The W-recess for 3D NAND replacement gate process currently has no in-line process control solution. W replacement renders the structure opaque in the ultraviolet/visible/near-infrared (UV/VIS/NIR) region beyond just a few tier layers. Additionally, increased wordline (WL) slit pitch scaling further reduces the already minimal optical signal from the top of the structure. Through finite-difference frequency-domain (FDFD) and optical critical dimension (OCD) simulations, we show that a specially designed, design rule-compliant (that is, possessing a slit pitch matching the device) ellipsometry target permits mid-IR light to completely penetrate through oxide metal (OM) pairs, enabling measurement of the W-recess Z-profile. Furthermore, recent experimental data measured on the designed target in >200 pair 3D NAND node prove that mid-IR light has sensitivity to the slit bottom.