The effects of a Si wafer cleaning method on the adhesion behavior of SU-8 microstructures were investigated. In particular, the role of adhesion energy was clearly demonstrated for fabricating high-aspect-ratio (HAR) SU-8 micropillar arrays with reduced contact area. The surface energy values of the Si wafers prepared through different cleaning techniques were calculated using contact angle measurements, revealing that the difference of adhesion energy was mainly determined by the presence of a SiO 2 layer. It was found that the adhesion energy of the SU-8 microstructure to the Si wafer was increased by approximately 72% through HF treatment that is required to remove the oxide layer. Furthermore, the dispersion component of the surface energy played a major role in the adhesion of SU-8 to various solid substrates. These results consistently indicate that thermodynamic adhesion energy needs to be high for the strong macroscopic adhesion strength of the SU-8 photoresist. Also, the calculated adhesion energy values were in good agreement with the physical measurements of macroscopic shear stress, suggesting that a simple thermodynamic approach used here can be used to examine the adhesion characteristics of the SU-8 photoresist. Using the optimized cleaning method, HAR SU micropillar arrays of 20 microm width and 300 microm height (AR = 15) were fabricated.
Hwang, K.-Y., C.-S. Park, J.-H. Kim, K.-Y. Suh, E.-C. Cho, N. Huh
Journal of Micromechanics and Microengineering