JP6894719B2 - Piezoelectric element - Google Patents

Piezoelectric element Download PDF

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JP6894719B2
JP6894719B2 JP2017029541A JP2017029541A JP6894719B2 JP 6894719 B2 JP6894719 B2 JP 6894719B2 JP 2017029541 A JP2017029541 A JP 2017029541A JP 2017029541 A JP2017029541 A JP 2017029541A JP 6894719 B2 JP6894719 B2 JP 6894719B2
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JP2018137297A (en
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王義 山崎
王義 山崎
尚己 桝本
尚己 桝本
博行 口地
博行 口地
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New Japan Radio Co Ltd
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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 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。 In recent years, MEMS (Micro Electro Mechanical System), which is small and thin and has high temperature processing resistance during solder flow, is often used for smartphones in huge demand. Most of them are capacitive MEMS microphones (acoustic transducers) that convert the vibration displacement of the diaphragm due to acoustic pressure into a voltage change and output it. However, in this capacitive type, it is becoming difficult to improve the signal-to-noise ratio due to the acoustic resistance generated by the flow of air in the gap between the diaphragm and the fixing plate. Therefore, attention is being paid to piezoelectric MEMS microphones that use a piezoelectric thin film and extract acoustic vibration as a voltage change due to the distortion of a diaphragm that is basically a single unit.

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。 As the piezoelectric MEMS microphone, for example, there is a cantilever structure as shown in FIGS. 8 and 9, FIG. 8 is a microphone having a quadrangular diaphragm, and FIG. 9 is a microphone having a triangular diaphragm.
In FIG. 8, reference numeral 2 is a diaphragm whose one end is supported by the substrate and has a piezoelectric thin film, and 3 is a hole formed in the central portion of the substrate. Two are provided so as to be supported at the left and right ends of the figure. Therefore, the two diaphragms 2 of this microphone have a cantilever structure in which the three sides of the quadrangle are open 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 is a triangular diaphragm, and four diaphragms 4 are provided so that one end thereof is supported by the left and right ends and the upper and lower ends of the holes 3 of the substrate. Therefore, the four diaphragms 4 of this microphone have a cantilever structure in which two sides of the triangle are open and one side is supported by a 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 above-mentioned piezoelectric MEMS microphone, although the sensitivity and signal-to-noise ratio equivalent to those of the capacitive MEMS microphone are obtained by adopting the cantilever structure, the characteristics have not been further improved. ..
In the microphone having the cantilever structure, there is a gap (g) between the two diaphragms 2 and between the diaphragm and the substrate in FIG. 8, and in FIG. 9, there is a gap (g) between each of the four diaphragms 4. Since there is g) and the acoustic pressure signal leaks through these gaps, the acoustic resistance of the gap portion that enters in parallel with the acoustic impedance for the diaphragms 2 and 4 becomes small. This decrease in the gap acoustic resistance has the disadvantage of increasing the low frequency roll-off frequency, that is, degrading the sensitivity at the low frequency, and is also a factor of deteriorating the signal 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, by adopting a structure in which the tips of each of the four are gathered at one place, the problem caused by the gap is improved to some extent, but it is not sufficient.
The width of this gap (g) is determined by the processing accuracy at the time of manufacturing and the in-plane uniformity of the residual stress of the thin films constituting the diaphragms 2 and 4, for example, the open ends facing each other across the gap are in the height direction. It is not easy to limit the width of the gap to a range that does not affect the characteristics of the microphone at the mass production level.
The above-mentioned problems also 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 an object of the present invention is to prevent deterioration of characteristics caused by a gap between each diaphragm or between a diaphragm and a substrate when a diaphragm having a cantilever structure is adopted. An object of the present invention is to provide a piezoelectric element suitable for mass productivity with high sensitivity, which has been solved and the signal-to-noise ratio has been improved.

