Reaction and Transport of Multiple Species during Plasma Etching

Transport and reaction in recesses and through-holes during O₂/CF₄ plasma etching of epoxy and photoresist films were investigated to determine the effect of operating conditions and design variables upon etch rate distribution and throughput. An experimental barrel etcher was used at 13.56 MHz and 1 torr, in which substrates were not contacted directly by the glow discharge. Preliminary experiments were performed on small etch specimens to characterize the chemistry of the etching reactions with minimal mass transport. Subsequently, measurement of local etch rate was made during etching of films on closely spaced wafers, and of drill smear in through holes. Diagnostic techniques included x-ray photoelectron spectroscopy (XPS) to measure the fluorination of the surface, optical emission spectroscopy (OES) and mass spectroscopy to determine gas composition, and laser interferometry to measure local etch rates. A steady-state continuum mathematical model describing diffusion and reaction of etchant species was formulated for comparison with experimental results. The preliminary study of the etching reaction kinetics provided reaction rate expressions and rate constants. The mathematical model of Alkire and Economou (1) was extended to treat multiple etchant species with nonlinear rate expressions. Dimensionless parameters for scaling results to other systems were identified. Experimental and theoretical results indicated that increased CF₄ content in the feed stream served to fluorinate the more accessible regions of polymer to a greater extent than the less accessible regions. Such behavior served to suppress the etch rate in more accessible regions, and to enhance etch rate in the less-accessible regions, thereby improving etch rate uniformity and reactor throughput significantly. For both geometries, the maximum in throughput occurred at a much higher level of CF₄ addition than that required to maximize the etch rates of unobstructed samples.