# Patent

## Title
Surface acoustic wave device including an IDT defined by a metal filled in grooves in a piezoelectric substrate

## Abstract

A surface acoustic wave device includes a LiTaO3 substrate, and an IDT having a high reflection coefficient, which has a relatively high electromechanical coupling coefficient k2, and which can obtain superior resonance characteristics and/or filter characteristics. In a surface acoustic wave device in which a plurality of grooves is provided in an upper surface of a LiTaO3 substrate and an IDT having a plurality of electrode fingers made of a metal filled in the plurality of grooves, the following equation is satisfied:
(ρ3×C44)1/2>1.95×1011, where ρ represents the density of the metal defining the IDT, and C44 represents the stiffness thereof.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device which is used as a resonator or a bandpass filter. Particularly, the present invention relates to a surface acoustic wave device including an IDT that is defined by a metal that is filled in grooves provided in a piezoelectric substrate.
2. Description of the Related Art
Surface acoustic wave devices have been widely used as a resonator and a bandpass filter. For example, WO2006/011417A1 discloses a surface acoustic wave device 1001 having a cross-sectional structure as schematically shown in FIG. 30. In the surface acoustic wave device 1001, a plurality of grooves 1002b is formed in an upper surface 1002a of a LiTaO3 substrate 1002. A metal is filled in the plurality of grooves 1002b, and an IDT 1003 having a plurality of electrode fingers is formed from the metal filled in the plurality of grooves 1002b. A SiO2 film 1004 is laminated so as to cover the upper surface 1002a of the LiTaO3 substrate 1002. Since the LiTaO3 substrate 1002 has a negative temperature coefficient of frequency TCF, the SiO2 film 1004 having a positive temperature coefficient of frequency TCF is laminated, and as a result, the absolute value of the temperature coefficient of frequency TCF of the surface acoustic wave device 1001 is decreased. In addition, since the IDT is formed using the metal filled in the plurality of grooves 1002b, a high reflection coefficient can be obtained in the IDT. In particular, when the wavelength of a surface acoustic wave is represented by λ, and when the thickness of Al filled in the grooves 1002b, that is, the thickness of the IDT made of Al, is set to 0.04λ, the reflection coefficient per electrode finger is 0.05, and a higher reflection coefficient can be obtained as the electrode thickness is increased.
Japanese Unexamined Patent Application Publication No. 2004-112748 discloses a surface acoustic wave device shown in FIG. 31. In a surface acoustic wave device 1101, an IDT 1103 is provided on a piezoelectric substrate 1102 made of LiTaO3 or LiNbO3. In addition, a protective film 1104 is arranged so as to cover the IDT 1103. Furthermore, in a remaining region other than a region in which the IDT 1103 and the protective film 1104 are provided, a first insulating layer 1105 made of SiO2 is formed to have a thickness equal to that of a laminated metal film in which the IDT 1103 and the protective film 1104 are laminated to each other. In addition, a second insulating layer 1106 made of SiO2 is laminated so as to cover the first insulating layer 1105. When a metal having a higher density than that of Al is used for the IDT 1103, the absolute value of the reflection coefficient can be increased, and undesirable ripples can be suppressed.
In the surface acoustic wave device 1001 described in WO2006/011417A1, as the thickness of the IDT made of Al is increased, the absolute value of the reflection coefficient can be increased. However, the inventors of the present invention discovered that superior resonance characteristics cannot be obtained by only increasing the absolute value of the reflection coefficient. That is, it was found that in the surface acoustic wave device described in WO2006/011417A1, although the absolute value of the reflection coefficient can be increased by increasing the thickness of the electrode made of Al, since the sign of the reflection coefficient is negative, many ripples are generated in a passband, and as a result, superior resonance characteristics cannot be obtained.
In WO2006/011417A1, the relationship between the thickness of the IDT and the reflection coefficient is described only with respect to the case in which the IDT made of Al is provided on the LiTaO3 substrate. In addition, paragraph [0129] of WO2006/011417A1 discloses that another metal, such as Au, may also be used to form the IDT. However, this description only discloses that when a LiNbO3 substrate is used, another metal, such as Au, may be used. That is, WO2006/011417A1 does not disclose that when a LiTaO3 substrate is used, the IDT may be formed using a metal other than Al.
On the other hand, Japanese Unexamined Patent Application Publication No. 2004-112748 discloses as described above that when the IDT made of a metal having a higher density than that of Al is used, the absolute value of the reflection coefficient can be increased. However, Japanese Unexamined Patent Application Publication No. 2004-112748 only discloses that ripples can be suppressed by increasing the absolute value of the reflection coefficient, and the sign of the reflection coefficient has not been considered. In addition, in the surface acoustic wave device described in Japanese Unexamined Patent Application Publication No. 2004-112748, there has been a problem of a low electromechanical coupling coefficient k2.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device in which a LiTaO3 substrate is used as a piezoelectric substrate, the reflection coefficient of an IDT is not only high, but the sign thereof is positive, the electromechanical coupling coefficient k2 is also high, and superior resonance characteristics and/or filter characteristics can be obtained.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate composed of LiTaO3 and including a plurality of grooves provided in the upper surface thereof, and an IDT including a plurality of electrode fingers which are defined by a metal filled in the plurality of grooves in the upper surface of the piezoelectric substrate, wherein when the density of the metal is represented by ρ (kg/m2), and the stiffness thereof is represented by C44 (N/m2), the metal satisfies the following equation (1).

