Evolution of patterned and unpatterned surfaces during high temperature annealing and plasma etching
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In this thesis we describe experiments designed to probe spontaneous and directed surface evolution during annealing and plasma etching of three materials of high technological interest: silicon, nanoporous silica and photoresist.
Vicinal Si(111) surfaces provide a source of steps whose configuration we control via the introduction of a topographic pattern; this is done using combination of photolithography and reactive ion etching. We study the length scale dependence of self-organization of step bunches during annealing at ~ 1273 °C in ultrahigh vacuum (UHV), resulting from sublimation and diffusion, and the competition between effects due to the intrinsic stiffness of steps and their mutual interactions. We also show the results of numerical simulations on these surfaces based upon a simple model of step motion, which we compare with our experimental observations.
Nanoporous silica (NPS) is a heterogeneous material which is of potential use in micro/nanoelectronic applications requiring an insulator with a small dielectric constant. We investigate the stability of the NPS-plasma interface during etching, comparing the tendency for spontaneous pattern formation with the persistence of patterned perturbations. We study samples with various porosity (0 ~ 50 vol.%) under low pressure C4F8/90%Ar plasma etching conditions. Our AFM characterization of unpatterned surfaces shows a monotonic increase in RMS roughness with etching time. Annealing etched NPS surfaces at temperatures over the range from 300 ~ 900 °C in UHV as well as in non-oxidizing environment produces no significant relaxation of etching-induced surface roughness. Statistical analysis using a height-height correlation function reveals that NPS surfaces do not show a simple scaling behavior during the technologically-relevant transient time regime. Etching of patterned surfaces reveals a persistent period of approximately 400 nm, which is ~ 4 times that which spontaneously appears during etching of unpatterned surfaces. Based upon this observation we investigate the possibility of period doubling, and find some evidence for it.
Lastly, we present preliminary results of surface morphological and compositional evolution of model photoresists for lithography at 193 nm and 248 nm wavelengths during etching using plasma and ion beam sputtering under a range of conditions. Surprisingly, our AFM characterization shows that there is no significant difference in roughness evolution between resists whose chemical backbone is qualitatively different, i.e. benzene-ring based 248 nm and acrylate-admatane based 193 nm polymers. The surface roughening however varies strongly with the position of a methyl group on the polymer backbone. Fourier transform infrared spectroscopy (FTIR), Ellipsometry and XPS characterizations show that the polymers become dense at the early stages of plasma / ion beam exposure, possibly due to graphitization, cross-linking and hydrogen loss. We compare these observations with molecular dynamic (MD) simulations of Ar+ ion beam sputtering.