Wafer visual inspection: higher resolution is not always better — 5–9 μm is the production sweet spot

A common myth in wafer inspection is that higher resolution is always better. In reality, doubling resolution quadruples the number of images for the same area, and scan time explodes. The real question is: how large are the defects your specification needs to catch?

Sample test: all four defect types detected

Using a 25 MP colour camera at about 2.5 μm with coaxial and high-angle ring lighting, we tested the customer's wafer samples:

• Edge chipping: detected by area, height and width; width about 0.5 mm
• Cracks: detected by area or length; up to about 1.86 mm (broken segments can still be linked)
• Contamination: detected by area
• Scratches: detected by area or width; width about 65 μm

All four could be imaged and detected. But 2.5 μm is far finer than the visual spec, making whole-wafer scans too slow.

Recommended configuration: 5–9 μm

• 5 μm: 25.6 × 25.6 mm field of view, detects about 25 μm defects, about 23.5 s with one light or 27.5 s with two
• 9 μm: 46 × 46 mm field of view, detects about 45 μm defects, about 14 s with one light or 15.5 s with two

In another semiconductor component case, 5 μm resolution detected defects of about 15 μm, and the roughly 2 mm depth of field of a 0.5× lens covered the component's 1 mm height difference. High-angle ring light is recommended for scratches on metal areas, and coaxial light for heavily textured areas, to avoid false calls.

More than a camera: a complete wafer inspection system

• Recipe management with automatic switching by work-order barcode
• Defect criteria set from your visual spec (area, width, length and more)
• NG distribution map, statistics and reports after each scan
• Cassette mapping with colour-coded results per wafer
• Optional star-pattern scanning for thickness and warpage