Oblique Sputtering Research Articles

Optimization of high frequency characteristics in Co-Ta thin films

Magnetic properties and high frequency characteristics of binary Co100−xTax (x = 0–24) thin films, prepared by using the oblique sputtering method under an external magnetic field of 1200 Oe have been studied. All the Co100−xTax films are easy to form amorphous phase, and thus exhibit strong in-plane uniaxial magnetic anisotropy, high resistivity, and low coercivity. With increasing x, the saturation magnetization (4πMs) and coercivity along the easy and hard axes (Hce and Hch) are decreased, while the magnetic anisotropy field (Hk) reaches the maximum value of 578 Oe for the films with the composition to become fully amorphous phase. Due to large Hk, high ferromagnetic-resonance frequency (fFMR) of 4.7–6.6 GHz is obtained for Co100−xTax (x = 11–24) films. The optimal magnetic properties of 4πMs = 10.9 kG, Hce = 6.2 Oe, Hch = 6.0 Oe, Hk = 507 Oe, and high resonance characteristics of fFMR = 6.4 GHz, respectively, are reached for Co84Ta16 films. Finally, high fFMR of 5.4–7.2 GHz and Hk of 434–617 Oe can be tuned by the modification of the sputtering parameters. This result reflects the benefit of Co100−xTax (x = 11–22) films for above 5 GHz applications.

Tailoring thermal stability behaviour of magnetic thin films by hybrid oblique gradient-composition sputtering

An experimental study of the magnetic and microwave properties together with the thermal stability of FeCoHf films fabricated by a novel hybrid oblique gradient-composition sputtering technique is presented. It was found that both ferromagnetic resonance frequency and thermal stability can be significantly enhanced by changing the oblique deposition angle. The result is tentatively explained by separating the role of each contribution of the magnetic anisotropy such as stress-induced anisotropy, shape anisotropy and pair-ordering anisotropy. On the basis of this experimental result, it is suggestive that this method is an effective way to fabricate thin films with good microwave performance oriented for high-frequency applications.

Magnetic anisotropy in Fe films deposited on SiO2/Si(001) and Si(001) substrates

The magnetic anisotropy of 10nm iron films deposited in an ultra high vacuum on the Si(001) surface and on the Si(001) over caped by 1.5nm layer of SiO2 was investigated. There is in-plane uniaxial magnetic anisotropy caused by oblique sputtering in the Fe films on a SiO2 buffer layer. The easy magnetization axis is always normal to the atomic flux direction but the value of the anisotropy field is different depending on the axial angle among sputtering direction and the substrate crystallographic axes. It is argued that the uniaxial magnetic anisotropy results from elongated surface roughness formation during film deposition. Several easy magnetization axes are found in Fe/Si(001) film without the SiO2 buffer layer. The mutual orientation of the main easy axes and Si crystallographic axes indicates that there is epitaxial growth of Fe/Si(001) film with the following orientation relative to the substrate: Fe[100] ∥Si[110]. The anisotropy energy of Fe/Si(001) film is estimated by simulation of angle dependence of remnant magnetization mr as the sum of the mr angle plot from uniaxial anisotropy (induced by oblique deposition) and the polar plot from biaxial magnetocrystalline anisotropy.

Structure and magnetic properties of Fe films with pyramid-like nanostructures deposited by oblique target direct current magnetron sputtering

