Non-flat Substrates Research Articles

Novel Pressure-less Adhesive Dispensing Strategy for Thin Die Applications

The automotive electronics market is facing increasingly demanding reliability requirements, along with restrictive European and international environmental legislations targeting the elimination of lead-based materials in electronics packages. As a result, pressure-less Ag-sinter adhesives have been identified as one of the most mature lead-free bonding materials, with thermal and electrical performances comparable or superior to lead-tin alloys, as well as reliability performances compliant with automotive mission profiles. Simultaneously, there is a clear trend towards using thinner dice (<200 µm) in the semiconductor packaging industry for automotive applications, driven primarily by the demand for devices with higher electrical performance. This poses a challenge for the use of pressure-less adhesives, as they have a natural tendency to climb up the vertical sides of the die, potentially causing adhesive to flow over the die surface and resulting in short circuits or yield loss during subsequent IC assembly process steps. Currently, one of the most established processes for controlling the rise of adhesive on the vertical sides of thin dice is 2D screen printing (also known as stencil printing). This process involves selectively printing a complete pattern of adhesive material onto a substrate, onto which the die can then be placed without the need to forcibly spread the pressure-less adhesive to achieve full die backside coverage. However, this process is associated with significant material wastage and is only applicable to flat substrates. The aim of this paper is to introduce an innovative dispensing strategy for thin dice, which dispenses the adhesive in a spiral pattern shape. This technique can be applied to both flat (2D) and non-flat (3D or downset) substrates and minimizes material waste by utilizing a standard time-pressure dispensing system. The paper provides an overview of the concept and the underlying dispensing strategy developed by Besi, as well as an example of its application for die thicknesses down to 110um and 3D substrates in some power packages manufactured by STMicroelectronics. Process capability is validated, and package reliability is demonstrated up to MSL3 + 1000 thermal cycles through scanning acoustic microscopy.

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing.

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

Characterization of lime based insulating mortars for existing buildings in the EU market

There are several lime mortars which have been developed to improve the thermal performance of the facades of existing buildings. They are used as the final coating of a building’s facade thanks to their insulating capacities, weather resistance, vapour permeability, easy application, non-flat substrates adaptability, and/or masonry compatibility. Normally these mortars come into a system to achieve both thermal properties and weathering resistance. The thermally improved mortars, which are lime based, contain an important amount of insulating particles such as cork, diatomaceous earth, expanded clay, expanded perlite or vermiculite, mullite, aerated glass, aerogel, EPS, wood fibres or others. They also may contain pozzolanas and other cementitious particles. They also may contain additives in order to improve its mechanical resistances and/or elasticity. The weathering resistance mortar, the more external one, is suited to complete the mechanical and outdoor resistance properties that the insulated layer lacks. Some of these systems may be obliged to use glass fibre meshes and External Thermal Insulation Composite Systems (ETICS) anchors so they are not as suitable as wanted in historical buildings. In this abstract our main goals are: Acknowledge the reasons to use these mortars, define the multiple layer system that are normally used within these mortars, find which mortars are normally available to be bought in the European Union market, analyse their declared properties and composition, and enlist them according to their potential capacities. Thanks to this paper we will have a better knowledge of the possibilities of lime mortars as insulating material.

Preparation of an Integrated Polarization Navigation Sensor via a Nanoimprint Photolithography Process

Based on the navigation strategy of insects utilizing the polarized skylight, an integrated polarization sensor for autonomous navigation is presented. The polarization sensor is fabricated using the proposed nanoimprint photolithography (NIPL) process by integrating a nanograting polarizer and an image chip. The NIPL process uses a UV-transparent variant template with nanoscale patterns and a microscale metal light-blocking layer. During the NIPL process, part of the resist material is pressed to fill into the nanofeatures of the variant template and is cured under UV exposure. At the same time, the other parts of the resist material create micropatterns according to the light-blocking layer. Polymer-based variant templates can be used for conformal contacts on non-flat substrates with excellent pattern transfer fidelity. The NIPL process is suitable for cross-scale micro–nano fabrication in wide applications. The measurement error of the polarization angle of the integrated polarization sensor is ±0.2°; thus, it will have a good application prospect in the polarization navigation application.

