Diagnostic and processing in SF6 RF remote plasma for silicon etching

Plasma diagnostics and Si etching were carried out in 13.56 MHz SF6 remote plasma generated in a hollow cathode discharge system. The plasma diagnostics were performed in the remote zone at a constant pressure of 40 Pa, at a constant applied RF power of 300 W and as a function of the SF6 flow rate (40-2000 sccm), where absolute concentrations of fluorine atoms were measured using actinometry optical emission spectroscopy; electron density, electron temperature and plasma potential were determined using single Langmuir probe, positive ion flux and negative ion fraction were determined using an electrostatic planar probe. The silicon etching process was studied at two distant values of flow rate, 80 and 1800 sccm and for three conditions of the substrate holder, namely the substrate is grounded, the substrate is negatively biased and the substrate is positively biased. The etched silicon was characterized for etch rate, optical reflectance and photoluminescence properties. It was found that the etch rate is relatively high (about 15 mg cm-2 min-1) and it is controlled mainly by the ratio of the ion flux over the reactive atomic fluorine flux, the highest etch rate was obtained at the higher flow rate (1800 sccm) and for positively biased substrate. The reflectance of the silicon surface was significantly reduced after etching and a reflectance as low as 0.2% was measured. A visible photoluminescence from the etched surface was recorded; it is centred at about 600 nm, and its intensity is inversely proportional to the measured reflectance.