Method for manufacturing resonator

A method for manufacturing a resonator of the present invention includes the steps of (a) forming a resonator film including a piezoelectric film made of piezoelectric material and (b) preparing a resonator substrate for supporting the resonator film. The method further comprises the step of (c) bonding the resonator film formed in the step (a) and the resonator substrate prepared in the step (b).

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2004-128921 filed in Japan on Apr. 23, 2004, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method for manufacturing a resonator which is applicable to filters for electronic circuits. In particular, it relates to a method for manufacturing a film bulk acoustic resonator.

(b) Description of Related Art

Film bulk acoustic resonators (hereinafter referred to as FBARs) have been used for high frequency filters of high performance. In recent years, with a decrease in size and an increase in frequency band of high frequency devices such as cellular phones, expectations have been placed on the application of the FBARs.

In the manufacture of the FBARs, the most important step is to form a structure for resonating acoustic waves. Such a structure has been formed by known methods, for example, by forming a trench from the bottom surface of a substrate to a lower electrode by etching, forming an air gap (air bridge) or forming a multilayer acoustic film.

Hereinafter, with reference toFIGS. 8 and 9, an explanation is given for examples of the conventional methods for forming the FBARs.

CONVENTIONAL EXAMPLE 1

FIGS. 8A to 8Eshow a method for manufacturing a resonator of Conventional Example 1 (see the specification of U.S. Pat. No. 6,384,697).

First, as shown inFIG. 8A, a resist film is applied to a resonator substrate100made of silicon and patterned into a resist pattern101. Then, using the resist pattern101as a mask, the resonator substrate100is etched to form a concave102.

The resist pattern101is removed and a sacrificial layer103made of silicon and oxygen is deposited on the resonator substrate100to fill the concave102as shown inFIG. 8B. Then, the sacrificial layer103is planarized by chemical mechanical polishing (CMP) as shown inFIG. 8Cto provide a sacrificial layer region104filled with the sacrificial layer103to be removed in a later step.

Further, a lower electrode105is formed on the resonator substrate100and the sacrificial layer region104as shown inFIG. 8Dand then a piezoelectric (PZ) film106and an upper electrode107are formed on the lower electrode105in this order. The lower electrode105and the piezoelectric film106are then etched to form a via hole108for forming an air gap.

Then, as shown inFIG. 8E, the sacrificial layer103in the sacrificial layer region104is removed at high speed using hydrofluoric acid (HF) through the via hole108. Thus, an air gap109is formed.

CONVENTIONAL EXAMPLE 2

FIGS. 9A to 9Fillustrate a method for manufacturing a resonator of Conventional Example2(see Japanese Unexamined Patent Publication No. 2002-509644).

First, as shown inFIG. 9A, a sacrificial layer201made of a polymer is deposited on a resonator substrate200. Then, the sacrificial layer201is etched using a resist pattern formed thereon as a mask to provide a sacrificial layer region202including the sacrificial layer201to be removed in a later step.

Then, an insulating protective film203is deposited over the resonator substrate200and the sacrificial layer region202as shown inFIG. 9C. Then, as shown inFIG. 9D, a lower electrode204is formed on a certain region of the insulating protective film203. Subsequently, a piezoelectric (PZ) film205and an upper electrode206are formed in this order on the insulating protective film203and the lower electrode204.

Further, the upper electrode206, piezoelectric film205, lower electrode204and insulating protective film203are etched to form a via hole207for forming an air gap as shown inFIG. 9E.

Then, as shown inFIG. 9F, the sacrificial layer region202is etched through the via hole207to remove the sacrificial layer201in the sacrificial layer region202. Thus, an air gap208is formed.

SUMMARY OF THE INVENTION

Both of the above-described conventional methods for forming the FBARs involve the complicated step of forming the piezoelectric film and the like on the bottommost sacrificial layer and then etching the sacrificial layer to form the air gap below the piezoelectric film. The manufacture of the FBARs requires a large number of complicated steps, thereby decreasing a step yield. Further, since the sacrificial layer is etched after the piezoelectric film has been formed, the piezoelectric film is damaged by the etching. This brings about deterioration in performance of the FBARs. Moreover, in the conventional methods for manufacturing the FBARs, the piezoelectric film is formed directly on the resonator substrate. Therefore, limitations are imposed on how to form the piezoelectric film.

