Nanoscale Metal Films Research Articles

A decoupling calculation method of separating temperature component from the nano-scale metal film thickness measurement output by the eddy current sensor

The eddy current method has been widely applied in the field of nano-scale metal film thickness detection. The output signal of an eddy current sensor is generally tiny in practice, and it is easily affected by the ambient temperature variation, which results in a decrease in the measurement accuracy. How to separate the temperature effect on film thickness measurement for achieving a high precision is a major problem. Therefore, a coupling model of an eddy current sensor with an electromagnetic field and a temperature field is established in this study, and the influence of the film thickness and temperature on coil impedance is calculated. It is found that the inductance and resistance of the coil vary monotonically as thickness and temperature with a measure of linearity in a certain range, as well as the real part and imaginary part of the output voltage by using an AC bridge. Then a film thickness-temperature decoupling calculation method is proposed, and a group of linear calibration intervals are further divided considering the linearity and measurement accuracy. According to the calculation results, it is confirmed that the method can decouple the two, and accomplish a higher accuracy. In addition, the method is verified by a series of experiments, and the variation trend of real and imaginary parts of output voltage with thickness and temperature is consistent with simulation results. At the same time, it is feasible to realize a synchronous detection of metal film thickness and ambient temperature in a certain range.

Determining effective refractive index and elasticity of nanoscale metal - thin, soft polymer bilayers using interference signatures of a glass probe

HypothesisThin metal films like silver (Ag) and gold (Au) on PDMS are perfect candidates for surface-enhanced Raman spectroscopy (SERS) active substrates whose optical and mechanical properties are often subjected to precise tuning. Measurement of such properties often requires multiple sophisticated experimental techniques and numerical analysis. When depositing a nanoscale thin metal film like Ag or Au on the polymer surface, the refractive index and elastic modulus of this bilayer structure could vary with the metal deposition time and properties of soft substrates. ExperimentsIn this work, we demonstrate a single experimental methodology to measure the refractive index and elasticity of thin metal films coated on PDMS. Using Dual-Wavelength Reflection Interference Contrast Microscopy (DW-RICM) measurements of a spherical glass probe of known shape, the refractive indices of Ag/PDMS and Au/PDMS substrates of varying metal deposition time, PDMS elasticity, and PDMS coating thickness are extracted corresponding to two different wavelengths. FindingsThe extracted refractive indices of metal/PDMS substrates decrease with increasing metal deposition time, decreasing PDMS elasticity, and increasing PDMS coating thickness. The extracted refractive indices are found to be in good agreement with corresponding UV/VIS spectroscopy measurements. Lastly, their elastic moduli are also obtained using Hertz's theory on the observed contact radius of the glass sphere on the aforementioned substrates.

Deposition of nanoscale metal films with the help of arc discharge

The method of magnetron sputtering using arc discharge is one of the most advanced and high-performance techniques for producing nanoscale films, especially metal films and, in particular, titanium and copper ones. The cathode spots that form during arc discharge are the discharge centers and they provide channels along which current travels from the cathode to the anode in the form of dense plasma jets of the cathode material forming a so-called plasma torch. The ion energy in such plasma torch is higher than in the conventional magnetron sputtering, which ensures a significantly better adhesion of the resulting films. At the same time, nanoscale titanium films are often used to create transition mating sublayers between the substrate and the main working film. Out of non-metallic films, titanium nitride films are of special interest. The main disadvantage of magnetron arc sputtering is the presence of a droplet phase in the plasma torch, which hinders the creation of nanoscale structures by forming condensate on the substrate. The resulting microdroplets are usually electrically neutral, therefore they cannot be removed and neutralized by means of electric or magnetic fields. The arc evaporator needs to be retrofitted so that the droplet phase was eliminated and thin-film coatings (including nanoscale coatings) could be produced. A plasma torch is formed with the help of a focusing coil, in which both ions of the sprayed substance and its droplets are present. A special flap prevents direct-flying droplets from getting in the flow of sprayed substance, and at least one more flap installed in the housing of the focusing coil protects the flow from droplets flying at an angle from the cathode. The flap is designed as a truncated cone made of non-magnetic metal, with the diameter of the larger base of the cone being equal to the inner diameter of the focusing coil, and the smaller diameter of the cone that faces the cooled cathode is equal to twice the diameter of the flap. The flap is coaxial with the cooled cathode of the arc evaporator.

