Wafer inspection and measurement is the set of systems in a fab that check every layer of a wafer for defects too small to see, and measure what was built. Measurement in a fab is called metrology. Together, these systems let a fab find a problem within hours, before it spoils thousands of chips.
A wafer is a thin disc of silicon, usually 300 millimetres across, on which hundreds of chips are made at once. Each chip is built from dozens of layers, and every layer can go wrong. A particle of dust, a scratch or a bridge between two wires can make a chip useless. Many such defects are only a few tens of nanometres across. A nanometre is a billionth of a metre, so these defects are thousands of times thinner than a human hair.
Inspection systems scan the wafer with light or with a beam of electrons. They compare the image of each chip with the chips next to it, because every chip on a wafer should look the same. Any difference is marked as a possible defect, with its exact position. A second machine, a review tool, then takes a close-up image of each marked point, and software sorts the defects by type.
Metrology systems measure. They check the thickness of each film, the width of each line, and whether each layer sits exactly on top of the layer below it. This alignment is called overlay. On a leading chip, two layers must line up to within a few nanometres.
Consider a memory fab in South Korea. After a polishing step, the inspection system scans a few wafers from each lot of 25. On a Tuesday morning, it finds 140 defects on one wafer, where the normal count is about 15. Most of them lie in an arc near the edge. The software groups the defects and draws them on a map of the wafer. An arc of scratches points to the polishing machine. So the engineers stop that machine and find a worn polishing pad. Only two lots, 50 wafers, went through it after the pad wore out. Without inspection, the problem would have shown up weeks later, when the finished chips failed their tests. By then, hundreds more wafers would have passed through the worn machine.
This is anomaly detection: the software learns the normal defect count for each step, and flags any wafer that departs from it. Every defect caught early protects the yield, the share of chips that work. The data also shows which manufacturing machine caused a problem. The semiconductor foundry uses the measurements to keep every step within its limits.
Inspection systems are complex machines, but much of their value lies in software: image processing, sorting defects, and analysing millions of measurements. The leading maker writes much of that software at its engineering centre in Chennai. The machines are used in fabs in Taiwan, South Korea, the United States and China, and the fab at Dholera, described on the page on semiconductor fabs in India, will need them too.