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9

Chip Scale Review September • October • 2019

[ChipScaleReview.com]

Results on electrical test vehicle

A silicon test vehicle was designed

to mimic bare dies. Two sizes of chips

were designed, 5x5mm² and 10x10mm²,

respectively. The test vehicle included

0.6µm-thick AlSi lines and passivation

layers of SiO

2

(0.5µm thick), and SiN

(0.6µm thick), respectively. Gold stud

to reduce the die thickness to ~40µm. An

additional 80µm-thick SINR layer was

laminated under vacuum to encapsulate

dies and the polymer was opened locally

to reach metal lines and allow external

connection. Finally, f lexible labels

were diced by laser and taken from the

wafer carrier.

bumps were formed on pads using standard

ball-bumping equipment. The stud bumps

were approximately 70µm in diameter and

30µm in height (

Figure 3

).

The wafer included 24 30x25mm² labels

and each one could receive one large and

one small die (

Figure 4

). The test vehicle

was designed to test the resistance of a

single contact between the die and the

flexible substrate thanks to a four-point

Kelvin pattern. In addition, the continuity

of daisy-chain structures, located at the

periphery and at the center of the dies, could

be measured (

Figure 5

). These patterns

include from 16 to 38 contacts according to

the size of dies and position.

Three wafers were fully populated and

electrically characterized. Wafers 1 and 2

included a bottom polymer layer 80µm thick.

Wafer 3 included a bottom polymer layer

30µm thick. For comparison, a fourth wafer

without bottom polymer was populated

only with small dies. Electrical tests were

performed during the manufacturing process

after the main steps, flip-chip bonding,

backside thinning and final encapsulation

(

Figure 6

). More than 90% of the Kelvin

structure was functional. Global average

values of Kelvin patterns are presented in

Figure 7

and details for each location are

shown in

Table 1

.

Figure 5:

Layout overview of a four-point Kelvin pattern and daisy chains in silicon dies.

Figure 4:

200mm test vehicle wafer with 24 labels including large and small dies.

Table 1:

Average resistance (in mOhm) of Kelvin patterns.

Figure 6:

Label including silicon dies flip-chip

bonded and thinned down to 40µm.