Bonding structures of silicon oxynitride prepared by oxidation of Si-rich silicon nitride

Thin oxynitride gate dielectric films were prepared by thermal oxidation of Low-Pressure Chemical Vapor Deposition (LPCVD) silicon-rich silicon nitride (SRN) at temperature ranging 850 to 1050 °C. X-ray photoelectron spectroscopy (XPS) was used to study the bonding structure of resulting oxynitride at the interface. We found that the Si atoms at the interface exist in the form of random bonding (RB) to oxygen and nitrogen atoms for samples oxidized at 850 and 950 °C. However, with heavy oxidation, the interface nitrogen atoms exist in the form of SiN₄ phase and the interface oxynitride layer is a random mixture (RM) of SiO₄ and SiN₄ phases for samples oxidized at 1050 °C. These results have great impact on the interface defect density. At interface with random mixture of SiO ₄ and SiN₄ phases, the constraint force of Si atom in SiN₄ phases is large and would result in higher interface trap density. Whereas in random bonding oxynitride/Si interface, the interface trap density is low because of less constraint force of Si atoms. In addition, the RB structure will help to minimize the composition, permittivity and bandgap fluctuations at the interface and improve the reliability of MOS devices.