JPH07193450A - Surface acoustic wave element and manufacture of the same - Google Patents

Abstract

PURPOSE:To easily manufacture high-reliability surface acoustic wave(SAW) elements by thickening the film of a pad for wire bonding without increasing the depositing process of a metal film. CONSTITUTION:A single resist layer 2 is formed by applying novolak resin resist to a tetrabolic lithium monocrystal transparent base plate 1. A resist pattern 3 is formed by covering this resist with an opening glass mask in a prescribed shape, and an aluminum thin film 4c, aluminum thin film 4a to be an interdigital electrode and aluminum thin film 4b of the pad for wire bonding are attached and formed on it. The pattern 3 is dissolved away and while remaining the thin films 4a and 4b, the aluminum thin film 4c is peeled off as well. Then, single layers of resist 5a, 5b and 5c are applied on it and covered with the glass mask and after the other resist is removed later while leaving only the resist 5b, the thin film 4a to be the interdigital electrode is etched to the set film thickness. Finally, the resist 5b is dissolved away, and an element S provided with the metal electrode pattern 4d of the desired interdigital electrode and the pad 4b for wire bonding is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device having an electrode pattern, and more particularly to a method of manufacturing a surface acoustic wave device by a lift-off process.

[0002]

2. Description of the Related Art At present, the main part of a high frequency filter using surface acoustic waves applied to TV, VTR, mobile communication, etc. is, for example, a piezoelectric substrate, comb electrodes on the substrate, and wire bonding pads. It is composed of. Conventionally, single crystals such as lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) have been widely used as substrate materials, but these materials are not enough to satisfy both the insertion loss and temperature characteristics of the filter. There is no electricity, so recently
Lithium tetraborate (Li2B4O7) single crystals, which have a large mechanical coupling constant and a zero temperature coefficient, are attracting attention.

However, the Li2B4O7 single crystal is easily attacked by acid, and when manufacturing a high frequency filter using a surface acoustic wave device having this single crystal as a substrate, the electrodes of the high frequency filter are formed by patterning with an acidic etching solution. It is impossible to do. Therefore, conventionally, a method of patterning by a lift-off process that does not require chemical etching using acid has been proposed (eg, M. Hatzak
is et.al, IBM J.RES.DEVELOP VOL.24, NO.4, JULY 1980 p
452-460).

Generally, a material having good conductivity is used as a material for the comb-shaped electrode and the wire bonding pad. For example, in a high frequency filter of 250 MHz band, an aluminum thin film having a thickness of about 0.2 μm is suitable. Has been done. In the conventional lift-off process described above, a resist is deposited on a substrate, the resist is patterned, an aluminum thin film is deposited on the patterned resist and the substrate, and then the resist is eliminated to form an electrode. However, in order to prevent the deformation of the overhang shape formed when patterning the resist, the substrate cannot be heated to a high temperature when the aluminum thin film is deposited, and the adhesion strength of the aluminum thin film to the substrate is reduced and wire bonding cannot be performed. There is a problem.

In order to solve the above problems, in order to reinforce only the wire bonding pad portion with an aluminum thin film having a film thickness (for example, about 0.5 to 1 μm for a 250 MHz band high frequency filter), A method of stacking two aluminum thin films on each other has been proposed (for example, JP-A-3-209909).

This is the method shown in FIGS. 2 (a) to 2 (i). First, on the substrate 11 of the cleaned piezoelectric material (see FIG.
(A)) In consideration of bondability of wire bonds, an aluminum thin film 12 for forming a bonding pad having a desired thickness is formed by vacuum deposition (FIG. 2B). Next, a resist 13 is applied and formed on the aluminum thin film 12 (FIG. 2C), the aluminum thin film 12 and the resist 13 are patterned by photoetching (FIG. 2D), and the resist 13 is removed. The bonding pad 14 is formed (FIG. 2E). Next, the aluminum thin film 15 for comb-shaped electrodes is deposited and formed on the bonding pad 14 and the substrate 11 by vacuum vapor deposition (FIG. 2F).
Then, in the same manner as in FIGS. 2C to 2E, the resist 1
6 (FIG. 2 (g)), the aluminum thin film 15 is patterned by photoetching (FIG. 2 (h)), and the resist 16 is removed to form a desired comb-shaped electrode 17 (FIG. 2 (i)).

