Low coherence interferometry for mid-infrared precision optics manufacturing

We report on an optical, non-contact, thickness measurement system for materials that are opaque in the Near infrared (NIR) through ultraviolet (UV) wavelength range. While measurement options do exist, they must either physically touch the sample or rely on an assumed bulk distribution of material. In addition, optical and semiconductor materials are often highly sensitive to contamination and greatly benefit from noncontact metrology. The authors have adapted techniques used for the Lumetrics Optigauge 2000 low coherence interferometry (LCI) system to build a 2.8 μm LCI system using non-silica optical fibers to direct the probe signal. A (100) Silicon (Si) test sample was selected as a control because it can be measured by both an Optigauge II and the MIR-LCI system. In this work, the authors successfully measured materials that are transparent in the midinfrared (MIR) range, such as Germanium. The authors speculate that MIR-LCI will enable wedge, thickness, flatness, and other measurements performed using an Optigauge II system for MIR transparent materials.