Ta Overlayer Research Articles

High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films

Lack of plasticity is one of the main disadvantages of metallic glasses. One of the solutions to this problem can be composite materials. Diffusion bonding is promising for composite fabrication. In the present work the diffusion process in glassy multilayer films was investigated. A combination of advanced transmission electron microscopy (TEM) methods and precision sputtering techniques allows visualization and study of diffusion in amorphous metallic layers with high resolution. Multilayered films were obtained by radio frequency sputter deposition of Zr-Cu and Zr-Pd. The multilayers were annealed under a high vacuum (10−5 Pa) for 1 and 5 h at 400 °C, that is, well below the crystallization temperatures but very close to the glass-transition temperatures of both types of the glassy layer. The structural evolution in the deposited films was investigated by high-resolution transmission electron microscopy. It was observed that, despite the big differences in the atomic mass and size, Pd and Cu have similar diffusion coefficients. Surprisingly, 1 h of annealing results in formation of metastable copper nanocrystals in the Zr-Cu layers which, however, disappear after 5 h of annealing. This effect may be connected with nanovoid formation under a complex stress state evolving upon annealing, and is related to the exceptionally slow relaxation of the glassy layers sealed with a Ta overlayer.

Modulation of Magnetism and Magnetic Anistropy at the Heavy-Metal/FeRh Interface

Herein, using first-principles calculations, we explore the magnetic-phase order and magnetic anisotropy energy (MAE) of FeRh (001) films with different heavy transition-metal contact layers (HTM = Hf, Ta, and W). While the antiferromagnetic ground state of FeRh still persists at the W/FeRh interface with the thicker FeRh films, the earlier HTMs of Hf and Ta favor the robust ferromagnetism in the interfacial FeRh layers. Moreover, in contrast to an in-plane magnetization of W/FeRh bilayer, the magnetization direction of FeRh reorients from in-plane to perpendicular in the presence of Hf and Ta overlayers. In particular, a large perpendicular MAE of 2 meV is identified at the Hf/FeRh interface, which is associated with the interfacial magnetic-phase transition and the large spin-orbit coupled 3d-5d hybridization.

Impact of boron diffusion at MgO grain boundaries on magneto-transport properties of MgO/CoFeB/W magnetic tunnel junctions

Controlling the B diffusion in CoFeB-MgO based magnetic tunnel junctions (MTJs) is important to realize the large tunneling magnetoresistance (TMR) ratio, which is required to realize high density magnetic random access memories (MRAMs). Here we investigate the detailed microstructure of the MgO-CoFeB MTJs having W underlayer and overlayer in comparison with well-studied MTJs with Ta underlayer and overlayer. Detailed microstructure analysis based on transmission electron microscopy and electron energy loss spectroscopy revealed that B diffused into Ta under-/overlayers and crystallization of CoFeB to CoFe efficiently occurred upon annealing at 300 °C and 400 °C in MTJs with Ta underlayers. In contrast, B diffused into the [001] tilt grain boundaries of MgO barrier in the case of MTJs with W under-/overlayers. These structural differences well explain the difference in post annealing temperature dependence of TMR ratio and resistance area product (RA) of MTJs with Ta and W under-/overlayers.

Enhanced spin-orbit torques in Pt/Co/Ta heterostructures

Spin-orbit torques (SOTs) are studied in perpendicularly magnetized ultrathin Co films sandwiched between two heavy metals, Pt and Ta. A significant enhancement of the Slonczewski-like torque is achieved by placing dissimilar metals with opposite spin Hall angles on opposite sides of the ferromagnet. SOTs were characterized through harmonic measurements and the contribution by the Ta overlayer was isolated by systematically varying its thickness. An effective spin Hall angle of up to 34% is observed, along with a sizable field-like torque that increases with increasing Ta layer thickness. Current-induced switching measurements reveal a corresponding increase in switching efficiency, suggesting that by engineering both interfaces in trilayer structures, the SOTs can be significantly improved.

