JP2006094567A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave device which not only has a frequency temperature characteristic improved by forming an SiO<SB>2</SB>film on an IDT, but also has hardly a crack in the surface of the SiO<SB>2</SB>film, can securely obtain a desired characteristic, and has a large coefficient of electromechanical coupling and a small attenuation constant α. <P>SOLUTION: The surface acoustic wave device has at least one IDT formed principally of Ag on an LiTaO<SB>3</SB>substrate of a 20-60° rotary Y plate and also has the SiO<SB>2</SB>film formed on the LiTaO<SB>3</SB>substrate while covering the IDT. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a surface acoustic wave device used for a resonator, a bandpass filter, and the like, and more particularly to a surface acoustic wave device using a rotating Y plate X propagation LiTaO ₃ substrate and a manufacturing method thereof.

In mobile communication devices such as mobile phones, surface acoustic wave filters are used as band filters and duplexers in the RF stage. As this type of surface acoustic wave filter, an IDT (interdigital transducer) made of Al is formed on a LiTaO ₃ substrate with 30 ° -50 ° rotated Y-plate X propagation, and an elastic surface using leaky acoustic waves Wave filters have been put into practical use.

However, this surface acoustic wave filter has a poor frequency temperature characteristic of −30 to −40 ppm / ° C., and its improvement has been demanded. Therefore, in order to improve the frequency temperature characteristics, a structure is proposed in which an IDT made of Al is formed on a 30 ° -50 ° rotated Y-plate X-propagating LiTaO ₃ substrate and then a SiO ₂ film is further laminated. By forming the SiO ₂ film, the frequency temperature characteristics are improved.

When an IDT made of Al is formed, the IDT electrode film thickness H / λ (where H is the film thickness and λ is the surface wave wavelength) is 0 in order to increase the reflection coefficient and the electromechanical coupling coefficient K ^2. 0.08 to 0.10 is considerably thickened. As described above, since the IDT made of Al is considerably thickened, the SiO ₂ film is formed on the portion shown in FIG. 16A in order to improve the frequency temperature characteristic. as shown in 16 (b), a large step in the SiO ₂ film occurs, there is a crack occurs in the SiO ₂ film. Therefore, the generation of cracks tends to deteriorate the filter characteristics of the surface acoustic wave filter.

In addition, since the electrode film thickness of the IDT made of Al is thick, the effect of covering the unevenness of the electrode surface of the IDT by the formation of the SiO ₂ film is not sufficient, and thereby the temperature characteristics may not be sufficiently improved. .

Further, the formation of the SiO ₂ film increases the attenuation constant, resulting in deterioration of the filter characteristics.

The object of the present invention is not only to improve the frequency-temperature characteristics by forming a SiO ₂ film in a surface acoustic wave device using a LiTaO ₃ substrate with a propagating rotating Y plate X in view of the current state of the prior art described above, The electrode film thickness of the IDT can be reduced, cracks in the SiO ₂ film can be prevented and the attenuation constant can be greatly reduced, so that the desired electrical characteristics such as filter characteristics can be obtained, and An object of the present invention is to provide a surface acoustic wave device and a method for manufacturing the same, in which the electromechanical coupling coefficient and the reflection coefficient in the IDT are sufficiently large.

According to a wide aspect of the present invention, it is formed on a LiTaO ₃ substrate having an Euler angle (0 ± 3 °, 113 ° to 142 °, 0 ± 3 °) and the LiTaO ₃ substrate, and is mainly composed of Ag. A film thickness that includes at least one IDT and a SiO ₂ film formed on the LiTaO ₃ substrate so as to cover the IDT and is normalized by the wavelength of the surface wave of the IDT is 0.01 to 0.08. The surface acoustic wave device is characterized in that the film thickness normalized by the wavelength of the surface wave of the SiO ₂ film is in the range of 0.10 to 0.45.

In the present invention, since the IDT is mainly composed of Ag, and the formation of the SiO ₂ film, the electromechanical coupling coefficient is increased and the frequency temperature characteristics are improved. Furthermore, since the LiTaO ₃ substrate having the specific Euler angle is used, the attenuation constant α is reduced.

