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S. Papazoglou and I. Zergioti
532 nm continuous wave (CW) laser source in ambient conditions and the reported
resistivities were 2–3 times lower than that of the bulk copper, similar to the values
for non-oxidizing atmospheres and linewidths of <5 µm, whereas the process was
monitored through reflectivity measurements that facilitated the evaluation of the
sintered structures. Moreover, a comparative study on the use of CW, ns and ps
pulsed laser sources has been demonstrated in [126], using both 532 and 1064 nm
wavelengths for the laser sintering of Ag NPs inks on PEN and showed that the
optimum electrical properties were obtained with 1064 nm and ns pulses due to the
lower optical absorption of the NPs at this wavelength. The laser wavelength effect on
the sintering of silver NPs inks was also reported by Paeng et al. [128]. In this work,
Ag NPs inks spin-coated on soda lime glass substrates were irradiated using three
different wavelengths namely 405, 514.5, and 817 nm at different laser intensities
and scanning speeds. One important finding was that while the optical response of
the processed inks was mainly determined by the laser wavelength it was the beam
intensity and the scanning speed that controlled the induced temperature fields. The
lowest electrical resistivity in this work was reported for the 514.5 nm wavelength
and 2 mm/s scanning speed (100 mW laser power) at 5.28 µ cm, while it was
slightly higher for the 405 and 817 nm wavelengths, where the resistivities were 5.7
and 8.9 µ cm respectively.
9.6 LIFT Towards 3D Printing and Additive
Manufacturing
The evolution of the laser printing technology during the last decade has allowed
for the printing of more complex structures towards the development of electrical
interconnects and rapid prototyping processes. The ability that the LIFT technique
offers relies on the fact that it can print virtually any material on any substrate and
this can be also exploited in applications where different surface topologies are used.
In these frames, Kaur et al. [131], have demonstrated the flip-chip bonding of vertical
cavity surface-emitting lasers (VCSEL), using LIFT (Fig. 9.13).
More specifically, this work involved the deposition of indium micro-bumps onto
bond pads including a successive deposition of additional printed layers so as to
create a final structure of 1.5 µm in height and 20 µm in diameter. Performance
evaluation of the developed chips showed a variation in optical power of less than
0.3 dB after 350 h. of testing at 85 °C and 85% relative humidity, a minimal degradation that could be attributed to moisture penetration. In a different work, Wang
et al. [132], reported the laser 3-dimensional printing of silver nanopastes for the
formation of interconnects including voxels crossing 100 µm wide silicon channels,
multilayer scaffold structures and high aspect ratio micro pyramids and micro pillars.
Furthermore, Zenou et al. [133], also showed the 3D printing of micro-objects using
a variation of LIFT entitled Thermal Induced Nozzle-LIFT (TIN-LIFT) that employs
sub-nanosecond pulses to enable the transfer of thick metal structures through the
S. Papazoglou and I. Zergioti
532 nm continuous wave (CW) laser source in ambient conditions and the reported
resistivities were 2–3 times lower than that of the bulk copper, similar to the values
for non-oxidizing atmospheres and linewidths of <5 µm, whereas the process was
monitored through reflectivity measurements that facilitated the evaluation of the
sintered structures. Moreover, a comparative study on the use of CW, ns and ps
pulsed laser sources has been demonstrated in [126], using both 532 and 1064 nm
wavelengths for the laser sintering of Ag NPs inks on PEN and showed that the
optimum electrical properties were obtained with 1064 nm and ns pulses due to the
lower optical absorption of the NPs at this wavelength. The laser wavelength effect on
the sintering of silver NPs inks was also reported by Paeng et al. [128]. In this work,
Ag NPs inks spin-coated on soda lime glass substrates were irradiated using three
different wavelengths namely 405, 514.5, and 817 nm at different laser intensities
and scanning speeds. One important finding was that while the optical response of
the processed inks was mainly determined by the laser wavelength it was the beam
intensity and the scanning speed that controlled the induced temperature fields. The
lowest electrical resistivity in this work was reported for the 514.5 nm wavelength
and 2 mm/s scanning speed (100 mW laser power) at 5.28 µ cm, while it was
slightly higher for the 405 and 817 nm wavelengths, where the resistivities were 5.7
and 8.9 µ cm respectively.
9.6 LIFT Towards 3D Printing and Additive
Manufacturing
The evolution of the laser printing technology during the last decade has allowed
for the printing of more complex structures towards the development of electrical
interconnects and rapid prototyping processes. The ability that the LIFT technique
offers relies on the fact that it can print virtually any material on any substrate and
this can be also exploited in applications where different surface topologies are used.
In these frames, Kaur et al. [131], have demonstrated the flip-chip bonding of vertical
cavity surface-emitting lasers (VCSEL), using LIFT (Fig. 9.13).
More specifically, this work involved the deposition of indium micro-bumps onto
bond pads including a successive deposition of additional printed layers so as to
create a final structure of 1.5 µm in height and 20 µm in diameter. Performance
evaluation of the developed chips showed a variation in optical power of less than
0.3 dB after 350 h. of testing at 85 °C and 85% relative humidity, a minimal degradation that could be attributed to moisture penetration. In a different work, Wang
et al. [132], reported the laser 3-dimensional printing of silver nanopastes for the
formation of interconnects including voxels crossing 100 µm wide silicon channels,
multilayer scaffold structures and high aspect ratio micro pyramids and micro pillars.
Furthermore, Zenou et al. [133], also showed the 3D printing of micro-objects using
a variation of LIFT entitled Thermal Induced Nozzle-LIFT (TIN-LIFT) that employs
sub-nanosecond pulses to enable the transfer of thick metal structures through the