JP2002261571A - Surface acoustic wave device and method of manufacturing the same - Google Patents

Abstract

[PROBLEMS] To provide a highly reliable surface acoustic wave device capable of improving adhesion strength and preventing peeling after bonding a bump or a wire as much as possible, and a method for manufacturing the same. The purpose is to: SOLUTION: An excitation electrode 4 for exciting a surface acoustic wave and a single-layer pad electrode 5 connected to the excitation electrode 4 and being thicker than the excitation electrode 4 are provided on a piezoelectric substrate 1, respectively.
The surface acoustic wave device has a protective film 2 formed on the excitation electrode 4 and on the outer periphery of the pad electrode 5.

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 suitably used as a high-frequency element in a mobile communication device such as a cellular phone or a cellular phone, and a method of manufacturing the same.

[0002]

2. Description of the Related Art FIGS. 4A and 4B show an example of the structure of a conventional surface acoustic wave device. For simplicity, FIG.
In (a), illustration of the protective film 2 is omitted. FIG. 4B is a cross-sectional view taken along line A4-A4 in FIG.

As shown in FIGS. 4A and 4B, on a piezoelectric substrate 1 made of, for example, a lithium tantalate single crystal,
A comb-shaped excitation electrode 4 for exciting a surface acoustic wave, a pad electrode 3 connected thereto, and a protective film 2 made of an oxide or the like formed in a region excluding a part of the upper surface of the pad electrode 3. Thus, a surface acoustic wave device is configured.

However, when a wire or a bump is joined to the pad electrode 3, a large external force may be applied at the time of joining, and there has been a problem that the pad electrode 3 is easily separated from the piezoelectric substrate 1. That is, the pad electrode 3 is generally formed at the same thickness as the excitation electrode 4 at the same time.
For example, since the thickness is as thin as about 200 nm, there is a problem that peeling occurs due to an external force after the formation of the bumps or the like, and that wires or bumps are difficult to bond to the pad electrode 3.

The excitation electrode 4 and the pad electrode are generally A
Although the same material of 1 (aluminum) and an alloy mainly composed of Al is formed, Au (gold) is generally used for wires and bumps. As described above, since the material of the pad electrode 3 is different from the material of the wire and the bump, and the compatibility of the two materials is poor (for example, the wettability is poor), the bonding strength between the wire or the bump and the pad electrode 3 is low.

FIG. 3A, which is shown in the same manner as FIG.
As shown in FIG. 3B (FIG. 3B is a sectional view taken along line A3-A3 in FIG. 3A), the pad electrode 3 is replaced with the lower layer 3A in the same configuration as the surface acoustic wave device shown in FIG. It is also known that the pad electrode 3 has a two-layer structure composed of a pad electrode 3 and an upper layer 3B.

However, since the pad electrode 3 has a laminated structure, the surface thereof is oxidized when, for example, the first lower layer 3A of the pad electrode is exposed to the air during the manufacturing process, whereby the lower layer 3A and the two lower layers are formed. There is a problem that the bonding strength with the upper layer 3B of the eye is reduced, and the adhesion strength at the interface between the two is reduced. In particular, Al or Al is formed on the second upper layer 3B.
When a material mainly composed of Au is selected, the bonding strength between Al or a material mainly composed of Al and the Au bump is reduced.

On the other hand, if Au or a material containing Au as a main component is selected for the upper layer 3 B, the pad electrode 3 and the excitation electrode 2
And the bonding strength with the metal becomes weak.

When manufacturing such a surface acoustic wave device, (1) a step of cleaning the surface of the piezoelectric substrate 1;
(2) a step of forming a photoresist in a desired shape by photolithography; (3) a step of etching the region of the lower layer 3A of the excitation electrode 4 and the pad electrode 3; and (4) a step of etching the excitation electrode 4 and the pad electrode. 3, a step of forming a photoresist in a desired shape by photolithography, a step of removing a region of the electrode pad 3B of the protection film 2 by etching, (7) a step of forming the upper layer 3B of the pad electrode 3 and (8) a step of forming the upper layer 3B of the pad electrode 3 by lift-off.

Here, in order to reduce the peeling of the lower layer 3B of the pad electrode 3, it is conceivable to improve the adhesion strength between the piezoelectric substrate 1 and the lower layer 3B. For this purpose, the surface of the piezoelectric substrate 1 in the step (1) is cleaned by UV irradiation, O ₂ plasma treatment, removal of organic substances and the like by an acid solution,
There is known a method in which the surface is activated by reverse sputtering of Ar, O ₂ or the like to increase the adhesion strength.

