Surface acoustic wave device

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface acoustic wave device for use, for example, as a resonator, band-pass filter, or other suitable device. In particular, the present invention relates to a surface acoustic wave device using a rotated Y plate X-propagating LiTaO₃ substrate and a method for manufacturing the same.

[0003] 2. Description of the Related Art

[0004] In mobile communication apparatuses, for example, cellular phones, surface acoustic wave filters have been used as RF stage band-pass filters and duplexers. As this sort of surface acoustic wave filter, the surface acoustic wave filter using a leaky surface acoustic wave has been in actual use, in which an IDT (interdigital transducer) made of Al is arranged on a 30 to 50-rotated Y plate X-propagating LiTaO₃ substrate.

[0005] However, this surface acoustic wave filter has a poor frequency temperature characteristic of 30 ppm/ C. to 40 ppm/ C. and, therefore, improvements thereto have been required. In order to improve the frequency temperature characteristic, a structure has been suggested, in which an IDT made of Al has been disposed on the 30 to 50-rotated Y plate X-propagating LiTaO₃ substrate and, thereafter, a SiO₂ film has been further laminated thereon. The frequency temperature characteristic is improved by arrangement of the SiO₂ film.

[0006] When the IDT made of Al is formed, the electrode film thickness H/ (H denotes a film thickness and denotes a wavelength of a surface acoustic wave) of the IDT is significantly increased to 0.08 to 0.10 in order to increase the reflection coefficient and electromechanical coupling coefficient K². Since the IDT made of Al was allowed to have a significant thickness as described above, regarding the portion shown in FIG. 16A, when the SiO₂ film was formed thereon in order to improve the frequency temperature characteristic, large height differences occurred in the SiO₂ film and, thereby, cracks sometimes occurred in the SiO₂ film as shown in FIG. 16B. Consequently, the filter characteristics of the surface acoustic wave filter were likely to degrade due to occurrence of the cracks.

[0007] In addition, since the electrode film thickness of the IDT made of Al is large, an effect of covering the disparities (concavity and convexity) on the electrode surface of the IDT based on the arrangement of the SiO₂ film was not adequate and, thereby, the temperature characteristic was not always improved adequately.

[0008] Furthermore, the attenuation constant was increased due to the arrangement of the SiO₂ film and, thereby, degradation of the filter characteristics occurred.

[0009] In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device and a method for manufacturing the same, the surface acoustic wave device includes a rotated Y plate X-propagating LiTaO₃ substrate and has a significantly improved frequency temperature characteristic due to the arrangement of the SiO₂ film and, in addition to this, the electrode film thickness of the IDT is greatly reduced, cracking in the SiO₂ film is reliably prevented and, furthermore, the attenuation constant is reduced by a large degree, so that targeted electrical characteristics, for example, filter characteristics, can be achieved, and the electromechanical coupling coefficient and reflection coefficient in the IDT achieve desired values.

[0010] According to a preferred embodiment of the present invention, a surface acoustic wave device includes a LiTaO₃ substrate having Euler angles (03, 110 to 150, 03), at least one IDT which is arranged on the LiTaO₃ substrate and which primarily includes Ag, and a SiO₂ film arranged on the LiTaO₃ substrate to cover the IDT.

[0011] In preferred embodiments of the present invention, since the IDT primarily includes Cu, and the SiO₂ film is provided, the electromechanical coupling coefficient is increased, and the frequency temperature characteristic is improved. Furthermore, since the LiTaO₃ substrate having the specific Euler angles is used, the attenuation constant is greatly reduced.

[0012] In a preferred embodiment of the present invention, preferably, the film thickness H/ (H represents a film thickness and represents a wavelength of a surface acoustic wave) of the IDT normalized by the wavelength of the surface acoustic wave is within the range of about 0.01 to about 0.08, and the film thickness H/ of the SiO₂ film normalized by the wavelength of the surface acoustic wave is within the range of about 0.15 to about 0.40. In that case, according to preferred embodiments of the present invention, a surface acoustic wave device which has a large electromechanical coupling coefficient and reflection coefficient, excellent frequency temperature characteristic, and adequately reduced attenuation constant nd in which cracking in the SiO₂ film are prevented from occurring can be provided with reliability.

[0013] In another preferred embodiment of the present invention, preferably, the film thickness H/ of the IDT is about 0.12 or less, and the combination of the normalized film thickness of the SiO₂ and the Euler angles of the LiTaO₃ substrate is specified to be any one of those indicated by (a) to (f) in the following Table 1.