(ρ3×C44)1/2>1.95×1011  Equation (1)

As the above metal material, at least one metal selected from the group consisting of Mo, Ta, Au, Pt, and W or an alloy primarily containing the above metal, for example, may preferably be used. In this case, the reflection coefficient of the IDT has a positive value with certainly, the absolute value thereof is sufficiently increased, and a high electromechanical coupling coefficient k2 is achieved.
The metal defining the IDT is not necessarily a single metal. According to a preferred embodiment of the present invention, the IDT may preferably be composed of a laminated metal film including metal films laminated to each other, and each of the metal films is composed of a metal selected from the group consisting of Al, Au, Ta, W, Pt, Cu, Ni, and Mo, for example. In this case, the above equation (1) is preferably satisfied when the quotient obtained by dividing the sum of the products of thicknesses T of the metal films and the densities of the metals forming the metal films by the sum of the thicknesses T of the metal films is defined as an average density and is represented by the above ρ (kg/m3), and when an average stiffness, which is the quotient obtained by dividing the sum of the products of the thicknesses T of the metal films and the stiffnesses of the metals forming the metal films by the sum of the thicknesses T of the metal films, and the metal films defining the laminated metal film, is represented by the above C44 (N/m2).
In addition, the surface acoustic wave device according to a preferred embodiment of the present invention preferably further includes a dielectric film which includes SiO2 or an inorganic material primarily composed of SiO2 and which covers the IDT and the piezoelectric substrate. Since the temperature coefficient of frequency of the dielectric film including SiO2 or an inorganic material primarily composed of SiO2 has a positive value, and the temperature coefficient of frequency of LiTaO3 has a negative value, a surface acoustic wave device having a small absolute value of the temperature coefficient of frequency TCF is provided.
In the surface acoustic wave device according to a preferred embodiment of the present invention, when the wavelength of a surface acoustic wave is represented by λ, a normalized thickness of the dielectric film which is normalized by λ is preferably set to about 0.05 to about 0.3, for example. With this configuration, a positive and a higher reflection coefficient and a high electromechanical coupling coefficient k2 are obtained.
In addition, the cut angle of the LiTaO3 substrate is preferably expressed in Euler angles (0°±10°, 70° ̃180°, 0°±10°), for example. With this configuration, by using a Rayleigh wave and an SH wave, a surface acoustic wave device having a high and a positive reflection coefficient and further having a high electromechanical coupling coefficient k2 is provided.
According to a preferred embodiment of the present invention, in an IDT having a plurality of electrode fingers defined by a metal filled in grooves provided in an upper surface of a LiTaO3 substrate, the metal satisfies the above equation (1). Thus, the absolute value of the reflection coefficient of the IDT is not only high, but also the reflection coefficient has a positive value, and a high electromechanical coupling coefficient k2 can be obtained. Accordingly, in the resonance characteristics and/or filter characteristics of the surface acoustic wave device, ripples are effectively suppressed and prevented from being generated in a passband, so that superior frequency characteristics can be obtained.
Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a partially schematic front cross-sectional view showing a portion of a surface acoustic wave device according to a preferred embodiment of the present invention, and
FIG. 1B is a schematic plan view of the surface acoustic wave device.
FIG. 2 is a view showing impedance-phase characteristics and frequency-phase characteristics according to the preferred embodiment shown in FIG. 1 in which the reflection coefficient of an IDT is about 0.13.
