JP4274201B2 - Edge reflection type surface acoustic wave filter - Google Patents

Description

  The present invention relates to a surface acoustic wave filter used as a bandpass filter, and more particularly to an end surface reflection type surface acoustic wave filter using an SH type surface wave such as a BGS wave or a Love wave.

  Conventionally, a longitudinally coupled resonator type surface acoustic wave filter or the like is known as an end surface reflection type surface acoustic wave device using SH type surface waves such as BGS waves and Love waves (Patent Document 1 below). In the end surface reflection type surface acoustic wave device, a surface wave is reflected between two opposing end surfaces. Accordingly, since no reflector is required, it is possible to reduce the size of a surface wave device such as a surface wave filter.

  FIG. 14 shows a conventional end face reflection type longitudinally coupled resonator type surface acoustic wave filter. The longitudinally coupled resonator type surface acoustic wave filter 101 has a piezoelectric substrate 102. On the upper surface of the piezoelectric substrate 102, first and second grooves 102a and 102b are formed in parallel with each other at a predetermined distance. Two IDTs 103 and 104 are formed between the grooves 102a and 102b in order to form a longitudinally coupled resonator type surface acoustic wave filter.

Note that the side surfaces of the first and second grooves 102a and 102b on the side where the IDTs 103 and 104 are formed constitute a reflection end surface that reflects SH type surface waves.
JP 2001-244789 A

  The above-described longitudinally coupled resonator type surface acoustic wave filter 101 does not require a reflector and can be miniaturized, but further expansion of out-of-band attenuation is required.

  An object of the present invention is to provide an end surface reflection type surface wave filter that can increase the out-of-passband attenuation and enhance selectivity in view of the above-described state of the prior art. It is to provide.

According to the present invention, the first and second reflective end surfaces, the upper surface, the lower surface, and the first and second extending from the lower ends of the first and second reflective end surfaces are arranged in parallel at a predetermined distance. First and second reflection end faces and first and second piezoelectric substrate portions, respectively, and first and second grooves or first and second grooves opened to the outside, respectively. and a second recess is formed, comprising: a piezoelectric substrate made of LiTaO _3, the first on the upper surface of the piezoelectric substrate, and a plurality of interdigital transducers which are formed in a region between the second grooves or recesses, A layer made of SiO ₂ formed so as to cover the plurality of interdigital transducers is further provided, and the electrode thickness H of the interdigital transducer is H / λ ≦ 0 when the wavelength of the surface wave to be used is λ. .06 An end face reflection type surface acoustic wave filter is provided.

Further, in the edge reflection type surface acoustic wave filter according to the present invention, preferably, the electrode thickness H of the Transducer (IDT) is an H /Ramuda≦0.045. In this case, the deterioration of insertion loss can be suppressed more effectively.

  In the end surface reflection type surface acoustic wave filter according to the present invention, preferably, the depth of the groove or the recess is configured to have a depth of one wavelength or more of the SH type surface wave to be used. The SH type surface wave propagates around a portion having a depth of one wavelength or less from the piezoelectric substrate surface. Therefore, by setting the depth of the groove or the concave portion within the above range, the SH type surface wave to be used is reliably reflected by the end face, and a surface wave filter having good characteristics can be configured.

  The specific structure of the end surface reflection type surface acoustic wave filter according to the present invention is not particularly limited, and may be a ladder type filter using an end surface reflection resonator, a longitudinally coupled resonator filter, or a lateral filter. A coupled resonator filter may be used.

In the end surface reflection type surface acoustic wave filter according to the present invention, since the layer made of SiO ₂ is formed on the upper surface of the piezoelectric substrate so as to cover the IDT, the insertion loss is deteriorated as in the first and second inventions. The attenuation outside the passband can be increased without incurring so much, and therefore, a small surface wave filter having excellent selectivity and not requiring a reflector can be obtained.

  When the depths of the first and second grooves or recesses are at least one wavelength of the SH type surface wave to be used, the IDTs of the first and second grooves are formed. The SH type surface wave to be used on the side surface of the side is reliably reflected, and an end surface reflection type surface wave filter having good characteristics can be obtained.

