Metal Films Research Articles

Tamm Plasmons: Properties, Applications, and Tuning with Help of Liquid Crystals

This article provides a brief overview of the research on localized optical states called Tamm plasmons (TPs) and their potential applications, which have been extensively studied in recent decades. These states arise under the influence of incident light at the interface between a metal film and a medium with the properties of a Bragg mirror, or between two media with the properties of a Bragg mirror. The localization of the states in the interfacial region is a consequence of the negative dielectric constant of the metal and the presence of a photonic band gap of the Bragg reflector. Optically, TPs appear as resonant reflection dips or peaks in the transmission and absorption spectra in the region corresponding to the photonic band gap. The relative simplicity of creating a Tamm structure and the significant sensitivity of TPs to its parameters make them attractive for applications. The formation of broadband and tunable TP modes in hybrid structures containing, in particular, rugate filters and porous distributed Bragg reflectors are considered. Considerable attention is paid to TP designs that include liquid crystals, which allow for the remote tuning of the TP spectrum without the mechanical restructuring of the system. The application of TPs in sensors, thermal emitters, absorbers, laser generation, and the experimental capabilities of TP-liquid crystal devices are also discussed.

Surface-Sensitive Waveguide Imaging for In Situ Analysis of Membrane Protein Binding Kinetics.

Ligand binding to membrane proteins initiates numerous therapeutic processes. Surface plasmon resonance (SPR), a popular method for analyzing molecular interactions, has emerged as a promising tool for in situ determination of membrane protein binding kinetics owing to its label-free detection, high surface sensitivity, and resistance to intracellular interference. However, the excitation of SPR relies on noble metal films, typically gold, which are biologically incompatible and can cause fluorescence quenching. Here we present a surface-sensitive waveguide imaging technique that retains the advantages of SPR and can be easily integrated into standard SPR devices by depositing additional dielectric layers onto SPR sensor chips using conventional vacuum evaporation. This novel sensor provides a silica surface, a well-known biologically compatible surface, for cell attachment and produces a sharper resonance curve and a stronger surface electric field than SPR, leading to improved measurement precision. After incorporating amplitude modulation, this approach increases measurement precision by approximately eight times compared to traditional SPR with the same optical setup. This high-precision method enables in situ single-cell analysis, revealing cell-to-cell heterogeneity that provides fuel for drug resistance. We believe this technique provides a streamlined strategy to enhance SPR for expanding their applications in biochemical research and drug screening.

Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals

The interplay of electronic charge, spin, and orbital currents, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz lightwave spintronics and orbitronics. The essential rules for how terahertz fields interact with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applicable electronic nonlinearity originating from spin-orbit interactions in conducting materials, wherein the interplay of light-induced spin and orbital textures manifests. We utilized terahertz harmonic generation spectroscopy to investigate the nonlinear dynamics over picosecond timescales in various transition metal films. We found that the terahertz harmonic generation efficiency scales with the spin Hall conductivity in the studied films, while the phase takes two possible values (shifted by π), depending on the d-shell filling. These findings elucidate the fundamental mechanisms governing non-equilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices.

Atomic Layer Deposition + Parylene Conformal Nanocoatings for Robust Corrosion Protection of Electronics

The use of flexible and miniaturized electronic devices is continuously growing with the advancement of interconnect, microvia and microBGA technologies. While these advancements have offered many benefits to high speed and complex electronics applications, they also have some design and reliability challenges, including corrosion protection. The drive for ultra-thin conformal coatings that can provide long-term robust and reliable protection for electronics and their components in harsh environments is continuing. While advancement of conformal coatings, including plasma polymerized coatings, nano liquid coatings and ultra-thin vapor phase coatings, have been able to meet the requirements up to a certain extent, the need to have an ultra-thin, lightweight conformal coating option that can provide the moisture barrier and corrosion resistance to a hermetic level is still unmet. The Atomic Layer Deposition (or ALD) technique, which was introduced in late 1990’s, has been advancing in many applications as it deposits ultra-thin films (a few nanometers) of metal and metal oxides with a precise thickness and conformality. However, its use in general electronics has been limited due to its high temperature processing conditions. This paper introduces an advanced ALD conformal coating - both alone and in combination with Parylenes - that is applied at room temperature, making it suitable for printed circuit boards and other components to provide robust and reliable protection from corrosion and other harsh environments. To understand the effectiveness of this coating on electronics against corrosion resistance, the mixed flow gas (MFG) testing was performed using Chlorine (Cl2), Hydrogen sulfide (H2S), Nitrogen dioxide (NO2) and Sulfur dioxide (SO2) for the period of 21 days. A long-term evaluation results demonstrates a robust and reliable protection against severe corrosive environments. In addition to these beneficial properties, experiments demonstrate excellent corrosion resistance, adhesion to the substrate, electrical insulation and a significant (sixty-three times) improvement on Parylene C’s water vapor transmission rate when combined with ALD.

