| This thesis presents novel methods of metal-based wafer bonding at back-end-of-the-line (BEOL) compatible conditions (≤450°C). For the first time to our knowledge, 200 mm diameter oxidized Si wafers are bonded with prime Si wafers using 10-300 nm thick Ti as bonding intermediate at 300-450°C. Nearly void-free bonding with strong mechanical integrity has been confirmed. Moreover, microcavity formation has been demonstrated by bonding of patterned wafers. Both Rutherford backscattering spectroscopy (RBS) and Auger electron spectroscopy (AES) show clear evidence of Si and Ti interdiffusion, whereas high-resolution transmission electron microscopy (HRTEM) reveals an approximately 8 nm thick amorphous layer at the bonding interface. Those results indicate that the strong adhesion at the Ti/Si bonding interface is attributed to a solid-state amorphization (SSA) assisted by interdiffusion.; A key effort is devoted to fundamental investigation of low-temperature transition metal(TM)/Si-based wafer bonding. With the extensive work on Ti/Si system, additional experiments are performed with six other TM/Si systems, namely Ni/Si, Co/Si, Pd/Si, Hf/Si, Au/Si and Ta/Si. The results indicate there are two principal requirements for TM/Si-based wafer bonding: (1) intimate contact (able to break through kinetic barriers), and (2) adequate chemical bonding. Three kinetic barriers addressed in this thesis are: (1) enclosed microvoids due to surface roughness, (2) gas molecules at the bonding interface, and (3) interfacial oxides. Presence of these barriers can prevent formation of intimate contact, consequently retarding or even blocking interfacial interactions for chemical bonding. The unique properties of Group IVA metals (e.g., Ti and Hf) to reduce native SiO2 on Si surfaces and their exceptionally large solid solubility for O2 and N2, help overcome those issues.; Once kinetic barriers are surmounted, the key for strong metal/Si-based wafer bonding is formation of chemical bonds, aided primarily by interdiffusion. According to their principal bonding mechanisms, the examined seven TM/Si-based wafer bonding can be divided into three groups: (1) silicidation bonding (Ni/Si, Co/Si and Pd/Si), (2) solid-state amorphization bonding (Ti/Si and Hf/Si), and (3) eutectic bonding (Au/Si). One of the major thesis contributions is the development and identification of a new type of metal-based wafer bonding, i.e. SSA bonding. |
