Detection of Electron- and Hole-traps in SiO2/Si by ESCA

Since very small amounts of defects or impurities influence Si MOS (Metal-Oxide-Semiconductor) device characteristics, it is usually difficult to detect them in the SiO2/Si structures used for these devices by ordinary surface characterization. We therefore tried to resolve this problem with a different approach. In an effort to detect electron and hole traps in SiO2/Si samples, the electric current that flows from the sample to the ground was measured as a function of the X-ray irradiation time during ESCA (Electron Spectroscopy for Chemical Analysis) measurements. The transient of this current was found to be due to electric charges supplied to the Si side of the SiO2/Si interface. The number of these charges increases as the electric charges of the opposite sign become trapped at oxide traps in the SiO2. This transient was found to be the sum of the exponential functions of the X-ray irradiation time with different time constants. Various types of traps can be distinguished by these time constants and the sign of the current. Our first experiment showed that traps whose concentration is of the order of 1014 m−2 can be detected by this method, whereas they cannot be easily detected with such sensitivity in an ESCA spectra because the ESCA detection limit is of the order of 1016 m−2. This new method was used to compare two different kinds of thermally oxidized silicon, namely, that oxidized in O2/N2 and that oxidized in pyrogenic steam. The present method is expected to be useful in the process development of MOS devices using ultra-thin SiO2 films.