JP7484069B2 - Manufacturing method of piezoelectric element - Google Patents

Description

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

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 is being paid to 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 vibration plate 2 with a cantilever structure, as shown in FIG. 11. In FIG. 11(a), two rectangular vibration plates 2 are formed, and in FIG. 11(b), four triangular vibration plates 2 are formed. This type of piezoelectric element is disclosed, for example, in Patent Document 1.

Figure 12 is a cross-sectional view of a piezoelectric element. As shown in Figure 12, 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 open.

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.

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 widen the opening width of the slit 1. If a piezoelectric element with an opening width of the slit 1 that is 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.

Therefore, the applicant proposed a piezoelectric element in which a thin film 9 is laminated on a piezoelectric film 5b as shown in FIG. 13 (Patent Document 2).

Japanese Patent No. 5936154 JP 2018-137297 A

The piezoelectric element previously proposed by the applicant of the present application can suppress the widening of the opening width of the slit 1 caused by the residual stress of the piezoelectric film by having a structure including a thin film 9, and can also suppress the associated deterioration of characteristics. However, in the manufacturing process, after the slit 1 is formed, an opening is formed in the thin film 9 so as to communicate with the slit 1, so it was necessary to form the opening width of the thin film 9 wider than the opening width of the slit 1, taking into consideration the variation in the manufacturing process (such as misalignment). Therefore, although the thin film 9 has the effect of suppressing the deformation of the diaphragm by increasing the elastic coefficient of the diaphragm, if the deformation of the diaphragm cannot be completely suppressed even when the thin film 9 is formed, the opening width of the slit 1 becomes larger than the design value, and when the piezoelectric element is used as a microphone, the acoustic resistance is reduced and deterioration of characteristics such as a decrease in sensitivity in the low frequency range cannot be suppressed. The present invention aims to solve such problems and provide a manufacturing method for a piezoelectric element that can suppress the influence of the residual stress of the piezoelectric film and suppress deterioration of characteristics.

In order to achieve the above object, the invention according to claim 1 of the present application is a method for manufacturing a piezoelectric element having a vibration plate separated by a slit and a coating film laminated on the vibration plate along the slit, the method comprising the steps of: preparing a support substrate on which a piezoelectric film constituting the vibration plate and a pair of electrodes arranged on either side of the piezoelectric film are laminated; forming the slit in the piezoelectric film; forming the coating film on the piezoelectric film; removing a portion of the support substrate to form a cavity and forming the vibration plate; and using the vibration plate exposed in the cavity as a mask film, removing a portion of the coating film to form an opening continuous with the slit.

The invention according to claim 2 of the present application is characterized in that in the method for manufacturing a piezoelectric element according to claim 1, a positive photosensitive film is formed as the coating film, and the vibration plate is used as an exposure mask film to expose the coating film, and then developed, thereby removing a portion of the coating film to form the opening.

The invention according to claim 3 of the present application is characterized in that in the method for manufacturing a piezoelectric element according to claim 1, the vibration plate is used as an etching mask film to dry etch the coating film, thereby removing a portion of the coating film to form the opening.

The method for manufacturing a piezoelectric element of the present invention can form an opening along the slit in a self-aligned manner by using the vibration plate exposed in the cavity as a mask film and removing a portion of the coating film. In particular, the method of forming an opening by using the vibration plate as an exposure mask or as a dry etching mask and removing a portion of the coating film has good controllability and can easily form a piezoelectric element.

Furthermore, if the coating film is formed from a positive photosensitive film, the patterning of the coating film and the formation of the openings can be performed simultaneously in a single development step, making this a simple method. When the coating film is removed by dry etching, it can also be removed by exposing and developing the positive or negative photosensitive film in areas other than the area on the piezoelectric film where the coating film is to be formed, which is preferable as it allows the coating film to be easily patterned.

In the method for manufacturing a piezoelectric element of the present invention, it is necessary to etch a thick support substrate to form a cavity. However, if the entire surface of the piezoelectric film or the like is covered with a coating film while etching, there is an advantage in that the occurrence of cracks in the piezoelectric film can be prevented. In this case, it is preferable to use a resin film containing a polyimide precursor as the coating film, as this has excellent heat resistance and allows subsequent patterning of the coating film and the formation of openings to be performed without any problems.

In addition, the manufacturing method of forming an opening width of about several micrometers using a resin film containing a polyimide precursor is widely used in the manufacturing process of ordinary electronic devices, and is a simple method with good controllability.

When a piezoelectric element formed by the manufacturing method of the present invention is used as an acoustic transducer, the adverse effects of residual stress can be mitigated and high acoustic resistance can be maintained, making it possible to suppress a decrease in sensitivity in the low-frequency range and a decrease in the signal-to-noise ratio.

