Ta Underlayer Research Articles

Magnetization reversal of Nd(Dy)–Fe–B thin films on Si(111) or Ta/Si(111)

Nd(Dy)–Fe–B films were prepared by dc magnetron sputtering on a Si(111) wafer with or without a Ta underlayer. The reversal magnetization process of Nd–Fe–B/Ta bilayer was found to be dominated by nucleation control model with the magnetic inhomogeneity coefficient αk=0.32 defined by Kronmuller’s formulation of micromagnetic theory. But the coercivity mechanism of Nd(Dy)–Fe–B single layer was fitted well to domain wall pinning behavior. The range factor (half width between pinning sites) r0 is equal to 6.9 nm as r0>δB, the width of the domain wall. The magnetization phenomena of the two films are also manifest from initial magnetization curves.

Magnetic properties of very thin single and multilayer NiFeCo and CoFe films deposited by sputtering

As the size of the giant magneto resistive (GMR) devices continues to decrease into submicrometer regime, the demagnetizing field from the magnetic layers will increase to very high values, prohibiting switching with a reasonably small current. The most feasible way to reduce it is to make the magnetic layers thinner. Single layers of NiFeCo and CoFe films as well as multilayers consisting of these with various underlayers have been deposited by rf diode sputtering in a magnetic field. NiFeCo becomes nonmagnetic at 15, 10, and 6 Å when using Ta, Si3N4, and Cu underlayers, respectively. The magnetization of NiFeCo films sandwiched with Cu decreases as the films become thinner, indicating that about one atomic layer loses its magnetic moment at each interface with Cu. The induced magnetic anisotropy is a strong function of the NiFeCo film thickness, changing from 7 Oe for 20 Å to 17 Oe for 100 Å. CoFe films lose very little magnetic moment at the interfaces with Cu. Multilayers of [NiFeCo/Cu/CoFe/Cu] with different Cu underlayer thickness have been made and the magnetic and GMR properties show a strong dependence on the Cu buffer layer thickness due to different interface roughnesses. With proper designing of the interfaces, these very thin magnetic films have high potential for ultrahigh-density magnetic memory and other applications.

Study of the Underlayer of a PdPtMn Spin-Valve Film

We investigated the PdPtMn bottom spin-valve, which had an high resistivity NiFeCr underlayer. The MR ratio of the NiFeCr-underlayer spin-valve became 1.14 times higher than that of a NiFe-underlayer, because the shunt loss of the underlayer was reduced, and the Hua of the pinned layer also became higher. We found by X-ray diffraction that a PdPtMn spin-valve with a Ta/NiFeCr underlayer had a strong (111) texture. Cross-sectional TEM observation revealed that PdPtMn on a NiFeCr underlayer had a larger grain than PdPtMn on a Ta underlayer. The initial layer of the PdPtMn with the NiFeCr underlayer had a stronger (111) orientation than the PdPtMn with the Ta underlayer. To achieve a large Hua, PdPtMn with a (111) texture and large grain size is required. We simulated the R-H curves of the spin-valve in the head shape. The shift in the quiescent operating point on the R-H curve was smaller for an NiFeCr underlayer than for an NiFe one.

Thermal relaxation effects observed in the giant magnetoresistance of NiFe/Ag degraded multilayers (abstract)

NiFe/Ag degraded multilayers have potential for use in sensors and data storage. The magnetoresistance (MR) observed in these materials, after thay have been optimized by postdeposition annealing, has been found to have a good field sensitivity. It is believed that the size and saturation field of the giant magnetoresistance (GMR) is a function of the size of magnetostatic interactions that occur between the magnetic layers in the system. In order to improve the field sensitivity of the GMR in these materials, the volume of the NiFe aligned antiparallel by magnetostatic coupling will have to be maximized. We have investigated time-dependent thermal relaxation effects in the GMR of these systems in an attempt to determine both the nature of the coupling and the distribution of blocking sizes. The multilayers were deposited onto glass substrates at the system ambient temperature, by dc sputtering in a sputtering pressure of 7.5 mTorr of Ar. Fifteen bilayer repeat multilayers were prepared by rotating the substrate over the NiFe and Ag targets, alternately. Both NiFe and Ta underlayers have been investigated. The postdeposition annealing strategy used was 315 °C for 10 min in a vacuum of better than 10−5 Torr. We have investigated the time dependence of the GMR using a linear four point probe and measurements have been made over a temperature range 10–450 K. The blocking temperature, and the particle/platelet size distribution, of the entities that cause the GMR can be determined from these measurements. In addition, we have found a number of interesting temperature-dependent MR effects, and it is seen that the blocking temperature is below 350 K in all cases (Fig. 1).

