Sputter-deposited Thin Films Research Articles

Sticking probabilities of Cu, Zn, Sn, and S atoms in magnetron sputtering plasmas employing a Cu2ZnSnS4 stoichiometric target

Investigations on the density decays in the afterglow of pulsed magnetron sputtering plasmas employing a Cu2ZnSnS4 (CZTS) stoichiometric target were performed to evaluate the sticking probabilities of Cu, Zn, Sn and S atoms. The sticking probabilities were evaluated from the linear relationships between the decay time constants of the atom densities and the discharge pressure. It has been found that the sticking probabilities of Cu, Zn, and Sn are almost the same. On the other hand, the sticking probability of S atom was found to be as low as 0.7 ± 0.1, if we assume unity for the sticking probabilities of Cu, Zn, and Sn. Therefore, it has been shown that the less-abundant composition of S atoms in sputter-deposited CZTS thin films is attributed to the small sticking probability of S.

Effects of post-annealing on the structural and nanomechanical properties of sputter-deposited FePd thin films

The effects of post-annealing on the microstructures, surface morphologies and nanomechanical characteristics of FePd thin films are investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) and nanoindentation techniques. The FePd thin films were deposited on the glass substrates at room temperature by magnetron sputtering system. Post-annealing was carried out at 500, 600 and 700 °C, respectively, resulting in progressive increase of the average grain size and surface roughness of FePd thin films, as well as the improved film crystallinity. XRD results show that FePd thin films are predominantly (111)-oriented, indicating a well-ordered microstructure. The hardness and Young's modulus of FePd thin films measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option indicated that both mechanical properties are decreased with increasing the annealing temperature. Furthermore, the relationship between the hardness and films grain size appears to follow closely with the Hall–Petch equation.

Structures, morphologies, and chemical states of sputter-deposited CrZrN thin films with various Zr contents

Chromium zirconium nitride (CrZrN) thin films were prepared on Si wafers and glasses at various Zr contents by reactive DC magnetron co-sputtering of Cr and Zr metals in Ar and N2 mixture without voltage biasing and external heating. Influences of the Zr contents on crystal structure, cross-section morphology, surface morphology, and chemical composition and chemical state were investigated by X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy, respectively. The results showed that N content in the films was increased when Zr content increased. Film microstructure changed from coarse columnar to finer-grain morphology and film surface became smooth caused by grain refinement. Zr–metal and Zr–N bonding fractions were increased with the increasing Zr content, whereas Cr–N was decreased due to being substituted by Zr of Cr atoms in the fcc B1 type crystal structure of (Cr, Zr)N. In addition to an increase in lattice parameter, the substitution of Zr could lead to an increase in interatomic distances which affected bonding length between metals and nitrogen atoms. According to the charge potential model, the increase in bonding distances between atoms resulted in negative shifts in binding energy of electrons of all elements that led to observation of lowering in the separation between photoelectron lines of Cr, Zr, and N elements. The lower separation with the increase of Zr content suggested that bonding between metals and nitrogen became stronger due to the dominance of the covalent character as evidenced by the enhanced hardness of the CrZrN materials.

Surface characteristics and electrochemical behaviour of sputter-deposited NiTi thin film

Thin nanocrystalline amorphous NiTi film was deposited on Si substrate using DC magnetron sputtering. The as-deposited NiTi thin film was crystallized by heat treatment at 500 °C for 1 h. The crystal structure, surface morphology, microstructure and surface chemistry of the deposited films were studied using X-ray diffraction, atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS), respectively. Corrosion behaviour was assessed in Ringer’s solution at 37 °C by open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy as a function of exposure time. OCP values indicate that the tendency for the formation of a spontaneous oxide film is greater for the NiTi thin films than the bulk NiTi. Long time exposure to Ringer’s solution was found to have a great effect on the corrosion behaviour of the samples. Significantly low corrosion current density was obtained for the annealed NiTi film from the potentiodynamic polarization curves indicating a typical passive behaviour, but as-deposited film and bulk NiTi alloy exhibited breakdown of passivity at potentials approximately +1.4 V (vs. SCE). XPS showed that the oxide film formed on the annealed NiTi thin film mainly composed of Ti oxides, and no evidence of Ni was found up to 8.2 nm beneath the top surface, suggesting the excellent corrosion resistance of this sample in Ringer’s solution.

