New tools for high-precision positioning of optical elements in high-NA microscope objectives

Joachim Heil; Tobias Bauer; Willi Mueller; Thomas Sure; Joachim Wesner

doi:10.1117/12.512835

26 February 2004 New tools for high-precision positioning of optical elements in high-NA microscope objectives

Joachim Heil, Tobias Bauer, Willi Mueller, Thomas Sure, Joachim Wesner

Proceedings Volume 5252, Optical Fabrication, Testing, and Metrology; (2004) https://doi.org/10.1117/12.512835
Event: Optical Systems Design, 2003, St. Etienne, France

Abstract

The precise positioning of the individual optical elements is essential for attaining diffraction limited performance in high-numerical-aperture (high-NA) microscope objectives. Tolerances are in the micron range or lower for high-end objectives, e.g. for broad-band scanning confocal applications, metrology objectives in general, and especially for deep ultraviolet (DUV) applications. The ever increasing demands on imaging performance ask for the continuous development and improvement of specialized measurement equipment for the production line. Our award-winning 150x/0.90-DUV-AT-infinity/0 objective for wafer inspection and metrology at 248nm employs air spacings in its doublets because of the instability of optical cements against DUV radiation. This comes however at the cost of a higher number of surfaces and even higher precision demands on their geometry, orientation and positioning. We present several tools enabling us to meet these requirements. A Fourier transform fringe analysis scheme is adapted to high-NA Fizeau interferometry for surface characterization. A white light Mirau interferometer for dimensional measurements on lens groups with sub-μm resolution enables us to keep surface distance errors lower than 2 μm. Residual aberrations of the objective are compensated for by translating special correction elements under observation of the wave-front using a DUV-Twyman-Green interferometer, which also incorporates a 903nm branch for the parfocal adjustment of the infrared (IR) autofocus feature of the objective. To adjust the shifting element for the elimination of on-axis coma, we compute an artificial (real-time) star test from the interferogram, allowing interactive manipulations of the element while monitoring their influence on the point spread function (PSF).

Citation Download Citation

Joachim Heil, Tobias Bauer, Willi Mueller, Thomas Sure, and Joachim Wesner "New tools for high-precision positioning of optical elements in high-NA microscope objectives", Proc. SPIE 5252, Optical Fabrication, Testing, and Metrology, (26 February 2004); https://doi.org/10.1117/12.512835

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