D. Mataras

Frequency variation under constant power conditions in hydrogen radio frequency discharges

The effect of driving frequency (13.56–50 MHz) on the electrical characteristics and the optical properties of hydrogen discharges has been studied, under constant power conditions. The determination of the discharge power and impedance was based on current and voltage wave form measurements, while at the same time spatially resolved Hα emission profiles were recorded. As frequency is increased, the rf voltage required for maintaining a constant power level is reduced, while the discharge current increases and the impedance decreases. Concurrently the overall Hα emission intensity decreases and its spatial distribution becomes more uniform. Further analysis of these measurements through a theoretical model reveals that frequency influences the motion of charged species as well as the electron energy and the electric field, resulting in a modification of their spatial distribution. Moreover, the loss rate of charged species is reduced, leading to an increase of the plasma density and to a decrease of the e...

Gas phase and surface kinetics in plasma enhanced chemical vapor deposition of microcrystalline silicon: The combined effect of rf power and hydrogen dilution

A gas phase and surface simulator of highly diluted silane in hydrogen rf discharges used for the deposition of microcrystalline silicon has been developed. The model uses the spatial density distribution of SiH (X 2Π) radicals measured using laser induced fluorescence and the total silane consumption for estimating the primary electron induced silane dissociation, thus avoiding fluid or statistical approaches commonly used for the prediction of electron impact rate coefficients. A critical analysis is made for the relative importance of all the parameters involved either in the gas phase chemistry or in the surface processes. The model results are compared to experimental data concerning disilane production and film growth rate over a wide range of rf power densities in 2% and 6% SiH4 in H2 discharges. The good agreement between experimental and model results allows for the extension of the discussion to the composition of the radical flux reaching the substrate, the relative contribution of each of the ...

Effect of frequency in the deposition of microcrystalline silicon from silane discharges

The influence of frequency in the range from 13.56 to 50 MHz, on the properties of 2% silane in hydrogen 0.5 Torr discharges used for the deposition of microcrystalline silicon thin films, has been investigated. The experiments were carried out under constant power conditions as determined through Fourier transform voltage and current measurements. The increase of frequency leads to a decrease of the rf field, an extension of the bulk, and a marked increase of the electron density and the amount of power consumed by electrons. These changes induce a decrease of the rate of high-energy electron–molecule collision processes (>10.5 eV) at higher frequencies and an enhancement of lower energy processes. Thus, there is a significant increase in the hydrogen flux toward surfaces, which can explain the beneficial effect of frequency to the crystallinity of μc-Si:H thin films. At the same time, SiH4 electron impact dissociation is enhanced mainly due to the increase of electron density. On the contrary, ionizatio...

Deposition rate optimization in SiH4/H2 PECVD of hydrogenated microcrystalline silicon

Abstract Intrinsic hydrogenated microcrystalline silicon films have been deposited by Plasma Enhanced Chemical Vapor Deposition using highly diluted SiH 4 in H 2 discharges, aiming at the increase of the deposition rate. Following a systematic optimization of the main process parameters, an increase of the film growth rate up to 7.5 A/s has been achieved, from 1 Torr 6% SiH 4 in H 2 dust-free discharges at a frequency of 30 MHz. The experimental results are combined to a mass transfer model that can very well predict the deposition rate, for revealing the main reasons leading to the fast growth of μc-Si:H.

Kinetics of Power Deposition and Silane Dissociation in Radio‐Frequency Discharges

Measurements of the actual power and silane consumption as a function of excitation voltage and pressure in a completely characterized radio-frequency, glow-discharge chamber used for the deposition of a-Si:H, are reported. The influence of these parameters on the discharge power and impedance is presented and discussed with consideration of the basic electron heating and power dissipation mechanisms. The discharge is more efficient,in terms of the amount of energy spent per dissociated silane molecule, at higher pressures and lower voltages. This amount tends toward the dissociation threshold of silane for lower power densities. The rate constant of electron impact dissociation of silane, calculated from silane consumption, increases with power with a different trend for each pressure, and is higher for lower pressures ranging from 0.08 to 0.2 s - .

