Paper PS1-ThA2
Time-Resolved Absorption Spectroscopy with LEDs as Light Source: Application to Etching Plasma Monitoring

Thursday, October 23, 2008, 2:20 pm, Room 304

Broad band absorption spectroscopy is widely used to measure the concentration of radicals, which is important to understand the physical chemistry of many plasmas. We show that it is possible to increase significantly the sensitivity of this technique and to perform time-resolved measurement by using Light Emitting Diodes (LEDs) as a light source. This is obtained thanks to the high stability of the LED intensity. By modulating the LED current and using a lock-in amplifier for light detection, it is possible to get rid of the plasma emission, which greatly enhances the reliability of the absorption spectroscopy technique in reactive plasmas. In particular, wavelength dependent absorption cross section can be measured without any distortion, inherent to baseline fluctuations when using other light sources such as Xe arcs. This is particularly important when the species absorbs over a broad band continuum. Finally, we show that it is possible to achieve time resolved measurements of radical density decay in the afterglow of pulsed discharges, giving insight into the gas phase and surface loss processes of these radicals. The method is applied to study radical loss kinetics in BCl3 and SiCl4 based high density plasmas. We concluded that UV absorption with LED is a new and powerful plasma diagnostics, which allows detecting several radicals with a small and low cost equipment, and which may be used for real time process monitoring applications.