Mechanism of reducing line edge roughness in ArF photoresist by using C F₃ i plasma

The roughening mechanism of ArF photoresist during etching was investigated to find out why C F₃ I gas reduces the line edge roughness (LER) in the photoresist pattern better than C F₄ gas. Since the plasma of reactive ion etching (RIE) consists of ultraviolet (UV) photons, radicals, and ions, the authors used a UV lamp and a neutral beam source for evaluating the effect of different plasma compositions on the photoresist roughness. The roughness was found not to increase only by UV photons or F radicals, but increase under the C F₄ RIE plasma which has both UV photons and F radicals. A C-F modified layer was generated on the resist surface because the UV damaged CO bonds reacted with F radicals and the resist surface became softer and shrank. Since C F₃ I plasma has a lower UV intensity and fewer F radicals compared with C F₄ plasma, the shrinkage on the sidewall of the photoresist was suppressed and resulted in a smaller LER when this plasma was used.