n-Feng Hsu

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.

Tailoring interfacial exchange coupling with low-energy ion beam bombardment: Tuning the interface roughness

By ascertaining NiO surface roughness in a Ni80Fe20/NiO film system, we were able to correlate the effects of altered interface roughness from low-energy ion-beam bombardment of the NiO layer and the different thermal instabilities in the NiO nanocrystallites. From experiment and by modelling the temperature dependence of the exchange bias field and coercivity, we have found that reducing the interface roughness and changing the interface texture from an irregular to striped conformation enhanced the exchange coupling strength. Our results were in good agreement with recent simulations using the domain state model that incorporated interface mixing.

Effects of humidity on nano-oxidation of silicon nitride thin film

Effects of humidity on nanometer-scale oxidation of silicon nitride thin film using atomic force microscope in contact mode are studied at various values of relative humidity (RH) (30-70%). The shape of oxide protrusion is determined by the concentration of oxyanions under the tip apex and oxyanions diffusion laterally on the surface. At low RH (<or= 60%), the kinetics of silicon nitride oxidation has a logarithmic relationship to oxide height versus oxidation time. The threshold time decreased and initial oxidation rate increased simultaneously as humidity increased because a high concentration of oxyanions at the oxide/silicon nitride interface was generated. When a high sample voltage (9-10 V) is applied at high RH (>or= 60%), the effective electric field is decreased because of the electron being trapped in the oxide and oxyanions accumulating on the oxide surface.

Shape Transition in the Initial Growth of Titanium Silicide Clusters on Si(111)

The growth of titanium silicide clusters has been observed for sub-monolayer Ti deposited on Si(111)-7×7. Irregular shape tetragonal Ti5Si4 islands were observed to grow at 600°C. On the other hand, the elongated orthorhombic-Ti5Si4 (O-Ti5Si4) clusters form at 700°C. The clusters underwent a rapid increase in length and slight reduction in width as the cluster area exceeds a critical size (~15 nm2). The elongated clusters are oriented along three equivalent Si directions of the Si(111) surface with O-Ti5Si4 (025)∥Si(2-20). The shape change is in agreement with theoretical prediction based on the strain relief mechanism.

Effects of Substrate Temperature on the Initial Growth of Titanium Silicides on Si(111)

The growth of titanium silicide was investigated in sub-monolayer Ti deposited on Si(111)-7×7. The formation of nanorod and cluster structures was observed for 600 and 700°C deposited samples. In samples heated to 800°C, titanium silicide structures transformed to clusters. C49-TiSi2 nanorods were found to orient along three equivalent Si directions with C49-TiSi2(200)∥Si(220). Deposition of sub-monolayer Ti at a high temperature was found to contribute to the lowering of the formation temperature of C49-TiSi2. The lowering of C49-TiSi2 growth temperature on heated substrate is attributed to smaller lattice mismatch between C49-TiSi2(200) and Si(220).

Fabrication of Ni-silicide/Si heterostructured nanowire arrays by glancing angle deposition and solid state reaction

This work develops a method for growing Ni-silicide/Si heterostructured nanowire arrays by glancing angle Ni deposition and solid state reaction on ordered Si nanowire arrays. Samples of ordered Si nanowire arrays were fabricated by nanosphere lithography and metal-induced catalytic etching. Glancing angle Ni deposition deposited Ni only on the top of Si nanowires. When the annealing temperature was 500°C, a Ni3Si2 phase was formed at the apex of the nanowires. The phase of silicide at the Ni-silicide/Si interface depended on the diameter of the Si nanowires, such that epitaxial NiSi2 with a {111} facet was formed at the Ni-silicide/Si interface in Si nanowires with large diameter, and NiSi was formed in Si nanowires with small diameter. A mechanism that is based on flux divergence and a nucleation-limited reaction is proposed to explain this phenomenon of size-dependent phase formation.

