  Low temperature covalent direct wafer-wafer bonding allows for the fusion of
multiple semiconductor wafers without any additional material at the bonding
interface. In the context of particle pixel detectors this might provide an
alternative to bump-bonding for joining sensors to readout chips. Previous
investigations have shown that the amorphous layer formed at the interface
during bonding is detrimental to charge propagation. To investigate the
influence of the bonding interface on signal collection we have fabricated
custom test structures by bonding high-resistivity N to high-resistivity P-type
silicon wafers thus forming P-N junctions. Scanning transmission electron
microscopy shows indeed the formation of ca. 3nm wide amorphous layer at the
interface. Using a scanning transient current technique (TCT) setup we were
able to record generated signals. Illuminating our sample with light of
different wavelengths and from different sides, indicates that the P side of
the bonded structures can be fully depleted, but not the N side. This indicates
a strongly asymmetric depletion behaviour which we attribute to the presence of
the bonding interface.