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、この振動板が同一幅で開放され複数に分割され、この分割された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、上記複数の振動板のそれぞれの開放端側の領域に、各振動板間の開放端に合わせたギャップを持つ付加薄膜を形成することにより、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, the invention of claim 1 includes a piezoelectric thin film and a diaphragm including a pair of electrodes arranged so as to sandwich the piezoelectric thin film, and the diaphragm is opened with the same width and divided into a plurality of parts. In a piezoelectric element in which one end of the divided diaphragms is supported by a substrate, an additional thin film having a gap in the region on the open end side of each of the plurality of diaphragms according to the open end between the diaphragms. By forming the diaphragm, 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.
According to another aspect of the invention, the upper Symbol diaphragm is characterized in that a diaphragm that vibrates by an 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 a cantilever structure in which one end of the diaphragm is supported by the substrate, more diaphragm open end side and child thicker than the support end side, the elastic modulus of the support end side of the diaphragm On the other hand, since the elastic modulus on the open end side is increased, the decrease in acoustic resistance due to the gap becomes small.
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 the distance from the support fixed end, that is, toward the open end of the diaphragm. In the present invention, since the elastic modulus on the open end side is increased, the widening 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 diaphragm having a cantilever structure is adopted, it is possible to suppress a decrease in acoustic resistance caused by a gap between each diaphragm or between a diaphragm and a substrate, and a low frequency roll limited by this can be suppressed. It is possible to effectively suppress an increase in the off-frequency, that is, a deterioration in sensitivity at a low frequency. Further, 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 high-sensitivity mass productivity can be obtained.
Furthermore, the deterioration of characteristics due to the widening of the gap width due to the residual stress during mass production is eliminated, and the requirements for processing accuracy and in-plane uniformity of the residual stress of the thin film constituting the diaphragm are relaxed. It has the advantage that it can be expected to improve the manufacturing yield.

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 the schematic structure of the quadrangular diaphragm type of the piezoelectric microphone which concerns on embodiment 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 the schematic structure of the triangular diaphragm type of the piezoelectric microphone which concerns on 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. 参考の第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 for reference. 参考の第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 of a reference. 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 a graph which showed the simulation value such as the maximum stress in an Example in comparison with the conventional. 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 standardized gap acoustic resistance axis in an 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 the schematic structure of the quadrangular 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 the 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 an outline of the quadrangular diaphragm type of the piezoelectric MEMS microphone of the embodiment, and FIG. 2 shows the outline of the triangular diaphragm type. Reference numeral 12 has a piezoelectric thin film and is divided into two at the center. Plate 3 is a hole made in the substrate. The two diaphragms 12 of the microphone of FIG. 1 have a cantilever structure in which three sides of a quadrangle are open and one side (supporting end) is supported by a substrate.
Further, reference numeral 14 in FIG. 2 is a triangular diaphragm in which a quadrangle is diagonally separated, and the diaphragm 14 has a shape in which one end thereof is supported by the left and right ends and the upper and lower ends of the holes 3 of the substrate. The four diaphragms 14 of the microphone are provided so as to have a cantilever structure in which two sides of the triangle are open and one side (supporting end) is supported by the 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 (FIG. 3 is a conceptual diagram, and the scale in the thickness direction and the scale in the lateral direction are different). Reference numeral 1 is a substrate made of silicon (Si), and the above-mentioned 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 7a is a lower piezoelectric thin film, 7b is an upper piezoelectric thin film, 8 is an electrode thin film, 9 is a wiring electrode, and 15 is an additional thin film, which constitute diaphragms 12 and 14. The piezoelectric thin films 7a and 7b are made of aluminum nitride or the like, the electrode thin films 8 are made of molybdenum (Mo) or the like, and the piezoelectric thin films 7a and 7b are sandwiched vertically by the three-layer electrode thin films 8. Each of the electrode thin films 8 is connected to each of the electrode thin films 8 of each layer by a connecting 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 operating principle of the piezoelectric microphone will be described.
For example, when acoustic pressure (considering quasi-DC pressure is considered for explanation) is applied from the holes 3 formed in the substrate 1 below FIG. 3 (or FIG. 5), the diaphragm 12 including the piezoelectric thin films 7a and 7b , 14 are curved and 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. Assuming that the crystal orientation indicating the piezoelectricity of the two-layer piezoelectric thin film is perpendicular to the stretching direction of the thin film and oriented in the same direction (for example, upward in FIG. Different voltages are generated, and the three-layer electrode thin film 8 and the wiring electrode 9 connected to the thin film 8 can be taken out to the outside. 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 to 0 at the tip because it is an open end. Therefore, 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 a large stress, and the stretching length is the signal energy (voltage squared × electrostatic) generated between the electrode thin films 8 by acoustic pressure. By setting the capacitance / 2) to be maximized, the signal-to-noise ratio of the 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 intentional or unintended imbalance in manufacturing of the residual stress of the thin film such as the piezoelectric thin film 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 becomes substantially large. In addition, the magnitude of deformation between adjacent diaphragms varies depending on the unintended non-uniformity of residual stress in manufacturing, and as a result, the gap (g) between adjacent diaphragms also effectively increases. As a matter of course, the tip of the diaphragms 12 and 14 is greatly deformed by the residual stress. It is known that the gap acoustic resistance decreases in inverse proportion to the cube of the width of the gap. The low frequency roll-off frequency, which indicates the deterioration of 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, taking advantage of the fact that the region from which the signal is taken out and the region where the deformation is large 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 By setting the region II on the open end (tip) side opposite to the support end and setting the elastic modulus (elastic modulus) of the region II with large deformation to a value substantially different from the region I on the support end side. , I try to optimize the signal energy and the gap. As shown in the examples described later, the region I from which the signal is taken out is relatively easy to bend (the effective elastic modulus is relatively small), the stress due to the acoustic signal is kept large, and the region II which 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 region II is made thicker than the film thickness of the region I, so that the region II is effective. Increases the elastic modulus. The curved spring constant increases in proportion to the secondary moment of the thin films constituting the diaphragms 12 and 14, that is, the cube of the film thickness. Assuming that the film thicknesses of Region I and 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, it is possible to eliminate the widening of the gap (g) due to the residual stress during manufacturing and to increase the acoustic resistance of the gap.

