〜４倍の弾性係数（弾性率）を有する高弾性率薄膜１８を使用することによって、領域IIの弾性係数を領域Ｉより大きくしている。なお、図５に示した振動板１２，１４の領域Ｉと領域IIの膜厚は等しいが、第１又は第２の実施例と併用することにより実効的弾性係数の比を大きくすることが可能である。また、領域IIとして、下層の電極薄膜８、下層の圧電薄膜７ａ及び高弾性率薄膜１８を誘電体支持薄膜の上に堆積させたものを用いてもよい。   FIG. 5 shows the configuration of the third embodiment (this FIG. 5 is also a conceptual diagram, the scale in the thickness direction is different from the scale in the horizontal direction). The region II is composed of a material having a high elastic modulus. In FIG. 5, the structure of the region I of the diaphragms 12 and 14 is the same as that of the first embodiment. For example, zinc oxide (ZnO) is used for the piezoelectric thin films 7a and 7b in the region I, and the diaphragm part of the region II By using a high modulus thin film 18 having a modulus of elasticity (modulus of elasticity) 2 to 4 times that of zinc oxide, such as silicon nitride, aluminum nitride, or metal molybdenum or tungsten Is larger than region I. Although the thicknesses of the regions I and II of the diaphragms 12 and 14 shown in FIG. 5 are equal, the effective elastic modulus ratio can be increased by using the diaphragms 12 and 14 together with the first or second embodiment. It is. Further, as the region II, a material obtained by depositing the lower electrode thin film 8, the lower piezoelectric thin film 7a, and the high modulus thin film 18 on the dielectric support thin film may be used.

以上のように、実施例によれば、片持ち梁構造の圧電型ＭＥＭＳマイクロフォン等において、各振動板間又は振動板−基板間のギャップ幅の残留応力による増大に伴う特性の劣化を解消し、高感度で信号雑音比を改善し、量産性に適した構造とすることができる。   As described above, according to the embodiment, in the piezoelectric MEMS microphone having a cantilever structure, the deterioration of characteristics due to the increase in the gap width between the diaphragms or between the diaphragm and the substrate due to the residual stress is eliminated, With high sensitivity, the signal to noise ratio can be improved, and a structure suitable for mass production can be obtained.

なお、実施例では、三角形振動板又は四角形振動板の片持ち梁構造の例を示したが、平面の外周形状が六角形や円形となる振動板を複数に分割した振動板を片持ち構造としたものに対しても、上記実施例を同様に適用することができる。   In the embodiment, an example of a cantilever structure of a triangular diaphragm or a quadrangular diaphragm has been shown. The above-described embodiment can be similarly applied to the above.

１…基板、 ２，４，１２，１４…振動板、
３…空孔、 ６…絶縁膜、
７ａ，７ｂ…圧電薄膜、 ８…電極薄膜、
９…配線電極、 １５…付加薄膜、
１６…開孔、 １８…高弾性率薄膜、
ｇ…ギャップ。
1 ... substrate, 2, 4, 12, 14 ... diaphragm,
3 ... hole, 6 ... insulating film,
7a, 7b ... piezoelectric thin film, 8 ... electrode thin film,
9 ... Wiring electrode, 15 ... Additional thin film,
16 ... opening, 18 ... high modulus thin film,
g ... Gap.

Claims (5)

圧電薄膜及びこの圧電薄膜を挟んで配置された一対の電極を含む振動板を備え、この振動板の一端が基板に支持されてなる圧電素子において、
上記振動板の開放端側の弾性係数を、該振動板の支持端側の弾性係数より大きくしたことを特徴とする圧電素子。 In a piezoelectric element comprising a diaphragm including a piezoelectric thin film and a pair of electrodes arranged with the piezoelectric thin film interposed therebetween, and one end of the diaphragm supported by a substrate,
A piezoelectric element characterized in that the elastic coefficient on the open end side of the diaphragm is larger than the elastic coefficient on the support end side of the diaphragm. 上記振動板の開放端側を上記支持端側より厚くすることにより、上記振動板の開放端側の弾性係数を大きくしたことを特徴とする請求項１記載の圧電素子。   2. The piezoelectric element according to claim 1, wherein the elastic coefficient on the open end side of the diaphragm is increased by making the open end side of the diaphragm thicker than the support end side. 上記振動板の支持端側に開孔を設けることにより、上記振動板の開放端側の弾性係数を大きくしたことを特徴とする請求項１又は２記載の圧電素子。   3. The piezoelectric element according to claim 1, wherein an elastic coefficient on the open end side of the diaphragm is increased by providing an opening on the support end side of the diaphragm. 上記振動板の開放端側を構成する材料の弾性係数を、上記支持端側を構成する材料の弾性係数より大きくすることにより、上記振動板の開放端側の弾性係数を大きくしたことを特徴とする請求項１乃至３のいずれかに記載の圧電素子。   The elastic coefficient of the material constituting the open end side of the diaphragm is made larger than the elastic coefficient of the material constituting the support end side, thereby increasing the elastic coefficient of the open end side of the diaphragm. The piezoelectric element according to any one of claims 1 to 3. 上記振動板は、音響圧力によって振動する振動板であることを特徴とする請求項１乃至４のいずれかに記載の圧電素子。

The piezoelectric element according to claim 1, wherein the diaphragm is a diaphragm that vibrates due to an acoustic pressure.

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