[0007]

However, in the above-mentioned conventional method, for example, the wire bonding pad having a film thickness of about 1 μm is formed first, but due to the step of 1 μm, the contact exposure at the fine processing of the comb-shaped electrode is performed. Is extremely non-uniform, which deteriorates the linearity of the comb-shaped electrodes that influence the device characteristics. Further, there is a problem that an extra step of forming and depositing an aluminum thin film is required.

Therefore, in order to solve the above-mentioned conventional problems, the present invention increases the surface acoustic wave device having a novel structure in which lithium tetraborate is used as a substrate and aluminum is used as a metal electrode, and the number of steps for depositing a metal thin film. It is an object of the present invention to provide an epoch-making method for manufacturing a surface acoustic wave device in which only the thickness of the wire bonding pad is increased.

[0009]

In order to solve the above problems, a surface acoustic wave device of the present invention comprises a metal electrode and a metal electrode comprising a single layer of aluminum or an alloy thereof on a substrate of lithium tetraborate. A feature is that a wire bonding pad thicker than the electrode is formed.

Further, the manufacturing method is characterized in that the metal electrode and the wire bonding pad are formed by the following steps. That is, (1) a step of depositing a metal film on a substrate of a piezoelectric body, (2) a step of forming a metal electrode region and a wire bonding pad by photolithography of the metal film, and (3) the wire bonding. After coating the pad for resist with a resist, the thickness of the metal film in the metal electrode region is reduced by etching to form a metal electrode.

[0011]

According to the present invention, it is possible to increase the film thickness of the wire bonding pad without increasing the number of additional steps for forming and depositing a metal thin film as in the conventional case. Further, for example, a metal film of aluminum or its alloy can be easily etched by a method using an organic alkaline developer, so that it can be applied to a material weak against acid such as lithium tetraborate (Li2B4O7) single crystal. As a result, defective wire bonding due to a decrease in the adhesion strength of the metal film to the substrate can be eliminated, and the manufacturing yield of the surface acoustic wave element can be improved. Further, even if the substrate cannot be heated to a high temperature at the time of vapor deposition of metal and the adhesion strength of the metal film is lowered by adopting the lift-off method, wire bonding failure does not occur by increasing the thickness of the wire bonding pad.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described in detail with reference to the drawings. A process of forming a fine pattern of a metal electrode (comb-shaped electrode in this embodiment) on the substrate 1 as shown in FIGS. 1A to 1I will be described. First, prepare a transparent substrate 1 of Li2B4O7 single crystal with a thickness of about 500 μm, and ultrasonically clean this with pure water, acetone, etc. (see FIG. 1).
(A)) A resist of novolac resin is applied to the substrate 1 by spin coating to form a single layer resist 2 having a thickness of about 1.5 μm (FIG. 1 (b)).

Next, the resist 2 is covered with a glass mask having openings of a predetermined shape (soda lime glass having a thickness of 2-3 mm and 5 inch square and patterned with chromium having a film thickness of about 0.86 μm). Ultraviolet light having a wavelength of about 405 nm is irradiated from above the glass mask, and thereafter, development is performed using an organic alkaline developer (eg, TMAH; tetramethylammonium hydroxide) to obtain a resist pattern 3 having a predetermined shape ( FIG. 1 (c)).

Then, an aluminum thin film 4c to be removed in a later step is formed on the resist pattern 3, an aluminum thin film 4a to be a comb-shaped electrode and an aluminum thin film 4b to be a wire bonding pad are formed on the substrate 1 by a vacuum deposition method to a thickness of about 1. Deposited on a single layer of 1.0 μm (Fig. 1 (d)),
The resist pattern 3 is dissolved and removed with acetone or the like, and the aluminum thin film 4b coated on the resist 3 is peeled off, leaving the aluminum thin films 4a and 4b to be the comb electrodes and the wire bonding pads (FIG. 1).
(E)).

