JP7514672B2 - Piezoelectric element and its manufacturing method - Google Patents

Description

The present invention relates to a piezoelectric element and a manufacturing method thereof, and in particular to a piezoelectric element that can be used for piezoelectric MEMS microphones that have high sensitivity and low noise, and a manufacturing method thereof.

In recent years, the demand for smartphones has been expanding rapidly, and many of them use microphones that use MEMS (Micro Electro Mechanical Systems) technology, which are small, thin, and can withstand the high temperatures of the solder reflow process during assembly. Furthermore, not only MEMS microphones, but other MEMS elements are rapidly becoming more common in a variety of fields.

Many of these types of MEMS elements are capacitive elements that detect the displacement of a diaphragm caused by acoustic pressure, etc., as a change in capacitance with the opposing fixed plate, and convert it into an electrical signal for output. However, capacitive elements are reaching their limit in improving the signal-to-noise ratio due to acoustic resistance caused by the flow of air in the gap between the diaphragm and the fixed plate. As a result, attention has been focused on piezoelectric elements that can extract acoustic pressure, etc. as a voltage change from the distortion of a single diaphragm made of a thin film (piezoelectric film) made of piezoelectric material.

In conventional piezoelectric elements, slits 1 of a desired shape are formed in a piezoelectric film to form a cantilever-structured vibration plate 2, as shown in FIG. 3. In FIG. 3(a), two rectangular vibration plates 2 are formed, and in FIG. 3(b), four triangular vibration plates 2 are formed. This type of piezoelectric element is disclosed, for example, in Patent Document 1.

Figure 4 is a cross-sectional view of a piezoelectric element. As shown in Figure 4, multi-layered piezoelectric films 5a and 5b are supported and fixed via an insulating film 4 on a support substrate 3 made of a silicon substrate, with the piezoelectric film 5a sandwiched between electrodes 6a and 6b, and the piezoelectric film 5b sandwiched between electrodes 6b and 6c. A cavity 7 is formed in the support substrate 3, and the piezoelectric film and electrodes separated by a slit 1 form a vibration plate 2 with one end fixed to the support substrate 3 and the other end being a free end.

In such a piezoelectric element, when the diaphragm 2 is subjected to acoustic pressure, etc., the piezoelectric film 5a is distorted, causing polarization inside it, making it possible to extract a voltage signal from the metal wiring 8a connected to electrode 6a and the metal wiring 8b connected to electrode 6b. Similarly, when the piezoelectric film 5b is distorted, polarization occurs inside it, making it possible to extract a voltage signal from the metal wiring 8a connected to electrode 6c and the metal wiring 8b connected to electrode 6b.

Japanese Patent No. 5936154

However, when the slit 1 is formed in the piezoelectric films 5a and 5b with such a cantilever structure, the residual stress is released, causing the film to warp and the opening width of the slit 1 to widen. If a piezoelectric element with an opening width of the slit 1 greater than the design value is used as a microphone, the acoustic resistance decreases, leading to deterioration of characteristics such as a decrease in sensitivity in the low-frequency range. The present invention aims to solve these problems and provide a piezoelectric element and a manufacturing method thereof that can suppress the effects of residual stress in the piezoelectric film and suppress deterioration of characteristics.

In order to achieve the above-mentioned object, the invention of claim 1 of the present application is a piezoelectric element comprising a vibration plate made of a piezoelectric film having one end fixed and the other end being a free end, and a pair of electrodes arranged on either side of the piezoelectric film, wherein the vibration plate is characterized in that a part of the piezoelectric film forms a stress relaxation region that relieves warping of the vibration plate in the vibration direction , and the stress relaxation region is an ion-implanted region .

The invention according to claim 2 of the present application is characterized in that in the piezoelectric element according to claim 1, the vibration plate is a piezoelectric film supported on a support substrate having a cavity, the piezoelectric film being partitioned by slits penetrating the piezoelectric film, and the stress relaxation region is disposed on the free end side of the vibration plate.