Controlling Ta phase in Ta/TaN bilayer by surface pre-treatment on TaN

Beyond 0.13 μm feature size, a bilayer of Ta/TaN is commonly used as a metal barrier and adhesive layer for copper multilevel interconnections. However, for the Ta films, cubic α-phase is preferred due to the lower resistivity, whereas high-resistive tetragonal β-phase should be avoided but easily formed by sputtering. Here, we demonstrate that the Ta overlayer phase can be effectively controlled by resputtering TaN surface using Ar prior to Ta growth. Upon surface treatment for enough time, α-phase forms instead of β-Ta. The intriguing phase change is attributed to the decrease of TaN lattice constant toward the surface, which renders a better lattice-matched template for α-Ta. From Auger depth profile, resputtering causes nitrogen deficiency in the surface layer of TaN to become TaN 0.1, leading to about 1.9% lattice mismatch. The mechanism for the phase change of the Ta overlayer was further verified by examining phase evolution of Ta deposited on various materials with different structure and lattice constants including W, TaN x ( x<1) and Ti.

Feasibility of Detecting Barrier Layer to Low-k Transition in Copper Cmp Using Raman Spectroscopy

AbstractIn copper CMP, transitions from copper to barrier as well as barrier to dielectric layer are typically sensed in situ using an optical reflectance technique. Spectroscopic techniques such as Raman, which allow monitoring the vibrational modes of silicon and low-k layers, have interesting potential for detecting these transitions. In this paper the use of Raman spectroscopy in detecting in situ removal of barrier layers from CDO materials is reported. Intensities of Raman peaks characteristic of Si-Si vibrations from Si substrate and C-H vibrations from low-k materials have been used for monitoring CDO layer thickness and detecting removal of Ta overlayer. An abrasion cell is integrated with a Raman spectrometer to demonstrate the feasibility of Raman monitoring in-situ. Capabilities and limitations of the Raman spectroscopic method are discussed.

Alteration of Cu conductivity in the size effect regime

The resistivity of thin Cu films depends on film thickness as the dimensions approach the electron mean-free-path for Cu of 39 nm. The key size-dependent contributions are from electron–surface scattering, grain boundary scattering, and surface roughness-induced scattering. Measurements with pseudoepitaxial Cu films deposited on Si have been undertaken to reduce effects of grain boundaries and surface roughness and suggest an electron-scattering parameter of p=0.12. Overlayers of metal films on the Cu generally increase the resistivity for Ta and Pt overlayers, and may reduce the resistivity for Au and Al. The resistivity increase may also be reversed if the overlayer oxidizes.

Ta metallization of Si–O–C substrate and Cu metallization of Ta/Si–O–C multilayer

Interfacial reactions of Ta with a Si–O–C low-dielectric constant (low-k) material and Cu/Ta/Si–O–C multilayers are investigated using x-ray photoelectron spectroscopy (XPS) and cross-sectional transmission electron microscopy (TEM). Data indicate that Ta deposition on the low-k substrate results in the initial formation of Ta oxide and TaC. Subsequent deposition of Ta eventually results in the formation of a metallic Ta overlayer at 300 K. The thickness of the initial Ta oxide/TaC-containing layer varies with the Ta deposition rate. At a deposition rate of ∼1 Å min−1, no metallic Ta is observed, even after 32 min sputter deposition time. In contrast, a film of roughly the same thickness, obtained after 15 s deposition at a rate of ∼2 Å s−1, is predominantly metallic Ta. Sputter deposition rates, derived from XPS data, are in agreement with film thicknesses derived from cross-sectional TEM data. Heating of Ta/low-k films in UHV results in no significant changes (as detected by XPS) up to 800 K. Cu deposited by sputter deposition onto a low-k surface covered with metallic Ta exhibits conformal growth, whereas 3d islanding is observed on a surface where TaC and Ta oxide are present. Cu diffusion into the bulk substrate is not observed at temperatures below 800 K in UHV.

