JP7624796B2 - Ladder Filters and Multiplexers - Google Patents

Description

The present invention relates to a ladder-type filter and a multiplexer, for example, to a ladder-type filter and a multiplexer having comb-type electrodes.

Surface acoustic wave resonators are known as acoustic wave resonators used in communication devices such as smartphones. It is known that a piezoelectric substrate forming a surface acoustic wave resonator is bonded to a support substrate. It is known that a silicon oxide film is provided between the support substrate and the piezoelectric substrate (e.g., Patent Document 1). It is known that the frequency temperature coefficients of the resonant frequency and the anti-resonant frequency in a surface acoustic wave resonator are different (e.g., Patent Document 2).

JP 2015-73331 A JP 2017-152868 A

In ladder filters, it is required to give the filter the desired characteristics. For example, it is required to set the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband independently.

The present invention has been developed in consideration of the above problems, and aims to set the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband independently.

The present invention relates to a piezoelectric substrate comprising: a support substrate; an insulating layer bonded directly or indirectly to the support substrate, the insulating layer having an average thickness in a first region greater than an average thickness in a second region, the insulating layer being mainly composed of silicon oxide; a piezoelectric substrate bonded directly or indirectly to the insulating layer, the insulating layer having a temperature coefficient of elastic constant whose sign is opposite to that of the temperature coefficient of elastic constant of the insulating layer, the piezoelectric substrate being a lithium tantalate substrate; a series resonator provided on the piezoelectric substrate in the first region, the series resonator having a pair of comb electrodes and connected in series between a first terminal and a second terminal , the series resonator forming a ladder-type filter ; and a series resonator provided on the piezoelectric substrate in the second region, the series resonator having a pair of comb electrodes, and a parallel resonator forming the ladder-type filter , the parallel resonator having one end connected to a line on which the series resonator is provided between the first terminal and the second terminal and the other end connected to ground, wherein one end of at least one of the parallel resonators is connected to the line between two of the series resonators, the thickness of the piezoelectric substrate in the first region and the second region is not more than twice the average pitch of the electrode fingers of the pair of comb electrodes, and the thickness of the insulating layer in the first region and the second region is not more than twice the average pitch of the electrode fingers of the pair of comb electrodes.

In the above configuration, the average thickness of the piezoelectric substrate in the first region can be smaller than the average thickness of the piezoelectric substrate in the second region.

In the above configuration, the average thickness of the piezoelectric substrate in the first region and the average thickness of the piezoelectric substrate in the second region can be equal .

The present invention relates to a piezoelectric substrate comprising: a support substrate; an insulating layer bonded directly or indirectly onto the support substrate, the insulating layer being mainly composed of silicon oxide; a piezoelectric substrate bonded directly or indirectly onto the insulating layer, the insulating layer having an average thickness in a first region smaller than an average thickness in a second region, the insulating layer having a temperature coefficient of elastic constant whose sign is opposite to that of the temperature coefficient of elastic constant of the insulating layer, the piezoelectric substrate being a lithium tantalate substrate; a series resonator formed on the piezoelectric substrate in the first region, the series resonator having a pair of comb electrodes and connected in series between a first terminal and a second terminal , the series resonator forming a ladder-type filter ; and a series resonator formed on the piezoelectric substrate in the second region, the series resonator having a pair of comb electrodes, and a parallel resonator forming the ladder-type filter , the parallel resonator having one end connected to a line on which the series resonator is provided between the first terminal and the second terminal and the other end connected to ground, wherein one end of at least one of the parallel resonators is connected to the line between two of the series resonators, the thickness of the piezoelectric substrate in the first region and the second region is not more than twice the average pitch of the electrode fingers of the pair of comb electrodes, and the thickness of the insulating layer in the first region and the second region is not more than twice the average pitch of the electrode fingers of the pair of comb electrodes.

In the above configuration, the average thickness of the insulating layer in the first region and the average thickness of the insulating layer in the second region may be equal .

