Introduction to VLSI Design

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Example: CMOS Inverter Layout

Enclosing the substrate contact


In the previous step we tried to place the n-type substrate contact in the n-well. Since we had drawn the n-well to cover the P-diffusion at minimum length, the well is not wide enough to accomodate the additional contact. We must enlarge the n-well, so that it also covers the substrate contact.

One way to do this would be to simply draw an adjoining rectangle using the n-well layer. Instead, we will try to modify the existing rectangle, so that it covers the contact.

1. Press F4 on the keyboard to toggle selection mode.

By default, the selection mode will only select whole objects. Pressing "F4" will change this default to partial selection. The information bar will start displaying "(P) Select" (P for partial) instead of "(F) Select" (F for Full).

2. Move cursor over the left edge of the n-well.

You'll notice that as soon as the cursor is close to the edge, only the edge line will be highlighted as a pale dashed line.

Click to enlarge

3. Click once to select the edge.

3. Move mouse over the selected edge (without pressing any mouse buttons).

You'll notice that the cursor changes shape when you are close to the edge.

4. Press and hold left mouse button when cursor changes above the selected edge.

You have grabbed the edge, and as long as you do not release the mouse button you can "stretch" the edge. Move the edge of the n-well so that all the of the substrate contact is covered by n-well.

Click to enlarge



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Last Updated by Ilhan Hatirnaz on 11/7/1998