High Capacitance MIM Capacitors with Crystallized TiO₂ Films by Plasma-Assisted Atomic Layer Annealing

MIM (metal-insulator-metal) capacitors have been widely used in memory devices (e.g., DRAM) as well as an energy storage device (e.g., electrostatic capacitor) these days. In both applications, one of the most important factor that determines the performance of capacitors is the capacitance density. Therefore, research on high-k materials which have high dielectric constant are actively conducted. In particular, titanium oxide (TiO₂) films have interesting characters that the dielectric properties of the films vary greatly depending on their crystallinity and phase; the dielectric constant of TiO₂ is lower than 30 in the amorphous films, but, on the other hand, is much higher in the anatase and rutile films (anatase: 30-75 and rutile: 90-170). However, the dielectric constant and the energy bandgap have a trade-off relationship: the higher the dielectric constant, the smaller the energy gap, which results in increased leakage current. Such trade-off can be partly solved by doping a higher energy bandgap material to a high-k material (e.g., Al-doped TiO₂) or making those two materials into a laminate structure (e.g., ZAZ(ZrO₂-Al₂O₃-ZrO₂) structure of DRAM capacitors).<br/>In the meantime, atomic layer deposition (ALD) is widely used for fabricating MIM capacitors because of its advantages such as precise thickness control, high conformality, and low-temperature process (&lt;300°C), etc. However, at a low process temperature, the thermal energy which is needed for crystallization is not sufficient, so it is difficult for a film to have high crystallinity. In addition, when the dielectric films’ thickness decreases, crystallization becomes even more difficult. In this aspect, plasma-assisted atomic layer annealing (ALA), the method to promote the crystallization of film by using high kinetic energy of ions in plasma, is being actively studied. Since the energy is transmitted only to a very thin layer of the surface, unlike general thermal annealing, only the very thin layers (5 atomic layers) of the surface is crystallized, not the entire film, in ALA process.<br/>In this study, the tri-layered structure of TiO₂-ZrO₂-TiO₂(TZT) with highly crystalline-TiO₂ layers even at relatively low deposition temperature (250°C) was fabricated by using ALD and plasma-assisted ALA. TZT dielectric film was deposited on Ru electrode using plasma-enhanced ALD (PEALD) process, and by controlling the plasma exposure time, the crystallinity of TiO₂ and ZrO₂ was determined as desired; that is, amorphous ZrO₂ for leakage current suppression and highly crystalline rutile-TiO₂ for high capacitance. It was notable that even the TiO₂ film deposited on top of the ZrO₂ layer, not only that deposited on the lattice-matching Ru electrode, had high crystallinity thanks to atomic layer annealing effect. The MIM capacitor showed high capacitance density of ~50nF/mm² with high dielectric constant of &gt;100 in the combined TZT layers.