4H-SiC纳米薄膜的微结构及其光电性质研究

采用新设计的电极结构的等离子体增强化学气相沉积(PECVD)技术,在高功率密度、高氢稀释比、低温、偏压及低反应气压的条件下,在衬底表面形成双等离子流,增加了衬底表面SiC的成核概率,增强成核作用,形成纳米晶.采用高H2等离子体刻蚀弱的、扭曲的、非晶Si—C及Si—Si和Si—H等键时,由于H等离子体对纳米SiC晶粒与非晶态键的差异刻蚀作用,产生自组织生长,发生晶化.Raman光谱和透射电子衍射(TEM)的测试结果表明,纳米晶SiC是4HSiC多型结构.电子显微照片表明平均粒径为16nm,形状为微柱体.实验结果指出,SiC纳米晶的形成必须经过偏压预处理成核,并且其晶化存在一个功率密度阈值;当低于这一功率密度阈值时,晶化消失;当超过这一阈值时,纳米晶含量随功率密度的提高而增加.随着晶化作用的加强,电导率增加. 关键词： 4H-SiC PECVD 纳米结构 多型薄膜 纳米电子学     

Thin film deposition from hexamethyldisiloxane fed glow discharges

Hexamethildisiloxane-oxygen fed radiofrequency discharges have been studied under high power density and 0.05÷0.1 torr pressure, i.e., experimental conditions of high monomer fragmentation. Actinometric Optical Emission Spectroscopy of plasma phase, X-Ray Photoelectron Spectroscopy and Infrared Spectroscopy analyses of the deposited films allowed to set the basis of the first “semi-quantitative” mechanism of deposition of siloxane films which can reasonably account for both deposition rates and film compositions. A by-product of the research is the use of CH-to-Ar emission ratio as a probe of carbon content in the film, an important practical parameter for process control.     

Characterization of Defects in PECVD-SiO2 Coatings on PET by Confocal Microscopy

Thin, transparent ceramic coatings on polymers are effective barriers against gas and vapor transmission. However, they always display some residual permeation, which can be attributed to defects. The main sources of these defects are dust particles on the polymer surface before deposition, and roughness of the polymer surface due to the presence of so-called antiblock particles. The transparency and extreme thinness of the films (d 50 nm) render the detection of defects virtually impossible by optical and even by electron microscopies. However, by using a technique based on reactive ion etching (RIE) in oxygen plasma, we are able to render defects visible, even by optical microscopy at relatively low magnification (100 ×). In the present article we present a confocal microscopy study, which has helped to better understand the effect of RIE at defect sites, as well as the origins of the defects in these coatings.     

Plasma-Enhanced CVD of Organosilicate Glass (OSG) Films Deposited from Octamethyltrisiloxane,Bis(trimethylsiloxy)methylsilane,and 1,1,3,3-Tetramethyldisiloxane

Octamethyltrisiloxane (OMTS), bis(trimethylsiloxy)methylsilane (BTMS), or 1,1,3,3-tetramethyldisiloxane (TMDS) mixed with oxygen and argon produced organosilicate glass (OSG) films having high methyl content under pulse plasma-enhanced chemical vapor deposition (PECVD). The hydrogen concentration in films deposited from OMTS, BTMS, and TMDS can exceed twice the maximum concentration in films grown from methylsilane precursors. However, the refractive indices and dielectric constants of the films grown from OMTS, BTMS, and TMDS were close to that of minimum values observed for the films grown from methylsilane precursors. This suggests that at high concentrations, methyl incorporation does not result in further reduction of film density. ²⁹Si nuclear magnetic resonance (NMR) resolves the complex differences in the structure of the OSG films that are not apparent from Fourier transform infrared spectroscopy (FTIR) and reveals that the stability of OSG materials is improved by eliminating M groups and favoring the incorporation of T groups. These films exhibit low dielectric constants in the range of 2.4–2.6.     

Plasma chemical preparation and characterization of perovskite-type mixed oxides

The microwave plasma enhanced chemical vapour deposition (MPECVD) was applied for the preparation of perovskite-like mixed oxides, using either the metal acetylacetonates or metal nitrates as precursors. The same materials were prepared by the citrate method as reference substances. The obtained mixed oxides were calcined at 1100 °C, tested in the methane oxidation up to 1000 °C and characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductive coupled plasma–optical emissions spectroscopy (ICP–OES) and specific surface area (BET) determination. These studies revealed that LaMnO₃ with La³⁺ cation partly substituted by Sr, Ce and Ba cations and La_0.9Sr_0.1Co_0.9Fe_0.1O₃ calcined at 1100 °C seem to be highly active and stable catalysts in methane oxidation. In the case of LaMnO₃ and La_0.9Ce_0.1MnO₃ lowest light-off temperatures were reached with the catalysts prepared by the plasma method.     

