JP4997439B2 - Piezoelectric element and method for manufacturing MEMS device - Google Patents

Description

  The present invention relates to a piezoelectric element and a method for manufacturing a MEMS device including the piezoelectric element.

  A piezoelectric element is an element based on a structure in which a piezoelectric body is sandwiched between two electrodes. The piezoelectric body deforms when a voltage is applied (reverse piezoelectric effect), and generates a voltage (positive piezoelectric effect) when deformed. Using this characteristic, it is used as an actuator or as various sensors.

Non-Patent Document 1 below describes a liquid viscosity / density sensor using a cantilever-type piezoelectric element. The piezoelectric cantilever described in this document is shown in FIG. The piezoelectric cantilever 101 has a structure in which a piezoelectric body 105 is sandwiched between a lower electrode 103 and upper electrodes 107 and 109. The upper electrode is separated into a U-shaped electrode 107 and a rectangular plate-shaped electrode 109. At the time of measurement, the tongue-shaped lower electrode 107 is immersed in the liquid, and an AC voltage is applied between the U-shaped electrode 107 and the lower electrode 109. Then, the piezoelectric body 105 is deformed by the inverse piezoelectric effect and vibrates at the same frequency as the applied voltage. The voltage of the piezoelectric body 105 generated by this vibration can be measured as a voltage fluctuation between the electrode 109 and the lower electrode 103. The amplitude of the voltage fluctuation to be measured becomes the largest when the applied voltage is the same as the resonance frequency determined by the piezoelectric cantilever 101 and the liquid to be measured. Is measuring.
JapaneseJournal of Applied Physics, vol.42, pp.6139-6142, (2003) Journal of Applied Physics, vol.89, pp.1497-1505, (2001) Applied PhysicsLetters, vol.75, pp.334-336, (1999)

  According to the above-mentioned Non-Patent Documents 1 and 2, the piezoelectric cantilever, the bridge, and the diaphragm can be provided with both driving and detecting functions by separating one electrode (upper electrode) sandwiching the piezoelectric element. Are listed. However, in this structure, since the piezoelectric thin film and the lower electrode are common, there is a problem in that noise of the drive signal enters the detection signal and lowers the S / N ratio of detection.

  As a result of intensive studies, the present inventor has found that the following piezoelectric element is effective in order to solve such a problem. In this piezoelectric element, a first structure in which a first piezoelectric body is sandwiched between a first upper electrode and a first lower electrode is disposed on an elastic base, and the first lower electrode is disposed on the elastic base. A second structure in which a second piezoelectric body is sandwiched between a second upper electrode and a second lower electrode, and the second lower electrode is connected to the elastic substrate side. And on the elastic substrate so that the first lower electrode and the second lower electrode are not in contact with each other, and the first piezoelectric body and the second piezoelectric body are not in contact with each other. It is characterized by providing in.

  According to the above-described piezoelectric element, it is possible to impart a drive and detection function to the first structure and the second structure. For example, when the first structure is used as an element for driving, the first piezoelectric body is vibrated at the same frequency as the AC voltage by applying an AC voltage to the first upper and lower electrodes. At this time, the vibration deforms the second piezoelectric body of the second structure through the elastic substrate. Since the deformation of the piezoelectric body is accompanied by generation of a voltage, the deformation of the second piezoelectric body can be measured as a voltage change between the second upper and lower electrodes. Therefore, the second structure can be used as a detection element.

  Furthermore, according to the above piezoelectric element, since the piezoelectric body and the lower electrode in the two structures are separated, the noise of the drive signal is less likely to be mixed into the detection signal, and the S / N ratio of the detection is improved. Can be made.

  Since the frequency characteristics of the piezoelectric element change depending on the environment in which the piezoelectric element is placed, the piezoelectric element can be used as any sensor that detects a change in the state of a physical quantity by detecting a change in frequency. It can be used as a momentum sensor such as angular velocity, a force sensor, a liquid viscosity sensor, a gas concentration sensor, an odor sensor, and a mass sensor.

  The piezoelectric element can have various shapes, such as a cantilever, a bridge, and a diaphragm.

