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Evaporator
Introduction
This model shows how to compute the film thickness deposited on a wafer from an evaporative source.
Model Definition
Gold is evaporated from a thermal source at a temperature of 2000 K onto a substrate held on a fixed surface. The deposited film thickness on the substrate and the chamber walls is computed. The model geometry is shown in Figure 1.
Figure 1: Model geometry. Various components of the evaporator are labeled.
Results and Discussion
Figure 2 shows the flux of gold molecules on the surfaces of the model. This constant flux determines the thickness of gold deposited (shown in Figure 3). Only the flux is required to compute the deposition rate, but in this instance, since most of the computational time is used to compute the view factors, solving the time-dependent problem adds little additional time to the solution process. Using a time-dependent model also allows for more advanced extensions of the model, for example, re-evaporation of gold from hot surfaces close to the evaporative source could be included.
Figure 2: Molecular flux on the surfaces of the system, during deposition.
Figure 3: Film thickness on the surfaces of the system, after 60 s of deposition.
Figure 4: Film thickness on the sample, after 60 s of deposition.
Figure 5: Film thickness vs time at a point on the corner of the sample.
The film thickness on the sample after 60 s of deposition is shown in Figure 4. The thickness varies between 34 nm and 39 nm across the sample, with radial symmetry about the midpoint of the source. As expected the film thickness at a point increases linearly with time, as shown in Figure 5.
Application Library path: Molecular_Flow_Module/Industrial_Applications/evaporator
Modeling Instructions
NOTE: This application requires at least 6 GB of RAM to run.
From the File menu, choose New.
New
In the New window, click  Model Wizard.
Model Wizard
1
In the Model Wizard window, click  3D.
2
In the Select Physics tree, select Fluid Flow>Rarefied Flow>Free Molecular Flow (fmf).
3
Click Add.
4
Click  Study.
5
In the Select Study tree, select General Studies>Time Dependent.
6
Geometry 1
Insert the prepared geometry sequence from file. You can read the instructions for creating the geometry in the appendix.
1
In the Geometry toolbar, click Insert Sequence and choose Insert Sequence.
2
3
In the Geometry toolbar, click  Build All.
4
Click the  Wireframe Rendering button in the Graphics toolbar.
In this model, wireframe rendering makes it easier to see inside the chamber.
Global Definitions
Parameters 1
1
In the Model Builder window, under Global Definitions click Parameters 1.
2
In the Settings window for Parameters, locate the Parameters section.
3
Definitions
Boat
1
In the Model Builder window, under Component 1 (comp1) right-click Definitions and choose Selections>Ball.
2
In the Settings window for Ball, type Boat in the Label text field.
3
Locate the Geometric Entity Level section. From the Level list, choose Boundary.
4
Locate the Ball Radius section. In the Radius text field, type 10.
All Boundaries
1
In the Definitions toolbar, click  Explicit.
2
In the Settings window for Explicit, type All Boundaries in the Label text field.
3
Locate the Input Entities section. From the Geometric entity level list, choose Boundary.
4
Select the All boundaries check box.
Boat and Shields
1
In the Definitions toolbar, click  Box.
2
In the Settings window for Box, type Boat and Shields in the Label text field.
3
Locate the Geometric Entity Level section. From the Level list, choose Boundary.
4
Locate the Box Limits section. In the x minimum text field, type -50.
5
In the x maximum text field, type 50.
6
In the y minimum text field, type -35.
7
In the y maximum text field, type 35.
8
In the z minimum text field, type -10.
9
In the z maximum text field, type 80.
Front Quadrant
1
In the Definitions toolbar, click  Explicit.
2
In the Settings window for Explicit, type Front Quadrant in the Label text field.
3
Locate the Input Entities section. From the Geometric entity level list, choose Boundary.
4
Sample Mount Back
1
In the Definitions toolbar, click  Explicit.
2
In the Settings window for Explicit, type Sample Mount Back in the Label text field.
3
Locate the Input Entities section. From the Geometric entity level list, choose Boundary.
4
Deposition Surfaces
1
In the Definitions toolbar, click  Difference.
2
In the Settings window for Difference, type Deposition Surfaces in the Label text field.
3
Locate the Geometric Entity Level section. From the Level list, choose Boundary.