The present invention is a ladder-type filter comprising: a supporting substrate; an insulating layer bonded directly or indirectly to the supporting substrate, the insulating layer having a temperature coefficient of elastic constant in a first region that has the same sign as and has a larger absolute value than the temperature coefficient of elastic constant in a second region, the insulating layer being mainly composed of silicon oxide; a piezoelectric substrate bonded directly or indirectly to the insulating layer, the insulating layer having a temperature coefficient of elastic constant that is opposite in sign to the temperature coefficient of elastic constant of the insulating layer, the piezoelectric substrate being a lithium tantalate substrate; a series resonator formed on the piezoelectric substrate in the first region, the series resonator having a pair of comb electrodes and connected in series between a first terminal and a second terminal , the series resonator forming the ladder-type filter ; and a parallel resonator formed on the piezoelectric substrate in the second region, the series resonator having a pair of comb electrodes and connected in parallel between the first terminal and the second terminal, the parallel resonator forming the ladder-type filter, the parallel resonator having a pair of comb electrodes and connected in parallel between the first terminal and the second terminal, wherein the thickness of the piezoelectric substrate in the first region and the second region is equal to or less than twice the average pitch of the electrode fingers of the pair of comb electrodes, and the thickness of the insulating layer in the first region and the second region is equal to or less than twice the average pitch of the electrode fingers of the pair of comb electrodes.

In the above configuration, the average thickness of the insulating layer in the first region and the average thickness of the insulating layer in the second region can be equal, and the average thickness of the piezoelectric substrate in the first region and the average thickness of the piezoelectric substrate in the second region can be equal .

In the above configuration, the ladder-type filter may be a ladder-type filter that does not have any resonators other than the series resonators and the parallel resonators.

The present invention is a multiplexer including the above ladder-type filter.

According to the present invention, the frequency temperature coefficients of the high and low frequency ends of the passband can be set independently.

FIG. 1A is a circuit diagram of a ladder-type filter in accordance with a first embodiment, and FIG. 1B is a schematic diagram showing the pass characteristic of the ladder-type filter. FIG. 2A is a plan view of the elastic wave resonator in the first embodiment, and FIG. 2B is a cross-sectional view taken along line AA of FIG. 3A and 3B are cross-sectional views of acoustic wave resonators used in the experiments and simulations, respectively. 4(a) and 4(b) are diagrams showing experimental and simulation results of the TCF of the resonant and anti-resonant frequencies. FIG. 5 is a plan view of the filter according to the first embodiment. FIG. 6 is a cross-sectional view of the filter according to the first embodiment. 7A and 7B are cross-sectional views of filters according to first and second modifications of the first embodiment, respectively. 8A and 8B are cross-sectional views of filters according to third and fourth modifications of the first embodiment, respectively. FIG. 9 is a circuit diagram of a duplexer according to the second embodiment.

The following describes an embodiment of the present invention with reference to the drawings.

Figure 1(a) is a circuit diagram of a ladder-type filter according to the first embodiment, and Figure 1(b) is a schematic diagram showing the pass characteristics of the ladder-type filter. As shown in Figure 1(a), the ladder-type filter includes series resonators S1 to S4 and parallel resonators P1 to P3. The series resonators S1 to S4 are connected in series between the input terminal Tin (first terminal) and the output terminal Tout (second terminal). The parallel resonators P1 to P3 are connected in parallel between the input terminal Tin and the output terminal Tout. One end of the parallel resonators P1 to P3 is connected to the ground terminal Tg. The number of series resonators and parallel resonators can be set appropriately.

As shown in FIG. 1(b), the ladder filter functions as a bandpass filter having a passband 48 and a stopband 49. The resonant frequency frs of the series resonators S1 to S4 is located in the center of the passband 48, and the anti-resonant frequency fas is located in the stopband 49 on the higher frequency side of the passband 48. The anti-resonant frequency fap of the parallel resonators P1 to P3 is located in the center of the passband 48, and the resonant frequency frp is located in the stopband 49 on the lower frequency side of the passband 48. The high-frequency end of the passband 48 is determined by the anti-resonant frequency fas of the series resonators S1 to S4, and the low-frequency end of the passband 48 is determined by the resonant frequency frp of the parallel resonators P1 to P3.

To reduce the temperature change of the passband 48, it is necessary to reduce the frequency temperature coefficient of the anti-resonant frequency fas of the series resonators S1 to S4 and the frequency temperature coefficient of the resonant frequency frp of the parallel resonators P1 to P3. Also, to keep the width of the passband unchanged, it is necessary to make the frequency temperature coefficient of the anti-resonant frequency fas of the series resonators S1 to S4 and the frequency temperature coefficient of the resonant frequency frp of the parallel resonators P1 to P3 approximately equal.

