An Ar‑H₂ plasma treatment was applied on an ultrathin RCA oxide to create well-passivated silicon wafers with symmetric c‑Si/SiO_x:H/a‑Si:H passivation layer stacks. The effective lifetime of these samples increased from 10 μs to 4 ms after annealing at 200 °C through Ar‑H₂ plasma treatment of the oxide. The results indicate that the plasma treatment can modify the RCA oxide and this enables atomic hydrogen diffusion at low annealing temperature, leading to a well passivated c‑Si/SiO_x:H interface. This might provide new possibilities to use wet chemical oxides in c-Si solar cells, for example as tunnel contacts.