Band Engineering of Ru/Rutile-TiO₂/Ru Capacitors by Doping Cobalt to Suppress Leakage Current

Abstract

A basic process to fabricate Ru/rutile-Co-doped TiO 2 /Ru capacitors for 20 nm-technology generation of DRAMs and beyond was developed. The aim of this study is that the basic process provides foresights into EOT and leakage-current density requirements. We chose rutile-TiO 2 for the insulators to meet the requirement of relative permittivity and Ru to suppress leakage-current density. We found the as-deposited lower electrode of Ru crystallized into the rutile phase of TiO 2 with a relative permittivity of 109 due to the similarity of crystal structures and lattice constants between rutile-TiO 2 and rutile-RuO 2 , which was generated by oxidizing Ru with ambient oxygen. Furthermore, we confirmed that doping elements that had a small-ionic radius, such as Co, decreased leakage-current density. The dependence of the leakage current of Ru/Co-doped-TiO 2 /Ru capacitors on temperature and analysis of the band structure with X-ray photoelectron spectroscopy revealed that leakage-current density is determined by a balance between thermionic current and tunneling current through the Schottky barrier. Through calculations using a theoretical equation for these currents, the optimum percentage of Co was estimated to range from 0.3-0.6 % to meet requirement of leakage current for 20 nmDRAMs. The major requirements of memory-cell capacitors for dynamic random access memories (DRAMs) have been set for equivalent oxide thickness (EOT) and leakage-current density. Capacitors need capacitance of 25fF regardless of what generation they are to avoid reading error.1 Less EOT is necessary to maintain capacitance with shrunken memory-cell areas. Twenty-nanometer DRAMs require capacitors with EOTs of less than 0.45 nm.1 Therefore, the permittivity of insulators must be larger than 60 assuming their physical thickness is 7 nm.1 The leakage-current density, on the other hand, needs to be less than 10 −7 A/cm 2 regardless of the generation when 1 V is applied between electrodes.1 This requirement is to maintain a refresh time as long as 64 ms.Major candidates for insulators fulfilling target permittivity are rutile-TiO 2 and perovskite-SrTiO 3 .1 We face difficulties with deposition despite the potential large permittivity of SrTiO 3 . Atomic layer deposition (ALD) is necessary to fabricate DRAM capacitors due to their high conformity and ability to control thickness. Although their precursors needs vapor pressures of more than 100 Torr at deposition temperatures around 200-400• C, the vapor pressures of strontium such as Sr(thd) 2 are insufficient being less than 1 Torr.2 Therefore, it is still difficult to commercialize SrTiO 3 . However, there are precursors of TiO 2 such as titanium tetraisopropoxide (TTIP) that have vapor pressures higher than that of strontium. Thus, TiO 2 is the most likely material for the capacitor insulators of 20 nm-technology generation of DRAMs and beyond.Insulators with large permittivity tend to have smaller band gaps. 3 The candidate material for electrodes should have a large work function, which is kn...

Influence of growth temperature on the structure and electrical properties of high‐permittivity TiO2 films in TiCl4‐H2O and TiCl4‐O3 atomic‐layer‐deposition processes

“…In previous works, very high‐ k values for SrTiO 3 have been measured but the growth process of this material in the proper stoichiometry by ALD is still quite complicated. Fortunately, high k values have been obtained also for rutile‐phase TiO 2 that can be synthesized by using more robust methods. For this reason, TiO 2 thin films with rutile structure are considered as promising candidates for DRAM capacitor dielectrics by the International Technology Roadmap for Semiconductors (ITRS) .…”

“…The further reduction of J L of these films, particularly at lower physical thicknesses of the dielectric layer, is still of great interest. Doping of TiO 2 with metals like Al or Co is one possible approach for that . Another way is to reduce the concentration of defects and/or undesirable impurities.…”

Continuous and Conformal Lithium Titanate Spinel Thin Films by Solid State Reaction

“…The band diagram is shown schematically in Figure 8 according to the metal work function, conduction band edge energies and bandgap summarized in Table S2. [30][31][32][33][34] Indeed, as the electrode materials are semiconductor materials, a proper ohmic contact between metal current collector and semiconductor is important and often overlooked. Interestingly, for LTO the band edges shift down in energy during lithiation (reduction of Ti(IV) to Ti(III)) while maintaining similar bandgap energy.…”

Leakage Current Control of SrTiO3 Thin Films through Al Doping at the Interface between Dielectric and Electrode Layers via Atomic Layer Deposition

“…Therefore, the thinner films are prone to have a high leakage current, especially due to the pinning of the Fermi level at a position close to the conduction band edge ( E c ) within the E g at the interface with the electrode. Another high‐ k dielectric thin film, rutile‐structure TiO 2 , also suffers from a similar problem . It was found, however, that Al doping, which acts as the acceptor in TiO 2 , could decrease the J level by almost six orders of magnitude at a t ox of ≈0.5 nm, by moving the Fermi‐level pinning position near the middle point of E g .…”