  Hereinafter, the present invention will be described in more detail by describing specific embodiments of the present invention.

FIG. 1 is a perspective view showing an end face reflection type longitudinally coupled resonator type surface acoustic wave filter 1 according to a reference example . In the longitudinally coupled resonator type surface acoustic wave filter 1, a rectangular plate-shaped piezoelectric substrate 2 is used. The piezoelectric substrate 2 is made of a piezoelectric single crystal such as LiTaO ₃ or LiNbO ₃ , or a piezoelectric ceramic such as a lead zirconate titanate ceramic. However, the piezoelectric substrate 2 may be a laminate of a piezoelectric thin film such as a ZnO thin film on an insulating substrate such as sapphire.

FIG. 2 is a perspective view showing a state in which the flexible resin layer 3 covering the upper surface of the piezoelectric substrate 2 is removed from the longitudinally coupled resonator type filter 1. As shown in FIG. 2, on the upper surface 2a of the piezoelectric substrate 2, first and second grooves 2b and 2c are formed in parallel with each other at a predetermined distance. The grooves 2b and 2c have a depth that does not reach the lower surface 2d. The inner side surfaces of the grooves 2b and 2c function as the first and second reflection end surfaces 2b ₁ and 2c ₁ . The depth of the first and second grooves 2b and 2c is set to a depth of one or more wavelengths of the surface wave to be used.

As shown in FIG. 11, first and second recesses 2x and 2y may be formed instead of the first and second grooves 2b and 2c. That is, in the piezoelectric substrate 2A shown in FIG. 11, the first and second reflection end faces 2b ₁ and 2c ₁ are arranged in parallel at a predetermined distance, and outward from the lower ends of the reflection end faces 2b ₁ and 2c _1. Extending piezoelectric substrate portions 2x ₁ and 2y ₁ are provided, thereby forming first and second recesses 2x and 2y that are opened outward. In the present invention, as described above, instead of the first and second grooves, the piezoelectric substrate 2A in which the first and second recesses opened outward are formed.

  A longitudinally coupled resonator type filter section is formed between the first and second grooves 2b and 2c. Each of the longitudinally coupled resonator type surface acoustic wave filter units has a structure in which a pair of IDTs 5 and 6 are arranged along the surface wave propagation direction.

However, the longitudinally coupled resonator type surface acoustic wave filter according to the reference example is not limited to the one-stage configuration, and may be a longitudinally coupled resonator type surface acoustic wave filter having two or more stages. A plurality of IDTs corresponding to the configuration of the wave filter are formed on the upper surface of the piezoelectric substrate 2.

A feature of the longitudinally coupled resonator type surface acoustic wave filter 1 of the present reference example is that a flexible resin coating layer 3 shown in FIG. 1 is formed so as to cover the upper surface 2 a of the piezoelectric substrate 2. The flexible resin coating layer 3 is formed so as to cover the IDTs 5 and 6 and the first and second grooves 2b and 2c. In the present reference example , the piezoelectric substrate portion outside the first and second grooves 2b and 2c is also covered with the upper surface of the piezoelectric substrate portion outside the first and second grooves 2b and 2c. The upper surface of the substrate portion may not necessarily be covered with the flexible resin coating layer 3.

  The flexible resin coating layer 3 is made of a resin having a flexibility that does not suppress the propagation of the surface wave excited when the longitudinally coupled resonator type surface acoustic wave filter 1 is operated. Various resins can be used as such a resin, but a gel-like resin is preferably used. Moreover, as said flexible resin, what consists of silicone rubber, an epoxy resin, or urethane rubber is used preferably, More preferably, silicone rubber is used. Note that more preferable characteristics and the like of the flexible resin will be described more specifically based on experimental examples later.

  Further, as shown in FIG. 3, in the first groove 2b, the resin coating layer 3 is formed so as to enter the groove.