5A to 5C are explanatory diagrams of a manufacturing method of the piezoelectric element according to the first embodiment of the present invention. 5A to 5C are explanatory diagrams of a manufacturing method of the piezoelectric element according to the first embodiment of the present invention. 5A to 5C are explanatory diagrams of a manufacturing method of the piezoelectric element according to the first embodiment of the present invention. 5A to 5C are explanatory diagrams of a manufacturing method of the piezoelectric element according to the first embodiment of the present invention. 5A to 5C are explanatory diagrams of a manufacturing method of the piezoelectric element according to the first embodiment of the present invention. FIG. 2 is a diagram for explaining the arrangement of a coating film according to the first embodiment of the present invention. 5A to 5C are explanatory diagrams of a method for manufacturing a piezoelectric element according to a second embodiment of the present invention. 5A to 5C are explanatory diagrams of a method for manufacturing a piezoelectric element according to a second embodiment of the present invention. 5A to 5C are explanatory diagrams of a method for manufacturing a piezoelectric element according to a second embodiment of the present invention. 11A to 11C are explanatory diagrams of a manufacturing method of a piezoelectric element according to a third 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. FIG. 13 is an explanatory diagram of another conventional piezoelectric element.

The method for manufacturing a piezoelectric element according to the present invention uses the diaphragm exposed in the cavity as a mask film to form a coating film having openings aligned with the slits in the diaphragm. Below, an embodiment of the present invention will be described in detail.

The first embodiment of the present invention will be described. The piezoelectric element of the present invention is a method for manufacturing a piezoelectric element having a vibration plate with a cantilever structure in which a slit 1 of a desired shape is formed, as shown in Figure 11.

On a support substrate 3 made of a silicon substrate, a multi-layered structure of piezoelectric films 5a and 5b is formed with an insulating film 4 between them, and electrodes 6a and 6b sandwich the piezoelectric film 5a, and electrodes 6b and 6c sandwich the piezoelectric film 5b. Slits 1 of a desired shape are formed in the piezoelectric films 5a and 5b. When a cavity is formed in the support substrate 3 as described below, the piezoelectric film and electrode defined by the slits 1 form a vibration plate 2 with one end fixed to the support substrate 3 and the other end open. Metal wiring 8a is formed to connect the electrodes 6a and 6c of each vibration plate 2, and metal wiring 8b is formed to connect the electrodes 6b (Figure 1).

A coating film 10 made of a photosensitive resin containing a polyimide precursor is applied to the entire surface. The coating film 10 also fills the inside of the slit 1. After a heat treatment such as pre-baking, the coating film 10 is exposed to light in order to pattern it. A positive type coating film 10 is used here, and light of a desired wavelength is irradiated to the area (area to be removed) other than the area where the coating film is to be formed along the opening of the slit 1, to form the exposed area 10a. The area where the coating film is to be formed becomes the unexposed area 10b (Figure 2).

Then, the support substrate 3 is removed by dry etching, and then the insulating film 4 is removed by wet etching to form a cavity 7 exposing the coating film 10 filling the slit 1 (Figure 3). As shown in Figure 3, the cavity 7 contains the piezoelectric film 5a and electrode 6a that will become the vibration plate 2, and a part of the unexposed region 10b of the coating film 10 is exposed in the slit 1. If the thick support substrate 3 and the insulating film 4 are etched without patterning the coating film 10, the coating film 10 will act as a surface protective film, which is preferable.

Next, the diaphragm 2 is used as an exposure mask film to expose the unexposed area 10b of the coating film 10 from the cavity 7 side. As a result, the unexposed area 10b is exposed to light, becoming the exposed area 10a (Figure 4).

The exposed regions 10a of the coating film 10 are then removed by development. The polyimide precursor is imidized by the necessary heat treatment, forming the coating film 10 made of polyimide. When performing the heat treatment, it is preferable to set the temperature range so that the openings 11 of the coating film 10 do not expand significantly.

Figure 5 is a cross-sectional view of a completed piezoelectric element. Figure 6 is a diagram explaining the positional relationship between the slit 1 and the coating film 10. In the example shown in Figure 6(a), the coating film 10 is arranged along the slit 1 between two opposing vibration plates 2. In this case, the coating film 10 is not formed on the slit 1 extending in the left-right direction of the drawing, because the opening width of this slit does not change significantly even if the residual stress of the vibration plate 2 is released and the vibration plate 2 is deformed. Of course, the coating film 10 may be formed. In the example shown in Figure 6(b), the coating film 10 is arranged along the slit 1 of the four opposing vibration plates 2. In this case, the warp of the vibration plate 2 is largest near the apex of the triangular vibration plate 2 and is small near the base, so the coating film 10 is formed only on the vibration plate 2 with the largest warp.

