Increased depth of field through wave-front coding: using an off-axis zone plate lens with cubic phase modulation in an EUV microscope

Abstract

The authors are extending the capabilities of the SHARP microscope (SEMATECH High-NA Actinic Reticle review Project) by implementing wave front coding as a complementary imaging mode. SHARP, using a single off-axis lens has a tilted focal plane, reducing the instrument’s field of view to a few micrometers. Wave-front coding increases the depth of field of an incoherent imaging system without affecting its resolution and light gathering power, rendering clear, large-field images for navigation and analysis. The resolution of the resulting image is close to the diffraction limit of the unmodified system. The authors have designed and nanofabricated zone plate lenses with a modified pattern that combines focusing power and wave front coding in a single optical element. The study clears the path to further applications of wave front coded zone plates in lab- and synchrotron-based microscopes and metrology tools. The authors have demonstrated wave-front coding in visible-light optical systems using Fresnel zone plate lenses in an offaxis configuration similar to SHARP. Simulations complementing the visible light experiments assess the performance of wave-front coded zone plates in the SHARP microscope.