Control of plasma process instabilities during thin silicon film deposition

Fourier transform infrared absorption spectroscopy (FTIR), optical emission spectroscopy (OES), self-bias voltage and plasma impedance controls were applied as in situ process diagnostics during the deposition of amorphous silicon thin-films. The diagnostic abilities of OES and FTIR are compared. The FTIR in-situ direct measurement of silane concentration in exhaust line is more precise than OES control. All in situ process diagnostics clearly indicates the inconsistency of plasma properties and therefore of deposition conditions. The drifts are comparable with the film deposition time. The FTIR measurement of reactant concentration in the process chamber evidence that the strong silane concentration drop (about 50%) in a plasma is the cause of the short-term drift of OES signals (SiH? emission), plasma impedance and self-bias voltage signals. The influences of the deposition chamber geometry and technological parameters on process drifts are considered. The decrease of the gas residence time in the reactor leads to a decrease of Initial Transient State phenomena. Finally, the improvement of solar cell performance based on thin silicon films is demonstrated when drifts are reduced.     

Hydrogenated microcrystalline silicon films prepared by VHF-PECVD and single junction solar cell

Intrinsic microcrystalline silicon films have been prepared with very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) from silane/hydrogen mixture at 180°C. The effect of silane concentration and discharge power on the growth of silicon films was investigated. Samples were investigated by Fourier transform infrared spectroscopy, Raman scattering and X-ray diffraction. The Raman spectrum shows that the morphological transition from microcrystalline to amorphous occurs under conditions of high silane concentration and low discharge power. X-ray diffraction spectra indicate a preferential growth direction of all microcrystalline silicon films in the (111) plane. In addition, a solar cell with an efficiency of 5.1% has been obtained with the intrinsic microcrystalline layer prepared at 10W.     

Synthesis of high purity silicon nanoparticles in a low pressure microwave reactor

The formation of pure single crystalline silicon nanoparticles by microwave induced decomposition of silane in a low pressure flow reactor is reported. The morphology and crystal structure of the particles are characterized in situ by particle mass spectrometry (PMS) and ex situ by means of X-ray diffraction, high resolution transmission electron microscopy, electron energy loss spectroscopy, and infrared spectroscopy. The preparation method allows for the adjustment of the mean particle diameter in the range 6 nm < or = dPM < or = 11 nm by controlling the precursor concentration, gas pressure, and microwave power. Spectroscopic investigations reveal that the particles are single crystal silicon. The potential on n- or p-type doping is in progress.     

Characterization of silicon oxide gas barrier films with controlling to the ion current density (ion flux) by plasma enhanced chemical vapor deposition

Silicon oxide gas barrier films were deposited on polyethylene terephthalate (PET) substrates by plasma enhanced chemical vapor deposition (PE-CVD) for applications to transparent barrier packaging. The barrier properties of the silicon oxide coated film were optimized by varying the bias conditions and input power in the radio frequency plasma. The plasma diagnostics, ion current density and substrate temperature were characterized by optical emission spectrometry (OES), an oscilloscope and thermometer, respectively. The coating properties were examined by Fourier transform infrared (FT-IR) spectroscopy and the water vapor transmission rate (WVTR). A high intensity of O and H ions and a high ion current density (ion flux) with a low temperature plasma process were found to be suitable for improving the barrier properties of the silicon oxide film coatings. The Si-O cage-like structure adversely affected the gas barrier properties of the deposited coating. The energy provided by ion bombardment (ion flux) can induce changes in the film density and composition similar to those that may occur by the increase in deposition temperature through rf bias. In addition, the film properties depend not only on a high ion current density (ion flux) and input power, but are also related to a silicon oxide film with a widely distributed planar ring size.     

Deposition of microcrystalline silicon in an integrated distributed electron cyclotron resonance PECVD reactor

The deposition of μc-Si in a low pressure high density plasma reactor is studied. Films were deposited either from pure silane or from the mixture of SiH₄ and H₂ onto glass substrates and deposition kinetics followed with kinetic phase modulated ellipsometry Growth rates of up to 0.8 nm/s were achieved with good quality material. Crystalline fraction shows a strong dependence on process pressure and exceeds 80% for samples grown at optimal conditions. It is found that hydrogen dilution is not needed for integrated distributed electron cyclotron resonance (IDECR) discharge to produce crystallized material. The grain size measured with X-ray diffraction was found to be between 10 and 15 nm and of single (111) orientation. Both ellipsometric data and Raman analysis show a strong dependence of crystallinity or hydrogen residence time in the reactor.     

