1 April 1975 New Concepts in Projection Mask Aligners
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Optical Engineering, 14(2), 142130 (1975). doi:10.1117/12.7978742
A new concept (the annular field optical system) makes it possible to avoid the extreme complexity of conventional projection mask alignment objectives. This has been done in a new, extremely simple, all-reflecting optical system that consists of two, concentric, spherical mirrors. A monolithic block consisting of three mutually perpendicular plane mirrors is used to fold the system into a configuration that makes possible an inherently synchronous, simple scanning arrangement. Partially coherent illumination is used to increase the effective MTF without decreasing the depth of focus.
A. Offner, "New Concepts in Projection Mask Aligners," Optical Engineering 14(2), 142130 (1 April 1975). https://doi.org/10.1117/12.7978742

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