Optical and mass spectrometric investigations of ions and neutral species in SF6 radio-frequency discharges.

Abstract

Radio-frequency (rf) discharges at 13.56 MHz were generated in pure ${\mathrm{SF}}_{6}$ using a capacitively coupled, parallel-plate GEC reference cell for gas pressures in the range of 4--33.3 Pa (30--250 mTorr) and for peak-to-peak applied rf voltages in the range of 100--300 V. The following measurements were made during operation of the discharge: (1) electrical characteristics which included power dissipation, voltage-current phase angle, and the dc self-bias; (2) time-averaged vertical and horizontal profiles of the optical emissions from neutral atomic fluorine for the 2${\mathit{p}}^{4}$3${\mathit{p}}^{\ensuremath{'}}$${\mathrm{}}^{2}$${\mathit{F}}_{7/2}^{0}$\ensuremath{\rightarrow}2${\mathit{p}}^{4}$3${\mathit{s}}^{\ensuremath{'}}$${\mathrm{}}^{2}$${\mathit{D}}_{5/2}$ and 2${\mathit{p}}^{4}$3p${\mathrm{}}^{2}$${\mathit{P}}_{3/2}^{0}$\ensuremath{\rightarrow}2${\mathit{p}}^{4}$3s${\mathrm{}}^{2}$${\mathit{P}}_{3/2}$ transitions; (3) spatially resolved, laser-induced fluorescence (LIF) utilizing the 2${\mathit{p}}^{4}$3s${\mathrm{}}^{4}$${\mathit{P}}_{5/2}$ metastable level of atomic fluorine; (4) mass spectra of neutral species in the plasma; and (5) kinetic-energy distributions and relative fluxes of mass-selected positive ions extracted from the plasma through a 0.1 mm diameter orifice in the grounded electrode. The dependence of the electrical characteristics on gas pressure confirms previous observations and model predictions which indicate, for example, that the plasma becomes more resistive as pressure increases. The optical-emission and LIF results are also consistent with previously reported pronounced peaks in emission intensity in front of the powered electrode and a complex double-layer formation at the plasma-sheath boundary, which can be attributed to the strong electron-attaching properties of the gas. From the mass-spectrometric observations of the neutral gas constituents, it can be inferred that a significant fraction (as much as 80% in some cases) of the ${\mathrm{SF}}_{6}$ in the cell can be dissociated or decomposed when the discharge is on for an input gas flow rate of 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ mol ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ (2.0 cubic centimeters per minute at standard temperature and pressure). The measured ion-energy distributions exhibit deviations from the simple ``rf-saddle structure'' that become more pronounced with decreasing ion mass. The ion-energy distributions also exhibit pronounced dependences on pressure and applied voltage that appear to be consistent with corresponding changes in the electrical characteristics and LIF. Changes in electrode-surface conditions produced by the discharge were found to dramatically affect the ion-energy distributions, LIF, and electrical characteristics. \textcopyright{} 1996 The American Physical Society.