Energy distributions of argon neutrals at the rf-powered electrode of a parallel-plate reactor

In a parallel-plate 13.56 MHz rf reactor energetic argon neutrals are created by charge-exchange collisions in the sheath adjacent to the powered electrode. Energetic argon neutrals are detected by sampling neutrals through a 100 Mym orifice in the cathode. Downstream the orifice neutrals are analyzed parallel to the cathode normal with a quadrupole mass spectrometer equipped with an energy filter having an angular acceptance of about 2.5 degree. The pressure range considered was between 5 and 50 m Torr with dc bias potentials between -100 and -400 V. The measured neutral energy distributions for low pressures and low dc bias potentials have a maximum intensity at low energies slowly decreasing toward higher energies. For higher pressures and higher dc bias potentials the energy distributions have minimal intensities at low energies, exhibit a maximum at roughly 40 eV, subsequently decreasing toward higher energies. This behavior is explained as being caused by multiple scattering of e nergetic neutrals in the sheath. Experimental neutral energy distributions are compared with Monte Carlo simulations. In the calculations charge exchange collisions, elastic scattering, and multiple collisions are taken into account. Experimental and theoretical neutral energy distributions are in fairly good agreement clearly indicating that multiple scattering of neutrals in the sheath is an important mechanism determining their energy distributions.