Mask Data Prep and Manhattan Layout
MOSIS Technical Notes
MOSIS strongly recommends that layout be Manhattan (all edges parallel to either the X or the Y axis). Should non-Manhattan layout be necessary, it should be confined to large objects, where minor adjustments in the edges are of no consequence.
Contacts and vias, being necessarily of minimum size, should always be Manhattan.
There are mask generation issues that make non-Manhattan layout subject to increasing distortion as feature sizes shrink. The issues all stem from the need to fracture the layout data, and put it on a grid of a given spot size (the spot size that will be used to write the mask on the e-beam or laser exposure tool). While the spot size has been shrinking too, it has not been shrinking as fast as the feature size, so there are fewer spots in a minimum size feature now than there were in the past. This means that off-grid layout is snapped by a relatively larger amount to force it onto the grid. And, necessarily, all 45-degree contacts are off-grid.
Furthermore, when any sizing of data is required (usually to compensate for sizing that occurs in the wafer processing), the algorithm used (which is based on extending or contracting the number of spots in each feature) exaggerates the sizing along 45-degree edges by 40% (square root of 2 - 1). Increasing the size of the 45-degree contacts is not the appropriate answer. Standing them up straight is the only good solution.