If contact of a substrate with water must be avoided during the patterning process, a two-component photoresist system composed of PMMA resist (bottom layer) and photoresist (top layer) offers an alternative to SX AR-N 4810/1.
Firstly, the PMMA layer is applied and tempered at 150 °C (if necessary, a temperature of 95 °C may also be sufficient). The photoresist layer is then deposited on top and dried as usual at approx. 95 °C, followed by exposure and aqueous-alkaline development of the photoresist structures. In this process, the PMMA protects the substrate against the aqueous solution. The through-developed PMMA can subsequently be removed using oxygen-plasma. Alternatively it is also possible to develop the PMMA layer with organic solvents (ethylbenzene). The solvent does not attack the photoresist, but effectively dissolves the PMMA. This alternative procedure is however only suited for larger structures (> 10 µm).
The PMMA layer should be as thin as possible (approx. 100 nm) in order to be able to complete the plasma etching step quickly. A film thickness of 0.5 – 2 µm is recommended for the photoresist. Suitable to be used is e.g. AR-P 672.02 as PMMA resist and AR-P 3540 as photoresist.