JP2011061743A - Surface acoustic wave device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate temperature characteristics in a resin mold state and prevent characteristic deterioration due to moisture absorption.
SOLUTION: An electrode 2 formed on a piezoelectric substrate 1, a first SiO ₂ layer 3 formed on the piezoelectric substrate 1 so as to cover the electrode 2, and an SiN formed on the first SiO ₂ layer 3 The laminate 7 includes a layer 4 and a second SiO ₂ layer 5 formed on the SiN layer 4. For example, LiTaO ₃ is used as the material of the piezoelectric substrate 1. The electrode 2 is, for example, a comb electrode using aluminum (Al).
[Selection] Figure 1

Description

  The present invention relates to a surface acoustic wave element capable of exciting an elastic wave such as a surface acoustic wave (SAW).

Conventionally, surface acoustic wave elements have been used in, for example, resonators and bandpass filters. In order to compensate the temperature characteristics of the surface acoustic wave element, there is a technique in which a comb-shaped electrode is formed on a piezoelectric substrate made of LiTaO ₃ or the like, and then a SiO ₂ (silicon oxide) layer is formed so as to cover the electrode. It has been proposed (see, for example, Patent Document 1). SiO ₂ has a temperature coefficient opposite to that of LiTaO ₃ or the like (LiTaO ₃ has a negative temperature coefficient, whereas SiO ₂ has a positive temperature coefficient), so that a temperature characteristic compensation effect is expected. Is done.

However, after laminating the SiO ₂ layer, resin sealing is performed, and when a surface acoustic wave element (SAW device) is cut out from the substrate and supplied as a bare chip, in this resin mold state, complete hermetic sealing is not possible. Therefore, it is inevitable that moisture enters the package. SiO ₂ is a material exhibiting hygroscopicity, and it has been found through tests by the inventors that the characteristics of the surface acoustic wave device including the SiO ₂ layer fluctuate due to the high temperature and high humidity test.

  In order to improve the weather resistance, a technique for providing a SiN film has been proposed (see, for example, Patent Document 2), but it is premised on the use of an expensive manufacturing apparatus and the film formation rate is also slow. Not practical.

Table 2005/083881 Japanese Patent No. 3925366

  The above prior art (Patent Document 1 and the like) is premised on a package having a hermetic structure that is completely hermetically sealed. Therefore, the problem of characteristic variation due to moisture absorption is not considered. On the other hand, there is a demand for handling a surface acoustic wave element in a bare chip state for downsizing and the like.

  An object of the present invention is to provide a surface acoustic wave element that solves the above-described problems and that can compensate for temperature characteristics and prevent characteristic deterioration due to moisture absorption, for example, in a resin mold state.

In order to solve the above-mentioned problem, the invention of claim 1 is a surface acoustic wave element in which an electrode layer is formed on a piezoelectric substrate, and is formed on the piezoelectric substrate so as to cover the electrode layer. the a first SiO ₂ layer for improving the frequency temperature characteristic of the surface acoustic wave element formed on the first SiO ₂ layer on, SiN for suppressing moisture absorption in the first SiO ₂ layer It is characterized in that it includes a laminate composed of a layer and a second SiO ₂ layer formed on the SiN layer and protecting the SiN layer.

Function According to a second aspect of the invention, a surface acoustic wave device according to claim 1, wherein the SiN layer, and the second SiO ₂ layer, which protects each function of suppressing moisture absorption, and the SiN layer It is characterized by being set to a thickness adapted to fulfill

The invention according to claim 3 is the surface acoustic wave element according to claim 1 or 2, wherein both the thickness of the SiN layer and the second SiO ₂ layer are set to about 100 nm or less. It is characterized by.

According to the first aspect of the present invention, the first SiO ₂ layer for improving the frequency temperature characteristic, the SiN layer for suppressing moisture absorption in the first SiO ₂ layer, and the first SiO ₂ layer for protecting the SiN layer. since they have a SiO ₂ layer, for example, in the resin molding condition, it performs compensation of the temperature characteristics, it is possible to prevent the characteristic degradation due to moisture absorption.

