Optical proximity correction (OPC) and assist feature rules are generated using a
process window (PW) analysis. A reference
pitch is chosen and the
mask bias is found that optimizes the
process window. This can be done using standard
process window analysis or through a weighted process window (WPW) analysis which accounts for focus and
dose distributions that are expected in a real process. The WPW analysis gives not only the optimum
mask bias, but also the center focus and
dose conditions for the optimum process centering. A series of other pitches and
mask biases are then analyzed by finding the common process window with the reference
pitch. For the standard PW analysis, a common process window is found. For the WPW analysis, the WPW is computed at the center focus and
dose conditions found for the reference
pitch. If mask or lens errors are to be accounted for, then multiple structures can be included in the analysis. Once the common process windows for the mask features of interest have been computed, functional fits to the data can be found. Once the functional forms have been found for each of the OPC parameters, the rules table can be determined by solving for the spacings of interest in the design.