JPH02295211A - Energy shut-up type surface acoustic wave element - Google Patents

Abstract

PURPOSE:To improve the resonance characteristic by changing the thickness of a dielectric film formed on a surface wave drive electrode and also the thickness of a dielectric film formed on a reflecting electrode in order to shut up the surface wave energy of a dielectric substrate set under the surface wave drive electrode. CONSTITUTION:An Al film having the fixed thickness of about 1mum is vacuum- deposited on a piezoelectric substrate 1, and a comb-line type drive electrode 2 and the reflecting electrodes 3 and 4 are formed at one time by a photolightographic process. Then these electrodes 2-4 are covered with a dielectric film 5 having different thickness with these electrodes respectively. Thus the surface acoustic wave energy is shut up on the surface of the substrate 1 under the electrode 2. The different transmitting speeds of surface waves are set between parts of comb-line type driving and reflecting electrodes by changing the thickness of the film 5. Thus the energy shut-up effect is increased. Then it is possible to obtain the excellent resonance characteristic free from the spurious resonance.

Description

[Detailed Description of the Invention] [Summary] Regarding the structure of an energy-trapped surface acoustic wave device, the thickness of the dielectric film formed on the surface wave drive electrode of the surface acoustic wave device and the thickness of the dielectric film formed on the reflective electrode are The purpose of this is to change the thickness of the dielectric film used to confine the surface wave energy to the surface of the piezoelectric substrate below the surface wave drive electrode and improve the resonance characteristics. A comb-shaped drive electrode and two reflective electrodes are provided on both sides of the comb-shaped drive electrode at a required interval on the piezoelectric substrate to be propagated, and the comb-shaped drive electrode and the two reflective electrodes are covered. and forming dielectric films having different thicknesses on the comb-shaped drive electrode and both reflective electrodes to confine surface acoustic wave energy to the surface of the piezoelectric substrate below the comb-shaped drive electrode. An energy-trapped surface acoustic wave device is constructed.

[Industrial application field]

The present invention relates to a surface acoustic wave device, and particularly to the structure of an energy trap type surface acoustic wave device.

In recent years, with the increase in speed of information processing equipment and communication equipment,
The frequency bands of carrier waves and signal waves are increasingly shifting to higher frequencies, and correspondingly, it is necessary to generate reference signals with high stability at high frequencies and elements for phase synchronization.Recently, elastic Surface wave devices, such as surface acoustic wave resonators, have come into use.

Surface acoustic wave resonators are generally characterized by a simple element configuration, but on the other hand, it is difficult to engineer the element performance. development was required.

[Conventional technology]

A surface acoustic wave element, for example, a surface acoustic wave resonator, uses a substrate that has a large electrical-mechanical coupling coefficient and a relatively small frequency temperature coefficient, for example, 36° rotation Y cut-X propagation LiTaO: + (36° Y LiTaO3)
A so-called short circuit is a method in which a comb-shaped drive electrode is provided on a single crystal substrate, reflective electrodes are formed on both sides of the comb-shaped drive electrode, for example, multiple strip-shaped conductor patterns are formed in parallel, and both ends of the conductor patterns are connected by a connecting conductor pattern. This is a two-terminal device with a strip-type reflective electrode.

The spacing between the comb-shaped drive electrode and the reflective electrode is 7/8λ, where λ is the wavelength of the surface wave propagating on the substrate surface. In the case of the energy trapping structure shown in FIG.

As the electrode material, aluminum (A
f) or an Af alloy mixed with a small amount of Cu to suppress electrode migration when an electric field is applied.

Conventionally, when a resonator is constructed by providing electrodes smaller than the outline size of the plate on both sides of a piezoelectric substrate or arranging multiple pairs of electrodes close to each other on the surface of a single piezoelectric substrate, the electrode portion It is known that a so-called engineered energy-confined piezoelectric resonator or energy-confined multimode piezoelectric filter (monolithic piezoelectric filter) is formed in which elastic wave energy is confined, and that stable resonance characteristics without spurious resonance can be obtained. ing. This is because the frequency at the bottom of the electrode is slightly lowered due to the mass of the electrode and the piezoelectric reaction, making it impossible for elastic waves to propagate to the non-electrode area (edited by IEICE, IEICE Handbook, Vol. 1). (See Volume 575, 1988).

