Surface acoustic wave device, and its fabrication process

Details Surface acoustic wave device, and its fabrication process Surface acoustic wave device, and its fabrication process

1999-05-24

2001-11-13

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S31300R, C310S364000

Reexamination Certificate

active

06316860

ABSTRACT:

ART FIELD
The present invention relates generally to a surface acoustic wave device, and its fabrication process, and more specifically to a surface acoustic wave device comprising a electrode film having improved power-durability, and its fabrication process.
BACKGROUND ART
Surface acoustic wave devices, i.e., surface acoustic wave filters or surface acoustic wave resonators are now increasingly used as an alternative to dielectric filters for RF-band filters used with mobile communications equipment such as portable telephones and cordless telephones. One reason for this is that a surface acoustic wave device, especially a surface acoustic wave filter is smaller in size than a dielectric filter. Another reason is that the surface acoustic wave filter is superior in electrical performance to the dielectric filter, if they are of the same size.
A surface acoustic wave device is made up of, at least, a piezoelectric substrate, a comb form of metal film electrode pattern formed on the surface of the piezoelectric substrate, and a package for housing both the piezoelectric substrate and the electrode pattern therein. Lithium niobate, lithium tantalate, quartz, etc. are used for the piezoelectric substrate. Lithium niobate, and lithium tantalate having a large electromechanical coupling coefficient are used especially for RF-band filters. Aluminum, etc. are used for the electrode pattern.
FIG. 13
illustrates a general process sequence of steps of fabricating a prior art surface acoustic wave device. At a step (b), a metal film
51
for an electrode material is first formed as by vapor deposition or sputtering on a piezoelectric substrate
50
pre-washed at a step (a). Following this, a photoresist is coated on the metal film
51
as by spin coating. Then, the photoresist is exposed to light according to a desired pattern using an exposure system, and developed to obtain a photoresist pattern
52
, as depicted at a step (c). Thereafter, the metal film is etched, either wet or dry, at a step (c) into a desired electrode pattern
53
. The photoresist used for pattern formation is removed at a step (e) using a stripping solution or by means of an ashing process. Thus, the pre-process called a photo-process finishes. After this, the piezoelectric substrate with the electrode pattern formed thereon is diced at a step (f) into individual chips. Then, each chip is fixed at a step (g) to a package using an bonding agent, after which bonding wires are interconnected at a step (h). Finally, a lid is welded at a step (i) to the package for ensuring airtightness, followed by performance inspection at a step (j). Thus, the so-called post-process comes to an end.
A problem with the surface acoustic wave device when used at an RF-band of about 1 GHz is that the lifetime becomes short because the electrode finger width of the comb electrode and the space between electrode fingers become as fine as about 1 &mgr;m. A key determinant of the lifetime of the surface acoustic wave device is the power-durability of the electrode film. At the beginning, aluminum or Al was used for the reasons of its small specific gravity, and its low electric resistance. However, a problem with using aluminum for the electrode film is that the degradation of the electrode film becomes more pronounced as the applied frequency becomes higher. When the surface acoustic wave device is in operation, repetitive stress proportional to frequency is applied on the electrode film on the piezoelectric substrate. The repetitive stress applied on the electrode film gives rise to migration of aluminum atoms, which in turn causes electrode film defects such as hillocks, and voids, resulting in some considerable degradation of the performance of the surface acoustic wave device. The degradation of the electrode film becomes more pronounced as the applied frequency becomes higher and the applied power becomes larger. In view of design consideration, the higher the frequency, the thinner the electrode film and the narrower the electrode width should be. Because of these and other factors, the electrode film is more likely to have defects as the applied frequency become higher. In other words, the power-durability of the surface acoustic wave device becomes low.
As an approach to reducing or preventing the degradation of the electrode film due to the migration of aluminum atoms, J. I. Latham et al have disclosed the use of an aluminum-copper (Al—Cu) alloy obtained by adding to aluminum a trace amount of a different type metal such as copper (Cu) (Thin Solid Films, 64, pp. 9-15, 1979), and showed that by use of such an aluminum alloy it is possible to prevent occurrence of hillocks or voids on the electrode film and so improve the power-durability of a surface acoustic wave device.
Other examples of improving the power-durability of electrodes by using aluminum alloys obtained by adding to aluminum trace amounts of different types of metals are disclosed in many publications inclusive of JP-B 7-107967 (Al—Ti alloy), U.S. Pat. No. 2,555,072 (Al—Ge alloy), and JP-A's 64-80113 (Al—Cu—Mg alloy) and 1-128607 (Al—Zn alloy). In any case, a trace amount of different metal is added to aluminum, so that the migration of aluminum atoms is inhibited, thereby preventing degradation of the electrode. However, the addition of a different metal to aluminum is not preferable for the reason that an inevitable increase in the electric resistance of the electrode film result in an increased loss in the surface acoustic wave device.
Incidentally, it is considered that the rate of diffusion of aluminum atoms is faster at grain boundaries than in crystal grains; that is, grain boundary diffusion occurs preferentially. It is thus believed that the migration of aluminum atoms due to repetitive stress applied on a surface acoustic wave device occurs predominantly at grain boundaries, as pointed out so far in the art.
FIG. 10
is a scanning electron microscope photograph showing how an aluminum electrode film is degraded due to repetitive stress applied on a surface acoustic wave device. The fact that voids are found at grain boundaries of the aluminum electrode film supports that the migration of aluminum atoms occurs predominately at grain boundaries.
Accordingly, if grain boundaries are removed out of an aluminum electrode film or if grain boundaries are substantially reduced, that is, if an aluminum electrode film close to a single crystal is achieved, great power-durability improvements are then expectable. As known in the art, one factor of the electric resistance of a electrode film is a scattering of electrons due to grain boundaries. In this regard, too, removal of grain boundaries is preferable because, in the absence of them, there is an electric resistance decrease and, hence, the loss in a surface acoustic wave device decreases.
Application of a substantial single-crystal form of material to an electrode film for a surface acoustic wave device has already been disclosed in JP-A 55-49014. The publication alleges that by use of an electrode material that is substantially a single crystal, it is possible to enhance the performance of a surface acoustic wave device, whatever material is used to make up the device. The publication shows that molecular beam epitaxy is preferred to obtain such an electrode film. However, the publication provides no disclosure of what is used for the substrate material, and how film-forming conditions are determined depending on the electrode material used. That is, what is disclosed therein is nothing else than general consideration to the effect that an improvement in the performance of a surface acoustic wave device is expectable by use of a single-crystal electrode film. Thus, the publication provides no illustrative disclosure of to what degree the Q value, and the aging properties are improved. To add to this, there are several problems in fabricating surface acoustic wave devices at low costs by molecular beam epitaxy. For instance, costly equipment is needed, and the film-forming rate is slow.
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