Wafer-level heterointegration process of SAW devices and LSI

The direct integration of surface acoustic wave (SAW) devices on top oflarge-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 onpiezoelectric crystals with special cut angle, which have different coefficientsof thermal expansion (CTE) from that of Si. In this paper, wafer-bonding-basedintegration technology for a one-chip SAW oscillator is described. A SAWresonator with a resonant frequency of 420 MHz fabricated on 128 Y cut lithiumniobate (LN) was integrated with a BiCMOS oscillator circuit. The key point ofthe developed process is to avoid thermal expansion mismatch problem between LNand 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 Geinterlayer. The developed process can be basically applied to not only SAWdevices but also a variety of non-Si devices.