Figure 2(a) is a plan view of the elastic wave resonator in Example 1, and Figure 2(b) is a cross-sectional view taken along the line A-A in Figure 2(a). The arrangement direction of the electrode fingers is the X direction, the extension direction of the electrode fingers is the Y direction, and the stacking direction of the support substrate and piezoelectric substrate is the Z direction. The X direction, Y direction, and Z direction do not necessarily correspond to the X-axis direction and Y-axis direction of the crystal orientation of the piezoelectric substrate. When the piezoelectric substrate is a rotated Y-cut X-propagation substrate, the X direction is the X-axis direction of the crystal orientation.

2(a) and 2(b), an insulating layer 11 is bonded onto a support substrate 10. A piezoelectric substrate 12 is bonded onto the insulating layer 11. The insulating layer 11 may be bonded directly onto the support substrate 10, or may be bonded indirectly via a bonding layer or the like. The piezoelectric substrate 12 may be bonded directly onto the insulating layer 11, or may be bonded indirectly via a bonding layer or the like. An elastic wave resonator 20 is provided on the piezoelectric substrate 12. The elastic wave resonator 20 has an IDT (Inter Digital Transducer) 22 and a reflector 24. The reflectors 24 are provided on both sides of the IDT 22 in the X direction. The IDT 22 and the reflector 24 are formed by a metal film 14 on the piezoelectric substrate 12.

The IDT 22 includes a pair of comb electrodes 18 facing each other. The comb electrodes 18 include a plurality of electrode fingers 15 and a bus bar 16 to which the plurality of electrode fingers 15 are connected. The region where the electrode fingers 15 of the pair of comb electrodes 18 intersect is the intersection region 25. The length of the intersection region 25 is the aperture length. The pair of comb electrodes 18 are provided facing each other so that the electrode fingers 15 are almost alternated in at least a part of the intersection region 25. The elastic wave excited by the plurality of electrode fingers 15 in the intersection region 25 propagates mainly in the X direction. The pitch of the electrode fingers 15 of one of the pair of comb electrodes 18 is approximately the wavelength λ of the elastic wave. The wavelength λ of the elastic wave is approximately the pitch of two electrode fingers 15. The reflector 24 reflects the elastic wave (surface acoustic wave) excited by the electrode fingers 15 of the IDT 22. As a result, the elastic wave is confined within the intersection region 25 of the IDT 22.

The piezoelectric substrate 12 is a single crystal lithium tantalate (LiTaO ₃ ) substrate, a single crystal lithium niobate (LiNbO ₃ ) substrate, or a single crystal quartz substrate, for example a rotated Y-cut X-propagation lithium tantalate substrate or a rotated Y-cut X-propagation lithium niobate substrate. The insulating layer 11 is, for example, an amorphous and/or polycrystalline layer mainly composed of silicon oxide (SiO ₂ ). The insulating layer 11 is mainly composed of silicon oxide and may contain impurities such as fluorine. The sign of the temperature coefficient of the elastic constant of the insulating layer 11 is opposite to the sign of the temperature coefficient of the elastic constant of the piezoelectric substrate 12. This allows the frequency temperature coefficient of the elastic wave resonator to be reduced.

The support substrate 10 has a linear expansion coefficient smaller than that of the piezoelectric substrate 12 in the X direction. This allows the frequency temperature coefficient of the elastic wave resonator to be small. The support substrate 10 is, for example, a sapphire substrate, an alumina substrate, a silicon substrate, or a silicon carbide substrate. The sapphire substrate is a single crystal aluminum oxide (Al ₂ O ₃ ) substrate with the r-plane, c-plane, or a-plane as the upper surface. The alumina substrate is a polycrystalline aluminum oxide (Al ₂ O ₃ ) substrate. The silicon substrate is a single crystal or polycrystalline silicon (Si) substrate. The silicon carbide substrate is a single crystal or polycrystalline silicon carbide (SiC) substrate.

The metal film 14 is a film whose main component is, for example, aluminum (Al), copper (Cu) or molybdenum (Mo), for example, an aluminum film, a copper film or a molybdenum film. An adhesive film such as a Ti (titanium) film or a Cr (chromium) film may be provided between the electrode fingers 15 and the piezoelectric substrate 12. The adhesive film is thinner than the electrode fingers 15. An insulating film may be provided so as to cover the electrode fingers 15. The insulating film functions as a protective film or a temperature compensation layer.

