JPS62128605A - Surface acoustic wave element and its production - Google Patents

Abstract

PURPOSE:To increase the reflection factor of a reflector and to reduce the size of a surface acoustic wave element by increasing the line width ratio of the reflector using metallic strips which are not connected with each other. CONSTITUTION:Here 0.5<etaR<1.0 is satisfied with etaR=LR/(LR+SR), where the metallic finger width is referred to as LR, the space between metallic fingers as SR and the line width ratio as etaR respectively with a reflector 3. As a result, the reflection factor using metallic strips is improved together with a small size of a surface acoustic wave element. Furthermore the capacity ratio of the wave element is reduced when it is used to a voltage controlled oscillator. Thus the frequency variable width is increased and the size of a filter containing a reflector for surface acoustic waves is reduced together with reduction of the insertion loss of the filter.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a resonator, a filter, a delay line, etc. having a reflector made of a metal strip, a comb-like electrode, etc. on a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates. The present invention relates to a surface acoustic wave device and a method for manufacturing the same.

"Prior art and its problems" Surface acoustic wave elements have traditionally been used for special purposes in military applications, but in recent years they have begun to be used in consumer equipment such as FM tuners and TVs, and have suddenly come into the spotlight. It has become. Specifically, I-type surface wave elements are commercialized as delay elements, oscillators, filters, and the like. The characteristics of these various surface acoustic wave devices are that they are small, lightweight, and highly reliable, and that their manufacturing process is similar to that of integrated circuits.
This includes being highly productive. Nowadays, it is mass-produced as an indispensable electronic component.

There are various types of surface acoustic waves that propagate on the surface of piezoelectric media, but
Rayleig is commonly used.
h) It is called a wave. Incidentally, there are a coupling coefficient and a temperature coefficient as indicators for evaluating the performance of a piezoelectric substrate. The coupling coefficient is an index representing the efficiency with which electrical energy is converted into vibration energy, and the temperature coefficient is an index representing the temperature coefficient of propagation delay time of a surface acoustic wave propagating through a piezoelectric medium. Furthermore, surface acoustic waves include a shear horizontal type surface acoustic wave in which particles are displaced in a direction perpendicular to the direction in which the surface acoustic waves propagate within the surface layer of the piezoelectric substrate through which the surface acoustic waves propagate, and the coupling coefficient is It is starting to attract attention due to its large size.

In recent applications of surface acoustic wave devices, surface acoustic wave resonators are being used in voltage-controlled oscillators, etc., as mobile radio devices such as car phones use higher frequencies, and are helping to reduce size, weight, and cost. There is a problem that the frequency variable width is narrow.

An example of a surface acoustic wave resonator is shown in FIG. 7 as an example of a conventional surface acoustic wave element having a reflector using a metal strip. That is, this surface acoustic wave resonator consists of a piezoelectric substrate l on which surface acoustic waves propagate, an interdigital electrode 2 for excitation of surface acoustic waves, and a large number of metal strips arranged periodically at right angles to the propagation direction of the surface acoustic waves. Reflectors 3 arranged in
．． It is configured to form a 3° angle. When a voltage of a specific frequency is applied to the interdigital electrode 2, the interdigital electrode 2
An electric field is applied to the surface of the piezoelectric substrate 1 in the gap, and the piezoelectricity of the piezoelectric substrate 1 causes a strain proportional to the voltage, and the strain propagates to both sides as a surface wave at a sound speed determined by the material of the piezoelectric substrate 1. This surface wave is reflected by the grating reflectors 3.3 on both sides, returns to the interdigital electrode 2 again, and resonates.

When such a surface acoustic wave resonator is expressed as an electrical equivalent circuit, it can be expressed as shown in FIG. 8, similar to the case of a crystal resonator. In FIG. 8, G.

is the capacitance of the interdigital electrode 2, which is an inductance, and the series resonant circuit of the capacitance C and the resistance R represents the resonance phenomenon of the resonator. Further, the ratio Go/C between Go and C is called a capacitance ratio and is expressed as γ. The capacitance ratio γ is one of the important parameters that determines the resonant frequency characteristics of a resonator, and when γ is small, the oscillation frequency can be varied over a wide frequency range when used in a voltage controlled oscillator.

The capacity ratio γ is y=(yr2/8 to 2)or r-・■(r
≧1) Te approximation is possible. Here, k2 is the aforementioned coupling coefficient, and r is a constant determined by the performance of the reflector.

r=1 represents the theoretical limit. ■As can be seen from the formula, the capacitance ratio γ is determined by the coupling coefficient of the piezoelectric substrate used and the performance of the reflector, but conventionally only the coupling coefficient of the piezoelectric substrate has been focused on, and the reflector performance has been influenced by the configuration of the reflector. There is no focus on improvement, the reflector becomes large, the element cannot be made smaller, the capacitance ratio is too high, and when used in a voltage controlled oscillator, the variable frequency cannot be increased, etc. was there.

