Hf Metal Research Articles

A study of HfO2 film interfaces with Si and SiO2

The interfaces studied were formed by the thermal oxidation of sputter-deposited Hf metal onto Si(100)- and SiO2-covered Si(100) wafers and were analyzed in situ and in real time using spectroscopic ellipsometry (SE) in the 1.5–4.5 photon energy range and with mass spectrometry of recoiled ions (MSRI). SE yielded optical properties and MSRI yielded film and interface composition. Reactivity between HfO2 and both substrates was found to be similar based upon the optical properties of the interface layer. Equivalent oxide thicknesses and interface trap levels were determined and significant reduction in interface traps was noticed for samples with a 2 nm SiO2 film on Si. In addition, hillock formation was seen to occur upon oxidation of Hf metal on the Si substrates and depended on the starting thickness of the metal layer and was not seen for the SiO2-covered substrates.

Effects of Additional Hf on the Thermal Stability and Mechanical Properties of Cu-Zr-Ag Bulk Glassy Alloys

The thermal stability, crystallization behavior and mechanical properties of the Cu45Zr45� xHfxAg10 (x ¼ 0 to 45 at%) glassy alloys have been investigated. The glass transition temperature (Tg), crystallization temperature (Tx), liquidus temperature (Tl) and the supercooled liquid regionTx (¼ TxTg) increase with increasing Hf content. The reduced glass transition temperature (Tg=Tl) values are in the range from 0.54 to 0.59. The Vicker's hardness (HV ), Young's modulus (E) and compressive fracture strength (� c,f) of the Cu45Zr45� xHfxAg10 (x ¼ 0 to 25 at%) bulk glassy alloys increase linearly with increasing Hf content and reach the maximum values of 554, 123 GPa and 2020 MPa, respectively, for Cu45Zr20Hf25Ag10 alloy. A different crystallization behavior is observed for Cu45Zr45Ag10 and Cu45Hf45Ag10 glassy alloys. (doi:10.2320/matertrans.47.1922) the thermal stability and mechanical properties of Cu-based Cu-Zr-Ag bulk glassy alloys. This paper presents the thermal stability, density and mechanical properties of the Cu-Zr-Hf- Ag glassy alloys. The effectiveness of substitution of Hf for Zr is also investigated. 2. Experimental Procedure Multi-component Cu-(Zr, Hf)-Ag alloy ingots with nom- inal compositions were prepared from the mixtures of pure Cu, Zr, Hf and Ag metals by arc melting in a Ti-gettered argon atmosphere. The purity of metals was over 99.9 mass%. Alloy ingots were re-melted four times to ensure chemical homogeneity. The glassy alloys in a ribbon form with a cross section of 0:02 � 1:5 mm 2 and in a cylindrical rod form with diameters up to 2.0 mm were prepared by melt spinning and copper mold casting, respectively. The glassy structure was identified by X-ray diffraction and the thermal stability associated with glass transition temperature (Tg) and crystallization temperature (Tx) was examined by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s. The liquidus temperature (Tl) was measured by differential thermal analyzer (DTA) at a heating rate of 0.33 K/s. The density was measured by Archimedes's principle by weigh- ing ingots in tetrabromoethane and in air. Vicker's hardness was measured using a microhardness tester. Mechanical properties were measured under a compressive load with an Instron testing machine. The gauge dimension of specimens was 2.0 mm in diameter and 4.0 mm in height and the strain rate was 5 � 10 � 4 s � 1 .

Saturated solid-solution hardening behavior of Ir–Hf–Nb refractory superalloys for ultra-high temperature applications

The principles of multi-component alloying and saturated solid-solution hardening were combined for the design and development of Ir-based alloys for ultra-high temperature applications. It is found that as a solute, the transition metal Hf combined with Nb exerts its full potential for solid-solution hardening on the solvent Ir at room temperature and higher. With a monolithic saturated face-centered cubic structure, the Ir–3Hf–5Nb alloy has a 0.2% yield strength of as high as 140 MPa even at 1950 °C.

The impact of Hf metal pre-deposition on the physical and electrical properties of ultrathin HfO2 films on Si0.9954C0.0046/Si heterolayers

The impact of Hf metal prior to the deposition of HfO2 films on tensile-strained SiC layers is demonstrated. The physical thicknesses are found to be ∼4.5 nm and ∼3.5 nm for HfO2/Hf gate stacks and HfO2 films, respectively, by a high-resolution transmission electron microscope. The formation of a Hf-silicate layer due to Si diffusion into the high-κ films is confirmed by x-ray photoelectron spectroscopy measurements. The effective dielectric constants of κ ∼ 6 for HfO2 films and κ ∼ 13 for HfO2/Hf gate stacks are calculated from the accumulation capacitance at the gate voltage of −2 V. The low leakage current density of ∼1 × 10−4 A cm−2 at −2 V, moderate interface state density of ∼1 × 1012 cm−2 eV−1 and low capacitance equivalent thickness (CET) of ∼1.4 nm for HfO2/Hf gate stacks on tensile-strained Si0.9954C0.0046 layers show a promising candidate for the new generation of complementary metal-oxide-semiconductor applications.