The thickness of the support substrate 10 is, for example, 50 μm to 500 μm. The thickness T1 of the insulating layer 11 is, for example, 0.1 μm to 10 μm, and is, for example, equal to or less than the wavelength λ of the elastic wave. The thickness T2 of the piezoelectric substrate 12 is, for example, 0.5 μm to 20 μm, and is, for example, equal to or less than the wavelength λ of the elastic wave. The wavelength λ of the elastic wave is, for example, 1 μm to 6 μm. When two electrode fingers 15 are considered as one pair, the number of pairs is, for example, 20 pairs to 300 pairs. The duty ratio of the IDT 22 is the thickness of the electrode finger 15/the pitch of the electrode finger 15, and is, for example, 30% to 80%. The aperture length of the IDT 22 is, for example, 10λ to 50λ.

When a material with a smaller linear expansion coefficient than the piezoelectric substrate 12 is used as the support substrate 10, the change in pitch of the electrode fingers 15 due to temperature change is smaller, and the change in the frequency temperature coefficient is also smaller. Furthermore, the positive and negative signs of the temperature coefficients of the elastic constants of the piezoelectric substrate 12 and the insulating layer 11 are reversed. This further reduces the frequency temperature coefficient.

[Experiments and Simulations]
The temperature coefficient of frequency (TCF) of the resonant frequency fr and the anti-resonant frequency was measured and simulated by changing the thicknesses T1 and T2 of the piezoelectric substrate 12 and the insulating layer 11. Figures 3(a) and 3(b) are cross-sectional views of the acoustic wave resonators used in the experiment and the simulation, respectively.

As shown in FIG. 3(a), in the experiment, a bonding layer 13 is provided between the insulating layer 11 and the piezoelectric substrate 12. The method of manufacturing the elastic wave resonator 20 will be described. The insulating layer 11 is formed on the support substrate 10 using a CVD (Chemical Vapor Deposition) method. The bonding layer 13 is formed on the insulating layer 11. The bonding layer 13 and the piezoelectric substrate 12 are bonded using a surface activation method. If a silicon oxide film is used as the insulating layer 11, the bonding strength with the piezoelectric substrate is weak, so the bonding layer 13 is provided. The upper surface of the piezoelectric substrate 12 is made to the desired thickness using a CMP (Chemical Mechanical Polishing) method. The IDT 22 and the reflector 24 are formed on the piezoelectric substrate 12 using a metal film 14.

The conditions for fabricating the acoustic wave resonator 20 are as follows.
Support substrate 10: sapphire substrate Insulating layer 11: silicon oxide layer with thickness T1 Bonding layer 13: aluminum oxide layer with thickness 10 nm Piezoelectric substrate 12: 42° Y-cut X-propagation lithium tantalate substrate with thickness T2 Metal film 14: aluminum film with thickness 500 nm Pitch of electrode fingers 15×2: 5 μm (wavelength λ of elastic wave)
Number of pairs of electrode fingers 15 (number of fingers x 2): 100 pairs Duty ratio: 50%
Opening length: 100μm (20λ)

As shown in FIG. 3(b), in the simulation, the support substrate 10 is provided on the damping material 46. An insulating layer 11 is provided on the support substrate 10, and a piezoelectric substrate 12 is provided on the insulating layer 11. Electrode fingers 15 are provided on the piezoelectric substrate 12. An air layer 44 is provided on the piezoelectric substrate 12 so as to cover the electrode fingers 15. The pitch of the electrode fingers 15 is λ/2. The width in the X direction is λ, and the boundary condition in the X direction is a periodic boundary condition. The width in the Y direction is λ/32, and the boundary condition in the Y direction is a periodic boundary condition. The other simulation conditions are the same as in the experiment.

Figures 4(a) and 4(b) show experimental and simulation results for the TCF of the resonant frequency and anti-resonant frequency. The horizontal axis is the thickness T2/λ of the piezoelectric substrate 12, and the vertical axis is the TCF. The thickness T2 of the piezoelectric substrate 12 was set to 0.2λ, 0.4λ, and 0.6λ, and the thickness T1 of the insulating layer 11 was set to 0.2λ, 0.4λ, and 0.6λ. The dots represent measurement and simulation points, and the curves are approximation curves. The estimated line of T1 = 0.1λ from the measurement points of T1 = 0.2λ, 0.4λ, and 0.6λ is shown as a solid line.

Comparing Figures 4(a) and 4(b), the TCF of the anti-resonant frequency fa is located on the negative side of the TCF of the resonant frequency fr. When the thickness T2 of the piezoelectric substrate 12 decreases, the TCF changes in the positive direction. The difference in TCF between T2 of 0.4λ and 0.2λ is greater than the difference in TCF between T2 of 0.6λ and 0.4λ. When the thickness T1 of the insulating layer 11 increases, the TCF changes in the positive direction.

