Effect of BST film thickness on the performance of tunable interdigital capacitors grown by MBE

摘要

Voltage-tunable, interdigital capacitors (IDCs) were fabricated on Ba_(0.29)Sr_(0.71)TiO_3 grown by hybrid molecular beam epitaxy (MBE). In this growth technique, we utilize the metal-organic precursor titanium tetraisopropoxide rather than solid-source Ti as with conventional MBE. Two samples of varying Ba_xSr_((1-X))TiO_3 (BST) thicknesses were fabricated and analyzed. High-quality, epitaxial Pt electrodes were deposited by sputtering from a high-purity Pt target at 825 ℃. The Pt electrodes were patterned and etched by argon ion milling, passivated with reactively sputtered SiO_2, and then metallized with lift-off Ti/Au. The fabricated devices consisted of two-port IDCs embedded in ground-signal-ground, coplanar waveguide (CPW) transmission lines to enable radio-frequency (RF) probing. The sample included open and thru de-embedding structures to remove pad and CPW parasitic impedances. Two-port RF scattering (S) parameters were measured from 100 MHz to 40 GHz while DC bias was stepped from 0 V to 100 V. The IDCs exhibit a high zero-bias radio-frequency (RF) quality factor (Q) approaching 200 at 1 GHz and better than 2.3:1 capacitance tuning for the 300-nm-thick sample. Differences in the Q(V) and C(V) response with varying thicknesses indicate that unknown higher order material phenomena are contributing to the loss and tuning characteristics of the material.