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a 5μm-thick film of the PS PBM was spin-coated onto a 100mm
Figure 3: Patterning capability of PS PBM: with a) a microscope; and b) cross-
section SEM inspections.
Figure 4: a) A patterned PS PBM wafer with electroplated Cu-Sn; b) a microscope
image of a Cu-Sn bump; and c) a schematic of the structure.
wafer. The wafers were contact-baked on hot plates at 60°C for
5 minutes and 120°C for an additional 10 minutes for soft bake.
Exposure was conducted by an i-line mask aligner at an exposure
energy of 100mJ/cm . The wafer was then developed using a
2
puddle develop process with cyclopentanone as the developer.
Figure 3 shows the fine-pitch patterning capability of the PS
PBM for a 4μm line/space feature with a 5μm thickness PS
PBM film (based on microscope inspection) and a steep sidewall
angle (~90°) based on a cross-section measured using a scanning
electron microscope (SEM) on a 10μm via pattern.
Cu-Sn electroplating. A silicon wafer with a patterned PS
PBM was fully cured at 200°C for 1 hour for the metallization
with electroplating. A Cu-Sn metal stack was selected for
bonding because of the low metal annealing temperature at
250°C. A critical condition for the experiment is the metal height
design and control because there is no surface planarization
applied before the wafer-level bonding. In this study, we used Cu-
Sn bumps with 2μm-thick Sn and 4μm-thick Cu electroplated on
the patterned wafer with the thickness of the PS PBM film being
5μm. The Cu-Sn electroplated PS PBM wafer, its microscope
image, and the schematic structure are shown in Figure 4.
PS PBM/Cu-Sn hybrid bonding. Finally, two PS PBM
patterned silicon wafers with Cu-Sn plated metals were bonded
together without CMP for surface planarization. The wafer-
level hybrid bonding was conducted at 250°C for 60min with a
bonding pressure of 20kN to form an interconnection between
the layers. The bonded wafer pair was further analyzed by
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