Optical Simulation and Modulation of Semitransparent Organic Solar Cells with Dual Ultrathin Ag Film Transparent Electrodes

The advancement of semitransparent organic solar cells utilizing narrow bandgap donor and acceptor materials has progressed rapidly in recent years. These semitransparent devices exhibit high absorption in the near‐infrared range and high transmission in the visible region, offering broad application potential. This research suggests employing dual ultrathin metal films as transparent electrodes to fabricate semitransparent photovoltaic devices. The investigation focuses on the spectral simulation and modulation of the transparent electrode structure, film thickness, optical coupling layer, and 1D photonic crystal utilizing the optical transfer matrix method. The primary goal of the integrated optical effects is to enhance the light absorption in the active layer while maintaining device visible transparency. Simulation results indicate the feasibility of a device structure consisting of Nb2O5/Ag/Nb2O5/PM6:BTP‐eC9:L8‐BO/MoO3/Ag/ZnSe/Na3AlF6/ZnSe, achieving an expected short‐circuit current density () of 17.10 mA cm−2, an average visible transmittance (AVT) of 50.40%, and a light utilization efficiency (LUE) of 5.49%. The incorporation of three nonperiodic dielectric layers shows the potential to further increase , AVT, and LUE to 17.40 mA cm−2, 51.49%, and 5.71%, respectively. This study introduces a novel device structure that optimizes active layer absorption and visible transmittance, aiming to advance semitransparent photovoltaic devices.

Symmetry-forbidden second-harmonic generation induced by magnetization dynamics in ferromagnetic/heavy metal films

Symmetry-forbidden second-harmonic generation induced by magnetization dynamics in ferromagnetic/heavy metal films

Origin of low-field microwave absorption in metallic magnetic films evidenced in FeRh/Ta/GaAs

Origin of low-field microwave absorption in metallic magnetic films evidenced in FeRh/Ta/GaAs

High-Aspect-Ratio Shape Replica Mold Fabrication Using Nanoimprinting and Silver Ink as Etching Mask

Effective high-aspect-ratio molds that minimize vacuum processes are becoming increasingly important for producing metalenses and other devices. To fabricate a high-aspect-ratio structure, a metal film must be used as a mask for dry etching, typically achieved via vacuum deposition. To avoid this vacuum process, we devised a method to develop an etching mask in the air using silver ink. The manufacturing method involved filling the mold with silver ink, baking it, removing silver from the convex parts of the mold with a polyethylene terephthalate film, and placing silver from the concave parts of the mold on top of the ultraviolet (UV)-cured resin using ultraviolet-nanoimprint lithography. The transferred pattern had silver on the convex parts, which was used as a mask for the oxygen dry etching of the UV-curable resin. Consequently, high-aspect-ratio resin shapes were obtained from three types of nano- and micromolds. Additionally, a high-aspect-ratio resin with silver was used as a replica mold to form a silver pattern. This process is effective and allows high-aspect-ratio patterns to be obtained from master molds.

Application of Picosecond Acoustic Metrology for monitoring Metal Films in Advanced Packaging

High-performance computing applications such as artificial intelligence, and machine learning require massive amount of parallel processing requiring significant memory bandwidth. High bandwidth memory (HBM) solutions have been optimized to enable such applications. HBM consists of core DRAM die and a logic buffer die in a vertically stacked 3D structure. Customers are adopting front-end solutions to meet the shrinking process tolerances and to meet the stringent reliability requirements. [1] Picosecond acoustic metrology is a well-established technique for in-line measurements [2-3] and can be applied to meet the challenges of advanced packaging applications. In this paper, we shall present system-level improvements and specific use cases that have accelerated adoption of the technology in fan-out wafer level packaging (FOWLP) and in the HBM process loop. In the FOWLP process, re-distribution lines (RDL) play a critical role in how I/Os are connected and measuring RDL thickness before and after copper seed etch is necessary as it impacts line resistance and leakage current. In the TSV process loop, multi-layer metal stack such as gold/nickel/copper is used to form pads for bonding to the micropillar. The thickness of each of the layers plays a critical role in reliability performance and hence in-line process control requires layer to layer thickness monitoring is. Similarly, in the microbump process flow, controlling copper seed/barrier thickness is critical as the relative bump heights will vary as a function of the copper seed thickness variation impacting bump co-planarity [4-5].

