Al Sputtering Research Articles

Eliminating surface cracks in metal film-polymer substrate for reliable flexible piezoelectric devices

In a flexible piezoelectric-metal-polymer device, the occurrence of surface cracks in metal-polymer interface may cause unstable metal connections, unreliable piezoelectric yield, poor passivation behaviour and metal electrode delamination. In this study, the properties of polydimethylsiloxane (PDMS) polymer substrate are optimised, and the sputter-deposition of aluminium (Al) and molybdenum (Mo) metal film layer is controlled to eradicate cracks and promote only surface wrinkles on large area metal-polymer interface. The PDMS substrate thickness of ∼123.8 µm, 20:1 prepolymer base to curing agent weight ratio, 50 °C PDMS curing temperature and 30 min of Al sputtering time have been discovered to produce crack-free wrinkle-only Al thin film layer on PDMS substrate. The Al thin film is 100 % conductive over large area and adhere well on PDMS with 0 % of Al removal. The grown zinc oxide (ZnO) and aluminium nitride (AlN) piezoelectric layer on the improved Al-PDMS are of good crystal quality, generating low leakage current and decent piezoelectric voltage. The attained crack-free wrinkled-only metal film-polymer substrate interface is vital for flexible piezoelectric-metal-polymer device to perform well as an acoustic sensor, energy harvester, or even as an insulation/buffer layer in stretchable electronics.

Enhancing piezoelectric response in (002)-Oriented TaxAl(1−x)N films by magnetron-sputtering composition-tunable AlTa alloys

Aluminum nitride (AlN) films suffer from poor piezoelectric response due to the local hexagonal symmetry of wurtzite crystal structure. The fundamental mechanism of improving piezoelectric response in Ta-doped AlN thin films has not been fully understood. Here, we report that composition-tunable AlTa alloys, instead of Al and Ta individual sputtering targets, can be employed to deposit (002)-oriented TaxAl(1-x)N films by magnetron-sputtering, where Ta-doping ratio is tunable by changing Al:Ta composition ratio of the AlTa alloys. Importantly, Ta-doping enhances spontaneous polarization to improve piezoelectricity significantly, as suggested by a threefold increase of the effective piezoelectric coefficients (d33,eff) upon doping 7 at.% Ta. A better alignment of spontaneous polarization for stronger piezoelectric response originates from the reduction in point defects and crystalline disorder in the reduced-symmetrical wurtzite structure by doping Ta in the TaxAl(1−x)N. This study provides significant insights in improving piezoelectric response in AlN for the development of high-performance film-bulk-acoustic-resonators.

Physics-separating artificial neural networks for predicting initial stages of Al sputtering and thin film deposition in Ar plasma discharges

Simulations of Al thin film sputter depositions rely on accurate plasma and surface interaction models. Establishing the latter commonly requires a higher level of abstraction and means to dismiss the fundamental atomic fidelity. Previous works on sputtering processes addressed this issue by establishing machine learning surrogate models, which include a basic surface state (i.e. stoichiometry) as static input. In this work, an evolving surface state and defect structure are introduced to jointly describe sputtering and growth with physics-separating artificial neural networks. The data describing the plasma–surface interactions (PSIs) stem from hybrid reactive molecular dynamics/time-stamped force bias Monte Carlo simulations of Al neutrals and Ar+ ions impinging onto Al(001) surfaces. It is demonstrated that the fundamental processes are comprehensively described by taking the surface state as well as defect structure into account. Hence, a machine learning PSI surrogate model is established that resolves the inherent kinetics with high physical fidelity. The resulting model is not restricted to input from modeling and simulation, but may similarly be applied to experimental input data.

