The bonding of metal electrodes and insulator hybrid interfaces is one of the key techniques in three-dimensional integration technology. Metal materials such as Cu or Al are easily directly bonded by surface activated bonding at room temperature, but insulator materials such as SiO2 or SiN are not. Using only Si ultrathin films, we propose a new bonding technique for SiO2/SiO2 bonding at room temperature. Two SiO2 surfaces, on which Si thin films were deposited, were contacted in vacuum. We confirmed that the thickness of the layer was about 7 nm by transmission electron microscopy observation and that the layer was non crystalline by electron energy loss spectroscopy analysis. No metal material was found in the bonding interface by energy-dispersive X-ray spectroscopy analysis. The surface energy was about 1 J/m2, and the bonding strength was more than 25 MPa. This bonding technique was successfully realized to enable SiO2/SiO2 bonding without a metal adhesion layer.
We have performed microscopy and electric measurements of the Ge/Ge interfaces bonded by surface-activated wafer bonding (SAB) technology. Similarly to the case of Si wafer bonding, two Ge wafers of 50 mm in diameter, both doped by Ga with a concentration of 2.2×1014 cm-3, were bonded by SAB at room temperature. The SAB process was performed in a high-vacuum chamber (10-4 Pa) at room temperature. The bonding was achieved by attaching and pressing the two wafers, the contact surfaces of which were activated by argon ion beam irradiation. The cross-sectional scanning electron microscopy (SEM) image of the Ge/Ge bonded sample apparently shows an interface that seems to be caused by crystallographic discontinuity. The measurement by transmission electron microscope (TEM) reveals an atomic-disordered layer structure of about 3 nm in thickness at the interface of the bonded Ge/Ge. The resistivity of bonded Ge/Ge samples across the interfaces was measured at 300 and 77 K. As compared with the result of similar measurements for non bonded bulk Ge samples, we find no significant difference in resistivity between the bulk Ge and bonded Ge/Ge samples.
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