Light Point Defect

A light point defect (LPD) is any particle, pit, or scratch on a wafer's bare surface that scatters a scanning laser beam strongly enough for an unpatterned-wafer inspection tool to register a bright pixel against the mirror-smooth silicon background. Before a wafer ever sees a patterned layer, it gets a blanket optical scan, and that scan is the industry's primary gate on surface quality:

Defect size, not raw defect count, is what actually decides a wafer's fate. A Surfscan-class tool can catalog thousands of LPDs on a single 300mm wafer in under a minute, but only the ones at or above the node's critical defect size matter — a logic fab running a sub-20nm design rule treats a 45nm particle as a yield-killer, while the same particle would have been noise on a 130nm process a generation earlier, so the pass/fail bin threshold is reset every time the design rule shrinks.

The scatter signature tells an engineer what kind of defect they are looking at before anyone touches a wafer under SEM. A raised particle scatters light forward and symmetrically in all directions; a pit — including the crystal-originated particle voids that show up on this same incoming-inspection step — scatters asymmetrically back toward the source because light is reflecting off a depression instead of bouncing off a bump; a scratch produces an elongated line of scatter events running in one direction instead of an isolated point.

LPD maps are the gatekeeper between a wafer lot and the fab floor. Foundries set incoming-wafer acceptance specs directly in LPD terms — for example a maximum of a few tenths of a defect per square centimeter at a given size bin — and a lot that fails the scan gets reworked, downgraded to a less critical layer, or rejected outright before it ever reaches a stepper, because a defect invisible to the eye at this stage becomes a dead die weeks later after dozens of patterned layers have been built on top of it.

Real tool and material suppliers sit on both sides of this check. KLA Corporation's Surfscan SP-series dominates unpatterned-wafer surface inspection and is the de facto reference instrument fabs calibrate against; wafer suppliers like Shin-Etsu, SUMCO, and GlobalWafers run the same class of scan before shipment so their own LPD counts match what the foundry will measure on receipt, and logic fabs such as TSMC, Samsung Foundry, and GlobalFoundries write LPD density directly into their incoming-material specifications rather than leaving it to a visual check.

Read an LPD count through a *scatter-signature* lens rather than a raw-count lens — the number on the report only means something once you know whether it came from a particle sitting on top of the wafer, a pit sitting in it, or a scratch dragged across it, because each one points the fab toward a different upstream fix: a tool venting particles, a crystal-growth defect, or a handling-robot problem.