4 Microstructure and Mechanical Reliability Issues of TSV
81
However, FE analysis of a stacked-die package shows that each die undergoes convex
curvature when the package is cooled from the stress-free temperature (150 °C),
which fundamentally alters the stresses and displacements relative to the free standing
die [41]. The convex curvature alters the radial interfacial stress to compressive,
reducing the proclivity towards interfacial fracture as well as dielectric cracking, and
shifting the critical failure location to the copper-pillar near the solder-microbump
interface in 3D packages [41]. In fact, the failure probability near the silicon–Cu
pillar-microbump junction (as indicated by the maximum equivalent strain), as well
as the required KOZ diameter, increase with increasing diameters of both the TSV
and the copper-pillar in a 3D package [43].
In addition to producing defects (e.g. voids) in TSVs, including complications in
electrical performance and potentially causing interfacial and or dielectric fracture,
induced stresses also give rise to a plasticity-related phenomenon commonly referred
to as copper-pumping, which can have serious reliability implications. Because of
the pervasiveness of this phenomenon, it is discussed separately in Sect. 4.3.1.3.
4.3.1.2 Microstructure and Stresses
Direct measurement of stresses in Cu-TSVs with X-ray micro-diffraction reveal a
significant tensile hydrostatic stress (∼ 234 MPa) at room temperature, which goes to
compression (−196 MPa) during annealing at 200 °C, and becomes a smaller tensile
stress when the sample is cooled to the ambient (167 MPa). The large initial tensile
stress is caused by grain boundary elimination during self-annealing and device
fabrication, and as noted previously, is undesirable from the reliability perspective,
since this causes large stresses in Si. A subsequent annealing treatment lowers the
tensile hydrostatic stress in Cu, even though the zone of initially larger grains expands
during annealing, as shown in Fig. 4.9 [29], possibly because of relaxation associated
with plasticity and creep at the high temperature.
EBSD studies have shown that during annealing, large grains remain stable when
there is a preponderance of 3 twin boundaries, but grains without twins and smaller
grains grow rapidly [44]. It has also been noted that the Cu-TSV has random texture
both before and after annealing. However, microvoids or small cracks have been
noted to form during annealing, thereby reducing stress, as shown in Fig. 4.10. This is
possibly because of vacancy diffusion to pre-existing defects under hydrostatic stress
gradients within the TSV during annealing, as noted in the discussion associated with
Fig. 4.5.
FEA based modeling work has also reported the linkage between the microstructure of copper grains and the stress in Cu-TSVs. The results from a linear elastic
mechanical model clearly demonstrate that the stress distribution is rather heterogeneous inside the TSV filler, as show in Fig. 4.11, considering the anisotropy of the
elastic compliance tensor of copper. Depending on the texture, morphology and distribution of the copper grains, stress concentrations may occur at the grain boundaries
[45]. Elastoplastic models have also been conducted directly on copper grain structures with the aim of explaining the formation of copper-pumping [46, 47]. However,
81
However, FE analysis of a stacked-die package shows that each die undergoes convex
curvature when the package is cooled from the stress-free temperature (150 °C),
which fundamentally alters the stresses and displacements relative to the free standing
die [41]. The convex curvature alters the radial interfacial stress to compressive,
reducing the proclivity towards interfacial fracture as well as dielectric cracking, and
shifting the critical failure location to the copper-pillar near the solder-microbump
interface in 3D packages [41]. In fact, the failure probability near the silicon–Cu
pillar-microbump junction (as indicated by the maximum equivalent strain), as well
as the required KOZ diameter, increase with increasing diameters of both the TSV
and the copper-pillar in a 3D package [43].
In addition to producing defects (e.g. voids) in TSVs, including complications in
electrical performance and potentially causing interfacial and or dielectric fracture,
induced stresses also give rise to a plasticity-related phenomenon commonly referred
to as copper-pumping, which can have serious reliability implications. Because of
the pervasiveness of this phenomenon, it is discussed separately in Sect. 4.3.1.3.
4.3.1.2 Microstructure and Stresses
Direct measurement of stresses in Cu-TSVs with X-ray micro-diffraction reveal a
significant tensile hydrostatic stress (∼ 234 MPa) at room temperature, which goes to
compression (−196 MPa) during annealing at 200 °C, and becomes a smaller tensile
stress when the sample is cooled to the ambient (167 MPa). The large initial tensile
stress is caused by grain boundary elimination during self-annealing and device
fabrication, and as noted previously, is undesirable from the reliability perspective,
since this causes large stresses in Si. A subsequent annealing treatment lowers the
tensile hydrostatic stress in Cu, even though the zone of initially larger grains expands
during annealing, as shown in Fig. 4.9 [29], possibly because of relaxation associated
with plasticity and creep at the high temperature.
EBSD studies have shown that during annealing, large grains remain stable when
there is a preponderance of 3 twin boundaries, but grains without twins and smaller
grains grow rapidly [44]. It has also been noted that the Cu-TSV has random texture
both before and after annealing. However, microvoids or small cracks have been
noted to form during annealing, thereby reducing stress, as shown in Fig. 4.10. This is
possibly because of vacancy diffusion to pre-existing defects under hydrostatic stress
gradients within the TSV during annealing, as noted in the discussion associated with
Fig. 4.5.
FEA based modeling work has also reported the linkage between the microstructure of copper grains and the stress in Cu-TSVs. The results from a linear elastic
mechanical model clearly demonstrate that the stress distribution is rather heterogeneous inside the TSV filler, as show in Fig. 4.11, considering the anisotropy of the
elastic compliance tensor of copper. Depending on the texture, morphology and distribution of the copper grains, stress concentrations may occur at the grain boundaries
[45]. Elastoplastic models have also been conducted directly on copper grain structures with the aim of explaining the formation of copper-pumping [46, 47]. However,