It is sectional drawing which shows typically the structure of the principal part of the surface acoustic wave element by one Embodiment of this invention. It is process drawing for demonstrating the manufacturing method of the surface acoustic wave element. It is sectional drawing which shows typically the structure of the laminated body of the sample used for the measurement for obtaining the same surface acoustic wave element. It is a characteristic view which shows the time change of the filter characteristic measured under high temperature, high humidity using the sample. It is a characteristic view which expands and shows the A section of FIG. It is a characteristic diagram showing temporal changes of the filter characteristics measured under high temperature and high humidity conditions by using the same sample with different thickness of SiO ₂ layer. It is a characteristic view which expands and shows the B section of FIG. It is a characteristic diagram showing temporal changes of the filter characteristics measured under high temperature and high humidity conditions by using the same sample with different thickness of SiO ₂ layer. It is a characteristic view which expands and shows the C section of FIG. It is a characteristic view which shows the time change of the filter characteristic measured in the thermostat using the sample. It is a characteristic view which expands and shows the D section of FIG. It is a characteristic view which shows the time change of the filter characteristic measured under high temperature, high humidity using another sample for obtaining the same surface acoustic wave element. It is a characteristic view which expands and shows the E section of FIG. It is a characteristic diagram showing temporal changes of the filter characteristics measured under high temperature and high humidity conditions by using the same separate samples with different thickness of SiO ₂ layer and a SiN layer. It is a characteristic view which expands and shows the F section of FIG. It is a characteristic view which shows the time change of the filter characteristic measured under high temperature, high humidity using the sample containing the laminated body of the same surface acoustic wave element. It is a characteristic view which expands and shows the G section of FIG. It is a characteristic diagram showing the measured filter temperature characteristics by using the same separate samples with different thickness of SiO ₂ layer and a SiN layer. It is a characteristic view which shows the filter temperature characteristic measured using the sample containing the laminated body of the same surface acoustic wave element.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view schematically showing a configuration of a main part of a surface acoustic wave element according to an embodiment of the present invention.

As shown in FIG. 1, the surface acoustic wave device of this embodiment includes an electrode 2 formed on a piezoelectric substrate 1 and a first SiO ₂ (formed on the piezoelectric substrate 1 so as to cover the electrode ₂ ( and silicon oxide) layer 3 includes a first 1SiO SiN formed on the _second layer 3 (silicon nitride) layer 4, the stacked body 7 consisting of the 2SiO ₂ layer 5 which is formed on the SiN layer 4. For example, LiTaO ₃ is used as the material of the piezoelectric substrate 1. The electrode 2 is, for example, a comb electrode using aluminum (Al).

The first SiO ₂ layer 3 is used as a temperature compensation layer for compensating temperature characteristics (frequency temperature characteristics) of the surface acoustic wave element. For this purpose, SiO ₂ having a temperature coefficient opposite to that of LiTaO ₃ is used. The SiN layer 4 is used as a moisture-proof layer. The second SiO ₂ layer 5 is used for preventing functional deterioration as a moisture-proof layer of the SiN layer 4.

The thickness of the first SiO ₂ layer 3 is not particularly limited (in this embodiment, for example, approximately 1800 mm (approximately 180 nm)). The thicknesses of both the SiN layer 4 and the second SiO ₂ layer 5 are preferably about 1000 mm (about 100 nm) or less (in this embodiment, both are about 500 mm (about 50 nm), for example. .)

  Although FIG. 1 shows a single surface acoustic wave element laminate 7, in practice, a plurality of surface acoustic wave elements are formed on a substrate (wafer) and then separated by dicing. .

Next, a method for manufacturing the surface acoustic wave device of this embodiment will be described. FIG. 2 is a process diagram for explaining the method of manufacturing the surface acoustic wave device. First, as shown in FIG. 2A, the electrode 2 is formed on the piezoelectric substrate 1, and the first SiO ₂ layer 3 is laminated so as to cover the electrode 2 on the piezoelectric substrate 1.

Here, for example, after the electrode 2 is formed on the piezoelectric substrate 1 by mask vapor deposition, photolithography, or the like, an insulating layer made of SiO ₂ is laminated as shown in FIG. _Two layers 3 are formed.

Next, as shown in FIG. 2B, the SiN layer 4 is laminated on the first SiO ₂ layer 3 by, for example, sputtering. Next, as shown in FIG. 2C, the second SiO ₂ layer 5 is laminated on the SiN layer 4 by, for example, a sputtering method to obtain a laminated body 7.

