Multilayer Thin Film Stacks Research Articles

Primary and Secondary Reflective Color Realized by Full‐Solution‐Processed Multi‐Layer Structures

AbstractA full solution‐based method is reported to fabricate multilayer thin film stacks for structural color applications. This is in contrast to the conventional fabrication methods that require high vacuum processes such as physical vapor deposition and magnetron sputtering, which significantly limits their practical use due to the high cost and long processing time. Copper/silicon dioxide/copper (Cu/SiO2/Cu) and copper/titanium dioxide/copper (Cu/TiO2/Cu) are chosen as the model system due to their simple structure and wide color tunability. A systematic investigation is carried out for each layer to ensure good film quality as well as its compatibility with all previous layers. Especially, a particulate Cu layer having optical properties distinct from that of bulk is obtained through such solution deposition, where the refractive index of Cu can be continuously tuned with deposition time. Both primary and secondary colors are achieved with a continuous manipulation of both the dielectric thickness and top Cu morphology on different substrates (e.g., silicon, glass, plastics, etc.).

Green Reflector with Predicted Chromatic Coordinates

The color reflector with multiple-layer thin film scheme has attracted much attention because of the potential for massive production by wafer-scale deposition and the possibility to integrate with photonics (semiconductor) devices. Here, an angle-insensitive green reflector with a simple multilayer dielectric thin film structure was reported, with predicted chromatic coordinates based on CIE 1931 standard. The SiN/SiO2 multilayer thin film stack, including a special silicon-rich nitride material with ultrahigh refractive index, was grown alternatively by an inductively coupled plasma chemical vapor deposition (ICPCVD) system at a low stage temperature of 80 °C. The green reflector showed a maximum reflectivity of 73% around 561 nm with a full width at half maximum (FWHM) of 87 nm in the visible wavelength range, which contributed significantly to its color appearance. The measurement by an angle-resolved spectrometer under the illumination of p/s-polarized light wave with a variable angle of incidence indicated that the reflectance spectrum blue-shifted slightly with the increasing of incident angle such that the green color could be kept.

Constructing coarse-grained skyrmion potentials from experimental data with Iterative Boltzmann Inversion

In an effort to understand skyrmion behavior on a coarse-grained level, skyrmions are often described as 2D quasiparticles evolving according to the Thiele equation. Interaction potentials are the key missing parameters for predictive modeling of experiments. Here, the Iterative Boltzmann Inversion technique commonly used in soft matter simulations is applied to construct potentials for skyrmion-skyrmion and skyrmion-magnetic material boundary interactions from a single experimental measurement without any prior assumptions of the potential form. It is found that the two interactions are purely repulsive and can be described by an exponential function for micrometer-sized skyrmions in a ferromagnetic thin film multilayer stack. This captures the physics on experimental length and time scales that are of interest for most skyrmion applications and typically inaccessible to atomistic or micromagnetic simulations.

Determining adhesion of critical interfaces in microelectronics – A reverse Finite Element Modelling approach based on nanoindentation

