Factors affecting the sensitivity of positive electron resists

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Abstract
Positive electron resists are polymeric film‐forming materials which undergo predominantly main‐chain scission upon exposure to high‐energy electrons. The irradiated regions of a selectively “patterned” film may be removed by fractional dissolution, enabling processes such as chemical etching to be carried out on the exposed substrate. Foremost in determining sensitivity (the minimum dose required to render an image developable, i.e., to produce a molecular weight distribution significantly removed from the original distribution) is the G(scission). Poly(methyl methacrylate) (PMMA) has a sensitivity of ∼10−5 C/cm2 (G(s) ∼ 1.9) compared with poly(butene‐1 sulfone) (PBS) whose sensitivity is ∼10−6 C/cm2 (G(s) ∼ 12). The molecular weight and molecular weight distribution are also of extreme importance, e.g., the sensitivity of PBS can be increased to ∼4–5 × 10−7 C/cm2 using molecular weights > 106 with low dispersitivity. Obviously, care must be taken when comparing sensitivities of two different materials.

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