Plasma-surface interactions of advanced photoresists with C4F8∕Ar discharges: Plasma parameter dependencies

Abstract

One recurring problem in nanoscale processing is roughening of photoresist (PR) materials during plasma etch. We studied the plasma etch behavior of 248nm PR, 193nm PR, and poly methyladamantyl methacrylate while changing the source power level (400–1200W), adjusting the bias power to change the self-bias voltage Vdc (−50to−150V), and varying the pressure (10–80mTorr) and the amount of fluorocarbon gas additive to the Ar discharge (0%–10% c-C4F8 in Ar). The authors found that the PR removal is dominated by the ion energy and fluence. Surface fluorination enhanced the removal rates. Two linked mechanisms for the roughening behavior of the films during processing were identified. Changes of PR top surface roughening behavior in response to variations of bias power and pressure could be interpreted by a model of roughness formation which is dominated by a physical pattern transfer mechanism, i.e., roughness amplification through selective ion-induced transfer. When the plasma source power was varied, they observed that roughness formation was linked to the surface energy density deposited during processing. As the energy required to volatilize a volume element from the surface increased, the surface roughening rates grew proportionally. This conversion of excess energy into roughening was found to depend on the molecular structure of the polymer, with adamantyl polymers having a very high roughening constant. Additional effects on the etch behavior arise from fluorination of the samples, as quantified by x-ray photoelectron spectroscopy. High F 2s∕F 1s intensity ratios, which indicate deeper fluorination, were measured for rough surface conditions. Smaller F 2s∕F 1s ratios indicate near-surface fluorination and correspond to smoother top surfaces and feature sidewalls. Molecular compounds showed roughening behavior relative to the respective cross-linking behavior even when processed in pure Ar discharges, suggesting that the measured surface fluorination mirrors surface morphology. When plasma etching three-dimensional trenches and contact holes patterned in PR, the authors found that the sidewall roughness changed with process parameters in a fashion similar to that seen for blanket surface roughness introduction using the same etch conditions. A close correlation between the surface and sidewall roughness results was obtained, suggesting that their model of polymer surface roughening also applies to resist sidewall evolution during plasma etch.