The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H, SiH3, and Si2H5 on a growing a-Si:H film, and CH3 and C2H5 on an a-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. For SiH3 on a-Si:H, β=0.28±0.03 in excellent agreement with previous determinations using other experimental approaches, and for Si2H5, 0.1<β<0.3. For H on a-Si:H, 0.4<β<1 and mostly consists of surface recombination as H2, while the etching probability of Si as SiH4 is only ε≈0.03 at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution of SiH4 in H2 the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surface recombination at the expense of sticking. For CH3 or C2H5 on a-C:H it is shown that β is not constant during the discharge afterglow, decreasing from about 0.01 down to 0.001. This reveals that chemisorption of these radicals on the H-saturated a-C:H surface is entirely governed by the competition between desorption and creation of active sites by ion bombardment or H atoms. The differences between the surface reaction kinetics of SiH3 on a-Si:H and CH3 on a-C:H are discussed within a unified model of precursor-mediated chemisorption.

Strong nonuniformities of plasma production are expected in capacitive discharges if the excitation wavelength becomes comparable to the reactor size (standing-wave effect) and/or if the plasma skin depth becomes comparable to the plate separation (skin effect) [M. A. Lieberman et al., Plasma Sources Sci. Technol. 11, 283 (2002)]. Ion flux uniformity measurements were carried out in a large-area square (40 cm×40 cm) capacitive discharge driven at frequencies between 13.56 MHz and 81.36 MHz in argon gas at 150 mTorr. At 13.56 MHz, the ion flux was uniform to ±5%. At 60 MHz (and above) and at low rf power, the standing-wave effect was seen (maximum of the ion flux at the center), in good quantitative agreement with theory. At higher rf power, maxima of the ion flux were observed at the edges, due either to the skin effect or to other edge effects.

Laser-induced fluorescence (LIF) techniques currently used as an optical diagnostic to study and characterize non-thermal equilibrium plasmas are reviewed. The general mechanisms occurring in one- or two-photon resonant absorption-induced fluorescence when used for the detection of species in their fundamental state are analysed. We emphasize the techniques based on the simultaneous absorption of two photons for the detection of light atoms and molecules and, particularly, two-photon absorption laser-induced fluorescence (TALIF) and derivative techniques, such as resonant enhanced multi-photo-ionization, two-photon absorption laser-induced stimulated emission (TALISE) and photo-fragment translational spectroscopy, in which molecules are photo-dissociated and the ejected atomic fragments are simultaneously detected by TALIF. The kinetics of one- and two-photon absorption are treated in detail including, for the latter, a modelling, which indicates the conditions of emergence of TALISE. The main calibration methods allowing one to obtain the absolute density of species detected by LIF and some examples of applications demonstrating the interest of these diagnostic techniques in plasma processing are presented.

This paper presents a systematic characterization of capacitively-coupled radio-frequency hydrogen discharges, produced within a parallel plate cylindrical setup at different rf applied voltages (V rf = 50 − 600 V), frequencies (f = 13.56 − 40.56 MHz), and pressures (p = 0.2 − 1 torr). A twodimensional, time-dependent fluid model for charged particle transport is self-consistently solved coupled to a homogeneous kinetic model for hydrogen, including vibrationally excited molecular species and electronically excited atomic species. Numerical simulations are compared with experimental measurements of various plasma parameters. A good quantitative agreement is found between simulations and experiment for the coupled electrical power and the plasma potential. The model underestimates the values of the electron density, the self-bias potential, and the H(n=1) atom density with respect to measurements, but agrees with experiment when predicting that all these parameters increase with either V rf , f , or p. The dissociation degree is about 10 −3 for the work conditions considered. Simulations adopt a wall-recombination probability for H atoms that was experimentally measured, thus accounting for surface modification with discharge operating conditions. Results show the key role played by the atomic wall-recombination mechanism in plasma description.

A two-dimensional numerical code, including three fluid modules to account for the description of electrical, thermal and chemical phenomena, has been developed for the modelling of hydrogenated amorphous silicon deposition from SiH 4 -H 2 radio-frequency glow discharges in a cylindrical PECVD reactor. The results of the model are compared to experimental data, obtained by different diagnostic techniques. The calculated radical densities are compared to those measured by threshold ionization mass spectrometry, at the centre of the substrate; the calculated SiH density profile between the electrodes is compared to those measured by laser-induced fluorescence and the radial distribution of the deposition rate on the substrate is compared to profilometry measurements. Globally, the model correctly predicts the main discharge characteristics for experimental conditions normally used for amorphous silicon deposition in the dust-free regime. The moderate agreement between model and experiment occurring for the hydrogen-dominated condition can be attributed to the simplified surface kinetics adopted in the model.

The analysis of the Doppler profile of the Balmer Halpha line in the cathode sheath and negative glow of a hydrogen glow discharge shows an important concentration of anisotropic energetic atoms. This translational energy is regarded as mainly resulting from reactions including ions accelerated by the cathode sheath electric field. The mechanisms involved are gas-phase charge-exchange collisions producing fast atoms moving toward the cathode and simultaneous neutralisation and backscattering of the ions at the cathode surface leading to energetic atoms moving in the other direction. Quantitative analysis shows that most of the emission of the fast atoms results from excitation directly produced by these ion impact reactions.

After reviewing the various in situ and non-intrusive diagnostics of radicals in low-pressure discharges, we present the technique of threshold ionization mass spectrometly. This technique has been applied to measure SiH,, CH, (n < 3) and H radical densities in radio frequency (RF) capacitively coupled parallel plate discharges of silane, methane and hydrogen. The procedure of deter-inltion o! &solute !adici! densities is det-i!ed. The comparison of %U3 and CH3 densities near the wall reveals large differences in surface reaction probabilities. Then, the contribution of SiH, and CH, ( n < 3) flux to a-Si:H and a-CH deposition is investigated and compared with the growth rate. Finally, for the first time, we report a measurement of H atom density by mass spectrometry in a hydrogen discharge.

This paper presents a systematic characterization of pure hydrogen capacitively coupled discharges, produced in a parallel plate cylindrical setup. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions H ϩ , H 2 ϩ , and H 3 ϩ , at different frequencies ͑13.56-60 MHz͒, pressures ͑0.2-8 Torr͒, rf applied voltages ͑50-450 V͒ and geometric dimensions ͑1.6-12.8 cm radii and 1.6-6.4 cm interelectrode distances͒. A good agreement is found between calculation results and experimental measurements for the coupled electrical power, the plasma potential, and the self-bias potential, at various frequencies and rf applied voltages. However, the model generally underestimates the electron density with respect to its measured values. The paper discusses different space-time events, such as the development of double-ionization structures or the occurrence of field inversion and field reversal phenomena. The dependencies on pressure and frequency of the time-average electric field distribution are analyzed and related to the electron displacement within space-charge sheaths. This study is later used to understand the variations of the hydrogen dissociation rate, with changes in discharge operating conditions. The influence of reactor dimensions on the spatial profiles of the plasma potential, the rf electric field, the electron density, and the electron mean energy are analyzed in terms of discharge symmetry. An investigation of the space-time averaged rf electric field variations, with changes in the applied voltage, pressure, and geometric dimensions is carried out. These variations are shown to follow a universal similarity curve, if an adequate normalization is used when plotting the rf electric field as a function of pressure. This innovative representation of rf discharges allows a univocal definition of a reactor working point, for given operating conditions.