Patent Publication Number: US-2016226464-A1

Title: Acoustic wave elements, and duplexers and electronic devices using same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application Serial No. PCT/JP2014/004929, filed Sep. 26, 2014, which claims priority to Japanese Application No. JP2013-211536, filed Oct. 9, 2013. 
    
    
     BACKGROUND 
       FIG. 7  is a cross-sectional schematic view of a conventional acoustic wave element  1 . The acoustic wave element  1  includes a piezoelectric body  2 , an oxide layer  130  disposed on the piezoelectric body  2 , an electrode  3  disposed on the oxide layer  130 , and a protection film  4  disposed on the oxide layer  130  to cover the electrode  3 . 
     For example, PCT publication WO2005/034347(A1) discloses a conventional acoustic wave element similar to the acoustic wave element  1 . 
     SUMMARY OF INVENTION 
     The present invention relates to an acoustic wave element, a duplexer, and an electronic device using the same. 
     An acoustic wave element includes a piezoelectric body, an aluminum oxide layer disposed on the piezoelectric body, an electrode disposed on the aluminum oxide layer, and a protection film disposed on the aluminum oxide layer to cover the electrode. The piezoelectric body is formed of a piezoelectric material based on lithium niobate having Euler angles (φ, θ, ψ). The aluminum oxide layer is formed of Al 2 O 3 . The electrode is configured to excite a main acoustic wave having a wavelength λ, and the protection film has a film thickness greater than 0.27λ. The Euler angles satisfy either ψ≦−2φ−3° or −2φ+3°≦ψ and both of −100°≦θ≦−60° and 2φ−2°≦ψ≦2φ+2°, and the acoustic wave element may suppress an unnecessary spurious signal to be generated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional schematic view of an acoustic wave element according to an embodiment. 
         FIG. 2  is a characteristic diagram of a comparative sample of the acoustic wave element. 
         FIG. 3  is a characteristic diagram of the acoustic wave element according to the embodiment. 
         FIG. 4  shows Euler angles of a piezoelectric body of the acoustic wave element according to the embodiment. 
         FIG. 5  is a block diagram of a duplexer mounted with the acoustic wave element according to the embodiment. 
         FIG. 6  is a block diagram of an electronic device mounted with the acoustic wave element according to the embodiment. 
         FIG. 7  is a cross-sectional schematic view of a conventional acoustic wave element. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional schematic view of an acoustic wave element  5  according to an embodiment. The acoustic wave element  5  includes a piezoelectric body  6 , an aluminum oxide layer  30  disposed on a surface  6 A of the piezoelectric body  6 , an electrode  7  disposed on a surface  30 A of the aluminum oxide layer  30 , and a protection film  8  disposed on the surface  30 A of the aluminum oxide layer  30  to cover the electrode  7 . A surface  30 B opposite to the surface  30 A of the aluminum oxide layer  30  abuts the surface  6 A of the piezoelectric body  6 . The protection film  8  has a surface  8 B abutting the surface  6 A of the piezoelectric body  6  and a surface  8 A opposite to the surface  8 B. The piezoelectric body  6  is a piezoelectric substrate formed of a piezoelectric material based on lithium niobate (LiNbO 3 ) and the Euler angles (φ, θ, ψ) of the piezoelectric body  6  may satisfy either ψ≦−2φ−3° or −2φ+3 °≦ψ and both of −100°≦θ≦−60° and 2φ−2°≦ψ≦2φ+2°. 
     The aluminum oxide layer  30  is formed of Al 2 O 3 , in particular of sapphire. The aluminum oxide layer  30  has the film thickness of 0.001λ or greater and 0.02λ or less. 
     The electrode  7  includes an elemental metal such as aluminum, copper, silver, gold, titanium, tungsten, molybdenum, platinum or chromium, or an alloy typically containing these elemental metals, or a laminated structure of these elemental metals. The electrode forms an IDT (Inter-Digital Transducer) electrode for exciting a main acoustic wave composed of a SH (Shear Horizontal) wave having a wavelength k, and the electrode is comb-shaped according to the embodiment. The total film thickness of the electrode  7  may generally range from 0.01λ to 0.15λ depending on the density of the electrode. 
     The protection film  8  is formed, for example, of a silicon oxide (SiO 2 ) film. In this case, the protection film  8  has a temperature characteristic reverse to that of the piezoelectric body  6  such that increasing the film thickness T 8  to be above 0.27λ may improve the frequency temperature characteristic of the acoustic wave element  5 . The protection film  8  may be formed of a material other than the silicon oxide film and can preferably protect the electrode  7  from the external environment. The film thickness T 8  of the protection film  8  is a film thickness of a portion where the electrode  7  is not formed, and corresponds to a distance from the surface  6 A of the piezoelectric body  6  that interfaces the piezoelectric body  6  with the protection film  8  to the surface  8 A of the protection film  8 . The wavelength λ of the main acoustic wave is twice the average pitch of electrode fingers of the electrode  7  that is comb-shaped. 
     An exemplary sample and a comparative sample were manufactured for the acoustic wave element  5  according to the embodiment. The comparative sample has a structure similar to that of the conventional acoustic wave element  1  shown in  FIG. 7 . In the comparative sample, the piezoelectric body  2  is formed of a piezoelectric material based on lithium niobate having the Euler angles (0°, −90°, 0°). The oxide layer  130  is formed of Al 2 O 3 . The electrode  3  is formed of metal such as copper and excites a main acoustic wave having the wavelength λ. The protection film  4  is formed of silicon oxide (SiO 2 ). 
     In particular, the oxide layer  130  is formed of a sapphire having the film thickness of 0.006λ. The electrode  3  has the film thickness of 0.062λ. The protection film  4  has the film thickness of 0.35λ. 
