JPS6090412A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To reduce reflected waves at electrode finger ends by providing an insulating film over the entire surface of a substrate except electrode parts of a surface acoustic wave element by a lift-off method. CONSTITUTION:The surface of the piezoelectric substrate 1 except an input electrode 3 and an output electrode 3 on it is covered with the insulating film 5 by the lift-off method. Acoustic impedance values of the electrode part 2 and insulating film part 5 are denoted as Z1 and Z2, and the electrode gap which is a free surface is charged with the insulating film 5 to approximate the ratio Z1/Z2 to 1. Thus, the difference in acoustic impedance between the electrode part and gap part is eliminated, so reflected waves at electrode finger ends are reduced to remove or reduce the influence of a triple transient echo wave upon the characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device with improved g % impedance for surface acoustic waves.

A conventional surface acoustic wave element is made of LiN, as shown in FIG.
b0a (= Op-Acid A) I On a piezoelectric substrate 1 made of LiTa03 (lithium tantalate), piezoelectric ceramics, etc., there is an input electrode 2 that converts an electric signal into an elastic wave 4, and a surface acoustic wave that has propagated. to electrical signals,
It consists of an output electrode 3 which is taken out.

Usually, a metal such as aluminum (A7) is deposited on the surface of the substrate for the 'a electrode part, and a photoetching technique is used to form a Qr regular-shaped 'I quadrupole pattern. A surface acoustic wave 4 excited from the input electrode 2 propagates along the substrate surface toward the output electrode 3 and is extracted as an output signal. At this time, a part of the surface acoustic wave 4 is reflected by the output electrode and returns to the input electrode 2, but reflection occurs here as well in the same manner as described above.

This wave is called a triple transit echo (hereinafter abbreviated as TTE) wave because it is received after a delay time three times that of the initially propagated wave, and for example, in a surface acoustic wave filter, it is This was the main cause of amplitude characteristic ripples and group delay characteristic ripples within the band.

It is well known that TTE consists of a reflected component due to discontinuity in acoustic inbeatance between the electrode section and a propagation path where no electrode exists, and a reflected component due to electrical interaction between the load and the electrode. In the latter case (1), depending on the matching conditions with the external load, it is possible to suppress the reflected intensity to a certain extent.

On the other hand, in the case of the former, there is a method of dividing the electrode finger into two (tuple 'Qj, polar structure), a method of tilting the output electrode with respect to the manual electrode so that the reflected wave does not return directly to the input end, or A method is used in which one of the input and output 11i poles is stepped.

Furthermore, using a small IjQfit phase material in the electrode,
By reducing the thickness of the electrode film, the amount of discontinuity in acoustic impedance can be reduced to ζ<<, and the reflection intensity can be reduced.

However, these methods require complicated opening patterns and manufacturing processes, and the electrodes occupy a large area due to the irradiation problem.
+J, T TE Sufficient suppression of E waves could not be obtained.

Furthermore, the method to reduce the electrode film thickness is to input 48
No. 11 (increasing resistance loss, electricity) had drawbacks such as deterioration of the efficiency of converting No. 11 into surface acoustic waves.

The present invention eliminates the above-mentioned drawbacks of the p1 elastic surface blood wave element. 13. The purpose of the present invention is to provide a surface acoustic wave element in which an insulating film is provided on the entire surface of the substrate except for the polar regions using a lift-off method, thereby reducing reflected waves at the ends of electrode fingers.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a diagram showing an embodiment of the manufacturing process of a surface acoustic wave device according to the present invention. First, as shown in the same figure (al), LiN
b0. , LiTaO3, piezoelectric ceramics, etc., an insulating film 5, for example, SiOt+ A It
's film etc. by vacuum evaporation method, sputtering method or CV
Formed by method D. Next, a positive type photoresist 6 is applied thereon, and the input/output WT and extreme portions (turns) are opened using a photo technique as shown in FIG. 2(b).