The positive and negative signs of the temperature coefficient of the elastic constant are opposite for lithium tantalate and silicon oxide. The energy of the surface acoustic wave excited by the electrode fingers 15 exists mainly in the piezoelectric substrate 12 and the insulating layer 11. When the piezoelectric substrate 12 is made thinner, the energy of the surface acoustic wave contained in the insulating layer 11 increases and the TCF changes in the positive direction. When the insulating layer 11 is made thicker, the energy of the surface acoustic wave contained in the insulating layer 11 increases and the TCF changes in the positive direction. In this way, by appropriately setting the thickness T1 of the piezoelectric substrate 12 and the thickness T2 of the insulating layer 11, the TCF at the high-frequency end and the TCF at the low-frequency end of the pass band of the ladder filter can be set independently.

For example, using simulation results as an example, when T1 = 0.6λ and T2 = 0.4λ in a series resonator, the TCF of the anti-resonant frequency fas of the series resonator is approximately -5 ppm/K. When T1 = 0.2λ and T2 = 0.4λ in a parallel resonator, the TCF of the resonant frequency frp of the parallel resonator is approximately -5 ppm/K. In this way, by making the thickness T2 of the piezoelectric substrate 12 of the series resonator and the parallel resonator approximately the same and making the thickness T1 of the insulating layer 11 of the series resonator greater than the thickness T1 of the insulating layer 11 of the parallel resonator, the TCF of the anti-resonant frequency fas of the series resonator and the TCF of the resonant frequency frp of the parallel resonator can be made approximately equal and close to 0.

In another example, for a series resonator, if T1 = 0.4λ and T2 = 0.2λ, the TCF of the anti-resonant frequency fas of the series resonator is approximately -10 ppm/K. For a parallel resonator, if T1 = 0.4λ and T2 = 0.6λ, the TCF of the resonant frequency frp of the parallel resonator is approximately -10 ppm/K. If the thickness T1 of the insulating layer 11 of the series resonator and the parallel resonator is approximately the same and the thickness T2 of the piezoelectric substrate 12 of the series resonator is smaller than the thickness T2 of the piezoelectric substrate 12 of the parallel resonator, the TCF of the anti-resonant frequency fas of the series resonator and the TCF of the resonant frequency frp of the parallel resonator can be approximately the same and approach 0.

Figure 5 is a plan view of the filter according to the first embodiment. As shown in Figure 5, an elastic wave resonator 20 and a wiring 26 are provided on a piezoelectric substrate 12. The elastic wave resonator 20 includes series resonators S1 to S4 and parallel resonators P1 to P3. The wiring 26 includes pads Pin, Pout, and Pg. The pads Pin, Pout, and Pg are electrically connected to the input terminal Tin, the output terminal Tout, and the ground terminal Tg, respectively. The series resonators S1 to S4 are connected in series between the pads Pin and Pout via the wiring 26, and the parallel resonators P1 to P3 are connected in parallel between the pads Pin and Pout via the wiring 26. The parallel resonators P1 to P3 are provided in an area 50, and the series resonators S1 to S4 are provided in an area 52.

Figure 6 is a cross-sectional view of the filter according to Example 1. As shown in Figure 6, the average thicknesses of the piezoelectric substrate 12 in regions 50 and 52 are T2p and T2s, respectively. A step is provided on the upper surface of the support substrate 10 between regions 50 and 52. The average thickness T1p of the insulating layer 11 in region 50 is smaller than the average thickness T1s of the insulating layer 11 in region 52. The average thicknesses T2p and T2s of the piezoelectric substrate 12 are approximately equal. The upper surface of the piezoelectric substrate 12 between regions 50 and 52 has no step and is approximately flat.

According to the first embodiment, the average thickness T1s of the insulating layer 11 in the region 52 (first region) where the series resonators S1 to S4 are provided is different from the average thickness T1p of the insulating layer 11 in the region 50 (second region) where the parallel resonators P1 to P3 are provided. This allows the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband to be set independently.

The average thickness T1s of the insulating layer in region 52 is greater than the average thickness T1p of the insulating layer 11 in region 50. This allows the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband to be closer together. T1s is preferably 1.1 times or more, and more preferably 1.5 times or more, of T1p. T1s is preferably 3 times or less of T1p.

The average thickness T2s of the piezoelectric substrate 12 in region 52 and the average thickness T2p of the piezoelectric substrate 12 in region 50 are approximately equal to a degree that allows for manufacturing errors. This allows the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband to be brought closer together.

