Optical performance comparison between negative tone development and positive tone development

A negative tone development (NTD) process has been considered as apromising candidate for the smaller contact solution due to the remarkable image quality over a positive tone develop (PTD) process. However, it has not been investigated why NTD has higher optical performance than PTD yet. In this paper, image log slope (ILS) and mask error enhancement factor (MEEF) of binary and phase shift masks (PSM) are investigated with considering mask bias, target critical dimension (CD) and pattern pitch. It is found that the irradiance slope is steep and wafer CD variation from mask CD variation is small when the target CD is relatively smaller than pattern pitch. Mathematical model is derived to analyze image quality of binary mask and PSM.Three-dimensional mask effect is also considered with rigorous simulation.