  In one embodiment, in the piezoelectric element, the first structure has two arms that are parallel to each other, and the second structure is disposed between the two arms. The structure can be as follows. With such a structure, since the first structure has a shape like a tuning fork, the amplitude of the elastic substrate at the resonance frequency can be increased. For this reason, it becomes possible to measure the frequency characteristics of the environment more sensitively.

The piezoelectric element according to the present invention can be manufactured by a semiconductor microfabrication technique using a photomask in which the upper electrode, the piezoelectric body, and the lower electrode are separated. In this case, if the upper electrode and the piezoelectric body are continuously etched with the same etching mask, the following problem may occur.
(1) When etching the piezoelectric layer, particularly wet etching, the piezoelectric layer is etched more than necessary, and as a result, the upper electrode hangs down to the lower electrode, causing an electrical short circuit.
(2) The side wall of the piezoelectric layer after etching becomes amorphous due to damage during etching, which deteriorates the piezoelectric characteristics of the piezoelectric body (see Non-Patent Document 3 listed above).
(3) Hydrogen enters from the side wall of the piezoelectric layer due to the plasma environment at the time of etching, and this reduces the piezoelectric body and degrades the piezoelectric characteristics.

Therefore, the present inventor has found that the following manufacturing method is effective in order to solve such problems as a result of intensive studies. This manufacturing method is a method of manufacturing a piezoelectric element in which a piezoelectric body is sandwiched between an upper electrode and a lower electrode,
A step of forming a lower conductive layer as a material for the lower electrode, a piezoelectric layer as a material for the piezoelectric material, and an upper conductive layer as a material for the upper electrode on the substrate in this order;
Etching the upper conductive layer using a first etching mask having a first mask pattern defining a shape of the upper electrode;
Etching the piezoelectric layer using a second etching mask having a second mask pattern that defines the shape of the piezoelectric thin film, and forming the piezoelectric body,
The second mask pattern has a shape that can accommodate the entire area of the first mask pattern.

  By adopting the above manufacturing method, since the piezoelectric body is formed to be slightly larger than the upper electrode, there is no problem that the upper electrode hangs down to the lower electrode and causes an electrical short circuit. The problem that the body deteriorates during or after etching can be avoided. As a result, it is possible to improve the piezoelectric characteristics of the element and to further improve the manufacturing yield.

  According to the manufacturing method described above, one photomask is necessary. Therefore, this manufacturing method is a factor that increases the cost of the photomask. For those skilled in the art who are required to make cost reduction efforts, it is easy to increase the number of photomasks without knowing the above problems. I can't think of it.

  In the case of manufacturing a self-supporting structure such as a cantilever, a bridge, or a diaphragm including the piezoelectric element according to the present invention disclosed above, a self-supporting structure is formed by penetrating a substrate such as silicon from the back surface by dry etching in the final process. It is often done to form the body.

  At this time, it is necessary to prevent the wafer installation portion inside the dry etching apparatus from being damaged after the wafer to be etched passes through by bonding the wafer to be etched to the dummy wafer. In order to bond the wafers together, photoresist or wax is used as shown in FIG. 6A, or diffusion pump oil is used as shown in FIG. 6B.

  However, when a photoresist is used, it is not easy to completely bond the wafers, and the wafers are often peeled off. If dry etching is performed in this state, the wafer to be etched is not sufficiently dissipated. (The heat is dissipated through the path of etched wafer → photoresist → dummy wafer.) Then, the temperature of the etched wafer rises and the etching mask evaporates. It becomes impossible to etch into the desired shape. Even if the etching mask does not evaporate, the photoresist may be solidified by overheating, and even if ashing with acetone or oxygen plasma is performed after the etching is completed, the photoresist may not be removed.

  When wax is used, the problem of peeling as in the case of a photoresist does not occur, but it is often difficult to remove the wax after etching.

  When the diffusion pump oil is used as shown in FIG. 6B, the problem of peeling as in the case of the photoresist does not occur, and the heat dissipation is better than that of the photoresist. However, although heat dissipation is good, the diffusion pump oil is solidified by the heat during etching, and it is difficult to remove the oil after the etching is completed, and in some cases, the device does not peel off from the dummy wafer.