However, prior to the step (7), the first lower layer 3 where the electrode pad 3A is exposed to the atmosphere is formed.
There is a problem that the surface of A is oxidized, and the bonding with the bump and the bonding with the pad electrode of the second layer are weakened.

Accordingly, the present invention solves the above-mentioned conventional problems, improves the adhesion strength, and prevents peeling after bonding of bumps and wires as much as possible. And a method for producing the same.

[0013]

In order to solve the above-mentioned problems, a surface acoustic wave device according to the present invention comprises an excitation electrode for exciting a surface acoustic wave on a piezoelectric substrate, and an excitation electrode connected to the excitation electrode. A thicker single-layer pad electrode is provided, and a protective film is formed on the excitation electrode and on the outer periphery of the pad electrode. Here, in particular, the excitation electrode and the pad electrode are made of different conductive materials.

Further, the method of manufacturing a surface acoustic wave device according to the present invention includes a step of forming an excitation electrode for exciting a surface acoustic wave on a piezoelectric substrate, and a step of laminating a protective film on a region including the excitation electrode. Laminating a photoresist film on a predetermined region excluding a pad electrode forming region on the protective film, and removing a pad electrode forming region of the protective film to expose a part of the piezoelectric substrate and a part of the excitation electrode; Performing a surface cleaning process by removing deposits on the surface of the exposed piezoelectric substrate; and forming a pad electrode on the photoresist film, on the exposed piezoelectric substrate, and on the exposed excitation electrode. The method includes a step of laminating materials and a step of forming a pad electrode by removing the photoresist film.

In particular, the surface cleaning of the piezoelectric substrate is performed by ultraviolet irradiation, gas phase reaction, or etching. Here, for the gas phase reaction, for example, plasma using oxygen, Ar (argon) or oxygen (O ₂ ) is used.
Is performed by reverse sputtering using The etching is performed by an acidic solution such as hydrofluoric acid, nitric acid or phosphoric acid.

According to the present invention, since the excitation electrode and the pad electrode are made of different materials, the material of the pad electrode suitable for the bonding wire and the bump can be selected, and the bonding strength of the wire and the bump can be increased. it can. Further, by increasing the thickness of the pad electrode irrespective of the thickness of the excitation electrode, the strength against external force is improved, and peeling is less likely to occur. Further, since the pad electrode is a single layer, the interposition of the oxide layer can be prevented.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a surface acoustic wave device and a method of manufacturing the same according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an example of the structure of a surface acoustic wave device according to the present invention.
(A) and (b) show. Note that, for simplicity, FIG.
, The illustration of the protective film 2 is omitted. FIG.
FIG. 2B is a sectional view taken along line A1-A1 in FIG.

As shown in FIG. 1, the surface acoustic wave device of the present invention has a thickness of about 0.35 mm made of, for example, a single crystal of lithium tantalate, a single crystal of lithium tetraborate, or a single crystal having a langasite-type crystal structure. A on the piezoelectric substrate 1
a plurality of excitation electrodes 4 of about 150 to 500 nm in thickness of a single layer or a plurality of layers,
A ladder-type circuit configuration is provided by arranging and connecting, and further connecting a single-layer pad electrode 5 formed thicker to each excitation electrode 4. The pad electrode 5 is provided on the piezoelectric substrate 1.
(In the outer peripheral portion of the pad electrode 5), a protective film 2 such as silicon oxide or silicon nitride is formed to a thickness of about 10 to 100 nm.

Here, each of the excitation electrodes 4 is constituted so that a pair of comb-shaped electrodes are engaged with each other, and an example in which reflector electrodes are disposed at both ends located in the propagation direction of the surface acoustic wave. Although shown, the reflector electrode may not be formed in some cases. For convenience of explanation, the excitation electrode 4 is referred to as an excitation electrode including the wiring connected to each comb-shaped electrode.

Here, the material forming the pad electrode 5 is a conductive material (particularly a metal material) different from Al generally used for the excitation electrode 4 or an alloy mainly composed of Al, that is, Au.
By selecting the same material as the bump or the bonding wire using the solder or the solder, both materials are well adapted, so that the bonding strength with the bump or the bonding wire can be increased.

The film thickness of the excitation electrode of the surface acoustic wave device is, for example, about 200 nm in a surface acoustic wave filter or the like used in the 1.9 GHz band. However, with this thickness, the strength at the time of forming the bump is not sufficient, so that the bump is peeled off or the bump cannot be joined. So, like this:
By making the pad electrode 5 thicker than the excitation electrode 4 and having the optimum thickness, the bonding of the bumps can be facilitated.

