Development of High-Power Durable SAW Devices with Epitaxial Aluminum Electrodes

Osamu Nakagawara

[Recipient of the 2010 Richard M. Fulrath Award from the American Ceramic Society]

Surface acoustic wave (SAW) devices are characterized by their capability for compact designs, which has led to their wide installation in mobile devices as frequency filters. Recently, the use of SAW devices in duplexers (antenna branching filters that can perform both transmitting and receiving) , which are an important passive element of communication equipment, has been studied, but a significant technical obstacle has been improvement of the reliability (high-power durability) of the transmission filter where the large power is applied.

In GHz-level high-frequency bands, in particular, microscopic electrode patterning at the 0.5µm level is required for the electrodes, and these designs are susceptible to pattern breakage when power flows. To overcome these practical obstacles in SAW devices, the pattern strength was increased by making the aluminum electrode material into epitaxial film having atomic orientation, and this was successful in enabling a dramatic improvement in the power durability. In lifetime tests that index the power durability, SAW devices that use epitaxial film exhibited a power durability that was a phenomenal 106 times longer than conventional polycrystalline electrodes (equivalent to a durability of 280 years), and these were developed into the world's first W-CDMA SAW duplexer product.

This technology has an extremely high versatility, and currently, it is widely employed for SAW duplexers in Japan and overseas for multiple frequency bands in the third-generation communication standards (UMTS), and it is used throughout the global communications market. This technology provides a significant contribution to the industry by enabling compact and low-profile designs for communication devices, particularly cellular phones.

Also, the discovery of this new crystalline growth pattern has drawn wide recognition as a valuable academic contribution to the field of crystallography, and for this reason, was recognized with the Richard M. Fulrath Award by the American Ceramic Society. Established in 1978 in memory of the great work by the late Professor Richard M. Fulrath, who made tremendous contributions to technological exchanges between Japan and the United States in the ceramic industry fields, the Fulrath award is given to individuals who have made important contributions to the development of ceramic science and technology. In 2010, there were five American and Japanese awardees, and an award symposium and banquet were held at the George Brown Convention Center in Houston, Texas in October 2010. This technology is described in this paper.

Typically, aluminum is used for the electrode film for SAW excitation because of its low resistance and low specific gravity. One drawback of aluminum, however, is that stress migration [1] tends to occur due to the application of repeated stress from SAW propagation, and this results in poor power durability. Because the SAW electrode width is inversely proportional to the operating frequency, the problem of stress migration*1 has become even more noticeable with the greater use of microscopic patterns, which are found in the design of higher frequency devices.

Stress migration is caused by diffusion along the grain boundary primarily consisting of aluminum atoms. In the bulk, the activation energy values for aluminum self-diffusion for single crystals without grain boundaries and for polycrystals with many grain boundaries are 135.1 and 67.55 kJ/mol, respectively, indicating that aluminum atoms in single crystals require larger amounts of energy for diffusion (less susceptible to diffusion) compared to polycrystals. For example, the time required for 100 nm self-diffusion of an aluminum atom at 100°C for single crystals is estimated to be a 109-fold longer than the time for polycrystals with many grain boundaries, which indicates extremely slow atomic movement. This was the reasoning behind our conjecture that the resistance to stress migration could be improved by changing to an epitaxial*2 film where the electrodes have an extremely small amount of grain boundaries within the crystal.

film where the electrodes have an extremely small amount of grain boundaries within the crystal.θrotated Y-cut X-propagation LiNbO3 substrates (called "θrotated Y-X LN substrates" below), which provide superior SAW propagation characteristics, and titanium is used as the intermediate layer which serves as the buffer for reducing the lattice mismatch between aluminum and the substrate. Figure 1 shows a comparison of the pole figure [3] measurement results when using X-ray diffraction (XRD) *3 in the aluminum [200] incidence direction on an aluminum/titanium film and an aluminum single-layer film formed by vacuum evaporation on a LN substrate at θ=64°. Figure 1 (a) shows a ring-shaped diffraction pattern, indicating that [111], which is a close-packed plane in the substrate perpendicular direction, grows for the aluminum single-layer film, and a random polycrystalline film is formed within the [111] plane.

On the other hand, in Fig. 1 (b), clearly-defined six-fold symmetrical spots were found, which indicates that a three-axis oriented epitaxial aluminum film was grown that also had regularity in the [111] plane in addition to the substrate perpendicular direction. In terms of crystallography, aluminum has a face-centered cubic (fcc) structure, and because the [200] pole figure for single-crystal aluminum has a three-fold symmetrical spot, the epitaxial film in Fig. 1 (b) has two single-crystal domains. Because they share the same center of symmetry, this suggests that the growth direction of the aluminum [111] axis is the same, and it has a double domain structure that is rotated 180° within the plane.

Fig. 1

Fig. 1

In addition, particular attention must be paid to the center of symmetry of the six-fold symmetrical spot, that is, the eccentricity of approximately 26° in the aluminum [111] orientation. Because the angle formed by the Z-axis of the θrotated Y-X LN substrate and the line perpendicular to the substrate is 90°-θ, this can be interpreted as a distinctive crystalline growth pattern where the aluminum [111] plane grows with the Z plane of the LN substrate serving as the epitaxial plane. Figure 2 shows a model of the epitaxial aluminum directional relationship on a 64° LN substrate, and this indicates that the aluminum [111] direction and Z-axis of the LN substrate are aligned. Aluminum/titanium films were also formed with different cut angles on the LN substrate, namely, θ=70°and 90° (Z-cut) . As a result, as shown in Fig. 3 (a) and (b) , the angles of eccentricity are 20° and 0°, respectively, and the above-described relationship of aluminum [111] and LN [001] is satisfied.

Fig. 2

Fig. 2

Fig. 3

Fig. 3

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