A bullseye is a wafer map where failures or parameter values form a center spot, a ring or a set of concentric bands. Many fab steps are radially symmetric, such as spin coating, single-wafer etch and deposition, CMP and wafer heating, so a bullseye usually points to one of them: center-fast etch, a chuck temperature gradient or a center-to-edge CMP profile. Its counterpart is the edge ring, with failures packed into the outer few millimeters.
Bullseye on the Vt map, center die are 20 millivolts high. Smells like the anneal chuck again.
Also heard as
- donut
- center spot
Related terms
Wafer map
Picture of a wafer showing each die's test result or measurement by location, used to spot spatial failure patterns.
Bin
Test category that each die or unit is sorted into at probe or final test, with bin 1 usually meaning good.
Yield
Fraction of units that come out good at a step or across a whole process, the headline number for any production line.
Excursion
Sudden departure of yield, defect counts or a key parameter from its normal band, big enough to trigger holds and an investigation.