Page 25 - ChipScale_Mar-Apr_2020-digital

Holistic approach to improve the reliability of

sub-5µm L/S Cu RDLs

By Ralf Schmidt, Jan Knaup, Ulrich Memmert, Jens Palm, Cornelia Jäger, Stefan Pieper, Uwe Kirbach, Thomas Beck [Atotech Group]
A d va n c e d p a c k a g i ng

t e c h n o l og ie s e n a b l e
f a s t er , t h i n n er , a n d
m o r e ef f ic ie n t m o bi le d e v ic e s .
In particular, fan-out wafer-level
packaging (FOWLP) allows for higher
performance, decreasing form factor,
and significant cost reduction [1].
Redistribution layers (RDLs) serve as
rerouting of the connections within
such packages (Figure 1). While
scaling of this technology appeared Figure 1: Schematic representation of a FOWLP. The circle highlights the areas of high stress impact upon the
to be straightforward in the past, thermal load.
next- generation FOWLP will require
new solutions. The need for higher
I/O counts requires miniaturization
a nd decreasi ng t h ick ness of t he
copper R DL traces. On the other
hand, artificial intelligence (AI),
high-performance computing, and
automotive applications will pose
challenges to the reliability and yield
of the packages. Decreasing copper
thickness and lines/spaces (L/S) result
in a strong decrease of the reliability
of t he R DL . FOW LP i nvolve s a
combination of different materials and
the respective mismatch in coefficients
of thermal expansion (CTEs) may
lead to stress impact between the
components upon thermal load. Figure 2: a) Stress-strain diagram of a copper deposit and schematic representation of the corresponding
In a typical FOWLP, die and mold elongation and necking during the tensile test experiment. b) Schematic Wöhler analysis for copper deposits
with high tensile strength and high ductility.
compounds make up the majority
of the device volume. Hence, these the deposit can be obtained. During the ductility of the material. The
components exert such large forces stretching of the sample, uniform maximum stress a material can sustain
upon the comparatively thin RDL elongation will take place initially. is referred to as its tensile strength.
stack that it must essentially follow In case the material is st retched
the dimensions of the former. From further, it will experience necking, Failure mechanism of copper RDL
the perspective of the RDLs, thermal which means that the elongation is traces
fluctuations impose a defined strain no longer uniform, but concentrated Thermal load may occur during
upon both copper and dielectric. in a small area. The localization of production or operation, both leading
Therefore, copper and dielect r ic the deformation may be caused by to repeated heating and cooling of
should exhibit excellent mechanical microscopic inhomogeneity of the the package. Such repeated thermal
proper ties in order to f ulf ill the material, which, in turn, may originate cycle s, t he refore, i nduce cycl ic
reliability requirements of the industry. from impurities within the deposit, mechanical loading and unloading,
Figure 2a illustrates a typical stress- voids, or grain boundaries. The sample i.e., cyclic mechanical strain. It has
strain curve of a high-quality copper becomes locally thinner and ultimately been known since at least the 19th
layer undergoing a tensile test from fracture will occur in this area. The century that cyclic mechanical loading
which the mechanical properties of total elongation at fracture is called can eventually lead to fracture of the

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