In view of the above problems, an object of the present invention is to allow the manufacture of a high performance resonator with high yield and low cost by simplifying the complicated steps of manufacturing the resonator including the step of forming the sacrificial layer.

To achieve the object, the present invention provides a method for manufacturing a resonator comprising the step of bonding a resonator film including a piezoelectric film made of piezoelectric material and a resonator substrate for supporting the resonator film.

More specifically, the method for manufacturing a resonator of the present invention comprises the steps of: (a) forming a resonator film including a piezoelectric film made of piezoelectric material; (b) preparing a resonator substrate for supporting the resonator film; and (c) bonding the resonator film and the resonator substrate.

The present method for manufacturing the resonator allows forming the resonator film on a substrate other than the support substrate. Therefore, the resonator film is formed with high crystallinity. Further, since the resonator substrate or the resonator film is formed with a part to be an air gap in advance, the air gap is easily formed below the resonator film without forming or removing the sacrificial layer. Thus, the resonator is obtained with great ease.

In the present method for manufacturing the resonator, the resonator film is preferably a resonator film for a film bulk acoustic resonator.

In the present method for manufacturing the resonator, it is preferable that the step (a) is the step of forming the resonator film on the main surface of a preparation substrate and the step (c) is the step of bonding the resonator film to the main surface of the resonator substrate.

Since the method is so constructed, the steps for manufacturing the resonator are drastically simplified. Further, since the air gap is formed in advance in the resonator substrate, there is no need of the etching step after the formation of the resonator film including the piezoelectric film. This prevents the piezoelectric film from being damaged. Moreover, since the piezoelectric film is formed on the preparation substrate different from the resonator substrate, a wide choice of the preparation substrate is offered. Thus, the piezoelectric film is obtained with high quality by epitaxial growth.

In the present method for manufacturing the resonator, the step (a) preferably includes the step of (d) forming the piezoelectric film made of piezoelectric material on the main surface of the preparation substrate. By so doing, the resonator film is obtained with reliability.

In the present method for manufacturing the resonator, the step (d) is preferably the step of forming the piezoelectric film by epitaxial growth. By so doing, the piezoelectric film is obtained with high quality.

In the present method for manufacturing the resonator, the step (d) is preferably the step of forming the piezoelectric film by sputtering. By so doing, the resonator film is obtained easily.

The piezoelectric film is preferably made of aluminum nitride (AlN), zinc oxide (ZnO) or lead zirconate titanate (PZT). If such material is used as the piezoelectric film, the piezoelectric film of high performance is obtained with reliability.

In the present method for manufacturing the resonator, the step (a) preferably includes the step of (e) forming a first thin film between the main surface of the preparation substrate and the piezoelectric film prior to the step (d) of forming the piezoelectric film. By so doing, the resonator film is surely peeled off the preparation substrate and damage to the resonator film caused upon peeling is reduced.

The step (e) may be the step of forming an insulating film or a conductive film. Further, the step (e) may be the step of forming a layered film of a conductive film and an insulating film.

In the present method for manufacturing the resonator, the step (a) preferably includes the step of (f) forming a second thin film on the piezoelectric film. This improves adhesion between the resonator film and the resonator substrate.

The step (f) may be the step of forming an insulating film or a conductive film. Further, the step (f) may be the step of forming a layered film of a conductive film and an insulating film.

In the present method for manufacturing the resonator, the step (a) preferably includes the step of removing part of the second thin film to form an opening in the second thin film. According to the thus constructed method, the air gap is formed upon bonding the resonator film and the resonator substrate without forming any sacrificial layer. Further, the air gap is provided with high dimensional precision.

In the present method for manufacturing the resonator, the step (b) preferably includes the step of (g) forming a third thin film on the main surface of the resonator substrate. This improves adhesion between the resonator film and the resonator substrate.

