Long-term Zernike lens aberration measurement

Monitoring long-term performance of projection optics in lithographic exposure systems will become more and more important, especially for 193nm wavelength. Various effects influence the quality and long-term stability of a lens projection system. Using the well known and established blazed phasegrating method, it is possible to identify lens degradation before it becomes a significant detractor in a manufacturing process. A two beam interferometer formed by a blazed grating reticle is used to measure the aberration values. This works for all DUV tools, and therefore it allows a comparison of tools from different suppliers. The test can be run after regular preventive maintenance or as daily monitor checks, in order to evaluate lens aberration over time. By storing the results, it is easy to generate a toot individual database. With this paper, we will show aberration data over time and the possibility to increase tool performance and stability.