Jien-Wei Yeh

High-Entropy Alloys: A Critical Review

High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.

Criterion for Sigma Phase Formation in Cr- and V-Containing High-Entropy Alloys

Formation of the σ phase has been observed in quite a few high-entropy alloys (HEAs) recently. The σ phase significantly enhances the hardness of the alloys but reduces their ductility. Thus, controlling the formation of σ phase through proper design is critical for HEAs. However, theories to predict the σ phase formation based on HEA composition are still not available. Here, we demonstrate that the σ phase formation is directly related to the valence electron concentration (VEC) of the alloy. The σ-phase-forming VEC range was systematically studied and revealed. Such finding is of crucial importance to the future design of HEAs.

Electroless Copper Deposition for Ultralarge-Scale Integration

The characteristics of electroless copper plating on different substrates of TiN/SiO 2 /Si, Cu seed /Ta/SiO 2 /Si, and Cu seed /TaN/SiO 2 /Si have been investigated. Continuous copper films with good surface morphology are obtained, and hydrogen-induced blister formation is inhibited by optimizing plating solution and conditions. Surface roughness of the electrolessly plated copper films increases with increasing film thickness, and the average roughness is 11 nm at a film thickness of 1 μm on Cu seed /TaN/SiO 2 /Si substrate. Conformal copper deposition with excellent step coverage completely fills deep subquarter-micrometer features of high aspect ratios up to five. Copper growth orientation depends on the underlayer structure. A copper film with strong (111) texture is plated on the (111) textured copper seed layer of Cu seed /TaN/SiO 2 /Si substrate, while no preferred orientation is found on the other substrates. After thermal annealing at 400°C in N 2 /H 2 for 1 h, Cu(111) texture is enhanced in all systems. By thermal annealing, defects in the plated copper are reduced, and the electrical resistivity of the plated copper is lowered to 1.75 μΩ cm at room temperature.

Thermally stable amorphous (AlMoNbSiTaTiVZr)50N50 nitride film as diffusion barrier in copper metallization

Results on copper metallization diffusion barriers using high-entropy alloy (HEA) nitride are reported. The HEA nitride (AlMoNbSiTaTiVZr)50N50 is amorphous in the as-deposited state and remains its noncrystallinity up to a high temperature of 850°C. To evaluate its diffusion barrier characteristics, Cu∕(AlMoNbSiTaTiVZr)50N50∕Si test structures were prepared and annealed under 750–900°C for 30min. The results show that the current nitride prevents the reaction between Cu and Si before its failure at 900°C. The outstanding barrier performance and high thermal stability of amorphous structure are suggested to originate from multiprincipal-element effects.

Sn/Pd Catalyzation and Electroless Cu Deposition on TaN Diffusion Barrier Layers

The catalyzation of TaN/SiO 2 /Si substrates was carried out by immersion in SnCl 2 /HCl and PdCl 2 /HCl solutions for electroless Cu deposition. The sizes and morphologies of the catalytic sites on the TaN layers were found to be a function of catalyzation conditions, including solution temperature, immersion time, and the surface oxides. The appropriate formula for catalyzation was obtained by considering both the quality and efficiency. The catalytic sites were composed of Sn and Pd, and the ratio of Sn/Pd was about 1.3. During electroless Cu deposition on the catalyzed TaN/SiO 2 /Si substrates. Cu nuclei first formed at the catalytic sites in the early stage, gradually agglomerated into dense islands, and finally merged to continuous deposition films. The Cu films were uniformly and smoothly deposited with a surface roughness of 6.2 nm under a film thickness of 210 nm. The lowest electrical resistivity of the Cu films was 2.5 μΩ cm, and the residual resistivity contributed to the participation of Sn-Pd catalyst and internal defects. Good gap-filling capability of electroless Cu deposition on Sn/Pd catalyzed, patterned substrates exhibited its high potential to act as a seed layer for Cu electrodeposition and even to completely fill submicrometer gaps in ultralarge-scale integrated metallization.

