Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics.

The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.

In etch plasmas used for semiconductor processing, concentrations of the precursor gas NF 3 and of the etch product SiF 4 are measured online and in situ using a new diagnostic arrangement, the Q-MACS Etch system, which is based on quantum cascade laser absorption spectroscopy (QCLAS). In addition, the etch rates of SiO 2 layers and of the silicon wafer are monitored including plasma-etching endpoint detection. For this purpose the Q-MACS Etch system is working as an interferometer arrangement. The experiments are performed in an industrial, dual-frequency, capacitively coupled, magnetically enhanced, reactive ion etcher (MERIE), which is a plasma reactor developed for dynamic random access memory (DRAM) technologies. In the spectral range 1028 ± 0.3 cm -1 , the absorption cross-sections of SiF 4 and NF 3 are determined to be r = (7.7 ± 0.7) × 10 -18 cm 2 molecule -1 and r = (8.7 ± 0.8) × 10 -20 cm 2 molecule -1 , respectively.

The method of spectroscopic determination of the degree of dissociation of hydrogen in non-equilibrium plasma, proposed in our previous paper, was verified under various plasma conditions. Three different gas discharges have been used, namely: (i) a DC-arc discharge, (ii) an RF discharge, f = 200 kHz and (iii) an MW discharge, f = 2.45 GHz, under pressure p = 0.3-8 mbar. Relative intensities of atomic and molecular spectroscopic lines, the H α and H β line and the (2-2)Q1 line of the Fulcher-α system, and the gas temperature were measured in a wide range of discharge conditions and used for the determination of the degree of dissociation of hydrogen. It is shown that the method leads to plausible results in the whole range of discharge conditions studied in this paper. In particular, for the first time an influence of the redistribution of the population density of excited hydrogen atoms among fine structure sublevels on values of the degree of dissociation of hydrogen obtained by the method was investigated experimentally. It is also shown, that the influence is rather significant. The assumption of Boltzmann populations of the sublevels is more appropriate than neglecting the redistribution after electron impact excitation for p > 1 mbar.

Trace gas sensing in the mid-infrared using quantum cascade lasers (QCLs) promises high specificity and sensitivity. We report on the performance of a simple cavity enhanced absorption spectroscopy (CEAS) sensor using a continuous wave external-cavity QCL at 7.4 μm. A noise-equivalent absorption coefficient αmin of 2.6 × 10–8 cm–1 in 625 s was achieved, which corresponds to a detection limit of 6 ± 1 ppb of CH4 in 15 millibars air for the R(3) transition at 1327.074 cm–1. This is the highest value of noise-equivalent absorption and among the longest effective path length (1780 m) reported to date with QCL-based CEAS.