A partially enlarged view of a conventional reflector is shown in FIG. That is,
The width of the metal strip used in the reflector and the spacing between the metal strips are approximately equal, and the line width ratio ηR of the reflector is made to be ηR=0.5. This deteriorates the performance of the reflector, specifically its reflectance, from its original characteristics.In order to increase the reflectance of the entire reflector, it is necessary to use a large number of metal strips, which increases the size of the element and increases the capacitance. The disadvantage was that the ratio γ was not reduced.

"Objective of the Invention" The object of the present invention is to solve the problems of the prior art described above, to improve the reflectance of a reflector using metal strips, to reduce the size of the element, and to reduce the capacitance ratio. To provide a surface acoustic wave element which, when used in a voltage controlled oscillator, has improved performance such as expansion of frequency variable width, miniaturization of a filter having a surface acoustic wave reflector, and reduction in insertion loss, and a method for manufacturing the same. It is in.

"Structure of the Invention" The surface acoustic wave element of the present invention has a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates, and a large number of interconnected elements whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave. If the reflector is equipped with a reflector made of a metal strip without a metal strip, the metal finger width of the reflector is LR5, the metal finger spacing is SR, and the line width ratio ηR is expressed as ηR=LR/(LR+SR),
It is characterized in that ηR<1.0.

The further improved surface acoustic wave element of the present invention has a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates, and a large number of interconnected elements whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave. a reflector made of a metal strip without a metal strip, and at least one set of interdigital electrodes, the metal finger width of the reflector is LR1, the metal finger spacing is SR, and the line width ratio ηR is ηR=LR/(LR+SR). When expressed as 0.5<ηR<1.0, and the electrode finger width of the interdigital electrode is LT, the electrode finger width interval is ST,
When one line width ratio is expressed as ηt=LT/(LT+ST), it is characterized in that ηT<ηR.

The method for manufacturing a surface acoustic wave device according to the present invention includes a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates, and a large number of interconnected devices whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave. A method for manufacturing a surface acoustic wave device having a reflector made of a metal strip, wherein the reflector has a metal finger width of LR1, a metal finger spacing of SR, and a line width ratio η.
When R is expressed as ηR=LR/(LR+SR),
It is characterized in that it is manufactured by photolithography using a photomask created by setting the line width ratio of the reflector pattern so that 0.5<ηR<1.0.

A further improved method of manufacturing a surface acoustic wave element of the present invention is to provide a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates, the center spacing being approximately 1/1/2 of the propagation wavelength of the surface acoustic wave.
2, a method for manufacturing a surface acoustic wave device comprising a reflector made of a large number of metal strips that are not connected to each other, and at least one set of interdigital electrodes, wherein the metal finger width of the reflector is set to LR1 metal finger spacing. is SR, and the line width ratio ηR is expressed as R=LR/(LR+SR), then 0.
5, so that ηR<1.0, and the electrode finger width of the interdigital electrode is LT, the electrode finger width interval is ST, and the line width ratio is expressed as ηT=LT/(LT+ST). case,
It is characterized in that it is manufactured by photolithography using a photomask created by setting the line width ratio of the reflector and interdigital electrode patterns so that η↑<ηR.

In the surface acoustic wave device of the present invention, the reflectance of the reflector can be improved by setting the pattern of the reflector as described above. In addition, by setting the patterns of the reflector and interdigital electrodes as described above, the resonance resistance and attenuation outside the passband are stabilized, and the design is compact and has a low capacitance ratio.
You can get the best. Further, in the method for manufacturing a surface acoustic wave device of the present invention, a surface acoustic wave device having the above-described pattern can be manufactured easily and with high precision.

Furthermore, by providing a photomask with an inspection pattern that effectively imitates the shape of the reflector or interdigital electrode, the line width can be easily and accurately measured using electrical, mechanical, or optical methods. The ratio can be measured, and the quality of the device can be determined immediately after etching.

In the present invention, it is preferable that the reflector made of a metal strip and the interdigital electrode be formed of the same metal, but from the viewpoint of the characteristics of the reflector, ease of process, cost, etc., A1 or A1 alloy is used as the metal. Preferably, or A
It is preferable to have a multilayer structure of I or A1 alloy and a high melting point metal.

"Embodiments of the Invention" Example 1 FIGS. 1, 2, and 3 show an embodiment in which the present invention is applied to a surface acoustic wave resonator, and FIG. 1 is a plan view, FIG. 2 is a sectional view, and FIG. 3 is a partially enlarged sectional view of the reflector portion.

