Suppression of Acoustic Resonances in BST-Based Bulk-Ceramic Varactors by Addition of Magnesium Borate

Prannoy Agrawal,Nicole Bohn,Kevin Häuser,Daniel Kienemund,Stipo Matic,Thomas Fink,Rolf Jakoby,Joachim R Binder,Mike Abrecht,Walter Bigler,Dominik Walk,Holger Maune

doi:10.3390/cryst11070786

Abstract

This work presents a method for reducing acoustic resonances in ferroelectric barium strontium titanate (BST)-based bulk ceramic varactors, which are capable of operation in high-power matching circuits. Two versions of parallel-plate varactors are manufactured here: one with pure BST and one with 10 vol-% magnesium borate, Mg3B2O6 (MBO). Each varactor includes a 0.85-mm-thick ferroelectric layer. Acoustic resonances that are present in the pure BST varactor are strongly suppressed in the BST-MBO varactor and, hence, the Q-factor is increased over a wide frequency range by the addition of small amounts of a low-dielectric-constant (LDK) MBO. Although the tunability is reduced due to the presence of non-tunable MBO, the increased Q-factor extends the varactor’s availability for low-loss and high-power applications.

Highlights

As minimum structure sizes reach atomic scale boundaries, the focus is on improving the manufacturing processes of integrated circuits
A notable presence of acoustics in pure barium strontium titanate (BST) varactor can be observed at 13.2 MHz, 16.4 MHz and 22.9 MHz, which decreases the Q-factor with increasing bias voltages
Due to the simplification with no dielectric losses, the simulation model cannot extract the absolute magnitudes of extracted equivalent series resistance (ESR) and the comparison can only be made for the relative changes

Summary

Introduction

As minimum structure sizes reach atomic scale boundaries, the focus is on improving the manufacturing processes of integrated circuits. The semiconductor varactor diodes suffer from a low Q-factor, low linearity and poor power handling [3,4], and the PIN-diode switched capacitor banks lack continuous tuning, as they provide discrete capacitances only [3]. BST-based varactors possess a high breakdown field strength in the kV/mm region These varactors can be based on three types of technology—thin-film, thick-film and bulk-ceramic. The main drawback of BST, as a solid-state dielectric, is its relatively high dielectric loss, which limits the varactor’s extensive use [9,10] Another disadvantage is the electric-field-induced electrostrictive strain across the ferroelectric layer, which results in acoustic waves that are reflected from adjacent layers, reducing the varactor’s Q-factor. Going forward, acoustic resonances methodology and a method for their suppression in BST-based bulkceramic varactor will be discussed using the composite ceramic approach

Theory

Simulative Analysis

Varactor Design and Characterization

Evaluation

Conclusions