Optimized plasma-deposited fluorocarbon coating for dry release and passivation of thin SU-8 cantilevers

Plasma-deposited fluorocarbon coatings are introduced as a convenient method for the dry release of polymer structures. In this method, the passivation process in a deep reactive ion etch reactor was used to deposit hydrophobic fluorocarbon films. Standard photolithography with the negative epoxy-based photoresist SU-8 was used to fabricate polymer structures such as cantilevers and membranes on top of the nonadhesive release layer. The authors identify the plasma density as the main parameter determining the surface properties of the deposited fluorocarbon films. They show that by modifying the pressure during fluorocarbon deposition, the surface free energy of the coating can be tuned to allow for uniform wetting during spin coating of arbitrary thin SU-8 films. Further, they define an optimal pressure regime for the release of thin polymer structures at high yield. They demonstrate the successful release of SU-8 cantilevers and membranes with thicknesses down to 2.3 and 1.7 mu m respectively, which is a considerable improvement to what has been achieved by dry release of all-polymer structures to date. Furthermore, chemical reaction of the SU-8 with the fluorocarbon coating during processing leads to a considerable increase of the surface free energy on one side of the released cantilevers. This process-integrated back side passivation is interesting for the use of the devices in biosensing applications. (c) 2007 American Vacuum Society.