Experimental and theoretical studies on N 1s levels of silicon oxynitride films

Nitridation of silicon dioxide layers at low temperatures by the use of nitrogen plasma generated by low energy electron impact has been investigated by means of X-ray photoelectron spectroscopy. The nitrogen concentration is increased by the application of a negative bias voltage to the specimen, indicating that N+ ions are the reacting species. When nitridation is performed above 450 °C, only one peak is observed at 397.9 eV, and it is attributed to N(–Si)3 (nitrogen atom bound to three Si atoms). For the nitridation below 400 °C, on the other hand, a peak appears at 399.2 eV in addition to the 397.9 eV-peak. When the film nitrided at 400 °C is heated at 700 °C in a vacuum, the intensity of the 399.2 eV-peak is slightly decreased, and new peaks appear at 399.9, 400.7 and 402.4 eV. The 399.9, 400.7, and 402.4 eV-species are not formed when the nitrided oxide layers containing no 399.2 eV-species are heated at 700 °C, showing that these species are produced from the 399.2 eV-species. Theoretical calculations using a density functional theory method show that O–N(–Si)2 (nitrogen atom bound to two Si atoms and one oxygen atom) has an N 1s level shifted by +1.3 eV from that of N(–Si)3, indicating that the 399.2 eV-peak is attributable to O–N(–Si)2. This species is likely to be easily formed by the insertion of N+ ions to the SiO2 surface without an extensive atomic rearrangement. The calculations also show that O–N–Si (nitrogen radical bound to both one oxygen and Si atoms) and ON–Si (nitrogen atom with an NO double bond and an Si–N single bond) have +1.8 and +3.1 eV energy shifts, respectively, from the N 1s peak of the N(–Si)3 species, leading to the attribution of the 399.9 eV-peak to O–N–Si and the 400.7 eV-peak to ON–Si. The calculations also show that the 402.4 eV-peak is attributable to [N(–Si)4]+ (a cation composed by a nitrogen atom bound to four Si atoms). The concentrations of the [N(–Si)4]+ and ON–Si species are almost constant throughout the nitrided oxide layers, showing that these species result from the reaction between O–N(–Si)2 and bulk SiO2. The concentration of O–N–Si species, on the other hand, is low at the surface and high near the interface, showing that O–N–Si is formed by the reaction with Si.