JPH0590865A - Central frequency adjusting method for acoustic surface wave filter - Google Patents

Abstract

PURPOSE:To accurately adjust the deviation of the central frequency of acoustic surface wave filter from a desired value without deteriorating frequency characteristics thereof by applying dry etching to the piezoelectric substrate with the pattern being masked until the measured value of the central frequency of a interdigital electrode pattern reaches to the desired value. CONSTITUTION:The central frequency (f) of a acoustic surface wave is measured. Next, whether or not the central frequency (f) coincides with a desired value is discriminated, and if the (f) does not coincide with the desired value, the dry etching process 13 is executed. A dry etching method, such as plasma etching or the like is used, and etching of the surface 1a of a piezoelectric substrate 1 is implemented under a predetermined etching condition while masking electrode fingers 2a and 3a of interdigital electrodes 2 and 3. The surface 1a of the piezoelectric substrate 1 of the section where no electrode fingers 2a and 3a exist is ground, while sections 2b and 3b under electrode fingers are left unground. The central frequency (f) is measured, whether or not the central frequency (f) coincides with a desired value is discriminated by a frequency discriminator 12, and if no coincidence is found, dry etching 13 is repeated until the central frequency (f) reaches the desired value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of adjusting a center frequency of a surface acoustic wave filter using surface acoustic waves propagating on a surface of a substrate.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field,
Some of them were described in the specification of Japanese Patent Application No. 2-39500. The configuration will be described below with reference to the drawings.

FIG. 2 is a schematic configuration diagram of the conventional surface acoustic wave filter described in the above-mentioned document.

In this surface acoustic wave filter, an input (or output side) interdigital transducer 2 for converting an electric signal into a surface acoustic wave and a surface acoustic wave into an electric signal are formed on the surface of a piezoelectric substrate 1 having a piezoelectric effect. The output-side (or input-side) interdigital transducer 3 for conversion is arranged to face. Each of the interdigital electrodes 2 and 3 is composed of a plurality of electrode fingers 2a and 3a. In FIG. 2, G is a high frequency signal, Z _s is an input resistance, Z _L is a load resistance, and s is a surface acoustic wave.

In this type of surface acoustic wave filter, when the high frequency signal G is applied to the interdigital transducer 2 via the input resistance Z _s , the interdigital transducer 2 excites the surface acoustic wave S. The surface acoustic wave S excited by the interdigital transducer 2 is the electrode finger 2
When it propagates in the direction perpendicular to a and reaches the interdigital electrode 3, a potential difference is generated between the electrode fingers 3a of the interdigital electrode 3, and an electric signal is induced. This electric signal is the load resistance Z
Output to _L side.

The center frequency f of the surface acoustic wave filter having a narrow band is the propagation velocity v of the surface acoustic wave S propagating on the piezoelectric substrate 1, the pitch d of the electrode fingers 2a and 3a (= the wavelength λ of the surface acoustic wave S). ), F = v / d is determined. However, since the propagation velocity v is determined by the type of the piezoelectric substrate 1,
Once the interdigital electrodes 2 and 3 are formed on the piezoelectric substrate 1, it becomes very difficult to change the center frequency f thereof.

Therefore, in the conventional center frequency adjusting method described in the above-mentioned document, the interdigital transducers 2, 2 are formed on the piezoelectric substrate 1.
3 is formed, then the electrode fingers 2a of the interdigital transducers 2, 3
The film thickness of 3a was measured, and the propagation speed v of the surface acoustic wave S was changed and adjusted by etching so that the film thickness became a desired thickness.

[0008]

However, the conventional method has the following problems. In the conventional center frequency adjusting method, the film thickness of the electrode fingers 2a and 3a of the interdigital transducers 2 and 3 is reduced mainly by wet etching to adjust the frequency. Therefore, the propagation velocity v of the surface acoustic wave S is increased. You can only adjust the frequency in the direction of making it. Therefore, there is a drawback in that the central frequency f can be adjusted for those having a frequency larger than a desired value, but the frequency cannot be adjusted for those having a central frequency f smaller than the desired value.

Further, since wet etching is mainly used, it is necessary to perform etching for a long time. However, if etching is performed for a long time, not only the film thickness of the electrode fingers 2a and 3a but also its width is reduced. , The frequency characteristics change. For this reason, there is a drawback in that it cannot be adjusted if the deviation from the center frequency f is large, and it is still difficult to provide a center frequency adjusting method that is technically sufficiently satisfactory. The present invention solves the problems that the above-mentioned conventional art has, that the center frequency can only be adjusted to be smaller than a desired value, and that the frequency characteristics change when the wet etching is performed for a long time. The present invention provides a method for adjusting the center frequency of the surface acoustic wave filter.