State-of-the-art microelectronic devices, especially power semiconductors, are usually exposed to harsh environmental conditions on the one hand, for example large temperature hubs, and to challenging application conditions on the other hand, such as pulsed current applications. Mechanical robustness and reliability have become more and more challenging in recent years. Along with a continuous shrinkage of dimensions, thin film materials are stacked and micro-structured for optimized performance of such devices. Major risks for device reliability can result from loss of adhesion at interfaces of these stacked thin films. Therefore, it is crucial for such devices to feature perfect initial adhesion of the said thin film layers, even after exposure to thermal treatment or in presence of intrinsic stress. In order to validate device robustness, the quality of interface adhesion between isolating and metallization layers should be carefully assessed. A particular focus is set on interfaces between the following, widely employed materials: Cu, Al, WTi, BPSG, SiN etc. Robustness by design at critical interfaces has relied more and more on advanced Finite Element Modelling (FEM) methods in recent years. This includes explicit modelling of adhesion and delamination. Accurate parameters that describe interfacial adhesion – critical adhesion energies in tensile and shear mode – need to be extracted by means of suitable experiments on tailored test vehicles. FEM is able in turn to verify the existing theoretical framework about extracting adhesion energies from experiments, given that the analytical models are often phenomenological and based on simplified assumptions. Experimentally, Nanoindentation on designed multi-layer thin film stacks has proven to be a suitable method to investigate inter-layer adhesion strength on test vehicle basis. We focused on layered stacks of tungsten‑titanium (WTi) and Boron-Phosphorous-Silicate Glass (BPSG) on Silicon substrates. The WTi layer exhibits compressive stress tailored by means of deposition. When an axis-symmetric conical or spherical indenter penetrates the WTi film, buckles around the indenter imprint arise upon loading and may propagate during unloading. Buckle geometry is measured by means of focused ion beam and Scanning Electron Microscopy. Afterwards the critical adhesion energies can be determined by existing analytical models. Combining experimental and simulation methods one may follow a Physics-of-Failure approach and assess the failure mechanisms related to thin film delamination and interface fracture. With experimental results on hand, we implemented an FE model of the same multi-layer system. Initiation and propagation of delamination was modeled by means of a Cohesive Zone Model (CZM) based on a cohesive surface contact approach. The model incorporates elastic-plastic deformation as well as intrinsic stresses of all layers, including the substrate. Following an inverse FEM approach, we reproduced the size of the delaminated area and load-displacement curves of the indenter by recursively readjusting Cohesive Zone parameters. The so-obtained adhesion parameters and modelling can be used elsewhere in delamination risk assessments of industrial semiconductor applications. This approach also gives insight into the accuracy of the formulae used for calculating the total energy release rate and extracting the further relevant parameters without the need to rely on phenomenological assumptions.

Switchable distributed Bragg reflector using GST phase change material.

We demonstrate the design, fabrication, and measurement of a switchable distributed Bragg reflector (DBR) that can be thermally switched from a close-to-zero reflective OFF state to a more than 70% reflection in its ON state. This is accomplished using a multilayer thin film stack using germanium (Ge) and the phase change material (PCM) Ge2Sb2Te5 (GST). The refractive indexes of Ge and GST in the amorphous state are closely matched, resulting in a nearly zero interface reflection. With appropriate antireflection coatings at the cavity ends, the overall reflection can be designed to be close to zero. When the GST is switched to the crystalline state, the refractive index contrast between the Ge and GST layers will increase dramatically contributing to the DBR reflection. Using this unique feature, we were able to design and experimentally demonstrate more than 70% reflection in the ON state and close to zero reflection in the OFF state at a wavelength of 2 µm.

Non-destructive depth profile evaluation of multi-layer thin film stack using simultaneous analysis of data from multiple X-ray photoelectron spectroscopy instruments

Non-destructive depth profile evaluation of multi-layer thin film stacks using simultaneous analysis of angle-resolved X-ray photoelectron spectroscopy data from multiple instruments is demonstrated. The data analysis algorithm, called the maximum smoothness method, was originally designed to handle data from a single XPS instrument with a single X-ray energy; in this work, the algorithm is extended to provide a simultaneous analysis tool which can handle data from multiple instruments with multiple X-ray energies. The analysis provides depth profiles for multilayer stacks that cannot be obtained by conventional data analysis methods. In this paper, metal multi-layer stack samples with total thickness greater than 10 nm are analyzed with the maximum smoothness method to non-destructively obtain depth profiles, with precise information on the chemical states of atoms in the surface layer (<2 nm) and the overall layer stack structure, which can only be obtained by analyzing the data from multiple instruments.

Ferromagnetic resonance and spin-wave exchange stiffness of FeGaB/Al2O3 multilayer thin film stack for microwave applications

In this article, we have illustrated the deposition of multilayer film stack of Ta (5 nm)/[FeGaB (15 nm)/Al2O3 (3 nm)] n/Ta (3 nm)/Al2O3 (2 nm); n = 1,5, and 10, onto Si substrates, and studied magnetodynamic properties with multilayer thickness and temperature dependence. The results of surface morphology of thin film stacks suggest dipolar coupling generates for n = 5 and 10 thin film stack, which influence the ferromagnetic absorption results. From results of ferromagnetic resonance spectroscopy (FMR), the resonance peak for each excitation frequency provides the effective magnetization, the gyromagnetic ratio and damping factor. The damping factor for multilayer stack n = 1 shows a lower value compared to the other two samples. The multilayer stack provides perpendicular surface magnetic anisotropy (ks) from thickness dependence of effective magnetization. The analysis of spin-wave resonance spectra at multiple frequencies allows manipulate the exchange stiffness (D) of multilayer film stack n = 5 and n = 10 and temperature dependence the exchange stiffness follows the itinerant-electron model (T2 law) and suggests electron - magnon scattering exists in the thin film stack.

