Characterization of silicon oxynitride thin films by x-ray photoelectron spectroscopy

There has been a considerable effort in the past decade to incorporate nitrogen into SiO₂ in order to improve the electrical properties of ultrathin (2-10 nm) gate oxides. Process conditions affect the nitrogen concentration, coordination, and depth distribution which, in turn, affect the electrical properties. X-ray photoelectron spectroscopy (XPS) is particularly well suited to obtaining the nitrogen coordination and, to a lesser extent, the nitrogen concentration in thin oxynitride films. To date, at least four different nitrogen coordinations have been reported in the XPS literature, all having the general formula: N(-Si_xO_yH_z), where x + y + z = 3 and x ≤ 3, y ≤ 1, z ≤ 2. In this article we review the XPS literature and report on a fifth nitrogen coordination, (O)₂=N-Si, with a nitrogen l.v binding energy of 402.8±0.1eV. Next nearest neighbor oxygen atoms shifted the N(-Si)₃ peak roughly 0.1 eV per oxygen atom. We also discuss results from a novel approach of determining the nitrogen areal density by XPS, the accuracy of which is dependent on the depth distribution of nitrogen. Secondary ion mass spectrometry is used to determine the depth N distribution, while nuclear reaction analysis is used to check the N concentration measured by XPS.