PIEZOELECTRIC DEVICE

A piezoelectric device that includes: a diaphragm; a supporting part configured to support at least a portion of an end of the diaphragm; a piezoelectric film disposed along a portion supported by the supporting part on the diaphragm, a width of the film along the supported portion being narrower than a width of the portion; a lower electrode disposed at a face of the piezoelectric film on a diaphragm side; and an upper electrode disposed on a face of the piezoelectric film on an opposite side to the diaphragm.

BACKGROUND OF THE INVENTION

The present invention relates to piezoelectric devices.

Electronic apparatuses such as cellular phones may be mounted with a plurality of microphones. For instance, a cellular phone may be provided with a microphone for detecting ambient sound (environmental sound) for a purpose of noise canceling in addition to a microphone for detecting a transmission voice during a call. As more and more electronic apparatuses are mounted with a plurality of microphones, downsizing of microphones is increasingly demanded.

Against a background like this, in recent years, a microphone manufactured using a micro electro mechanical systems (MEMS) technology (hereinafter, referred to as a MEMS microphone) has been drawing an attention (for instance, Patent Publication JP 2004-506394 A).

In mounting a microphone onto an electronic apparatus, not only downsizing of a microphone but also sensitivity enhancement of the microphone is required. Sensitivity enhancement is required also for a MEMS microphone.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of such circumstances, and an object thereof is to enhance the sensitivity of a MEMS microphone.

A piezoelectric device related to one aspect of the present invention includes: a diaphragm; a supporting part configured to support at least a part of an end of the diaphragm; a piezoelectric film disposed on the diaphragm along a portion supported by the supporting part, a width of the film along the supported portion being narrower than a width of the portion; a lower electrode disposed on a surface of a diaphragm side of the piezoelectric film; and an upper electrode disposed on a face of the piezoelectric film on an opposite side to the diaphragm.

According to the present invention, the sensitivity of a MEMS microphone can be improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention is described below with reference to the drawings.FIG. 1is a view showing an appearance of a piezoelectric device of one embodiment of the present invention. A piezoelectric device100is a device for configuring a MEMS microphone that converts sound pressure into an electrical signal, and includes a diaphragm110, a supporting part111, and piezoelectric parts112. The piezoelectric device100is divided into two by a minute slit113of about 1 μm or less wide for instance.

The diaphragm110is a thin film that vibrates due to sound pressure and can be formed of silicon (Si). The diaphragm110has a substantially rectangular shape, wherein lower parts of a facing set of sides114and115are supported by the supporting part111. In other words, the diaphragm110has a both-end supported beam structure. It is beneficial that the diaphragm110is degenerate silicon, and has a function as a lower electrode of the piezoelectric part112as described later. What is called a degenerate silicon or degenerate semiconductor doped with a dopant in high concentration (1×1019cm−3or over). To be more precise, by doping phosphorus (P), arsenic (As), or antimony (Sb) at a concentration of 1×1019cm−3or over into Si as an n-type dopant (a donor), a degenerate semiconductor can be formed. A degenerate semiconductor may also be formed by doping a p-type dopant (an acceptor) into Si.

The piezoelectric parts112are disposed along the portion supported by the supporting part111on the diaphragm110. As shown inFIG. 1, a width (A) of the piezoelectric part112(a width of a piezoelectric film210as described later) is narrower than a width (B) of the portion supported by the supporting part111on the diaphragm110(in other words, a width of the side114). For instance, the width (A) of the piezoelectric part112may be about 100 μm, and the width (B) of the portion supported by the supporting part111on the diaphragm110may be about 300 μm. Although four piezoelectric parts112are disposed on the diaphragm110in the configuration shown inFIG. 1, the number of piezoelectric parts112is not limited to this. In the configuration shown inFIG. 1, although ends of the piezoelectric parts112are disposed on the sides114and115, ends may be disposed separately from the sides114and115.

FIG. 2is a cross-sectional view of the piezoelectric device100at a line X-Y shown inFIG. 1. The supporting part111includes a substrate200and an insulating layer201.

The substrate200is formed of silicon (Si) for instance. The insulating layer201is formed of silicon oxide (SiO2) for instance. The diaphragm110is formed on the supporting part111formed in this manner.

Each of the piezoelectric parts112disposed along the portion supported by the supporting part111on the diaphragm110includes a piezoelectric film210, an upper electrode211, and wirings212and213.

