Thin film transfer technology for tunable SAW filter using integrated ferroelectric varactors

A tunable RF bandpass filter with low insertion loss, sharp cut-off characteristics and small size is strongly required for advanced wireless systems. The frequency tuning of a ladder type filter is possible using variable capacitors (VCs) connected in parallel and/or series with surface acoustic wave (SAW) or bulk acoustic wave (BAW) resonators. VCs made of ferroelectric materials such as barium strontium titanate (BST) are preferable for this use due to high tuning ratio, small size and good linearity at GHz range. However, the direct deposition of the ferroelectric films on a SAW substrate such as lithium niobate (LN) is difficult. In this study, we have developed BST film transfer technology for the monolithic integration of SAW filters and BST VCs. A BST film deposited and patterned on a Si wafer was transferred to a LN wafer by Au-Au bonding and Si lost wafer process. The transferred BST worked as a VC, and the resonant frequency of a 1 GHz SAW resonator was shifted downward by 7 MHz by applying a DC bias voltage of 3 V to the VC. This is the clear evidence that the frequency tuning of a SAW filter is possible by the transferred BST VCs.