Wall
The Wall feature provides boundary conditions for modeling different kinds of solid surfaces within vacuum systems.
Wall Type
Select Wall type: Wall (the default), Outgassing wall, Adsorption/desorption, or Deposition. For Wall it sets the incoming molecular flux for each species, G, equal to the flux emitted from the wall, J. Therefore all the incident molecules are diffusely reflected.
Flux
For Outgassing wall it allows an additional outgassing flux J0,G to be added to the incident flux, so the total emitted flux for each species is given by J = G+J0,G. Select an Outgoing flux: Specify directly (the default), Number of SCCM units, Mass flux, Total mass flow, or Thermal desorption rate.
Specify Directly
For Specify directly, and for each species, enter a value or expression for J0,G (SI unit: 1/(m2⋅s)).
Number of SCCM (Standard Cubic Centimeters Per Minute) Units
For Number of SCCM units, and for each species, enter Qsccm,G (dimensionless). This quantity is automatically converted to a standardized volumetric flow rate in SI units. Select an option from the Standard flow rate defined by list — Standard density (the default) or Standard pressure and temperature.
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For Standard density enter a Standard molar volume Vm (SI unit: m3/mol). The default is 0.0224136 m3/mol (in accordance with SEMI standard E12-0303).
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For Standard pressure and temperature enter a Standard pressure Pst (SI unit: Pa) and Standard temperature Tst (SI unit: K). For 2D components, also enter a Channel thickness dbc (SI unit: m).
Mass Flux
For Mass flux, and for each species, enter a value or expression for Mf,G (SI unit: kg/(m2⋅s)). The default is 0 kg/(m2⋅s).
Total Mass Flow
For Total mass flow, and for each species, enter a value or expression for Qm,G (SI unit: kg/s). The default is 0 kg/s. For 2D components, also enter a Channel thickness dbc (SI unit: m). This is used to specify the depth of the boundary in the out-of-plane direction.
Thermal Desorption Rate
For Thermal desorption rate, and for each species, enter a value or expression for Dr,G (SI unit: W/m2). The default is 3e-12[torr*l/(cm^2*s)]. The outgassing flux, J0,G is related to the thermal desorption rate via J0,G = NADr,G/RT, where NA is Avogadro’s number, R is the universal gas constant, and T is the temperature.
Adsorption/Desorption
For Adsorption/desorption it is used in transient models to model adsorption and desorption of gas molecules on the surface. For each of the species enter the following information.
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 Sticking coefficient SG (dimensionless). This defines the probability that molecules incident on the surface are adsorbed.
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 Desorption rate DG (SI unit: mol/(m2⋅s)). This defines the rate of desorption. The boundary condition tracks the number of moles of gas adsorbed per unit area of the surface (fmf.n_ads) as a function of time.
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 Initial adsorbent concentration nads,0,G (SI unit: mol/m2).
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 Additional molar flux ΓG (SI unit: mol/(m2⋅s)) can be added to the surface, enabling, for example, the modeling of molecules arriving at the surface by diffusion through the walls of the chamber from the outside (these molecules are added to the adsorbed molecules on the surface). Note that molar units are used for the adsorbed molecules, but the total number of molecules per unit area is available as the variable fmf.N_ads.
Solving for the absorbent concentration can cause convergence issues if the concentration is close to zero, as its value can sometimes go negative during the solution process. If this happens when using this boundary condition, tighten the relative solver tolerance (see the settings for the Study 1>Time-Dependent Solver node) to 1e-3. Then in the settings for the Study 1>Solver Configurations>Time-Dependent Solver node change the absolute tolerance to 1e-4 (if this node is not visible right-click the Study 1 node and select Show Default Solver).
See also Adsorption and Desorption in the section Theory for the Free Molecular Flow Interface.
Deposition
Deposition is used for simulations in which the molecular flow of a deposited species is tracked. It is assumed that all the molecules arriving at the surface are deposited.
For each species, enter a Film density ρfilm,G (SI unit: kg/m3) and Initial film thickness hfilm,0,G (SI unit: m) to compute the film thickness (fmf.h_film) as a function of time.