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Title - Materials Processing - David Hash's Figures
The figures presented here are SEMS (Semiconductor Equipment Modeling Software) predictions of a CF4 - ICP (inductively coupled plasma) with a flow rate of 20 sccm, an absorbed power of 240 W, and a pressure of 50 mTorr. The SEMS code has been recently developed to model semiconductor processing including neutral gas coupling to the plasma. Traditionally, this coupling is neglected, and in the simplest case the drift-diffusion approximation is used in which a constant neutral gas temperature and total density is assumed. The first four figures show the differences between SEMS (labeled "gas heating") and drift-diffusion approximation in the
(a) neutral gas temperature
(b) electron temperature
(c) CF4 consumption,
(d) the negative ion production
The final two figures of (e) neutral gas and (f) electron temperature show the effect of the effect of negative ions in the SEMS model. The first four figures include negative ions. The last two show the results with and without F-.
This figure depicts neutral gas temperature This figure depicts electron temperature
a) The drift-diffusion assumes a constant gas temperature (400 K). The SEMS results more accurately model the physics showing the heating effect of charge exchange collisions, dissociative recombination, and ion-ion recombination. b) The increased gas temperature due to charge exchange collisions, dissociative recombination, and ion-ion recombination directly directly results in a decrease in neutral gas densities since the pressure drop across the reactor is negligible at this flow rate and pressure. This results in lower electron impact production rates leading to a reduced electron density. For a fixed power, this will result in an increase in electron temperature, relative to the drift-diffusion approximation.
This figure depicts CF4 consumption This figure depicts negative ion production
c) The increased electron temperature in the SEMS results leads to a larger predicted consumption of CF4 when compared to the drift-diffusion results. d) Similarly, the higher electron temperature results in a larger production of negative ions.
This figure depicts neutral gas with F- This figure depicts electron temperature without F-
e) The addition of negative ions adds the effect of additional gas heating from the highly exothermic ion-ion recombinations which efficiently pump energy from the ions to the neutral gas. f) The addition of negative ions has the effect of lowering the electron density resulting in a higher predicted electron temperature.
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