Contact mask optimization and SRAF design

Contact hole patterning and especially sub resolution assist feature (SRAF) insertion and optical proximity correction (OPC) have become an extremely critical element of enabling continued shrink of CMOS technology. These elements of mask generation are fundamental to the success of technology execution. As off-axis illumination modes have been introduced to resolve smaller pitches, forbidden pitches emerge that need to be considered in random logic layouts. Optimized placement of assist features for these pitches is extremely important for overall process window and tolerance budget considerations. Several techniques have recently been developed for model based SRAF optimization. These typically focus only on optimizing the aerial image through focus, but may not include sensitivity to mask error as well. These approaches will be evaluated and discussed. Total CD uniformity is presented as a metric for evaluation of mask solutions. This includes the impact of dose and focus, but also masks error in estimating the total CD variation of a contact patterning process. The SRAF solution with the lowest overall variation is the winner. This methodology is presented for parametric through pitch features, and logic patterns.