  Further, as described above, the longitudinally coupled resonator type surface acoustic wave filter 1 is flexible so as to cover the regions where the IDTs 5 and 6 are formed and the grooves 2b and 2c and to enter the grooves 2b. Since the resin coating layer 3 is formed, it is possible to increase the out-of-band attenuation. This will be described based on a specific experimental example.

As the longitudinally coupled resonator type surface acoustic wave filter of the above reference example , a piezoelectric substrate made of LiTaO ₃ and having outer dimensions of 0.9 mm × 1.7 mm × thickness 0.38 mm is prepared, and IDT5 is formed on the upper surface of the piezoelectric substrate. , 6 were formed. In IDTs 5 and 6, the total number of electrode fingers is 30 pairs, and the wavelength is 21.8 μm. Further, after forming IDTs 5 and 6, first and second grooves 2a and 2b each having a width of 0.17 mm and a depth of 0.1 mm were formed. The longitudinally coupled resonator type surface acoustic wave filter thus prepared is used as a conventional example, and a flexible resin coating layer 3 made of a gel-like silicone resin is formed on the upper surface of the longitudinally coupled resonator type surface acoustic wave filter. This was used as a longitudinally coupled resonator type surface acoustic wave filter as a reference example .

The attenuation-frequency characteristics of the longitudinally coupled resonator type surface acoustic wave filters of the conventional example and the reference example were measured. The results are shown in FIG. The solid line in FIG. 4 shows the characteristic of the reference example , and the broken line shows the characteristic of the conventional example. Also, a solid line P and a broken line Q shown in the center of FIG. 4 are characteristics obtained by enlarging the characteristics of the reference example and the conventional example on the right scale of the vertical axis.

As is clear from FIG. 4, the longitudinally coupled resonator type surface acoustic wave filter of the reference example provided with the flexible resin coating layer 3 slightly increases the loss in the passband, but greatly increases the attenuation outside the passband. It can be seen that it can be improved. This is thought to be due to the fact that the flexible resin coating layer 3 is formed so as to enter the first groove 2b, so that reflection of unwanted waves at the reflection end face is suppressed.

  Based on such a result, the inventor variously changes the application area of the flexible resin coating layer 3 on the upper surface of the piezoelectric substrate 2 and suppresses how much the spurious indicated by the arrows A1 and A2 in FIG. 4 is suppressed. I investigated. The results are shown in FIG.

  Note that the ratio (%) of the coated area on the horizontal axis in FIG. 5 refers to the ratio of the coated area of the resin coating layer 3 to the entire upper surface of the piezoelectric substrate 2.

  As can be seen from FIG. 5, as the coating area of the insulating resin coating layer increases, the spurious indicated by the arrows A1 and A2 is suppressed, whereby the attenuation outside the passband can be increased.

  Further, the amount of penetration of the flexible resin coating layer 3 into the first and second grooves, the insertion loss in the band, and the level of spurious outside the pass band described above were measured. The results are shown in FIGS.

  In addition, 0 in the penetration | invasion amount to the groove | channel in FIG.6 and FIG.7 shows the case where the flexible resin coating layer is provided only to the area | region in which IDT5 and 6 are provided, and the range of 0 to 100% is The ratio of the corresponding cross-sectional area of the flexible resin coating layer entering the groove 2b with respect to the cross-sectional area of the groove 2b is shown.

  As is apparent from FIG. 6, the insertion loss of the flexible resin coating layer 3 into one groove 2b is 50% or less, although the insertion loss decreases as the penetration amount of the flexible resin coating layer into the groove increases. Then, it can be seen that the deterioration of the insertion loss can be suppressed to 2 dB or less. Therefore, if the ratio of the flexible resin coating layer to one groove 2b is 50% or less, spurious in the vicinity of the pass band can be suppressed as shown in FIG. 7 without causing much deterioration in insertion loss. It can be seen that the out-of-band attenuation can be increased.

  Therefore, preferably, the flexible resin coating layer is formed so that the flexible resin coating layer enters at a ratio of 50% or less in one of the first and second grooves 2b and 2c.