Study of Microstructure Regulation and In Situ Tensile Performance of Ni-Al Films

In this paper, Ni-Al films were prepared using magnetron sputtering technology. The microstructure of the films and the relationship between the residual stress and the adhesion strength were studied. More importantly, the mechanical strength–ductility properties of Ni-Al films were evaluated by in situ tensile testing. The results showed that the film mainly consisted of Ni3Al phase and Ni-based solid solution at the low power of Al target. The phase transition occurred, and the NiAl phase appeared when the Al sputtering power was increased. The complex structure, with the coexistence of Ni-based solid solution, Ni3Al, and NiAl phases, possessed increased residual stress and reduced adhesion strength. Meanwhile, the crack was easily penetrated through the inside and outside of the film, falling off during in situ tensile testing. While the small residual stress and large adhesion strength were obtained, only Ni3Al and NiAl coexisted, and the film fell off with difficulty. When the Al sputtering power was 400 W, the film showed the largest adhesion strength and the smallest residual stress. The best comprehensive performance was achieved with a tensile strength of 854 MPa and a yield strength of 90 MPa. The Al content of the film was up to 23.03 at.%, which was beneficial to the application and performance improvement of the film in molten salt corrosion resistance.

Tailoring the electrical properties of Cu2ZnSnS4 thin film by heterovalent Al3+ doping

Kesterite Cu2ZnSnS4 (CZTS) thin film is an attractive photovoltaic material. Harmful vacancy and anti-site defects in CZTS can be reduced by equivalent doping. However, equivalent doping leads to a reduction in the carrier concentration that is essential to the built-in electric field and built-in potential of the device. In this work, we propose heterovalent Al doping to provide hole conduction in CZTS thin film. The chemical valence of doping cation Al3+ is different from that of the substituted cation Sn4+. Al-doped CZTS thin films were fabricated by magnetron sputtering and sulfurization. The Al content was controlled by the Al sputtering time of the precursor. The experimental results revealed the successful fabrication of heterovalent Al-doped CZTS with the preferred orientation of the (112) plane. The partial substitution of Sn by Al led to a right shift of the CZTS diffraction peaks. Al doping could improve the crystalline quality of CZTS, reduce the dislocation density and microstrain of CZTS, and enhance the compactness of the thin-film surface. With the increase in Al content, the distance decreased between the Fermi level and the valence band maximum of CZTS, and the hole concentration and conductivity of CZTS increased monotonously while the non-radiative recombination of carriers was suppressed. The conductivity of CZTS reached 1.47 × 102 S/cm. Therefore, heterovalent Al doping was found to be a useful method to regulate the electrical properties of CZTS thin film.

Optimal Growth Conditions for Forming c-Axis (002) Aluminum Nitride Thin Films as a Buffer Layer for Hexagonal Gallium Nitride Thin Films Produced with In Situ Continual Radio Frequency Sputtering.

Aluminum nitride (AlN) thin-film materials possess a wide energy gap; thus, they are suitable for use in various optoelectronic devices. In this study, AlN thin films were deposited using radio frequency magnetron sputtering with an Al sputtering target and N2 as the reactive gas. The N2 working gas flow rate was varied among 20, 30, and 40 sccm to optimize the AlN thin film growth. The optimal AlN thin film was produced with 40 sccm N2 flow at 500 W under 100% N2 gas and at 600 °C. The films were studied using X-ray diffraction and had (002) phase orientation. X-ray photoelectron spectroscopy was used to determine the atomic content of the optimal film to be Al, 32%; N, 52%; and O, 12% at 100 nm beneath the surface of the thin film. The film was also investigated through atomic force microscopy and had a root mean square roughness of 2.57 nm and a hardness of 76.21 GPa. Finally, in situ continual sputtering was used to produce a gallium nitride (GaN) layer on Si with the AlN thin film as a buffer layer. The AlN thin films investigated in this study have excellent material properties, and the proposed process could be a less expensive method of growing high-quality GaN thin films for various applications in GaN-based power transistors and Si integrated circuits.

Aluminum-black silicon plasmonic nano-eggs structure for deep-UV surface-enhanced resonance Raman spectroscopy

By combining surface-enhanced Raman spectroscopy together with resonance Raman effects in the deep-UV region, ultra-sensitive and selective molecule detection can be achieved by deep-UV surface-enhanced resonance Raman spectroscopy (SERRS). Here, we report a deep-UV plasmonic nano-eggs structure consisting of elongated Al nanoparticles on black Si (BSi) for use in deep-UV SERRS characterization of biomolecules. The Al/BSi nano-eggs structure can be easily fabricated over a large area via conventional techniques including inductively coupled-plasma reactive ion etching on a Si substrate and Al sputtering without the need for accurate thickness control. A home-built deep-UV SERRS setup with the excitation wavelength of 266 nm is used to characterize adenine deposited on Al/BSi nano-eggs structures. High-intensity and reproducible Raman signals for adenine are obtained. A low-cost and easy-to-fabricate Al/BSi nano-eggs structure provides a convenient means to achieve deep-UV SERRS characterization, and it is thought to be beneficial for the development of ultra-sensitive molecule detection schemes.

