A golden wafer is a reference wafer with precisely known and stable properties used to calibrate metrology tools, verify equipment performance, and ensure measurement consistency. Purpose: Provides a fixed reference point against which metrology tool performance is measured. Eliminates process variation from tool qualification. Calibration: Metrology tool measures golden wafer periodically. Results compared to certified reference values. Any drift indicates tool problem requiring recalibration. Properties: Certified thickness, CD, overlay marks, reflectivity, sheet resistance, or other relevant parameters. Values determined by reference lab measurements (NIST-traceable when possible). Stability: Golden wafers must have extremely stable properties over time. Stored in controlled conditions. Properties verified periodically. Types: Film thickness reference: Oxide or nitride of known thickness for ellipsometer/reflectometer calibration. CD reference: Precisely measured features for CD-SEM calibration. Overlay reference: Known offset patterns for overlay tool calibration. Sheet resistance: Known Rs value for four-point probe verification. Tool matching: Golden wafer measured on multiple tools ensures consistent measurements across the fab. Identifies tool-to-tool offsets. Lifetime: Golden wafers degrade over time from handling, contamination, and oxide growth. Must be replaced and re-certified periodically. Handling: Special handling protocols to minimize surface changes. Clean storage, limited measurements, careful transport. Cost: Certification and maintenance of golden wafer program is significant but essential investment for metrology quality.