JP2012175315A - Surface acoustic wave filter device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface acoustic wave filter device having a steep filter property, a wide passband, and a small manufacturing variation in filter property.SOLUTION: A surface acoustic wave filter device 1 includes a ladder type surface acoustic wave filter part 20, in which a thickness t1 of a dielectric layer 33 of a surface acoustic wave resonator 30 constituting series arm resonators is different from a thickness t2 of a dielectric layer 43 of a surface acoustic wave resonator 40 constituting parallel arm resonators, and a duty ratio of an IDT electrode 32 of the surface acoustic wave resonator 30 haing the thicker dielectric layer is smaller than a duty ratio of an IDT electrode 42 of the surface acoustic wave resonator 40 having the thinner dielectric layer.

Description

  The present invention relates to a surface acoustic wave filter device. In particular, the present invention relates to a surface acoustic wave filter device having a ladder type surface acoustic wave filter section.

  2. Description of the Related Art Conventionally, a surface acoustic wave filter device using a surface acoustic wave is used as a band-pass filter or a duplexer mounted on an RF (Radio Frequency) circuit in a communication device such as a mobile phone. As an example of such a surface acoustic wave filter device, for example, Patent Document 1 below describes a surface acoustic wave filter device having a ladder-type surface acoustic wave filter unit. In Patent Document 1, in a plurality of surface acoustic wave resonators constituting a ladder-type surface acoustic wave filter unit, a protective film is provided on the IDT electrode of each surface acoustic wave resonator to constitute a parallel arm resonator. By making the thickness of the protective film on the IDT electrode in the surface acoustic wave resonator different from the thickness of the protective film on the IDT electrode in the surface acoustic wave resonator constituting the series arm resonator, the filter It describes that the characteristics are steep and the frequency characteristics in the passband are flat.

JP 2000-196409 A

  However, when the surface acoustic wave filter device described in Patent Document 1 is to be manufactured, there is a problem that the manufacturing variation of the filter characteristics becomes large.

  The present invention has been made in view of the above points, and an object thereof is to provide a surface acoustic wave filter device having a steep filter characteristic, a wide pass band, and a small manufacturing variation in filter characteristics. There is.

  The surface acoustic wave filter device according to the present invention includes a ladder type surface acoustic wave filter section. The ladder-type surface acoustic wave filter unit includes a series arm, a series arm resonator, a parallel arm, and a parallel arm resonator. The series arm resonator is connected in the series arm. The parallel arm connects the serial arm and the ground. The parallel arm resonator is provided on the parallel arm. Each of the series arm resonator and the parallel arm resonator is constituted by a surface acoustic wave resonator having a piezoelectric substrate, an IDT electrode, and a dielectric layer. The IDT electrode is formed on the piezoelectric substrate. The dielectric layer is formed so as to cover the IDT electrode. The thickness of the dielectric layer of the surface acoustic wave resonator constituting the series arm resonator is different from the thickness of the dielectric layer of the surface acoustic wave resonator constituting the parallel arm resonator. The duty ratio of the IDT electrode of the surface acoustic wave resonator having the thick dielectric layer is smaller than the duty ratio of the IDT electrode of the surface acoustic wave resonator having the thin dielectric layer.

  In a specific aspect of the surface acoustic wave filter device according to the present invention, each of the series arm resonator and the parallel arm resonator includes a plurality of surface acoustic wave resonators, and the thickness of the dielectric layer The average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator having a large thickness is smaller than the average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator having a thin dielectric layer.

  In another specific aspect of the surface acoustic wave filter device according to the present invention, the duty ratio of the IDT electrode of the surface acoustic wave resonator constituting each of the series arm resonator and the parallel arm resonator is 0.4 to 0.6. It is in the range.

  In another specific aspect of the surface acoustic wave filter device according to the present invention, the surface acoustic wave filter device forms a series arm resonator or a parallel arm resonator, and the surface acoustic wave has a thick dielectric layer. A surface acoustic wave resonator in which the thickness normalized by the surface acoustic wave wavelength of the dielectric layer of the resonator is t1, a series arm resonator or a parallel arm resonator is formed, and the dielectric layer is thin. The thickness of the dielectric layer of the surface acoustic wave that is normalized by the wavelength of the surface acoustic wave is t2, which constitutes a series arm resonator or a parallel arm resonator, and the IDT of the surface acoustic wave resonator having a thick dielectric layer. The average value of the duty ratio of the electrode is D1, and the average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator in which the series arm resonator or the parallel arm resonator is formed and the thickness of the dielectric layer is thin is D2. Then, D1 <D2 <D1 + 0.465 × (t1−t2) is satisfied.

In still another specific aspect of the surface acoustic wave filter device according to the present invention, the piezoelectric substrate is made of a LiNbO ₃ substrate. The surface acoustic wave filter device uses Rayleigh waves as the main mode.

In still another specific aspect of the surface acoustic wave filter device according to the present invention, the dielectric layer includes a SiO ₂ layer. The dielectric layer may be composed of only the SiO ₂ layer, it may be constituted by a laminate including a SiO ₂ layer.

  According to the present invention, it is possible to provide a surface acoustic wave filter device having steep filter characteristics, a wide pass band, and small manufacturing variations in filter characteristics.

