JPH10200354A - Production of surface acoustic wave element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a surface acoustic wave(SAW) device such as a SAW filter enhanced in adhesion electrodes and having a uniform and accurate interdigital electrode at low cost by depositing an oxidized zinc film and depositing a metal film for electrode on that film through the same filming device later. SOLUTION: On a silicon wafer 4 forming an oxide film on its surface, a ZnO film 3 is formed by sputtering. Next, an Al film 6 to become an electrode material is sputtered. Since the ZnO film 2 and the electrode material can be formed by the same sputtering device, continuous sputtering is enabled. Then, an electrode pattern is formed by performing etching with an etching solution pH adjusted so as to selectively etch only the electrode material. As a result, a depositing device required conventionally is reduced, the number of processes and man-hours can be reduced and cost reduction is enabled. Further, adhesion is enhanced and even in a thick electrode film, the electrode can be formed with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface acoustic wave (Su)
rface Acoustic Wave (hereinafter abbreviated as SAW)) The present invention relates to a method for manufacturing an element, and particularly to a method for manufacturing a comb-shaped electrode on a piezoelectric body.

[0002]

2. Description of the Related Art A high-frequency filter is one of the indispensable components for reducing the cost, size, and weight of mobile communication equipment. Among them, monolithic SAW filters that form electrodes on thin-film piezoelectrics to excite SAW are lower in cost and smaller in size than elastic SAW filters using bulk waves, such as dielectric filters and quartz filters. It is gaining attention because of its advantages. SAW similar in structure to a monolithic SAW filter
Convolvers are also attracting attention as components suitable for reducing the cost, size, and weight of data communication equipment.

FIG. 4 is a perspective view of a typical example of a monolithic SAW filter. A piezoelectric film of zinc oxide (hereinafter referred to as ZnO) is formed on a substrate 4 of a silicon wafer by a high frequency sputtering method, and an input side comb electrode 1 and an output side comb electrode 2 are formed on the ZnO film 3. . SA
The structure of the W filter is comb-shaped electrode / piezoelectric body / substrate, piezoelectric body /
Although various structures such as a comb-shaped electrode / substrate have been considered, the present invention mainly relates to a structure of a comb-shaped electrode / piezoelectric body / (electrode) / substrate, and therefore, description will be limited to this.

[0004] The principle of operation of this SAW filter will be described below. When an electric signal is applied to the comb electrode 1 on the input side, the electric signal is converted into mechanical distortion by the piezoelectric effect, and the SAW
5 and propagates on the surface of the ZnO film 3 and in the piezoelectric film. The SAW 5 arriving at the output side comb electrode 2 is converted from mechanical distortion into an electric signal, and is extracted as an output signal. FIG. 6 is a flowchart of a manufacturing process of a conventional monolithic SAW filter. A ZnO film 3 is formed on a substrate 4. As a method for forming the ZnO film 3, a high frequency sputtering method is mainly used. A photoresist pattern opposite to the electrode patterns of the input and output comb electrodes 1 and 2 is formed by photolithography. An electrode material is formed on the entire surface of the substrate by a vapor deposition method. The comb-shaped electrodes 1 and 2 on the input and output sides are produced by lift-off in which the photoresist pattern is peeled off together with the electrode material thereon. Thereafter, the substrate is formed into a chip by, for example, dicing, mounted on a case, and passed to a device forming step of bonding leads.

[0005] As described above, the method of fabricating a comb-shaped electrode by using lift-off has advantages that the electrode can be finely processed easily, and that the electrode is formed by vapor deposition, so that damage to the piezoelectric film can be reduced. is there.

[0006]

However, in order to fabricate a comb-shaped electrode using lift-off, an electrode material must be vertically adhered to a substrate, so that electrode formation by a vapor deposition apparatus is indispensable. That is, the piezoelectric substance ZnO
While the film is formed by a sputtering device, expensive equipment for vertically depositing the electrode material on the substrate is required,
Product costs increase.

In order to increase the adhesion strength of the electrode, the higher the substrate temperature, the better. However, if the substrate temperature is raised to 100 ° C. or higher, the photoresist is thermally deformed, and a precise pattern cannot be obtained. Further, when the electrode has a thickness of 1 μm or more, the stripping solution at the time of lift-off becomes difficult to penetrate into the photoresist under the electrode, and there is a problem that the lift-off cannot be performed uniformly.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a method for manufacturing a SAW device such as a SAW filter having a uniform and highly accurate comb-shaped electrode with a high electrode adhesion strength at low cost. .

[0009]

According to the present invention, there is provided a method for manufacturing a surface acoustic wave device comprising a ZnO film deposited on a substrate and an Al or Al alloy electrode formed thereon. After depositing a zinc film, a metal film for an electrode is deposited thereon by the same film forming apparatus. In such a case, the ZnO film and the electrode material can be formed by the same sputtering device, and thus, a vapor deposition device for vapor-depositing the electrode material is unnecessary.

