Investigation of surface modifications of 193 and 248nm photoresist materials during low-pressure plasma etching

Abstract

Plasma-based pattern transfer of lithographically produced nanoscale patterns in advanced photoresist materials is often accompanied by photoresist surface roughening and line edge roughening due to factors which are not well understood. We have studied the evolution of surface roughening in prototypical 193 and 248nm photoresist materials during plasma processing as a function of plasma operating parameters. We used real-time ellipsometry and mass spectrometry, along with atomic force microscopy, x-ray photoemission spectroscopy and time-of-flight secondary ion mass spectrometry in an effort to understand the morphological and chemical changes of the photoresist materials as a function of plasma–surface interactions parameters, e.g., maximum ion energy, total energy flux, and plasma chemistry, and photoresist material. A comparison of 248nm photoresist with 193nm photoresist shows that significantly more surface roughness is introduced in the 193nm photoresist for most plasma processing conditions investigated. We also find a dramatic dependence of surface roughening on the chemistry of the plasma process, e.g., for Ar–C4F8 a modified photoresist surface layer with an extent of about 50nm is produced in 193nm photoresist, whereas for C4F8 discharges the surface modification is much less for otherwise similar conditions. We show that one important reason for these differences may be ion-enhanced selective volatilization of carbonyl groups of the 193nm photoresist polymer backbone which is absent for the 248nm material, along with modulation of the ion-interaction with the photoresist material by fluorocarbon surface passivation.