Reducing the effects of mismatch between zinc oxide and silicon by silane plasma modification

We investigated the mismatch between zinc oxide (ZnO) and silicon (Si) upon reduction by silane plasma modification in a plasma-enhanced chemical vapor deposition system. This plasma treatment was only carried out for 10?s and the Si–H bonds that were provided by the silane plasma modification as dangling bonds on the Si wafer in addition to functioning as a conjunction layer to reduce the defects. The X-ray diffraction analysis of the ZnO/p-type silicon structure produced by silane plasma modification has a slightly lower full width at the half maximum, which improved the ZnO film’s crystalline properties. After the silane plasma modification ZnO/Si diode is produced, the measured current–voltage characteristics gave favorable rectifying properties and reverse bias had a low leakage current. The ZnO/Si diode under illumination increased the short-circuit current (I_sc) from 7.32 to 19.75?mA/cm², which is an improvement compared with a conventional bare ZnO/Si diode because of the reduced ZnO/Si interface states. Therefore, the silane plasma modification diminishes the effects of the interface and improves the ZnO/Si diode performance.     

Microcrystalline silicon deposition: Process stability and process control

Applying in situ process diagnostics, we identified several process drifts occurring in the parallel plate plasma deposition of microcrystalline silicon (μc-Si:H). These process drifts are powder formation (visible from diminishing dc-bias and changing spatial emission profile on a time scale of 10⁰ s), transient SiH₄ depletion (visible from a decreasing SiH emission intensity on a time scale of 10² s), plasma heating (visible from an increasing substrate temperature on a time scale of 10³ s) and a still puzzling long-term drift (visible from a decreasing SiH emission intensity on a time scale of 10⁴ s). The effect of these drifts on the crystalline volume fraction in the deposited films is investigated by selected area electron diffraction and depth-profiled Raman spectroscopy. An example shows how the transient depletion and long-term drift can be prevented by suitable process control. Solar cells deposited using this process control show enhanced performance. Options for process control of plasma heating and powder formation are discussed.     

Microwave plasma enhanced CVD of aluminum oxide films: OES diagnostics and influence of the RF bias

Aluminum oxide films are obtained in a remote-microwave-plasma-enhanced chemical vapour deposition (RMPECVD) reactor. In this technique, only oxygen gas plasma is generated while trimethylaluminum (TMA) carried by argon is fed near the substrate holder which can be RF biased and deposition occurs at the surface of a silicon wafer. Optical emission spectroscopy (OES) is used in order to gain information on the plasma excitation of various species. The presence of an inert gas (argon) allows an evaluation of the relative densities of reactive species by the actinometry technique. A comparison has been done with and without RF biasing or TMA injection at different microwave powers in the same conditions of Ar and O₂ flow rates. The RF biasing creates an additional plasma near the substrate holder but has a weak influence on the OES signal. The microwave plasma is the major mode in the present experimental set up. Less than 2% of TMA modifies the plasma chemistry by improving the atomic oxygen formation. In addition, the TMA decomposition involves a cooling effect of the electron energy. Although this dual mode configuration of the plasma production induces a slight decrease of the deposition rate, it limits the incorporation of impurities (O, H) in the layer by improving the ion bombardment and etching of the growing layer.     

LOW FREQUENCY GLOW DISCHARGE HYDROGENATED AMORPHOUS SILICON CARBIDE FILMS

Hydrogenated amorphous Si_xC_1-x:H films with various compositions were prepared by low frequency (110 kHz) glow discharge decomposition of a silane and methane mixture diluted by helium. The deposition rate, the composition, the structure and the refractive index of the films were studied using SEM, AES and IB analyses. Only the deposition rate was found to be influenced by the position of the silicon substrate in the reactor. From infrared analyses we estimate that the structure of the films is a tetrahedral network where carbon atoms are randomly replaced by silicon atoms and where only one single hydrogen is bound to silicon. The rather dense and inorganic character of these films is confirmed by the high values of their refractive index ranging from 2.1 for x=0 to 3.4 for x=1.     