Effect of substrate bias on the structure and properties of multi-element (AlCrTaTiZr)N coatings

Multi-element (AlCrTaTiZr)N coatings are prepared by reactive RF magnetron sputtering. The influence of substrate bias (0 to −200 V) on the deposition rate, composition, structure and mechanical properties of these coatings is investigated. A reduction in both deposition rate and Al concentration is observed with increasing substrate bias. The grounded substrate is found to possess a columnar structure. The columnar structure is not so apparent in the denser coatings deposited at an applied substrate bias of −150 V. Furthermore, a minimum in coating roughness is found to occur at the intermediate substrate bias of −100 V. All the (AlCrTaTiZr)N coatings appear to have a single FCC structure from XRD analysis. Furthermore, it is determined from XRD that there is an increase in both (111) peak intensity and grain size, and a decrease in the lattice strain, for increasing substrate bias. The residual stress, hardness, elastic modulus and toughness of the coatings are found to be greatly enhanced by the substrate bias. The highest hardness and elastic modulus of approximately 36 Gpa and 360 GPa, respectively, were obtained at a substrate bias of −150 V.

Displacement Activation of Tantalum Diffusion Barrier Layer for Electroless Copper Deposition

Activation of Ta/SiO 2 /Si substrates using a PdCl 2 buffered oxide etch (BOE)/HNO 3 displacement solution and electroless Cu deposition have been investigated in this paper. Surface morphologies and distribution of Pd nuclei on the Ta layer were strongly dependent on the ratio of BOE/HNO 3 during activation as confirmed by scanning electron microscopy and atomic force microscopy surface observation. Using an activation solution with a ratio of BOE/HNO 3 of 3:2, electrolessly deposited Cu films with good surface coverage and low electrical resistivity were obtained. The surface coverage of electroless Cu grains on the Ta layer degraded with longer activation time and other activation solutions due to the Pd removal resulting from lateral under-substrate cut during activation, No preferred orientation of the electrolessly deposited Cu films was found from X-ray diffraction analysis. Good selectivity of electroless Cu deposition contributed to the preferred nucleation of Cu on Pd nuclei which deposited only on the Ta layer rather than on SiO 2 during activation.

Structural and Thermodynamic Factors of Suppressed Interdiffusion Kinetics in Multi-component High-entropy Materials

We report multi-component high-entropy materials as extraordinarily robust diffusion barriers and clarify the highly suppressed interdiffusion kinetics in the multi-component materials from structural and thermodynamic perspectives. The failures of six alloy barriers with different numbers of elements, from unitary Ti to senary TiTaCrZrAlRu, against the interdiffusion of Cu and Si were characterized, and experimental results indicated that, with more elements incorporated, the failure temperature of the barriers increased from 550 to 900°C. The activation energy of Cu diffusion through the alloy barriers was determined to increase from 110 to 163 kJ/mole. Mechanistic analyses suggest that, structurally, severe lattice distortion strains and a high packing density caused by different atom sizes, and, thermodynamically, a strengthened cohesion provide a total increase of 55 kJ/mole in the activation energy of substitutional Cu diffusion, and are believed to be the dominant factors of suppressed interdiffusion kinetics through the multi-component barrier materials.

Broadband antireflective poly-Si nanosponge for thin film solar cells

Antireflective nanosponges are fabricated on polycrystalline silicon (poly-Si) thin films using Ag-nanoparticles (NPs) assisted etching. Crystal orientations and grain sizes of the poly-Si thin films are investigated for the poly-Si nanosponge formation and the resultant optical properties. The Ag-NPs assisted etching preferentially etches the poly-Si thin films along crystal orientation of [110]. A 400 nm thick poly-Si nanosponge reduces effective optical reflection of the poly-Si thin film with substrate crystal orientation of (110) and averaged grain size of 250 nm from 26 % to 3 % at the wavelengths ranging from 400 nm to 1000 nm. Carrier lifetimes were found to be 41 and 36 mus for poly-Si thin film and RTO-passivated nanosponges, respectively.

In situ epitaxial growth of TiO2 on RuO2 nanorods with reactive sputtering

In this work, TiO2 deposition on RuO2 nanorods with reactive sputtering was studied. The TiO2 deposition was performed in situ after the RuO2 nanorod deposition at the same substrate temperature of 450 °C. The morphology examination and structure analysis have indicated a uniform and pure rutile TiO2 deposition on RuO2 nanorods. High-resolution transmission electron microscopy images also revealed an epitaxial growth of TiO2 on RuO2 nanorods. Such a low-temperature fabrication technique for one-dimensional (1D) heteronanostructure may apply to other functional materials. Since RuO2 is a good electric conductor, 1D heteronanostructures made from RuO2 nanorods are expected to exhibit enhanced functionality particularly in electrical and electrochemical applications.