Chip Scale Review May • June • 2019[ChipScaleReview.com]
the detection of shorts, opens, and
distortions in redistribution lines.
• (PI)/(PR)/Metal residue: Metal grains
generate a random “grainy” texture
i n convent ional opt ical images
that complicates the detection of
low contrast organic defects. CF
images suppress this graininess.
This suppression, combined with the
bright signal generated by organic
mater ials in CF images, makes
organic residues on critical metal
surfaces easy to detect (
Solder bumps deposited on contact
pa d s p r ov ide t he l a s t l aye r i n t he
connection chain for flip-chip packages or
package-on-package (POP) applications.
The height a nd copla na r it y of t he
bumps are critical to ensuring reliable
electrical connections. High-speed laser
triangulation (LT) can measure height
and coplanarity for every bump on the
wafer, as follows:
• Adva nc e d bump i ng p r o c e s s e s
use a PI layer to add mechanical
strength to the solder connection.
This reduces the number of process
steps by eliminating the under-
bump metal but complicates the
measurement of bump height. The
PI layer is added after the bump is
formed. PI is semitransparent and
varies in thickness across the wafer, so
it interferes with conventional bump
height measurements. Combining
VTSS measurements of PI thickness
with LT measurements of bump height
yields the true height of the bump above
the surface on which it was deposited
• Bump residue: Similar to the problems
described for RDL, organic residues
on bumps can degrade connection
reliability, and the graininess of
metal bumps in conventional optical
images makes those residues hard to
detect. The bright signal of organics
a nd t h e s u p p r e s s i on of me t a l
graininess makes them easy to see in
CF images (
Copper pillar bumps
Copper pillar bumps consist of a
cylindrical copper pillar topped by a
solder bump. They allow manufacturers
to increase bump density (by decreasing
diameter and pitch) while preserving
enough height to underfill with insulating
adhesives. Copper pillar bumps have
additional inspection and metrology
requirements, such as top and bottom
critical dimension (CD). With the pitch and
density of pillar interconnects increasing to
address scaling and latency, a single wafer
may have more than 50 million bumps
with heights down to 5 microns or less.
The ability to measure CD and capture,
process, and analyze the associated data
are critical to process monitoring.
a) (left) Illustration of organic residue on metal contact; b) (middle) Conventional optical images
of clear (upper) and contaminated (lower) metal contacts; and c) (right) CF images of clear (upper) and
contaminated (lower) metal contacts.
(left) Laser triangulation measurements of bump top and between bump data points; and (right)
Bump height measurement through the PI layer.
(top to bottom) Bright field, dark field, and CF images of solder bumps with organic residue.