MASS-SPECTROMETRIC MEASUREMENTS OF NEUTRAL REACTANT AND PRODUCT DENSITIES DURING SI ETCHING IN A HIGH-DENSITY HELICAL RESONATOR CL-2 PLASMA

Line-of-sight mass spectrometry was used to sample both stable and rea ctive neutral species near the walls in the downstream region of a hig h-density helical resonator Cl-2 plasma during etching of Si. In this region, where the positive ion density is 1-2X10(11) cm(-3), the Cl-2 number density at a pressure of 8 mTorr decreases by about 20% when th e plasma is ignited. At constant pumping speed, this percentage increa ses with decreasing pressure, reaching 30% at 1 mTorr. A decrease of a bout 10% is due to expansion of the gas, heated by the plasma to a mea sured temperature of 400+/-50 K, integrated over a distance of one mea n-free path from the wall. This, therefore, accounts for about one-hal f to one-third of the drop in Cl-2 number density. The remaining half to two-thirds of the decrease in Cl-2 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 spee d. SiCl4 is the main etching product in the chamber, though not necess arily a primary product. Smaller amounts of SiCl2 and possibly SiCl ar e 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 dissociatio ns measured in this study are much higher than earlier values derived from Cl-atom measurements, and are more in line with recent measuremen ts and model predictions for high-density plasmas. Relationships betwe en the mass spectrometer geometry and detection efficiency were also i nvestigated. The detection configuration with the quadrupole axis perp endicular to the line of sight was found to be superior to one in whic h the quadrupole axis was parallel to the line of sight. In the latter configuration, signals from Cl and Cl-2 are anomolously large due to charge exchange, producing a collimated beam of fast neutrals at the s ampling orifice. (C) 1996 American Institute of Physics.