In this thesis, we investigate the electrical properties and reliability of novel metal-oxide-semiconductor field-effect transistors (MOSFETs) for 65 nm technology node and below. Roughly, we divide the thesis into two parts, strained-silicon channel engineering and high-k/metal gate stacks respectively. Firstly, to study the influence of stress on carrier transport properties, we proposed an approach to get uniaxial compressive/tensile stress from the channel by bending silicon substrate to enhance device performance. By applying uniaxial longitudinal tensile/compressive stress, the drain current and mobility were found to increase obviously in n/p-type MOSFETs, respectively. The enhancement can be attributed to the reduction of effective transport mass and to the suppression of inter-valley scattering. However, we found that the external mechanical stress aggravated hot carrier effects in n-type MOSFETs. Therefore, in n-type MOSFETs, the behaviors of the substrate current and the impact ionization rate under mechanical stress are investigated. It was found that the substrate current and gate voltage corresponding to the maximum impact ionization current has significantly increased by increasing external mechanical stress. According to the relationship to the strain-induced mobility enhancement, the increase in impact ionization efficiency resulted from the decrease in threshold energy for impact ionization which was due to the narrowing of the band gap.
In p-type MOSFETs, the reliability issue, named negative bias temperature instability, is the dominant degradation mechanism during ON-state operation. Therefore, we investigate the NBTI characteristics of strained p-type MOSFETs with external uniaxial tensile/compressive stress. The results indicate that uniaxial compressive stress not only enhances drive current but also reduces NBTI degradation. On the contrary, uniaxial tensile stress leads to a significant degradation in both of drive current and NBTI behavior. The observed Cgc-Vg curve shows the inversion capacitance is strongly dependent on mechanical strain, meaning that the probability of electrochemical reaction decreases/or increases due to the changes in inversion carrier density according to the Nit generation rate of the reaction-diffusion model. Moreover, the charge pumping result is also consistent with the threshold voltage shift of the strained device, which means the degradation is mainly due to trap generation at the Si/SiO2 interface.
In addition, to investigate the influences of biaxial compressive stress on p-MOSFETs, we attempts to combine intrinsic and external mechanical stress. It was found that drain current and hole mobility of p-type MOSFET with Si1-xGex raised Source/Drain and external applied mechanical stress significantly decreased due to the increase of effective conductive mass at room temperature. However, this phenomenon was inverted above 363K. Because hole can gain enough thermal energy to transit to higher energy level by inter-valley scattering, its transport mechanism was dominated by lower effective mass at higher energy level. Besides, the model is also evidenced that the mobility degradation under biaxial compressive stress becomes aggravated while temperature decreases from 300 K to 100 K, which is mainly due to the increase of the ratio of carriers occupied in lowest band.
On the other hand, the SiO2 dielectric and poly-gate are unsuitable for CMOS application below 65 nm technology node due to unacceptable gate leakage current. Therefore, in the second section of this thesis, we established the electrical characteristics and physical mechanisms of MOSFETs with HfO2 dielectric/TiN gate by analyzing experimental data from charge pumping, split C-V, DC Id-Vg, and pulse Id-Vg. It is found that the threshold voltage (Vth) has a significant decrease as titanium increases in metal gate for n-MOSFETs, whereas the Vth increases in p-MOSFETs. By examining flat band voltage, we found the Vth shift was resulted from metal gate work function (£pm) which became smaller as titanium increased in metal gate. In addition,the dependence of effective mobility on temperature from 100K to 300K was entirely analyzed, which indicated HfO2 remote phonon scattering as the dominant cause of the mobility degradation in n- and p-type MOSFETs when titanium decreased.
However, the gate leakage current is also strongly dependent on the nitrogen in metal
gate. It is proved that the nitrogen can assivate the traps in HfO2 by pulse I-V,leading to the decrease in gate leakage dominated by Frenkel- Poole mechanism.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-1220110-174355 |
| Date | 20 December 2010 |
| Creators | Kuo, Yuan-Jui |
| Contributors | Tzyy-Ming Cheng, Ting-Chang Chang, MEi-Ying Cheng, WEN-YAO Huang, Tsung-Ming Tasi |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-1220110-174355 |
| Rights | withheld, Copyright information available at source archive |
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