L. Sansonnens

A voltage uniformity study in large-area reactors for RF plasma deposition

Non-uniform voltage distribution across the electrode area results in inhomogeneous thin-film RF plasma deposition in large-area reactors. In this work, a two-dimensional analytic model for the calculation of the voltage distribution across the electrode area is presented. The results of this model are in good agreement with measurements performed without plasma at 13.56 MHz and 70 MHz in a large-area reactor. The principal voltage inhomogeneities are caused by logarithmic singularities in the vicinity of RF connections and not by standing waves. These singularities are only described by a two-dimensional model and cannot be intuitively predicted by analogy to a one-dimensional case. Plasma light emission measurements and thickness homogeneity studies of a-Si:H deposited films show that the plasma reproduces these voltage inhomogeneities. Improvement of the voltage uniformity is investigated by changing the number and position of the RF connections.

Anionic clusters in dusty hydrocarbon and silane plasmas

Measurements of anions and cations are reported for hydrocarbon and silane radio frequency capacitive glow discharges. Series of anions were observed by quadrupole mass spectrometry using power‐modulated plasmas, and their structures are interpreted from the form of the mass spectra. Various experiments in silane plasmas show that anion confinement results in particles and conversely, anion detrapping can inhibit particle formation. In contrast, the polymerized neutral flux magnitudes, mass spectra and dynamics are independent of the powder formation. Powder is known to form readily in deposition plasmas containing electronegative free radicals, and the general role of anions in particle formation is discussed in the light of these experiments.

The Role of Metastable Atoms in Argon-Diluted Silane Radiofrequency Plasmas

The evolution of the argon metastable states density has been studied by absorption spectroscopy in power-modulated plasmas of argon and a mixture of 4% silane in argon. A small concentration of silane suppresses the argon metastable states density by molecular quenching. This molecular quenching adds to the electronic collisional dissociation to increase the silane dissociation rate as compared with pure silane plasmas. Using time-resolved emission spectroscopy, the role of metastable states in excitation to the argon 2P2 state has been determined in comparison with production from the ground state. In silane plasmas, emission from SiH* is due essentially to electron impact dissociation of silane, whereas in 4% silane-in-argon plasmas, emission from SiH* seems to be due to electron impact excitation of the SiH ground state. These studies demonstrate that argon is not simply a buffer gas but has an influence on the dissociation rate in plasma-assisted deposition of amorphous silicon using argon-diluted silane plasmas.

Fast equilibration of silane/hydrogen plasmas in large area RF capacitive reactors monitored by optical emission spectroscopy

The optimal plasma parameters for plasma processing, such as deposition of microcrystalline silicon from silane and hydrogen, are generally chosen in steady-state discharge conditions. However, this steady state must be reached in a short time after plasma ignition to avoid significant film deposition in non-optimal conditions during the plasma transient phase.Simple and inexpensive time-resolved optical emission spectroscopy has been used to measure the plasma time evolution from ignition to steady-state conditions in a large area RF capacitive plasma reactor. Absolute values of silane and hydrogen molecular number densities, relative values of electron density, and qualitative information on electron temperature were obtained without the need for absolute intensity calibration. Apart from the experimental verification of constant electron temperature, the particular condition here is that the emission intensities should be followed from the instant of ignition, since the molecular densities are known at this instant.A plasma model for the reactor, and a dispersive axial flow model for the pumping line, were used to show why the plasma chemistry in a well-designed large area reactor generally reaches steady-state conditions in less than one second. The optimal design for fast equilibration is a closed, directly-pumped showerhead reactor with a uniform plasma which fills the whole reactor volume.

Laser-Scribing Patterning for the Production of Organometallic Halide Perovskite Solar Modules

Efficiencies of solar cells based on organometallic halide perovskite absorber material have dramatically increased over the past few years. Most of efficiencies reported so far have, however, been obtained on solar cells with very small lab-scale area of less than 0.3 cm2. Only a handful of studies addressed the performances of minimodules based on perovskite, and all of them showed relatively large dead areas between the solar cell segments. In this study, we used laser-scribing techniques to pattern the module segment, reduce the dead area, and optimize the aperture area efficiency. The fraction of the dead area in the module is less than 16%, which proves that the laser-scribing technology can be adopted for monolithic serial interconnected perovskite modules and paves the way to improving module efficiency.

Degree of dissociation measured by FTIR absorption spectroscopy applied to VHF silane plasmas

In situ Fourier transform infrared (FTIR) absorption spectroscopy has been used to determine the fractional depletion of silane in a radio-frequency (rf) glow discharge. The technique used a simple single-pass arrangement and was implemented in a large-area industrial reactor for deposition of amorphous silicon. Measurements were made on silane plasmas for a range of excitation frequencies. It was observed that, at constant plasma power, the fractional depletion increased from 35% at 13.56 MHz to 70% at 70 MHz. With a simple model based on the density continuity equations in the gas phase, it was shown that this increase is due to a higher dissociation rate and is largely responsible for the observed increase in the deposition rate with the frequency.

Attachment-induced ionization instability in electronegative capacitive RF discharges

Attachment-induced ionization instability has been experimentally observed in O2 and CF4 capacitive RF discharges using time-resolved voltage probe, Langmuir probe, optical emission and mass spectrometry measurements. This instability occurs under specific conditions of power and pressure, and produces synchronized oscillations in the kilohertz range on potentials, emission intensity and positive ion fluxes. In contrast, the SF6 plasma was observed to remain stable under all experimental conditions. This can be understood by considering attachment and ionization cross sections of these gases and applying the theoretical criterion of instability. Contrary to O2 and CF4, the attachment rate coefficient of SF6 is very high at low energy and has a negative dependence on the electronic temperature. The application of the criterion shows clearly that O2 and CF4 plasmas are unstable at low electronic temperature, and that the SF6 plasma is stable due to its particular low-energy attachment cross section.

Optimization of the microcrystalline silicon deposition efficiency

Cost reduction constraints for microcrystalline silicon thin film photovoltaic solar cells require high deposition rates and high silane gas utilization efficiencies. If the requirements in deposition rate have sometimes been fulfilled, it is generally not the case for the silane utilization. In this work, a reactor-independent methodology has been developed to determine the optimum plasma parameters in terms of deposition rate, silane utilization, and material microstructure. Using this optimization method, a microcrystalline layer has been deposited over a large area at a rate of 10.9A∕s, with a silane utilization efficiency above 80%.

Development of a numerical simulation tool to study uniformity of large area PECVD film processing

A numerical two dimensional model to calculate the deposition uniformity over the whole electrode surface in large area rectangular plasma enhanced chemical vapour deposition reactors is presented. In this model, the three dimensional mass and species continuity equations are averaged over the electrode gap, which is small compared to the lateral dimensions of the plasma reactor, to obtain the two dimensional averaged transport equations. The model was applied to the particular case of silicon nitride deposition by selecting a limited chemistry model, including 8 neutral species and nine gas phase reactions. The results are compared with uniformity profiles obtained in a UNAXIS KAI-1 800 Plasmabox® reactor.

Fast Deposition of a-Si:H Layers and Solar Cells in a Large-Area (40 x40 cm2) VHF-GD Reactor

Note: IMT-NE Number: 299 Reference PV-LAB-CONF-1999-001 Record created on 2009-02-10, modified on 2017-05-10