Capacitance-voltage properties of thin Ta2O5 films on silicon
Abstract
A comprehensive study of the high frequency (1 MHz) capacitance-voltage (C-V) properties of Al/Ta2O5/p-Si capacitors is described. The dependence of the dielectric constant of thermally oxidized Ta2O5 on the oxidation temperature and the silicon substrate temperature during the deposition of tantalum metal was studied. Charge trapping or generation processes in the Ta2O5 bulk or in the Ta2O5-Si interface region were investigated on the basis of shifts in the flat-band voltage of 1 MHz C-V curves. Attempts to measure the quasi-static or static C-V characteristics of the Al/Ta2O5/p-Si capacitors by the ramp voltage or charge-voltage method were unsuccessful since the leakage current of the capacitors is higher than the displacement current for most gate voltages. The Ta2O5 thin films on p-type silicon substrates were prepared by thermal oxidation at 430-675°C of electron-beam-deposited tantalum. The silicon substrate temperature during tantalum evaporation was either close to room temperature or 150°C. The effective dielectric constant of Ta2O5 films formed from tantalum which had been deposited onto silicon substrates heated to 150°C is in the range 22-23, the value depending on the oxidation temperature. It is higher than that of comparable Ta2O5 films formed from tantalum deposited onto silicon substrates close to room temperature which varies from 18-26. The sign and magnitude of the flat-band voltage depends on the oxidation temperature. It is about +1 V for Ta2O5 formed at 430°C and decreases monotonically with increasing oxidation temperature to about -1.5 V for Ta2O5 formed at 675°C. The flat-band voltage exhibits an instability with respect to voltage-time stresses at room temperature because of charge trapping and/or generation phenomena either in the Ta2O5 bulk or in the Ta2O5-Si interface region. At least three different charge trapping and/or generation processes have been observed. © 1988.