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 the result of a trial calculation of the increase in the gap acoustic resistance, which is a thick film for the length of the vibrating plate 14 for the vibrating plate 14 having a triangular cantilever structure as shown in FIG. Using the length of region II as a variable and the absence of region II as a reference (value 1.0), a three-dimensional simulator, etc., can be used to determine how the deformation due to maximum stress, resonance frequency, and residual stress changes. It was calculated using. The materials constituting the diaphragm 14 are the same, and the film thickness of the region II is twice the film thickness of the region I. As expected, the deformation due to residual stress decreased as the length of region II increased. 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, but as the length of region II increases, the overall spring constant increases due to the thickening of the film. It shows the characteristic that the resonance frequency becomes higher on the contrary.

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 change of the maximum stress corresponding to the output voltage and the change of the gap corresponding to the gap acoustic resistance while keeping the film thickness of the region I constant. Practically, the resonance frequency is kept constant (about 20 kHz in the case of voice), and the signal energy between the electrode thin films representing the signal-to-noise ratio and the cube of the gap width corresponding to the gap acoustic resistance are standardized and compared. Is preferable. The resonance frequency can be kept constant regardless of the length ratio of the region II by adjusting the film thickness of the region I while keeping the film thickness ratio of the region I and the region II constant. Then, the product of the gap acoustic resistance and the signal-to-noise ratio is calculated as a figure of merit (normalized acoustic resistance) and plotted against the length ratio of region II, which is shown in FIG. As in FIG. 6, the case where there is no region II is used as a reference (value 1). This figure of merit begins to increase when the ratio of the length of region II exceeds 0.5, and becomes approximately 3 at 0.7. Since it is not necessary to increase the gap acoustic resistance unnecessarily, the ratio of the lengths of the region II may be about 0.6 to 0.8 in practical use.

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 film thickness of the region II, the effective elastic modulus can be increased, the gap widening due to the residual stress, and the decrease in the gap acoustic resistance can be suppressed. The increase in roll-off frequency at low frequencies 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 of FIG. 2 is shown below. The length of each support end is 900 μm, a two-layer aluminum nitride thin film having a thickness of 0.6 μm is used as the piezoelectric thin films 7a and 7b, and a molybdenum thin film having a thickness of 50 nm is used as the electrode thin film 8. 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 360 μm. All thin films are deposited by sputtering. Further, the pattern width of the gap (g) (the width of the gap when there is no residual stress) is set to 0.6 μm. As a result, the resonance frequency can be set to about 20 kHz and the roll-off frequency can be set to 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. If scandium aluminum nitride (Al 1-x Sc x N) having a large transverse piezoelectric strain coefficient is used as the piezoelectric thin films 7a and 7b in which scandium is added instead of aluminum nitride, the performance is further improved. As the material of the electrode thin film 8, in addition to the molybdenum thin film, a platinum (Pt) thin film, a titanium (Ti) film film, an iridium (Ir) thin film, and a ruthenium (Ru) thin film can also be used.

