Misalignment modes in high-performance optical systems

Misalignment modes are combinations of rigid-body perturtbations to the opt ical elements that make up an optical system. Comparison of misalignment mo des associated with arbitrary metrology data and optical-system performance can be used to increase the sensitivity of those metrology measurements to specific optical-system performance specifications such as centroid distor tion. Misalignment modes are compared using a mean squared error (MSE) metr ic. The MSE value can serve as a figure of merit by which metrology measure ment parameters are optimized. Selection of measurable misalignment modes i n the case of metrology and interesting misalignment modes in the case of o ptical-system performance is based on a determination of whether a mode can fit into a projection camera, given actuator-stroke and mirror-tilt bounds . As an example of this type of analysis, we find that in the case of an ex treme ultraviolet lithography (EUVL) projection camera, exit-pupil wavefron t measurements can be made more sensitive to centroid distortion if these m easurements are collected at optimized field positions both inside and outs ide the ring field of view of such a camera. (C) 2000 Society of Photo-Opti cal instrumentation Engineers. [S0091-3286(00)01007-2].