JPH0815247B2 - Surface acoustic wave device - Google Patents

Description

The present invention relates to a surface acoustic wave device having an interdigital electrode and a grating reflector on a piezoelectric substrate, and more particularly, to a periodic pitch width of lines and spaces of the grating. The present invention relates to a surface acoustic wave device in which desired reflection characteristics are obtained by changing to

[Conventional technology]

A surface acoustic wave resonator has been put into practical use as a conventional surface acoustic wave device using a grating reflector. As a technology for improving the performance of this surface acoustic wave resonator, a configuration in which the electrode period pitch of the grating reflector is slightly larger than the electrode period pitch of the interdigitated electrodes is disclosed in, for example, T. Uno.
Et al. “Optimization of quartz SAW resorator str
ucture with groove gratings ", IEEE.Traus vol.SU-29,
No. 6, P299 (1982).

[Problems to be solved by the invention]

In manufacturing a photomask for a surface acoustic wave resonator having such a conventional configuration, it is necessary to make a slight dimensional difference between the grating reflector and the interdigitated electrodes, which requires extremely high accuracy, and the manufacturing is difficult. It has the drawback of being difficult. Normal,
As a photomask for these surface acoustic wave resonators, 1: 1
A (1: 1) mask and a 5: 1 or 10: 1 magnified mask (reticle) are used. In detail, when making these high precision photomasks, electron beam exposure is usually used, but the pattern size difference is quantized because the process requires pattern data, and as a result,
A rounding error occurs. In order to avoid this, patterns that normally require delicate dimensions are subjected to double exposure at different reduction rates to produce a slight difference in dimension. However, in this process, double exposure causes an error in the position of each pattern. Further, since the double exposure is performed, the time required becomes long and the price of the photomask inevitably rises.

[Means for solving problems]

An object of the present invention is to provide a surface acoustic wave device which eliminates the above-mentioned drawbacks.

According to the present invention, in a surface acoustic wave device having a crossed finger-shaped electrode on a piezoelectric substrate and a grating reflector composed of a plurality of grating lines and spaces arranged on both sides of the crossed or finger-shaped electrode and adjacent to each other, The grating reflector is arranged at the same interval as a first line period row having grating lines arranged at the same intervals as the lines of the interdigitated electrode and a line interval in the first line period array. A second line period sequence having a grating line, and the first and second line period sequences have an interval between adjacent grating lines of a first,
A surface acoustic wave device is obtained which is different from the spacing between the grating lines of the second line period array and has no electrodes formed between the first and second line period arrays.

〔Example〕

 Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a part of a grating reflector according to an embodiment of the present invention. In this example, the line
Spaces 13 having different pitches are interposed between a periodic row 11 and a periodic row 12 having the same pitch of 14 and a space 15. First
Assuming that the grating shown in the figure is one section, the grating reflector is configured by arranging a number of this one section. This one
In the section, the central space 13 vector-synthesizes the periodic sequences 11 and 12, and the central frequency of the reflection characteristic can be moved.

FIG. 2 shows a surface acoustic wave device of the present invention. Piezoelectric substrate 21
The lines and spaces of the upper grating reflectors 22 and 23 also use a width of the same size as the lines and spaces of the interdigital electrode 24 to make the apparent periodic pitch different from that of the interdigital electrode 24. .

Now, the substrate material is ST cut quartz, and the grating reflector is made of an aluminum film, and the film thickness of this aluminum film is h / λ.
= 1.6 (%) and the center frequency _{0 of the} stop band is 1
FIG. 3 and FIG. 4 show the reflection characteristics in the period L of each electrode of the grating reflector when there are two types near 00 (MHz) and 1 (GHz).

FIG. 3 is a characteristic diagram showing the case of using the conventional equally spaced periodic grating electrode and the case of using the grating electrode of the present invention. The conventional period La is 20.1 (μ
m). In the case of the present invention, the period Lb is 20.0 (μm) and 20.5.
(Μm) combination and the number of grating electrodes is 100
It was a book. As is clear from the figure, the characteristics of both were completely in agreement.

FIG. 4 shows the characteristics when the conditions are the same as those in FIG. 3 and the number of grating electrodes is the same except for the number. Here, the number of electrodes is 500. In this case as well, the characteristics of the conventional case and the case of the present invention completely matched.