First-principles calculations for the structural and magnetic properties of ordered NiFe(001) thin films with and without a Ta overlayer

The first-principles calculations within the local density approximation using the full potential linearized augmented plane wave (FLAPW) method were performed to investigate the structural and magnetic properties of the Ta/NiFe interface for both clean NixFe1−x (001) thin films and with a Ta overlayer. To study the composition dependence, we adopted x=0.5 (L10 structure with either Fe or Ni layers on the surface) and 0.75 (L12 structure with mixed Fe-Ni layers or with Ni on the surface). The equilibrium overlayer/substrate distance and the preferred site position of Ta were obtained by structural optimization employing atomic-force calculations and total energy comparisons for several possible adsorption sites of Ta. By comparing with results for the clean surface of five-layer NixFe1−x (001) films, we found that Ta has a significant detrimental effect on the magnetic properties of NiFe with its induced magnetic moment coupled ferro- or antiferro-magnetically with the substrate depending sensitively on the surface layer.

Impurity effects on the adhesion of aluminum films on sapphire substrates

The adhesion of aluminum (Al) films onto sapphire substrates in the presence of controlled contaminants is being investigated. In this study, adhesion strength is evaluated by continuous scratch and nanoindentation tests to induce delamination of the Al film from the sapphire substrate. If delamination blisters or spallations can be induced, then fracture mechanics based models can be used to calculate the fracture energy or work of adhesion based on the radius of the blister. Initial specimens of 178 nm thick Al films were vapor deposited onto (0001) oriented sapphire substrates with a 5--19 nm layer of carbon sputter deposited onto the sapphire surface of selected samples. Continuous scratch tests promoted blistering of the film in specimens with carbon on the sapphire surface. Delamination blisters could not be induced by continuous indentation testing in samples with or without carbon at the interface. An overlayer of sputtered tantalum (Ta) was then used on a second set of 500 nm thick Al films with and without 10--20 nm of sputtered carbon on the sapphire surface to promote delaminations. With Ta overlayers, continuous nanoindentation techniques induced larger diameter delamination blisters in the specimens with carbon, than in the specimens without carbon. Resistance to blistering, or smaller induced blisters, indicates a higher interfacial strength.

Can 5 d transition-metal monolayers BE magnetic on ferromagnetic substrate?

The magnetic properties of X/5Fe/X (X: Hf, Ta, W, Re, Os, Ir, Pt) have been studied by means of the full-potential linearized augmented-plane-wave method, in which the core states are treated fully relativistically and the valence states are treated semirelativistically and the generalized gradient approximations' (GCA) exchange-correlation potentials are employed for 5 d elements. It has been found that Hf, Ta, W, Re and Os overlayers on Fe(0 0 1) are almost nonmagnetic. Following this trend, if the spin–orbit interactions are taken into account, the magnetism of Ir and Pt overlayers is expected to be killed as for the Ag(0 0 1) and Au(0 0 1) substrate. So the nonexistence of the magnetism for 5 d overlayers is furtherly verified on ferromagnetic substrate besides on nonmagnetic substrate. Moreover, due to the more extended 5 d orbitals as compared to those of 3 d and 4 d, much stronger hybridizations between the 5 d states of the overlayer and the Fe substrate exist, which greatly suppress the magnetic moment of the interface Fe layer.

Transmission electron microscopy of the sequence of phase formation in the interfacial solid-phase reactions in Ta/Si systems

The interfacial reaction of Ta thin films on (100) Si was investigated by high-resolution transmission electron microscopy. An amorphous layer was observed at the as-deposited Ta/Si interface. A phase of Ta5Si3 was first found to form at the interface between a Ta overlayer and the amorphous layer after annealing at 560 °C for 1 h. Annealing at 630 °C for 1 h led to the formation of another interlayer due to the outdiffusion of Si between the amorphous layer and Si. The phase in this interlayer transformed from a metastable one to TaSi2 due to annealing at 680 °C for 1 h. The first nucleation of Ta5Si3 at the interface between Ta and the amorphous layer implies that the initially formed amorphous layer has a metal-rich composition close to Ta5Si3. The formation of the interlayer between the amorphous layer and Si prior to the nucleation of TaSi2 was considered as a result of a kinetic constraint to favor the nucleation of TaSi2.

