Monolithic Integration of BST Thin Film Varactors and Au Electroplated Thick Film Inductors above IC

Abstract

This paper presents an integration process of barium strontium titanate (BST) thin film varactors and Au thick film inductors above an integrated circuit (IC) by film transfer technology and Au electroplating process. A high-quality BST film grown on a Si substrate at 650°C was patterned into MIM (Metal-Insulator-Metal) structures, and transferred to an IC substrate at 270°C by BCB (Benzocyclobutene) polymer bonding and Si lost wafer process. Thick film inductors fabricated by Au electroplating were also integrated above the IC substrate to realize high Q factor. The capacitance tunability of the BST varactors did not decrease by transfer process. The Q factor of the fabricated inductor was higher than that of conventional inductors on IC chips. The resonant frequency of an LC series resonance circuit composed of the transferred BST varactor and the Au electroplated inductor changed from 0.76 GHz to 1.19 GHz by applying a DC bias voltage of 8 V to the BST varactor. Monolithic integration technology developed in this research will be useful for tunable radio frequency circuits like tunable power amplifiers.