JP4742764B2 - Piezoelectric device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a piezoelectric device using longitudinal vibration and a method for manufacturing the same, and more particularly to an improvement for further improving the crystal orientation of a piezoelectric thin film provided in the piezoelectric device.

An interesting piezoelectric device for this invention is a bulk acoustic wave (BAW) element. The BAW element includes a substrate made of lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), or the like, and a piezoelectric thin film made of zinc oxide (ZnO), aluminum nitride (AlN), or the like formed thereon. This is an element utilizing thickness longitudinal vibration having a piezoelectric thin film as a main vibration part.

  A resonator using a BAW element generally includes a substrate, a piezoelectric thin film supported by the substrate, and an upper electrode and a lower electrode formed so as to face each other with the piezoelectric thin film sandwiched in the thickness direction ( For example, see Patent Document 1).

  For example, a bandpass filter is configured by combining a plurality of resonators using the BAW element as described above. With the recent expansion of the wireless communication equipment market, there is a demand for a wider bandwidth for the filter. In order to increase the bandwidth of a filter using a BAW element, it is necessary to increase the electromechanical coupling coefficient of the resonator. In order to increase the electromechanical coupling coefficient, it is necessary to improve the crystal orientation of the piezoelectric thin film.

  For example, in a resonator using a BAW element described in Patent Document 2, a c-axis alignment film in which crystals are aligned in the thickness direction is used as a piezoelectric thin film. Thus, in the piezoelectric thin film, by orienting the crystal in the same c-axis direction as the vibration direction, the propagation loss of vibration in the piezoelectric thin film can be reduced, and as a result, the electromechanical coupling coefficient is increased. be able to.

However, in the resonator using the BAW element described in Patent Document 2, when a typical AlN or ZnO is used as a material for the piezoelectric thin film, the piezoelectric thin film is formed when the piezoelectric thin film is formed on a normal polycrystalline electrode. While the c-axis, which is the polarization axis, grows in the normal direction of the substrate, it becomes a random orientation in the c-plane, resulting in a uniaxial alignment film. As a result, there are many grain boundaries in the c-plane (in the direction of the main surface of the piezoelectric thin film), the bonding force between grains is weakened, and bulk elastic waves are scattered. There is a problem that the propagation loss in the direction (the main surface direction of the piezoelectric thin film) cannot be reduced.
JP-A-60-217711 JP 2003-158309 A

  Therefore, an object of the present invention is to provide a piezoelectric device and a method for manufacturing the same that can solve the above-described problems.

  In short, in the present invention, the grain boundary is reduced by the triaxial orientation of the piezoelectric thin film, thereby suppressing the scattering of bulk elastic waves in the piezoelectric thin film, resulting in improved piezoelectric characteristics. An improved piezoelectric device is provided.

More specifically, the present invention includes a piezoelectric substrate including a substrate made of Si, a piezoelectric thin film supported by the substrate, and an upper electrode and a lower electrode formed so as to face each other with the piezoelectric thin film sandwiched in the thickness direction. First directed to the device. In such a piezoelectric device, in the present invention, a buffer layer for relaxing crystal lattice irregularities is formed between the substrate and the lower electrode, and this buffer layer has a laminated structure, and an epitaxial Cu film is formed from the substrate side . It has a Ti film and a conductor film , and at least the lower electrode and the piezoelectric thin film are formed of an epitaxial film.

  In the first embodiment of the piezoelectric device according to the present invention, an air gap is formed between the lower electrode and the buffer layer, thereby forming an air gap type piezoelectric device.

Above conductive film is, Al, Pt, Au, Pd , Ag, Rh, γ-Mn, Ir, α-Sn, Cu, preferably includes one selected from gamma-Fe and Ni.

  In addition, it is preferable that the lower electrode and the buffer layer are in contact with each other in the peripheral region of the gap and thereby electrically connected to each other.

  In the first embodiment, the lower electrode includes one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. Is preferred.

  In the second embodiment of the piezoelectric device according to the present invention, the lower electrode is formed in contact with the buffer layer without any gap.

  In the second embodiment, the lower electrode preferably includes a Ti film formed on the buffer layer and a conductor film formed on the Ti film. In this case, the conductor film preferably contains one kind selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni.

  In the second embodiment, a cavity that exposes at least a part of the surface of the buffer layer on the substrate side is formed in the substrate, whereby a diaphragm type piezoelectric device may be configured.

  In the piezoelectric device according to the present invention, the piezoelectric thin film is preferably made of AlN or ZnO.

  Further, it is preferable that the upper electrode is composed of an epitaxial film.

  The present invention is also directed to a method for manufacturing a piezoelectric device having a structure as described above.

  In a first aspect, a method for manufacturing a piezoelectric device according to the present invention is directed to the above-described method for manufacturing an air gap type piezoelectric device.