Therefore, the present inventor has found that the following manufacturing method is effective in order to solve such problems as a result of intensive studies. This manufacturing method is a method for manufacturing a MEMS device by causing a wafer having a predetermined structure formed on the front surface to penetrate by dry etching from the back surface,
A first step of coating the surface of the wafer to be etched with a photoresist;
A second step of applying diffusion pump oil to a dummy wafer used for installation in a dry etching apparatus;
A third step of bonding the dummy wafer to the wafer to be etched by bonding the surface coated with the photoresist and the surface coated with the diffusion pump oil;
A fourth step of dry etching the wafer to be etched by the dry etching apparatus;
It is characterized by having.

  By bonding the dummy wafers by such a method, the adhesion between the wafer to be etched and the dummy wafers is improved, so that the thermal conductivity is improved and the etching into a desired shape is facilitated. Furthermore, since the thermal conductivity is improved and the heat radiation is performed well, the photoresist can be easily removed by ashing with acetone and oxygen plasma after the etching is completed, and the photoresist is solidified by being overheated and removed. The situation where it becomes difficult can be prevented to a minimum. Alkyl naphthalene (having an alkyl group in the naphthalene ring) is known as a diffusion pump oil, and these can be used.

  In another embodiment, a photoresist may be coated on the dummy wafer, and a diffusion pump oil may be applied thereon. According to such an embodiment, the adhesion and thermal conductivity between the wafer to be etched and the dummy wafer can be further improved.

  This method can solve the problem of photoresist removal that follows in the manufacture of any MEMS device.

  Hereinafter, more specific embodiments of the present invention will be described with reference to the accompanying drawings. First, an example of an embodiment of a piezoelectric element according to the present invention will be described using the piezoelectric element 201 depicted in FIG.

  The piezoelectric element 201 has two piezoelectric element structures in which a piezoelectric body is sandwiched between two electrodes on the piezoelectric element 201 so that they do not directly contact each other. The first piezoelectric element structure includes a U-shaped (U-shaped) lower electrode 205, a piezoelectric thin film 207, and an upper electrode 209, respectively. The second piezoelectric element structure is disposed between the U-shaped arms of the first piezoelectric element structure, and includes a lower electrode 211, a piezoelectric thin film 213, and an upper electrode 215. The portions 205a and 211a where the lower electrodes 205 and 211 protrude from the piezoelectric thin films 207 and 209 are portions that are intentionally exposed in order to ensure conduction to the lower electrodes 205 and 211, respectively.

  The upper electrode 209 is slightly smaller than the piezoelectric thin film 207 on the surface where the upper electrode 209 contacts the piezoelectric thin film 207. Therefore, the upper electrode 209 and the piezoelectric thin film 207 have a step structure over the entire outer periphery of the upper electrode 209. Similarly, the upper electrode 215 is slightly smaller than the piezoelectric thin film 213 on the surface where the upper electrode 215 and the piezoelectric thin film 213 are in contact, and the upper electrode 215 and the piezoelectric thin film 213 are stepped over the entire outer periphery of the upper electrode 215. It has a structure. In both the first piezoelectric element structure and the second piezoelectric element structure, the protruding width of the piezoelectric thin film 213 is set to 0.5 μm or more. This step structure is a structure formed in order to prevent problems that may occur when the piezoelectric element 201 is manufactured. Details will be described later.

  Various materials can be used as the material for the elastic substrate. For example, silicon, silicon oxide film, silicon nitride film, silicon carbide, gallium arsenide and other compound semiconductors, alumina, aluminum, stainless steel, etc. can be used. Is possible. Various known materials such as lead zirconate titanate (PZT) can be used as the material of the piezoelectric thin film.

