Boron diffusion and penetration in ultrathin oxide with poly-Si gate

Abstract

Boron penetration through thin gate oxide down to 17 /spl Aring/ is investigated in this work. Boron penetration is characterized by the amount of flat band shift in a MOS capacitor. The effective diffusion coefficient of boron in these thin oxides is found to be higher than in thicker oxides. The introduction of a moderate dose of fluorine (1/spl times/10/sup 15/ cm/sup -2/) during gate doping enhances boron penetration in these thin oxides. Compared to as-deposited polycrystalline silicon (poly-Si), crystallized amorphous silicon (/spl alpha/-Si) films display slower boron diffusion in the gate and reduce enhancement of boron penetration due to fluorine. However, crystallized /spl alpha/-Si gate also reduces the amount of dopant activation and leads to extra gate depletion. The tradeoff between dopant activation and boron penetration is discussed.