ABSTRACT
Mid-infrared Cavity RingDown Spectroscopy using a CW tunable diode laser system in a RF plasma for Silane dust particles location and density determination. *
Jérôme Remy, Marcel Hemerik, Winfred Stoffels, Gerrit Kroesen, University of Technology of Eindhoven, Applied Physics Department Den Dolech 2 , P.O.Box 513, 5600 MB EINDHOVEN, The Netherlands – EU
The CRDS experiment currently being improved at the University of Eindhoven is part of an International Microgravity Plasma Facility project aimed at studying the basics of the trapping, nucleation, coagulation and growth of particles in silane plasmas. Determining location and density of particles is needed in plasma technologies for surface processing, since their generation can lead to defects. Various plasma diagnostics are available and used on the ground for that purpose but cavity ringdown spectroscopy proved to be a very sensitive method for detecting even the smallest particles. In fact, it is our goal to understand and control the dust cloud in the plasma, with an emphasis on small (up to 5 nm) particles. We present recent progress on the cavity ringdown set-up and the latest results obtained in silane (SiH4) plasmas. Our 0.1 mW CW tunable diode laser system covers the wavelength range between 1800 and 2400 cm-1 and its linewidth is about 50 MHz. The invisible beam generated has been characterized by a beam profiler using a mercury-cadmium-telluride infrared detector. The set-up uses a one-meter stable confocal cavity suspended in a vacuum vessel, with two high reflective mirrors (reflectivity between 0.997 and 0.999 over the wavelength range 4.2 – 5.6 mm). Our ringdown cavity experiment has a finesse of 1000 and a minimum preliminary detection limit of 10-5. It is already two orders of magnitude more sensitive than other infrared absorption measurements on the SiHx radicals. However, the upper limit of the detectable absorbance is determined by the fact that for a too strong absorption no ring-up can occur in the optical cavity. This is the reason why a new diagnostic is currently under investigation, using more conventional infrared absorption spectroscopy.
* For more information about this research work, please email: [email protected]
