Isamu Asano

Ruthenium Films Prepared by Liquid Source Metalorganic Chemical Vapor Deposition Using Ru(dpm)3 Dissolved with Tetrahydrofuran Solvent

Ruthenium films were prepared on SiO2/Si substrates by liquid source metalorganic chemical vapor deposition (LSMOCVD) using Ru(C11H19O2)3 (tris (dipivaloylmethanato) ruthenium: Ru(dpm)3) precursor dissolved in C4H8O (tetrahydrofuran: THF) solvent as a liquid source. The deposited films consisted of metallic ruthenium in single phase under all deposition conditions studied and could be directly deposited on SiO2 without having a distinct incubation time. We elucidated that the deposition mechanism was controlled by an oxygen-controlled surface reaction on the basis of its strong dependence on the oxygen concentration above 300°C. Good step coverage of over 70% for film deposited on a concave structure with an aspect ratio of ten was obtained in the oxygen-controlled surface reaction region. Therefore, the Ru films deposited by LSMOCVD using Ru(dpm)3 dissolved in THF solvent are effective for use as capacitor electrodes in gigabit dynamic random access memories (DRAMs).

Low temperature metal-based cell integration technology for gigabit and embedded DRAMs

An advanced memory cell structure with poly/metal word lines and metal bit lines is proposed. The thermal processes are carefully designed for the metal-based cell to be consistent with narrow gap filling, wet cleaning, planarity, and the contact process. The extremely low temperature process also helps suppress the short channel effect of the MOS transistors. The fully self-aligned contact and via-hole technology provides the minimum memory cell area. This technology is promising for future gigabit DRAMs and embedded DRAMs.

Properties of Ruthenium Films Prepared by Liquid Source Metalorganic Chemical Vapor Deposition Using Ru(EtCp)2 with Tetrahydrofuran Solvent.

Pure Ru thin films were deposited on SiO2/Si substrates, using Ru(C2H5C5H4)2 with C4H8O (THF) solvent, by liquid source metalorganic chemical vapor deposition. The Ru single phase could be obtained under all growth conditions. For temperatures below 350°C, deposition occurred in the surface reaction region because the kinetics increased exponentially as a function of the deposition temperature with an activation energy of about 1.1 eV. Above 350°C, the deposition was controlled by the mass transport process. Step coverage for the Ru thin films deposited at 300–325°C with an aspect ratio of 3.3 was about 100%. The Ru thin films grown at 325°C showed a dense and smooth microstructure and had resistivities of <100 µΩcm.

Combining Ta2O5 and Nb2O5 in Bilayered Structures and Solid Solutions for Use in MIM Capacitors

Binary systems of Ta 2 O 5 and Nb 2 O 5 are investigated as dielectric materials that allow low-temperature fabrication and are suitable for use in metal-insulator-metal (MIM) capacitors. This investigation shows that a Nb 2 O 5 film is crystallized in a hexagonal symmetry at a temperature of less than 500°C, which is more than 250°C lower than the corresponding temperature for a Ta 2 O 5 film. Using Nb 2 O 5 as a nucleation layer lowers the crystallization temperature for Ta 2 O 5 to less than 500°C. This is because hexagonal Ta 2 O 5 is stably formed on the hexagonal Nb 2 O 5 by heteroepitaxy. A Ta 2 O 5 /Nb 2 O 5 bilayer with a high dielectric constant of 50 can therefore be prepared by a 500°C fabrication process. Solid solutions of Nb 2 O 5 and Ta 2 O 5 are also investigated. The substitution of 10% Nb 2 O 5 in Ta 2 O 5 to form (Ta 0 . 9 Nb 0 . 1 ) 2 O 5 was found to decrease the temperature of crystallization from 700°C for pure Ta 2 O 5 to 550°C. The dielectric constant of the (Ta 0 . 9 Nb 0 . 1 ) 2 O 5 film is approximately 20% greater than that of the Ta 2 O 5 film. This is because the substitutional Nb 2 O 5 stabilizes the material in the hexagonal phase, which corresponds to a high dielectric constant, at lower temperatures.

