Conventional Sputtering Techniques Research Articles

Phase coexistence and transitions between antiferromagnetic and ferromagnetic states in a synthetic antiferromagnet

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field ∼100 mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields ∼ 40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices whose operation is based on spin textures in perpendicularly magnetized SAFs. Published by the American Physical Society 2024

Atomic Layer Deposition of Ultrathin ZnO Films for Hybrid Window Layers for Cu(Inx,Ga1-x)Se2 Solar Cells.

The efficiency of thin-film chalcogenide solar cells is dependent on their window layer thickness. However, the application of an ultrathin window layer is difficult because of the limited capability of the deposition process. This paper reports the use of atomic layer deposition (ALD) processes for fabrication of thin window layers for Cu(Inx,Ga1−x)Se2 (CIGS) thin-film solar cells, replacing conventional sputtering techniques. We fabricated a viable ultrathin 12 nm window layer on a CdS buffer layer from the uniform conformal coating provided by ALD. CIGS solar cells with an ALD ZnO window layer exhibited superior photovoltaic performances to those of cells with a sputtered intrinsic ZnO (i-ZnO) window layer. The short-circuit current of the former solar cells improved with the reduction in light loss caused by using a thinner ZnO window layer with a wider band gap. Ultrathin uniform A-ZnO window layers also proved more effective than sputtered i-ZnO layers at improving the open-circuit voltage of the CIGS solar cells, because of the additional buffering effect caused by their semiconducting nature. In addition, because of the precise control of the material structure provided by ALD, CIGS solar cells with A-ZnO window layers exhibited a narrow deviation of photovoltaic properties, advantageous for large-scale mass production purposes.

Magnetic and Electrical (GMR) Properties of Rh(IrMn)/Co/Cu/Ni(Py) Multilayered Thin Films

The magnetic and magnetoresistance properties of multilayered structures consisting of two different combinations of magnetic and nonmagnetic IrMn, Co, Cu, Ni, Py thin layers were studied in the temperature range of 10–300 K. Thin films were prepared by using conventional magnetic sputtering techniques in ultra-high vacuum conditions. The structural characterization was examined by X-ray reflectivity. Magnetic and electrical properties were studied by conventional dc magnetization and 4-probe resistivity measurements. A close relationship between magneto resistivity and the relative behavior of magnetic hysteresis of adjacent magnetic layers was observed. Very abrupt stepwise changes at coercive field regions of the soft magnetic layers were attributed to the so-called giant magnetoresistance effect at low temperatures in both combinations of the layers.

Modification of magnetron sputter deposition of nc-WC/a-C(:H) coatings with an additional RF discharge

It is known that the degree of ionization of the coating precursors is one of the key factors in a control of the structure and properties of sputter deposited carbon-based coatings. Numerous modifications of a conventional magnetron sputtering technique were elaborated in order to increase the degree of plasma ionization. The most effective of them were based on an application of an additional source of plasma excitation, independent of magnetron discharge. This paper presents results of the investigations of nc-WC/a-C(:H) coatings, deposited by means of magnetron sputtering of graphite and tungsten targets, enhanced by an additional radio-frequency discharge of a maximum power of 150 W, limited to a volume directly adjacent to the specimens. In order to assess thickness, surface morphology and chemical composition of the coatings, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques were used, while Raman Shift Spectroscopy (RSS) together with XPS was applied to determine the types of bonds between carbon atoms. Tribological properties of the coatings were investigated using a ‘pin-on-disc’ method, with the friction couple comprising bearing steel counterbody. It has been found that additional RF discharge has a significant effect on the type of bonds of carbon atoms in the coating. In the case of non‑hydrogenated nc-WC/a-C coatings, an introduction of additional RF discharge caused an increase of a contribution of sp3 hybridized carbon atoms by 32%, as well as an increase of friction coefficient. The highest share of sp3 hybridization was obtained in the case of hydrogenated nc-WC/a-C:H coatings deposited in Ar + CH4 atmosphere with an enhancement of additional RF discharge maintained at 150 W of power. These conditions brought about a decreased friction coefficient (to a value of 0.05) and a decreased wear rate (to an approximate value of 10−17 m3 N−1 m−1).

