Current Understanding of the Transport Behavior of Hydrogen Species in MOS Stacks and Their Relation to Reliability Degradation

We review recent experiments that suggest a comprehensive model for the reversible hydrogen (H) transport between the poly-Si interface and the oxide/Si interface of MOS stacks. The H diffusion in intact model MOS structures is probed by H depth profiling via resonant N-15-H nuclear reaction analysis (NRA). It is demonstrated that MOS device degradation correlates with H accumulation in the oxide/Si interface region. A specific ultra-thin oxynitride is discovered in the poly-Si/oxynitride interface as well as in the near-surface region of N-2-annealed nitride films and shown to function as a potential H-storage layer. The interfacial storage layer between the poly-Si gate and the oxynitride dielectric of MOS transistors is found to contain two kinds of H species, which are mobile and stable, respectively, versus irradiation-induced relocation. The mobile H, if stimulated by energetic carriers, migrates across the gate films and relocates to the SiO2/Si interface, causing device instabilities. Understanding this basic hydrogen transport behavior allows conceiving fabrication countermeasures to improve the device reliability.