Nobuyoshi Awaya

TiO2 anatase nanolayer on TiN thin film exhibiting high-speed bipolar resistive switching

The surface oxidized layer of a TiN barrier metal thin film grown on a Pt electrode was used as a resistive switching material. The fabricated memory cell shows bipolar resistive switching on a nanosecond order. A TiO2 anatase layer of about 2.5nm thick on TiN thin film was characterized by high-resolution scanning transmission electron microscopy. The results suggested that the high-speed resistive change was derived from the Mott transition in the TiO2 anatase nanolayer, and the obtained results could relate to the formation of filament paths previously reported in binary transition metal oxide thin films exhibiting resistive switching.

Evaluation of a copper metallization process and the electrical characteristics of copper-interconnected quarter-micron CMOS

Copper metallization was applied to quarter-micron CMOS circuits using copper chemical vapor deposition (CVD) and chemical mechanical polishing (CMP). Both the metallization process and the electrical characteristics of CMOS devices/circuits were evaluated. Process-induced metal contamination on both sides of the wafer were quantitatively evaluated and reduced to about of 10/sup 11/ atoms/cm/sup 2/ by using an optimized cleaning sequence. The ability of borophosphosilicate-glass (BPSG) to act as a copper diffusion barrier was discovered and the ability of TiN to do so was also confirmed. Electrical characteristics of n and p MOSFETs with copper interconnections were stable even after annealing at 550/spl deg/C. The leakage current of the pn junction, capacitance-voltage characteristics and time-dependent dielectric breakdown characteristics of the MOS diode indicate that the copper metallization process did not deteriorate the pn junction and the gate oxide. Normal operation of a 53-stage quarter-micron CMOS inverter ring oscillator with copper metallization was successfully achieved.

Plasma-Enhanced Chemical Vapor Deposition of Copper

Plasma enhanced copper chemical vapor deposition (CVD) using acetyl-acetonato copper as a source compound is investigated for VLSI metallization. Results show that electrical resistivity and film morphology are greatly dependent on substrate temperature, and that plasma enhancement results in smooth copper film formation. Hydrogen is essential for reducing oxygen and carbon incorporation of copper film. A smooth surface film of low electrical resistivity (1.8 µΩcm) is obtained by plasma-enhanced CVD with hydrogen carrier gas at the substrate temperatures between 200°C and 280°C. Investigation of the step coverage of the film results in the coverage of the trench being superior to that of the sputtering method.

Double-level copper interconnections using selective copper CVD

Selective copper CVD technique involving hydrogen reduction of hexafluoro acetylacetonate copper has been used to fill vias for fabricating double-level copper interconnect structure. The surface morphology of selectively deposited copper on copper substrate of the via bottom depends strongly on via opening process. A two-step via opening process consisting of an reactive ion etching of the insulating interlayer and a wet removal of the interlayer metal results in smooth copper plug formation by CVD. Double-level copper interconnect structures have been fabricated using this technique and a via resistance as low as 100 mΩ has been obtained for a 1 μ diameter via.

The Effect of Adding Hexafluoroacetylacetone on Chemical Vapor Deposition of Copper Using Cu(I) and Cu(II) Precursor Systems

The effect of adding hexafluoroacetylacetone [H(hfac)] on the characteristics of copper chemical vapor deposition (CVD) with beta-ketonate precursors was studied. A significant difference in this effect is found for a CVD system using Cu(II) and Cu(I) precursors. In the reaction system using copper(II) bis-hexafluoroacetylacetonate (Cu(hfac) 2 ) as the Cu(II) precursor, H(hfac) addition improves deposition selectivity on substrate surfaces by eliminating copper nuclei growth on the insulator. In the system using copper(I)-hexafluoroacetylacetonate trimethylvinylsilane [Cu(hfac)(tmvs)] as a Cu(I) precursor, selectivity is lost by H(hfac) addition but void formation in blanket CVD films in subquarter micron trenches has been suppressed. These results can be explained by the role of the H(hfac) species in the two deposition reactions. In the Cu(II) precursor system, H(hfac) poisons hydrogen-induced active surface sites on an insulator. In the Cu(I) system, it acts as a catalytic reagent for the disproportional reaction of Cu(hfac) and enhances nucleation at the first stage of the deposition reaction.