Magnetic and structural properties of Sm/sub 2/Fe/sub 10/Co/sub 7//M/sub x/C/sub y/ thin films

The structural and magnetic properties of Sm/sub 2/Fe/sub 10/Co/sub 7//M/sub x/C/sub y/ (M=Al and Si; x/spl ges/0 and y/spl ges/0) multilayer films with Ta underlayers and overlayers before and after annealing at 700/spl deg/C for 5-12 min have been fabricated and studied. Structural studies show evidence of layer diffusion upon annealing. X-ray diffraction shows that the samples after annealing consist of a soft phase, /spl alpha/-Fe, and a hard phase with the 2-17-type structure. The samples studied have in-plane anisotropy with single hysteresis loops indicating that the two phases are strongly exchange coupled. After annealing, the coercivity of samples with Al/sub x/C/sub y/ is found to increase with increasing Al/sub x/C/sub y/ composition to a maximum of 3.3 kOe. The energy products for these samples are found to increase from 1.4 MGOe for SmFeCo to 8 MGOe for SmFeCo with Al/sub x/C/sub y/.

Fabrication of Nb/Ta - O/Ta/epitaxial Nb Josephson junctions

Tunnelling junctions which had an epitaxial Nb base electrode grown on a sapphire substrate with a Ta overlayer were studied. X-ray diffraction and resistance measurements revealed that the overlayer Ta was in the bcc phase and had good crystallinity. To investigate the coverage of the Ta layer, the surface morphology was observed by atomic force microscopy and was compared with that of the Al overlayer. We successfully fabricated Nb/Ta - O/Ta/epitaxial Nb junctions with a Josephson critical current of . These junctions showed relatively large excess currents at the subgap voltage region.

Magnetotransport properties of annealed (FeCo)/Ag multilayers

The dependence of the giant magnetoresistance (GMR) on the sputtering pressure, the presence of a Ta underlayer and overlayer, the annealing temperature and the bilayer thickness has been studied in sputtered (Fe-Co)/Ag multilayers. The optimum sputtering pressure was found to be 5 mTorr. The Ta underlayer and overlayer influence greatly the microstructural changes during heat treatment and consequently the magnitude of magnetoresistance. The magnetoresistance increases initially with increasing annealing temperature, reaches a maximum in the region between 250 and 325 °C and after that decreases. The maximum GMR was 8.43% at 46 K and 5.29% at room temperature for films with Ta layers annealed at 275 °C. The magnetic properties are also affected by the annealing. The as-sputtered samples show coercivity in the range 60–220 Oe. After heat treatment in vacuum for 10 min the coercivity drops dramatically in all samples. The films with the highest GMR are believed to be superparamagnetic.

X-ray photoelectron spectroscopy study for Nb Josephson junctions with overlayer structure

We studied metal/Nb structures with Al, Zr, Ta, Hf, V, and Ti as thin overlayers (all transition elements except Al) using x-ray photoelectron spectroscopy. The metal thickness sufficient to protect Nb from the formation of Nb native oxide was 2.0 nm. In structures with Al, Zr, Ta, and Hf overlayers, the oxide peaks in the photoelectron spectra, which resulted from native oxide grown on the metal surface, shifted toward higher binding energies with increasing metal overlayer thickness. These shifts were attributed to the metal coverage and the insulating oxide layers. The oxide insulating properties of the native oxides and the metal coverage of the Nb surface were studied at the same time, nondestructively, using XPS. These are essential factors for fabricating Nb junctions with excellent current-voltage characteristics.

