JPWO2005036744A1 - Boundary acoustic wave device - Google Patents

Abstract

Provided is a boundary acoustic wave device which can obtain good resonance characteristics and filter characteristics and can reduce a manufacturing deviation of frequency. In the boundary acoustic wave device in which the interdigital electrode 4 and / or the reflectors 5 and 6 are arranged at the boundary between the LiNbO3 substrate 2 as the first medium layer and the SiO2 film 3 as the second medium layer, A boundary acoustic wave device in which 0.33 <d <0.49 is satisfied, where d is the duty ratio of the interdigital electrode 4 and / or the reflectors 5 and 6.

Description

  The present invention relates to a boundary acoustic wave device using a boundary acoustic wave propagating through a boundary between first and second medium layers having different sound speeds, and more particularly to a boundary acoustic wave device having an improved electrode structure.

  A surface acoustic wave device using surface acoustic waves such as Rayleigh waves and first leakage waves can be reduced in size and weight, and does not require adjustment.

  Therefore, the surface acoustic wave device is widely used for an RF filter or IF filter for mobile phones, a VCO resonator, a television VIF filter, or the like.

  However, since the surface acoustic wave has the property of propagating on the surface of the medium, it is sensitive to changes in the surface state of the medium. Therefore, in a chip in which surface acoustic waves propagate, the chip surface in which surface acoustic waves propagate must be protected. For this reason, it is necessary to hermetically seal the surface acoustic wave device using a package provided with a cavity so that the surface of the surface acoustic wave chip faces the cavity. Such a package is generally expensive, and the size of the package has to be much larger than the size of the surface acoustic wave chip.

  A boundary acoustic wave device has been proposed as a device that eliminates the need for a package having a cavity as described above.

  FIG. 5 is a schematic partial cutaway front sectional view showing an example of a conventional boundary acoustic wave device. In the boundary acoustic wave device 101, first and second medium layers 102 and 103 having different sound speeds are stacked. An IDT 104 as an electroacoustic transducer is disposed at a boundary A between the first and second medium layers 102 and 103. In addition, reflectors (not shown) are arranged on both sides of the IDT 104 in the boundary acoustic wave propagation direction.

  The boundary acoustic wave device 101 excites a boundary acoustic wave by applying an input signal to the IDT 104. The boundary acoustic wave propagates along the boundary A of the boundary acoustic wave device 101 as schematically shown by an arrow B in FIG.

Non-Patent Document 1 below shows an example of such a boundary acoustic wave device. Here, SiO ₂ film is formed to a predetermined thickness to 126 ° rotation Y plate X has IDT is formed LiTaO ₃ substrate of propagation, the LiTaO ₃ substrate so as to cover the IDT. In this structure, it is shown that an SV + P type elastic boundary wave called a Stoneley wave propagates. Non-Patent Document 1 states that when the thickness of the SiO ₂ film is 1.0λ (where λ is the wavelength of the boundary acoustic wave), an electromechanical coupling coefficient of 2% is obtained.

On the other hand, Patent Document 1 below discloses a boundary acoustic wave device having a structure in which an IDT made of Al is formed on a 126 ° rotated Y-plate X-propagating LiTaO ₃ substrate. Here, when the film thickness of the electrode fingers of the interdigital electrode is H, the width of the electrode fingers is L, and the interval between the electrode fingers is S, L = S = 1 μm. A polycrystalline silicon oxide film constituting the second medium is formed on the IDT. Here, the duty ratio L / (L + S) of the interdigital electrode is 0.5.
WO98-52279 “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and ultrason. , VOL. SU-25, No. 6,1978 IEEE

  In the boundary acoustic wave device as well, like the surface acoustic wave device, it is strongly demanded that the frequency variation is small. However, the conventional boundary acoustic wave device cannot sufficiently satisfy this requirement, and it has been difficult to increase the yield rate.

