Yasushi Nakasaki

Electromigration in a single crystalline submicron width aluminum interconnection

Electromigration properties in a single crystalline submicron width aluminum interconnection formed on Si(111) have been examined by resistance change measurements and in situ observations using scanning electron microscopy. It was observed that single crystalline aluminum has an extremely high resistance to electromigration‐induced open circuit failures, when compared to polycrystalline copper and aluminum. The mechanism for the high resistance is considered to be a large activation energy, resulting from lattice diffusion. A tendency for void formation to become parallel to the longitudinal direction of the interconnection assisted the life time prolongation.

Water absorption properties of fluorine-doped SiO2 films using plasma-enhanced chemical vapor deposition

The water absorption properties of a PE-CVD (plasma-enhanced chemical vapor deposition) fluorine-doped SiO2 film with a low dielectric constant were studied. It was concluded that highly stable F-doped SiO2 film was obtained at F contents from 2.0% to 4.2% (3.2≤k≤3.6) using high-density plasma CVD. However, at F contents higher than 4.2% (k<3.2), the amount of water absorption was markedly increased due to the presence of Si–F bonds, such as Si(–F)2 bonds, which are highly reactive with water. On the other hand, water absorption was observed at every F content for conventional plasma CVD films. Through gas phase component analysis and investigation of the incident ion energy distribution using a quadrupole mass spectrometer, it was confirmed that a high efficiency of gas dissociation and high-energy ion bombardment are the keys to obtaining high-quality films with a high resistance to water absorption.

Self‐aligned passivation on copper interconnection durability against oxidizing ambient annealing

A self‐aligned niobium (Nb) passivation method has been developed in order to improve the stability of copper (Cu) in an oxidizing ambient. A Cu/Nb/SiO2/(100)Si structure was annealed between 400 and 850 °C for 30 min in a gas mixture of H2 and N2. The underlying Nb diffused to the Cu surface and turned into its nitride at 750 °C. The surface Nb nitride layer acted as a passivation layer against oxidation. The passivated Cu was found to retain its resistivity of 2.0 μΩ cm even after oxidation at 400 °C for 30 min in a dry oxygen ambient.

Improvement of charge-to-breakdown distribution by fluorine incorporation into thin gate oxides

This paper reports on the effect of fluorine incorporation on gate-oxide reliability, especially the spatial distribution of charge-to-breakdown (Q/sub BD/). Fluorine atoms were implanted into gate electrodes and introduced into gate-oxide films by annealing. Excess fluorine incorporation increased the oxide thickness and degraded not only the reliability of Si/SiO/sub 2/ interfaces but also dielectric-breakdown immunity. However, it was found, for the first time, that appropriate fluorine incorporation into gate-oxide films could dramatically improve Q/sub BD/-distribution tails in Weibull plots, while maintaining both Si/SiO/sub 2/ interface characteristics and average Q/sub BD/ values. The experimental result for a depth profile of fluorine atoms indicated that fluorine atoms are located dominantly at the two interfaces of the gate-oxide film. In addition, the results of infrared (IR) absorption analysis indicated that the strain of SiO/sub 2/ structures is reduced with increasing fluorine doses. We proposed that both strain release and restructuring of the SiO/sub 2/ network by fluorine incorporation are responsible for improving the Q/sub BD/ of weaker oxide films.

High aspect ratio hole filling by tungsten chemical vapor deposition combined with a silicon sidewall and barrier metal for multilevel interconnection

A newly developed processing for high aspect ratio hole filling by tungsten chemical vapor deposition, combined with a Si sidewall technique and resist etch back is proposed. A high aspect ratio hole (around 3) was completely filled with W and W‐Si alloy without voids. It is also proposed to interpose a TiN/TiSi2 layer between W and Si, in order to suppress rapid silicidation of W at high temperatures above 800 °C. Silicidation rates for W/TiN/TiSi2/Si systems were 2–2.5 orders of magnitude lower than W/Si systems. Electrical contact resistivity was kept to be lower than 1×10−5 Ω cm2 even after 900 °C annealing by suppressing rapid silicidation of W.

