JP2009232338A - Surface-acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a SAW (surface-acoustic wave) device which suppresses spuriousness generated at the side of high zone in the passing zone of SAW device employing LBO for a substrate while standing against thermal shock test and drop test. <P>SOLUTION: In the SAW device, the LBO substrate 2, in which an IDT electrode is formed on the front side and a groove 3 of depth d is formed on the rear side thereof, is mounted on a package by ultraviolet curing type adhesive. In this case, when the thickness of the LBO substrate 2 is shown by t (μm), the wavelength of elastic surface wave oscillated by the IDT electrode is shown by λ (μm) and the depth of the groove 3 is shown by d (μm), the depth d of the groove 3 is predetermined so as to be within the range of 8λ≤d≤(t-150). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a surface acoustic wave device using a piezoelectric substrate with an uneven back surface, and in particular, suppresses spurious generated on the high frequency side of a pass band, and is resistant to thermal shock and dropping, and a surface acoustic wave device. The present invention relates to a surface acoustic wave device in which an ultraviolet curable adhesive can be used for bonding to a package bottom surface.

In recent years, surface acoustic wave devices (SAW devices) have been widely used in the communication field, and are widely used especially for cellular phones, LANs, and the like because they have excellent characteristics such as high performance, small size, and mass productivity. In a SAW device using a piezoelectric material as a substrate, a surface wave is excited and a bulk wave is radiated into the substrate, which is received and spurious, degrading the characteristics of the SAW device.
Various means for suppressing the bulk wave have been proposed for a long time. Patent Document 1 discloses a TV filter in which a groove is formed on the back surface of a lithium tantalate substrate. In Patent Document 1, grooves are formed on the back surface of the 112.2 ° Y-cut LiTaO ₃ substrate in a direction inclined by an angle θ1 (47.5 °) from the first cleavage line, and the groove pitch is 0.3 mm or less. The groove width is 80 μm to 90 μm, and the groove depth is 80 μm or more.
Patent Document 2 discloses a surface wave device in which a substrate bottom surface is grooved to reduce amplitude ripple and phase ripple.
12A and 12B are diagrams showing a conventional surface acoustic wave device disclosed in Patent Document 2. FIG. 12A is a bottom view, and FIG. 12B is a cross-sectional view taken along line XX in FIG.
Although not shown, a BGS wave IDT is formed on the surface of the SAW substrate 51 shown in FIGS. On the other hand, as shown in FIG. 12A, the back surface of the SAW substrate 51 has a depth of 33% or more with respect to the thickness T of the SAW substrate 51 at a 45 ° angle with the BGS wave traveling direction (arrow direction). A plurality of grooves 51a each having a length d are formed.
In this case, the material of the substrate is PZT, the width of the groove 51a is 100 μm, the thickness T of the substrate is 450 μm, the interval between the grooves 51a is 300 μm, and the depth d of the grooves 51a is set to 0 to 180 μm. Patent Document 2 discloses that as the depth ratio (d / T) of the groove 51a is increased, the amplitude ripple decreases and the group delay ripple also decreases.