High-Performance Thin-Film Transistors of Quaternary Indium-Zinc-Tin Oxide Films Grown by Atomic Layer Deposition.

A new deposition technique is required to grow the active oxide semiconductor layer for emerging oxide electronics beyond the conventional sputtering technique. Atomic layer deposition (ALD) has the benefits of versatile composition control, low defect density in films, and conformal growth over a complex structure, which can hardly be obtained with sputtering. This study demonstrates the feasibility of growing amorphous In-Zn-Sn-O (a-IZTO) through ALD for oxide thin-film transistor (TFT) applications. In the ALD of the a-IZTO film, the growth behavior indicates that there exists a growth correlation between the precursor molecules and the film surface where the ALD reaction occurs. This provides a detailed understanding of the ALD process that is required for precise composition control. The a-IZTO film with In/Zn/Sn = 10:70:20 was chosen for high-performance TFTs, among other compositions, regarding the field-effect mobility (μFE), turn-on voltage ( Von), and subthreshold swing (SS) voltage. The optimized TFT device with the a-IZTO film thickness of 8 nm revealed a high performance with a μFE of 22 cm2 V-1 s-1, Von of 0.8 V, and SS of 0.15 V dec-1 after annealing at 400 °C for 30 min. Furthermore, an emerging device such as a vertical channel TFT was demonstrated. Thus, the a-IZTO ALD process could offer promising opportunities for a variety of emerging oxide electronics beyond planar TFTs.

Removing overhang and increasing atom re-deposition of sputtering to enable gap-filling scalability

Semiconductor manufacturing has reached sub-10-nm technology nodes, and nanoscale gap filling has become an emergency critical requirement for memory with high density and low power consumption. However, it is limited by the non-conformal deposition of current sputtering techniques, such as physical vapor deposition, despite its high quality, low impurity level, and good composition controlling. Here, we present an excellent void-free gap filling results for sub-10-nm nano-pores, and extend the gap-filling limitation of the conventional sputtering technique by the deposit-etch-deposit (DED) method. Coupled with investigating the DED mechanism via the experimental measurements on the nano-pores with different sizes, results indicate that the overhang is removed and atoms are re-deposited during the etching step, which are critical factors for DED process. DED parameters including deposition, etching, and cycling times are investigated and optimized comprehensively. Furthermore, element uniformity and electrical performance are comparable between DED and conventional sputtering technique and have little difference. These results demonstrate that the DED technique, owing to its flexibility and low-cost application compared to conventional sputtering tools, is a potential nanoscale gap-filling solution for sub-10-nm technology nodes.

Enhanced properties of tungsten thin films deposited with a novel HiPIMS approach

Abstract Despite the tremendous potential for industrial use of tungsten (W), very few studies have been reported so far on controlling and tailoring the properties of W thin films obtained by physical vapor deposition techniques and, even less, for those deposited by High Power Impulse Magnetron Sputtering (HiPIMS). This study presents results on the deposition process and properties characterization of nanocrystalline W thin films deposited on silicon and molybdenum substrates (100 W average sputtering power) by conventional dc magnetron sputtering (dcMS) and HiPIMS techniques. Topological, structural, mechanical and tribological properties of the deposited thin films were investigated. It was found that in HiPIMS, both deposition process and coatings properties may be optimized by using an appropriate magnetic field configuration and pulsing design. Compared to the other deposited samples, the W films grown in multi-pulse (5 × 3 μs) HiPIMS assisted by an additional magnetic field, created with a toroidal-shaped permanent magnet placed in front of the magnetron cathode, show significantly enhanced properties, such as: smoother surfaces, higher homogeneity and denser microstructure, higher hardness and Young’s modulus values, better adhesion to the silicon substrate and lower coefficient of friction. Mechanical behaviour and structural changes are discussed based on plasma diagnostics results.

