Surface roughness has a significant effect on adhesion. We used a singleasperity model to describe a smooth tip in contact with a rough surface and predicted that an optimal size of asperity will yield a minimum of adhesion. Experimentally, adhesive forces on silicon wafers with varying roughness were measured using AFM cantilevers with varying tip radii. It was found that minima do exist, and for all tip radii, the adhesion falls significantly for roughness greater than 1-2 nm and drops at higher roughness for larger tips. In addition to RMS roughness, the roughness exponent is another important parameter for the characterization of rough surfaces and its effect on adhesion was also investigated. We developed computer programs to simulate a set of fractal rough surfaces with differing roughness exponents. The adhesive forces between an AFM tip and the fractal surfaces were calculated and the adhesion was seen to decrease as the roughness exponent increases. This work should help minimize MEMS stiction (adhesion) and progress the understanding of nanoscale contact mechanics. Abstract I. Effect of RMS Roughness on AdhesionA single-asperity model was used to describe a smooth tip in contact with a rough surface and the total interaction force for all molecules interacting with the sample was obtained. Adhesive force measurementAFM cantilevers with varying tip radii (75.0 nm to 9.08 µm) were used to measure adhesive force on silicon wafers with varying roughness (0.2 nm to 39 nm). Stiction occurs when surface adhesion forces are higher than the mechanical restoring force of the microstructure and it is a notorious cause of malfunctioning in microelectromechanical systems (MEMS) due to their large surface area-tovolume ratio. To avoid in-use stiction, adhesion forces should be minimized. Although it is clear that roughness affects adhesion, a fundamental understanding of the role of nanoscale roughness in adhesion of the surfaces of modern microdevices is still lacking.

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Al to Al bonding was successfully demonstrated for hermetic sealing of MEMS devices and three-dimensional interconnects. On a MEMS device wafer, 2 µm thick Al (with 2% Cu) was patterned at the perimeters of the individual dies as well as the input/output bond pads. On a cap wafer, after forming polycrystalline-Si filled vias, the seal rings and bond pads were also patterned with the Al described above. The two wafers were then bonded at ~ 450 °C with various bond forces up to 80 kN. The leak detection on the capped device showed the superb hermeticity of ~10 -12 cm 3 atm/sec He leak rate with an Al seal width as narrow as 3 µm. And the electrical contact resistance of the Al to Al bonded interface measured less than 1 Ω.

Various categories of bonding technologies were investigated for MEMS encapsulation applications. The bonding processes presented in this paper include Al to Al, Si to Si, and metal (Al or Au) to Si. Among the above different bonding schemes, the Al to Al bonding gave the highest process yield and bond strength. In addition, actual MEMS accelerometers were successfully integrated with Al to Al bonding with high yield all the way through plastic over-mold packaging and assembly.