The invention of claim 3 of the present application is a manufacturing method for a piezoelectric element having a vibration plate made of a piezoelectric film having one end fixed and the other end being a free end, and a pair of electrodes arranged on either side of the piezoelectric film, the method comprising the steps of: forming the piezoelectric film on a supporting substrate; forming the pair of electrodes arranged on either side of the piezoelectric film; partitioning the piezoelectric film so that at least one end is fixed to the supporting substrate and the other end is a free end, thereby forming the vibration plate; and injecting ions into a part of the piezoelectric film of the vibration plate where warping occurs in the vibration direction of the vibration plate, to form a stress relaxation region that relieves the warping .

The piezoelectric element of the present invention can be made into a vibration plate without warping by providing a stress relief region in part of the piezoelectric film, and acoustic pressure, etc. can be efficiently transmitted to the vibration plate 2, making it possible to obtain a large output signal.

In particular, in a diaphragm in which the piezoelectric film is divided by slits, by making the diaphragm warp-free, the opening width of the slits can be as designed, making it possible to create a piezoelectric element with the desired characteristics. When such a piezoelectric element is used as an acoustic transducer, the acoustic resistance can be maintained high, making it possible to suppress a decrease in sensitivity in the low-frequency range and a decrease in the signal-to-noise ratio.

The method for manufacturing a piezoelectric element of the present invention is a simple and easy-to-control method that can form a stress relief region by injecting ions into the piezoelectric film. In particular, by changing the injection conditions, the stress relief region can be adjusted to the desired stress, making it easy to relieve the warping of the diaphragm in accordance with the degree of warping.

5A to 5C are explanatory diagrams of a method for manufacturing a piezoelectric element according to an embodiment of the present invention. 5A to 5C are explanatory diagrams of a method for manufacturing a piezoelectric element according to an embodiment of the present invention. FIG. 1 is an explanatory diagram of a conventional piezoelectric element. FIG. 1 is an explanatory diagram of a conventional piezoelectric element.

The piezoelectric element according to the present invention has a portion of the diaphragm as a stress relief region, and is a piezoelectric element with a diaphragm that is free of warping. Below, an embodiment of the present invention will be described in detail according to the manufacturing process.

The piezoelectric element of the present invention is a piezoelectric element equipped with a cantilever-structured vibration plate having a slit 1 of the desired shape as shown in FIG. 3.

First, a metal film that will become an electrode is formed on a support substrate 3 made of a silicon substrate via an insulating film 4, and an electrode 6a is formed by a normal photolithography method, and a piezoelectric film 5a is formed on the electrode 6a and the insulating film 4. Next, a metal film that will become an electrode is formed on the piezoelectric film 5a, and an electrode 6b is formed by a normal photolithography method, and a piezoelectric film 5b is formed on the electrode 6b and the piezoelectric film 5a. Next, a metal film that will become an electrode is formed on the piezoelectric film 5b, and an electrode 6c is formed by a normal photolithography method. After that, a metal wiring 8a that connects to the electrode 6a and the electrode 6c, and a metal wiring 8b that connects to the electrode 6b are formed.

Part of the piezoelectric films 5b and 5a is etched away to form a slit 1. Also, part of the support substrate 3 and insulating film 4 is removed to form a cavity 7. In this state, the piezoelectric films 5a and 5b become a vibration plate 2 with one end supported on the support substrate 3 and insulating film 4. The above formation method is the same as the manufacturing method for conventional piezoelectric elements.

