Wafer warpage is bowing or distortion of the wafer caused by stress or temperature effects, impacting lithography focus, handling, and process uniformity. Warpage types: (1) Bow—spherical curvature (concave or convex); (2) Warp—non-spherical deviation from flat; (3) SFQR/SBIR—site-level flatness metrics for lithography. Causes: (1) Film stress—compressive or tensile stress from deposited films; (2) CTE mismatch—different thermal expansion between film and substrate; (3) Dopant stress—ion implant damage or concentration; (4) Thermal history—non-uniform heating/cooling; (5) Back-side processing—asymmetric films on front vs. back. Measurement: (1) Bow/warp—capacitive or optical scanning across full wafer; (2) Site flatness—high-density mapping for lithography qualification; (3) Stress calculation—from bow change and Stoney equation (σ = Esds²/6Rf). Impact: (1) Lithography—defocus if warpage exceeds DOF (depth of focus); (2) Wafer handling—excessive bow causes chuck failures, robot drops; (3) Bonding—wafer-to-wafer bonding requires flat surfaces; (4) CMP—non-uniform removal on warped wafers. Specification: SEMI standards define bow (<50 μm typical) and warp (<60 μm) limits for incoming wafers. Mitigation: stress compensation films (deposit opposing stress on backside), process optimization (reduce film stress), anneal conditions tuning, backside film removal. Advanced node challenge: more film layers and higher aspect ratio structures increase cumulative stress and warpage risk.