FIG. 3 is a view showing impedance-frequency characteristics and phase-frequency characteristics of a surface acoustic wave device prepared for comparison purpose in which a reflection coefficient γ of an IDT is about −0.13.
FIG. 4 is a view showing the impedance-phase characteristics and the frequency-phase characteristics according to the preferred embodiment shown in FIGS. 1A and 1B in which the reflection coefficient of the IDT is about 0.09.
FIG. 5 is a view showing the impedance-frequency characteristics and the phase-frequency characteristics of a surface acoustic wave device prepared for comparison purpose in which the reflection coefficient γ of an IDT is about −0.09.
FIG. 6 is a view showing the relationship between a normalized thickness H/λ of an IDT and an electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.2, and IDTs made of various metals are each provided on upper surface of a LiTaO3 substrate having Euler angles (0°, 126°, 0°).
FIG. 7 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.25, and IDTs made of various metals are each provided on upper surface of a LiTaO3 substrate having Euler angles (0°, 126°, 0°).
FIG. 8 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.3, and IDTs made of various metals are each provided on upper surface of a LiTaO3 substrate having Euler angles (0°, 126°, 0°).
FIG. 9 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.05, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 10 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.1, and IDTs are defined by filling various metals in grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 11 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.15, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 12 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.25, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 13 is a view showing the relationship between the normalized thickness H/λ of an IDT and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.3, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 14 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.05, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates.
FIG. 15 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.1, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 16 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.15, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 17 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.25, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 18 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which the normalized thicknesses of SiO2 films are each about 0.3, and IDTs are defined by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 19 is a view showing the relationship between the normalized thickness of a SiO2 film and the reflection coefficient in each of surface acoustic wave devices in which IDTs formed to have a normalized thickness of about 0.005 by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 20 is a view showing the relationship between the normalized thickness of a SiO2 film and the reflection coefficient in each of surface acoustic wave devices in which IDTs are formed to have a normalized thickness of about 0.03 by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 21 is a view showing the relationship between the normalized thickness of a SiO2 film and the reflection coefficient in each of surface acoustic wave devices in which IDTs are formed to have a normalized thickness of about 0.05 by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°).
FIG. 22 is a view showing the relationship between the normalized thickness of a SiO2 film and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which IDTs are formed to have a normalized thickness of about 0.005 by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°) and in which SiO2 films are further laminated.
FIG. 23 is a view showing the relationship between the normalized thickness of a SiO2 film and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which IDTs are formed to have a normalized thickness of about 0.03 by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°) and in which SiO2 films are further laminated.
