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A Grid Reactor with Low Ion Bombardment Energy for Large Area PECVD of Thin Film Silicon Solar Cells

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... Humans artificially create plasmas in their everyday life: neon tubes and plasma screens are everyday examples. Plasmas are also widely used in industry, for instance to build solar panels [4] or aerospace propulsion systems [5,6]. Since the beginning of the 1950's, the idea of inducing thermonuclear fusion in a controlled manner has driven an enormous research effort in the field of plasma physics [7]. ...
... Electrostatic potential in open field lines 4 ...
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Controlled nuclear fusion could provide our society with a clean, safe, and virtually inexhaustible source of electric power production. The tokamak has proven to be capable of producing large amounts of fusion reactions by confining magnetically the fusion fuel at sufficiently high density and temperature, thus in the plasma state. Because of turbulence, however, high temperature plasma reaches the outermost region of the tokamak, the Scrape-Off Layer (SOL), which features open magnetic field lines that channel particles and heat into a dedicated region of the vacuum vessel. The plasma dynamics in the SOL is crucial in determining the performance of tokamak devices, and constitutes one of the greatest uncertainties in the success of the fusion program. In the last few years, the development of numerical codes based on reduced fluid models has provided a tool to study turbulence in open field line configurations. In particular, the GBS (Global Braginskii Solver) code has been developed at CRPP and is used to perform global, three-dimensional, full-n, flux-driven simulations of plasma turbulence in open field lines. Reaching predictive capabilities is an outstanding challenge that involves a proper treatment of the plasma-wall interactions at the end of the field lines, to well describe the particle and energy losses. This involves the study of plasma sheaths, namely the layers forming at the interface between plasmas and solid surfaces, where the drift and quasineutrality approximations break down. This is an investigation of general interest, as sheaths are present in all laboratory plasmas. This thesis presents progress in the understanding of plasma sheaths and their coupling with the turbulence in the main plasma. A kinetic code is developed to study the magnetized plasma-wall transition region and derive a complete set of analytical boundary conditions that supply the sheath physics to fluid codes. These boundary conditions are implemented in the GBS code and simulations of SOL turbulence are carried out to investigate the importance of the sheath in determining the equilibrium electric fields, intrinsic toroidal rotation, and SOL width, in different limited configurations. For each study carried out in this thesis, simple analytical models are developed to interpret the simulation results and reveal the fundamental mechanisms underlying the system dynamics. The electrostatic potential appears to be determined by a combined effect of sheath physics and electron adiabaticity. Intrinsic flows are driven by the sheath, while turbulence provides the mechanism for radial momentum transport. The position of the limiter can modify the turbulence properties in the SOL, thus playing an important role in setting the SOL width.
... This high-quality a-Si:H absorber layer is one of the key achievements of AIST enabling world-record solar cells mentioned in Section 2. The deposition rate, being typically one order of magnitude below that of a diode reactor, is the main drawback of this reactor type. The grid reactor design as presented in [126] is related to the triode reactor design. The third reactor concept (Fig. 3(c)) uses a plasma box as in Kai reactors built by Oerlikon Solar/Tokyo Electron, or IRFE electrodes as in Octopus systems built by INDEOtec, and is related to the reactor concept presented in [127]. ...
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One challenge for microcrystalline silicon (μc-Si:H) deposition is to achieve high deposition rates while maintaining high quality films. In this work, an inductively-coupled plasma (ICP) is used to deposit μc-Si:H on glass substrates by means of a novel planar resonant antenna at 13.56 MHz. No particle formation occurs in the low pressure (5 Pa) plasma, but the films suffer post-oxidation. By embedding a 5 MHz RF-biased substrate, films deposited simultaneously with and without RF bias are compared. It is shown that large area, low pressure (5 Pa), particle-free ICP deposition at 1 nm/s of μc-Si:H films can be obtained without post-oxidation by means of a planar resonant antenna, provided that RF substrate bias is included for independent control of the ion energy.
