Plasma-assisted direct bonding has been investigated for wafer scale encapsulation of microelectromechanical systems ͑MEMS͒. Direct bonding requires smooth and flat wafer surfaces, which is seldom the case after fabrication of MEMS devices. Therefore, we have used polished chemical vapor deposited oxide as an intermediate bonding layer. The oxide layer is polished prior to bonding the MEMS wafer to cap silicon wafer. The bonding is carried out with plasma-assisted direct wafer bonding at a low

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... emperature ͑Ͻ300°C͒. Two different methods to form electrical contacts to the encapsulated device are presented. In the first method trenches are etched on the surface of the cap wafer before the bonding. During the bonding the trenches are aligned to the contact pads of the device wafer. After bonding the cap wafer is thinned down with grinding until the path to the contact pads is opened. In the second method one or both of the wafers are thinned down to around 100 m after bonding. The electrical path to contact pads is formed using V-groove sawing, metal sputtering, and lithography. To test the viability of the developed methods for MEMS encapsulation, we have sealed polysilicon resonator structures at a wafer level.