	TABLE 1
	SiO2		More
	film thickness	Euler angles of LiTaO3	preferable Euler angles
(a)	0.15 to 0.18	0 3, 117 to 137, 0 3	0 3, 120 to 135, 0 3
(b)	0.18 to 0.23	0 3, 117 to 136, 0 3	0 3, 118 to 133, 0 3
(c)	0.23 to 0.28	0 3, 115 to 135, 0 3	0 3, 117 to 133, 0 3
(d)	0.28 to 0.33	0 3, 113 to 133, 0 3	0 3, 115 to 132, 0 3
(e)	0.33 to 0.38	0 3, 113 to 135, 0 3	0 3, 115 to 133, 0 3
(f)	0.38 to 0.40	0 3, 113 to 132, 0 3	0 3, 115 to 130, 0 3

[0014] In the surface acoustic wave device according to preferred embodiments of the present invention, preferably, a leaky surface acoustic wave is preferably used as the surface acoustic wave. According to preferred embodiments of the present invention, a surface acoustic wave device having an excellent frequency temperature characteristic, an IDT with a large electromechanical coupling coefficient and a reflection coefficient, and a small propagation loss can be provided.

[0015] Other features, elements, characteristics and advantages of the present invention will be more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

[0016] FIG. 1 is a plan view showing a surface acoustic wave device according to a preferred embodiment of the present invention.

[0017] FIG. 2 is a diagram showing the relationship between the and the electromechanical coupling coefficient K² on the LiTaO₃ substrate having Euler angles (0, , 0) when the film thickness of an electrode is 0.

[0018] FIG. 3 is a diagram showing the relationship between the and the attenuation constant on the LiTaO₃ substrate having Euler angles (0, , 0) when the film thickness of an electrode is 0.

[0019] FIG. 4 is a diagram showing the relationship between the normalized film thickness H/ of Ag film and the electromechanical coupling coefficient K² when electrodes made of Ag films having various film thicknesses are arranged on respective LiTaO₃ substrates having Euler angles (0, 126, 0).

[0020] FIG. 5 is a diagram showing the relationship between the normalized film thickness H/ of a SiO₂ film and the temperature coefficient of resonant frequency TCF when SiO₂ films having various film thicknesses are arranged on three types of LiTaO₃ substrate, each having Euler angles (0, 113, 0), (0, 126, 0), or (0, 129, 0), while an electrode film thickness is 0.

[0021] FIG. 6 is a diagram showing changes in the attenuation constant when Ag films having normalized film thicknesses of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, 120, 0), and respective SiO₂ films having normalized film thicknesses of about 0 to about 0.5 are arranged.

[0022] FIG. 7 is a diagram showing changes in the attenuation constant when Ag films having normalized film thicknesses of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, 140, 0), and respective SiO₂ films having normalized film thicknesses of about 0 to about 0.5 are arranged.

[0023] FIG. 8 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.1 are laminated.

[0024] FIG. 9 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.15 are laminated.

[0025] FIG. 10 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, 0, 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.2 are laminated.

[0026] FIG. 11 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.25 are laminated.

[0027] FIG. 12 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.3 are laminated.

[0028] FIG. 13 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.35 are laminated.

[0029] FIG. 14 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.4 are laminated.

[0030] FIG. 15 is a diagram showing changes in the attenuation constant when various Ag films having normalized film thicknesses H/ of about 0.1 or less are arranged on respective LiTaO₃ substrates having Euler angles (0, , 0), and respective SiO₂ films having a normalized film thickness H/ of about 0.45 are laminated.

[0031] FIGS. 16A and 16B are scanning electron micrographs for explaining problems in a conventional surface acoustic wave device where FIG. 16A shows the condition before formation of a SiO₂ film, and FIG. 16B shows the surface condition of the SiO₂ film after formation of the SiO₂ film.

[0032] The present invention will be made clear by describing specific preferred embodiments of the present invention below with reference to the drawings.

[0033] FIG. 1 is a plan view for explaining a longitudinally coupled resonator filter as a surface acoustic wave device according to a preferred embodiment of the present invention.

[0034] A surface acoustic wave device 11 has a structure in which IDTs 13a and 13b and reflectors 14a and 14b are arranged on the top surface of a LiTaO₃ substrate 12. A SiO₂ film 15 is arranged while covering the IDTs 13a and 13b and reflectors 14a and 14b. As the LiTaO₃ substrate 12, a 20 to 60-rotated Y plate LiTaO₃ substrate is preferably used. Regarding rotated Y plate X-propagating LiTaO₃ substrates having cut angles outside this range, attenuation constants are increased and TCFs are degraded.

