Improved Low-Temperature Si ­ Si Hydrophilic Wafer Bonding

Low-temperature processing is important for bond and exfoliation and materials integration techniques. Of particular importance for hydrophilic wafer bonding is the reduction and removal of thermally generated voids at the bond interface, i.e., voids not caused by particulates. Possible causes of thermally generated voids are excess water and hydrocarbon contamination, both at the bond interface. Several bonding preparation techniques were explored, and the effects on interfacial void density and bond strength were recorded. After bonding, all wafer pairs were annealed to 250°C. Infrared imaging was used to monitor void formation, and bond strength was measured using the blade insertion method. Microvoids with lateral dimensions greater than 30 μm were imaged using acoustic microscopy. The highest bond strength was 1260 mJ/m 2 for plasma-cleaned wafers followed closely by 1150 mJ/m 2 for an HF oxide strip before hydrophilization. In addition to these techniques, bonding in a vacuum or the use of a prebond anneal were able to eliminate interfacial voids up to the anneal temperature.

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