In a more limited aspect of the present invention, the film thickness H / λ normalized by the wavelength of the surface wave of the SiO ₂ film is in the range of 0.15 to 0.40, in which case, according to the present invention, A surface acoustic wave device having a large electromechanical coupling coefficient and reflection coefficient, good frequency temperature characteristics, a sufficiently small attenuation constant, and hardly causing cracks in the SiO ₂ film can be provided reliably.

In still another aspect of the present invention, the film thickness H / λ of the IDT is 0.12 or less, and the normalized film thickness of SiO ₂ and the Euler angle of the LiTaO ₃ substrate are the following (a) to (f ).

In another specific aspect of the surface acoustic wave device according to the present invention, the film thickness H / λ of the IDT is 0.02 to 0.06, the normalized film thickness of the SiO ₂ and the Euler angle of the LiTaO ₃ substrate. Is one of the combinations represented by the following (g) to (l).

In another specific aspect of the surface acoustic wave device according to the present invention, the film thickness H / λ of the IDT is 0.03 to 0.05, the normalized film thickness of the SiO ₂ and the Euler angle of the LiTaO ₃ substrate. Is any of the combinations represented by the following (m) to (r).

In the surface acoustic wave device according to the present invention, at least one IDT mainly composed of Ag is formed on a LiTaO ₃ substrate having Euler angles (0 ± 3 °, 113 ° to 142 °, 0 ± 3 °). Since the SiO ₂ film is formed on the LiTaO ₃ substrate so as to cover the IDT, the electromechanical coupling coefficient is large, the temperature characteristic is excellent, and the damping constant α is reduced. A surface wave device can be provided.

In the present invention, when the normalized film thickness of the SiO ₂ film is in the range of 0.15 to 0.40, the electromechanical coupling coefficient can be further increased and good temperature characteristics are realized. be able to.

Further, the film thickness H / λ of the IDT is 0.01 to 0.08, and the Euler angle θ of the LiTaO ₃ substrate and the normalization of the SiO ₂ film are shown in Table 1 (a) to (f). When the film thickness H / λ is selected, it is possible to provide a surface acoustic wave device that has a larger electromechanical coupling coefficient, a smaller attenuation constant α, and an excellent frequency-temperature characteristic.

  Hereinafter, the present invention will be clarified by describing specific embodiments of the present invention with reference to the drawings.

  FIG. 1 is a plan view for explaining a longitudinally coupled resonator filter as a surface acoustic wave device according to an embodiment of the present invention.

The surface acoustic wave device 11 has a structure in which IDTs 13 a and 13 b and reflectors 14 a and 14 b are formed on the upper surface of a LiTaO ₃ substrate 12. An SiO ₂ film 15 is formed so as to cover the IDTs 13a and 13b and the reflectors 14a and 14b. As the LiTaO ₃ substrate 12, a 20 ° to 60 ° rotated Y-plate LiTaO ₃ substrate is used. In the rotating Y plate X propagation LiTaO ₃ substrate having a cut angle outside this range, the attenuation constant is large and the TCF is also deteriorated.

  The IDTs 13a and 13b and the reflectors 14a and 14b are made of Ag having a higher density than Al.

  As described above, since the IDTs 13a and 13b and the reflectors 14a and 14b are made of Ag having a higher density than that of Al, the film thicknesses of the IDTs 13a and 13b and the reflectors 14a and 14b are compared with those using Al. Even when the thickness is reduced, the electromechanical coupling coefficient and the reflection coefficient can be increased.

Since the electrode film thickness can be reduced as described above, it is possible to reliably suppress the occurrence of cracks based on the aforementioned steps in the SiO ₂ film 15 formed on the IDTs 13a and 13b. Regarding the thickness of the SiO ₂ film 15, it is preferable that the film thickness H / λ normalized by the wavelength of the surface wave is in the range of 0.15 to 0.40, as will be apparent from the experimental examples described later. By setting it within this range, the attenuation constant can be made much smaller than when the SiO ₂ film is not formed, and the loss can be reduced.

  As will be described later, the film thickness H / λ normalized by the surface wave wavelengths of the IDTs 13a and 13b is preferably 0.01 to 0.08.

