Tribology of Su-8 Micro-Dot Patterns on Silicon Surface

Abstract

Micro-Electro-Mechanical Systems (MEMS) are miniaturized devices that are made from Si or SU-8 (an epoxy polymer) materials. As Si is a material compatible with Integrated-Circuit (IC) technology, it has been vastly employed in the fabrication of MEMS devices. SU-8 is an important negative photoresist material that has excellent thermal stability, making it well suited for permanently used applications. However, Si and SU-8 materials have several surfaces related tribological issues such as high friction, adhesion and wear during the sliding contacts. The reason for these behaviors is due to the nature of their brittleness. The tribological properties of SU-8 can be improved by some chemical/physical surface treatments (e.g. O2 plasma treatment coupled with an overcoat of perfluoropolyether). Hence, it is proposed that Si can be protected by a thin layer of SU-8 which has been surface treated. The wear life of Si can be further improved by a suitable pattern of SU-8 on Si substrate. The main objective of this thesis is to develop an alternative method to fabricate micro-dot patterns using polymer jet printing on Si substrate. Hence, in this thesis, we propose and investigate low friction and high wear-resistant SU-8 micro-dots pattern on Si surface. Tribological properties of optimized SU-8 patterns (micro-dots with varying pitch) on Si were evaluated using both ball-on-disc and flat-on-flat tribometers. Sliding tests on the patterns were conducted against a 2 mm diameter Si3N4 ball and SU-8 spin-coated 2 mm x 2 mm Si wafer at varying normal loads and constant sliding speeds. For the ball-on-disk configuration, it was observed that the pitch of the SU-8 pattern on Si substrate had a significant effect on the initial coefficient of friction and wear durability. Initial coefficient of friction studies have concluded that the SU-8 polymeric micro-dots improved the tribological properties by sharing the normal force and reducing the contact area. The optimized pitch length of 150 ?m has shown an initial coefficient of friction of 0.13 as compared to 0.6 for bare Si and 0.22 for SU-8 spin-coated film. For the wear durability test, ultra-thin layer of perfluoropolyether (PFPE) was over-coated onto SU-8 micro-dot specimens, and the optimized pitch (450 ?m) specimens have shown wear durability of more than 100,000 cycles at a normal load of 350 mN. Under the same conditions, bare Si over-coated with PFPE and spin-coated SU-8 film with PFPE over-coated would show wear lives of 2,500 cycles and 3,600 cycles respectively. Extensive experiments were conducted on flat-on-flat contact surfaces, a geometry likely to be encountered in actual microsystems. The pitch of the SU-8 pattern on Si substrate had significant effect on the wear durability. Ultra-thin layer of PFPE was over-coated onto SU-8 micro-dot patterned specimens for enhanced wear durability, and similarly, the optimized pitch length of 450 ?m has shown a wear life of more than 100,000 cycles at a normal load of 650 mN.