Transfer etching of bilayer resists in oxygen-based plasmas

Thin film imaging offers the possibility of extending 248 nm lithography to sub 150 nm resolution. We have been working on a 248 nm bilayer imaging scheme which utilizes a thin Si-containing resist on top of a thick, planarizing underlayer. The image is developed in the top layer and transferred to the underlayer via O₂-based plasma etching. This article focuses on three aspects of the critical transfer etch process: etch resistance of the imaging resist, profile control and resist roughening. The imaging resist thickness loss is very fast during the first few seconds of the etch after which the rate diminishes. The relative importance of three phenomena that can explain this nonlinear behavior: oxidation of silicon, deprotection of resist moieties, and plasma etching of resist, are discussed. Fourier transform infrared studies on imaging resist films indicate minimal deprotection-related film thickness losses. X-ray photoelectron spectroscopy analyses of etched films indicate that the extent of surface oxidation increases initially and then becomes constant. Thus, the etching of this category of resists can be described as a combination of the oxidation of the silicon species and sputtering of the oxide-like layer formed. Post-transfer etch profiles using an O₂ plasma are shown, and methods to reduce imaging resist faceting and thickness loss either by modifying the imaging layer silicon content or by using passivating plasma chemistries are discussed. The effect of different etching chemistries and processing conditions on imaging layer roughening and striation formation on underlayer sidewalls are explained with the aid of scanning electron microscopy micrographs and atomic force microscopy images of etched feature sidewalls. It is shown that the SO₂–O_{2<- - /inf>} etch significantly reduces the sidewall roughness from the postlithograpy values. The ∼3.5 nm rms sidewall roughness observed postetch is comparable to postdeveloped roughness values measured for mature single layer resists. The printing of 125 nm line/space patterns and 150 nm trench features with 10:1 aspect ratios in the underlayer is also demonstrated. © 2000 American Vacuum Society.