From 120 to 32 nm CMOS technology: development of OPC and RET to rescue optical lithography

Starting from the 120 nm CMOS technology node down to the 32 nm node, we have entered into a new lithographic regime. The wavelength has not changed (only 193 nm), and we move closer and closer to the theoretical optical resolution limit. Therefore, Resolution Enhancement Techniques (RET) have been developed in order to print all shapes properly and close the resolution gap. The primary RET developed are off-axis illumination, sub-resolution assist features and a phase shift mask. Moreover, working closer to the resolution limit implies bigger image distortion between the mask and the silicon. For this purpose OPC (Optical Proximity Correction) has been widely used by making mask pre-compensation of all non linear effects, optical diffraction and interference effects, resist and etch. RET and OPC are also fundamentally linked. RET such as off-axis illumination generates more distortion, and therefore justifies the need of more aggressive OPC, and RET techniques like Alt PSM and sub-resolution assist features are generated through the OPC infrastructure. From its first industrial utilization for 120 nm node to 32 nm prospectively, many evolutions have been seen for OPC. These include the generalisation to all lithographic layers, moving to pixel based simulation, usage of full chip simulation and verification, the incorporation of process window effects like Energy Latitude or Depth of Focus into the OPC algorithm, and inverse lithography approach. For RET, we have seen huge differentiation depending on the type of application, such as logic or memory. In conclusion, we need to consider design as a third party that is playing a key role in this RET–OPC synergy. To use more aggressive RET and reduce the cycle time of OPC recipe development, more regular designs are considered as a key enabler for the future: they will allow logic makers to consider RET options that are pushed as far as those used by memory makers. To cite this article: Y. Trouiller, C. R. Physique 7 (2006).