The manufacturing method according to the first aspect includes a step of preparing a substrate made of Si, a step of forming a buffer layer for alleviating crystal lattice irregularities on a portion of the substrate, and at least a portion of the buffer layer. Forming a sacrificial layer that is intended to be selectively removed later on a portion of the substrate to cover, and at least sacrificing so that at least a portion is located above the buffer layer Forming a lower electrode on the layer, forming a piezoelectric thin film on the substrate so as to cover the sacrificial layer, and forming the upper electrode on the piezoelectric thin film so as to face the lower electrode through the piezoelectric thin film And removing the sacrificial layer, thereby forming a gap between the lower electrode and the buffer layer. The buffer layer includes an epitaxial Cu film , a Ti film, and a conductor film . The buffer layer forming step includes a step of forming an epitaxial Cu film on the substrate, and a step of forming a Ti film on the epitaxial Cu film. Forming a conductor film on the Ti film , wherein the sacrificial layer forming step, the lower electrode forming step, and the piezoelectric thin film forming step are performed so that the sacrificial layer, the lower electrode, and the piezoelectric thin film are epitaxially grown, respectively. It is characterized by being implemented.

  In the second aspect, the method for manufacturing a piezoelectric device according to the present invention is directed to the above-described method for manufacturing a diaphragm type piezoelectric device.

The manufacturing method according to the second aspect includes a step of preparing a substrate made of Si, a step of forming a buffer layer for alleviating crystal lattice irregularities on a portion of the substrate, and at least a portion of the buffer layer. A step of forming a lower electrode on the substrate so as to cover, a step of forming a piezoelectric thin film on the substrate so as to cover the lower electrode at a position above the buffer layer, and a lower electrode facing the lower electrode via the piezoelectric thin film As described above, the method includes a step of forming an upper electrode on the piezoelectric thin film, and a step of forming a cavity in a portion of the substrate where the buffer layer is formed. The buffer layer has an epitaxial Cu film, and the buffer layer forming step is performed to form an epitaxial Cu film on the substrate, and the lower electrode forming step and the piezoelectric thin film forming step are respectively The lower electrode forming step is performed so as to epitaxially grow a piezoelectric thin film, and includes a step of forming a Ti film on the buffer layer and a step of forming a conductor film on the Ti film .

  In the method for manufacturing a piezoelectric device according to the present invention, a substrate having a hydrogen-terminated Si (100) surface or Si (111) surface is advantageously used as the substrate.

  When the substrate has a hydrogen-terminated Si (100) surface, in the buffer layer forming step, an epitaxial Cu film having a crystal orientation of Cu (100) is formed on the Si (100) surface. On the other hand, when the substrate has a Si (111) surface terminated with hydrogen, in the buffer layer forming step, an epitaxial Cu film having a crystal orientation of Cu (111) is formed on the Si (111) surface. .

  In the method for manufacturing a piezoelectric device according to the present invention, it is preferable that the upper electrode forming step is performed so that the upper electrode is epitaxially grown.

For example, in the field of surface acoustic wave devices, when an electrode made of a metal having a face-centered cubic structure such as Al is formed on a substrate made of an oxide single crystal such as LiTaO ₃ or LiNbO ₃ , this electrode material is epitaxially grown. It is known that it is effective to use a Ti film as an intermediate layer. In contrast, in the present invention, a polycrystalline substrate made of Si is used. Unlike the oxide single crystals such as LiTaO ₃ and LiNbO ₃ described above, Si has poor lattice matching with Ti. Furthermore, since amorphous SiO ₂ generated on the surface by reaction with O ₂ in the atmosphere inhibits crystal growth, interface control becomes important.

  In this invention, it is possible to epitaxially grow an electrode having a face-centered cubic structure such as Al by controlling the interface of Cu having good lattice matching with Si as a buffer layer. Further, it is possible to epitaxially grow a piezoelectric thin film using this electrode as a base. it can.

  More specifically, according to the present invention, the buffer layer is formed between the substrate made of Si and the lower electrode, and the surface of the buffer layer on the substrate side is provided by the epitaxial Cu film. The piezoelectric thin film can be composed of an epitaxial film.

  When comparing the lattice constants of Si and Cu, Si is 0.543 nm and Cu is 0.3614 nm, which is not matched as it is, but 0.543 × 2 = 1.086 and 0.3614. Since x3 = 1.084 is matched with a mismatch of 0.18%, it is possible to perform epitaxial growth in which Si is 2 cycles and Cu is 3 cycles. When an epitaxial Cu film is formed on the Si (100) surface, an epitaxial Cu film having a crystal orientation of Cu (100) is formed, and an epitaxial Cu film is formed on the Si (111) surface. In some cases, an epitaxial Cu film having a crystal orientation of Cu (111) is formed. The lower electrode epitaxially grown by controlling the interface using the epitaxial Cu film as described above as a buffer layer is highly oriented. As a result, the grain boundary which is a high resistance component is reduced, and the wiring resistance can be reduced. The resonance characteristics are improved.