Multipurpose nanocomposite resist for free-standing transparent conductive thin films

Nanocomposites formed by silver nanowires (AgNWs) embedded in a polymer matrix are a convenient way to deposit thin films with electrical conductance and high transparency on different substrates. Nanocomposite resists containing AgNWs in a poly(methyl methacrylate) solution can be effectively used to produce conductive coatings in a straightforward manner. Here, we show that by adding a sacrificial layer of polyvinylpyrrolidone on a glass substrate, prior to the nanocomposite resist, it is possible to obtain large-area free-standing films of about 450 nm with electrical conductance and high transparency. The films can be transferred to different surfaces and materials including non-flat substrates. The formation of conductive stacks by piling two layers was also demonstrated. The optical, electrical, and structural properties of these free-standing films were studied obtaining films with transmittance T(%) = 78% at 550 nm, sheet resistance Rs = (670 ± 40) Ω sq−1 and surface roughness Ra = (50 ± 10) nm. We studied the strain resistance behavior of films transferred to polyethylene terephthalate sheets under bending tests finding a sensitivity of (0.51 ± 0.01) Ω deg−1 and a gradual increase in the resistance during cycling. In addition, thin flexible supports can be added by covering the nanocomposite film with polydimethylsiloxane (PDMS) prior to its release, enhancing the mechanical robustness and improving the manipulation of the films.

Violent droplet impacts with non-flat surfaces

The application of Wagner theory to idealised two-dimensional inertially dominated droplet impacts is extended to incorporate non-flat substrates that are continuous functions of distance along the surface. Mixed boundary value problems are solved for the displacement and velocity potentials for both a single impact with an asymmetric substrate and a pair of impacts. The droplet free-surface position and the pressure on the wetted surface are calculated, along with the load and moment on the substrate. For double impacts a void may be formed between the substrate and the droplet free surface. Double impacts are compared with a single asymmetric impact with one half of the equivalent substrate geometry to assess how the free surface, loads and moments are affected by the separation between impact sites. Interactions between symmetric double impacts enhance the liquid penetration between the impact sites and increase the load and moment on each substrate element compared with the corresponding single impact. The time taken for the void between droplet and substrate to become saturated is found assuming the gas pressure build-up and capillary forces are negligible, giving an estimate for the transition time from a partially wetted to a fully wetted surface close to the initial impact site. After the void becomes saturated, the subsequent free-surface evolution is determined and the effect of periodic roughness on the contact line evolution is calculated. For surfaces formed of an array of asperities, secondary impacts which both traps further voids and completely wet the surface are found.

Atomistic modeling of physical vapor deposition on complex topology substrates

Molecular Dynamics simulations of Cu physical vapor deposition are employed to investigate the effects of substrate topology and line of sight on the synthesis of thin films on Cu substrates. The deposition and surface relaxation processes of sputtering Cu ions are simulated considering various angles of incidence (0–60°) and sputtering energies (1, 10, 25, and 50 eV). Cu substrates are constructed with three different surface geometries: i) a flat Cu (111) surface; ii) a sinusoidal shaped substrate with a period of 127.6 Å and amplitude 20 Å; and iii) a combined substrate composed of a flat Cu (111) surface and a cylinder of radius 63.8 Å, floating 50 Å above the flat surface. Results from simulations with low impact energy (1 eV) show a strong effect of the angle of incidence on the roughness of the thin film produced on the flat surface. For non-flat substrates, areas with no line of sight have diminished film coverage, which is offset by increasing the deposition energy. Higher energy atoms have greater adatom mobility enabling the coverage of regions with no line of sight and the reduction of surface roughness. The results show a lower sticking probability for highly energetic deposition at high angles of deposition, effectively promoting film coverage in areas with no line of sight through physical redeposition processes, which is augmented by particle-induced sputtering. These atomistic insights are relevant for the understanding of physical vapor deposition processes on convoluted topology substrates, such as magnetron sputtering on metamaterial lattices.

Simultaneous Intelligent Imaging of Nanoscale Reactivity and Topography by Scanning Electrochemical Microscopy.

