Candidate: Hassan Kianmehr
Title: Reconfigurable Microwave and Millimeter-Wave Devices Using Liquid Crystal Technology
Date: June 14, 2024
Time: 2:30 PM
Place: EIT 3142
Supervisor(s): Mansour, Raafat
Abstract:
In recent years, microwave liquid crystal (LC) 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 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, utilizing either a pair of permanent magnets or a pair of coil magnets.
The design and fabrication of three silicon-micromachined variable capacitors is presented utilizing nematic LC technology: a shunt capacitor, a series capacitor with an integrated bias line, and another series capacitor. Enclosed within a thin micromachined housing, the LC material enables electronic control over its dielectric properties, offering versatility across a broad spectrum of RF reconfigurable applications requiring analog tuning. Measurement and simulation outcomes for the chip LC shunt variable capacitor reveal a measured quality factor ranging from 44 to 123 at 1 GHz. With a biasing voltage adjustable from 0 V to 40 V, the fabricated micromachined capacitor demonstrates an 18% capacitance shift. The LC-based series capacitor, featuring 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. Another LC-integrated series capacitor, tailored for applications necessitating a series capacitor without isolating bias voltage from RF terminals, assumes a crucial role. Demonstrating a notable 24% shift in capacitance and achieving a quality factor of up to 105 at 1 GHz, the capacitor's performance undergoes comprehensive evaluation through measurement and simulation. Unlike the shunt capacitor, the two 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. A single tuning element, employing only one pair of magnets, reduces tuning complexity and enhances reliability. Alternatively, tuning using a pair of coil magnets could eliminate moving parts in the filter. The filter concept holds promise for higher frequencies like mm-Wave, where spectrum usage efficiency relies on reconfigurability.
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 a phase variation of 114 and 118 , respectively, as the bias voltage ranges from 0V to 25V. 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.