A mirror-polished lithium tantalate with a Y-axis cut rotated at 36 degrees was used as the piezoelectric substrate 1 on which shear horizontal surface acoustic waves propagate, and AI was sputter-deposited thereon to a thickness of 2000 nm. Then reflector 3.3
Using a photomask prepared by changing the line width ratio ηR of ' and the line width ratio ηT of the interdigital electrode 2, the pattern is transferred to the photoresist, and the reflector 3.3' and the interdigital interdigitation are formed by a normal wet etching method. The shaped electrode 2 was patterned. At this time, a test pattern imitating the reflector 3.3° and the interdigital electrode 2 was also formed. Table 1 shows the characteristics of the surface acoustic wave resonator according to the line width ratio measured using an optical method on this inspection pattern. Note that the propagation direction was approximately the X-axis direction. In addition, this surface acoustic wave resonator is
The number of unconnected metal strips of reflector 3.3° is 200, the number of pairs of interdigital electrodes 2 is lO pairs, and the pitch of reflector 3.3° and interdigital electrode 2 is about 4. , the resonant frequency was approximately 510 MHz.

Table 1 According to Table 1, the capacitance ratio is the line width ratio η of the reflector 3.3゛.
It can be seen that as R becomes larger, it becomes smaller. In other words, this indicates that the reflectance of the reflector is increasing.

As another example, the structure was similar to that of the previous example, but the number of metal strips constituting the 3.3° reflector was reduced to 150. When the line width ratio of the interdigital electrode 2 is set to 0.5 as in the previous embodiment, the reflector 3.
When the 3° line width ratio ηR was 0.5, the capacitance ratio increased by about 15%, and when ηR was 0.7, there was almost no change.

From this, it can be seen that the reflectance of the reflector of 3.3 degrees according to the present invention is much higher than that of the conventional example, and it can be said that it is extremely effective in lowering the capacitance ratio and downsizing. In this case, ηR
The magnitude of is within the range of the present invention, 0, 5<ηR<1.0
If so, it is clear that the characteristics are superior to the conventional example, but in order to fully obtain the effects of the present invention, ηR is preferably 0.55 or more. On the other hand, if ηR is too large, the metal strip constituting the reflector There is an increased possibility that they will come into contact with each other,
Since the yield decreases, ηR is preferably 0.9 or less.

The above characteristics are Al-9i (Si 1-3%), A
I-Cu (CuO, 5-4%), AI-Ti (Ti
O, 5 to 2%)
20 to 4 high melting point metals such as r, No, W, Ti
The electrode material has a multilayer structure formed to a thickness of 0.00 mm, on which AI or the above-mentioned A1 alloy is formed, and on which Cr, Cr, etc. are formed.
50 to 500 of high melting point metals such as No, W, Ti, etc.
A similar tendency was observed when using a multilayered electrode material formed to a human thickness. Therefore, it can be seen that the effects obtained by the present invention do not depend on the electrode material.

Example 2 Regarding a surface acoustic wave element manufactured in such a manner that the piezoelectric substrate 1 of Example 1 on which the shear horizontal type surface acoustic waves propagate is propagated approximately in the X-axis direction by replacing the piezoelectric substrate 1 with 41-degree rotated Y-axis cut lithium niobate. As a result of measuring the characteristics in the same manner as above, the characteristics showed the same tendency as in Example 1.

Example 3 Regarding a surface acoustic wave element manufactured in such a manner that the piezoelectric substrate l on which shear horizontal type surface acoustic waves propagates in Example 1 is replaced with 84-degree rotated Y-axis cut lithium niobate so that the waves propagate approximately in the X-axis direction. As a result of measuring the characteristics in the same manner as above, the characteristics showed the same tendency as in Example 1.

As described above, in the present invention, as the piezoelectric substrate l on which shear horizontal type surface acoustic waves propagate, the cut angle,
Various types with different propagation directions can be adopted.

Embodiment 4 FIG. 4 shows still another embodiment in which the present invention is applied to a resonance filter. This resonant filter is
It has two sets of interdigital electrodes 2.2', and a reflector 3.3' is formed on the outside thereof.

In this resonant filter, the interdigital electrode 2.2'
Table 2 shows the results of measuring the characteristics by keeping the line width ratio of 1 line constant at 0.5 and changing the line width ratio ηR of the reflector 3.3°.

As is clear from Table 2, the line width ratio η of the reflector 3.3'
It can be seen that as R increases, the reflectance of the reflectors 3 and 3' increases, and the insertion loss of the filter decreases.

The piezoelectric substrate l of this embodiment is made of lithium tantalate with a Y cut of 38 degrees, and the propagation direction of surface acoustic waves is approximately in the X-axis direction. However, similar trends were obtained when other piezoelectric substrates were used.

Incidentally, the line width ratio ηT of the interdigital electrodes is preferably subject to the following limitations.

