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ZEISS ForTune is a photo mask tuning system for the semiconductor manufacturing industry. The tuning process improves critical dimension uniformity and mask image placement errors via intra–volume material modification at different depths within the mask material. For these applications, diffraction–limited spots of an ultrashort pulse laser are to be generated throughout the volume of the mask. In the present paper, we present an optical design concept for generation of diffraction–limited laser foci in thick specimens with refractive index n2. The optical design includes a microscope objective of medium–sized numerical aperture NA. A medium with refractive index n1 ≠ n2 is in between the microscope objective and the specimen – this index mismatch is the root cause for appearance of spherical aberration when diffraction-limited laser foci in different depths are to be generated. Thus, in addition to a proper paraxial focus position also a certain amount of spherical aberration must be corrected for. The amount of spherical aberration increases with focus position variation within the specimen, numerical aperture NA and the refractive index mismatch Δn = n2 − n1 of the specimen. The design concept includes an adaptive optical element such as a deformable mirror or a spatial light modulator. Due to limitations in their achievable optical path difference, it is essential to combine the adaptive element with an appropriate conventional focusing mechanism. Optical designs will be shown allowing for surprisingly thick specimens having a thickness corresponding to several thousands of Rayleigh ranges dR. Besides photo mask tuning, the presented optical design concepts can be used for different applications such as laser scanning microscopes, 3D printers using two-photon polymerization or vitrography.
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