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Particle sources depending on surface state

Particle sources may change their behavior depending on their surface coverage. Examples, which are implemented in the DSMC algorithm are

Sputtering sources with given ion flux

A sputtering source with a given ion flux is given by the command

add_surface_emission("material", ion_flux, ["rp_1", "rp_2", ...], [yield_1, yield_2, ...]);

Parameters are as in the following:

  • material → Material phase in the surface layer. For different surface materials different sputtering processes can be defined.
  • ion_flux → The effective ion flux is given in sccm.
  • [“rp_1”, “rp_2”, …] → list of gaseous reaction products
  • [yield_1, yield_2, …] → list of partial yield of the gaseous reaction products

The sputtering yield of a gasous reaction product is the product of the reaction probability and its partial yield. As an example, the surface chemistry and sputtering source for the reactive TiOx process is given:

# Specification of surface material phases and initial coverage:
add_material("Ti_metallic", 9e18);
add_material("TiO2",        9e18);
set_initial_coverage("TiO2", 0.9);
# Surface reactions:
add_surface_reaction("O2", 1, 1, 1.0, "Ti_metallic", "TiO2", [], []);
add_cover_deposition("Ti", 1, 1, "Ti_metallic", [], []);
# Sputtering source:
add_surface_emission("Ti_metallic", ion_flux_sccm, ["Ti"], [0.5]);
set_emission_sputter("Ti", 4.89, 1.5);
add_surface_emission("TiO2", ion_flux_sccm, ["Ti", "O2"], [0.05, 0.05]);
set_emission_sputter("Ti", 6.0, 1.5);
set_emission_sputter("O2", 6.0, 1.5);

In the above example, sputtering of metallic Ti and oxidized TiO2 is handled separately. The sputtering yield of TiO2 is by factor of 10 lower compared to metallic Ti. The energy distributions of the sputtered gaseous species are to be specified afterwards for each case.

Note: When sputtering the surface layer, it is being replaced by the material beneath the surface layer.

  • If there is deposition on the sputtering surface, the replaced surface fractions are according to the stoichiometry of the material in the deposition layer.
  • If there is no deposited material beneath the surface layer, the replacement material will be the material which is firstly defined in the deposition chemistry (i.e. Ti_metallic in the above example).

Thermal desorption of coated material

If surfaces are coated with materials with high vapour pressure, there is the possibility of thermal desorption of that material. For this purpose, a desorption source is implemented. It can be declared via the syntax

add_desorption("species", pressure, "material", amount);

The parameters are as follows:

  • species: The species, which is to be emitted from the wall into the gas phase
  • pressure: The vapor pressure in Pa. The vapor pressure generally depends on the wall temperature. For many materials, the vapor pressure curve as a function of temperature is available in literature1).
  • material: The associated wall material.
  • amount: The number of gas molecules needed to form a bond site of the wall material

If the surface material does not completely desorb but leaves behind some remainder, modified command is available:

add_desorption_r("species", pressure, "material", amount_gas_species, "remainder", amount_remainder);

Here, we have the following parameters:

  • species: The species, which is to be emitted from the wall into the gas phase
  • pressure: The vapor pressure in Pa.
  • material: The associated wall material.
  • amount_gas_species: The number of gas molecules needed to form a bond site of the wall material.
  • remainder: The wall material which is left behind by the desorption process.
  • amount_remainder: The number of wall sites of the remaining material created by the desorption process.

Example: If we have a metallic ZnAl mixed site and assume that only Zn can desorp and leave behind a half Al2 site, the command would look like follows:

add_desorption_r("Zn", vapor_pressure, "ZnAl", 1, "Al2", 0.5);
For metals see e. g. Alcock et al.; Vapour pressure equations for the metallic elements: 298-2500 K, Canadian Metallurgy Quaterly, 1984, 23, 309-313