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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 (

Figure 4

).

Solder bumps

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

(

Figure 5

).

• 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 (

Figure 6

).

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.

Figure 4:

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.

Figure 5:

(left) Laser triangulation measurements of bump top and between bump data points; and (right)

Bump height measurement through the PI layer.

Figure 6:

(top to bottom) Bright field, dark field, and CF images of solder bumps with organic residue.