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/.

Effects of Ta/Cu Double Structured Underlayers on GMR in CoFe/IrMn Spin-Valves

By inserting only 0.75 nm-thick Cu underlayer between the IrMn/CoFe/Cu/CoFe spin-valve stack and Ta underlayer, the MR ratio was about 1.8 times as large as that without the Cu layer. XRD and magnetic measurements suggest that the Cu islands formed in the initial growth stage modify the interfaces of the spin-Valve, resulting in the enhancement of the spin-dependent scattering. When the Cu thickness (t) was larger than 1.5 nm, the MR ratio and coercivity of the free were increased and decreased, respectively, in comparison with those of t=0 nm. This was due to the considerable increase in the crystallinity of the spin-valve films. Namely, Cu is expected to function as the buffer layer in this thickness range. The inductive/spin-valve composite heads using a material developed in this study were stable and quiet in the dynamic performance.

Effect of interface on exchange coupling in NiFe/FeMn system

The influence of the interface between NiFe and FeMn layers on the exchange coupling in the NiFe/FeMn system was investigated experimentally. The state of the interface between NiFe and FeMn thin films depends on the sequence of the NiFe and FeMn layers, deposition conditions and underlayer. The exchange coupling was exhibited only for samples with the FeMn film crystallized into the /spl gamma/-phase. The value of an exchange bias field barely depends on the amount of the /spl gamma/-FeMn phase. Even a small amount of the /spl gamma/-FeMn phase localized at the interface can provide a strong enough exchange coupling in the NiFe/FeMn system. The samples deposited on the Ta underlayer at room temperature exhibited the lowest roughness, smallest crystallite size and highest exchange coupling. At these conditions the interface between the NiFe and FeMn layers is smooth and sharp and results in a homogeneous and strong enough exchange bias field and low coercivity. The annealing changes the exchange coupling in the NiFe/FeMn system due to modification of the interface between the layers.

Magnetic properties and interaction effect in sputtered Nd/sub 2/Fe/sub 14/B films

Nd/sub 2/Fe/sub 14/B/Ta bilayers with individual layer thickness between 40-400 nm have been grown on Si(111) and polycrystalline alumina substrates, respectively, by dc magnetron sputtering. The coercivity of Nd/sub 2/Fe/sub 14/B film increases with increasing Ta thickness. Room temperature coercivity of 7-10 kOe was routinely obtained with Ta underlayer thicknesses greater than 40 nm. Exchange coupling among Nd/sub 2/Fe/sub 14/B nanocrystalline grains in the hard magnetic layer with higher coercivity was studied by the Henkel plot, which shows positive and large deviation from the linear Wohlfarth relationship, depicting strong coupling among Nd/sub 2/Fe/sub 14/B nano-grains.

Room temperature GMR studies in (Cr,Ta)/[Au/NiFe]/sub n/ multilayers

Multilayered samples of Au/NiFe were sputtered using dc magnetron sputtering, for room temperature giant magnetoresistance (GMR) studies. Two characteristic GMR peaks were observed at 15 and 28 /spl Aring/ of Au spacer thickness, with values of 7.4% and 5.4%, respectively. The large magnetoresistive effect at low applied field has been attributed to the weak antiferromagnetic coupling of the magnetic layers as indicated by their calculated exchange constant values. The effect of a buffer layer on the GMR hysteresis, along with its field sensitivity were investigated for both Cr and Ta precursor layers. The Ta underlayer showed a significantly reduced GMR hysteresis. Also reported are GMR values as a function of applied magnetic field during multilayer deposition, and substrate/buffer layer effects. The multilayer structures were high quality and textured along the <111> direction.

GMR properties of spin valves using multilayered Co90Fe10 for free magnetic layer

To achieve higher GMR ratios in spin valves, Co90Fe10 films were used as the free and the pinned layer. A single layer of CoFe 100 Å and a multilayer of CoFe 18 Å/(Cu 8 Å/CoFe 18 Å)5 were used as the free layer. Although the single layer CoFe showed large coercivities of 58 Oe along the easy axis and 40 Oe along the hard axis, the multilayer CoFe 18 Å/(Cu 8 Å/CoFe 18 Å)5 showed smaller coercivities of 10 Oe along the easy axis and 1.5 Oe along the hard axis on a 100 Å thick Ta underlayer. The anisotropy field of the multilayer was 20 Oe. The film structures of the free layer were evaluated and it was found that the multilayer had higher oriented (111) texture than the single layer. The reason why the multilayer had higher oriented (111) texture was that the Cu layers between CoFe layers helped to have highly oriented (111) texture. To reduce the anisotropy field, the film was annealed in a magnetic field perpendicular to the initial easy axis at several temperatures. As the result, the anisotropy field was reduced to 3 Oe and the coercivity was 4 Oe after annealing at 140 °C. The GMR ratio of the spin valve of Ta 100 Å/CoFe 18 Å/(Cu 8 Å/CoFe 18 Å)4/Cu 24 Å/CoFe 30 Å/FeMn 150 Å/Ta 50 Å was 7% after the annealing.