Comparison of Thermal Stability and Chemical Bonding Configurations of Plasma Oxynitrided Hf and Zr Thin Films

In this work, we study the characteristics of plasma oxynitrided Hf and Zr thin films. A 5-nm-thick Hf or Zr metal film is deposited on the bare Si substrate, followed by plasma oxynitridation on these metal films in a or ambient. Incorporation of O and N leads to the formation of and films. The high nitrogen content in the films prepared by plasma oxynitridation is found to increase the onset of the crystallization temperature, as compared to films prepared by plasma oxynitridation. Nevertheless, the difference in crystallization temperature is not seen for films. The interlayer (IL) between (or ) and Si is found to be thinner for the films with plasma oxynitridation than those with plasma oxynitridation. However, the nitrogen incorporated by plasma oxynitridation appears to be depleted after rapid thermal oxidation annealing and is not effective to inhibit the growth of the IL. The activation energy of the IL growth for and oxynitrided is 0.23 and 0.13 eV, respectively. The activation energy of the IL growth for and oxynitrided is 0.19 and 0.14 eV, respectively.

Aggressively scaled ultra thin undoped HfO/sub 2/ gate dielectric (EOT<0.7 nm) with TaN gate electrode using engineered interface layer

The ultrathin HfO/sub 2/ gate dielectric (EOT<0.7 nm) has been achieved by using a novel "oxygen-scavenging effect" technique without incorporation of nitrogen or other "dopants" such as Al, Ti, or La. Interfacial oxidation growth was suppressed by Hf scavenging layer on HfO/sub 2/ gate dielectric with appropriate annealing, leading to thinner EOT. As the scavenging layer thickness increases, EOT becomes thinner. This scavenging technique produced a EOT of 7.1 /spl Aring/, the thinnest EOT value reported to date for "undoped" HfO/sub 2/ with acceptable leakage current, while EOT of 12.5 /spl Aring/ was obtained for the control HfO/sub 2/ film with the same physical thickness after 450/spl deg/C anneal for 30 min at forming gas ambient. This reduced EOT is attributed to "scavenging effect" that Hf metal layer consumes oxygen during anneal and suppresses interfacial reaction effectively, making thinner interface layer. Using this fabrication approach, EOT of /spl sim/ 0.9 nm after conventional self-aligned MOSFETs process was successfully obtained.

Orientation and Composition Dependences of the Surface Energy and Work Function Observed by First-Principles Calculation for the Mo–Hf System

First-principles calculations were employed firstly to identify the D0 3 , B2 (and/or L2 a ) and D0 3 structures are relatively stable for the possible non-equilibrium Mo 3 Hf, MoHf and MoHf 3 alloys, respectively, and then to determine the surface energies and work functions of three low index crystalline surfaces for the identified Mo-Hf alloys as well as for the pure Mo and Hf metals. In the present calculations, two atomic configurations are considered for each crystalline surface, i.e., an ideal as-set surface and a relaxed surface, which corresponds to a more or less real situation. It is found that with increasing the openness of the crystalline surface, the surface energy increases while the work function decreases, suggesting an anisotropic hehavior, and the influence of relaxation on these surface properties is enhanced considerably. Besides, based on an analytical analysis, an approximately linear dependence between (he surface property and Mo-Hf alloy composition is found that the surface energy as well as the work function of the alloys decreases with increasing of the Hf concentration.

Fabrication of V-Based Laves Phase Compound Multifilamentary Wires by Applying a Rapidly-Heating/Quenching Process to PIT Precursors and Using a V Tube

We fabricated V-based Laves phase compound such as V/sub 2/(Hf,Zr) multifilamentary wire by applying a rapidly-heating/quenching (RHQ) process. Pure Hf and Zr metal powders were mixed and it put in high pure V tube composed with tantalum (Ta), and it was drawn in order to make wire (powder-in-tube (PIT) method). The wires were heat-treated by RHQ process, and some were annealed additionally in vacuum. Since Ta has a higher melting point than V, it is expected that the alloying prevents from breakage of the precursor wire during RHQ treatment because the maximum temperature exceeds 2500 K. SEM observation confirmed that the thick reaction layer within about 10 microns was formed on the boundary of the powder mixture filament and the V matrix after the RHQ. T/sub c/ values of the as-quenched samples increased with increasing maximum temperature during RHQ process, and the maximum mid point T/sub c/ value was about 8.0 K. The transport J/sub c/ properties under magnetic fields were also investigated.

