Rah Richard Engeln

In situ reaction mechanism studies of plasma-assisted atomic layer deposition of Al2O3

Reaction mechanisms during plasma-assisted atomic layer deposition (ALD) of Al2O3 from Al(CH3)3 and O2 plasma were studied by time-resolved quartz crystal microbalance measurements, mass spectrometry, and optical emission spectroscopy. Al(CH3)3 chemisorption on the oxide surface after the plasma pulse releases CH4 products while from the detection of CO, CO2, and H2O in the O2 plasma it is established that surface –CH3 groups are predominantly removed by O radical-driven combustionlike reactions. Also a second pathway exists for –CH3 removal driven by H2O generated in this plasma step. These reaction pathways are expected to be generic for plasma-assisted ALD of oxides from metal organic precursors.

The argon-hydrogen expanding plasma: model and experiments

An argon expanding cascaded arc plasma, with small amounts (0-10 vol.%) of hydrogen added to the flow, is investigated by means of Thomson-Rayleigh scattering and optical emission spectroscopy. The results, especially the electron density behaviour as a function of the distance from the onset of the expansion, are interpreted by comparison with results of a quasi one-dimensional model. The associative charge exchange reaction between Ar+ ions and H2 molecules plays a dominant role in the model. Assuming that H2 molecules from the wall enter the plasma in the shock region, the large ionization loss can be explained. Good agreement between model and experiment is found for the electron and neutral density and the electron temperature behaviour. This makes plausible the existence of a recirculation flow inside the vacuum vessel, which transports wall-associated hydrogen molecules towards the plasma.

Fourier transform phase shift cavity ring down spectroscopy

Abstract The spin-forbidden b 1 Σ g + (v ′ =0)← X 3 Σ g − (v″=0) band of molecular oxygen in air has been measured to show that broad band continuous wave phase shift cavity ring down spectroscopy can be combined with Fourier transform spectroscopy. In contrast to ordinary cavity ring down spectroscopy, where a laser is used to excite the ring down cavity, here, an incoherent continuous broad band light source is used. This opens the way to build a Fourier transform spectrometer with which absorption measurements can be performed at a resolution determined by the Michelson interferometer and over a wavelength range from 10 μm to 250 nm with a sensitivity increase of about three orders of magnitude, as compared to standard Fourier transform spectrometers.

B-spline parametrization of the dielectric function applied to spectroscopic ellipsometry on amorphous carbon

The remote plasma deposition of hydrogenated amorphous carbon (a-C:H) thin films is investigated by in situ spectroscopic ellipsometry (SE). The dielectric function of the a-C:H film is in this paper parametrized by means of B-splines. In contrast with the commonly used Tauc–Lorentz oscillator, B-splines are a purely mathematical description of the dielectric function. We will show that the B-spline parametrization, which requires no prior knowledge about the film or its interaction with light, is a fast and simple-to-apply method that accurately determines thickness, surface roughness, and the dielectric constants of hydrogenated amorphous carbon thin films. Analysis of the deposition process provides us with information about the high deposition rate, the nucleation stage, and the homogeneity in depth of the deposited film. Finally, we show that the B-spline parametrization can serve as a stepping stone to physics-based models, such as the Tauc–Lorentz oscillator.

DETECTION OF CH IN AN EXPANDING ARGON/ACETYLENE PLASMA USING CAVITY RING DOWN ABSORPTION SPECTROSCOPY

Abstract Cavity ring down (CRD) absorption spectroscopy is used to measure the methylidyne (CH) radical in an Ar/C 2 H 2 plasma. The rotational spectrum of the A 2 Δ (v′=0) ← X 2 Π (v′′=0) transition around 430 nm is recorded to determine the total CH ground state density, both as a function of the current through the arc producing the low-pressure Ar plasma and as a function of the injected acetylene flow. Total ground state densities between 5×10 15 and 8×10 16 m −3 are detected. The trends show that the methylidyne radical plays a minor role in the growing mechanism of hydrogenated amorphous carbon films and is predominantly formed in the charge exchange/dissociative recombination channel starting from the C 2 H radical.