[0035] The IDTs 13a and 13b and reflectors 14a and 14b are made of Ag having a density higher than that of Al.

[0036] Since the IDTs 13a and 13b and reflectors 14a and 14b are made of Ag having a density higher than that of Al as described above, even when the film thicknesses of the IDTs 13a and 13b and reflectors 14a and 14b are reduced compared with those in the case where Al is used, the electromechanical coupling coefficient and the reflection coefficient can be increased.

[0037] Since the electrode film thickness can be reduced as described above, occurrence of cracks due to the height differences of the SiO₂ film 15 arranged on the IDTs 13a and 13b can be reliably prevented. Regarding the thickness of the SiO₂ film 15, as is clear from experimental examples described later, the film thickness H/ normalized by the wavelength of the surface acoustic wave is preferably within the range of about 0.15 to about 0.40. When the film thickness falls within this range, the attenuation constant can be decreased by a large degree compared with that in the case where the SiO₂ film is not arranged and, therefore, reduction of loss can be achieved.

[0038] As described later, the film thicknesses H/ of the IDTs 13a and 13b normalized by the wavelength of the surface acoustic wave are preferably about 0.01 to about 0.08.

[0039] In the surface acoustic wave device according to preferred embodiments of the present invention, as described above, since the IDTs 13a and 13b include Ag on the LiTaO₃ substrate 12, the electrode film thicknesses of the IDTs 13a and 13b can be decreased. Consequently, occurrence of height differences in the SiO₂ film can be prevented, and cracking can be prevented with reliability. Furthermore, since the LiTaO₃ substrate having specific Euler angles is used, the attenuation constant can be reduced by a large degree, and reduction of loss can be achieved. In addition, excellent frequency temperature characteristic can be realized by arrangement of the SiO₂ film 15. This will be described based on the specific experimental examples.

[0040] Surface acoustic waves which propagate through the LiTaO₃ substrate preferably includes a leaky surface acoustic wave (LSAW) other than a Rayleigh wave. The leaky surface acoustic wave has a large sound velocity and a large electromechanical coupling coefficient compared with those of the Rayleigh wave, and propagates while releasing energy in the substrate. Consequently, the leaky surface acoustic wave has an attenuation constant which causes propagation loss.

[0041] FIG. 3 shows the relationship between the of the Euler angles (0, , 0) and the attenuation constant when the substrate surface is electrically short-circuited regarding the rotated Y plate X-propagating LiTaO₃ substrate. There is a relationship that rotation angle0-90.

[0042] As is clear from FIG. 3, the attenuation constant is small when the of the Euler angles is within the range of about 124 to about 126, and the attenuation constant is increased outside this range. It is known that in the case where an IDT made of Al having relatively large thickness is used, the attenuation constant becomes small when the is about 129 to about 136. Therefore, conventionally, when the IDT made of Al was used, a LiTaO₃ substrate in which the of the Euler angles was within the range of about 129 to about 136 was used.

[0043] FIG. 2 shows the relationship between the of the Euler angles and the electromechanical coupling coefficient K². As is clear from FIG. 2, a large electromechanical coupling coefficient K² can be achieved when the of the Euler angles is within the range of about 100 to about 120. However, as is clear from FIG. 3, since the attenuation constant is large when the is within the range of about 100 to about 120, the LiTaO₃ substrate having a of about 100 to about 120 cannot be used for the surface acoustic wave device.

[0044] FIG. 4 shows the relationship between the normalized film thickness H/ of an Ag film and the electromechanical coupling coefficient K when an IDT made of Ag is arranged on a 36-rotated Y plate X-propagating LiTaO₃ substrate (Euler angles (0, 126, 0)). Here, denotes a wavelength at a center frequency of a surface acoustic wave device.

[0045] As is clear from FIG. 4, when the film thickness H/ of the Ag film is within the range of about 0.01 to about 0.08, the electromechanical coupling coefficient K² is approximately 1.5 times or more than that in the case where no Ag film is arranged (H/0), when the film thickness H/ of the Ag film is within the range of about 0.02 to about 0.06, the electromechanical coupling coefficient K² takes on a value that is approximately 1.7 times or more than that in the case where no Ag film is arranged, and when the film thickness H/ of the Ag film is within the range of about 0.03 to about 0.05, the electromechanical coupling coefficient K takes on a value that is approximately 1.8 times or more than that in the case where no Ag film is arranged.