Polarization-dependent vibrational strong coupling in a metamaterial

Abstract We report on polarization-dependent vibrational strong coupling between vibrational transitions and the electromagnetic mode of a molecule-embedded metamaterial cavity. Our metamaterial cavity consists of a modified trilayer composed of a metal-insulator-metal, where an infrared-activated polymer layer is sandwiched between an array of linear antennae on top and a continuous metal film at the bottom. In comparison with previous reports, the coupling between the metamaterial resonance and the vibrational modes of the polymer spacer layer reveals three distinct dips corresponding to upper and lower polaritons, and an additional dip attributed to the emergence of dark states. Using polarization-selective reflectivity simulation of the cavity, we analyze their response to different light polarization and find two orthogonally polarized optical systems with different coupling strengths, one strongly coupled and one weakly coupled. The strong coupling gives rise to the formation of lower and upper polaritons, while dark states emerge because of weak coupling. The ability to engineer new physical systems with tunable properties can pave the way for ultrasensitive vibrational spectroscopy, infrared photonic devices, chemical reaction control, and quantum information processing.

Plasmon-Driven Gold Nanopillar Multiarrayed Gene Amplification Methodology for the High-Throughput Discrimination of Pathogens.

Molecular diagnosis limitations, including complex treatment processes, low cost-effectiveness, and operator-dependent low reproducibility, interrupt the timely prevention of disease spread and the development of medical devices for home and outdoor uses. A newly fabricated gold nanopillar array-based film is presented for superior photothermal energy conversion. Magnifying the metal film surface-to-volume ratio increases the photothermal energy conversion efficiency, resulting in a swift reduction in the gene amplification reaction time. Plasmonic energy-based ultrafast gene amplification and facile confirmation methodology offer a rapid disease discrimination platform for high-throughput multiplexed diagnosis. The superior performance of the gold nanopillar arrayed film is demonstrated by measuring the amount of pathogen (Vibrio cholerae) with a sensitivity of 101 cfu mL-1 in 5.5min. The newly engineered gold nanopillar arrayed film can be utilized to diagnose universal pathogens to achieve an increasingly successful complete cure.

Sacrificial Layer for Sintering Enhancements of Ceramic, Semiconductor, and Metal Films: A Universal Strategy.

The manufacturing of thin films through selective laser sintering of micro/nanoparticles is an emerging technology that has been developing rapidly over the last two decades owing to its digitization, efficiency, and good adaptability to various materials. However, high-quality laser sintering of different materials remains a challenge: ceramic particles are difficult to be sintered due to low absorbance; metallic particles are prone to oxidation; semiconductor particles are difficult to process for performance enhancement due to high stress. In this work, a new approach is proposed that employs an additional Indium Tin Oxide (ITO) sacrificial layer to assist laser sintering of different functional materials, which detaches after sintering without contaminating the target material. As a laser absorber, the ITO layer can raise the sintering temperature up to 2950 K, resulting in well coarsening of ceramic grains. As an oxygen barrier, the ITO layer maintains the oxidation level of the metal die below 25%. As a temperature homogenizer, the ITO layer delays the cracking and improves the performance of the semiconductor material, which in turn increases its Seebeck factor to 1.4-fold. Therefore, the ITO sacrificial layer is a material-friendly, purity-neutral laser sintering strategy, which supports laser sintering of multi-materials and high-performance thin-film devices.

An Ultra-Wide Range D-Shaped Fiber SPR Sensor with a Nanostructure of Gold-MoS2 and Sodium for the Simultaneous Measurement of Refractive Index and Temperature.

Refractive index (RI) and temperature (T) are both critical environmental parameters for environmental monitoring, food production, and medical testing. The paper develops a D-shaped photonic crystal fiber (PCF) sensor to measure RI and T simultaneously. Its cross-sectional structure encompasses a hexagonal-hole lattice, with one hole selectively filled with toluene for temperature sensing. By coating the D-shaped surface of the PCF with a metal film and a MoS2 film, the refractive index-detection channel is formed. Numerical results demonstrate that RI and T can be reflected in the same spectrum, without any interference caused by the two parameters with each other. At an environmental RI of 1.26-1.38, its maximum RI sensitivity is up to 5400 nm/RIU. At a temperature of 20-80 °C, its temperature sensitivity reaches -1.2 nm/°C. This design allows for a broad operational spectrum and an extensive measurement range, which makes it particularly suitable for applications requiring low-RI detection. Moreover, the resonance strength of the sensor is significantly enhanced by introducing a two-dimensional material MoS2 on the D-surface. Specifically, it reaches 195,149 dB/m when RI = 1.34 at 30 °C. This is much higher than that of most previous studies, and the requirements for inspection equipment can be lowered in this case. These results are essential for progress in simultaneously detecting RI and T.