1 is an equivalent circuit diagram of a surface acoustic wave filter device according to an embodiment of the present invention. 1 is a schematic cross-sectional view of a part of a surface acoustic wave resonator constituting a series arm resonator according to an embodiment of the present invention. 1 is a schematic cross-sectional view of a part of a surface acoustic wave resonator constituting a parallel arm resonator according to an embodiment of the present invention. 5 is a graph showing the filter characteristics of the transmission filter of the surface acoustic wave filter device of Comparative Example 1 and the filter characteristics of the transmission filter of the surface acoustic wave filter device of Comparative Example 2. 5 is a graph showing filter characteristics of a transmission filter of a surface acoustic wave filter device according to an embodiment of the present invention and filter characteristics of a transmission filter of a surface acoustic wave filter device of Comparative Example 1. 5 is a graph showing filter characteristics of a transmission filter of a surface acoustic wave filter device according to an embodiment of the present invention and filter characteristics of a transmission filter of a surface acoustic wave filter device of Comparative Example 2. It is a graph showing the relationship between the duty ratio of an IDT electrode and the electrode finger width dependence of the frequency of an IDT electrode. In FIG. 7, the data indicated by diamonds is data when the thickness normalized by the surface acoustic wave wavelength of the SiO ₂ layer of the dielectric layer formed on the IDT electrode is 0.33. It is. In FIG. 7, the data indicated by squares is data in the case where the thickness normalized by the surface acoustic wave wavelength of the SiO ₂ layer of the dielectric layer formed on the IDT electrode is 0.29. . In FIG. 7, the data indicated by triangles is data when the thickness normalized by the surface acoustic wave wavelength of the SiO ₂ layer of the dielectric layer formed on the IDT electrode is 0.24. . In FIG. 7, the data indicated by x is data in the case where the thickness normalized by the wavelength of the surface acoustic wave of the SiO ₂ layer of the dielectric layer formed on the IDT electrode is 0.20. . It is a graph showing the filter characteristic of the transmission filter of the surface acoustic wave filter apparatus of the 1st pattern of the comparative example 3, and the filter characteristic of the transmission filter of the surface acoustic wave filter apparatus of the 2nd pattern. It is a graph showing the filter characteristic of the transmission filter of the surface acoustic wave filter apparatus of the 1st pattern of the Example of this invention, and the filter characteristic of the transmission filter of the surface acoustic wave filter apparatus of a 2nd pattern.

  Hereinafter, a preferred embodiment in which the present invention is implemented will be described taking the surface acoustic wave filter device 1 shown in FIG. 1 as an example. The surface acoustic wave filter device 1 of this embodiment is a duplexer having a ladder type surface acoustic wave filter section. A duplexer is a type of duplexer. However, the surface acoustic wave filter device according to the present invention is not limited to a duplexer. The surface acoustic wave filter device according to the present invention is not particularly limited as long as it has a ladder type surface acoustic wave filter section, and may be another duplexer such as a triplexer, an input signal terminal or an output. It may include a plurality of filter portions in which at least one of the signal terminals is not connected in common, or may include only one ladder type surface acoustic wave filter portion.

  FIG. 1 is an equivalent circuit diagram of a surface acoustic wave filter device 1 according to this embodiment. FIG. 2 is a schematic cross-sectional view of a part of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, and S3 in the present embodiment. FIG. 3 is a schematic cross-sectional view of a part of the surface acoustic wave resonator 40 constituting the parallel arm resonators P1, P2 and P3 in the present embodiment.

  The surface acoustic wave filter device 1 according to the present embodiment is mounted on an RF circuit such as a mobile phone compatible with a CDMA system such as UMTS. Specifically, the surface acoustic wave filter device 1 is a duplexer corresponding to UMTS-BAND3. The transmission frequency band of UMTS-BAND3 is 1710 MHz to 1785 MHz, and the reception frequency band is 1805 MHz to 1880 MHz. Thus, in UMTS-BAND3, the transmission side frequency band is located on the lower frequency side than the reception side frequency band.

  As shown in FIG. 1, the surface acoustic wave filter device 1 includes an antenna terminal 10 connected to an antenna, a transmission-side signal terminal 11, and a reception-side signal terminal 12. A reception filter 13 is connected between the antenna terminal 10 and the reception-side signal terminal 12. In the present embodiment, the configuration of the reception filter 13 is not particularly limited. The reception filter 13 may be configured by, for example, a longitudinally coupled resonator type surface acoustic wave filter unit, or may be configured by a ladder type surface acoustic wave filter unit. In FIG. 1, only one reception-side signal terminal 12 is drawn. However, when the reception filter 13 is composed of a balanced filter unit having a balance-unbalance conversion function, two reception signals are received. A side signal terminal is provided.

  A transmission filter 20 is connected between the antenna terminal 10 and the transmission-side signal terminal 11. An inductor L as a matching circuit is connected between a connection point between the transmission filter 20 and the reception filter 13 and the connection point between the antenna terminal 10 and the ground.

  The transmission filter 20 includes a ladder type surface acoustic wave filter unit. The transmission filter 20 includes a series arm 21 that connects the antenna terminal 10 and the transmission-side signal terminal 11. A plurality of series arm resonators S1, S2, S3 are connected to the series arm 21 in series. Each of the series arm resonators S1, S2, and S3 includes a plurality of surface acoustic wave resonators that function as a single resonator. Thereby, the power durability of the transmission filter 20 is improved. However, in the present invention, the series arm resonator may be constituted by a plurality of surface acoustic wave resonators functioning as one resonator, or may be constituted by one surface acoustic wave resonator. .

  The transmission filter 20 has a plurality of parallel arms 22, 23, and 24 connected between the series arm 21 and the ground. Each of the parallel arms 22, 23, 24 is provided with parallel arm resonators P1, P2, P3. Each of the parallel arm resonators P1, P2, and P3 is composed of a plurality of surface acoustic wave resonators that function as one resonator. Thereby, the power durability of the transmission filter 20 is improved. However, in the present invention, the parallel arm resonator may be constituted by a plurality of surface acoustic wave resonators functioning as one resonator, or may be constituted by one surface acoustic wave resonator. .

