Process Development and Characterization of Al2O3 High-k Gate Dielectrics Prepared by Anodization and Nitric Acid Oxidation for MOS Devices

Room-temperature anodic oxidation (anodization) is introduced to prepare ultra-thin aluminum oxide (Al2O3) high-k gate dielectrics on p-Si with equivalent oxide thickness (EOT) down to 14 Å. Both DC and DAC (DC superimposed with AC) anodization techniques were investigated. Effective dielectric constant of k~7.5 and leakage current with 2~3 orders of magnitude lower than SiO2 are observed. The conduction mechanism in Al2O3 under negatively biased region is shown to be Fowler-Nordheim (F-N) tunneling with an effective barrier height of 1.6 eV. It is found that the positively biased current is sensitive to interface trap capacitance (Cit) and can be used as an efficient way to evaluate the Al2O3/Si interfacial property. An optimal process control with minimized Cit via monitoring the positively biased current is demonstrated. Also, ultra-thin Al2O3 gate dielectric on n-type 4H-SiC was prepared by room-temperature nitric acid (HNO3) oxidation. The k~9.4 and EOT of 26 Å are produced, and the interfacial layer and carbon cluster are not observed. The electrical responses of MOS capacitors under heating and illumination are used to identify the conduction mechanisms. For the positively biased case, the conduction mechanism is shown to be dominated by Schottky emission with an effective barrier height of 1.12eV. For the negatively biased case, the gate current is shown to be due to the generation-recombination process in depletion region and is limited by the minority carrier generation rate.
The high frequency Terman’s method for interface state density (Dit) extraction is applied to inspect the lateral nonuniformity (LNU) of effective oxide charges (Qeff) in MOS capacitors. The two parallel capacitors model is constructed to simulate the LNU charges, and it was shown that negative effective Dit (NED) appears as the LNU occurs in gate oxide. This technique was first used to examine the Qeff distribution in Al2O3 prepared by anodization and HNO3 oxidation. It was found that LNU in Al2O3 is sensitive to oxidation mechanisms and can be eliminated by appropriate oxidation process.