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

Figure 4 shows a transmission electron microscope (TEM) image with cross-sections of the aluminum, titanium, and 64° LN planes. This enables confirmation of clear lattice images for LN, titanium, and aluminum. Also, the aluminum/titanium and titanium/LN borders do not show a boundary layer, such as an amorphous layer, indicating that the epitaxial film grows continuously. The incidence (observation) direction of the aluminum film is [-1-12], and the aluminum [111] direction calculated from this nearly matches the direction of the lattice fringe shown in the figure. Also, the pitches in the lattice fringe of the titanium and LN substrate indicate that the planes parallel to the aluminum [111] plane are the titanium [001] plane and LN [001] plane. Furthermore, because the angle formed by the aluminum [111] plane and titanium [001] plane for the film and substrate boundary was 25° to 26°, it was possible to visually confirm the epitaxial growth for the Z plane of LN. The reasons for this type of directional relationship are described below.

Fig. 4

Fig. 4

The consistency of the crystalline lattice for the titanium [001] plane and aluminum [111] plane with respect to the Z plane of LN was considered. The lattice arrangement and interatomic spacing for each plane are shown in Fig. 5. The Z plane of LN, titanium [001] plane having a hexagonal close-packed (hcp) structure, and aluminum [111] plane having a face-centered cubic structure are all close-packed structures*4 and have similar atomic arrangements to those shown in the figure. Furthermore, when the interatomic distances are compared, titanium uses an intermediate value between the LN substrate and aluminum, and so this reduces the mismatch between LN and aluminum. It is thought that the reactivity properties of titanium are added to this lattice consistency for enabling epitaxial growth of metal film on an oxide single-crystalline substrate.

Fig. 5

Fig. 5

Next, the epitaxial aluminum film formed on 64° LN is used to manufacture the SAW duplexer, and its power durability was evaluated. Ladder-type filters were used for both the transmitting and receiving sides. Figure 6 shows a schematic diagram of the evaluation system. An acceleration test based on the input power and chip temperature was used for the evaluation. The power-on point was the frequency at the upper-frequency side where the insertion loss becomes 2.5 dB in an atmosphere of 85°C. The power durability evaluation results are shown in Fig. 7. The breakdown times are plotted for values calculated from actually-measured values using the temperature acceleration coefficient. For purposes of comparison, this is also shown together with the power durability for a polycrystalline aluminum electrode, which represents the conventional structure. The triaxial-oriented epitaxial electrode exhibited a breakdown time that was 106 times longer than a conventional electrode (equivalent to a durability of 280 years) , and this value is high enough to enable practical usage. Based on this technology, the world's first SAW duplexer for W-CDMA, which is the third-generation standards, was developed as a product in 2004.

And we successfully developed epitaxial electrodes on LiTaO3 substrates, which are generally used as the LN described in this paper and feature superior temperature characteristics. Currently, this is widely used in SAW duplexers in Japan and overseas for the 2GHz, 1.7GHz, and 800MHz bands for the third-generation communication standards (UMTS) , and more than three million units continue to be produced and sold per month (cumulative total of more than 100million units) .

Fig. 6

Fig. 6

Fig. 7

Fig. 7

The epitaxial aluminum obtained here does not show crystal grain boundaries within the field of view in Fig. 4, and the grain boundary is thought to exist in a very small degree at the boundary of grains formed with the double domain formation. The dramatic reduction in the crystal grain boundary due to epitaxial growth is thought to increase the activation energy for the system, resulting in the substantial boost in power durability. The Fulrath Award was presented in recognition of both the high practicability of this technology and the academic value of the discovery of this unique crystalline growth pattern.

Glossary

*1 Stress Migration: Stress migration is a phenomenon similar to electromigration that occurs in the aluminum thin film of integrated circuits. In stress migration, aluminum atoms move due to internal stress in piezoelectric substrates caused by the propagation of surface acoustic waves, which results in the formation of hillocks, voids, and other defects.
*2 Epitaxial: Epitaxial refers to growth on a crystal that maintains a fixed orientational relationship on the crystalline substrate which serves as the base. In most thin films, this forms a highly-oriented film where the atoms are extremely aligned.
*3 X-Ray Diffraction (XRD) Pole Figure:

X-Ray Diffraction (XRD) Pole Figure: This is an X-ray diffraction method for obtaining crystal in-plane orientation information. As shown in the figure, the X-ray and detector positions (θ-2θ) are fixed on a certain crystalline plane. The "aluminum [200] incidence direction" mentioned in this paper means that (θ-2θ) are fixed on a certain crystalline plane. The "aluminum [200] incidence direction" mentioned in this paper means that θ and 2θare aligned on the reflective surface of the aluminum [200] . This is rotated to φ=0~360°as needed by changing the tilt ψ of the crystalline plane. The stereographic projection of this series of scans is a pole figure.

X-Ray Diffraction (XRD) Pole Figure

*4 Face-Centered Cubic (fcc) Structure, Hexagonal Close-Packed (hcp) Structure:

The close-packed structures where atoms are the most tightly packed is the face-centered cubic (fcc) structure for a cubic crystal lattice and the hexagonal close-packed (hcp) structure for a hexagonal crystal lattice. The [111] plane of the face-centered cubic structure and the [001] plane of the hexagonal close-packed structure are both close-packed planes (shown as aluminum [111] and titanium [001] in the figure).

Face-Centered Cubic (fcc) Structure, Hexagonal Close-Packed (hcp) Structure

Original Paper

  1. "Epitaxially grown aluminum films with titanium intermediate layer on θ rotated Y-X LiNbO3 piezoelectric single crystal substrates." J. Crystal Growth 249 (2003) 497-501.
  2. "High power durable SAW antenna duplexers for W-CDMA with epitaxially grown aluminum electrodes." 2002 IEEE Ultrasonics Symposium Proc. (2002) 43-46.
  3. "High Power Durable SAW Filter with Epitaxial Aluminum Electrodes on 38.5°rotated Y-X LiTaO3 by Two-step Process Sequence in Titanium Intermediate Layer." 2003 IEEE Ultrasonics Symposium Proc. (2003) P2L-2.


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