Ultrathin silicon oxynitride formed by low-energy electron impact plasma nitridation and chemical oxidation methods

Abstract

A formation method of ultrathin silicon oxynitride layers with high-nitrogen concentrations and good electrical characteristics has been developed. This method consists of nitridation by the use of nitrogen plasma generated by low-energy electron impact and chemical oxidation. Without annealing after the nitridation or oxidation, the metal-nitrided oxide-semiconductor diodes possess poor electrical characteristics with a high-density leakage current, a large flat band shift in the negative bias direction, and a large hysteresis in the capacitance-voltage curves. With annealing in nitrogen, on the other hand, the leakage current density is markedly decreased, the flat-band voltage shifts to nearly zero gate bias, and the hysteresis is eliminated. X-ray photoelectron spectroscopy measurements show that the electrical characteristics of the oxynitride layers containing only nitrogen atoms bound to one oxygen atom and two Si atoms each, O–N(–Si)2, as nitrogen species are excellent, while the electrical characteristics become poor when the oxynitride layers contain charged species, N–(Si)4+ or high concentrations of nitrogen atoms bound to three Si atoms, N–(Si)3 or N–[Si(–O3)]3. Oxynitride layers in which nitridation is initially performed followed by chemical oxidation by immersion in nitric acid possess almost uniform nitrogen atomic concentration, N/(O+N), of 10–12% throughout the layer and high-relative dielectric constant, εr of 5.2. This εr is higher than that expected from the nitrogen concentration, indicating that nitrogen atoms have long-range interaction. When chemical oxide layers are initially formed and then nitridation is performed, the nitrogen concentration is lower (i.e., ∼8% for the surface region) and decreases with the depth, resulting in a lower εr of 4.1.