There are two reasons why we must construct the Nwell mask in a non-standard way. First, our convention for drawing nwell differs from VTI's. Second, we have chosen a more agressive design rule for the minimum spacing permitted between p+ diffusion inside nwell and n+ diffusion outside nwell, and the placement of the nwell boundary must be acceptable to both VTI and ICL (see [1], rules 6.3.5., 6.3.7., and 6.3.12.). VTI has agreed to support this non-standard requirement. (1-28-87: Even though ICL is no longer a foundry source, we'll retain the non-standard derivation equation, for the other reasons cited.)

Nwell for the mask is constructed from "Dragon-drawn" nwell (i.e.., nwell drawn according to the Dragon convention, which is supported by the Chipndale layout system) by undersizing it by 1.0 microns per side, or 2000 nm diameter. In VTI's notation:

VTI's standard derivation includes a CR layer, which they use for drawing nwell resistors. We don't use such resistors, nor do we have such a layer.

By the way, do we need any more formal citing of the agreement between PARC and VTI that permits us to do this non-standard thing?

The intent of this construction is to put the final electrical boundary between nwell and substrate at least 7 microns from p+ diffusions in nwell and at least 3 microns from n+ diffusions outside nwell. This "7-and-3" position is a compromise between VTI and ICL. VTI's standard requirement is "8-and-4." To meet our requirement for more aggressive spacing between p+ and n+, VTI first proposed "8-and-2," but this was not acceptable to ICL. ICL's standard requirement was "4-and-6," but they felt that anything between "3-and-7" and "7-and-3" would be acceptable. VTI then agreed to do "7-and-3." (1-28-87: Even though ICL is no longer a foundry source, we'll retain the non-standard derivation equation.)

The construction of the P Field Dope mask is logically related to the construction of the Nwell mask, and is hence covered by the same agreement. It is constructed from "Dragon-drawn" nwell by oversizing it by 0.5 microns per side, or 1000 nm diameter. In VTI's notation:

Again, since we don't have a CR layer, it doesn't appear in our derivation equation. Also, we don't require a denotch threshold, since two constructed shapes can never become close enough to each other that merging them would be a good idea.

Our derivation of the via mask differs from VTI's derivation, but we hope the differences are too minor to worry about. Both derivations oversize vias over diffusion, by 0.25 microns per side. But VTI oversizes only vias of minimum size, i.e., 2 x 2 microns.

Our scribe line is constructed asymmetrically around the "user data" rectangle. All edges have a minimum amount of scribe line geometries. The left and bottom edges also have all of the rest of the scribe line area. This is done so that holes can be cut out of this scribe line "middle" area, and artifacts such as alignment keys and CD structures can be inserted. The precise location of these artifacts in the scribe line is not important.

(VTI's spec says its scribe line is 3-sided with the open end to the right. They also drop artifacts into the scribe line area.)