Phase-stepping interferometry for parallelism measurement of step gauge faces

The article presents a measurement method driven with a non-contact optical system consisting of a mirror array by employing phase stepping interferometry to determine the surface parallelism of step gauge faces. The array houses a prism mirror to allow simultaneous interferometric measurement of two opposite step gauge faces relative to the front surface. Mechanical movement of the mirror array is detected using an alignment monitoring mirror to compensate the tilt angle of the system. The front surface of the step gauge and the alignment mirror, located at the measurement arm of the interferometer, are adjusted normal to the collimated laser beam of 50 mm diameter. Phase stepping is performed by a piezo controlled mirror at the reference arm. A theoretical approach is described to verify the experimentally obtained sensitivities of the system due to yaw and pitch misalignments of the mirror array and the step gauge. The technique offers improved measurement accuracy to measure the parallelism of the step gauge faces. Main uncertainty components are caused by the plane fitting analysis of non-ideal surfaces and repeatability of the results. The combined standard uncertainty of parallelism measurement is 18 µrad.