Buried oxynitride layers were formed in silica glass by N+only or both N+‐ and Si+‐ion implantation. These samples were characterized by secondary‐ion mass spectroscopy, Rutherford backscattering spectrometry, x‐ray photoelectron spectroscopy, and scanning electron microscope. The depth profile of implanted N without Si implantation is a trapezoidal shape at a high‐dose condition (more than about 3 × 1016ions/cm2). In the N+implantation only, a part of implanted N reacts with Si, and forms a SiON layer which is thermally unstable. In both Si+(100 keV, 1 × 1017ions/cm2) and N+(50 keV, 11× 1017cm2) implantation, the depth profile of N is a Gaussian distribution, and the thermal stability of a SiON layer is dramatically improved. However, bubbles were generated near the projected range of Si and N at a high‐dose condition (the total dose was 4 × 1017ion/cm2and the dose ratio Si/N=3/4). The generation of bubbles was suppressed by two‐step N+implantation (40 and 60 keV).