CO and byproduct formation during CO2 reduction in dielectric barrier discharges

The dissociation of CO2 and the formation of CO, O3, and O2 were studied in a dielectric barrier discharge (DBD) at atmospheric pressure by means of ex-situ infrared absorption spectroscopy. CO mixing ratios of 0.1%–4.4% were determined for specific injected energies between 0.1 and 20 eV per molecule (0.3–70 kJ/l). A lower limit of the gas temperature of 320–480 K was estimated from the wall temperature of the quartz reactor as measured with an infrared camera. The formation of CO in the DBD could be described as function of the total number of transferred charges during the residence time of the gas in the active plasma zone. An almost stoichiometric CO:O2 ratio of 2:1 was observed along with a strongly temperature dependent O3 production up to 0.075%. Although the ideal range for an efficient CO2 dissociation in plasmas of 1 eV per molecule for the specific injected energy was covered, the energy efficiency remained below 5% for all conditions. The present results indicate a reaction mechanism which is...

Trace gas measurements using optically resonant cavities and quantum cascade lasers operating at room temperature

Achieving the high sensitivity necessary for trace gas detection in the midinfrared molecular fingerprint region generally requires long absorption path lengths. In addition, for wider application, especially for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelectrically (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ∼0.5 m long cavity having a small sample volume of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 μm yielded path lengths of 1080 m and a noise equivalent absorption of 2×10−7 cm−1 Hz−1/2. The molecular concentration detection limit with a 20 s integration time was found to be 6×108 molecules/cm3 for N2O a...

Detailed study of the plasma-activated catalytic generation of ammonia in N2-H2 plasmas

We investigated the efficiency and formation mechanism of ammonia generation in recombining plasmas generated from mixtures of N2 and H2 under various plasma conditions. In contrast to the Haber-Bosch process, in which the molecules are dissociated on a catalytic surface, under these plasma conditions the precursor molecules, N2 and H2, are already dissociated in the gas phase. Surfaces are thus exposed to large fluxes of atomic N and H radicals. The ammonia production turns out to be strongly dependent on the fluxes of atomic N and H radicals to the surface. By optimizing the atomic N and H fluxes to the surface using an atomic nitrogen and hydrogen source ammonia can be formed efficiently, i.e., more than 10% of the total background pressure is measured to be ammonia. The results obtained show a strong similarity with results reported in literature, which were explained by the production of ammonia at the surface by stepwise addition reactions between adsorbed nitrogen and hydrogen containing radicals a...

Flow dynamics and invasion by background gas of a supersonically expanding thermal plasma

The transport of neutral argon atoms in an expanding thermal argon/hydrogen plasma is studied by means of laser-induced fluorescence spectroscopy around 811 nm, on the long living Ar[4s] atoms. Although the Doppler shifted laser-induced fluorescence measurements are performed on argon atoms in the metastable Ar*(3P2) and resonant Ar*(3P1) states, it is argued that in the plasma jet the velocity distribution function of these Ar[4s] atoms images the velocity distribution functions of the ground-state argon atoms. From the results it is inferred that the velocity behaviour of the supersonically expanding argon gas can be predicted from the momentum balance, and the temperature from the adiabatic relation between density and temperature. However, the adiabatic constant is found to be 1.4±0.1, smaller than the adiabatic constant of a neutral argon gas expansion which is (5/3). Both in the axial and in the radial directions the velocity distributions measured in the shock region show clear departures from thermodynamic equilibrium. From the radial velocity distribution it is concluded that background gas invades the supersonic part of the expanding plasma jet. The results on temperature and velocity in the subsonic region show that the radius of the plasma jet hardly increases after the stationary shock front, indicating that the flow pattern is geometrically determined.

Density and temperature of N atoms in the afterglow of a microwave discharge measured by a two-photon laser-induced fluorescence technique

Both the axial density and temperature profiles of ground-state nitrogen atoms have been measured in a microwave discharge and its afterglow in the presence of the so-called short-lived afterglow by means of two-photon absorption laser-induced fluorescence (TALIF). The temperature is obtained from the Doppler broadening of the spectral profile, after deconvolution with the laser profile. The N atom temperature decreases from about 1400 K in the end of the discharge zone to about 300 K in the downstream part of the afterglow. The sharp temperature decrease immediately behind the discharge zone can reasonably be explained by heat transfer to the flow tube wall. The absolute N atom density is obtained by calibrating the fluorescence yield with a TALIF signal from krypton atoms. The N density increases from 1.5×1021 m-3 in the discharge zone to about 3.5×1021 m-3 in the late afterglow. However, the N atom flux is conserved along the flow tube, indicating negligible consumption or production of N atoms in the short-lived afterglow.