CD-SEM (Critical Dimension Scanning Electron Microscope) is a specialized SEM optimized for automated, high-throughput measurement of feature linewidths on semiconductor wafers. Principle: Electron beam scans across feature edge. Secondary electron signal profile shows edges as bright peaks. Distance between edges = CD measurement. Resolution: Sub-nanometer measurement precision. Beam landing energy typically 300-800 eV to minimize charging and damage. Automation: Fully automated pattern recognition, navigation, and measurement on production wafers. Measures hundreds of sites per wafer. Recipe-driven: Measurement recipes define sites, features, and measurement algorithms. Run unattended in production. Measurement types: Line width, space width, line-edge roughness (LER), line-width roughness (LWR), hole/contact diameter. Top-down imaging: Views wafer from above. Measures in-plane dimensions. Cannot directly measure 3D profiles (height, sidewall angle). Accuracy vs precision: High precision (repeatability) for process monitoring. Absolute accuracy requires calibration to reference standards or TEM. Charging effects: Low beam energy and charge compensation (flood gun) needed for insulating surfaces. Applications: After-develop inspection (ADI), after-etch inspection (AEI), process monitoring, OPC verification. Vendors: Hitachi High-Tech, Applied Materials (formerly KLA), ASML. Throughput: 30-60 wafers per hour depending on measurement density.