Fabrication development of microelectro mechanical switches with nanocrystalline graphite contact

Abstract

This study introduces a novel design and fabrication process for MEMS (Micro-Electro-Mechanical Systems) switches, focusing on improving performance and reliability, especially under various temperature conditions. The research presents a novel in-plane MEMS switch coupled with dual awl meander springs, achieving consistent low pull-in voltage across different temperatures and surpassing traditional transistor temperature limitations. A unique fabrication process for MEMS switches with nanocrystalline graphite contact was developed, simplifying fabrication and enhancing repeatability and stability. Nanocrystalline graphite (NCG) is deposited using plasma-enhanced chemical vapour deposition (PECVD), which has emerged as a promising material for enhancing the hot-cycling lifetime of switch contacts. However, bridging air gaps with the deposited NCG causes unwanted leakage. This project proposes a process with a single lithography step and a maskless directional etch step for fabricating in-plane MEM switches with NCG-coated sidewall contacts. A 600nm undoped NCG coating was deposited using PECVD on a double-clamped in-plane MEMS switch anchored by dual awl meander springs. The NCG coating was removed from the top surface of the Si layer using a directional reactive ion etch (RIE) in O2 plasma, leaving the NCG coating on the sidewall. The thesis includes a comprehensive analysis of mechanical structure optimization, the introduction of the indirect etching method, and an in-depth evaluation of mechanical and electrical reliability through cold and hot cycle electrical measurements. We demonstrated on-state times of $8s$ per cycle over 45 hot-switching cycles, with a pull-in voltage of 27.7V and a pull-out voltage of 5.9V. The ON current of the device was maintained at 1nA during the hot-switching cycles. We also demonstrated rapid switching behaviour over 7000 cold-switching cycles, with a stable pull-in voltage of 36V and a pull-out voltage of 13V. Fabricated on high-resistivity silicon devices, the NCG-coated switch maintained the current at a 1nA-level under high drain voltages, retaining stable high-operation voltage hot-switching cycles. This finding confirms the substantial potential of NCG coatings to provide excellent electrical performance in MEMS devices, offering valuable insights for commercial MEMS switch applications. A high-temperature measurement system was also implemented to evaluate the MEMS switch's mechanical and electrical performance under various temperature conditions. In tests reaching 125℃, the switch preserved good mechanical switching behaviour and maintained the same low pull-in voltage and 1nA ON current. When at 100℃, the switch endured multiple hot-switching cycles with the same pull-in voltage and same ON current at 25℃, proving the suitability of NCG coatings for long-term conductive state switching operations at high temperatures. The results confirm the novel switch design's significant potential for high-temperature resilience, stable mechanical switching behaviour, and excellent electrical performance. This shows that NCG can be deposited as a sidewall coating for in-plane MEM switches without a patterned etch. This research lays the groundwork for further exploration and potential commercial applications of MEMS switches, demonstrating their value in various technological domains, and has the potential as a switch contact material for hot-cycling applications that require relatively long on-state times.

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Identifiers

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

ORCID for Suan Pu: orcid.org/0000-0002-3335-8880

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