Atomic Layer Deposition Of High Dielectric Constant Gate Dielectric

With the rapid development of microelectronics industry, feature size of metal-oxide-semiconductor-field-effective-transistor (MOSFET) as the key part of Si-based integrated circuits is scaling down at a speed of Moore's law. However, when the thickness of equivalent oxide of conventional SiO₂ gate dielectric is reduced to atomic level, the electron tunneling is becoming serious enough to endanger the stability and reliability of devices. To overcome these problems, many new processes and materials are currently under investigation. There are mainly two ways to find the suitable materials: 1) high dielectric constant (High-k) materials to replace SiO₂; 2) high carrier mobility semiconductor as substrates, such as III-V compound semiconductors, Ge, SiGe.All of the High-k materials must meet a set of criteria to perform as successful gate dielectric. A summary of the appropriate materials properties for the selection of materials for gate dielectric applications is: 1) high permittivity and barrier height; 2) thermodynamic stability on Si; 3) film morphology; 4) good interface quality; 5) process compatibility, etc. Among III-V compound semiconductors with high carrier mobility, GaAs is the leading one being studied. Although GaAs-based devices have become a mature technology they suffer from a lack of a suitable oxide that can be used in the fabrication of MOSFET devices, thereby limiting the implementation of logic circuitry. Any gate dielectric on GaAs has to be able to unpin the Fermi level and be thermodynamically stable with the semiconductor.As technology requires smaller devices, newer processes have to be developed to fabricate these devices. A promising technique currently being researched for its use in the formation of gate dielectrics is atomic layer deposition or ALD. The self-limiting chemisorption reaction of ALD allows the deposition of a material through highly uniform and conformal growth, with thickness control at the atomic layer level. As a result, the growth mechanism and surface chemistry of ALD are introduced firstly in this thesis. Al₂O₃ is then deposited by atomic layer deposition on Si and GaAs wafers using trimethylaluminum (TMA) and H₂O as precursors. As many reports indicated, the direct contact of High-k: materials and semiconductor substrates will be imperfect and have many issues, such as interface layer increasing the equivalent oxide thickness, interface states leading todegradation of devices, etc. Pretreatment of substrate surface before the deposition of High-k materials has been shown to be an effective way to improve these properties. Moreover, it is hard to grow high-quality thin films on HF-last semiconductors in the atomic layer deposition. As a result, we studied the effect of NH3 plasma surface pretreatments on interface quality during atomic layer deposition in this thesis.The growth rates of Al₂O₃ films are 1.1 A/cycle and 1.3A/cycle on HF-last and NH₃ plasma-treated Si (100) surfaces, respectively. The interlayer thicknesses increase with the growth cycles for the HF-last samples, whereas keep constant for the NH3 plasma-treated case due to the diffusion of oxygen is hindered by the initial SiO_xN_y layer. And Al₂O₃ layer densities are rather independent of the number of growth cycles in all cases, but lower than the bulk value. These low densities are probably due to carbon and hydrogen contaminants or excess oxygen in the films by incomplete oxidation of aluminum hydroxide (Al(OH)₃). These impurities are decreased with the rapid thermal annealing(RTA). In addition, the surface roughness is improved obviously after RTA at higher temperatures.Al₂O₃ thin films were also synthesized by atomic layer deposition on HF-last and NH₃ plasma-treated GaAs substrates. The thicknesses of interfacial layer are 0.9nm and 0.3 run from the HRTEM results, respectively. Effects of the NH3 plasma pretreatment on the chemical and electrical properties of the Al₂O₃/GaAs interface have been investigated through XPS and C-V measurements. XPS analyses show that an interlayer including Ga oxides and excess As is obtained from the ALD of Al₂O₃ on the HF-last GaAs, whereas an absence of GaAs oxides and the elemental As is accomplished with the interface between Al₂O₃ dielectric and NH3-treated GaAs. The result indicated that NH3 plasma pretreatment can efficiently decrease GaAs oxides and suppress the interfacial layer regrowth. The absence of GaAs oxides and elemental As are primarily responsible for the improvement in the electrical properties which is evaluated by the C-V analysis. Our results provide a key step to fabricate MOSFET based GaAs substrate.In addition, the principle of ALD leads to one monolayer deposited in each cycle. In practice, the full monolayer growth per cycle is hard to obtain. Therefore, we also investigated the initial surface reaction of atomic layer deposition of Al₂O₃, HfO₂ and ZrO₂ High-k; films on the hydroxylated GaAs surface using density functional theory except for theexperimental works. The calculation results show that both half-reactions of ALD-Al₂O₃ using TMA and H₂O as precursors are exothermic. All intermediate complexes have lower energies than the reactants through the reaction route. The byproduct CH4 can desorb from substrate spontaneously with on addition energy required. As for ALD-Hf(Zr)O₂ using Hf(Zr)Cl₄ and H₂O as precursors, the chemisorbed states have the lowest energies. As a result, there is a high probability that they will be trapped molecularly instead of dissociating to from the products. Furthermore, the energies of HCl physisorbed states are all lower than that of the dissociated products. As a result, additional energies are still necessary for their desorption to drive the reactions towards the final products, respectively. Longer HCl purges are therefore needed to complete the corresponding half-reactions over the entire substrate surface. These results are of great importance and are instructive for the application of ALD.