  In addition, since the piezoelectric thin film can be epitaxially grown by taking over the crystal orientation of the lower electrode, the crystallinity of the piezoelectric thin film is improved. More specifically, when the piezoelectric thin film is made of AlN or ZnO, an epitaxial triaxially oriented film in which the c-axis is oriented in the normal direction of the substrate and the orientation in the c-plane is aligned. Compared with a piezoelectric device including a piezoelectric thin film that is a conventional uniaxially oriented film, a piezoelectric device including a piezoelectric thin film that is such an epitaxial triaxially oriented film has a dramatically reduced grain boundary. As a result, the piezoelectric characteristics can be remarkably improved.

  In the present invention, the material of the conductor film or the lower electrode which becomes a part of the buffer layer is selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe and Ni. When one selected is used, these have a face-centered cubic structure, so that an epitaxial structure can be easily obtained in the conductor film or the lower electrode.

Further, according to the present invention, the buffer layer is, as described above, includes a conductive film, between Runode, the epitaxial Cu film and the conductive film is a Ti film is formed between the conductive film and the epitaxial Cu film It is possible to increase the adhesive strength at. Moreover, since Cu and Ti have high activation energy for mutual diffusion, the Ti film can function as a diffusion barrier layer. The same applies to the case where the lower electrode is formed so as to be in contact with the buffer layer and the Ti film is formed between the conductor film provided in the lower electrode and the buffer layer.

  When the piezoelectric device according to the present invention is an air gap type piezoelectric device in which a gap is formed between the lower electrode and the buffer layer, the surface of the buffer layer opposite to the substrate side has conductivity. In addition, when a structure in which the lower electrode and the buffer layer are in contact with each other in the peripheral region of the gap and are electrically connected to each other is employed, stray capacitance due to the presence of the buffer layer should not be generated. It is possible to prevent deterioration of resonance characteristics due to stray capacitance.

  In the piezoelectric device according to the present invention, when the upper electrode is also composed of an epitaxial film, the upper electrode is also highly oriented, grain boundaries that are high resistance components are reduced, and wiring resistance can be reduced. Resonance characteristics can be improved. Further, the film configuration of the vibration part is approximately symmetrical about the piezoelectric thin film, and the material constants of the upper electrode and the lower electrode can be made equal, so that the resonance characteristics can be further improved.

  FIG. 1 is a sectional view showing a piezoelectric device 1 according to a first embodiment of the present invention. This piezoelectric device 1 utilizes thickness longitudinal vibration and constitutes a resonator. The piezoelectric device 1 is an air gap type.

  The piezoelectric device 1 includes a substrate 2 made of Si. As the substrate 2, for example, a Si (100) substrate whose surface crystal plane is a Si (100) plane is used. Instead of this, a Si (111) substrate having a Si (111) plane may be used.

  The piezoelectric device 1 also includes a piezoelectric thin film 3 supported by the substrate 2 and an upper electrode 4 and a lower electrode 5 formed so as to face each other with the piezoelectric thin film 3 sandwiched in the thickness direction. The piezoelectric thin film 3 is made of a piezoelectric material such as AlN or ZnO. The upper electrode 4 and the lower electrode 5 include, for example, one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. The These materials have a face-centered cubic structure and facilitate the construction of the upper electrode 4 and the lower electrode 5 from an epitaxial film.

  Each of the upper electrode 4 and the lower electrode 5 may have a multilayer structure. For example, the upper electrode 4 may employ a multilayer structure including a Ti film formed on the piezoelectric thin film 3, a Pt film formed thereon, and an Au film formed thereon. On the other hand, for the lower electrode 5, for example, a multilayer structure including a Ti film formed on the substrate 2, an Au film formed thereon, and a Pt film formed thereon may be employed.

  The piezoelectric device 1 further includes a buffer layer 6. The buffer layer 6 is for reducing crystal lattice irregularities that may be encountered when a manufacturing method described later is performed, and is formed on the substrate 2 between the substrate 2 and the lower electrode 5. Since the piezoelectric device 1 is an air gap type as described above, a gap 7 is formed between the lower electrode 5 and the buffer layer 6.

  In this embodiment, the buffer layer 6 has a cross-sectional structure as shown in FIG. That is, the buffer layer 6 has a laminated structure including an epitaxial Cu film 8 formed on the substrate 2, an epitaxial Ti film 9 formed thereon, and an epitaxial Al film 10 formed thereon, The surface of the buffer layer 6 on the substrate 2 side is provided by the epitaxial Cu film 8.

In the buffer layer 6 described above, the Ti film 9 acts to improve the adhesion of the Al film 10 to the Cu film 8. The Al film 10 disposed on the opposite side of the buffer layer 6 from the substrate 2 side may be replaced with a conductor film made of a conductor other than Al. As the conductors constituting this conductor film, as in the case of the upper electrode 4 and the lower electrode 5 described above, in addition to Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, A face-centered cubic structure such as γ-Fe or Ni can be advantageously used.