[Modification 1 of Example 1]
7A is a cross-sectional view of a filter according to a first modified example of the first embodiment. As shown in FIG. 7A, a convex portion 54 and a concave portion 56 are provided at the interface between the support substrate 10 and the insulating layer 11. The thicknesses T4p and T4s of the insulating layer 11 at the concave portions 56 in the regions 50 and 52 are substantially equal. The heights T5p and T5s of the convex portions 54 in the regions 50 and 52 are different from each other. T5p is greater than T5s. As a result, the average thickness T1p of the insulating layer 11 in the region 50 is smaller than the average thickness T1s of the insulating layer 11 in the region 52. The other configurations are the same as those of the first embodiment, and therefore description thereof will be omitted.

As in Variation 1 of Example 1, the height of the protrusions 54 may be varied to make the average thicknesses T1p and T1s of the insulating layer 11 different. The average thicknesses T1p and T1s of the insulating layer 11 can be calculated from the volume per unit area of the insulating layer 11 in a plan view.

[Modification 2 of Example 1]
Fig. 7B is a cross-sectional view of a filter according to Modification 2 of Example 1. As shown in Fig. 7B, there is no step between regions 50 and 52 on the upper surface of the support substrate 10 and the upper surface of the piezoelectric substrate 12, and the upper surface is substantially flat. A step is provided on the upper surface of the insulating layer 11 between regions 50 and 52. T2p is larger than T2s, and T1p is smaller than T1s. The other configurations are the same as those of Example 1, and therefore description thereof will be omitted.

As in the second modification of the first embodiment, the average thickness T2s of the piezoelectric substrate 12 in the region 52 is different from the average thickness T2p of the piezoelectric substrate 12 in the region 50. This allows the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband to be set independently.

The average thickness T1s of the insulating layer 11 in region 52 is greater than the average thickness T1p of the insulating layer 11 in region 50, and the average thickness T2s of the piezoelectric substrate 12 in region 52 is smaller than the average thickness T2p of the piezoelectric substrate 12 in region 50. This allows the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband to be closer together. T2p is preferably 1.1 times or more than T2s, and more preferably 1.5 times or more. T2p is preferably 3 times or less than T2s.

[Modification 3 of Example 1]
Fig. 8(a) is a cross-sectional view of a filter according to Modification 3 of Example 1. As shown in Fig. 8(a), there is no step between areas 50 and 52 on the upper surface of the piezoelectric substrate 12, which is substantially flat. Steps are provided on the upper surface of the support substrate 10 and the upper surface of the insulating layer 11 between areas 50 and 52. T2p is larger than T2s, and T1p is substantially equal to T1s. The other configurations are the same as those of Example 1, and therefore description thereof will be omitted.

As in the third modification of the first embodiment, the average thickness T1s of the insulating layer 11 in the region 52 is approximately equal to the average thickness T1p of the insulating layer 11 in the region 50 within the manufacturing error range, and the average thickness T2s of the piezoelectric substrate 12 in the region 52 is smaller than the average thickness T2p of the piezoelectric substrate 12 in the region 50. This makes it possible to bring the frequency temperature coefficients of the high-frequency end and the low-frequency end of the pass band closer together.

[Fourth Modification of the First Embodiment]
FIG. 8B is a cross-sectional view of a filter according to a fourth modified example of the first embodiment. As shown in FIG. 8B, an insulating layer 11p is provided between the support substrate 10 and the piezoelectric substrate 12 in the region 50, and an insulating layer 11s is provided between the support substrate 10 and the piezoelectric substrate 12 in the region 52. T2p is approximately equal to T2s, and T1p is approximately equal to T1s. The temperature coefficient of the elastic constant of the insulating layer 11p is different from the temperature coefficient of the elastic constant of the insulating layer 11s. For example, the fluorine concentration of the insulating layer 11s is higher than that of the insulating layer 11p. For example, fluorine is not intentionally added to the insulating layer 11p, and about 5 atomic % of fluorine is added to the insulating layer 11s. The other configurations are the same as those of the first embodiment, and a description thereof will be omitted.

When the fluorine concentration in silicon oxide increases, the amount of change in the temperature coefficient of the elastic constant increases. Therefore, the fluorine concentration of insulating layer 11s in region 52 is made higher than the fluorine concentration of insulating layer 11p in region 50. This makes it possible to bring the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband closer together.

According to the fourth modification of the first embodiment, the temperature coefficient of the elastic constant of the insulating layer 11s in the region 52 is different from the temperature coefficient of the elastic constant of the insulating layer 11p in the region 50. This allows the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband to be set independently.

