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EE734 Example
A simple simulation of a LOCOS process |
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EE734
Back to the Table of Contents for the example |
Grid Definition |
The first step is to specify the starting grid. The simulator uses a finite element approach to solve the equations relevant to the process. Values for the various parameters and variables are stored for each point of the finite element grid. There is a trade-off between the accuracy of the simulation and the amount of memory used and run time. If the grid is too sparse the accuracy of the simulation will be compromised. If the grid is too dense the simulation will consume a lot of memory and take too long to run, with little benefit in improved accuracy.
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To set up the initial grid I selected Commands ® Mesh Define. I started by defining the mesh in the y (vertical) direction. I first set the slider bar to Location = 0 mm and Spacing = 0.05 mm and clicked on Insert. I then set Location = 1.00 mm and Spacing = 0.50 mm and clicked on Insert again. The simulator will use these values to set up a grid that starts at a spacing of 0.05 mm near the silicon surface but increases toward 0.50 mm as the depth below the surface increases to 1 mm. Finally I set Location = 3.00 mm and Spacing = 1.00 mm and clicked on Insert one more time. This will increase the grid spacing toward 1.00 mm in the region between 1.00 mm and 3 mm. For this example the main thing that will be simulated in the silicon will be the redistribution of dopant atoms as the oxidation takes place and silicon is consumed. I therefore want a denser grid near the surface of the silicon, where most of the changes will be. |
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In the width dimension I expect all the interesting stuff to occur near the edge of the silicon nitride. I therefore set the grid in the x direction as shown in the adjacent figure. This gives a denser grid near the center of the sample being simulated. |
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I then clicked on Write to incorporate these grid settings into the simulation deck.
One thing to bear in mind is that there is a pretty good chance you will not get the grid exactly right on your first attempt, and will need to edit the simulation deck after you see the results of the first run to improve the accuracy of the simulation. The simulator will also adjust the grid, for example gridlines will be added in an oxide layer as it grows, or in a deposited layer.
Table of Contents
Substrate Definition
EE734|Electrical Engineering
Please report problems with this page to George Valco (valco@ee.eng.ohio-state.edu)