[0010]

In order to solve the above-mentioned problems, a first aspect of the present invention provides a comb-shaped electrode pattern for converting an electric signal into a surface acoustic wave and then converting the surface acoustic wave into an electric signal again. In the narrow band surface acoustic wave filter formed on the piezoelectric substrate, the center frequency is measured by energizing the interdigital electrode pattern, and the interdigital electrode pattern is used as a mask until the measured value reaches a desired value. The piezoelectric substrate is dry-etched under predetermined etching conditions.

According to a second invention, in the first invention, the piezoelectric substrate is made of quartz, and the interdigital electrode pattern is made of aluminum or an aluminum alloy.

[0012]

According to the first aspect of the invention, since the center frequency adjusting method for the surface acoustic wave filter is constructed as described above, the center frequency is measured by energizing the interdigital electrode pattern of the manufactured surface acoustic wave filter. Thus, the frequency difference from the desired value can be obtained. Therefore, when the dry etching is performed using the interdigital electrode pattern as a mask so that the frequency difference becomes 0, only the surface of the piezoelectric substrate is etched,
The center frequency can be adjusted by changing the propagation velocity of the surface acoustic wave.

According to the second invention, since the etching rate of the piezoelectric substrate made of quartz is higher than that of aluminum or aluminum alloy forming the interdigital electrode pattern, only the surface of the piezoelectric substrate is masked with the interdigital electrode pattern. Can be accurately etched by dry etching,
The propagation velocity of surface acoustic waves can be increased or decreased. Therefore, the above problem can be solved.

[0014]

1 (a) and 1 (b) are explanatory views of a frequency adjusting method in a surface acoustic wave filter showing an embodiment of the present invention. FIG. 1 (a) is a flow chart of frequency adjusting processing,
FIG. 6B shows the change in mass load due to dry etching.

In the frequency adjusting method of this embodiment, as shown in FIG. 2 of the related art, a piezoelectric substrate 1 is formed by using, for example, crystal, and aluminum or aluminum alloy is used as an electrode material on the piezoelectric substrate 1. Interdigital electrodes 2, 3
It is formed and to produce a narrowband timing extraction SAW filter of the example 2.4 GHz _z.

Using the surface acoustic wave filter manufactured in this manner, as shown in FIG. 1A, in the frequency measurement process 11, the center frequency f of the surface acoustic wave filter is measured using a network analyzer or the like. .. Next, in the frequency determination process 12, it is determined whether the center frequency f of the surface acoustic wave filter matches a desired value, and if the center frequency f matches the desired value, the center frequency adjustment process ends.
If they do not match, the process proceeds to the dry etching process 13.

In the dry etching process 13, as shown in FIG.
As shown in (b), by using a dry etching method such as plasma etching, the electrode fingers 2a of the interdigital electrodes 2 and 3,
The surface 1a of the piezoelectric substrate 1 is etched under predetermined etching conditions using 3a as a mask.

Piezoelectric substrate 1 where electrode fingers 2a and 3a are not present
The surface 1a of the above is abraded, but the portions 2b and 3b under the electrode fingers 2a and 3a remain without being abraded. These parts 2a and 3b are
Since a mass load is applied to the substrate surface 1b after etching, it has an effect of reducing the propagation velocity v of the surface acoustic wave S propagating on the substrate surface 1b. Moreover, since the mass load increases with the progress of etching, the surface acoustic wave S
The propagation velocity v of V is also decelerated. Therefore, the frequency of the surface acoustic wave S propagating on the surface 1b of the piezoelectric substrate 1, that is, the center frequency f of the surface acoustic wave filter decreases.

After performing the dry etching process 13 as described above, the frequency measurement process 11 shown in FIG. 1A is performed to measure the center frequency f, and the center frequency f coincides with a desired value in the frequency determination process 12. It is determined whether or not to do so, and if they do not match, the dry etching process 13 is performed again, and the above process is repeated until the center frequency f matches a desired value. 3A to 3C are views showing changes in the substrate surface due to the dry etching of FIG. FIG. 4 shows a dry etching process 13 using a plasma etching method for 30 seconds under the conditions of etching gas CF ₄ , film thickness 430Å of the interdigital electrodes 2, 3, gas flow rate 178 sccm, pressure 10 Pa, and discharge power 100 W. Seconds, 90 seconds, 120 seconds, 1
It is a diagram showing the relationship between the shift amount of the etching time and the center frequency f Δf (MH _z) when performing 35 seconds.

In FIGS. 3A to 3C, FIG.
Before the etching of 1), the amorphous layer 1A is present on the surface 1a of the piezoelectric substrate 1 and is exposed. At the beginning of etching in FIG. 3B, the electrode fingers 2a and 3
Since the amorphous layer 1A is dry-etched using a as a mask, the crystal lattice of the surface 1a of the piezoelectric substrate 1 approaches a single crystal. Therefore, for the surface acoustic wave S propagating on the surface 1a, the mechanical resistance decreases, and conversely the propagation velocity v increases. Therefore, as shown in FIG. 4, the center frequency f rises at the beginning of etching.