4
Locate the Input Entities section. Under Selections to add, click  Add.
5
In the Add dialog box, select All Boundaries in the Selections to add list.
6
7
In the Settings window for Difference, locate the Input Entities section.
8
Under Selections to subtract, click  Add.
9
In the Add dialog box, in the Selections to subtract list, choose Boat and Shields, Front Quadrant, and Sample Mount Back.
10
Now set up the physics. Since the angular distribution is of particular interest in this model, increase the integration resolution to the maximum value to ensure the most accurate angular resolution when computing the flux.
Free Molecular Flow (fmf)
1
In the Model Builder window, under Component 1 (comp1) click Free Molecular Flow (fmf).
2
In the Settings window for Free Molecular Flow, locate the Integration Settings section.
3
From the Integration resolution list, choose 4096.
In this model the number density and pressure of the deposited species are of limited interest and will not be computed.
4
Locate the Compute section. Clear the Number density check box.
5
Clear the Pressure check box.
Molecular Flow 1
1
In the Model Builder window, under Component 1 (comp1)>Free Molecular Flow (fmf) click Molecular Flow 1.
2
In the Settings window for Molecular Flow, locate the Molecular Weight of Species section.
3
In the Mn,G text field, type Mn0.
Surface Temperature 1
1
In the Model Builder window, click Surface Temperature 1.
2
In the Settings window for Surface Temperature, locate the Surface Temperature section.
3
In the T text field, type Tamb.
Wall 1
1
In the Model Builder window, click Wall 1.
2
In the Settings window for Wall, locate the Wall Type section.
3
From the Wall type list, choose Deposition.
4
Locate the Deposition section. In the ρfilm,G text field, type rho0.
Evaporation 1
1
In the Physics toolbar, click  Boundaries and choose Evaporation.
2
3
In the Settings window for Evaporation, locate the Evaporation section.
4
In the pvap,G text field, type pvap.
An additional Surface Temperature node is required to specify the temperature of the evaporating gold.
Surface Temperature 2
1
In the Physics toolbar, click  Boundaries and choose Surface Temperature.
2
In the Settings window for Surface Temperature, locate the Boundary Selection section.
3
From the Selection list, choose Boat.
4
Locate the Surface Temperature section. In the T text field, type Tevap.
Mesh 1
Size 1
1
In the Model Builder window, under Component 1 (comp1) right-click Mesh 1 and choose Size.
2
In the Settings window for Size, locate the Geometric Entity Selection section.
3
From the Geometric entity level list, choose Boundary.
4
From the Selection list, choose Boat.
5
Locate the Element Size section. From the Predefined list, choose Extra fine.
Size 2
1
In the Model Builder window, right-click Mesh 1 and choose Size.
2
In the Settings window for Size, locate the Geometric Entity Selection section.
3
From the Geometric entity level list, choose Boundary.
4
From the Selection list, choose Meshing 1.
5
Locate the Element Size section. From the Predefined list, choose Extra fine.
Size 3
1
Right-click Mesh 1 and choose Size.
2
In the Settings window for Size, locate the Geometric Entity Selection section.
3
From the Geometric entity level list, choose Boundary.
4
From the Selection list, choose Meshing 2.
5
Locate the Element Size section. From the Predefined list, choose Extremely fine.
6
Click the Custom button.
7
Locate the Element Size Parameters section.
8
Select the Maximum element size check box. In the associated text field, type 3.
Free Triangular 1
1
In the Mesh toolbar, click  Boundary and choose Free Triangular.
2
In the Settings window for Free Triangular, locate the Boundary Selection section.
3
From the Geometric entity level list, choose Remaining.
4
Click  Build All.
Study 1
Step 1: Time Dependent
1
In the Model Builder window, under Study 1 click Step 1: Time Dependent.
2
In the Settings window for Time Dependent, locate the Study Settings section.
3
In the Output times text field, type range(0,6,60).
4
In the Home toolbar, click  Compute.
Results
Study 1/Solution 1 (sol1)
In the Model Builder window, expand the Results>Datasets node, then click Study 1/Solution 1 (sol1).
Selection
1
In the Results toolbar, click  Attributes and choose Selection.
2
In the Settings window for Selection, locate the Geometric Entity Selection section.
3
From the Geometric entity level list, choose Boundary.
