Sequential plasma activation process for microfluidics packaging at room temperature

A sequential plasma activation process consisting of oxygen reactive ion etching (RIE) plasma and nitrogen radical activation was applied for microfluidics packaging at room temperature. Si/glass and glass/glass wafers were activated by the oxygen RIE plasma and nitrogen microwave radicals one after another. The activated wafers by the two-step process were brought into contact in air followed by keeping them in air for 24 h. The wafers were bonded throughout the entire area and the bonding strength of the interface was as strong as silicon and glass bulk without any post-annealing process and wet chemical cleaning steps. Bonding strength considerably increased with the nitrogen radical treatment after oxygen RIE activation prior to bonding. Chemical reliability tests showed that the bonded interfaces were significantly worked with various chemicals. Si/glass and glass/glass cavities formed by the sequential plasma activation process indicated hermetic sealing behavior. High bonding strength was thought to be due to a diffusion of absorbing water into wafer surface and a reaction between silicon oxynitride layers on the mating wafers. An amorphous layer of 7 nm thick was found at the Si/Si interface. Activation with N₂ radical for 1200 s after O₂ RIE plasma treatment for 60 s generated a new phase in the Si wafers across the amorphous layer, probably due to the implantation effect of N2 radical in Si wafer. T-shape microfluidics channels were fabricated on glass wafers by bulk micromachining and bonded by using the sequential plasma activation process at room temperature.