In the same way, set the location of a second X line to 1.0 with a spacing of 0.1. You can either set the values by dragging a slider or by entering a number directly.
Now, select the Y direction and set the lines with the same values as the X direction. You can now add the comments at the Comment line. The ATHENA Mesh Define menu should appear as shown in Figure 7.17.
You can now write the menu-prepared mesh information into the input file. But first, preview the rectangular grid by selecting the View... button and the View Grid window (Figure 7.18) will appear. Notice that vertical and horizontal grid lines are distributed uniformly, and the 121 points and the 200 triangles will be generated.
A uniform grid such as the one shown in Figure 7.18 is inefficient for performing complex simulations. Therefore, the grid must be improved. First, make a better grid in the y-direction. Usually, it’s necessary to get better resolution for the depth profile after the ion implantation step. When adaptive gridding capability isn’t used, apply preliminary knowledge of the process you are going to simulate.
Suppose you want to perform a 60 keV boron implant so that the implant peak would be around 0.2 µm. It is reasonable to make a finer grid at this depth. To achieve this, simply add one more Y-line by setting the Location to 0.2 and the Spacing to 0.02. The new rectangular grid (Figure 7.19) will now appear. Notice the number of points and triangles have increased to 231 and 400 respectively.
Finally, write the Mesh Define information to the file by pressing the Write button. A set of lines like these will appear:
GO ATHENA
# NON-UNIFORM GRID
LINE X LOC=0.00 SPAC=0.1
LINE X LOC=0.3 SPAC=0.02
LINE X LOC=1 SPAC=0.1
LINE Y LOC=0.00 SPAC=0.03
LINE Y LOC=0.2 SPAC=0.02
LINE Y LOC=1 SPAC=0.1
The first line (GO ATHENA) is called an autointerface statement and tells DECKBUILD that the following file should be run by ATHENA
The grid in ATHENA consists of points connected to form a number of triangles. Each point has one or more nodes associated with it. A point within a material region has one node, while a point which belongs to several regions has several nodes. A node represents the solution (e.g., doping concentration) in a particular material region at the point. For example, a given node may represent solution values in silicon at a point with coordinates (0.0, 0.0); an entirely different node may represent solution values in oxide at the same point (0.0, 0.0).
So, the previous INIT statement creates the<100>silicon region of 1.0 µm x 1.0 µm size, which is uniformly doped with boron concentration of 3e14 atom/cm3. This simulation structure is ready for any process step (e.g., implant, diffusion, Reactive Ion Etching). Before discussing the simulation of physical processing using SSUPREM4, ELITE or OPTOLITH modules, it’s important to discuss structure manipulation statements that can precede or alternate with physical process steps.
7.7.2.3. Defining the initial substrate.
The LINE statements specified by the Mesh Define menu set only the rectangular base for the ATHENA simulation structure. The next step is the initialization of the substrate region with its points, nodes, triangles, background doping, substrate orientation, and some additional parameters. To initialize the simulation structure, select ATHENA Command Menu®Mesh Initialize... and the Mesh Initialize Menu will appear (see Figure 7-20)