In-line metrology encompasses all measurements performed during wafer processing to monitor, control, and optimize the manufacturing process in real-time. Philosophy: Measure during manufacturing, not just at the end. Catch problems early before they propagate through subsequent process steps. Key measurements: CD (by CD-SEM, OCD), film thickness (ellipsometry, reflectometry), overlay (IBO, DBO), defect inspection, sheet resistance, particle counts. Sampling: Not every wafer measured at every step. Sampling plans balance process control needs with metrology throughput and cost. Feed-forward: Measurements from one step used to adjust subsequent steps. Example: measured CD after litho used to adjust etch recipe. Feedback: Measurements after processing used to adjust the same process on next lot. Example: post-etch CD fed back to litho dose. SPC integration: All inline measurements feed into SPC system. Control charts detect trends and excursions. Automation: Fully automated measurement recipes. Wafers loaded, measured, and returned to process without operator intervention. Metrology tool matching: Multiple metrology tools must give consistent results. Tool-to-tool matching regularly verified. Data volume: Modern fabs generate enormous metrology data. Big data analytics increasingly used for process optimization. APC integration: Inline metrology data drives APC systems for automatic recipe adjustment. Cost of metrology: Balance between measurement cost and value of information. Over-measurement wastes throughput, under-measurement risks yield loss.