GB2260023A

GB2260023A - Elastic wave device having silica film

Info

Publication number: GB2260023A
Application number: GB9219756A
Authority: GB
Inventors: Takumi Suetsugu; Kazuyasu Hikita; Tokio Kai
Original assignee: Mitsubishi Materials Corp
Current assignee: Mitsubishi Materials Corp
Priority date: 1991-09-25
Filing date: 1992-09-18
Publication date: 1993-03-31
Also published as: FR2685831B1; GB9219756D0; JPH0590864A; DE4232046A1; FR2685831A1

Abstract

An elastic wave device e.g. a SAW filter or a single crystal resonator (Figs 3-5), is produced by forming an electrode 12 for exciting an elastic wave on a surface of a lithium tetraborate single crystal substrate 10, and by covering the surface of said substrate, including the electrodes 12 with a silica film 14. The elastic wave device has excellent frequency- characteristics with respect to temperature, and the insertion loss does not increase even under high humidity. <IMAGE>

Description

? ? C"-" T ELASTIC WAVE DEVICE AND METHOD OF PRODUCING THE SAME

BACKGROUND OF THE INVENTION

This invention relates to an elastic wave device such as a surface acoustic wave filter, IDT (Inter digital Transducer) type resonator and a single crystal resonator, and to a method of producing the same. In more particular, it relates to an elastic wave device using a lithium tetraborate (Li2BN407) single crystal substrate and to a method of producing the same.

An elastic wave device of this kind is produced in such a way that an electrode for exciting an elastic wave is formed on a piezoelectric substrate, followed by dicing to make a chip unit and the chips thus f ormed are sealed in a metallic container or the like. The substrate employed in this case is f ormed of a single crystal of lithium tetraborate, lithium niobate, lithium tantalate and the like. In particular, the elastic wave device made of a lithium tetraborate single crystal substrate has a smaller variation of frequency characteristic toward temperature compared to those of devices using other kinds of substrates, and exhibits a stable frequency characteristic at the equivalent temperature as in the device made of crystal.

However, the lithium tetraborate single crystal substrate is soluble in water, acid and the like, which is a drawback since desirable electrical characteristics are not obtained due to the roughened of the surf ace of the substrates during the process of producing the elastic wave device.

To solve these problems, Unexamined Published Japanese Patent Application No. 63-178615 discloses a method f or producing a surf ace acoustic wave element in which a silica (Si02) f ilm is f ormed on the surf ace of the lithium tetraborate single crystal substrate and then aluminum is evaporated onto the silica film, followed by wet etching to f orm, an aluminum electrode. This application also discloses another method for producing a surf ace acoustic wave element comprising: the steps of evaporating aluminum onto the substrate surf ace; f orming an electrode by dry etching so that a plurality of surface acoustic wave element sections are formed; coating the substrate surface including the electrode with a protective f ilm f ormed of synthetic resin petroleum paraffin, natural fats and oils, natural wax or the like; separating the surface acoustic wave element sections; and removing the protective film.

According to these methods of production, corrosion of the single crystal substrate from water, acid and the like, during forming of the electrode, pattern etching, and dicing, is prevented However, in the former method, the silica film lies between the lithium tetraborate single crystal substrate and the electrode. This makes it impossible to obtain an increased electromechanical coupling factor of 3 the surface acoustic wave elements. In the latter method, the substrate surface tends to deliquesce from reaction of the lithium, tetraborate with atmospheric water-vapor because the surface of the lithium tetraborate single crystal substrate is exposed in air during the period from removal of the protective f ilm to the time it is sealed in a container. As a result, the substrate surface is roughened and the surface of the acoustic wave element is not stable to insertion-loss increase.

To avoid this, use of the substrate without removing the protective film has been considered. However, since the protective f ilm used in the conventional method is formed from synthetic resin, petroleum paraffin, natural f ats and soil, natural wax or the like, and the protective film itself may adsorb the elastic wave, the essential function of the elastic wave device deteriorates.

SUMMARY OF THE INVENTION

An object of this invention is to provide a highly reliable elastic wave device with an upgraded frequency characteristic toward temperature capable of preventing the increase of insertion loss caused by a roughened substrate-surf ace even under high humidity, and to provide a method of producing such a device.

We have discovered that the foregoing object is achieved with an elastic wave device comprising a lithium - 4 tetraborate single crystal substrate, an electrode f ormed on the surface of the substrate for exiting an elastic wave, and a silica f ilm coating the surface of the substrate including the electrode.