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Thin-film silicon solar cells are one possible answer to the increasing energy demand of today. Hydrogenated amorphous silicon (a-Si:H) plays a crucial role therein - as absorber layers, but also as doped layers to build p-i -n junctions. This thesis is devoted to a-Si:H, with the main focus on thin-film silicon solar cells, but also with applications for opto-electronic devices, detectors, and other types of solar cells such as heterojunction solar cells. We discuss models of a-Si:H and develop further the representation of defects by amphoteric states. Using a simple model, we show - in agreement with layer-by-layer simulations and experimental results - that trapped electrons tend to dominate the electric field deformation in the initial state, whereas positively charged defects dominate in the degraded state. Experimentally, we define the deposition parameter space accessible by plasma-enhanced chemical vapor deposition (PECVD) and explore that space by varying the deposition temperature, pressure, excitation frequency, power, and H2/SiH4 ratio for intrinsic absorber layers. This leads to a catalog of a-Si:H absorber layers with tunable properties and we incorporate these materials into solar cells. For every pressure, we find an optimum hydrogen dilution where the light-induced degradation of solar cells is minimal and comparable for all pressures. Using narrow-bandgap absorbers, we demonstrate short-circuit current densities of Jsc=18.2 mA/cm2 with a 300-nm-thick absorber layer and extract more than 20 mA/cm2 from a cell with a 1000-nm-thick absorber layer. Using wide-bandgap absorbers, we achieve open-circuit voltages (Voc) of 1.04 V and Voc-fill factor products of 739mV. For such materials, we find an increased Voc dependence on substrate roughness. This is investigated by transmission electron microscopy and is attributed to porous a-Si:H material grown above peaks on the textured substrates. Depositing absorber layers in a triode reactor, we achieve efficiencies of 10.0% after light soaking. Further, we describe observations of a reversible, light-induced Voc increase of solar cells with thin p-type layers, and decrease with thick p-type layers, with a magnified effect on rough substrates. Based on layer measurements and simulations, we attribute the Voc increase to the degradation of the p-layer and the Voc decrease to the degradation of the absorber layer.
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The generation of directed energetic electrons by the expanding sheath is observed in asymmetric capacitively coupled radio frequency discharges at low pressures (≤ 1 Pa) in different gases. The phenomenon of such electron beams is investigated by a combination of experimental diagnostics, an analytical model and simulations. At sufficiently low pressures multiple reflections of electron beams at the plasma boundaries are observed. An analytical model shows how these beams lead to an enhanced high energy tail of the electron energy distribution function. Thus, stochastic heating is closely related to electron beams.
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A reactor using localized remote plasma in a grid electrode is presented in this study. The aim is to reduce the ion bombardment energy inherent in RF capacitively coupled parallel plate reactors used to deposit large area thin film silicon solar cells. High ion bombardment energy could cause defects in silicon layers and deteriorate electrical interfaces, therefore, by reducing the ion bombardment energy, lower defect density might be obtained. In this study, the low ion bombardment energy results from the reactor design. By inserting a grounded grid close to the RF electrode of a parallel plate reactor, the electrode area asymmetry is increased while retaining the lateral uniformity required for large area deposition. This asymmetry causes a strong negative self-bias voltage, which reduces the time-averaged plasma potential and thus lowers the ion bombardment energy. In addition to the self-bias, the time evolution of plasma light emission and plasma potential RF waveform are also affected by the grid, thereby further reducing the time-averaged plasma potential and ion bombardment energy. Finally, a good correlation between the measured time-averaged plasma potential and measured low ion bombardment energy is found in a broad range of RF voltages.
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Depositing microcrystalline intrinsic silicon films is an important step for the production of thin silicon tandem junction solar cells. Due to the high cost of capital equipment, it is becoming increasingly important to improve the processing speed of thin silicon films for continued commercial viability. In this work, a combination of the excitation frequencies 13.56MHz + 27.12 MHz was used for thin silicon film deposition. According to the electrical asymmetry, the DC self bias on the RF electrode was varied by adjusting the phase between the two applied frequencies. A single junction microcrystalline cell with above 5.5% efficiency was deposited in a Gen5 PECVD process using the Electrical Asymmetry Effect (EAE). The deposition rate was higher than 0.8 nm/s. A similar increase of the deposition rate in a pure 13.56 MHz discharge led to a strong degradation of the μc-Si:H quality and the single junction cell performance fell to 4% efficiency. It was found that layers deposited using the EAE have a better uniformity compared to layers deposited in a pure 27.12 MHz discharge. In comparison to traditional RF-PECVD processes, electrically asymmetric discharges allow to achieve a regime of plasma conditions with low ion energies and high electron densities.
