In a recent paper15, we presented a novel method for fully automated model-based generation and optimization
of sub-resolution assist features which, when placed on a contact layer photomask, minimize the variations in the printed
pattern with respect to focus change. Here we extend that methodology to improve the contrast of the light intensity in
addition to minimizing variations caused by focus change.
We present a model-based method of generating and optimizing sub-resolution assist features. Assist feature
generation is based on a focus sensitivity map derived from a cost function that minimizes the variations in the printed
pattern with respect to focus change. We also demonstrate a method to generate mask-friendly SRAF polygons from the
focus sensitivity map. After model-based placement, assist features and the main polygons are optimized together by
moving their edge segments. One of the optimization goals is that side-lobes and assist features should not print. This is
enforced by computing image on a two dimensional grid. We demonstrate the process window improvement for a
contact layer example.
In recent years, mask critical dimension (CD) linearity and uniformity has become increasingly important. The ITRS roadmap shows the mask CD control requirements exceeding those of the wafer side beyond the 45nm node. Measurements show that there are systematic, uncorrected proximity effects even when a state-of-the-art proximity effect correction (PEC) algorithm is used. The uncorrected proximity effect is predictable with a computational model. The model for e-beam lithography and etch process contains terms to model short-range pattern density effects and plasma shadowing effect in Cr-etch. The model is calibrated using CD measurements on a test mask. The model is valid for arbitrary 2-D patterns. We present a model-based mask process compensation (MPC) method which applies geometric changes to polygons as in OPC. We discuss the goodness of model fit to the calibration data; verification of the calibrated model by SEM images; and the improvement obtained by MPC. The mask writing error, i.e. final inspection CD minus incoming database CD, was reduced by a factor of 2 through the use of MPC.