HiPIMS-deposited thermochromic VO2 films on polymeric substrates

Thermochromic vanadium dioxide, VO2, is a semi-conductor of major interest for energy control applications. Generally, the fabrication of high-quality VO2 films requires a high deposition temperature over 400°C which limits its large scale implementation as well as its compatibility with temperature-sensitive substrates. In this work, we demonstrate the deposition of thermochromic VO2 thin films on flexible polymer substrates using High Power Impulse Magnetron Sputtering (HiPIMS); this process is known for its high degree of ionization, high plasma density and hence high ion flux towards the substrate that allows one to deposit crystalline films at lower temperatures in comparison with conventional sputtering techniques. We first describe the optimization approach of the HiPIMS process followed by the optical performance study of the resulting VO2 films on Kapton polyimide substrates. The films deposited at temperatures as low as 275°C are then analyzed by means of temperature-resolved spectrophotometry, XPS, ERD, RBS, Raman spectroscopy and spectroscopic ellipsometry. Optimized films show an infrared modulation ΔT2500 nm of up to 50% with a transition temperature Tc close to the bulk temperature of 68°C.

Protection of Si photocathode using TiO2 deposited by high power impulse magnetron sputtering for H2 evolution in alkaline media

Si is an excellent absorber material for use in photoelectrochemical (PEC) hydrogen production. Only a few studies have been done using Si in alkaline electrolyte for hydrogen evolution due to its poor chemical stability in high pH electrolyte, indicating that a chemically stable protection layer is essential. Here we investigate thin TiO2 films deposited by high power impulse magnetron sputtering (HiPIMS) as a protection layer for a p-type silicon photocathode for photoelectrochemical H2 evolution in a high pH electrolyte. The X-ray reflectometry analysis reveals that the HiPIMS process provides improved film density for TiO2 films (4.15g/cm3), and consequently results in a significantly less corroded Si surface. The Si photocathode protected by the HiPIMS grown TiO2 film along with Pt as co-catalyst produced a photocurrent onset potential of ~0.5V vs. RHE in 1M KOH and showed a 4% decay over 24h in KOH. In contrast, the sample with the TiO2 deposited using conventional DC sputtering technique of similar thickness shows 20% loss in photocurrent for the same time interval. Considering the fact that the experiments were carried out not in the cleanroom, much less corrosion loss can be obtained if done in dust-free condition. Hence, these results suggest the HiPIMS technique as an improved approach for the protection of photoelectrodes, which are unstable in alkaline solution.

Argon plasma treatment on Cu surface for Cu bonding in 3D integration and their characteristics

3D integration enhances RC delay mitigation, improves inter-die bandwidth, and has routing advantages for the next generation integrated circuit technology. To realize the advantages of 3D integration, metallic bonding between different dies or wafers is necessary. So, Cu-to-Cu metallic bonding is, without doubt, a key process needed for 3D integration. In this study, Ar plasma treatment on the Cu surface for Cu thermo-compression bonding temperature less than 400°C was investigated. Ar plasma treatment on the Cu thin film was performed using a conventional DC sputtering technique. The effect of Cu surface modified by Ar plasma was studied for Cu-to-Cu bonding. Also, the influence of Ar plasma treatment on the Cu surface was evaluated structurally and electrically.

Effect of annealing time on the structural, optical and electrical characteristics of DC sputtered ITO thin films