4
From the Selection list, choose Deposition Surfaces.
Surface
When constant elements are used it is necessary to turn off the refinement in the postprocessing settings. This has been done automatically in the default plot, but in subsequent plots this setting will need to be changed manually.
1
In the Model Builder window, expand the Results>Incident Molecular Flux (fmf) node, then click Surface.
2
In the Settings window for Surface, click to expand the Quality section.
Note that the resolution is set to No refinement.
3
In the Incident Molecular Flux (fmf) toolbar, click  Plot.
Compare the resulting plot with that in Figure 2.
Film Thickness
1
In the Home toolbar, click  Add Plot Group and choose 3D Plot Group.
2
In the Settings window for 3D Plot Group, type Film Thickness in the Label text field.
Surface 1
1
Right-click Film Thickness and choose Surface.
2
In the Settings window for Surface, click Replace Expression in the upper-right corner of the Expression section. From the menu, choose Component 1 (comp1)>Free Molecular Flow>Adsorbed/Deposited species>fmf.h_film_G - Film thickness - m.
3
Locate the Expression section. From the Unit list, choose nm.
4
Locate the Coloring and Style section. Click  Change Color Table.
5
In the Color Table dialog box, select Linear>Viridis in the tree.
6
7
In the Settings window for Surface, locate the Quality section.
8
From the Resolution list, choose No refinement.
9
In the Film Thickness toolbar, click  Plot.
Compare the resulting plot with that in Figure 3.
10
In the Results toolbar, click  More Datasets and choose Surface.
Surface 1
1
In the Settings window for Surface, locate the Selection section.
2
From the Selection list, choose Meshing 2.
3
Locate the Parameterization section. From the x- and y-axes list, choose XY-plane.
Film Thickness on Sample
1
In the Results toolbar, click  2D Plot Group.
2
In the Settings window for 2D Plot Group, type Film Thickness on Sample in the Label text field.
Surface 1
1
Right-click Film Thickness on Sample and choose Surface.
2
In the Settings window for Surface, click Replace Expression in the upper-right corner of the Expression section. From the menu, choose fmf.h_film_G - Film thickness - m.
3
Locate the Expression section. From the Unit list, choose nm.
4
Locate the Coloring and Style section. Click  Change Color Table.
5
In the Color Table dialog box, select Linear>Viridis in the tree.
6
7
In the Settings window for Surface, click to expand the Quality section.
8
From the Resolution list, choose No refinement.
9
In the Film Thickness on Sample toolbar, click  Plot.
Compare the resulting plot with that in Figure 4.
Film Thickness vs. Time
1
In the Home toolbar, click  Add Plot Group and choose 1D Plot Group.
2
In the Settings window for 1D Plot Group, type Film Thickness vs. Time in the Label text field.
3
Locate the Plot Settings section.
4
Select the x-axis label check box. In the associated text field, type Time (s).
Point Graph 1
1
Right-click Film Thickness vs. Time and choose Point Graph.
2
3
In the Settings window for Point Graph, click Replace Expression in the upper-right corner of the y-Axis Data section. From the menu, choose Component 1 (comp1)>Free Molecular Flow>Adsorbed/Deposited species>fmf.h_film_G - Film thickness - m.
4
Locate the y-Axis Data section. From the Unit list, choose nm.
5
In the Film Thickness vs. Time toolbar, click  Plot.
Compare the resulting plot with that in Figure 5.
Appendix - Geometry Instructions
From the File menu, choose New.
New
In the New window, click  Blank Model.
Add Component
In the Home toolbar, click  Add Component and choose 3D.
Geometry 1
1
In the Settings window for Geometry, locate the Units section.
2
From the Length unit list, choose mm.
Cylinder 1 (cyl1)
1
In the Geometry toolbar, click  Cylinder.
2
In the Settings window for Cylinder, locate the Size and Shape section.
3
In the Radius text field, type 254.
4
In the Height text field, type 381.
5
Locate the Position section. In the y text field, type 25.4.
6
In the z text field, type -50.8.
Sphere 1 (sph1)
1
In the Geometry toolbar, click  Sphere.
2
In the Settings window for Sphere, locate the Size section.
3
In the Radius text field, type 254.
4
Locate the Position section. In the y text field, type 25.4.
