Characterization of silicon wafer surfaces after hydrophilic and hydrophobic treatments by atomic force microscopy

The surface properties of silicon are the key parameters to the success of the silicon direct bonding technique in the semiconductor industry. The hydrophilicity and roughness of silicon surfaces processed with three typical surface treatments in the direct bonding technique are systematically evaluated by atomic force microscopy (AFM). Hydrofluoric (HF)-treated silicon surfaces used for hydrophobic bonding show negligible change in adhesion forces with humidity. But HF treatment results in a large roughness increase, suggesting additional processes are desired. RCA 1 treated and thermal oxidized silicon surfaces show strong increases in adhesion forces when the environmental humidity increases from 10% to around 60%. Further rise of humidity results in a drop of the adhesion force. This is believed to be due to a change in the structure of the absorbed water layer on the surfaces. As hydrophilic treated surfaces exhibit strong adhesion forces over a wide range of humidity, the direct bonding should be done at low humidity to reduce the water bubble forming at the bonding interface. Also, the hydrophilic treatments can decrease surface roughness significantly, which is favorable for the direct bonding.