Resist freezing process for double exposure lithography

In this study, we have developed a thermal freezing process to prevent intermixing between 1st patterned positive resist and 2nd positive resist. Based on solvent solubility switch characteristic of polymer after higher temperature bake, a prototype of polymer consisting of methyladmantane mathacrylate, norbornanecarbo lactone mathacrylate and hydroxyl admantane mathacrylate was selected for resist-on-resist double exposure experiment to prevent the intermixing between layers. Photo sensitivity shifting of this prototype resist after post develop bake further facilitates the design by preventing 1st layer resist distortion from 2nd exposure. Lastly, through composition and formulation optimization, 35nm L/S patterns were successfully demonstrated by using a 1.2NA stepper.