The Use of Langmuir Probe Measurements to Study Reaction Kinetics in Remote Plasma-Enhanced Chemical Vapor Deposition of
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THE USE OF LANGMUIR PROBE MEASUREMENTS TO STUDY REACTION KINETICS IN REMOTE PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION OF SILICON B. ANTHONY, T. HSU, L. BREAUX, R. QIAN, S. BANERJEE, AND A. TASCH
The University of Texas, Microelectronics Research Center, Department of Electrical and Computer Engineering, Austin, TX 78712. ABSTRACT The reaction kinetics of Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) have been studied using Langmuir probe measurements of plasma density and plasma potential combined with growth rate data as a function of r-f power, chamber pressure, and substrate bias. An observed increase in growth rate for negative substrate bias suggests that argon ions drive the reaction. The variation of ion density, and plasma potential with r-f power suggests that the flux of argon ions, not their kinetic energy is responsible for increased growth rates at higher r-f powers. INTRODUCTION Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) has been successfully applied to the deposition of homoepitaxial silicon films at temperatures between 150 and 480"C [1,2], heteroepitaxial Ge films on Si at 300°C [3], and high quality SiO 2 on Si at 400°C [4]. Despite striking advances in terms of successful deposition of a variety of films, our understanding of the fundamental physical mechanisms of deposition remains incomplete, although significant progress has been made recently. A substantial obstacle to developing an understanding of the RPCVD reaction kinetics is that the reactions are plasma-enhanced making them more difficult to study than purely thermal reactions. One method for studying plasmas is the use of a Langmuir probe [5]. Langmuir probes can be used to measure the plasma potential, electron and ion densities, and electron temperature [6-9]. In this work, a Langmuir probe has been used to measure the variation of these plasma parameters with r-f power and chamber pressure. The observed dependence of plasma parameters on r-f power has been correlated with the dependence of growth rate on r-f power in order to infer the role of plasma excited species and to suggest how best to optimized the deposition process. EXPERIMENTAL The Langmuir probe used in this study was mounted in the deposition chamber -5cm below the gas ring as shown in Figure 1. In this configuration, the probe is within the glow region for all values of r-f power used in this study. A single probe design is used with a probe diameter of 0.5 mm and a probe length of 1.5 mm corresponding to a probe area, Ap, of 2.4 mm 2 . Deposition conditions have been simulated as closely as possible except that 100% He was substituted for 98%He/2%SiH 4 in order to prevent silicon deposition on the probe. Probe measurements have been made for chamber pressures between 100 and 450 mTorr, He flows of 15 and 20 sccm through the gas ring, and r-f powers between 4 and 8 W. The plasma parameters were found to be independent of the He flow so that the values presented are the average of measurements at 15 and 20 sccm He. Measurements at 200mTorr were
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