hard bake

Hard bake is a high-temperature treatment that hardens photoresist after development, preparing it to withstand etch processes. **Temperature**: 100-150 degrees C typical. Higher than soft bake. **Purpose**: Cross-links resist, drives out remaining solvent, improves etch resistance, improves adhesion. **Timing**: After develop, before etch. Protection step for pattern. **CD change**: Some CD shrinkage may occur due to thermal flow. Process sensitive. **Duration**: Several minutes. May be convection oven or hot plate. **Process variations**: Some modern processes skip hard bake if resist is sufficiently stable. **UV cure**: Alternative to thermal hard bake. UV radiation cross-links resist surface. **Ion implant hardening**: For implant, very hard crust required to prevent resist popping during implant. Higher temperature or UV cure. **Reflow limitation**: Too high temperature causes resist reflow, rounding features. Stay below glass transition. **Etch selectivity**: Well-baked resist has better selectivity (slower etch rate in plasma) than poorly baked.