Using an Indium tin oxide (ITO) ceramic target (In2O3:SnO2, 90:10 wt%), ITO thin films were deposited by conventional direct current magnetron sputtering technique onto glass substrates at room temperature. The obtained ITO films were annealed at 400 °C for different annealing times (1, 2, 5, 7, and 9 h). The effect of annealing time on their structural, optical and electrical properties was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microcopy (AFM), ultra violet–visible (UV–Vis) spectrometer, and temperature dependence Hall measurements. XRD data of obtained ITO films reveal that the films were polycrystalline with cubic structure and exhibit (222), (400) and (440) crystallographic planes of In2O3. AFM and Scanning Electron Microscopy SEM have been used to probe the surface roughness and the morphology of the films. The refractive index (n), thickness and porosity (%) of the films were evaluated from transmittance spectra obtained in the range 350–700 nm by UV–Vis. The optical band gap of ITO film was found to be varying from 3.35 to 3.47 eV with the annealing time. The annealing time dependence of resistivity, carrier concentration, carrier mobility, sheet resistance, and figure of merit values of the films at room temperature were discussed. The carrier concentration of the films increased from 1.21 × 1020 to 1.90 × 1020 cm−3, the Hall mobility increased from 11.38 to 18 cm2 V−1 s−1 and electrical resistivity decreased from 3.97 × 10−3 to 2.13 × 10−3 Ω cm with the increase of annealing time from 1 to 9 h. Additionally, the temperature dependence of the carrier concentration, and carrier mobility for the as-deposited and 400 °C annealed ITO films for 2 and 9 h were analysed in the temperature range of 80–350 K.

Sputtering deposition of P-type SnO films using robust Sn/SnO2 mixed target

Although SnO is a promising p-type oxide material for thin-film transistors (TFTs), the development of the TFT-industry-compatible deposition technique and materials for p-type SnO films is still an issue. In this work, we demonstrate the preparation of pure and p-type SnO films using the Sn/SnO2 mixed target and the conventional magnetron sputtering technique. By controlling the sputtering conditions, the deposited films can be tuned from pure n-type SnO2 to pure p-type SnO. Compared to other sputtering target materials (e.g., pure SnO and Sn), the Sn/SnO2 mixed targets can be fabricated by the high-temperature high-pressure pressing/sintering technique and have higher density and robustness more suitable for real uses. The p-type mobility of SnO films obtained here is comparable to results of other approaches, showing the feasibility of this method.

Electrical, optical and etching properties of Zn-Sn-O thin films deposited by combinatorial sputtering

Zn-Sn-O (ZTO) films are known to be able to form an amorphous phase, which provides a smooth surface morphology as well as etched side wall, when deposited by using the conventional sputtering technique and, therefore, to have a potential to be applied as transparent thin film transistors. In this study, ZTO thin films were prepared by using combined sputtering of ZnO and SnO2 targets, and the dependences of their electrical and optical properties on the composition and the deposition parameters were examined. The Sn content in the films was varied in the range of 35 ∼ 85 at.%. The deposition was carried out at room temperature, 150 and 300 °C, and the oxygen content in sputtering gas was varied from 0 to 1 vol.%. Sn-rich films had better electrical properties, but showed large oxygen deficiency when deposited at low oxygen partial pressures. ZTO films with Sn contents lower than 55 at.% had good optical transmission, but the electrical properties were poor due to very low carrier concentrations. A high Hall mobility of larger than 10 cm2/Vs could be obtained in the carrier density range 1017∼1020 cm−3, and the etching rate was measurable for films with Sn content up to 70 at.% when using a dilute HCl solution, indicating a good possibility of utilizing ZTO films for device applications.

A novel method to construct 3D electrodes at the sidewall of microfluidic channel

We report a simple, low-cost and novel method for constructing three-dimensional (3D) microelectrodes in microfluidic system by utilizing low melting point metal alloy. Three-dimensional electrodes have unique properties in application of cell lysis, electro-osmosis, electroporation and dielectrophoresis. The fabrication process involves conventional photolithography and sputtering techniques to fabricate planar electrodes, positioning bismuth (Bi) alloy microspheres at the sidewall of PDMS channel, plasma bonding and low temperature annealing to improve electrical connection between metal microspheres and planar electrodes. Compared to other fabrication methods for 3D electrodes, the presented one does not require rigorous experimental conditions, cumbersome processes and expensive equipments. Numerical analysis on electric field distribution with different electrode configurations was presented to verify the unique field distribution of arc-shaped electrodes. The application of 3D electrode configuration with high-conductive alloy microspheres was confirmed by particle manipulation based on dielectrophoresis. The proposed technique offers alternatives to construct 3D electrodes from 2D electrodes. More importantly, the simplicity of the fabrication process provides easy ways to fabricate electrodes fast with arc-shaped geometry at the sidewall of microchannel.