Patent Document 3 discloses a ladder-type SAW filter configured by forming a plurality of SAW resonators on a 36 ° Y-cut-X propagation LiTaO ₃ substrate and connecting the SAW resonators with wiring. . The SAW resonator is composed of a comb-shaped electrode and a reflector, and a groove having a semicircular cross section is formed in parallel with the IDT electrode, just below the first-stage parallel resonator from the input side of the ladder-type SAW filter. The thickness of the thinnest part of the substrate is reduced to about 150 μm. It is described that a SAW device in which the influence of bulk wave reflection is suppressed can be obtained by a structure in which a part of the substrate is thinned into a groove shape.
Patent Document 4 discloses a surface acoustic wave device in which a groove is formed on the back surface of a 45 ° X-cut Z-propagating lithium four-tartrate (45 ° X-Z Li ₂ B ₄ O ₇ ) substrate.
FIG. 13 is a cross-sectional view showing a conventional surface acoustic wave element disclosed in Patent Document 4. In FIG.
The surface acoustic wave element 61 shown in FIG. 13 has comb-teeth electrodes (IDT electrodes) 63 and 64 made up of a plurality of electrode fingers for excitation on the surface of a 45 ° X-Z Li ₂ B ₄ O ₇ substrate 62. Is formed. In the surface acoustic wave element 61, a plurality of grooves 65 are formed by dicer cutting so as to be orthogonal to the propagation direction B of the surface acoustic wave, and bulk waves are suppressed by the side wall surface 65a of each groove 65. The reception of bulk waves at the receiving side IDT electrode is suppressed.
Conditions for shielding bulk waves (solid arrows: reflected once, broken arrows: reflected twice) radiated from the IDT electrodes 63, 64 toward the back surface of the piezoelectric substrate by the side wall surfaces 65a of the grooves 65, that is, The arrangement position of each groove 65 is determined for each of the back surface reflection once and the back surface reflection twice. When the distance between the centers of the IDT electrodes 63 and 64 is set to 2L, the formation position of the groove 65 for suppressing the bulk wave reflected once on the back surface as shown by the solid line is the center portion of the IDT electrode 63. It is assumed that the distance L is a distance L from the position in the propagation direction B. Parts where the formation positions of the two grooves 65-2 and 65-3 for suppressing the bulk wave reflected twice on the back surface indicated by the broken line are separated from the position A in the propagation direction B by distances L / 2 and 3L / 2. It is said.
As shown in FIG. 13, when the internally propagating bulk wave is transmitted and received by causing equiangular reflection and reflection on the back surface of the substrate, it is considered that the influence of spurious is the greatest. Therefore, in Patent Document 4, the formation position of each groove is determined, and a groove is formed at each position by dicer cutting, thereby suppressing bulk wave spurious by each groove.
JP 60-153616 A JP-A-8-32403 Japanese Patent Laid-Open No. 11-330899 JP 2002-246876 A

However, a groove is formed on the back surface of the lithium tetratartrate Li ₂ B ₄ O ₇ substrate to suppress bulk waves as disclosed in Patent Documents 1 to 4, thereby generating a high frequency side of the passband. Although it is possible to reduce spurious, there is a risk that the mechanical strength of the substrate deteriorates and breakage occurs due to a thermal shock test (heat cycle test) or a drop test.
It has also been proposed to suppress the bulk wave by roughening the back surface of the substrate. In such a SAW device, an ultraviolet curable adhesive is used when bonding and fixing the SAW device to the bottom surface of the package. When used, there is a problem that ultraviolet rays are scattered by the back surface and the curing of the adhesive is inhibited.
The present invention has been made to solve the above problems, and provides a SAW device using an LBO substrate that is resistant to thermal shock and dropping while suppressing spurious due to bulk waves. Another object of the present invention is to provide a SAW device using an LBO substrate that can use an ultraviolet curable adhesive for fixing to a package or the like while suppressing spurious due to bulk waves.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  [Application Example 1] In the SAW device of the present invention, a comb-shaped electrode is formed on the other surface of a lithium tetraborate substrate having one surface roughened, and one surface of the lithium tetraborate substrate is used as a package for ultraviolet light. A SAW device mounted with a curable adhesive, wherein a groove is formed on one surface of the lithium tetraborate substrate, the thickness of the lithium tetraborate substrate is t (μm), and is excited by the comb electrode. When the surface acoustic wave wavelength is λ (μm) and the groove depth is d (μm), the groove depth d is in the range of 8λ ≦ d ≦ (t−150). It is characterized by.

  If comprised as mentioned above, the SAW device which suppresses the spurious resulting from the bulk wave which arises in the high region side of a passage zone, and withstands environmental tests, such as a thermal shock test and a drop test, is realizable. In addition, since the groove is provided on the back surface of the lithium tetraborate substrate, the groove portion becomes thin and the ultraviolet ray can be transmitted. Therefore, when the lithium tetraborate substrate is mounted on the package, an ultraviolet curable adhesive may be used. There is an advantage that you can.

  Application Example 2 The SAW device according to Application Example 1 is characterized in that one surface of the lithium tetraborate substrate on which the groove is formed is polished.

  If comprised as mentioned above, while suppressing the spurious resulting from the bulk wave which arises in the high region side of a passage region, the SAW device which can be used for a thermal shock test and a drop test can be constituted. In addition, since the back side is also polished, the transmission of ultraviolet rays is further improved, and the curing of the ultraviolet curable adhesive is promoted.