UV Absorptance of Titanium Dioxide Thin Films by Plasma Enhanced Deposition from Mixtures of Oxygen and Titanium-Tetrakis-Isopropoxide

A low pressure radio frequency discharge was used to deposit films by mixtures of oxygen and titanium (IV) isopropoxide (TTIP) at powers of 200 W on films of polyethylene-terephthalat and samples of quartz glass. In the non-thermal plasma, films of rather pure TiO₂ could be deposited as revealed by X-ray photo-electron spectroscopy. Besides the film growth rate and the chemical composition, the spectral behaviour of the spectral transmittance of visually transparent films was determined in the range from 200 to 500 nm. Furthermore, the absorptance of films has been derived at characteristic spectral positions of the transmission spectra of the films. Accordingly, cut-off wavelength was found to increase with deposition time from 5 to 10 min as well as with the concentration of TTIP in a range below 1.7%. At 310 nm, the spectral absorption coefficient (extinction coefficient × concentration) was 12 μm⁻¹. While keeping other parameters constant, this coefficient decreased by 4 μm⁻¹ due to an increase of the concentration of TTIP from 1.7% to 8%. Simultaneously, the surface roughness increased as revealed by profilometry. Thus, since the chemical structure of films was found to change only marginally, a decrease of the film density is likely to cause the observed dependence of the absorption coefficient with increasing precursor concentration.     

Influence of interfaces density and thermal processes on mechanical stress of PECVD silicon nitride

The paper focuses on a particular silicon nitride thin film (SiN_x) produced by plasma enahanced chemical vapor deposition (PECVD) technique with high deposition rate (26 nm/min) and low values of mechanical stress (<100 MPa). This was perfomed with mixed frequency procedure varying the modulation of high frequency at 13.56 MHz and low frequency at 308 kHz of RF power supply during the deposition, without changing the ratio of reaction gases. Low stress silicon nitride is commonly obtained by tailoring the thickness ratio of high frequency vs. low frequency silicon nitride layers.The attention of this work was directed to the influence of the number of interfaces per thickness unit on the stress characteristics of the deposited material. Two sets of wafer samples were deposited with low stress silicon nitride, with a thickness of 260 nm and 2 μm, respectively. Thermal annealing processes at 380 and 520 °C in a inert enviroment were also performed on the wafers.The Stoney–Hoffman model was used to estimate the stress values by wafer curvature measurement with a mechanical surface profilometer: the stress was calculated for the as-deposited layer, and after each annealing process.The thickness and the refractive index of the SiN_x were also measured and charaterized by variable angle spectra elliposometry (VASE) techinique.The experimental measurements were performed at the MT-LAB, IRST (Istituto per la Ricerca Scientifica e Tecnologica) of Bruno Kessler Foundation for Research in Trento.     

Cross‐Linking and Degradation Properties of Plasma Enhanced Chemical Vapor Deposited Poly(2‐hydroxyethyl methacrylate)

Plasma Enhanced Chemical Vapor Deposition (PECVD) of poly‐2‐hydroxyethyl methacrylate (pHEMA) biocompatible, biodegradable polymer films were produced alone and cross‐linked with ethylene glycol diacrylate (EGDA). Degree of cross‐linking was controlled via manipulation of the EGDA flow rate, which influenced the amount of swelling and the extent of degradation of the films in an aqueous solution over time. Noncross‐linked pHEMA films swelled 10% more than cross‐linked films after 24 h of incubation in an aqueous environment. Increasing degree of film cross‐linking decreased degradation over time. Thus, PECVD pHEMA films with variable cross‐linking properties enable tuning of gel formation and degradation properties, making these films useful in a variety of biologically significant applications.

     

SiCl4浓度对微晶硅薄膜生长及光电特性的影响

研究了SiCl4浓度对等离子体增强化学气相沉积(PECVD)系统中以SiCl4/H2为反应气体的微晶硅薄膜生长及光电特性的影响.结果表明,微晶硅薄膜的沉积速率和晶化率均随SiCl4浓度的增加而增大,而晶粒平均尺寸在SiCl4浓度小于65;时呈增大趋势,在SiCl4浓度大于65;时呈减小趋势;此外,光照实验表明制备的微晶硅薄膜具有较稳定的微观结构,具有类稳恒光电导效应,且样品的电导率依赖于SiCl4浓度的变化.此外,还讨论了Cl基基团在微晶硅薄膜生长过程中所起的作用.     

VHF-PECVD沉积本征微晶硅薄膜激活能特性研究

采用激活能测试装置测量VHF-PECVD高速沉积的本征微晶硅薄膜,并对不同晶化率的样品和不同沉积功率、不同沉积压强条件下沉积制备的样品的激活能进行了分析研究.结果表明:在非晶-微晶相变域附近,激活能随着晶化率的升高而降低;随着沉积功率的增大和沉积气压的增大,沉积速率提高,样品的激活能升高,通过提高沉积功率和沉积气压可以有效的抑制氧污染.