  Next, the inventor of the present application made various changes to the material constituting the flexible resin coating layer in the above experimental example, and examined the change in insertion loss and the change in the degree of suppression of spurious indicated by the arrows A1 and A2. The results are shown in FIG. The horizontal axis in FIG. 8 represents the density of the flexible resin coating layer. In this case, the application area of the flexible resin coating layer was 100%. In FIG. 8, ● indicates the result of insertion loss when a resin coating layer made of epoxy resin is used, + indicates the result of spurious suppression, and ○ indicates insertion when silicon resin is used. As for the result of loss, x shows the result of the spurious suppression degree at the time of using silicon resin.

  Further, FIG. 9 shows the Young's modulus of the material constituting the flexible resin coating layer, the change in the insertion loss, and the change in the spurious suppression degree. In FIG. 9, ●, +, ○, and x represent the same meaning as in FIG.

Further, FIG. 10 shows the linear expansion coefficient (× 10 ⁻⁴ / ° C.), the change in insertion loss, and the change in spurious suppression of the resin coating layer. In FIG. 10, ●, +, ○, and x represent the same meaning as in FIG.

As is apparent from FIG. 8, when the density of the flexible resin coating layer after curing is 1.2 g / cm ³ or less, more preferably 1.0 g / cm ³ or less, spurious is suppressed while suppressing deterioration of insertion loss. It can be seen that it can be effectively suppressed.

  Similarly, as apparent from FIG. 9, when the Young's modulus after curing of the flexible resin coating layer is 1 MPa or less, the spurious can be effectively suppressed while similarly suppressing the deterioration of insertion loss. Recognize.

Moreover, from the result of FIG. 10, the linear expansion coefficient after hardening of the flexible resin coating layer is 1.9 × 10 ⁻⁴ (/ ° C.) or more, more preferably 2.3 × 10 ⁻⁴ (/ ° C.) or more. In this case, it can be seen that spurious can be effectively suppressed while suppressing deterioration of insertion loss. Therefore, preferably, the resin constituting the flexible resin coating layer is a gel-like resin, the density after curing is 1.0 g / cm ³ or less, the Young's modulus after curing is 1 MPa or less, and the line after curing Those having an expansion coefficient of 2.3 × 10 ⁻⁴ (/ ° C.) or more are preferably used.

In one embodiment of the present invention, a protective layer 11 made of SiO ₂ is provided on the upper surface 2 a of the piezoelectric substrate 2 so as to cover the IDT 5. In FIG. 12A, the portion on the side where the IDT 6 is provided is not shown, but the protective layer 11 is provided so as to cover the IDT 6 as well.

In FIG. 12B, the attenuation-frequency characteristic of the longitudinally coupled resonator type surface acoustic wave filter provided with such a protective layer 11 is shown by a solid line. For comparison, the attenuation-frequency characteristics of a longitudinally coupled resonator-type surface acoustic wave filter that is configured in the same manner except that no protective layer is provided are indicated by broken lines. Note that 36 ° LiTaO ₃ was used as the piezoelectric substrate, and Al was used as the metal constituting the IDTs 5 and 6. Alternatively, the thickness h of the SiO ₂ film was set to h / λ = 0.30, where λ is the wavelength of the surface wave.

  As is apparent from FIG. 12B, it can be seen that the provision of the protective layer 11 can effectively reduce out-of-band spurious without causing a large deterioration in insertion loss. Furthermore, by forming the protective layer, the frequency temperature coefficient can be reduced as compared with a surface acoustic wave filter not provided with a protective layer.

  FIG. 13 shows the deterioration of the insertion loss when the IDT film thickness is changed in the surface acoustic wave filter having the above configuration. As is clear from FIG. 13, the thickness H of the IDT electrode is in the range of H / λ ≦ 0.06, more preferably H / λ ≦ 0.045, where λ is the wavelength of the surface wave to be used. By setting the range, it is possible to suppress the deterioration of insertion loss.