  In the piezoelectric device 1 as described above, at least the lower electrode 5 and the piezoelectric thin film 3 are formed of an epitaxial film, and preferably the upper electrode 4 is also formed of an epitaxial film.

  Next, a method for manufacturing the piezoelectric device 1 will be described with reference to FIG. FIG. 3 is a view corresponding to FIG. 1 and showing a structure obtained in the middle of manufacturing the piezoelectric device 1. In FIG. 3, elements corresponding to those shown in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.

In order to manufacture the piezoelectric device 1, first, a substrate 2 made of Si is prepared. As the substrate 2, a Si (100) substrate whose surface crystal plane is the Si (100) plane or a Si (111) substrate whose surface crystal plane is the Si (111) plane is prepared. Then, the substrate 2 is immersed in, for example, a buffered hydrofluoric acid (BHF) solution, the surface SiO ₂ oxide film is removed, and dangling bonds on the outermost surface are terminated with hydrogen atoms.

  Next, the buffer layer 6 is formed on a part of the substrate 2, more specifically, in a region larger than a region where the vibration part is formed. The buffer layer 6 is formed by, for example, a method such as CVD, sputtering, or vacuum deposition, and is patterned by a photolithography technique. The formation region of the buffer layer 6 may be the same size as the vibration part region. Further, it may be on the entire surface of the substrate 2.

When the buffer layer 6 having a laminated structure as shown in FIG. 2 is formed, for example, the epitaxial Cu film 8 has a thickness of 0.2 μm, and the epitaxial Ti film 9 has a thickness of 0.1 μm. The epitaxial Al film 10 has a thickness of 0.1 μm. When such a buffer layer 6 is formed by the lift-off method, a photoresist is applied, and then general patterning steps such as heat treatment, exposure, development, and descum are performed, and then the buffer layer 6 of the substrate 2 is formed. The SiO ₂ oxide film on the surface of the region where the film is to be formed is removed with a BHF solution or the like. Thereafter, lift-off is performed with a stripping solution such as an organic solvent. After lift-off, the organic matter adhering to the surface of the buffer layer 6 may be removed by carbonizing it with oxygen plasma or the like.

  In forming the buffer layer 6 as described above, as shown in FIG. 2, when the substrate 2 has a hydrogen-terminated Si (100) surface, the crystal orientation is Cu on this Si (100) surface. An (100) epitaxial Cu film 8 is formed. Then, taking over this crystal orientation, an epitaxial Ti film 9 having a crystal orientation of Ti (001) is formed on the epitaxial Cu film 8, and an epitaxial Al film 10 having a crystal orientation of Al (111) is formed thereon. Is done.

  Although not shown, when the substrate 2 has a hydrogen-terminated Si (111) surface, an epitaxial Cu film 8 having a crystal orientation of Cu (111) is formed on the Si (111) surface.

  Next, a sacrificial layer 11 is formed on a part of the substrate 2 so as to cover the buffer layer 6 and is scheduled to be selectively removed later for forming the gap 7. The sacrificial layer 11 is then given a shape that should provide the voids 7 using a method such as a photoetching method.

  The material of the sacrificial layer 11 is preferably one that is epitaxially grown on the buffer layer 6 and can be easily removed in a subsequent removal step. As a material for such a sacrificial layer 11, for example, zinc oxide (ZnO) that is easily chemically dissolved can be advantageously used.

When ZnO is used as the material of the sacrificial layer 11, the sacrificial layer 11 made of ZnO is formed by sputtering. After vacuuming to a pressure on the order of 10 ⁻⁴ Pa, an Ar / O ₂ mixed gas is introduced, and a sacrificial layer 11 made of, for example, ZnO having a thickness of about μm is formed by reactive sputtering. It is.

  The thickness of the sacrificial layer 11 needs to be selected so that a gap 7 having a thickness that does not come into contact with the substrate 2 even when the piezoelectric thin film 3 as the vibrating portion and the upper and lower electrodes 4 and 5 are bent is formed. It is preferable that it is 50 nm or more and 10 micrometers or less from the ease of. Further, the minimum value of the distance between the end of the sacrificial layer 11 and the vibration part is preferably 50 times or less the thickness of the vibration part.

  As described above, it has been found that the sacrificial layer 11 formed of ZnO on the buffer layer 6 is a triaxially oriented epitaxial film even when the temperature of the substrate 2 is changed from room temperature to about 200 ° C.

  Next, the lower electrode 5 is formed on at least the sacrificial layer 11 so that at least a part thereof is positioned above the buffer layer 6. In this embodiment, the lower electrode 5 extends from the sacrificial layer 11 to the substrate 2. The lower electrode 5 must be a thin film epitaxially grown on the sacrificial layer 11 made of ZnO on the buffer layer 6 and is preferably an electrode that can provide good resonance characteristics. The material suitable for constituting the lower electrode 5 is as described above. The lower electrode 5 is formed through a film forming process such as CVD, sputtering, or electron beam evaporation and a patterning process using a photolithography technique.