  When an AC voltage is applied between the U-shaped lower electrode 205 and the upper electrode 209, the piezoelectric thin film 207 is deformed by the inverse piezoelectric effect and vibrates at the same frequency as the AC voltage. This vibration is transmitted to the elastic substrate 203 and shakes the elastic substrate 203. Further, the vibration of the elastic substrate 203 is transmitted to the piezoelectric thin film 213 having the second piezoelectric element structure, and the piezoelectric thin film 213 is also vibrated. When the piezoelectric thin film 213 is deformed by this vibration, the piezoelectric thin film 213 generates a voltage due to a positive piezoelectric effect. This voltage fluctuation can be measured as a voltage fluctuation between the lower electrode 211 and the upper electrode 215. The amplitude of the voltage fluctuation detected from the second piezoelectric element structures 211 to 215 is such that the frequency of the alternating voltage applied to the first piezoelectric element structures 205 to 209 is that of the elastic substrate 203 (or the entire piezoelectric element 201). It becomes the largest when it matches the resonance frequency. Since the resonance frequency of the elastic substrate 203 (or the entire piezoelectric element 201) varies depending on the environment where the elastic substrate 203 (or the piezoelectric element 201) is located, the piezoelectric element 201 detects the change in the physical quantity by detecting the frequency change. It can be used as a sensor to detect. For example, the piezoelectric element 201 can be used as a momentum sensor such as acceleration and angular velocity, a force sensor, a liquid viscosity sensor, a gas concentration sensor, an odor sensor, a mass sensor, and the like. As described above, the piezoelectric element 201 is a single element having both functions of driving and detection.

  In the piezoelectric element 201, as illustrated in FIG. 2, the lower electrode 205 and the piezoelectric body 207 of the first piezoelectric element structure are not in contact with the lower electrode 211 and the piezoelectric body 213 of the second piezoelectric element structure. For this reason, in the piezoelectric element 201, the drive signal applied to the first piezoelectric element structure is less likely to be mixed into the detection signal obtained from the second piezoelectric element structure, enabling measurement with less noise. It has become.

  As illustrated in FIG. 2, when the first piezoelectric element structure has a U shape, the amplitude at the time of resonance can be increased in the same manner as a tuning fork, so that the resonance frequency can be detected more easily. Can do. However, one piezoelectric element structure provided in a piezoelectric element using the present invention does not necessarily have a U-shape. In FIG. 2, the first piezoelectric element structure and the second piezoelectric element structure are formed on the same surface of the elastic substrate. However, depending on the embodiment, each of the first piezoelectric element structure and the second piezoelectric element structure may be formed on different surfaces. It may be.

  Next, a method for manufacturing the piezoelectric element 201 will be described. The piezoelectric element 201 is generally manufactured by a process similar to a normal semiconductor manufacturing process, but a part thereof is manufactured by using a specific manufacturing method provided by one aspect of the present invention. Hereinafter, a description of a manufacturing process similar to a normal semiconductor manufacturing process will be briefly described, and only a manufacturing process unique to the present invention will be described in detail.

  The manufacturing process of the piezoelectric element 201 is roughly divided into four stages as shown in the flowchart of FIG. In the first step 301, on the elastic substrate 203, the lower conductive layer that is the material of the lower electrodes 205 and 211, the piezoelectric layer that is the material of the piezoelectric thin films 207 and 213, and the material of the upper electrodes 209 and 215. The upper conductive layer is formed in this order. In the next step 302, the upper electrode layer is formed by resist patterning by lithography and etching. Subsequently, in step 303, the piezoelectric layer is formed, and in step 304, the lower electrode and the substrate are formed by resist patterning and etching by lithography.

If the surface shape of the upper electrodes 209 and 215 and the surface shape of the piezoelectric thin films 207 and 213 are the same, they can be photoetched with the same photomask. However, this may cause the following problems.
(1) When etching the piezoelectric layer, particularly wet etching, the piezoelectric layer is etched more than necessary, and as a result, the upper electrode hangs down to the lower electrode, causing an electrical short circuit.
(2) The side wall of the piezoelectric layer after etching becomes amorphous due to damage during etching, which degrades the piezoelectric properties of the piezoelectric body (Applied Physics Letters, vol.75, pp.334- 336, (1999)).
(3) Hydrogen enters from the side wall of the piezoelectric layer due to the plasma environment at the time of etching, and this reduces the piezoelectric body and degrades the piezoelectric characteristics.

