| AVS 60th International Symposium and Exhibition | |
| Plasma Science and Technology | Tuesday Sessions |
| Session PS1-TuA |
| Session: | Plasma Diagnostics, Sensors and Control |
| Presenter: | A.E. Wendt, University of Wisconsin-Madison |
| Authors: | S.B. Radovanov, Varian Semiconductor Equipment, Silicon Systems Group, Applied Materials Inc. H.M. Persing, Applied Materials Inc. Varian Semiconductor Equipment, Silicon Systems Group, Applied Materials Inc. J.B. Boffard, University of Wisconsin-Madison C.L. Culver, University of Wisconsin-Madison S. Wang, University of Wisconsin-Madison C.C. Lin, University of Wisconsin-Madison A.E. Wendt, University of Wisconsin-Madison |
| Correspondent: | Click to Email |
The spectrum of light emitted by plasmas used in materials processing applications includes vacuum ultraviolet (VUV) photons, which are known to play a significant role in critical surface reactions under certain process conditions. Monitoring of the surface flux of VUV photons emitted from the inductively coupled plasma (ICP) and its dependence on discharge parameters is thus highly desirable. However, non-invasive direct detection of VUV photons is generally difficult, as few window materials transmit in the VUV. We thus examine the argon resonance level atom concentration as a prospective proxy for VUV emission, as 106.7 and 104.8 nm VUV photons are produced in the spontaneous radiative decay from Ar resonance levels to the ground state. Argon resonance level concentrations have been measured in the center of an ICP with a planar spiral induction antenna through “branching fraction” analysis of the visible optical emission (OES) spectrum.* Measured concentrations are subsequently used as inputs to a VUV radiation transport model developed to determine the corresponding axial VUV photon flux. Reabsorption of VUV photons by ground state atoms is significant even at pressures as low as 1 mTorr, and a proper accounting in the model thus requires accurate representation of the gas temperature. Model results based on the resonance level concentrations over a range of pressures (1-25 mTorr) and RF (13.56 MHz) power (up to 1000 W) compare favorably with the axial VUV photon flux sampled directly through a small hole at the center of an electrode located opposite the ICP antenna. Absolute VUV fluxes were measured with a windowless aluminum oxide photodiode sensitive to wavelengths below ~110 nm. Additionally, relative VUV fluxes were also obtained using a sodium salicylate coating on the inside of a side port vacuum window. The sodium salicylate converts VUV into a detectable visible light signal through fluorescence, and, unlike the photodiode, is sensitive in the wavelength range of H atom VUV emissions (122 nm). Preliminary results suggest that a combination of photodiode and sodium salicylate signals thus allows discrimination between hydrogen and argon contributions to the VUV flux in Ar/H2 gas mixtures.
Support from NSF grant PHY-1068670 and the Applied Materials Corporation is gratefully acknowledged.
*Plasma Sources Sci. Technol. 18 (2009) 035017.