Friedel–Crafts alkylation of aromatics with benzyl alcohols catalyzed by rare earth metal triflates supported on MCM-41 mesoporous silica

Friedel–Crafts alkylation of aromatics with benzyl alcohol as alkylating agent has been investigated over rare earth metal triflates, Sc(OTf) 3, Hf(OTf) 4, La(OTf) 3, and Yb(OTf) 3 supported on MCM-41. The catalytic activity of triflates, particularly, Sc(OTf) 3 and Hf(OTf) 4 was enhanced by being loaded onto MCM-41 because of increased dispersion, and gave the benzylated product in high yield. They were reusable without the loss of the activity for their recycling. Water generated during the benzylation with benzyl alcohol (BnOH) did not affect on the catalytic activity of the Sc(OTf) 3 supported on MCM-41 (Sc(OTf) 3/MCM-41). The catalytic activity of Sc(OTf) 3/MCM-41 was not affected by the further addition of water equimolar to BnOH. The benzylation of aromatics (toluene, p-xylene, mesitylene, and naphthalene) with benzyl alcohol, and of benzene with benzyl alcohols (4-methylbenzyl alcohol and 4-chlorobenzyl alcohol) were studied over Sc(OTf) 3/MCM-41 at 353 K. The rate of the benzylation of benzene over Sc(OTf) 3/MCM-41 was accelerated by electron donating groups and retarded by electron withdrawing groups. The benzylation of aromatics proceeds with direct benzylation with benzyl alcohol, and the dehydration product, i.e., dibenzyl ether (DBE), which is a by-product formed during the benzylation of aromatics; however, it participates in the benzylation with benzene via BnOH with the decrease in BnOH.

Effects of annealing temperature on the characteristics of silicate/HfO2 insulator formed on the p-Si/Si0.8Ge0.2 and p-Si/Si0.8Ge0.2/intrinsic-Si (20Å) substrates

We successfully deposited the reliable silicate/HfO2 insulator films on both p-Si/Si0.8Ge0.2 and p-Si/Si0.8Ge0.2/intrinsic-Si substrates via the oxidation and annealing of thin Hf film deposited by rf-magnetron sputtering. The oxidation of Hf metal films results in the electrically stable silicate/HfO2 stacked layers. The silicate films (i.e., a layer of Hf–Si–Ge–O) were formed between the HfO2 film and the p-Si/Si0.8Ge0.2 substrate. We found that the Ge segregation is suppressed by the insertion of a 2-nm-thick Si-overlayer on the Si0.8Ge0.2 substrate, resulting in better electrical properties than those obtained from the samples without the Si-overlayer. High-temperature annealing at 700 °C causes the diffusion of Ge into the silicate/HfO2 stacked insulator films only for the samples without the Si-overlayer. In addition, 700 °C annealing results in the formation of a thin SiOx layer at the silicate–Si0.8Ge0.2 substrate regardless of the presence of the Si-overlayer.

Novel Fabrication Process for HfO2 Thin Film for Gate Dielectric

A novel process for fabricating a high-k dielectric/Si structure is proposed. The structure is fabricated by ultrahigh vacuum (UHV) sputtering of Hf metal followed by postannealing in atmospheric oxygen ambient. In the fabrication of structures by this method, an interfacial layer such as SiO2 is scarcely formed because the Si surface is already covered with Hf metal from UHV sputtering, before heating in oxygen ambient. Also a postoxidation condition is set up so that the metal Hf, but not the substrate Si, is oxidized completely. Consequently, the HfO2/Si structure was fabricated with an interfacial layer thickness of as low as 0.2 nm. In addition, thermal stability of the HfO2 film fabricated by this method was maintained up to 800°C for 30 min, which is a standard thermal budget for the impurity activation process. These experimental facts show that this novel process is effective for fabricating the high-k dielectric/Si stacked structure with ultrasmall equivalent oxide thickness (EOT) for the next-generation metal-oxide-semiconductor field effect transistor (MOSFET).

