The current conduction mechanisms in Hf-based gate dielectrics should be identified. They are generally attributed to Schottky emission (SE), Frenkel-Poole (F-P) emission, and Fowler-Nordheim (F-N) tunneling. Each current transport mechanisms will be introduced in the following.
1.3.1 Schottky emission (SE)
Electrons get enough thermal energy to overcome the potential barrier of the dielectric and transport to the anode, called Schottky emission or thermionic emission.
The charge transport process of S.E is shown in Fig. 1-9. The standard Schottky emission could be expressed as
Schottky barrier height, k is Boltzmann’s constant, ε0 is the permittivity of free space, εr is the dynamic dielectric constant, m* is the electron effective mass, and m0 is the free electron mass.9
1.3.2 Frenkel-Poole (F-P) emission
When gate under negative bias, electrons will inject from gate into the dielectric layer and will be trapped into shallow trap levels. Thereafter, the electrons transported through the dielectric layer by hopping between these trap levels, leading to leakage current, called Frenkel-Poole (F-P) emission. The charge transport process of F-P is shown in Fig. 1-10. The standard F-P emission could be expressed as
dominate conduction mechanism in the region of medium to high electric fields1.3.3 Fowler-Nordheim (F-N) tunneling
In higher electric field, the Fowler-Nordheim (F-N) tunneling dominated the conduction mechanism. The injection of electrons from the gate entered the conduction band of HfO2 by tunneling through a triangular potential barrier. The charge transport process of F-N is shown in Fig. 1-11. The standard F-N emission could be expressed as
where JFN is the current density, h is the Plunk’s constant, m* is the electron effective mass in oxide, and qf is the potential barrier height.
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1.4 Motivation
The rapid progress of complementary metal oxide semiconductor (CMOS) integrated circuit technology has met several serious technological challenges over the past few years. According to the prediction of the International Technology Roadmap for Semiconductor (ITRS), the conventional gate dielectric layer will reach its physical limits. Gate dielectric scaling of CMOS will increase the speed and the packing density of modern circuits. However, the aggressive shrinking of the gate length and gate dielectric thickness accompanies excessive leakage current and reliability problems. To solve these problems, a major solution is to replace the traditional SiO2 or SiON by other higher dielectric constant material. Using high dielectric constant material for gate dielectric could have larger physical thickness and maintain smaller equivalent oxide thickness (EOT). As a result, high-dielectric-constant (high-κ) thin films have been considered as suitable gate dielectric for modern CMOS technology. There are various high-κ thin film has been investigated. Among these high-κ materials, HfO2 is considered as the most promising candidate because of high dielectric constant (~25), wide band gap (~5.7 eV), and large band offset with Si conduction band (~1.5 eV). Nevertheless, there are still some issues which need to be considered, such as the reliability and thermal stability of the dielectrics.
1.4.1 Plasma nitridation
It has been reported that nitrogen incorporated into HfO2 gate dielectrics has beneficial effect on performance [43]. As reported in previous study, nitrogen incorporation can suppress crystallization during high temperature treatment, reduce
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dopant penetration, increase dielectric constant, and reduce leakage current by about 3-4 orders of magnitude [44]. Umezawa et al [45] noted that nitrogen could deactivate the oxygen vacancy related states within HfO2 band gap. The absence of gap states leads to the removal of electron leakage path. Fig. 1-12 illustrates the leakage reduction mechanism of the nitrogen incorporated in HfO2 thin film. When two nitrogen (N) atoms are located nsearby the oxygen vacancy (Vo), two electrons which trapped at the Vo level are transferred to N atoms. As shown in Fig. 1-12(b), the relaxation of Hf atoms causes the elimination of Vo level state, resulting to the removal of electron path in the band gap.
1.4.2 Plasma fluorination
In recently years, the formation of an interfacial layer (IL) at HfO2/Si interface during the growth of dielectric thin film and post processing appears to be a critical issue. Because of low dielectric constant of IL, IL limits the reduction of the effective oxide thickness (EOT). The quality of interfacial layer (IL) becomes more and more important due to gate dielectric scaling. Wang et al noted that the applied electric field would be largely distributed at low-κ layer in high-κ/low-κ stack layer; as a result, the first breakdown happened in the low-κ layer [46].
Fluorine incorporation also has some improvement on electrical characteristics [47, 48]. Fluorine incorporated into dielectric layer could improve IL quality because of stronger bonding energy of Si-F bonds (5.73 eV) compared to Si-H bonds (3.18eV) [49]. Furthermore, IL at HfO2/Si interface could be suppressed by CF4 pre-treatment [50]. The hysteresis also could be suppressed by fluorine, which could be explained
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by inner-interface model as shown in Fig. 1-13 and Fig. 1-14.
1.4.2 Dual plasma treatment
There have been many studies using fluorine plasma treatment or nitrogen plasma treatment to improve the performance of the device. In this thesis, we proposed to combine two kinds of plasma treatment (CF4 pre-treatment and nitrogen post-treatment) in order to further improve the electrical characteristics and strength the reliability characteristics of Hf-based MIS capacitor and Low-Temperature Polycrystalline-Silicon (LTPS) Thin-Film Transistor (TFT).
This method that combined CF4 pre-treatment and nitrogen post-treatment is called dual plasma treatment. The purpose of dual plasma treatment is to combine the advantages of two kinds of plasma treatment. We intended to employ CF4 plasma pre-treatment in order to eliminate the low dielectric constant interfacial layer and improved the quality of silicon substrate surface. Moreover, we used the NH3 and N2
plasma post-treatment to reduce oxygen vacancies, increase the permittivity, and increase the crystallization temperature of Hf-based device.