Reconfigurable Microwave/Millimeter-Wave Devices Using Liquid Crystal Technology

Abstract

In recent years, microwave liquid crystal (LC) technology has attracted significant attention from researchers due to its tunability and low-loss characteristics, extending up to terahertz frequencies. However, existing state-of-the-art LC-based devices face limitations in fabrication processes, resulting in relatively large sizes compared to other available tunable technologies. This thesis seeks to address this challenge by proposing a fabrication process for chip-scale and miniaturized LC-integrated devices. To achieve this goal, an alignment method is adopted for silicon micromachined devices, along with a comprehensive fabrication process for chip capacitors, reflective loads, and reflective-type phase shifters. Additionally, a tunable waveguide filter is designed and fabricated based on a control mechanism using a static magnetic field. The design and fabrication of silicon-micromachined variable capacitors are presented, utilizing nematic LC technology: shunt and series capacitors with and without integrated bias lines. The LC material enables electronic control over its dielectric properties, offering versatility across a broad spectrum of RF reconfigurable applications that require analog tuning. Measurement and simulation results for the chip LC shunt variable capacitor reveal a measured quality factor ranging from 44 to 123 at 1 GHz. With a biasing control voltage from 0 V to 40 V, the fabricated micromachined capacitor demonstrates an 18% capacitance shift. The LC-based series capacitor, demonstrated with an integrated bias line, isolates voltage control from RF terminals, serving a pivotal role in devices where series capacitors are essential. With a 21% shift in capacitance and a quality factor of up to 45 at 1 GHz, the capacitor’s performance is evaluated comprehensively through measurement and simulation. LC-integrated series capacitors tailored for applications without isolating bias voltage from RF terminals are demonstrated, yielding a notable 24% shift in capacitance and achieving a quality factor of up to 105 at 1 GHz. The demonstrated series capacitors feature a 10 μm thin layer of LC material, contributing to lower control voltage requirements and faster response times. These devices are manufactured through an in-house multi-layer microfabrication process. The thesis introduces a tunable waveguide filter that integrates LC material within quartz glass tubes, actuated by a static magnetic field. This innovative filter demonstrates a 7% tuning range with minimal bandwidth variation. Experimental validation for a 7.5 GHz filter with a 2.5% bandwidth confirms the concept’s viability. The quality factor of this filter varies between 108 and 288 in the fabricated sample. Tuning of the filter is first demonstrated using both a pair of rotating magnets; then, a pair of coil magnets is used to eliminate moving parts. Finally, two monolithically integrated reflective loads and two reflective-type phase shifters are presented, employing LC material as a reconfigurable element. The LC material is confined within a micromachined space, and its dielectric properties are controlled through an applied bias voltage. The tunable reflective loads find applicability in RTPSs. Operating at frequencies of 28 GHz and 62 GHz, the reflective loads exhibit phase variations of 113◦ and 118◦, respectively, as the bias voltage ranges from 0 V to 25 V. The 28 GHz and 62 GHz devices demonstrate reflective insertion losses of 3.9 dB and 4.3 dB, respectively, indicating figures of merit of 29◦/dB and 27.5◦/dB, respectively. Employing tandem hybrids at the operating frequency alongside two identical reflective loads has led to two reflective-type phase shifters at 28 GHz and 62 GHz. While the phase shift remains the same as the corresponding reflective loads, the insertion loss increases due to the use of hybrids. The insertion loss is measured at 5.95 dB and 7 dB for the 28 GHz and 62 GHz samples, respectively. Fabrication of these devices is conducted in-house using a multi-layer microfabrication process. To the best of our knowledge, this marks the first time a fully silicon-made, chip-level LC integrated reflective load and RTPS phase shifter is presented.