A unique challenge has emerged in the Channel Hole process module of advanced 3D NAND manufacturing: control of the lateral SiN recess post Channel Hole etch. Compared traditional optical critical dimension (OCD) technology based on ultraviolet, visible, and near-IR light, the IRCD system exploits unique optical properties of common semiconductor fab materials in the mid-IR to enable accurate measurements of high-aspect-ratio etches with high Z dimensional fidelity. Utilizing the mid-IR wavelength range, a robust and unique measurement methodology is demonstrated to measure the lateral SiN recess that occurs post channel hole etch due to etch bias between SiO2 and SiN.