JP2008022408A - Piezoelectric thin film resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an SMR type piezoelectric thin film resonator that can resolve or reduce a warpage of a substrate due to film stress, suppress deformation or destruction, or characteristic deterioration of a device, and be industrially put to practical use and manufactured at low cost. <P>SOLUTION: A piezoelectric thin film resonator 11 includes: an acoustic multilayer film 13 on an upper surface of a substrate 12, in which a high acoustic impedance layer and a low acoustic impedance layer are alternately stacked; and a lower electrode 14, a piezoelectric thin film 15; and an upper electrode 16 which are stacked on the film 13, and further includes a compression stress film 17 on a lower surface of the substrate. The compressive stress film is formed in the same film configuration and in the same dimension and thickness as the acoustic multilayer film so as to have a compression stress of a level offset by a compressive stress caused by the acoustic multilayer film in the substrate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a piezoelectric thin film resonator including a piezoelectric thin film provided on a substrate via an acoustic multilayer film.

  Recently, particularly in the field of mobile communication devices such as mobile phones, the frequency of use has been increased. Correspondingly, a piezoelectric element used as a filter or a resonator in such electronic devices is required to have a very high frequency of several GHz or more. Piezoelectric vibrators that use longitudinal vibration of piezoelectric substrates that have been widely used in the past have to reduce the thickness of the piezoelectric substrate in order to increase the resonance frequency. However, in practice, a frequency of about several tens to several hundreds of MHz is the limit due to difficulty in machining.

  Therefore, a piezoelectric thin film resonator using the longitudinal vibration of the thickness of the piezoelectric thin film has been proposed (see, for example, Non-Patent Document 1). In this piezoelectric thin film resonator, two different structures are known. One of them is a diaphragm type piezoelectric thin film resonator in which a base film, a lower electrode, a piezoelectric thin film, and an upper electrode are formed on a substrate, and an opening is formed by removing the substrate portion below the vibrating portion from the back surface of the substrate. (FBAR) (see, for example, Patent Documents 1 to 3). The other one is a piezoelectric thin film resonator (SMR) in which a piezoelectric thin film, lower and upper electrodes are provided on a substrate via an acoustic multilayer film (see, for example, Patent Documents 4 and 5).

FIG. 4 schematically shows a general configuration of a piezoelectric thin film resonator having an SMR structure. The piezoelectric thin film resonator 1 has an acoustic multilayer film 3 formed on a substrate 2. The acoustic multilayer film 3 is configured by alternately laminating high acoustic impedance layers 3 a _{1 to} 3 a _n and low acoustic impedance layers 3 b _{1 to} 3 b _n . On the acoustic multilayer film 3, a lower electrode film 4, a piezoelectric thin film 5, and an upper electrode film 6 are laminated. The elastic wave (bulk wave) generated by the piezoelectric thin film 5 is confined in the piezoelectric thin film by the acoustic multilayer film 3 and is not affected by the substrate 2, so that a high Q value can be obtained. In addition, since the entire lower surface of the piezoelectric thin film 5 is fixed by the acoustic multilayer film 3, stable operation is possible.

The low acoustic impedance layer of the acoustic multilayer film 3 uses, for example, SiO ₂ or Mg, and the high acoustic impedance layer uses, for example, AlN, ZnO, Al ₂ O ₃ , Ta ₂ O ₅ , W, using a sputtering method or the like. A film is formed on the substrate (see, for example, Patent Document 6). As the substrate 2, for example, a single crystal substrate such as Si, quartz, GaAs or sapphire, a polycrystalline ceramic substrate such as Al ₂ O ₃ or Al ₂ O ₃ .TiC, quartz or glass is used.

K.M.Lakin, "Thin Film Resonators and Filters", 1999 IEEE Ultrasonics Symposium, p.895-906 JP 2002-76824 A JP 2003-51732 A JP 2003-60478 A JP 2004-235886 A Japanese Patent Laid-Open No. 2004-235887 Japanese Patent Laid-Open No. 2005-136761

However, in the SMR type piezoelectric thin film resonator, the SiO ₂ film of the low acoustic impedance layer formed by sputtering on the surface of the substrate has a large compressive stress as shown by an arrow in FIG. This film stress may cause the substrate to warp, which may cause the element to be deformed or broken, or deteriorate its vibration characteristics. Further, if the substrate is warped in the manufacturing process, there is a problem that it may be difficult to suck with a vacuum chuck during transportation or the element may be destroyed, and the handleability is greatly reduced. Furthermore, these problems greatly hinder the practical application of SMR type piezoelectric thin film resonators.

