Metrology and inspection are the two measurement disciplines that keep a semiconductor fab in control — they are how a foundry knows, wafer by wafer, whether hundreds of process steps are producing the right structures and whether anything has gone wrong. The two answer different questions. Metrology measures dimensions and material properties: is the feature the right size, is the film the right thickness, are the layers aligned? Inspection hunts for defects: is there a particle, a bridge, a missing pattern, a scratch? Together they generate the data that feeds statistical process control and the feedback loops that hold yield, and they are the core business of companies like KLA, alongside Applied Materials, Hitachi High-Tech, and ASML.\n\nMetrology measures — CD, film thickness, profile, and overlay — non-destructively and in-line. The central number is critical dimension (CD): the width of the smallest features, measured either by a CD-SEM (a scanning electron microscope tuned for linewidth) or by optical scatterometry / OCD, which fits the diffraction from a periodic grating to a physical model to extract CD, height, and sidewall angle at high throughput. Film thickness and optical properties come from ellipsometry and X-ray reflectometry; layer registration comes from overlay metrology on scribe-line targets. Because these tools run on production wafers between process steps, they must be fast and non-destructive — trading some absolute accuracy for the throughput needed to sample every lot without slowing the line.\n\nInspection finds defects, trading throughput against sensitivity. Inspection tools scan the wafer and flag anything that should not be there, usually by comparing supposedly identical dies (or repeating cells) and treating any difference as a candidate defect. Optical inspection is fast and covers whole wafers — brightfield for many defect types, darkfield for scattering particles — but its resolution is limited by the wavelength of light. Electron-beam inspection is far more sensitive, catching tiny or buried defects and even electrical faults through voltage contrast, but it is slow, so it is reserved for the hardest layers and for root-cause work. Flagged defects are then passed to a review SEM that images and classifies each one, separating true yield-killers from harmless nuisance defects.\n\n| | Metrology (measure) | Inspection (find defects) |\n|---|---|---|\n| Question | is it the right size / thickness? | is anything wrong? |\n| Measures | CD, thickness, profile, overlay | particles, bridges, opens, pattern defects |\n| Tools | CD-SEM, OCD, ellipsometry, XRR | brightfield/darkfield optical, e-beam |\n| Method | fit an indirect signal to a model | die-to-die comparison |\n| Trade | accuracy vs throughput | throughput vs sensitivity |\n| Feeds | SPC + APC (tune next run) | defect review, root cause, yield |\n\n``svg\n\n``\n\nBoth feed process control, closing the loop that protects yield. The measurements don't merely grade wafers; they drive control. Statistical process control (SPC) charts each parameter against control limits so that drift or an out-of-spec excursion triggers a hold before bad wafers pile up, and advanced process control (APC) feeds metrology results back to tune the next run's litho dose, etch time, or deposition. This is why sampling strategy matters: measure too little and defects escape, measure too much and throughput and cost suffer, so fabs carefully optimize where and how often to look. As features shrink, the metrology and inspection budgets tighten faster than resolution improves, which is why the field leans ever harder on e-beam, actinic (EUV-wavelength) tools, and machine-learning defect classification.\n\nRead metrology and inspection through a quant lens rather than a 'check the wafer' lens: they convert the physical wafer into two streams of numbers — a distribution of dimensions (CD, thickness, overlay) and a catalog of defects — and everything downstream is statistics on those streams. Metrology's game is an inverse problem: infer a structure's true profile from an indirect signal (electrons, diffracted light) fast enough to sample production. Inspection's game is a detection problem: maximize the probability of catching a real killer defect while holding false alarms and scan time down. Yield is ultimately governed by how tightly you hold the first distribution and how completely you enumerate the second — which is why a leading fab spends nearly as much on seeing the chip as on making it.