Rendering SU-8 hydrophilic to facilitate use in micro channel fabrication

Abstract

A simple method is presented to reduce the contact angle of the photo-resist SU-8. A low contact angle is valuable in micro total analysis systems for the fabrication of micro channels where the capillary pressure is linearly related to the cosine of the contact angle, θ. If the surface of the channel is hydrophilic, the capillary pressure can be used as the only means to direct the liquid through the channels and the need for external pumps can be avoided. This is very useful, especially in the fabrication of devices for 'lab-on-a-chip' where it is important to keep the design as simple as possible. A commonly used technique for releasing structures fabricated on Si wafers is to use a sacrificial Cr layer. It is shown that the contact angle of SU-8 decreases by 40° after etching this layer. A further reduction in contact angle is desirable and can be achieved by treating the sample with ethanolamine at 50 °C for only 10 min. The resulting contact angle is 23° ± 7°. Using a wet chemical treatment, a selective change in contact angle between different areas of a micro channel system can be achieved, without the need to involve different materials in the fabrication process.