  An object of the present invention is to provide a boundary acoustic wave device having good resonance characteristics and filter characteristics and having small frequency variations in view of the current state of the prior art.

  The present invention relates to a boundary acoustic wave device including a first medium layer, a second medium layer, and an interdigital electrode and / or a reflector disposed at a boundary between the first and second medium layers. The boundary acoustic wave device is characterized in that 0.33 <d <0.49, where d is a duty ratio of the interdigital electrode and / or the reflector.

In a specific aspect of the boundary acoustic wave device according to the present invention, the first medium layer is configured using a substrate whose main component is LiNbO ₃ .

In another specific aspect of the boundary acoustic wave device according to the present invention, the second medium layer is configured using a dielectric mainly composed of SiO ₂ .

  In the boundary acoustic wave device according to the present invention, the duty ratio d of the interdigital electrode and / or the reflector is greater than 0.33 and less than 0.49. Therefore, as will be apparent from specific experimental examples described later, it is possible to reduce frequency variations during manufacturing.

  That is, the present invention is characterized in that in the boundary acoustic wave device, the duty ratio is set in the specific range, thereby reducing the frequency variation during the manufacturing.

FIG. 1 is a schematic plan view showing an electrode structure of a boundary acoustic wave device according to an embodiment of the present invention. FIG. 2 is a surface cross-sectional view showing a main part of the boundary acoustic wave device shown in FIG. FIG. 3 is a diagram showing a change in the manufacturing deviation σ of the resonance frequency when the duty ratio of the interdigital electrode and the reflector of the boundary acoustic wave device is changed. FIG. 4 is a diagram showing a change in the manufacturing deviation σ of the antiresonance frequency when the duty ratio of the interdigital electrode and the reflector of the boundary acoustic wave device is changed. FIG. 5 is a schematic partial cutaway front sectional view for explaining a conventional boundary acoustic wave device.

Explanation of symbols

1 ... boundary acoustic wave device 2 ... LiNbO ₃ substrate (first medium layer)
3 ... SiO ₂ film (second medium layer)
4 ... Interdigital electrode 5, 6 ... Reflector 1

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

  FIG. 1 is a schematic cross-sectional view showing an electrode structure of a boundary acoustic wave device according to an embodiment of the present invention.

In the boundary acoustic wave device 1, a rectangular plate-like LiNbO ₃ substrate 2 is used as the first medium layer. Here, the LiNbO ₃ substrate 2 is configured using a 15 ° rotated Y-plate X propagation LiNbO ₃ substrate. However, in the present invention, it may be used rotated Y plate X propagating LiNbO ₃ substrate of another rotation angle.

On the LiNbO ₃ substrate 2, as shown in a schematic cross-sectional view in FIG. 2, a SiO ₂ film 3 is laminated as a _second medium layer. In FIG. 1, the illustration of the SiO ₂ film is omitted, and an electrode structure arranged at the boundary between the SiO ₂ film 3 and the LiNbO ₃ substrate 2 is shown. That is, the interdigital electrode 4 and the reflectors 5 and 6 are formed on the LiNbO ₃ substrate 2. The interdigital electrode 4 has a plurality of electrode fingers 4a and 4b that are inserted into each other. Here, the interdigital electrode 4 is weighted so that the crossing width changes in the surface wave propagation direction as shown.

  The reason why the interdigital electrode 4 is weighted in the cross width as described above is to suppress the transverse mode spurious.

  Further, grating reflectors 5 and 6 are disposed on both sides of the interdigital electrode 4 in the surface wave propagation direction. The reflectors 5 and 6 have a structure in which a plurality of electrode fingers 5a and 6a are short-circuited at both ends.

  In the present embodiment, since the duty ratio of the interdigital electrode 4 and the reflectors 5 and 6 is larger than 0.33 and smaller than 0.49, it is possible to reduce frequency variation during manufacturing. This will be described based on a specific experimental example. The duty ratio in the present invention is a value determined by duty ratio = L / P, where P is the distance between electrode finger centers in the space between electrode fingers adjacent to the left and right, and L is the width of the electrode finger. is there.