Simulation of the SiH (A2Δ → X 2Π) emission spectrum in a silane glow discharge and derivation of an improved set of molecular constants

Abstract An improved simulation of the rotational intensity distribution of the 0-0 emission band of the A 2 Δ-X 2 Π transition of SiH, that is experimentally obtained by optical emission spectroscopy from a silane rf glow-discharge, is reported. The improvements consist in the use of a new term value formula, a recent set of molecular constants and an extended least squares fitting analysis for the micro-optimization of the set of constants. Thus, the rotational temperature of SiH is determined by utilizing a sufficient spectral resolution together with an experimentally determined instrument function of the optical system. The optimum fit of the emission spectrum is obtained for a rotational temperature T ROT = 2840 ± 50 K which is significantly higher compared to those previously reported. Furthermore, an abnormal behaviour of the observed splittings compared to the theoretical calculations is observed, while in two cases experimental measurements of λ-doublets were performed.

INFLUENCE OF PLASMA CONDITIONS ON THE DEFECT FORMATION MECHANISM IN AMORPHOUS HYDROGENATED SILICON

The variation of a‐Si:H film quality, deposited by a rf glow discharge of pure silane, is examined as a function of the interelectrode distance for two different pressures. Constant photocurrent and modulated photocurrent methods are used to estimate the magnitude and the shape of the defect states in the valence band and the conduction band, respectively. An effort is made to correlate the film quality parameters and the defect formation with the plasma macroscopic and microscopic parameters. The results suggest that, at low interelectrode distances, high sticking coefficient radicals modify the film growth and the defect formation mechanisms, leading to the deterioration of the film quality. The conclusions drawn are compared with the predictions of recent theoretical models concerning the defect formation in a‐Si:H.

Simulation of the Electrical Properties of SiH4/H2 RF Discharges

The results of a two-dimensional fluid simulator were validated against the experimentally measured electrical properties (power dissipation and discharge current) of SiH4/H2 RF discharges used for the deposition of microcrystalline or amorphous silicon thin films. The use of the typical values for the electron–SiH4 and H2 collision cross sections found in the literature, results in a significant underestimation of the calculated power dissipated in the discharge and an overestimation of the current flow. A study of the main parameters affecting the model results showed that this deviation is mainly due to the H2 ionization rate. An improved calculation of the power dissipated in the discharge was then possible, leading to a significant improvement in the prediction of the deposition rate. This result underlines the importance of H2 chemistry in SiH4/H2 discharges and also indicates that the correct estimation of the discharge electrical properties is a necessary first step in the development of a code for simulating the deposition of silicon films from SiH4/H2 discharges.

Spatial rotational temperature and emission intensity profiles in silane plasmas

The spatial variation of the rotational temperature of the 0-0 band of the A transition of SiH in radio frequency pure silane discharges is investigated by utilizing a method of improved accuracy for the simulation of the emission spectrum. Furthermore, the variation of the rotational temperature as a function of the pressure at the maximum intensity position of the interelectrode space is examined. The results are used to extract information about the relative variations of the electron energy distribution function and the effective electron density in the interelectrode space. Thus, it is considered that there is a sensitive relation of the observed high rotational temperatures to the mean electron energy, provided that the electron energy distribution function does not change. More specifically, a decrease of the rotational temperature is observed on moving away from the powered electrode sheath region and a similar decrease is also detected with increasing pressure. This behaviour is compared with the spatial emission intensity profiles. The variation of the space-integrated emission intensity under the various discharge conditions, related to the modification of the effective electron density, is also presented.

Influence of discharge geometry on power dissipation and sheath impedances in silane discharges

The influence of the interelectrode distance on the power consumed in rf silane glow discharges, for two different electrode sizes and at two different silane pressures is reported. The effect of the increasing grounded surface area in contact with the discharge is analysed by using a simplified calculation of the rf and grounded sheath impedances. At all cases the power consumed in the discharge increases with interelectrode distance and is higher for higher pressures, for a constant excitation voltage. The sheath impedances are always higher for lower pressures except for the case of the ground sheath when using the large electrode set.