A reaction mechanism for the etching of silicon nitride layers in aqueous hydrofluoric acid solutions is proposed. The surface of Si 3 N 4 consists of SiNH 2 groups that are etched from the solid matrix via three possible routes. Depending on the pH, these SiNH 2 groups are protonated (pK a ϭ 1.4) to SiNH 3 ϩ . At ϽpH 4, the rate-limiting step consists of an elimination of NH 3 and a subsequent addition of F Ϫ or HF to the vacant surface site to form Si-F. At ϾpH 3, the elimination of NH 2 Ϫ is assisted by HF 2 Ϫ , followed by a transfer of one of the fluorides of HF 2 Ϫ to the vacant site. All subsequent reaction steps to remove the SiF unit are nucleophilic substitution reactions with low activation energies. The etch rates and mechanism of different types of silicon nitride films are compared with that of SiO 2 etching. Therefore, etch selectivity between these two materials can be explained. The theory is also applicable for silicon hydrogen passivation.

IEEE 802.11s is the task group in the IEEE that is in the process of standardizing wireless mesh networks. A hot topic in this standardization effort concerns the need for additional medium access functionality beyond the basic IEEE 802.11 carrier sense multiple access with collision avoidance (CSMA/CA). In this paper, we discuss the connection between CSMA/CA and Dijkstra's classical Philosophers' problem, and its implications for the debate inside IEEE 802.11s. In an alternative view of this paper, we state some new mathematical models, theorems and conjectures related to the Philosophers' problem.

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