The Effect of Intra-field CD Uniformity Control (CDC) on Mask Birefringence

摘要

Mask and Wafer CD Uniformity (CDU) improvement by utilizing an ultrafast laser system for writing shading elements inside the bulk of Quartz (Qz) Photomasks has previously been shown to be an effective and practical application . The CD Control ( CDC) Process is working in production environments for 90 and 65 nm design rule processes which utilize KrF and ArF scanners.Advanced design rule nodes at 45 and 32 nm will utilize high and hyper NA immersion lithography, which require highly polarized light and immersion technology. Maintaining a high degree of polarization requires low birefringence (BF) of the optical path and specifically of the mask. Current mask blanks contribute between 5 to 20 nm of BF which is too high for polarized systems. This lead to the recent introduction of special low BF blanks which provide ＜ lnm BF per mask.The CDC Process which introduces an optical element inside the quartz (Qz) mask performs a local change of the bulk Qz morphology which causes a local change in refractive index of the Qz and may induce some local BF. The induced BF, if too high, may potentially cause depolarization of the highly polarized light of hyper NA scanners. Depolarizing the light by a high degree has the potential to degrade the image contrast in the litho process. The current study examined the effect of the CDC Process on the mask BF at 193 nm by writing controlled attenuation shading elements inside special low BF Qz blanks and measuring the BF induced by the CDC Process. Results: It was found that BF induced by the CDC Process is so small that its effect on loss of CDU is negligible compared to the gain in CDU. This will allow mask and IC manufactures to take advantage of Pixer's CDC Process in hyper NA litho processes at 45 and 32 nm nodes.