We developed a ray tracing simulation tool for imaging systems including a Fresnel lens with a quasi-arbitrary sidewall structure. One issue with Fresnel lens is that noise in the image plane can appear from rays passing through or reflected at its sidewalls. One way to reduce it is to modify the orientation of the sidewalls so that rays will not reach the image plane. To find the best sidewall orientations, we developed a method where locally, a sidewall can freely be oriented. We could then derive the best modulation scheme for each Fresnel lens sidewall. In the case of a single imaging Fresnel lens, relative parasite noise intensity could mostly be prevented. To experimentally check our method, snapshot images were taken with single Fresnel lenses and a single spherical lens. No noticeable differences in image quality could be observed using a standard C-MOS camera. However, parasite noise could experimentally be detected with a Fresnel lens prototype when using a very high-dynamic range C-MOS camera.
In wafer steppers, the alignment of an exposure field to the reticle being imaged is known to affect the success or failure of that field's circuit(s). Because of this relationship between alignment accuracy and device yield, much emphasis is placed on obtaining and consistently maintaining an alignment accuracy within tight design rules.
For the purpose of measuring aspheric surfaces, an improved type of computer generated hologram, named the inclined bar type hologram, is proposed. This hologram is characterized by inclined bars. The inclination angle for each bar is obtained from the calculated wavefront aberration of the test lens. A few problems with these computer generated holograms are analyzed, and the advantage of the inclined bar type hologram over the conventional one is demonstrated by experiment.
In order to produce an accurate aspheric surface, it is necessary to know the exact shape of that surface in the process of polishing. For this purpose, we have developed an interferometer using computer generated holograms. Experiments were made on that apparatus with three kinds of computer generated holograms. They are 1) interferogram, 2) Lohmann type hologram and 3) inclined bar type hologram.
A software has been developed to draw interferogram, but generally computer time to draw it was larger than that to obtain a Lohmann type hologram. The deterioration of the interference pattern when using a Lohmann type hologram could be remedied by using a proposed “inclined bar” type hologram to some extent. Therefore, the inclined bar type hologram is more practical for the purpose of testing an aspheric surface.
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