Influence of deposition conditions on the composition, texture and microstructure of RF-magnetron sputter-deposited hydroxyapatite thin films

In this study, nanostructured hydroxyapatite (HA) coatings were deposited at a constant power via radio frequency magnetron sputtering. Substrate rotation was applied to increase the uniformity of the thin film properties. The crystallographic orientation, composition and microstructure of the coatings were examined. The characteristics of the deposited coatings associated with the applied deposition conditions were evaluated as a function of time that the samples spent under the target racetrack. The obtained thin films exhibit a change in out-of-plane orientation from preferential (002) to (300) and an increase in the Ca/P molar ratio when exposed for a longer time under the racetrack. This introduces the possibility of tuning crystallographic orientations of HA coatings.

Crystallization dynamics and interface stability of strontium titanate thin films on silicon.

Different physical vapor deposition methods have been used to fabricate strontium titanate thin films. Within the binary phase diagram of SrO and TiO2 the stoichiometry ranges from Ti rich to Sr rich, respectively. The crystallization of these amorphous SrTiO3 layers is investigated by in situ grazing-incidence X-ray diffraction using synchrotron radiation. The crystallization dynamics and evolution of the lattice constants as well as crystallite sizes of the SrTiO3 layers were determined for temperatures up to 1223 K under atmospheric conditions applying different heating rates. At approximately 473 K, crystallization of perovskite-type SrTiO3 is initiated for Sr-rich electron beam evaporated layers, whereas Sr-depleted sputter-deposited thin films crystallize at 739 K. During annealing, a significant diffusion of Si from the substrate into the SrTiO3 layers occurs in the case of Sr-rich composition. This leads to the formation of secondary silicate phases which are observed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.

Spontaneous growth of bismuth nanowires on a sputter-deposited thin bismuth film

Thin films of bismuth of thickness 100nm have been deposited onto Si(111) substrates held at 120°C using magnetron sputter deposition. The three-inch magnetrons were operated at DC powers between 30W and 50W to give deposition rates between 0.03 and 0.09nms−1. Examination of the surface of the thin films using scanning electron microscopy revealed nanowires of diameter 80–120nm and length between 3 and 16μm emerging from the film. An investigation of the effect of deposition rate found little influence on the nanowire diameter, but indicated that some longer nanowires could be found on the film deposited at the lowest rate. A 20nm film grown on glass at 110°C shows a nanowire with dimension of around 50nm. Using a relatively higher rate for the first 25nm followed by a lower rate to 100nm produced some very straight and regular nanorods.

Fabrication of SiGeSb heating electrodes and their application for four-terminal chalcogenide programmable switches

This paper reports on sputter-deposited SiGeSb thin films and their application for four-terminal chalcogenide switch devices. The microstructures and electrical properties of the SiGeSb films were highly dependent on antimony concentration and annealing temperature. Microstructural changes such as surface roughening and formation of antimony grains were observed only for the Sb-rich SiGeSb films after annealing at 400 °C and higher. The sheet resistance of the SiGeSb films containing a small amount of antimony changed sporadically with annealing temperature because of a trade-off between activation and surface depletion of antimony. The resistance of the SiGeSb heating electrodes was varied by changing sputtering power for the antimony target and by changing the annealing temperature. Four-terminal chalcogenide switch devices were fabricated with SiGeSb heating electrodes of varying resistance. It was found that the switching voltage of the fabricated switch device was proportional to the resistance of the SiGeSb heating electrode. This indicates that the SiGeSb films with tunable sheet resistance are of great importance in fabricating chalcogenide switch devices and the optimization of the resistance of the SiGeSb film is essential to ensure proper switch operation.

