JPH04365212A - Manufacture of surface acoustic wave device - Google Patents

Abstract

PURPOSE:To reduce the dispersion of a resonance frequency and to improve yield by forming plural electrode patterns differing in the electrode duty corresponding to the thickness of a conductive film of a wafer. CONSTITUTION:Metal such as aluminum or the like is deposited on the surface of a conductive material wafer 2 by vacuum deposition. When a conductive film 6 is formed by the vacuum deposition, the conductive film 6 shows the thickness distribution of a thick center and a thin outer periphery. Next, the surface of the conductive film 6 is uniformity coated with a resist film 5 and exposed by using the mask of the prescribed electrode pattern. In that case, the exposure amount is increased at the center corresponding to the thickness distribution of the conductive film 6 and gradually reduced toward the peripheral part. Therefore, the electrode patterns formed by etching have the various duties for each part of the wafer corresponding to the thickness distribution of the conductive film 6. Namely, since the electrode pattern is formed so that the electrode duty is larger at the central part and smaller at the peripheral part, the dispersion of the resonance frequency is reduced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a surface acoustic wave device.

0002

[Prior Art] Generally, in surface acoustic wave devices used as filters or oscillators for VHF or UHF bands, a conductive film such as aluminum is deposited on a piezoelectric material wafer made of LiTaO3, LiNbO3, SiO2, etc. After forming the piezoelectric substrate by the method described above, a plurality of identical electrode patterns for surface acoustic wave devices are formed on the piezoelectric substrate by photolithography, and the wafer is cut to produce a plurality of surface acoustic wave devices.

FIG. 5 shows a schematic plan view of a general surface acoustic wave device. In the figure, 3 is a piezoelectric substrate, and 4 is a comb-shaped electrode formed on the piezoelectric substrate 3. FIG. 6 is a cross-sectional view of this surface acoustic wave device taken along the line XX', where W is the electrode finger 4.
A indicates the width, h indicates the film thickness of the electrode finger 4A, and P indicates the electrode pitch.

The resonance frequency of the surface acoustic wave device is determined by the propagation velocity V of the surface acoustic wave propagating on the surface of the piezoelectric substrate 3 and the interelectrode pitch P of the interdigitated electrodes 4 formed on the piezoelectric substrate 3.
Determined by

[0005] The propagation velocity V of the surface acoustic wave is approximately constant depending on the piezoelectric material, but the electrode film thickness h and the electrode pitch P
It changes depending on the ratio of the line width W of the electrode to the line width W/P=D (hereinafter referred to as duty). For example, when the film thickness h of the electrode increases, the propagation speed decreases due to the mass effect. The amount of change ΔV in the propagation velocity of the surface acoustic wave when the film thickness h of the electrode changes is expressed by the following equation.

[0006]

[Math 1]

In equation (1), V represents the propagation velocity of the surface acoustic wave in the substrate, and K1 and K2 represent the electromechanical coupling coefficients.

[0008] In this way, in the surface acoustic wave device,
Since the propagation speed of the surface acoustic wave propagating on the substrate surface changes depending on the film thickness of the electrode, the resonant frequency changes accordingly. For example, as the film thickness of the electrode increases, the resonant frequency decreases.

Therefore, when a plurality of the same electrode patterns are formed on a wafer, if the thickness of the conductive film is not uniform, the resonant frequency of the completed surface acoustic wave device will differ depending on the location on the wafer.

Generally, when a conductive film is formed on a substrate by vacuum deposition, the thickness of the formed conductive film is not uniform. As shown in Figure 7, a small hole 1a is made on the surface of the evaporation furnace.
If we consider the case where evaporation is performed by placing the wafer 2 on top of the evaporation source 1 from which evaporated molecules fly out, the wafer is separated by a distance r from the evaporation source 1, and the direction and normal of the evaporated molecules are inclined by θ. The mass m of a substance adhering to a unit area of a surface is expressed by the following formula.

[0011]

[Math 2]

In equation (2), s is a quantity called the adhesion coefficient, which represents the proportion of molecules that evaporated and collided with the wafer 2 and remains on the wafer 2 without being reflected, and M is the amount of molecules that evaporated in a certain period of time. The total mass of ψ is the angle between the normal to the surface of the small hole 1a of the evaporation source and the evaporation direction of the evaporated molecules.