  The laminated body 7 is resin-sealed, and a bump base metal and a bump are formed on an electrode pad drawn out from a comb-shaped electrode. Thereafter, a surface acoustic wave element is cut out from the substrate and supplied as a bare chip. In addition, after mounting a surface acoustic wave element on a PC board, resin may be apply | coated and cut out, and after mounting on a PC board with other electronic components, the whole may be molded with resin.

Next, the laminated structure shown in FIG. 1 and the basis for obtaining the optimum film thickness of each layer and the process leading to this will be described. FIG. 3 is a cross-sectional view schematically showing the structure of a laminate of samples used for measurement for obtaining the surface acoustic wave element, and FIG. 4 shows filter characteristics measured using the sample at high temperature and high humidity. FIG. 5 is a characteristic diagram showing an enlarged view of part A in FIG. 4, and FIG. 6 is a filter measured under high temperature and high humidity using the same sample with a different thickness of the SiO ₂ layer. FIG. 7 is a characteristic diagram showing an enlarged view of part B in FIG. 6, and FIG. 8 is a measurement under high temperature and high humidity using the same sample with a different thickness of the SiO ₂ layer. FIG. 9 is a characteristic diagram showing an enlarged view of part C of FIG. 8, and FIG. 10 shows a time change of the filter characteristics measured in the thermostatic chamber using the same sample. FIG. 11 is a characteristic diagram showing the D portion of FIG. 10 in an enlarged manner, and FIG. 12 shows another sample for obtaining the surface acoustic wave device. Characteristic diagram showing temporal changes of the filter characteristics measured at high temperature and high humidity, 13, the characteristic diagram showing the enlarged portion E of FIG. 12, FIG. 14, changed the thickness of the SiO ₂ layer and a SiN layer FIG. 15 is a characteristic diagram showing an enlarged portion F of FIG. 14, and FIG. 16 is a surface acoustic wave device. FIG. 15 is a characteristic diagram showing a time change of filter characteristics measured using a different sample under high temperature and high humidity. FIG. 17 is a characteristic diagram showing a time change of the filter characteristics measured under high temperature and high humidity using a sample including the laminate of FIG. 16, FIG. 17 is a characteristic diagram showing an enlarged G part of FIG. 16, and FIG. 18 is a SiO ₂ layer FIG. 19 is a characteristic diagram showing the filter temperature characteristics measured using the same sample with different thicknesses of the SiN layer, and FIG. 19 shows the filter temperature characteristics measured using the sample including the laminate of the surface acoustic wave elements. FIG.

The inventors first used the sample (resin-sealed package sample) including the laminate 13 as shown in FIG. 3 to determine the thickness of the SiO ₂ layer formed on the piezoelectric substrate and the temperature characteristics of the filter. The relationship was measured. The relationship between the temperature (−30 ° C. to 80 ° C.) and the frequency shift (frequency temperature characteristics) was examined by gradually increasing the thickness of the SiO ₂ layer. As a result, when the thickness of the SiO ₂ layer is increased, the negative temperature coefficient of the right-sloping characteristic line decreases (becomes flat), and the thickness is about 20% λ (λ: surface acoustic wave wavelength). It was found that the temperature was almost flat and the temperature compensation was almost complete. From this, when it increased, it became a positive temperature coefficient.

  Next, the inventors put a sample (resin-sealed package sample) with various thicknesses into a high-temperature and high-humidity tank at 85 ° C. and 95%, and filter characteristics (between frequency and insertion loss). (Relation)) over time. 4 and 5 show the measurement results when the thickness is 13% λ, FIGS. 6 and 7 show the measurement results when the thickness is 20% λ, and FIGS. 8 and 9 show the measurement results when the thickness is 25% λ.

  4 and 5, the curve a1 is after application of the resin, the curve a2 is after 100 hours in the high-temperature and high-humidity tank, the curve a3 is after 500 hours in the high-temperature and high-humidity tank, and the curve a4 is high-temperature and high-humidity. The measurement result 1000 hours after charging in the tank is shown.

  6 and 7, curve b1 is after resin application, curve b2 is 100 hours after charging in the high-temperature and high-humidity tank, curve b3 is 500 hours after charging in the high-temperature and high-humidity tank, and curve b4 is high-temperature. The measurement result 1000 hours after throwing in a high-humidity tank is shown.

