Enhancing the dry etch resistance of polymethyl methacrylate patterned with electron beam lithography

Acrylic resists are used for both electron beam lithography and for deep-ultraviolet (UV) lithography at 193 nm wavelength. Polymethyl methacrylate (PMMA) is the most widely used acrylic positive tone electron beam resist. While it offers superb resolution in this role, its dry etch resistance is quite poor. Here, the authors present a new technique for enhancing the dry etch resistance of PMMA. This involves adding Irgacure 651—a photo-cross-linking agent to PMMA. Irgacure-containing PMMA can be spin-coated onto substrates in exactly the same way as pure PMMA. Addition of Irgacure does not impair the chain scissioning properties of PMMA under electron beam irradiation. Electron beam lithography can be carried out with this resist in exactly the same manner as with pure PMMA, although at a higher dose. After electron beam exposure, the exposed sample can be developed in diluted methyl isobutyl ketone solvent, again just as with pure PMMA. A postlithography UV exposure step then cross-links the patterned resist; substantially enhancing its dry etch resistance. This technique enables the fabrication of deeper etched structures than is possible with PMMA alone.