Surface acoustic wave filter and multiplexer
A filter includes a LiTaO 3 piezoelectric layer having a cut angle, a high acoustic velocity support substrate, a low acoustic velocity film, and an IDT electrode. A cut angle θ B of the piezoelectric layer at which a Rayleigh wave spurious response is locally reduced or minimized is determined from a wavelength of the IDT electrode, a film thickness of the IDT electrode, a specific gravity of the IDT electrode, an electrode duty, a thickness of the piezoelectric layer, and a film thickness of the low acoustic velocity film. The Cut angle of the piezoelectric layer satisfies a relationship of θ B −4≤θ≤θ B +4.
1. A surface acoustic wave filter comprising:
a LiTaO 3 piezoelectric layer having a cut angle θ°;
a high acoustic velocity support substrate in which a bulk wave propagates at a higher acoustic velocity than an acoustic wave propagating in the LiTaO 3 piezoelectric layer;
a low acoustic velocity film between the high acoustic velocity support substrate and the LiTaO 3 piezoelectric layer, and in which a bulk wave propagates at a lower acoustic velocity than the acoustic wave propagating in the LiTaO 3 piezoelectric film; and
an InterDigital Transducer (IDT) electrode on the LiTaO 3 piezoelectric layer; wherein a repetition period of a pair of comb-shaped electrodes defining the IDT electrode is denoted by a wavelength λ (μm), a film thickness of the pair of comb-shaped electrodes is denoted by T IDT (μm), a specific gravity of the IDT electrode is denoted by ρ (g/cm 3 ), an electrode duty of the pair of comb-shaped electrodes is denoted by D, a thickness of the LiTaO 3 piezoelectric layer is denoted by T LT (μm), a film thickness of the low acoustic velocity film is denoted by T VL (μm), and a cut angle θ B (°) of the LiTaO 3 piezoelectric layer at which a Rayleigh wave spurious response is locally reduced or minimized is specified by:
θ
B
=
43.09
-
27.5
×
{
(
T
IDT
λ
)
×
ρ
-
0.1956
}
-
19.2
×
(
D
-
0.5
)
+
17.4578
×
{
(
T
LT
λ
)
-
0.35
}
+
5.619
×
{
(
T
VL
λ
)
-
0.3
}
+
32.66
×
[
{
(
T
LT
λ
)
-
0.35
}
2
-
0.0125
]
+
24.63
×
{
(
T
LT
λ
)
-
0.35
}
×
{
(
T
VL
λ
)
-
0.3
}
and, the cut angle θ(°) of the LiTaO 3 piezoelectric layer satisfies a relationship of:
θ B −4≤≤θ B +4.
2. The surface acoustic wave filter according to claim 1 , wherein the surface acoustic wave filter includes a ladder filter structure defined by a serial arm resonator and a parallel arm resonator each including the LiTaO 3 piezoelectric layer and the IDT electrode.
3. The surface acoustic wave filter according to claim 1 , wherein the surface acoustic wave filter has a longitudinally-coupled filter structure including a plurality of acoustic wave resonators each including the LiTaO 3 piezoelectric layer and the IDT electrode arranged side-by-side in a propagation direction of the acoustic wave.
4. A multiplexer comprising:
a common terminal;
a first input/output terminal;
a second input/output terminal;
the surface acoustic wave filter according to claim 1 , which is connected between the common terminal and the first input/output terminal; and
a lower-frequency-side filter connected between the common terminal and the second input/output terminal, and having a pass band on lower frequency side than a pass band of the surface acoustic wave filter.
5. The multiplexer according to claim 4 , wherein
the pass band of the surface acoustic wave filter is a frequency band of Long Term Evolution (LTE) Band41; and
the pass band of the lower-frequency-side filter is a frequency band of LTE Band25.
6. The multiplexer according to claim 4 , further comprising an additional surface acoustic wave filter having a pass band with a frequency band of LTE Band66.
7. The multiplexer according to claim 6 , wherein
the surface acoustic wave filter is connected between a first input/output terminal and a common terminal;
the lower-frequency-side filter is connected between a second input/output terminal and the common terminal; and
an additional surface acoustic wave filter is connected between a third input/output terminal and the common terminal.
8. The surface acoustic wave filter according to claim 1 , wherein the surface acoustic wave filter includes a ladder filter structure defined by a plurality of serial arm resonators, a plurality of parallel arm resonators, and a matching inductor.
9. The surface acoustic wave filter according to claim 8 , wherein
the plurality of serial arm resonators are connected in series;
the plurality of parallel arm resonators are respectively connected between pairs of the serial arm resonators and a ground; and
the inductor is connected between at least one of the plurality of parallel arm resonators and the ground.
10. The surface acoustic wave filter according to claim 9 , wherein one of the plurality of parallel arm resonators is connected directly to the ground.
11. The surface acoustic wave filter according to claim 2 , wherein the serial arm resonator and the parallel arm resonator are both surface acoustic wave resonators.
12. The surface acoustic wave filter according to claim 1 , wherein the IDT electrode is defined by opposed electrode fingers having a multilayer structure including a close contact layer and a main electrode layer.
13. The surface acoustic wave filter according to claim 12 , wherein
the close contact layer is made of Ti; and
a thickness of the close contact layer is about 12 nm.
14. The surface acoustic wave filter according to claim 12 , wherein
the main electrode layer is made of at least of Al and Cu; and
a thickness of the main electrode layer is about 141 nm.
15. The surface acoustic wave filter according to claim 12 , wherein the opposed electrode fingers are covered by a protective film containing silicon dioxide.
16. The surface acoustic wave filter according to claim 1 , wherein the high acoustic velocity support substrate is a silicon substrate having a thickness of about 125 μm.
17. The surface acoustic wave filter according to claim 1 , wherein the low acoustic velocity film is made of silicon dioxide as a main ingredient and has a thickness of about 600 nm.
18. The surface acoustic wave filter according to claim 8 , wherein resonant frequencies of the plurality of serial arm resonators are different from one another.
19. The multiplexer according to claim 4 , wherein the multiplexer is a pentaplexer for LTE Band25, LTE Band66, and LTE Band41.
20. The surface acoustic wave filter according to claim 1 , wherein a spurious response level in the pass band of the lower-frequency-side filter is about 0.