In general, the excitation electrode of the surface acoustic wave filter is made of Al or an alloy containing Al as a main component, but may be made of other Cu or Ag in the future. Also in that case, in the structure of the present invention, since most of the pad electrodes are formed on the piezoelectric substrate, it is not necessary to consider the problem that the adhesion strength between different types of films is weak. Can be selected.

The method of manufacturing the surface acoustic wave device according to the present invention will be described in detail with reference to the process charts shown in FIGS.

First, as shown in FIG. 2A, an excitation electrode material is formed on a piezoelectric substrate 1 to a predetermined thickness by a thin film forming method such as a vapor deposition method or a sputtering method.

Next, as shown in FIG. 2B, after the excitation electrode 4 having a predetermined pattern shape is formed by photolithography, RIE, or the like, a region including the excitation electrode 4, ie, the piezoelectric substrate 1 and on the excitation electrode 4
The protective film 12 is laminated on almost the entire surface of the piezoelectric substrate 1 by a thin film forming method such as the D method.

Next, as shown in FIG.
The photoresist film 6 is laminated on a predetermined area on the second electrode 2 excluding the pad electrode formation area 12a.

Next, as shown in FIG. 2D, the pad electrode formation region 12a of the protective film 12 in FIG. 2C is removed by photolithography, RIE, or the like. A part (surface 1a) and a part of the excitation electrode 4 (wiring end 4a) are exposed, and a protective film 2 having a predetermined pattern is formed.

Next, as shown in FIG. 2 (e), a surface cleaning treatment is performed by removing the deposits such as the resist which have adhered to the exposed surface of the piezoelectric substrate 1. Here, this surface cleaning treatment is performed by ultraviolet irradiation, gas phase reaction, or etching.

Next, as shown in FIG. 2 (f), the sputtering is performed on the photoresist film 6, on the exposed surface 1a of the piezoelectric substrate 1, and on the exposed wiring end 4a of the excitation electrode 4. The pad electrode material 15 is laminated on almost the entire surface of the piezoelectric substrate 1 by using a thin film forming method such as a deposition method or a vapor deposition method.

Then, as shown in FIG. 2G, the photoresist film 6 is removed by a lift-off method, and a pad electrode 5 having a predetermined shape is formed to manufacture a surface acoustic wave device.

Here, before the pad electrode 5 is formed, the surface of the piezoelectric substrate 1 on which the pad electrode 5 is formed is cleaned, so that the adhesion strength between the pad electrode 5 and the piezoelectric substrate 1 is increased. Peeling or the like at the time of joining the wires can be prevented as much as possible.

Similarly, the surface cleaning treatment may be performed by UV irradiation, gas phase reaction (plasma treatment with O 2, or reaction with reverse sputtered particles of Ar or O 2), thin hydrofluoric acid, nitric acid, phosphoric acid or other acid. A similar effect can be obtained by etching and cleaning the surface of the piezoelectric substrate 1 with a solution.

[0034]

Next, an embodiment of the present invention will be described.

On a 0.35 mm-thick piezoelectric substrate made of lithium tantalate single crystal, an Al—Cu (1 wt%) alloy was formed to a thickness of about 200 nm using a DC sputtering device (1.9 GHz). 200 nm is suitable for a band SAW filter).

Next, a photoresist film was coated using a spin coater, and exposed by a reduction projection exposure machine (stepper).

The pattern of the excitation electrode is a ladder type 1.9G.
The circuit configuration is suitable for the Hz band.

After exposure, development and RIE were performed to form an excitation electrode pattern having a desired shape.

Thereafter, a protective film made of SiO ₂ is formed by CVD.
Film formation was performed at 300 ° C. using an apparatus. Note that SiO ₂ is preferable as the material of the protective film. This is because SiO ₂ has a small coefficient of thermal expansion, so that the expansion and contraction of the substrate is suppressed, and the SAW
This is because the temperature characteristics of the filter are improved. The preferred thickness of the protective film is 15 nm to 100 nm.
The reason is that if it is thinner than 15 nm, the insulating effect of the protective film is lost, and if it is thicker than 100 nm, the insertion loss, which is an electrical characteristic of the filter, becomes 5 dB or more, and the characteristics are deteriorated.

Thereafter, photolithography is performed again,
The photoresist was developed into the shape of the pad electrode and removed.

Next, dry etching was performed on SiO ₂ with O ₂ and CF ₄ gas.

At this time, a part of the surface of the lithium tantalate single crystal substrate and a part of the excitation electrode were exposed in a pad electrode shape.

