9 Copper Micro and Nano Particles Mixture for 3D Interconnection …
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The reason for the lower transition temperature of micro-particles paste compared
to the nano-particles paste is that the micro-particles were washed to remove the oxide
prior to paste formulation. Since the micro—and nano—particle mixed well in the
mixture, the porosity is reduced and the mixture is able to fuse at lower temperature
due to the heat localization. Since the nano-particles fill in the interstitials formed by
micro-particles, the connection between particles is well in mixed paste. This is likely
the reason leading to a low resistance after sintering. Since the sudden resistance
increase is observed in the nano-particles paste, the cracks are suspected to happen
at the 5th minute during sintering. The solvents evaporation during sintering might
result in residual stress in the paste due to the different evaporation rates. The nanoparticles are unable to overcome the stress because of agglomeration so the cracks
occur. This is further investigated by the sheet resistance and SEM image analysis.
Sheet Resistance
Sheet resistance is a better way to measure the paste electrical conductivity as it is
independent of the paste thickness. Here, we tested 7 samples of each kind of paste
with Sheet Resistance Measurement System (CMT-SR2000N). Results are shown in
Fig. 9.14. The line in the box is resistance mean of all the samples. It shows that the
sheet resistance value of nano-particles paste is out of the normal range. It is as high
as 5 k/sq. The micro- and mixed paste test results are shown in the inset figure.
The average sheet resistance of the micro- and mixed paste are 3.1 and 0.25 /sq,
respectively.
The sheet resistance measurement illustrates that the mixed paste indeed has
improved electrical conductivity as compared to the micro- and nano-particle pastes.
The results are consistent with the in situ measurement. The reason for the high
Fig. 9.14 Sheet resistance of nano-, micro- and mixture paste (3:1)
245
The reason for the lower transition temperature of micro-particles paste compared
to the nano-particles paste is that the micro-particles were washed to remove the oxide
prior to paste formulation. Since the micro—and nano—particle mixed well in the
mixture, the porosity is reduced and the mixture is able to fuse at lower temperature
due to the heat localization. Since the nano-particles fill in the interstitials formed by
micro-particles, the connection between particles is well in mixed paste. This is likely
the reason leading to a low resistance after sintering. Since the sudden resistance
increase is observed in the nano-particles paste, the cracks are suspected to happen
at the 5th minute during sintering. The solvents evaporation during sintering might
result in residual stress in the paste due to the different evaporation rates. The nanoparticles are unable to overcome the stress because of agglomeration so the cracks
occur. This is further investigated by the sheet resistance and SEM image analysis.
Sheet Resistance
Sheet resistance is a better way to measure the paste electrical conductivity as it is
independent of the paste thickness. Here, we tested 7 samples of each kind of paste
with Sheet Resistance Measurement System (CMT-SR2000N). Results are shown in
Fig. 9.14. The line in the box is resistance mean of all the samples. It shows that the
sheet resistance value of nano-particles paste is out of the normal range. It is as high
as 5 k/sq. The micro- and mixed paste test results are shown in the inset figure.
The average sheet resistance of the micro- and mixed paste are 3.1 and 0.25 /sq,
respectively.
The sheet resistance measurement illustrates that the mixed paste indeed has
improved electrical conductivity as compared to the micro- and nano-particle pastes.
The results are consistent with the in situ measurement. The reason for the high
Fig. 9.14 Sheet resistance of nano-, micro- and mixture paste (3:1)