The elastic wave device according to the inven tion is produced by evaporating a metal on the surf ace of the lithium tetraborate single crystal substrate; forming the electrode f or exciting the elastic wave by means of etching; and forming a continuous silica film on the surface of the substrate including the electrodes.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 is a sectional view taken along line A-A in Fig. 2.

Fig. 2 is an entire perspective view of a surface acoustic wave filter according to the invention.

Fig. 3 is a sectional view'taken along line B-B in Fig. 4.

Fig. 4 is a perspective view of a lithium tetraborate single crystal resonator of another embodiment according to the invention viewed from the top.

Fig. 5 is a perspective view of the resonator of Fig. 4 viewed from the bottom.

c DESCRIPTION OF THE PREFERRED EMBODIMENT i An elastic wave device according to this invention comprises a surface acoustic wave filter using a lithium tetraborate single crystal substrate, an IDT (Inter digital Transducer) type resonator, a single crystal resonator and the like. The elastic wave includes not only a surface acoustic wave, but also a bulk acoustic wave.

An electrode for exciting an elastic wave accord- ng to the invention may be produced by forming a film of a metal, such as, aluminum, gold, or the like on the surface of a lithium tetraborate single crystal substrate by means of vacuum evaporation, high-frequency sputtering evaporation, ion-beam evaporation, etc., and then forming a desired pattern by a photolithography. The metal is preferably aluminum which is light weight and has good distortion transfer characteristics.

The substrate surface having the electrodes thereon is coated with a silica film which coats the electrodes as well as the substrate. The silica film is continuous and covers the surface of the substrate surface having the electrode thereon. If the electrodes have a thickness of, for example, 0.5jam, the silica film has a thickness of about lMm on the surface of the substrate.

The silica f ilm may be formed by a variety of methods, such as, highfrequency sputtering; CVD; plasma CVD; or a sol-gel method. Highfrequency sputtering is a method wherein Si02 is evaporated in an argon gas atmosphere. In the sol- gel method, the substrate surface with the electrodes thereon is coated with a silicon alkoxide solution, followed by heating to remove the solvent. The coating may be effected by dip-coating, spin-coating, and the like. Spin-coating in which a spinning substrate surface is coated with a silicon alkoxide solution may preferably be used to obtain an even film-thickness.

According to this invention, the deliquescent property of the lithium tetraborate single crystal substrate presents no problem at the time of forming the electrodes if they are dipped in pure water f or a very short time. However, contact with atmospheric water- vapor for a relatively extended time until sealing in a container is after f orming the electrode, results in deterioration f or use as an elastic wave device because of the reaction of the substrate surface with water, which results in an increased insertion loss.

In the inventive elastic wave device, a lithium tetraborate single crystal substrate having good frequency characteristics with respect to temperature is coated by the silica f ilm. This isolates the substrate from atmospheric water-vapor, the substrate surface is prevented from being roughened and the insertion loss does not increase. The silica film does not adsorb the elastic wave so that it does not adversely affect the characteristics of the elastic wave device.

T As described, in the conventional elastic wave device, the provision of the silica film between the lithium tetraborate single crystal substrate and the electrode makes it impossible to obtain a large electromechanical coupling factor in the surface acoustic wave element. Moveover, the electrode is formed directly on the lithium tetraborate single crystal substrate, and the insertion loss is increased due to the roughened surface of the substrate during the time until it is sealed into a container. With the present invention, the silica film permanently coats and protects the lithium tetraborate single crystal substrate having good temperature frequencycharacteristics. This together with the electrodes provided directly on the substrate, provides an excellent elastic wave device wherein the protective film does not adsorb the elastic wave and the insertion loss does not increase over an extended period.

The present invention is described with reference to the following examples and comparative examples based on the accompanying drawings, although it is not limited thereto.

EXAMPLE 1

Figs. 1 and 2, show an inventive embodiment wherein the elastic wave device is a surf ace acoustic wave f ilter 18. The surface acoustic wave f ilter is was produced by forming a plurality of aluminum electrodes 12 with a thickness of about 0.5pm in the form of two combined combs on a surf ace of a lithium tetraborate single crystal substrate 10 having a thickness of 500 gm by photolithogra- phy. The surf ace of the substrate 10 including electrodes 12 was coated with a continuous silica f ilm 14 of a thickness of about 0. igm, followed by dicing to make a chip unit.

