Frequency variation under constant power conditions in hydrogen radio frequency discharges

Abstract

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 electric field. Under these conditions, the total power spend for electron acceleration increases with frequency, but combined to the higher electron density, leads to a drop of the average energy gained per electron, a drop of the mean electron energy, and an enhancement of the low-energy electron-molecule collision processes against high energy ones.

Exploration of the deposition limits of microcrystalline silicon

“…We believe that the only way to evaluate the effects and exploit the possible benefits is to vary the frequency while keeping a constant power density in the discharge, otherwise the inevitable decrease of the necessary sustaining voltage with increasing frequency leads to false impressions since the power going into the discharge increases with frequency [22,24,37]. VHF is associated with very high power losses in the feed lines, as well as with deposition non-uniformities owing to standing and evanescent waves on large-area electrodes [36].…”

Comparative study of plasma-deposited fluorocarbon coatings on different substrates

“…1b). The discharge current and more precisely the current flow in the bulk of the plasma presents a linear relation to the electron drift velocity and the electron density [17]. The increase of pressure will result in an electron mobility drop and consequently in a drop of electron drift…”

“…An estimation of the variation of electron density with pressure can be done by using these electrical measurements and more precisely the discharge phase impedance and the resistive part of the total discharge impedance [18,19]. Application of the analysis of refs [17,19] . Therefore, it appears that the lower the fragmentation of the fluorinated gas precursor, obtained by lowering both pressure and power, the higher the water contact angles of the treated textiles.…”

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

“…The method for the measurement of the power consumed in 13.56 MHz discharges and the extension to include higher frequencies have been presented in detail elsewhere. 13,15 The setup used for recording emission intensities of SiH (A 2 ⌬→X 2 ⌸) as well as ␣ and ␤ Balmer lines of atomic hydrogen, has been described in detail elsewhere. 16,17 Mass spectrometric measurements have been performed using a Hidden analytical ͑HAL 301͒ quadrupole mass spectrometer connected at the exhaust port of the reactor.…”

“…12 The proper combinations of the electrical parameters ͑voltage, current, and impedance͒ ensuring constant power dissipation at each frequency were then used in an analytical discharge model. The model considers the sheath dynamics as well as the mass transport of the charged particles in the bulk plasma, 13 aiming mainly at the prediction of the ion flux toward the surfaces and at the distinction of the fraction of power spend either for ion or for electron acceleration. Furthermore, the paths where electrons consume the fraction of the total power gained, are investigated by recording emission profiles of electronically excited species and by applying mass-spectrometric measurements of the total SiH 4 consumption in combination with a mass transfer model.…”