Ta Underlayer Research Articles

Effects of underlayer materials and substrate temperatures on the structural and magnetic properties of Nd 2 Fe 14 B films

Thin films of Nd2Fe14B were fabricated on heated glass substrates by dc magnetron sputtering. Different material underlayers (Ta, Mo, or W) were used to examine the underlayer influence on the structural and magnetic properties of the NdFeB films. Deposited on a Ta buffer layer at 420 degrees C, the 300 nm thick NdFeB films were shown to be isotropic. But when the substrate temperature Ts was elevated to 520 degrees C, the Nd2Fe14B crystallites of (00l) plane were epitaxially grown on Ta (110) underlayer. In contrast, Mo (110) buffer layer could not induce any preferential orientation in NdFeB film irrespective of the substrate temperature or film thickness. The W buffer layer was found to be most effective for the nucleation of Nd2Fe14B crystallites with c-axis alignment perpendicular to the film plane when Ts<490 degrees C. But at Ts=490 degrees C the magnetic layer became isotropic. The maximum coercivity obtained was about 995 kA/m for the 100nm film deposited on W underlayer at 490 degrees C. These variations were tentatively explained in terms of the lattice misfit between the underlayer and the magnetic layer, combined with the considerations of underlayer morphologies.

Interlayer exchange coupling in Co/Ru/Co trilayers

The dependence of the interlayer exchange coupling in Si(001)/Ta/Co/Ru/Co/Ta trilayer on the thickness of the top Co layer is investigated. A strong in-plane uniaxial anisotropy is created in the magnetic layers by the shape anisotropy of an obliquely sputtered Ta underlayer, as confirmed by magnetometry and Brillouin light scattering measurements. The evolution of the magnetic properties, such as the magnetization at remanence and the saturation fields of the hysteresis loops and the giant magnetoresistance, is discussed in terms of the bilinear and biquadratic exchange coupling coefficients.

Effects of underlayer on one-directional anisotropy in spin-valve films without any antiferromagnetic layers

The magnetoresistive (MR) characteristics of spin-valve films with no antiferromagnetic layer were investigated. The films were deposited with various underlayer materials and the one-directional anisotropy in MR curves of the film with Ta underlayer turned out very strong anisotropy compared with that of other materials. It seemed that the strong one-directional anisotropy was caused by increase of coercivity and magnetic anisotropy induced in the pinned layer by external dc magnetic field during deposition. The calculated MR curves by using a torque equation clarified a relation between the magnetic characteristics of the pinned layer and the one-directional anisotropy in measured MR curves.

Variation of magnetization and the Landé g factor with thickness in Ni–Fe films

We have measured the Landé g factor, the effective magnetization Meff, the uniaxial anisotropy Hk, and the Gilbert damping parameter α, as a function of Permalloy film thickness from 2.5 to 50 nm. We used a pulsed inductive microwave magnetometer capable of generating dc bias fields of 35.2 kA/m (440 Oe). A significant decrease in g is observed with decreasing thickness below 10 nm. Also, Meff decreases with decreasing thickness consistent with a surface anisotropy constant of 0.196±0.025 mJ/m2. The decrease in g can arise from the orbital motion of the electrons at the interface not being quenched by the crystal field. We also compare our data to a model of an effective g factor suggesting that the decrease in g factor might also stem from the Ni–Fe interface with a Ta underlayer.

Effect of Interface Conditions on Yield Behavior of Passivated Copper Thin Films

Wafer curvature was used to study the thermal–mechanical behavior of 1-μm Cu thin films capped with a 100-nm-thick Si3N4 layer. These films were grown with either a Ta or a Si3N4 underlayer. Films on Si3N4 that were exposed to oxygen at the film/capping layer interface or at the center of the copper layer exhibited Bauschinger-like yielding at low stress. Stacks deposited under continuous vacuum, with a Ta underlayer, with carbon exposure at the upper surface of the copper film, or with oxygen exposure of only the underlayer did not demonstrate the anomalous yielding. Preferential diffusion of oxygen into copper grain boundaries or interfaces is the likely cause of the early yield behavior. Possible mechanisms include an increase in interface adhesion due to the presence of oxygen in solution and diffusion-induced dislocation glide as an additional driving force for dislocation motion at low applied stress.

