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 sc heme which utilizes a thin Si-containing resist on top of a thick, planariz ing underlayer. The image is developed in the top layer and transferred to the underlayer via O-2-based plasma etching. This article focuses on three aspects of the critical transfer etch process: etch resistance of the imagi ng resist, profile control and resist roughening. The imaging resist thickn ess 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 ex plain this nonlinear behavior: oxidation of silicon, deprotection of resist moieties, and plasma etching of resist, are discussed. Fourier transform i nfrared studies on imaging resist films indicate minimal deprotection-relat ed film thickness losses. X-ray photoelectron spectroscopy analyses of etch ed films indicate that the extent of surface oxidation increases initially and then becomes constant. Thus, the etching of this category of resists ca n be described as a combination of the oxidation of the silicon species and sputtering of the oxide-like layer formed. Post-transfer etch profiles usi ng an O-2 plasma are shown, and methods to reduce imaging resist faceting a nd thickness loss either by modifying the imaging layer silicon content or by using passivating plasma chemistries are discussed. The effect of differ ent etching chemistries and processing conditions on imaging layer rougheni ng and striation formation on underlayer sidewalls are explained with the a id of scanning electron microscopy micrographs and atomic force microscopy images of etched feature sidewalls. It is shown that the SO2-O-2 etch signi ficantly reduces the sidewall roughness from the postlithograpy values. The similar to 3.5 nm rms sidewall roughness observed postetch is comparable t o 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. (C) 2000 America n Vacuum Society. [S0734-2101(00)15104-8].