Preoxidation UV Treatment of Silicon Wafers

As the thickness of gate oxides employed in high density MOS integrated circuits decreases, more attention must be given to the condition of the silicon surface prior to the thermal gate oxidation process. Among others, organic contaminants are of particular concern in this case. In this paper a UV/O₂ treatment, which is aimed at the removal of organics, is studied as a step prior to thermal growth of very thin (<20 nm) gate oxides. The results obtained show that such a treatment causes: (i) growth of up to 1.5 nm thick oxide which stabilizes the silicon surface before oxidation, (ii) reduction of carbon content in the subsequently grown thermal oxide, and (iii) improved dielectric strength in gate oxides provided that water vapor is added to oxygen during the exposure of silicon to the UV treatment. On the other hand, no impact of the preoxidation UV treatment was found on interface trap density and oxide fixed charge in the subsequently formed Si-SiO₂ structure. Considering these findings, the preoxidation UV/O₂ treatment is recommended as an effective and easy to implement addition to the standard wafer cleaning process.