Superior Dynamic Penetration Resistance of Nanoscale Multilayer Polymer/Metal Films

Abstract Recent advances in experimental techniques have enabled impact tests of ultrathin films. For example, microprojectile impact tests of ultrathin polymer films have revealed that their specific penetration energy is about ten times more than that of the conventional armor materials. On the other hand, metallic nanostructures have demonstrated extraordinary mechanical properties. These observations suggest that multilayer arrangements of nanoscale polymer and metal films could possess superior ballistic impact resistance. In order to test this hypothesis, we simulated the impact tests of multilayer aluminum-polyurea nanostructures using molecular dynamics (MD). Our simulations demonstrate that the ballistic limit velocity (V50) and the specific penetration energy of the multilayers and aluminum nanofilms are significantly higher than the experimentally measured values for any material. In order to further investigate the mechanisms associated with the observed superior ballistic performance of multilayers, we computed their V50 using an existing membrane model and another analytical model reflecting a two-stage penetration process. Our results demonstrate a potential bottom-up design pathway for developing flexible barrier materials with superior dynamic penetration resistance.

Enhanced near-Infrared Photoresponse from Nanoscale Ag-Au Alloyed Films

Alloying of metals provides a vast parameter space for tuning of material, chemical, and mechanical properties, impacting disciplines ranging from photonics and catalysis to aerospace. From an optical point-of-view, pure thin metal films yield enhanced light absorption due to their cavity effects. However, an ideal metal–semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. Here we demonstrate that Ag-Au alloys provide an ideal model system for controlling the optical and electrical responses in nanoscale thin metal films for hot carrier photodetectors with improved performance. While pure Ag and Au have long hot carrier attenuation lengths >20 nm, their optical absorption is insufficient for high efficiency devices. Instead, we find that alloying Ag and Au enhances the absorption by ∼50% while maintaining attenuation lengths >15 nm, currently limited by grain boundary scattering, although the electron attenuation length of pure Au outperforms pure Ag as well as all of the alloys investigated here. Further, our density functional theory analysis shows that the addition of small amounts of Au to the Ag lattice significantly enhances the hot hole generation rate. Combined, these findings suggest a route to high efficiency hot carrier devices based on metallic alloying with potential applications ranging from photodetectors and sensors to improved catalytic materials.

Numerical simulation of fractional non-Fourier heat transfer in thin metal films under short-pulse laser

In this paper, heat transfer is investigated in thin metal films subjected to a short-pulse laser. Due to the short temporal scales and high intensity of laser pulses during the heating process, the non-Fourier phenomenon can be observed in the temperature response of thin metal films. Therefore, the fractional non-Fourier model has been applied to investigate the heat transport in thin metal films to capture the thermal behaviors of nano-scale metal films. The fractional non-Fourier model is presented to investigate the heat transfer in single-layer gold film and gold‑chromium double-layer films under short-pulse laser. The governing fractional heat conduction equation is solved numerically using the finite difference method based on implicit scheme, and the obtained results are compared to the experimental data. The numerical results obtained from the fractional model represent its accuracy and suitability to predict the thermal behavior of the thin metal films irradiated by short-pulse laser. In addition, upon comparing the results of the fractional model with the results of other models which have been employed in this field, it can be seen that the fractional model has a higher capability than others to examine the non-Fourier effects during laser heating of thin metal films.

Giant enhancement of silicon plasmonic shortwave infrared photodetection using nanoscale self-organized metallic films

Many consumer technologies rely on photodetection of infrared light, such as lidar, low visibility imaging, proximity sensors/range finders, etc. However, silicon, the standard material of the semiconductor industry, becomes transparent for wavelengths above 1.1 µm, as the photons no longer have sufficient energy to stimulate direct band-to-band absorption. We report here a Schottky photodetector design that extends silicon’s optical detection range beyond this 1.1 µm limit, by utilizing internal photoemission of hot carriers. Our design relies on an ultra-thin fractally nanostructured aluminum optical absorber and yet remarkably achieves over 50% absorption of incident light. We demonstrate 2 orders of magnitude improvements of responsivity, noise-equivalent power, and specific detectivity as compared to a reference Schottky photodetector made of bulk metal films. We attribute this to the combination of superior transport and momentum relaxation processes from the nanoscale fractal geometries. Specifically, we show a direct link between internal quantum efficiency enhancement and structural parameters such as perimeter-to-surface ratio. Finally, our devices also function as bulk refractive index sensors. Our approach uses an exceedingly simple complementary metal-oxide-semiconductor (CMOS)-compatible “bottom up” fabrication that is cheap and scalable and is a promising candidate for future cost-effective and robust shortwave infrared photodetection and sensing applications.