In the present method for manufacturing the resonator, the step (b) preferably includes the step of removing part of the third thin film to form an opening in the third thin film. According to the thus constructed method, the air gap is formed upon bonding the resonator film and the resonator substrate without forming any sacrificial layer. Further, the air gap is provided with high dimensional precision.

The step (g) may be the step of forming an insulating film or a conductive film. Further, the step (g) may be the step of forming a layered film of a conductive film and an insulating film.

In the present method for manufacturing the resonator, the step (b) preferably includes the step of (h) forming a hole or a concave in the resonator substrate. This allows forming the air gap in the resonator with reliability.

The step (h) may be the step of selectively irradiating the resonator substrate with a laser beam or the step of selectively wet-etching the resonator substrate.

In the present method for manufacturing the resonator, the step (b) preferably includes the step of forming a multilayer acoustic film made of fourth thin films and fifth thin films alternately stacked on the main surface of the resonator substrate and the fifth thin films are lower in acoustic impedance than the fourth thin films.

According to the thus constructed method, the resonator is obtained easily without forming the air gap below the resonator film.

The fourth thin films and the fifth thin films preferably have a thickness corresponding to one fourth of the resonance wavelength of the resonator, respectively. This allows forming the multilayer acoustic film with reliability.

In this case, the piezoelectric film preferably has a thickness corresponding to one half of the resonance wavelength of the resonator. By so doing, the multilayer acoustic film confines elastic waves in the piezoelectric film with high efficiency. Thus, the resonator is provided with high performance.

The fourth thin films are preferably made of silicon oxide (SiO2) and the fifth thin films are preferably made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT) or tungsten (W). By so doing, the multilayer acoustic film is provided with high performance.

It is preferred that the present method for manufacturing the resonator further comprises the step of (i) peeling the preparation substrate prior to the step (c). This allows providing a simple method for manufacturing the resonator.

The step (i) is preferably carried out by laser lift-off, wet etching or dry etching.

In the present method for manufacturing the resonator, the air gap is provided without forming any sacrificial layer. Therefore, a film bulk acoustic resonator is obtained with great ease. Further, since the piezoelectric film may be formed by epitaxial growth, the obtained piezoelectric film is given with uniform crystallinity and thickness. As a result, the step of adjusting the resonator is simplified.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1A to 1E, an explanation is given for Embodiment 1 of the present invention.FIGS. 1A to 1Eschematically illustrate the steps of manufacturing a film bulk acoustic resonator (FBAR) of this embodiment.

First, as shown inFIG. 1A, a resonator substrate1made of silicon (Si) is irradiated with the third harmonic of a yttrium aluminum garnet (YAG) laser (wavelength: 355 nm, output: 300 J/cm2) to form a through hole1apenetrating the resonator substrate1.

Then, as shown inFIG. 1B, a buffer layer3made of gallium nitride (GaN) is formed on a preparation substrate2made of silicon carbide (SiC). Then, a piezoelectric film4made of aluminum nitride (AlN) is deposited thereon by epitaxial growth. In this embodiment, the piezoelectric film4solely serves as a resonator film10.

The resonator film10is brought into close contact with the main surface of the resonator substrate1and then heated at 375° C. for 10 minutes so that the resonator film10is bonded to the resonator substrate1as shown inFIG. 1C. The resulting structure includes an air gap1bformed below the resonator film10.

Then, as shown inFIG. 1D, the preparation substrate2is peeled off the resonator film10by laser lift-off. For example, a laser of such a wavelength that is absorbed not in the preparation substrate2but in the GaN buffer layer3(e.g., a YAG laser of 355 nm wavelength) is applied to the preparation substrate2to fuse the GaN buffer layer3into Ga and N. Thus, the preparation substrate2is peeled off the resonator film10.

Finally, as shown inFIG. 1E, electrodes5made of molybdenum (Mo) are formed on the upper surface and the lower surface of the resulting resonator film10, respectively. The electrodes5in this embodiment include an upper electrode5A and a lower electrode5B formed on the top surface and the bottom surface of the resonator film10, respectively.