Three-Layer laminated metal gate electrodes with tunable work functions for CMOS applications

This letter presents a novel technique for tuning the work function of a metal gate electrode. Laminated metal gate electrodes consisting of three ultrathin (/spl sim/1-nm) layers, with metal nitrides (HfN, TiN, or TaN) as the bottom and top layers and element metals (Hf, Ti, or Ta) as the middle layer, were sequentially deposited on SiO/sub 2/, followed by rapid thermal annealing annealing. Annealing of the laminated metal gate stacks at high temperatures (800/spl deg/C-1000/spl deg/C) drastically increased their work functions (as much as 1 eV for HfN-Ti-TaN at 1000/spl deg/C). On the contrary, the bulk metal gate electrodes (HfN, TiN and TaN) exhibited consistent midgap work functions with only slight variation under identical annealing conditions. The work function change of the laminated metal electrodes is attributed to the crystallization and the grain boundary effect of the laminated structures after annealing. This change is stable and not affected by subsequent high-temperature process. The three-layer laminated metal gate technique provides PMOS-compatible work functions and excellent thermal stability even after annealing at 1000/spl deg/C.

Binary and ternary metal carbides based upon Ti, Zr and Hf

Ab initio electronic structure calculations are employed to study the behavior of ternary alloy carbides based upon the metals Ti, Zr and Hf. Significant variations of the bulk modulus are found. The relative importance of each of the metals in influencing the cohesive properties of both binary and ternary metal carbides is discussed. A miscibility concept of ternary systems relative to the binary counterparts is considered at length and where it is argued that miscibility of the equilibrated or stable system is far different than for the metastable system as would be the case, for example, in a pressure synthesized alloy. From comparisons of two extremes of stable and metastable miscibility, speculation is made of extreme conditions as to how the ternary alloys can be achieved.

Hafnium Oxide and Zirconium Oxide Atomic Layer Deposition: Initial Precursor and Potential Side-Reaction Product Pathways with H/Si(100)-2×1

Hybrid density functional calculations have been carried out using cluster models of the H/Si(100)-2 x 1 surface to investigate the mechanistic details of the initial surface reactions occurring in the atomic layer deposition of hafnium and zirconium oxides (HfO2 and ZrO2). Reaction pathways involving the metal precursors ZrCl4, Zr(CH3)4, HfCl4, and Hf(CH3)4 have been examined. Pathways leading to the formation of a Zr-Si or Hf-Si linkage show a significant sensitivity to the identity of the leaving group, with chloride loss reactions being both kinetically and thermodynamically less favorable than reactions leading to the loss of a methyl group. The energetics of the Zr(CH3)4 and Hf(CH3)4 reactions are similar with an overall exothermicity of 0.3-0.4 eV and a classical barrier height of 1.1-1.2 eV. For the reaction between H2O and the H/Si(100)-2 x 1 surface, the activation energy and overall reaction enthalpy are 1.6 and -0.8 eV, respectively. Due to contamination, trace amounts of H2O may be encountered by metal precursors, leading to the formation of minor species that can lead to unanticipated side-reaction pathways. Such gas-phase reactions between the halogenated and alkylated metal precursors and H2O are exothermic with small or no reaction barriers, allowing for the possibility of metal precursor hydroxylation before the H/Si surface is encountered. Of the contaminant surface reaction pathways, the most kinetically favorable corresponds to the surface -OH deposition. Interestingly, for the hydroxylated metal precursors, a unique reaction pathway resulting in the direct formation of Si-O-Zr and Si-O-Hf linkages has been identified and found to be the most thermodynamically stable pathway available, being exothermic by approximately 1.0 eV.

Influence of thickness of Hf buffer layer on the interfacial structures of sputtered HfO 2 on SiO 2/Si

The influence of thickness of Hf–metal buffer layer on the interfacial diffusion and reaction was investigated using in situ X-ray photoelectron spectroscopy, scanning Auger microscope and grazing incident X-ray reflectivity. Hf–metal firstly reacted with native Si oxide forming Hf silicates, and all Si–O was further reduced to be Si 0 after 1 nm Hf–metal deposition. The Hf–metal and Hf-suboxide in Hf(1 nm)/SiO 2/Si structure were further oxidized to be Hf 4+ during HfO 2 sputtering deposition and post-deposition annealing. Si diffused out and reacted with HfO 2 during annealing. The Hf(1 nm) buffer layer exhibited a better performance than the Hf(0.3 nm) buffer layer in suppressing the diffusion of Si and the reaction between diffused Si and HfO 2.

X-ray photoelectron spectroscopic analysis of HfO 2/Hf/SiO 2/Si structure

A stoichiometric HfO 2 film was successfully prepared using direct reactive sputtering deposition on a native SiO 2/Si wafer, before which an ultrathin Hf metal film was deposited as a buffer layer. X-ray photoelectron spectroscopy depth profiling technique was employed to investigate the interface chemistry of the obtained structure. It was observed that Hf silicates were formed in the interfacial layer. The binding energies of Hf 4f and O 1s shifted to higher binding energy side synchronously during the removal of the contaminant layer. These shifts were attributed to the modification of the surface potential by Ar + sputtering.