5
In the z text field, type 330.2.
Union 1 (uni1)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Click in the Graphics window and then press Ctrl+A to select both objects.
3
In the Settings window for Union, locate the Union section.
4
Clear the Keep interior boundaries check box.
Cylinder 2 (cyl2)
1
In the Geometry toolbar, click  Cylinder.
2
In the Settings window for Cylinder, locate the Size and Shape section.
3
In the Radius text field, type 82.55.
4
In the Height text field, type 12.7.
5
Locate the Position section. In the z text field, type 279.4.
Cylinder 3 (cyl3)
1
In the Geometry toolbar, click  Cylinder.
2
In the Settings window for Cylinder, locate the Size and Shape section.
3
In the Radius text field, type 19.05.
4
In the Height text field, type 50.8.
5
Locate the Position section. In the x text field, type -79.
6
In the y text field, type -34.8.
7
In the z text field, type -50.8.
Rotate 1 (rot1)
1
In the Geometry toolbar, click  Transforms and choose Rotate.
2
In the Settings window for Rotate, locate the Input section.
3
Select the Keep input objects check box.
4
Click the  Wireframe Rendering button in the Graphics toolbar.
5
6
Locate the Rotation section. In the Angle text field, type range(0,30,60).
7
Locate the Point on Axis of Rotation section. In the y text field, type 102.03.
Block 1 (blk1)
1
In the Geometry toolbar, click  Block.
2
In the Settings window for Block, locate the Size and Shape section.
3
In the Width text field, type 2.54.
4
In the Depth text field, type 63.5.
5
In the Height text field, type 76.2.
6
Locate the Position section. In the x text field, type -34.5.
7
In the y text field, type -27.2.
8
In the z text field, type -50.8.
9
Locate the Rotation Angle section. In the Rotation text field, type -15.
10
Locate the Selections of Resulting Entities section. Find the Cumulative selection subsection. Click New.
11
In the New Cumulative Selection dialog box, type Shield in the Name text field.
12
Mirror 1 (mir1)
1
In the Geometry toolbar, click  Transforms and choose Mirror.
2
In the Settings window for Mirror, locate the Input section.
3
From the Input objects list, choose Shield.
4
Locate the Selections of Resulting Entities section. Find the Cumulative selection subsection. From the Contribute to list, choose Shield.
5
Locate the Input section. Select the Keep input objects check box.
6
Locate the Normal Vector to Plane of Reflection section. In the x text field, type 1.
7
In the z text field, type 0.
Block 2 (blk2)
1
In the Geometry toolbar, click  Block.
2
In the Settings window for Block, locate the Size and Shape section.
3
In the Width text field, type 12.7.
4
In the Depth text field, type 85.
5
In the Height text field, type 1.27.
6
Locate the Position section. In the x text field, type -6.35.
7
In the y text field, type -40.
8
In the z text field, type -1.27.
Sphere 2 (sph2)
1
In the Geometry toolbar, click  Sphere.
2
In the Settings window for Sphere, locate the Size section.
3
In the Radius text field, type 6.35.
4
Locate the Position section. In the z text field, type 2.54.
Copy 1 (copy1)
1
In the Geometry toolbar, click  Transforms and choose Copy.
2
3
In the Settings window for Copy, locate the Displacement section.
4
In the z text field, type 1.27.
Block 3 (blk3)
1
In the Geometry toolbar, click  Block.
2
In the Settings window for Block, locate the Size and Shape section.
3
In the Width text field, type 120.
4
In the Depth text field, type 34.35.
5
In the Height text field, type 1.27.
6
Locate the Position section. In the x text field, type -60.
7
In the y text field, type 55.2.
8
In the z text field, type -1.27.
Cylinder 4 (cyl4)
1
In the Geometry toolbar, click  Cylinder.
2
In the Settings window for Cylinder, locate the Size and Shape section.
3
In the Radius text field, type 101.6.
4
In the Height text field, type 1.27.
5
Locate the Position section. In the y text field, type 137.2.
6
In the z text field, type -1.27.
Union 2 (uni2)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Select the objects blk3 and cyl4 only.
3
In the Settings window for Union, click  Build Selected.
Delete Entities 1 (del1)
1
In the Geometry toolbar, click  Delete.
