JPS6235704A - Measuring method for frequency characteristics of surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To measure the center frequency of a surface acoustic wave filter in a wafer state by taking a measurement by using a surface acoustic wave filter having cross finger type electrodes which are larger in electrode width at the peripheral parts than at the center part. CONSTITUTION:The surface acoustic wave filter for measurement is constituted by forming a cross finger type electrode 11 which is equal in electrode width w-h and electrode interval s+h to a surface acoustic wave filter for manufacture used actually at the center part and the 2nd cross finger type electrodes 12 which have electrode width aw-h and electrode intervals as+h larger than the electrode width w-h and electrode intervals s+h of the cross finger type electrode 11 outside the cross finger type electrode 11. The use of the surface acoustic wave filter for manufacture the center frequency f0 of the surface acoustic wave filter for manufacture to be measured indirectly at a frequency lower than it, so even if the center frequency approximates to microwaves, the center frequency of a surface acoustic wave filter in a water state is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for measuring frequency characteristics of a surface acoustic wave filter, in which the electrode width and the electrode spacing at the periphery are different from each other at the center. By measuring using a surface acoustic wave filter having interdigital electrodes wider than those of the wafer, it is possible to measure the center frequency of the surface acoustic wave filter with the wafer lying down.

[Conventional technology]

A surface acoustic wave filter is constructed by forming input and output interdigital electrodes on a piezoelectric substrate. Generally, after forming interdigital electrodes on a piezoelectric wafer (large substrate) and forming a large number of surface acoustic wave filters using photolithography technology, the frequency characteristics of the surface acoustic wave filters, and hence the center frequency, are measured. It is then cut to produce individual surface acoustic wave filters. The center frequency fo of a surface acoustic wave filter constituted by regular interdigital electrodes, that is, interdigital electrodes with constant electrode width, electrode spacing, and intersecting width, is expressed as follows.

That is, normally, regular interdigital electrodes are produced by photolithography by transferring a photomask (1) as shown in FIG. 4, but as shown in FIG. The width W' of the electrode (2) is smaller than the width W corresponding to the electrode in the photomask (1). This is due to overetch, but
If the sum of the overetching amounts on both sides of the electrode is h, then E;
・The width W' of the electrode after lithography is w-h, and the width corresponding to the electrode spacing in photomask (1) is S
Then, the width between the electrodes becomes s+h. Therefore, the length λ of one period of the interdigital electrode (2) does not change before and after photolithography, and λ=2 (w+s) ... (A1)
becomes. In the interdigital electrodes, the time to for the surface acoustic wave to propagate over a distance of λ is given by: Here, VM and VF are the surface acoustic wave velocities under the electrode and at the free surface, respectively. And the center frequency fO of the interdigital electrode is fo=□
...(A3)t.

Therefore, the previous equation (1) is derived from equations (A2) and (A3).

Note that it is difficult to express the center frequency of a surface acoustic wave filter configured with interdigital electrodes having a complicated shape other than regular interdigital electrodes using a simple equation. However, the frequency characteristics of the surface acoustic wave filter are proportional to the center frequency of the interdigital electrodes expressed by equation N.

In interdigital electrodes manufactured by photolithography, the values of the electrode width wh and the electrode spacing s+h cannot be changed after the interdigital electrodes are manufactured. Therefore, when changing the center frequency after manufacturing the interdigital electrodes, this is done by changing the speed of the surface acoustic waves. One method for changing the speed of this surface acoustic wave is to implant ions such as helium He in a vacuum. Ion implantation reduces the surface acoustic wave velocity vF, particularly of the free surface, so that the center frequency of equation (1) can also be reduced.

[Problem that the invention seeks to solve]

When performing ion implantation, it is necessary to measure the center frequency of the interdigital electrodes after fabrication in advance and determine the amount of ion implantation based on the measured value. Conventionally, a part of the photomask is replaced with a surface acoustic wave filter consisting of regular type interdigital electrodes with the same electrode width and electrode spacing as the manufacturing surface acoustic wave filter, and the center frequency of the regular type interdigital electrodes is changed. was measuring.

However, as the center frequency approaches microwaves, it has become difficult to accurately measure wafers due to stray capacitance.

