New aqueous-alkali soluble polymers may even outperform novolacs in certain features. Standard novolacs generally melt in a range between 115–130°C. The thicker the resist film, the higher is the impact of this feature on the resist structure. Ridges with a height of 10 µm often simply melt away at 120°C. A high thermal stability could prevent this problem which particularly occurs during ion or plasma etching steps.
Investigations with an aqueous-alkaline developable copolymer of PMMA and polystyrene showed a high thermal resistance of these films. This polymer produced a chemically amplified negative-tone resist after addition of acidifies and aminic cross-linkers. First determinations of the thermal stability indicated no roundening of edges at a film thickness of 4 µm up to a temperature of 220°C. As to be expected for a chemically amplified resist, films are highly sensitive. In addition to a use as one-layer resist, also applications for two-layer systems are possible (see SX AR-N 4700/1).