  Therefore, when manufacturing the piezoelectric element 201 according to the present invention, as shown in FIG. 4, the mask pattern of the photomask that defines the shape of the piezoelectric thin film is changed from the mask pattern of the photomask that defines the shape of the upper electrode. Use a bigger one. That is, the mask pattern for the piezoelectric body has a shape that can accommodate the entire area of the mask pattern for the upper electrode. (See FIG. 4C.) As a result, the piezoelectric thin films 207 and 213 have a shape slightly larger than the upper electrodes 209 and 215, and the piezoelectric thin films 207 and 213, the upper electrodes 209 and 215, and As shown in FIG. 2, a step-like structure is exhibited. The width of the step portion of the piezoelectric element is preferably 0.5 μm or more. By adopting such a manufacturing method, it is possible to prevent the above-described problems from occurring, and to improve the yield and the piezoelectric characteristics of the element in manufacturing the piezoelectric element.

  5a and 5b show more specific embodiments of the piezoelectric element according to the present invention. In this embodiment, the piezoelectric element according to the present invention is implemented as a cantilever. The cantilever 501 has a structure in which two piezoelectric element structures in which a piezoelectric body is sandwiched between two electrodes are formed on an SOI substrate so that they do not directly contact each other. The SOI substrate includes a silicon substrate 503, a buried silicon oxide film 505, and a surface silicon layer 507. The surface silicon layer 507 plays a role of the elastic substrate 203 in the piezoelectric element 201 together with the silicon oxide film 509 formed thereon. . The structure corresponding to the first piezoelectric element structure of the piezoelectric element 201 is a lower electrode 511, a piezoelectric thin film 513, and an upper electrode 515, which are connected to the lower electrode 205, the piezoelectric thin film 207, and the upper electrode 209 of the piezoelectric element 201. Each corresponds. Similarly, the structure corresponding to the second piezoelectric element structure of the piezoelectric element 201 is the lower electrode 519, the piezoelectric thin film 521, and the upper electrode 523, which are the lower electrode 211, the piezoelectric thin film 213, and the upper part of the piezoelectric element 201. Each corresponds to the electrode 215. Since the first piezoelectric element structures 511 to 515 and the second piezoelectric element structures 519 to 523 are not in direct contact with each other, the drive signal applied to the first piezoelectric element structure is the same as that of the piezoelectric element 201. Therefore, it is difficult to mix in the detection signal obtained from the piezoelectric element structure, and measurement with less noise can be performed. Further, like the piezoelectric element 201, the upper electrode is formed to be slightly smaller than the piezoelectric thin film, and the upper electrode and the lower electrode are in contact with each other at the time of manufacture, or the piezoelectric characteristics of the piezoelectric body are degraded. It is avoiding. The silicon oxide film 509 is provided to prevent mutual diffusion between the electrode and silicon when heat treatment is performed in the formation of the piezoelectric body.

  The cantilever 501 is a final component that is formed by penetrating the silicon substrate 503 from the silicon substrate 503 side by dry etching in the final step after the piezoelectric element portion is formed by the manufacturing process as shown in the flowchart of FIG. To be completed.

  At this time, it is necessary to prevent the wafer installation portion inside the dry etching apparatus from being damaged after the wafer to be etched is pasted by attaching the wear wafer to the surface on which the piezoelectric element is formed.

  In this process, when the cantilever 501 is manufactured, it is preferable to bond the dummy wafer by the following method according to one aspect of the present invention. First, a photoresist is applied to the surface opposite to the surface to be etched of the wafer (the surface on which the piezoelectric element is formed), and the coating is cured by baking at an appropriate temperature. Next, diffusion pump oil such as alkylnaphthalene is applied to the dummy wafer by spin coating or the like. Then, a dummy wafer is bonded to the wafer to be etched by bonding the surface coated with the photoresist and the surface coated with the diffusion pump oil. This is shown in FIG.

  By bonding the dummy wafers by such a method, the adhesion between the wafer to be etched and the dummy wafers is improved, so that the thermal conductivity is improved and the etching into a desired shape is facilitated. Furthermore, since the thermal conductivity is improved and the heat radiation is performed well, the photoresist can be easily removed by ashing with acetone and oxygen plasma after the etching is completed, and the photoresist is solidified by being overheated and removed. The situation where it becomes difficult can be prevented to a minimum.

