Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-10-26
2002-06-18
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06407486
ABSTRACT:
TECHNICAL FIELD
This invention relates to a surface acoustic wave device and more particularly, to a surface acoustic wave device having an electrode film with improved power durability.
BACKGROUND ART
Surface acoustic wave (SAW) devices, typically SAW filters and SAW resonators are prevalently utilized instead of dielectric filters as RF filters in mobile communication equipment such as mobile phones and cordless phones. The reason is that the SAW devices, especially SAW filters have a smaller size than the dielectric filters and when devices of the identical size are compared, the former have better electrical characteristics.
The SAW device includes at least a piezoelectric substrate, a comb-shaped electrode pattern in the form of a metal film formed on a surface of the piezoelectric substrate, and a package accommodating the piezoelectric substrate and the electrode pattern. As the piezoelectric substrate, lithium niobate, lithium tantalate and quartz crystal are used. Especially for RF band filters, lithium niobate and lithium tantalate having a large electro-mechanical coupling constant are often used. Aluminum or the like is used as the electrode pattern.
FIG. 5
illustrates one customary process for preparing a conventional SAW device. First, a piezo-electric substrate
50
is cleaned in step (a). In step (b), a metal film
51
as electrode material is deposited on the piezoelectric substrate
50
as by evaporation or sputtering. A photoresist is applied onto the metal film
51
, for example, by spin coating. As shown in step (c), the photoresist is exposed in a desired pattern in an aligner and developed, obtaining a photoresist pattern
52
. Thereafter, in step (d), the metal film is processed into a desired electrode pattern
53
by wet etching or dry etching. In step (e), the photoresist used for patterning is removed by using a liquid remover or ashing. At this point, the early process known as photolithographic process is completed. Thereafter, in step (f), the piezoelectric substrate having the electrode pattern formed thereon is cut into plural chips by dicing. Then in step (g), each diced chip is attached to a package with an adhesive. The next step (h) is wiring with bonding wire. Finally, a lid is welded to the package in step (i) in order to ensure air tightness. Step (j) is to inspect properties, with which the late process is completed. Recently, for the purposes of achieving further miniaturization and removing the residual inductance associated with wire bonding, a face-down connection method of facing the SAW chip to the package and accomplishing connection with metal bumps is sometimes used instead of the steps (g) and (h).
The SAW devices for use in the RF band of approximately 1 GHz have the problem of a short lifetime because the width of fingers and the distance between fingers of the comb-shaped electrode are as narrow as 1 micron. The lifetime of SAW devices mainly depends on the power durability of electrode film. As the electrode material in SAW devices, aluminum (Al) is originally used because of its low specific gravity and low electric resistance. However, when aluminum is applied to the electrode film, it is an important task to increase the power durability of the electrode film to comply with the increasing frequency of SAW devices. During the operation of SAW devices, repetitive stresses in proportion to the frequency are applied to the electrode film on the piezoelectric substrate. The repetitive stresses applied to the electrode film invite migration of aluminum atoms, which causes defects such as hillocks and voids to generate in the electrode film, significantly degrading the properties of SAW devices. This degradation phenomenon of the electrode film becomes more outstanding as the operating frequency becomes higher and as the supplied power becomes higher. It is necessary from the design requirement to reduce the thickness of the electrode film and narrow the width of electrode fingers as the frequency becomes higher. With these factors combined, the electrode film is more susceptible to defects as the frequency becomes higher. That is, a drop of the power durability is a serious problem.
As the means for improving the degradation phenomenon of the electrode film due to migration of aluminum atoms, J. I. Latham et al. disclosed to add a minor amount of a hetero metal such as copper (Cu) to aluminum to form an aluminum-copper (Al—Cu) alloy (Thin Solid Films, 64, pp. 9-15, 1979). The alloying of aluminum suppresses the generation of hillocks and voids in the electrode film and improves the power durability of SAW devices.
Many other examples of adding a minor amount of a hetero metal to aluminum to form an aluminum alloy for improving the power durability of electrodes are disclosed in JP-B 7-107967 (Al—Ti alloy), Japanese Patent No. 2,555,072 (Al—Ge alloy), JP-A 64-80113 (Al—Cu—Mg alloy), and JP-A 1-128607 (Al—Zn alloy). In these cases, by adding a minor amount of a hetero metal to aluminum, the migration of aluminum atoms is suppressed to thereby restrain the electrode from degradation. The addition of a hetero metal to aluminum, however, is not regarded preferable because this necessarily leads to an increase of the electric resistance of the electrode film, resulting in a SAW device having an increased loss.
It is believed that the diffusion rate of aluminum atoms is higher along grain boundaries than within grains, that is, grain boundary diffusion is preferential. It is thus supposed that the migration of aluminum atoms induced by repetitive stresses in SAW devices is predominant along grain boundaries, and this is already pointed out in the prior art. It is, therefore, expected that the power durability would be significantly improved if grain boundaries can be eliminated from the aluminum electrode film or substantially reduced, that is, if an aluminum electrode film can be made approximately single crystal. Since one factor of introducing an electric resistance in the electrode film is scattering of electrons at grain boundaries, the elimination of grain boundaries is preferable in that the electric resistance is reduced and hence, the loss of SAW devices is reduced.
It is already disclosed in JP-A 55-49014 that a substantially single crystal electrode material is applied to the electrode film of SAW device. Allegedly, the use of a substantially single crystal electrode material can enhance the performance of the SAW device independent of the type of material of which the SAW device is constructed. The publication describes that a molecular beam epitaxy method is recommended as the method for forming such an electrode film. The publication is silent about what electrode material is deposited on what substrate material under what conditions to form a film, but merely discloses the general discussion that a performance improvement of SAW device is expectable from the use of the above-described single crystal electrode film. It is unclear how much Q value and aging properties are improved. Additionally stated, the molecular beam epitaxy method has problems against the low cost manufacture of SAW devices including an expensive system and a slow film deposition rate.
An illustrative example of applying a single crystal or crystallographically unidirectionally oriented aluminum film to the electrode film of SAW device is disclosed in Japanese Patent No. 2,545,983. In this patent, by using a rotated Y cut quartz crystal substrate in the range of 25 degree rotated Y cut to 39 degree rotated Y cut as the piezoelectric substrate, and effecting evaporation at a high rate (deposition rate 40 Å/sec) and a low temperature (substrate temperature 80° C.), there is obtained a (311) oriented film, which is allegedly an epitaxially grown film close to single crystal. Although it is likely in the case of low temperature evaporation that the adhesion between the underlying quartz crystal substrate and the aluminum electrode film (that is, film bond strength) becomes a problem, the patent indicates that a very thin Ti or Cr film is formed
Kimura Noritoshi
Nakano Masahiro
Sato Katsuo
Dougherty Thomas M.
TDK Corporation
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