Nitrogen implantation effect on SiO2/Si interface roughness is analyzed and a new method for growing ultrathin gate oxide is proposed. Interface roughness is improved by nitrogen implantation; the root-mean-square values measured by atomic force microscopy are 2.72-2.79 Angstrom, 1.7-2.12 Angstrom, and 2.12-2.33 Angstrom for 0, 1.0 x 10(14)/cm(2), and 2.0 x 10(14)/cm2 nitrogen implantation, respectively. Initial O-2 injection method is composed of O-2 injection/N-2 anneal/main oxidation, and the control process is composed of N-2 anneal/main oxidation. The purpose of O-2 injection is to form an extremely thin oxide layer that provides a Si/SiO2 interface at which nitrogen is accumulated during N2 annealing process. Time of flight secondary ion mass spectroscopy data show that the initial O-2 injection method has a higher peak than that of the control process. The oxidation reduction rates of the control process were: 0.6% (15.4 Angstrom /15.5 Angstrom), 18.2%(26.9 Angstrom /32.9 Angstrom), and 16.4% (34.1 Angstrom /37.4 Angstrom) for the wafers of 1, 10, and 20 min oxidation time, respectively. And the rates of the initial O-2 injection method were 7.6% (19.4 Angstrom /21 Angstrom), 25% (23.1 Angstrom /30.8 Angstrom ), and 29.5% (28.4 Angstrom /40.3 Angstrom). It is clear evidence that the nitrogen profile can affect oxidation retardation effect as well as the nitrogen dosage. Metal-oxide-semiconductor capacitors with gate oxide thickness of 2 nm have been fabricated. Boron penetration prevention ability is intensified by the initial O-2 injection process for the same dose of nitrogen implantation. The oxide reliability was investigated using time-zero dielectric breakdown (TZDB) measurement. The initial O-2 injection method shows improved TZDB.