Background doping can be set by clicking on the desired impurity box (e.g., Boron). The background impurity concentration specification will then become active. If the None box is checked, the concentration information will become inactive and will appear grayed out from the rest of the menu. Select the desired concentration using the slider (e.g., 3.0) and select an exponent from the Exp : menu (e.g., 14). This will give a background concentration of 3.0e14 atom/cm3. You can set background concentration using the By Resistivity specification in Ohm•cm. For this tutorial, check the 2D box in the Dimensionality field. This will run the simulation in a two-dimensional calculation.
Note: Two-dimensional mode is used in this tutorial to demonstrate 2D grid generation and manipulation. In most cases, however, it is unnecessary to change the Auto default in the Dimensionality item of the Mesh Initialize menu. ATHENA will begin in 1D and will automatically switch to 2D mode at the first statement, which disrupts the lateral uniformity of the device structure. This generally results in massive savings of computation time.
You can now write the mesh initialization information into the file by pressing the Write button. The following two lines will appear in the Deckbuild Text Subwindow:
# INITIAL SILICON STRUCTURE
INIT SILICON C.BORON=3.0E14 ORIENTATION=100 TWO.D
So, the previous INIT statement creates the<100>silicon region of 1.0 mm x 1.0 mm 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.
7.7.2.4. Simple Film Depositions
Conformal deposition can be used to generate multi-layered structures. Conformal deposition is the simplest deposit model and can be used in all cases when the exact shape of the deposited layer is not critical. Conformal deposition can also be used in place of oxidation of planar or quasi-planar semiconductor regions when doping redistribution during the oxidation process is negligible.
To set the conformal deposition step, select the menu items Process→Deposit→Deposit... from the Commands menu in DECKBUILD and the ATHENA Deposit Menu (Figure 7-21) will appear.
As shown, Conformal Deposition is the default. If it is known that the oxide layer thickness grown in a process is 200 Angstroms, you can substitute this with conformal oxide deposition. Select Oxide from the Material menu and set its thickness to 0.02 µm. It is always useful to set several grid layers in a deposited layer. In this case, at least two grid layers are needed to simulate impurity transport through the oxide layer. In some other cases (e.g., photoresist deposition over a non-planar structure), a sufficiently fine grid is needed to accurately simulate processes within the deposited layer. There are also situations (e.g., spacer formation) when several grid layers in a deposited material region are needed to properly represent the geometrical shape of the region.
The grid in the deposited layer is controlled by Grid Specification parameters in the ATHENA Deposit Menu. Set the Total number of grid layers to 2, add a Comment , and click on the Write button. The following lines will then appear in the Deckbuild Text Subwindow:
# GATE OXIDE DEPOSITION
DEPOSIT OXIDE THICK=0.02 DIVISIONS=2
The next step will be to deposit a phosphorus doped polysilicon layer of 0.5µm thickness. Select the material Polysilicon, and set the thickness to 0.5. To add doping, select the Impurities box. The Impurity Concentration section will be immediately added to the ATHENA Deposit Menu (See Figure 7-22).
Figure 7.22.Impurity section of the ATHENA Deposit Menu.
Click on the Phosphorus checkbox and set the doping level (e.g., 5.0x10 19 ) using the slider and the Exp menu. You can set a non-uniform grid in the deposited layer by changing the Nominal grid spacing(DY) and the Grid spacing location(YDY) parameters. To create a finer grid at the polysilicon surface, set the total number of grid layers to 10, the Nominal grid spacing(DY) to 0.02 µm and the Grid spacing location(YdY) to 0.0 (at the surface). Then, click on the Write button and the following deposition statement will be written in the input file as:
DEPOSIT POLY THICK=0.5 C.PHOSPHOR=5.0E19 DIVISIONS=10 \ DY=0.02 YDY=0.0 MIN.SPACING=0.001
Use the Cont button to continue the ATHENA simulation. This will create the three layer structure shown in the left plot of Figure 7.23 (Topmost layer is PolySi=0.5µm, Gate Oxide=0.02µm, Silicon = 1µm). The MIN.SPACING parameter preserves the horizontal mesh spacing for high aspect ratio grids. ATHENA tries to reduce high aspect ratio grids and MIN.SPACING stops this. To get a finer grid not at the polysilicon surface but in the middle of polysilicon layer, change YDY to 0.2. This puts on a finer grid at a distance of 0.2µm from the surface of the structure. You can do this by positioning the cursor in the input file and backspacing over existing text, or entering new text. For example:
DEPOSIT POLY THICK=0.5 C.PHOSPHOR=5.0E19 DIVISIONS=10 \ DY=0.02 YDY=0.2
It is possible to see the effect of changing the YDY. parameter within the polysilicon without rerunning the whole input file. To do this, highlight the previous statement (DEPOSIT OXIDE...), select Main Control→Init from History button, and press the Cont button. The new history file can then be loaded into TONYPLOT (see the right plot in Figure 7.23).