Gap-fill type HSQ/ZEP520A bilayer resist process-(III): optimal process window for HSQ air-tip formation

Abstract

In previous study HSQ air-tip high density array with sub-20 nm radius of curvature were obtained by stripping ZEP520A after thermal reflow of ultra-thin HSQ (hydrogen silsesquioxane) gap-filled ZEP520A contact holes (C/H). And, the mechanical strength of HSQ spacer to resist shrinkage and thermal reflow of ZEP520A was found to play a dual role on the deformation of HSQ-coated C/H and thus the formation of HSQ air-tip. In this paper, effects of HSQ spacer width and thermal reflow of ZEP520A for HSQ air-tip formation are further studied for optimal process window. The effects of pattern and process parameters on the HSQ spacer width and shrink rate of thermal reflow are also evaluated. In short, thicker HSQ spacer is obtained for smaller C/H array size, looser pattern density, larger C/H CD, lower HSQ dilution ratio and thinner resist thickness. Dependence of HSQ spacer width on HSQ dilution ratio is stronger for thicker ZEP520A which implies that HSQ is deficient to fill the sidewall for deeper C/H. Lower shrink rate of diluted HSQ-coated ZEP520A under reflow is obtained for smaller C/H array size, looser pattern density, larger C/H CD, lower HSQ dilution ratio and thinner resist thickness. All of these relationships reflect the dependence of thermal reflow on the resistant effect of HSQ spacer width. Optimal process and pattern conditions for determining critical HSQ spacer width to form HSQ air-tip without bending or HSQ air-rod without shrunk of hole are described in detail.