  For example, when the piezoelectric device 1 to be obtained is a 2 GHz band BAW resonator, if the lower electrode 5 is composed of a Ti film, an Au film thereon, and a Pt film thereon, the thickness of the lower electrode 5 is It is preferably about 0.1 to 0.3 μm.

  Next, the piezoelectric thin film 3 is formed on the substrate 2 so as to cover the sacrificial layer 11. The piezoelectric thin film 3 is a film that is epitaxially grown by successively taking over the crystal orientations of the buffer layer 6, the sacrificial layer 11, and the lower electrode 5 at a position above the buffer layer 6. As the material constituting the piezoelectric thin film 3, it is preferable to use AlN or ZnO as described above.

For example, when AlN is used as the material of the piezoelectric thin film 3, if the piezoelectric device 2 is a 2 GHz band BAW resonator, the thickness of the piezoelectric thin film 3 is about 1.6 μm. Such a piezoelectric thin film 3 is evacuated to a pressure on the order of 10 ⁻⁴ Pa, and then an Ar / N ₂ mixed gas is introduced to form an AlN film by reactive sputtering. After patterning a resistant resin, the AlN film is patterned by performing wet etching with an alkaline solution using the resin as a mask.

  Next, the upper electrode 4 is formed on the piezoelectric thin film 3 so as to face the lower electrode 5 through the piezoelectric thin film 3. The upper electrode 4 is formed through a film forming process using CVD, sputtering, electron beam evaporation, or the like and a patterning process using a photolithography technique. The material suitable for constituting the upper electrode 4 is as described above. The upper electrode 4 is not essential, but is preferably an epitaxial film grown by taking over the crystal orientation of the piezoelectric thin film 3.

  As an example, when the piezoelectric device 1 is a 2 GHz band BAW resonator, when the upper electrode 4 is composed of a Ti film, a Pt film thereon, and an Au film thereon, the thickness of the upper electrode 4 is about 0. 0.1 to 0.3 μm is preferable.

  Next, the sacrificial layer 11 is removed, whereby a gap 7 is formed between the lower electrode 5 and the buffer layer 6 as shown in FIG. For example, etching is applied to remove the sacrificial layer 11. When the sacrificial layer 11 is made of ZnO, the sacrificial layer 11 is etched away using an acidic aqueous solution such as a phosphoric acid / acetic acid mixed aqueous solution as an etchant.

  In addition, when the material etched by an acid like Al etc. is used for the upper and lower electrodes 4 and 5, it is necessary to protect these electrodes 4 and 5 by patterning a photoresist etc. by photolithography. In this case, it is necessary to remove the photoresist and the like after removing the sacrificial layer 11. In removing the photoresist, wet removal using an organic solvent, which is generally performed, dry removal using oxygen plasma, or a combination thereof can be applied. On the other hand, when the upper and lower electrodes 4 and 5 are made of a material that is not corroded by acid, such as Au, Pt, Pd, etc., the above-described protection by the photoresist and the removal step thereof are not necessary.

  As described above, the piezoelectric device 1 shown in FIG. 1 is obtained.

  FIG. 4 is a sectional view showing a piezoelectric device 1a according to the second embodiment of the present invention, and corresponds to FIG. In FIG. 4, elements corresponding to the elements shown in FIG.

The piezoelectric device 1a shown in FIG. 4 differs from the piezoelectric device 1 shown in FIG. That is, the buffer layer 6 is formed so as to be in contact with the lower electrode 5 in the peripheral region of the gap 7. As a result, a conductor film such as an Al film 10 is disposed on the opposite side of the buffer layer 6 to the substrate 2 side, as shown in FIG. 2, and the surface opposite to the substrate 2 side has conductivity. Therefore, the lower electrode 5 and the buffer layer 6 are electrically connected to each other. When such a structure is employed, stray capacitance generated between the lower electrode 5 and the buffer layer 6 can be prevented from being generated, and deterioration of resonance characteristics due to stray capacitance can be prevented.

  FIG. 5 is a sectional view showing a piezoelectric device 21 according to a third embodiment of the present invention. Similarly to the piezoelectric devices 1 and 1a described above, the piezoelectric device 21 also uses thickness longitudinal vibration and constitutes a resonator. The piezoelectric device 21 is a diaphragm type.

  Unless otherwise specified, the material of each element included in the piezoelectric device 21 and each process performed for manufacturing are the same as those of the piezoelectric device 1 according to the first embodiment described above.

  The piezoelectric device 21 includes a substrate 22 made of Si, a piezoelectric thin film 23 supported by the substrate 22, and an upper electrode 24 and a lower electrode 25 formed so as to face each other with the piezoelectric thin film 23 sandwiched in the thickness direction. Yes.

