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P. Kumar et al.
Fig. 3.11 The sequence of events showing cross-section of a TSV after (a) metal filling [52] (b)
planarization of Si wafer on the surface having via mouth or opening (left and right picture shows
top and profile views, respectively) (redrawn from [55]), and (c) thinning of Si wafer from the
back-side [56]
the side with the TSV-mouth or opening (i.e., from the side where TSV was created
and filled). In addition, the Si wafer may also need to be thinned down to the desired
thickness. It is convenient to thin down the Si wafer from the backside, where TSV
processing was not performed, primarily to avoid polishing the ductile metal and
brittle Si at the same time. Figure 3.11 shows the process flow of these two processes
and their impact on the TSV assembly.
As mentioned above, Cu pumping, which manifests itself in protrusion of Cu relative to Si [54, 57, 58], may take place during high temperature processing of front-end
and back-end structures, and deposition of redistribution layers (RDL). Since FEOL
and BEOL processes, as well as soldering and die stacking steps typically require
heating the device to temperatures in the range of 250–450
◦ C, the as-deposited Cu
TSV structures are often annealed at temperatures moderately higher than that used
in subsequent fabrication steps, after filling process and before chemical-mechanical
planarization (CMP) [58]. Such pre-CMP annealing reduces the adverse effects of
Cu-pumping during subsequent service, and also improves the adhesion of the viametal to the via-walls, and stabilizes the microstructure of the metal filler [59].
Mechanical grinding and chemo-mechanical polishing (CMP) are subsequently
used to planarize the Si-die or wafer at the via-opening side, as well as to thin the
Si-wafer. Following planarization and thinning, the Si surfaces possess a smooth
finish (typically better than 1 µm), as well as small residual stresses in TSV. For
performing mechanical grinding and CMP, the Si wafer containing TSV assembly
can be temporarily bonded with a glass sheet or another thick wafer using epoxy, etc.
P. Kumar et al.
Fig. 3.11 The sequence of events showing cross-section of a TSV after (a) metal filling [52] (b)
planarization of Si wafer on the surface having via mouth or opening (left and right picture shows
top and profile views, respectively) (redrawn from [55]), and (c) thinning of Si wafer from the
back-side [56]
the side with the TSV-mouth or opening (i.e., from the side where TSV was created
and filled). In addition, the Si wafer may also need to be thinned down to the desired
thickness. It is convenient to thin down the Si wafer from the backside, where TSV
processing was not performed, primarily to avoid polishing the ductile metal and
brittle Si at the same time. Figure 3.11 shows the process flow of these two processes
and their impact on the TSV assembly.
As mentioned above, Cu pumping, which manifests itself in protrusion of Cu relative to Si [54, 57, 58], may take place during high temperature processing of front-end
and back-end structures, and deposition of redistribution layers (RDL). Since FEOL
and BEOL processes, as well as soldering and die stacking steps typically require
heating the device to temperatures in the range of 250–450
◦ C, the as-deposited Cu
TSV structures are often annealed at temperatures moderately higher than that used
in subsequent fabrication steps, after filling process and before chemical-mechanical
planarization (CMP) [58]. Such pre-CMP annealing reduces the adverse effects of
Cu-pumping during subsequent service, and also improves the adhesion of the viametal to the via-walls, and stabilizes the microstructure of the metal filler [59].
Mechanical grinding and chemo-mechanical polishing (CMP) are subsequently
used to planarize the Si-die or wafer at the via-opening side, as well as to thin the
Si-wafer. Following planarization and thinning, the Si surfaces possess a smooth
finish (typically better than 1 µm), as well as small residual stresses in TSV. For
performing mechanical grinding and CMP, the Si wafer containing TSV assembly
can be temporarily bonded with a glass sheet or another thick wafer using epoxy, etc.