Time-fractional subdiffusion model for thin metal films under femtosecond laser pulses based on Caputo fractional derivative to examine anomalous diffusion process

In this paper, the non-Fourier effects are examined in thin metal films exposed to femtosecond laser pulses. For the first time, the time-fractional subdiffusion model is presented based on the Caputo fractional derivative to examine the anomalous diffusion process in nano-scale metal films. The fractional heat conduction equation is solved numerically, using the finite difference method based on implicit scheme and the numerical results are compared to experimental data. The comparison implies that the numerical results well coincide with experimental data, proving the accuracy and suitability of fractional model to capture thermal behaviors of thin metal films heated by short-pulse laser. Furthermore, the results of time-fractional model are compared to other models including parabolic and hyperbolic two-step models. Having compared, one can be found is that the fractional model is more reliable than other models, because its results are well matched with experimental data. The use of Fourier's law in parabolic and hyperbolic two-step model leads to deviation from experimental data which can be addressed upon applying the proposed time-fractional model in this work. Moreover, the presented time-fractional model can be effectively utilized for studying the heat transport in thin metal films under laser pulses as well as the laser heating process and its applications in engineering. Ultimately, the effect of changes in various parameters such as order of fractional derivative, laser pulse duration, metal materials (gold and chromium), gold film thickness, relaxation time and absorption depth of laser pulses are investigated on temperature response of thin metal films.

Liquid-State Dewetting of Pulsed-Laser-Heated Nanoscale Metal Films and Other Geometries

Metal films of nanoscale thickness, deposited on substrates and exposed to laser heating, provide systems that involve several interesting multiphysics effects. In addition to fluid mechanical aspects associated with a free boundary setup, other relevant physical effects include phase change, thermal flow, and liquid–solid interactions. Such films are challenging to model, in particular because inertial effects may be relevant, and large contact angles require care when considering the long-wave formulation. Applications of nanoscale metal films are numerous, and the materials science community is actively pursuing more complex setups involving templated films and substrates, bimetallic films and alloys, and a variety of elemental film geometries. The goal of this review is to discuss our current understanding of thin metal film systems, while also providing an overview of the challenges in this research area, which stands at the intersection of fluid mechanics, materials science, and thermal physics.

Laser‐Triggered Large Magnetoresistance Change Observed in Corbino Disk of Cu/SiO2/Si

AbstractThe Corbino disk is used to study magnetoresistance as it eliminates Hall‐voltage interference and allows for direct probing of bulk properties. A laser‐triggered magnetoresistance effect in the Corbino disk of a Cu/SiO2/Si structure is reported. When a laser is applied, the magnetoresistance is greatly improved, by 360 times, under a 635 nm laser and a 2.1T magnetic field. In addition, the effect shows a close connection with the geometry of the nanoscale metal‐film structure. This effect is attributed to an increase in carrier concentration and bending of the motion path under the laser and magnetic field. This work suggests a new approach for the exploration of the magnetoelectric properties of nanoscale metal films and opens a new window on the use of light to adjust magnetoresistance devices.

Ultrastable cyclic bending response of carbon nanotube/copper laminate composite film

Nanostructured metal thin films have high strength and high electrical conductivity, but their fatigue resistance is relative low due to the weak plasticity and ductility deformed by localization at grain boundary or interface. Here we report a carbon nanotube/copper laminate composite film with ultrahigh fatigue resistance, which exhibits history-independent cyclic bending response. The high dense wrinkling of composite film becomes the dominant cyclic deformation behavior. The cracks initiation and formation are voided by the wrinkling process releasing the bending stress. The residual stress and resistivity of composite film no longer increase even if the number of cycle reaches as high as105. This result provides us a novel insight to design nanoscale metal films with ultrahigh fatigue resistance and superior service ability.