When the wafer is placed as shown in FIG. 7, θ and ψ are 0 at the center of the wafer and increase from the center to the periphery. Moreover, the distance r from the evaporation source 1 changes similarly. Therefore, as is clear from equation (2), the thickness of the conductive film formed is thicker at the center of the wafer and becomes thinner toward the periphery. Therefore, when a conductive film is formed on the wafer 2 by vacuum deposition and a plurality of the same electrode patterns are formed, the resonant frequency of the completed surface acoustic wave device will vary.

[0014] Conventionally, there is a method of frequency adjustment to account for such variations in resonance frequency. For example, JP-A-1-
No. 24007 (H03H 3/10) discloses a method of etching a piezoelectric substrate by dry etching to adjust the frequency. Such a method requires a dry etching device, which poses a problem in that the number of manufacturing steps increases.

[0015] Also, Japanese Patent Application Laid-Open No. 61-157110 (H0
3H 3/08) Publication states that a plurality of patterning masks that can obtain predetermined frequency characteristics within the expected range of variation in the propagation velocity of surface acoustic waves are prepared, and surface acoustic wave A method is disclosed in which a predetermined patterning mask is selected depending on the propagation speed of the patterning mask, and a predetermined resist film pattern is formed on a conductive film of a wafer using the mask. However, although this method can accommodate variations in the propagation speed of surface acoustic waves from wafer to wafer,
It is not possible to compensate for variations depending on location on a single wafer.

[0016]

SUMMARY OF THE INVENTION It is an object of the present invention to reduce variations in resonance frequency in a surface acoustic wave device without requiring a device for adjusting the resonance frequency and without increasing manufacturing man-hours, thereby improving yield. The present invention provides an improved manufacturing method.

[0017]

[Means for Solving the Problems] In view of the above-mentioned points, the method for manufacturing a surface acoustic wave device of the present invention provides a method for manufacturing a surface acoustic wave device according to the thickness of the conductive film so that the resonant frequency of each surface acoustic wave device becomes equal. This method is characterized in that a plurality of electrode patterns are formed on a piezoelectric material wafer with different electrode duties.

[0018]

[Operation] By changing the duty of the electrode pattern formed on the wafer to compensate for the fact that the mass of the electrode, which affects the propagation speed of surface acoustic waves, differs depending on the part of the wafer due to the difference in film thickness, it is possible to The variation in resonance frequency in the surface acoustic wave device inside is reduced.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the method for manufacturing a surface acoustic wave device according to the present invention will be described below with reference to the drawings.

The manufacturing process of the electrode pattern will be briefly described. First, several thousand Å of metal such as aluminum is deposited on the surface of a piezoelectric material wafer by vacuum deposition. When a conductive film is formed by vacuum evaporation as described above, the formed conductive film exhibits a film thickness distribution such that it is thickest at the center of the wafer and becomes thinner toward the outer periphery. Next, a resist is uniformly coated on the conductive film. Then, by exposing the resist using a mask with a predetermined electrode pattern and developing it, a resist film having a predetermined electrode pattern shape is formed on the conductive film. This wafer is etched to remove unnecessary portions of the conductive film, and then the resist film is removed to form electrodes in a predetermined pattern on the wafer.

In the present invention, the electrode duty of each electrode pattern to be formed is changed for each part of the wafer in accordance with the film thickness distribution of the conductive film so that the resonance frequency is constant. For this purpose, a resist film formed on the conductive film is processed into such an electrode pattern shape.

The method is to change the amount of exposure when exposing a resist-coated wafer with an exposure machine using a patterning mask. The exposure machine used is of a type that moves at a predetermined repeat pitch and exposes the mask pattern in a reduced size.

FIG. 2 is a diagram for explaining this exposure process, and shows an example of equal-magnification exposure for simplicity. If this exposure step is performed with a constant exposure amount, the duty of the electrodes formed on the wafer 2 will be constant, but by changing the exposure amount for each portion of the wafer 2, the electrode duty is varied. In other words, when the resist used is a negative type, the amount of exposure is increased in the central part where the thickness of the conductive film 5 is thick, and the amount of exposure is increased as it goes to the periphery, depending on the film thickness distribution of the conductive film 5. Reduce quantity.