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Very high effective minority carrier lifetime (6.3 ms) and very low surface recombination velocity (2.6 cm/s) have been demonstrated on float-zone 1–2 Ω cm crystalline silicon (c-Si) wafers by depositing a-Si:H films using grid-biased remote radio-frequency plasma enhanced chemical vapor deposition (RF PECVD). This method employs a semi-transparent DC biased-grid within the conventional RF PECVD configuration. The DC-biased grid is positioned between the RF electrodes in order to develop a remote plasma and thus allow control of the flux and type of precursors involved in the growth of hydrogenated amorphous silicon (a-Si:H) film. It is shown that compared to conventional RF diode, the grid-biased remote RF PECVD method produces a-Si:H films with superior passivating properties as well as significantly lower concentrations of void and SiH2 bonding and a lower overall hydrogen content, all of which contribute to a higher quality a-Si:H–c-Si heterostructure.
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The interplay of gas flow and depletion by plasma dissociation determines the spatial distribution of species and the deposition uniformity in a plasma source. Many plasma reactors use a gas showerhead and the design of the flow dynamics is a critical aspect of the reactor performance. In this paper, plasma deposition is considered as chemically reacting gas flow in an ideal showerhead reactor. The gas fluid flow is described by finite-gap stagnation-point creeping flow. The distribution of neutral species across the electrode gap is determined by diffusion equations, whereas their lateral transport is purely convective. Parameters relevant to large-area radio-frequency plasma deposition are particularly suitable for a complete analytical solution of the multi-component transport. A representative reaction scheme for hydrogen/silane plasma deposition is used for an analytical example from first principles which shows good agreement with numerical simulation. For a laterally uniform plasma, apart from edge effects, the deposition uniformity is limited only by the lateral uniformity of the pressure: if the electrode gap is very small in a large-area reactor, the pressure and deposition rate will be non-uniform even for a uniform showerhead. The deposition mass flux is self-consistently accounted for by the Stefan velocity for arbitrary levels of gas concentration and depletion, and its influence on streamlines and fluid velocity is shown.
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Measurements of total cross sections for collision induced dissociation (CID) and proton abstraction have been made for the reactants H3++H2, Ar, and He. The laboratory collision energies range from a few up to 400 eV and D3+ has been substituted to investigate possible isotope effects. The CID cross sections of H3+↠H++H2 or H2++H maximize at a value of a few Å2. Proton abstraction is the dominant process for relative collision energies below 5 eV and is observed to lead to a highly excited H3+ product which often autodissociates. Dissociative charge transfer at higher collision energies is responsible for producing H2+ product ions. The role of internal energy contained within the H3+ primaries in previous experiments and the effects of internal energy on the cross section measurements presented here are also discussed.
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A meshgrid is installed to study the effect of mesh bias on the lateral conductivity properties of intrinsic microcrystalline silicon films deposited by low frequency inductively coupled plasma. When a mesh bias is increased from 0 to −15 V, the dark conductivity remarkably decreases by three orders of magnitude, whereas the ratio of the photo and dark conductivity improves by one order. On contrary, the applied substrate bias has only a marginal effect on the lateral conductivity. It is revealed from the measured electron energy distribution functions that the sheath layer induced ion bombardment is responsible for the drastic change.
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Wavelengths for about 47,000 spectral lines of atoms and atomic ions, as well as transition probabilities A for about 5000 lines, are tabulated. The data were selected in such a way as to include the prominent lines over a wide spectra region. Wavelengths of lines of neutral through quadruply ionized atoms in the range 40 to 40,000 A are first presented, then transition probability data for atoms in various stages of ionization is given with emphasis on the neutral and singly ionized species. Estimates of the accuracies of the A-values are provided. Wavelengths, energy levels, and statistical weights serve to identify the lines and to provide useful data for plasma spectroscopy applications.
