Monitoring of Direct Reactions During Etching of Silicon
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of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena
CA 91125, [email protected] **Division of Chemistry and Biochemistry, Montana State University, Bozeman, MO 59717 ABSTRACT We present evidence of a direct reaction occurring when a hyperthermal fluorine atom beam (4.8 eV) impinges on a fluorinated silicon surface under steady-state etching conditions. When monitoring in-situ and in real time reactive scattering products by means of a quadrupole mass spectrometer, SiF 3 + and SiF 2 + are detected with bimodal time of flight distributions. The slow component can be described by a Maxwell-Boltzmann distribution at the surface temperature. However, the fast component is leaving the surface with velocities substantially higher than thermal and with a flux which does not obey the cosine law. Its translational energy increases with the angle of incidence of the hyperthermal fluorine beam. Etching in the direct reaction mode should result in highly anisotropic profiles by overcoming product desorption limitations. INTRODUCTION The etching of semiconductor surfaces with halogens is a major process for directional pattern transfer in microelectronics. The etching of silicon, in particular, is the most widespread application, due to the importance of the material. Fluorine- and chlorine-based chemistries have been studied widely over the past 20 years, and a recent authoritative review of the basic surface science aspects of these chemistries captured the progress and current understanding of the reaction mechanisms involved [1]. In previous studies of the etching process, the emphasis has always been on the synergism between ions and neutrals which imparts directionality to the process and helps achieve high etch rates. However, energetic ion beams, as typically employed in plasma etching, have translational energies in the 50-500 eV regime and may lead to lattice damage, mask sputtering, and charge damage [2]. It has been suggested that low-energy (1-12 eV) neutral beams of halogen atoms or molecules may overcome these problems improving thereby the etch process [1]. Indeed, it has already been demonstrated that anisotropic etching of Si with a translationally hot Cl2 beam is feasible [3,4]. It has also been shown that a direct reaction between hyperthermal chlorine atoms and a chlorinated Si surface takes place at cryogenic temperatures through collision-induced desorption leading to removal of weakly bound species from the etched surface [5]. Such direct reactions, facilitated by hyperthermal beams, bear promise for new etching techniques. However, these reactions must be identified, understood and controlled before their full potential can be realized. In this study, we focus on the reactive dynamics occurring during the interaction of translationally hot neutral fluorine atoms with a fluorinated silicon surface. We are trying to determine how etching proceeds under realistic surface conditions in a regime of translational energies where "collision-cascade" does not occur. Our exp
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