  Next, referring to FIGS. 2 and 3, the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 and the elasticity constituting the parallel arm resonators P1, P2, P3 are illustrated. A configuration with the surface acoustic wave resonator 40 will be described.

The surface acoustic wave resonators 30 and 40 include a piezoelectric substrate 31. That is, the surface acoustic wave resonators 30 and 40 share the piezoelectric substrate 31. In the transmission filter 20 of the present embodiment, the Rayleigh wave is the main mode. Therefore, the piezoelectric substrate 31 is configured by a LiNbO ₃ substrate. Specifically, in the present embodiment, the piezoelectric substrate 31 is composed of a LiNbO ₃ substrate in which θ of Euler angles (φ, θ, ψ) is in the range of 25 ° to 45 °. Note that φ is preferably in the range of 0 ° ± 5 °.

Hereinafter, in the present embodiment, an example in which the piezoelectric substrate 31 is configured by a LiNbO ₃ substrate having Euler angles (0 °, 37.5 °, 0 °) will be described. In this case, the transmission filter 20 sets the Rayleigh wave propagating in the direction of ψ as the main mode. That is, ψ = 0 ° is the propagation direction. Note that a LiNbO ₃ substrate having Euler angles of (0 °, 37.5 °, 0 °) is a 127.5 ° Y-cut X-propagation LiNbO ₃ substrate in another expression. However, in the present invention, the piezoelectric substrate may not be composed by the LiNbO ₃ substrate. The piezoelectric substrate may be composed of, for example, a LiTaO ₃ substrate or a quartz substrate.

  The surface acoustic wave resonator 30 includes an IDT electrode 32 and a dielectric layer 33. The IDT electrode 32 is formed on the main surface 31 a of the piezoelectric substrate 31. The dielectric layer 33 is formed on the IDT electrode 32 and the main surface 31 a of the piezoelectric substrate 31. That is, the IDT electrode 32 is covered with the dielectric layer 33. The surface acoustic wave resonator 40 includes an IDT electrode 42 and a dielectric layer 43. The IDT electrode 42 is formed on the main surface 31 a of the piezoelectric substrate 31. The dielectric layer 43 is formed on the IDT electrode 42 and the main surface 31 a of the piezoelectric substrate 31. That is, the IDT electrode 42 is covered with the dielectric layer 43.

  The IDT electrodes 32 and 42 are composed of a pair of comb-like electrodes. The comb-like electrode has a plurality of electrode fingers and a bus bar to which the plurality of electrode fingers are connected. 2 and 3, only the electrode finger portion of the IDT electrodes 32 and 42 is shown. The IDT electrodes 32 and 42 are not particularly limited. The IDT electrodes 32 and 42 are, for example, a metal selected from the group consisting of Al, Pt, Cu, Au, Ag, W, Ni, Cr, Ti, Co, and Ta, or Al, Pt, Cu, Au, Ag. , W, Ni, Cr, Ti, Co, and Ta, and an alloy containing one or more metals selected from the group consisting of Ta, Co, and Ta. Further, the IDT electrodes 32 and 42 may be formed of a laminate of a plurality of conductive films made of the above metals or alloys.

  Specifically, in the present embodiment, the IDT electrodes 32 and 42 are both NiCr layer (thickness 10 nm), Pt layer (thickness 40 nm), and Ti layer (thickness 10 nm) from the piezoelectric substrate 31 side. And the Al layer (thickness 150 nm) and the Ti layer (thickness 10 nm) are comprised by the laminated body laminated | stacked in this order. That is, the IDT electrode 32 and the IDT electrode 42 have the same layer configuration and the same film thickness. In the present embodiment, the IDT electrodes 32 and 42 are configured so that Rayleigh waves can be set as the main mode.

  In the present embodiment, the surface acoustic wave resonator 30 and the surface acoustic wave resonator 40 have different IDT electrode duty ratios (metallization ratios). Specifically, the duty ratio of the IDT electrode 32 is 0.475, and the duty ratio of the IDT electrode 42 is 0.52.

The dielectric layers 33 and 43 can be formed of various dielectric materials such as SiO ₂ , SiN, SiON, Ta ₂ O ₅ , AlN, Al ₂ O ₃ , and ZnO. The dielectric layers 33 and 43 may be formed of a single dielectric layer, but may be formed of a laminate of a plurality of dielectric layers. The dielectric layers 33 and 43 preferably include a SiO ₂ layer. In this case, since the SiO ₂ layer has a positive frequency temperature coefficient (TCF) and the LiNbO ₃ substrate has a negative frequency temperature coefficient, the frequency temperature coefficient of the dielectric layers 33 and 43, Since the sign of the frequency temperature coefficient of the piezoelectric substrate 31 is reversed, the frequency temperature characteristic of the transmission filter 20 can be improved.

The dielectric layer 33 and 43, for example, may be constituted by only the SiO ₂ layer but may be constituted by a laminate including a SiO ₂ layer. Specifically, in the present embodiment, the dielectric layers 33 and 43 are configured by a laminate of SiO ₂ layers 33a and 43a and SiN layers 33b and 43b. The SiO ₂ layers 33 a and 43 a are formed so as to cover the IDT electrodes 32 and 42 and the main surface 31 a of the piezoelectric substrate 31. The SiN layers 33b and 43b are formed on the SiO ₂ layers 33a and 43a. The SiN layers 33b and 43b function as a frequency adjustment film for adjusting the frequency of the transmission filter 20 by adjusting the thickness thereof. The thickness of the SiN layers 33b and 43b can be set to about 20 nm, for example.

