Determination of the gas-phase Si atom density in radio frequency discharges by means of cavity ring-down spectroscopy

Absolute densities of eroded silicon in the gas phase from the interaction of a target with a radio frequency (rf) plasma have been measured by cavity ring-down (CRD) spectroscopy. As a target either a quartz plate or a silicon wafer was used, which was attached on the powered electrode of an asymetrically driven argon rf discharge. The Si density was determined from the decrease in the CRD decay time when tuning the laser wavelength across one of the transitions of the silicon resonance line multiplet (centred at 252 nm). Typical Si densities were in the range from 107 to 108 cm-3 only, demonstrating the sensitivity of our method. The Si distribution in the plasma extended over a wider area than the dimensions of the target and the densities increased with rf power and argon pressure, but saturated at higher pressures and rf powers. Similar densities of Si were measured when using the quartz plate or the Si wafer.