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Deposited copper physical
properties
Physical properties of the plated copper
deposit are essential for the reliability of
the substrate. A few of the most important
physical properties are tensile strength,
elongation percentage, and internal stress.
These properties show the tolerance of the
deposit for thermal stress and warpage.
Standard testing equipment was used to
measure the tensile strength, elongation
Figure 4: Cross sections and measured data of fine lines and filled vias. percentage, and internal stress of the
deposit. The copper plated from the
and 7µm linewidth panels. This data is at the top, center, and bottom allowed process has a greater than 36,000psi tensile
shown in Table 2. across-panel uniformity to be assessed. strength and more than 18% elongation.
In addition to the uniformity control This data is shown in Table 3. Because of the extraordinarily thin
benefit, insoluble anodes are also easy dimensions required by modern packaging
to maintain and allow the plater to apply 2-in-1 simultaneous fine-line plating RDL, internal stress of the copper metal
higher plating current density. For larger and via filling is an important parameter. With high
linewidths, the process has even tighter For RDL applications containing both internal stress, the deposit may warp, and
control. For fine lines of 10µm width, vias and fine lines, it is a challenge for an warpage may get worse with time or with
the plated copper height variation is electrolyte to maintain great via filling temperature. The plated deposits from
below 0.5µm when plating at a current capability while also obtaining good the process at various current densities all
density of 1.5 ASD. At a higher current uniformity and coplanarity on fine lines. exhibit low stress—below 1.0Kg/mm .
2
density of 4 ASD, the thickness variation We found that by adjusting the electrolyte
was approximately 1µm. It is common VMS to have copper sulfate at 220g/ Deposited copper grain structure
knowledge that plating uniformity L and sulfuric acid at 50g/L, it enabled An X-ray diffraction (XRD) study was
can be improved when plating current the process to fill vias of 60 x 40µm with performed for the plated deposits at a
density is reduced. a dimple of less than 5µm (Table 4). current density of 1.5 ASD to identify the
The variation of plated thickness Under these conditions, the plated height crystal phase and different planes. The
across a panel-level substrate that is variation between fine lines of 15µm width diffraction pattern of the copper grains
410mm x 510mm in size was below to the larger pads was approximately obtained was the same as the standard
0.5µm. Measurements taken on the panel 1.0µm (Figure 4). reported for copper in the literature. In
addition to relative intensities of the crystal
orientations, the crystallographic density
and the lattice constant is also of interest
in determining whether there is a preferred
orientation. The data indicates that the
deposits from the bath have preferred [111]
planes as shown in Table 5.
Focused ion beam-scanning electron
microscopy (FIB-SEM) photos showed
that the plated copper deposit had
equiaxial grain structure, which does
not have much variation under different
plating current densities (Figure 5).
Table 5: Copper deposit XRD data.
Summary
Panel-level packaging presents
unique challenges for electrolytic
copper metallization systems. With
proper selection of VMS electrolyte
and equipment, advanced packaging
fabricators can achieve the required level
of coplanarity for embedded trace and 2-in-
1 plating for RDL build-up. Commercial
electrolyte systems for embedded trace
plating are available that can provide good
performance and, with adjustment, are
capable of 2-in-1 fine-line plating and via
Figure 5: Copper deposit FIB/SEM pictures under different plating current densities.

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