In this configuration with the coating film 10, when the diaphragm 2 warps in the opposite direction to the cavity 7, the opening width of the slit 1 widens, but the acoustic resistance of the opening 11 of the coating film 10 that continues from the slit 1 becomes larger than when the coating film 10 is not present, and the effect of suppressing deterioration of characteristics is increased.

The cross-sectional shape of the opening 11 in the coating film 10 can be changed by appropriately adjusting the exposure conditions and/or development conditions. Specifically, the opening width of the opening 11 can be made equal to the opening width of the slit 1, or can be made narrower than the opening width of the slit 1. In the latter case, the acoustic resistance of the opening 11 is expected to increase, which is expected to have a greater effect in suppressing deterioration of characteristics.

On the other hand, if the diaphragm 2 is warped toward the cavity 7, the opening width of the slit 1 increases, but the opening width of the coating film 10 decreases, so deterioration of characteristics can be suppressed.

Next, a second embodiment will be described. In the above first embodiment, the case where a coating film 10 made of a photosensitive resin containing a positive polyimide precursor is used has been described, but it is also possible to use a coating film 10A made of a photosensitive resin containing a negative polyimide precursor. In this case, the method of forming the opening 11 in the coating film 10A is different.

First, as in Example 1, a multi-layered piezoelectric film 5a, 5b is laminated on a support substrate 3 made of a silicon substrate with an insulating film 4 between it, electrodes 6a and 6b sandwiching the piezoelectric film 5a, and electrodes 6b and 6c sandwiching the piezoelectric film 5b. Slits 1 of a desired shape are formed in the piezoelectric films 5a and 5b. Metal wiring 8a is formed to connect the electrodes 6a and 6c of each of the vibration plates 2 partitioned by the slits 1, and metal wiring 8b is formed to connect the electrodes 6b (Figure 1).

A coating film 10A made of a photosensitive resin containing a negative polyimide precursor is applied to the entire surface. The coating film 10A also fills the inside of the slit 1. After a heat treatment such as pre-baking, the coating film 10A is exposed to light for patterning. The coating film 10A used here is a negative type, and the area along the opening of the slit 1 where the coating film is to be formed is irradiated with light of a desired wavelength to form an exposed area 10a. The area other than the area where the coating film is to be formed (the area to be removed) becomes an unexposed area 10b (Figure 7).

Then, the support substrate 3 is removed by dry etching, and then the insulating film 4 is removed by wet etching to form a cavity 7 that exposes the coating film 10A that fills the slit 1 (Figure 8). As shown in Figure 8, the cavity 7 contains the piezoelectric film 5a and electrode 6a that will become the vibration plate 2, and a part of the exposed area 10a of the coating film 10A is exposed in the slit 1.

Next, using the diaphragm 2 as an etching mask film, a portion of the exposed region 10a of the coating film 10A is dry etched from the cavity 7 side to form an opening 11 (Figure 9). The cross-sectional shape of the opening 11 in the coating film 10A can be changed by appropriately adjusting the etching conditions. Specifically, the opening width of the opening 11 can be made equal to the opening width of the slit 1, or the opening width of the opening 11 can be made narrower than the opening width of the slit 1.

Then, the unexposed regions 10b of the coating film 10A are removed by development. The polyimide precursor is imidized by the necessary heat treatment, and the coating film 10A made of polyimide is formed. It is preferable to set this heat treatment in a temperature range that does not cause the opening 11 of the coating film 10A to expand significantly.

In this embodiment, as in the first embodiment, a piezoelectric element similar to the piezoelectric element having the structure shown in FIG. 5 can be completed. The positional relationship between the slit 1 and the coating film 10A can also be arranged as shown in FIG. 6.

Next, a third embodiment will be described. In the above second embodiment, the case where the diaphragm 2 is used as an etching mask film when forming the opening 11 in the coating film 10A has been described, but in the above first embodiment, it is also possible to use the diaphragm 2 as an etching mask film. That is, in the above second embodiment, the case where the coating film 10A made of a photosensitive resin containing a negative polyimide precursor has been described, but it is also possible to use a coating film 10 made of a photosensitive resin containing a positive polyimide precursor.

In this case, instead of using the diaphragm 2 shown in FIG. 4 as an exposure mask film in the first embodiment, it can be used as an etching mask film.

First, as in Example 1, a multi-layered piezoelectric film 5a, 5b is laminated on a support substrate 3 made of a silicon substrate with an insulating film 4 between it, electrodes 6a and 6b sandwiching the piezoelectric film 5a, and electrodes 6b and 6c sandwiching the piezoelectric film 5b. Slits 1 of a desired shape are formed in the piezoelectric films 5a and 5b. Metal wiring 8a is formed to connect the electrodes 6a and 6c of each of the vibration plates 2 partitioned by the slits 1, and metal wiring 8b is formed to connect the electrodes 6b (Figure 1).

