JPS5892117A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave filter with good characteristics through high position accuracy and a thick sound absorbing film, by forming the sound absorbing film pattern through the use of a photopolymer film. CONSTITUTION:After a pattern consisting of input and output transducers 1, 4 and a multistrip coupler 3 is formed on a piezoelectric crystal substrate 2 with a vacuum deposition method and photoetching technology, a photopolymer film is press-stuck onto the substrate, sound absotbing film patterns 7, 8, 9 are sensitized with the close or projective exposure. When the photopolymer film is of negative type, the pattern is formed by using a phenomenon that the light irradiated part becomes insoluble to a developer, and when the film is of positive type, a phenomenon that the light irradiated part becomes soluble is used.

Description

Detailed Description of the Invention (1) Technical Field of the Invention The present invention relates to an elastic surface acoustic wave filter formed by forming a sound-absorbing film pattern using a photopolymer film. The filter is made of lithium niobate (LiNbOs)
) A piezoelectric crystal such as lithium tantalate (LiT10m) is used as a plate, and an input cadence changer and an output cadence changer consisting of comb-shaped electrodes with interlocking combs are arranged on both sides of the piezoelectric substrate. In fact, it is an element that has a blind-shaped multi-strip coupler between the two transformers, and both transformers 1'? Sound-absorbing false patterns are arranged alternately on the back 4 and on the sides of the filter. Figure 1 is a configuration diagram showing the arrangement of electrodes and sound-absorbing membranes in a surface acoustic wave filter. When a high frequency electric signal tube is applied to the piezoelectric crystal 2, the frequency characteristics determined by the shape of the electric piece - trtFi comb-shaped turtle etc. are changed to 9 surface mK & piezoelectric crystal 2
It propagates above the piezoelectric crystal 2, is converted into an electrical signal by the multi-strip coupler 3, is converted again into a surface wave by the multi-strip coupler 3, propagates above the piezoelectric crystal 2, and is converted into an electrical signal again by the output transducer 4. MR is issued as.

In a filter with such a configuration, there are various disadvantages other than the surface waves of signal propagation.

For example, there is a bulk wave transmitted through a piezoelectric coil, and even in the surface wave propagated from the large input quadrangle sensor 4, it is reflected by the output quadrangle sensor 4 and returns to the input quadrangle sensor 19, and is re-reflected at this \. The surface wave from the incoming quadrature resistor 1 propagates in the opposite direction and reaches the end face 5 of the crystal substrate, where it is reflected and reaches the output quadrature resistor 4. There are various types of non-t' waves, such as waves that reach the output tube, and waves that are reflected from the output radiator 4 and reach the end face 6 of the crystal substrate, where they are reflected again and propagate in the direction of the input radiator IK. In this case, countermeasures have been taken such as changing the electrode arrangement of the transformer or changing the shape of the comb-shaped electrode to cancel out the reflected waves. Measures are taken to absorb unnecessary waves propagating in directions other than the same electrode using a sound absorbing film.

For example, in Figure 1, zig-sag-shaped sound-absorbing films 7 and 8 are provided on the backs of the input and output transducers 1 and 4 to absorb unnecessary waves propagating in the opposite direction 11 from the transducer. The reason why pattern 7.8 has a zigzag shape is that it is rounded so that unnecessary waves that are not absorbed by the sound absorbing film but are reflected are propagated in a direction other than the transducer. The position and pattern accuracy of position 10ell have a large influence on filter characteristics.0 Figure 2 is an enlarged view of the bending point lO of the sound absorbing film 7, and the design position f117 of the sound absorbing film indicated by diagonal lines in the lower left corner is where the pattern is formed. Due to misalignment of the mask, the part that should form the sound absorbing film 7' at position 11 indicated by the diagonal line downward to the right, and the part that should form the bending point 10, was damaged by the K' drip during the drying and oxidation of the paint film. Considering the case of curved bending point 10', the surface wave 1 is reflected at the point avoiding the transducer, so there is no shadow @IFi, but the surface wave b11 is reflected in the original propagation direction, and is reflected in the original direction. It becomes impossible to remove the undesired J& waves propagated from the transducer 1. In Figure 1, the transducer 1.
This sound absorbing film 9 The deviation from the designed position has a sensitive effect on the filter's far-range threat.

