Alternating PSM optimization using model-based OPC

We describe proximity correction methods for alternating phase shift mask (APSM) designs, including graduated phase transition PSM, phase-conjugate, also known as dual trench PSM, and double exposure clear and dark field PSM. We determine the magnitude and characteristics of proximity errors inherent to APSM, and compare them with the corresponding characteristics of binary masks. We present our investigations on integrating APSM and proximity correction, including CD control improvements at nominal conditions and through focus. We examine the limitations of each APSM/proximity correction alternative. All correction methods and proximity error characterizations were done using TAURUS - OPC. This includes phase shift mask model generation, boolean operations for generating intermediate correction layers, proximity correction, and boolean post- processing to generate the final output layers. We show that large imaging distortions near phase transitions regions require proximity correction and the size of the proximity correction serifs is large due to a Mask Error Factor less than 1.0.