However, in the latter stage of etching shown in FIG. 3C, when the amorphous layer 1A is completely etched,
After that, only the substrate crystal is scraped off, and the electrode fingers 2a, 3
The film thickness of the portions 2b and 3b below a becomes thicker and the mass load (mechanical resistance) increases. Therefore, as shown in FIG. 4, the center frequency f decreases.

On the basis of the data of FIG. 4, the starting center frequency f is 2.497695 GH under the etching conditions of FIG.
_The following results were obtained when dry etching of the surface acoustic wave filter for timing extraction, which was _z , was performed for 30 seconds, 90 seconds, and 125 seconds. Center frequency f with the progress of etching 2.50182GH _z, 2.4992
575H _z and change a final desired frequency 2.
It was consistent with the 48832GH _z. At this time, it was also confirmed that the transmission characteristics did not change with the progress of etching, and the effectiveness of the method of this example could be observed.

As described above, in this embodiment, the center frequency f is obtained by dry-etching the piezoelectric substrate 1 using the interdigital electrodes 2 and 3 formed on the piezoelectric substrate 1 as a mask.
Can be shifted higher or lower.
Therefore, a surface acoustic wave filter having a center frequency f higher than a desired value, which cannot be adjusted by the conventional method of wet-etching the interdigital transducer, can be made into a good product. Moreover, compared with aluminum or aluminum alloy that forms the interdigital electrodes 2, 3, the piezoelectric substrate 1
Since the etching rate of the crystal that constitutes
Even if the etching time is lengthened, the width of the electrode fingers 2a and 3a does not change. Therefore, the desired center frequency f can be easily and accurately obtained without deterioration of the frequency characteristic.

The invention is not limited to the above embodiment, but various modifications can be made. Examples of such modifications include the following.

(I) In the above embodiment, the piezoelectric substrate 1 was made of quartz and the interdigital electrodes 2 and 3 were made of aluminum or aluminum alloy. However, the etching rate of the piezoelectric substrate 1 is higher than that of the interdigital electrodes 2 and 3. Any other material may be used as long as it is an electrode material and a substrate material that increase the thickness. Further, the etching conditions may be changed to other conditions depending on the non-etching material, and other dry etching methods such as sputter etching and ion beam etching may be used.

(Ii) A metal film of chromium or the like having a low etching rate is previously deposited on the surfaces of the interdigital electrodes 2 and 3, and the surface 1a of the piezoelectric substrate 1 is etched by the dry etching process 13. For example, the electrode fingers 2a,
It is possible to suppress the decrease of the width of 3a as much as possible and prevent the deterioration of the frequency characteristic more accurately. The surface acoustic wave filter is not limited to the shape shown in FIG. 2 and may be modified into various shapes and structures.

[0027]

As described in detail above, according to the first invention, the center frequency is adjusted by dry etching the piezoelectric substrate using the interdigital electrode pattern formed on the piezoelectric substrate as a mask. Since this is done, it is possible to shift the center frequency to either the higher side or the lower side. Therefore, a surface acoustic wave filter having a center frequency higher than a desired value, which cannot be adjusted by the conventional method, can be made a good product.

Moreover, by performing dry etching under optimum etching conditions such as using a piezoelectric substrate material having an etching rate higher than that of the electrode material, etching processing can be performed in a shorter time than wet etching.
Therefore, there is no change in the electrode width, it is possible to accurately prevent deterioration of the frequency characteristics, and it is possible to accurately adjust the frequency even for a large deviation from the center frequency.

According to the second invention, since the piezoelectric substrate is made of quartz and the interdigital electrode pattern is made of aluminum or aluminum alloy, the etching rate of the piezoelectric substrate is higher than that of the interdigital electrode pattern, and the electrode is formed. Only the surface of the piezoelectric substrate can be accurately etched without reducing the width of the pattern.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a frequency adjusting method according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a conventional surface acoustic wave filter.

FIG. 3 is a diagram showing changes in the substrate surface due to the dry etching of FIG.

FIG. 4 is a diagram showing the relationship between the etching time and the center frequency shift amount of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2,3 Interdigital electrode 2a, 3a Electrode finger 11 Frequency measurement process 12 Frequency determination process 13 Dry etching process

Claims (2)

[Claims] 1. A narrow band surface acoustic wave filter formed on a piezoelectric substrate with a comb-shaped electrode pattern for converting an electric signal into a surface acoustic wave and then converting the surface acoustic wave into an electric signal again. An electric current is applied to the electrode pattern to measure the center frequency, and the piezoelectric substrate is dry-etched under predetermined etching conditions using the interdigital electrode pattern as a mask until the measured value reaches a desired value. A method for adjusting the center frequency of a surface acoustic wave filter. 2. The center frequency adjusting method for a surface acoustic wave filter according to claim 1, wherein the piezoelectric substrate is made of quartz, and the interdigital electrode pattern is made of aluminum or an aluminum alloy. Adjustment method.