The interdigital electrode 4 and the reflectors 5 and 6 were formed by the following thin film formation method. That is, a NiCr film having a thickness of 0.001λ was formed on the LiNbO ₃ substrate 2 as an underlayer by a vapor deposition method. Here, λ indicates the arrangement of surface waves defined by the electrode finger period of the interdigital electrode 4.

  An Au film having a thickness of 0.06λ was formed on the NiCr film by the same vapor deposition method. Thereafter, patterning was performed by the lift-off method to form the interdigital electrode 4 and the reflectors 5 and 6.

  The wavelength λ in the interdigital electrode 4 was 3 μm. Further, the number of electrode fingers in the interdigital electrode 4 was 50, the number of electrode fingers in the reflector was 50, and the aperture length A in the reflector was 30λ.

After the interdigital electrode 4 and the reflectors 5 and 6 were formed, the SiO ₂ film 3 was formed by RF magnetron sputtering. The temperature during film formation was in the range of 200 ± 50 ° C., and the thickness of the SiO ₂ film was 1.5λ.

  In the boundary acoustic wave device 1 configured as described above, the SH type boundary acoustic wave is excited and confined between the reflectors 5 and 6, and the resonance characteristics by the SH type boundary acoustic wave can be obtained.

  In the boundary acoustic wave device 1, various types of boundary acoustic wave devices were manufactured by varying the duty ratio of the interdigital electrode 4.

  40 pieces of each of the plurality of types of boundary acoustic wave devices prepared in this manner were taken out, and the frequency manufacturing deviation σ of the resonance frequency and the anti-resonance frequency was evaluated. The deviation σ is a standard deviation when it is assumed that the frequency variation has a normal distribution.

  FIG. 3 shows the relationship between the frequency manufacturing deviation of the resonance frequency obtained as described above and the duty ratio, and FIG. 4 shows the relationship between the frequency manufacturing deviation of the anti-resonance frequency and the duty ratio.

  As apparent from FIG. 3, when the duty ratio is 0.5, that is, in the conventional boundary acoustic wave device, the manufacturing deviation of the resonance frequency is σ2530 ppm, whereas the duty ratio is smaller than 0.49 and 0. In a range larger than .33, it can be seen that the manufacturing deviation σ is smaller than 2330 ppm. Furthermore, it can be seen that in the range where the duty ratio is smaller than 0.44 and larger than 0.33, the manufacturing deviation σ is further reduced to 2050 ppm or less.

  On the other hand, when the duty ratio is 0.5, the manufacturing deviation σ of the anti-resonance frequency is 2580 ppm, while the duty ratio is smaller than 0.5 and larger than 0.31 in FIG. It can be seen that the manufacturing deviation σ of the resonance frequency is smaller than 2410 ppm. Further, it can be seen that when the duty ratio is smaller than 0.44 and larger than 0.33, the manufacturing deviation σ of the anti-resonance frequency becomes even better at 1970 ppm or less.

  Therefore, in the boundary acoustic wave device 1, by making the duty ratio of the electrode larger than 0.33 and smaller than 0.49, the frequency manufacturing deviation can be effectively reduced. It can be seen that the rate can be significantly increased.

  When a ladder filter using the boundary acoustic wave device 1 as a resonator is configured, the manufacturing variation of the frequency at the lower end of the passband is determined by the manufacturing deviation σ of the resonance frequency of the boundary acoustic wave resonator, and the passband The manufacturing deviation of the upper end frequency is determined by the manufacturing deviation σ of the anti-resonance frequency. Therefore, when the boundary acoustic wave device 1 is used as a resonator for forming a ladder type filter or the like, the frequency variation of the filter can be effectively reduced.

