Top-down SEM imaging captures the wafer surface from directly above, providing plan-view measurements of CD, pattern shape, and defect inspection. Perspective: Electron beam perpendicular to wafer surface. Images show x-y dimensions but not depth/height. CD measurement: Measures linewidth and space width from edge-to-edge distance in top-down view. Standard approach for CD-SEM inline metrology. Edge detection: Secondary electron intensity peaks at feature edges due to topographic and material contrast. Algorithm extracts edge positions from intensity profiles. Pattern verification: Confirms lithography and etch patterns match design intent. Detects pattern defects (bridging, missing features, CD excursions). LER/LWR measurement: Line Edge Roughness and Line Width Roughness measured from top-down SEM images. Statistical analysis of edge position variation along line. Tilted imaging: Some CD-SEMs can tilt beam or stage slightly (e.g., 5-10 degrees) to gain limited 3D information about sidewall profile. Resolution: Modern CD-SEMs resolve features <10nm. Beam size ~3-5nm. Limitations: Cannot measure feature height, sidewall angle, or undercut directly. Cross-section or scatterometry needed for 3D profile. Defect review: Top-down SEM used for defect review after optical inspection identifies defect coordinates. Sampling: Top-down SEM typically measures subset of features for statistical process monitoring rather than 100% inspection.