Application of quantum cascade laser absorption spectroscopy to studies of fluorocarbon molecules

Abstract

Abstract. The recent advent of quantum cascade lasers (QCLs) enables room-temperature mid-infrared spectrometer operation which is particularly favourable for industrial process monitoring and control, i.e. the detection of transient and stable molecular species. Conversely, fluorocarbon containing radio-frequency discharges are of special interest for plasma etching and deposition as well as for fundamental studies on gas phase and plasma surface reactions. The application of QCL absorption spectroscopy to such low pressure plasmas is typically hampered by non-linear effects connected with the pulsed mode of the lasers. Nevertheless, adequate calibration can eliminate such effects, especially in the case of complex spectra where single line parameters are not available. In order to facilitate measurements in fluorocarbon plasmas, studies on complex spectra of CF 4 and C 3 F 8 at 7.86 µm (1269 -1275 cm -1 ) under low pressure conditions have been performed. The intra-pulse mode, i.e. pulses of up to 300 ns, was applied yielding highly resolved spectral scans of ~ 1 cm -1 coverage. Effective absorption cross sections were determined and their temperature dependence was studied in the relevant range up to 400 K and found to be non-negligible.

Quantum Cascade Laser Absorption Spectroscopy as a Plasma Diagnostic Tool: An Overview

“…For two of these intra pulse sweeps ( R 1 and R 2 ) spectral micro-windows were defined, namely A 1 , A 2 and A 3 ( Figure 5c ), for which effective cross sections could be determined. Although the spectral coverage of the micro-windows is different the σ eff <ν> values are in good agreement ( Figure 5a ) [ 58 ].…”

“…The absorption spectra are commonly complex in nature and as a result, they lack any appearance of the rapid passage phenomena (see Figure 5 or [32,57,58]). It has been demonstrated that defining effective absorption cross sections σ eff within a spectral micro-window <ν> in both the inter or intra pulse method may also lead to an adequate calibration (case ii) [32,58]. Absolute values for σ eff are obtained from a measurement of the incident and transmitted intensity, I 0 and I , of a standardized gas sample of known number density n and absorption length L : ${n\sigma }_{\mathrm{italiceff}}(\langle \nu \rangle )=\frac{1}{L}\mathit{ln}\left(\frac{I_0(\langle \nu \rangle )}{I(\langle \nu \rangle )}\right).$…”

“…For the majority of relevant polyatomic molecules neither high resolution absorption cross sections nor spectral data for additional calculations are reported in the literature. The absorption spectra are commonly complex in nature and as a result, they lack any appearance of the rapid passage phenomena (see Figure 5 or [ 32 , 57 , 58 ]). It has been demonstrated that defining effective absorption cross sections σ eff within a spectral micro-window <ν> in both the inter or intra pulse method may also lead to an adequate calibration (case ii) [ 32 , 58 ].…”

In Situ Monitoring Capabiities of Quantum Cascade Laser Absorption Spectroscopy in Industrial Plasma Processes

“…It is known, that σ ef f may have a strong temperature dependence, e. g. for CF 4 and C 3 F 8 [17]. Therefore, calibration is preferably carried out close to the experimental conditions to avoid inaccuracies due to line broadening and non-linear absorption effects, since σ ef f factors in such phenomena.…”

“…In addition, in the case of pulsed QCLs whose instrumental line widths are significant larger than typical for lead salt diode lasers, the resulting absorption spectra may become structureless as shown by Hempel et al, who compared both laser based spectroscopical techniques for the detection of BCl 3 [10] (a typical spectrum is given below in figure 7). It has been demonstrated that defining effective absorption cross sections σ ef f within a spectral micro-window ν may also lead to an adequate calibration [7,16,17]. Absolute values for σ ef f are obtained from a separate measurement of the incident and transmitted intensity, I 0 and I, for different gas samples of known number density N and absorption length l:…”