Tungsten/titanium nitride low-resistance interconnections durable for high-temperature processing

Newly developed W/TiN bilayer interconnections, able to withstand high‐temperature processing after metalization, are described. Minimum TiN thickness was found to be 100 A in order to retain adhesion between W and SiO2, while being a reaction barrier between W and Si. Silicidation rate in the W/TiN/Si system was reduced by about 4.5 orders of magnitude, in comparison with a W/Si system at 950 °C. TiN/p+‐Si contact resistivity on the order of 10−6 Ω cm2 was realized by boron ion implantation through a TiN layer and rapid thermal annealing for boron activation at temperatures above 1000 °C.

Structural Studies of High-Performance Low-k Dielectric Materials Improved by Electron-Beam Curing

With the use of a newly developed electron beam (EB) curing process, an advanced methylsilsesquioxane (MSQ) low-k dielectric (LKD) film of k=2.9 was developed. It is noteworthy that the EB curing process can drastically improve the mechanical strength of LKD film and reduces the thermal budget without increasing the k value. The X-ray absorption fine structure (XAFS) study on the LKD was conducted to clarify the structural change upon EB curing. The structure of the film was compared with those of two different types of other MSQ films, the ladder-network structure and the random-network structure, and a chemical vapor deposition (CVD) film. The Si–O–Si bond angle and Si–O (Si–C) bond length were determined by fitting the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra. Si–O–Si bond angle of LKD film was found to be between those of the ladder and the random structure, which are 135° and 147°, respectively. The X-ray absorption near-edge structure (XANES) spectra of LKD film revealed two broad features corresponding to a mixture of the two structures. In contrast, Si–O–Si angles of the EB-cured LKD film and the CVD film were similar, and the XANES features of both films were almost identical with those of the random structure. The electronic structure as determined from XANES spectra was also discussed by comparing three-dimensional-linkage models obtained by ab initio calculations. We confirmed that the EB curing process of LKD film causes a drastic structural change. The change from the mixture of ladder and random structures to the completely random structure was caused by C–H bond breaking followed by the formation of new polymer-like clusters with C–C bonds.

Novel fabrication process to realize ultra-thin (EOT = 0.7nm) and ultra-low leakage SiON gate dielectrics

We achieved 0.73 nm EOT, 88 A/cm/sup 2/ J/sub g/, and 92% G/sub m/ of that for SiO/sub 2/ by just oxidation of SiN films. In addition, we achieved further improvement of EOT-J/sub g/ characteristics by re-nitridation process with minimum degradation of G/sub m/. EOT of 0.70 nm and J/sub g/ as 95 A/cm/sup 2/ is realized with superior suppression of boron penetration (/spl Delta/V/sub th/=0.04 V).

High-Performance SiOF Film Fabricated Using a Dual-Frequency-Plasma Chemical Vapor Deposition system

With the use of a newly developed dual-frequency-plasma chemical vapor deposition (DFP-CVD) system, an advanced SiOF film of k = 3.4, which exhibits excellent resistance for moisture absorption, was developed. The physical and chemical properties of the SiOF film were compared to those of typical SiOF films deposited by both conventional high-density-plasma CVD (HDP-CVD) and plasma-enhanced CVD (PE-CVD) systems, with the same k value. The DFP-CVD SiOF film appears to be significantly superior to the HDP-CVD SiOF film, as revealed by the following results. The moisture absorption rate measured by thermal desorption spectroscopy (TDS) (after 4 days of air exposure) is about 5 times lower, the hardness was 1.8 times higher, and the hygroscopicity (after 1 hour of boiling) was 2.6 times lower. These results confirm that the DFP-CVD SiOF film is applicable to Al and Cu interconnect structures for devices of the 130 nm scale and beyond.

Impact of metal gate electrode on Weibull distribution of TDDB in HfSiON gate dielectrics

The slope parameter of Weibull plot of Tbd, β, strongly depends on gate electrode material for metalgate/HfSiON gate stacks in n-FETs. Furthermore β of Tbd under bipolar stress is larger than that under DC stress. From these results, it is found that the balance of injected carriers is strongly related to β in terms of the origin of large β for metal-gate/high-k.