FIG. 24 is a view showing the relationship between the normalized thickness of a SiO2 film and the electromechanical coupling coefficient k2 of each of surface acoustic wave devices in which IDTs are formed to have a normalized thickness of about 0.05 by filling various metals in upper surface grooves of LiTaO3 substrates each having Euler angles (0°, 126°, 0°) and in which SiO2 films are further laminated.
FIG. 25 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which LiTaO3 substrates each having Euler angles (0°, 90°, 0°) are used, various metals are filled in upper surface grooves of the LiTaO3 substrates, and SiO2 films are each further formed to have a normalized thickness of 0.25.
FIG. 26 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which LiTaO3 substrates each having Euler angles (0°, 110°, 0°) are used, various metals are filled in upper surface grooves of the LiTaO3 substrates, and SiO2 films are each further formed to have a normalized thickness of about 0.25.
FIG. 27 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which LiTaO3 substrates each having Euler angles (0°, 150°, 0°) are used, various metals are filled in upper surface grooves of the LiTaO3 substrates, and SiO2 films are each further formed to have a normalized thickness of about 0.25.
FIG. 28 is a view showing the relationship between the normalized thickness H/λ of an IDT and the reflection coefficient in each of surface acoustic wave devices in which LiTaO3 substrates each having Euler angles (0°, 170°, 0°) are used, various metals are filled in upper surface grooves of the LiTaO3 substrates, and SiO2 films are each further formed to have a normalized thickness of about 0.25.
FIG. 29 is a view showing various metal elements and A values shown in Table 1.
FIG. 30 is a schematic front cross-sectional view illustrating one example of a conventional surface acoustic wave device.
FIG. 31 is a partially cut-off front cross-sectional view showing another example of a conventional surface acoustic wave device.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings so as to disclose the present invention.
FIG. 1A is a partial schematic front cross-sectional view showing a portion at which an IDT of a surface acoustic wave device according to a first preferred embodiment of the present invention is provided, and FIG. 1B is a schematic plan view of the surface acoustic wave device.
As shown in FIG. 1A, a surface acoustic wave device 1 includes a LiTaO3 substrate 2. In an upper surface 2a of the LiTaO3 substrate 2, a plurality of grooves 2b is provided. By filling a metal in the plurality of grooves 2b, an IDT 3 having a plurality of electrode fingers is formed. An upper surface of the IDT 3 and the upper surface 2a of the LiTaO3 substrate 2 are flush or substantially flush with one another.
A SiO2 film 4 is arranged so as to cover the upper surface 2a and the IDT 3.
As shown in FIG. 1B, the surface acoustic wave device 1 preferably is a one-port type surface acoustic wave resonator which includes the IDT 3 and first and second reflectors 5 and 6 disposed at both sides of the IDT 3 in a surface acoustic wave propagation direction. In addition, the reflectors 5 and 6 are preferably grating reflectors in which two ends of the electrode fingers are short-circuited.
As in the case of the IDT 3, the reflectors 5 and 6 are each formed such that the same metal as that used for the IDT is filled in grooves provided in the upper surface 2a of the LiTaO3 substrate 2. Thus, in the reflectors 5 and 6, the electrode surface and the upper surface 2a of the LiTaO3 substrate 2 are flush or substantially flush with one another. Accordingly, the upper surface of the SiO2 film 4 is substantially planarized over the entire surface acoustic wave device 1.
Although the temperature coefficient of frequency TCF of the LiTaO3 substrate 2 has a negative value, the temperature coefficient of frequency TCF of the SiO2 film 4 has a positive value, and thus, the absolute value of the temperature coefficient of frequency TCF is decreased overall. As a result, in the surface acoustic wave device 1, changes in frequency characteristics with changes in temperature are very small.
In the present preferred embodiment, the following equation (1) is preferably satisfied when the density of a metal forming the IDT is represented by ρ (kg/m3), and the stiffness is represented by C44 (N/m2):

