72
P. Kumar et al.
4.2 Microstructural Characterization and Stress
Measurement
4.2.1 Microstructural Characterization
The microstructure of TSV can be characterized at four levels, i.e., crystal structure,
phase structure, grain structure and defect structure. Bunge introduced a function
G(x) =
i(x), phase
g(x), orientation
D(x), defects, lattice strain
(4.1)
and mathematical methods to quantitatively describe microstructure [1, 2]. For experimental characterization, standard metallographic techniques can also be applied to
TSV structures. However, preparing TSV samples for microstructural characterization to investigate precise locations through the TSV sample structures using standard
metallographic polishing is tedious and unpredictable due to the small size of TSVs
and presence of soft materials (e.g., Cu, Ag paste, etc.) adjacent to hard and brittle
Si. Focused ion beam (FIB) milling, which allows precise machining at small length
scales, is widely used for preparing TSV samples for microstructural characterization. FIB can also be used to prepare ultra-thin samples, suitable for observation under
transmission electron microscope (TEM). Machining samples using FIB is both a
slow and an expensive process. Emerging Xe
+ plasma FIB systems, with optimized
control over milling parameters, promise faster removal rates [3]. Another method
for characterization of the microstructure of filler material is by via revealing: in this
process, Si is wet-etched using hydroxides, which do not affect metal fillers, thus
revealing the pillar [4, 5]. However, this is also a slow process and it does not allow
observation of the Si-filler interface. Therefore, sample preparation, especially over
the entire height of the TSV, for microstructural characterization is often very slow.
The important microstructural features of TSV assemblies that are often characterized and quantified are the size and shape of filler grains, micro-texture and grain
orientation, the presence of twin boundaries in Cu fillers, shapes and sizes of voids
or cavities in the filler, micro-cracks, interfacial structure, etc.
Grain size, which can be measured using scanning electron microscope (SEM) and
TEM, is important for the estimation of the strength of the metal filler (generally, the
smaller the grain size, the greater the strength of the material). The hardness and hence
strength of the TSV fillers can be measured using nano-indentation. Interestingly, as
shown in Fig. 4.1, nano-indentation studies have shown that TSV fillers with smaller
grain size may not always show an enhanced strength. Furthermore, as also shown in
Fig. 4.1, the strength versus grain size behavior often does not follow the Hall-Petch
relationship; in other words, the increase in the strength is not strictly proportional to
the inverse of the square root of the grain size [6, 7]. These deviations can be attributed
to the fact that TSV fillers contain relatively few grains across the via-diameter, as
P. Kumar et al.
4.2 Microstructural Characterization and Stress
Measurement
4.2.1 Microstructural Characterization
The microstructure of TSV can be characterized at four levels, i.e., crystal structure,
phase structure, grain structure and defect structure. Bunge introduced a function
G(x) =
i(x), phase
g(x), orientation
D(x), defects, lattice strain
(4.1)
and mathematical methods to quantitatively describe microstructure [1, 2]. For experimental characterization, standard metallographic techniques can also be applied to
TSV structures. However, preparing TSV samples for microstructural characterization to investigate precise locations through the TSV sample structures using standard
metallographic polishing is tedious and unpredictable due to the small size of TSVs
and presence of soft materials (e.g., Cu, Ag paste, etc.) adjacent to hard and brittle
Si. Focused ion beam (FIB) milling, which allows precise machining at small length
scales, is widely used for preparing TSV samples for microstructural characterization. FIB can also be used to prepare ultra-thin samples, suitable for observation under
transmission electron microscope (TEM). Machining samples using FIB is both a
slow and an expensive process. Emerging Xe
+ plasma FIB systems, with optimized
control over milling parameters, promise faster removal rates [3]. Another method
for characterization of the microstructure of filler material is by via revealing: in this
process, Si is wet-etched using hydroxides, which do not affect metal fillers, thus
revealing the pillar [4, 5]. However, this is also a slow process and it does not allow
observation of the Si-filler interface. Therefore, sample preparation, especially over
the entire height of the TSV, for microstructural characterization is often very slow.
The important microstructural features of TSV assemblies that are often characterized and quantified are the size and shape of filler grains, micro-texture and grain
orientation, the presence of twin boundaries in Cu fillers, shapes and sizes of voids
or cavities in the filler, micro-cracks, interfacial structure, etc.
Grain size, which can be measured using scanning electron microscope (SEM) and
TEM, is important for the estimation of the strength of the metal filler (generally, the
smaller the grain size, the greater the strength of the material). The hardness and hence
strength of the TSV fillers can be measured using nano-indentation. Interestingly, as
shown in Fig. 4.1, nano-indentation studies have shown that TSV fillers with smaller
grain size may not always show an enhanced strength. Furthermore, as also shown in
Fig. 4.1, the strength versus grain size behavior often does not follow the Hall-Petch
relationship; in other words, the increase in the strength is not strictly proportional to
the inverse of the square root of the grain size [6, 7]. These deviations can be attributed
to the fact that TSV fillers contain relatively few grains across the via-diameter, as