Tunable Microwave Devices Using BST( Barium Strontium Titanate)And Base Metal Electrodes

Abstract

Barium strontium titanate,(BST), a solid solution perovskite, is a potential candidate for integration into microwave devices. BST ferroelectric thin films are attractive for radio frequency and microwave applications due to its high figure of merit, thermal stability and ease of integration into microelectronic circuits. However, for many non-military uses, the high cost of conventionally processed ferroelectric thin film / BST based devices is a limiting factor. This high cost stems from single-crystalline sapphire, MgO, or LAO substrates, and Pt or Au metallization commonly used in microwave devices. Here we present a device process and materials complement offering a low cost alternative.

Planar interdigitated capacitors Ba[subscript 0.6]Sr[subscript 0.4]TiO₃ (BST) thin films with Cu top electrodes were fabricated on polycrystalline alumina substrates using a single step photolithographic technique and lift-off process. RF magnetron sputtering was used for fabrication of BST thin films while Cu thin films were thermally evaporated. The dielectric tunability of the IDCs was 40 % for an applied electric field of 12 V / &#181m, which corresponds to 3 µm electrode gap spacing and a 35 volt dc bias. Low frequency (1MHz) loss measurements reveal a dielectric Q (Quality factor) ~ 100 while a device Q of ~ 30 is obtained at 26 GHz. Leakage current measurements of the BST planar varactors show current densities of 1.0 x 10⁻⁶ A / cm² for an electric field of 10 V / µm. These dielectric characteristics (tunability and Q value) are comparable to numerous reports of IDCs with BST films prepared on expensive single crystalline substrates using noble metallization. As such, this technology is significantly less expensive, and amenable to large volume manufacturing.

A tunable 3rd order combline bandpass microwave filter based on BST thin films on polycrystalline alumina substrate and Cu electrodes was fabricated and characterized at room temperature. Fabrication was done using a single step photolithographic technique and metal lift off process. Tuning was achieved using a interdigitated varactor configuration (Cu / BST / Alumina).The center frequency of the filter was 1.85 GHz and was tuned to 2.05 GHz upon application of 125 V. The insertion loss was 4.5 dB at 0 V and this decreased to 3.5 dB at 125 V. The return loss was found to be better than 9 dB at all applied fields. In addition, the filter also exhibited low power consumption (< 6 &#181;W) and low intermodulation distortion (IP3 = 38 dBm).

A microwave phase shifter based on Cu transmission lines on BST thin film/alumina substrate was fabricated and tested. The X &#8212; band (8 - 12 GHz) phase shifter showed a phase shift of 18 degree for an applied bias of 130 V at 10 GHz and had an insertion loss of only 1.1 dB at zero bias at 10 GHz. The return loss was better than 19 dB for all bias states. This insertion loss is among the best reported to date for a microwave phase shifter. The initial phase shifter results look promising and it exhibits a figure of merit of 17 degree / dB.

In this work we report the fabrication, characterization, and process optimization for tunable microwave devices using low cost materials, simple and inexpensive processing routes entirely compatible with large volume manufacturing. This thesis represents the first comprehensive demonstration of integrated microwave devices using ceramic substrates and base metallization incorporating ferroelectric thin film technology at room temperature.