Patterning, electroplating and removal of SU-8 moulds by excimer laser micromachining

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Abstract
The ablation characteristics of the SU-8 photoresist (spun on Si wafers) under 248 KrF excimer pulsed laser radiation have been studied. The variation of etch rate with fluence has been investigated in the range 0.05-3.01 J cm-2. The threshold fluence for ablation of SU-8 is measured to be about 0.05 J cm-2. The etch rate of SU-8 is found to be higher than that of polyimide (previously reported) under similar conditions. We have investigated the effects of different prebake temperatures (90, 110, 120 and 200 °C) on ablation characteristics, which are found to be similar for all temperatures. The effect of increasing the number of laser shots (from 10 to 10 000) has been examined at different fluences in order to understand the etch-rate variation near the `end of film' stage of ablation. The results of our analysis using scanning electron microscopy, profilometry and optical microscopy reveal the very smooth morphology of the etched surfaces with no significant debris, no noticeable damage to underlying silicon and the gradual build-up of a carbonaceous film outside and around the etch pits. We find SU-8 very suitable for excimer ablation lithography and have demonstrated this by patterning a gear structure in an SU-8 resist layer with an aspect ratio of 4.5. For the first time, we have shown that the laser micromachining technique has the potential to cleanly remove SU-8 after electroplating a microstructure with copper.