(ρ3×C44)1/2>1.95×1011  Equation (1)

As a result, in the surface acoustic wave device 1 according to the present preferred embodiment, the absolute value of the reflection coefficient of the IDT 3 is not only increased, but also the reflection coefficient has a positive value, and a high electromechanical coupling coefficient k2 can be obtained, such that superior resonance characteristics having small ripples can be obtained.
In surface acoustic wave devices, although it has been understood that the absolute value of the reflection coefficient of an IDT is preferably increased, attention has not been paid to the polarity of the reflection coefficient. However, the inventors of the present invention discovered that when the polarity of the reflection coefficient has a negative value, ripples are generated, and superior resonance characteristics and/or filter characteristics cannot be obtained. Hereinafter, this finding will be described with reference to particular experimental examples.
FIG. 2 is a view showing impedance-frequency characteristics and phase-frequency characteristics obtained when a reflection coefficient γ of the IDT is set to about 0.13. In this case, the grooves 2b having a depth of about 0.05λ were formed in a LiTaO3 substrate of Euler angles (0°, 126°, 0°) to have a groove width of about 0.5 μm, for example. Gold (Au) was used as an electrode material that is filled in the grooves, so that the IDT 3 was formed in which the number of pairs was 100 and the intersection width was about 30 μm. In addition, the reflectors 5 and 6 were formed so that the width of the electrode finger and the electrode thickness were similar to those of the IDT 3, and the number of electrode fingers of each reflector was set to 100. In the surface acoustic wave device 1, the reflection coefficient of the electrode finger of the IDT was about 0.13.
In addition, for comparison, a surface acoustic wave device was formed in which the metal filled in the grooves of the LiTaO3 substrate was changed to Al, the reflection coefficient λ of the IDT was set to about −0.13, and the reflection coefficient λ of each of the reflectors 5 and 6 was also set to about −0.13. The impedance-frequency characteristics and the phase-frequency characteristics of the surface acoustic wave device of a first comparative example are shown in FIG. 3.
As shown by comparing FIG. 2 to FIG. 3, although many ripples are generated in a passband in the surface acoustic wave device of the first comparative example, similar ripples are not generated in the present preferred embodiment, and thus, superior resonance characteristics are obtained.
In addition, a surface acoustic wave device was formed in which the thickness of the Au was changed from about 0.05λ to about 0.3λ, and the reflection coefficient γ was about 0.09, and the impedance-frequency characteristics and the phase-frequency characteristics were measured. The results are shown in FIG. 4.
In addition, for comparison, a surface acoustic wave device was formed as a second comparative example in a manner similar to that described above except that the metal forming the IDT and the reflectors was changed to Al and the reflection coefficient γ was set to about −0.09. The impedance-frequency characteristics and the phase-frequency characteristics of the surface acoustic wave device of the second comparative example are shown in FIG. 5.
As shown in FIG. 5, although many ripples are generated in a passband, according to the present preferred embodiment shown in FIG. 4, the ripples are not generated.
Accordingly, as shown by comparing FIGS. 2 and 3 and by comparing FIGS. 4 and 5, when the polarity of the reflection coefficient of the IDT 3 is negative, a plurality of undesirable ripples is generated in a passband. However, according to the present preferred embodiment in which the reflection coefficient γ has a positive value, a plurality of ripples is not generated, and thus, superior resonance characteristics are obtained.
In addition, FIGS. 6 to 8 are views showing the relationship between the normalized thickness of an IDT and an electromechanical coupling coefficient k2 of a surface acoustic wave device of a third comparative example in which as in the conventional surface acoustic wave device 1101 shown in FIG. 31, an IDT made of a metal is provided on an upper surface of the LiTaO3 substrate 1002, and a SiO2 film having a flat upper surface is arranged so as to cover the IDT. In this case, the Euler angles of LiTaO3 are (0°, 126°, 0°). In addition, the normalized thickness is a value obtained by dividing the thickness by a wavelength λ of a surface acoustic wave. FIG. 6 shows the results obtained when the normalized thickness of the SiO2 film is about 0.2, FIG. 7 shows the results obtained when the normalized thickness of the SiO2 film is about 0.25, and FIG. 8 shows the results obtained when the normalized thickness of the SiO2 film is about 0.3.
