In this dissertation, we concentrate our effort to examine the effect of dual plasma treatment (CF4 pre-treatment and nitrogen post-treatment) on the electrical characteristics and the reliability of the Hf-based device. There are five chapters in this dissertation, and the content of each chapter are described as following.
In chapter 1, we introduce the general background of the high-κ materials,
13
including the principle of choosing high-κ materials, and the most candidate high-κ material. Then, we describe the current conduction mechanisms in high-κ dielectric, such as Schottky emission (SE), Frenkel-Poole (F-P) emission, and Fowler-Nordheim (F-N) tunneling. The motivation that new method called dual plasma treatment is launched to combine the advantages of two kinds of plasma treatment (CF4 pre-treatment and nitrogen post-treatment).
In chapter 2, we propose to combine two kinds of plasma treatment (denoted as dual plasma treatment), CF4 pre-treatment and nitrogen post-treatment, in order to achieve further improvement. We have examined the reliability properties and the current conduction mechanism of HfO2 MIS capacitor structure. First of all, the capacitance-voltage (C-V) characteristics and current-voltage (J-V) characteristics will be briefly described. Second, the frequency dispersion and constant voltage stress (CVS) characteristics of the samples will be analyzed to estimate the improvement.
Finally, current conduction mechanisms, such as Schottky emission, Frenkel-Poole (F-P) emission, and Fowler-Nordheim (F-N) tunneling will be discussed. Schottky barrier height, F-P barrier height, and F-N barrier height will be extracted.
In chapter 3, Al/Ti/ HfAlOx/Si MIS capacitor structure would be fabricated. We propose to combine CF4 pre-treatment and N2 post-treatment (denoted as dual plasma treatment) to examine interface quality and reliability properties of HfAlOx MIS capacitor.
In chapter 4, according to chapter 2 and chapter 3, we have demonstrated the dual plasma treatment on MIS capacitor with high-κ gate dielectric. In this chapter, dual plasma treatment (CF4 pre-treatment and N2 post-treatment) will be utilized on LTPS TFTs to reduce defects in poly-Si channel and HfO2 gate dielectric. The
14
electrical improvement would be studied, including the hysteresis and the I-V characteristics. The device parameters, such as Vth, S.S., Gm,μeff,Dit, and Ntrap are extracted to study the improvement effect. Furthermore, the reliability properties and mechanisms of high performance HfO2 gate dielectric LTPS-TFT with dual plasma treatment are investigated, including PBS, NBS, HCS.
Finally, in chapter 5, the summarizations of experimental results in this dissertation and future recommendations are described.
15
Table 1-1 Main high-κ gate dielectric materials with their parameters: ε is the permittivity, Eg is the band gap, CBO is the conduction band offset, and VBO is the valence band offset [23].
16
Fig. 1-1 International Technology Roadmap for Semiconductors (ITRS). Predictions of the gate oxide (SiO2) thickness for future technology generations, which were defined by the critical device size [10].
Fig. 1-2 The direct tunneling gate current vs. gate voltage with different gate oxide thickness [11].
17
Fig. 1-3 Schematic of direct tunneling through a SiO2 and the thicker high-k gate dielectric layer.
Fig. 1-4 Power consumption and gate leakage current density for a chip which has a 15 Å thick SiO2 gate dielectric compared to the potential reduction in leakage current by an alternate gate dielectric exhibiting the same EOT. Total gate area of 0.1 cm2 [3].
18
Fig. 1-5 Dielectric constant versus band gap for candidate high-κ materials [24].
Fig. 1-6 Three types of M (metal)-Si-O phase diagrams at fixed temperature and pressure: (a) SiO2 dominant, (b) No phase dominant, and (c) Metal oxide dominant [26].
19
Fig. 1-7 The relation of band offsets with carrier injection of electrons and holes in gate oxide band states.
Fig. 1-8 The band alignments of typical high-k gate dielectrics [24].
20
Fig. 1-9 Energy band diagram of Schottky emission current conduction mechanism.
Fig. 1-10 Energy band diagram of Frenkel-Poole emission current conduction mechanism.
Gate e
-E
FE
FE
CE
VGate
e
-E
FE
FE
CE
VGate e
-E
FE
FE
CE
VGate
e
-E
FE
FE
CE
V21
Fig. 1-11 Energy band diagram of Fowler-Nordheim tunneling current conduction mechanism.
Fig. 1-12 Schematic illustration of N incorporation effects :
(a) N-induced atomistic relaxation around Vo: ① Electron transfer from Vo to N atoms,② Outward movement of Hf4+ ions due to the increase in Hf4+- Hf4+
Coulomb repulsion, (b) N-induced elimination of leakage paths: ③ Vo level elevation due to the decrease in attractive Coulomb interaction fromHf4+ ions around Vo,④ removal of leakage paths due to the elimination of a Vo level [45]
Gate e -EF
EF EC
EV Gate
e -EF
EF EC
EV
22
Fig. 1-13 Inner-interface trapping model of HfO2 for sweeping voltage from (a) accumulation to (b) inversion [48].
Fig. 1-14 Inner-interface trapping model of HfO2 after CF4 plasma treatment for sweeping voltage from (a) accumulation to (b) inversion [48].
23
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