Postirradiation polymerization of e-beam negative resists: Theoretical analysis and method of inhibition

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Abstract
Many e‐beam negative resists show postirradiation polymerization, i.e., even after e‐beam irradiation termination, crosslinking reaction continues. This phenomenon shows an unfavorable effect in precise pattern fabrication; the first delineated portion of the resist undergoes a dimensional change with respect to the last‐delineated portion. A method to inhibit postirradiation polymerization is presented based on a theoretical analysis of the postpolymerization mechanism. In the model used for analysis, active radicals are not extinguished in the vacuum and polymerization proceeds at a rate proportional to the radical concentration (first order reaction). A radical loses its activity due to collision with another radical (second‐order reaction ). This model successfully explained experimental results. Postirradiation polymerization inhibition is achieved by adding a radical scavenger to resists. Active radical collision with a scavenger also extinguishes radical activity. Thus resist pattern width and thickness are freed from time dependence after irradiation. Resolution is also improved by postpolymerization inhibition as chain reactions in resists are suppressed. Although sensitivity is decreased, a highly sensitive negative resist can meet today’s requirement (around 1 μC/cm2), even after the addition of a scavenger. To demonstrate the benefit of postirradiation polymerization inhibition, a Cr mask was made with submicron features. It has 7‐μm period Y‐Y propagation patterns for a bubble memory, in which all gaps are designed as 0.5 μm. Adding a radical scavenger, 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH), and SEL‐N resist, precise pattern fabrication was achieved, irrespective of the time after exposure.

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