By comparison with PMMA resists, CSAR 62 is characterized by a higher sensitivity and a significantly better plasma etch resistance. The main components of the resist are poly(α-methylstyrene-co-chloromethacrylic acid methyl ester), an acid generator and the safer solvent anisole. The higher sensitivity results from the addition of halogen atoms to the polymer chain. Generally, chlorine is used (this is true for CSAR 62 -chloromethacrylic acid methyl ester), but bromine or iodine would be an option as well. The chlorine atom facilitates the breaking of the polymer chain during the exposure with electrons. Besides that, there is the supporting effect of a halogenated acid generator. By adding further reactive halogens, the attack on the polymer chain is accelerated once more. Thus, less energy (smaller dose) is needed for the breakdown of the high-molecular polymer in small fragments. These fragments quickly dissolve in the developer, while the unexposed, still high-molecular resist areas remain intact.
The support of the chain break by a temperature-stable acid generator gave the resist its name – Chemical Semi Amplified Resist. Since the activation of the generator occurs at the same time as the electron exposure, the layer does not need to be tempered after exposure. Further experiments and users’ feedback, however, show that the high sensitivity is almost reached without the acid generator anyway. On behalf of a higher long-term stability, CSAR 62 is now produced without addition of acid generator.
The better plasma etch resistance results from the addition of aromatic substituents into the polymer, such as phenyl, naphthyl or anthracyl groups. The CSAR contains α-methylstyrene for its etch stability. Due to their π-electrons, aromates are significantly more stable against different plasmas in comparison to aliphatic polymers such as PMMA and are on the same level as photo resists. These mainly consist of novolaks (formalin condensed cresols), which due to the cresols possess a high portion of aromates and thus are plasma etch stable.
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