J. Campmany

Effects of plasma processing on the microstructural properties of silicon powders

The effects of plasma processing conditions on the microstructural properties of silicon powders are presented. Hydrogenated nanophase silicon powders were prepared using low-pressure and low-temperature square wave modulated RF plasma (13.56 MHz) using pure silane gas. Plasma parameters such as pressure, RF power, plasma modulation frequency, and gas flow rate were varied. In situ analysis by quadrupolar mass spectroscopy and ex situ analysis of the silicon powders by Fourier transform infrared spectroscopy (FTIR) and thermal desorption spectrometry of hydrogen were performed. The thermal desorption spectrometry results show the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films. The FTIR analysis also determined the microstructural characteristics of powders and hence their volume/surface ratio. This parameter was determined from the balance of Pj probabilities of having one of the Hj-Si-Si4-j bond arrangements in the powder particles. These results reveal an increase in hydrogen content and a reduction in volume/surface ratio as the modulation frequency of RF power increases. In consequence, higher compactness of silicon powders is associated with long particle residence times inside the plasma as a result of ion bombardment. TEM analysis indicated a considerable dispersion of particle size and some degree of structure of the silicon powder characterized by intergrain linkage. We point out the dominant presence of hydrogen on the particle surfaces (external voids), which may cause the high reactivity of grains, increasing the degree of intergrain linkage.

Preparation of nanoscale amorphous silicon based powder in a square-wave-modulated rf plasma reactor

Abstract Nanoscale amorphous silicon powder was prepared in a specially designed plasma reactor, operating with square-wave modulation (SQWM) of the rf power at low pressure and low temperature. We have observed that in order to increase the powder yield, the process temperature must be lower and the pressure must be higher than that normally used for thin film deposition of a-Si: H in plasma enhanced chemical vapour deposition (PECVD). The square-wave modulation (SQWM) of the rf generator at low frequencies also improves the generation of powder. The experimental set-up was equipped with a quadrupole mass spectrometer (QMS), which directly analyses the species present in the plasma, and with a time- and spatially- resolved optical-emission spectroscopy system (OES). The characterization of the amorphous silicon powders by electron microscopy (TEM) and thermal desorption spectroscopy (TDS) of hydrogen provides the nanoscale powder size, microstructure and chemical composition.

Properties of amorphous silicon thin films grown in square wave modulated silane rf discharges

Hydrogenated amorphous silicon (a‐Si:H) thin films have been obtained from pure SiH4 rf discharges by using the square wave modulation (SQWM) method. Film properties have been studied by means of spectroellipsometry, thermal desorption spectrometry, photothermal deflection spectroscopy and electrical conductivity measurements, as a function of the modulation frequency of the rf power amplitude (0.2–4000 Hz). The films deposited at frequencies about 1 kHz show the best structural and optoelectronic characteristics. Based upon the experimental results, a qualitative model is presented, which points up the importance of plasma negative ions in the deposition of a‐Si:H from SQWM rf discharges through their influence on powder particle formation.Hydrogenated amorphous silicon (a‐Si:H) thin films have been obtained from pure SiH4 rf discharges by using the square wave modulation (SQWM) method. Film properties have been studied by means of spectroellipsometry, thermal desorption spectrometry, photothermal deflection spectroscopy and electrical conductivity measurements, as a function of the modulation frequency of the rf power amplitude (0.2–4000 Hz). The films deposited at frequencies about 1 kHz show the best structural and optoelectronic characteristics. Based upon the experimental results, a qualitative model is presented, which points up the importance of plasma negative ions in the deposition of a‐Si:H from SQWM rf discharges through their influence on powder particle formation.

Spectroscopic ellipsometric characterization of transparent thin film amorphous electronic materials : Integrated analysis

Abstract Silicon dioxide and silicon oxynitride thin films deposited on crystalline silicon substrates were characterized by phase modulated spectroscopic ellipsometry in the UV-visible range. The thickness and the refractive index of the films were calculated by assuming an ambient/film/substrate system with three different approaches: modeling the dielectric function with Cauchys dispersion formula, using the Bruggeman effective medium approximation and using a direct inversion algorithm method. The comparison of the results obtained by these approaches along with the subsequent qualitative study of possible weak absorption and depth inhomogeneities in the samples allows us to evaluate the accuracy of the experimental data as well as the reliability of the model and results.