Anomalous switching behavior of antiparallel-coupled Co layers separated by a super thin Ru spacer

The details of the magnetization reversal in coupled ferromagnetic Co/Ru/Co trilayers deposited on obliquely sputtered Ta underlayers were studied using the magneto-optical indicator film technique. The ground states of the sandwich are characterized by noncollinear magnetization orientations in the two Co layers, which can be remagnetized by the motion of 180° and non-180° domain walls. In the latter case, the angle between the magnetization vectors in the adjacent domains was revealed to be about ±100°, and an anomalous magnetization reversal was observed. The canted magnetization states and their mutual transformations are discussed in terms of the competition between ferromagnetic coupling through pinholes and antiferromagnetic coupling across the Ru layer.

Magnetic properties of Co films deposited onto obliquely sputtered Ta underlayers

Gold-capped Co films deposited on obliquely sputtered Ta underlayers have produced coercivities in excess of 400 Oe, and anisotropy fields of 800 Oe. The coercivity increases with Ta deposition angle and thickness. The coercivity and anisotropy can be increased further to values >600 and 900 Oe, respectively, by replacing the Au capping layer with Cu. Films with an obliquely deposited underlayer exhibit a reduced density and an increased interface roughness compared to normally deposited films. The trilayers fabricated in this study show promising results for use in giant magnetoresistance sensors.

Texture and hillocking in sputter-deposited copper thin films

Copper thin films have been deposited by magnetron sputtering under a wide variety of conditions. In nearly all cases, (1 1 1) texture is predominant, although its extent varies with the conditions. Greater texture appears to be associated with abnormal grain growth during deposition. For films deposited on novel amorphous SiN x , barriers made by in-situ nitriding of SiO2, the texture is significantly weaker than on conventional Ta or TaN underlayers. Pronounced thermal hillocking is observed. As has been found for other ccp metals, strong (1 1 1) texture suppresses the tendency to hillocking.

Features of domain nucleation and growth in Co/Ru/Co synthetic antiferromagnets deposited on obliquely sputtered Ta underlayers

Obliquely sputtered Ta underlayers were used to stabilize antiferromagnetically coupled Co/Ru/Co/Ta structures with a strong uniaxial anisotropy. Magneto-optical indicator film imaging revealed nucleation and growth of domains during remagnetization. For 0.5 nm Ru thickness and 10.6 nm Ta thickness, the top film magnetization is canted 82° from bottom film. The canting is attributed to ferromagnetic coupling through pinholes.

Temperature dependent specular scattering enhancement in Cu-wedge spin valves

Magneto-transport properties have been systematically studied in Cu-wedge regular and specular spin valves prepared by magnetron sputtering. In the specular spin valve, a thin oxide layer is inserted between the Ta underlayer and the free layer to provide a specular interface. Up to 50% enhancement in giant magnetoresistance (GMR) is realized by the specular interface at room temperature. The observed Cu layer thickness dependence of both the resistivity and GMR in both types of the spin valves has been satisfactorily modeled using a semiclassical Boltzmann approach. Temperature dependence of the GMR has been measured from 325 to 15 K for several Cu-layer thicknesses. The specular scattering enhancement initially increases as the temperature is lowered from 325 K, but rapidly decreases below 100 K. This decrease is correlated with the reduced degree of the full antiparallel alignment between the magnetic moments of the pinned and free layers in the specular spin valve, which is caused by the antiferromagnetic ordering of the oxide layer.

Seed-layer effect on the microstructure and magnetic properties of Co/Pd multilayers

We have investigated Co/Pd multilayers deposited on either Ta or indium tin oxide (ITO) seed layers as a potential perpendicular recording media. We have examined the microstructural evolution of the films deposited on the two different types of seed layers and related it to the magnetic properties of the films. Ta underlayer produces a strong 〈111〉 fiber texture in the multilayer while ITO produces randomly oriented grains. Transmission electron microscopy reveals a microstructure of columnar grains separated by less dense material at the boundaries for the multilayers with an ITO underlayer. However, the less dense material is absent when using a Ta underlayer. The films exhibited strong perpendicular anisotropy and a higher coercivity of ∼6800 Oe and squareness of ∼0.99 are obtained for the films deposited on an ITO seed layer. The differences in the value of coercivity and squareness in the films can be correlated with the differences in the evolution of microstructures for different seed layers.

