Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 136

FIGURE 4.10

Fabrication of covered channels with SU-8: (a) with embedded mask (after [62] ); (b) selective proton writing
(after [65] ); and (c) with sacrificial layer (after [63] ).

into the SU-8 layer and forms a thin polymerized layer ( Fig. 4.10 (b,2)). Next, the proton beam with
higher energy polymerizes the sidewalls of the channel. The high energy allows the beam to penetrate
through the SU-8 layer down to the substrate surface ( Fig. 4.10 (b,3)). In the final step, the exposed SU-
8 is developed, resulting in a covered microchannel ( Fig. 4.10 (b,4)).
Another method uses a sacrificial layer [63,64] to fabricate a closed SU-8 channel. The first SU-
8 layer is coated, exposed, and developed to form the bottom of the channel ( Fig. 4.10 (c1)).
Subsequently, a sacrificial structure is deposited and patterned ( Fig. 4.10 (c2)). The sacrificial
material can be thermoplastics, waxes, epoxies [63] , or positive photoresist [65] . Because of the
self-planarizing nature of an unexposed SU-8 film, the sidewalls and the channel ceiling are formed
with a single coating of a second SU-8 layer ( Fig. 4.10 (c3)). After developing the second layer, the
sacrificial material inside the channel is removed, leaving a closed SU-8 microchannel

( Fig. 4.10 (c4)).