Fe thin films were deposited by oblique target direct current magnetron sputtering on Si (100) and (111) substrates. The structure, surface morphology and magnetic properties of the thin films were characterized using X-ray diffraction, field emission scanning electron microscopy, and superconducting quantum interference device magnetometer, respectively. The results reveal that the structure of the as-deposited Fe thin films is body-centered cubic with the preferential [110] crystalline orientation. A pyramid-like nanostructure with sharp tip was formed on the surfaces of Fe thin films under appropriate sputtering power. Formation of the pyramid-like nanostructure is mainly owed to the enhancement of atomic mobility and the bombardment effect with increasing of sputtering power. Meanwhile, the crystalline orientation of Si substrate and the intrinsic stress in the films are expected to have little contribution to the formation of the pyramid-like nanostructure. The magnetic anisotropy was found in the as-deposited Fe thin films, and varies with the thickness of the Fe thin films. As the film thickness increases from 604 to 1,786 nm, the magnetic anisotropy field and the uniaxial anisotropy constant increase from 3.8 to 5.6 kOe, and from 0.4 × 106 to 1.1 × 106 erg/cm3, respectively, which indicates that besides magnetocrystalline anisotropy, stress induced anisotropy and shape anisotropy also exist in the as-deposited Fe thin films.

A study on structural, optical and hydrophobic properties of oblique angle sputter deposited HfO2 films

HfO2 thin films have been synthesized by oblique angle reactive DC magnetron sputtering technique. Present study reports the effect of deposition angle on the hydrophobic, structural, surface morphological and optical properties of HfO2 thin films. All the films were deposited at room temperature. HfO2 thin films were found to possess monoclinic crystal structure. Significant changes in topography of the films, with change in deposition angle, have been observed. Surface roughness increases with decrease in deposition angle as observed from AFM image analysis. UV–vis spectroscopy has been used to study the optical properties of the films. Small changes in transmission and refractive index have been observed. The films are hydrophobic in nature and the contact angle is strongly influenced by deposition angle due to change in its surface roughness. Contact angle of 106.3° has been achieved for deposition angle of 30° which is highest so far for HfO2 thin films prepared by magnetron sputtering. The results of this study reflect that contact angle of the films can be tuned by deposition angle. Good optical transmission along with hydrophobic character make HfO2 thin films of great use as optical coatings for lenses, windshields and optoelectronic devices.

Shape-anisotropic enhanced damping in CoZr periodic arrays of nanohill structure

The preparation of CoZr nanostructure films to replicate the order of anodized aluminum oxide template with barrier layer was described. Coercivity and in-plane magnetic anisotropy were increased with the increase of oblique sputtering angle. Resonance frequency and damping factor had the same tendency also. Note that larger damping factor in nanostructure films was observed compared with that of continuous films in Si substrate, which was induced by magnetic anisotropy distribution particularly with a significant out-of-plane contribution due to the competition of shape anisotropy.PACS75.75.-c, 75.70.-i, 07.57.Pt

Fabrication and magnetic properties of NiFe–ZnO nano-granular films

Excellent soft magnetic and high frequency properties were obtained successfully in the (Ni75Fe25)x(ZnO)1−x granular films fabricated on the glass substrate by RF magnetron oblique sputtering. The microstructure, magnetic and high frequency properties were investigated systematically. High resolution transmission electron micrographs show that the film consists of fcc Ni75Fe25 particles uniformly embedded in an amorphous insulating matrix ZnO with particle size a few nanometers. The (Ni75Fe25)x(ZnO)1−x films exhibit excellent soft magnetic properties in a wide x range from 0.50 to 0.80 with coercivity not exceeding 5 × 10−4 T, which is ascribed to the exchange coupling between magnetic particles. Especially for the sample with x = 0.64, coercivities in hard and easy axes are 5.0 × 10−5 and 3.6 × 10−4 T, respectively, and the electric resistivity ρ reaches 1,790 μΩ·cm. The dependence of complex permeability u = u′ − ju″ on frequency f shows that the real part u′ is more than 130 below 500 MHz, and the ferromagnetic resonance frequency fr reaches 1.32 GHz, implying the promising for high frequency application.

