Objective  To fulfill the stringent high-resolution prerequisites of the imaging system deployed for semiconductor defect detection, a catadioptric objective lens with a large numerical aperture, specialized for near-ultraviolet to visible light wavelengths, has been meticulously devised in accordance with the distinct characteristics and design parameters of the system. The dispersive traits of optical glass materials within the near-ultraviolet to visible light spectrum are subjected to comprehensive analysis. In order to ameliorate the secondary spectrum of the optical system, a judicious selection of appropriate glass material is undertaken based on rigorous theoretical calculations of the second-order spectrum. The constructed objective lens, comprising an assemblage of 11 spherical lenses, exhibits a compact architectural arrangement. Specifically engineered for spectral coverage spanning 360-520 nm, the objective lens boasts a numerical aperture of 0.9, a focal length of 5.65 mm, a field of view measuring 0.8 mm, and an operative distance of 0.8 mm. This innovative design embodies an infinite conjugate catadioptric configuration. The results of this design endeavor reveal a favorable Modulation Transfer Function (MTF) for the objective lens, with wave aberrations across the entire field of view quantified at less than 0.09λ (λ=632.8 nm). Furthermore, diverse geometric aberrations are effectively rectified, aligning with the stringent apochromatic prerequisites. The resultant objective lens configuration not only conforms to these demanding optical standards but also underscores simplicity in its architectural layout and an extensive operational working distance. This facilitates convenience in practical production assembly and application.

  Methods  In the context of wide-spectrum large numerical aperture objectives, the mitigation of field curvature and secondary spectrum emerges as a focal point meriting dedicated consideration. Consequently, meticulous assessment of the field curvature and axial chromatic aberration within the system is conducted in accordance with foundational principles governing primary aberrations intrinsic to thin lens configurations. Subsequently, optimization of the objective lens ensues through a judicious amalgamation of suitable glass materials and structural arrangements, with a particular emphasis on addressing secondary spectrum intricacies. In the realm of catadioptric mirror assemblies, the formulation of the catadioptric mirror group is informed by the principles underlying the Mangin mirror.

  Results and Discussions   Following optimization procedures, the imaging fidelity of the objective lens has satisfactorily adhered to the stipulated image quality criteria. The graphical representation illustrated in Figure 2 showcases the near-optimal alignment of the system's full field of view with the theoretical limits imposed. Evident in Figure 3, the intricate challenge of axial chromatic aberration within the optical system has been effectively ameliorated, with the secondary spectral chromatic aberration quantified at approximately 0.18 μm—this accomplishment aligns harmoniously with the prescriptive image quality prerequisites governing the system. According to Figure 4 and Figure 5, field curvature, distortion, and astigmatism are well corrected, the distortion is only 0.073% at 520 nm, and the all-encompassing wave aberration well below 0.09λ, it is judicious to assert that the optical system demonstrates commendable imaging fidelity. The objective lens, embracing an infinity-conjugate configuration, accommodates a maximal angular field of view for emitted parallel light of approximately 3°. To cater to various magnification needs, a tube lens, appropriately matched in parameters, can be affixed posteriorly to the objective lens. For instance, coupling the objective lens with a standard tube lens possessing a focal length of 200 mm, as delineated by the formula (2), facilitates the realization of a magnification factor of 35×.

  Conclusions  In response to the high-resolution demands inherent to optical systems within the domain of semiconductor defect detection, this paper introduces a catadioptric objective lens featuring a numerical aperture of 0.9 and an operational wavelength range spanning 360 nm to 520 nm. With a remarkably streamlined configuration composed of just 11 lenses, the lens design effectively meets the stringent criteria for high-resolution output. By effectively addressing challenges related to field curvature and secondary spectral chromatic aberration, the lens achieves comprehensive correction. Wave aberration analysis demonstrates that the entire field of view remains well below 0.09λ. The integration of a catadioptric structure serves to condense the optical path while concurrently simplifying its intricacy. Notably, the designed objective lens can be seamlessly paired with commercially available standard tube lenses of varying focal lengths, thereby offering the flexibility to attain diverse magnification levels as required for inspection imaging purposes.