Plasma-activated direct bonding of patterned silicon-on-insulator wafers to diamond-coated wafers under vacuum

• Unpatterned wafers make good plasma-activated direct bonding with a SiO2 layer.
• Diamond-coated wafers having UNCD, NCD and MCD with low residual stress are used.
• Patterned SOI wafer makes poor bonding to diamond-coated wafer.
• CMP of already patterned SOI wafer makes good bonding to diamond-coated wafer.
• Sequential plasma activation and lower annealing temperature improve the bond.

Direct wafer bonding requires the surfaces to have low surface roughness (Ra < 0.5 nm) as well as to be free of any particles or contaminants. Meeting these requirements for wafers patterned with lithography and dry etching presents a serious problem in terms of removal of photoresist residue and etch-related particles, which would require expensive additional equipment to be removed. In this study, we propose the use of chemical mechanical polishing (CMP) to be performed after all lithography and dry etch process steps involving several masks are completed. To reduce the adverse effect of any remaining slurry that might reside in the etched structures, we also propose to reduce the maximum annealing temperature from 550 °C down to 300 °C. The effect of lower annealing temperature on bonding is compensated using a sequential plasma activation with 60 s of O2 followed by 90 s of N2 on contacting surfaces made of silicon dioxide to achieve successful wafer bonding. Initial plasma activation with O2 additionally serves as a final cleaning step whereas the following activation with N2 for an extended duration is to fully activate the surface for direct bonding. This proposed technique can motivate the use of direct wafer bonding for microfabrication of advanced MEMS devices.

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