Low-temperature RF Plasma Research Articles

Photoluminescence and paramagnetic defects in silicon-implanted silicon dioxide layers

Thermally grown SiO 2 layers on Si substrates implanted with Si + ions with a dose of 6×10 16 cm −2 were studied by the techniques of photoluminescence, electron paramagnetic resonance (EPR), and low-frequency Raman scattering. Distinct oxygen-vacancy associated defects in SiO 2 and non-bridging oxygen hole centers were identified by EPR. The luminescence intensity in the 620 nm range was found to correlate with the number of these defects. The low-frequency Raman scattering technique was used to estimate the average size of the Si nanocrystallites formed after the implantation and thermal annealing at T>1100°C, which are responsible for the photoluminescence band with a maximum at 740 nm. The intensity of this band can be significantly enhanced by an additional treatment of the samples in a low-temperature RF plasma.

Reaction Processes for Low Temperature (<150°C) Plasma Enhanced Deposition of Hydrogenated Amorphous Silicon Thin-Film Transistors on Transparent Plastic Substrates

AbstractThis article presents mechanisms for low temperature (<150°C) rf plasma enhanced chemical vapor deposition of silicon and silicon nitride thin films that lead to sufficient electronic quality for thin film transistor (TFT) fabrication and operation. For silicon deposition, hydrogen abstraction and etching, and silicon disproportionation reactions are identified that can lead to optimized hydrogen concentration and bonding environments at <150°C. Nitrogen dilution of SiH4/NH3 mixtures during silicon nitride deposition at low temperatures helps promote N-H bonding, leading to reduced charge trapping. Good quality amorphous silicon TFT's fabricated with a maximum processing temperature of 110 °C are demonstrated on flexible transparent plastic substrates. Transistors formed with the same process on glass and plastic show linear mobilities of 0.33 and 0.12 cm2/Vs, respectively, with ION/IOFF ratios > 106.

Reaction Processes for Low Temperature (<150°C) Plasma Enhanced Deposition of Hydrogenated Amorphous Silicon Thin Film Transistors on Transparent Plastic Substrates

ABSTRACTThis article presents mechanisms for low temperature (<150°C) rf plasma enhanced chemical vapor deposition of silicon and silicon nitride thin films that lead to sufficient electronic quality for thin film transistor (TFT) fabrication and operation. For silicon deposition, hydrogen abstraction and etching, and silicon disproportionation reactions are identified that can lead to optimized hydrogen concentration and bonding environments at <150°C. Nitrogen dilution of SiH4/NH3 mixtures during silicon nitride deposition at low temperatures helps promote N-H bonding, leading to reduced charge trapping. Good quality amorphous silicon TFT's fabricated with a maximum processing temperature of 110°C are demonstrated on flexible transparent plastic substrates. Transistors formed with the same process on glass and plastic show linear mobilities of 0.33 and 0.12 cm2/Vs, respectively, with ION/IOFF ratios > 106.

Ion-assisted etching and profile development of silicon in molecular chlorine

An ion beam etching study designed to characterize the kinetic and transport processes important in the ion-assisted etching of silicon in molecular chlorine was performed. Monoenergetic argon ion beams were directed normal to a silicon wafer that was simultaneously exposed to a background of molecular chlorine, thereby simulating a low temperature rf plasma etching process. The ion-induced etching yield scaled with the square root of the ion energy for a surface saturated with adsorbed chlorine. Moreover, the yield was found to depend strongly on the ion to neutral flux ratio which was varied over two orders of magnitude. A kinetic model in which the dissociative sticking probability and the yield scale linearly with surface coverage was developed and was found to be consistent with the yield data in the ion energy range of 90–300 eV. Its applicability to etching topography was tested with additional experiments where patterned silicon wafers (with oxide masks and with a range of features and linewidths) were etched at saturation in the apparatus. Minimal lag effects and some microtrenching and sidewall sloping were encountered. The etching selectivity of silicon over oxide at 100 eV was greater than 30. Computer simulations of the etching process and profile development were performed using the kinetic model and a line-of-sight re-emission model for the chlorine transport. Knudsen diffusion was found to be important in the transport of neutrals to the base of the feature, with the aspect ratio dependence of the etch rate becoming significant only at high values of the ion to neutral flux ratio. The agreement between simulation and experiment was good, with the initial sticking probability on a clean surface and the average product stoichiometry as parameters comparing favorably to published results. However, the observed microtrenching and sidewall sloping were not predicted by these simulations.

Microstructural and Vibrational Characterization of the Hydrogenated Amorphous Silicon Powders

Hydrogenated nanophase silicon powders prepared by low pressure and low temperature rf plasma using pure silane gas, have been characterized by transmission electron microscopy (TEM), Fourier transform IR spectroscopy (FTIR) and thermal desorption spectrometry (TDS) of hydrogen. By means of these analysis, the evolution of the hydrogen bonds and the oxidation processes as a consequence of annealing of the silicon powders under vacuum or atmospheric conditions have been investigated. The TDS results reveal the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films, and the FTIR spectra evidence the oxidation process taking place in the silicon powders as a consequence of the annealing. These results along with the TEM analysis show that silicon powder particles present intergrain linkage. We also study the silicon powder particles considering their microstructure and vibrational characteristics as well as their degree of polymerization.