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It is important to reduce the hydrogen concentration, in particular, the Si-H2 bond concentration, in a hydrogenated amorphous silicon film to improve its light-soaking stability. In a previous study, we found that a triode configuration plasma enhanced chemical vapor deposition method provides high quality and a very low hydrogen concentration film; however, the origin of the hydrogen reduction has been unknown. In this article, we investigate the essential factor causing the very low hydrogen concentrations observed in the triode system. In several experiments, we observed strong influences of deposition precursors on the resulting hydrogen concentrations. We propose that due to a steric hindrance, the hydrogen elimination process during film growth is disturbed when higher silane radicals stick to a growth surface. In a triode system, corresponding with the separation of the film growth surface from the precursor generation region, the contribution of higher silane radicals to film growth is suppressed due to their short diffusion length and frequent collisions with silane molecules.
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The sheath dynamics of helium and hydrogen RF-discharges at 13.56 MHz in the GEC reference cell are studied by laser spectroscopic electric field measurements. In case of helium, the Stark splitting of the n = 11 Rydberg state is measured by LIF spectroscopy applied to metastable helium atoms in the 2s1S0 state. A novel laser spectroscopic technique in atomic hydrogen allows the sensitive measurements of electric fields down to about 10 V cm-1. Two-dimensional space and time resolved results are presented. Sheath voltages, sheath ion and net charge densities and displacement current densities are directly derived from the measured electric fields. Under certain assumptions, also the electron conduction and ion current densities, the power dissipated in the discharge and the ion energy distribution function can be inferred. The experimental results show that the common step-model for the spatial electron distribution in the sheath is a reasonable approximation in the case of helium but is less appropriate in hydrogen. In the hydrogen RF-discharge, field reversal during the anodic phase of the applied voltage is observed and investigated in detail. A simple analytic model for the field reversal effect is developed that describes quantitatively the experimental results.
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The energy distribution of ions striking the cathode in a glow discharge has been measured for a series of gases including hydrogen, helium, neon, and argon. Standard ultrahigh vacuum techniques were used to maintain a high degree of gas purity. The ions were detected by placing a small pinhole in the cathode of a glow discharge and analyzed for energy by means of a sector-type electrostatic analyzer. The ion species was determined with a conventional magnetic analyzer and the resulting ion beam intensity measured with an electron multiplier. An oscilloscope display gave the ion-energy distribution for a particular ion species. A simple theory involving assumptions that the ions originated in the negative glow and undergo primarily symmetrical charge transfer as they pass through the cathode dark space to the cathode gives results that are in most cases in good agreement with experiment. The charge-transfer cross sections determined are in reasonable agreement with other published data.
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This letter presents the low temperature silicon epitaxial growth on p-type, 〈100〉 Si wafers by plasma-enhanced chemical vapor deposition with a stainless steel mesh. Following a modified exsitu spin-etch cleaning and an insitu H2 baking step, the epitaxial layer was grown at 313 °C using SiH4 (30 sccm)/H2 (22 sccm) with a pressure of 61 mTorr and a rf power of 10 W. Epitaxial layers were also grown at 323 °C with different silane flow rates. The epitaxial film contains higher defect density when the silane flow rate is low.
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Attention is given to the preparation of a-Si:F:H films from SiF4 and H2 mixture gas by three discharging methods (DC, CSDC, and RF). The fluorine concentrations for the DC, CSDC and RF films are 15 at. percent, 1.5 at. percent, and 0.5 at. percent, respectively. Optical emission spectra from the discharging gas suggest that the kinetics of the chemical reaction on the substrate surface is essential for the film formation and the cathode screen considerably affects the plasma condition. Even 1.5 at. percent incorporated fluorine atoms have the effect to reduce the optically induced conductivity change. When the nondoped a-Si:F:H layer is deposited onto the doped film, considerable amount of doped atoms are incorporated into the nondoped layer.