Compositional and structural properties of nanostructured ZnO thin films grown by oblique angle reactive sputtering deposition: effect on the refractive index

In this work, we report the successful growth of ZnO nanostructured films by oblique angle magnetron sputtering deposition (OAD) from a Zn target in a mixture gas of Ar/O2. The film microstructure and the surface morphology of the samples were explored by HRSEM. The crystalline structure and the composition were determined by XRD and RBS, respectively. The optical properties of the ZnO films (refractive index, n, and extinction coefficient, k) were also studied by spectroscopic ellipsometry. Films were found to be porous and consisting of an inclined columnar structure, with columns tilting in the direction of the incident flux. The experimental results reveal that the deposition angle, the Ar/O2 gas ratio and the distance between the target and the substrate play a significant role in the composition, crystalline structure and optical and electrical properties of the ZnO thin films. In particular, it has been found that using different deposition angles in the range 0°–85° it is possible to control the refractive index varying from 1.9 to 1.5, due to the high porosity of the films as a result of the self-shadowing columnar structure produced during the process.

Enhancing exchange bias and tailoring microwave properties of FeCo/MnIr multilayers by oblique deposition

A systematic study of the static and dynamic magnetic properties with regards to temperature for FeCo/MnIr multilayered thin films fabricated by oblique sputtering technique was carried out. Compared with the film produced by conventional non-oblique sputtering method, those films grown by oblique deposition show a significant increment of exchange bias. The magnetic anisotropy field and the ferromagnetic resonance frequency can also be enhanced and tailored by changing oblique deposition angle. In addition, thermal stability of the dynamic characteristics of the films was presented and discussed from application-oriented perspective.

Observation of rotatable stripe domain in permalloy films with oblique sputtering

Stripe domain (SD) in obliquely sputtered permalloy films were investigated by comparing with normally sputtered ones. The critical thickness for SD formation of obliquely sputtered films was about 100 nm thinner than that of normally sputtered films. The hysteresis loops of obliquely sputtered films showed a peculiar shape. A rotation of SD towards easy axis was observed in the obliquely sputtered films, which was confirmed by permeability spectra under a bias field. The origin of the rotation could result from in-plane uniaxial anisotropy, which is induced by the shape effect of the oblique columnar growth of permalloy grains.

Important Role of Nanopore Morphology in Superoleophobic Hierarchical Surfaces

This work reports the importance of nanopore morphology in designing super liquid-repellent submicropillar/nanopore hierarchical surfaces. The hierarchical surfaces were fabricated using a combined process of oblique angle sputter deposition of aluminum with subsequent anodizing, and the surfaces were coated with a fluorinated alkyl phosphate layer to reduce the surface energy. The size of the nanopores, the interpore distance, and the porosity of the anodic films on the submicrometer pillars were controlled by varying the anodizing and pore-widening conditions. The present study demonstrates that super liquid repellency can be achieved on intrinsically oleophilic surfaces by introducing hierarchical submicropillar/nanopore morphology even for oils with surface energies as low as ∼25 mN m–1. The porosity in the submicrometer pillars was a key factor in influencing the contact angle hysteresis; higher porosity is needed to reduce the contact angle hysteresis.

Enhanced microwave absorption in columnar structured magnetic materials

CoZr columnar structured magnetic films were fabricated by oblique sputtering onto porous aluminum oxide substrates with different oblique angles. The scanning electron microscope images showed the formation of columnar structure and nanoporous structure was disappeared with increasing oblique angle. The static magnetic properties showed larger coercivity and lower magnetization squareness due to columnar structure resulted from nanopores. Dynamic magnetic properties were determined by effect of holes and oblique sputtering. Enhanced microwave absorption was obtained by complex permeability measurement, which line width of columnar film increased 1.4 GHz comparing with 0.5 GHz of continuous film. Therefore, it is an effective way to get the adjustable anisotropy field and line width, which is desirable for obtaining the high resonance and high permeability ferromagnetic film materials for high frequency application.

