Use of the ultraviolet absorption spectrum of CF2 to determine the spatially resolved absolute CF2 density, rotational temperature, and vibrational distribution in a plasma etching reactor

Abstract

Broadband ultraviolet absorption spectroscopy has been used to determine CF2 densities in a plasma etch reactor used for industrial wafer processing, using the CF2 à 1B1 ? X 1A1 absorption spectrum. Attempts to fit the experimental spectra using previously published Franck–Condon factors gave poor results, and values for the higher vibrational levels of the à state [(0,v2,0), with v>6] from the ground state were missing; hence new values were calculated. These were computed for transitions between low-lying vibrational levels of CF2 X 1A1 to vibrational levels of CF2 à 1B1 (v,v,0) up to high values of the vibrational quantum numbers using high level ab initio calculations combined with an anharmonic Franck Condon factor method. The Franck Condon factors were used to determine the absorption cross sections of CF2 at selected wavelengths, which in turn were used to calculate number densities from the experimental spectra. Number densities of CF2 have been determined in different regions of the plasma, including the center of the plasma and outside the plasma volume, and CF2 rotational temperatures and vibrational energy distributions were estimated. For absorption spectra obtained outside the confined plasma volume, the CF2 density was determined as (0.39±0.08)×1013 molecule cm–3 and the vibrational and rotational temperatures were determined as 303 and 350 K, respectively. In the center of the plasma reactor, the CF2 density is estimated as (3.0±0.6)×1013 molecules cm–3 with Trot ? 500 K. The fitted vibrational distribution in the CF2 ground state corresponds to two Boltzmann distributions with Tvib ? 300 and Tvib ? 1000 K, indicating that CF2 molecules are initially produced highly vibrationally excited, but are partially relaxed in the plasma by collision.

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ORCID for John M. Dyke: orcid.org/0000-0002-9808-303X

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