Reduction of diffraction effect for fabrication of very high aspect ratio microchannels in SU-8 over large area by soft cushion technology

Edge bead due to spin coating has been found to cause an air gap as large as 53 m for a 99-m thick (measured at the wafer center) SU-8 coating over a 4-in. wafer. This caused poor mask width replication and non-uniformity of SU-8 pattern width. We have devised a process using a soft cushion technology to improve mask dimension replication and large-area pattern uniformity. A soft cushion was placed beneath the substrate to produce convex bending of the wafer in order to improve the contact between the mask and photoresist top surface during UV exposure. Dramatic improvements in pattern uniformity, from over 30% variation in SU-8 width across the wafer to less than 10%, and mask width replication, from 54% deviation from mask width to 20%, have been demonstrated. Numerically calculated increases in SU-8 width from the wafer center to the edge bead using the Fresnel diffraction equation match well with observed values. Further, employment of this technique with a narrow (10 m) dark-field mask enabled the fabrication over the entire surface of 4-in. diameter wafers of dense SU-8 gratings separated by microchannels with aspect ratio over 18.