Planarization and edge bead reduction of spin-coated polydimethylsiloxane

Films of polydimethylsiloxane (PDMS) can be obtained by spin coating for a range of applications in microengineering and microsystems. However, the spin coating of PDMS films in the 100-1000 mu m thickness range invariably results in the formation of non-negligible 'edge beads'. If not removed, such features could cause problems if the PDMS films were to form part of a multi-step planar process involving e.g. contact photolithography. Here, we study the reflow planarization and edge bead reduction of spin-coated PDMS films obtained in the spin-speed range (250-500 rpm). We provide an optimized technical solution for the planarization and associated edge bead removal-along with useful spin speed versus thickness data of PDMS films. Our approach takes into consideration the reflow planarization time and the polymerization time. We demonstrate the usefulness of our method by conducting photolithography on the planarized PDMS films by using a commercial spin-coated photoresist (AZ (R) 1518)-clearly showing that the photolithographic resolution is improved for planarized PDMS films compared to non-planarized PDMS films. Finally, the findings suggest that if a planarized PDMS film having a thickness of similar to 1000 mu m is required, a single very low spin speed coating followed by an enhanced reflow planarization is preferable to multiple higher spin speed coatings and planarization steps. The findings should be useful to those working to develop stretchable technologies using planar means who wish to integrate PDMS films into more complex systems.