In the surface acoustic wave device according to the present invention, as described above, the IDTs 13a and 13b are made of Ag on the LiTaO ₃ substrate 12, and the electrode film thickness of the IDTs 13a and 13b can be reduced. Therefore, the generation of a step in the SiO ₂ film can be suppressed, and cracks can be reliably prevented. Further, since the LiTaO ₃ substrate having the specific Euler angle is used, the attenuation constant can be greatly reduced, and the loss can be reduced. In addition, the formation of the SiO ₂ film 15 realizes good frequency temperature characteristics. This will be described based on a specific experimental example.

In addition to Rayleigh waves, surface acoustic waves that propagate through the LiTaO ₃ substrate include leaky surface acoustic waves (LSAW). The leaky surface acoustic wave has a faster sound speed and a larger electromechanical coupling coefficient than the Rayleigh wave, but propagates while radiating energy into the substrate. Therefore, the leaky surface acoustic wave has an attenuation constant that causes a propagation loss.

FIG. 2 shows the relationship between the Euler angles (0 °, θ, 0 °) θ in the rotating Y-plate X-propagating LiTaO ₃ substrate and the attenuation constant α when the substrate surface is electrically short-circuited. Note that there is a relationship of rotation angle = θ−90 °.

As is apparent from FIG. 2, the attenuation constant α is small when the Euler angle θ is in the range of 124 ° to 126 °, but the attenuation constant α is large outside this range. Further, it is known that when an IDT made of Al having a relatively large film thickness is used, the attenuation constant becomes small at θ = 129 ° to 136 °. Therefore, conventionally, when an IDT made of Al is used, a LiTaO ₃ substrate having an Euler angle θ in the range of 129 ° to 136 ° has been used.

FIG. 3 shows the relationship between the Euler angle θ and the electromechanical coupling coefficient K ² . As is clear from FIG. 3, a large electromechanical coupling coefficient K ² is obtained when the Euler angle θ is in the range of 100 ° to 120 °. However, as is apparent from FIG. 2, the attenuation constant α is large in the range of θ = 100 ° to 120 °, and the LiTaO ₃ substrate of θ = 100 ° to 120 ° cannot be used for the surface acoustic wave device.

FIG. 4 shows the normalized film thickness H / of the Ag film when an IDT made of Ag is formed on a 36 ° rotated Y-plate X-propagating LiTaO ₃ substrate (Euler angles (0 °, 126 °, 0 °)). The relationship between λ and the electromechanical coupling coefficient K ² is shown. Note that λ represents the wavelength at the center frequency of the surface acoustic wave device.

As is apparent from FIG. 4, when the Ag film thickness H / λ is in the range of 0.01 to 0.08, the electromechanical coupling coefficient K ² is such that the Ag film is not formed (H / λ = 0). It can be seen that it is 1.5 times or more. Further, when the thickness of the Ag film is in the range of H / λ = 0.02 to 0.06, the electromechanical coupling coefficient K ² is 1.7 times or more compared to the case where the Ag film is not formed. It can be seen that when the film thickness H / λ of the Ag film is in the range of 0.03 to 0.05, the value is 1.8 times or more that when the Ag film is not formed.

  When the normalized film thickness H / λ of the Ag film exceeds 0.08, it is difficult to produce an IDT made of the Ag film. Therefore, since a large electromechanical coupling coefficient can be obtained and the IDT can be easily manufactured, the thickness of the IDT made of the Ag film is desirably in the range of 0.01 to 0.08, and more preferably The range is 0.02 to 0.06, more preferably 0.03 to 0.05.

Next, FIG. 5 shows changes in the frequency temperature coefficient TCF when a SiO ₂ film is formed on a LiTaO ₃ substrate. FIG. 5 shows SiO ₂ films on three types of LiTaO ₃ substrates of Euler angles (0 °, 113 °, 0 °), (0 °, 126 °, 0 °) and (0 °, 129 °, 0 °). The relationship between the normalized film thickness H / λ of the SiO ₂ film and TCF in the case where is formed is shown. Here, no electrode is formed.

As is apparent from FIG. 5, the TCF is within the range of the normalized film thickness H / λ of the SiO ₂ film of 0.15 to 0.45 regardless of whether θ is 113 °, 126 ° or 129 °. It can be seen that the range is from -20 to +17 ppm / ° C. However, since the formation of the SiO ₂ film takes time, the film thickness H / lambda of the SiO ₂ film 0.15 to 0.40 is desirable.