Efforts are being made to improve the resonance characteristics of surface acoustic wave devices as well. For example, by changing the thickness of the barrel-shaped drive electrode and the reflective electrode, surface acoustic wave energy is transferred to the bottom of the comb-shaped drive electrode. Attempts have been made to confine the area (Shimizu, Suzuki: "A significantly smaller and broadband surface acoustic wave resonator", paper presented at the 16th EM Symposium (1987).
3.11)).

FIG. 6 is a diagram showing a conventional example of an energy-trapped surface acoustic wave resonator. In the figure, 1 is a piezoelectric substrate, 2"a,
2'b is AI1. The comb-shaped drive electrodes are formed by interposing the electrodes alternately between the comb-shaped electrodes made by the manufacturer. 3' and 4' are so-called short strip type in which a plurality of strip-like A2 patterns are formed in parallel on both sides of the comb-shaped drive electrode, and both ends of these A/2 patterns are connected by a connection/l pattern. It is a reflective electrode. The distance between the comb-shaped drive electrode and the reflective electrode and the pitch of each electrode are λ/2, where λ is the wavelength of the surface wave propagating on the substrate surface.In other words, the comb-shaped drive electrode and the reflective electrode are arranged at a continuous equal pitch of λ/2. It is located in

Both the electrode width and the electrode spacing are usually designed to be λ/4.

The thickness of the Af electrode pattern ranges from several hundred nm to about 1 μm and is formed by vacuum evaporation or the like.

Figure (a) shows a frequency-increasing energy trapping structure, in which the thickness of the comb-shaped electrodes 2'a and 2'b is less than half that of the reflective electrodes 3° and 4°. Therefore, the mass effect of the electrode in the comb-shaped drive electrode part is smaller than that in the reflective electrode part, and the velocity of the surface wave is higher.In other words, in the comb-shaped drive electrode part, the frequency increases and the surface wave energy It will be trapped under the electrode.

On the other hand, the figure (opening) shows a frequency lowering type energy trapping structure, and contrary to the case of the figure (a), the thickness of the comb-shaped electrode 2'a + 2'b is the same as that of the reflective electrode 3' and Therefore, the mass effect of the electrode in the comb-shaped drive electrode part is larger than that in the reflective electrode part, and the velocity of the surface wave is smaller. The frequency decreases in the lower part, and the surface wave energy is similarly confined to the lower part of the comb-shaped drive electrode.

[Problem to be solved by the invention]

However, in the above conventional example, the thickness of the Affi electrode is 1 μm.
It is difficult to stably manufacture a thin film with a difference in film thickness between the comb-shaped drive electrode and the reflective electrode, and the change in surface acoustic wave velocity due to the difference in film thickness of the Aj2 electrode is extremely small, and the energy confinement effect is There was a problem that the desired resonance characteristics could not be obtained due to insufficient results, and a solution to this problem was needed.

[Means to solve the problem]

The above problem involves surface acoustic wave excitation. On the piezoelectric substrate 1 to be propagated, a comb-shaped drive electrode 2 and reflective electrodes 3.4 are arranged on both sides of the comb-shaped drive electrode 2 with a required spacing, and the barrel-shaped drive electrode 2 and the reflection A dielectric film 5 covering the electrodes 3 and 4 and having different film thicknesses on the comb-shaped drive electrode 2 and the reflective electrode 3.4 is deposited to improve the piezoelectricity of the lower part of the comb-shaped drive electrode 2. This problem can be solved by an energy-trapped surface acoustic wave device that confines surface acoustic wave energy to the surface of a substrate.

[Effect]

The energy trapping type surface acoustic wave device of the present invention has a small mass effect of A! Rather than changing the thickness of the electrode, the thickness of the thick dielectric film formed on the comb-shaped drive electrode and the reflective electrode is changed to increase the propagation velocity of the surface wave on the comb-shaped drive electrode and the reflective electrode. Since it changes the propagation speed of surface waves, its energy trapping effect is large. Furthermore, since the propagation speed of surface waves in a dielectric differs depending on the properties of the dielectric, the energy trapping effect can be widely selected depending on the combination of the type and thickness, and can be applied to various surface acoustic wave devices.

