Dependence of photoresist surface modifications during plasma-based pattern transfer on choice of feedgas composition: Comparison of C4F8- and CF4-based discharges
Authors: Engelmann S. , Bruce R. L. , Weilnboeck F. , Sumiya M. , Kwon T. , Phaneuf R. , Oehrlein G. S. , Andes C. , Graves D. , Nest D. , Hudson E. A. 
Published: May 2009

The authors report a study of the impact of surface chemical factors on etch rates along with surface and line edge roughness introduction for prototypical photoresist (PR) materials and structures during plasma-based pattern transfer employing fluorocarbon (FC) discharges. For selected photoresist materials and model polymers (193nm PR, 248nm, PR, and polymethyladamantyl methacrylate), the influence of bulk polymer properties on plasma durability was clarified by comparing etch rates, surface roughness introduction, and profile evolution of nanostructures. They studied the effects of both fluorocarbon fragment deposition and polymer surface fluorination by gas phase fluorine atoms on plasma etching resistance and surface roughness evolution of the organic materials by comparing discharges fed with C4F8∕Ar or CF4∕Ar∕H2 gas mixtures. The spatial frequency distribution of surface roughness was obtained using fast Fourier transformation of atomic force microscopy data. A graphitic layer was formed for Ar containing discharges on the polymer surfaces. Fluorocarbon deposition on the damaged photoresist affected roughening in two opposing ways: ion-induced mixing with the damaged polymer layer increased surface roughening, whereas for simple FC precursor deposition a reduction in surface roughness was seen. The latter effect was especially important during profile evolution of three-dimensional structures. Fluorination of the photoresist surfaces by fluorine-rich plasma increased polymer etching yields, and for highly fluorinated surfaces inhibited the formation of the graphitic surface layer. The destruction of the adamantyl structure is usually found in fluorocarbon/argon discharges and is a major origin of roughness evolution for 193nm PR materials. Process conditions having high etch yields were found to improve the roughness results of 193nm PR after etch. The fluorination of the photoresist materials prevented the formation of characteristic small scale roughness features at the cost of large scale roughness introduction. Use of low energy density process conditions prevented the introduction of large scale roughness and can be explained by suppression of surface roughness growth by an ion-induced transfer mechanism. Alternatively, CF4∕H2 processing also showed improved roughness results due to a separate layer deposition of the fluorocarbon film on top of the photoresist material. The etch results for all process conditions can be combined in a surface roughening model where the roughening behavior of the film scales linearly with the energy density delivered to the polymer surface during processing. Even for a range of feedgas chemistries, adamantyl-containing polymers show enhanced roughening rates, suggesting that the instability of the adamantyl structure used in 193nm PR polymers is the performance limiting factor for processing these PR materials.