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Hot cathode dc multipolar plasma sources are very efficient but have lifetime and contamination problems when they are operated with chemically active gases. As an alternative solution the rf excitation of a triode structure immersed in a multicusp magnetic field has been developed. The structure has an internal cathode, an anode around which are the magnet lines, and a third electrode which is either the target electrode in the case of ‘plasma processing’ or the beam forming electrode in the case of ‘ion beam processing’. The source has been operated with oxygen and fluorocarbon gases without any lifetime problems. The discharge may be run down to 10−4 torr (within source chamber) and creates a plasma which is homogeneous to ± 1.5% over a 175 mm diameter section and which delivers at the beam forming electrode a current density of about 1 mAcm−2 for 500 W rf power.
Article
We have developed a highly stabilized hydrogenated amorphous silicon solar cell with extremely low Si–H2 bond density in the i layer using a triode-plasma CVD method. Si–H2 bond density decreased from 1.7 to 0.6 at.%, and correspondingly the degradation ratio of the cell efficiency decreased from 13% to 10% in comparison to the conventional diode system. It was also found that the hydrogen dilution mainly affects Si–H bond density rather than Si–H2 bond density in the films. Further optimization for the TCO/p interface results in the stabilized cell efficiency of 9.22%.
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We developed a novel technique for high-rate growth of microcrystalline silicon films by plasma-enhanced chemical vapor deposition, designing a novel cathode with interconnected multi-holes, which leads to generate uniformly flat-distributed stable high-density-plasma spots near cathode-surface. Improvement of quality of high-rate grown films was discussed, and microcrystalline silicon films with a low defect density of 1.2×1016 cm−3 were obtained at a high rate of 7.7 nm/s, demonstrating the efficient gas dissociation and the effectiveness of the novel cathode. The spatial distribution of plasma at cathode-surface holes was analyzed using optical emission spectroscopy for further optimization of plasma conditions.
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The present paper summarizes the main features of the hollow cathode discharges generated by a radio frequency (r.f.) instead of a d.c. field. The pressure of gas inside the hollow cathode is almost independent on the reactor pressure, which allows to generate discharge at high collision frequency and transport it into the low pressure reactor. The discharge forced out from the hollow cathode forms a decaying plasma channel with extraordinary properties. Gas metastables excited inside the cathode can act in selected gas mixtures as a source of additional heat, thereby enhancing thermionic electron emission and ionization of the gas. An arc regime can be started from the glow discharge simply by increasing the r.f. power. Hollow cathode arc in the cathode metal vapor can be sustained even without working gas. Examples of utilization of hollow cathodes in the film deposition and dry etching technology are presented. Small size cylindrical r.f. cathodes allow special applications inside narrow (
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A highly uniform and dense plasma was successfully produced in a parallel-plate reactor with a uniformity of ± 2%. The approach is based on modifying the distribution of the current density over the flat electrode surface by incorporating hollow cathodes in order to compensate for the non-uniform part of the current distribution. Experiments were carried out in a vacuum chamber of outer diameter 300mm with two parallel electrodes (200mm in diameter, Al) separated by 35 mm, one of which is a hollow-cathode type. The working gas pressure ranged from 0.01 to 0.2 Torr (1 Torr= 133.3 Pa) of Ar. The radial ion current distribution was measured at distances of 10, 20 and 30 mm from the surface of the hollow cathode using a negatively biased double probe.
Article
The deposition of hydrogenated microcrystalline silicon (µc-Si:H) at a relatively high working pressure is performed using a conventional radio-frequency plasma-enhanced chemical vapor deposition method. Correlation of the deposition rate and crystallinity with deposition parameters, such as working pressure, flow rate, dilution ratio and input RF power, are studied. It was found that the deposition rate exhibits a maximum at around 4 Torr and that the crystallinity of films decreases monotonically with increasing pressure. The combination of SiH4 depletion and high working pressure in the plasma is necessary to improve the crystallinity of films deposited at a high rate. Consequently, a high deposition rate of 9.3 Å/s is achieved with high crystallinity and low defect density.
