New Concepts in Projection Mask Aligners

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.