As described above, according to the method for manufacturing the FBAR of this embodiment the resonator film10formed on the preparation substrate2is bonded to the resonator substrate1which has been provided with the air gap1bin advance. Therefore, there is no need of forming a sacrificial layer for forming the air gap1bin the resonator substrate10or the resonator film10and the manufacturing steps are simplified. For example, a conventional method using the sacrificial layer requires at least 100 steps to form a resonator of the same structure as that obtained by the manufacturing method of this embodiment. However, the method of this embodiment reduces the number of steps to 80, realizing a 20% or more decrease in the number of steps.

Further, since there is no need of removing the sacrificial layer by etching, the resonator film10will not be damaged. Therefore, a high performance resonator is obtained with ease.

In the method for manufacturing the FBAR of this embodiment, the epitaxially grown piezoelectric film4made of AlN shows less variations in crystallinity and thickness. Therefore, an adjustment of the resonance frequency to the desired value, which is carried out after the formation of the resonator, is simplified.

For the epitaxial growth of the piezoelectric film made of AlN, it is necessary for the substrate to be made of SiC. However, since SiC is considerably expensive as compared with commonly used Si, the use of SiC for forming the FBAR is not practical on the cost front. Further, because of its poor workability, SiC is not suitable as a substrate for forming the FBAR in which an air gap shall be formed with high precision. Therefore, in the conventional method for forming the FBAR which involves direct deposition of the piezoelectric film on the resonator substrate, the epitaxially grown piezoelectric film cannot be employed.

On the other hand, the method for manufacturing the FBAR of this embodiment employs a bonding technique. Therefore, the preparation substrate2for forming the piezoelectric film4thereon may be made of other material than that of the resonator substrate1. Accordingly, inexpensive Si having high workability is used as the resonator substrate1, while SiC suitable for epitaxial growth of the piezoelectric film4is used as the preparation substrate2. Further, the preparation substrate2can be reused after peeling the piezoelectric film4.

In the method for manufacturing the resonator of this embodiment, the electrodes5are formed after the epitaxial growth of the piezoelectric film4. This eliminates the need of considering the heat resistance of the electrode material. Therefore, although the electrodes5of this embodiment are made of Mo, any material can be used as the electrodes5as required.

For example, if tungsten (W) or iridium (Ir) having high acoustic impedance is used as the electrode material, the Q value of the obtained resonator increases. On the other hand, if aluminum (Al) or copper (Cu) having low resistance is used as the electrode material, loss by electrical resistance is reduced and the electrodes are thinned down, thereby increasing the frequency band of the resonator. Further, if stable metal such as gold (Au) is used as the electrode material, the electrodes5are prevented from deterioration in characteristic and adhesion to the film is enhanced. Thus, the resonator is obtained with high reliability.

In this embodiment, the electrodes5are single-layered. However, the electrodes5may be multilayered using the above-mentioned electrode materials in combination. In this case, the electrodes are given with a high Q value and low resistance. Further, a dielectric film may be interposed between the piezoelectric film4and the electrodes5.

In this embodiment, the through hole1ais formed using the YAG laser. However, it may be formed with a krypton fluoride (KrF) excimer laser (wavelength: 248 nm, output: 600 mJ/cm2).

The through hole1amay also be formed by patterning the resonator substrate using a silicon oxide (SiO2) film as a mask and then wet-etching the resonator substrate using a high temperature potassium hydroxide (KOH) solution. Or alternatively, the through hole1amay be formed by dry etching using a fluorine-based gas such as SF6or CF4as an etching gas. Instead of forming the through hole1a,a concave which does not penetrate the substrate may be formed by controlling the etching time.

In this embodiment, the air gap1bis provided as a round through hole. However, the shape of the air gap1bmay be changed easily using the above-described methods. For example, if the air gap1bis in the form of a rectangle or a pentagon whose sides are not parallel to each other so that it reduces unwanted resonant oscillation in the lateral mode, deterioration in characteristic due to the unwanted resonance is eliminated (e.g., an increase in loss due to energy loss or deterioration in filtering characteristic due to unwanted resonance in a desired frequency band).