Design, Simulation, Fabrication, and Characterization of an Electrothermal Tip-Tilt-Piston Large Angle Micromirror for High Fill Factor Segmented Optical Arrays.

Micro-electromechanical system (MEMS) micromirrors have been in development for many years, but the ability to steer beams to angles larger than 20° remains a challenging endeavor. This paper details a MEMS micromirror device capable of achieving large motion for both tip/tilt angles and piston motion. The device consists of an electrothermal actuation assembly fabricated from a carefully patterned multilayer thin-film stack (SiO2/Al/SiO2) that is epoxy bonded to a 1 mm2 Au coated micromirror fabricated from an SOI wafer. The actuation assembly consists of four identical actuators, each comprised of a series of beams that use the inherent residual stresses and coefficient of thermal expansion (CTE) mismatches of the selected thin films to enable the large, upward, out-of-plane deflections necessary for large-angle beamsteering. Finite element simulations were performed (COMSOL v5.5) to capture initial elevations and tip/tilt motion displacements and achieved <10% variance in comparison to the experiment. The measured performance metrics of the micromirror include tip/tilt angles of ±23°, piston motion of 127 µm at sub-resonance, and dynamics characterization with observed resonant frequencies at ~145 Hz and ~226 Hz, for tip/tilt and piston motion, respectively. This unique single element design can readily be scaled into a full segmented micromirror array exhibiting an optical fill-factor >85%, making it suitable for optical phased array beam control applications.

Effect of Ta insertion between Pt and CoFeB on interfacial magnetic anisotropy in Pt/CoFeB/MgO multilayer thin-film stack

The effect of a thin Ta layer inserted between Pt and CoFeB layers on perpendicular magnetic anisotropy (PMA) with MgO barrier layer has been studied. The crystallinity was studied by performing high-resolution x-ray diffraction (HR-XRD) technique. While the crystal peak of Pt is observed in all sample stack that is oriented as (111) face-centered cubic crystal structure, a very weak and broad α-Ta (110) peak is observed when Ta layer thickness is above 0.8 nm. Magneto-optical Kerr effect (MOKE) measurements show that the PMA could be enhanced by inserting Ta layer between Pt and CoFeB layer. The magnetically dead layer thickness (tdead) and the interfacial anisotropy energy density (Ki) of the various Ta-inserted layer thickness were found as 0.135 nm; 0.75 erg/cm2, 0.141 nm; 1.02 erg/cm2, 0.187 nm; 1.15 erg/cm2, for tTa = 0.0, 0.5, and 1.0 nm, respectively. Both tdead and Ki increase with Ta-inserted layer thickness in Pt/Ta(t)/CoFeB/MgO multilayer film stack. The sources of interfacial magnetic anisotropy can be hybridization, crystallinity properties, and/or B/Ta diffusion effect at the interfaces in Pt/Ta(t)/CoFeB/MgO multilayer film stack.

Passive radiative cooling and other photonic approaches for the temperature control of photovoltaics: a comparative study for crystalline silicon-based architectures.

