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 silic on wafer was used, which was attached on the powered electrode of an asymet rically driven argon rf discharge. The Si density was determined from the d ecrease in the CRD decay time when tuning the laser wavelength across one o f the transitions of the silicon resonance line multiplet (centred at 252 n m). Typical Si densities were in the range from 10(7) to 10(8) cm(-3) only, demonstrating the sensitivity of our method. The Si distribution in the pl asma extended over a wider area than the dimensions of the target and the d ensities increased with rf power and argon pressure, but saturated at highe r pressures and rf powers. Similar densities of Si were measured when using the quartz plate or the Si wafer.