Mass spectrometric measurements of neutral reactant and product densities during Si etching in a high-density helical resonator Cl2 plasma

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Abstract
Line-of-sight mass spectrometry was used to sample both stable and reactive neutral species near the walls in the downstream region of a high-density helical resonator Cl2 plasma during etching of Si. In this region, where the positive ion density is 1–2×1011 cm−3, the Cl2 number density at a pressure of 8 mTorr decreases by about 20% when the plasma is ignited. At constant pumping speed, this percentage increases with decreasing pressure, reaching 30% at 1 mTorr. A decrease of about 10% is due to expansion of the gas, heated by the plasma to a measured temperature of 400±50 K, integrated over a distance of one mean-free path from the wall. This, therefore, accounts for about one-half to one-third of the drop in Cl2 number density. The remaining half to two-thirds of the decrease in Cl2 number density upon ignition of the plasma can be ascribed to the formation of Cl atoms and SiClx etch products. Cl atoms are detected throughout this pressure range; their percentage increases at the higher pressures at constant pumping speed. SiCl4 is the main etching product in the chamber, though not necessarily a primary product. Smaller amounts of SiCl2 and possibly SiCl are also present in the plasma. Within experimental error, chlorine mass balance is found at all pressures, indicating an overall consistency in the mass spectrometric calibration methods. The percent dissociations measured in this study are much higher than earlier values derived from Cl-atom measurements, and are more in line with recent measurements and model predictions for high-density plasmas. Relationships between the mass spectrometer geometry and detection efficiency were also investigated. The detection configuration with the quadrupole axis perpendicular to the line of sight was found to be superior to one in which the quadrupole axis was parallel to the line of sight. In the latter configuration, signals from Cl and Cl2 are anomolously large due to charge exchange, producing a collimated beam of fast neutrals at the sampling orifice.

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