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Optical Inspection

Method used to find defects on a wafer.
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Description

Optical inspection is split into two main segments, brightfield and darkfield. Darkfield inspection tools measure light reflected at a lower angle. Brightfield inspection, the workhorse technology in the fab, measures light at a higher angle. It uses broadband light to illuminate a wafer. Then, the light is collected and an image is digitized.

Brightfield measures light reflected between 25 to 30 degrees. Greyfield is between 30 to 40 degrees. And then the rest is darkfield.

Optical inspection has a physical limit, although this depends on the system. Brightfield has sensitivities down to <5nm in SRAM, while darkfield is <15nm, according to data from ASML and others. Typically, these systems have a throughput of 1 or more wafers an hour, although this varies as well.

These figures are a moving target as vendors continue to make improvements in optical. Optical is also moving into new directions, such as wafer inspection tools based on a combination of brightfield, greyfield and darkfield techniques.