NEM Relay Scaling for Ultra-low Power Digital Logic

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. CMOS has been the building block for modern digital logic for decades and the performance and energy efficiency of CMOS continues to improve as technology develops, mainly through scaling. However due to the 60 mV/dec limit on MOS transistors, continue to reduce the power supply voltage would result in an increase in offstage leakage current that would eventually dominate and increase the energy per operation of a transistor. In order to overcome this barrier, mechanical switches are proposed. Mechanical relays has the benefit of no leakage current through the air gap in the off state, which potentially enables further scaling of power supply voltage that can surpass MOS transistors. Several groups have been able to demonstrate mechanical switches with no leakage and abrupt on-off switching characteristics, however both the sizes of switches (~10 4 µm 2) and the operation voltages (> 10 V) are huge, causing the switching energy to be significantly larger than MOS transistors. Scaling efforts are needed to minimize the switching energy of a mechanical switch. In this thesis, prototype relay devices demonstrated by previous studies are discussed and the key factors that need to be addressed in order to minimize the switching energy are pointed out. The minimum switching energy is found to be limited by the contact adhesive force between contacts; studies on prototype devices with different contact areas shows that van der Waals force between contacts are the main source of adhesion in the prototype mechanical relays. Experiments show that by adding surface coating materials with low Hamaker constant can lower the contact adhesive force. In order to lower the structural stiffness, process development of poly-SiGe is optimized through multiple post-deposition processing techniques for reduced stress gradient of 100 nm thin films, a 10X reduction from the prototype devices. The combined learning from adhesive force work and thin film processing leads to the experiments of fabricating scaled devices of 500X smaller footprint. The results show that scaled relays can potentially operate at low voltages (~2V), but …