Effect of oxygen/argon gas ratio on the structure and optical properties of sputter-deposited nanocrystalline HfO2 thin films

Nanocrystalline hafnium oxide (HfO2) thin films have been produced under variable reactive oxygen (O2) fractionation (Г) employing Hf metal for reactive sputter-deposition. The effect of Г on the HfO2 compound formation, structure, morphology and optical properties has been evaluated. Without oxygen, the films of hexagonal phase of Hf metal were grown. Films grown at different O2 pressure are nanocrystalline, monoclinic HfO2 with (1¯11) texturing. The optical properties of HfO2 films have been evaluated using spectroscopic ellipsometry (SE). The optical constants and their dispersion profiles indicate that the best optical-quality HfO2 films are formed at O2 ratio of ≥0.2. The index of refraction (n) profiles derived from SE measurements follow the Cauchy dispersion relation. The correlation between oxygen-fraction and optical properties in HfO2 films is established.

Free-standing NiTi alloy nanowires fabricated by nanoskiving.

We report on free-standing NiTi alloy nanowires (120 nm × 75 nm) fabricated using a technique referred to as "nanoskiving", which complements conventional thin film sputter deposition with ultramicrotomy for thin sectioning. To date, the technique has been limited to pure metals without exploring metallic alloys. Leveraging the technique for the fabrication of shape memory alloy (SMA) nanostructures meets two critical requirements: compositional control (via film deposition) and controlled dimensions (via film deposition and programmable sectioning). Microstructure and composition analysis confirm continuity of the produced nanowires and Ni and Ti elemental uniformity. Free-standing NiTi nanowires are robust and remain intact throughout physical manipulation. The fabrication of NiTi alloy nanowires by nanoskiving will advance fundamental characterization of small scale SMA behavior.

Crystalline Size Effects on Texture Coefficient, Electrical and Optical Properties of Sputter-deposited Ga-doped ZnO Thin Films

C -axis oriented Ga-doped ZnO (GZO) films with various thicknesses were deposited on glass substrate by radio frequency (RF) magnetron sputtering. The dependence of crystal structure, electrical, and optical properties of the GZO films on crystalline size were systematically studied. The results showed that the texture coefficient of (002) peak ( TC (002) ) decreases with increasing crystalline size. The Hall mobility μ was reciprocal to electron effective mass and the fitted relaxation time τ was 0.11 ± 0.01 μs. With the increase of average crystalline size, the resistivity increased slightly, which is caused by the competition of (002) and (101) plane, introducing in some defects and leading to carrier density reduction. The optical band gap was in the range from 3.454 to 3.319 eV with increasing crystalline size from 26.96 to 30.88 nm, showing a negative relationship. The dependence of optical band gap ( E g op ) on the crystalline size ( R ) can be qualitatively explained by a quantum confinement effect. The relationship between E g op and R of GZO films suggests that tuning up optical properties for desired applications can be achieved by controlling the crystalline size.

Development of a sputter-deposited nanocarbon thin film electrode for use as a biosensor

We have developed nanocarbon film electrodes formed by an electron cyclotron resonance (ECR) sputtering method. The nanocarbon film contains a nano-crystalline sp2 and sp3 mixed bond structure with an atomically flat surface and has a wide potential window, relatively high electrochemical activity and a low background noise level similar to a boron-doped diamond electrode. Surface conditions such as hydrophilicity on the nanocarbon film electrode can be controlled by electrochemical treatments while maintaining the wide potential window and surface flatness, and, therefore, in measurements of various biomolecules including oxidatively damaged DNA and oligonucleotides, their adsorption onto the electrode surface is suppressed. For example, cytosine and methylated cytosine in oligonucleotides can be distinguished by square wave voltammetry. A carbon electrode with a thorn-like nanostructure could be fabricated by UV/ozone treatment using different etching rates depending on nanometer-order differences in the local sp3 content of the nanocarbon film. This nanostructured surface was very effective in improving efficient direct electron transfer (DET) with enzymes such as bilirubin oxidase. Carbon film electrodes doped with metal nanoparticles have been also developed by co-sputtering metal and carbon. A Pt-nanoparticle-dispersed carbon electrode shows high electrochemical activity and stability for detecting hydrogen peroxide indicating that it is suitable for use as a biosensor. [TANSO 2014 (No. 264), 133–9.]

