The nature of titanium-containing films formed on polymer surfaces by reactions of sorbed water with TiCl 4

Abstract

Recently, we reported a new method for functionalizing polymer surfaces with etch resistant inorganic species such as Ti. Such methods are of interest because of their potential use in near-surface imaging schemes which may provide resolution superior to that obtained with conventional resist materials. Our technique involved reaction of TiCl 4 with water sorbed on hydrophilic polymer surfaces. The extremely high O 2 reactive ion etch (RIE) selectivity (>1000:1) of the Ti-containing films enabled the study of new organic-on-organic O 2 RIE bilayer resist schemes. One tone-retaining version having a hydrophilic, top, positive-resist layer over a bottom, negative, crosslinked-resist layer was realized. However, the exact nature of the Ti-containing films was not determined nor was the scope of the humidity-dependent Ti deposition at very low relative humidity (RH) values. This paper reports the reactions of gaseous TiCl 4 with various polymers equilibrated at 50% and 0% RH. The reactions were studied using thickness measurements, O 2 RIE rates, Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). The results show that at 0%RH, hydrophilic polymers such as hard-baked HPR-206 resist (HB206) have very little (<2 x 10 14 atoms/cm 2) Ti at the polymer surface and are readily etched by O 2 RIE. In contrast, hydrophobic polymers such as polystyrene (PS) react readily with TiCl 4 to give a measurable thickness increase, are etched at a reduced O 2 RIE rate and appear to have Ti distributed throughout the film. At 50% RH, the hydrophilic HB206 film reacts with TiCl 4 only at the surface to provide a film which is tenacious in its O 2 RIE resistance and is much thicker (1 x 10 16 atoms/cm 2) than that on the surface of the 0% RH sample. The hydrophobic polystyrene, in comparison, contains more Ti than the HB206 sample but much less than the 0% RH polystyrene sample and etches at a normal rate of O 2 RIE. XPS and RBS analyses reveal that the Ti on the 50% RH HB206 sample is present entirely as TiO 2 at the polymer surface while its composition on PS is TiO 2-like only on the surface and covers only a fraction of the surface. In the latter the Ti is distributed throughout the film and is present at too low a concentration to form the continuous TiO 2 layer needed to lower the O 2 RIE rate. XPS with depth profiling of the highly etch resistant TiO 2 layers on HB206 reveal TiO 2 thickness of 100–200Å with a surface roughness of ∼ 50Å. Application of these thin TiO 2 films to microlithography is currently under investigation.