2
In the Settings window for Delete Entities, locate the Entities or Objects to Delete section.
3
From the Geometric entity level list, choose Domain.
4
On the object uni2, select Domains 1, 2, and 5 only.
Union 3 (uni3)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Select the objects blk2 and del1 only.
3
In the Settings window for Union, locate the Union section.
4
Clear the Keep interior boundaries check box.
Union 4 (uni4)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
Select the objects copy1, sph2, and uni3 only.
3
In the Settings window for Union, click  Build Selected.
Delete Entities 2 (del2)
1
In the Geometry toolbar, click  Delete.
2
In the Settings window for Delete Entities, locate the Entities or Objects to Delete section.
3
From the Geometric entity level list, choose Domain.
4
On the object uni4, select Domains 2–4 and 7 only.
Union 5 (uni5)
1
In the Geometry toolbar, click  Booleans and Partitions and choose Union.
2
3
In the Settings window for Union, locate the Union section.
4
Clear the Keep interior boundaries check box.
Work Plane 1 (wp1)
1
In the Geometry toolbar, click  Work Plane.
2
In the Settings window for Work Plane, locate the Plane Definition section.
3
In the z-coordinate text field, type -1.27.
Work Plane 1 (wp1)>Plane Geometry
In the Model Builder window, click Plane Geometry.
Work Plane 1 (wp1)>Polygon 1 (pol1)
1
In the Work Plane toolbar, click  Polygon.
2
In the Settings window for Polygon, locate the Coordinates section.
3
From the Data source list, choose Vectors.
4
In the xw text field, type -19.05 0 0 19.05 19.05 6.35 6.35 0 0 -6.35 -6.35 -19.05.
5
In the yw text field, type 63.5 82.55 82.55 63.5 63.5 45.55 45.55 44.45 44.45 45.54 45.54 63.5.
Extrude 1 (ext1)
1
In the Model Builder window, under Component 1 (comp1)>Geometry 1 right-click Work Plane 1 (wp1) and choose Extrude.
2
In the Settings window for Extrude, locate the Distances section.
3
4
Select the Reverse direction check box.
Work Plane 2 (wp2)
1
In the Geometry toolbar, click  Work Plane.
2
In the Settings window for Work Plane, locate the Plane Definition section.
3
In the z-coordinate text field, type -50.8.
Work Plane 2 (wp2)>Plane Geometry
In the Model Builder window, click Plane Geometry.
Work Plane 2 (wp2)>Circle 1 (c1)
1
In the Work Plane toolbar, click  Circle.
2
In the Settings window for Circle, locate the Size and Shape section.
3
In the Radius text field, type 63.5.
4
Locate the Position section. In the yw text field, type 203.2.
Work Plane 3 (wp3)
1
In the Model Builder window, under Component 1 (comp1)>Geometry 1 right-click Work Plane 2 (wp2) and choose Work Plane.
2
In the Settings window for Work Plane, locate the Plane Definition section.
3
In the z-coordinate text field, type 279.4.
Work Plane 3 (wp3)>Plane Geometry
In the Model Builder window, click Plane Geometry.
Work Plane 3 (wp3)>Circle 1 (c1)
1
In the Work Plane toolbar, click  Circle.
2
In the Settings window for Circle, locate the Size and Shape section.
3
In the Radius text field, type 101.6.
4
In the Sector angle text field, type 90.
5
Locate the Position section. In the xw text field, type -38.1.
6
In the yw text field, type 38.1.
7
Locate the Rotation Angle section. In the Rotation text field, type -90.
Difference 1 (dif1)
1
In the Model Builder window, under Component 1 (comp1)>Geometry 1 right-click Work Plane 3 (wp3) and choose Booleans and Partitions>Difference.
2
3
In the Settings window for Difference, locate the Difference section.
4
Find the Objects to subtract subsection. Click to select the  Activate Selection toggle button.
5
Select the objects blk1, cyl2, cyl3, ext1, mir1, rot1(1), rot1(2), rot1(3), uni5, wp2, and wp3 only.
Form Composite Faces 1 (cmf1)
1
In the Geometry toolbar, click  Virtual Operations and choose Form Composite Faces.
2
On the object fin, select Boundaries 34 and 36 only.
Form Composite Faces 2 (cmf2)
1
In the Geometry toolbar, click  Virtual Operations and choose Form Composite Faces.
