Ultra high resolution DUV water immersion microscope objective

In semiconductor manufacturing DUV microscopy at 248 nm wavelength is well established for wafer inspection and mask metrology. The next step in wavelength to 193 nm for higher resolution is hampered among other things by the fact that so far no really suitable light sources are available for microscopy in this wavelength range. During the past few years immersion lithography has emerged as a new approach for printing smaller feature sizes and in this context also water immersion for mask metrology was accepted by the mask manufacturers. In the paper the very latest development of a 248 nm wavelength 200x DUV microscope objective with NA = 1.25 and half pitch resolution of 60 nm is presented. Because the main application is metrology, an outstanding production performance is of particular importance. For this, new mounting schemes had to be developed. The paper covers the aspects of optical design and tolerancing, mounting techniques as well as applications.