The above explanation is an example in which the line of the interdigital electrode or the width of the space different from the space of the interdigital electrode is slightly increased in order to make the pitch of the grating reflector slightly larger than the pitch of the interdigital electrode. However, this large and small conditions are large and small of the acoustic impedance of the aluminum thin film electrodes forming the interdigital electrodes for the piezoelectric substrate (for example, in the case of the above example, for the ST cut quartz substrate,
The acoustic impedance of the aluminum electrode is large). In the case of this crystal, due to the influence of the reflected wave reflected by each λ / 4 electrode finger that constitutes the interdigital electrode due to the relationship of the acoustic impedance, the frequency of the interdigital shape with the highest excitation efficiency is lower than the center frequency. Moving.

In order to maximize the reflectance of the reflector at that frequency, the pitch of the grating reflector may be set slightly larger than the pitch of the interdigital electrodes. As a substrate that is often used other than the ST-cut quartz substrate, there is a 128-degree rotated Y-cut X-propagation LiNbO ₃ substrate. In that case, contrary to the crystal, the impedance of the aluminum electrode is smaller than that of the LiNbO ₃ substrate. As described above, the pitch of the grating reflectors needs to be slightly smaller than the pitch of the interdigital electrodes in order to satisfy the optimum resonator configuration condition. For this purpose, the width of the line or space having a width different from the line or space of the interdigitated electrodes may be made smaller than that of quartz. The magnitude and sign of this acoustic impedance are made wide, for example, as described on pages 166 to 168 of the acoustic wave device technology handbook by Ohmsha.

〔The invention's effect〕

As described above, according to the present invention, it is possible to achieve the movement of the center frequency of the reflection characteristic, and in particular, the center frequency of the reflection characteristic can be moved by using the same pitch as the periodic pitch of the interdigital electrodes. There is an effect that a double exposure process is not necessary, high-precision and short-time manufacturing can be achieved, and a high Q characteristic can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view showing one section of a grating reflector of the present invention, FIG. 2 is a plan view showing an embodiment of the present invention, and FIG.
FIG. 4 and FIG. 4 are diagrams showing the reflection characteristics of the device of the present invention. 11,12 …… Grating, 13 …… Space, 21 …… Piezoelectric substrate, 22,23 Grating reflector, 24 …… Cross finger electrodes.

Claims (3)

[Claims] 1. A surface acoustic wave device comprising at least one pair of interdigital electrodes on a piezoelectric substrate, and a grating reflector disposed outside the interdigital electrodes, wherein the grating reflector is made of an elastic material. A plurality of periodic rows of the same pattern, which are composed of first lines and first spaces having the same line width and space width of the interdigitated electrodes in the propagation direction of the surface wave, are arranged between the periodic rows. A second line or a second space having a different width from the line or the space of the interdigital electrode is inserted, and the pattern of the interdigital electrode and the grating reflector is a beam spot without double exposure. A surface acoustic wave device, characterized in that it is formed in a direct drawing pattern drawn by an electron beam with a fixed size diameter. 2. A surface acoustic wave device comprising at least one pair of interdigitated electrodes on a piezoelectric substrate and a grating reflector arranged outside the interdigitated electrodes, wherein the grating reflector is elastic. A plurality of periodic rows of the same pattern, which are composed of first lines and first spaces having the same line width and space width of the interdigitated electrodes in the propagation direction of the surface wave, are arranged between the periodic rows. A second line or a second space having a different width from the line or the space of the interdigital electrode is inserted, and the pattern of the interdigital electrode and the grating reflector is a beam spot without double exposure. A surface acoustic wave device formed through a photomask formed by electron beam drawing with a fixed size diameter. 3. A surface acoustic wave device comprising at least one pair of interdigital electrodes on a piezoelectric substrate and a grating reflector disposed outside the interdigital electrodes, wherein the grating reflector is made of elastic material. A plurality of periodic rows of the same pattern, which are composed of first lines and first spaces having the same line width and space width of the interdigitated electrodes in the propagation direction of the surface wave, are arranged between the periodic rows. A second line or a second space having a different width from the line or the space of the interdigital electrode is inserted, and the pattern of the interdigital electrode and the grating reflector is a beam spot without double exposure. The width of the second line or the second space is formed through a reticle formed by electron beam drawing with a fixed size diameter. A surface acoustic wave device characterized in that it is a value obtained by multiplying a positive integer multiple of the beam spot size diameter of the electron beam writing by a reduction ratio in reduction exposure by the electron beam writing through the reticle. .