First Alternative Example of Embodiment 1

Hereinafter, with reference toFIGS. 2A to 2D, an explanation is given for a first alternative example of Embodiment 1.FIGS. 2A to 2Dschematically illustrate the method for manufacturing the resonator of this alternative example. InFIGS. 2A to 2D, the same components as those shown inFIGS. 1A to 1Eare given with the same reference numerals. In this alternative example, the steps prior to the step of forming the through hole1ain the resonator substrate1are the same as those in Embodiment 1 and therefore the explanation thereof is omitted.

First, as shown inFIG. 2A, a buffer layer3made of GaN is formed on a preparation substrate2made of SiC. Then, an insulating film made of silicon oxide (SiO2) as a first thin film8and a piezoelectric film4made of AlN are deposited thereon to form a resonator film10.

Then, the main surface of the resonator substrate1and the main surface of the preparation substrate2are faced to each other. The resonator film10is brought into close contact with the resonator substrate1and heated at 375° C. for 10 minutes, thereby bonding the resonator film10to the resonator substrate1as shown inFIG. 2B. The resulting structure includes an air gap1bformed below the resonator film10.

Then, a YAG laser or the like is applied to the preparation substrate2to fuse the buffer layer3, thereby peeling the preparation substrate2off the resonator film10as shown inFIG. 2C.

Finally, as shown inFIG. 2D, an upper electrode5A and a lower electrode5B made of Mo are formed on the upper and lower surfaces of the resonator film10, respectively, by common vacuum deposition.

In this alternative example, the first thin film8is formed between the buffer layer3and the piezoelectric film4. Therefore, the piezoelectric film4is prevented from being damaged upon peeling the preparation substrate2off the resonator film10. Further, upon peeling of the preparation substrate2off the resonator film10, the unwanted buffer layer3will not remain on the resonator film10, i.e., a so-called film residue is prevented.

If the piezoelectric film4is made of material having a negative temperature coefficient such as zinc oxide (ZnO), SiO2or the like having a positive temperature coefficient is used as the first thin film8. This suppresses variations in resonance frequency due to temperature.

In this alternative example, the first thin film8is made of SiO2, but it may be made of silicon nitride (SiN) or the like. Further, the upper electrode5A is formed on the first thin film8, but it may be formed after the first thin film8is etched away. Or alternatively, the etch amount of the first thin film8may be adjusted to control the thickness of the resonator film10, thereby making a fine adjustment to the resonant frequency of the resonator.

If the first thin film8is a conductive film of Mo or the like, it may be used as the upper electrode5A. The first thin film8may be formed of a conductive film and an insulating film.

Second Alternative Example of Embodiment 1

Hereinafter, with reference toFIGS. 3A to 3D, an explanation is given for a second alternative example of Embodiment 1 of the present invention.FIGS. 3A to 3Dschematically illustrate the method for manufacturing the resonator of this alternative example. InFIGS. 3A to 3D, the same components as those shown inFIGS. 1A to 1Eare given with the same reference numerals. In this alternative example, the steps prior to the step of forming the piezoelectric film4on the resonator substrate1are the same as those in Embodiment 1 and therefore the explanation thereof is omitted.

In this alternative example, a conductive film made of Mo is formed as a second thin film9on the piezoelectric film4by vacuum deposition to provide a resonator film10as shown inFIG. 3A. Then, as shown inFIG. 3B, the resonator film10is brought into close contact with the main surface of the resonator substrate1and then heated at 375° C. for 10 minutes, thereby bonding the resonator film10to the main surface of the resonator substrate1. Thus, an air gap1bis formed below the resonator film10.

Then, a YAG laser or the like is applied to the preparation substrate2to fuse the buffer layer3, thereby peeling the preparation substrate2off the resonator film10as shown inFIG. 3C. Finally, as shown inFIG. 3D, an upper electrode5A made of Mo is formed by common vacuum deposition. In this alternative example, a lower electrode5B is formed on the rear surface of the resonator substrate1to be in contact with the second thin film9.

