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UBM panel. A repeating rectangular
pattern matches the reticle layout
of t he l it hog r aphy process. T he
defects comprising the pattern were
determined to be undersized RDL
lines, which were ultimately traced
to a problem with the condenser lens
of the lithography tool that produced
thinner RDL lines on the die at the
lower left corner of the reticle. Figure
9a shows CF images of thinner (8µm)
R DL lines and Figure 9b shows
images of normal (14µm) RDL lines.
Five panels were in the lot and all of
them had the same thin RDL problems
caused by the lithography tool. All
Figure 7: a) (left) BF and b) (right) CF images of RDL and UBM pads captured with 4X objective (1.75µm/pixel) were inspected with both BF and
CF illu m i nat ion. T he BF
nuisance defect counts without finding inspections found none of
real process issues on the wafer. the thin RDL defects while
The CF image (Figure 7b) shows a the CF inspections fou nd
clear contrast difference between the the reticle pattern on all five
previous metal layer in the background panels.
and the overlying RDL and the UBM
structures. The texture and graininess Summary
of the metal are not visible, permitting CF illumination technology
high-sensitivit y i nspection with provides clear advantages
detection of single-pixel defects. in detecting defects that are
Figure 8 shows a composite map ch a r a c t e r i s t ic of FOPL P
from CF inspection of the RDL and processes on large panels. It
readily detects organic residues
t h a t a r e t r a n s p a r e nt a n d
essentially invisible under BF
and DF illumination. Its ability
to eliminate the metal graininess
and texture that obscure real
defects and generate false-
positive nuisance defects in
BF/DF inspections allows CF
technology to deliver high-
sensitivity single-pixel defect
detection.

Reference
1. G. Si ng h , K . Best , R .
M c Cl e a r y “A d v a n c e d
packaging lithography and
inspection solution for next-
generation FOWLP-FOPLP
Figure 8: CF panel map of defects on the RDL/UBM
sample reveals a repeating pattern that matches the processing,” IEEE Xplore,
lithography reticle layout. Figure 9: CF images of thin (top) and normal (bottom) RDL lines. Oct. 2016.

Biography
Woo Young Han is an applications engineering manager at Onto Innovation, Richardson, Texas. He joined
the company in 2000 and is currently managing the applications engineering group for the inspection business.
He holds an Electrical Engineering degree from U. of Toronto. Email wooyoung.han@ontoinnovation.com

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