Since the diaphragm type piezoelectric thin film resonator also forms a SiO ₂ thin film as a base film on the substrate surface, there is a possibility that the element is similarly deformed or broken due to the compressive stress, and the characteristics are deteriorated. For this reason, the piezoelectric thin film resonators described in Patent Documents 1 and ₂ balance the tensile stress of the AlN thin film constituting the piezoelectric thin film with the compressive stress of the SiO ₂ thin film. In the piezoelectric thin film resonator described in Patent Document 3, the piezoelectric layer is composed of a first layer for ensuring required piezoelectricity and a second layer for adjusting stress in the entire resonator. However, it is difficult to make the stress of the entire element completely zero by these configurations, and it is also difficult to apply to the SMR type piezoelectric thin film resonator as it is.

  Accordingly, the present invention has been made in view of the above-described conventional problems, and an object thereof is to reduce the manufacturing cost of an SMR type piezoelectric thin film resonator in which a piezoelectric thin film is provided on a substrate via an acoustic multilayer film. It is to eliminate or reduce the warpage of the substrate due to the film stress while reducing it, thereby suppressing the deformation and destruction of the element or the deterioration of the vibration characteristics, and industrially promoting its practical use.

  According to the present invention, in order to achieve the above object, an acoustic multilayer film in which a plurality of high acoustic impedance layers and low acoustic impedance layers are alternately laminated on one surface of a substrate, and formed on the acoustic multilayer film A lower electrode, a piezoelectric thin film formed on the lower electrode, an upper electrode formed on the piezoelectric thin film, and a compressive stress film formed on the other surface of the substrate. There is provided a piezoelectric thin film resonator formed to have a compressive stress of a magnitude that cancels out the compressive stress generated by an acoustic multilayer film in a substrate.

  Thus, by canceling the compressive stress on both the front and back sides of the substrate, the warp of the substrate can be eliminated, and the deformation and destruction of the element or the deterioration of the vibration characteristics can be suppressed. Since such a compressive stress film can be easily formed industrially using conventional techniques, an SMR type piezoelectric thin film resonator can be commercialized industrially.

  In one embodiment, the compressive stress film has the same film configuration, the same plane and the same thickness as the acoustic multilayer film, and is formed symmetrically with the acoustic multilayer film across the substrate. Since this compressive stress film can be formed under the same film forming conditions using the same manufacturing process and equipment as the acoustic multilayer film as it is, it can be manufactured at a relatively low cost and easily.

  In another embodiment, the compressive stress film has the same planar dimensions as the acoustic multilayer film, is disposed symmetrically with the acoustic multilayer film across the substrate, and is made of the same material as the high acoustic impedance layer; A single first layer having the total thickness of the high acoustic impedance layers and a single layer having the total thickness of the low acoustic impedance layers made of the same material as the low acoustic impedance layer. It is formed by laminating one second layer. Since this compressive stress film can be formed by using exactly the same manufacturing process and equipment as the acoustic multilayer film, it can be easily manufactured at a relatively low cost, and can be formed by two film forming processes. Man-hours can be reduced and productivity is improved.

  In still another embodiment, the compressive stress film is formed of a single layer having the same plane size as the acoustic multilayer film and made of a single material, and is disposed symmetrically with the acoustic multilayer film across the substrate. . Since this compressive stress film can be formed in a single film formation process, the number of steps can be reduced, and higher productivity can be obtained.

Hereinafter, preferred embodiments of a piezoelectric thin film resonator according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 schematically shows the configuration of a piezoelectric thin film resonator according to a first embodiment of the present invention. The piezoelectric thin film resonator 11 of the present embodiment has a substrate 12 made of, for example, Si, quartz, or glass material. As the material of the other substrate 12, a single crystal substrate such as GaAs or sapphire, a polycrystalline ceramic substrate such as Al ₂ O ₃ or Al ₂ O ₃ .TiC, quartz, or the like can be used.

On the upper surface of the substrate 12, an acoustic multi-layer film 13 formed by alternately laminating a high acoustic impedance layer _13a 1 _{~13a n} and a low acoustic impedance layer _13b 1 13 b _n are formed. Each of the high acoustic impedance layers is formed of a piezoelectric thin film such as AlN or ZnO. As other high acoustic impedance materials, Al ₂ O ₃ , Ta ₂ O ₅ , W, or the like can be used. The low acoustic impedance layer is formed of, for example, a SiO ₂ thin film. Mg or the like can be used as another low acoustic impedance material.

  On the acoustic multilayer film 13, a lower electrode film 14, a piezoelectric thin film 15 and an upper electrode film 16 are laminated. The piezoelectric thin film 15 is formed of various known piezoelectric materials such as AlN, ZnO, and CdS. The lower and upper electrode films 14 and 16 can be formed of, for example, a Cr / Au film, and various other known conductive materials such as Pt, Au, Al, W, and Mo can be used as other materials.

