Dynamic performance of DUV step-and-scan systems and process latitude

Abstract

All lithographic step and scan exposure tools require accurate synchronization between the reticle stage and wafer stage during scanning operation. Lateral vibrations between the two stages can be detrimental to process latitude and CD control. The magnitude of these vibrations is expressed as the moving standard deviation (MSD). This paper gives a theoretical framework to predict the impact of MSD on process latitude. The theory is verified by experiments performed on ASML 248 nm step and scan systems. Vibrations were induced between the reticle stage and wafer stage. Focus exposure matrices were exposed for 130 nm and 180 nm features at different noise levels. This establishes the relation between the process latitude and MSD level. The theoretical predictions agreed with the experimental result. Both revealed that the process latitude is most critical towards MSD for dense lines. The exposure latitude and depth of focus decrease rapidly when the MSD level increases. The tolerance towards MSD diminishes when resolution goes down. For 180 nm dense lines, 25 nm MSD can be tolerated to keep loss in EL limited to 10 percent. For 100 nm dense lines this is only 15 nm. Base don the current MSD level of ASML step and scan tools, which is approximately 10 nm, MSD will not deteriorate process latitude of 100 nm features significantly. It was found that for the chosen resist and process conditions, MSD did not influence the best energy and isofocal CD for the dense liens. A decline in Best Energy was observed for the 180 nm isolated lines when the MSD level exceeded 30 nm. This may have serious implications for CD control of isolated lines when the MSD level varies broadly within a batch.