Integration of BST varactors with surface acoustic wave device by film transfer technology for tunable RF filters

Abstract

This paper presents a film transfer process to integrate barium strontium titanate (BST) metal-insulator-metal (MIM) structures with surface acoustic wave (SAW) devices on a lithium niobate (LN) substrate. A high-quality BST film grown on a Si substrate above 650 • C was patterned into the MIM structures, and transferred to a LN substrate below 130 • C by Ar-plasma-activated Au-Au bonding and the Si lost wafer process. Simple test SAW devices with the transferred BST variable capacitors (VCs) were fabricated and characterized. The resonance frequency of a one-port SAW resonator with the VC connected in series changed from 999 to 1018 MHz, when a dc bias voltage of 3 V was applied to the VC. Although the observed frequency tuning range was smaller than expected due to the degradation of BST in the process, the experimental result demonstrated that a tunable SAW filter with the transferred BST VCs was feasible.

Wafer-level Packaging, Equipment Made in House, and Heterogeneous Integration

“…Tunable bandwidth SAW filters were fabricated by bonding variable capacitors to a LiNbO 3 wafer via selective transfer. (18) The variable capacitors are ferroelectric capacitors made of barium strontium titanate [Ba(SrTi)O 3 , BST], the capacitance of which can be controlled by applied voltage.…”

Piezoelectric acoustic wave devices based on heterogeneous integration technology

“…Although the basic operation of the tunable SAW filter was demonstrated, the MEMS varactors were much larger than SAW resonators. If the MEMS varactor is replaced with a barium strontium titanate (BST) varactor, the size decreases to a few thousandths or smaller, because the dielectric constant of BST is typically several hundreds without bias voltage [11]. BST has excellent voltage tunability of dielectric constant (several hundred % or higher) and low dielectric loss (tan δ) even in GHz range.…”

Structures and dielectric properties of multilayer BST/ST/BST and ST/BST/ST sandwich structures

“…As nonlinear ferroelectric materials, Ba 1-x Sr x TiO 3 (BST) films have been the most promising candidates for applications in radio frequency/microwave tunable devices including phase shifters, tunable filters, sensors, resonators, and varactors [1][2][3][4] since the dielectric constant (ε r ) of the materials can be adjusted with an applied electric field. However, it is difficult for pure BST films to simultaneously achieve the required dielectric properties, especially low dielectric loss and high tunability.…”