  In the present invention, it is considered that the reason why the frequency manufacturing deviation decreases as the duty ratio decreases is as follows.

In order to excite or reflect the boundary wave on the LiNbO ₃ substrate, the structure in which the IDT electrode 4 and the reflectors 5 and 6 are arranged and the SiO ₂ film is laminated thereon was examined. The IDT 4 and the reflectors 5 and 6 have a grating structure in which the sections represented by the interval P described above are continuously arranged adjacent to each other. This grating structure has a portion where the electrode fingers are present and a portion where the electrode fingers are not present. When the SiO ₂ film 3 is formed, if the gap S between the electrode fingers is reduced, the filling state of the SiO ₂ film 3 in the gap becomes unstable. Therefore, it is considered that when the duty ratio becomes small, the filling state of the SiO ₂ film becomes stable and the frequency manufacturing deviation becomes small.

However, even when the duty ratio is around 0.9, the frequency manufacturing deviation is small. On the contrary, since the SiO ₂ film is stable in a state where the gap is not filled, it is considered that the frequency manufacturing deviation is small.

In the surface acoustic wave, when the SiO ₂ film is formed so as to cover the IDT electrode, the thickness of the SiO ₂ film is thin, so that the surface of the SiO ₂ film is uneven. The surface wave is affected by the unevenness.

However, in the boundary acoustic wave device, the thickness of the SiO ₂ film is sufficiently thick, and the unevenness of the surface of the SiO ₂ film 3 is reduced. Further, even if such irregularities are generated, the vibration energy of the boundary acoustic wave is hardly distributed on the surface of the SiO ₂ film, and thus is hardly affected by the irregularities.

  In the above embodiment, the electrode material constituting the interdigital electrode and the reflectors 5 and 6 is configured by laminating an Au film on a base layer made of NiCr. However, the electrode material is not limited to a material mainly composed of Au, and metals, alloys such as Ag, Cu, Al, Fe, Ni, W, Ta, Pt, Mo, Cr, Ti, ZnO, and ITO, and oxides A conductor may be used. Further, it is not always necessary to form the adhesion layer such as NiCr as a base layer. That is, the electrode may be composed only of a single metal material.

Further, the material constituting the first medium layer is not limited to the LiNbO ₃ substrate 2 but may be composed of other piezoelectric substrates such as a LiTaO ₃ substrate, a quartz substrate, and a langasite substrate. Further, the first medium layer may be made of glass or the like.

Also, the second medium layer is not limited to the SiO ₂ film, and may be another material such as an Al film, a SiN film, a polycrystalline Si film, and the second medium layer is not necessarily a thin film. There is no need to be done.

  In addition, the boundary portion constituted by the first and second medium layers is particularly limited as long as the interdigital electrode 4 and the gap between the electrode fingers of the reflectors 5 and 6 are filled with the medium. is not.

  Furthermore, in the above embodiment, the interdigital electrode 4 and the reflectors 5 and 6 are provided in the boundary acoustic wave device 1, but the number of interdigital electrodes and the number of reflectors are not particularly limited. In other words, a longitudinally coupled boundary acoustic wave filter in which a plurality of interdigital electrodes are arranged between a pair of reflectors may be configured, and a plurality of boundary acoustic wave devices and boundary acoustic wave filters are connected in series. They may be parallel or cascaded.

Claims (3)

In the boundary acoustic wave device including the first medium layer, the second medium layer, and the interdigital electrode and / or the reflector disposed at the boundary between the first and second medium layers,
The boundary acoustic wave device according to claim 3, wherein 0.33 <d <0.49, where d is a duty ratio of the interdigital electrode and / or the reflector. The boundary acoustic wave device according to claim 1, wherein the first medium layer is made of a substrate mainly composed of LiNbO ₃ . 3. The boundary acoustic wave device according to claim 1, wherein the second medium layer is made of a dielectric having SiO ₂ as a main component.

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