Synthesis of ZnO: Ge nanocomposite thin films by plasma gas condensation

In this work, we introduce a new method for the synthesis of Ge nanoparticles embedded ZnO thin films that are considered to be a potential candidate for photovoltaic applications. As opposed to current techniques, for the independent preparation of Ge nanoparticles, we propose using Cluster Deposition Source (CDS), which utilizes gas condensation of sputtered Ge atoms. For the synthesis of ZnO thin film host material conventional sputtering technique is employed. In the proposed technique independently synthesized Ge nanoparticles and ZnO thin films are combined into a composite structure on (100) oriented Si substrates. X-ray diffraction (XRD) patterns of the samples have revealed that Ge nanoparticles preferentially settle on (113) planes on top of the (002) oriented ZnO layer. It is realized that Ge nanoparticles with sizes ranging from 16nm to 20nm could be embedded into a well-defined ZnO matrix. In fact, transmission electron microscopy (TEM) studies performed on Ge nanoparticles captured on a Cu grids placed just above the substrate during deposition for about 60s have manifested that Ge nanoparticles reach to ZnO matrix as clusters composed of particles with sizes of about 7–8nm and then eventually grow larger due to substrate heating implemented during capping layer deposition. Optical absorption measurements have revealed that Ge nanoparticle inclusion lead to an additional absorption edge at about 2.75eV along with 3.17eV edge resulting from ZnO host.

Pt3Y electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

We report here that significant electrocatalysis occurs during oxygen reduction reaction (ORR) at the Pt3Y alloy thin film electrodes. In addition, we synthesized Pt3Y alloy nanocatalysts for use in proton exchange membrane fuel cells, fabricated by using a high pressure sputtering technique in a gaseous mixture of Ar and He. Rather than the dense film deposited by conventional sputtering techniques, the resulting structure was comprised of a Pt3 Y alloy nanocatalyst layer with an average particle size of 10–12 nm. The Pt3Y alloy nanocatalysts were characterized by scanning electron microscopy, transmission electron microscopy, high-resolution X-ray photoelectron spectroscopy, and X-ray absorption near edge spectroscopy. The cell performance of the membrane electrode assembly with multiple layers of sputter-deposited Pt3Y alloy nanoparticles and spray-deposited Nafion–carbon-ink improved dramatically compared to that obtained with the Pt only nanoparticles. The high performance of Pt3Y alloy nanocatalysts fabricated at a sputtering pressure of 200 mTorr (Ar/He = 1) was due to miniaturization of the Pt3Y alloy particles, formation of the porous catalyst layer, and enhancement of the kinetic activity for ORR.

Bilinear Temperature Gradient Focusing in a Hybrid PDMS/Glass Microfluidic Chip Integrated with Planar Heaters for Generating Temperature Gradients

Temperature gradient focusing (TGF) is a counterflow gradient focusing technique, which utilizes a temperature gradient across a microchannel or capillary to separate analytes. With an appropriate buffer, the temperature gradient creates a gradient in both the electric field and electrophoretic velocity. Combined with a bulk counter flow, ionic species concentrate at a unique point where the total velocity sums to zero and separate from each other. Scanning TGF uses varying bulk flow so that a large number of analytes that have large differences in electrophoretic mobility can be sequentially focused and passed by a single detection point. Up to now, scanning TGF examples have been performed using a linear temperature gradient which has limitations in improving peak capacity and resolution at the same time. In this work, we develop a bilinear temperature gradient along the separation channel that improves both peak capacity and separation resolution simultaneously. The temperature profile along the channel consists of a very sharp gradient used to preconcentrate the sample followed by a shallow gradient that increases separation resolution. A specialized design is developed for the heaters to achieve the bilinear profile using both analytical and numerical modeling. The heaters are integrated onto a hybrid PDMS/glass chip fabricated using conventional sputtering and soft-lithography techniques. Separation performance is characterized by separating several different dyes and amino acids that have close electrophoretic mobilities. Experiments show a dramatic improvement in peak capacity and resolution in comparison to the standard linear temperature gradient.