[0010] An etching solution for selectively etching only the electrode metal film is used. For example,
As another method of electrode etching, carbon tetrachloride (CC
There is a reactive ion etching method using l ₄ ),
The selectivity is not good and the ZnO film is slightly etched. In the case of wet etching, the temperature is low and the surface of the ZnO film is not damaged.

In particular, the pH of the etching solution is adjusted to pH 9 to p.
Control is performed in the range of H13. Al is susceptible to corrosion below pH 4 and above pH 9, whereas ZnO is stable between pH 7 and pH 13. By utilizing this fact, by using an etchant adjusted to a pH range of 9 to 13 with a buffer solution, only the Al or Al alloy can be etched without etching the ZnO film.

As an etching solution, an aqueous solution of potassium borate and an aqueous solution of sodium hydroxide are used. It is easy to adjust the pH to 9 or more with an aqueous solution of sodium hydroxide using an aqueous solution of potassium borate as a buffer.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. 1 is a process flow chart of a manufacturing process of a SAW filter having a synchronous frequency = 215 [MHz] according to a manufacturing method of the present invention. FIG. 2 is a cross-sectional view of each of the main manufacturing steps in the manufacturing steps. Hereinafter, description will be made with reference to FIG.

A ZnO film 3 is formed by sputtering on a silicon substrate 4 having a 1 μm-thick oxide film formed on the surface [FIG. 2 (a)]. FIG. 3 is a schematic sectional view of a sputtering device for forming a ZnO film. The ZnO film is formed by a sputtering method using a ZnO sintered target as a sputtering target or a reactive sputtering method using a metal zinc (Zn) target to form a ZnO film while reacting with oxygen (O ₂ ). There is. Here, the latter reactive sputtering method was used.

The chamber 11 of the sputtering apparatus is connected to a vacuum pump (not shown), and is evacuated via a valve 17. Reference numeral 4 denotes a substrate to be sputtered, and 12 denotes a Zn target for sputtering. Ar and O ₂ were used as the sputtering gas 14. Reference numeral 15 denotes a radio frequency (RF) power supply for biasing the target. In order to form an insulator such as ZnO, a high frequency power supply is desirable.

Table 1 shows the conditions for forming the ZnO film. ZnO
The film thickness was 6 μm.

[0017]

[Table 1] The ZnO film 3 formed on the substrate 4 needs to have a c-axis oriented film in order to have piezoelectricity. However, the ZnO film formed under the above conditions has a good piezoelectric property with a dispersion σ of 6 ° or less. showed that.

Next, an Al film 6 serving as an electrode material is sputtered using the same sputtering apparatus shown in FIG. 3 (FIG. 3B). Although Al was used as the electrode material, an Al alloy may be used. Reference numeral 13 in FIG. 3 is an Al target for sputtering. Ar sputtering is used as the sputtering gas 14 for Al sputtering.
Was used. In addition, direct current (D
C) Although the power supply 16 is used, a high frequency power supply may be used.

Table 1 also shows the conditions for forming the electrode material. A
The thickness of the 1 film 6 was 1 μm. Here, the substrate temperature is set to 100
When the film is formed by heating from about 200 ° C. to about 200 ° C., the adhesion strength between the Al film 6 and the ZnO film 3 is improved. Since the ZnO film 3 and the electrode material can be formed by the same sputtering apparatus, continuous sputtering is possible. In the past, a vapor deposition apparatus was required to form an electrode pattern by lift-off, but in the manufacturing method of the present invention, it is possible to produce with the same sputtering apparatus, and not only the vapor deposition apparatus is unnecessary but also required for the process. Time has been greatly reduced.

Next, an electrode pattern of a comb-shaped electrode is formed by photolithography. Subsequently, a photoresist 7 [TSMR-890, manufactured by Tokyo Ohka Co., Ltd.] is formed on the Al film 6.
0] is applied and pre-baking is performed [FIG.
Next, exposure is performed using a photomask. By developing with a developing solution, a pattern of the photoresist 7 is formed [FIG.

Then, the electrode material is etched [FIG. Table 2 shows examples of the etching solution used at that time.

[0022]

[Table 2] This etchant is an example in which the pH is adjusted to 10. The sodium hydroxide solution may be reduced and the amount of water may be increased accordingly. If the sodium hydroxide solution is 21.30 ml or more, the pH is kept at 9 or more. By using an etchant adjusted to a range higher than pH 9 and lower than pH 13, only the Al film 6 can be etched without etching the ZnO film 3.