In this alternative example, the second thin film9of high adhesion is formed on the surface of the resonator film10. Therefore, the resonator film10and the resonator substrate1are bonded with improved adhesion stability.

In this alternative example, the second thin film9is a conductive film made of Mo, but it may be made of tungsten (W), aluminum (Al), gold (Au), copper (Cu), titanium (Ti), iridium (Ir) or lanthanum hexaboride (LaB6). Or alternatively, the second thin film9may be a layered film of Ti and Au. In place of the conductive film, an insulating film made of SiO2or SiN may be used. Also in this case, the adhesion stability improves in a like manner as the above. The second thin film9may be formed of a conductive film and an insulating film formed on the piezoelectric film4.

Further, in the same manner as in the first alternative example, the first thin film8may be formed between the main surface of the preparation substrate2and the piezoelectric film4.

Hereinafter, with reference toFIGS. 4A to 4F, an explanation is given for Embodiment 2 of the present invention.FIGS. 4A to 4Fschematically illustrate the method for manufacturing the resonator of Embodiment 2. InFIGS. 4A to 4F, the same components as those shown inFIGS. 1A to 1Eare given with the same reference numerals.

First, as shown inFIG. 4A, an insulating film made of silicon oxide (SiO2) is formed as a third thin film12on a resonator substrate1made of Si. Then, as shown inFIG. 4B, part of the third thin film12is wet-etched using a suitable mask to form an opening12a.

Then, as shown inFIG. 4C, a buffer layer3made of GaN is formed on a preparation substrate2made of SiC and a piezoelectric film4made of AlN and a conductive film made of Mo as a second thin film9are deposited thereon to form a resonator film10.

Then, as shown inFIG. 4D, the resonator film10is bonded to the resonator substrate1provided with the third thin film12including the opening12a,thereby forming an air gap12bbelow the resonator film10.

Then, a YAG laser or the like is applied to the preparation substrate2to fuse the buffer layer3, thereby peeling the preparation substrate2off the resonator film10as shown inFIG. 4E.

Finally, as shown inFIG. 4F, an upper electrode5A made of Mo is formed by common vacuum deposition. In this embodiment, the second thin film9is used as a lower electrode5B.

In this embodiment, the third thin film12deposited on the resonator substrate1is etched to form the opening12a,thereby providing the air gap12b.Therefore, the resonator is obtained easily without forming any sacrificial layer and the air gap12bis provided with high dimensional precision.

In this embodiment, the third thin film12is made of SiO2, but it may be made of SiN or the like. Or alternatively, the third thin film12may be a conductive film or a layered structure of a conductive film and an insulating film.

In this embodiment, the opening12ais formed by wet etching, but it may be formed by dry etching or lift-off depending on the material of the third thin film12.

First Alternative Example of Embodiment 2

Hereinafter, with reference toFIGS. 5A to 5E, an explanation is given for a first alternative example of Embodiment 2 of the present invention.FIGS. 5A to 5Eschematically illustrate the method for manufacturing the resonator of this alternative example. InFIGS. 5A to 5E, the same components as those shown inFIGS. 1A to 1Eare given with the same reference numerals.

First, as shown inFIG. 5A, a buffer layer3made of GaN is formed on a preparation substrate2made of SiC. Then, a piezoelectric film4made of AlN and a second thin film9including two films, i.e., a conductive film9A made of Mo and an insulating film9B made of SiO2, are deposited thereon to form a resonator film10.

Then, part of the second thin film9is etched away using a suitable mask to form an opening9ain the second thin film9as shown inFIG. 5B.

Then, as shown inFIG. 5C, the resonator film10provided with the opening9ais bonded to the main surface of the resonator substrate1, thereby forming an air gap9bbelow the resonator film10.

Then, a YAG laser or the like is applied to the preparation substrate2to fuse the buffer layer3, thereby peeling the preparation substrate2off the resonator film10as shown inFIG. 5D.

Finally, as shown inFIG. 5E, an upper electrode5A made of Mo is formed by common vacuum deposition. In this alternative example, the conductive film9A in the second thin film9is used as a lower electrode5B.

In this alternative example, the second thin film9at the outermost surface of the resonator film10is etched to form the opening9a,thereby providing the air gap9b.Therefore, the resonator is obtained easily without forming any sacrificial layer and the air gap9bis provided with high dimensional precision.

In this alternative example, the second thin film9is formed of the conductive film9A and the insulating film9B deposited in this order, but it may be formed of either a conductive film or an insulating film. Further, instead of etching, the opening9amay be formed by lift-off or the like depending on the material of the thin film.

Second Alternative Example of Embodiment 2

Hereinafter, with reference toFIGS. 6A to 6E, an explanation is given for a second alternative example of Embodiment 2 of the present invention.FIGS. 6A to 6Eschematically illustrate the method for manufacturing the resonator of this alternative example. InFIGS. 6A to 6E, the same components as those shown inFIGS. 1A to 1Eare given with the same reference numerals.

First, as shown inFIG. 6A, a third thin film12having an opening12ais formed on a resonator substrate1made of Si by common electron beam deposition and lift-off. The third thin film12includes a50nm thick Ti film and a 50 nm thick Au film.

Then, as shown inFIG. 6B, a buffer layer3including a 40 nm thick AlN film3A and a 500 nm thick GaN film3B is formed on a preparation substrate2made of SiC. Then, a 1 μm thick piezoelectric film4made of AlN is formed. Further, a second thin film9including a 50 nm thick Ti film, a 500 nm thick Au film, a 200 nm thick gold-tin (Au—Sn) alloy film and a 5 nm thick Au film is formed to obtain a resonator film10.

Subsequently, the resonator film10is brought into close contact with the third thin film12formed on the surface of the resonator substrate1and heated at 375° C. for 10 minutes, thereby bonding the resonator film10to the resonator substrate1. Since the resonator film10is bonded to the resonator substrate1provided with the opening12a,an air gap12bis provided below the resonator film10as shown inFIG. 6C.

Then, a YAG laser or the like is applied to the preparation substrate2to fuse the buffer layer3, thereby peeling the preparation substrate2off the resonator film10as shown inFIG. 6D.

Finally, as shown inFIG. 6E, an upper electrode5A including a 50 nm thick Ti film and a 200 nm thick Au film is formed on the resonator film10by electron beam deposition and lift-off. In this alternative example, the second and third thin films9and12are used as a lower electrode5B.

In this alternative example, the Sn-containing second thin film9is formed on the surface of the resonator film10and the Au-containing third thin film12is formed on the surface of the resonator substrate1. Therefore, upon bonding the resonator film10to the resonator substrate1, Sn contained in the second thin film9is dispersed into Au in the third thin film12to cause eutectic bonding. Thus, the resonator film10is firmly bonded to the resonator substrate1. Further, since the air gap12bis formed of the opening12awhich has been provided in advance in the surface of the resonator substrate1, the resonator is obtained easily without forming any sacrificial layer. Moreover, since the opening12ais formed by lift-off, the air gap12bis provided with high dimensional precision.

Hereinafter, with reference toFIGS. 7A to 7E, an explanation is given for Embodiment 3 of the present invention.FIGS. 7A to 7Eschematically illustrate the method for manufacturing the resonator of this alternative example. InFIGS. 7A to 7E, the same components as those shown inFIGS. 1A to 1Eare given with the same reference numerals.

First, as shown inFIG. 7A, 6 fourth thin films15which are made of SiO2and have a low acoustic impedance and 6 fifth thin films16which are made of AlN and have a high acoustic impedance are alternately stacked on a resonator substrate1made of Si to form a multilayer acoustic film14. Each of the stacked thin films has a thickness corresponding to one fourth of the resonant wavelength (λ) of the resonator.

Then, as shown inFIG. 7B, a buffer layer3made of GaN is formed on a preparation substrate2made of SiC, and then a piezoelectric film4made of AlN and a conductive film made of Mo as a second thin film9are deposited thereon to form a resonator film10.