It will be shown below that the center frequency cannot be accurately measured even if only the electrode width and electrode spacing of the regular interdigital electrode are increased.

Consider a case in which the electrode width and electrode spacing of regular interdigital electrodes in a photomask are set to be a times (a>1) that of a manufacturing surface acoustic wave filter. In this case, the electrode width and spacing of the regular interdigital electrode after photolithography are aw-h and as+
h. Therefore, the center frequency f of the regular interdigital electrode
1 becomes...(B1). From equation (1) and equation (Bl), it is obtained from the following equation.

(B2) In the microwave surface acoustic wave filter, the value of h is quite large, so the second term in equation (B2) cannot be ignored. Therefore, afl does not match fo, and as a becomes larger, the difference between the two becomes larger.

The present invention provides a measurement method that enables measurement in a wafer state by indirectly measuring the center frequency of a manufactured surface acoustic wave filter at a lower frequency.

[Means for solving problems]

In the present invention, a first electrode portion formed with a predetermined first electrode width and electrode spacing, and a second electrode portion disposed outside the first electrode portion and wider than the first electrode width and electrode spacing are provided. Using a surface acoustic wave filter consisting of interdigital electrodes formed with a second electrode portion having a second electrode width and an electrode spacing, interdigital electrodes are formed with a first electrode width and an electrode spacing. Measure the center frequency of a surface acoustic wave filter constructed from

This first electrode part can be an interdigital electrode (11) or a finger electrode (13). As the input and output interdigital electrodes, interdigital electrodes both formed of the first electrode part and the second electrode part may be used, or one of the input or output may be the interdigital electrode described above. , the other may be a regular interdigital electrode with a second electrode portion.

[Effect]

Conventionally, it has been difficult to measure the center frequency of a surface acoustic wave filter in a wafer state using microwaves, but according to the present invention, measurement can be performed in a wafer state by indirectly measuring at a lower frequency. become.

〔Example〕

In the present invention, in order to measure the center frequency of the surface acoustic wave filter in the wafer state before cutting, the interdigital electrode (15) shown in FIG. 1 or the interdigital electrode (16) shown in FIG.
) is used for measurement on the input and output sides. That is, the surface acoustic wave filter for measurement shown in FIG. 1 has an interdigital electrode (11) in the center part having the same electrode width w-h and electrode spacing s+h as the surface acoustic wave filter for manufacturing actually used. At the same time, an electrode width w of this interdigital electrode (11) is formed on the outside of this interdigital electrode (11).
-h and the electrode spacing s+h, the second interdigital electrode (12
). In addition, the surface acoustic wave filter for measurement shown in FIG. 2 has a finger-shaped electrode (13) formed in the center with the same electrode width w-h and electrode spacing s+h as the surface acoustic wave filter for manufacturing that is actually used. At the same time, an electrode width aw- which is wider than the electrode width w-h and the electrode spacing s+h of this finger-like electrode (13) is provided on the outside of this finger-like electrode (13).
interdigital electrodes (14) with h and electrode spacing as+h
Construct by forming. Note that W is the width corresponding to the electrode in the photomask, S is the width corresponding to the electrode spacing in the photomask, and h is the sum of the overetch amount on both sides of the electrode.

For example, the frequency characteristics of a surface acoustic wave filter with the interdigitated type 1 (16) shown in FIG. 2 on the input and output sides are as follows.
As shown in FIG. 3, the vicinity of the center frequency almost coincides with the vicinity of the center frequency of a surface acoustic wave filter constituted by regular interdigital electrodes that are equal to the interdigital electrodes (14) at the periphery. If the frequency between adjacent traps in FIG. 3 is r, then the center frequency fo of the interdigital electrodes of the surface acoustic wave filter for manufacture is as follows.

That is, in FIGS. 1 and 2, the time to for the surface acoustic wave to propagate the distance d between the broken lines is given by (C1). Here, n is the central interdigital electrode (1
1) or the number of periods of the finger electrode (13), m is the number of periods of one outer interdigital electrode (12) or finger electrode (14), and in FIGS. 1 and 2, n=10 , m formula 1.

Also, d=2 (n+ma)(w+s)...” (
C2). Then, the frequency fB between adjacent traps in FIG. 3 becomes fB=□...(C3)D. d) In the case of ma(w+s), the second of equation (C1)
This term is very small compared to the first term and can be ignored. Here, (1), (C2).

