13. Tribo-mechanical Aspects in Micro-electro Mechanical Systems (MEMS)

Micro Electro-Mechanical System (MEMS) devices continue to find new applications in technology. Some of the successful MEMS are micro-reservoir, micro-pumps, cantilever, micro-pillars for holding the mirror devices in projectors, rotors, channels, valves, sensors etc. In majority of the cases, polycrystalline silicon is used as the structural material for MEMS fabrication because of the micro fabrication process knowledge acquired from the semi-conductor industry. Recently, polymer materials have also been used for MEMS fabrication. Some of the polymers used as structural material are acrylic (PMMA), PDMS and the epoxy-based SU-8. SU-8 is a negative photoresist which is UV curable and has excellent mechanical properties over other polymers. However, when compared to silicon, SU-8 is mechanically inferior. SU-8 has excellent thermal stability. Despite many processing advantages, the bulk mechanical and tribological properties of SU-8 are the main limitations in making it is a versatile MEMS material. Therefore, it is important to increase mechanical strength and toughness, and reduce friction and wear of SU-8 without affecting its micro-fabrication efficiency. Tribology of MEMS, in general, is important as these machines do contain moving parts with relative motion in contact with each other. Smaller parts have high surface area to volume ratio which increases the adhesion and friction forces between two surfaces. In MEMS, wear is a result of high friction.