ＮｉＦｅ／ＦｅＭｎ膜の交換結合における熱処理の効果

The effects of thermal annealing (from 25°C to 300°C ) on exchange-coupling fields in NiFe/FeMn films were investigated. The exchange-coupling fields decrease with an increase in the temperature from 90°C to 210°C but remain constant at 210°C or higher in Ta/NiFe/FeMn films with a Ta underlayer. However, the exchange-coupling fields in these annealed Ta/NiFe/FeMn films make a dramatic recovery if the FeMn layers are thinned by ion-milling and annealed again at 210°C or higher. These changes in the exchange-coupling fields are explained as thermal fluctuations in FeMn spins. On the other hand, the exchangecoupling fields increase at 210°C or higher in NiFe/FeMn films without Ta underlayers. XPS depth-profile results reveal interdiffusion in the NiFe-FeMn interface after annealing at higher than 210°C in the NiFe/FeMn films.

Formation of copper interconnects by the reflow of sputtered copper films

AbstractThe formation of Cu interconnects using conventional sputtering and reflow processes has been studied. The use of W, Mo, and TiN underlayers makes it possible to fill grooves with Cu at reflow temperatures as low as 450°C. There is a close relationship between filling characteristics and agglomeration. Cu films agglomerate more easily on W, Mo, and TiN underlayers than on TiW and Ta underlayers because of their low Cu wetta‐bility. It was found also that Cu agglomeration was affected drastically by underlayer surface roughness. By adding the CMP process to the conventional sputtering and reflow processes, Cu interconnects down to 0.6 jim in width have been fabricated successfully. The resistivity of the interconnects is about 1.9 μω.cm.

High coercivity SmFeSiC films fabricated by multilayer sputtering

SmFeSiC thin films have been obtained by sputtering SmFe/C(Si) multilayers with a Ta underlayer on Si substrates and subsequently annealing at 700/spl deg/C. The coercivity of the SmFeSiC films strongly depends on the sputtering and annealing conditions. The influence of the thickness of the Ta underlayer, the thickness ratio of SmFe to C(Si), the argon pressure and heat treatment were studied. In-plane coercivities up to 7.2 kOe and squareness of 0.94 were obtained.

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.

Effects of barrier layer and processing conditions on thin film Cu microstructure

The crystallographic texture and grain size of sputtered Cu films were characterized as a function of deposition temperature, barrier layer material, and vacuum conditions. For Cu deposited in a HV chamber, (111) Cu texture was found to weaken with increasing deposition temperatures on W, amorphous C and Ta barrier layers, each deposited at 30°C. Conversely, under identical Cu deposition conditions, texture was found to strengthen with increasing deposition temperature on Ta deposited at 100°C. Median Cu grain size varied parabolically with deposition temperature on all barrier layers and was slightly higher on the 100°C Ta at a given Cu deposition temperature, relative to the other underlayers. For depositions in an UHV chamber, Cu texture was found to strengthen with increasing Cu deposition temperature, independent of Ta deposition temperature. Median Cu grain size, however, was still higher on 100°C Ta than on 30°C Ta. The observed differences between the two different chambers suggest that the trend of weak texture at elevated deposition temperatures may be related to contamination. Characterization of the Ta underlayers revealed that the strengthened texture of Cu films deposited on 100°C Ta is likely related to textural inheritance.

Magnetic properties of ion-beam sputter deposited NiFe ultrathin films

Studies have been made of the magnetic properties of NiFe ultrathin films 15–100 Å thick. The NiFe films were deposited on Cu, Ti, and Ta underlayers by an ion-beam sputtering method. In the NiFe films deposited on the Cu layers, the saturation magnetization shows little change with thickness down to 60 Å, but below this it shows a gradual decrease with decreasing thickness. The coercivity in the hard axis is almost unaltered with the film thickness, whereas the coercivity in the easy axis and the anisotropy field decrease with decreasing film thickness. Both values greatly decrease below 30 Å. Thickness dependence of the induced uniaxial anisotropy constant and the direct-current permeability were obtained from these parameters. Remarkable decreases of these parameters were observed in the films on the Ti and Ta layers likely owing to the formation of fine islands. Magnetic properties and giant magnetoresistance in multilayers are discussed on the basis of the present experimental results.

