Candidate: Tejinder Singh
Title: Miniaturized Phase Change Material Based Switches and Switch Matrices for Millimeter Wave Applications
Date: July 17, 2019
Time: 4:30 PM
Place: EIT 3145
Supervisor(s): Mansour, Raafat R.
Microwave switching networks, the major building blocks in wireless communication systems and satellite payloads, are used for signal routing and provide system redundancy. In addition of allowing flexible interconnections between various ports and channels. Switches have the ability to optimize bandwidth allocation. Mechanical, semiconductor and MEMS (microelectromechanical systems) are the prominent candidates for realizing microwave switch matrices. Mechanical switches have the advantage of providing exceptional RF performance but are bulky and expensive. On the other hand, semiconductor switches produce poor RF performance, have leakage issues and are highly non-linear. MEMS-based switches outperform the semiconductor based in terms of optimal RF performance, unfortunately, these switches have reliability issues and need high dc voltage to operate.
Current state-of-the-art microwave switches lack the performance, size or cost in order to enable the flexibility at large scale as seen in modern system-on-chips (SoC). Chalcogenide phase change materials (PCMs), which have been used widely in optical data storage, and in digital non-volatile memory devices, take advantage of the resistivity change of material with the application of short thermal pulses. The use of PCMs for RF and microwave applications has been exploited only over the past five years. PCM technology has the potential to highly miniaturize complex microwave switch matrices while offering adequate RF performance for millimeter-wave applications. In addition, PCM-based switches are latching switches, they consume very little dc power allowing the realization of highly miniaturized switch matrices.
The objective of the research described in this thesis is to investigate the feasibility of RF PCM technology to develop multi-port RF switches and switch matrices. Most of the papers reported in literature are focused on single-pole single-throw (SPST) switch configurations. The focus of the thesis is to: (a) develop and optimize an in-house fabrication process for the development of PCM based RF switches; (b) develop multi-port PCM based RF switches and switch matrices, and (c) demonstrate possibility of monolithically integrate RF-MEMS technology with PCM from reliability standpoint.
Micro-fabrication process development for PCM based devices involves characterization, optimization and fabrication of PCM germanium telluride (GeTe) based RF switches. It also involves investigating the materials’ aspect and design parameters of the switches and their impact on the RF performance. Surface properties of GeTe thin-films are investigated through atomic force microscopy (AFM), scanning electron microscopy (SEM) and cross-wafer resistance mapping measurements. Optimized GeTe thin-films exhibit over five orders of resistance change. Various GeTe switch design constraints are studied via cross-sectioning of the fabricated device using focused ion beam (FIB)-SEM. Current-carrying capacity and resistance of micro-heaters is extracted using electrical characterization. The RF performance of the PCM switches is optimized using diverse design parameters and characterization of PCM thin films. Methods to reduce parasitic elements in PCM switches are discussed. The RF performance of the optimized PCM based switch is simulated and measured demonstrating better than 0.4 dB of insertion loss and a return loss better than 20 dB from DC to 67 GHz.
Thermoreflectance based transient thermal imaging technique is applied to study the heat distribution, hot spots and thermal cross-coupling in PCM based switches and switch matrices for the first time. Device failure analysis is reported from materials’ degradation and possible methods are reported to minimize such failures. A miniaturized scalable four-port monolithically integrated unit-cell is developed with two operational states with primary applications in the realization of multi-band switchable band reject filters.
For the first time, monolithically integrated RF-PCM GeTe based T-type switch is demonstrated for millimeter wave applications and an approach to monolithically implement T-type switch as a switching unit cell for millimeter-wave redundancy switch matrix applications is implemented. A 4 x 6 PCM-based redundancy switch matrix is developed by monolithically integrating four T-type switches in cascade configuration. Various monolithically integrated multi-port PCM based RF devices have been developed, including single-pole multi-throw (SPNT) switches, C-type, R-type switches, 6-bit and 8-bit switched capacitor bank, switched attenuators, impedance matching networks, and switch matrices.
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