Plasma pretreatment to improve resist properties by reduction of resist flow during postbake

Abstract

Softening and subsequent flowing of resist during postexposure baking, used to promote resist adhesion, often results in reduced resolution and linewidth control. This is unacceptable for the fabrication of very fine feature VLSI devices since linewidth variations of 0.5–1.5 μm can be observed under certain baking conditions. Treatment of positive and negative resists with a nitrogen plasma prior to postbaking reduces resist flow significantly and enables linewidth integrity to be maintained. Minimal resist erosion occurs upon plasma treatment. In one example a series of 1.4-μm-diam windows were exposed and developed in 0.8-μm-thick HPR-204 positive photoresist. Samples treated with a 400 W unshielded nitrogen plasma for 10 min exhibited no change in feature width whereas the untreated control, baked as above, exhibited completely closed windows due to excessive resist flow. A model is proposed to account for the improved flow characteristics.