PDF/月刊誌論文/code:pg_0411_03 マテリアル インテグレーション 2004年11月号
ELECTRONIC CERAMICS 機能性単結晶の最近の進展

PyroFreeTM Lithium Tantalate (LiTaO3) for Surface Acoustic Waves Applications
■著者
Silicon Light Machines Gisele Foulon

■要約
Lithium Tantalate (LT) wafers are widely used for surface acoustic waves (SAW) devices such as RF filters and duplexers for mobile communications. Over the past years, the trend in mobile communication has been towards multi-functional and high-functional terminals, where miniaturization and modularization are becoming a must. SAW filters have had of course to follow this trend, facing more and more challenging size and weight requirements. Silicon Light Machines, has focused on two areas to solve these challenges: preventing Lithium Tantalate wafers from sparking during processing, thus enabling SAW manufacturers to use thinner substrates to make the filters and we also have invented the first, to our knowledge, packaging technology at wafer level that yields the smallest possible form factor for SAW filters with the lowest profile. This paper focuses on our PyroFree LT technology. Standard, untreated Lithium Tantalate wafers are pyroelectric and build up an electrical charge, leading to discharging and arcing, during thermal cycling that normally occurs during SAW manufacturing processes. This sparking damages SAW electrode structures or even breaks the LT wafer. This phenomenon is due to a combination of the high pyroelectric constant of LT and a low electrical conductivity. A way to dissipate the charge faster to avoid arcing is to increase the conductivity of the material. It has been shown in Lithium Niobate (LN) that the reduced state of the crystal exhibits higher conductivity. However, because of much slower ionic diffusion in the LT crystal, the reduction processes that can be applied for LN are inefficient for LT. Silicon Light Machines developed the PyroFree process to increase the conductivity of Lithium Tantalate in such a way that the material becomes free of pyroelectric charging, under any temperature cycle. The properties of this improved material are discussed in this paper, in terms of surface charging, bulk conductivity and SAW properties. A systematic comparison with "standard" Lithium Tantalate will be presented. PyroFree LiTaO3 shows an increase in bulk conductivity up to 5 orders of magnitude compared to unprocessed LT, while the SAW properties remain very similar to both materials.