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10

Chip Scale Review January • February • 2018

[ChipScaleReview.com]

Wa f e r- l e ve l f a n - o u t p a c k a g i n g

represents a substantially lower price

point compared to TSV [4], but it is

still cost prohibitive for many products

especially for mult i- ch ip packages

that are typically greater than 20mm

in size. An increase in substrate size

has been one way the integrated circuit

(IC) industry has delivered significant

cost reductions historically [5]. Fan-

out technologies would enable this

trend to continue at the package level

even though wafer subst rates seem

to have reached a plateau at 300mm.

Panel substrates 510 x 515mm in size

represent a g reater than th ree -fold

increase in substrate area for less than

a 50% increase in cost. This multiplier

in number of die or packages per substrate for panel compared to

wafers increases as the size of the unit grows. This is particularly

important for multi-chip fan-out structures, which by their nature

are larger than single-die packages. This is plotted in

Figure 2

for

6mm die up to 32mm die.

The cost reduction is compelling for a transition from a 300mm

wafer to a 510 x 515mm panel, but meeting the technology

requirements now and in the future [6] for high-yielding panel-

level fan-out must be proven. Cu line formation is one of the more

demanding applications prohibiting the transition from wafer- to

panel-scale fan-out manufacturing. High density interconnect (HDI)

printed circuit board type panel-scale electroplating tools can support

a 10µm line/space (L/S) feature size, but 5µm L/S is now required.

Also gaining traction in the next few years, multi-chip fan-out

technical requirements will include a feature size reduction from

10 to 2µm L/S and cost reduction achieved through a substrate

transition from wafer to panel substrates. For this reason, the design

features commonly found on wafer-level ECD tools will need to

be ported to panel-scale equipment. Thin-seed plating hardware

(<5000Å) compatible with advanced chemistries will be critical to

deliver improved within-die performance.

Thin seed plating

There is a trend towards greater heterogeneity in advanced packaging

structures, which leads to pattern and density variations that present

challenges to uniform deposition across the panel substrate. Concurrently,

the market is moving towards smaller features, which require thinner

seeds. However, these thinner seeds can pose greater uniformity challenges

for deposition because there are higher resistances in thin seed plating

that lead to greater terminal effect. Two strategies can combat these panel

plating challenges: 1) multi-zone anodes, which provide the capability to

fine tune currents on specific areas of the panel, and 2) the use of dummy

die, which move higher deposition areas away from active die.

Terminal effect.

The terminal effect is a phenomenon that occurs due

to the resistance of the seed layer on the substrate. There are typically

localized higher current densities at the edge compared to the center.

These higher current densities lead to thicker deposition.

As linewidths shrink below 10µm, Cu seed layer thicknesses in ECD

must reach sub-micron levels. This transition causes what is known

as a terminal effect, where the thickness near the contact area has a

significantly lower resistance than the areas located farther from the

contacts resulting in increased ECD thickness. This phenomenon is

modeled in

Figure 3

.

Multi-zone anodes.

Multi-zone anodes provide the ability to adjust current

density to fine tune deposition to specific sections of the panel to accommodate a

variety of patterns.

The terminal effect in conjunction with changes linked to the dummy die near the edge

of the wafer can cause large shifts in the plated Cu thickness near the contact region.

Figure 3

shows the variation in near-edge film thickness for a thin-seed and thick-seed

condition. Because the productivity of panel-scale ECD tools for multi-chip fan-out is so

high, it is unlikely that there will be enough manufacturing demand for dedicated process

tools that can be statically configured to support a single seed thickness or dummy

structure. Therefore, there is a need to dynamically adjust the ECD cell to provide

uniform plating thickness independent of seed thickness or dummy structure.

One way to deliver this dynamic tuning capability is with multi-zone anodes.

Multi-zone anodes provide the ability to adjust the current density within a certain

area of the substrate independent of current crowding effects observed with shielding.

Multi-zone anodes also offer the flexibility to make panel-to-panel adjustments or

even achieve a desired plated thickness profile during the plating of a single panel. A

five-zone anode array may be arranged as shown in

Figure 4

.

Figure 2:

Productivity comparison of a 300mm wafer with a 510 x 515mm panel

for various die/package sizes.

Figure 3:

Current density profile across 250mm half-panels (i.e., from center of

panel to edge) plated at the same average current; 30µm seed and 0.2µm seed.