In addition, FIGS. 9 to 13 are views showing the relationship between the normalized thickness of an IDT and the electromechanical coupling coefficient k2 of a surface acoustic wave device having the structure in which, as in FIG. 1A, the IDT is formed by filling a metal in a plurality of grooves provided in an upper surface of a LiTaO3 substrate, and a SiO2 film is laminated on the LiTaO3 substrate. In this case, the Euler angles of the LiTaO3 substrate are (0°, 126°, 0°). FIGS. 9 to 13 show the results of the cases in which the normalized thicknesses of SiO2 are about 0.05, about 0.1, about 0.15, about 0.25, and about 0.3, respectively.
As is clear from a comparison of the results shown in FIGS. 6 to 8 and those shown in FIGS. 9 to 13, when the metal is filled in the plurality of grooves provided in the upper surface of the LiTaO3 substrate, that is, in the cases shown in FIGS. 9 to 13, the electromechanical coupling coefficient k2 is increased. In particular, as shown in FIGS. 9 to 13 which are obtained when the IDT is formed by filling the metal in the grooves provided in the upper surface of the LiTaO3 substrate, even if the thickness of the SiO2 film is increased, a decrease in electromechanical coupling coefficient k2 is small.
Accordingly, in order to obtain a higher electromechanical coupling coefficient k2, an IDT which includes a plurality of electrode fingers that are formed by filling a metal material in a plurality of grooves provided in a LiTaO3 substrate is preferable.
In addition, FIGS. 14 to 18 are views showing the relationship between a normalized thickness H/λ of an IDT and the reflection coefficient in surface acoustic wave devices in which, as in FIGS. 9 to 13, IDTs are provided which have a plurality of finger electrodes formed by filling various metals in grooves provided in upper surfaces of LiTaO3 substrates having Euler angles (0°, 126°, 0°), and SiO2 films having various normalized thicknesses are laminated on the upper surfaces. FIGS. 14 to 18 show the results of the cases in which the normalized thicknesses of the SiO2 films are about 0.05, about 0.1, about 0.15, about 0.25, and about 0.3, respectively.
As shown in FIGS. 14 to 18, when Al is used for the IDT, and when Ag or Cu is used for the IDT, the reflection coefficient tends to have a negative value, and that even if it has a positive value, the absolute value thereof is relatively small. Furthermore, when the normalized thickness of the IDT is increased, and Al, Ag, or Cu is used for the IDT, the reflection coefficient is shifted from a positive value side to a negative value side.
On the other hand, when a metal which is heavier than Ag is used, regardless of the thickness of the SiO2 film and the thickness of the IDT, the reflection coefficient has a positive value, and in addition, as the normalized thickness of the IDT is increased, the absolute value of the reflection coefficient can be increased.
Thus, from the results shown in FIGS. 14 to 18, a metal selected from the group consisting of Mo, Ta, Au, Pt, and W, for example, which are each heavier than Ag, is preferably used. That is, when at least one metal selected from the group consisting of the metals mentioned above or an alloy primarily composed of the metals mentioned above is used, the reflection coefficient has a positive value, and in addition, the absolute value of the reflection coefficient can be increased by increasing the normalized thickness of the IDT.
In addition, FIGS. 19 to 21 are views showing the relationship between the normalized thickness of a SiO2 film and the reflection coefficient in configurations in which IDTs are formed by filling various metals in grooves provided in upper surfaces of LiTaO3 substrates each having Euler angles (0°, 126°, 0°) and SiO2 films are further laminated thereon. FIGS. 19 to 21 show the results obtained when the normalized thicknesses of the IDTs are about 0.005, about 0.03, and about 0.05, respectively.
As shown in FIGS. 19 to 21, in each case in which the normalized thickness of the IDT is about 0.005, about 0.03, or about 0.05, even if the thickness of the SiO2 film is changed, as long as Al, Cu, or Ag is used, the reflection coefficient is small when it has a positive value and may have a negative value in some cases. On the other hand, when the metals described above which are heavier than Ag are used, even if the thickness of the SiO2 film is changed, the reflection coefficient has a positive value, and even if the thickness of the SiO2 film is increased, the reflection coefficient is not substantially decreased.
