Surface roughness characterization of silicon and SU-8 microcomponents

Surface characterization of microcomponents provides key information to help understand and predict the performance of microdevices. In a microgear transmission, the surface roughness has a strong effect on the friction, running life and power consumption. In a static fluid microdevice, the liquid distribution is influenced by the surface tension and capillary force, which are primarily determined by the surface roughness. In a flowing microchannel case, surface roughness results in unsteady secondary flows. In this paper, a study is presented to characterize the surface roughness of silicon and SU-8 microcomponents. The silicon components studied are fabricated using STS ICP plasma etching system. The sidewall roughness of the component is measured using atomic force microscopy. Repeated measurements have been conducted at different sidewall depths of the microstructure. The AFM images of the measurements is given. The measurement results show that the sidewall is smoother at the lower level than that at the upper level in a Si microstrucutre, and the average roughness R_a obtained throughout the depth is 151.11 nm. The UltraThick SU-8 Process (UTSP) provides another way to fabricate microstructures as thick as 1 mm with a very vertical sidewall. The roughness contour of the sidewall shows that the surface topography is similar throughout the depth. The average roughness R_a is 46.46 nm. Other surface parameters, such as R_q, R_p-p, R_pk and R_sk, are also obtained and analysed. The implication of the smooth surface roughness of SU-8 structures to their applications is discussed in terms of transmission efficiency, the changes in friction to flowing liquid in a microchannel and the changes in the surface tension and capillary effect.