Cracking and Toughening Mechanisms in Nanoscale Metallic Multilayer Films: A Brief Review

Nanoscale metallic multilayer films (NMMFs) have captured scientific interests on their mechanical responses. Compared with the properties of monolithic films, multilayers possess unique high strength as the individual layer thickness reduces to the nanoscale, which is benefited from the plentiful hetero-interfaces. However, NMMFs always exhibit a low fracture toughness and ductility, which seriously hinders their practical applications. While there have been reviews on the strengthening and deformation mechanisms of microlaminate, rapid developments in nanotechnology have brought an urgent requirement for an overview focused on the cracking and toughening mechanisms in nanoscale metallic multilayers. This article provides an extensive review on the structure, standard methodology and fracture mechanisms of NMMFs. A number of issues about the crack-related properties of NMMFs have been displayed, such as fracture toughness, wear resistance, adhesion energy, and plastic instability. Taken together, it is hoped that this review will achieve the following two purposes: (1) introducing the size-dependent cracking and toughness performance in NMMFs; and (2) offer a better understanding of the role interfaces displayed in toughening mechanisms. Finally, we list a few questions we concerned, which may shed light on further development.

A novel evaluation strategy for fatigue reliability of flexible nanoscale films

In order to evaluate fatigue reliability of nanoscale metal films on flexible substrates, here we proposed an effective evaluation way to obtain critical fatigue cracking strain based on the direct observation of fatigue damage sites through conventional dynamic bending testing technique. By this method, fatigue properties and damage behaviors of 930 nm-thick Au films and 600 nm-thick Mo-W multilayers with individual layer thickness 100 nm on flexible polyimide substrates were investigated. Coffin-Manson relationship between the fatigue life and the applied strain range was obtained for the Au films and Mo-W multilayers. The characterization of fatigue damage behaviors verifies the feasibility of this method, which seems easier and more effective comparing with the other testing methods.

Surface tension and viscosity play significant roles in metal nanofilm heat destabilization

Self-consistent simulations have started to unravel the mysteries of how heat affects the evolution of nanoscale metal films and filaments.

The effect of surface step and twin boundary on deformation twinning in nanoscale metallic systems

The effect of surface step and twin boundary on the mechanical twinning process in nanoscale face-centred cubic metallic films was studied using atomistic simulations. Aluminium was considered as a model material but comparisons were made with silver and copper. Surface steps were identified as privileged sites for twin nucleation at lower stresses, leading to the formation of only one large twin in defect-free films. In presence of a coherent twin boundary which acts as a strong barrier to the propagation of dislocations, the extension of nucleated twins is much more limited but the density of secondary twin boundaries is found higher. The key role played by Lomer dislocations, resulting from the interaction between incipient twins and the coherent twin boundary, on the nucleation of new twins was demonstrated. These findings shed light on some elementary mechanisms that can be involved in the elaboration of nanotwinned materials with interesting mechanical properties.

Evaluating nanoscale ultra-thin metal films by means of lateral photovoltaic effect in metal–semiconductor structure

Nanoscale metal–semiconductor (MS) structure materials occupy an important position in semiconductor and microelectronic field due to their abundant physical phenomena and effects. The thickness of metal films is a critical factor in determining characteristics of MS devices. How to detect or evaluate the metal thickness is always a key issue for realizing high performance MS devices. In this work, we propose a direct surface detection by use of the lateral photovoltaic effect (LPE) in MS structure, which can not only measure nanoscale thickness, but also detect the fluctuation of metal films. This method is based on the fact that the output of lateral photovoltaic voltage (LPV) is closely linked with the metal thickness at the laser spot. We believe this laser-based contact-free detection is a useful supplement to the traditional methods, such as AFM, SEM, TEM or step profiler. This is because these traditional methods are always incapable of directly detecting ultra-thin metal films in MS structure materials.

