Particle sources

Constant sources

Particle sources can either have Maxwellian or Sputter type energy emission, or a given emission energy interval can be specified. The declaration syntax is add_source("species", amount, "unit");

Example:

add_source("O2", 50, "sccm");
add_source("Al", 10, "Pa");

Both commands declare a particle source. The first argument is the gas species to be created, the second argument the amount. The unit argument can be either "sccm" or "Pa" specifying a mass flow or pressure controlled source, respectively.

Sources with inscribed profile

For the source surface it may be required to have a non-uniform profile over the source surface.

An example is magnetron sputtering where the sputtering emission distribution from the cathode depends on the Ar⁺ ion bombardment profile. The Ar⁺ ion bombardment profile can be stored in an absorption plot by putting Arplus into the ABSORPTION plot list.

In a subsequent DSMC neutral transport simulation, e.g. in order to evaluate the film thickness profile of the sputtered material, there is now an easy way to use the Ar⁺ ion bombardment profile for defining the sputtering source:

  add_source("Ti", 50, "sccm");           # First, set Ti source with a given particle flux
  set_emission_sputter("Ti", 4.89, 1.0);  # Apply sputtering energy emission profile with 4.89 eV binding energy
  set_profile("Arplus-absorption.pos");   # Apply erosion profile according to Arplus absorption profile from
                                          # previous PIC-MC plasma simulation

For the file Arplus-absorption.pos has to be copied from one of the Ar⁺ absorption volume and mesh plots, respectively from PIC-MC plasma simulation. Alternatively, a time-averaged file can be made from several absorption plots via post processing. In the set_profile command, the filename has to be given as relative path with respect to the location of the parameter file (in the example above, Arplus-absorption.pos must be in the same directory as the parameter file).

A more accurate way is to model the sputtering process within the PIC-MC plasma simulation by taking into account the angular and energy dependency of the sputtering yield. In this case, the desorption_XXX.pos files or their mesh plot equivalents can be taken as profile for the subsequent DSMC simulation. However the assumption that the local sputtering rate is proportional to the ion flux is a good starting approximation in most cases.

Vapor pressure source depending on surface temperature

In some cases, a source surface might have a temperature profile. This is especially relevant in connection with imported wall temperature distributions. In case of a pressure source - e.g. for an evaporation surface - the vapor pressure is temperature dependent. Most temperature-dependent vapor-pressure curves can be fitted with a simple exponential expression:

$p(T) = exp(-a/T + b)$

The syntax of the temperature-dependent pressure source is:

add_vapor_source("species", a, b);

The parameters a and b are material dependent. For Aluminum, suitable values are a=36070 and b=24.469. Thus, a temperature-dependent Al evaporation source can be defined by:

add_vapor_source("Al", 36070, 24.469);