Experimental evaluation of out-of-plane distortion of electrostatically chucked EUV reticle

Reticle protection during storage, handling and use is one of the critical issues of EUV lithography because no practical pellicle has been found for EUV reticles as yet. The front surface of an EUV reticle has to be protected from particles larger than 20-30 rim. to maintain the image quality on the wafer plane, and the backside also has to be protected to maintain the flatness of the reticle chucked on an electrostatic chuck (17-SQ. In this paper, we are focusing on particles on the backside of the reticle. If a particle lies between the reticle and the chuck, it has a strong impact on the flatness of the reticle, and the wafer overlay is degraded by out-of-plane distortion (OPD) and in-plane distortion (IPD) due to the particle 1-5. From this point of view, we need to know the maximum pen-nissible size of particles on the backside of the reticle. MIR-M-Selete introduced an experimental setup that can measure the flatness of the chucked reticle in a vacuum. An electrostatic chuck is installed in the vacuum chamber of Mask Protection Engineering Tool (MPE Tool)6, a reticle is automatically carried from a reticle pod to the chuck in the tool. The flatness of the reticle can be measured by an interferometer through a viewport underneath the chamber. We can measure the reticle flatness with 3-rm@rirns reproducibility using this setup. We report results of experimental evaluation about the relationship between the reticle OPD, the size of particle and the chucking force of ESC.