In the present embodiment, the thickness of the SiO ₂ layer 33a is 670 nm, and the thickness of the SiO ₂ layer 43a is 370 nm. For this reason, the thickness t1 of the dielectric layer 33 in the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 is the surface acoustic wave constituting the parallel arm resonators P1, P2, P3. It is larger than the thickness t2 of the dielectric layer 43 in the resonator 40 (t1> t2).

  In the surface acoustic wave filter device 1 of the present embodiment, the thickness of the dielectric layer of the surface acoustic wave resonator constituting the series arm resonator is equal to the dielectric layer of the surface acoustic wave resonator constituting the parallel arm resonator. The duty ratio of the IDT electrode of the surface acoustic wave resonator having a thick dielectric layer is different from the duty ratio of the IDT electrode of the surface acoustic wave resonator having a thin dielectric layer. Is also small.

  That is, in the surface acoustic wave filter device 1 of this embodiment, the thickness of the dielectric layer 33 of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 is equal to the parallel arm resonator P1. , P2 and P3 are different from the thickness of the dielectric layer 43 of the surface acoustic wave resonator 40, and the duty ratio of the IDT electrode 32 of the surface acoustic wave resonator 30 is thick. However, the duty ratio of the IDT electrode 42 of the surface acoustic wave resonator 40 having a thin dielectric layer is smaller.

  In UMTS-BAND3, the transmission frequency band and the reception frequency band are separated by only 20 MHz. As described above, in a duplexer corresponding to a system in which the interval between the transmission frequency band and the reception frequency band is narrow, at least one of the transmission filter and the reception filter has high steep filter characteristics and excellent frequency temperature characteristics. It is required to be. More specifically, when the transmission frequency band is located on the lower frequency side than the reception frequency band, the transmission filter is required to have a filter characteristic having high steepness on the high frequency band side. Further, when the transmission frequency band is located on the higher frequency side than the reception frequency band, the transmission filter is required to have a filter characteristic having high steepness on the low frequency band side.

  Here, when the transmission filter is constituted by a ladder-type surface acoustic wave filter unit, in order to realize a filter characteristic having high steepness on the high side of the pass band, the elastic surface constituting the series arm resonator is used. It is necessary to reduce the Δf (frequency difference between the resonance frequency and the antiresonance frequency) of the wave resonator. This is because Δf of the surface acoustic wave resonator constituting the series arm resonator greatly affects the steepness on the high side of the passband. On the other hand, in order to realize filter characteristics with high steepness on the low pass band side, it is necessary to reduce Δf of the surface acoustic wave resonator constituting the parallel arm resonator. This is because Δf of the surface acoustic wave resonator constituting the parallel arm resonator greatly affects the steepness on the low pass band side.

In the surface acoustic wave resonator, Δf can be reduced by increasing the thickness of the dielectric layer formed so as to cover the IDT electrode and the main surface of the piezoelectric substrate. In particular, in the case of a surface acoustic wave resonator in which the piezoelectric substrate is a LiNbO ₃ substrate or a LiTaO ₃ substrate, Δf can be reduced by using a dielectric layer including a SiO ₂ layer and increasing the thickness of the dielectric layer. The frequency temperature characteristic can be improved. From the viewpoint of improving both the steepness on the high band side of the pass band and the steepness on the low band side of the transmission filter, the surface acoustic wave resonance that constitutes both the series arm resonator and the parallel arm resonator. It is conceivable to increase the thickness of the dielectric layer in the child.

  A surface acoustic wave filter device in which the thicknesses of the dielectric layers 33 and 43 in the surface acoustic wave resonators 30 and 40 constituting the transmission filter 20 are 370 nm is prepared as Comparative Example 1, and the transmission filter 20 is configured. A surface acoustic wave filter device in which the thicknesses of the dielectric layers 33 and 43 in the surface acoustic wave resonators 30 and 40 are 670 nm was prepared as Comparative Example 2. FIG. 4 shows the filter characteristics of the transmission filter of the surface acoustic wave filter device of Comparative Example 1 and the filter characteristics of the transmission filter of the surface acoustic wave filter device of Comparative Example 2. In FIG. 4, the solid line indicates Comparative Example 1, and the broken line indicates Comparative Example 2.

  In the first comparative example, in the surface acoustic wave resonator constituting both the series arm resonator and the parallel arm resonator of the transmission filter, the dielectric layer is thinned, so that Δf of all the surface acoustic wave resonators is increased. . For this reason, as can be seen from FIG. 4, while the pass band width is widened, both the steepness on the high side of the pass band and the steepness on the low side of the pass band are low. Therefore, in Comparative Example 1, the amount of attenuation in the reception frequency band of UMTS-BAND3 is insufficient.

  In Comparative Example 2, since the dielectric layer is thickened in the surface acoustic wave resonators constituting both the serial arm resonator and the parallel arm resonator of the transmission filter, Δf of all the surface acoustic wave resonators is Get smaller. For this reason, as can be seen from FIG. 4, both the steepness on the high side of the passband and the steepness on the low side of the passband are high, while the passband width is narrow. Therefore, in Comparative Example 2, a passband width corresponding to the transmission frequency band of UMTS-BAND3 cannot be realized.

  In this way, when the thickness of the dielectric layer in the surface acoustic wave resonator constituting the series arm resonator is equal to the thickness of the dielectric layer in the surface acoustic wave resonator constituting the parallel arm resonator It is difficult to realize a filter characteristic having a high steepness and a wide pass bandwidth.

