Abstract
Plasma activated wafer bonding is a low-temperature process for joining
similar and dissimilar materials. We have measured oxide degradation
caused by a plasma activation process intended for bonding of silicon
wafers. The fixed oxide charge increased by 2.9E10 cm-2 and the
interface trap density saturated at 7.2E10 cm-2eV-1 after plasma
activation, independent of gate geometry. The increase in interface
trap density was reversed by a forming gas anneal. Bonding experiments
were performed with wafers that were subjected to forming gas anneal
between plasma activation and wafer mating. We measured a fracture
surface energy of ~ 0.3 Jm-2, compared to ~ 0.7 Jm-2 for samples that
had not been subjected to the anneal. Pull tests yielded a lower limit
for the mean bond strength in both set of samples in the 3-5 MPa range.
Contact angle measurements showed the plasma activated wafers to be
slightly less hydrophilic after forming gas anneal, but still much more
hydrophilic than before plasma activation. Our results indicate that Qf
and Dit may be reduced without eliminating the bonding capability of
the surface. With appropriate process optimisation, strong plasma
activated wafer bonding without oxide damage can be feasible.
similar and dissimilar materials. We have measured oxide degradation
caused by a plasma activation process intended for bonding of silicon
wafers. The fixed oxide charge increased by 2.9E10 cm-2 and the
interface trap density saturated at 7.2E10 cm-2eV-1 after plasma
activation, independent of gate geometry. The increase in interface
trap density was reversed by a forming gas anneal. Bonding experiments
were performed with wafers that were subjected to forming gas anneal
between plasma activation and wafer mating. We measured a fracture
surface energy of ~ 0.3 Jm-2, compared to ~ 0.7 Jm-2 for samples that
had not been subjected to the anneal. Pull tests yielded a lower limit
for the mean bond strength in both set of samples in the 3-5 MPa range.
Contact angle measurements showed the plasma activated wafers to be
slightly less hydrophilic after forming gas anneal, but still much more
hydrophilic than before plasma activation. Our results indicate that Qf
and Dit may be reduced without eliminating the bonding capability of
the surface. With appropriate process optimisation, strong plasma
activated wafer bonding without oxide damage can be feasible.