Article
The separated plasma triode (SPT) method has been developed to deposit high-quality a-Si:H films at high deposition rates. Plasma properties were measured by a probe method and an optical emission spectroscopy (OES) method. It is possible to improve the electron density in a plasma without increasing the plasma potential near the substrate by the SPT method. High-quality a-Si:H films and high-performance a-Si solar cells were obtained at high deposition rates using the SPT method.
Article
A plasma gun has been developed which projects ionized matter (metallic and deuterium ions) at speeds up to 2×107 cm per second. There is some evidence to support the hypothesis that the plasma projected by this gun comes off in an expanding torus which is shaped by its own magnetic field. When the plasma gun is fired into a dc magnetic field, the plasma forms a compact geometrical configuration (a plasma-magnetic entity called a plasmoid) which proceeds across the magnetic field. Plasmoids appear to be plasma cylinders elongated in the direction of the magnetic field. Plasmoids possess a measurable magnetic moment, a measurable translational speed, a transverse electric field, and a measurable size. Plasmoids can interact with each other, seemingly by reflecting off one another. Their orbits can also be made to curve toward one another. Plasmoids can be made to spiral to a stop if projected into a gas at about 10-3 mm Hg pressure. Plasmoids can also be made to smash each other into fragments. There is some scant evidence to support the hypothesis that they undergo fission and possess spin.
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We present measurements on typical silane-hydrogen RF/VHF deposition plasmas and the corresponding a-Si:H films deposited from these plasmas. A range of process settings was used, covering both the α and the γ′ regime of the discharge. Mass resolved ion energy distributions were measured at the grounded electrode to determine the ion flux at the growing surface. Although the main precursors are radicals, in the lower pressure α regime the flux of ions towards the surface can account for at least 10% of the observed growth rate. In the γ′ regime this contribution to the growth of the film is less. We measured internal stress, hydrogen concentration, hydrogen bonding configuration, and refractive index to determine the effects of the ion bombardment on the structure of the deposited a-Si:H films. Good structural properties, i.e. a refractive index of about 4.25 at 600 nm and a minimum number of SiH2 bonds, are found above a threshold energy of 5 eV per deposited atom. This observation is explained in terms of knock-on processes of the deposited atoms by ions and an increased mobility of the growth precursors at the surface. Both these processes promote the formation of a dense film.
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Plasmoidal discharges. A new type of vacuum tube discharge is described, excited by comparatively low voltage high-frequency oscillations (1.7 meters up to 30 meters wave-length) in which remarkable luminous balls, spindles and pear-shaped bodies are formed. These have been reported in earlier papers in Nature and the Phil. Mag. A study of the conditions under which these bodies are formed and their behavior in magnetic fields has lead to the conclusion that surface charges play a part in their formation. They have been shown to be built of excited molecules, as their spectra are molecular band spectra (O2 or CO) and since Langmuir has seen in their formation the possibility of plasma oscillations, such as he observed in the mercury arc; they have been named "plasmoids" provisionally.
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High-rate growth of microcrystalline-silicon (μc-Si) using very-high-frequency plasma-enhanced chemical vapor deposition in combination with a kind of triode method was investigated. In this method, a mesh electrode is placed near substrates, and discharge spots are generated on the mesh and moving around. It was found that the moving spots are generated under limited conditions of deposition pressure and flow rates of silane and hydrogen, and they are required to obtain high-rate and high-quality materials. Under such conditions, optical emission of Si* significantly increases near the mesh and it increases more by increasing hydrogen-dilution-ratio. It is assumed that the spots are generated by hollow-cathode-like discharges in the cavities of charged mesh, and such discharge promotes the decomposition of silane to realize high-rate growth of μc-Si.
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We report on the observation of fast hydrogen atoms in a capacitively coupled RF reactor by optical emission spec- troscopy. For the analysis we use the prominent H emission line of atomic hydrogen in combination with other lines from molecular hydrogen and argon. Several characteristic emis- sion structures can be identified. One of these structures is related to fast hydrogen atoms traveling from the surface of the powered electrode to the plasma bulk. From the appear- ance time within the RF period we conclude that this feature is originated from ion bombardment of the electrode surface. Measured pressure dependencies and a simple model for the ion dynamics support this assumption.
