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- Ions respond only to the time-averaged electric field and the ion motion is collisional with a constant mean free path. This assumption limits the model application to total gas pressures higher than 200 mTorr. - The time-averaged values of the electric field and the electron density in the RF sheath have been calculated assuming that the sheath-plasma boundary oscillates as d - An ambipolar transport in the bulk has been assumed and although the electron distribution function is not expected to be Maxwellian, a use of electron temperature has been made for an initial estimation of the ambipolar dc field and the determination of ambipolar diffusion coefficient.
The formulation of the model is not self-consistent in the sense that it requires the self-bias voltage, the discharge current, the ohmic part of the discharge impedance, and the spatial distribution of the ionization rate.
The output parameters of the model are the spatial distribution of electrons and ions, the distribution of the electric field, the power dissipation to ions and electrons, and the mean electron energy.
- Ion transport in the sheaths The transport of ions in the sheath is usually described as a collisionless or collisional, space charge limited motion. The charge conservation condition and the assumption of no ionization or recombination in the sheath impose the requirement of a constant ion flux. In reality, ion density gradually decreases while ion drift velocity increases, from the plasma sheath boundary towards the RF electrode: (1) where, n The mobility solution of the Child-Langmuir law for a collisional sheath has been used to calculate the ion flux thus, the ion mean free path λ (2) By combining eq. 2 to Poisson’s equation (dE/dx=en (3) where E is the time averaged value of the sheath field at the position x, and E (4) where d is the mean sheath thickness and E The distribution of the electric field can then be combined to Poisson’s equation to calculate the ion density distribution (5) and the voltage distribution in the sheath (6) Furthermore, the electrons in the sheath are assumed to be in Boltzmann equilibrium and have a Maxwellian distribution. Therefore, the electron density distribution is of the form: (7) where n The time-averaged values of the electric field and the sheath thickness, required for the solution of the set of equations (3-7), are calculated as follows: The mean values of the powered and grounded sheath lengths are found assuming a pure capacitive nature of the two sheaths. The power and grounded sheath reactive impedances can be distinguished, and the sheath lengths can be then calculated using: (8) where I The values of the sheath fields E (9) For the values of the field at x=0 (sheath-bulk interface) the Godyak-Sternberg model for the pre-sheath is used and E (10) The necessary electron temperature estimation results in this case from equation (11) with V
- Ion transport in the bulk In order to relate the time-averaged values of the field and the charged particle densities in the sheaths to their values in the bulk, ambipolar diffusion dominated ion transport is considered: (12) where S(x) is the time averaged ionization rate in space and Dais the ambipolar diffusion coefficient. The ambipolar diffusion coefficient D (13) Since, the values of the electric field in the bulk are expected to be rather weak, a low field mobility is used instead of the high field mobility k used to describe ion drift motion in the sheaths. Thus, the ambipolar diffusion equation (12) is solved for d (14) where, d (15)
- Power dissipation The calculation of the ion flux density in both sheaths permits the estimation of the power consumed for ion acceleration (16) The power consumed for electron heating can then be distinguished as the difference between the total power consumed in the discharge and the power consumed for ion acceleration (17) In order to relate the different paths of power dissipation with the ohmic part of the discharge impedance, an in series equivalent circuit is assumed for the discharge. The resistance related with ion acceleration in the powered and grounded sheath is calculated using the formulas (18) The resistive component Rν related with electron-molecule collisions is then calculated using the expression (19) where, R the total resistive component, is strictly related to the frequency at which the electrons relax their momentum (20) where n
- Electron heating and mean electron energy The power consumed for electron heating Pe that has been calculated above represents the total power transferred to electrons. However, significant variations are expected in the spatial distribution of electron heating. In order to follow the changes in the spatial variation of electron heating, the time-averaged power transferred per electron θ can be used. This parameter can be expressed as a function of the time averaged electric field and the electron-molecule collision frequency (21) The changes in electron heating will be reflected in the value of the mean electron energy, resulting from the balance between the energy gained from the field and the energy lost per collision: (22) where, the value of the effective (ohmic) part of the RF bulk electric field has been used: (23) u Assuming a sinusoidal RF field and substituting electron drift velocity as (24) where, φ=tan (25) that has the solution (26) where νe is the mean collision frequency for energy transfer, resulting from the product of k times νm. The term can be attributed to the mean electron energy that would be observed in a dc electric field of strength Eeff.
The complete model formulation has been presented in the following work: E. Amanatides and D. Mataras while the extension of the model to include reactive gases and mixtures was published in: E. Amanatides, D. Mataras, D. E. Rapakoulias Application of the model to H
_{2} 0.5 Torr discharges and constant power dissipation of 40 mW/cm^{2}. The model predicts an increase of electron density and a shift of the peak of electron density towards the center of the discharge with frequency
_{2} 0.5 Torr discharges and constant power dissipation of 40 mW/cm^{2}. The model predicts a drop of mean electron energy and a small time variation with frequency
_{2} 0.5 Torr discharges and constant power dissipation of 40 mW/cm^{2}. The model predicts an drop of electric field intensity with frequency
Ion flux towards both powered and grounded electrodes as a function of frequency - The conditions are: pure H _{2} 0.5 Torr discharges and constant power dissipation of 40 mW/cm^{2}. The model predicts a clear drop of the ion flux for the powered electrode and a slight decrease for the grounded electrode with frequency
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