Confined CVD Synthesis and Temperature-Dependent Spectroscopic Properties of Bilayer Graphene Ribbon Arrays with Bifunctional Modulation of Adhesion Metal.

Bilayer graphene ribbons (GRs) hold great promise for the fabrication of next-generation nanodevices, thanks to unparalleled electronic properties, especially the tunable bandgap in association with twist angle, ribbon width, edge structure, and interlayer coupling. A common challenge in manufacturing bilayer GRs via templated chemical vapor deposition (CVD) approach is uncontrollable dewetting of micro- and nano-scaled patterned metal substrates. Herein, a confined CVD synthetic strategy of bilayer GR arrays is proposed, by utilizing the bifunctional Ni as a buffered adhesion layer to regulate the anisotropic dewetting of metal film in the V-groove and as a carbon-dissolution regulated metal to initiate the bilayer nucleation. Using C2H4 as direct donor of C dimer species, high-quality bilayer GR arrays are synthesized on regular CuNi ribbons with twist angles at 900 °C, harnessing the non-equilibrium jointly induced by confined V-groove and C dimer species. The nucleation and growth mechanism of bilayer GR are investigated with density functional theory (DFT) calculations. The as-grown bilayer GRs display distinctive variable temperature Raman and photoluminescence properties. Our results contribute to a highly controllable technique for fabricating twisted bilayer GR arrays and deep insights into the optical properties of bilayer GRs for potential optoelectronics applications.

An Investigation of the Indentation Elastic Modulus for Metal Films on Flexible Substrates Considering the Substrate Effect.

The accurate measurement of the elastic modulus of thin metal films on flexible substrates is critical for understanding the mechanical reliability of flexible electronics. However, conventional methods, such as the Oliver-Pharr model, often underestimate the modulus due to substrate effects, particularly with low-modulus substrates like polyimide (PI). In this study, we propose an improved weighting model that replaces the empirical weighting factor with a variable X to better account for substrate contributions. Nanoindentation experiments were performed on Cu and Al films with thicknesses of 0.5, 1, and 1.5 μm, deposited on PI and silicon substrates. The results show a significant underestimation of the elastic modulus when traditional methods were applied, especially on PI substrates, where values decreased by up to 95%. Using the proposed X-based model, the corrected elastic modulus aligned with the inherent properties of the films, with errors reduced to within 2%. A finite element analysis (FEA) validated the stress and displacement distributions, demonstrating the substrate's influence on indentation behavior. This study provides a robust framework for accurately measuring the elastic modulus of thin films on flexible substrates, paving the way for a more reliable mechanical characterization in flexible electronics.

Tailoring the structure of Al50Mo50 alloy films towards realizing the well match between mechanical and electrical performance by controlling the growth mode

Metal films that combine superior mechanical properties with good conductivity are crucial for fulfilling the structural and functional requirements for MEMS applications. To achieve this, single-phase supersaturated solid solution Al50Mo50 alloy films were synthesized using magnetron sputtering at room temperature. By varying the deposition rate, the growth mode of these films was systematically controlled. The results demonstrate that there is no segregation in any of the films, which maintain homogeneity in composition and exhibit unexpected thermal stability at 550℃. As the deposition rate increases, the growth mode of the films transitions from a nano-columnar crystal structure to a layered + island structure. The supersaturated solid solution Al50Mo50 alloy film featuring a layered + island structure exhibits notable properties: high hardness (11.86GPa), high elastic modulus (194.27GPa), excellent ductility with a critical strain of 1.5%, and exceptionally low resistivity (100.95 μΩ cm). This combination of mechanical and electrical properties ensures a well-balanced performance suitable for MEMS applications. Moreover, the controlled process of the growth model and the optimization mechanism for enhanced ductility are systematically elucidated. These findings identify Al50Mo50 alloy films as an ideal candidate material for the design and development of MEMS/NEMS devices.

Enhanced two-temperature model and its application in comprehensive analysis of femtosecond laser melting of gold, copper and their alloys.