  Application Example 3 The SAW device according to Application Example 1 or 2, wherein an angle formed between the groove and the propagation direction of the surface acoustic wave is approximately 45 °.

  If comprised as mentioned above, the spurious resulting from the bulk wave which arises in the high region side of a passband can be suppressed most.

Hereinafter, embodiments of a surface acoustic wave device (hereinafter referred to as a SAW device) of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing a schematic configuration of a SAW device according to an embodiment of the present invention.
The SAW device 1 according to this embodiment shown in FIG. 1 has a lithium tetraborate Li ₂ B ₄ O ₇ (hereinafter referred to as LBO) having a comb-shaped electrode (hereinafter referred to as IDT electrode) 10 formed on the surface. The substrate 2 is mounted (fixed) to the package 20 with an ultraviolet curable adhesive 11. A plurality of grooves 3 are formed on the back surface of the LBO substrate 2. If necessary, a reflector (not shown) is disposed on the main surface of the LBO substrate 2. The package 20 is constituted by a ceramic substrate, for example.

2A and 2B are views showing the configuration of the LBO substrate of the SAW device according to the present embodiment. FIG. 2A is a bottom view thereof, and FIG. 2B is a cross-sectional view taken along the line QQ in FIG. A groove 3 having a width w2 and a depth d is formed with a period w1 on the back surface of the LBO substrate having a thickness t and having the surface polished and the other surface roughened.
A plurality of grooves 3 are formed on the back surface of the LBO substrate 2 at an angle of approximately 45 ° with respect to the propagation direction of the surface wave. Further, as shown in FIG. 2B, a groove 3 having a width w2 and a depth d is formed at a period w1 on the back surface of the LBO substrate 2 having a thickness t whose both surfaces are polished.
The purpose of forming the groove 3 on the back surface of the SAW device 1 using the LBO substrate 2 is, for example, in the case of a SAW filter, to suppress spurious generated on the high frequency side of the pass band, but the SAW device is required. It is necessary to sufficiently withstand environmental tests such as thermal shock tests (heat cycle tests) and drop tests.
Therefore, a 250 MHz band (wavelength λ = 13.61 μm) SAW filter was prototyped on the LBO substrate 2, and the relationship between the depth of the groove formed on the back surface and spurious suppression, the amount of insertion loss variation due to thermal shock tests, and frequency variation. The relationship with the amount, the amount of variation in insertion loss due to a drop test, and the amount of frequency variation were experimentally determined. The plate thickness t of the LBO substrate was 350 μm, the groove width w2 was 150 μm, the groove period w1 was 550 μm, and the groove depth d was a parameter, and was changed from 90 μm to 240 μm. Since the LBO substrate is common to all experiments, description thereof is omitted.

FIG. 3 is a diagram showing the relationship between the number of thermal shock tests (number of cycles) and the insertion loss variation (dB) of the SAW filter in which the depth d of the groove 3 is 120 μm.
It can be seen that the SAW filter in which the depth d of the groove 3 is 120 μm has a very small insertion loss fluctuation amount after the thermal shock test of 3000 cycles.
FIG. 4A is a diagram showing the relationship between the number of thermal shock tests (number of cycles) and the amount of frequency fluctuation (ppm) of the SAW filter in which the depth d of the groove 3 is 120 μm.
It can be seen that the SAW filter in which the depth d of the groove 3 is 120 μm has a small amount of frequency fluctuation after the thermal shock test of 3000 cycles.
4 (b) and 4 (c) show the relationship between the number of drop tests, insertion loss fluctuation (dB) and frequency fluctuation (ppm), respectively, for SAW filters with a groove 3 depth d of 120 μm. It is a figure.
The drop test was conducted by dropping naturally onto concrete from a height of 1.5 m. It can be seen from FIG. 4B that the insertion loss fluctuation amount (dB) after 27 drop tests is extremely small.
Further, it was found from FIG. 4C that the frequency fluctuation amount (ppm) after 27 drop tests was small.

