Two-step rapid thermal diffusion of boron into silicon using a boron nitride solid diffusion source

A two-step rapid thermal diffusion process of boron into silicon using a boron nitride solid diffusion source is described. During the first step, HBO² glass is transferred onto the silicon wafer from the diffusion source by keeping the temperature of the silicon wafer at 750° C while the diffusion source is at about 900° C. Boron is, then, diffused into the silicon wafer from HBO² glass at 1000° C or 1100° C in N₂ during the second step. Extremely shallow junctions with junction depths of about 20 nanometers and sheet resistances of about 350 ohms/sq can be achieved with this method as well as relatively deep junctions with junction depths of about 175 nanometers and sheet re-sistances of about 55 ohms/sq. When the diffusion is performed at 1100° C, both the junction depth and electrically active boron concentration at the surface increase as the ambient gas is changed from N₂ to O₂ while the sheet resistance decreases. A boron rich layer which has high resistivity is not formed at the surface when the diffusion is performed at 1100° C in O₂ ambient. This work was supported by Ministry of Science and Technology, Korea