The magnetic properties and microstructure of Co-Pt thin films using wet etching process.

Perpendicular magnetic recording (PMR) is a promising candidate for high density magnetic recording and has already been applied to hard disk drive (HDD) systems. However, media noise still limits the recording density. To reduce the media noise and achieve a high signal-to-noise ratio (SNR) in hard disk media, the grains of the magnetic layer must be magnetically isolated from each other. This study examined whether sputter-deposited Co-Pt thin films can have adjacent grains that are physically isolated. To accomplish this, the effects of the sputtering conditions and wet etching process on magnetic properties and the microstructure of the films were investigated. The film structure was Co-Pt (30 nm)/Ru (30 nm)/NiFe (10 nm)/Ta (5 nm). The composition of the Co-Pt thin films was Co-30.7 at.% Pt. The Co-Pt thin films were deposited in Ar gas at 5, 10, 12.5, and 15 mTorr. Wet etching process was performed using 7% nitric acid solution at room temperature. These films had high out-of-plane coercivity of up to 7032 Oe, which is twice that of the as-deposited film. These results suggest that wet etched Co-Pt thin films have weaker exchange coupling and enhanced out-of-plane coercivity, which would reduce the medium noise.

Characterization of spectral selectivity of tin-doped In2O3 thin films using spectroscopic ellipsometry

Transparent conductive oxide (TCO) films on cover glass are one of the key materials for solar thermal collector. Currently, the benefit of using TCO single layer is limited due to its insufficient optical selectivity. The objective of this paper is to increase the spectral selectivity of ITO film with different thickness by whole solar spectrum antireflection treatment. The sputter-deposited ITO thin films on glass substrate are characterized by variable-angle spectroscopic ellipsometry in the spectral range of 300nm to 2500nm. Based on suitable dielectric function models such as Drude, Gaussian, and Tauc–Lorentz, the theoretical spectra for the transmittance and reflectance of ITO film with different thickness in the spectral range from 300nm to 50μm are calculated. Also, the multi-layered AR coatings for ITO film with different thickness in the broad solar spectrum are designed using optical software. For ITO film with thickness from 40 to 120nm, an increase of the solar transmittance more than 6% is achieved, without compromise in the infrared emissivity. Following such AR designs, ITO three-layer AR coating (glass/Al2O3/ITO (120nm)/SiO2) was fabricated. The solar transmittance has been improved from 79.1% without any AR coatings to 84.7% with a three-layer AR coating, and the IR emissivity remains about 0.2 unchanged.

Tunable exchange bias-like effect in patterned hard-soft two-dimensional lateral composites with perpendicular magnetic anisotropy

Patterned hard-soft 2D magnetic lateral composites have been fabricated by e-beam lithography plus dry etching techniques on sputter-deposited NdCo5 thin films with perpendicular magnetic anisotropy. Their magnetic behavior is strongly thickness dependent due to the interplay between out-of-plane anisotropy and magnetostatic energy. Thus, the spatial modulation of thicknesses leads to an exchange coupled system with hard/soft magnetic regions in which rotatable anisotropy of the thicker elements provides an extra tool to design the global magnetic behavior of the patterned lateral composite. Kerr microscopy studies (domain imaging and magneto-optical Kerr effect magnetometry) reveal that the resulting hysteresis loops exhibit a tunable exchange bias-like shift that can be switched on/off by the applied magnetic field.

Effect of KCN Treatment on Cu-Se Secondary Phase of One-step Sputter-deposited CIGS Thin Films Using Quaternary Target