Furthermore, as shown in FIG.
For example, after coating the entire surface with aluminum using a vacuum evaporation method or sputtering method as shown in Figure 2(d), the lift-off method is used to remove the gold Jj5 from areas other than the input and output electrodes, and the device is completed (Silk Figure 2). e)).

By the above process, the parts other than the input/output electrodes 2 and 3 are
A saw surface acoustic wave element covered with 145 is obtained.

Next, the operation of the present invention will be explained.

FIG. 3 is a partially enlarged view of the manual electrode 2 shown in FIG. 2. Here, ff11. The acoustic impedances of the pole part 2 and the insulating film part 5 are respectively zl and 2. shall be. °By filling the IT electrode gap, which was previously a free surface, with the insulating film 5, the zi
and 2. Bringing the ratio of ν to close to 1 becomes the ν1 function.

Generally, the intensity of the reflected wave generated at the end of the electrode finger is expressed as a function of the amount of discontinuity ξ in the acoustic impedance between the electrode 1 and the gap. Here, even if U′, 1ξ1≦i1.0
5, and when the electrode finger width and electrode gap are equal, the amplitude reflectance 1 rJ 1 of the reflected wave is 1 P 1-w 1 tanh (Nξ) 1 where, N:
This is the logarithm of the electrodes. According to the surface acoustic wave device of the present invention, 1ξ1≧0
According to the above equation, the reflectance lp1 of the reflected wave caused by the discontinuity of the acoustic impedance can also be made almost zero.

The film thickness h2 of the insulating film 5 is approximated by =-shih based on the amount 7711 of the metal frame material used for the electrode and the mass ηL2 of the insulating film material.

2. In the above embodiment, the electrode film 7 was provided on the piezoelectric substrate 1 by the step-off method after the insulating film 5 was deposited. Alternatively, the insulating film 5 may be provided by a lift-off method.

In FIG. 3, if the height of the electrode section 7 is h11 and the peak of the insulating film 5, then if hz>hl, the total insulating film thickness is greater than the electrode film thickness h1, then lift-off occurs. It is desirable to adopt the former manufacturing process because of the ease of pattern formation using the method. On the other hand, if 11s>ht, the latter manufacturing process is preferable.

On the other hand, when hl>h2, the latter manufacturing process is preferable. In the case of h,=h, the process of dora may be adopted.

Furthermore, in the above embodiment, an insulating film is provided on the entire surface of the substrate except for the electrode portions, but an insulating film may be provided only on the input/output electrode portions where reflected waves are generated. , the same effect as the above embodiment is achieved.

As described above, in the surface acoustic wave device of the present invention, an insulating film is provided on the entire surface of the piezoelectric substrate except for the input/output electrode portions by a very simple method called the lift-off method, and the acoustic Since the configuration eliminates the impedance difference, the generation of elastic reflected waves at the electrode finger ends is reduced, and TTE
It is possible to eliminate or significantly reduce the influence on characteristics caused by

In addition, when the insulation film thickness h2≧electrode film thickness h1, the insulation film will retain the 11! There is no problem of the electrode becoming thinner due to etching.

[Brief explanation of drawings]

Figure 1 r/i A diagram showing a conventional surface acoustic wave device, Figure 2 (
Figures at to 82 (el are cross-sectional views for each manufacturing process of the surface acoustic wave device according to the present invention, and Figure 3 t is an enlarged cross-sectional view showing a part of the electrode in Figure 2 (e) formed according to the present invention. The diagram shows: 1...Piezoelectric substrate 2...Input electrode 3...Output electrode 4...Surface acoustic wave 5...Insulation Membrane 6...Resist 7...Metal film Applicant: Pioneer Corporation Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] An input electrode and an output electrode are formed on a piezoelectric substrate to face each other, and an insulating film is formed on the substrate to partially tilt the exposed electrode finger end, and the reflected wave at the exposed electrode finger end is suppressed. A surface acoustic wave device characterized by suppression.