Specifically, an aluminum film was evaporated on the entire surface of the substrate 10 by vacuum evapora tion, followed by applying a photoresist on the aluminum film. The aluminum film was the covered with a mask having an attached electrode pattern followed by exposure to light. The exposed substrate was developed in an alkaline solution and rinsed with pure water. The aluminum film was then wet- etched using an etching solution composed of an aqueous solution of NaOH to form electrodes 12 for exciting the elastic wave. After removing the photoresist with acetone, the electrode pad sections 13 were covered with a metallic mask, and a continuous silica film 14 was formed uniformly on the surface of the substrate 10, including electrodes 12, by high- frequency sputtering. After remov- 1, ing the metallic mask from substrate 10, it was diced to make a chip unit.

Finally, leads 16 were connected to electrode pad sections 13 to provide a surface acoustic wave filter 18.

COMPARATIVE EXAMPLE 1 A surface acoustic wave filter was produced by the same method as in the Example 1 excepting that the 10 silica film 14 was not formed.

EXAMPLE 2

This embodiment is shown in Figs. 3 to 5. The 15 elastic wave device is a lithium tetraborate single crystal resonator 30. The resonator 30 has a plurality of circular aluminum electrodes 22 each having a diameter of about 15mm and having a thickness of about 0.5pm formed on the upper and lower surfaces of a lithium tetraborate single crystal substrate 20 by photolithography. The substrate 20 has a thickness of 30Ogm. Silica films 24 each having a thickness of about 0 - lmm were f ormed on the surf aces of the substrate 20 including those electrodes 22. Thereafter dicing was carried out to make chip units. Two circular shaped electrodes on both surfaces of the substrate were formed on the positions where the electrodes overlap with each other.

Specifically, electrodes 22 for exciting an elastic wave were f ormed in the same manner as in Example 1. Af ter removing the photoresist with acetone, the electrode pad sections 23 were covered with a metallic mask and then, a continuous silica f ilm 24 was f ormed uniformly on the surface of substrate 20 including the electrodes 22 by high-f requency sputtering. The metallic mask was removed and dicing was performed to make a chip unit. Finally, a lithium tetraborate single crystal resonator 30 was ob- tained by connecting leads 26 with the electrode pad sections 23 of a pair of electrodes 22 of the chip thus produced.

COMPARATIVE EXAMPLE 2 A lithium tetraborate single crystal resonator was produced by the same method as in the Example 2 except that the silica film 24 was not formed.

The reliability of each of the surface acoustic wave filters of Example 1 and Comparative Example 1 and the lithium tetraborate single crystal resonators of Example 2 and Comparative Example 2 were tested by allowing the elastic wave devices to stand under a humidity of 85%, and temperature of 850C for 48 hours and the insertion losses before and after standing were measured. The result of the tests are shown in the following Table.

1 c j, Insertion Loss (Unit: dB) Before After Humidity Test Humidity Test Example 1 2.7 3.0 Comparative Example 1 2.8 11.4 Example 2 0.4 0.4 Comparative Example 2 0.4 7.5 As is apparent from the Table, the insertion loss of the elastic wave device after the test in the Comparative Examples 1 and 2 wherein the silica f ilms were not formed is greatly increased as compared with that before the test. on the other hand, the insertion loss of the elastic wave device in Examples 1 and 2 wherein the silica films were formed varies little and exhibits a higher reliability.

12

Claims

CLAIMS:

1. An elastic wave device comprising: a lithium tetraborate single crystal substrate; an electrode for exciting an elastic wave, formed on a surface of the substrate; and a coating of a continuous silica film over the said surface of the substrate and over the electrode.

2. The device as claimed in claim 1 wherein the electrode is made of gold or aluminium.

3. A method of producing an elastic wave device comprising: evaporating a metal film on an outer surface of a lithium tetraborate single crystal substrate; etching the metal film to form an electrode for exciting an elastic wave; and forming a continuous silica film over the said outer surface of the substrate and over the electrode.

4. The method as claimed in claim 3, wherein the silica film is formed by sputtering evaporation.

1,1115 1 13

5. The method as claimed in claim 3, wherein the silica film is formed by coating a silicon alkoxide solution on the said surface of the substrate and over the electrode thereon, and heating to remove solvent from the solution.

6. The method as claimed in claim 3, 4 or 5 in which the electrode metal is gold or aluminium.

7. An elastic wave device produced by the method of claims 3, 4 or 5.