Numerical simulation of nanoparticle-generating electronegative plasmas in the PECVD of nanostructured silicon film

The results of numerical simulation of the equilibrium parameters of a low pressure nanopowder-generating discharge in silane for the plasma enhanced chemical vapor deposition (PECVD) of nanostructured silicon-based films are presented. It is shown that a low electron temperature and a low density of negative SiH₃⁻ ions are favorable for the PECVD process. This opens a possibility to predict the main parameters of the reactive plasma and plasma-nucleated nanoparticles, and hence, to control the quality of silicon nanofilms.     

Influence of dopant concentration and type of substrate on the local organization of low-pressure chemical vapour deposition in situ boron doped silicon films from silane and boron trichloride

The deposition of in situ boron doped silicon films from boron trichloride BCl₃ and silane SiH₄ in a conventional low-pressure chemical vapour deposition reactor has been studied for high boron doping levels and two kinds of substrates (SiO₂ and Si₃N₄). On the basis of transmission electron microscopy and X-ray photoelectron spectroscopy results, these films appear to be highly sensitive to the local electronic environment of both substrate and deposited atoms. Indeed, beyond a critical doping level, this material becomes more and more amorphous, due to the occurrence of a particular organization of boron atoms in the silicon matrix. This behaviour results in a lowering of the well-known boron enhancement effect for deposition rate and crystalline fraction.     

Preparation and properties of novel epoxy/graphene oxide nanosheets (GON) composites functionalized with flame retardant containing phosphorus and silicon

2-(Diphenylphosphino)ethyltriethoxy silane (DPPES) was grafted onto the surface of graphene oxide nanosheets (GON) via a condensation reaction. X-ray photoelectron spectroscopy, X-ray diffractometry, Fourier transform infrared spectroscopy and Raman spectroscopy verify that DPPES did not only covalently bond to GON as a functionalization moiety, but partly restored its conjugated structure as a reducing agent. DPPES on graphene sheets oxide was observed by transmission electron microscopy, and contributed to the favorable dispersion of DPPES-GON in nonpolar toluene. Additionally, the flame retardancy and thermal stability of epoxy/DPPES-GON nanocomposites that contain various weight fractions of DPPES-GON were studied using the limiting oxygen index test, UL-94 test and by thermogravimetric analysis in nitrogen. The composites containing 10 wt% DPPES-GON can pass V-0 rating in UL-94 test. Adding 10 wt% DPPES-GON in epoxy greatly increased the char yield and LOI by 42% and 80%, respectively. Epoxy/DPPES-GON nanocomposites with phosphorus, silicon and graphene layer structures were found to exhibit much greater flame retardancy than neat epoxy. The synergistic effects among silicon, phosphorus and GON can improve the flame retardancy of epoxy resin.     

Effect of silane flow rate on structural,electrical and optical properties of silicon thin films grown by VHF PECVD technique

Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow rates have been investigated. The silane flow rate was varied from 5 sccm to 30 sccm, maintaining all other parameters constant. The electrical, structural and optical properties of these films were systematically studied as a function of silane flow rate. These films were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and UV–visible (UV–Vis) spectroscopy. Different crystalline volume fraction (22%–60%) and band gap (∼1.58 eV–∼1.96 eV) were achieved for silicon thin films by varying the silane concentration. A transition from amorphous to nanocrystalline silicon has been confirmed by Raman and FTIR analysis. The film grown at this transition region shows the high conductivity in the order of 10⁻⁴ Ω⁻¹ cm⁻¹.     

Comparing single-walled carbon nanotubes and samarium oxide as strain sensors for model glass-fibre/epoxy composites

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm₂O₃, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre–matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm³⁺ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm³⁺ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites.     

流态床CVD法纳米氮化硅粉体的制备

采用硅烷和氨气在立式双温区流态床中化学气相沉积, 制备了形状规则的球状无定形氮化硅纳米粉体, 并利用X射线衍射(XRD)、透射电镜(TEM)和傅立叶红外(FTIR)研究了该纳米粉体的 形貌、成分和物相. 讨论了该流态床法制备纳米氮化硅的关键因素, 并得到流态床中制备氮化硅纳米粉体的优化工艺参数.     