Further, a resist is formed on the substrate 1 and the aluminum thin film 4a formed on the substrate 1 by spin coating to form a single layer resist 5a, 5b, 5c having a thickness of about 1.5 μm.
Is formed by coating (FIG. 1 (f)), and the resists 5a, 5b, 5c are covered with a glass mask having a predetermined pattern in the same manner as in the step shown in FIG. 1 (c), and ultraviolet light is irradiated from above. Then, the resist is developed with the organic alkaline developer described above (FIG. 1 (g)) to leave only the resist 5b on the wire bonding pad 4b, and to form the resist 5a on the aluminum thin film 4a to be the comb-shaped electrode and the substrate. After the resist 5c on 1 was removed by dissolution, the aluminum thin film 4a to be a comb-shaped electrode was etched to a set film thickness of 0.27 μm by utilizing the fact that aluminum is soluble in an alkaline solution (FIG. 1 (h)). Finally, the resist 5b is dissolved and removed by acetone, and a surface acoustic wave element S having a desired fine comb-shaped metal electrode pattern 4d and a wire bonding pad 4b is formed. Can be prepared (FIG. 1 (i)).

Here, the step between the wire bonding pad and the comb-shaped electrode is the thickness of the comb-shaped electrode (0.5 μm or more; 0.5
If the thickness of the wire bonding pad is poorer and the thickness of the wire bonding pad (about 1.0 to 1.5 μm) is taken into consideration, it is 0.
It is required to be 5 μm or more, preferably 0.5 to 1.0 μm.

As described above, it is possible to manufacture a surface acoustic wave device in which only the wire bonding pad has a large film thickness, and it is possible to perform wire bonding without increasing the number of steps for forming and depositing a metal thin film as in the conventional case. It is extremely easy to form the pad to be thicker than the metal electrode, and it is possible to perform reliable wire bonding.

In this embodiment, the method of using aluminum as the metal electrode and using the organic alkaline developing solution as the etching solution has been described. However, the etching solution is not limited to this, and sodium hydroxide may be used.
It is possible to use a (NaOH) aqueous solution, a potassium hydroxide (KOH) aqueous solution, or the like. Further, as the metal electrode, an alloy containing aluminum as a main component other than aluminum, for example, titanium
Any material that is soluble in an alkaline solution such as an aluminum alloy containing (Ti) or copper (Cu) may be used. Further, the metal electrode is not limited to the comb-shaped electrode, but may be applied to various well-known metal electrodes having various shapes, and may be appropriately modified and implemented without departing from the scope of the present invention.

[0019]

As described above, according to the surface acoustic wave device and the method for manufacturing the same of the present invention, the film thickness of the wire bonding pad can be increased without increasing the number of steps of vapor deposition of the metal film unlike the conventional case. Therefore, a highly reliable method of manufacturing a surface acoustic wave element can be provided very simply. In addition, for example, when the material of the metal electrode film is aluminum, it can be etched using an organic alkaline developing solution, and particularly, it can be sufficiently applied to a material that is weak against acid such as Li2B4O7 single crystal.

[Brief description of drawings]

1A to 1I are views showing a lift-off process of an embodiment according to the present invention.

2 (a) to (i) are diagrams showing a manufacturing process of a conventional surface acoustic wave device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Resist 4b ... Wire bonding pad 4d ... Metal electrode pattern (comb-shaped electrode) S ... Surface acoustic wave element

Claims (2)

[Claims] 1. A surface acoustic wave device comprising a lithium tetraborate substrate on which a metal electrode made of a single layer of aluminum or an alloy thereof and a wire bonding pad thicker than the metal electrode are formed. 2. A method of manufacturing a surface acoustic wave device, comprising forming a metal electrode and a wire bonding pad on a piezoelectric substrate, wherein the metal electrode and the wire bonding pad have the following (1) to (3). A method of manufacturing a surface acoustic wave device, which is characterized by being formed by a process. (1) A step of depositing a metal film on the substrate of the piezoelectric body. (2) A step of forming a metal electrode region and a wire bonding pad by photolithography on the metal film. (3) A step of coating the wire bonding pad with a resist and then thinning the metal film in the metal electrode region by etching to form a metal electrode.