  FIG. 5 shows the filter characteristics of the transmission filter 20 of the surface acoustic wave filter device 1 of the present embodiment and the filter characteristics of the transmission filter of the surface acoustic wave filter device of the first comparative example. In FIG. 5, the solid line indicates the present embodiment, and the broken line indicates Comparative Example 1. FIG. 6 shows the filter characteristics of the transmission filter 20 of the surface acoustic wave filter device 1 of the present embodiment and the filter characteristics of the transmission filter of the surface acoustic wave filter device of Comparative Example 2. In FIG. 6, the solid line indicates this embodiment, and the broken line indicates Comparative Example 2.

  As described above, in the transmission filter 20 of the surface acoustic wave filter device 1 of the present embodiment, the thickness of the dielectric layer 33 in the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, and S3. t1 is made thicker than the thickness t2 of the dielectric layer 43 in the surface acoustic wave resonator 40 constituting the parallel arm resonators P1, P2, and P3. Therefore, in this embodiment, as shown in FIG. 5, a pass bandwidth having a width almost the same as that of Comparative Example 1 can be obtained. Furthermore, as shown in FIG. 6, the steepness on the high side of the pass band equivalent to that of Comparative Example 2 can be obtained.

  In the present embodiment, since the dielectric layer 33 in the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 is thickened, Δf of the surface acoustic wave resonator 30 is reduced, The steepness on the high side of the passband is high. On the other hand, since the dielectric layer 43 in the surface acoustic wave resonator 40 constituting the parallel arm resonators P1, P2, and P3 is thinned, Δf of the surface acoustic wave resonator 40 is increased, and the passband width is increased. Is getting wider. That is, the thickness t1 of the dielectric layer 33 in the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 is the surface acoustic wave constituting the parallel arm resonators P1, P2, P3. In the present embodiment in which the thickness of the dielectric layer 43 in the resonator 40 is greater than the thickness t2, it is possible to achieve filter characteristics having high steepness and a wide pass bandwidth.

  In the present embodiment, the transmission side frequency band is located on the lower side of the reception side frequency band, and a transmission filter having a filter characteristic with high steepness on the high side of the pass band is required. UMTS-BAND3 Therefore, the thickness t1 of the dielectric layer 33 in the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 constitutes the parallel arm resonators P1, P2, P3. The thickness of the dielectric layer 43 in the surface acoustic wave resonator 40 is greater than t2.

  In the present invention, the transmission-side frequency band is located higher than the reception-side frequency band, and a transmission filter having a filter characteristic with high steepness in the low pass band is required. In the surface acoustic wave filter device, the thickness of the dielectric layer in the surface acoustic wave resonator constituting the series arm resonator is larger than the thickness of the dielectric layer in the surface acoustic wave resonator constituting the parallel arm resonator. Is also thinned. In this case, since the dielectric layer in the surface acoustic wave resonator constituting the parallel arm resonator is thickened, Δf of the surface acoustic wave resonator is reduced, and the steepness on the low pass band side is high. On the other hand, since the dielectric layer in the surface acoustic wave resonator constituting the series arm resonator is thinned, Δf of the surface acoustic wave resonator is increased and the passband width is increased.

  On the other hand, as a result of earnest research, the present inventors have found that the thickness of the dielectric layer in the surface acoustic wave resonator that constitutes the series arm resonator and the surface acoustic wave resonator that constitutes the parallel arm resonator. When the thickness of the dielectric layer is different, the amount of change in the frequency of the surface acoustic wave resonator constituting the series arm resonator when the width of the electrode finger of the IDT electrode changes, and the parallel arm resonator It has been found that the amount of change in the frequency of the surface acoustic wave resonator is different. For this reason, when the width of the electrode finger of the IDT electrode changes, the obtained filter characteristics change. Therefore, when the manufacturing variation occurs in the width of the electrode finger of the IDT electrode, a large variation occurs in the obtained filter characteristics.

  Based on this knowledge, the present inventors have conducted further research, and as a result, the thickness of the dielectric layer in the surface acoustic wave resonator constituting the series arm resonator and the elastic surface constituting the parallel arm resonator. Even if the thickness of the dielectric layer in the wave resonator is different, the frequency of the surface acoustic wave resonator when the width of the electrode finger of the IDT electrode is changed by changing the duty ratio of the IDT electrode. It has been found that the variation in the filter characteristics can be reduced by adjusting the amount of change.

  FIG. 7 is a graph showing the relationship between the duty ratio of the IDT electrode and the electrode finger width dependency of the frequency of the IDT electrode. Here, the electrode finger width dependency of the frequency of the IDT electrode indicates a change amount of the frequency when the width of the electrode finger of the IDT electrode is changed. As the electrode finger width dependency of the frequency of the IDT electrode is closer to 0.00, the amount of change in the frequency of the surface acoustic wave resonator when the width of the electrode finger of the IDT electrode changes is smaller.

