AVS 61st International Symposium & Exhibition
    Plasma Science and Technology Wednesday Sessions
       Session PS-WeA

Paper PS-WeA1
Diagnostics of Cl2/O2 Inductively-Coupled Plasmas by Ultra-High Sensitivity Broad-Band Absorption Spectroscopy

Wednesday, November 12, 2014, 2:20 pm, Room 308

Session: Plasma Diagnostics, Sensors, and Control
Presenter: Mickaël Foucher, LPP-CNRS, Ecole Polytechnique, France
Authors: M. Foucher, LPP-CNRS, Ecole Polytechnique, France
E. Carbone, LTM - MINATEC - CEA/LETI, France
J.-P. Booth, LPP-CNRS, Ecole Polytechnique, France
Correspondent: Click to Email
Inductively-coupled plasmas (ICP) containing O2 and Cl2 (and often HBr) are widely used for etching, for example of silicon transistor gate. Simulations, including global and two-dimensional fluid models have been developed over the years, but validation against experimental measurements of absolute densities remains sparse.

Absorption spectroscopy is a powerful diagnostic for reactive plasmas, providing absolute density measurements of numerous atoms, molecules and free radicals in ground and various excited states. The sensitivity is determined in practice by the characteristics of the light source used: spectral range, stability and intensity. Previously Xe arc lamps have been used but they suffer from spatiotemporal fluctuations, limiting the sensitivity to about 10-3 in absorption. More recently UV light-emitting diodes have been used, giving greatly increased stability, but these have very limited spectral ranges (a few 10’s of nm), necessitating the use of specific diodes for each species detected.

We have constructed a new absorption bench that overcomes these difficulties. The light from a broad-band (200-1000) nm laser-induced plasma source (Energetiq-LDLS EQ-99) is collimated and steered with reflective (achromatic) optics. After passing through the reactor the beam is analyzed with an aberration-corrected spectrograph (Acton SCT-320) equipped with a 1024 element photodiode array detector. Three gratings allow spectral ranges of 32, 63 or 250 nm to be analyzed at one time. This setup gives spectra in minutes with random noise and baseline stability better than 10-4, allowing detection of species which only show weak absorption bands over wide spectral ranges.

The gases (O2, Cl2 and mixtures) are flowed through a cylindrical aluminum chamber (55 cm diameter, 10 cm height). The plasma is excited at 13.56 MHz by a 4-turn planar coil through a dielectric window. We observed molecular absorption bands from ground state Cl2, vibrationally excited O2 and of various OxCly reaction products, allowing the densities, vibrational and rotation temperatures to be determined. As far as we know this is the first time oxychlorides densities have been measured in plasmas. This data is complemented by measurements of absolute atom densities (Cl and O) by TALIF and hairpin probe measurements of electron density. The interpretation of this data set will be discussed.