9 Copper Micro and Nano Particles Mixture for 3D Interconnection …
255
9.5 Summary and Outlook
9.5.1 Summary
This chapter introduces a copper mixed paste consisting of copper micro-particles
and copper nano-particles for 3D interconnections. This mixed paste is proposed
to solve the low packing density issue of micro-particles paste and cracks in nanoparticles paste. The micro-particles are able to prevent the cracks in nano-particles
paste and the nano-particles can fill the interstice of the micro-particles paste. To
develop a mixed paste with maximum packing density, a model is first established
to determine the optimal ratio between micro—and nano—particles based on the
Monte Carlo Method. The simulation results show that the best ratio to be 6.3:1
(micro: nano) by weight.
The additives and solvents used during paste preparation have low boiling points
to lower the sinter temperature. The paste s surface morphologies are characterized
by optical microscope and SEM. In situ resistance measurement is used to determine
transition temperature and monitor the real-time resistance change. Sheet resistance
is used to test the electrical conductivity. SEM images and ImageJ software are
incorporated to observe the paste packing density. The main experimental results list
as follows:
• The in situ resistance measurement shows that the mixed paste has the lowest transition temperature and lowest resistance after sintering. No irreversible resistance
increase as nano-particles paste was observed;
• The mixed paste has lowest sheet resistance;
• SEM images show that the mixed paste has lowest porosity and the nano-particles
paste indeed has large amount of cracks after sintering. The 3:1 paste has improved
performance on both thermal and electrical among all the trial ratios;
• Die-to-wafer bonding test shows that mixed paste having the highest bond strength
of 0.7 MPa on average.
In conclusion, the experiment results could be a useful reference for future studies
on copper particles for 3D interconnection and bonding. The copper micro—and
nano—particles mixed paste have an obvious improvement compared to the microparticles and nano-particles pastes. Table 9.4 shows the specific comparison of
different properties.
This kind of mixed paste would also be beneficial for process scale up by significantly reducing the production cost. The mixed paste with the optimum ratio reported
here can reduce the cost by at least ~4 times on an estimate, comparing to the nanoparticles paste. Specifically, the mixed paste can be used for LED die attachment,
wafer stacking, alloy joints in power electronics devices and other low temperature
3D integration interconnection applications.
255
9.5 Summary and Outlook
9.5.1 Summary
This chapter introduces a copper mixed paste consisting of copper micro-particles
and copper nano-particles for 3D interconnections. This mixed paste is proposed
to solve the low packing density issue of micro-particles paste and cracks in nanoparticles paste. The micro-particles are able to prevent the cracks in nano-particles
paste and the nano-particles can fill the interstice of the micro-particles paste. To
develop a mixed paste with maximum packing density, a model is first established
to determine the optimal ratio between micro—and nano—particles based on the
Monte Carlo Method. The simulation results show that the best ratio to be 6.3:1
(micro: nano) by weight.
The additives and solvents used during paste preparation have low boiling points
to lower the sinter temperature. The paste s surface morphologies are characterized
by optical microscope and SEM. In situ resistance measurement is used to determine
transition temperature and monitor the real-time resistance change. Sheet resistance
is used to test the electrical conductivity. SEM images and ImageJ software are
incorporated to observe the paste packing density. The main experimental results list
as follows:
• The in situ resistance measurement shows that the mixed paste has the lowest transition temperature and lowest resistance after sintering. No irreversible resistance
increase as nano-particles paste was observed;
• The mixed paste has lowest sheet resistance;
• SEM images show that the mixed paste has lowest porosity and the nano-particles
paste indeed has large amount of cracks after sintering. The 3:1 paste has improved
performance on both thermal and electrical among all the trial ratios;
• Die-to-wafer bonding test shows that mixed paste having the highest bond strength
of 0.7 MPa on average.
In conclusion, the experiment results could be a useful reference for future studies
on copper particles for 3D interconnection and bonding. The copper micro—and
nano—particles mixed paste have an obvious improvement compared to the microparticles and nano-particles pastes. Table 9.4 shows the specific comparison of
different properties.
This kind of mixed paste would also be beneficial for process scale up by significantly reducing the production cost. The mixed paste with the optimum ratio reported
here can reduce the cost by at least ~4 times on an estimate, comparing to the nanoparticles paste. Specifically, the mixed paste can be used for LED die attachment,
wafer stacking, alloy joints in power electronics devices and other low temperature
3D integration interconnection applications.