  8 and 9, curve c1 is after resin application, curve c2 is 100 hours after charging in the high-temperature and high-humidity tank, curve c3 is 500 hours after charging in the high-temperature and high-humidity tank, and curve c4 is high-temperature. The measurement result 1000 hours after throwing in a high-humidity tank is shown.

From the above measurement results, with the characteristics due to moisture absorption in the SiO ₂ layer is varied, as the thickness of the SiO ₂ layer, it can be seen that the deterioration of the insertion loss moisture absorption is increased becomes larger.

  Next, the inventors put the sample after moisture absorption into a thermostatic bath at 130 ° C. for 20 hours, and examined changes in filter characteristics. 10 and 11, the curve d1 shows the measurement result before charging in the thermostat, and the curve d2 shows the measurement result after 20 hours in charging in the thermostat.

  From this measurement result, the insertion loss was 11.5 dB before charging the thermostat, but it recovered to 7.4 dB after charging the thermostat. It is considered that the propagation loss of about 4 dB has been recovered.

Next, the inventors have prepared a sample including a laminate in which a SiO ₂ layer (3500 mm) and a SiN layer (1000 mm) are stacked on a piezoelectric substrate, a SiO ₂ layer (1800 mm) on a piezoelectric substrate, and a SiN layer (500 Å) and a sample containing a laminate comprising a laminated, and the 1SiO ₂ layer on the piezoelectric substrate and the (1800 Å), SiN layer (500 Å), laminates first 2SiO ₂ layer and (500 Å) laminated ( A sample including the surface acoustic wave element laminate of this embodiment was placed in a high-temperature and high-humidity tank at 85 ° C. and 95%, and the time change of filter characteristics (relationship between frequency and insertion loss) was examined. . 12 and 13 show a sample including a laminate in which an SiO ₂ layer (3500 mm) and an SiN layer (1000 mm) are stacked, and FIGS. 14 and 15 show an SiO ₂ layer (1800 mm) and an SiN layer (500 mm). FIG. 16 and FIG. 17 show the measurement results of the sample including the stacked body in which the SiN layer (500 Å) and the second SiO ₂ layer (500 Å) are stacked.

  12 and 13, the curve e1 shows the measurement result after resin application, the curve e3 shows the measurement result after 500 hours in the high temperature and high humidity tank, and the curve e4 shows the measurement result after 1000 hours in the high temperature and high humidity tank.

  14 and 15, the curve f1 is after resin application, the curve f2 is 100 hours after charging in the high-temperature and high-humidity tank, the curve f3 is 500 hours after charging in the high-temperature and high-humidity tank, and the curve f4 is high-temperature and high-humidity. The measurement result 1000 hours after charging in the tank is shown.

  In FIGS. 16 and 17, curve g1 is after resin application, curve g2 is 100 hours after charging in the high-temperature and high-humidity tank, curve g3 is 500 hours after charging in the high-temperature and high-humidity tank, and curve g4 is high-temperature and high-humidity. The measurement result 1000 hours after charging in the tank is shown.

From the above measurement results, the 1SiO ₂ layer on the piezoelectric substrate and the (1800 Å), SiN layer (500 Å), is more of a sample containing a laminate formed by laminating a first 2SiO ₂ layer (500 Å), a piezoelectric substrate A sample including a laminate in which a SiO ₂ layer (3500 mm) and a SiN layer (1000 mm) are stacked on top, and a laminate in which a SiO ₂ layer (1800 mm) and a SiN layer (500 mm) are stacked on a piezoelectric substrate. It can be seen that the minimum insertion loss before the high temperature and high humidity test is smaller than that of the containing sample.

Generally, since SiN is a film having an internal stress larger than that of SiO ₂ , the SiN film having a large stress is sandwiched between the first SiO ₂ layer (1800 mm) and the _second SiO ₂ layer (500 mm), so that the entire laminated film can be obtained. It is considered that the stress is relieved, the laminated film behaves as if it is a single SiO ₂ layer, and the propagation loss is improved.

