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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.

Low-pressure pulsed DC N2-H2 plasmas with admixtures of CH4 were investigated in a comparative study in two different types of reactors, (i) a cylindrical industrial scale active screen plasma nitriding (ASPN) reactor and (ii) a laboratory scale plasma nitriding monitoring reactor (PLANIMOR) with a linear electrode configuration. Applying infrared laser absorption spectroscopy (IRLAS) the evolution of the molecular concentrations of four stable molecules, CH4, HCN, NH3, and C2H2 has been monitored. The degree of dissociation of the carbon containing precursor, CH4, varied between 80 and 95%. The concentrations of the molecular reaction products have been found in a range of 1011 to 1016 molecules cm-3. By analyzing the development of the molecular concentrations at changes of the screen plasma power, the gas mixture and the admixture of CH4, a similar behavior of the monitored species depending on the varied parameters has been found in both reactors with NH3 and HCN as the main products of plasma conversion. The fragmentation efficiency of methane has been determined, decreasing from 2.4 to 0.8 × 1015 molecules J-1 with the increasing power of the screen plasmas in both reactors. With the help of optical emission spectroscopy (OES) the rotational temperature of the N+2 ions in the plasma could be determined, being in a range of 650...920 K where the rotational temperature of the screen plasma in the ASPN reactor is approximately 100 K higher than that in PLANIMOR. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the N+2 -(0-0) band of the first negative system, which has been used as the reference parameter for comparing both reactors, increases strongly in relation to the intensity of the neutral component, represented by the N2-(0-0) band of the second positive system. In addition, steel samples have been treated in both the ASPN reactor and PLANIMOR leading to comparable nitriding results. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Low-pressure rf plasmas have been applied for etching of ultra-low-k SiCOH wafers using an Oxford Plasmalab System 100. In pure CF plasmas, SiCOH layers have been etched for different power values. Using quantum cascade laser absorption spectroscopy in the mid-infrared spectral range, the correlation of online and in situ measured concentrations of two etching products, CO and SiF, with the ex situ determined etching rates has been studied. The concentration of SiF was found to range between 0.6 and 1.4 × 10 molecules cm. In contrast the concentrations of CO were measured to be only about 50 % of the SiF density with 7 × 10 molecules cm in maximum. The production rate of SiF, determined from the time behavior of its concentration after plasma ignition, was found to be between 1 and 5 × 10 cm s. The etching rates varied between 2 and 7 nm s. Both parameters increase nearly linearly with the applied rf power. It was found that for power values of up to 1.1 kW, the etching rate depends nearly linearly on the in situ monitored concentrations of both etching products. Therefore, the concentration of the etching products can be directly used as a measure of the etching rate. [ABSTRACT FROM AUTHOR]

Low-pressure pulsed dc H2–N2 plasmas with admixtures of CH4 or CO2 used for conventional (CPN) and active screen plasma nitriding (ASPN) were studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were performed in two modes: (i) the plasma at an internal model probe, driven by a bias voltage operated only, representing a CPN approach, and (ii) the screen plasma operated only, which corresponds to an ASPN experiment. Combining in situ tunable diode laser absorption spectroscopy with ex situ Fourier transform infrared spectroscopy the evolution of the concentrations of the methyl radical and of eight stable molecules, C2H2, CH4, C2H4, CO, CO2, NH3, HCN and H2O was monitored. The degree of dissociation of the carbon-containing precursor molecules varied between 40% and 98%. The methyl radical concentration was found to be in the range 1011–1012 molecules cm−3. By analysing the development of molecular concentrations with changes in gas mixtures and plasma power values, it was found that (i) C2H2, HCN and NH3 were the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (RF (CH4) ≈ (0.5–4) × 1016 molecules J−1, RF (CO2) ≈ (0.5–3.4) × 1016 molecules J−1) and the respective conversion efficiencies to the product molecules (RC (product) ≈ 1013–1015 molecules J−1) were determined for different gas mixtures and plasma power values. With the help of OES the rotational temperature of the screen plasma could be determined, which increased with power from 600 to 850 K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the -(0–0)-band, increased strongly in relation to the intensity of the neutral component, represented by the N2-(0–2)-band. [ABSTRACT FROM AUTHOR]

A compact and transportable three channel quantum cascade laser system (TRIPLE Q) based on mid-infrared absorption spectroscopy has been developed for time-resolved plasma diagnostics. The TRIPLE Q spectrometer encompasses three independently controlled quantum cascade lasers (QCLs), which can be used for chemical sensing, particularly for gas phase analysis of plasmas. All three QCLs are operated in the intra-pulse mode with typical pulse lengths of the order of 150 ns. Using a multiplexed detection, a time resolution shorter than 1 μs can be achieved. Hence, the spectrometer is well suited to study kinetic processes of multiple infrared active compounds in reactive plasmas. A special data processing and analysis technique has been established to account for time jitter effects of the infrared emission of the QCLs. The performance of the TRIPLE Q system has been validated in pulsed direct current plasmas containing N2O/air and NO2/air. [ABSTRACT FROM AUTHOR]

The effect of combining plasma and photocatalyst for Volatile Organic Compounds (VOC) removal was investigated in a pulsed low-pressure dc discharge. The photocatalyst was TiO2 while the VOC was acetylene (1000 ppm) diluted in dry air. The temporal evolution of C2H2 concentration was measured by Tunable Diode Laser Absorption Spectroscopy (TDLAS) in the mid-infrared region during the plasma pulse (one second). The contribution of external ultraviolet radiation and plasma exposure were quantified, both with and without a photocatalyst. The synergetic effect was clearly demonstrated. [ABSTRACT FROM AUTHOR]

The potential of an approach combining a pulsed microwave discharge with a catalytic packed bed reactor for the oxidation of volatile organic compounds (VOC) has been studied. The ignition of the plasma has been made possible by inserting ferroelectric BaTiO3 pellets inside the microwave excitator. As a test VOC molecule, acetylene (C2H2) in an amount of 200 ppm in dry air was used. The total oxidation products CO and CO2 have been monitored downstream of the plasma region by tuneable diode laser absorption spectroscopy in the infrared region. The oxidative efficiency was found to increase linearly with the pulse repetition rate. The oxidative efficiency and the CO2/CO selectivity were considerably enhanced when the catalyst was located inside the plasma region, which shows the synergetic effect of the plasma-catalyst combination. [ABSTRACT FROM AUTHOR]

Abstract: The active screen (AS) nitriding is an advanced technology for the plasma nitriding of steel components providing a number of advantages over conventional plasma nitriding. Developed in the last decade, the AS technology has found its industrial application, however the understanding of the fundamental mechanisms and their relationship is not complete. For the first time the mid infrared tunable diode laser absorption technique in combination with the quadruple mass spectrometry has been applied in this work. In-situ diagnostics of chemical phenomena in the N2–H2 plasmas under variation of the process conditions such as H2 to N2 gas ratio, process temperature and bias activation power provided valuable information about the concentration of active nitrogen species in the vicinity of the samples'' surfaces. The thickness of the compound layer representing the nitriding efficiency was used as a response to nitriding conditions for the evaluation of selected process parameters derived from the plasma diagnostic studies. A 1:1 gas mixture of the N2–H2 process gases seems to be most effective to get thick compound layers. [Copyright &y& Elsevier]