40-$\mu{\rm m}$ Cu/Au Flip-Chip Joints Made by 200$^{\circ}{\rm C}$ Solid-State Bonding Process

Flip-chip interconnect joints of copper/gold (Cu/Au) with 40-
$\mu{\rm m}$
diameter and 100-
$\mu{\rm m}$
pitch were made between silicon (Si) chips and Cu substrates using solid-state bonding at 200
$^{\circ}{\rm C}$
with a static pressure of 250–400 psi (1.7–2.7 MPa). The array of 50
$\,\times\,$
50 Cu/Au columns was first created. In fabrication, photoresist with 50
$\,\times\,$
50 cavities of 40-
$\mu{\rm m}$
diameter and 45-
$\mu{\rm m}$
depth were produced on Si wafers, which were first coated with 30 nm chromium and 100 nm Au films. Cu of 25-
$\mu{\rm m}$
thickness was electroplated in the cavities, followed by 10
$\mu{\rm m}$
of Au. After stripping the photoresist, the array of 50
$\,\times\,$
50 Cu/Au columns was obtained on a chip region of the wafer. The 50
$\,\times\,$
50 Cu/Au columns on the chip were bonded to a Cu substrate by solid-state bonding. No molten phase was involved and no flux was used. No underfill was applied. The corresponding load for each column was only 0.22–0.35 g. Cross-section scanning electron microscopy images show that Cu/Au columns were well bonded to the Cu substrate. Despite the large mismatch in the coefficient of thermal expansion between Si and Cu, no joint breakage was observed. The pull test was performed and the fracture modes were evaluated. The fracture force and fracture strength obtained were 11.2–14.2 kg and 35–44 MPa (5000–6400 psi), respectively. The measured fracture force is four times larger than the criterion of the pull-off test in MIL-STD-883E.