Caractérisation de décharges à barrières diélectriques atmosphériques et sub-atmosphériques et application à la déposition de couches d'oxyde de silicium

Dielectric Barrier Discharges (DBD) have been used for more than a century, especially for ozone production. Research conducted within the last twenty years has investigated the discharge mechanisms involved and the different discharge regimes observable (filamentary, glow, Townsned and multi-peaks). These studies highlight the fundamental role of metastable species to establish and maintain a homogeneous discharge. These recent improvements in understanding the physics of DBD's open perspectives for new applications and new interests in atmospheric pressure surface treatment. Working at atmospheric pressure for silicon oxide deposition is of great interest : the possibility of continuous process, no vacuum component costs and maintenance, no loading/unloading time. However, in comparison with a classical plasma enhanced chemical vapor deposition (PECVD) process, the high pressure and thus the high gas density may result in a gas phase chemistry and a larger formation of dust particles. Exploring a new pressure range from 10 to 1000 mbar could be an alternative for this process. In the first part of this work the effect of the pressure on a DBD in non-reactive gases (helium, argon and nitrogen), then in neutral gas/oxygen mixture has been investigated with electrical measurements (discharge current and applied voltage), with high-speed imaging and with time-resolved optical emission spectroscopy. The second part of this work is dedicated to SiOx barrier coating characterization (FTIR-ATR, XPS, AFM and SEM) as a function of pressure in oxygen/hexamethyldisiloxane (HMDSO) gas mixture highly diluted in nitrogen. The exploration of discharge regimes as a function of pressure shows, in nitrogen, a progressive transition from Townsend to multi-peaks regime between 320 and 160 mbar. A detailed study of this regime in helium and nitrogen with high-speed imaging shows that each multi-peak corresponds to a new spatially homogeneous discharge. However, the discharge is not completely extinguished between each pulse and the remaining light emission reveals the metastable activity (excitation transfer or Penning effect). Paschen's curves obtained from electrical characterization of the discharge show an inversion (compared to standard cuves) of argon and helium cuves. This inversion shows the importance of metastable energies and capabilities to ionize almost all impurities, in the case of helium. This explains why in helium a breakdown under a lower electric field than in argon is possible. A detailed study of a glow discharge in helium as a function of pressure and impurities with time-resolved spectroscopy showed the metastables evolution within a discharge and the role of impurities in quenching or creation rate of metastables. This study also shows a 4 minutes time for thermal stabilization of the discharge (electrode heating and thermosdesorption). In helium and nitrogen, the very first microseconds of discharge are filamentary and change after 2-3 periods (∼ 200 µs) to the glow or Townsend regime respectively. Adding oxygen, an electronegative and metastable quencher gas, make the discharge change to filamentary when a proportion of more than 1500 ppm is added at atmospheric pressure. This rate is increased until 2 % in nitrogen at 350 mbar. The presence of a polymer substrate reduced this Townsend working domain due to the increase of impurities in the discharge caused by polymer etching. However, this process in pure nitrogen is very efficient for implanting nitrogen functional groups on the surface of polymer films. An incorporation of 23 % of nitrogen onto a PET surface has been reached. Regarding SiOx thin film deposition, adding HMDSO, even for ∼ 100 ppm make the discharge change to filamentary. A pressure below 40 mbar must be reached to obtain a multi-peak regime. This high pressure process is fast and deposition rate of 17 nm/s could be obtain at 500 mbar. FTIR, AFM and SEM characterisation of the coatings showed an inhomogeneous composition and structure of the layer between entrance and exit of the discharge along the gas flow. These conditions could explain that the best oxygen barrier obtained was 40.5 cm3/(m2· atm · day). Depending on the discharge parameters (frequency, residence time, power and pressure) the coatings are more or less organic. A progressive decarbonification of the layer due to progressive monomer depletion explains this behavior. A higher oxygen rate allows a better film composition homogeneity along the discharge, while a longer deposition time results in a rougher coating but has no effect on the layer composition. The particular geometry of the discharge cell (6 cm by 6 cm electrodes and 2 mm gap) with gas injection from one side, leads to a different chemistry along the gas flow. At the entrance, the coating is smooth (Ra ∼ 5 nm) and dense but organic whereas at the exit it is rough (Ra ∼ 15 nm) but has a quasi-stoechiometric composition. These differences are explained by heterogeneous reactions, comparable to PECVD process (surface chemistry) close to the gas input and a progressive transition to homogeneous reactions at the exit of the discharge (volume chemistry) which result in particle formation of nanometric size. Analysis of these particles by laser light scattering (LLS) shows a pressure threshold of 200 mbar with a constant gas mixture and flow within all the pressure range. At this pressure, the first detectable particles appear at the end of the discharge. From 200 to 1000 mbar, this threshold becomes closer and closer to the discharge entrance, but it always corresponds to a residence time of the gas in the discharge of around 30 ms. Thus this time corresponds to the characteristic formation time of detectable particles in the discharge. We also showed that this threshold varies linearly with the power injected in the discharge, the higher the power the faster the particles appear. Spatio-temporal LLS measurements show a cyclic (Τ ∼ 1-2 s) formation of particles. This behavior is linked to a rapid growth and a trapping of the particle in the discharge when they reach a 200 nm size. Then, when their size or density increases (∼ 240 nm) they are collected on the electrodes or expelled by the flow drag force which becomes preponderant in comparison with the electrostatic trapping force. Finally, they agglomerate at the exit of the discharge up to ∼ 300 nm size. Then a new cycle starts. This global approach of SiOx deposition process by DBD opens new perspectives of applications in a new pressure range and show the key parameters to be adjusted for an industrial application.