The radiative cooling of objects during daytime under direct sunlight has recently been shown to be significantly enhanced by utilizing nanophotonic coatings. Multilayer thin film stacks, 2D photonic crystals, etc. as coating structures improved the thermal emission rate of a device in the infrared atmospheric transparency window reducing considerably devices' temperature. Due to the increased heating in photovoltaic (PV) devices - that has significant adverse consequences on both their efficiency and life-time - and inspired by the recent advances in daytime radiative cooling, we developed a coupled thermal-electrical modeling to examine the physical mechanisms on how a radiative cooler affects the overall efficiency of commercial photovoltaic modules and how the radiative cooling impact is compared with the impact of other photonic strategies for reducing heat generation within PVs, such as ultraviolet and sub-bandgap reflection. Employing our modeling, which takes into account all the major intrinsic processes affected by the temperature variation in a PV device, we additionally identified the validity regimes of the currently existing PV-cooling models which treat the PV coolers as simple thermal emitters. Finally, we assessed some realistic photonic coolers from the literature, compatible with photovoltaics, to implement the radiative cooling requirements and the requirements related to the reduction of heat generation, and demonstrated their associated impact on the temperature reduction and PV efficiency. Consistent with previous works, we showed that combining radiative cooling with sub-bandgap reflection proves to be more promising for increasing PVs' efficiency. Providing the physical mechanisms and requirements for reducing PV operating temperature, our study provides guidelines for utilizing suitable photonic structures for enhancing the efficiency and the lifetime of PV devices.

Dynamic control of spontaneous emission rate using tunable hyperbolic metamaterials.

We numerically investigate the dynamic control over the spontaneous emission rate of quantum emitters using tunable hyperbolic metamaterials (HMMs). The dispersion of a metal-dielectric thin-film stack at a given frequency can undergo a topological transition from an elliptical to a hyperbolic dispersion by incorporating a tunable metal or dielectric film in the HMM. This transition modifies the local density of optical states of the emitter and, hence, its emission rate. In the visible range, we use an HMM consisting of TiN and ${{\rm Sb}_2}{{\rm S}_3}$Sb2S3 and show considerable tunability in the Purcell enhancement and quantum efficiency as ${{\rm Sb}_2}{{\rm S}_3}$Sb2S3 phase changes from amorphous to crystalline. Similarly, we show tunable Purcell enhancement in the telecommunication wavelength range using a ${\rm TiN}/{{\rm VO}_2}$TiN/VO2- HMM. Finally, tunable spontaneous emission rate in the mid-IR range is obtained using a ${\rm graphene}/{\rm MgF}_2$graphene/MgF2 HMM by modifying the graphene conductivity through changing its chemical potential. We show that using a metal nitride (for the visible and NIR HMMs) and a fluoride (for the mid-IR HMM) is important to get an appreciable change in the effective permittivity of the thin-film multilayer stack.

Origin of Interfacial Magnetic Anisotropy in Ta/CoFeB/MgO and Pt/CoFeB/MgO Multilayer Thin Film Stacks

Interfacial perpendicular magnetic anisotropy (PMA) in Pt/Co40Fe40B20/MgO and Ta/Co40Fe40B20/MgO multilayer structures has been researched at various CoFeB layer thicknesses. Magneto-optical Kerr effect (MOKE) and vibrating sample magnetometer (VSM) measurements show that while strong PMA is achieved in Ta/Co40Fe40B20/MgO structure for the thickness of Co40Fe40B20 range from 0.5 to 1.0 nm, a very weak PMA is observed in Pt/Co40Fe40B20/MgO film stack for the same Co40Fe40B20 thicknesses. Based on the experimental results, the interfacial anisotropy energy in Ta/Co40Fe40B20/MgO (1.17 erg/cm2) was found to be almost 2.5× larger than that in Pt/Co40Fe40B20/MgO (0.43 erg/cm2). It is accepted that the PMA comes from the seed/ferromagnetic and ferromagnetic/oxide layer interfaces. Thus, the origin of interfacial magnetic anisotropy can be hybridization and/or crystallinity properties at the interfaces in Ta- and Pt-seeded stacks. While the PMA is strong in Pt/Co/MgO structure, it becomes in-plane in Fe-rich Pt/CoFeB/MgO structure. It is deduced that hybridization between 3d-5d (Co-Pt) orbitals is more dominant in Pt/CoFeB interface than 2p-3d (O-Fe) hybridization at CoFeB/MgO interface in Pt-seeded stacks. In addition, the crystallinity was studied by performing high-resolution x-ray diffraction (HR-XRD) technique. The crystal orientations of Ta and Pt are found as (002) and (111), respectively. The ferromagnetic layer might be induced out-of-plane orientation in Ta-seeded stack due to observing highly crystallized MgO (001). Thus, another reason for in-plane magnetic anisotropy in Pt/Co40Fe40B20/MgO might be the absence of MgO crystallization because Pt is crystallized fcc (111) orientation.

