SMO applied to contact layers at the 32nm node and below with consideration of MEEF and MRC

An illuminator and mask patterns were optimized (SMO) to minimize CD variation of a set of contact patterns selected from logic layouts and an array of SRAM cells. MEEF and defocus characteristics of the target patterns were modeled as functions of constraints on minimum mask features and spaces (MRC). This process was then repeated after linearly shrinking the input patterns by 10%. Common statistical measures of CD control worsen as MRC becomes more restrictive, but these are weak indicators compared to behavior at points in the image that exhibit high MEEF or low depth of focus. SMO solutions for minimum MEEF and maximum depth of focus are different, so some compromise is necessary. By including exposure time among the variables to be optimized, some control over local mask bias is made available to minimize MEEF and loss of litho quality due to MRC.