2
On the object cmf1, select Boundary 1 only.
3
In the Settings window for Form Composite Faces, locate the Input section.
4
Click  Clear Selection.
5
On the object cmf1, select Boundaries 18, 19, 22, 23, 39, 40, and 60 only.
Form Composite Faces 3 (cmf3)
1
In the Geometry toolbar, click  Virtual Operations and choose Form Composite Faces.
2
On the object cmf2, select Boundary 1 only.
3
In the Settings window for Form Composite Faces, locate the Input section.
4
Click  Clear Selection.
5
On the object cmf2, select Boundaries 36 and 38 only.
Collapse Faces 1 (clf1)
1
In the Geometry toolbar, click  Virtual Operations and choose Collapse Faces.
2
On the object cmf3, select Boundaries 17 and 64 only.
Boat
1
In the Geometry toolbar, click  Selections and choose Ball Selection.
2
In the Settings window for Ball Selection, locate the Geometric Entity Level section.
3
From the Level list, choose Boundary.
4
Locate the Ball Radius section. In the Radius text field, type 10.
5
In the Label text field, type Boat.
Geometry
1
In the Geometry toolbar, click  Selections and choose Explicit Selection.
2
In the Settings window for Explicit Selection, type Geometry in the Label text field.
3
Locate the Entities to Select section. From the Geometric entity level list, choose Object.
4
All Boundaries
1
In the Geometry toolbar, click  Selections and choose Adjacent Selection.
2
In the Settings window for Adjacent Selection, type All Boundaries in the Label text field.
3
Locate the Input Entities section. Click  Add.
4
In the Add dialog box, select Geometry in the Input selections list.
5
6
In the Settings window for Adjacent Selection, locate the Output Entities section.
7
Select the Interior boundaries check box.
Boat and Shields
1
In the Geometry toolbar, click  Selections and choose Union Selection.
2
In the Settings window for Union Selection, locate the Geometric Entity Level section.
3
From the Level list, choose Boundary.
4
In the Label text field, type Boat and Shields.
5
Locate the Input Entities section. Click  Add.
6
In the Add dialog box, in the Selections to add list, choose Shield and Boat.
7
Front Quadrant
1
In the Geometry toolbar, click  Selections and choose Explicit Selection.
2
In the Settings window for Explicit Selection, type Front Quadrant in the Label text field.
3
Locate the Entities to Select section. From the Geometric entity level list, choose Boundary.
4
On the object clf1, select Boundaries 1 and 4 only.
Sample Mount Back
1
In the Geometry toolbar, click  Selections and choose Explicit Selection.
2
In the Settings window for Explicit Selection, type Sample Mount Back in the Label text field.
3
Locate the Entities to Select section. From the Geometric entity level list, choose Boundary.
4
Select the Group by continuous tangent check box.
5
On the object clf1, select Boundaries 9, 10, 12, 44, and 52 only.
Deposition Surfaces
1
In the Geometry toolbar, click  Selections and choose Difference Selection.
2
In the Settings window for Difference Selection, type Deposition Surfaces in the Label text field.
3
Locate the Geometric Entity Level section. From the Level list, choose Boundary.
4
Locate the Input Entities section. Click  Add.
5
In the Add dialog box, select All Boundaries in the Selections to add list.
6
7
In the Settings window for Difference Selection, locate the Input Entities section.
8
9
In the Add dialog box, in the Selections to subtract list, choose Boat and Shields, Front Quadrant, and Sample Mount Back.
10
Meshing 1
1
In the Geometry toolbar, click  Selections and choose Explicit Selection.
2
In the Settings window for Explicit Selection, type Meshing 1 in the Label text field.
3
Locate the Entities to Select section. From the Geometric entity level list, choose Boundary.
4
Select the Group by continuous tangent check box.
5
On the object clf1, select Boundaries 1, 2, 4, 5, 25, 42, 43, 49, 53, and 59 only.
Meshing 2
1
In the Geometry toolbar, click  Selections and choose Explicit Selection.
2
In the Settings window for Explicit Selection, type Meshing 2 in the Label text field.
3
Locate the Entities to Select section. From the Geometric entity level list, choose Boundary.
4
On the object clf1, select Boundary 21 only.