Biosensing with the singular phase of an ultrathin metal-dielectric nanophotonic cavity

The concept of point of darkness has received much attention for biosensing based on phase-sensitive detection and perfect absorption of light. The maximum phase change is possible at the point of darkness where the reflection is almost zero. To date, this has been experimentally realized using different material systems through the concept of topological darkness. However, complex nanopatterning techniques are required to realize topological darkness. Here, we report an approach to realize perfect absorption and extreme phase singularity using a simple metal-dielectric multilayer thin-film stack. The multilayer stack works on the principle of an asymmetric Fabry–Perot cavity and shows an abrupt phase change at the reflectionless point due to the presence of a highly absorbing ultrathin film of germanium in the stack. In the proof-of-concept phase-sensitive biosensing experiments, we functionalize the film surface with an ultrathin layer of biotin-thiol to capture streptavidin at a low concentration of 1 pM.

Transverse magnetooptic effect in multilayers applied to mapping of microwave currents

A sensor capable of mapping microwave (mw) currents in semiconductor circuits can be realized by exploiting magneto-optic effects (MO) at transverse magnetization (M⊥) in ultrathin ferromagnetic or ferrimagnetic films. In the sensor, M⊥would be induced in the magnetic film by the fringing fields of mw currents flowing in the semiconductor circuit along the plane of incidence. In this work, an evaluation of MO sensor performance was made for nanostructures consisting of ultrathin Fe layers sandwiched between AlN dielectric layers. The multilayer thin film stacks were grown on Si wafer substrates. The performance of the sensor systems is characterized in terms of magnetization-induced changes in the MO multilayer reflection coefficients, expressed analytically. Sensor configurations which optimize the operation at the laser wavelength of 410 nm, and which are still easy to fabricate, are proposed. Modeling predicts the strongest MO enhancement in a sensor incorporating two Fe nanolayers, each of a different thickness, formed by the layer sequence AlN/Fe/AlN/Fe/AlN/Au/Si. The use of ferrimagnetic hexagonal ferrite films with the in-plane c-axis as an alternative sensor material is also discussed.

Daytime Radiative Cooling Using Near-Black Infrared Emitters

Recent works have demonstrated that daytime radiative cooling under direct sunlight can be achieved using multilayer thin films designed to emit in the infrared atmospheric transparency window while reflecting visible light. Here, we demonstrate that a polymer-coated fused silica mirror, as a near-ideal blackbody in the mid-infrared and near-ideal reflector in the solar spectrum, achieves radiative cooling below ambient air temperature under direct sunlight (8.2 °C) and at night (8.4 °C). Its performance exceeds that of a multilayer thin film stack fabricated using vacuum deposition methods by nearly 3 °C. Furthermore, we estimate the cooler has an average net cooling power of about 127 W m–2 during daytime at ambient temperature even considering the significant influence of external conduction and convection, more than twice that reported previously. Our work demonstrates that abundant materials and straightforward fabrication can be used to achieve daytime radiative cooling, advancing applications such ...

Graded-index thin-film stack for cladding and coupling.

A graded-index multilayer thin-film stack is optimized to act as a cladding layer on top of a silicon (Si) nanowaveguide and also a collimator for chip coupling where the waveguide ends. The numerical example shows an optimized graded-index profile from 2.35 to 1.45 provides an optical coupling to the standard single-mode fiber with efficiency close to 90% while retaining tight light confinement for the Si nanowaveguide. The corresponding material realization of a graded-index profile with a Si-rich nitride SiNx/SiON/SiO2 system is explored using inductively coupled plasma chemical vapor deposition, and a SiNx cladded Si waveguide is demonstrated.