Accordingly, as shown in FIGS. 14 to 18 and FIGS. 19 to 21, even if the thickness of the SiO2 film is changed, when the IDT is made using a metal that is heavier than Ag, the reflection coefficient has a positive value, and a reflection coefficient having a high absolute value is obtained.
In addition, FIGS. 22 to 24 are views showing the relationship between the normalized thickness of a SiO2 film and the electromechanical coupling coefficient of the configurations in which IDTs are made by filling various metals in grooves provided in upper surfaces of LiTaO3 substrates each having Euler angles (0°, 126°, 0°), and SiO2 films are further laminated thereof. FIGS. 22 to 24 show the results obtained when the normalized thicknesses of the IDTs are about 0.005, about 0.03, and about 0.05, respectively. In the configuration in which a SiO2 film is provided, the electromechanical coupling coefficient k2 usually tends to decrease due to the lamination of the SiO2 film. However, in the configuration described above in which the IDT is made using a metal heavier than Ag, as shown in FIGS. 22 to 24, the electromechanical coupling coefficient k2 is not likely to decrease. That is, a sufficiently high electromechanical coupling coefficient k2 can be obtained.
FIG. 25 is a view showing changes in the reflection coefficient with changes in the normalized thickness H/λ of IDTs that are made of various metals of surface acoustic wave devices in which various metals are filled in grooves provided in upper surfaces of LiTaO3 substrates, and SiO2 films are further laminated thereon except that the Euler angles are changed to (0°, 90°, 0°). The normalized thickness of the SiO2 film was set to about 0.25.
As shown in FIG. 25, even if the Euler angles are changed to (0°, 90°, 0°), as in the results shown in FIGS. 6 to 24, when a metal heavier than Ag is used, the reflection coefficient has a positive value, and furthermore, when the normalized thickness H/λ of the IDT is increased, the absolute value of the reflection coefficient is increased.
Accordingly, as in the case in which the Euler angles are (0°, 126°, 0°), when the Euler angles are (0°, 90°, 0°), the same or similar results as those in the preferred embodiments described above can also be obtained.
In addition, FIGS. 26 to 28 are views showing changes in the reflection coefficient with changes in the normalized thickness H/λ of an IDT which are obtained when LiTaO3 substrates having Euler angles (0°, 110°, 0°), (0°, 150°, 0°), and (0°, 170°, 0°) are used. The normalized thickness of the SiO2 film was set to about 0.25.
As shown in FIGS. 26 to 28, as in the cases in which the Euler angles are (0°, 126°, 0°) and (0°, 90°, 0°), when the Euler angles are (0°, 110°, 0°), (0°, 150°, 0°), and (0°, 170°, 0°), when a metal heavier than Ag is used, the reflection coefficient has a positive value, and the absolute value of the reflection coefficient is increased as the thickness of the IDT is increased.
That is, when the IDT is formed by filling a metal heavier than Ag in grooves provided in a LiTaO3 substrate without depending on the Euler angles, as in the preferred embodiments described above, the reflection coefficient has a positive value, the absolute value thereof is increased, and the electromechanical coupling coefficient k2 can be increased.
In addition, in the preferred embodiments described above, the reason that the metal is selected so as to satisfy the equation (1) is as follows. That is, in order to obtain the advantages described above, a metal that is merely heavier than Ag may not be suitable for use, however, a metal that satisfies the above equation (1) is preferably used. That is, as shown in FIGS. 6 to 24, in the case of Mo having a density of 10,219 kg/m3 which is less than the density of 10,500 kg/m3 of Ag, an advantageous result can also be obtained. That is, when Mo is used, the absolute value of the reflection coefficient is higher than the reflection coefficient that is obtained by using Ag, and a positive reflection coefficient can also be obtained. Thus, in addition to the density of the metal, other parameters were also investigated. As a result, since the densities of Al, Cu, Ag, Mo, Au, Ta, Pt, and W, the stiffnesses C44, and the left-hand side of equation (1), that is, A=(ρ3×C44)1/2>1.95×1011, have the relationship shown in the following Table 1 and in FIG. 29, it was discovered that the A value may be greater than 1.95×1011, that is, a metal which can satisfy the above equation (1) may be used.