The piezoelectric device 21 further includes a buffer layer 26. The buffer layer 26 is formed between the substrate 22 and the lower electrode 25. In this embodiment, the lower electrode 25 is formed in contact with the buffer layer 26 without a gap. Buffer layer 26 has the multilayer structure as shown in FIG. 2, at least, of the buffer layer 26, the surface of the substrate 22 side is given by the epitaxial Cu film.

  As described above, since the piezoelectric device 21 is a diaphragm type, the substrate 22 has a cavity 27 that exposes at least a part of the surface of the buffer layer 26 on the substrate 22 side.

  In the piezoelectric device 21 as described above, at least the lower electrode 25 and the piezoelectric thin film 23 are made of an epitaxial film, and preferably, the upper electrode 24 is also made of an epitaxial film.

  Although not shown in detail, the lower electrode 25 in the piezoelectric device 21 may have a laminated structure including a Ti film formed on the buffer layer 26 and a conductor film formed on the Ti film. In this case, the conductor film preferably contains one kind selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni.

  Next, a method for manufacturing the piezoelectric device 21 will be described with reference to FIG. FIG. 6 is a view corresponding to FIG. 5, showing a structure obtained in the middle of manufacturing the piezoelectric device 21. In FIG. 6, elements corresponding to the elements shown in FIG.

  In order to manufacture the piezoelectric device 21, first, a substrate 22 made of Si is prepared. Next, the buffer layer 26 is formed on a part of the substrate 22. Next, the lower electrode 25 is formed on the substrate 22 so as to cover at least a part of the buffer layer 26. Next, the piezoelectric thin film 23 is formed on the substrate 22 so as to cover the lower electrode 25 at a position above the buffer layer 26. Next, the upper electrode 24 is formed on the piezoelectric thin film 23 so as to face the lower electrode 25 through the piezoelectric thin film 23. Then, a cavity 27 is formed in the portion of the substrate 22 where the buffer layer 26 is formed, that is, the portion corresponding to the vibrating portion, for example, by reactive ion etching, whereby the diaphragm type piezoelectric device 21 is obtained.

  In the step of forming the cavity 27 described above, not only a part of the substrate 22 but also at least a part of the buffer layer 26 may be removed.

  Next, experimental examples carried out to confirm the effects of the present invention will be described.

(Experimental example 1)
A Si (100) substrate was prepared, which was immersed in a BHF solution, the surface SiO ₂ oxide film was removed, and dangling bonds on the outermost surface were terminated with hydrogen atoms.

Next, the processed substrate was set in an electron beam vapor deposition apparatus having a multi-source-compatible crucible, and evacuated by a high vacuum pump until the pressure reached 10 ⁻⁵ Pa level.

  Next, in the above state, a Cu film serving as a buffer layer is formed to a thickness of 200 nm, and then left for about 15 minutes to cool the crucible that has become high temperature by electron beam irradiation, and then the crucible is rotated. With the vacuum maintained, a Ti film as a base film for the lower electrode was formed to a thickness of 100 nm, and then an Al film that was a main part of the lower electrode was formed to a thickness of 200 nm. At this time, the substrate was rotated and revolved so that a uniform film thickness distribution was obtained. A shutter is provided above the crucible, and the thickness of each of the Cu film, Ti film and Al film was controlled by opening / closing the shutter and adjusting the film formation time. In addition, for any of the Cu film, Ti film, and Al film, room temperature was applied as the film formation temperature, and no heating was performed.

  The crystallinity of the buffer layer thus prepared was analyzed by X-ray diffraction. As a result, in the XRD pole figure measurement with Cu (111) incidence, as shown in FIG. 7, a four-fold symmetric spot with the substrate normal direction as the symmetric center was detected, and it was found that epitaxial growth occurred. Further, in the pole figure measurement with Ti (102) incidence, 12-fold symmetric spots are detected as shown in FIG. 8, and in the pole figure measurement with Al (111) incidence, as shown in FIG. was detected. From these facts, epitaxial growth was also confirmed for the Ti film and the Al film.

Next, as described above, the substrate on which the epitaxial Cu film as the buffer layer and the epitaxial Ti film and the epitaxial Al film as the lower electrode are formed is set in the sputtering chamber until the pressure reaches 10 ⁻⁵ Pa level. After evacuation, an Ar / O ₂ mixed gas was introduced to form a ZnO film as a piezoelectric thin film. At this time, the substrate temperature was about 200 ° C., and the thickness of the formed ZnO film was about 1 μm.

  When crystallinity analysis was performed by X-ray diffraction at this stage, the incident pole figures of Cu (111), Ti (102), and Al (200) are as shown in FIGS. 10, 11, and 12, respectively. Results were obtained. As for the ZnO (102) pole figure, as shown in FIG. 13, a 12-fold symmetric spot was detected, and it was confirmed that the ZnO film was a triaxially oriented film on which epitaxial growth occurred.

