Critical-Dimension Scanning Electron Microscope

CD-SEM and CD-AFM both measure a pattern's critical dimension directly, by imaging the actual feature rather than inferring its size from an optical model the way scatterometry does — CD-SEM with a scanning electron beam, CD-AFM with a physical probe tip tracing the feature's profile.

Patterned feature needs direct CD measurement
Not an optical-model fit, an actual image or trace of the feature
Direct CD measurement method
CD-SEM
Electron beam images top-down feature width, fast, widely deployed
CD-AFM
Probe tip traces full 3D sidewall profile, including undercut
Feature CD and/or full profile measured
Direct, model-independent dimensional data

CD-AFM reaches a sidewall profile CD-SEM fundamentally cannot, because an electron beam looking down from above simply can't see a feature's hidden undercut. CD-SEM images from essentially a top-down view, so it measures top-surface width well but has limited ability to characterize a sidewall angle, footing, or undercut hidden beneath an overhang; CD-AFM's physical probe tip traces the actual 3D surface contour as it moves, capturing sidewall shape and undercut geometry that a top-down electron image can't resolve — at the cost of being far slower per measurement than CD-SEM's fast electron scan.

CD-SEM and CD-AFM — Direct Measurement, Different Blind Spots top-down speed vs. full 3D sidewall profile CD-SEM — top-down electron beam undercut beneath overhang — not visible from above fast top-down scan CD-AFM — physical probe trace probe traces undercut directly, slower per point CD-SEM handles routine, high-volume CD checks; CD-AFM is reserved for detailed sidewall characterization

That speed gap is exactly why CD-SEM runs as the routine, high-volume CD monitor while CD-AFM stays reserved for targeted characterization work. A fab needs to check critical dimension on a large sample of production wafers continuously, which only CD-SEM's throughput realistically supports; CD-AFM's much slower point-by-point probe trace makes it impractical for that volume, so it gets used selectively — qualifying a new process, investigating a specific sidewall-shape problem, or providing ground-truth validation for a faster, optically-modeled technique like scatterometry.

CD-SEM and CD-AFM both measure dimension directly rather than inferring it from a model, and the choice between them comes down to exactly the tradeoff CD metrology always runs into: how much of the feature's full 3D shape you actually need to see, against how fast you need to see it.

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