Selective Si epitaxial growth by plasma-enhanced chemical vapor deposition at very low temperature

A rf-plasma chemical vapor deposition process for selective epitaxial Si growth from SiH4 and SiF4 at a deposition temperature of 300–400 °C is described. Selective epitaxial growth is obtained as a balance of deposition precursors versus etching by F species. Also, the results indicate that a high H-surface coverage is not essential to deposit crystalline Si films by very low temperature rf-plasma chemical vapor deposition. P-doped films with a mobility of 80 cm2/V s and a carrier concentration of 3×1018 cm−3 are reported.

Optical and electrical properties of a-Si xN y:H films prepared by rf plasma using N 2+SiH 4 gas mixtures

Hydrogenated amorphous silicon nitride (a-SixNy:H) films, prepared from SiH4 and N2 gas mixture by a low temperature rf plasma process, create new perspectives to obtain highly transparent films with low electrical conductivity. Films were deposited on c-Si substrates at a temperature of 350°C using different gas concentrations and several rf power densities. The films were optically characterized by ellipsometry during the process. The results show dependences of the refractive index and the extinction coefficient on silane concentration and rf power. Ellipsometric data point to an abrupt interface film-substrate and an evolution of the surface roughness associated with the enhancement of the ion bombardment. Electrical measurements on Al/a-SixNy:H/c-Si MIS structures show conductivity based on the Poole-Frenkel transport mechanism for Si/N ratios lower than 1.2. The Poole-Frenkel constant and the trap energy values agree with those previously reported taking into account the concentration of Si-H bonds and the temperature dependence of the conductivity. The electrical parameters, such as electrical conductivity and density of states, show critical dependence on the gas mixture and on the rf power density, which corroborates the ellipsometric results.

Novel low temperature RF plasma annealing using NH 3 − 2 gas mixture

A novel low temperature (350°C) RF plasma technique using a NH3−N2 gas mixture was used to anneal bipolar structures. Vertical pnp transistors made with high energy ion implantation and possessing poor electrical characteristics have been dramatically improved after 30min annealing with this new technique. The value of the ideality factor of the base current which was about 1.4 before annealing approached the ideal value of 1.0 after 30min annealing.

Low Temperature RF Plasma Annealing Using A NH3-N2 Gas Mixture

Techniques such as plasma etching, electron-beam lithography, X-ray lithography, ion etching, sputtering and ion implantation are used to Increase the density of Integration of semiconductor devices. All these techniques introduce undesirable radiation damage into the processed device. Annealing techniques are used to reduce or remove completely the radiation induced defects. The conventional postmetallization low temperature (400–450 °C) annealing using Forming gas (10% H2-90% N2) does not anneal all the charge pentres in many device structures. We have employed a novel low temperature (350°C) rf plasma technique using a NH3-N2 gas mixture to anneal bipolar structures. Vertical pnp transistors made with high energy ion implantation and having poor electrical characteristics have been dramatically improved after 30 min annealing with this new technique. The value of the ideality factor of the base current which was about 1.4 before annealing approached the ideal value of 1.0 after 30 min annealing. Optical emission spectroscopy of the NH3-N2 glow discharge shows the presence of NH radicals, atomic hydrogen and nitrogen ion molecules during the plasma annealing. The atomic hydrogen passivates electrically active defects in the oxide and at the Si/SiO2 interface. A nitridation process occurs at the surface of the top BPSG layer, where a thin silicon nitride film is formed and plays the role of a capping layer which inhibits saturation phenomena. Angle-resolved XPS and ellipsometry have been used to analyze the surface of a silicon wafer exposed to this plasma annealing process. A qualitative model is also proposed to explain the mechanisms involved in this novel NH3-N2 rf plasma annealing process.

Thin silicon oxides grown by low-temperature rf plasma anodization and deposition

We have studied the low-temperature (<500 °C) growth and deposition of thin silicon oxides in an rf plasma system. Oxides were grown by plasma anodization, sputter deposition, and combinations of these two processes. The oxides were electrically characterized with high-frequency and low-frequency capacitance-voltage techniques and the quality was found to depend strongly on plasma process parameters. In particular, we found strong dependences on bias current, plasma potential, and oxygen partial pressure. Under presently optimal conditions we have fabricated oxides with oxide charge densities Qox<1×1011 charges/cm2, interface state densities Dss<2×1011 states/cm2/eV, and breakdown fields Eb>13 MV/cm. Finally, we have studied the reduction of fast surface states in the plasma oxides by rapid thermal annealing.

Studies on microarcing

Introduction of impurities into plasma discharges by unipolar arcs has been observed in various plasma experiments. However, the mechanisms which lead to microarcing are not well understood. To study the conditions under which micro-arcing occurs, as well as the resulting erosion rates, a small scale experiment has been initiated. Electrodes are immersed in a low-temperature RF-plasma. A capacitor is connected between them and biased by an external voltage supply. Currents between 10 and 200 A with pulse duration between 10 and 20 μs are measured when arcs strike across the space charge sheath between the negative electrode and the plasma. These arcs produce craters on the negative electrode with diameters between 20 and 50 μm. Relating the removed material to the integrated current of the arcs yields erosion rates of about 10 −8 kg/C for 304 stainless steel. The present results suggest that arcing on the first wall of tokamaks is a transient phenomenon which may be controlled by proper surface conditioning.

Survey of low temperature rf plasma polymerization and processing

Plasma-induced polymerization of both conventional and unconventional monomers has produced materials of unique properties suitable for applications in diverse fields. These fields include permselective and biological membranes, barrier coatings, dielectric layers, antireflection optical coatings, light guides, and adhesive bonding of otherwise incompatable materials. The science behind these applications is developing into predictions of properties based on defined variables but has not resolved some basic questions (such as reaction mechanism or residual activity within the films). Solutions to these problems will expand the applications horizon.