Thermal stability of Ta-pinned spin valves

It has recently been found that large uniaxial anisotropy fields in excess of 120 kA/m (1500 Oe) can be created in thin (3–5 nm) films of Co by obliquely sputtered Ta underlayers. This anisotropy can be used to pin the bottom film of a spin valve while having only a modest effect on the top “free” film, separated by a 2.5 nm Cu spacer layer. This article describes measurements of thermal stability in these Ta-pinned spin valves. Using room temperature giant magnetoresistance (GMR) as a measure, we find that the structure is stable under cumulative 20 min anneals at 25 °C intervals up to 300 °C; GMR decreases to zero upon further anneals up to 450 °C. Measurements taken at elevated temperatures reveal that GMR decreases linearly with temperature, extrapolating to zero at approximately 425 °C, while the anisotropy field is much less temperature dependent, remaining nearly constant up to 150 °C and gradually decreasing to 50% of its room temperature value at 325 °C.

Strong anisotropy in thin magnetic films deposited on obliquely sputtered Ta underlayers

Anisotropy fields in excess of 120 kA/m (1500 Oe) have been produced in 3–5-nm-thick polycrystalline films of Co by oblique sputtering of Ta underlayers. The unusually high anisotropy is magnetostatic in origin and is induced by corrugations on the surface of an obliquely sputtered Ta underlayer. Cross-sectional transmission electron microscopy reveals 4 nm columnar grains of Ta tilted toward the Ta source and elongated perpendicular to the Ta flux in the film plane. The anisotropy field of the Co film increases with both the underlayer thickness and the angle between the Ta source and the film normal. In spin valve samples, the anisotropy is attenuated by more than an order of magnitude across a 4-nm-thick Cu spacer. Magnetoresistance measurements on a spin valve indicate less than 2° dispersion in hard axis directions, and despite the nanometer-scale roughness of the underlayer there is weak broadening of the ferromagnetic resonance line.

Strong anisotropy in thin magnetic films deposited on obliquely sputtered Ta underlayers

Anisotropy fields in excess of 120 kA/m (1500 Oe) have been produced in 3–5-nm-thick polycrystalline films of Co by oblique sputtering of Ta underlayers. The unusually high anisotropy is magnetostatic in origin and is induced by corrugations on the surface of an obliquely sputtered Ta underlayer. Cross-sectional transmission electron microscopy reveals 4 nm columnar grains of Ta tilted toward the Ta source and elongated perpendicular to the Ta flux in the film plane. The anisotropy field of the Co film increases with both the underlayer thickness and the angle between the Ta source and the film normal. In spin valve samples, the anisotropy is attenuated by more than an order of magnitude across a 4-nm-thick Cu spacer. Magnetoresistance measurements on a spin valve indicate less than 2° dispersion in hard axis directions, and despite the nanometer-scale roughness of the underlayer there is weak broadening of the ferromagnetic resonance line.

Electromigration in Epitaxial Copper Lines

AbstractCopper is becoming the metal of choice for interconnect applications in integrated circuits due to its lower electrical resistivity and higher electromigration (EM) resistance. The effects of grain size, preferred orientation, and interfacial layers, on electromigration in Cu lines are, however, not yet well understood. In this paper, we compare the EM characteristics of epitaxial Cu(111) lines with that of polycrystalline lines with a (111) texture. We also investigate the effects of Ta, TaN and TiN interfacial underlayers and Cu crystal quality on the EM response of epitaxial Cu(001) lines. EM tests were carried out on 2-μm-wide Cu lines at a constant current density between 0.02 and 3.5 MA/cm2 in an Ar ambient at 300 °C. Our results indicate that EM resistance of both (111) and (001) epitaxial lines have a higher mean time to failure (MTTF) than polycrystalline Cu lines. The presence of a TiN or TaN interfacial layer increases the MTTF in Cu(001) films, while a Ta underlayer degrades EM resistance. X-ray diffractograms indicating a smaller full-width-at-half-maximum of Cu(001) films on nitride underlayers suggest that lower defect densities in these Cu films is the likely reason for improved EM resistance. Both (111) and (100) orientations fail by formation of faceted voids.

