Tutorials Manual

Chapter 4: Materials Problems
Tutorials Manual
4.3 Plasma Etching
The CHEMKIN software includes a number of special features for modeling plasmas,
most of which are demonstrated in these examples. For treating non-equilibrium
plasmas, the plasma models allow you to specify different temperatures for the
neutral gas, electrons, ions, and surfaces. A gas-phase reaction rate can be
designated to depend on the electron temperature rather than the default neutral gas
temperature. An electron-impact reaction can also be designated to involve a
specified energy loss per collision. This option is used to describe excitation energy
losses for electrons in solving the electron energy equation, without explicitly defining
each vibrationally or electronically excited-state as a separate species within the
corresponding thermodynamic data. For surface reactions, ion-assisted reactions can
have yields that depend on the ion energy. Such reactions can also be designated as
Bohm reactions, which means that the ion flux is limited by the Bohm velocity of the
ions, rather than by the ion's thermal speed. The implementation of the Bohm criterion
is described in detail in Section 8.5.3 of the CHEMKIN Theory Manual. This factor has
been introduced to account for the fact that the ion interaction with the surface will be
subject to transport limitations, such that a gradient near the walls in the ion density
will occur. A zero-dimensional model cannot capture this effect, such that a Bohm-flux
correction model is used.
4.3.1 Steady-state Chlorine Plasma
4.3.1.1 Project Description
This user tutorial provides an example of modeling a low-pressure plasma reactor as
a steady-state Perfectly Stirred Reactor, using the chlorine plasma chemistry set
described in Figure 4.4.5. The plasma pressure is 5 mtorr and pure chlorine gas flows
into the reactor at 35 sccm. The energy equation and the electron energy equation
are solved, and a heat-transfer correlation is used to account for heat loss through the
wall of the reactor.
4.3.1.2 Project Setup
The project file is called plasma_psr__chlorine.ckprj. The data files used for this
sample are located in the samples\plasma_psr__chlorine directory. This reactor
diagram contains one gas inlet, one plasma PSR, and one outlet.
RD0411-C20-000-001 141 © 2007 Reaction Design