The structural, optical and electrical properties of sputter-deposited CIGS films were directly influenced by the sputtering process parameters such as substrate temperature, working pressure, RF power and distance between target and substrate. CIGS thin films deposited by using a quaternary target revealed to be Se deficient due to Se low vapor pressure. This Se deficiency affected the overall stoichiometry of the films, causing the films to be Cu-rich. Current tends to pass through the Cu-Se channels which act as the shunting path increasing the film conductivity. The crystal structure of CIGS thin films depends on the substrate orientation due to the influence of surface morphology, grain size and stress of Mo substrate. The excess of Cu was removed from the CIGS films by KCN treatment, achieving a suitable Cu concentration (referred as Cu-poor) for the fabrication of solar cell. Due to high Cu concentrations on the CIGS film surface induced by Cu-Se phases after CIGS film deposition, KCN treatment proved to be necessary for the fabrication of high efficiency solar cells. Also during KCN treatment, dislocation density and lattice parameter decreased as excess Cu was removed, resulting in increase of bandgap and the decrease of conductivity of CIGS films. It was revealed that Cu-Se secondary phase could be removed by KCN wet etching of CIGS films, allowing the fabrication of high efficiency absorber layer.

On the evolution of residual stress at different substrate temperatures in sputter-deposited polycrystalline Mo thin films by x-ray diffraction

The evolution of in-plane biaxial residual stress in polycrystalline Mo thin films deposited on Si at different substrate temperature were investigated using x-ray diffraction methods. The analyses were performed using both single and multi hkl reflection peak shifts methods in the asymmetric grazing incidence geometry. High compressive stress builds up in films synthesized at low substrate temperature while tensile stresses which are relatively low in magnitude manifest themselves at high substrate temperature. While such an evolutionary pattern is a direct consequence of competition between the intrinsic and thermal components of stress and varies as a function of temperature, the predominant cause of intrinsic compressive stress in Mo films deposited at low substrate temperature was traced to oxygen impurities incorporated in the film during the growth process.

Controlling the preferred orientation in sputter-deposited Cu2O thin films: Influence of the initial growth stage and homoepitaxial growth mechanism

We report the homoepitaxial growth of Cu2O thin films with tunable growth orientations on glass and silicon substrates using reactive sputtering. The importance of the initial growth stage on the subsequent development of specific orientations is detailed. Among the various deposition conditions, the total pressure is found to have a strong effect on the preferred orientation of Cu2O films: 〈100〉 at low pressure and 〈111〉 at high pressure. Furthermore, the texture of the deposited films is also influenced by that of a first Cu2O layer acting as a continuous seed layer. Effectively, using a two-step deposition procedure, a highly textured Cu2O thin film with controlled orientation can be formed independently of the deposition conditions. Provided that it has a sufficient thickness, this layer acts as a seed layer that determines the crystal orientation of a second layer grown even after air exposure and/or without specific cleaning of the surface prior to deposition. High-resolution transmission electron microscopy analyses evidence that the top layer grows in homoepitaxy with the seed layer with a microstructure consisting of single crystal columns crossing the interface between the layers.

Modification of optical and mechanical surface properties of sputter-deposited aluminum thin films through ion implantation

Aluminum (Al) thin films are used widely as an electronic material in a variety of applications because of their high conductivity, optical reflectance and low cost. In the present study, helium (He) and nitrogen (N2) ions were implanted in sputter-deposited Al thin films with different doses and energies, and the changes in the film properties, such as the surface roughness, optical reflectance, hardness and Young’s modulus, were investigated. The results showed that the implantation of both ions smooth the surface of Al thin films by decreasing the hillock density, resulting in low global surface roughness. In particular, in the case of He ion implantation, the moderated degradation of optical reflectance was observed compared to the film implanted with nitrogen ion. On the other hand, excessive ion implantation increased the local (short-range) surface roughness, which deteriorated the optical reflectance of the Al films. The continuous stiffness measurement technique in nano-indentation showed that ion implantation increased the hardness near the surface. He ion implantation hardened the surface of the Al thin film, and almost 2 times higher surface hardness was achieved with an ion dose of 1018 ions/cm2 at 40 KeV.

Finite Element Modelling of Stress-Induced Fracture in Ti-Si-N Films

Nanocomposite coating films have been increasingly used in industrial applications because of their unique mechanical and physical properties. Residual stresses generated during the growth of sputter-deposited thin films due to a strain mismatch between the film and the substrate may lead to significant failure problems. Large residual stresses may generate buckling, delamination and film fracture. Although buckles with cracks in thin films have been experimentally observed, their origins are still not well understood.