Monolithically Integrated Phase Change Material GeTe-Based RF Components for Millimeter Wave Applications

Abstract

RF switches are the fundamental building blocks for realizing reconfigurable front-ends in communication devices. Currently available RF switches are dominated by semiconductor technology which, while performing adequately up to a few GHz, suffer from signal leakage issues at millimeter-wave (mmWave) frequencies. On the other hand, mechanical RF components provide exceptional RF performance and reliability, but are bulky and expensive, whereas switches based on microelectromechanical systems (MEMS) have reliability issues and require high actuation voltage. Therefore, there is a clear need to develop reliable miniature components in order to deliver cost-effective and superior RF performance for various applications at mmWave frequencies.

Chalcogenide phase change materials (PCMs) have been widely used in optical storage media and non-volatile memories. PCM especially germanium telluride (GeTe) exhibits more than five-orders of resistance change with the application of short nano- microsecond thermal pulses. PCM’s property of resistance change is exploited to develop highly miniaturized and latching (non-volatile) RF switches with negligible DC power consumption. RF PCM technology carries a potential to highly miniaturize and monolithically integrate complex reconfigurable microwave components.

This thesis reports the development of miniaturized and reliable PCM GeTe-based RF switch as a fundamental unit-cell for reconfigurable mmWave devices. RF switches that exhibit exceptional RF performance from DC to 67 GHz are developed using an optimized in-house eight layer microfabrication process. Design parameters of the switches and their impact on RF performance is investigated along with material characterization and optimization of GeTe thin films. High power handling and linearity of the switches has been experimentally investigated. The developed GeTe-based RF switches are cycled for more than 1 million times demonstrating high reliability. The non-volatility of the PCM switches has been validated by studying the variation in ON-state and OFF-state resistance over time.

Miniaturization of reconfigurable RF components requires dense integration of RF switches. PCM GeTe-based switch matrices utilizing RF switches as a unit-cell have been demonstrated for the first time. Compact single-port multiple-throw (SPNT) switches are developed in SP2T, SP3T, SP8T, and SP16T configurations. A monolithically integrated scalable four-port RF switch unit-cell is demonstrated with two operational states. A reconfigurable band rejection module is realized utilizing a scalable switch matrix. A broadband mmWave T-type RF switch with three operational states is demonstrated from DC to 67 GHz. An approach to monolithically cascade T-type switches for redundancy applications is used for the development of a 4×6 redundancy switch matrix. Multiple compact PCM GeTe-based scalable crossbar switch matrices are developed for mmWave applications. Crossbar switch matrices up to 16×16 are also developed for non-volatile low frequency signal routing applications.

This thesis reports first demonstration of various PCM-based reconfigurable RF components. Utilizing multi-port switches and switch matrices, RF components such as switched capacitor banks, reconfigurable switched variable attenuators, true-time-delay switched phase shifters and reflective type phase shifters are developed for mmWave applications. Broadband on-chip integrated resistors and matched terminations are developed. A technique to improve the self-resonance frequency of on-chip capacitors by design optimization has been discussed.

PCM-based switches allow extremely tight integration in reconfigurable RF circuits despite their requirement of higher than 725 °C for melt-quench switching action. Heat distribution and thermal cross-coupling (actuation crosstalk) in GeTe-based switches has been experimentally studied for the first time using transient thermal imaging. Intermediate non-volatile resistance states in GeTe are observed at cryogenic and room temperatures. This can be seen as an opportunity to improve the reliability of PCM GeTe-based devices at superconducting temperatures. Possibility of monolithic integration and miniaturization capabilities of PCM technology for reconfigurable RF components demonstrated that are in this doctoral research prove the vast potential of this technology for future wireless networks.