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Chapter 8:
Photoresists for Extreme Ultraviolet Lithography
Extreme ultraviolet lithography (EUVL) has advanced dramatically since its inception in 1986. Current EUV technology is challenging conventional 193-nm lithography for the 32-nm node and is the likely technology that will be used for the 22-nm node. Progress in EUV research has required an interdisciplinary approach involving partnerships of scientists and engineers within both industry and academics. Contributions from the fields of physics, optics, computer science, business, materials science, and chemistry have been critical to all phases of EUV technical advances. One of the most important components of EUVL is the technological advancement of photoresists. Although EUV photoresist performance has made dramatic advances over the years, resist requirements for 2013 are extremely demanding [Fig. 8.1 (a)]. In the future, resists must show considerable improvement in resolution, line edge roughness (LER), and sensitivity. Unfortunately, these three resist properties are in opposition to each other and have been described as the RLS (resolution, LER, and sensitivity) tradeoff typified by the triangular image in Fig. 8.1(b) and evaluated theoretically by Gallatin and van Steenwinckel. While resists exist that can demonstrate each requirement individually, there are currently no resists that can meet all three requirements at once. The purpose of this review is to describe the history of the first resists imaged by EUV light up to present-day resist technology. The emphasis will be on describing early EUV challenges and questions, and on chemical approaches that were applied to further EUV resist technology. The development of photoresists has occurred through the successful collaboration of industry and academia. Typically, the best-performing resists come from industry; however, most industrial research is proprietary, so technical details are generally not publicly disclosed. Fortunately, the role played by academics lends balance to the industry by advancing and evaluating new chemical approaches and publishing the results. This review will cover contributions from both industry and academia but will primarily emphasize publications that reveal the most information available during each period of research.
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