In FIG. 7, the data indicated by diamonds is a surface acoustic wave resonator having the same structure as the surface acoustic wave resonators 30 and 40 of the present embodiment, and is a dielectric layer formed on the IDT electrode. This is data when the thickness normalized with the wavelength of the surface acoustic wave of the SiO ₂ layer is 0.33. In FIG. 7, the data indicated by squares are surface acoustic wave resonators having the same configuration as the surface acoustic wave resonators 30 and 40 of the present embodiment, and the dielectric layer SiO formed on the IDT electrode. _This is data when the thickness normalized by the wavelength of the _two- layer surface acoustic wave is 0.29. In FIG. 7, the data indicated by triangles is a surface acoustic wave resonator having the same configuration as the surface acoustic wave resonators 30 and 40 of the present embodiment, and is the SiO of the dielectric layer formed on the IDT electrode. _This is data when the thickness normalized by the wavelength of the _two- layer surface acoustic wave is 0.24. In FIG. 7, the data indicated by x is a surface acoustic wave resonator having the same configuration as the surface acoustic wave resonators 30 and 40 of the present embodiment, and is the SiO of the dielectric layer formed on the IDT electrode. _This is data when the thickness normalized by the wavelength of the _two- layer surface acoustic wave is 0.20.

  From the results shown in FIG. 7, it can be seen that when the duty ratio of the IDT electrode is constant, the dependency of the frequency of the IDT electrode on the electrode finger width increases as the dielectric layer becomes thinner. It can also be seen that when the thickness of the dielectric layer is constant, the dependency of the IDT electrode frequency on the electrode finger width increases as the duty ratio of the IDT electrode decreases.

For example, in a ladder-type surface acoustic wave filter unit, the standard is determined by the surface acoustic wave wavelength of the SiO ₂ layer of the dielectric layer formed on the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator. The thickness is 0.33, and the wavelength of the surface acoustic wave of the SiO ₂ layer of the dielectric layer formed on the IDT electrode of the surface acoustic wave resonator constituting the parallel arm resonator is When the standardized thickness is 0.20 and the duty ratio of each IDT electrode of the surface acoustic wave resonator constituting the series arm resonator and the parallel arm resonator is unified to 0.5 The electrode finger width dependency of the frequency of the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator and the electrode of the frequency of the IDT electrode of the surface acoustic wave resonator constituting the parallel arm resonator The finger width dependency is about 0.05 different from That. Therefore, when the thickness of the dielectric layer in the surface acoustic wave resonator constituting the series arm resonator differs from the thickness of the dielectric layer in the surface acoustic wave resonator constituting the parallel arm resonator, If the duty ratios of the IDT electrodes of the surface acoustic wave resonators constituting the series arm resonator and the parallel arm resonator are the same, when the manufacturing variation occurs in the electrode finger width of the IDT electrode, the series arm The amount of change in the frequency of the surface acoustic wave resonator that makes up the resonator is different from the amount of change in the frequency of the surface acoustic wave resonator that makes up the parallel arm resonator. Result. Specific examples are shown below.

  The present embodiment except that the duty ratio of the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator is equal to the duty ratio of the IDT electrode of the surface acoustic wave resonator constituting the parallel arm resonator As a comparative example 3, a surface acoustic wave filter device having a ladder type surface acoustic wave filter section composed of surface acoustic wave resonators having the same configuration as the surface acoustic wave resonators 30 and 40 of FIG. Assuming a case where manufacturing variation occurs in the electrode finger width of the IDT electrode, the IDT electrode electrode finger width of 500 nm is used as the first pattern of Comparative Example 3, and the IDT electrode electrode finger width is 520 nm. Using this as the second pattern of Comparative Example 3, the filter characteristics of the transmission filter were measured.

  FIG. 8 shows the filter characteristics of the transmission filter of the surface acoustic wave filter device of the first pattern of Comparative Example 3 and the filter characteristics of the transmission filter of the surface acoustic wave filter device of the second pattern. In FIG. 8, the solid line indicates the first pattern of Comparative Example 3, and the broken line indicates the second pattern of Comparative Example 3.

  As illustrated in FIG. 8, the filter characteristics of the transmission filter are different between the first pattern and the second pattern of Comparative Example 3. Specifically, in the first pattern and the second pattern of Comparative Example 3, not only the frequency of the transmission filter but also the waveform is different. In the second pattern of Comparative Example 3, ripples are generated in the passband. As described above, when the duty ratio of the IDT electrode is equal between the surface acoustic wave resonator constituting the series arm resonator and the surface acoustic wave resonator constituting the parallel arm resonator, the manufacturing variation is caused in the electrode finger width of the IDT electrode. When this occurs, the amount of change in the frequency of the surface acoustic wave resonator that constitutes the series arm resonator is different from the amount of change in the frequency of the surface acoustic wave resonator that constitutes the parallel arm resonator. Manufacturing variations occur in the filter characteristics of the transmission filter.

  Here, in the surface acoustic wave filter device 1 of the present embodiment, the thickness of the dielectric layer 33 of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 is equal to the parallel arm resonator P1. , P2 and P3 are different from the thickness of the dielectric layer 43 of the surface acoustic wave resonator 40, and the duty ratio of the IDT electrode 32 of the surface acoustic wave resonator 30 is thick. However, the duty ratio of the IDT electrode 42 of the surface acoustic wave resonator 40 having a thin dielectric layer is made smaller. For this reason, in this embodiment, the electrode finger width dependence of the frequency of the IDT electrode 32 of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3, and the parallel arm resonators P1, P2, The difference between the frequency of the IDT electrode 42 of the surface acoustic wave resonator 40 constituting the P3 and the electrode finger width dependency is small. Therefore, even if manufacturing variation occurs in the electrode finger width of the IDT electrode, manufacturing variation in filter characteristics can be suppressed.

