Characteristics of titanium dioxide thin film depending on the seed layer and post-treatment

Abstract

Recently, scaling down of memory devices is progressing rapidly. To response scaling down, research to change the structure of cell transistors and capacitors has been continuously conducted for high integration of DRAM. However, there is a limit to improving the integration by changing the structure of the cell. Therefore, researchers are conducting research to reduce the size of the cell itself. However, as the size of the cell decreases, the thickness of the thin film constituting the semiconductor device becomes thinner, and thus faces many challenges. Typically, as the dielectric thin film constituting the device becomes thinner, there is a problem in that the leakage current increases. Therefore, in order to reduce leakage current, the thickness of the dielectric film must be maintained above a certain level. However, if the thin film thickness is kept thick, a problem arises that the capacitance value becomes small and the size of the device increases. Therefore, research on high-k metal oxide thin films is being conducted to produce highly integrated device and deposit thin films with low leakage current and high capacitance. As a next-generation high-k thin film, thin films such as ZrO2, HfO2, and TiO2 are in the spotlight. These materials have the advantage of having a higher dielectric constant compared to the SiO2 thin film, and thus the capacitance value is maintained even when deposited over a certain thickness. Among these materials, research on TiO2 thin films is being actively conducted. TiO2 thin film can be deposited using ALD process, and rutile structure TiO2 thin film has a high dielectric constant of 80 or more. However, the TiO2 thin film has several problems. In order to deposit a rutile structure TiO2 thin film, heat treatment of 700 ℃ or higher is required after the TiO2 thin film is deposited. Therefore, it is difficult to apply it to the semiconductor industry. In addition, the TiO2 thin film has a low energy band gap. Therefore, it has a high leakage current value because it has a small band-offset value due to low bandgap energy. In addition, it has a high leakage current value due to oxygen vacancy present in the TiO2 thin film. Currently, many research teams are conducting research on reducing leakage current through the Al and Si doping process in the TiO2 thin film, but doping process has the disadvantage of lowering the crystallinity of the TiO2 thin film. Additionally, when depositing TiO2 thin films in research institutes and semiconductor industry, halide Ti precursor is used for deposition. In this case, the halide material such as Cl remains in the thin film, exacerbating the electrical properties, and the thin film and chamber are corroded by the by-product. In this study, a TiO2 thin film deposition process was established using a new metal organic Ti precursor (halide-free), and leakage current was reduced by controlling the oxygen vacancy concentration in the thin film through annealing & post-plasma treatment process. Additionally, we developed a process for depositing rutile structure TiO2 thin films even at low temperatures through SnO2 seed layer and low-temperature annealing processes. First, a TiO2 thin film deposition process was established using the atomic layer deposition method and metal organic Ti precursor & H2O reactant. By analyzing TiO2 thin film thickness according to process conditions using ellipsometry, process conditions such as precursor feeding time, reactant feeding time, and ALD temperature window could be established. In addition, it was confirmed that a high-quality TiO2 thin film without impurities was deposited through XPS analysis. Second, a post-plasma treatment process was performed to obtain low leakage current and crystallinity of the deposited TiO2 thin film. In the past, elemental doping processes such as Si and Al were mainly studied to reduce the leakage current of the TiO2 thin film, but this process has the disadvantage of reducing the crystallinity of the TiO2 thin film. XPS analysis was used to analyze the effect of O2 plasma post-treatment, and as a result, it was confirmed that the oxygen vacancy concentration in the TiO2 thin film decreased as the O2 plasma power increased. As the oxygen vacancy concentration in the TiO2 thin film decreased, the crystallinity of the TiO2 thin film was improved, and it was confirmed that the dielectric constant value increased and the leakage current value decreased. XPS, AFM, GIXRD, TEM, and I-V/C-V analysis were used to investigate the properties of the as-dep TiO2 thin film and O2 post plasma treated TiO2 thin film. Third, in the case of post-plasma treatment, it is difficult to apply to high aspect ratio devices. Therefore, the properties of the TiO2 thin film were improved through the post-annealing process applicable to high aspect ratio devices. Post annealing process was carried out at 400 ℃, and the annealing process was carried out in vacuum, N2, O2, and air atmospheres. XPS analysis was used to analyze the oxygen vacancy concentration in the thin film according to the atmosphere of the annealing process. In addition, as oxygen vacancy decreased, the dielectric constant of the TiO2 thin film increased, and it was confirmed that the leakage current decreased. XPS, GIXRD, AFM, and I-V/C-V analysis were used to investigate the properties of TiO2 thin films according to the annealing process conditions. Finally, a low-temperature rutile structure TiO2 thin film deposition process was developed to increase the dielectric constant of the TiO2 thin film. In this study, a SnO2 thin film was deposited as a seed layer before TiO2 thin film deposition. After the SnO2 thin film was deposited, the TiO2 thin film was deposited, and then a 400 ℃ annealing process was performed to obtain a rutile structure TiO2 thin film. In addition, it was confirmed that the leakage current was reduced because the 400 ℃ annealing process was performed during the rutile structure TiO2 thin film deposition process. XPS, GIXRD, TEM, AES, and I-V/C-V analysis were used to confirm the properties of the deposited rutile structure TiO¬2 thin film.