Ultra-thin silicon oxide (a-SiOx:H) films have been grown by means of plasma enhanced chemical vapor deposition (PECVD) to replace the standard hydrogenated amorphous silicon (a-Si:H) passivation layer for silicon heterojunction solar cells to reduce parasitic absorption. Additionally, silicon oxide surfaces are well known as superior substrates for the nucleation enhancement for nanocrystalline silicon doped films. Symmetrical passivation samples were fabricated with variable a-SiOx:H layers with a thickness of 10-1.5 nm and characterized after several annealing steps (25-650 °C). The best value reached so far on <100> oriented Si wafers is: implied open circuit voltage of 686 mV and minority carrier lifetime of 1.6 ms after annealing at 300 °C. Such values were found to be reproducible even for ultra-thin a-SiOx:H layers (1.5 nm).