| Summary: | 博士 === 國立交通大學 === 電子工程系 === 91 === The use of gate oxides in direct tunneling regime is required for sub-100nm CMOS devices. While, a great reliability concern induced by direct tunneling in such thin oxides is being aroused.
The objective of this dissertation is to investigate direct tunneling caused reliability issues in ultra-thin oxide devices. First of all, enhanced hot carrier degradation in nMOSFETs with a forward substrate bias is observed. The enhanced degradation cannot be simply explained by conventional hot carrier theory. In this work, an Auger recombination assisted electron energy gain mechanism is proposed to explain this phenomenon. Characterization of hot electron gate injection current and hot carrier light emission is performed to confirm the proposed theory. The role of the Auger effect in terms of operating voltages is also identified. As opposed to conventional hot carrier degradation, this Auger enhanced degradation exhibits positive temperature dependence. This new degradation mode may bring about a major reliability concern in high temperature operation and imposes a limitation on the applied substrate bias.
Based on above understanding, we found that Auger recombination also plays an important role when an ultra-thin oxide is stressed in the valence-band tunneling regime. In ultra-thin oxide nMOSFETs, holes created by valence-band tunneling provide for Auger recombination with channel electrons, thus increasing hot electron energy and hot carrier degradation. The degradation features are discussed, and the numerical calculation for the Auger recombination rate is performed. Since valence holes can be swept out by a reverse substrate bias, the degradation exhibits positive substrate bias dependence. Such dependence is particularly obvious at a reduced drain bias, where the Auger effect becomes dominant. The valence-band tunneling induced degradation may cause a severe reliability issue in forward biased substrate or floating substrate devices.
Furthermore, a large direct tunneling current has been known to decrease oxide time-to-breakdown and limit oxide further scaling. Actually in most circuits, the failure criterion is determined by the hardness of oxide breakdown (BD). In this part, forward substrate bias enhanced breakdown progression in ultra-thin oxide pMOS is proposed. The enhanced progression is attributed to the increase of hole tunneling current resulting from breakdown induced channel carrier heating. The carrier temperature extracted from the spectral distribution of hot carrier luminescence is around 1300K. The substrate bias dependence of post-breakdown hole tunneling current is confirmed through the calculation of channel hole distribution in sub-bands. This observed phenomenon is significant to ultra-thin gate oxide reliability in floating substrate and forward-biased substrate devices.
Finally, a new flicker noise degradation mode in pMOSFETs caused by oxide breakdown is proposed. Breakdown (BD) induced channel carrier heating not only accelerates the BD progression, but also enhances the reaction for negative bias temperature instability (NBTI) in the local BD spot. The flicker noise degradation can be explained by the non-uniform threshold voltage distribution resulting from NBTI created positive and localized oxide charges. Substrate bias effect on the NBT degradation rate is also evaluated for analog applications.
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