The interaction of organic and inorganic resist materials with photons of wavelengths between the x-ray region of the spectrum (0.2 and 2 nm) through the EUV (13.5 nm), DUV (193 and 248 nm), mid-UV (365 nm), and visible region of the spectrum (405, 436, and 700 nm), and with charged particles such as electrons have been exploited in lithographic imaging. This chapter attempts to describe the most important organic and inorganic photochemical and radiochemical imaging mechanisms that underlie these lithographies.
Listed in Fig. 4.1 are the main photochemical and radiochemical imaging mechanisms that underlie advanced lithography. As discussed in Chapter 1 and shown in Fig. 1.1, resists are in two tones: negative and positive. For positive resists, the exposed part of the resist film is dissolved away during development, reproducing a positive image of the mask; in negative resists, the unexposed part of the resist is dissolved away during development, leaving the exposed part, and reproducing a negative image of the mask. The photochemical and radiochemical reactions that engender negative-tone resist imaging mechanisms include photo- and radiation-initiated polymerization, crosslinking, and polarity switching reactions such as condensation, pinacol rearrangement, esterification, and ligand exchange. Relative to the starting unexposed resist material, the negative-tone imaging mechanisms either lead to significant increase in molecular weight of the exposed resist material (as in polymerization and crosslinking imaging mechanisms), or polarity switching (as in condensation, pinacol rearrangement, esterification, and ligand exchange imaging mechanisms).
In contrast, the photochemical and radiochemical reactions that engender positive-tone resist imaging mechanisms include photolysis and radiolysis, as well as radiation-induced main-chain scissioning, and depolymerization. The photolysis and radiolysis imaging mechanisms include dissolution inhibition, deprotection, and Claisen and photo-Fries rearrangement reactions. Relative to the starting unexposed resist material, the positive-tone imaging mechanisms lead to either a significant decrease in molecular weight (as in main-chain scissioning or depolymerization imaging mechanisms) or polarity switching (as in photolysis and radiolysis imaging mechanisms). Before we delve into the specifics of the above imaging mechanisms, we provide preliminary remarks on the photochemistry and photophysics associated with the interaction of radiation and resist materials.
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