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Crystallite size and crystal-axis orientation have been widely controlled in glow-discharge deposited microcrystalline silicon (muc-Si: H) by setting a third electrode in a capacitively coupled glow-discharge system. Crystallite size of microcrystals has varied continuously from 80 A to 350 A in diameter as the substrate bias voltage is scanned from +100 V to -150 V when the third electrode is kept grounded. Strong preferential orientation to the (220) crystallographic axis of crystallite has also been observed when the positive bias voltage was applied to the substrate heated at 350°C. It has been demonstrated on the basis of mass spectrometric measurement that the crystallite size is strongly affected by the amount of ionic species impinging on the growing surface of muc-Si: H.
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A one-dimensional fluid model for radio-frequency glow discharges is presented which describes silane/hydrogen discharges that are used for the deposition of amorphous silicon (a-Si:H). The model is used to investigate the relation between the external settings (such as pressure, gas inlet, applied power, and frequency) and the resulting composition of the gas and the deposition rate. In the model, discharge quantities such as the electric field, densities, and fluxes of the particles are calculated self-consistently. Look-up tables of the rates of the electron impact collisions as a function of the average electron energy are obtained by solving the Boltzmann equation in a two term approximation for a sequence of values of the reduced electric field. These tables are updated as the composition of the background neutral gas evolves under the influence of chemical reactions and pumping. Pumping configuration and gas inlet are taken into account by adding source terms in the density balance equations. The effect of pumping is represented by an average residence time. The gas inlet is represented by uniformly distributed particle sources. Also the radial transport of neutrals from the discharge volume into the discharge-free volume is important. As the fluid model is one dimensional, this radial transport is taken into account by an additional source term in the density balance equations. Plasma–wall interaction of the radicals (i.e., the growth of a-Si:H) is included through the use of sticking coefficients. A sensitivity study has been used to find a minimum set of different particles and reactions needed to describe the discharge adequately and to reduce the computational effort. This study has also been used to identify the most important plasma-chemical processes and resulted in a minimum set of 24 species, 15 electron-neutral reactions, and 22 chemical reactions. In order to verify the model, including the chemistry used, the results are compared with data from experiments. The partial pressures of silane, hydrogen, disilane, and the growth rate of amorphous silicon are compared for various combinations of the operating pressure (10–50 Pa), the power (2.5–10 W), and the frequency (13.56–65 MHz). The model shows good agreement with the experimental data in the dust free α regime. Discharges in the γ′ regime, where dust has a significant influence, could not be used to validate the model. © 1997 American Institute of Physics.
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Capacitively coupled plasmas (CCPs) generated using high frequency (3–30 MHz) and very high frequency (30–300 MHz) radio-frequency (rf) sources are used for many plasma processing applications including thin film etching and deposition. When chamber dimensions become commensurate with the effective rf wavelength in the plasma, electromagnetic wave effects impose a significant influence on plasma behavior. Because the effective rf wavelength in plasma depends upon both rf and plasma process conditions (e.g., rf power and gas pressure), a self-consistent model including both the rf power delivery system and the plasma discharge is highly desirable to capture a more complete physical picture of the plasma behavior. A three-dimensional model for self-consistently studying both electrodynamic and plasma dynamic behavior of large-area (Gen 10, >8 m2) CCP is described in this paper. This model includes Maxwell’s equations and transport equations for charged and neutral species, which are coupled and solved in the time domain. The complete rf plasma discharge chamber including the rf power delivery subsystem, rf feed, electrodes, and the plasma domain is modeled as an integrated system. Based on this full-wave solution model, important limitations for processing uniformity imposed by electromagnetic wave propagation effects in a large-area CCP (3.05×2.85 m2 electrode size) are studied. The behavior of H2 plasmas in such a reactor is examined from 13.56 to 200 MHz. It is shown that various rectangular harmonics of electromagnetic fields can be excited in a large-area rectangular reactor as the rf or power is increased. The rectangular harmonics can create not only center-high plasma distribution but also high plasma density at the corners and along the edges of the reactor.
