1-10-6: IC Layout Tutorial: Adding Contacts to a Transistor
Although the default transistor is 2x3 in size, most people want them to be wider. For the purposes of this example, make the transistor be 12 wide. To do this, select the node and use the Object Properties command (in menu Edit / Properties).
|Two easier ways to see the objects properties are to double-click on the node, or select it and type Control-I. When the "node Properties" dialog appears, make the width 12 and click OK.
|Next we need a contact. Choose a "Metal-1-N-Active-Con" to connect the N-Active to Metal-1. Make its size be 5x12 instead of the default 5x5. Notice that contacts are "smart" about the cuts, and add them to fill the node. Note also that the port (the inner rectangle) grows with the node.
Designers who have used polygon-based systems will be tempted to move these two nodes together so that they form the desired structure:
Electric is a connectivity-oriented system, and insists that these components be wired together.
The easiest way to connect the contact to the transistor is to spread the nodes apart, wire them, and then push them back together. These two figures show the transistor and contact nodes, spread apart, and connected by an arc.
The arc was made by selecting one node, clicking and HOLDING the right button, dragging the mouse over the other component, and then releasing the button to create the arc.
Notice that the ends of an arc are centered and indented from the edge by half of the arc's width (the ends are illustrated by "+" on the right). The ends of an arc must sit inside of the ports. If an arc moves such that its ends are still in the ports, then the nodes don't have to move. See Section 5-4-3 for more on arc geometry.
Now that the nodes are wired together, bring the contact in close. Notice that the arc has shrunk down to a square, with the endpoints very close together. If you make the arc rigid, the two nodes will be held together in this configuration. To do this, use the Rigid command (in menu Edit / Arc). As shown here, the "R" on the selected arc tells you that it has been made rigid. See Section 5-2-1 for more arc constraints.
Another common situation in making contacts meet transistors is when the sizes are not the same. In this example, the contact is the default size. The arc runs from the center of the contact's port to the top of the transistor's port. The finished layout is shown on the right.
Here are some points about connecting nodes with arcs:
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