Application of nitrogen implantation to ULSI

Nitrogen is not a commonly used ion species in Si ULSI. It cannot be used as an n-type dopant because of its low solubility in Si. However, it shows interesting properties such as the suppression of boron diffusion when applied to source/drain doping and the nitridation of gate oxide when applied to gate doping. In this report, first, the effects of nitrogen preimplantation to the formation of boron-doped shallow p⁺n junctions are described. The technique is successfully applied to 0.25 μm PMOSFETs, forming shallow junctions and thus suppressing short channel effects. Next, the effects of nitrogen implantation into p⁺ poly-Si gates are studied. The implanted nitrogen diffuses to the gate oxide during annealing and nitrides the gate oxide. As a result, boron penetration through the gate oxide is suppressed and the reliability and hot carrier resistance are improved.