〔Example〕

FIG. 1 is a perspective view illustrating an embodiment of the present invention. In the figure, 1 is a piezoelectric substrate, for example, a 36° Y-cut Li
TaO, single crystal substrate, dimensions are 0.35 mm thick and 2 wide.
mm, length 6 mm, surface polished smooth. The electrodes were arranged so that the surface waves propagated in the X direction. Reference numeral 2 denotes a comb-shaped drive electrode, and a pair of comb-shaped electrodes 2a and 2b each having 50 comb teeth are arranged with the comb teeth interposed between them. Reference numeral 3.4 denotes a reflective electrode, which is a so-called short strip type in which 100 strip-shaped conductor patterns are formed in parallel, and both ends of these conductor patterns are connected by a connecting conductor pattern. Specifically, to form these electrode patterns, an A2 film having a constant thickness of about 1 μm is vacuum-deposited on the substrate 1, and the comb-shaped drive electrode 2 and reflective electrodes 3 and 4 are simultaneously formed using a normal photolithography method. Formed.

The electrode spacing was λ/2, including the gap between the comb-shaped drive electrode and both reflective electrodes, that is, the electrodes were arranged at continuous equal pitches.

? In order to obtain a resonance frequency of 181 MHz, from the sound speed of the X-propagating surface wave of the substrate 1 of 4090 m/s, λ
The electrode pitch was calculated as λ/2, and the electrode width and electrode spacing were designed as λ/4.

5 is a dielectric film, for example, in this example, the temperature coefficient of surface propagation velocity is opposite in sign to the temperature coefficient of LiTaO=, and it is made of silicon dioxide (S), which is also effective for improving frequency temperature characteristics.
(see Japanese Patent Application No. 55-159612 and Japanese Patent Application No. 63-187705) to a thickness of 5 μm by RF sputtering.

Reference numerals 6a and 6b are external lead lead-out portions for connecting a power source for driving the comb-shaped drive electrodes 2, which are exposed electrodes provided at a part of each connection portion of a pair of comb-shaped electrodes not shown in FIG. Department. That is, the silicon dioxide film thereon is coated with, for example, CF. It was removed by ion etching to form a hole.

7 is a concave portion on the drive electrode, in which half the thickness of the silicon dioxide film on the comb-shaped drive electrode 2 is the same <CF. The silicon dioxide film was formed by removing the silicon dioxide film by ion etching in the silicon dioxide film, and therefore, the thickness of the silicon dioxide film at that portion is approximately 2.5 μm.

FIG. 2 is a configuration diagram of a surface acoustic wave device according to an embodiment of the present invention. The figure (a) is a plan view, and the figure (opening) is a cross-sectional view taken along the line AA', which supplements the perspective view of FIG. 1 to make it easier to understand. In the figure, comb-shaped electrodes 2a and 2b have comb teeth interposed between them to form a comb-shaped drive electrode 2. 6
The shaded portions a and 6b indicate that the Affi electrode surface is exposed to form an external lead lead-out portion.

FIG. 3 is a conceptual diagram illustrating the energy confinement state of the present invention. 3 shows the energy distribution before the recess 7 is formed on the comb-shaped drive electrode, and the surface wave energy propagates almost entirely on the reflective electrode as well. On the other hand, ■ shows the energy distribution after forming the recess 7 on the comb-shaped drive electrode, where almost no surface wave energy propagation is seen on the reflective electrode, and the lower part of the comb-shaped drive electrode This shows the state in which surface wave energy is effectively confined.

FIG. 4 is a diagram showing impedance characteristics of an embodiment of the present invention. In the figure, ■ is the resonance characteristic before the recess 7 is formed in the silicon dioxide film on the comb-shaped drive electrode, as described above, and ■ is the resonance characteristic after the recess 7 is formed in the silicon dioxide film on the comb-shaped drive electrode. It shows the characteristics. Note that the measurement was carried out according to the usual method using a network analyzer.

