240
Y. Dai and C. S. Tan
9.3.1 Paste Formation and Sintering Profile
Sintering profile is critical to the paste physical property. Sintering temperature and
time have an effect on the bond strength. The shear strength can vary from 0.5 to
40 Mpa [35]. The environment during bonding also has an influence on the bonding
characteristics. Basically, when the pastes sinter in H 2 , the oxide on the film surface
can be removed [36]. The sintering temperature is then decided by the solvents used
during the synthesis process. Since it is complicated and costly to use H 2 in industry,
researchers are looking for solvents that can be removed in N 2 or vacuum. Yue et al.
found that using polyethylene glycol (PEG) as the removing oxidize solvent, paste
sintering in N 2 environment using a bonding pressure of 0.4 MPa at 350 °C has
shear bond strength of 40 MPa, which is higher than that sintered in vacuum [37]. lAscorbic acid (AA, vitamin C) is then reported to be helpful to the Cu paste property
sintered in N 2 atmosphere and lower temperature [38].
To find a better sintering environment, different ambient include vacuum, N 2 and
H 2 , has been attempted in our experiment. Pastes sintering in H 2 has a more uniform
surface than the other two kinds of environment from the SEM and microscope
images. However, to meet the reliability and cost requirements, N 2 is used as the
ambient and the solvents used to synthesize the paste was investigated to ensure it
could be evaporated in N 2 at a lower temperature.
The particles used in the experiments are: (1) commercial micro-particles
purchased from American Elements (1–2 μm in diameter, 99% purity); (2) Cu
nano-particles (40–80 nm in diameter, passivated with a thin layer of amines) are
CuantumFuse
TM solder materials provided by Lockheed Martin Advanced Technology Center [39]. Before the experiments, TEM (Transmission Electron Microscope) was used to identify the nano-particle size. Taking the average size of 69 nm
into the model, it is calculated that the best weight ratio between the micro- and
nano-particles is ~6:1.
The nano-particles were washed and centrifuged before adding the necessary
additives to form a paste. The micro-particles were dispersed in the same solvents
and additives used in formulating the nano-particles paste. These pastes were applied
onto glass substrates via doctor blade technique before sintering. The mixed paste
was first formulated with a weight ratio of 6:1 (wt%, micro-particles: nano-particles,
the ratio mentioned below are in this sequence consistently).
The three kinds of paste were first observed under wide-field microscope.
Figure 9.7 shows the images of the micro-, nano- and mixed paste respectively.
The micro-particle paste surface has more organic while the nano-particles paste has
obvious cracks, which are likely due to the nano-particle agglomeration phenomenon
mentioned in the introduction. This image proves the initial assessment and potential issues to be addressed. The microscope image of mixed paste seems to have no
cracks, with a uniform and clear surface.
Pastes with three different kinds of ratios were also made for comparison: 1:1,
6:1 and 15: 1 (micro: nano). The ratio gap is significant to ensure that any differences could be readily observed. These mixture paste were prepared using the same
Y. Dai and C. S. Tan
9.3.1 Paste Formation and Sintering Profile
Sintering profile is critical to the paste physical property. Sintering temperature and
time have an effect on the bond strength. The shear strength can vary from 0.5 to
40 Mpa [35]. The environment during bonding also has an influence on the bonding
characteristics. Basically, when the pastes sinter in H 2 , the oxide on the film surface
can be removed [36]. The sintering temperature is then decided by the solvents used
during the synthesis process. Since it is complicated and costly to use H 2 in industry,
researchers are looking for solvents that can be removed in N 2 or vacuum. Yue et al.
found that using polyethylene glycol (PEG) as the removing oxidize solvent, paste
sintering in N 2 environment using a bonding pressure of 0.4 MPa at 350 °C has
shear bond strength of 40 MPa, which is higher than that sintered in vacuum [37]. lAscorbic acid (AA, vitamin C) is then reported to be helpful to the Cu paste property
sintered in N 2 atmosphere and lower temperature [38].
To find a better sintering environment, different ambient include vacuum, N 2 and
H 2 , has been attempted in our experiment. Pastes sintering in H 2 has a more uniform
surface than the other two kinds of environment from the SEM and microscope
images. However, to meet the reliability and cost requirements, N 2 is used as the
ambient and the solvents used to synthesize the paste was investigated to ensure it
could be evaporated in N 2 at a lower temperature.
The particles used in the experiments are: (1) commercial micro-particles
purchased from American Elements (1–2 μm in diameter, 99% purity); (2) Cu
nano-particles (40–80 nm in diameter, passivated with a thin layer of amines) are
CuantumFuse
TM solder materials provided by Lockheed Martin Advanced Technology Center [39]. Before the experiments, TEM (Transmission Electron Microscope) was used to identify the nano-particle size. Taking the average size of 69 nm
into the model, it is calculated that the best weight ratio between the micro- and
nano-particles is ~6:1.
The nano-particles were washed and centrifuged before adding the necessary
additives to form a paste. The micro-particles were dispersed in the same solvents
and additives used in formulating the nano-particles paste. These pastes were applied
onto glass substrates via doctor blade technique before sintering. The mixed paste
was first formulated with a weight ratio of 6:1 (wt%, micro-particles: nano-particles,
the ratio mentioned below are in this sequence consistently).
The three kinds of paste were first observed under wide-field microscope.
Figure 9.7 shows the images of the micro-, nano- and mixed paste respectively.
The micro-particle paste surface has more organic while the nano-particles paste has
obvious cracks, which are likely due to the nano-particle agglomeration phenomenon
mentioned in the introduction. This image proves the initial assessment and potential issues to be addressed. The microscope image of mixed paste seems to have no
cracks, with a uniform and clear surface.
Pastes with three different kinds of ratios were also made for comparison: 1:1,
6:1 and 15: 1 (micro: nano). The ratio gap is significant to ensure that any differences could be readily observed. These mixture paste were prepared using the same