Photoresist processing for high-resolution DUV lithography at 257 nm

In this paper, we describe the studies conducted at Etec Systems, Inc., an Applied Materials company, using various resists from photoresist manufacturers. Etec undertook this investigation with the objective of creating 257-nm resist processes that would allow the continued use of precoated mask blanks from commercial suppliers. Successful delivery of this technology would allow the industry to adopt next-generation maskmaking systems without having to change current business practices, avoiding being forced to invest heavily in capital equipment, facilities, and the associated technology development programs necessary for local coating of mask blanks. The main criteria used to select an appropriate resist candidate were commercial availability, environmental stability, and across-plate critical dimension (CD) uniformity. The aging data provides an indication of shelf life-pre-exposure ranging from hours to months of delay. Methods for measuring the incoming plate quality are highlighted. The importance of a highly consistent, tightly controlled bake process-post-exposure bake (PEB) and post-apply bake (PAB)-is illustrated in detail. This paper also shows controlled CD structures below the wavelength: 257 nm. The authors describe the processing difficulties encountered while attempting to create a photomask manufacturing-capable process and the techniques used to address the difficulties. Bottom antireflective coatings, photobase, and chemical amplification are areas of focus, as are their resulting effects on the final photomask.