| Summary: | 碩士 === 國立暨南國際大學 === 電機工程學系 === 99 === This study mainly discusses the degradation and breakdown phenomenon of thin silicon dioxide gate layer at nanoscale by applying uniaxial mechanical stress. In the past, many studies have shown that the degradation behavior of thin silicon oxide layer is a highly localized phenomenon, and a single breakdown phenomenon cannot be observed directy from the traditional measurement methods. Therefore, the aim of this study is to realize the nano-scale breakdown and degradation behavior of thin silcion oxide layer subjected to uni-axial strain by using a measurement tool with nano-scale resolution.
By taking advantage of nanoscale measurement capability of conductive atomic force microscopy (C-AFM) and the excellent performance in electrical characterization of semiconductor parameter analyzer Agilent 4156C, we combined the two to apply the electrical stress onto the bared thin silicon dioxide films through C-AFM tip which acted as the metal gate electrode in the traditional metal-oxide-semiconductor (MOS) capacitor. The contact area between the probe and the oxide layer is about several dozens to several hundred square nanometers, and the lateral resolution could reach a range of several nm.
On the hand, strain has been applied to MOS devices to improve their performances. However, most of the papers in the literature only discuss about the carrier mobility and operation frequency improvement of MOS devices after the application of strain, rarely has addressed the influences of strain on the oxide degradation and breakdown behaviors at nanoscale. Although, it was reported that the tunneling leakage current through the thin gate oxide would occur if stain was applied to the gate oxide, it is hardly found in the literature that discusses the relationship between gate leakage current and the location on gate oxide subjected to mechanical stress. Therefore, the emphasis of this thesis is to explore the degradation and breakdown properties of thin gate oxide at nanoscale at different position after strain is applied.
In this work, a self-made bending tool was used to apply mechanical uni-axial stress to the samples. C-AFM in combining with semiconductor parameter analyzer Agilent 4156C was used to detect the gate oxide leakage current at different position on the oxide surface. In addition to the nanoscale I-V characteristics were deternmined, we also measured the Weibull distribution of the strained samples by applying constant-voltage stress as well as -ray irradiation to the samples.
Both upward and downward stresses were applied to the thin SiO2 sample. Our experimental results show that the gate leakage current at either end of the strained samples is higher, while it is lower at the central region of the samples. This is attributed to greater curvature and hence higher strain at the center region of the stressed samples. Weibull distribution of breakdown voltage was also determined by applying constant voltage stress to the strained samples. We found that the breakdown voltage Weibull distribution at either end is higher than that at the central region of the strained samples. We concluded that the samples subjected to the mechanical stress, either bended upward or downward, were affected by the strain and exhibited higher leakage current and lower breakdown voltage at the position with higher strain. This result reflects that the bending of the samples caused by applied mechanical strain would deteriorate the bonding and produce weak spots inside the oxide samples. After irradiation, we found that the greater the irradiation dose the worse the drgradation behaviors would occur in the bended oxide samples, which indicates that higher irradiation dose produced more destruction of the oxide bonding of the bended oxide.
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