The temperature coefficient of the elastic constant of the insulating layer 11s in region 52 has the same sign as the temperature coefficient of the elastic constant of the insulating layer 11p in region 50 and has a larger absolute value. This makes it possible to bring the frequency temperature coefficients of the high-frequency end and the low-frequency end of the passband closer together.

In the first embodiment and its first to third modifications, the temperature coefficient of the elastic constant of the insulating layer 11 may be different between regions 50 and 52. It is preferable that the temperature coefficient of the elastic constant of the insulating layer 11 in region 52 has the same sign as the temperature coefficient of the elastic constant of the insulating layer 11 in region 50 and has a larger absolute value.

In the first embodiment and its modified example, the energy of the surface acoustic wave is mainly present between the surface of the piezoelectric substrate 12 and a depth of about 2λ from the surface. Therefore, the average thicknesses T2p and T2s of the piezoelectric substrate in the regions 50 and 52 are set to be less than twice the average pitch of the electrode fingers 15, and the average thicknesses T1p and T1s of the insulating layer 11 in the regions 50 and 52 are set to be less than twice the average pitch of the electrode fingers 15. This distributes the energy of the surface acoustic wave to both the piezoelectric substrate 12 and the insulating layer 11. Therefore, the TCF can be set by setting the thicknesses of the piezoelectric substrate 12 and the insulating layer 11. T2p, T2s, T1p, and T1s are preferably 1.5 times or less, more preferably 1.2 times or less, of the average pitch of the electrode fingers 15. T2p, T2s, T1p, and T1s are preferably 0.2 times or more of the average pitch of the electrode fingers 15.

When the IDT 22 excites SH (Shear Horizontal), bulk waves are likely to be generated. When the piezoelectric substrate 12 is a Y-cut lithium tantalate substrate rotated 36° or more and 48° or less, SH waves are excited. At this time, when the thickness of the piezoelectric substrate 12 is equal to or less than the wavelength λ of the elastic wave, that is, when it is equal to or less than twice the average pitch of the electrode fingers 15, loss is suppressed. In addition, when the distance from the top surface of the support substrate 10 to the top surface of the piezoelectric substrate 12 is equal to or less than four times the average pitch of the electrode fingers 15, loss is suppressed.

To prevent the elastic wave from leaking to the support substrate 10, it is preferable that the acoustic impedance of the support substrate 10 is higher than that of the piezoelectric substrate 12 (i.e., the sound velocity of the support substrate 10 is faster than that of the piezoelectric substrate 12). In addition, since the elastic wave propagates within the insulating layer 11, it is preferable that the acoustic impedance of the insulating layer 11 is lower than that of the piezoelectric substrate 12 and the support substrate 10 (i.e., the sound velocity of the insulating layer 11 is slower than that of the piezoelectric substrate 12 and the support substrate 10). Aluminum oxide, aluminum nitride, or silicon nitride can be used as the bonding layer 13 in FIG. 3(a). The thickness of the bonding layer 13 is 1 nm or more and 10 nm or less.

Figure 9 is a circuit diagram of a duplexer according to the second embodiment. As shown in Figure 9, a transmission filter 40 is connected between the common terminal Ant and the transmission terminal Tx. A reception filter 42 is connected between the common terminal Ant and the reception terminal Rx. The transmission filter 40 passes a signal in the transmission band among the high-frequency signals input from the transmission terminal Tx to the common terminal Ant as a transmission signal, and suppresses signals of other frequencies. The reception filter 42 passes a signal in the reception band among the high-frequency signals input from the common terminal Ant to the reception terminal Rx as a reception signal, and suppresses signals of other frequencies. At least one of the transmission filter 40 and the reception filter 42 can be a filter according to the first embodiment and its modified examples.

Although a duplexer has been used as an example of a multiplexer, a triplexer or quadplexer may also be used.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

REFERENCE SIGNS LIST 10 Support substrate 11, 11p, 11s Insulating layer 12 Piezoelectric substrate 13 Bonding layer 15 Electrode finger 18 Interdigital electrode 20 Acoustic wave resonator 22 IDT
50, 52 Area

Claims (9)