(3) Prior art and problems The metal film parts such as the strip coupler 3 were vacuum IIA-viewed and photo-etched as shown in Photo 11! iIntuitive technology (Photographic Roughing) Since the patterns are formed at the same time, there is no misalignment of the patterns.The first-class sound membrane is formed by screen printing after these patterns are formed, so the position is slightly different from the designed position. 69 It is necessary to heat and harden the screen after the screen is removed. 69 For this light removal, a refractive S full valve with a pig angle should be added depending on the location of "who" explained in Fig. 2 Lfia*t, , <These are the causes of variations in the passband bone quality of surface acoustic wave filters.

In the field, the essential features of a single-absorption film are that it has a large sound-absorbing effect and long-term stability.The sound-absorbing effect depends on the material used and the plastic thickness. The thickness must be at least AIO or SM ◎ Therefore, conventionally, oleo antidermatology was formed using a screen printing method using a polymeric material such as epoxy 111 fat. It is impossible to form a pattern using the screen printing method, and the positional accuracy of the screen printing method is approximately 50 μm.If the film is thickened by printing repeatedly, the positional accuracy will be significantly inferior, resulting in a problem with the thickness and position. (4) Purpose of the Invention The purpose of the present invention is to form a sound absorbing film with a high position f1111t and a high thickness. This is used to form a film pattern using @&.

(5) Structure of the Invention A photopolymer film is used to carry out the present invention. This film #i is a film made of a synthetic resin that has photosensitivity and can be used by arbitrarily spreading it onto an object. It is commercially available as a photopolymer. (
For example, Dupond's photopolymer film) In the present invention, a pattern consisting of a human output transformer and a multi-strip coupler is formed on a piezoelectric crystal substrate by vacuum evaporation and photolithography, and then a photopolymer film is pressure-bonded onto the substrate and exposed to close contact. The sound-absorbing film pattern is exposed to light by Fi projection exposure, and if the photopolymer film is a negative type, the light irradiated part becomes insoluble in the developer, and if it is a positive type, the light irradiated part becomes soluble. Then, a pattern f is formed.

It goes without saying that if a photopolymer film with the required thickness is commercially available, it is convenient to use it as is. Patterns can be created using photolithography using such a ready-made film. When forming, there is no misalignment unlike when forming using a screen printing method.
A sound-absorbing film pattern with a high position ffIIt can be created by extending it to the sixth position without the occurrence of II.

(6) Example 3 of the Invention A surface acoustic wave filter having a frequency of 2 to 38 MHz KJ was produced using a sound-absorbing layered bottom polymer film, and its effects were fully evaluated. 1) Film 730FR manufactured by Upond Co., Ltd. was used, and the thickness of I4 was 60±5 μm.

As a result, the position PII between the input/output quadrangle resistor and the multi-strip coupler pattern and the 'lfm pattern can be sufficiently created within ±30 m, which is the target value, and a filter 1 with a main signal ratio of -43 dB can be obtained as a sound absorption effect. ◎ On the other hand, with the conventional screen printing method, the film thickness was 15 to 30 μm.
When forming a sound absorbing film, the position l1lk? *pL is ±50
The μm trench signal ratio is 1↓1 to 42 dB, and the filter characteristics have been improved by implementing the present invention.

(7) Effects of the Invention The present invention shows that in the conventional manufacturing of filter collectors, the sound-absorbing film pattern is formed by screen printing, but because the dimensions of the element eye are as small as 11x3a1, the alignment accuracy is confusing. While the formation of a thick f-absorbing film caused by large scratches was difficult, the photopolymer film
By using this method of forming patterned tubes (by photo-etching), the drawbacks of the conventional methods can be corrected, and it is now possible to manufacture an elastic surface tlI filter with good bendability.

[Brief explanation of drawings]

Figure 1 is a configuration diagram showing the electric power distribution of the surface acoustic wave filter and the sound-absorbing film, and Figure 2 is an enlarged view of the refraction part of the sound-absorbing film pattern. Strip coupler, 4 is output transformer, 7.
8.9#-j sound absorbing membrane pattern.

Claims (1)

[Claims] In a surface acoustic wave filter in which a pattern consisting of at least an input cadence detector, an output radiator detector, a sound absorbing film, etc. is provided on a piezoelectric crystal substrate, the elasticity table thlrI assumes that the sound absorption error pattern is formed by using the photopolymer film 1. ! filter 0