Surface Reaction Group
The Surface Reaction Group node () allows input of a large number of surface reactions. More than one Surface Reaction Group can be added to a component.
Import Reactions from File
This section allows to import surface reactions from a text file into the table in the Reaction Table section. This section allows to import reactions from a text file into the table in the Reaction Table section. The reaction data must be in a specific format, referred to as data blocks. A surface reaction group data block starts with the keyword SURFACEREACTIONGROUP. After supplying the reaction rates data in different ways, all the data blocks need to be ended with the keyword END.
Specify which surface reaction group block to read from file
There are multiple choices to specify which surface reaction group block(s) to read from the target data file.
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If All is selected, all the SURFACEREACTIONGROUP data blocks within the specified data file will be read into this feature and their data will be combined in the Reaction Table.
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If By label is selected, a Label input box will show up and only the data blocks with matching labels (see below for customizing labels for data blocks in the data file) will be read, where multiple blocks could have the same label and their data will be combined.
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If All unlabeled is selected, all the data blocks with no customized labels will be read and their data will be combined.
The following examples show the format used to define surface reactions within a surface reaction group data block.
%-----------------------------------------------
% Surface Reaction Groups
%-----------------------------------------------
@label(etching)
SurfaceReactionGroup
% regular data line
Cl+SiCl(s)=>SiCl2(s) ! Sticking 0.4 0 0
Cl++SiCl2(s)=>Cl+SiCl2 ! Rate 1 0.05 5.8
 
% data line with optional yield section
Cl2++SiCl(s)=>Cl2+SiCl ! Rate 1 0.046 5.8 ! Yield 0.005 33 25
 
% data line with optional global section
Cl++Si(s)=>Cl+Si ! Sticking 1.0 0.05 5.8 ! None 0.25
Cl2++SiCl3(s)=>Cl2+SiCl3 ! Rate 1 0.046 5.8 ! UseBohm 0.25
Cl+SiCl2(s)=>SiCl3(s) ! Sticking 0.3 0 0 ! AddDiffusion 0.25 0.1
 
% data line with both optional yield section and optional global section
Cl2++SiCl2(s)=>Cl2+SiCl2 ! Rate 1 0.046 5.8 ! AddDiffusion 0.25 0.1 ! Yield 0.005 33 25
Cl++Si(s)=>SiCl(s) ! Sticking 0.99 0 0 ! Yield 0.005 50 25 ! UseBohm 0.25
Cl++SiCl(s)=>Cl+SiCl ! Sticking 1 0.05 5.8 ! Yield 0.005 50 25 ! AddDiffusion 0.25 0.1
END
 
The rules used to define surface reactions within a surface reaction group data block are listed here:
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Each SURFACEREACTIONGROUP data block can optionally have a customized label, defined using the annotation @label(AAA) right above the block keyword, where AAA is the label itself. This label will be used when reading data blocks By label, and data blocks with the same label will be combined in the same Reaction Table.
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Each surface reaction is generally defined using a single line of data, and each single data line is separated into sections using the delimiter “!”. There can be 2 to 4 sections including Formula section, Regular data section, Yield section (optional), Global section (optional) from left to right, where the ordering of the last two optional sections can be switched.
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The items within each section are separated by space.
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For the Formula section, there is exactly one item, reaction formula. This formula follows the same syntax as in Reaction Group features and Surface Reaction features, and surface reactions are irreversible.
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For the regular data section, there are exactly four items. From left to right these are: 1) keyword used to specify how the reaction rate is defined, either Sticking for using sticking coefficient or Rate for using rate constant; 2) the value corresponding the option selected in 1); 3) Secondary emission coefficient; 4) Mean energy of secondary electrons.
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For the optional Yield section, there are exactly four items. From left to right these are: 1) identifier used to show this is a Yield section, therefore the value is always Yield; 2) Proportionality constant; 3) Incoming species energy; 4) Threshold energy.
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The optional Global section is used to provide extra data for global model, which is useful when space-dependent model and global model share the same plasma chemistry data file. For this section, there are either two or three items. From left to right these are: 1) keyword used to override/modify the selection in the regular data section (Sticking or Rate), where the options are None for no effect, UseBohm for using Bohm velocity to calculate the reaction rates (override Sticking or Rate), or AddDiffusion for adding a small correction term due to diffusion to the reaction rate (modify Sticking or Rate); 2) Correction factor; 3) Effective diffusion length (optional) that is only needed when the keyword in 1) is set to AddDiffusion.
The same rules applied in the Reaction Group feature still apply here:
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Any line that starts with “%” is treated as a comment.
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It is possible to use model variables to define expressions. The following keywords can be used: TE is the electron temperature in eV, NE is the electron density in m−3, TGAS is the gas temperature in K, and EN is the reduced electric field in V·m2.
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It is also possible to use any variable in the model using the scope plasma before the variable..
If one wants to automatically put different surface reaction sets into different Surface Reaction Group features and label them accordingly, it is recommended to use the Plasma Chemistry Import feature.
Reaction Table
Based on the selection of Diffusion Model in the main interface node, the Reaction Table has different appearance with different number of columns.
If Diffusion Model is set to Mixture-averaged or Fick’s law, which means the model is a space-dependent model, the columns from left to right are for Reaction sequence number, Reactant, Product, Specify reaction using option, Rate constant, Sticking coefficient, Secondary emission coefficient (γi), Mean energy of secondary electrons (εi), Include yield option, Proportionality constant (A0), incoming species Energy (Ei), Threshold energy (Eth).
If Diffusion Model is set to Global, which means the model is a global model, most of the columns stay the same except two changes: 1) the two columns corresponding to secondary emissions will be hidden as these are not needed for global models; 2) extra columns including Global model options, Correction factor and Effective diffusion length will appear at the rightmost end of the table, which are dedicated to global models and match the optional Global section in the data file described above.
Apart from these column-wise behaviors, the table also has some cell-wise behaviors where certain cells will be enabled and disabled when the selection of other cells on the same row changed. These behaviors are independent across different rows as they are different surface reactions. Here is a list of these behaviors:
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If the Specify reaction using option is set to Rate coefficient, the Sticking coefficient cell will be disabled, and vice versa.
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The three cells following the Include yield cell will only be enabled if the Include yield option is selected.
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For the extra cells for global models, the Correction factor cell is always enabled. The Effective diffusion length cell will only be enabled if the Global model options is set to Add diffusion. And if the Global model options is set to Use Bohm, all the three cells starting from the Specify reaction using option will be disabled as the Bohm velocity is used to calculate the reaction rate and therefore both Rate constant and Sticking coefficient are no longer needed.
The general rules are that if the cells are grayed out and cannot be modified, they are disabled and their values are not being used.
To manually add a reaction to the Reaction Table, click the Add button () and a new row will appear which represents a new reaction. Trivial default values will be provided to the reaction except the Reaction sequence number. The Reactant and Product of the reaction can then be entered, and COMSOL automatically update the list of species features and create new species features if it finds new species.
To make sure the surface reactions are defined properly, certain rules are imposed to check the reaction formula match the options selected. These rules are:
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To use Sticking coefficient there should be exactly one gaseous species in the reactants.
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To Include yield there should be exactly two reactant species including one gaseous species and one surface species.
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To Use Bohm velocity there should be exactly one charged gaseous species in the reactants.
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To Add diffusion there should be exactly one gaseous species in the reactants.
Error messages will be thrown if some surface reactions entered in the table do not follow these rules..
It is recommended to use the Surface Reaction Group feature with the Import Reactions from File functionality or with the Plasma Chemistry Import feature, rather than manually entering all the reactions one by one.