Geometrical Effects on Ultrasonic Al Bump Direct Bonding for Microsystem Integration: Simulation and Experiments.

This study employed finite element analysis to simulate ultrasonic metal bump direct bonding. The stress distribution on bonding interfaces in metal bump arrays made of Al, Cu, and Ni/Pd/Au was simulated by adjusting geometrical parameters of the bumps, including the shape, size, and height; the bonding was performed with ultrasonic vibration with a frequency of 35 kHz under a force of 200 N, temperature of 200 °C, and duration of 5 s. The simulation results revealed that the maximum stress of square bumps was greater than that of round bumps. The maximum stress of little square bumps was at least 15% greater than those of little round bumps and big round bumps. An experimental demonstration was performed in which bumps were created on Si chips through Al sputtering and lithography processes. Subtractive lithography etching was the only effective process for the bonding of bumps, and Ar plasma treatment magnified the joint strength. The actual joint shear strength was positively proportional to the simulated maximum stress. Specifically, the shear strength reached 44.6 MPa in the case of ultrasonic bonding for the little Al square bumps.

Highly-Conformal Sputtered Through-Silicon Vias With Sharp Superconducting Transition

This paper describes the microfabrication and electrical characterization of aluminum-coated superconducting through-silicon vias (TSVs) with sharp superconducting transition above 1 K. The sharp superconducting transition was achieved by means of fully conformal and void-free DC-sputtering of the TSVs with Al, and is here demonstrated in up to $500~\mu \text{m}$ -deep vias. Full conformality of Al sputtering was made possible by shaping the vias with a tailored hourglass profile, which allowed a metallic layer as thick as 430 nm to be deposited in the center of the vias. Single-via electric resistance as low as 160 $\text{m}\Omega $ at room temperature and superconductivity at 1.27 K were measured by a three-dimensional (3D) cross-bridge Kelvin resistor structure. This work establishes a CMOS-compatible fabrication process suitable for arrays of superconducting TSVs and 3D integration of superconducting silicon-based devices. [2020-0354]

Effect of Techniques for Polishing Molybdenum Mirrors on their Optical Stability under Cleaning D2–N2 Plasma

The effect of cleaning with D2–(2–4) mol % N2 glow discharge plasma on the reflectivity of Mo(111) single-crystal mirrors is studied. The surface of each mirror is prepared by mechanical polishing with diamond pastes or diamond powder. Polishing of the mirrors is accompanied by the formation of a layer of material up to 3 microns thick, different in structure from the bulk metal. The peculiarity of this layer is a gradual improvement in its structure further away from the surface of the mirror and transition from an amorphous layer to a layer with nanoscale crystallites, and then a gradual transformation into the structure of an unbroken single crystal. When polishing, the diamond abrasive is pressed into the surface layer of the mirror. During the plasma-cleaning process, carbon particles pressed into the surface layer of the mirror initiate the development of negative roughness (pits or depressions). At the same time, a 5–10-nm layer of molybdenum nitride is formed. In disordered layers of the mirrors, the probability of the origination and growth of blisters is reduced. Aluminum is chosen as the metal that changes the total reflection coefficient of Mo. The use of a D2–N2 mixture to increase the rate of Al sputtering in the last stage of cleaning is replaced by plasma exposure in deuterium with $${\text{D}}_{3}^{ + }$$ ions, which initiates the dissociation of molybdenum nitride and the removal of nitrogen from the surface layer of the mirror. As a result of prolonged plasma exposure, the reflectivity of the mirrors polished with diamond paste increases to a steady-state value close to the reflection of the reference mirror.