Fabrication of size-controllable nanofluidic channels using angled physical vapor deposition

► Sealed nanochannels with well-controlled dimensions are fabricated using NIL and PVD. ► The width of nanochannels are controlled by the imprint depth ( h ) as well as the incident angle ( θ ). ► Silicon oxide film is chosen as deposition cover material for its optics transparency. ► Ultrasmall nanochannels downing size to about 50 nm is fabricated in large area. ► The method is applicable for most substrates and cover layer. In this paper, we demonstrate a new and simple method to achieve ultrasmall nanochannels downing size to about 50 nm with well-controlled dimensions in large area by using NIL and angled physical vapor deposition (PVD). The polymer SU-8 nanogratings substrate is fabricated using NIL. After NIL, oblique angle ion beam sputtering of silicon oxide was used, in a single step, to reduce the cross section of the SU-8 imprinted trenches and seal the channels at the same time. During this process, the sidewalls of any trench will shadow the evaporation and most of the silicon oxide film will be deposited on the sidewalls at the top of the trench, therefore, the shadow area will be protected and ultimately formed as sealed nanochannels on substrate. The advantage of this process is that the line width can be adjustable simply by controlling the deposition as well as the imprint depth, offering an alternative solution for high-resolution and low-cost fabrication of nanofluidic chip. The method is applicable for most substrates and cover layer, therefore, it provides a new way to produce size reduction and size controlling of nanopatterns for various material.

A Unified Spatio-Temporal Framework of the Cuerno—Barabasi Stochastic Continuum Model of Surface Sputtering

The nonlinear continuum model proposed by Cuerno and Barabasi is the most successful and widely acceptable theoretical description of oblique incidence ion sputtered surfaces to date and is quite robust in its predictions of the time evolution and scaling of interfaces driven by ion bombardment. However, this theory has thus far predicted only ripple topographies and rough surfaces for short and large scales, respectively. As a result, its application to the interpretation and study of nanodots, predicted by Monte Carlo simulations for, and observed in experiments of, oblique incidence sputtering is still unclear and, hence, an open problem. In this paper, we provide a new insight to the theory, within the same length scale, that explains nanodot formation on off-normal incidence sputtered surfaces, among others, and propose ways of observing the predicted topographies of the MC simulations, as well as possible control of the size of the nanodots, in the framework of the Cuerno—Barabasi continuum theory.

Incident flux angle induced crystal texture transformation in nanostructured molybdenum films

Molybdenum films were observed to undergo a dramatic change in crystal texture orientation when the incident flux angle was varied in an oblique angle sputter deposition on amorphous substrates. Reflection high-energy electron diffraction pole figure and scanning electron microscopy were used to analyze in detail the texture orientation of the films. The normal incident deposition resulted in a fiber texture film with the minimum energy (110) crystal plane parallel to the substrate surface. A (110)[11¯0] biaxial texture was observed for the samples grown with low incident angles of less than 45°, with respect to the surface normal. On the other hand, for an oblique angle deposition of larger than 60°, a (111)[112¯] biaxial texture was observed and appeared to be consistent with a zone T structure where the geometrically fastest growth [001] direction of a crystal plays a dominant role in defining the texture. We argue that a structural transition had occurred when the incident flux was varied from near normal incidence to a large angle.

Tunable misalignment of ferromagnetic and antiferromagnetic easy axes in exchange biased bilayers

In this paper we report experiments that show how to tune the unidirectional anisotropy field in exchange biased Ni81Fe19/Ir20Mn80 bilayers grown by sputtering. During growth the samples are held in an obliquely inclined stage, and simultaneously a static magnetic field is applied along an arbitrary direction in the film plane. While the direction of the ferromagnetic anisotropy field is given by the tilted columnar microstructures induced by the oblique sputtering, the direction of the unidirectional field can be tuned by the application of the in situ magnetic field. The magnetic properties were investigated using the ferromagnetic resonance technique.

