Low-temperature bonding of Si wafers has been studied utilizing reactive ion etching-mode plasma activation. The hydrophilic Si and thermally oxidized Si wafers were exposed to N 2 , Ar, or O 2 plasma prior to bonding in air or vacuum. After plasma treatment the wafers were cleaned in RCA-1 solution and/or deionized water. Strong bonding was achieved at 200°C with all the investigated plasma gases, if proper bonding and cleaning procedures were used. Extended RCA-1 cleaning deteriorated the bond strength, but a short cleaning improved bonding. We found that the activation of the thermal oxide has a larger influence on the bond strength than the activation of the native oxide surface in Si/oxide wafer pairs. We suggest that the plasma treatment induces a highly disordered surface structure, which enhances the diffusion of the water from the bonded interface. As a result of the plasma exposure the number of the surface OH groups is greatly increased enabling strong bonding at a low temperature.

Hermetic packaging is often an essential requirement to enable proper functionality throughout the device's lifetime and ensure the optimal performance of a micro electronic mechanical system (MEMS) device. Solid-liquid interdiffusion (SLID) bonding is a novel and attractive way to encapsulate MEMS devices at a wafer level. SLID bonding utilizes a low-melting-point metal to reduce the bonding process temperature; and metallic seal rings take out less of the valuable surface area and have a lower gas permeability compared to polymer or glassbased sealing materials. In addition, ductile metals can adopt mechanical and thermo-mechanical stresses during their service lifetime, which improves their reliability. In this study, the principles of Au-Sn and Cu-Sn SLID bonding are presented, which are meant to be used for wafer-level hermetic sealing of MEMS resonators. Seal rings in 15.24 cm silicon wafers were bonded at a width of 60 lm, electroplated, and used with Au-Sn and Cu-Sn layer structures. The wafer bonding temperature varied between 300°C and 350°C, and the bonding force was 3.5 kN under the ambient pressure, that is, it was less than 0.1 Pa. A shear test was used to compare the mechanical properties of the interconnections between both material systems. In addition, important factors pertaining to bond ring design are discussed according to their effects on the failure mechanisms. The results show that the design of metal structures can significantly affect the reliability of bond rings.

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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 temperature ͑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.

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