Article
We have established, through time correlated plasma emission and electrode and plasma potential measurements, that the near electrode emission observed in asymmetric capacitively coupled 13.56 MHz-driven hydrogen plasmas is caused by field reversal that leads to sheath collapse. Near-electrode emission has now been observed in Ar and He. The field reversal appears to be due to collision-induced electron drag. © 1997 American Institute of Physics.
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It is demonstrated that the signature of bulk hydrogen stretching modes in the infrared of microcrystalline silicon (μc-Si:H) deposited at high deposition rates can be used for solar cell optimization in the high pressure depletion regime. A relation between the performance of a p-i-n solar cell and the hydride stretching modes corresponding to hydrogenated crystalline grain boundaries is observed. These crystalline surfaces show postdeposition oxidation and the absence of these surfaces in the μc-Si:H matrix reflects device grade microcrystalline material.
Article
The ion-bombardment induced surface and bulk processes during hydrogenated amorphous silicon (a‐Si:H) deposition have been studied by employing an external rf substrate bias (ERFSB) in a remote Ar–H2–SiH4 expanding thermal plasma (ETP). The comparison of the ETP chemical vapor deposition without and with ERFSB enables us to identify some important ion-surface and ion-bulk interactions responsible for film property modifications. Employing ERFSB creates an additional growth flux and the low energetic ions deliver an extra 5–10 eV per Si atom deposited at typical deposition rates of 10–42 Å/s which is a sufficient ion dose to modify the film growth. It is demonstrated that the extra surface and bulk process during a‐Si:H growth, induced by the additional ion bombardment, provide an extra degree of freedom to manipulate the a‐Si:H microstructure. An ion-film interaction diagram is introduced, which is used to discriminate ion-surface interactions from ion-bulk interactions. According to this ion-film interaction diagram, the a‐Si:H grown with ERFSB can be roughly classified in three phases. In phase I the only ion-surface process activated is Si surface atom displacement. In phase II also ion-induced Si bulk atom displacement is sufficiently activated, whereas in phase III ion-induced Si atom sputtering is significant. Phase I is characterized by a reduction in the nanosized void density, a reduction in defect density, and an improvement of the photoresponse. We find that the Si surface displacement is the process responsible for various improvements of the material properties via the enhanced surface migration. Phase II is characterized by an enhancement of vacancy incorporation. In accordance with the introduced ion-film interaction diagram, the Si atom bulk displacement process is responsible for the incorporation of additional vacancies. Phase III is characterized by the decrease in growth flux and the increase in void density. The significant contribution of ion-sputtering processes is responsible for the effects observed in phase III.
Article
Triode-type radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) equipment has been developed in order to grow well-aligned carbon nanotubes on Si and glass substrates at 550 °C. The CVD equipment employs a grid electrode in addition to the cathode and anode electrodes. The grid electrode allows the growth of a well-aligned carbon nanotube with an inside and an outside diameter of 7 and 17 nm, respectively. Moreover, the patterning growth of the well-aligned CNT on a glass substrate was also demonstrated.
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In this paper we present a parametric study of the effect of discharge voltage on capacitively coupled, parallel plate (CCPP) radio frequency discharges in pure hydrogen at low pressure, performed using a 1D(r)2D(v) particle in cell/Monte Carlo collision model with self-consistent neutral kinetics and also compare our results with experimental and theoretical ones reported in the literature. In the first part of the paper, we review the essential features of the numerical code, together with the database of plasma particles and neutral kinetics data. Results are discussed, in particular, for charged particle density and energy, the appearance of the double layer phenomenon, the plasma potential and the atom density. A possible role of photoelectric emission in the charged particle balance is also discussed.
Article
The effects of space charge on the energy resolution obtainable with deflection type analyzers have been considered previously for cylindrical beams. In the present analysis the problem of the electric field in a retarding potential analyzer is considered. This electric field is larger than the transverse field which causes radial expansion. Extension of the analysis to two dimensions in a simple case shows, however, that the longitudinal field is much reduced when the beam has a diameter small compared to its length.