Abstract: By using very dry HCl gas as an additive species in thermal oxidation ambient, we obtained an aluminium film MOS structure with an almast zero fixed oxide charge, an interface trap density below 1×10¹⁰ no./cm²-eV and a passivation efficiency above 99.5% at an intentional high sodium contamination of about 3×10¹³ ions/cm². Reasons responsible for these results are discussed.