Tunable resonance frequency of FeNi films by oblique sputtering

FeNi thin films were fabricated by radio frequency magnetron sputtering on Si(111). Dynamic properties at remanence of the films were systematically investigated in a wide frequency range from 100MHz to 5GHz. The results show that both thickness of FeNi films and oblique angle have important effects on the magnetic properties of the films, the magnetic resonant frequency of the films can also be adjusted by the two factors. The in-plane uniaxial magnetic anisotropy field can be adjusted from 82Oe to 220Oe by increasing the oblique angle. As a consequence, the magnetic resonant frequency of the films increased from 2.7GHz to 4.2GHz.

Effect of nanoparticle size on magnetic damping parameter in Co92Zr8 soft magnetic films

Co92Zr8(50nm)/Ag(x) soft magnetic films have been prepared on Si (111) substrates by oblique sputtering at 45°. Nanoparticle size of Co92Zr8 soft magnetic films can be tuned by thickening Ag buffer layer from 9nm to 96nm. The static and dynamic magnetic properties show great dependence on Ag buffer layer thickness. The coercivity and effective damping parameter of Co92Zr8 films increase with thickening Ag buffer layer. The intrinsic and extrinsic parts of damping were extracted from the effective damping parameter. For x=96nm film, the extrinsic damping parameter is 0.028, which is significantly larger than 0.004 for x=9nm film. The origin of the enhancement of extrinsic damping can be explained by increased inhomogeneity of anisotropy. Therefore, it is an effective method to tailor magnetic damping parameter of thin magnetic films, which is desirable for high frequency application.

Laterally grown ZnO nanorod arrays on an obliquely deposited seed layer and its UV photocurrent response

Laterally grown ZnO nanorods have advantages over vertically grown ones in terms of process complexity in ultraviolet photodetector fabrication. A ZnO seed layer was deposited on a glass substrate by an oblique-angle sputtering method and Cr thin film was deposited and patterned as the metal contact with the ZnO nanorods. Cr lines also acted as a growth barrier to suppress vertical ZnO growth. Oblique sputter deposition of ZnO thin films resulted in an inclined columnar growth, and the ZnO nanorods were grown laterally and unidirectionally by a Cr growth barrier in a hydrothermal bath. With 80^o oblique deposition, the c-axis of the ZnO seed layer inclined by 5^o from the vertical direction. The photocurrent of the detector in 365nm illumination increased by 100 times over that of the dark current. Passivation of the ZnO nanorod surface by polydimethyl siloxane (PDMS) showed an even higher photocurrent.

Nanostructures and sensing properties of ZnO prepared using normal and oblique angle deposition techniques

Nanostructured zinc oxide (ZnO) for gas sensing application has been prepared by using normal and oblique angle sputtering deposition techniques under different substrate temperatures. Oblique angle plasma beam deposition is demonstrated effectively growing large-area uniformly aligned and inclined ZnO nanorod arrays on catalyst-free silicon substrate due to a self-shadowing mechanism, whereas normal radio frequency sputtering deposition yields nanoparticles as island growth mode. Furthermore, the density of the nanorod arrays is dependent on the incident angle of ZnO plasma beam. With an increase of the incident flux angle, large inter spacing was induced, resulting in sparser nanorod arrays. The nanorod arrays grown with an incident angle of 70° have an average diameter of ∼150–300nm and length of ∼700–750nm. The experimental data from characterization of the samples indicates that the obtained samples at different substrate temperatures and incident angles have wurtzite structure with a c-axis orientation.Sensing characterization reveals that the nanorod-based sensor shows higher sensitivity, faster response and recovery time, as well as better reproducibility than that of nanoparticle-based gas sensor to 100ppm hydrogen and methane at low operating temperature below 150°C due to the porosity and large grain boundaries of the nanorod arrays. It demonstrates that oblique angle of sputtering deposition is a simple, inexpensive synthesis process to get high-porosity nanostructures and as a result, improves the sensing properties of fabricated ZnO sensors, which permits us to obtain sensors with high sensitivity, low operating temperature and stability.