In general, it is known that the thickness of the piezoelectric thin film 15 is ½ of the wavelength λ when the sound wave having the resonance frequency f ₀ propagates through the piezoelectric thin film, using this as a single thin film. The thickness of the high acoustic impedance layer _13a 1 _{~13a n} and the low acoustic impedance layer _13b 1 13 b _n is the wavelength λ when the sound waves of the resonant frequency _{f 0} thereof as each single layer is propagated 1 / Set to 4.

  The number of layers of the acoustic multilayer film 13 is normally set to 10 or more. As the number of layers of the acoustic multilayer film 13 increases, the load impedance when viewing the direction from the piezoelectric thin film 15 to the substrate 12 is reduced. Therefore, the piezoelectric thin film can be acoustically separated from the substrate, and resonance with a high Q value can be obtained. Further, since the entire lower surface of the piezoelectric thin film 15 is supported by the acoustic multilayer film 13, a stable operation can be obtained.

A compressive stress film 17 is formed on the lower surface of the substrate 12. The compressive stress film 17 has the same configuration as the acoustic multilayer film 13 and is formed symmetrically with the substrate 12 interposed therebetween. In other words, the compressive stress film 17, a first layer _17a 1 _{~17a n} having the same dimensions of the same material as the high acoustic impedance layers _13a 1 _{~13a n,} the same as the low acoustic impedance layer _13b 1 13 b _n material The second layers 17b _{1 to} 17b _n having the same dimensions are stacked alternately with the same number of layers as the acoustic multilayer film 13.

  The compressive stress film 17 can be formed under the same film formation conditions by using the same manufacturing process and equipment as those of the acoustic multilayer film 13 with the substrate 12 turned upside down. Therefore, the piezoelectric thin film resonator 11 of the first embodiment can be easily manufactured at a relatively low cost, and its industrial practical use can be easily realized.

On the upper surface side of the substrate 12, compressive stress generated by the acoustic multilayer film 13 acts as shown by arrows in FIG. By providing the compressive stress film 17 on the lower surface side of the substrate 12, a compressive stress having the same magnitude as the compressive stress caused by the acoustic multilayer film 13 acts in the opposite direction. Although tensile stress due to the piezoelectric thin film 15 is also generated on the upper surface side of the substrate 12, it is significantly smaller than the compressive stress of the SiO ₂ thin film. Accordingly, the stress acting on both the front and back surfaces of the substrate 12 can be offset and the warpage of the substrate can be eliminated.

FIG. 2 schematically shows the configuration of a piezoelectric thin film resonator according to a second embodiment of the present invention. In the piezoelectric thin film resonator 21 of the second embodiment, as in the first embodiment, an acoustic multilayer film 23 is formed on the upper surface of a substrate 22 made of Si, quartz or glass material, and a lower electrode film is formed thereon. 24, the piezoelectric thin film 25 and the upper electrode film 26 are laminated. Acoustic multilayer film 23, AlN, etc. 1 and ～23A _n high acoustic impedance layer _23a made of ZnO, and a low acoustic impedance layer _23b 1 _{~23b n} made of SiO ₂ film are laminated alternately.

  A compressive stress film 27 having the same plane dimensions as the acoustic multilayer film 23 is formed on the lower surface of the substrate 22 symmetrically with the acoustic multilayer film 23 with the substrate interposed therebetween. The compressive stress film 27 of the second embodiment includes a single first layer 27a formed of the same material as the high acoustic impedance layer of the acoustic multilayer film 23 and a single layer formed of the same material as the low acoustic impedance layer. The second embodiment is different from the first embodiment in that it has a two-layer structure in which one second layer 27b is laminated.

Thickness Ta of the first layer 27a is, _ta 1 the thickness of each high acoustic impedance layer _23a 1 _{~23a n} respectively, ..., when the ta _n, set so that _{Ta = ta 1 + ... + ta} n To do. Thickness Tb of the second layer 27b, the thickness of each _tb 1 of each low acoustic impedance layer _23b 1 _{~23b n,} ..., when the tb _n, set so that _{Tb = tb 1 + ... + tb} n To do. By this compressive stress film 27, a compressive stress having approximately the same magnitude as the compressive stress generated by the acoustic multilayer film 23 is applied to the lower surface side of the substrate 22 in the opposite direction. Accordingly, it is possible to cancel or reduce the warpage of the substrate by canceling out the stress acting on both the front and back surfaces of the substrate 22.

  The compressive stress film 27 can be formed by using the same manufacturing process and equipment as the acoustic multilayer film 23 with the substrate 22 upside down. At this time, the film formation conditions of the first layer 27a and the second layer 27b are adjusted so that the above-described film thicknesses are obtained. In this way, the piezoelectric thin film resonator 21 of the second embodiment can also be easily manufactured at a relatively low cost, and its industrial practical use can be realized. In addition, since the compressive stress film 27 can be formed by two film forming steps, the number of steps can be reduced and the productivity is improved.

