ABSTRACT
Mid-infrared Cavity Ring Down Spectroscopy on a RF Silane plasma
Jérôme Remy, Gerrit Kroesen, Technische Universiteit Eindhoven, Applied Physics Department, den Dolech 2, P.O Box 513, 5600 MB Eindhoven, The Netherlands - EU
We are currently improving a CRDS experiment at the University of Eindhoven. It is part of the International Microgravity Plasma Facility project, aimed at studying the basics of the nucleation, growth and trapping of dust particles in Silane plasmas. Locating these particles and determining their density can prevent defects in plasma technologies for surface processing. Various plasma diagnostics are used for that purpose but Cavity Ring Down Spectroscopy proved to be a very sensitive method for detecting even the smallest particles. In fact, we would like to understand the dust cloud in the plasma, with an emphasis on small - up to 5 nm - particles. Therefore, we use a CW tunable diode laser system that covers the wavelength range between 1800 and 2400 cm-1 and whose line width is about 50 MHz. We characterize the invisible beam 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. The latest results show our Ring Down Cavity experiment has a finesse of 1000 and a minimum preliminary absorbance 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 depends whether the absorption is too strong or not, i.e. whether a ring-up can occur in the optical cavity or not. This is the reason why an additional diagnostic is currently under investigation, using more conventional infrared absorption spectroscopy.
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