A reduced intermodulation distortion tunable ferroelectric capacitor-architecture and demonstration

A ferroelectric tunable capacitor device architecture is presented that allows for a reduction of intermodulation distortion (IMD), while maintaining high tunability at low bias voltages. The tunable capacitor is fabricated from epitaxial thin-film barium-strontium-titanate deposited on a sapphire substrate. The RF portion of the capacitor is a conventional planar gap capacitor with a 12-14-/spl mu/m gap. However, rather than superimposing the dc bias on the RF pads, a separate bias structure is fabricated within the RF gap. The interdigital bias structure has narrowly spaced high resistance (2-3/spl times/10/sup 4/ /spl Omega//sq) oxide conductor electrodes, such as indium-tin-oxide electrodes or lanthanum-strontium-cobalt-oxide electrodes spaced 1-2 /spl mu/m apart. The high resistivity of the bias electrodes decouples the dc bias from the RF signal path. This bias structure allows high dc fields to be developed with less than 30 V applied to tune the material permittivity (1 : 1.4), but is sufficiently resistive to avoid affecting the Q factor of the RF capacitor. Since the RF gap is wide, the IMD performance remains good, even at modest tuning voltages. The following three classes of gap capacitor have been fabricated for concept verification: 1) a conventional gap structure (without additional bias structure); 2) the proposed RF gap capacitor with the dc-bias structure; and 3) a narrower conventional RF gap-capacitor structure used as an IMD reference. The proposed RF gap capacitor with dc-bias structure has been fabricated in two versions: one in which the highly resistive bias electrodes are electrically connected to the RF electrodes (the attached-bias-electrode (ABE) scheme) and one in which the highly resistive electrodes are provided with a separate port for further control (the isolated-bias-electrode (IBE) scheme). In addition, parallel and perpendicular orientation of the bias electrodes relative to the RF field is investigated. The frequency response of the proposed gap capacitor with the dc-bias structure is characterized and its analysis shows that the highly resistive bias lines are serving as a dc-bias path for high tunability, but are not attenuating the RF signal. While the IBE structure has more degrees of freedom for biasing as compared to the ABE structure, the overall tunability at 30 V and IMD performance of both the ABE and IBE structures are similar. Two-tone IMD tests show that the IMD performance for the gap capacitor with the bias structure is improved by 6 dB over the conventional reference structure at the same tunability.