[0046] When the normalized film thickness H/ of the Ag film exceeds about 0.08, manufacture of an IDT made of the Ag film becomes difficult. Consequently, in order that a large electromechanical coupling coefficient can be achieved, and the IDT is manufactured with ease, the thickness of the IDT made of the Ag film is desirably within the range of about 0.01 to about 0.08, more preferably, is specified to be within the range of about 0.02 to about 0.06, and further preferably, be within the range of about 0.03 to about 0.05.

[0047] Changes in the temperature coefficient of frequency TCF when a SiO₂ film is arranged on a LiTaO₃ substrate are shown in FIG. 5. FIG. 5 shows the relationship between the normalized film thickness H/ of the SiO₂ film and the TCF when SiO₂ films are arranged on three types of LiTaO₃ substrates, each having Euler angles (0, 113, 0), (0, 126, 0), or (0, 129, 0). No electrode is provided in this example.

[0048] As is clear from FIG. 5, in each of the cases where the is 113, 126, and 129, the TCF falls within the range of 20 ppm/ C. to 17 ppm/ C. when the normalized film thickness H/ of the SiO₂ film is within the range of about 0.15 to about 0.45. However, since film formation of the SiO₂ film takes much time, the film thickness H/ of the SiO₂ film is desirably in the range of about 0.15 to about 0.40.

[0049] It has been known that the TCFs of the Rayleigh wave, etc., have been improved by arrangement of the SiO₂ film on the LiTaO₃ substrate. However, regarding a structure in which the electrode made of Ag is arranged on the LiTaO₃ substrate, and the SiO₂ film is further laminated, it is not reported that any actual experiment was performed in consideration of the thickness of the electrode made of Ag, the film thickness of the SiO₂, the cut angle, and the attenuation constant of the leaky elastic wave.

[0050] FIG. 6 shows changes in the attenuation constant when an electrode made of Ag having a normalized film thickness H/ of about 0.10 or less and a SiO₂ film having a normalized film thickness H/ of 0 to about 0.5 are arranged on a LiTaO₃ substrate having Euler angles (0, 120, 0). As is clear from FIG. 6, when the film thickness H/ of the SiO₂ film is about 0.2 to about 0.40, and the film thickness H/ of the Ag film is about 0.01 to about 0.10, the attenuation constant becomes small.

[0051] On the other hand, FIG. 7 shows changes in the attenuation constant when an Ag film having a normalized film thickness H/ of about 0 to about 0.10 is arranged on a LiTaO₃ substrate having Euler angles (0, 140, 0), and a SiO₂ film having a normalized film thickness H/ of about 0 to about 0.5 is further arranged.

[0052] As is clear from FIG. 7, in the case where the LiTaO₃ substrate having the of 140 is used, even when the film thickness of the SiO₂ film is changed as described above while the film thickness of the Ag film is about 0.06 or less, the attenuation constants a are large.

[0053] Consequently, it is clear that in order to realize an excellent TCF, large electromechanical coupling coefficient, and small attenuation constant, the cut angle of the LiTaO₃ substrate, that is, the Euler angles, the film thickness of the SiO₂ film, and the film thickness of the electrode made of Ag must be combined while each of them is adjusted to be optimum.

[0054] Each of FIGS. 8 to 15 shows the relationship between the and the attenuation constant when an Ag film having a normalized film thickness H/ of about 0.1 or less is arranged on a LiTaO₃ substrate while the normalized film thickness H/ of a SiO₂ film is approximately 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 or 0.45.

[0055] As is clear from FIGS. 8 to 15, in the case where the thickness H/ of the Ag film is with the range of about 0.01 to about 0.08, when the thickness of the SiO₂ film and the of the Euler angles are chosen in order to satisfy (a) to (f) shown in the following Table 2, the frequency temperature characteristic TCF becomes excellent, the electromechanical coupling coefficient is increased, and the attenuation constant can be effectively controlled. Desirably, further excellent characteristics can be attained when more preferable Euler angles shown in the right column of the following Table 2 are chosen.

TABLE 2
Ag film thickness H/: about 0.01 to about 0.08
	SiO2	Euler angles	More preferable
	film thickness	of LiTaO3	Euler angles
	(a)	0.15 to 0.18	0, 117 to 137, 0	0, 120 to 135, 0
	(b)	0.18 to 0.23	0, 117 to 136, 0	0, 118 to 133, 0
	(c)	0.23 to 0.28	0, 115 to 135, 0	0, 117 to 133, 0
	(d)	0.28 to 0.33	0, 113 to 133, 0	0, 115 to 132, 0
	(e)	0.33 to 0.38	0, 113 to 135, 0	0, 115 to 133, 0
	(f)	0.38 to 0.40	0, 113 to 132, 0	0, 115 to 130, 0

[0056] More preferably, in the case where the normalized thickness H/ of the Ag film is about 0.02 to about 0.06, when the thickness of the SiO₂ film and the of the Euler angles are chosen in order to satisfy (g) to (1) shown in the following Table 3, more preferable characteristics can be attained. Desirably, further excellent characteristics can be attained when more preferable Euler angles shown in the right column of the following Table 3 are chosen.

