D. E. Rapakoulias

Power dissipation and impedance measurements in radio‐frequency discharges

An improved method for the measurement of the power consumed in low pressure, radio frequency discharges is presented. The method involves the measurement of current and voltage waveforms outside the reactor, and the determination of the discharge impedance and the network of parasitics. The measured waveforms are transformed to the equivalent ones at the powered electrode, by using an electrical circuit model of the stray impedance of the cell, with experimentally determined components. A tunable shunt circuit is used for minimizing displacement currents. The equivalent circuit contains elements which account also for resistive power losses in the cell‐shunt circuit. The obtained discharge power is compared with measurements of the total power output of the generator made by a power meter. Results concerning power consumption and impedance in argon and silane discharges are presented as a function of the excitation voltage and the pressure. In both cases there is a discharge impedance drop, for higher vo...

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.

Simulation of energy transfer from a glow discharge to a solid surface

In this paper, a model simulating the energy transfer from a glow discharge to a metal surface is presented. The model takes into account separately the transfer of each excitation mode of the plasma species: vibrational, translational, rotational, electronic, ionic, secondary electrons, and atom recombination. The case studied is that of a N2 glow discharge heating a W surface. Vibrational excitation transfer appears to be the main channel for the heating of the surface. Recombination on the surface also has an important contribution to the energy transfer from the plasma to the solid. Ions and metastable molecules play a minor role.

Combined effect of electrode gap and radio frequency on power deposition and film growth kinetics in SiH4/H2 discharges

The combined effect of the variation of the interelectrode gap (1.3–2.5 cm) and radio frequency (13.56–50 MHz) on the properties of highly diluted silane in hydrogen discharges used for the deposition of microcrystalline silicon thin films is presented. The investigation included electrical and optical discharge measurements as well as the in situ determination of the film growth rate. In the lower frequencies regime, the increase of the interelectrode gap for the same applied voltage results in higher current flows and higher total power dissipation. On the other hand, at 50 MHz the variation of the interelectrode space has only a slight effect on the total power dissipation, due to the low excitation voltage. However, at all frequencies, the increase of the interelectrode space results in a drop of the power dissipation per discharge volume. This is related to the less effective energy transfer to the electrons that is due to the enhancement of the bulk relative to the sheath ohmic heating. The variatio...

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 - .

Power dissipation mechanisms in radio‐frequency driven silane discharges: The influence of discharge geometry

The influence of chamber geometry on the electron impact dissociation and dissociative excitation processes, in a capacitively coupled radio‐frequency (rf) silane discharge, is presented. This is achieved by examining the influence of geometry, in conjunction with other macroscopic parameters, on the concentrations of ground state SiH and electronically excited SiH* radicals, recorded simultaneously by spatially resolved laser induced fluorescence and optical emission spectroscopy. The geometry of the discharge is modified, by changing either the interelectrode distance or the electrode diameters, whereas the electric symmetry of the discharge is monitored by the change of the self‐bias direct‐current potential of the rf electrode. Our observations indicate that the specific path and power consumption of the generation processes of the two radicals are spatially differentiated and sensitive to geometry variations. Thus, emission and fluorescence spatial intensity profiles are differently influenced. In th...

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.

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.