First stage of the formation of refractory metal thin films on Si(111)

XPS, work-function and LEED have been used to study the interface formation of the heavy refractory metals (Nb, Mo, Ta and W) with Si substrates. Metal evaporation has been carried out in uhv, on clean Si(111) (7 × 7) surfaces maintained at room temperature. Deposit thickness θ has ranged from a fraction of a monolayer (ML) to ≈ 100 ML. Intensity variations of the substrate and deposit XPS signals, work-function changes and core level binding energy shifts as a function of θ agree so as to indicate the different natures of the interfaces, and different growth modes of the thin films. Mo and W form abrupt interfaces with Si(111) and grow layer-by-layer. On the contrary, Nb and Ta give very reactive interfaces; interdiffusion takes place, after the completion of one 2D disordered chemisorbed metal layer, leading to mixed (1 metal: 2 Si) phases until ≈ 6 ML of Nb or ≈ 4 ML of Ta are deposited. Then, pure Nb condenses on top of the mixed layer, while the Ta overlayer presents a gradually decreasing concentration of Si. These different interfacial reactions can be explained by a qualitative model which takes into account both the Hiraki electrostatic screening mechanism and the Brillson metal-semiconductor bond strength concept.

Fabrication and dc characteristics of small-area tantalum and niobium superconducting tunnel junctions

We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.

Electron tunneling in tantalum surface layers on niobium.

We have performed electron tunneling measurements on tantalum surface layers on niobium. The tunnel junctions comprise 2000-A\r{} Nb base electrodes with 10--100-A\r{} in situ--deposited Ta overlayers, an oxide barrier, and Ag, Pb, or Pb-Bi alloy counterelectrodes. The base electrodes were prepared by ion-beam sputter deposition. The characteristics of these junctions have been studied as a function of Ta-layer thickness. These include the critical current, bound-state energy, phonon structure, and oxide barrier shape. We have compared our results for the product ${I}_{c}$R versus tantalum-layer thickness with an extended version of the Gallagher theory which accounts for both the finite mean free path in the Ta overlayers and suppression of the ${I}_{c}$R product due to strong-coupling effects. Excellent fits to the data yield a value of the intrinsic scattering probability for electrons at the Ta/Nb interface of ${r}^{2}$=0.01. This is consistent with the value expected from simple scattering off the potential step created by the difference between the Fermi energies of Ta and Nb. We have found a universal empirical correlation in average barrier height \ensuremath{\varphi}\ifmmode\bar\else\textasciimacron\fi{} and width s in the form \ensuremath{\varphi}\ifmmode\bar\else\textasciimacron\fi{}=6 eV/(s-10 A\r{}) for measured junctions which holds both for our data and results for available data in the literature for oxide-barrier junctions. The latter are composed of a wide variety of base and counterelectrode materials. These results are discussed in the general context of oxide growth and compared with results for artificial tunnel barriers.

Critical current behavior and oxide barrier properties of Ta surface layers on Nb

We have investigated the critical current, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> R, and oxide barrier shape in Nb/Ta/Ox/C.E. tunnel junctions. Here, layers of Ta in the thickness range O<D <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ta</inf> <1000Å were deposited, in situ, on 2000Å thick Nb underlayers. Junctions were completed with Pb, PbBi, and Ag counter-electrodes. We find that as D <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ta</inf> is increased, there is a more rapid decrease of I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> R compared with the effective energy gap, in accord with an extended version of the Gallagher theory including strong-coupling and electron-scattering effects. In addition, we have investigated the average barrier height, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\bar{\phi}</tex> , and width, s, of the oxide barriers which form on the Ta overlayers. It is observed empirically that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\bar{\phi} \sim 6/(s-s_{o})</tex> where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s_{o} \sim10</tex> Å and φ is measured in eV. This relationship is also found to hold for barrier formation on a wide variety of pure and composite metallic systems. These results are discussed in conjunction with the Fromhold-Mott-Cabrera theory for self-limiting oxide growth on metal surfaces.