One-layer water vapor poly(olefin) barriers compete metal sputtering onto flexible substrates

The effect of one-layer polymeric barrier coatings onto flexible and thin polyethylene terephthalate (PET) film is herein examined towards the blockage of water vapor. It is observed that well-known semi-crystalline polymers such as poly(ethylene) (PE), partially chlorinated PE (47% Cl-PE), poly(propylene) (PP), and poly(vinylidene fluoride) (PVDF) completely fail (0–4%) to reduce PET's water vapor transmission rate (WVTR). A similar trend is also shown for amorphous hydrophobic polymers such as poly(dimethyl siloxane) (PDMS) and commercial poly(silazanes). Poly(vinylidene chloride) (PVDC) containing halogen groups blocks water vapor, but its high viscosity and film inhomogeneity at higher thickness restricts it's WVTR reduction up to 40% onto PET. On the other hand, alkylated poly(olefins) of controlled molecular weight and low dispersity, prepared by living cationic polymerization, act as highly effective transparent polymeric barrier coatings. Reduction in PET's WVTR up to 62% is shown for one-layer barrier, and 69% for a two layer coating, values highly comparable with the costly Al sputtering (74% reduction; completely metallic coating). The incorporation of nanomaterials into high polymeric barriers is also discussed. Lastly, mechanical tests on elongation of PET/barrier coatings reveal the necessity to examine polymeric barriers in a broader spectrum, covering WVTR reduction, transparency, and coating-induced resistance towards cracking/deformation.

Effects of Al Sputtering Film on the Oxidation Behavior of NiCrAlY Bondcoat

In this study, the oxidation behavior of Al coated NiCrAlY bondcoat is investigated. It is known that many methods are applied to improve the lifetime of bondcoat in thermal barrier coatings. Herein, the Al sputtering method is selected to increase the Al content, which does not change the structure of bondcoat. Thin Al film of ~2 µm was sputtered on the surface of bondcoat, which improved the oxidation resistance of NiCrAlY bondcoat. Experimental results showed that, after oxidation for 200 h at 1200 °C, the formation of a dense and continuous α-Al2O3/Cr2O3 multilayer was observed on the Al coated bondcoat surface. In contrast, a mixed oxides (NiO, Cr2O3 and spinel oxides) layer formed on the surface of the as-sprayed bondcoat samples. Results of the cyclic oxidation at 1050 °C within 204 h indicated that the Al sputtering method can improve the oxidation resistance of bondcoat. This study offers a potential way to prolong the lifetime of thermal barrier coatings and provides analysis of the oxidation mechanism.

Characterization of Al-doped Mn–Co–Ni–O NTC thermistor films prepared by the magnetron co-sputtering approach

Abstract A serial of Al-doped Mn–Co–Ni–O thermistor thin films were prepared by a magnetron co-sputtering system using an Mn1.4Co1.3Ni0.3 alloy target and an Al target. The composition and electrical properties of the thin films could be well controlled by adjusting the sputtering power of the Al target. It was found that the Mn3+, Mn4+, and Co2+ cations at the octahedral sites of the compact and flat films were gradually replaced by the Al3+ cation during doping. The amount of Al dopant in the compound film was easily adjusted by controlling the sputtering power applied to the doping Al target. The experimental results demonstrate that the NTC characters were found in all films. The optimum point (15 W) of Al sputtering power was determined, at which both the maximum material constant B value (4112 K) and the lowest carrier concentration (2.46 × 1014 cm−3) was characterized in the film. The significant enhancement of the B value for NTC thin films can potentially widen the temperature measurement range for sensing applications.

Hydrogen generation from ammonia borane by chemically dealloyed platinum nanoparticles

In this study, platinum catalysts were prepared from cosputtered platinum-aluminum alloys by dealloying method and utilized in hydrogen generation from ammonia borane. Aluminum was selectively removed from the alloy by sodium hydroxide. The resultant platinum particles obtained after dealloying was found to have nanoporous structure. The effect of Al sputtering power on hydrogen generation performance of particles was evaluated. The catalysts were characterized by SEM, TEM, and Zeta-Sizer. The maximum hydrogen generation rate of the catalysts was measured as 82.8 (± 2.2) L H2 min−1 g−1catalyst. The effect of catalyst amount was also evaluated. The activation energy of the reaction was calculated as 20.4 (± 0.8) kJ mol−1. Furthermore, the catalysts could preserve their catalytic activity upon consecutive runs with effective washing steps.

