4 Microstructure and Mechanical Reliability Issues of TSV
83
Fig. 4.10 (a) Voids and cracks form at various locations in a TSV after annealing (red circles).
(b) Voids near the top of the TSV, and (c) grain boundary cracks near one of the side-walls [44]
When heated up relatively rapidly and kept at a constant elevated temperature for
a period, the protrusion observed is generally associated with plastic deformation of
the copper, with no relative displacement observed at the interface between Cu and Si,
as observed in Fig. 4.12. Generally, the extrusion increases with increasing temperature, and occurs during any pre or post-CMP annealing, as well as heating associated
with dielectric-deposition for fabricating the back end of line (BEOL) or redistribution layer (RDL). FEA shows that following heating to a high annealing/fabrication
temperature (425 °C) and cooling down, the Von Mises stress exceeds the yield
strength over the entire upper part of the TSV, resulting in yielding of the top part of
the via (Fig. 4.13a). As a result, inelastic (plastic and creep) strains accrue in the top
83
Fig. 4.10 (a) Voids and cracks form at various locations in a TSV after annealing (red circles).
(b) Voids near the top of the TSV, and (c) grain boundary cracks near one of the side-walls [44]
When heated up relatively rapidly and kept at a constant elevated temperature for
a period, the protrusion observed is generally associated with plastic deformation of
the copper, with no relative displacement observed at the interface between Cu and Si,
as observed in Fig. 4.12. Generally, the extrusion increases with increasing temperature, and occurs during any pre or post-CMP annealing, as well as heating associated
with dielectric-deposition for fabricating the back end of line (BEOL) or redistribution layer (RDL). FEA shows that following heating to a high annealing/fabrication
temperature (425 °C) and cooling down, the Von Mises stress exceeds the yield
strength over the entire upper part of the TSV, resulting in yielding of the top part of
the via (Fig. 4.13a). As a result, inelastic (plastic and creep) strains accrue in the top