As a result, each surface acoustic wave device electrode pattern formed on the wafer 2 has a duty distribution as shown in FIG. That is, in the central part A, by increasing the exposure amount, the resist film 5 is exposed to the dotted line A shown in FIG. 2, wider than the width of the transparent part 7A of the mask 7, and is removed up to the dotted line A in the development process. . In addition, in the B section and C section of the wafer 2, FIG.
Adjust the exposure amount so that the dotted lines B and C are exposed.

By doing this, the resist pattern formed after developing the resist film 5 becomes a pattern with a different duty for each part of the wafer 2. Therefore, the electrode pattern formed by etching has a different duty for each part of the wafer 2, depending on the thickness distribution of the conductive film 6.

[0026] In this way, an electrode pattern is formed in which the electrode duty is large in the center and becomes smaller toward the periphery by photolithography of a conductive film formed by vacuum evaporation and exhibiting a constant film thickness distribution. By doing so, it is possible to obtain a surface acoustic wave device with less variation in resonance frequency.

Another method for forming the resist film 5 in a pattern with a different duty for each portion of the wafer 2 is to use a constant exposure amount and perform the pattern in a developing process. That is, when dropping the developer onto the exposed wafer, more of the developer is dropped at the center so that more of the resist film 5 is removed from the center during the development process, and the method is similar to the method described above. Form a resist pattern.

FIG. 3 is a diagram showing the distribution of resonant frequencies of a surface acoustic wave device manufactured from a single wafer manufactured by the method described above. Compared to the frequency distribution shown in FIG. 4 when electrode patterns with the same duty are formed, the variation in resonance frequency is reduced.

[0029]

As described above, according to the present invention, it is possible to reduce variations in the resonance frequency of surface acoustic wave devices manufactured from the same wafer, and it is possible to improve the yield.

[Brief explanation of the drawing]

FIG. 1 schematically shows the duty distribution of electrodes formed on a wafer by the manufacturing method of the present invention.

FIG. 2 is a diagram showing a resist film exposure process of the present invention.

FIG. 3 is a diagram showing the distribution of resonant frequencies of a surface acoustic wave device made from a single wafer according to the present invention.

FIG. 4 is a diagram showing the distribution of resonant frequencies of a surface acoustic wave device manufactured from a single wafer by a conventional method.

FIG. 5 is a schematic diagram showing a surface acoustic wave device.

6 is a sectional view taken along line XX' of the surface acoustic wave device of FIG. 5. FIG.

FIG. 7 is a diagram for explaining the formation of a conductive film by vacuum evaporation.

[Explanation of symbols]

2 Piezoelectric material wafer 5 Resist film 6 Conductive film 7 Patterning mask

Claims (3)

[Claims] 1. Manufacture of a surface acoustic wave device in which electrode patterns for a plurality of surface acoustic wave devices are formed on a piezoelectric material wafer with a conductive film formed on one surface by processing the wafer using a photolithography method. In the method, the plurality of electrode patterns are formed by varying the ratio of the line width of the electrode to the electrode pitch in each electrode pattern according to the thickness of the conductive film so that the resonance frequency of each surface acoustic wave device is equal. 1. A method of manufacturing a surface acoustic wave device, comprising: forming a surface acoustic wave device. 2. A process of forming a conductive film on a piezoelectric material wafer, a process of forming a resist film on the conductive film, and patterning so that the resonance frequencies of each surface acoustic wave device are equal. a step of exposing the resist film in a portion of the conductive film at a different exposure amount depending on the film thickness of the portion where the electrode pattern is to be formed using a mask; 2. The method of manufacturing a surface acoustic wave device according to claim 1, further comprising the steps of forming a resist pattern in the shape of a plurality of electrode patterns and removing unnecessary portions of the conductive film by etching. 3. A step of forming a conductive film on a piezoelectric material wafer, a step of forming a resist film on the conductive film, and a resist film on each portion of the conductive film where an electrode pattern is formed. A step of exposing the conductive film at a constant exposure amount using a patterning mask, and adjusting the thickness of the conductive film at a portion where the electrode pattern is formed so that the resonance frequency of each surface acoustic wave device is equal. forming resist patterns in the shape of a plurality of electrode patterns on the conductive film by changing the development amount of the resist film in the portion, and removing unnecessary portions of the conductive film by etching; The method for manufacturing a surface acoustic wave device according to claim 1, characterized in that: