Using Phase Shift Interferometry to Measure the Topography of Thin Metal Films

Abstract

A low-cost phase-shifting laser interferometer is being constructed at Houghton University to measure the thickness and topography of thin metal films produced with a variety of deposition parameters. The modified Twyman-Green Interferometer uses a 632 nm laser. Three piezoelectric ceramic stacks move a reference mirror up to 883 nm along the direction of the beam with a precision of <1 nm. The interference pattern is captured by a 2 MP camera. The system is suspended by springs and uses eddy current damping to decrease the movement of the system. A housing mounted to the outer frame blocks wind produced by the building’s air handling system. A LabVIEW program controls the mirror movement and fits a sine function to the intensity of each pixel vs. reference mirror position. An intensity plot of the fitted phase shifts represents the topography. Initial tests using two λ/10 flat mirrors indicate that the individual pixel height measurements are repeatable within about 1.6 nm. Assuming the film surface is smooth, the maximum repeatability of the overall topography should therefore be <1 nm.