TABLE 1







Density
C44
A =



kg/m3
[N/m2]
√(ρ3 × C44)





















Al
2699
2.61E+10
2.27E+10



Cu
8930
5.14E+10
1.91E+11



Ag
10500
3.29E+10
1.95E+11



Mo
10219
1.07E+11
3.38E+11



Au
19300
2.99E+10
4.64E+11



Ta
16678
8.25E+10
6.19E+11



Pt
21400
5.97E+10
7.65E+11



W
19265
1.60E+11
1.07E+12










In addition, according to preferred embodiments of the present invention, although the Euler angles (φ, θ, Ψ) of LiTaO3 are not particularly limited as described above, when a Rayleigh wave and an SH wave are used as the surface acoustic wave, it is preferable that the Euler angle φ be set in the range of 0°±10°, for example, the Euler angle θ be set in the range of 70° ̃180°, for example, and the Euler angle Ψ be set in the range of 0°±10°, for example. That is, when the Euler angles are in the range of (0°±10°, 70° ̃180°, 0°±10°), for example, a Rayleigh wave and an SH wave can preferably be used. More preferably, in the range of (0°±10°, 90° ̃180°, 0°±10°), for example, an SH wave can be preferably used.
In addition, a LSAW wave may also preferably be used, and in this case, the Euler angles may preferably be set in the range of (0°±10°, 110° ̃160°, 0°±10°), for example. In addition, in the preferred embodiments described above, although the electrode structure of the one-port type SAW resonator is shown, as an example, the surface acoustic wave device according to preferred embodiments of the present invention may be widely applied to other resonator structures or other resonator type surface acoustic wave filters.
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.

## Claims

1. A surface acoustic wave device comprising:
a piezoelectric substrate composed of a LiTaO3 substrate and including a plurality of grooves provided in an upper surface thereof;
an IDT including a plurality of electrode fingers which are defined by a metal filled in the plurality of grooves in the upper surface of the piezoelectric substrate; wherein
the metal is at least one metal selected from the group consisting of Mo, Ta, Au, and Pt or an alloy primarily composed of the metal; and
the metal satisfies the following equation (1):


(ρ3×C44)1/2>1.95×1011  Equation (1)


where ρ (kg/m3) represents the density of the metal, and C44 (N/M2) represents the stiffness thereof.

2. The surface acoustic wave device according to claim 1, further comprising a dielectric film including SiO2 or an inorganic material primarily including SiO2 and being arranged to cover the IDT and the piezoelectric substrate.

3. The surface acoustic wave device according to claim 2, wherein when a wavelength of a surface acoustic wave is represented by λ, a normalized thickness of the dielectric film which is normalized by λ is about 0.05 to about 0.3.

4. The surface acoustic wave device according to claim 1, wherein the cut angle of the LiTaO3 substrate is expressed in Euler angles (0°±10°, 70° ̃180°, 0°±10°).

5. A surface acoustic wave device comprising:
a piezoelectric substrate composed of a LiTaO3 substrate and including a plurality of grooves provided in an upper surface thereof;
an IDT including a plurality of electrode fingers which are disposed in the plurality of grooves in the upper surface of the piezoelectric substrate; wherein
the IDT is defined by a laminated metal film including metal films laminated to each other, and each of the metal films is composed of a metal selected from the group consisting of Al, Au, Ta, W, Pt, Cu, Ni, and Mo; and
the laminate metal film satisfies the following equation (1):


(ρ3×C44)1/2>1.95×1011  Equation (1)


where the quotient obtained by dividing the sum of the products of thicknesses T of the metal films and the densities of the metals composing the metal films by the sum of the thicknesses T of the metal films is defined as an average density and is represented by the ρ (kg/m3), and where an average stiffness, which is the quotient obtained by dividing the sum of the products of the thicknesses T of the metal films and the stiffnesses of the metals composing the metal films by the sum of the thicknesses T of the metal films, and the metal films defining the laminated metal film, is represented by the C44 (N/M2).