Wafer-level heterointegration process of SAW devices and LSI

The direct integration of surface acoustic wave (SAW) devices on top of large-scale integrated circuits (LSI) will enable multiband wireless front-ends, one-chip wireless systems, high-performance one-chip oscillators etc. However, this has been difficult, because SAW devices are often fabricated on piezoelectric crystals with special cut angle, which have different coefficients of thermal expansion (CTE) from that of Si. In this paper, wafer-bonding-based integration technology for a one-chip SAW oscillator is described. A SAW resonator with a resonant frequency of 420 MHz fabricated on 128 ° Y cut lithium niobate (LN) was integrated with a BiCMOS oscillator circuit. The key point of the developed process is to avoid thermal expansion mismatch problem between LN and Si. The SAW resonators were supported with a Si wafer in a half-diced form, and then bonded to a LSI wafer by Au-Au bonding. Before the Au-Au bonding, bonding surfaces were activated by Ar plasma to reduce bonding temperature. Finally, the Si support wafer was removed by the sacrificial etching of a Ge interlayer. The developed process can be basically applied to not only SAW devices but also a variety of non-Si devices.