Sputter-Deposition of Silver Nanoparticles into Ionic Liquid as a Sacrificial Reservoir in Antimicrobial Organosilicate Nanocomposite Coatings

We present a new approach for fabricating robust, regenerable antimicrobial coatings containing an ionic liquid (IL) phase incorporating silver nanoparticles (AgNPs) as a reservoir for Ag(0)/Ag(+) species within sol-gel-derived nanocomposite films integrating organosilicate nanoparticles. The IL serves as an ultralow volatility (vacuum-compatible) liquid target, allowing for the direct deposition and dispersion of a high-density AgNP "ionosol" following conventional sputtering techniques. Two like-anion ILs were investigated in this work: methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, [N(8881)][Tf(2)N], and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][Tf(2)N]. Silver ionosols derived from these two ILs were incorporated into silica-based sol-gel films and the resultant antimicrobial activity evaluated against Pseudomonas aeruginosa bacteria. Imaging of the surface morphologies of the as-prepared films established a link between an open macroporous film architecture and the observation of high activity. Nanocomposites based on [N(8881)][Tf(2)N] displayed excellent antimicrobial activity against P. aeruginosa over multiple cycles, reducing cell viability by 6 log units within 4 h of contact. Surprisingly, similar films prepared from [emim][Tf(2)N] presented negligible antimicrobial activity, an observation we attribute to the differing abilities of these IL cations to infiltrate the cell wall, regulating the influx of silver ions to the bacterium's interior.

A simple and cost-effective method for fabrication of integrated electronic-microfluidic devices using a laser-patterned PDMS layer

We report a simple and cost-effective method for fabricating integrated electronic-microfluidic devices with multilayer configurations. A CO2 laser plotter was employed to directly write patterns on a transferred polydimethylsiloxane (PDMS) layer, which served as both a bonding and a working layer. The integration of electronics in microfluidic devices was achieved by an alignment bonding of top and bottom electrode-patterned substrates fabricated with conventional lithography, sputtering and lift-off techniques. Processes of the developed fabrication method were illustrated. Major issues associated with this method as PDMS surface treatment and characterization, thickness-control of the transferred PDMS layer, and laser parameters optimization were discussed, along with the examination and testing of bonding with two representative materials (glass and silicon). The capability of this method was further demonstrated by fabricating a microfluidic chip with sputter-coated electrodes on the top and bottom substrates. The device functioning as a microparticle focusing and trapping chip was experimentally verified. It is confirmed that the proposed method has many advantages, including simple and fast fabrication process, low cost, easy integration of electronics, strong bonding strength, chemical and biological compatibility, etc.

High-frequency magnetic characteristics of Fe-Co-based nanocrystalline alloy films

Magnetically soft Fe-Co-based nanocrystalline alloy films were produced by two preparation methods: One using a new energetic cluster deposition technique and another using a conventional magnetron sputtering technique. Their structural, static magnetic properties and high-frequency magnetic characteristics were investigated. In the energetic cluster deposition method, by applying a high-bias voltage to a substrate, positively charged clusters in a cluster beam were accelerated electrically and deposited onto a negatively biased substrate together with neutral clusters from the same cluster source, to form a high-density Fe-Co alloy cluster-assembled film with good high-frequency magnetic characteristics. In the conventional magnetron sputtering method, only by rotating substrate holder and without applying a static inducing magnetic field on the substrates, we produced Fe-Co-based nanocrystalline alloy films with a remarkable in-plane uniaxial magnetic anisotropy and a good soft magnetic property. The obtained Fe-Co-O, Fe-Co-Ti-N, and Fe-Co-Cr-N films all revealed a high real permeability exceeding 500 at a frequency up to 1.2 GHz. This makes Fe-Co-based nanocrystalline alloy films potential candidates as soft magnetic thin film materials for the high-frequency applications.