Development of a conformable electronic skin based on silver nanowires and PDMS

This paper presented the designed and tested a flexible and stretchable pressure sensor array that could be used to cover 3D surface to measure contact pressure. The sensor array is laminated into a thin film with 1 mm in thickness and can easily be stretched without losing its functionality. The fabricated sensor array contained 8×8 sensing elements, each could measure the pressure up to 180 kPa. An improved sandwich structure is used to build the sensor array. The upper and lower layers were PDMS thin films embedded with conductor strips formed by PDMS-based silver nanowires (AgNWs) networks covered with nano-scale thin metal film. The middle layer was formed a porous PDMS film inserted with circular conductive rubber. The sensor array could detect the contact pressure within 30% stretching rate. In this paper, the performance of the pressure sensor array was systematically studied. With the corresponding scanning power-supply circuit and data acquisition system, it is demonstrated that the system can successfully capture the tactile images induced by objects of different shapes. Such sensor system could be applied on complex surfaces in robots or medical devices for contact pressure detection and feedback.

Substrate melting during laser heating of nanoscale metal films

We investigate heat transfer mechanisms relevant to metal films of nanoscale thickness deposited on a silicon (Si) substrate coated by a silicon oxide (SiO2) layer and exposed to laser irradiation. Such a setup is commonly used in the experiments exploring self and directed assembly of metal films that melt when irradiated by laser and then evolve on time scale measured in nanoseconds. We show that in a common experimental setting, not only the metal but also the SiO2 layer may melt. Our study of the effect of the laser parameters, including energy density and pulse duration, shows that melting of the substrate occurs on spatial and temporal scales that are of experimental relevance. Furthermore, we discuss how the thicknesses of metal and of the substrate itself influence the maximum depth and liquid lifetime of the melted SiO2 layer. In particular, we find that there is a minimum thickness of SiO2 layer for which this layer melts and furthermore, the melting occurs only for metal films of thickness in a specified range. In the experiments, substrate melting is of practical importance since it may significantly modify the evolution of the deposited nanoscale metal films or other geometries on nanoscale.

Analysis of removal region in nanoscale metal film processed by ultrafast-pulse laser

The ultrafast-pulse laser can be employed for the micromachining of nanoscale optical and electronic devices due to hardly diffusive heat-induced zone in very short period. In order to effectively control the processing quality, the ultrafast-pulse laser with certain wavelength and absorptivity for material is preferred. The processing characteristics of nanoscale devices patterned by ultrafast-pulse laser are different from these of traditional large scale device and long pulse laser. The traditional model cannot correctly account for processing characteristics. Therefore, this study utilizes an improved electron kinetic theory to analyze the processing characteristics of the nanoscale metal film ablated by ultrafast-pulse laser. The ablated depth per pulse and crater diameter in nanoscale metal film are predicted by this work and compared with the available measured data. This study theoretically validates that the first regime is determined by directly optical penetration absorption for low laser fluences and the second regime is governed by the thermal diffusion for high laser fluences. The effects of material properties on the ablation rate and squared diameter are also discussed.

Residual stress within nanoscale metallic multilayer systems during thermal cycling

Projected applications for nanoscale metallic multilayers will include wide temperature ranges. Since film residual stress has been known to alter system reliability, stress development within new film structures with high interfacial densities should be characterized to identify potential long-term performance barriers. To understand factors contributing to thermal stress evolution within nanoscale metallic multilayers, stress in Cu/Nb systems adhered to Si substrates was calculated from curvature measurements collected during cycling between 25°C and 400°C. Additionally, stress within each type of component layers was calculated from shifts in the primary peak position from in-situ heated X-ray diffraction. The effects of both film architecture (layer thickness) and layer order in metallic multilayers were tracked and compared with monolithic Cu and Nb films (1µm total thickness). Analysis indicated that the thermoelastic slope of nanoscale metallic multilayer films (with 20nm and 100nm individual layer thicknesses) depends on thermal expansion mismatch, elastic modulus of the components, and also interfacial density. The layer thickness (i.e. interfacial density) affected thermoelastic slope magnitude (−1.23±0.09MPa/°C for 20nmCu/Nb vs. −0.89±0.03MPa/°C for 100nmCu/Nb) while layer order had minimal impact on stress responses after the initial thermal cycle (−0.82±0.07MPa/°C for 100nm Cu/Nb). When comparing stress responses of monolithic Cu and Nb films to those of the Cu/Nb systems, the nanoscale metallic multilayers show a similar increase in stress above 200°C to the Nb monolithic films, indicating that Nb components play a larger role in stress development than Cu. Phase specific stress calculations (Cu vs. Nb) from X-ray diffraction peak shifts in 20nm Cu/Nb collected during heating reveal that the component layers within a multilayer film respond similarly to their monolithic counterparts.