Substituting equation (C3) into equation (C1), we get is obtained, and the previous equation (n) is derived from equation (C4).

Incidentally, here, the propagation time t shown by the above-mentioned equation (C3).

The relationship between the frequency r and the frequency r between adjacent traps in FIG. 3 will be explained. At the trap frequency shown in FIG. 3 where the insertion loss is infinite, the number of standing waves existing between the distance d is a half-integer. Let this number be 1 1/2 (k is an integer), and the trap frequency at this time is fT.

Then, it becomes . Here, vA is the average speed of surface acoustic waves at distance d, and is expressed as follows. If the adjacent trap frequency on the high frequency side of fTe is fTΦ, the standing wave number at this time is +1/2
Therefore, and fB=fT, -fT, ...-
(C8). (2) is obtained from formulas (C5) to (C8). That is, the propagation time tp is equal to the reciprocal of the frequency r between adjacent traps.

From the above equation (II), the measurement of the frequency f between adjacent traps of the surface acoustic wave filter for measurement can be performed at a frequency lower than the center frequency of the surface acoustic wave filter for manufacturing that is actually used, Measurement in wafer state becomes easier. Further, the value of w+s can be easily measured using a photomask.

In the upper peeling, interdigital electrodes (16) shown in Fig. 2 were used on the input and output sides, but other interdigital electrodes (16) were used on the input side and output side.
The frequency characteristics of the surface acoustic wave filter in which 6) is one of the input side or the output side and the other is a normal type interdigital electrode equal to the interdigital electrode (14) in the peripheral part are also the same as in Fig. 3. . Also, the structure is similar to that described above, and instead of the interdigital electrodes shown in FIG. 2, the interdigital electrodes (15
) also has frequency characteristics similar to those shown in FIG. 3, although the shape changes somewhat due to the influence of the interdigital electrode (11) in the center.

〔Effect of the invention〕

According to the present invention, the electrode width and electrode spacing of the peripheral electrode portion on at least one of the input side and the output side are the same as the electrode width and electrode spacing of the central portion (the electrode width and electrode spacing of the interdigital electrode of the surface elastic plate filter for manufacturing). By using a surface acoustic wave filter for measurement consisting of interdigital electrodes with a wider width (same width and electrode spacing), the frequency Since fo can be measured, it is possible to measure the center frequency of a surface acoustic wave filter in a wafer state even if the center frequency fo approaches a microwave.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a surface acoustic wave filter for measurement according to the present invention, FIG. 2 is a plan view of a surface acoustic wave filter for measurement according to another embodiment, and FIG. 3 is a relationship between trap frequency and insertion loss. FIG. 4 is a plan view of the photomask, and FIG. 5 is a plan view of interdigital electrodes. (11), (12), and (14) are interdigital electrodes, and (13) is a finger-like electrode. Same Hidemori Matsukuma 72.1゛7. Fig. 4 Plan view of crossed palm-shaped electric scraper Fig. 5 Knee Ding 2 Successor l↑Nira EpF Display of November 13th 1985 1, gl> lno1 1985 Patent Application No. 174724 No. 3, relationship with Jl Jl) Iz1 which is a supplementary city Single person address of Patent I 6-chome, Kitashinyo, Tokyo Parts Ward [No. 17-35 Name (218) Sony Corporation Representative Director Noriyoshi Ohga 4. The number of inventions increases due to Agent 6, Capture 11- [Field of Industrial Application] The present invention relates to a method for measuring the frequency characteristics of surface acoustic wave filters, especially surface acoustic wave filters in the wafer state of iM. .Yo or more

Claims (1)

[Scope of Claims] A first electrode portion formed with a predetermined first electrode width and electrode spacing; and a first electrode portion disposed outside the first electrode portion and wider than the first electrode width and electrode spacing. Using a surface acoustic wave filter consisting of interdigital electrodes formed with a second electrode portion formed by the second electrode width and electrode spacing, the intersection formed by the first electrode width and electrode spacing is A method for measuring the frequency characteristics of a surface acoustic wave filter, the method comprising measuring the center frequency of a surface acoustic wave filter composed of finger-shaped electrodes.