Large Anisotropy Via Oblique Sputtering of Ta Underlayers

ABSTRACTAnisotropy fields in excesss of 120kA/m(1500Oe)have been produced in 3 nm to 5 nm thick polycrystalline films of Co with Ta underlayers. The unusualy high anisotropy is magnetostatic in origin, and is induced by corrugations on the surface of an obliquely sputtered Ta underlayer. Cross-sectional TEM reveas 8 nm columnar grains of Ta tilted toward the Ta source and elongated perpendicuar to the Ta flux in the film plane. The anisotropy field of the Co film increases with both the underlayer thickness and the angle between theTa source and the film normal. In spin valve samples, the anisotropy is attenuated by more than an order of magnitude across a 4nm thick Cu spacer. Magnetoresistance measurements on a spin valve indicate less than 2° dispersion in hard axis directions, and despite the nanometer-scale roughness of the underlayer, there is weak broadening of the ferromagnetic resonance line.

Solid-state amorphization at tetragonal-Ta/Cu interfaces

This letter describes the formation of a thin amorphous layer at the tetragonal-Ta/Cu interfaces, which appear in copper metallization structures of microelectronic devices. The disordered layer grows up to 4 nm when annealed at between 400 and 600 °C. Since Ta and Cu are immiscible according to thermodynamic data, this is an unusual observation. A mechanism for the amorphous phase formation is proposed using both physical and chemical considerations. A high content of Cu is detected in the Ta layer up to 5 nm from the interface when annealed at 600 °C. Although the adhesion is promoted by the interface reaction, a sufficiently thick Ta underlayer is recommended for efficient blocking of Cu diffusion. Neither solid-state amorphization nor Cu diffusion into Ta is observed at bcc-Ta/Cu interfaces.

Underlayer effect on magnetoresistance of top- and bottom-type spin valves

The effect of underlayers on top- and bottom-type spin valves with PtMn and NiO is investigated. For the top-type spin valves with PtMn, magnetoresistance (MR) ratios are enhanced by using Zr underlayers compared to those for Ta underlayers. The bottom-type spin valves with PtMn showed larger MR ratios than the top-type spin valves as the free layer thickness decreased. The improved giant magnetoresistance results mainly from the increased Δρ due to the smoother interface for the spin valves. Spin valves with NiO underlayers showed smaller resistivity and an almost constant Δρ for a wide range of free layer thicknesses.

CoFe/IrMn spin-valves prepared on Cu islands

By inserting only a 0.75 nm-thick Cu layer between the CoFe/Cu/CoFe/IrMn spin-valve stack and Ta underlayer, the magnetoresistance (MR) ratio and /spl Delta/R were about 1.8 and 1.4 times, respectively, as large as those without the Cu Atomic force microscopy, X-ray diffraction, and MR measurements suggest that the Cu islands formed in the initial growth stage modified the interfaces of the spin-valve and the structure of the component layers, resulting in the enhancement of the spin-dependent electron scattering. When the inserting Cu thickness (t) was larger than 1.5 nm, not only the MR characteristics but the magnetic properties, such as coercivity of the free layer, was improved in comparison with that of t=0 nm. This is due to the considerable increase in the crystallinity of the spin-valve films. In this thickness range, Cu is expected to function as the buffer layer.

Influence of microstructure on thermal stability of spin-valve multilayers

We investigated the influence of microstructure on the thermal degradation of giant magnetoresistance ΔR on spin-valve multilayers constructed by IrMn/CoFe/Cu/CoFe/NiFe, using transmission electron microscopy and energy dispersive x-ray spectroscopy. We found that the main cause of thermal degradation at about 250 °C is the interlayer diffusion along the grain boundary, which had more influence on ΔR than the compositional mixing at the multilayer interface. The polycrystal spin-valve multilayers deposited on Ta and CoZrTa underlayers had rapid degradation of ΔR above the annealing temperature of 225 °C, resulting in 16% and 20% degradation at 275 °C, respectively. A Ta underlayer with amorphous structure has the effect of reducing the density of the grain boundary by improving the surface smoothness, as compared with a crystalline CoZrTa underlayer, leading to slightly less degradation. By contrast, a spin-valve epitaxial layer deposited on a MgO(111) substrate brought about a marked reduction of the thermal degradation of ΔR to within 3%, even at 275 °C, due to a remarkable reduction of crystallographic defects like the grain boundary.