Ion Assisted Deposition Of HfO2 Thin Films For CMOS Gate Dielectric Applications

Abstract

The scaling down of Complementary Metal Oxide Semiconductor (CMOS) transistors to sub-100nm requires replacement of conventional Silicon dioxide layer with high dielectric constant (K) material for gate dielectric. Among the various high-K dielectrics that have been studied, HfO2 is found to be a promising candidate because of its high dielectric constant (~25), large band gap (5.68 eV), thermodynamic stability and good interface with Si. The HfO2 films have already been deposited using different growth techniques such as Atomic layer Deposition (ALD), Metalorgonic Chemical Vapor Deposition (MOCVD) and Pulsed Laser Deposition (PLD). Ion Assisted Deposition (IAD) is a novel technique that has been successfully employed to produce optical coatings of required quality. This growth technique presents many advantages over the other techniques such as formation from solid oxide sources, low growth temperatures (25-3000C) and film densification by ion bombardment. Hence this technique has been used to prepare HfO2 films in the present investigations. This thesis presents the structural, optical and electrical properties of HfO2 thin films prepared by Ion assisted deposition (IAD). The suitability of Ion assisted deposition process and the importance of investigations on the influence of process parameters on the film characteristics have been brought out in the process parameters-structure-composition and properties correlation presented in this thesis. The aim of this work is to process and characterize HfO2 films and investigate the influence of process parameters on the structure, composition and properties of the films to identify their suitability for CMOS gate applications. HfO2 films were deposited on p-type Si (100) wafers by Ion assisted deposition in an electron beam evaporation (Leybold,L-560) system. Pre-bombardment of the substrates with Argon ions has been done to remove any native oxide layer formation on Silicon by using a hallow cathode ion source (DENTON VACUUM CC103). During the film deposition a collimated oxygen ion beam, generated from the ion source is directed towards the substrate. The oxygen ion current is controlled by adjusting the voltage applied to the ion source and the oxygen flow through the ion source. The oxygen ions bombard the film as it grows and in that process improves its packing density as well as its stoichiometry. Keeping the deposition rate and thickness constant, HfO2 films have been deposited by varying Ion Current, Ion energy and substrate temperature. MOS capacitors were fabricated with Aluminum as gate electrode deposited by thermal evaporation. Ellipsometry techniques have been used to measure the optical thickness of the films. The interfacial layer (IL) formed at the HfO2/ Si interface was investigated by using Fourier transform Infrared spectroscopy (FT-IR). The structural characterization was carried out by X-ray diffraction technique. The high frequency capacitance-voltage and DC leakage current characteristics were measured to analyze the electrical characteristics of MOS capacitors. The effect of post deposition annealing (PDA) of the films at 600°C and 700ºC in Forming Gas (15%H2+85%N2) ambient and Post metallization annealing (PMA) at 400ºC in the same ambient was also investigated to observe the changes in electrical characteristics. The initial step of this work was to compare the characteristics of the films deposited by reactive evaporation and Ion assisted deposition which confirmed the superiority of the quality of IAD coatings and justified the need to proceed further with a more detailed study on the influence of various parameters on the properties of IAD coatings. HfO2 films deposited on substrates maintained at 1000C exhibited better structural, Optical and Electrical properties. The leakage current in these films were lower which has been attributed to silicate free interface as confirmed by XRD studies. Investigations on films deposited with oxygen ion beams of different currents in the range 20 to 40mA indicated that the films deposited at 20mA ion current showed better electrical properties. Better stoichiometry of these films as indicated by FT IR studies was one of the reasons for their improved performance. Annealing of these films at 6000C and 7000C in FGA medium resulted in creation of silicates and silicides at the interface thereby increasing the leakage currents and degraded the film properties. The films deposited with oxygen ion beams generated with a driving voltage 265V showed better structural and optical properties with silicate free interface compared with low and high driving voltages. Among all the films, the maximum dielectric constant of about 21.9 with a minimum EOT of 5.5 nm corresponding to a film deposited at ion current 20mA with PMA 400°C in FG ambient for 20minites is achieved. The lowest value of interface charge density achieved is 2.7 x1012 per cm-2 eV-1 corresponding to the sample deposited at substrate temperature 100°C with deposition rate of 0.5Å/sec followed by post metallization annealing at 400°C in forming gas for 20minutes. The range of Dit values that were obtained are varying from 2.7x 1012 – 16.7x1012 cm-2eV-1.It was also found that, the samples deposited at higher ion currents show lower Dit values than the samples deposited at lower ion currents. From the I−V analysis, the leakage current density is found to be comparatively less in IAD than in reactive evaporation. Leakage current increases with increase in substrate temperature and the same trend is observed with annealed films also. The lowest leakage current density of 1.05x10–8 A/cm2 at a gate bias of 1V was observed in the films deposited at substrate temperature 1000C. The present thesis focused on the suitability of the Ion Assisted deposition process for the preparation of HfO2 films for high-K gate dielectric application and the importance of investigations on the influence of process parameters on the film characteristics.