  In addition, referring to FIG. 10, the Cu peak disappears. This is considered to be due to the fact that interdiffusion occurred between Cu and Si during the high-temperature ZnO film formation, and amorphous Cu silicide was generated. However, as shown in FIGS. 11 and 12, a diffraction spot is detected from the Ti film and the Al film, and the ZnO (102) pole figure is detected as a 12-fold symmetric spot as shown in FIG. As described above, the ZnO film was found to be an epitaxially grown triaxially oriented film. Therefore, it is considered that the high orientation of the Al film that forms the interface that is most important for the epitaxialization of the piezoelectric thin film is not lost, thereby enabling the epitaxial growth of the ZnO film.

Thereafter, an upper electrode forming step was performed on the sample obtained as described above to produce a BAW filter using ZnO as a piezoelectric thin film. As a result, the piezoelectric thin film was replaced with a conventional uniaxially oriented film. compared to the electromechanical coupling coefficient _{K 2} is increased by 0.6%.

(Experimental example 2)
In place of the ZnO film forming process as the piezoelectric thin film in Experimental Example 1, the same process as in Experimental Example 1 was performed except that the following AlN film forming process was performed, and the sample BAW filter was obtained. Produced.

In the sputtering process for forming the AlN film, the substrate temperature is set to about 250 ° C., the ultimate vacuum is set to about 10 ⁻⁵ Pa, an Ar / N ₂ mixed gas as a sputtering gas is introduced, and the film thickness is about 1 μm. An AlN film was formed.

  When the crystallinity of this AlN film is analyzed by X-ray diffraction, the AlN (102) pole figure shows a 12-fold symmetric spot similar to that shown in FIG. 13, and an epitaxial film having the same orientation as that of the ZnO film. Found out growing.

  Further, when the surface morphology of the AlN film was measured using an atomic force microscope, the case where the AlN film was a uniaxially oriented film was about Ra = 2.0 nm. In the case of the obtained epitaxial AlN film, Ra = 1.5 nm, and it was found that the flatness was improved.

  Furthermore, when the characteristics as a resonator and a filter were evaluated, propagation loss was significantly reduced as compared with the case where the AlN film was a uniaxially oriented film.

(Experimental example 3)
Instead of Al used as the electrode material in Experimental Example 1, Au and Pt were used, respectively, and in each case of Au and Pt, a ZnO film and an AlN film were formed as piezoelectric thin films, respectively. The BAW filter used as each sample was produced through the same process.

  As a result, in both the pole figure of Au (111) incidence and the pole figure of Pt (111) incidence, a 12-fold symmetric spot similar to that in the case of Al was detected, and an epitaxial satisfying the same crystal orientation relationship as Al. It was confirmed to be a membrane. In addition, with regard to the ZnO film and the AlN film as piezoelectric thin films, 12-fold symmetric spots were detected, and it was confirmed that they were epitaxially grown.

1 is a cross-sectional view showing a piezoelectric device 1 according to a first embodiment of the present invention. It is sectional drawing which expands and shows the buffer layer 6 shown in FIG. It is sectional drawing which shows the structure obtained in the stage in the middle of manufacturing the piezoelectric apparatus 1 shown in FIG. It is sectional drawing which shows the piezoelectric apparatus 1a by 2nd Embodiment of this invention. It is sectional drawing which shows the piezoelectric apparatus 21 by 3rd Embodiment of this invention. It is sectional drawing which shows the structure obtained in the step in the middle of manufacturing the piezoelectric device 21 shown in FIG. It is a XRD pole figure of Cu (111) incidence about a Cu film before formation of a ZnO film of a sample produced in example 1 of an experiment. 6 is an XRD pole figure of Ti (102) incidence on a Ti film before formation of a ZnO film of a sample manufactured in Experimental Example 1. FIG. 6 is an XRD pole figure of Al (111) incidence on an Al film before formation of a ZnO film of a sample manufactured in Experimental Example 1. FIG. It is a XRD pole figure of Cu (111) incidence about a Cu film after formation of a ZnO film of a sample produced in example 1 of an experiment. 6 is an XRD pole figure of Ti (102) incidence on a Ti film after formation of a ZnO film of a sample manufactured in Experimental Example 1. FIG. 6 is an XRD pole figure of Al (200) incidence on an Al film after formation of a ZnO film produced in Experimental Example 1. FIG. 6 is an XRD pole figure of ZnO (102) incidence on a sample ZnO film fabricated in Experimental Example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1a, 21 Piezoelectric device 2,22 Substrate 3,23 Piezoelectric thin film 4,24 Upper electrode 5,25 Lower electrode 6,26 Buffer layer 7 Void 8 Epitaxial Cu film 9 Epitaxial Ti film 10 Epitaxial Al film 11 Sacrificial layer 27 Cavity

Claims (19)

