Improved inclined multi-lithography using water as exposure medium and its 3D mixing microchannel application

This paper presents the inclined UV lithography in water medium and 3D mixing microchannel application. The theoretical limit of the inclination angle of the structure is determined by the difference of the refractive index between air and SU-8. In order to reduce the mismatch of the refractive index between air and photoresist, DI water was used as exposure medium instead of air. As a result, the maximum angle of the inclined structure was improved from 38.7° (in air) to 56.2° (in water). Fine and high aspect ratio SU-8 structures were obtained using UV exposure in water medium. As a microfluidic application, a microchannel having 3D slanted grooves was designed and fabricated. 45° slanted grooves were formed successfully on the sidewalls of the microchannel by using inclined UV exposure in water medium. Mixing efficiency of this microchannel was evaluated by CFD simulation and cross-sectional fluorescence image detection using confocal microscope.