Specifically, for example, in the ladder-type surface acoustic wave filter unit, the SiO ₂ layer elastic surface of the dielectric layer formed on the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator The elasticity of the SiO ₂ layer of the dielectric layer formed on the IDT electrode of the surface acoustic wave resonator having a thickness normalized by the wave wavelength of 0.33 and constituting the parallel arm resonator When the thickness normalized by the wavelength of the surface wave is 0.20, the parallel arm resonator has a duty ratio of the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator of 0.55. Of the IDT electrodes of the surface acoustic wave resonators constituting the series arm resonator by making the duty ratio of the IDT electrodes of the surface acoustic wave resonators constituting the IDT 0.60 And the elasticity constituting the parallel arm resonator And an electrode finger width dependence of the frequency of the IDT electrode of the surface acoustic wave resonator may be substantially equal. Therefore, even if manufacturing variation occurs in the electrode finger width of the IDT electrode, manufacturing variation in filter characteristics can be suppressed. Specific examples are shown below.

  A surface acoustic wave filter device having a ladder-type surface acoustic wave filter unit composed of surface acoustic wave resonators having the same configuration as the surface acoustic wave resonators 30 and 40 of this embodiment was prepared as an example. Assuming a case where manufacturing variations occur in the electrode finger width of the IDT electrode, the IDT electrode electrode finger width of 500 nm is used as the first pattern of the example, and the IDT electrode electrode finger width is 520 nm. As the second pattern of the example, the filter characteristics of the transmission filter were measured.

  FIG. 9 shows the filter characteristics of the transmission filter of the surface acoustic wave filter device having the first pattern according to the embodiment and the filter characteristics of the transmission filter of the surface acoustic wave filter device having the second pattern. In FIG. 9, the solid line indicates the first pattern of the embodiment, and the broken line indicates the second pattern of the embodiment.

  As shown in FIG. 9, the filter characteristics of the transmission filter are almost the same between the first pattern and the second pattern of the embodiment. Specifically, the first pattern and the second pattern of the embodiment have almost the same waveform, although the frequency of the transmission filter is different. As described above, in the surface acoustic wave resonator constituting the series arm resonator and the parallel arm resonator, the duty ratio of the IDT electrode 32 of the surface acoustic wave resonator 30 having a thick dielectric layer is set to be equal to that of the dielectric layer. When the thickness ratio is smaller than the duty ratio of the IDT electrode 42 of the surface acoustic wave resonator 40, the surface acoustic wave resonance constituting the series arm resonator is produced even if manufacturing variation occurs in the electrode finger width of the IDT electrode. Since the amount of change in the frequency of the child and the amount of change in the frequency of the surface acoustic wave resonator constituting the parallel arm resonator are substantially equal, manufacturing variation hardly occurs in the filter characteristics of the transmission filter.

  In FIG. 9, although the frequency of the transmission filter is different between the first pattern and the second pattern of the embodiment, the thickness of the SiN layers 33b and 43b of the dielectric layers 33 and 43 can be adjusted by adjusting the thickness. Frequency.

  In the present embodiment, the thickness t1 of the dielectric layer 33 in the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 constitutes the parallel arm resonators P1, P2, P3. Since the thickness t2 of the dielectric layer 43 in the surface acoustic wave resonator 40 is thicker, the IDT electrode 32 of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3. The duty ratio is made smaller than the duty ratio of the IDT electrode 42 of the surface acoustic wave resonator 40 constituting the parallel arm resonators P1, P2, P3.

  In the present invention, the thickness of the dielectric layer in the surface acoustic wave resonator constituting the parallel arm resonator is thicker than the thickness of the dielectric layer in the surface acoustic wave resonator constituting the series arm resonator. In the surface acoustic wave filter device, the duty ratio of the IDT electrode of the surface acoustic wave resonator constituting the parallel arm resonator is equal to that of the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator. The duty ratio is made smaller. Even in this case, manufacturing variations hardly occur in the filter characteristics of the transmission filter.

  In this embodiment, the duty ratio of the IDT electrode 32 of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, S3 constitutes the parallel arm resonators P1, P2, P3. The IDTs of all the surface acoustic wave resonators of the surface acoustic wave resonators constituting the series arm resonator or the parallel arm resonator are made smaller than the duty ratio of the IDT electrode 42 of the surface acoustic wave resonator 40. It is not necessary for the duty ratio of the electrodes to be the same. For example, among the surface acoustic wave resonators constituting the series arm resonator or the parallel arm resonator, the duty ratio of the IDT electrode of a specific surface acoustic wave resonator is set to the duty ratio of the IDT electrode of another surface acoustic wave resonator. It may be different from the ratio. In particular, among the surface acoustic wave resonators constituting the series arm resonator or the parallel arm resonator, the duty ratio of the IDT electrode of a specific surface acoustic wave resonator is set to the duty ratio of the IDT electrode of another surface acoustic wave resonator. The average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator constituting the series arm resonator and the duty ratio of the IDT electrode of the surface acoustic wave resonator constituting the parallel arm resonator It is sufficient if the average value is different.

  When FIG. 7 is examined in more detail, it can be seen that the following relational expression (1) holds.

  (Dependence of IDT electrode frequency on electrode finger width) = − 0.656 + 0.337 × (thickness normalized by wavelength of surface acoustic wave of dielectric layer) + 0.724 × (duty ratio of IDT electrode) (1)

  For this reason, it is ideal to set the duty ratio of the IDT electrode so as to satisfy the following expression (2).

D2 = D1 + 0.465 × (t1-t2) (2)
However,
t1: a thickness standardized by the wavelength of the surface acoustic wave of the dielectric layer of the surface acoustic wave resonator constituting the series arm resonator or the parallel arm resonator and having a thick dielectric layer,
t2: a thickness that is a series arm resonator or a parallel arm resonator, the thickness of which is normalized by the wavelength of the surface acoustic wave of the dielectric layer of the surface acoustic wave resonator in which the thickness of the dielectric layer is thin,
D1: An average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator that constitutes a series arm resonator or a parallel arm resonator and has a thick dielectric layer,
D2: An average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator that constitutes a series arm resonator or a parallel arm resonator and has a thin dielectric layer,
It is.