The piezoelectric film210is disposed on the diaphragm110so as to be vibrated in association with vibration of the diaphragm110. The piezoelectric film210is a thin film of a piezoelectric body that converts force applied by the vibration to voltage, and is formed of scandium doped aluminum nitride (ScAlN) for instance. ScAlN is formed by substituting a part of aluminum (Al) in aluminum nitride (AlN) with scandium (Sc). For instance, ScAlN used for the piezoelectric film210may be formed by substituting Al with Sc so that Sc becomes about 40 atom % when atomic concentration that is a sum of the number of Al atoms and the number of Sc atoms is assumed to be 100 atom %. The thickness of the piezoelectric film210may be about 500 nm for instance. A ratio of a width (D) of a vibration portion of the piezoelectric film210to a width (C) from a center of the diaphragm110to the supporting part111may be about 40% for instance. The width (C) may be about 300 μm and the width (D) may be about 120 μm for instance.

The upper electrode211is disposed on an upper side of the piezoelectric film210. The upper electrode211is a metal electrode and may be formed of aluminum (Al) for instance, and may have a thickness of about 50 nm. The upper electrode211may have tensile stress. Since the piezoelectric film210formed of ScAlN has a compressive stress, by allowing the upper electrode211to have a tensile stress, a stress at the piezoelectric part112is corrected and a deformation of the diaphragm110can be suppressed.

The wiring212is electrically coupled to the upper electrode211. The wiring213is electrically coupled to the lower electrode (the diaphragm110). The wirings212and213are formed by using gold (Au), platinum (Pt), titanium (Ti), aluminum (Al), or the like for instance.

In the piezoelectric device100of the configuration described above, the piezoelectric film210vibrates in association with a vibration of the diaphragm110caused by sound pressure. Voltage corresponding to the vibration of the piezoelectric film210is output through the wirings212and213of the piezoelectric body112. As shown inFIG. 1, four piezoelectric bodies112are provided in the piezoelectric device100. The four piezoelectric bodies112can be electrically coupled in parallel as shown inFIG. 3for instance. The coupling shown inFIG. 3is just an example and a form of coupling of the piezoelectric bodies112is not limited to this.

In the piezoelectric device100, as shown inFIG. 1, the width (A) of the piezoelectric part112(the width of the piezoelectric film210) is narrower than the width (B) of the portion supported by the supporting part111of the diaphragm110(in other words, the width of the side114). Such a structure increases the stress applied to the piezoelectric part112by the vibration of the diaphragm110caused by sound pressure compared to the case where the piezoelectric part112has the same width as the diaphragm110(in other words, the width (A) of the piezoelectric part112=the width (B) of the diaphragm110). Consequently, the stress applied to a unit area of the piezoelectric part112becomes large, and the voltage sensitivity and the generated energy of the piezoelectric part112can be increased. In other words, sensitivity of the MEMS microphone configured by using the piezoelectric device100can be improved.

FIG. 4is a graph showing one example of a relation between the width of the piezoelectric part112and the voltage sensitivity at the piezoelectric device100. InFIG. 4, the horizontal axis represents a ratio (%) of the width (A) of the piezoelectric part112to the width (B) of the diaphragm110, and the vertical axis represents the voltage sensitivity (mV/Pa) showing output voltage (mV) per sound pressure (Pa) at the piezoelectric part112. As shown inFIG. 4, the voltage sensitivity increases as the ratio of the width of the piezoelectric part112to that of the diaphragm110decreases. Accordingly, at the piezoelectric device100of the embodiment, the voltage sensitivity can be improved.

FIG. 5is a graph showing one example of a relation between the width of the piezoelectric part112and the generated energy. InFIG. 5, the horizontal axis represents the ratio (%) of the width (A) of the piezoelectric part112to the width (B) of the diaphragm110, and the vertical axis represents the generated energy (fJ/Pa) per sound pressure at the piezoelectric part112. As shown inFIG. 5, the generated energy increases as the ratio of the width of the piezoelectric part112to that of the diaphragm110decreases. Accordingly, at the piezoelectric device100of the embodiment, the generated energy can be enlarged.

When the width of the piezoelectric part112becomes narrow, a capacitance value of the piezoelectric part112becomes small. When the capacitance value becomes small, impedance mismatching with an amplifier circuit may be likely to occur due to an impedance increase, or an influence of parasitic capacitance may be likely to become large. Therefore, the width (A) of the piezoelectric part112is determined by taking account of a trade-off between improvement of the voltage sensitivity and an increase in the impedance and such.