FIGS. 5A and 5B show the number of thermal shock tests (number of cycles), the amount of insertion loss fluctuation (dB), and the amount of frequency fluctuation (with a depth d of the groove 3 of 160 μm, respectively). (ppm).
FIG. 5A shows that the SAW filter having a groove depth d of 160 μm has a very small insertion loss fluctuation amount after the thermal shock test of 3000 cycles.
Further, from FIG. 5B, it was found that the SAW filter in which the depth d of the groove 3 is 160 μm has a small amount of frequency fluctuation (ppm) after a 3000 cycle thermal shock test.
FIG. 5C is a diagram showing the relationship between the number of drop tests and the insertion loss fluctuation amount (dB) of the SAW filter in which the depth d of the groove 3 is 160 μm.
From this figure, it can be seen that the insertion loss fluctuation amount (dB) after 27 drop tests is extremely small.

FIG. 6 is a diagram showing the relationship between the number of drop tests and the frequency fluctuation amount (ppm) of the SAW filter in which the depth d of the groove 3 is 160 μm.
From this figure, it was found that the frequency fluctuation amount (ppm) after 27 drop tests was small.
FIGS. 7A and 7B show the number of thermal shock tests (number of cycles), insertion loss fluctuation amount (dB), and frequency fluctuation of the SAW filter in which the depth d of the groove 3 is 200 μm, respectively. It is the figure which showed the relationship with quantity (ppm).
FIG. 7A shows that the SAW filter having the groove 3 having a depth d of 200 μm has a very small insertion loss fluctuation (dB) after a 3000-cycle thermal shock test.
Further, as shown in FIG. 7B, the SAW filter having a groove 3 with a depth d of 200 μm has a frequency fluctuation amount (ppm) after a 3000-cycle thermal shock test, but it is in a range where there is no practical problem. I understand.

8A and 8B show the relationship between the number of drop tests, the amount of insertion loss fluctuation (dB), and the amount of frequency fluctuation (ppm) of a SAW filter in which the depth d of the groove 3 is 200 μm, respectively. FIG.
FIG. 8A shows that the insertion loss fluctuation amount (dB) after 27 drop tests is extremely small.
Further, it can be seen from FIG. 8B that the frequency fluctuation amount (ppm) after 27 drop tests is small.
FIG. 9 is a diagram showing the relationship between the number of thermal shock tests (number of cycles) and the insertion loss fluctuation amount (dB) of the SAW filter in which the depth d of the groove 3 is 240 μm.
From this figure, it was found that when the depth d of the groove is 240 μm, that is, the remaining thickness of the substrate in the groove portion is approximately 110 μm, a slight defect occurs at 1000, 2000, and 3000 cycles.

FIG. 10A is a diagram showing the relationship between the number of thermal shock tests (number of cycles) and the frequency fluctuation amount (ppm) of the SAW filter in which the depth d of the groove 3 is 240 μm.
From this figure, it was found that some defects occurred in 1000, 2000, and 3000 cycles.
FIGS. 10B and 10C show the relationship between the number of drop tests, the amount of insertion loss fluctuation (dB), and the amount of frequency fluctuation (ppm) of the SAW filter in which the depth d of the groove 3 is 240 μm, respectively. FIG.
As is clear from FIGS. 10B and 10C, when the groove depth d is 240 μm, the insertion loss fluctuation amount and the frequency fluctuation amount are not defective even after 27 natural drop tests. I understand.

FIG. 11 shows that the groove 3 is formed on the back surface of the SAW filter, and the depth d of the groove 3 and the size of the spurious due to the bulk wave (attenuation amount (dB)) generated on the high frequency side of the pass band of the SAW filter. ) And a so-called box-and-whisker diagram.
As is well known, the minimum, median, maximum, 25% point, and 75% point can be visually read from the boxplot.
The horizontal axis represents the depth d (mm) of the groove 3 and was changed to 0.09 mm, 0.11 mm, 0.13 mm, 0.15 mm, and 0.17 mm. The vertical axis (dB) shows the magnitude of spurious generated on the high frequency side of the passband at that time.
From FIG. 11, it was found that if the depth d of the groove 3 is 0.11 mm or more, the required standard 35.5 dB is satisfied, and the spurious size does not change even if the groove is deeper than 0.11 mm.
Since the spurious due to the bulk wave generated on the high frequency side of the pass band of the SAW filter depends on the wavelength λ of the surface acoustic wave, the depth d of the groove 3 is 0.11 mm (110 μm) and the wavelength λ (13.61 μm). When it is standardized by λ, it becomes 8λ, and if there is a depth d larger than this, spurious can be suppressed and the standard (35.5 dB) can be satisfied.