Plasma-deposited silicon nitride films with low hydrogen content for amorphous silicon thin-film transistors application

Abstract A comparative study of the vibrational properties of PECVD amorphous silicon nitride films obtained from SiH 4 + NH 3 and SiH 4 + N 2 precursor gas mixtures has been performed by FT-IR transmission spectroscopy. The bonded hydrogen, calculated from the absorption spectra, shows important quantitative and qualitative differences depending on the precursor gas mixtures used. The hydrogen content of near-stoichiometric films obtained from SiH 4 + N 2 mixture is 10 times lower than that of films prepared from SiH 4 + NH 3 mixture. In addition, hydrogen is mainly bonded to nitrogen atoms in films from SiH 4 + NH 3 , whereas it is mainly bonded to silicon atoms in films from SiH 4 + N 2 . These low-hydrogenated silicon nitride films, obtained from mixtures containing N 2 , have been applied as insulator layers in the preparation of amorphous silicon thin-film transistors (a-Si TFTs). The TFTs were of normal staggered type composed of the structure Al/a-SiN:H/a-Si:H grown on NiCr source and drain electrodes deposited on glass substrates. TFTs with a 0.2 μm thick a-Si:H layer and 10 μm channel length have on-off current ratios of 5 × 10 4 , electron field-effect mobilities of about 1.5 cm 2 /V s and threshold voltages around 5 V.

Effect of substrate temperature on deposition rate of rf plasma‐deposited hydrogenated amorphous silicon thin films

We present a study about the influence of substrate temperature on deposition rate of hydrogenated amorphous silicon thin films prepared by rf glow discharge decomposition of pure silane gas in a capacitively coupled plasma reactor. Two different behaviors are observed depending on deposition pressure conditions. At high pressure (30 Pa) the influence of substrate temperature on deposition rate is mainly through a modification of gas density, in such a way that the substrate temperature of deposition rate is similar to pressure dependence at constant temperature. On the contrary, at low pressure (3 Pa), a gas density effect cannot account for the observed increase of deposition rate as substrate temperature rises above 450 K with an activation energy of 1.1 kcal/mole. In accordance with laser‐induced fluorescence measurements reported in the literature, this rise has been ascribed to an increase of secondary electron emission from the growing film surface as a result of molecular hydrogen desorption.

Error minimization method for spectroscopic and phase-modulated ellipsometric measurements on highly transparent thin films

We point out that there is an intrinsic magnification of error in the measurement of transparent or semitransparent thin films by the usual method of phase-modulated ellipsometry. This procedure is suitable for absorbing materials, but for nonabsorbing materials it gives a great amount of error in the measurement of ellipsometric angles at some critical values. We propose a new methodology for the phase-modulated ellipsometric measurements that avoids this magnification. We illustrate the advantages of this new method by measuring the index of refraction of a low-pressure chemical-vapor-deposited a-SiO2 thin film with greater accuracy than that achieved by the usual method.

Effect of hydrogen dilution on the growth of hydrogenated amorphous silicon studied by in-situ phase-modulated ellipsometry

Abstract We have conducted a study of the growth of hydrogenated amorphous silicon (a-Si:H) thin films, deposited in SiH 4 H 2 r.f. discharges, through spectroscopic and real-time phase-modulated ellipsometry. The study focused on the microstructure of the a-Si:H-crystalline Si interface and the a-Si:H bulk layer. Real-time data were analysed using a theoretical growth model, previously proposed, which assumes cylindrical nucleation, coalesence and long-term growth phases. For high silane fractions (above 60%) the results are compatible with the existence of an abrupt interface, whereas for low silane fractions (20%) the real-time ellipsometric data can be modelled assuming the existence of an interface, 1.2 nm thick, composed of SiO 2 (95%) and a-Si:H (5%). The ellipsometric results indicate an optimal degree of silane fraction for which the bulk void fraction is minimized. The results are discussed in terms of growth kinetics.

Optical, vibrational and compositional study of amorphous silicon oxynitride thin films grown by an RF plasma using N2O + SiH4 gas mixtures

Abstract Highly transparent hydrogenated amorphous silicon oxynitride (a-SixOy:H) thin films were obtained in an RF plasma reactor from SiH4 and N2O gases at 300°C of substrate temperature. This paper presents a comparative study of the optical, vibrational and compositional properties of the a-SiNxOy:H films through spectroscopic ellipsometry, FT-IR and XPS techniques. Ellipsometric results are modeled assuming a simple structure air/film/substrate and applying the Bruggeman effective medium approximation for a mixture of stoichiometric Si3N4, SiO2 and SiO materials. These results are interpreted in terms of the chemical reactions taking place in the plasma and the microstructure of the films. Those obtained from N2O/SiH4 ≥ 20 can be interpreted as a mixture of phases. Below this ratio, the films obtained can be interpreted in terms of random models.

Spectroscopic ellipsometry measurements of the diamond-crystalline Si interface in chemically vapour-deposited polycrystalline diamond films

Abstract Optical characterization of polycrystalline diamond films has been performed by spectroscopic phase-modulated ellipsometry measurements in the UV-visible range. The films were obtained from a gas mixture of methane diluted in hydrogen in a hot-filament chemical vapour deposition system on crystalline silicon wafers previously polished with diamond powder. The ellipsometric measurements were obtained from the structure interface-layer/diamond-film after a chemical etching of the substrate. The back surface of the diamond films exhibits a specular surface with high reflectivity, allowing precise ellipsometric measurements. A two-layer structure is deduced from the ellipsometric data and it is discussed together with the X-ray diffraction results.