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Our results show that the defect distribution within a nanometer size percolation path is nonuniform. The defects, which are shown as oxygen vacancies, spread out radially from the center of the percolation path. The conduction band edges of the defective oxide are lowered for 0.14-0.78 eV when the Si-O composition changes from SiO 1.76 to SiO 0.7 .Over the past years, efforts have been made to understand the trap/defect generation, 1-3 percolation path formation, 4-6 and degradation 7-9 in amorphous SiO 2 in attempts to predict the occurrence of the time dependent dielectric breakdown accurately under device operating conditions. There are a few well accepted physical models 2,10,11 which describe the importance of different trap/defect generation processes as well as their impacts to the dielectric breakdown ͑BD͒. It is commonly agreed that the defects generated during pre-and post-BD stress are responsible for the various leakage current profiles observed in the accelerated tests. Among the proposed defects, oxygen vacancy and its related species serve as strong candidates responsible for the oxide wearing-out process, from stress induced leakage current 1,2 to progressive breakdown ͑or soft-BD͒. 8,12,13 It is therefore important to know the distribution of oxygen deficiency in a BD path and understand its role in the oxide degradation process. In this letter, we study the defective oxide using scanning transmission electron microscope ͑STEM͒ with electron energy loss spectroscopy ͑EELS͒, and the distribution of oxygen deficiency in a percolation path is mapped and discussed. Figure 1͑a͒ shows the high angle annular dark field ͑HAADF͒ image of a typical metal oxide semiconductor ͑MOS͒ gate stack ͑L ϫ W = 0.5ϫ 0.15 m 2 ͒ after dielectric breakdown, which is isolated using focused ion beam ͑FIB͒ milling. The initial BD was created using a constant voltage stress of V gstress = 4.1 V and compliance current limit I gl = 1.0 A. It was further stressed to the post-BD phase using a lower voltage V gstress = 3.1 V with I gl = 2.0 A. The dielectric layer is a 22 Å nitrided amorphous SiO 2 ͑N% ϳ 3%͒. A dielectric breakdown induced epitaxy 14 ͑DBIE͒ nanomarker is identified at the BD location. Electron energy loss spectra, both Si L 2,3 edge and O K edge, were acquired at positions 1-6 in the oxide layer to get the information of the breakdown. To access the local properties from a 22 Å layer, a fine probe size as well as a good energy resolution is needed. In this experiment, the STEM probe size was set to be around 3 Å in diameter and the EELS energy resolution was 0.7 eV. The plots of Si L 2,3 edge spectra are shown in Fig. 1͑b͒; only spectra at positions 1-3 ͑half of the symmetrical DBIE͒ are displayed for discussion. The different Si oxidation states are labeled in the figure. The Si 4+ signals ͑onsets at 105 eV͒ are originated from the bulk SiO 2 bonding in the central region of the oxide layer. It reveals the electronic structures at the bottom of the oxide conduction band. 15 The changes in the peak shape and int...

Our results show that the physical origin of the digital telegraph noise observed in the early stage of the progressive breakdown is originated from the defective oxide with low oxygen concentrations. The outer shells of the percolation path contribute significantly to the random switching of current levels as a result of the ON/OFF state of percolation path. The formation of a nanosize conductive Si path in the inner shell of the percolation path pushes the oxide to a high leakage state and suppresses the visibility of the digital noise.