In addition, from the results of the thermal shock test and the drop test described above, it was found that the depth d of the groove 3 is desirably 200 μm or less.
If the thickness of the LBO substrate 2 is t (μm) and the thickness (t−d) of the remaining portion of the groove 3 is larger than (350−200) = 150 μm, it is possible to withstand the thermal shock test and the drop test. It turned out more. That is, the depth d of the groove needs to be smaller than (t−150 μm).
Therefore, the range of the depth d of the groove 3 satisfying both the spurious on the high band side of the pass band, the thermal shock test and the drop test is 8λ ≦ d ≦ (t−150 μm).
One surface (back surface) of the LBO substrate, that is, the surface to be bonded to the inner surface of the package is roughened, and the groove 3 for suppressing bulk waves is formed on the surface. It was sufficient to cure the UV curable adhesive.
In addition, the SAW device in which the groove 3 was formed on the back surface of the LBO substrate whose both main surfaces were polished was also tested, but spurious due to the bulk wave generated on the high frequency side of the passband could be suppressed. Moreover, since the back side is also polished, the ultraviolet ray transmittance is good and the ultraviolet curable adhesive is sufficiently cured.

The figure which showed schematic structure of the SAW device which concerns on embodiment of this invention. It is the figure which showed the structure of the LBO board | substrate of the SAW device which concerns on this embodiment, (a) is a bottom view, (b) is sectional drawing in the QQ line of (a). The figure which shows the relationship between the frequency | count of a thermal shock test, and insertion loss fluctuation amount. (A) is a thermal shock test and frequency fluctuation amount, (b), (c) is a figure which shows the relationship between a drop test and insertion loss fluctuation amount and frequency fluctuation amount, respectively. (A), (b) is a figure which shows the relationship between a thermal shock test and the amount of fluctuations of insertion loss, and the amount of frequency fluctuation, respectively, (c) is a figure which shows the relationship between a drop test and the amount of fluctuation of insertion loss. The figure which shows the relationship between a drop test and the amount of frequency fluctuations. (A), (b) is a figure which shows the relationship between a thermal shock test, and insertion loss fluctuation amount and frequency fluctuation amount, respectively. (A), (b) is a figure which shows the relationship between a drop test and an insertion loss fluctuation amount and a frequency fluctuation amount, respectively. The figure which shows the relationship between a thermal shock test and an insertion loss fluctuation amount. (A) is a figure which shows the relationship between a thermal shock test and a frequency fluctuation amount, (b), (c) is a figure which shows the relationship between a drop test and an insertion loss fluctuation amount and a frequency fluctuation amount, respectively. (A) is a box-and-whisker diagram showing the relationship between the depth of the groove and the spurious, and (b) is a diagram showing the depth of the groove, its measured value, the minimum, maximum, average value and standard deviation of the spurious. It is the figure which showed the structure of the conventional SAW device, (a) is a bottom view, (b) is sectional drawing. Sectional drawing which shows the other structure of the conventional SAW device.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... SAW device, 2 ... LBO substrate, 3 ... Groove, 4 ... Back surface, 10 ... IDT electrode, 11 ... Ultraviolet curable adhesive, 20 ... Package, t ... Thickness of LBO substrate, w1 ... Groove period, w2 ... groove width, d ... groove depth, θ ... angle between surface wave propagation direction and groove

Claims (3)

A surface acoustic wave device in which a comb-shaped electrode is formed on the other surface of a lithium tetraborate substrate roughened on one surface, and one surface of the lithium tetraborate substrate is mounted on a package with an ultraviolet curable adhesive. There,
Forming a groove on one surface of the lithium tetraborate substrate;
When the thickness of the lithium tetraborate substrate is t (μm), the wavelength of the surface acoustic wave excited by the comb electrode is λ (μm), and the depth of the groove is d (μm),
A surface acoustic wave device, wherein a depth d of the groove is in a range of 8λ ≦ d ≦ (t−150).   2. The surface acoustic wave device according to claim 1, wherein one surface of the lithium tetraborate substrate forming the groove is polished.   3. The surface acoustic wave device according to claim 1, wherein an angle between the groove and a propagation direction of the surface acoustic wave is approximately 45 °.

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