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Title: Advanced CMOS technologies (high-k/metal gate stacks) for sub-22nm node
Authors: Wu, Ling.
Keywords: DRNTU::Engineering::Electrical and electronic engineering::Microelectronics
Issue Date: 2013
Source: Wu, L. (2013). Advanced CMOS technologies (high-k/metal gate stacks) for sub-22nm node. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: A thermally grown silicon dioxide (SiO2), which forms the insulating layer in the metal-oxide-semiconductor field effect transistor (MOSFET), is considered as the heart of a MOSFET. It has been essential for the microelectronics revolution due to the following outstanding properties: high resistivity (~1015 Ω cm), excellent dielectric strength (~1×107 V/cm), large band gap (9 eV), high melting point (1713 °C), native and low defect density interface with Si (~1010 eV-1 cm-2). However, high-K/metal gate technology is now replacing conventional SiO2 (SiON)/Poly-Si gate in state-of-the-art transistors for both high performance and low-power applications to overcome the unacceptable large gate leakage current resulting from the extremely thin SiO2 thickness (<1nm), as well as Fermi level pinning, poly depletion and dopant penetration associated with Poly gate/HfO2 interface. In this report, several advanced topics in high-k/metal gate stack are studied in order to address various issues, such as band-edge metal gate work function tunning, thermal stability of metal gate with different composition and deposition methods, and novel post deposition treatment and/or deposition technique to improve high-k quality. The topics studied including: 1. Electrical and Physical Properties of Er doped HfO2 high-k dielectrics prepared by Atomic Layer Deposition A metal gate/Hf-based high-k dielectric gate stack with an appropriate work function is considered as one of the critical technology solutions for sub-45 nm complementary metal oxide semiconductor technology. One of the major problems for HfO2 is Fermi-level pinning between HfO2 and the metal gate. Recently, HfO2 incorporated with lanthanide (e.g., La, Dy, Er, etc.) received considerable attention due to its capability to tune the metal work function toward the Si conduction band-edge, enabling its n-FET application. In this work, Er-doped HfO2 high-k dielectrics (with 4 and 7% Er) were prepared by atomic layer deposition (ALD), which is more compatible with the industrial needs due to excellent process controllability and is also more suitable for sub-1 nm equivalent oxide thickness scaling due to superb scalability. With 7% of Er incorporated into HfO2, (1) the TiN metal gate work function can be modulated to a value of ~4.18 eV; (2) the thermal stability of the HfO2 film is improved, as evidenced by X-ray photoelectron spectroscopy and X-ray diffraction studies; and (3) the K-value and leakage properties of HfO2 are maintained after Er doping. 2. Thermal Stability of TiN Metal Gate Prepared by Atomic Layer Deposition or Physical Vapor Deposition on HfO2 High-k Dielectric The thermal stability of TiN metal gate with various composition prepared by either ALD or PVD on HfO2 high-k dielectric is investigated and compared by electrical (Capacitance, leakage current) and physical (X-ray Photoelectron Spectroscopy, High Resolution Transmission Electron Microscopy, and Electron Energy Loss Spectroscopy) analysis. After annealing of the TiN/HfO2 stack at 1000 oC for 30 s, it is observed that: 1) Nitrogen tends to out-diffuse from TiN for all the samples; 2) Oxygen from the interfacial layer (IL) between HfO2 and Si tends to diffuse towards TiN. PVD Ti-rich TiN shows a wider oxygen distribution in the gate stack and also a thinner IL than the N-rich sample. Besides, the oxygen out-diffusion can be significantly suppressed for ALD TiN compared to the PVD TiN samples. The work function of TiN metal gate is correlated with its thermal stability. 3. A Novel Multi Deposition Multi Room-Temperature Annealing Technique via Ultraviolet-Ozone to Improve High-k/Metal (HfZrO/TiN) Gate Stack Integrity for a Gate-Last Process ALD HfZrO high-k fabricated by novel multi deposition multi annealing (MDMA) technique at room temperature in Ultraviolet-Ozone (UVO) ambient is systematically investigated for the first time via both physical and electrical characterization. As compared to the reference gate stack treated by conventional rapid thermal annealing (RTA) @ 600 oC for 30 s (with PVD TiN electrode), the devices receiving MDMA in UVO demonstrates: 1) more than one order of magnitude leakage reduction without EOT penalty at both room temperature and an elevated temperature of 125 oC; 2) much improved stress induced degradation in term of leakage increase and flat band voltage shift (both room temperature and 125 oC); 3) enhanced dielectrics break-down strength and time-dependent-dielectric-breakdown (TDDB) life time. The improvement strongly correlates with the cycle number of deposition and annealing (D & A, while keeping the total annealing time and total dielectrics thickness as the same). Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) analysis suggest both oxygen vacancies (Vo) and grain boundaries suppression in the MDMA treated samples are likely responsible for the device improvement. Besides, the nMOSFETs with UVO MDMA show superior properties, in terms of enhanced channel electron mobility, improved immunity to biased temperature instability, and reduced gate dielectric relaxation current. This is explained by the reduction of bulk oxide trap and interface trap density because of healing of Vo after UVO MDMA annealing. The novel room temperature UVO annealing is promising for the gate stack technology in a gate last integration scheme.
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