Wafer to wafer direct bonding using surfaces activated by hydrogen plasma treatment

Abstract

This paper describes a method of direct wafer bonding using surfaces activated by RF hydrogen plasma for silicon on insulator (SOI) applications. The hydrogen plasma cleaning of silicon in the RIE mode is investigated as a pretreatment for Si direct bonding. Gas-phase cleaning using hydrogen plasma is considered an especially effective way of removing hydrocarbon and fluorocarbon contaminants from the Si surface, and the cleaned Si surface is successfully terminated by hydrogen. The hydrogen-terminated surfaces are rendered hydrophilic. The effect of plasma treatment on the surface chemistry is investigated by Auger electron spectrometry (AES). It is revealed that the carbon impurity concentration at the surface can be reduced below the 5 at.% level. Volatile hydrogen compounds are formed during the plasma procedure and pumped away, resulting in substrate surfaces free from carbon contamination. Atomic force microscopy (AFM) is used to estimate the surface roughness and morphology as a function of plasma exposure time and power. The surface becomes smoother with decreasing plasma exposure time and power. When the exposure time exceeds 10 min at constant plasma power of 100 W and the plasma power increases over 130 W at a constant exposure time of 3 min, roughness increases rapidly. The increase in roughness is caused by Si{111} platelet defects and the Si etching reaction occurs preferentially at positions where the Si{111} platelets intersect the surface. The initial interfacial energy measured by the crack propagation method is 506 mJ/m/sup 2/, which is 3 times stronger than that of a conventional RCA cleaning method.