A support substrate;
an insulating layer that is directly or indirectly bonded to the support substrate, the insulating layer having an average thickness in a first region greater than an average thickness in a second region, and that is mainly composed of silicon oxide;
a piezoelectric substrate that is directly or indirectly bonded onto the insulating layer, the piezoelectric substrate having a temperature coefficient of elastic constant that is opposite in sign to the temperature coefficient of elastic constant of the insulating layer, the piezoelectric substrate being a lithium tantalate substrate;
a series resonator that is provided on the piezoelectric substrate in the first region, has a pair of comb-shaped electrodes, and is connected in series between a first terminal and a second terminal to form a ladder-type filter ;
a parallel resonator forming the ladder-type filter, the parallel resonator having a pair of comb-shaped electrodes and one end connected to a line on which the series resonator is provided between the first terminal and the second terminal and the other end connected to ground , the parallel resonator being provided on the piezoelectric substrate in the second region ;
Equipped with
one end of at least one of the parallel resonators is connected to the line between two of the series resonators;
A ladder-type filter, wherein a thickness of the piezoelectric substrate in the first region and the second region is less than or equal to twice the average pitch of the electrode fingers of the pair of comb electrodes, and a thickness of the insulating layer in the first region and the second region is less than or equal to twice the average pitch of the electrode fingers of the pair of comb electrodes. 2. The ladder-type filter according to claim 1, wherein an average thickness of the piezoelectric substrate in the first region is smaller than an average thickness of the piezoelectric substrate in the second region. 2. The ladder-type filter according to claim 1, wherein an average thickness of the piezoelectric substrate in the first region is equal to an average thickness of the piezoelectric substrate in the second region. A support substrate;
an insulating layer that is directly or indirectly bonded to the support substrate and is mainly composed of silicon oxide;
a piezoelectric substrate bonded directly or indirectly onto the insulating layer, the piezoelectric substrate having an average thickness in a first region smaller than an average thickness in a second region, a temperature coefficient of elastic constant having a sign opposite to that of the insulating layer, the piezoelectric substrate being a lithium tantalate substrate;
a series resonator that is provided on the piezoelectric substrate in the first region, has a pair of comb-shaped electrodes, and is connected in series between a first terminal and a second terminal to form a ladder-type filter ;
a parallel resonator forming the ladder-type filter, the parallel resonator having a pair of comb-shaped electrodes and one end connected to a line on which the series resonator is provided between the first terminal and the second terminal and the other end connected to ground , the parallel resonator being provided on the piezoelectric substrate in the second region ;
Equipped with
one end of at least one of the parallel resonators is connected to the line between two of the series resonators;
A ladder-type filter, wherein a thickness of the piezoelectric substrate in the first region and the second region is less than or equal to twice the average pitch of the electrode fingers of the pair of comb electrodes, and a thickness of the insulating layer in the first region and the second region is less than or equal to twice the average pitch of the electrode fingers of the pair of comb electrodes. 5. The ladder-type filter according to claim 4, wherein an average thickness of the insulating layer in the first region is equal to an average thickness of the insulating layer in the second region. A support substrate;
an insulating layer that is directly or indirectly bonded to the support substrate, the temperature coefficient of elastic constant in the first region having the same sign as the temperature coefficient of elastic constant in the second region and a larger absolute value, and that is mainly composed of silicon oxide;
a piezoelectric substrate that is directly or indirectly bonded onto the insulating layer, the piezoelectric substrate having a temperature coefficient of elastic constant that is opposite in sign to the temperature coefficient of elastic constant of the insulating layer, the piezoelectric substrate being a lithium tantalate substrate;
a series resonator that is provided on the piezoelectric substrate in the first region, has a pair of comb-shaped electrodes, and is connected in series between a first terminal and a second terminal to form a ladder-type filter ;
a parallel resonator forming the ladder filter, the parallel resonator being provided on the piezoelectric substrate in the second region, the parallel resonator having a pair of comb-shaped electrodes and connected in parallel between the first terminal and the second terminal;
Equipped with
A ladder-type filter, wherein a thickness of the piezoelectric substrate in the first region and the second region is less than or equal to twice the average pitch of the electrode fingers of the pair of comb electrodes, and a thickness of the insulating layer in the first region and the second region is less than or equal to twice the average pitch of the electrode fingers of the pair of comb electrodes. an average thickness of the insulating layer in the first region and an average thickness of the insulating layer in the second region are equal;
7. The ladder-type filter according to claim 6, wherein an average thickness of the piezoelectric substrate in the first region is equal to an average thickness of the piezoelectric substrate in the second region. 8. The ladder-type filter according to claim 1, wherein the ladder-type filter does not include any resonator other than the series resonator and the parallel resonator. A multiplexer comprising the ladder filter according to any one of claims 1 to 8.

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