NBI Optical Filters in Minimally Invasive Medical Devices

The integration of the narrow band imaging (NBI) technique in highly miniaturized minimally invasive medical devices is presented. NBI provides a more reliable sensing performance for in vivo studies on tissues as compared to approaches based on white light illumination. NBI uses a selective filtered light with peak transmission at 415 (blue) and 540 nm (green). The blue light improves the visualization of the superficial mucosal layer, while the deeper penetration of the green light highlights the vascular patterns of the subepithelial vessels. The optical filters are based on a multilayer thin-film stack, using the Fabry–Perot configuration with titanium dioxide (TiO2) and silicon dioxide (SiO2). The blue light-emitting diode (LED) combined with the blue filter results in a maximum central wavelength at 414 nm, full-width half-maximum (FWHM) of 19 nm and maximum relative transmittance of 21%. The green LED combined with the green filter yields maximum peak intensity at 536 nm, FWHM of 30 nm, and maximum relative transmittance of 35%. RF-sputtering was used for the deposition of NBI optical filters. The refractive index and extinction coefficient of the TiO2 and SiO2 thin films were characterized and the green and blue filter designs were experimentally validated.

Nanomechanical investigation of thin-film electroceramic/metal-organic framework multilayers

Thin-film multilayer stacks of mechanically hard magnetron sputtered indium tin oxide (ITO) and mechanically soft highly porous surface anchored metal-organic framework (SURMOF) HKUST-1 were studied using nanoindentation. Crystalline, continuous, and monolithic surface anchored MOF thin films were fabricated using a liquid-phase epitaxial growth method. Control over respective fabrication processes allowed for tuning of the thickness of the thin film systems with a high degree of precision. It was found that the mechanical indentation of such thin films is significantly affected by the substrate properties; however, elastic parameters were able to be decoupled for constituent thin-film materials (EITO ≈ 96.7 GPa, EHKUST−1 ≈ 22.0 GPa). For indentation of multilayer stacks, it was found that as the layer thicknesses were increased, while holding the relative thickness of ITO and HKUST-1 constant, the resistance to deformation was significantly altered. Such an observation is likely due to small, albeit significant, changes in film texture, interfacial roughness, size effects, and controlling deformation mechanism as a result of increasing material deposition during processing. Such effects may have consequences regarding the rational mechanical design and utilization of MOF-based hybrid thin-film devices.

Four Point Bending Test for Adhesion Testing of Packaging Strictures: A Review

Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA(Received December 10, 2014: Corrected December 15, 2014: Accepted December 24, 2014)Abstract: To establish the reliability of a packaging structures, adhesion testing of key interfaces is a critical task. Due tothe material mismatch, the interface may be prone to delamination failure due to conditions during the manufacturing of theproduct or just from the day-to-day use. To assess the reliability of the interface adhesion strength testing can be performedduring the design phase of the product. One test method of interest is the four-point bending (4PB) adhesion strength testmethod. This test method has been implemented in a variety of situations to evaluate the adhesion strength of interfaces inbimaterial structures to the interfaces within thin film multilayer stacks. This article presents a review of the 4PB adhesionstrength testing method and key implementations of the technique in regards to semiconductor packaging. Keywords: four-point bending (4PB), adhesion, thin structures, multi-layers, packaging

Influence of Annealing Temperature on Magnetoelectric Properties of CoFe2O4/Pt/Pb(Zr0.3Ti0.7)O3 Thin Films

CoFe2O4/Pt/Pb(Zr0.3Ti0.7)O3 thin films were grown on Pt/Ti/SiO2/Si substrate in order to investigate the magnetoelectric properties of ferromagnetic/ferroelectric multilayer thin films. Thin Pt layer was introduced to prevent inter-diffusion between CoFe2O4 and Pb(Zr0.3Ti0.7)O3 (PZT) layers. PZT thin film was grown directly on top of Pt substrate by utilizing sol-gel spin coating technique. In order to investigate the possible annealing effect on film microstructure and magnetoelectric properties, multilayer thin film stack was heat-treated at different temperatures ranging from 550°C to 650°C. The structural properties of the films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ferroelectric and ferromagnetic behaviors were analyzed by measuring polarization and magnetization – electric and magnetic field hysteresis. Magnetoelectric coefficients were calculated by measuring magnetoelectric voltages using magnetoelectric measurement system. Both the magnetoelectric properties and the coupling effect of CoFe2O4/Pt/PZT films on ferromagnetic and magnetoelectric properties are discussed as a function of heat-treatment temperature.