LAYOUT CONSIDERATIONS DUE TO PROCESS CONSTRAINTS:ANTENNA RULES

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ANTENNA RULES

A side effect of the manufacturing process that leads to damaged parts is known as the antenna effect. Under certain conditions, plasma etchers or ion implanters induce charge onto various structures that connect to a gate of a transistor. The induced charge threatens to overstress and irreparably damage the thin gate oxides of the transistor, causing unreliable operation.

Charge is readily induced during the manufacturing process if a structure is built in such a way that it acts like an antenna. An example of an antenna structure is shown in Figure 8.9, where the ratio between poly over field (thick oxide)

Layout Design Techniques to Address Electrical Characteristics-0137

and poly over gate (thin oxide) is large enough. There are others specific to each process, and some include metal.

As the gate size gets smaller and more metals are added to a chip, and as process engineers reduce the thickness of the oxides, the antenna effect can have a greater impact on the yield of a wafer.

Approach 1 shows a technique to eliminate the antenna shown in the example by breaking up the long poly that acts like an antenna.

Approach 2 shows a diode placed near the transistor in danger to eliminate the effect of the metal antenna. As soon as enough charge is induced onto the metal by the antenna effect, the diode diverts the charge to the substrate.

In many advanced processes, the antenna rule checking is required for structures made of any number of routing layers, i.e., metal1, metal2, etc. Some of the automated routers know how to limit the routing in one metal for long distances in such a way that they can avoid the antenna problem.

Explicit and separate diodes attached to certain nodes may not be necessary if an inherent diode is attached to the line somewhere along the path. For example, a source or drain area of another transistor may be sufficient to act as a diode to divert unwanted charge.

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