Periodic Error Elimination Through New Displacement Measuring Interferometer Design

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The research objective of this study is to construct, test, and verify a new concept for displacement measuring interferometer design. The new heterodyne (two frequency) interferometer design will eliminate periodic error, which currently limits the accuracy of traditional configurations. Periodic error is the result of "polarization coding", where the two collinear optical frequencies are separated within the interferometer using perpendicular linear polarization states and polarization-dependent optics. Due to inherent imperfections in the optics and light polarizations, mixing occurs because the frequencies do not travel only along their intended paths. The error appears as a cyclical (or periodic) oscillation of the measured displacement about the true value during motion, typically with an amplitude of several nanometers. In the new design, the two frequencies are generated and spatially separated using an acousto-optic modulator. By removing the potential for overlap and frequency mixing within the interferometer, periodic error is removed. The primary education objective is development of a team-taught graduate seminar course that will include: 1) creative writing; 2) technical writing; 3) oral presentation; and 4) proposal writing.The new design concept will improve displacement measurement accuracy to the sub-nanometer level for various applications, including position feedback for lithographic stepper stages, machine tools, and coordinate measuring machines. In stepper stages for integrated circuit production, for example, improved positioning accuracy is an important component in the continuous push toward increased feature density and, subsequently, computing power. In addition to the research benefits, education impacts will include improved communication skills and creativity for graduate students through the seminar course.