Next, UV irradiation was performed for 10 minutes to remove organic substances such as resist on the surface of the substrate and to clean the substrate. Then, an Au film was formed to a thickness of 800 nm by an evaporation method. Here, the material of the pad electrode is preferably the same as the material of the bump to be added later, and when the material of the bump or wire is Au,
Au was selected and formed. The thickness is 800 nm
It is preferable that it is above. This is because the thicker the thinner the thinner the film is, the harder it is torn, and thus the higher the resistance to pulling after forming the bump or wire. The photoresist film and the Au film formed thereon were peeled in a peeling solution at 90 ° C. to form a pad electrode.

Next, Au bumps were formed on these pad electrodes. This joining state was good.

Thus, according to the present embodiment, as a result of measuring the die shear strength, a very high value of 1.0 N or more was obtained, while the conventional structure was 0.5 N or less.

[0046]

As described above, according to the surface acoustic wave device and the method of manufacturing the same of the present invention, the thickness of the pad electrode is arbitrarily increased as compared with the prior art, so that it is resistant to peeling and has high reliability. An excellent surface acoustic wave device can be provided. Also,
Since the pad electrode portion is a single layer and does not have a laminated structure, it is possible to provide a highly reliable surface acoustic wave device which does not peel off from the laminated interface as in the related art.

Further, by making the material of the pad electrode a material suitable for bonding with the bump or the bonding wire, the bonding of the bump or the wire can be surely and firmly performed, and it is highly reliable and is not easily peeled off. Surface acoustic wave device can be provided.

When the strength against the peeling of the pad electrode and the bonding strength of the bumps and the bonding wires are sufficient, the pad electrode can be made thin, and an excellent surface acoustic wave capable of saving film forming time and material can be obtained. Equipment can be provided.

Conventionally, when a pad electrode made of a material different from that of the excitation electrode is used, it is necessary to provide an intermediate layer.
In the present invention, such consideration is completely unnecessary, and since a single-layer pad electrode is provided directly on the piezoelectric substrate, the adhesion strength to the piezoelectric substrate and the bumps and wires can be increased even if any material is selected. An excellent surface acoustic wave device with high reliability can be provided.

Further, by performing the surface cleaning treatment of the piezoelectric substrate,
It is possible to provide a highly reliable surface acoustic wave device that can improve the adhesion strength between the piezoelectric substrate and the pad electrode and can prevent peeling after bonding of bumps and wires as much as possible.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams schematically illustrating an embodiment of a surface acoustic wave device according to the present invention, wherein FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line A1-A1 of FIG.

FIGS. 2A to 2G are cross-sectional views schematically illustrating a manufacturing process of the surface acoustic wave device according to the present invention.

3A and 3B are diagrams schematically illustrating a conventional surface acoustic wave device, wherein FIG. 3A is a plan view, and FIG. 3B is a cross-sectional view taken along line A3-A3 of FIG.

4A and 4B are diagrams schematically illustrating another conventional surface acoustic wave device, wherein FIG. 4A is a plan view, and FIG. 4B is A4-A of FIG.
FIG. 4 is a sectional view taken along line 4.

[Explanation of symbols]

 1: piezoelectric substrate 2, 12: protective film 3: pad electrode 4: excitation electrode 5: pad electrode

Claims (4)

[Claims] An excitation electrode for exciting a surface acoustic wave and a single-layer pad electrode connected to the excitation electrode and being thicker than the excitation electrode are provided on a piezoelectric substrate, respectively. A surface acoustic wave device comprising a protective film formed on an outer peripheral portion of the pad electrode. 2. An excitation electrode for exciting a surface acoustic wave and a single-layer pad electrode made of a material different from the excitation electrode connected to the excitation electrode are provided on a piezoelectric substrate, respectively. A surface acoustic wave device comprising a protective film formed on the pad electrode and on the outer periphery of the pad electrode. 3. A step of forming an excitation electrode for exciting a surface acoustic wave on a piezoelectric substrate, a step of laminating a protective film on a region including the excitation electrode, and a step of forming a pad electrode formation region on the protective film. Laminating a photoresist film on a predetermined region except for removing the pad electrode forming region of the protective film to expose a part of the piezoelectric substrate and a part of the excitation electrode; and Performing a surface cleaning treatment of the substrate, and on the photoresist film, on the exposed piezoelectric substrate, and on the exposed excitation electrode,
3. The method for manufacturing a surface acoustic wave device according to claim 1, further comprising: a step of laminating a pad electrode material; and a step of removing the photoresist film to form a pad electrode. 4. The method for manufacturing a surface acoustic wave device according to claim 3, wherein the surface cleaning of the piezoelectric substrate is performed by ultraviolet irradiation, gas phase reaction, or etching.