Atomic Layer Deposition of HfO2 Using Alkoxides as Precursors

We have used density functional theory to investigate the atomic layer deposition (ALD) of hafnium oxide (HfO2) thin films using metal alkoxides as both metal and oxygen precursors. In particular, we have predicted chemical mechanisms for the ALD of HfO2 using tetraethoxyl hafnium [Hf(OEt)4] and water. We find that the ALD half-reactions involve the formation of stable adsorbed complexes and precursor-mediated ligand-exchange mechanisms. When Hf(OEt)4 is used as the metal precursor and water is used as the oxygen source, both the metal and oxygen ligand-exchange reactions are unactivated with respect to the reactants. However, the reactions have moderate barriers relative to the adsorbed complexes, and the removal of the ethanol byproducts requires substantial desorption energies. However, when HfCl4 is used as the metal precursor and Hf(OEt)4 is used as the oxygen source, both the metal and oxygen half-reactions possess high activation barriers (∼30−40 kcal/mol). Our results imply that although Hf(OEt)4 may be a suitable metal precursor it has relatively slow kinetics when used as an ALD oxygen source. We have also investigated several alternative reaction pathways that may affect the overall growth rate, including incomplete elimination surface reactions and nongrowth ligand exchange reactions. Finally, we discuss the self-limiting nature of the half-reactions and chemical strategies for adjusting the reactivity of alkoxide ALD precursors.

Preparation of ultrathin HfO2 films and comparison of HfO2/SiO2/Si interfacial structures

AbstractStoichiometric HfO2 films were successfully prepared using both thermal oxidation of Hf metal film and direct reactive sputtering deposited on native SiO2/Si wafer. Angle‐resolved x‐ray photoelectron spectroscopy, Auger depth profiling and grazing incident x‐ray reflectivity have been exploited to study the interfacial structures of the generated HfO2 films. A thinner interface (about 1.2 nm) of Hf‐rich silicates is formed using thermal oxidation, while the reactive sputtering deposition leads to a thicker (about 3.2 nm) Si‐rich interface. The results of binding energy, chemical composition, thickness and density all indicate that both interfaces have a graded structure where the concentration of Hf in the silicate interlayer decreases gradually along the depth. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Catalytic Oligomerization of Ethylene to Higher Linear α-Olefins Promoted by Cationic Group 4 Cyclopentadienyl-Arene Active Catalysts: A DFT Investigation Exploring the Influence of Electronic Factors on the Catalytic Properties by Modification of the Hemilabile Arene Functionality

Hessen and co-workers described Ti-based precatalysts of the title class bearing a hemilabile ancillary arene functionality for ethylene trimerization. The analogues with the heavier group 4 metals Zr and Hf as the active center have been recently explored computationally. This study suggested the Zr-based system as a promising catalyst for production of an oligomer mixture containing 1-octene besides the prevalent 1-hexene. Herein, we have presented a detailed computational analysis of the influence of modifications of the hemilabile arene functionality on the catalytic abilities of cationic group 4 [(η5-Cp-(CMe2R‘-bridge)-C6HyRx)MII(C2H4)2]+ active species (M = Zr, Hf) for linear ethylene oligomerization, employing a gradient-corrected DFT method. This comprised the arene substitution with prototypical electron-releasing (R = Me) and electron-withdrawing (R = F, CF3) groups in various combinations and also the enlargement of the Cp-arene connecting bridge by an additional methylene group (R‘ = CH2). The...

High-quality HfSixOy gate dielectrics fabricated by solid phase interface reaction between physical-vapor-deposited metal–Hf and SiO2 underlayer

We fabricated high-quality Hf–silicate (HfSixOy) gate dielectrics by utilizing the solid phase interface reaction between physical-vapor-deposited metal–Hf (typically 0.5nm thick) and SiO2 underlayers. Metal diffusion to the SiO2 layer increases the permittivity of the underlayer, while preservation of the initial SiO2∕Si bottom interface ensures good electrical properties of the gate dielectrics. The Hf–silicate layer remains amorphous and the poly-Si∕HfSixOy gate stack endures activation annealing at 1000°C. The interface trap density was comparable to that of conventional SiO2 dielectrics and the hysteresis of capacitance–voltage curves was as low as 4mV for a bias swing between −2 and +2.5V. Moreover, high electron mobility, equal to 89% of the universal mobility, was obtained for the high-k gate transistor.