Also, the 1SiO ₂ layer on the piezoelectric substrate and the (1800 Å), SiN layer (500 Å), is more of a sample containing a laminate formed by laminating a first 2SiO ₂ layer (500 Å), SiO ₂ on the piezoelectric substrate Compared to a sample including a laminate in which a layer (3500 mm) and a SiN layer (1000 mm) are stacked, and a sample including a stack in which a SiO ₂ layer (1800 mm) and a SiN layer (500 mm) are stacked on a piezoelectric substrate. It can be seen that the deterioration due to the high temperature and high humidity test was reduced.

It was found that when the pinhole in the SiN layer generated when the SiN layer is thin is covered with the _second SiO ₂ layer (500 mm), the function as a moisture-proof film is not impaired. This pinholes SiN layer is filled by the 2SiO ₂ layer, and the 2SiO ₂ layers, absorbs moisture before the first 1SiO ₂ layer, further, the SiN layer as a moisture-proof film, direct sales By not being exposed to moisture, even if pinholes are present in the SiN layer, it is prevented from permeating through the first SiO ₂ layer.

Next, the inventors have a sample including a laminate in which a SiO ₂ layer (1800 mm) and a SiN layer (500 mm) are stacked on a piezoelectric substrate, and a first SiO ₂ layer (1800 mm) on the piezoelectric substrate. The temperature characteristics of the filter (temperature (−30 ° C.)) for a sample including a laminate (a laminate of the surface acoustic wave device of this embodiment) in which a SiN layer (500 Å) and a second SiO ₂ layer (500 Å) are laminated. ˜80 ° C.) and the frequency shift). 18 shows a sample including a laminate in which a SiO ₂ layer (1800 mm) and a SiN layer (500 mm) are stacked on a piezoelectric substrate, and FIG. 19 shows a first SiO ₂ layer (1800 mm) on the piezoelectric substrate, SiN layer (500 Å), showing a case where a sample containing a laminate formed by laminating a first 2SiO ₂ layer (500 Å). As can be seen from the straight lines h1 and h2, the temperature characteristics are substantially the same. That is, it can be seen that the second SiO ₂ layer (500 mm) does not affect the compensation of the temperature characteristics.

Thus, according to the configuration of this embodiment, the temperature characteristic can be compensated for by the first SiO ₂ layer 3. In addition, the SiN layer 4 can prevent deterioration of characteristics due to moisture absorption. Furthermore, since the SiN layer 4 can be thinned by the _second SiO ₂ layer 5, the propagation loss can be improved. Therefore, it can be mounted in a bare chip state by resin sealing, which can contribute to downsizing and cost reduction. In addition, film formation can be performed at a relatively high speed without using an expensive manufacturing apparatus.

The embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to these embodiments, and the design can be changed without departing from the gist of the present invention. Is included in the present invention. In the above-described embodiment, the case where LiTaO ₃ is used as the material of the piezoelectric substrate has been described. However, LiNbO ₃ , Li ₂ B ₄ O ₇ , quartz, or the like may be used in addition to LiTaO ₃ . Also, and the 1SiO ₂ layer 3, SiN (silicon nitride) layer 4, the deposition of the 2SiO ₂ layer 5, in addition to the sputtering method may be used CVD method, or the like. A protective film may be formed between the piezoelectric substrate and the electrode.

  As an electrode material, in addition to aluminum, for example, an alloy containing aluminum, copper (Cu), or the like can be used.

1 Piezoelectric substrate 2 Electrode (electrode layer)
3 First SiO ₂ layer (first SiO ₂ layer)
4 SiN layer 5 first 2SiO ₂ layer (second SiO ₂ layer)
7 Laminate

Claims (3)

A surface acoustic wave device in which an electrode layer is formed on a piezoelectric substrate,
Said to cover the electrode layer is formed on a piezoelectric substrate, a first SiO ₂ layer for improving the frequency temperature characteristic of said surface acoustic wave element formed on the first SiO ₂ layer on, wherein the SiN layer for suppressing moisture absorption of the first SiO ₂ layer, is formed on the SiN layer, comprising a laminate consisting of the second SiO ₂ layer for protecting the SiN layer A surface acoustic wave device. The SiN layer and the second SiO ₂ layer are set to thicknesses that are adapted to perform a function of suppressing moisture absorption and a function of protecting the SiN layer, respectively. 2. The surface acoustic wave device according to 1. 3. The surface acoustic wave device according to claim 1, wherein thicknesses of the SiN layer and the second SiO ₂ layer are both set to about 100 nm or less.