  However, in practice, it is not always necessary to satisfy the above formula (2). This is because some surface acoustic wave resonators constituting a series arm resonator or a parallel arm resonator have a large influence on the filter waveform and a small influence on the filter waveform. . Therefore, among the surface acoustic wave resonators that constitute the series arm resonator, the surface acoustic wave resonator that has a large influence on the filter waveform, and the surface acoustic wave resonators that constitute the parallel arm resonator, It is preferable that the above formula (2) is established with a surface acoustic wave resonator having a large influence on the filter waveform. From such a viewpoint, it is preferable that the following formula (3) is satisfied.

  D1 <D2 <D1 + 0.465 × (t1-t2) (3)

  In addition, it is preferable that the duty ratio of each IDT electrode of the surface acoustic wave resonator which comprises a series arm resonator or a parallel arm resonator exists in the range of 0.4-0.6. By doing so, steep filter characteristics can be realized, the passband can be widened, and manufacturing variations in filter characteristics can be reduced.

  Next, an example of a method for manufacturing the transmission filter 20 of this embodiment will be described.

  First, IDT electrodes 32 and 42 are formed on the piezoelectric substrate 31. The IDT electrodes 32 and 42 can be formed by, for example, a vapor deposition method or a sputtering method.

Next, a SiO ₂ layer is formed on the piezoelectric substrate 31 so as to cover the IDT electrodes 32 and 42. The SiO ₂ layer can be formed by, for example, a bias sputtering method. Next, the surface of the formed SiO ₂ layer is flattened by an etch back method.

Next, a mask made of resist or the like is formed on the IDT electrode 32 of the surface acoustic wave resonator 30 constituting the series arm resonators S1, S2, and S3, and the parallel arm resonators P1, P2, and P3 are formed. The SiO ₂ layers 33 a and 43 a are formed by etching the SiO ₂ layer on the IDT electrode 42 of the surface acoustic wave resonator 40 that is formed. Thereafter, the mask is peeled off.

  Finally, the transmission filter 20 can be formed by forming the SiN layers 33b and 43b.

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave filter apparatus 10 ... Antenna terminal 11 ... Transmission side signal terminal 12 ... Reception side signal terminal 13 ... Reception filter 20 ... Transmission filter 21 ... Series arms 22, 23, 24 ... Parallel arms 30, 40 ... Surface acoustic waves Resonator 31 ... piezoelectric substrate 31a ... main surfaces 32, 42 ... IDT electrodes 33, 43 ... dielectric layers 33a, 43a ... SiO ₂ layers 33b, 43b ... SiN layers P1, P2, P3 ... parallel arm resonators S1, S2, S3: Series arm resonator

Claims (6)

With serial arms,
A series arm resonator connected in the series arm;
A parallel arm connecting the series arm and the ground;
A parallel arm resonator provided in the parallel arm;
A surface acoustic wave filter device including a ladder-type surface acoustic wave filter unit having:
Each of the series arm resonator and the parallel arm resonator includes a piezoelectric substrate, an IDT electrode formed on the piezoelectric substrate, and an elastic layer formed to cover the IDT electrode. It is composed of surface wave resonators,
The thickness of the dielectric layer of the surface acoustic wave resonator constituting the series arm resonator is different from the thickness of the dielectric layer of the surface acoustic wave resonator constituting the parallel arm resonator, and the dielectric A surface acoustic wave filter device, wherein a duty ratio of an IDT electrode of a surface acoustic wave resonator having a thick body layer is smaller than a duty ratio of an IDT electrode of a surface acoustic wave resonator having a thin dielectric layer.   Each of the series arm resonator and the parallel arm resonator is composed of a plurality of surface acoustic wave resonators, and the duty ratio of the IDT electrode of the surface acoustic wave resonator in which the dielectric layer is thick is formed. The surface acoustic wave filter device according to claim 1, wherein an average value is smaller than an average value of duty ratios of IDT electrodes of the surface acoustic wave resonator in which the dielectric layer is thin.   The elasticity according to claim 1 or 2, wherein a duty ratio of an IDT electrode of a surface acoustic wave resonator constituting each of the series arm resonator and the parallel arm resonator is in a range of 0.4 to 0.6. Surface wave filter device. The series arm resonator or the parallel arm resonator is configured, and the thickness normalized by the wavelength of the surface acoustic wave of the dielectric layer of the surface acoustic wave resonator having a thick dielectric layer is t1, A series arm resonator or a parallel arm resonator is formed, and the thickness normalized by the wavelength of the surface acoustic wave of the dielectric layer of the surface acoustic wave resonator having a thin dielectric layer is t2. The arm resonator or the parallel arm resonator is configured, and the average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator having a thick dielectric layer is D1, and the series arm resonator or the parallel arm resonator is When the average value of the duty ratio of the IDT electrode of the surface acoustic wave resonator in which the thickness of the dielectric layer is thin is D2,
D1 <D2 <D1 + 0.465 × (t1-t2)
The surface acoustic wave filter device according to any one of claims 1 to 3, wherein The surface acoustic wave filter device according to any one of claims 1 to 4, wherein the piezoelectric substrate is made of a LiNbO ₃ substrate and uses a Rayleigh wave as a main mode. The surface acoustic wave filter device according to claim 1, wherein the dielectric layer includes a SiO ₂ layer.

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