The use of alternating phase shift masks (Alt-PSMs) for poly gate patterning is becoming a well-established method for reducing gate critical dimension (CD) and variability. The application of alt-PSM for other device layers and for improving resolution (minimum pitch) is less developed due to more complex layouts, more stringent mask constraints and cost of ownership restrictions. Resolution of contact pairs and nested contacts is found to be improved using alt-PSM compared to embedded PSMs (EPSMs). To improve the process window of semi-nested and isolated contacts, sub-resolution phase-shifted assist features are employed on the mask. Square assist features, rather than rectangular assist features, are used to reduce mask fabrication requirements as one can use a larger minimum assist feature dimension. Because of high mask error enhancement factors (MEEFs), assist features with dimensions as large as 75% of the nominal contact size can be used without patterning on the wafer. Compared to using alt-PSM for poly gate patterning the use of alt-PSM for tight pitch patterning places additional constraints on mask manufacturing. The smaller phase regions intrinsic to tight pitch patterning result in tighter phase uniformity and mask defect requirements.