| Summary: | 博士 === 國立交通大學 === 電子工程系所 === 93 === We have investigared the device characteristics and the reliability of the MOSFETs fabricated by advanced deep sub-micron technologies. To reduce the intolerable leakage current of the ultra-thin gate oxide, the nitrided oxides and high-k gate dielectrics are introduced to place the conventional gate oxide; the strained SiGe layer is applied to be the device channel for enhancing the device performance; various surface treatments are performed to improve the quality of high-k HfO2 film. Hence, our studies are focused on three main topics. Firstly, we have investigated the effect of hot-carrier degradation on device reliability and the low-frequency flicker noise characteristics for the deep sub-micron nMOSFETs with ultra-thin nitrided gate oxides. Secondly, the degradation mechanism of high voltage stressing and the channel thickness effect on device characteristics for the deep sub-micron pMOSFETs with ultra-thin N2O-annealed SiN gate dielectric and strained Si0.85Ge0.15 channel have been studied. Finally, we have also investigated the effect of pre-deposition surface treatment on the electrical characteristics for the ultra-thin HfO2 gate dielectrics.
We have investigated the device degradation caused by the hot-electron-induced electron trapping in various ultra-thin (EOT = 1.6 nm) nitrided gate oxides for 0.13 um nMOSFETs. It has been found that the nitrogen-incorporated gate dielectrics by a variety of popular techniques including Si3N4/SiO2 (N/O) stack, NO annealing, and plasma nitridation result in enhanced hot-electron-induced device degradations as compared to the conventional gate oxide counterpart. The exacerbated hot-electron degradations are attributed to the electron trap generation in the ultra-thin gate dielectric rather than the interface state generation as a result of nitrogen incorporation, and the mechanism has also been confirmed by several aspects: the positive shift of threshold voltage, the insignificant variation of subthreshold swing, the reduction of gate leakage current, no slope change of the Ib-Vcb curves for DCIV measurement, and a small exponent (n ~ 0.3) of �幀t versus stress time after the nitrided gate oxide devices were stressed. Moreover, the nitrogen incorporation into the ultra-thin gate oxide has been demonstrated to be more vulnerable to the hot-electron degradation as considering the long-term reliability issues, and the plasma nitridation has be shown to be the most promising technique of ultra-thin gate oxide nitridation for the sub-100nm device applications.
The low-frequency flicker noise of the 0.15 um nMOSFETs with ultra-thin (EOT = 1.6 nm) thermal oxide, Si3N4/SiO2 (N/O) stack, NO oxynitride, and plasma nitrided oxide has been demonstrated. We have found that the nitrogen incorporation in the ultra-thin gate oxide will increase the flicker noise by introducing more electron traps. It is due to the fact that the low-frequency flicker noise is mainly generated by the trapping/detrapping of channel electrons with the interface states and the electron traps. However, the nitrogen incorporation can improve the device immunity against the hot-carrier degradation in the flicker noise because the hot-electron-induced electron trapping may suppress the effective electron traps for generating flicker noise. Moreover, moderate increase of noise level is obtained when the nitrided oxide is suffering breakdown comparing with the thermal oxide even though a significant amount of electron traps are created when oxide breakdown is occurred. We also found that the frequency index of the noise spectrum is varied with the gate bias and it is strong related to the oxide traps. Hot-carrier degradation and oxide breakdown may lower the frequency index for both thermal oxide and nitrided oxide devices. For considering the flicker noise characteristics, the plasma nitrided oxide has been demonstrated its potential for sub-100 nm MOSFET devices in analog and RF applications because of its higher oxide quality.
The pMOSFET with 50-nm thick Si0.85Ge0.15 channel and ultra-thin (EOT = 3.1 nm) N2O-annealed SiN gate dielectric has been shown to have well-performing on/off and output characteristics. Several methodologies for the device reliability characterization, such as stress-induced leakage current (SILC), drain avalanche hot-carrier (DAHC) injection, channel hot-carrier (CHC) injection and negative-bias temperature-instability (NBTI), have been used and the results are compared. In terms of the long-term degradation, the excellent quality of the N2O-annealed SiN gate dielectric can be firmly obtained because only negligible degradations have been found after stressing no matter which technique was employed. Even so, the experimental results have been compared and we found that the HC degradation is worse than the NBTI degradation and the channel hot-carrier (CHC) stressing is the worst case for all kinds of reliability testing. Meanwhile, we have also verified that the interface state generation is the dominant mechanism responsible for the HC-induced degradation while the electron trapping dominates the device degradation for the NBTI stressing.
We also have found that the thickness of SiGe channel has a great impact on the device characteristics. With controlling the SiGe layer thickness thinner than 15 nm, the device depicts a subthreshold swing of 68 mV/dec, the interface state density of 1×1011 eV-1cm-2, acceptable junction leakage, and more than 50% hole mobility improvement comparing to the Si channel device. Therefore, high quality interface between the gate dielectric and the strained SiGe channel can be achieved by using the N2O-annealed SiN gate dielectric and the device performance can be improved.
Finally, we have investigated the effects that various pre-deposition surface treatments, such as HF dipping (HF-dipped), NH3 surface nitridation (NH3-annealed), and rapid thermal oxidation (RTO-treated), have on the electrical properties of HfO2 gate dielectrics. The NH3-annealed technique is far superior to the others because the dielectric subjecting to the NH3 surface nitridation possesses a tremendously reduced leakage current, the lowest equivalent oxide thickness (EOT), and a moderate hysteresis width. In contrast, the RTO-treated preparation can only effectively reduce the leakage current by its resultant increased physical thickness and displays considerably severe hysteresis. The dependence of hysteresis on the initial inversion bias (Vinv), temperature, and frequency are also investigated for all splits. The hysteresis width increases upon increasing the initial inversion bias and decreasing the temperature, but it is rather insensitive to the measuring frequency. Our experimental results indicate that the hysteresis width depends exponentially on both the initial inversion bias and the temperature, and it can be described well by a general empirical relationship with the form . In addition, the conduction currents through the dielectrics are probably dominated by trap-assisted tunneling (TAT) because the current densities display stronger temperature dependence at low voltage than they do at higher voltages. Based on the trap-assisted tunneling model, the corresponding parameters have been extracted and are presented.
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