  In the example of FIG. 13, Al is used as the IDT, but it has been confirmed by the inventors that similar characteristics are exhibited even when Cu, Au, or the like is used as the IDT.

The perspective view which shows the end surface reflection type longitudinal coupling resonator type | mold surface wave filter which concerns on a reference example . The perspective view which shows the state which removed the resin coating layer which consists of resin which has a softness | flexibility in the longitudinal coupling resonator type | mold surface wave filter shown in FIG. FIG. 2 is a partially cutaway cross-sectional view for explaining a main part of the longitudinally coupled resonator type surface acoustic wave filter of the reference example shown in FIG. 1. The figure which shows the attenuation amount-frequency characteristic of a reference example and a prior art example longitudinally coupled resonator type | mold surface acoustic wave filter. The figure which shows the relationship between the application area of a resin coating layer, and the suppression degree of the spurious by formation of a resin coating layer. The figure which shows the relationship between the penetration amount of the resin coating layer to a 1st groove | channel, and insertion loss. The figure which shows the relationship between the penetration | invasion amount of the resin coating layer in a 1st groove | channel, and the improvement degree of an out-of-band spurious. The figure which shows the relationship between the density after hardening of a resin coating layer, the change of insertion loss, and the change of a spurious suppression degree. The figure which shows the relationship between the Young's modulus after hardening of a resin coating layer, the change of insertion loss, and the change of a spurious suppression degree. The figure which shows the relationship between the linear expansion coefficient after hardening of a resin coating layer, the change of insertion loss, and the change of a spurious suppression degree. The perspective view which shows the modification of the longitudinally coupled resonator type | mold surface acoustic wave filter of a reference example . (A) And (b) is a partial cutaway sectional view showing one embodiment of the longitudinally coupled resonator type surface acoustic wave filter of the present invention, and each attenuation of the surface acoustic wave filter prepared for comparison with the one embodiment . The figure which shows quantity-frequency characteristic. The figure which shows the change of the insertion loss at the time of changing the film thickness of IDT in the surface wave filter shown to Fig.12 (a). The perspective view which shows the conventional longitudinal coupling resonator type | mold surface acoustic wave filter.

Explanation of symbols

1 ... longitudinally coupled resonator type surface acoustic wave filter 2 ... piezoelectric substrate 2a ... top 2b, 2c ... first and second grooves 2b _1, 2b ₂ ... side 2d ... underside 2x, 2y ... first, second recess 3 ... resin coating layers 5, 6 ... IDT
11 ... Protective layer

Claims (6)

First and second reflective end faces, a top face, a bottom face, and first and second piezoelectric substrate portions extending outward from the lower ends of the first and second reflective end faces, arranged in parallel at a predetermined distance. The first and second reflective end faces and the first and second piezoelectric substrate portions form first and second grooves or first and second recesses that are opened outward, respectively. It is, a piezoelectric substrate made of LiTaO _3,
A plurality of interdigital transducers formed in a region between the first and second grooves or recesses on the upper surface of the piezoelectric substrate;
A layer made of SiO ₂ formed to cover the plurality of interdigital transducers;
An end face reflection type surface acoustic wave filter, wherein the electrode thickness H of the interdigital transducer is such that H / λ ≦ 0.06, where λ is the wavelength of the surface wave to be used.   2. The end face reflection type surface acoustic wave filter according to claim 1, wherein an electrode thickness H of the interdigital transducer is H / λ ≦ 0.045, where λ is a wavelength of the surface wave to be used.   The end surface reflection type surface acoustic wave filter according to claim 1 or 2, wherein the depth of the groove or the concave portion has a depth of one wavelength or more of the SH type surface acoustic wave to be used.   The end face reflection type surface acoustic wave filter according to claim 1, wherein the end face reflection type surface wave filter is a ladder type filter using an end face reflection resonator.   The end face reflection type surface acoustic wave filter according to claim 1, which is a longitudinally coupled resonator filter.   The end face reflection type surface acoustic wave filter according to claim 1, which is a laterally coupled resonator filter.

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