ＮｉＦｅ膜の比抵抗と面粗さに及ぼす下地，基板温度の影響

This paper studies the resistivity and surface roughness of NiFe films on a Ta underlayer at various substrate temperatures. In an experiment, the bulk resistivity ρ∞ decreased with increasing substrate temperature on glass. but decreased with decreasing substrate temperature on a Ta underlayer. The NiFe film density agreed with the bulk value. On glass. the surface roughness of NiFe films increased remarkably with increasing substrate temperature and thickness. On a Ta underlayer, however, it slightly depended on the substrate temperature and thickness.

The Effects of Processing on the Microstructure of Copper Thin Films on Tantalum Barrier Layers

abstractPreferred crystallographic orientation and grain size distribution were characterized as a function of processing for sputtered Cu films on Ta underlayers. The Ta barrier layer was deposited at two temperatures, 30 and 100 °C. Cu was deposited at 30, 150 and 250 °C on the 30 °C Ta, and at 100, 150, 200 and 250 °C on the 100 °C Ta. In the first set of samples, with increasing deposition temperature, the Cu (111) fiber texture grew weaker and the volume fraction of randomly oriented grains increased from 0.23 to 0.74. In contrast, for the films deposited on the 100 °C Ta, with increasing deposition temperature, Cu (111) fiber texture strengthened and the fractions of randomly oriented and twinned grains decreased. Grain size was lognormally distributed in all samples and varied approximately parabolically with deposition temperature. At a given deposition temperature, median grain size in the Cu was larger in the films deposited on the 100 °C Ta. These results will be related to the microstructure of the Ta underlayers. Cu microstructure on the 100 °C Ta is shown to be influenced by textural inheritance from the Ta underlayer. Microstructure of the Cu on 30 °C Ta is discussed in terms of trace contaminants.

Magnetic induced uniaxial anisotropy in NiFe and NiFeCr films (abstract)

We report the experimental results of magnetic uniaxial anisotropy induced during sputtering (K1) and annealing (K2) of NiFe and NiFeCr films. The NiFe and NiFeCr films of around 30 nm thickness were deposited using dc magnetron sputtering. Glass and Ta (20 nm) coated glass were used as substrates. The magnetic properties were determined using a B-H looper at 10 Hz. The crystal orientation was determined using x-ray diffraction. The results show that the predictions from a two-component anisotropy model (that is K2/[K1+K2]) for a 3 h 310 °C anneal are in good agreement with experimental data for both NiFe/glass and NiFeCr/glass, but not for NiFe/Ta/glass and NiFeCr/Ta/glass. This can be explained by the orientation effect from x-ray diffraction studies. The NiFe and NiFeCr have higher (111) preferred orientation when deposited on Ta coated glass as compared to samples deposited on glass only. The effects of annealing temperature and Ta underlayer will be discussed. The activation energy for magnetic annealing induced anisotropy will also be discussed.

Giant Magnetoresistance in As-Sputter-Deposited Au/NiFe Multilayer Thin Films

ABSTRACTGiant magnetoresistance with low saturation fields (Hs’s) is reported in Au and permalloy (Ni0.82Fe0.18) or Co-doped permalloy multilayer thin films as-deposited on Ta-overcoated Si and glass substrates. A ΔR/R as high as 4.0% with ≈25 Oe Hs was observed at 295 K for the film consisting of 10 layers of 24 Å Au/13 Å Ni0.82Fe0.18 deposited on a 3 Å Ta-overcoated glass at 50 °C. A Hs value as low as ≈20 Oe with a 15% smaller ΔR/R has been observed for the films with a thicker (e.g., 50 Å) Ta underlayer. Magnetic hysteresis loops of these films indicate the presence of antiferromagnetic exchange coupling between the Ni0.82Fe0.18 layers. This exchange coupling is much smaller for the multilayer films without the Ta underlayer, resulting in a 6x smaller ΔR/R and lOx larger Hs observed for these films. Results of x-ray diffraction analysis indicate stronger (111) texturing for the multilayer films with a Ta underlayer, consistent with the stronger antiferromagnetic coupling between the the Ni0.82Fe0.18 layers in the film. The addition of 2–10 % Co moderately increases the ΔR/R value, but also increases substantially the Hs (up to ≈200 Oe).