A novel strategy for ideal MOS stack of high dielectric reliability

In this paper we review recent experimental results on the hydrogen (H) impurity diffusion behavior in MOS structures that suggest a new approach to improve their dielectric reliability. The most desirable MOS stacks feature a specific hydrogen-retaining cover layer in an upper section that prevents H impurity leaking into the dielectric films underneath. The hydrogen diffusion behavior in intact model MOS stacks as well as in the basic SiO₂/Si system is probed by H depth profiling via resonant ¹⁵N-H nuclear reaction analysis combined with a variety of surface-sensitive spectroscopies. It is found that almost all thin film materials that comprise the MOS devices are permeable to H impurities. Diffusion of the hydrogen, however, can be suppressed by a specific ultra-thin oxynitride layer, which has exceptionally stable H retention properties. Since the degradation of MOS devices was demonstrated to correlate with H accumulation in the oxide/Si interface region, we suggest that not merely the well-investigated buried SiO₂/Si interface but also the top surface of the MOS stack is of critical importance for the reliability. In other words, guarding the entire MOS stack from H impurity diffusion (such as by an H-retaining oxynitride interlayer) will be instrumental in realizing highly reliable dielectric films.