Optical emission spectroscopy as a process control tool during plasma enhanced chemical vapor deposition of microcrystalline silicon thin films

The decisive criterion associated with the species emission intensity ratio (Hα/SiH*) which characterizes the crystallinity of microcrystalline silicon (μc-Si) film was found to display an unstable behavior resulting from species concentration variation during μc-Si film growth with optical emission spectroscopy (OES) tool. In this study, a real-time process control system i.e. closed-loop system was developed. It aims to control the species intensity ratio with OES device in a very high frequency (VHF) plasma enhanced chemical vapor deposition reactor, via modulating the VHF power and silane dilution to improve μc-Si film growth for high efficiency a-Si/μc-Si tandem solar cell. The experiment results show that the closed-loop system stabilized the Hα/SiH* intensity ratio within a variation of 5% during the μc-Si film deposition process. Higher growth rate of μc-Si film with the same crystallinity was obtained in the closed loop system which consumed less power and SiH4 gas than in the open loop system, i.e. without process control.     

PCVD法对碳化硅陶瓷的表面改性研究

利用等离子辉光放电化学气相沉积技术（ＰＣＶＤ），控制甲烷与磋烷质量流量比、硅烷与氨质量流量比，在碳化硅基体表面分别沉积上无定形碳化硅和氨化硅薄膜，研究其膜的组成、沉积工艺、厚度等对碳化硅陶瓷的强度改性影响．在一定的沉积条件下沉积的碳化硅薄膜可以使基体强度提高２０％达到８５０ＭＰａ，沉积氮化硅薄膜使强度提高３０％达到９００ＭＰａ，改性的效果很明显．     

Chemical-reaction dependence of plasma parameter in reactive silane plasma

Electron temperature in a silane glow-discharge plasma, being an important plasma parameter for determining photo-induced instability in the resulting hydrogenated amorphous silicon (a-Si:H), has been studied under various film-preparation conditions. We have used an optical-emission-intensity ratio of Si^* to SiH^* (I_si*/I_siH^*) which corresponds to the high-energy-tail slope of the electron-energy-distribution function in the plasma as a measure of electron temperature in a reactive silane glow-discharge plasma. We have found quite differently from the conventional non-reactive glow-discharge plasma such as hydrogen plasma that the electron temperature in the silane plasma is strongly modified by the substrate temperature (gas temperature) especially under high silane-gas partial-pressure condition. This anomalous behavior of the electron temperature in the silane plasma has been explained by means of gas-phase-polymerization reaction and electron-attachment process to the polymers in the plasma. The electron temperature has been remarkably reduced when a hydrogen-dilution method and a cathode-heating method are used which are considered to control polymer-formation reactions in the silane plasma together with utilization of conventional electron-temperature-controlling methods such as a very high plasma-excitation frequency and an application of magnetic field for electron-confinement. As a consequence of the reduction of electron temperature in the silane plasma, highly stabilized a-Si:H has been successfully obtained even under high growth rate conditions of 1.5 nm s^-1.     

Silicon nanocrystal production through non-thermal plasma synthesis: a comparative study between silicon tetrachloride and silane precursors

Silicon nanocrystals with sizes between 5 and 10?nm have been produced in a non-thermal plasma reactor using silicon tetrachloride as precursor. We demonstrate that high-quality material can be produced with this method and that production rates as high as 140?mg?h(-1) can be obtained, with a maximum precursor utilization rate of roughly 50%. Compared to the case in which particles are produced using silane as the main precursor, the gas composition needs to be modified and hydrogen needs to be added to the mixture to enable the nucleation and growth of the powder. The presence of chlorine in the system leads to the production of nanoparticles with a chlorine terminated surface which is significantly less robust against oxidation in air compared to the case of a hydrogen terminated surface. We also observe that significantly higher power input is needed to guarantee the formation of crystalline particles, which is a consequence not only of the different gas-phase composition, but also of the influence of chlorine on the stability of the crystalline structure.     

Evolution of structural and optical properties of nanostructured silicon carbon films deposited by plasma enhanced chemical vapour deposition

Nanostructured silicon carbon films composed of silicon nanocrystallites embedded in hydrogenated amorphous silicon carbon matrix have been deposited by plasma enhanced chemical vapour deposition technique using silane and methane gas mixture highly diluted in hydrogen. The structural and optical properties of the films have been investigated by X-ray diffraction, Raman, Fourier transform infrared, ultra violet-visible-near infrared and photoluminescence spectroscopies while the composition of the films has been obtained from nuclear techniques. The study has demonstrated that the structure of the films evolves from microcrystalline to nanocrystalline phase with the increase in radio frequency (rf) power. Further, it is shown that with increasing the rf power the size of silicon nanocrystallites decreases while the optical gap increases and a blueshift of visible room temperature photoluminescence peak can be observed.