When Young's modulus of the diaphragm110changes with temperature, the voltage sensitivity of the piezoelectric device100also changes. In this respect, in the embodiment, since the diaphragm110is formed of a degenerate semiconductor, a change of Young's modulus of the diaphragm110with temperature can be suppressed. In other words, a change of the voltage sensitivity of the piezoelectric device100can be suppressed.

FIG. 6is a graph showing one example of a relation between temperature and Young's modulus. InFIG. 6, the horizontal axis represents temperature (° C.) and the vertical axis represents Young's modulus (GPa). InFIG. 6, four temperature characteristics (P1) to (P4) of the diaphragm110with different doping concentrations are shown. P1 inFIG. 6shows a temperature characteristic when nothing is doped into Si. P2 inFIG. 6shows a temperature characteristic when an n-type dopant is doped into Si at a concentration of 1×1019cm−3. P3 inFIG. 6shows a temperature characteristic when the n-type dopant is doped into Si at a concentration of 5×1019cm−3. P4 inFIG. 6shows a temperature characteristic when the n-type dopant is doped into Si at a concentration of 8×1019cm−3. As shown inFIG. 6, in comparison with Young's modulus in a case P1 where nothing is doped into Si, by making the doping concentration in Si 1×1019cm−3or more, in other words, by making Si into a degenerate semiconductor, the change in Young's modulus due to temperature change can be suppressed. With the diaphragm110being formed of a degenerate semiconductor, the change of the voltage sensitivity of the piezoelectric device100can be suppressed.

FIG. 7is a view showing another configuration example of the piezoelectric device. As for the same components as those of the piezoelectric device100shown inFIG. 1, the same reference numerals are given and descriptions are omitted. As shown inFIG. 7, in the piezoelectric device700, lower parts of all sides710to713of the diaphragm110are supported by the supporting part111. In other words, the diaphragm110has an entire circumference supported beam structure. A configuration of a cross-section at an X-Y line shown inFIG. 7is equivalent to the configuration shown inFIG. 2.

The piezoelectric device100shown inFIG. 1has the both-end supported beam structure, wherein the diaphragm110is provided with the slit113. For that reason, in the piezoelectric device100, while the diaphragm110is easily bendable and the voltage sensitivity can be improved, the diaphragm110may be deformed due to stresses of the piezoelectric films210and the upper electrodes211, which may change an amount of sound leakage from the slit113and may cause variations in the voltage sensitivity. On the contrary, as shown inFIG. 7, the piezoelectric device700is not provided with the slit113of the piezoelectric device100. Therefore, in the piezoelectric device700, no sound leakage from the slit occurs and the variations in the voltage sensitivity can be suppressed.

FIG. 8is a view showing another configuration example of the piezoelectric device. As for the same components as those of the piezoelectric device100shown inFIG. 1, the same reference numerals are given and descriptions are omitted. As shown inFIG. 8, in a piezoelectric device800, a slit810is provided along a substantial center line810substantially parallel to the sides114and115supported by the supporting part111in addition to the slit113. The diaphragm110is divided into four by the slits113and810. In other words, the diaphragm110has a cantilever structure. By making the diaphragm110the cantilever structure, the diaphragm110becomes more flexible than in the case of the piezoelectric device100shown inFIG. 1, and the voltage sensitivity can be improved.

FIG. 9is a view showing one configuration example in which a vicinity of a center of the diaphragm110is made thin in the piezoelectric device100shown inFIG. 1. As shown inFIG. 9, a thickness of a region901in the vicinity of the center of the diaphragm110can be formed thinner than a thickness of a region900of the diaphragm110, where the piezoelectric film210is disposed. In this manner, by thinning the thickness of the region901in the vicinity of the center of the diaphragm110, the diaphragm110is made to be easily bendable, and the voltage sensitivity can be improved. Since the piezoelectric device800shown in theFIG. 8is not provided with the slit810, the deformation of the diaphragm110is suppressed, and the variations in the voltage sensitivity are suppressed.

Changing the thickness of the region900in the diaphragm110, where the piezoelectric film210is disposed, changes a state of expansion and contraction of the piezoelectric film210resulting from the vibration of the diaphragm110and changes voltage output characteristics of the piezoelectric part112. More specifically, thinning the thickness of the region900in the diaphragm110, where the piezoelectric film210is disposed, when the piezoelectric film210bends downwardly for instance, may increase an amount of contraction of an under side of the piezoelectric film210, may cancel out voltage output resulting from expansion of an upper side of the piezoelectric film210, and may reduce the output voltage from the piezoelectric part112. For this reason, thinning only the region901in a vicinity of a center without changing the thickness of the region900can improve the voltage sensitivity without influencing the voltage output characteristics of the piezoelectric part112.