Extensive research on ultrashort laser-induced melting of noble metals like Au, Ag and Cu is available. However, studies on laser energy deposition and thermal damage of their alloys, which are currently attracting interest for energy harvesting and storage devices, are limited. This study investigates the melting damage threshold (DT) of three intermetallic alloys of Au and Cu (Au3Cu, AuCu and AuCu3) subjected to single-pulse femtosecond laser irradiation, comparing them with their constituent metals. This is accomplished by extending an earlier-developed two-temperature model (TTM)-based code with several improvements, including precise modeling of temperature-dependent optical properties and ballistic electron transport. The dynamic optical model inclusive of ballistic effects is demonstrated to reproduce the experimental DT of pure metals with minimal variation and is therefore adopted for further investigation. Our simulations reveal that the alloy films have significantly lower incipient and complete melting thresholds compared to the pure metals due to their low thermal conductivity and high electron-phonon coupling strength. Theoretical studies on varying the thickness of metal and alloy films unveil the usual trend of a rapid increase in DT up to a certain thickness, followed by a saturation region. This universal DT profile is elucidated by proposing a first-of-its-kind analytical function. Excellent agreement between the coefficients of the function with optical and electron diffusion parameters derived from the comprehensive theory proposed here reinforces the robustness of the model. The novelty of this study also lies in introducing the concept of a critical film thickness for which the entire film attains the melting temperature at its complete melting threshold.

Continuous and Extremely Flat Ag Electrode by Tailoring Surface Energy for Organic Light‐Emitting Diodes

AbstractThe exploration of alternative transparent electrodes to indium tin oxide (ITO) in organic light‐emitting diodes (OLEDs) has been a topic of interest. Among various candidates, ultrathin and continuous metal films have garnered significant interest for their ability to offer both transparency and conductivity. However, due to the higher surface energy of metals compared to substrates, metal films tend to grow in a discrete island shape, resulting in poor conductivity and even low transparency due to a localized surface plasmon by metal islands. In this work, a way to produce homogeneous ultrathin and continuous Ag film (UCAF) achieved through primary NF3 plasma treatment on a glass substrate is proposed. This treatment introduces F bonds on the surface, enhancing the hydrophilicity of the glass surface and facilitating the formation of UCAF with high optical transmittance (>70%) and low sheet resistance (<10 Ω/ϒ). By incorporating UCAF into OLEDs as a bottom electrode, the current efficiency showed an 86% enhancement compared to an ITO bottom electrode at an operating current density of 10 mA cm−2. Numerical simulation results elucidated superior light extraction efficiency in OLED with UCAF compared to that with ITO, attributed to both cavity effect and reduced waveguide mode at the organic/electrode interface.

Thermogravimetric analysis of commercial tungsten molecular precursors for vapor phase deposition processes.

Thin films and coatings based on Group 6 metal tungsten (W) have garnered intense interest for applications including catalysis, lubrication, and solar energy. Due to its selectivity and conformality, atomic layer deposition (ALD) has emerged as a key route towards oxides, dichalcogenides, and elemental metal films of W. A key component of the ALD process is the appropriate selection of molecular precursors. Thermogravimetric analysis (TGA) is the primary sorting criterion for precursor suitability and helps delineate probable ALD temperature windows, sublimation/vaporization kinetics, and parameters of decomposition. Currently, a majority of the W materials growth literature is traced to a grouping of commercially available volatile molecular precursors. However, no comprehensive thermochemical analysis exists for all of these precursors, hampering a meaningful direct comparison. Herein, we present an extensive thermogravimetric analysis and direct comparison of commercial volatile W molecular precursors. We report probable ALD temperature windows, enthalpies of sublimation (ΔH sub), activation energies (E a), and evaluate thermal stress. Our findings highlight several commercial precursors yet to be reported for ALD growth, but featuring thermochemical properties falling within our suitable observed parametric ranges indicative of potential/viable deposition processes.

Sensing Thermophoretic Forces by Nanoplasmonic Actuators with Interferometric Scattering Readout.

Noble metal nanoparticles (NPs) represent nanoscale, optically addressable heat sources whose temperature gradients give rise to thermophoretic forces that can act back on the NPs. Herein we investigate 20 nm Ag NPs bound via molecular tethers to a 20 nm thin Au film as nanoplasmonic actuators that generate a local temperature gradient and simultaneously act as optical sensors of forces that induce their displacement from their equilibrium position. Forces of sufficient magnitude to affect the NP-film distance modulate the interferometric scattering (iSCAT) signal of the individual NPs and become detectable due to the distance-dependent damping of the NP scattering in the vicinity of the metal film. With total incident power densities within a range between 1.40 and 4.80 kW cm-2, the experiments reveal a continuous decay in the NP iSCAT signal, consistent with a decrease in the NP-film separation due to an attractive thermophoretic force.