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 CF 2 densities in a plasma etch reactor used for industrial wafer processing, using the CF 2 à 1 B 1 ←X 1 A 1 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,v 2 ,0), with v 2 ЈϾ6] from the ground state were missing; hence new values were calculated. These were computed for transitions between low-lying vibrational levels of CF 2 X 1 A 1 to vibrational levels of CF 2 à 1 B 1 (v 1 Ј ,v 2 Ј,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 CF 2 at selected wavelengths, which in turn were used to calculate number densities from the experimental spectra. Number densities of CF 2 have been determined in different regions of the plasma, including the center of the plasma and outside the plasma volume, and CF 2 rotational temperatures and vibrational energy distributions were estimated. For absorption spectra obtained outside the confined plasma volume, the CF 2 density was determined as (0.39Ϯ0.08)ϫ10 13 molecule cm Ϫ3 and the vibrational and rotational temperatures were determined as 303 and 350 K, respectively. In the center of the plasma reactor, the CF 2 density is estimated as (3.0Ϯ0.6)ϫ1013 molecules cm Ϫ3 with T rot Ϸ500 K. The fitted vibrational distribution in the CF 2 ground state corresponds to two Boltzmann distributions with T vib Ϸ300 and T vib Ϸ1000 K, indicating that CF 2 molecules are initially produced highly vibrationally excited, but are partially relaxed in the plasma by collision.

Time resolved measurements of the CF2rotational temperature in pulsed fluorocarbon rf plasmas

“…In the shown example the rotational temperature of CF 2 was calculated to be 390±40 K. This is a typical value for the rotational temperature of CF 2 , which was also measured by others with broadband spectroscopic methods, i.e. 350 K in a 13.56 MHz rf plasma with FTIR [20] and 500±100 K in a 2/27 MHz rf plasma with UV absorption spectroscopy [16].…”

New Compact Continuous Spectrum Light Source Using Atmospheric Pressure Microplasma with High-Velocity Ar Gas Flow

“…Recently, arc lamps have been employed as the light source for ultraviolet absorption spectroscopic (UVAS) measurement of molecular radical density. [1][2][3][4] Since the measurement did not affect the plasma process itself or the radical densities within the plasma, absorption spectroscopy could potentially be a useful tool for plasma diagnosis. Unfortunately, the conventional Xe arc lamp has power supply and thermal issues to be overcome, as discussed above.…”

Laser-induced fluorescence ion diagnostics in light of plasma processing

“…This ensures that the measured density closely represents the total amount of that chemical species, though corrections for vibrational and rotational temperature may be needed. Broadband absorption has been used in this manner to determine CF and CF 2 in industrial etch reactors [1,2]. For some important atomic radicals, such as fluorine, no suitable absorption line is available above the vacuum limit.…”

“…Instead an optical emission line intensity may be used to measure relative densities through careful calibration to a rare gas emission line (actinometry) [3]. Both methods, absorption and emission, as described [1][2][3], were used to measure radical densities in the present study. influence of source gas upon plasma composition, under realistic etch conditions.…”