Enhanced Structural and Magnetic Ordering of FePt/Mn-Oxide Bilayers by Ion-Beam Bombardment and Annealing

Structural and magnetic properties of FePt thin films were affected strongly by capped MnOx layers prepared by ion-beam bom-bardment and post-annealing. As-deposited FePt/MnOx bilayer exhibited a magnetically soft fee phase, and it turned to an ordered fct FePt phase with large coercivity (~8 kOe) after annealing at 550°C. Increasing the %02/Ar in capped MnOx layer during de position resulted in smaller ordered FePt grains separated by grain boundaries of MnOx. We found that the superlattice (001) peak is broadened considerably with larger amount of MnOx incorporated into FePt, likely due to the hindered formation of hard phase. Our results indicate that FePt/MnOx films deposited with lower %02/Ar, the oxygen atoms may occupy the interstitial positions in the FePt lattice to induce a local strain thus enhancing the FePt ordering. Further increased %02/Ar in capped MnOx layer, the excess oxygen atoms act a diffusion barrier effectively to inhibit the FePt grain growth and ordering.

Microstructures and Magnetism of Different Oxides Separating FePt Grains via Ion-Beam Bombardment and Annealing

The effects of the fabrication methods and different capped oxide (SiO2 and TiO2) layers on the microstructure and magnetism of FePt thin films were studied. Both structural ordering (S � 0:7) from the fcc FePt phase to the fct FePt phase and magnetic hardening were observed in the annealed FePt/SiO2 thin films with a low substrate rotation speed (Sr ¼ 1 rpm). However, only the annealed FePt/SiO2 thin films prepared with a high Sr (10 rpm) exhibited isolated FePt grains separated by the grain boundary SiO2, as revealed by transmission electron microscopy and magnetometry. Furthermore, similar results in microstructures and magnetic properties were obtained after replacing the capped layer with TiO2. However, an enhanced order parameter (S � 0:85) and a smaller FePt grain size (� 6:8 nm), which are promising characteristics for ultrahighdensity magnetic recording, were achieved in the annealed FePt/TiO2 thin films; however, the annealed FePt/SiO2 thin films exhibited a larger grain size (� 15 nm). This indicates that TiO2 inhibits the grain growth of FePt more effectively than SiO2. # 2010 The Japan Society of Applied Physics

Effect of NH3 / He Plasma Treatment on Electrical Reliability and Early-Stage Electromigration Behavior of Copper Interconnects

Electromigration reliability tests of dual-damascene Cu interconnect structures with or without a NH 3 /He plasma treatment have been performed in this study at 400°C under a high current density of 8 MA/cm 2 to clarify the effect of plasma treatment on the interface structure between Cu wires and SiCN capping layers and on the early-stage electromigration behavior and electrical reliability of the interconnects. From cumulative failure probabilities, small shape factors and large median time to failure were obtained for the interconnects with the plasma treatment, indicating an improved electromigration resistance. The oxide layer on the Cu surface was removed by plasma treatment, and an alternative CuSiN interlayer was formed at the Cu/SiCN interface, enhancing interface adhesion and reducing the diffusion paths of Cu atoms. Therefore, voids nucleated inside the Cu wires, rather than at the interface, at the early stage of the electromigration tests. The change in the Cu/SiCN interface structure varied the early-stage electromigration-induced voiding behavior and then effectively improved the electrical reliability of the Cu interconnect structures.

Effects of Asymmetric Local Joule Heating on Silicon Nanowire-Based Devices Formed by Dielectrophoresis Alignment Across Pt Electrodes

We demonstrate the fabrication and characterization of silicon nanowire-based devices in metal-nanowire-metal configuration using direct current dielectrophoresis. The current-voltage characteristics of the devices were found rectifying, and their direction of rectification could be determined by voltage sweep direction due to the asymmetric Joule heating effect that occurred in the electrical measurement process. The photosensing properties of the rectifying devices were investigated. It reveals that when the rectifying device was in reverse-biased mode, the excellent photoresponse was achieved due to the strong built-in electric field at the junction interface. It is expected that rectifying silicon nanowire-based devices through this novel and facile method can be potentially applied to other applications such as logic gates and sensors.