TABLE 3
Ag film thickness H/: about 0.02 to about 0.06
	SiO2	Euler angles	More preferable
	film thickness	of LiTaO3	Euler angles
	(g)	0.15 to 0.18	0, 120 to 133, 0	0, 122 to 130, 0
	(h)	0.18 to 0.23	0, 120 to 137, 0	0, 122 to 136, 0
	(i)	0.23 to 0.28	0, 120 to 135, 0	0, 122 to 133, 0
	(j)	0.28 to 0.33	0, 118 to 135, 0	0, 120 to 133, 0
	(k)	0.33 to 0.38	0, 115 to 133, 0	0, 117 to 130, 0
	(l)	0.38 to 0.40	0, 113 to 130, 0	0, 115 to 128, 0

[0057] Further preferably, in the case where the normalized thickness of the Ag film is about 0.03 to about 0.05, when the thickness of the SiO₂ film and the of the Euler angles are chosen in order to satisfy (m) to (r) shown in the following Table 4, further excellent characteristics can be attained. In this case as well, the characteristics can be further improved when more preferable Euler angles shown in the right column of the following Table 4 are chosen.

TABLE 4
Ag film thickness H/: about 0.03 to about 0.05
	SiO2 film	Euler angles	More preferable
	thickness	of LiTaO3	Euler angles
	(m)	0.15 to 0.18	0, 122 to 142, 0	0, 123 to 140, 0
	(n)	0.18 to 0.23	0, 120 to 140, 0	0, 122 to 137, 0
	(o)	0.23 to 0.28	0, 117 to 138, 0	0, 120 to 135, 0
	(p)	0.28 to 0.33	0, 116 to 136, 0	0, 118 to 134, 0
	(q)	0.33 to 0.38	0, 114 to 135, 0	0, 117 to 133, 0
	(r)	0.38 to 0.40	0, 113 to 130, 0	0, 115 to 128, 0

[0058] In preferred embodiments of the present invention, the IDT may include Ag alone, or include an Ag alloy or a laminate of Ag and other metal as long as Ag is primarily included. The IDT primarily containing Ag essentially includes approximately 80% by weight or more of Ag relative to the total IDT. Consequently, an Al thin film or Ti thin film may be arranged as the substrate of Al, and in this case, it is also preferable that Ag includes approximately 80% by weight or more of the total of the substrate thin film and Ag.

[0059] In the aforementioned experiment, the LiTaO₃ substrate having Euler angles (0, , 0) was used. In general, variations of 03 occur regarding the Euler angles of the substrate material. However, the effects of preferred embodiments of the present invention can be achieved even within the range of such variations, that is, regarding the LiTaO₃ substrate of (03, 110 to 150, 030).

[0060] The present invention can be applied to not only the longitudinally coupled resonator type surface acoustic wave filter shown in FIG. 1, but also various surface acoustic wave devices, for example, surface acoustic wave resonators, transversely coupled surface acoustic wave filters, ladder filters, and lattice filters.

[0061] In the surface acoustic wave device according to preferred embodiments of the present invention, since at least one IDT primarily including Ag is arranged on the LiTaO₃ substrate having Euler angles (03, 110 to 150, 03), and the SiO₂ film is arranged on the LiTaO₃ substrate to cover the IDT, the surface acoustic wave device having a large electromechanical coupling coefficient, excellent temperature characteristics, a reduced attenuation constant , and a reduced propagation loss can be provided.

[0062] In preferred embodiments of the present invention, when the film thickness normalized by the wavelength of the surface acoustic wave of the IDT is within the range of about 0.01 to about 0.08, and the normalized film thickness of the SiO₂ film is within the range of about 0.15 to about 0.40, the electromechanical coupling coefficient can be further increased, and excellent temperature characteristics can be realized.

[0063] Furthermore, when the film thickness H/ of the IDT is about 0.01 to about 0.08, and the of the Euler angles of the LiTaO₃ substrate and the normalized film thickness H/ of the SiO₂ film are chosen as indicated by (a) to (f) shown in Table 5, the surface acoustic wave device having a larger electromechanical coupling coefficient, further reduced attenuation constant , and further excellent frequency temperature characteristic can be provided.

[0064] It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.