FIG. 3 schematically shows the configuration of a piezoelectric thin film resonator according to a third embodiment of the present invention. As in the first and second embodiments, the piezoelectric thin film resonator 31 of the third embodiment has an acoustic multilayer film 33 formed on the upper surface of a substrate 32 made of Si, quartz, glass, or the like, on which the acoustic multilayer film 33 is formed. A lower electrode film 34, a piezoelectric thin film 35, and an upper electrode film 36 are laminated. Acoustic multilayer 33, AlN, etc. 1 and ～33A _n high acoustic impedance layer _33a made of ZnO, and a low acoustic impedance layer _33b 1 _{~33b n} made of SiO ₂ film are laminated alternately.

  A compressive stress film 37 having the same plane dimensions as the acoustic multilayer film 33 is formed on the lower surface of the substrate 32 symmetrically with the acoustic multilayer film 33 with the substrate interposed therebetween. The compressive stress film 7 of the third embodiment is different from the first and second embodiments in that it consists of a single layer formed of a single material.

Since the compressive stress of the acoustic multilayer film 33 can be calculated in advance, the material and film thickness of the compressive stress film 37 are selected so as to have the same compressive stress as that. The material of the compressive stress film 37 may be any material as long as it has a compressive stress when it is formed on the substrate, for example, the same SiO ₂ as the low acoustic impedance layer. As a result, the stress acting on both the front and back surfaces of the substrate 32 can be offset, and the warpage of the substrate can be eliminated or reduced.

  As described above, the piezoelectric thin film resonator 31 of the third embodiment can also be manufactured at a relatively low cost and easily, and its industrial practical use can be realized. In addition, since the compressive stress film 37 can be formed in a single film formation step, the number of steps can be reduced, and higher productivity than the first and second embodiments can be obtained.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiments, and various modifications and changes can be made within the technical scope. For example, in the second embodiment, the first layer and the second layer of the compressive stress film can be laminated in reverse order on the substrate.

1 is a schematic cross-sectional view showing a first embodiment of a piezoelectric thin film resonator according to the present invention. FIG. 3 is a schematic sectional view showing a second embodiment of the piezoelectric thin film resonator according to the present invention. FIG. 4 is a schematic sectional view showing a third embodiment of the piezoelectric thin film resonator according to the present invention. FIG. 6 is a schematic cross-sectional view showing a conventional piezoelectric thin film resonator.

Explanation of symbols

1, 11, 21, 31 ... piezoelectric thin film resonator, 2,12,22,32 ... substrate, 3,13,23,33 ... acoustic _{multilayer, 3a 1 ~3a n, 13a 1} ~13a n, 23a 1 ~ 23a _{n, 33a} 1 ~33a _{n ...} high acoustic impedance _{layer, 3b 1 ~3b n, 13b 1} ~13b n, 23b 1 ~23b n, 33b 1 ~33b n ... low acoustic impedance layer, 4,14,24,34 ... lower electrode film, 5,15,25,35 ... piezoelectric thin film, 6,16,26,36 ... upper electrode film, 17, 27, 37 ... compression stress _film, 17a 1 _{~17a n,} 27a ... first layer, 17b _{1 to} 17b _n , 27b... Second layer.

Claims (4)

  An acoustic multilayer film in which a plurality of high acoustic impedance layers and low acoustic impedance layers are alternately laminated on one surface of a substrate, a lower electrode formed on the acoustic multilayer film, and a piezoelectric formed on the lower electrode A thin film, an upper electrode formed on the piezoelectric thin film, and a compressive stress film formed on the other surface of the substrate, wherein the compressive stress film is generated by the acoustic multilayer film on the substrate; A piezoelectric thin film resonator formed to have a compressive stress of a magnitude that cancels out.   The compressive stress film has the same film configuration, the same plane and the same thickness as the acoustic multilayer film, and is formed symmetrically with the acoustic multilayer film with the substrate interposed therebetween. 2. A piezoelectric thin film resonator according to 1.   The compressive stress film has the same planar dimensions as the acoustic multilayer film, is disposed symmetrically with the acoustic multilayer film across the substrate, is made of the same material as the high acoustic impedance layer, and each of the high acoustic impedance layers A single first layer having a total film thickness and a single second layer made of the same material as the low acoustic impedance layer and having a total film thickness of the low acoustic impedance layers The piezoelectric thin film resonator according to claim 1, wherein the piezoelectric thin film resonator is formed by laminating a layer.   The compressive stress film is formed of a single layer having the same plane dimension as the acoustic multilayer film and made of a single material, and is disposed symmetrically with the acoustic multilayer film with the substrate interposed therebetween. The piezoelectric thin film resonator according to claim 1.

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