The effect of plasma pretreatments (reactive ion etching in SF₆ and SF₆/O₂) on Si/Si wafer direct bonding was investigated. Etching in SF6 caused a bonding behaviour generally known from hydrophobic (HF etched) samples, whereas adding O₂ to the feed gas caused the Si(100) surfaces to become hydrophilic and spontaneous bonding was achieved. The structure of the bonded interfaces were analysed by high-resolution electron microscopy, ellipsometry, multiple internal reflection spectroscopy, and secondary ion mass spectroscopy. All the plasma treatments result in an interface structure analogous to that known from bonded hydrophobic wafer pairs. The interface does not involve an additional layer such as an SiO₂ or an amorphous Si layer but small one-dimensional defects forming a disturbed layer about 1 to 2 nm thick.

Keywords: plasma activation, Si(100)-surfaces, wafer direct bonding

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