  In another embodiment, as shown in FIG. 7B, after a photoresist is applied to the dummy wafer, a diffusion pump oil may be applied thereon. According to such an embodiment, the adhesion and thermal conductivity between the wafer to be etched and the dummy wafer can be further improved.

  Although the present invention has been described using preferred examples so that the present invention can be better understood, it is obvious that the embodiments of the present invention are not limited to these examples. It goes without saying that the present invention can take various embodiments without departing from the scope thereof.

It is a figure for demonstrating the piezoelectric cantilever by a prior art. It is a figure for demonstrating an example of the embodiment of the piezoelectric element provided by this invention. It is a figure for demonstrating an example of the manufacturing method of the piezoelectric element provided by this invention. It is an example of the photomask used with said manufacturing method. It is the figure on which the more concrete Example of the piezoelectric element by this invention was drawn. It is the figure on which the more concrete Example of the piezoelectric element by this invention was drawn. It is a figure for demonstrating the MEMS device manufacturing method by a prior art. It is a figure for demonstrating an example of the manufacturing method of the MEMS device provided by this invention.

Explanation of symbols

201 Piezoelectric element 203 Elastic substrate 205 Lower electrode 207 Piezoelectric thin film 209 having the first piezoelectric element structure Upper electrode 211 having the first piezoelectric element structure Lower electrode 213 having the second piezoelectric element structure Piezoelectric body having the second piezoelectric element structure Thin film 215 Upper electrode 501 of second piezoelectric element structure Cantilever 503 Silicon substrate 505 Embedded silicon oxide film 507 Surface silicon layer 509 Silicon oxide film

Claims (6)

A first structure in which a first piezoelectric body is sandwiched between a first upper electrode and a first lower electrode is placed on an elastic substrate so that the first lower electrode is on the elastic substrate side. A second structure in which a second piezoelectric body is sandwiched between a second upper electrode and a second lower electrode, such that the second lower electrode is on the elastic substrate side, and A piezoelectric element provided on the elastic base so that the first lower electrode and the second lower electrode do not contact each other, and the first piezoelectric body and the second piezoelectric body do not contact each other. Because
The piezoelectric element, wherein the first structure has two arm portions that are parallel to each other, and the second structure is disposed between the two arm portions.   In the surface of the first structure and / or the second structure where the piezoelectric body faces the upper electrode, the piezoelectric body has a larger surface area than the upper electrode, and the upper electrode and the piezoelectric body 2. The piezoelectric element according to claim 1, wherein the body exhibits a step structure over the entire outer periphery of the upper electrode. The piezoelectric element according to claim 2 , wherein a width of the step portion of the piezoelectric element in the step structure is 0.5 μm or more. A method of manufacturing the piezoelectric element according to claim 1 ,
A lower conductive layer serving as a material for the first lower electrode and a second lower electrode on the elastic substrate , a piezoelectric layer serving as a material for the first piezoelectric body and the second piezoelectric body , and the first upper portion. Forming an upper conductive layer as a material of the electrode and the second upper electrode in this order;
Etching the upper conductive layer using a first etching mask having a first mask pattern defining a shape of the first upper electrode and the second upper electrode ;
The piezoelectric layer is etched by using a second etching mask having a second mask pattern that defines the shape of the first piezoelectric body and the second piezoelectric body, and the first piezoelectric body and the second piezoelectric body are etched . the second mask pattern comprises the steps, the forming the piezoelectric body, the whole area of the first mask pattern has a shape that can accommodated in its internal,
The manufacturing method characterized by the above-mentioned. A method of manufacturing a MEMS device by causing a wafer on which the piezoelectric element according to claim 1 is formed to penetrate from a back surface by dry etching,
A first step of coating the surface of the wafer to be etched with a photoresist;
A second step of applying diffusion pump oil to a dummy wafer used for installation in a dry etching apparatus;
A third step of bonding the dummy wafer to the wafer to be etched by bonding the surface coated with the photoresist and the surface coated with the diffusion pump oil;
A fourth step of performing dry etching of the wafer to be etched by the dry etching apparatus;
The manufacturing method characterized by having.   6. The manufacturing method according to claim 5, wherein, in the second step, after the dummy wafer is coated with a photoresist, the diffusion pump oil is applied to the coated surface.