       FIG. 2  is a characteristic diagram of the comparative sample of the acoustic wave element. In  FIG. 2 , the vertical axis represents normalized admittance to the matched value, whereas the horizontal axis represents frequency. In the comparative sample of the acoustic wave element, when the film thickness of the protection film  4  is set, for example, to 0.35λ to improve the temperature characteristic of the acoustic wave element formed of silicon oxide, an unnecessary spurious signal S 1  is generated at a frequency approximately 1.3 times the resonant frequency as shown in  FIG. 2 . Transverse waves having various acoustic velocities are generated in the comparative sample of the acoustic wave element. The unnecessary spurious signal S 1  may be attributable to the fastest transverse wave of the transverse waves generated in the acoustic wave element. 
     The aforementioned fastest transverse wave may degrade the characteristic quality of a filter or a duplexer to which the acoustic wave element of the comparative sample is applied. In order to suppress the unnecessary spurious signal S 1 , the Euler angles (φ, θ, ψ) of the piezoelectric body  2  are changed via the angles φ and ψ. The unnecessary spurious signal S 1  caused by a faster transverse wave can be suppressed no matter whether the angle φ or ψ is changed. This, in turn, would conversely generate another unnecessary spurious signal S 1  different from the aforementioned one at a frequency band slightly lower than the resonant frequency. This unnecessary spurious signal S 1  may be attributable to a Rayleigh wave. 
     When the film thickness of the protection film  8  is set thicker than 0.27λ to improve the frequency temperature characteristic of the acoustic wave element  5  according to the embodiment, an unnecessary spurious signal S 1  caused by the Rayleigh wave can be suppressed while another unnecessary spurious signal S 1  generated around the frequency by the faster transverse wave can be suppressed by setting the angles φ and ψ of the Euler angles (φ, θ, ψ) of the piezoelectric body  6  greater than a predetermined angle and changing the angle φ from 0° to follow ψ=2φ to some extent. 
       FIG. 3  is a characteristic diagram of the acoustic wave element  5 . In  FIG. 3 , the vertical axis represents normalized admittance (dB), which is a ratio of an admittance value to a value matched during the resonance, whereas the horizontal axis represents frequency (MHz). In the sample of the acoustic wave element  5 , the piezoelectric body  6  is formed of a lithium niobate having the Euler angles (−3°, −90°, −3°). The aluminum oxide layer  30  is formed of a sapphire having the film thickness of 0.006λ. The electrode  7  is formed of copper having the film thickness of 0.062λ. The protection film  8  is formed of a silicon oxide (SiO 2 ) having the film thickness of 0.35λ. As shown in  FIG. 3 , the acoustic wave element  5  according to the embodiment can suppress an unnecessary spurious signal S 1  caused by the Rayleigh wave in the comparative sample as shown in  FIG. 2 , while suppressing another unnecessary spurious signal S 1  generated around a frequency band by the faster transverse wave. 
     The hatched lines of  FIG. 4  show ranges R 1  and R 2  that the angles y and w of the Euler angles (φ, θ, ψ) can take for the piezoelectric body  6  formed of a piezoelectric material based on lithium niobate. It is to be appreciated that: the angle θ satisfies −100°≦θ≦−60°; the film thickness T 8  of the protection film  8  is greater than 0.27λ; and the electrode  7  has a normalized film thickness of 0.062λ and is formed of copper. The line L 1  representing the relationship of ψ=2φ shown in  FIG. 4  can be construed as representing the relationship between the angles y and ψ especially when the spurious signal S 1  ( FIG. 2 ) caused by the Rayleigh wave is suppressed. As shown in  FIG. 2 , the spurious signal S 1  caused by the Rayleigh wave can be suppressed within the range of either ψ≦−2φ−3° or −2φ+3°≦ψ and within the range of the angle ψ of ±2° centered to the line L 1 , i.e., the range of 2φ−2°≦ψ≦2φ+2°. 
       FIG. 5  is a block diagram of a duplexer  33  mounted with the acoustic wave element  5  according to the embodiment. The duplexer  33  includes a filter  31 , a filter  32  having a passband higher than that of the filter  31 , a terminal  36  connected to the filter  31 , a terminal  35  connected to the filter  32 , and a terminal  34  connected between the filters  31  and  32 . The acoustic wave element  5  according to the embodiment is preferably used in the filter  31 . There is a possibility that a spurious signal caused by a faster transverse wave in the filter  31  may degrade the characteristic of the filter  32  in a higher passband. Accordingly, configuring the filter  31  by the acoustic wave element  5  according to the embodiment can prevent the characteristic degradation of the filter  32 . If the filter  31  is a transmission filter and the filter  32  is a reception filter, the terminal  36  would be an input terminal connected to the transmitter, the terminal  35  would be an output terminal connected to the receiver, and the terminal  34  would be an antenna terminal connected to an antenna. 
     The acoustic wave element  5  according to the embodiment may be applied to a resonator, and may be applied to a filter such as a ladder-type filter or a DMS filter. 
       FIG. 6  is a block diagram of an electronic device  40  mounted with the acoustic wave element  5  according to the embodiment. The electronic device  40  includes a filter  37 , a semiconductor integrated circuit element  38  connected to the filter  37 , and a reproduction device  39  connected to the semiconductor integrated circuit element  38 . The filter  37  is configured by the acoustic wave element  5  according to the embodiment. The acoustic wave element  5  may improve the telecommunications quality in the aforementioned resonator, filter, and electronic device  40 . 
     The acoustic wave element of the present invention can suppress the generation of an unnecessary spurious signal and is applicable to an electronic device such as a mobile phone.