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

Nowadays thin films play an important role in everyday life and in industries. Thin film technology has been developed primarily for the growing demand for development of smaller and smaller devices that require advanced materials and new processing techniques suitable for micro and nano technology. One of the methods of producing thin films is by means of plasma technology. This is a technique by which plasma, also known as 'the fourth state of matter', is used to generate ions, electrons and radicals which deposit at a surface where they will form a thin film. The source of these depositing species could be either gaseous, liquid or solid in origin. Most of the time a plasma is generated by means of an electrical discharge. Plasma technologies are used for various applications. One of these is Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD systems have widespread applications in the electronic sector, because of their exibility in depositing different films such as silicon oxide thin films (SiOx). These films, currently deposited by organosilicon compounds, such as hexamethyldisiloxane (HMDSO), have dielectric and good barrier properties. The aim of the work at CRPP consists in optimizing PECVD processes for the deposition of these films on polymers. In this study, the species in gaseous phase and the powder produced in oxygen-diluted HMDSO plasmas were experimentally characterized in a radiofrequency (RF) capacitively-coupled reactor at 13.56 MHz. The gaseous phase of these plasmas and the particle formation were studied by in-situ infrared absorption spectroscopy and optical emission spectroscopy. The study of the powder formation is important because the particle deposition imposes an upper limit on the deposition rate. Particle formation was firstly studied as a function of HMDSO/O2 ratio. Analysis of the time evolution of the infrared absorption spectra gives the development of the size, the number density, the structure and the particle composition. At high oxygen dilution the formation of large particles is observed. The analysis of HMDSO/O2 plasma at low O2 content suggests that at the beginning of the process very small particles are formed by polymerization; the same particles do not grow much and keep sizes around 50 nm. The admixture of inert or non-reactant gases, such as Ar or N2, into the plasma promotes polymerization of the monomer; a high value of one of these inert or non-reactant gases leads to small powder particle size, since only polymerization takes place in this case. Particle size around 50 nm is also found in this case. At the same time we see a decrease of the carbon content and a weak reduction of the stoichiometry in SiOx indicated by the shift of the SiOSi asymmetric stretching vibration. This reduction also influences the deposited layers, less oxidized and, thus, quite far from a stoichiometric character. Hexamethyldisilazane (HMDSN) and tetramethylsilane (TMS) are other reagents that have been identified as alternatives to HMDSO. The in-situ infrared absorption spectra show that HMDSO/O2 and HMDSN/O2 plasmas are quite similar, but in the second case a lower SiOSi stretching peak intensity, due to NH and SiN peaks, is seen. TMS/O2 and HMDSN/O2 plasma spectra show a low SiOSi bending and stretching peak intensity, due also to the oxygen atom absence in their chemical structure. Two separate chapters are respectively devoted to the silicon nitride thin films (SiNx) and plasma crystal formation. SiNx layers are deposited by three different organosilicon compounds, such as HMDSN, TMS and bis(dimethylamino)dimethylsilane (BDMADMS). No powder formation is visualized in this contest. The ex-situ infrared absorption spectra show the difficulty to produce stable SiNx films; they present a low SiN peak intensity and are quite hygroscopic. Furthermore, in HMDSO/O2 plasmas at high oxygen dilution and high pressure, later in time into the discharge, accretion leads to larger and larger particles which can finish up in particles forming a plasma crystal. The study of RF harmonics shows the presence of instabilities, clear indications of the passage from small particulate to plasma crystal structure.

