THESIS
2020
xvii, 128 pages : color illustrations ; 30 cm
Abstract
Cu pillar micro-bump technology constitutes a new generation of interconnection
technology for high-density fine-pitch flip-chip die stacking and 3D IC integration,
which inherits the advantages from solder bump technology while overcoming the
drawbacks. However, due to the significant mismatch of coefficient of thermal
expansion (CTE) between a silicon chip and an organic substrate, the Cu pillar micro-bumps
may suffer from high thermomechanical stress during device packaging and
service, which may shorten lifespan and limit commercialization.
Cu pillar micro-bumps with polymer cores have been demonstrated to effectively
reduce thermomechanical stress and improve joint reliability. Unfortunately, current
fabrication techniques of polymer core use traditional semiconductor fa...[
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Cu pillar micro-bump technology constitutes a new generation of interconnection
technology for high-density fine-pitch flip-chip die stacking and 3D IC integration,
which inherits the advantages from solder bump technology while overcoming the
drawbacks. However, due to the significant mismatch of coefficient of thermal
expansion (CTE) between a silicon chip and an organic substrate, the Cu pillar micro-bumps
may suffer from high thermomechanical stress during device packaging and
service, which may shorten lifespan and limit commercialization.
Cu pillar micro-bumps with polymer cores have been demonstrated to effectively
reduce thermomechanical stress and improve joint reliability. Unfortunately, current
fabrication techniques of polymer core use traditional semiconductor fabrication
processes and suffer from issues related to the chip size, pillar height, substrate topology
and grass effect. Thus, it is important to investigate an improved method for polymer
core fabrication in order to broaden the application. Therefore, the aim of this thesis is
to 1) develop a new polymer core fabrication method to overcome the current
limitations, 2) establish a polymer core surface metallization method to form the
designed joint structure, 3) introduce bonding technology to form joints which are made
of Cu pillar micro-bumps with printed polymer cores, and 4) investigate the reliability
performance of the joints.
An additive manufacturing process by in-situ UV LED curing during aerosol
printing was developed for polymer core fabrication. To fulfill the dimension
requirement of the Cu pillar joint, a high-intensity, focused emission UV LED package
was designed, fabricated and optimized for in-situ UV LED curing. Micro-scale
cylindrical polymer cores with a diameter of 20μm and a height of 30μm were achieved
by synchronized UV curable acrylic resin printing and in-situ UV LED curing using an
aerosol jet printer. From the printing results, it can be concluded that the printed
polymer cores are uniform, and that the fabrication process is stable and reliable.
Cu pillar micro-bumps with printed polymer cores were successfully achieved by
surface metallization. Seed layer sputtering and electroplating on printed acrylic
polymer were found to be the key processes for polymer core surface metallization. The
diameter of the structure was 35μm while the height was 35μm. For comparison,
conventional Cu pillars with similar dimensions were prepared using the similar
processes. Shear test results showed that the shear strength of Cu pillar micro-bumps
with printed polymer cores was 20% higher than that of conventional Cu pillars, this is
because adhesion between UV-cured polymer and Al pads is superior to the adhesion
between the TiW and Cu.
The joints made of Cu pillar micro-bumps with printed polymer cores were
achieved by flip-chip bonding technology. Silicon chips with Cu pillar micro-bumps
with printed polymer cores were bonded to BT substrates to form the joints. Stress
analysis by finite element simulation demonstrated that the Cu pillar micro-bumps with
printed polymer cores exhibited reduced stresses in various test conditions, which
indicated a better reliability performance than conventional copper pillars.
The great improvement in reliability performance of the joints of Cu pillar micro-bumps
with printed polymer cores was reported. The reliability of joints made of Cu
pillar micro-bumps with printed polymer cores and joints made of conventional Cu
pillars was investigated under temperature cycling condition and drop condition.
Printed polymer cores increased the characteristic life by 32% in temperature cycling
test (0℃-100℃), while the drop test showed that printed polymer cores increased the
characteristic life by 4 times. It can be concluded that Cu pillar micro-bumps with
printed polymer cores reduce stress and improve joint reliability, based on the results
from mechanical and reliability tests.
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