By depositing SiO ₂ film on the LiTaO ₃ substrate, but it has been known that TCF such Rayleigh waves is improved, the LiTaO ₃ substrate, and forming an electrode made of Ag, further SiO ₂ film In the structure in which the layers are laminated, there has been no report that has actually been tested in consideration of the film thickness of the electrode made of Ag, the film thickness of SiO ₂ , the cut angle, and the damping constant of the leaky elastic wave.

FIG. 6 shows an electrode made of Ag having a normalized film thickness H / λ of 0.10 or less on a LiTaO ₃ substrate with Euler angles (0 °, 120 °, 0 °), and a normalized film thickness H / λ of 0. The change of the attenuation constant α when an SiO ₂ film of ˜0.5 is formed is shown. As apparent from FIG. 6, when the film thickness H / λ of the SiO ₂ film is 0.2 to 0.40 and the film thickness H / λ of the Ag film is 0.01 to 0.10, the attenuation constant α is You can see that it is getting smaller.

On the other hand, FIG. 7 shows that an Ag film having a normalized film thickness H / λ of 0 to 0.10 is formed on a LiTaO ₃ substrate having an Euler angle of (0 °, 140 °, 0 °). A change in the attenuation constant α when a SiO ₂ film having a chemical thickness H / λ of 0 to 0.5 is formed is shown.

As is apparent from FIG. 7, when a LiTaO ₃ substrate with θ = 140 ° is used, even if the thickness of the Ag ₂ film is 0.06 or less and the thickness of the SiO ₂ film is changed as described above, It can be seen that the attenuation constant α is large.

That is, in order to realize a good TCF, a large electromechanical coupling coefficient, and a small attenuation constant, the cut angle of the LiTaO ₃ substrate, that is, the Euler angle, the film thickness of the SiO ₂ film, and the film thickness of the electrode made of Ag, respectively. It can be seen that it is necessary to combine them optimally.

8 to 15, the normalized film thickness H / λ of the SiO ₂ film is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 or The relationship between θ and the attenuation constant α when an Ag film having a normalized film thickness H / λ of 0.1 or less on a LiTaO ₃ substrate is 0.45 is shown.

As apparent from FIGS. 8 to 15, when the thickness H / λ of the Ag film is 0.01 to 0.08, the thickness of the SiO ₂ film and the Euler angle θ are shown in Table 4 below. If it is selected so as to satisfy (a) to (f), it can be seen that the frequency temperature characteristic TCF is good, the electromechanical coupling coefficient is large, and the attenuation constant α can be effectively suppressed. Desirably, even better characteristics can be obtained by selecting a more preferable Euler angle on the right side of Table 4 below.

More preferably, when the normalized film thickness of the Ag film is 0.02 to 0.06, the thickness of the SiO ₂ film and the Euler angle θ are shown in Table 5 below (g) to If it is selected so as to satisfy (l), it is more preferable, and more desirably, by selecting a more preferable Euler angle on the right side of Table 5 below, even better characteristics can be obtained.

More preferably, when the normalized film thickness of the Ag film is 0.03 to 0.05, the thickness of the SiO ₂ film and the Euler angle θ are (m) to (r) shown in Table 6 below. If it is selected so as to satisfy the above, even better characteristics can be obtained. Even in this case, the characteristics can be further improved by selecting a more preferable Euler angle shown on the right side of Table 6 below.

  In the present invention, the IDT may be composed only of Ag, but may be composed of an Ag alloy or a laminate of Ag and another metal as long as Ag is mainly used. The IDT mainly composed of Ag may be 80% by weight or more of the entire IDT. Accordingly, an Al thin film or a Ti thin film may be formed on the Ag base, and in this case as well, 80% by weight or more of the total of the base thin film and Ag may be composed of Ag.

In the above experiment, a LiTaO ₃ substrate with Euler angles (0 °, θ, 0 °) was used, but variations of 0 ± 3 ° usually occur in Euler angles of the substrate material. The effect of the present invention can be obtained even in a LiTaO ₃ substrate within such a variation range, that is, (0 ± 3 °, 110 ° to 150 °, 0 ± 3 °).