After etching of Al using this etching solution, the photoresist is peeled off with acetone and dried to complete the photolithography (FIG. 1F). After that, a device process is performed in which the substrate is chipped by, for example, dicing, mounted on a case, and leads are bonded. As another method of electrode etching, there is a reactive ion etching method using carbon tetrachloride (CCl ₄ ). However, as described above, the selectivity is not good, and the etching of the ZnO film occurs a little.
It is not suitable for producing a SAW filter. In the above wet etching, only Al can be easily etched without damaging the ZnO film.

As shown in FIG. 1, a ZnO film and an Al film were continuously formed by the same sputtering apparatus, and photolithography and etching could be simplified from four steps to three steps. . In addition, an intricate comb-shaped electrode pattern having a line width of 6 μm and an interval of 6 μm could be formed uniformly and with high precision. FIG. 5 shows the filter characteristics of a SAW filter using this process. The horizontal axis is the frequency, and the vertical axis is the signal attenuation rate. The SAW filter manufactured using this method is:
The filter characteristics were equal to or higher than those of the conventional SAW filter manufactured by the lift-off method. [Example 2] Next, a SAW convolver used as a key device in spread spectrum communication was manufactured. FIG. 6 shows a perspective view of the SAW convolver. A ZnO film 23, which is a piezoelectric material, is formed on a substrate 24, and an input side comb electrode 21, a reference signal comb electrode 28, and an output electrode 24 are formed on the surface thereof.

An input signal is input to the input side comb electrode 21 and a reference signal is input to the other reference signal comb electrode 28.
The SAW 25 excited by each signal propagates to the central output electrode 22, where it is spatially time-integrated and obtained as an output signal. By using this element on the transmission / reception side, data communication with high programmability becomes possible. S
Since the AW convolver is similar to the element structure of the SAW filter, it can be manufactured by the same manufacturing method as that of the SAW filter if the ZnO film thickness is optimized and the electrode pattern is changed.

A SAW convolver was prototyped in the same process as in the first embodiment. As a result, an output signal of −40 dBm was obtained with respect to the input signal of 0 dBm. This is almost the highest value of the SAW convolver obtained by the conventional manufacturing method,
Considering the reduction in the number of vapor deposition devices and the number of steps, the manufacturing method of the present invention has a great advantage.

[0027]

As described above, the present invention relates to a method of manufacturing a monolithic type SAW filter, in which a Z
An electrode pattern was formed by forming an nO film and Al or an Al alloy as an electrode material with the same sputtering apparatus, and performing etching with an etching solution whose pH was adjusted so that only the electrode material could be selectively etched. As a result, the number of deposition apparatuses conventionally required was reduced, and the number of steps and man-hours was greatly reduced, thereby enabling cost reduction. In addition, the adhesion strength is increased, and a highly accurate electrode can be formed even with a thick electrode film.

[Brief description of the drawings]

FIG. 1 is a process flow chart of a manufacturing process according to a manufacturing method of a monolithic SAW filter of the present invention.

FIG. 2 is a sectional view in the order of steps for explaining a method of manufacturing a SAW filter according to the present invention.

FIG. 3 is a schematic diagram of a sputtering apparatus.

FIG. 4 is a filter characteristic diagram of a SAW filter according to the manufacturing method of the present invention.

FIG. 5 is a perspective view of an example of a monolithic SAW filter.

FIG. 6 is a perspective view of an example of a SAW convolver.

FIG. 7 is a manufacturing process flow chart of a conventional monolithic SAW filter.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 input side comb electrode 2 output side comb electrode 3 ZnO film 4 substrate 5 SAW 6 Al film 7 photoresist 11 chamber 12 Zn target 13 Al target or Al alloy target 14 Ar gas and O ₂ gas 15 High frequency power supply 16 DC power supply 17 Vacuum Valve 21 Input side comb electrode 22 Output electrode 23 ZnO film 24 Substrate 25 SAW 28 Reference signal comb electrode

Claims (4)

[Claims] A zinc oxide film is deposited on a substrate, and an aluminum oxide film is formed on the zinc oxide film.
Alternatively, in a method of manufacturing a surface acoustic wave device having an electrode of an Al alloy, a zinc oxide film is deposited, and then a metal film for an electrode is deposited on the zinc oxide film by the same film forming apparatus. Production method. 2. A zinc oxide film is deposited on a substrate, and A
A method for manufacturing a surface acoustic wave device having a comb-shaped electrode made of l or Al alloy, wherein an etching solution for selectively etching only the electrode metal film is used. 3. The pH of the etching solution is from pH 9 to pH 13.
3. The method for manufacturing a surface acoustic wave device according to claim 2, wherein the surface acoustic wave device is controlled to fall within the range. 4. The method according to claim 3, wherein a potassium borate solution and a sodium hydroxide solution are used as the etching solution.