FIG. 5 shows the relationship between the line width ratio ηT of the interdigital electrode and the electrode capacitance. According to this, when the line width ratio of the interdigital electrodes is increased, the electrode capacitance increases, and as a result, the capacitance ratio increases.

In addition, the amount of out-of-band attenuation also deteriorates.

FIG. 6 shows the relationship between the line width ratio of the interdigital electrode and the radiation conductance. Here, the radiation conductance is inversely proportional to, for example, the resonance resistance of the surface acoustic wave resonator. In general, the resonance resistance of a surface acoustic wave resonator should be small, but for this purpose, the radiation conductance of the interdigital electrode should be large.

From the above, the line width ratio of one interdigital electrode is 0.3
It can be said that the range of ≦ηT≦0.7 is preferable from the viewpoint of capacitance ratio and resonance resistance.

"Effects of the Invention" As explained above, according to the present invention, the reflectance of the reflector can be increased by increasing the line width ratio of the reflector using metal strips that are not connected to each other. . Therefore, when applied to a surface acoustic wave resonator, the capacitance ratio and resonance resistance are reduced, making it possible to downsize the element, and when applied to voltage-controlled oscillators, the frequency variable range is much wider than conventional examples. Become. In addition, if applied to a resonant filter, an element that is smaller and has lower loss than conventional filters can be obtained9.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an embodiment in which the present invention is applied to a surface acoustic wave resonator, and FIG. 2 is a sectional view of the same surface acoustic wave resonator.
Fig. 3 is a partially enlarged sectional view of the surface acoustic wave resonator, Fig. 4 is a plan view showing another embodiment in which the present invention is applied to a resonant filter, and Fig. 5 shows the line width ratio of the interdigital electrode. Figure 6 is a characteristic chart showing the change in radiation conductance when the line width ratio of the interdigital electrodes is changed, and Figure 7 is a characteristic chart showing the change in the interelectrode capacitance when changing the line width ratio of the interdigital electrode. FIG. 8 is a partially enlarged sectional view of the surface acoustic wave resonator, and FIG. 8 is an electrical equivalent circuit diagram of the surface acoustic wave element. In the figure, l is the piezoelectric substrate, 2.2' is the interdigital electrode, and 3.3
° is a reflector.

Claims (6)

[Claims] (1) A reflector consisting of a large number of metal strips that are not connected to each other and whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave is provided on a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates. In the surface acoustic wave device, when the metal finger width of the reflector is LR, the metal finger interval is SR, and the line width ratio ηR is expressed as ηR=LR/(LR+SR), 0.5<ηR<1.0. A surface acoustic wave device characterized by: (2) In claim 1, 0.55≦ηR
A surface acoustic wave element that is ≦0.9. (3) a reflector consisting of a large number of metal strips that are not connected to each other and whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave, on a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates; In a surface acoustic wave device comprising at least one set of interdigital electrodes, the width of the metal fingers of the reflector is LR, the distance between the metal fingers is SR, and the line width ratio ηR is ηR=
When expressed as LR/(LR+SR), 0.5<ηR<
1.0, and the electrode finger width of the interdigital electrode is LT, the electrode finger width interval is ST, and the line width ratio ηT is ηT.
A surface acoustic wave element characterized in that ηT<ηR when expressed as =LT/(LT+ST). (4) In claim 3, 0.55≦ηR
≦0.9 and 0.3≦ηT≦0.7. (5) A reflector consisting of a large number of metal strips that are not connected to each other and whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave is provided on the piezoelectric substrate on which the shear horizontal type surface acoustic wave propagates. In the method for manufacturing a surface acoustic wave device, the metal finger width of the reflector is LR, and the metal finger spacing is S.
A photomask created by setting the line width ratio of the reflector pattern so that 0.5<ηR<1.0, where R and line width ratio ηR are expressed as ηR=LR/(LR+SR). 1. A method for manufacturing a surface acoustic wave device, characterized in that the device is manufactured by photolithography using. (6) A reflector consisting of a large number of metal strips that are not connected to each other and whose center spacing is approximately 1/2 of the propagation wavelength of the surface acoustic wave is placed on a piezoelectric substrate on which a shear horizontal type surface acoustic wave propagates. , and at least one set of interdigital electrodes, where the metal finger width of the reflector is LR, the metal finger spacing is SR, and the line width ratio ηR is ηR=LR/(LR+SR). When expressed as 0
．． 5<ηR<1.0, and when the electrode finger width of the interdigital electrode is LT, the electrode finger width interval is ST, and the line width ratio ηT is expressed as ηT=LT/(LT+ST),
1. A method of manufacturing a surface acoustic wave device, characterized in that the device is manufactured by photolithography using a photomask created by setting the linear radiation ratio of the pattern of the reflector and interdigital electrode so that ηT<ηR.