Then, the resonator film10is bonded to the resonator substrate1on which the multilayer acoustic film14has been formed. The obtained structure includes the multilayer acoustic film14below the resonator film10as shown inFIG. 7C.

Further, as shown inFIG. 7D, a YAG laser or the like is applied to the preparation substrate2to fuse the buffer layer3, thereby peeling the preparation substrate2off the resonator film10.

Finally, as shown inFIG. 7E, an upper electrode5A made of Mo is formed on the resonator film10by common vacuum deposition. In this embodiment, the second thin film9is used as a lower electrode5B.

In this embodiment, the multilayer acoustic film14is provided below the resonator film10. Therefore, even if an air gap is not formed below the resonator film10, elastic waves generated in the piezoelectric film4are confined in the piezoelectric film4. This eliminates the need of forming a through hole in the resonator substrate1or etching the thin film formed on the resonator substrate1or the resonator film10for forming the air gap. Thus, the resonator is obtained with ease.

In this embodiment, the resonator film10including the piezoelectric film4formed on the preparation substrate2is bonded to the multilayer acoustic film14formed on the resonator substrate1made of Si. Therefore, the multilayer acoustic film and the piezoelectric film are obtained easily with higher quality as compared with the multilayer acoustic film and the piezoelectric film formed in this order on the resonator substrate1made of Si. As a result, the resonator is obtained easily with high performance.

In this embodiment, 6 fourth thin films15and 6 fifth thin films16are alternately stacked. However, if at least 2 fourth thin films15and 2 fifth thin films16are alternately stacked, the reflectance becomes 90% or more to give the above-described effect. Further, if the piezoelectric film4has a thickness corresponding to one half of the resonance wavelength (λ) of the resonator, elastic waves are confined in the piezoelectric film4with high efficiency.

In this embodiment, the fourth thin films15are made of SiO2and the fifth thin films16are made of AlN. However, a combination of SiO2and ZnO, SiO2and PZT or SiO2and W may be used to form the thin films15and16.

Also in this embodiment, the first thin film8may be interposed between the buffer layer3and the piezoelectric film4as in the first alternative example of Embodiment 1. This allows obtaining the same effect as that of the first alternative example of Embodiment 1.

In Embodiments 1 to 3 and their alternative examples, the piezoelectric film is deposited by epitaxial growth, but it may be formed by sputtering. In this case, the step of depositing the piezoelectric film is simplified. The piezoelectric film may be made of other materials than AlN such as zinc oxide (ZnO) or lead zirconium titanate (PZT). Further, the buffer layer may be made of aluminum gallium nitride (AlGaN) or the like.

The preparation substrate may be removed by wet-etching any layer sandwiched between the resonator substrate and the preparation substrate. For example, if a buffer layer made of SiO2is formed, buffered hydrofluoric acid is used as a wet etchant to fuse SiO2, thereby peeling off the preparation substrate easily. In this case, a conductive film made of Mo or Pt is formed on the SiO2buffer layer and then the piezoelectric film made of AlN or the like is formed thereon. By so doing, the piezoelectric film is given with high quality and the upper electrode is formed with ease.

The preparation substrate may also be removed by dry etching. In this case, the piezoelectric film may be formed directly on the preparation substrate and bonded to the resonator substrate without forming the buffer layer on the preparation substrate. For example, a piezoelectric film of AlN or the like is formed directly on a substrate made of highly crystalline sapphire and then bonded to the main surface of the resonator substrate. Thereafter, the sapphire substrate only is removed by dry etching using a chlorine-based gas such as boron trichloride (BCl3). In this way, the resonator is obtained without forming the buffer layer.

The method for manufacturing the resonator of the present invention allows obtaining a film bulk acoustic resonator with great ease because the air gap is provided without forming any sacrificial layer. Further, since the piezoelectric film is formed by epitaxial growth the obtained piezoelectric film is given with uniform crystallinity and thickness, thereby simplifying the step of adjusting the resonator. Thus, the method of the present invention is useful for manufacturing resonators applicable to filters for electronic circuits, especially film bulk acoustic resonators.