EPFL2011

Microfabricated solid oxide fuel cells

Samuel Rey-Mermet

Micro-fabricated solid oxide fuel cells (µSOFCs) are finding an increasing interest as potential power sources for portable devices such as MP3 players or laptops. The aim of this work was to fabricate a µSOFC demonstrator that works at 500°C and is fuelled by hydrogen. This thesis was divided into two parts. The first one was devoted to the development of an electrolyte and electrodes in form of sputtered thin films with electrical and mechanical properties suitable for the implementation in a real cell. YSZ and CGO electrolyte have been reactively sputtered from metallic targets. Both films are dense and have a columnar microstructure. The ionic conductivity of these films was of 0.5 S/m at 550°C for the CGO and of 5.5 x 10-3 S/m at 500°C for the yttria stabilized zirconia (YSZ) . Albeit the ceria doped gadolinia (CGO) was a better ionic conductor at low temperature, it was not possible to obtain an open circuit voltage (OCV) with a CGO electrolyte film. Most likely, the reduction of the Ce+4 ion into Ce+3 in a hydrogen atmosphere creates an electrical leakage. Better results were obtained with YSZ layers. Single, (111) textured columnar films showed OCV's of 200 mV. Best results were obtained with a double layer of two different microstructures. The first one exhibited a dense, columnar microstructure with (111) texture. The second layer was porous with nanocrystalline grains and preferential (200) orientation. The improved properties are ascribed to the absence of film crossing grain boundaries. Of special interest is the mechanical stress behaviour upon heating to the operation temperature. The stress was investigated as a function of temperature up to 700°C. An anomalous, hysteretic behaviour was found during the first heating cycle in YSZ as well as CGO thin films. This phenomenon could be modelled as an oxygen uptake to fill up excess oxygen vacancies created during the sputtering process. The model allowed to derive a diffusion activation energy of 0.6 eV for these excess vacancies in YSZ. Annealing in air at 700 °C permits to reduce stress and to stabilize the YSZ membrane. As electrode materials, sputter deposited, porous platinum, porous Ni-CGO composites and dense LaxSr1-xCoO3-y (LSCO) thin films were developed and characterized. The PEN (Positive electrode-Electrolyte-Negative electrode) layer processes were combined with Micro Electro Mechanical System (MEMS) process technology to fabricate µSOFC test devices. The PEN membranes were liberated by deep silicon dry etching. The cell diameter was varied between 0.5 and 5 mm, the electrolyte thickness between 500 and 700 nm. A nickel grid grown by electroplating was used to support the electrolyte layer and to serves as current collector for the anode. The cell with a 5 mm diameter shows a very good mechanical stability up to 600°C in SOFC operating conditions and for several heating cycles. The functionality of the fuel cell with two 20 nm thick porous platinum electrodes and a YSZ bilayer electrolyte (500 nm) has been demonstrated. An OCV of 850 mV was measured at 500°C with hydrogen fuel. Unfortunately, a too high cathode contact resistance reduced the current to very low values. The achieved maximal power density was only 19 µW/cm2. A simple design change should remedy the problem.

EPFL2008

Arcing in Very Large Area Plasma-Enhanced Chemical Vapour Deposition Reactors

Boris Legradic

This thesis presents an investigation into arcing and parasitic plasmas in large area plasma enhanced chemical vapour deposition reactors. Two types were investigated: RF breakdown in millimetric gaps in absence of plasma (e.g. dark space shielding), in a millibar pressure range, and RF hollow cathodes in glow discharges. RF breakdown curves (voltage vs. pressure) for parallel plate electrodes generally show a steep left-hand branch at low pressures and a flatter right hand branch at higher pressures. Introducing protrusions or holes in parallel plate electrodes will lower the breakdown voltage in certain conditions. This is, however, not due to the increased electric field at sharp edges or ridges. Instead, both experiments and simulation show that breakdown at high pressure will occur at the protrusion providing the smallest gap, while breakdown at low pressure will occur in the aperture providing the largest gap. This holds true as long as the feature in question is wide enough: Features that are too narrow will lose too many electrons due to diffusion, either to the walls of the apertures or to the surroundings of the protrusion, which negates the effect on the breakdown voltage. An analytical approximation of breakdown in parallel plates with cylindrical protrusions supports this argument. The simulation developed to study breakdown in structured parallel plate electrodes also presents a tool to aid the design of complex RF parts for dark-space shielding. A method was developed to measure the pressure-limits of ignition for RF hollow cathodes, and it could be shown that these limits not only depend on gas type, diameter and depth of the hollow cathode, but also on the presence and/or absence of other hollow cathodes in the vicinity. It could also be conclusively shown that hollow cathodes damage the electrode by sputtering and/or evaporation.

EPFL2011

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

EPFL2011

Microfabricated solid oxide fuel cells

Samuel Rey-Mermet

EPFL2008