A substrate made of Si, a piezoelectric thin film supported by the substrate, and an upper electrode and a lower electrode formed so as to face each other with the piezoelectric thin film sandwiched in the thickness direction,
A buffer layer for relaxing crystal lattice irregularities is formed between the substrate and the lower electrode, and the buffer layer has a laminated structure, and has an epitaxial Cu film , a Ti film, and a conductor film from the substrate side. ,And,
A piezoelectric device, wherein at least the lower electrode and the piezoelectric thin film are made of an epitaxial film.   The piezoelectric device according to claim 1, wherein a gap is formed between the lower electrode and the buffer layer. 3. The piezoelectric device according to claim 2 , wherein the conductive film includes one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. . 4. The piezoelectric device according to claim 2 , wherein the lower electrode and the buffer layer are in contact with each other in a peripheral region of the gap and thereby electrically connected to each other. The lower electrode, Al, Pt, Au, Pd , Ag, Rh, γ-Mn, Ir, α-Sn, Cu, including one selected from gamma-Fe and Ni, any one of claims 2 to 4 The piezoelectric device described in 1.   The piezoelectric device according to claim 1, wherein the lower electrode is formed so as to be in contact with the buffer layer without a gap. The piezoelectric device according to claim 6 , wherein the lower electrode includes a Ti film formed on the buffer layer and a conductor film formed on the Ti film. The piezoelectric device according to claim 7 , wherein the conductive film includes one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. . On the substrate, the buffer layer, the cavity exposing at least a portion of said substrate side face is formed, a piezoelectric device according to any one of claims 6 to 8. The piezoelectric thin film is made of AlN or ZnO, a piezoelectric device according to any one of claims 1 to 9. The upper electrode is comprised of an epitaxial film, a piezoelectric device according to any one of claims 1 to 10. Preparing a substrate made of Si;
Forming a buffer layer for alleviating crystal lattice irregularities on a part of the substrate;
Forming a sacrificial layer on a part of the substrate so as to cover at least a part of the buffer layer;
Forming a lower electrode on at least the sacrificial layer so that at least a portion is located at a position above the buffer layer; and
Forming a piezoelectric thin film on the substrate so as to cover the sacrificial layer;
Forming an upper electrode on the piezoelectric thin film so as to face the lower electrode through the piezoelectric thin film;
Removing the sacrificial layer, thereby forming a gap between the lower electrode and the buffer layer,
The buffer layer has an epitaxial Cu film , a Ti film and a conductor film , and the buffer layer forming step includes a step of forming the epitaxial Cu film on the substrate and a step of forming a Ti film on the epitaxial Cu film. And a step of forming a conductor film on the Ti film ,
The sacrificial layer forming step, the lower electrode forming step, and the piezoelectric thin film forming step are performed to epitaxially grow the sacrificial layer, the lower electrode, and the piezoelectric thin film, respectively.
A method for manufacturing a piezoelectric device. The step of forming the conductor film includes forming the conductor film including one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. The method for manufacturing a piezoelectric device according to claim 12 , comprising the step of: The lower electrode forming step is a step of forming the lower electrode including one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. A method for manufacturing a piezoelectric device according to claim 12 or 13 , comprising: Preparing a substrate made of Si;
Forming a buffer layer for alleviating crystal lattice irregularities on a part of the substrate;
Forming a lower electrode on the substrate so as to cover at least a part of the buffer layer;
Forming a piezoelectric thin film on the substrate so as to cover the lower electrode at a position above the buffer layer;
Forming an upper electrode on the piezoelectric thin film so as to face the lower electrode through the piezoelectric thin film;
A cavity forming step of forming a cavity in a portion of the substrate where the buffer layer is formed,
The buffer layer has an epitaxial Cu film, and the buffer layer forming step is performed to form the epitaxial Cu film on the substrate,
The lower electrode forming step and the piezoelectric thin film forming step are performed so as to epitaxially grow the lower electrode and the piezoelectric thin film, respectively .
The lower electrode forming step includes a step of forming a Ti film on the buffer layer and a step of forming a conductor film on the Ti film.
A method for manufacturing a piezoelectric device. The step of forming the conductor film includes forming the conductor film including one selected from Al, Pt, Au, Pd, Ag, Rh, γ-Mn, Ir, α-Sn, Cu, γ-Fe, and Ni. The method for manufacturing a piezoelectric device according to claim 15 , comprising the step of: The substrate has a hydrogen-terminated Si (100) surface, and the buffer layer forming step includes a step of forming the epitaxial Cu film having a crystal orientation of Cu (100) on the Si (100) surface. A method for manufacturing a piezoelectric device according to any one of claims 12 to 16 . The substrate has a hydrogen-terminated Si (111) surface, and the buffer layer forming step includes a step of forming the epitaxial Cu film having a crystal orientation of Cu (111) on the Si (111) surface. A method for manufacturing a piezoelectric device according to any one of claims 12 to 16 . The upper electrode forming step is carried out the upper electrode so as to epitaxially grow, the method for manufacturing a piezoelectric device according to any one of claims 12 to 18.

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