Artificial star test by real-time video holography for the adjustment of high-numerical-aperture micro-objectives

The star test is a useful tool for fast visual inspection of the aberrations of micro-objectives during final adjustment. One of its most valuable properties is that it permits instantaneous observation of the effect of adjustments of lens groups, for instance, the shifting element during on-axis coma adjustment. Sometimes, however, it is difficult to perform the star test, e.g., in the ultraviolet region, which represents a field of growing interest driven by applications in semiconductor inspection and metrology. In addition, it is difficult to display the point-spread functions with video cameras because of the high dynamic range needed. We present a simple work-around with which to overcome these problems. If an interferometer is available for quantitative wave-front analysis on the production line, the point-spread function may quite easily be computed from an interferogram of the wave front in the back focal plane of a micro-objective. We describe the achievement and application of such a simulated star test in various spectral regions, together with some of its useful applications, including real-time wave-front manipulation.