Enhancement of Photoresist Etch Rates by Argon Metastables in a Plasma Afterglow Reactor
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ENHANCEMENT OF PHOTORESIST ETCH RATES BY ARGON METASTABLES IN A PLASMA AFTERGLOW REACTOR. Milo D. Koretsky and Jeffrey A. Reimer. Department of Chemical Engineering, University of California, Berkeley. ABSTRACT The etching of a novolak-based positive photoresist was studied downstream of 02/CF4 and Ar/02/CF4 plasmas. Gas phase electron resonance was used to quantitatively track the two primary etchant species, atomic oxygen and atomic fluorine. These species were then correlated to etch rates as determined by laser interferometry. In addition to being industrially relevant, the downstream reactor allows the influence of neutral chemistry and surface activation to be deconvoluted from that of ion bombardment and strong electric fields, which traditionally make processes occurring in the body of the plasma so difficult to understand. The maximum in etch rate with respect to CF4 addition to an oxygen plasma yields an optimum ratio of O/F of 20 at 150 0 C. The addition of argon to the 02/CF4 gas mixture unexpectedly leads to an increase in etch rate by a factor of at least two. Moreover, the optimum O/F ratio decreases by a factor of three and, correspondingly, the optimum CF4 mole fraction decieases. Analysis of oxygen and fluorine densities suggests argon metastables alter the homogeneous chemistry for fluorine atom production as well as the heterogeneous chemistry of photoresist chain scission. Overall activation energies are 10.6, 10.8, 4.1 and 0.7 kcal/mole for 02, Ar/02, 02/CF4 and Ar/02/CF4 mixtures, respectively. The optimal CF4 mole fraction increases with increasing substrate temperature. A HougenWatson model for the heterogeneous chemistry on the wafer surface reproduces the data very well. INTRODUCTION Glow discharge plasmas are used routinely in the microelectronics industry to remove polymers from solid surfaces. This so-called "dry processing" has a number of advantages; yet empirical improvements in enhancing etch rates (and thus manufacturing throughput) are nearly exhausted. The goal of this research is to examine the fundamental chemical mechanisms by which polymers are etched in a glow discharge plasma. Mat. Res. Soc. Symp. Proc. Vol. 236. @1992 Materials Research Society
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The etch rate of photoresist in a glow discharge resuits from a convolution of three distinct chemistries. First, the reactive radicals produced in the discharge react with the polymer film leading to volatile surface species (neutral heterogeneous chemistry). Second, energy must be transferred to the polymer surface in order for product desorption to occur (surface chemistry). The source of this energy may be either energetic ions that derive from the plasma environment or thermal energy provided by heating the substrate, or some combination of these two. Finally, the volatile etch products can be dissociated in the discharge before they are pumped away; this can create additional etchant species which could not be obtained from the feed gases alone. In general, a change in plasma parameters, such as pressure, power, or eve
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