Correlation Between Development of Leakage Current and Hydrogen Ionization in Ultrathin Silicon Dioxide Layers
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ABSTRACT The generation of leakage current across 3-6-nm thick thermal oxides on (100)Si under electrical stress or irradiation with 10-eV photons is compared with the radiation-induced defect generation in 35-66-nm thick SiO 2 layers. The degradation of both ultrathin and conventional oxides appears correlated with the concentration of atomic hydrogen in the layer. Both the leakage currents and the irradiation-induced defects were found to have two components: one thermally unstable that correlates with the H-induced donor states, and another related to the permanent oxide network damage ascribed to H-assisted Si-O bond break. As both degradation processes involve a proton formed in the oxide, we suggest that H ionization either by electron emission or by trapping a hole triggers oxide degradation. INTRODUCTION Development of electrical conductivity of ultrathin oxides on silicon as result of electrical stress indicates that there is a generation of electron states in the SiO 2 layer with energy levels providing an underbarrier pathway for charge carriers (electrons and/or holes) between the electrodes of the metal-oxide-semiconductor (MOS) structure. It is now well established that the leakage current is related to some kind of electron traps generated in the oxide by injected charge carriers [1-5] with possible involvement of hydrogen [1,6]. However, these traps are hardly accessible for investigation because of the local nature of the leakage current or dielectric breakdown. This hampers attempts to analyze the relationship between the degradation phenomena and the oxide processing which is needed to improve the reliability of MOS devices. As demonstrated recently, a leakage current similar to that observed after an electrical stress can be produced in Si MOS structures with ultrathin oxides by ionizing radiation: 7-rays [7], Xrays [8], and vacuum-ultraviolet (VUV) photons [9]. In contrast to the local injection-induced oxide degradation, the radiation-induced one occurs in a laterally uniform manner thanks to the uniformity of the radiation adsorption. Particularly, in the case of low-energy VUV irradiation it is possible to directly determine the density of the electron-hole pairs injected into the SiO 2 layer and to estimate the effective cross section of the defect generation process accounting for the creation of leakage current. In the present work we will compare the VUV-induced generation of leakage current in ultrathin (3-6-nm thick) oxides with defect creation in thicker (35-66 nm) SiO 2 layers studied by charge injection spectroscopy [10]. It will be shown that the development of leakage current in ultrathin SiO 2 correlates with the concentration of radiolytic H in the oxide which gives rise to defect states similar to the donor-like slow interface states and the broken bond oxide traps in thicker oxides. Because formation of the latter two proceeds with participation of a proton, we suggest the ionization of hydrogen to be the trigger of the oxide degradation.
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Mat. Res. Soc. Symp. Proc. Vol.
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