In general, a piezoelectric film has residual stress, so that a warp may occur in the vibration plate 2 having a free end as shown in FIG. 1. When the vibration plate 2 is warped in the opposite direction to the cavity 7 as shown in FIG. 1, the piezoelectric film 5b has a larger compressive stress than the piezoelectric film 5a. Therefore, in order to alleviate the warp of the vibration plate 2, specifically to reduce the compressive stress of the piezoelectric film 5b, ions are injected into the surface of the vibration plate 2 from the direction of the arrow shown in FIG. 2 to form a stress relaxation region 9. The surface of the vibration plate 2 (piezoelectric film 5b) on which this stress relaxation region 9 is formed has the crystallinity of the piezoelectric film disturbed by the injection of ions, and the compressive stress is relieved. Since it is not desirable to recover the crystallinity disturbance, it is preferable not to perform a heat treatment after the injection of ions. By forming the stress relaxation region 9 in this way, the warp of the vibration plate 2 is eliminated.

The stress relaxation region 9 is formed, for example, using a focused ion beam device with gallium (Ga) as the ion species under conditions of an acceleration energy of 30 keV and a current of 30 nA. It was confirmed that the warping of the piezoelectric film surface on the free end side of the diaphragm 2, which has a warp of about 30 μm, can be alleviated by injecting ions under the above conditions. At this time, it was also confirmed that the free end side of the diaphragm 2 has a small contribution to the signal output from the piezoelectric element, and that forming the stress relaxation region 9 in this region causes no problems with the output signal of the piezoelectric element.

Naturally, the degree of warping of the diaphragm 2 varies depending on the magnitude of the residual stress in the piezoelectric film. Therefore, the ion injection conditions, such as acceleration energy, current, injection time, injection area, and scanning direction, can be set appropriately depending on the degree of warping of the diaphragm 2. It is also effective to repeat the ion injection multiple times depending on the degree of warping of the diaphragm 2.

The stress relief region 9 is not limited to being formed on a portion of the surface of the piezoelectric film 5b as shown in FIG. 1, but can also be formed on the entire piezoelectric film 5b, or on both the piezoelectric film 5b and the piezoelectric film 5a.

In the example shown in FIG. 1, the stress relaxation region 9 is formed from the surface side of the piezoelectric film 5b, but it is also possible to form the stress relaxation region 9 in the piezoelectric film 5a from the cavity 7 side.

By configuring the device with the stress relief area 9 in this way, the warping of the diaphragm 2 is mitigated, the slit width can be set as designed, and the effect of suppressing deterioration of the characteristics of the piezoelectric element is greatly improved.

Although the embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the above embodiments. For example, the ions to be implanted are not limited to gallium. Furthermore, the present invention is not limited to a piezoelectric film with a multi-layer structure.

1: Slit, 2: Vibration plate, 3: Support substrate, 4: Insulating film, 5a, 5b: Piezoelectric film, 6a, 6b, 6c: Electrodes, 7: Cavity, 8a, 8b: Wiring metal, 9: Stress relaxation area

Claims (3)

A piezoelectric element including a vibration plate made of a piezoelectric film having one fixed end and the other free end, and a pair of electrodes disposed on either side of the piezoelectric film,
a part of the piezoelectric film of the vibration plate serves as a stress relaxation region that relaxes warping of the vibration plate in the vibration direction ;
The stress relaxation region is an ion-implanted region . 2. The piezoelectric element according to claim 1,
The vibration plate includes a piezoelectric film supported on a support substrate having a cavity, the piezoelectric film being partitioned by slits passing through the piezoelectric film,
A piezoelectric element, characterized in that the stress relaxation region is disposed on the free end side of the vibration plate. A method for manufacturing a piezoelectric element including a vibration plate made of a piezoelectric film having one fixed end and the other free end, and a pair of electrodes disposed on either side of the piezoelectric film, comprising:
forming the piezoelectric film on a supporting substrate;
forming the pair of electrodes disposed on either side of the piezoelectric film;
forming the vibration plate by dividing the piezoelectric film so that at least one end is fixed to the support substrate and the other end is a free end;
a step of injecting ions into a portion of the piezoelectric film of the vibration plate where warping occurs in the vibration direction of the vibration plate, to form a stress relief region that relieves the warping.

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