The fiber-optic frequency space low coherence interferometers (FSLCI) became a very important tool for monitoring of back-grinding and chemical wafer thinning processes in semiconductor industry.The FSLCI tool measures geometry of the wafer, by analyzing the optical signal fringes spaced in k space by the amount proportional to 1/(Lref - Lsig), where Lsig is the length optical path of the portion of the beam reflected from the sample, while Lref is the optical length of the reference path).The FSLCI tools suffers from the range limitation due to limited spectral resolution of spectrometers. This limitation can be relaxed by placing inside the optical path a free space Fabry Perot filter (FSFPF) having similar optical thickness as (Lref - Lsig) . By measuring resulting beats in the frequency space one can extend range of FSLCI without using higher resolution spectrometers [1].In this place I replaced FSFPF by a novel all fiber optic filter. The novel proprietary and patent pending filter transmission is almost perfectly sinusoidal function of the light frequency. New filter unlike FSFPF does not exhibit Fresnel losses, and is more mechanically robust. We also improved signal analysis considering also phase of the fringes. The new filter and new data processing algorithm result in improved repeatability of the system by one order of magnitude.

References: 1. Walecki, Wojtek J., et al. In Applied Optical Metrology II, vol. 10373, p. 103730N. International Society for Optics and Photonics, 2017.