Global Modeling of Time Modulated Electronegative Plasmas

Sungjin Kim
(Professors Allan J. Lichtenberg and Michael A. Lieberman)
(UC-SMART) 97-01

In this research, we are developing a model to investigate the properties of electronegative plasma driven by time modulated power. We have started with a study on the O2 plasma to develop a general model for electronegative discharges. The calculations are done utilizing a global model of a cylindrical plasma discharge [1,2]. This model assumes uniform spatial distribution of plasma parameters over the volume of bulk plasma, with the plasma density in the bulk dropping sharply to edge values at the walls. The global model consists of particle balance and power balance equations. In the particle balance equations, the reaction rates of O2+, O- ions, and electrons have been included for simplicity. The O+ ions and metastable O atoms will also be included later. We are developing a simulation code that can be used in the simulation with variable power modulation periods, duty ratios, and modulation pulse shapes.

As a preliminary examination of the simulation codes, calculations for a sinusoidal modulation of power with a duty ratio of 50% have been performed. The pulse period was varied from 1 microsecond to 1 millisecond. In the shorter modulation period, we observed that the electron temperature falls rapidly during the off cycle whereas the plasma density does not. And the time-average plasma density and the time-average electron temperature showed dependency on the modulation period.

We are working on the optimization of the simulation codes to include more reaction chemistry and are planning to do more simulation with various duty ratios or modulation shapes. Later we will apply the results from this research to the modeling of dual frequency discharge.

Figure 1: Plasma densities averaged over one period of the modulated power with various modulation periods. The dotted line represents plasma density of the CW driven plasma.

Figure 2: Electron temperatures averaged over one period of the modulated power with various modulation periods. The dotted line represents electron temperature of the CW driven plasma.

C. Lee, D. B. Graves, M. A. Lieberman, and D. W. Hess, "Global Model of Plasma Chemistry in a High Density Oxygen Discharge," J. Electrochemical Soc., Vol. 141, 1994.
S. Ashida, C. Lee, and M. A. Lieberman, "Spatially Averaged (Global) Model of Time Modulated High Density Argon Plasmas," J. Vacuum Science Technology A, Vol. 13, 1995.

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