Microstructure and Properties of CrAlSiN Coatings Deposited by HiPIMS and Direct-Current Magnetron Sputtering

Four CrAlSiN coatings with different Al contents were prepared by varying Al sputtering power using a combined method of high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). Microstructure, chemical compositions and mechanical properties of the four coatings were characterized by XRD, XPS, SEM, TEM, nano-indentation etc. Results showed that the four coatings possessed face-centered cubic (fcc) CrN phase with Al and part of Si dissolved in it. Another part of Si existed as amorphous phase in the coatings. All the four coatings had columnar structure and formed in epitaxy growth by template effect. With increasing Al sputtering power, the Al content increased from 11.4 to 17.6 at.% gradually, while the Cr content decreased a little. The hardness of the CrAlSiN coatings increased initially probably due to the refinement of the coating grain size, while it decreased afterwards mainly caused by the aggregation of small particles leading to rougher surface and defects formed in the coatings. The elastic modulus and adhesive strength possessed the same variation tendency as the hardness versus Al sputtering power. The CrAlSiN coating with Al sputtering power of about 0.8 kW had the highest hardness of about 26.7 GPa and adhesive strength of about 27.2 N.

Enhanced metallic targets prepared by spark plasma sintering for sputtering deposition of protective coatings

Metallic Ti, Al and Ti-Al sputtering targets with diameter of 50.8 mm and thickness of 3.175 mm were developed successfully by spark plasma sintering (SPS) process. SPS experiments were conducted in vacuum under 50 MPa and sintering temperatures of 1250 °C for Ti, 560 °C for Al and 1200 °C for Ti-Al, dwell time of 5 min, 12:2 DC pulse pattern, heating and cooling rates of 50 °C min−1. The surface roughness Ra of about 0.2 μm was obtained after mechanical polishing of the SPSed samples. The raw materials consisted of pure Ti, Al and Ti-Al 75–25 at% fine powders. XRD analysis revealed the polycrystalline nature of all the powders and SPSed samples. Ti and Ti-Al powders and SPSed Ti and Ti-Al (α2-Ti3Al) samples had hexagonal structure while Al powder and SPSed Al sample had cubic structure. SEM analysis showed irregular shaped powder particles that changed in the SPSed samples due to the recrystallization process. EDS analysis confirmed the presence of Ti and/or Al elements and minor amounts of O, Fe, Cu or Ag. The relative density of all the SPSed samples was over 97% that is close to the theoretical density. The SPSed Al sample yielded the highest electrical conductivity. The SPSed Ti-Al (Ti3Al) sample exhibited superior mechanical properties comparatively with the other samples. The SPSed targets were tested in vacuum magnetron sputtering installations proving very good behavior within deposition of thin films on various substrates. The obtained results are encouraging to use the developed targets in practical applications.

Measurement and modeling of aluminum sputtering and ionization in the DIII-D divertor including magnetic pre-sheath effects

We present analysis and modeling of Al sputtering and ionization in attached, low-power L-mode plasmas near the outer divertor strike point of the DIII-D tokamak. Al serves as a useful proxy for Be, the low-Z main wall material for ITER and JET that will undergo significant divertor plasma contact upon migrating from the first wall to the divertor. Al is easily distinguishable from background sources in DIII-D (namely C and B), has a high physical sputtering yield similar to Be, and has a long ionization mean free path compared to its gyro radius (). Using neutral Al emission imaging techniques, we measured a toroidal and radial asymmetry in the shape of the photo-emission plumes of sputtered neutral Al that was consistent with previously observed asymmetry in the distribution of redeposited Al in these experiments. We propose that the main cause of the emission and redeposition asymmetry is due to a sputtering anisotropy caused by near-grazing angle incident ions. The observed emission asymmetry was reproduced using a simple emission/ionization model that included full angular distributions of sputtering yield and energy calculated by SDTRIM.SP, but not when symmetric, mono-energetic cosine sputtering distributions were assumed. We used an ion orbit tracking model to calculate the distributions of ion impact energies through the potential gradient in the magnetic pre-sheath and Debye sheath. We found that with the magnetic field pitch angle (1.5°–2° with respect to the surface plane), the majority of ions strike the surface at <15° with respect to the surface plane, leading to angular sputtering yield and energy distributions with significant forward-scattering bias. We also observed surface microstructure consistent with directional sputtering and ion flux shadowing expected from the calculated ion incidence angles.