Carrier-gas effects on characteristics of copper chemical vapor deposition using hexafluoro-acetylacetonate-copper (1) trimethylvinylsilane

Abstract The deposition characteristics of copper chemical vapor deposition using the liquid precursor hexafluoro-acetylacetonate-copper (1) trimethylvinylsilane are investigated. The precursor supply is directly controlled by a liquid-injection system. The deposition rate and selectivity on the substrate surface strongly depend on the carrier gas. A hydrogen carrier-gas system has a higher deposition rate and lower selectivity than an argon carrier gas. The deposition rate of copper is proportional to the square root of the precursor partial pressure under reaction-rate-limited conditions at a substrate temperature below 200 °C. In these conditions, a deposition rate of up to 70 nm min −1 and an apparent activation energy of 11 kcal mol −1 has been obtained. Surface morphology and step coverage improves with decreasing substrate temperature and increasing precursor supply. A smooth film with a low resistivity of 2.0 Ω cm can be formed superconformally on the step coverage at a substrate temperature below 180 °C and a high aspect ratio vias can be filled under the optimum deposition conditions.

Resistivity and resistive switching properties of Pr0.7Ca0.3MnO3 thin films

The authors studied the relationship between electrical resistivity and resistive switching properties in various stoichiometric and nonstoichiometric Pr1−xCaxMnO3 (PCMO) (x=0.3) thin films fabricated by pulsed laser deposition. The resistivity of Pt/PCMO/Pt structured thin films depended mainly on the PCMO deposition temperature, which was related to the crystallinity of the thin films. The highest resistivity was obtained from the lowest deposition temperature (300°C) specimen, and it was amorphous, while higher temperature deposition specimens (500–800°C) showed specific crystallographic orientation depending on the deposition temperature but showed quite low resistivity. Amorphous film deposited at 350°C exhibited monopolar resistive switching when pulses were applied.

Accelerated-Deposition Rate and High-Quality Film Copper Chemical Vapor Deposition Using a Water Vapor Addition to a Hydrogen and Cu(HFA)2 Reaction System

Accelerated-deposition-rate chemical vapor deposition (CVD) of copper is studied using hydrogen reduction of bis-hexafluoro acetylacetonate copper (Cu(HFA)2). An in-situ hydrated Cu(HFA)2 formation system using gaseous H2O addition to the hydrogen carrier gas increases the deposition rate to 90 nm/min. This is about ten times larger than that of the conventional reaction system. It also improves the surface morphology and the electrical resistivity of the deposited copper film. A high-aspect-ratio via filling by selective CVD and excellent step coverage by blanket CVD are successfully obtained with this technique.

Effect of Thin-Film Texture and Zirconium Diffusion on Reliability against Electromigration in Chemical-Vapor-Deposited Copper Interconnects

The effects of film texture and zirconium diffusion on the reliability of chemical-vapor-deposited (CVD) copper interconnects were examined. The crystallographic orientation of CVD copper depends strongly on the substrate structure and pre-treatment sequences. Sputter pre-etching of TiN liner greatly enhanced the (111) orientation of sputter deposited copper film. The CVD copper grew epitaxially on the (111) oriented sputtered copper seed layer. Dominant impurities in CVD copper are oxygen and fluorine, which can be reduced by hydrogen annealing at 400°C. By improving the texture, the lifetime against electromigration extends about twice longer than that of as-deposited CVD copper film. Copper zirconium alloy wire with a surface layer of the inter-metallic compound CuZr2 further extended the electromigration lifetime due to thermal diffusion of zirconium into copper. The electromigration lifetime of this alloy wire was significantly improved with only a small increase in electrical resistivity.

Controllability of Electrical Conductivity by Oxygen Vacancies and Charge Carrier Trapping at Interface between CoO and Electrodes

Recently, the role of resistance random access memory (RRAM) is becoming extremely important in the development of nonvolatile memories. RRAM works by changing the resistance of the transition metal oxide contained in RRAM after the application of a sufficiently high voltage, however, this switching mechanism has not been fully clarified. In this study, by performing first principles calculations based on the density functional theory, we first investigate the change in the property of bulk CoO resulting from oxygen vacancies and charge carrier trapping in the vicinity of the oxygen vacancies. Next, we perform calculations for slab models of CoO in contact with Ta, W, and Pt electrodes and hence investigate the effects of oxygen vacancies at the interface between the CoO layer and the electrode layer. On the basis of the obtained results, we conclude that W is the most suitable electrode material compared with Ta and Pt.