Not only in the configuration shown inFIG. 1but also in the configurations shown inFIG. 7andFIG. 8, a region in the vicinity of the center of the diaphragm110may be thinned.

FIG. 10is a view showing another configuration example of the piezoelectric device. As for the same components as those of the piezoelectric device100shown inFIG. 1, the same reference numerals are given and descriptions are omitted. As shown inFIG. 10, the diaphragm110has a substantially circular shape in a piezoelectric device1000. In this case also, the width (A) of the piezoelectric part112is made narrower than the width (B) of the portion supported by supporting part111in the diaphragm110of the piezoelectric part112. In this manner, the diaphragm110may have not only a substantially rectangular shape but also an arbitrary shape. Although four piezoelectric parts112are disposed on the diaphragm110inFIG. 10, the number of the piezoelectric parts112is not limited to this but may be any number. For instance, as shown inFIG. 11three piezoelectric parts112may be disposed on the diaphragm110having a substantially circular shape.

FIG. 12is a view showing another configuration example of the piezoelectric device. As for the same components as those of the piezoelectric device100shown inFIG. 2, the same reference numerals are given and descriptions are omitted. As shown inFIG. 2, the diaphragm110is used as a lower electrode of the piezoelectric part112in the piezoelectric device100. On the other hand, in a piezoelectric device shown inFIG. 12, a lower electrode1210is provided separately from the diaphragm110. The lower electrode1210is a metal electrode and may be formed of aluminum (Al) for instance, and may have a thickness of about 50 nm.

The embodiments of the present invention have been described above. According to the embodiments, the piezoelectric device is formed so that the width (A) of the piezoelectric part112is narrower than the width (B) of the portion supported by the supporting part111in the diaphragm110. This increases a stress applied to a unit area of the piezoelectric part112, and enables enhancement of the voltage sensitivity and the generated energy in the piezoelectric part112. In other words, the sensitivity of a MEMS microphone configured by using the piezoelectric device100can be improved.

According to the embodiment, the diaphragm110can be formed of a degenerate semiconductor. Thereby, variations in the Young's modulus of the diaphragm110with temperature can be suppressed, and variations in the voltage sensitivity of the piezoelectric device with temperature can be suppressed.

According to the embodiment, the diaphragm110formed of a degenerate semiconductor can be used as a lower electrode of the piezoelectric part112. Thereby, the piezoelectric device100can be downsized as compared with a case where the lower electrode is formed separately from the diaphragm110.

According to the embodiment, as shown inFIG. 1, making the diaphragm110have the both-end supported beam structure makes it possible to make the diaphragm110easily bendable and to improve the voltage sensitivity of the piezoelectric device.

According to the embodiment, as shown inFIG. 8, when making the diaphragm110have the both-end supported beam structure, the substantial center line810substantially parallel to the sides114and115of the supporting part111makes the diaphragm110have a separate configuration, thereby, making the diaphragm110more bendable and being able to improve the voltage sensitivity of the piezoelectric device.

According to the embodiment, as shown inFIG. 7, making the diaphragm110have the entire circumference supported beam structure makes it possible to suppress the deformation of the diaphragm110and to suppress the variations in the voltage sensitivity caused by the deformation of the diaphragm110.

According to the embodiment, as shown inFIG. 9, making the region901in the vicinity of the center of the diaphragm110thin makes it possible to make the diaphragm110more bendable and to improve the voltage sensitivity of the piezoelectric device.

According to the embodiment, it is possible to make the upper electrode211formed at an upper side of the piezoelectric film210having a compressive stress have a tensile stress. Thereby, the stress at the piezoelectric part112is corrected, and the deformation of the diaphragm110is suppressed.

The embodiment is for facilitating comprehension of the present invention, and not for comprehending by limiting the present invention. The present invention can be changed and/or improved without being deviated from the gist thereof. The present invention includes also equivalents thereof.

For instance, in the embodiment, although an example is described in which the piezoelectric device is used as a MEMS microphone by vibrating the diaphragm by sound pressure, uses of the piezoelectric device is not limited to this, and is usable for a sensor that detects vibration of a medium in the surrounding of the piezoelectric device.