Tailoring electrical and optical properties of Al-doped ZnO thin films grown at room temperature by reactive magnetron co-sputtering: From band gap to near infrared

Al-doped zinc oxide (AZO) thin films were deposited at room temperature (RT) using reactive magnetron co-sputtering of two metallic Zn and Al targets in the presence of oxygen. The structural, optical and electrical properties of the AZO thin films were tailored by varying the Al content through changing Al sputtering power from 0 to 30 W. The AZO thin films deposited at the optimal deposition conditions with Al power of 26–28 W exhibited high transparency in the visible range, shorter plasma wavelength, lower electrical resistivity, lower transmittance and higher reflectance in the near IR range. The results domenstrate that the Al-doping content is strongly correlated to the band gap, electrical and near infrared optical properties using various characterization techniques. Tailoring the infrared optical properties of AZO films can be achieved and applied for desired optical coating applications.

Nanoporous Pt and Ru catalysts by chemical dealloying of Pt-Al and Ru-Al alloys for ultrafast hydrogen generation

Novel nanoporous catalysts from selective dissolution of alloys have been prepared for fast hydrogen generation from chemical hydrides. Pt and Ru have been used as precious metals in the alloys, while the nonprecious metal is Al. The alloy composition has been arranged by sputtering power of Al, which is varied from 200 to 400 W for Pt-Al and Ru-Al alloys. The influence of particle size on the sputtering power of Al has been demonstrated. The particle size of the Pt nanoparticle decreases from 900 to 90 nm when the sputtering power of Al is increased from 200 to 400 W. However, the particle size of the Ru nanoparticle decreased from 610 to 55 with the same increase in Al sputtering power. The hydrolysis reaction rate of NaBH4 scales with the sputtering power of Al and has been raised from 20 to 90 L min−1gcatalyst−1 for Pt while it increases from 23 to 110 L min−1gcatalyst−1 for Ru. The BET analysis has demonstrated that the measured surface areas of the Pt and Ru nanoparticles are approximately 55.5 and 151 m2 g−1, respectively. Joint like morphology providing the porous structure has been clearly concluded by TEM analysis. The average pore sizes are 1 and 10 nm for the Ru and Pt nanoparticles, respectively. In this study, fast hydrogen generation with lower activation energies of 31.7 and 32.1 kJ mol−1 has been achieved for the Pt and Ru nanoparticles, respectively. The critical parameters demonstrating the high catalytic activity in the hydrolysis reaction of NaBH4 are reported for the catalysts developed by the dealloying process.

On the potential of Er-doped AlN film as luminescence sensing layer for multilayer Al/AlN coating health monitoring

Luminescence sensing is an attractive approach for in-situ monitoring the health and integrity of multilayered surface protective coatings on highly active metals. In this work, we demonstrate the potential of erbium (Er) doped aluminum nitride (AlN) films to be applied as luminescence sensing layer in the Al/AlN multilayered coating system. The AlN:Er films were prepared by radio-frequency magnetron sputtering from Al targets doped with varied concentrations (from 0.5 at.% to 2.5 at.%) of Er. The chemical composition, surface morphology, crystal structure, chemical state and optical properties of the deposited AlN:Er films were investigated. We found that the crystal structure, surface roughness and optical properties of the AlN:Er films have strong correlations with the Er doping level. The as-deposited AlN:Er films showed the characteristic photoluminescence emission lines of the trivalent Er ions in the visible frequency range and quenching was observed when the Er doping level increased to 2.5 at.%. This work signifies the viability of AlN:Er film as luminescence sensing layer and the optimal Er doping concentration of the Al sputtering target is 2.0 at.% for practical applications.