Thin Film Processing with Ecr and Downstream Oxygen Plasmas
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THIN FILM PROCESSING WITH ECR AND DOWNSTREAM OXYGEN PLASMAS
DENNIS W. HESS Department of Chemical Engineering,
Lehigh University,
Bethlehem,
PA
18015
ABSTRACT Downstream oxygen plasmas are used routinely to etch and strip resist Differences in the materials in integrated circuit (IC) process sequences. electrical damage incurred in thin (26 nm) silicon dioxide films on silicon due to the afterglow of rf and microwave oxygen plasmas appear to be due to higher energy VUV emission and to a greater density of positive ions in the case of rf excitation at downstream distances less than 35 cm. The high flux of oxygen ions in electron cyclotron resonance (ECR) plasmas also permits Oxidation rates of reasonable silicon oxidation rates at low temperature. silicon in ECR oxygen plasmas at temperatures between 250 and 450 C are comparable to those obtained for thermal oxidation in dry oxygen at temperatures above 850 C. Although oxidation rates are adequate under floating, anodic or cathodic bias conditions, only oxides grown under floating or anodic conditions display comparable chemical, physical and electrical properties to those of thermal oxides.
INTRODUCTION Oxygen plasmas have been used extensively in the fabrication of microelectronic devices and integrated circuits. For instance, resist materials are patterned and/or stripped routinely using oxygen or oxygencontaining plasmas (for a recent review, see Ref. 1). Typically, oxygen pressures above 1 torr are used to strip resist materials in a downstream reactor configuration, while a low pressure (1 - 10 mtorr) plasma is invoked for pattern delineation. Oxygen discharges can also be used to form oxygen ions and to assist ion transport so that oxidation of silicon can be performed at reasonable rates at low (30 nm grown under substrate floating conditions, the refractive index is 1.43 or below, and breakdown fields are reduced (-3 MV/cm). These data indicate that the oxide quality is unacceptable for device fabrication. Therefore, this bias regime has not been studied further. The ECR oxides with the best thickness uniformity and the best physical and electrical characteristics have been grown under anodic (substrate holder positive) bias (8). Anodizations performed under constant current and constant voltage conditions yield identical oxide properties. For constant current anodization (20 mA/cm ), no change in growth rate occurs between temperatures of 250 and 400 C. At pressures between 0.13 and 1.3 Pa, three growth regimes are evident as shown in Fig. 4: an initial parabolic oxidation region (100 nm) with growth rates below that of the linear regime where space charge effects appear to dominate. Indeed, such non-linear growth kinetics during constant current anodization have been previously ascribed to space-charge limitations [3,13). The high growth rates (relative to those of thermal oxidation at equivalent temperatures) under anodic conditions are believed to be due to the transport of 0- across the growing oxide film by the applied field (8). Since it is un
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