Before the recesses 7 were formed in the silicon dioxide film on the comb-shaped drive electrode, many spurious resonances occurred near the resonance point (■); It can be seen that after forming , single mode resonance was achieved without any spurious (■).

Furthermore, as can be seen from the figure, when a portion of the silicon dioxide film is removed to make the film thinner, the resonant frequency increases slightly. This is because the propagation velocity of surface waves in the silicon dioxide film is smaller than that in the LiTaO substrate, and therefore the propagation velocity when the recess 7 is formed is relatively higher than that when the recess 7 is not formed. As it becomes larger, the resonant frequency increases. That is, this example has a frequency increasing type energy confinement structure.

On the other hand, FIG. 5 is a diagram illustrating another embodiment of the present invention. In the figure, 7' is a platform above the drive electrode, and the thickness of the silicon dioxide film on the comb-shaped drive electrode is made thicker than other parts. In this case, contrary to the embodiment shown in FIG.
The resonant frequency is lower than that in the case where no . In other words, it becomes a frequency lowering type energy confinement structure.

In the above example, the thickness of the silicon dioxide film on the comb-shaped drive electrode and the reflective electrode was changed, but the same effect can be obtained even if a silicon dioxide film of the required thickness is formed on only one of them. can get.

In addition, the dielectric layer 5 is not limited to a silicon dioxide film, but also tantalum pentoxide (RA.OS), niobium pentoxide (Nb
It goes without saying that other dielectrics such as ZOS), tungsten oxide (WO3), and silicon nitride (SiJ4) may be used as appropriate to realize an energy trap type surface acoustic wave element with the desired performance.

〔Effect of the invention〕

As described in detail above, according to the present invention, by changing the thickness of the dielectric film formed on the comb-shaped drive electrode and the reflective electrode, the propagation velocity of the surface wave in the comb-shaped drive electrode portion and the reflective electrode portion can be adjusted. Since it changes the propagation velocity of surface waves, its energy trapping effect is large.

As a result, excellent resonance characteristics without spurious resonance can be obtained, which greatly contributes to improving the performance and quality of surface acoustic wave resonators.

[Brief explanation of drawings]

FIG. 1 is a perspective view illustrating an embodiment of the present invention, FIG. 2 is a configuration diagram of a surface acoustic wave element according to an embodiment of the present invention, and FIG. 3 is a conceptual diagram illustrating an energy confinement state of the present invention. 4 is a diagram showing impedance characteristics of an embodiment of the present invention, FIG. 5 is a diagram illustrating another embodiment of the present invention, and FIG. 6 is a diagram illustrating a conventional example of an energy-trapped surface acoustic wave resonator. In the figure, 1 is a piezoelectric substrate, 2 is a comb-shaped drive electrode, 3 and 4 are reflective electrodes, 5 is a dielectric film, 6a and 6b are external lead lead-out portions, 7 is a recess on the drive electrode, and 7゛ is a drive This is the pedestal above the electrode. ? Roma ■ Goshuumeihikuda 1/) In σ-da/Slip/Kojishi 1 Old Part
Doki dagger a81Z by A81Z's main example θ bullet l surface f Aoko n loss eight figure 1
2 Kuchira tζ≧≠〉Sun/Monday/G2Izer()寅め到. +I 8λ
A"'Fzu〉Figure 5 B (A) Weight loss phlegm (hill waste {L (mouth) Insei F or A lying down next energy I legi 'one step υ included and shi L Zen/lowering face He who finished the selection [7 examples of VF Zuhoki 6 notes]

Claims (1)

[Claims] On the piezoelectric substrate (1) on which surface acoustic waves are excited and propagated, a comb-shaped drive electrode (2) and reflective electrodes (3, 4) are arranged at a required interval on both sides of the comb-shaped drive electrode (2). ), covering the comb-shaped drive electrode (2) and reflective electrodes (3, 4), and having film thicknesses on the comb-shaped drive electrode (2) and reflective electrodes (3, 4) mutually. Depositing and forming a different dielectric film (5),
An energy trap type surface acoustic wave element characterized in that surface acoustic wave energy is confined in the surface of a piezoelectric substrate below the comb-shaped drive electrode (2).