A coating film 10 made of a photosensitive resin containing a polyimide precursor is applied to the entire surface. The coating film 10 also fills the inside of the slit 1. After a heat treatment such as pre-baking, the coating film 10 is exposed to light in order to pattern it. A positive type coating film 10 is used here, and light of a desired wavelength is irradiated to the area (area to be removed) other than the area where the coating film is to be formed along the opening of the slit 1 to form the exposed area 10a. The area where the coating film is to be formed becomes the unexposed area 10b (Figure 2).

Then, the support substrate 3 is removed by dry etching, and then the insulating film 4 is removed by wet etching to form a cavity 7 that exposes the coating film 10 that fills the slit 1 (Figure 3). As shown in Figure 3, the piezoelectric film 5a and electrode 6a that form the vibration plate 2 are exposed in the cavity 7, and part of the unexposed region 10b of the coating film 10 is exposed in the slit 1.

Next, using the diaphragm 2 as an etching mask film, a portion of the unexposed region 10b of the coating film 10 is dry etched from the cavity 7 side to form an opening 11 (Figure 10). The cross-sectional shape of the opening 11 in the coating film 10 can be changed by appropriately adjusting the etching conditions. Specifically, the opening width of the opening 11 can be made equal to the opening width of the slit 1, or the opening width of the opening 11 can be made narrower than the opening width of the slit 1.

The exposed area 10a of the coating film 10 is then removed by development. The polyimide precursor is imidized by the necessary heat treatment, forming the coating film 10 made of polyimide. It is preferable to set this heat treatment in a temperature range that does not cause the opening 11 of the coating film 10 to expand significantly.

In this embodiment, as in the first embodiment, a piezoelectric element having the structure shown in FIG. 5 can be completed. The positional relationship between the slit 1 and the coating film 10 can also be arranged as shown in FIG. 6.

Although the embodiment of the present invention has been described above, it goes without saying that the present invention is not limited to the above embodiment. For example, in the above embodiment, the coating film 10, 10A is formed of a photosensitive polyimide film, but as long as the opening 11 can be formed using the diaphragm 2 as an etching mask and the desired shape can be patterned on the diaphragm 2, a non-photosensitive film may be used, and an organic film or an inorganic film other than a polyimide film may be used. In addition, the coating film 10, 10A is formed with a narrow width along the extension direction of the slit 1, but by widening the area of the coating film 10, 10A and adjusting the position of the coating film, the deformation of the piezoelectric film can be controlled, and a piezoelectric element with excellent characteristics can be formed. In addition, the coating film 10 can be divided into multiple pieces and arranged. Furthermore, the present invention is not limited to a piezoelectric film with a multi-layer structure.

In the above embodiment, the coating films 10 and 10A are used as protective films, and the openings 11 are formed before patterning the coating films 10 and 10A. In embodiment 2 or 3, the coating films 10 and 10A can be exposed and developed, and then the cavities 7 and the openings 11 can be formed. Alternatively, after exposure to form the openings in embodiment 1, and after forming the openings 11 in embodiments 2 and 3, exposure and development to pattern the coating films 10 and 10A can be performed.

Furthermore, the process of forming the slit 1 may be performed by forming the cavity 7 and then removing a portion of the piezoelectric films 5a and 5b to form the slit 1. In this case, the coating films 10 and 10A are formed on the piezoelectric film 5b etc. that does not have a slit.

1: Slit, 2: Vibration plate, 3: Support substrate, 4: Insulating film, 5a, 5b: Piezoelectric film, 6a, 6b, 6c: Electrode, 7: Cavity, 8a, 8b: Wiring metal, 9: Thin film, 10, 10A: Coating film, 10a: Exposed area, 10b: Unexposed area, 11: Opening

Claims (3)

A method for manufacturing a piezoelectric element having a vibration plate separated by slits and a coating film laminated on the vibration plate along the slits, comprising the steps of:
preparing a support substrate on which a piezoelectric film constituting the diaphragm and a pair of electrodes arranged to sandwich the piezoelectric film are laminated;
forming the slits in the piezoelectric film;
forming the covering film on the piezoelectric film;
removing a portion of the support substrate to form a cavity and form the diaphragm;
and a step of using the vibration plate exposed in the cavity as a mask film and removing a portion of the coating film to form an opening continuous with the slit. 2. The method for producing a piezoelectric element according to claim 1,
forming a positive photosensitive film as the coating film;
A method for manufacturing a piezoelectric element, comprising the steps of: exposing the coating film using the vibration plate as an exposure mask; and developing the coating film to remove a portion of the coating film to form the opening. 2. The method for producing a piezoelectric element according to claim 1,
A method for manufacturing a piezoelectric element, comprising the steps of: dry-etching the coating film using the vibration plate as an etching mask film, thereby removing a portion of the coating film to form the opening portion.

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