図４に、参考の第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 for reference, in which the diaphragm 12 of the region I is provided with an opening. In FIG. 4, the vibrating plate 12 having the region I and the region II has the same configuration as that of FIG. 3 in which the additional thin film 15 is not provided, and many openings penetrating the region I of the vibrating plate 12 up to the holes 3 16 or a part of the piezoelectric thin film or the electrode thin film is thinned to provide a non-penetrating hole, so that the elastic modulus of the region I is lower than that of the region II, and the effective elastic modulus of this region II is made higher than that of the region I. By reducing the diameter of the opening 16 to about 0.5 μm, it is possible to slightly suppress a decrease in the gap acoustic resistance through the opening 16. It can also be used in combination with the configuration of the first embodiment, in which case the ratio of the effective elastic modulus of the region I and the region II can be made larger. The configuration of the second embodiment is the same even in the case of the triangular diaphragm (14).

図５に、参考の第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 for reference (this FIG. 5 is also a conceptual diagram, and the scale in the thickness direction and the scale in the lateral direction are different). Region II of the diaphragm is composed of a material with a high elastic modulus. In FIG. 5, the configuration of the region I of the vibrating plates 12 and 14 is the same as that of the first embodiment, but zinc oxide (ZnO) is used for the piezoelectric thin films 7a and 7b of this region I, and the vibrating plate portion of the region II is used. By using a high elastic modulus thin film 18 having an elastic modulus (modulus) of 2 to 4 times that of zinc oxide, such as silicon nitride, aluminum nitride, or metal molybdenum or tungsten, the elastic modulus of region II Is larger than the area I. Although the film thicknesses of the regions I and II of the diaphragms 12 and 14 shown in FIG. 5 are the same, the ratio of the effective elastic modulus can be increased by using the diaphragms 12 and 14 in combination with the first or second embodiment. Is. Further, as the region II, an electrode thin film 8 in the lower layer, a piezoelectric thin film 7a in the lower layer, and a high elastic modulus thin film 18 deposited 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 or the like, the deterioration of the characteristics due to the increase in the gap width between each diaphragm or between the diaphragm and the substrate due to the residual stress is eliminated. The signal-to-noise ratio can be improved with high sensitivity, and the structure can be made suitable for mass productivity.

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, but a diaphragm obtained by dividing a diaphragm having a hexagonal or circular outer peripheral shape into a plurality of parts is defined as a cantilever structure. The above-mentioned embodiment can be similarly applied to the above-mentioned one.

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 ... holes, 6 ... insulating film,
7a, 7b ... Piezoelectric thin film, 8 ... Electrode thin film,
9 ... Wiring electrode, 15 ... Additional thin film,
16 ... Opening, 18 ... High elastic modulus thin film,
g ... Gap.

Claims (2)

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、この振動板が同一幅で開放され複数に分割され、この分割された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、
上記複数の振動板のそれぞれの開放端側の領域に、各振動板間の開放端に合わせたギャップを持つ付加薄膜を形成することにより、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。 A diaphragm including a piezoelectric thin film and a pair of electrodes arranged across the piezoelectric thin film is provided, and the diaphragm is opened with the same width and divided into a plurality of diaphragms, and one end of the divided diaphragms is supported by a substrate. In the piezoelectric element
By forming an additional thin film having a gap matched to the open end between the diaphragms in the region on the open end side of each of the plurality of diaphragms, the elastic coefficient on the open end side of the diaphragms is vibrated. A piezoelectric element characterized by having a larger elastic coefficient than the support end side of the plate. 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記載の圧電素子。 The piezoelectric element according to claim 1 , wherein the diaphragm is a diaphragm that vibrates due to acoustic pressure.
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