  The present invention is not limited to the longitudinally coupled resonator type surface acoustic wave filter shown in FIG. 1, but various surface waves such as a surface acoustic wave resonator, a laterally coupled type surface wave filter, a ladder type filter, and a lattice type filter. It can be applied to the device.

1 is a plan view showing a surface acoustic wave device according to an embodiment of the present invention. The thickness of the electrode is Euler angle when the 0 (0 °, θ, 0 °) view showing the relationship between theta and the electromechanical coupling coefficient K ² in the LiTaO ₃ substrate of. Euler angle when the film thickness of the electrodes is 0 (0 °, θ, 0 °) LiTaO 3 shows a relationship between theta and the attenuation constant α on the substrate. Normalized film thickness H / λ of an Ag film and electromechanical coupling coefficient when electrodes made of Ag films of various film thicknesses are formed on a LiTaO ₃ substrate with Euler angles (0 °, 126 °, 0 °) diagram showing the relationship between K ^2. In three types of LiTaO ₃ substrates of Euler angles (0 °, 113 °, 0 °), (0 °, 126 °, 0 °) and (0 °, 129 °, 0 °), the electrode film thickness is 0, The figure which shows the relationship between the normalized film thickness H / λ of the SiO ₂ film and the frequency temperature coefficient TCF when the SiO ₂ film having various thicknesses is formed. On a LiTaO ₃ substrate with Euler angles (0 °, 120 °, 0 °), an Ag film having a normalized film thickness of 0.1 or less is formed, and an SiO ₂ film having a normalized film thickness of 0 to 0.5 is formed. The figure which shows the change of the attenuation constant (alpha) at the time of forming into a film. On a LiTaO ₃ substrate with Euler angles (0 °, 140 °, 0 °), an Ag film having a normalized film thickness of 0.1 or less is formed, and an SiO ₂ film having a normalized film thickness of 0 to 0.5 is formed. The figure which shows the change of the attenuation constant (alpha) at the time of forming into a film. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.1. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.15. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.2. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate with Euler angles (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.25. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.3. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.35. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.4. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. Various Ag films having a normalized film thickness H / λ of 0.1 or less are formed on a LiTaO ₃ substrate having an Euler angle (0 °, θ, 0 °), and the normalized film thickness H / λ is 0.45. shows the case of stacking an SiO ₂ film, the variation of the attenuation constant alpha. (A) and (b) are diagrams for explaining a problem of the conventional surface acoustic wave device, before forming the SiO ₂ film and (a), the SiO ₂ film after film formation (b) The figure which shows the scanning electron micrograph which shows the state of the surface.

Explanation of symbols

11 ... surface acoustic wave device 12 ... LiTaO ₃ substrate 13a, 13b ... IDT
15 ... SiO ₂ film

Claims (5)

LiTaO ₃ substrate with Euler angles (0 ± 3 °, 113 ° -142 °, 0 ± 3 °),
Formed on the LiTaO ₃ substrate, and at least one IDT mainly composed of Ag;
An SiO ₂ film formed on the LiTaO ₃ substrate so as to cover the IDT,
The film thickness normalized by the wavelength of the surface wave of the IDT is 0.01 to 0.08,
A surface acoustic wave device characterized in that the thickness of the SiO ₂ film normalized by the surface wave wavelength is in the range of 0.10 to 0.45. The surface acoustic wave device according to claim 1, wherein the normalized film thickness of the SiO ₂ and the Euler angle of the LiTaO ₃ substrate are any of the combinations represented by the following (a) to (f).
The IDT film thickness H / λ is 0.02 to 0.06, and the normalized film thickness of the SiO ₂ and the Euler angle of the LiTaO ₃ substrate are represented by the following (g) to (l): The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is any one of the above.
The IDT film thickness H / λ is 0.03 to 0.05, and the normalized film thickness of the SiO ₂ and the Euler angle of the LiTaO ₃ substrate are represented by the following (m) to (r): The surface acoustic wave device according to claim 1, wherein the surface acoustic wave device is any one of the above.
The surface acoustic wave device according to claim 1, wherein a leaky surface acoustic wave mainly composed of an SH wave is used as the surface acoustic wave.