Formation and oxidation properties of (Ti1−xAlx)N thin films prepared by dc reactive sputtering

We prepared (Ti1−xAlx)N thin films by dc reactive sputtering using alloy targets and investigated their oxidation properties. The oxidation rate of the (Ti1−xAlx)N film decreases with increasing aluminum content because Al2O3, with a low oxygen diffusion coefficient, is segregated at the film surface. In particular, the phase separation into a double layer of Al2O3 and TiO2 is more noticeable at high temperature. On the other hand, at an initial stage of oxidation, a mixed oxide of titanium and aluminum (>5 nm thick) is formed after oxidation at 550 °C for 1 min. This mixed-oxide formation is a result of insufficient growth of the Al2O3 layer, which causes fast oxygen diffusion and hence high oxidation rate. We thus conclude that (Ti1−xAlx)N is difficult to use as a barrier metal for metal-insulator-metal capacitors as long as the oxidation resistance is based on the growth of a protective Al2O3 layer on the film surface.

Niobia-stabilized tantalum pentoxide (NST) - novel high-k dielectrics for low-temperature process of MIM capacitors

A novel dielectric - niobia-stabilized tantalum pentoxide (NST) - has been developed and applied in MIM capacitors. A 10% NST film has the effect of decreasing crystallization temperature by more than 150/spl deg/C and enhancing permittivity by 20%, while the leakage current stays low, compared with the properties of conventional tantalum pentoxide. These improved properties result from the fact that the doped niobia stabilizes a low-temperature hexagonal phase with high permittivity.

1.5 nm equivalent thickness Ta/sub 2/O/sub 5/ high-k dielectric with rugged Si suited for mass production of high density DRAMs

A 1.5 nm equivalent thickness Ta/sub 2/O/sub 5//rugged Si capacitor is demonstrated for mass production of high density DRAMs (Dynamic Random Access Memories). More than 10 years breakdown lifetime of CVD-TiN/Ta/sub 2/O/sub 5//rugged Si capacitor is experimentally clarified for the first time. An excellent pause refresh property is confirmed by using 0.40 mm/sup 2/ DRAM cell as well. This system is applicable to a 0.16 mm/sup 2/ cell for production.

A conformal ruthenium electrode for MIM capacitors in Gbit DRAMs using the CVD technology based on oxygen-controlled surface reaction

We have developed a novel CVD-Ru technique, clarified the growth mechanism and fabricated BST capacitors. The growth mechanism is dominated by the surface reaction which is rate-determined by the oxygen supply. Well-tuned conditions enable fabrication of any type of storage node: a concave type with a uniform 20-nm film thickness and a pillar type from a buried film. The electrode/BST interface is degraded by the reduction-oxidation reaction during the Ru-CVD, but post-annealing restores the ideal I-V characteristics.

Novel Storage-Node Contacts with Stacked Point-Cusp Magnetron Sp-TiN Barrier for Metal–Insulator–Metal Ru/Ta2O5/Ru Capacitors in Gigabit Dynamic Random Access Memories

We have investigated a mechanism for increasing the contact resistance (Rc) of the ruthenium (Ru) and the titanium nitride (TiN) barrier during the tantalum oxide (Ta2O5) oxidation annealing in the metal–insulator–metal (MIM) capacitors. It has been found that controlling the atomic ratio of Ti to N (Ti/N ratio) is a key to keeping the contact resistance low. Controlling the Ti/N ratio is easy for the sp-TiN, while it is difficult for the chemical-vapor-deposited TiN. On the basis of this result, we have proposed a novel Ru/TiN contact with a stacked-barrier structure fabricated by using the point-cusp magnetron (PCM) sputtering method. We have applied this structure to the MIM-Ru/Ta2O5/Ru capacitor in a gigabit DRAM with 0.10 µm design. In this capacitor, we have obtained the contact resistance of 10 kΩbit and the capacitor leakage current of 10-17 A/bit in the range of -1 to +1 V.

Oxidation-resistant amorphous TaN barrier for MIM-Ta/sub 2/O/sub 5/ capacitors in giga-bit DRAMs

We demonstrate that an amorphous TaN layer with no grain boundaries shows a good oxidation-resistant performance after forming and annealing the Ta/sub 2/O/sub 5/ dielectric of MIM capacitors for DRAM applications at 550/spl deg/C in O/sub 2/ ambient. We fabricated an MIM-Ta/sub 2/O/sub 5/ capacitor with a concave-type Ru storage node on the TaN barrier metal. This showed a contact resistivity of 0.27 k/spl Omega//spl middot//spl mu/m/sup 2/, a capacitance of 20 fF/bit, and a leakage current of 0.9 fA/bit (-1 to 1 V). We further fabricated a crown-type Ru electrode to demonstrate scalability to 0.10 /spl mu/m design rules.