Scanning electrochemical microscopy (SECM) enables reactivity and topography imaging of single nanostructures in the electrolyte solution. The in situ reactivity and topography, however, are convoluted in the real-time image, thus requiring another imaging method for subsequent deconvolution. Herein, we develop an intelligent mode of nanoscale SECM to simultaneously obtain separate reactivity and topography images of non-flat substrates with reactive and inert regions. Specifically, an ∼0.5 μm-diameter Pt tip approaches a substrate with an ∼0.15 μm-height active Au band adjacent to an ∼0.4 μm-wide slope of the inactive glass surface followed by a flat inactive glass region. The amperometric tip current versus tip-substrate distance is measured to observe feedback effects including redox-mediated electron tunneling from the substrate. The intelligent SECM software automatically terminates the tip approach depending on the local reactivity and topography of the substrate under the tip. The resultant short tip-substrate distances allow for non-contact and high-resolution imaging in contrast to other imaging modes based on approach curves. The numerical post-analysis of each approach curve locates the substrate under the tip for quantitative topography imaging and determines the tip current at a constant distance for topography-independent reactivity imaging. The nanoscale grooves are revealed by intelligent topography SECM imaging as compared to scanning electron microscopy and atomic force microscopy without reactivity information and as unnoticed by constant-height SECM imaging owing to the convolution of topography with reactivity. Additionally, intelligent reactivity imaging traces abrupt changes in the constant-distance tip current across the Au/glass boundary, which prevents constant-current SECM imaging.

The MoSeS dynamic omnigami paradigm for smart shape and composition programmable 2D materials

The properties of 2D materials can be broadly tuned through alloying and phase and strain engineering. Shape programmable materials offer tremendous functionality, but sub-micron objects are typically unachievable with conventional thin films. Here we propose a new approach, combining phase/strain engineering with shape programming, to form 3D objects by patterned alloying of 2D transition metal dichalcogenide (TMD) monolayers. Conjugately, monolayers can be compositionally patterned using non-flat substrates. For concreteness, we focus on the TMD alloy MoSe{}_{2c}S{}_{2(1-c)}; i.e., MoSeS. These 2D materials down-scale shape/composition programming to nanoscale objects/patterns, provide control of both bending and stretching deformations, are reversibly actuatable with electric fields, and possess the extraordinary and diverse properties of TMDs. Utilizing a first principles-informed continuum model, we demonstrate how a variety of shapes/composition patterns can be programmed and reversibly modulated across length scales. The vast space of possible designs and scales enables novel material properties and thus new applications spanning flexible electronics/optics, catalysis, responsive coatings, and soft robotics.

Nanoscale Intelligent Imaging Based on Real-Time Analysis of Approach Curve by Scanning Electrochemical Microscopy.

Scanning electrochemical microscopy (SECM) enables high-resolution imaging by examining the amperometric response of an ultramicroelectrode tip near a substrate. Spatial resolution, however, is compromised for nonflat substrates, where distances from a tip far exceed the tip size to avoid artifacts caused by the tip-substrate contact. Herein, we propose a new imaging mode of SECM based on real-time analysis of the approach curve to actively control nanoscale tip-substrate distances without contact. The power of this software-based method is demonstrated by imaging an insulating substrate with step edges using standard instrumentation without combination of another method for distance measurement, e.g., atomic force microscopy. An ∼500 nm diameter Pt tip approaches down to ∼50 nm from upper and lower terraces of a 500 nm height step edge, which are located by real-time theoretical fitting of an experimental approach curve to ensure the lack of electrochemical reactivity. The tip approach to the step edge can be terminated at <20 nm prior to the tip-substrate contact as soon as the theory deviates from the tip current, which is analyzed numerically afterward to locate the inert edge. The advantageous local adjustment of tip height and tip current at the final point of tip approach distinguishes the proposed imaging mode from other modes based on standard instrumentation. In addition, the glass sheath of the Pt tip is thinned to ∼150 nm to rarely contact the step edge, which is unavoidable and instantaneously detected as an abrupt change in the slope of approach curve to prevent damage of the fragile nanotip.

Prediction of a flying droplet landing over a non-flat substrates for ink-jet applications

Printing with inkjet technology has found new forms of application in the industry and in this article we study this technology focused on printing on non-flat surfaces. Since there is no print history over distances greater than 1 mm due to the rupture phenomenon, an initial quality standard is defined to measure achievements in a relative manner. An interactive method is used that requires the user to approach the machine in multiple analyzes of different types. The first approach is a mathematical model this model was constructed to predict the drop distance of the drop in the non-planar substrate with respect to the planned one in the flat substrate, taking into account that most of the drops fall to different heights presenting a greater or lesser state of development the phenomena present in the flight. The results allow to initiate a process of compensation that avoids the distortion of the figure to improve the printing resolution. The results are validated using a relative quality through industrial ink-jet printer with heads capable of injecting functional fluids. The initial result indicates that in standard surface printing with print relative quality already defined, it can be used only for low resolution formats with thick lines, and the result can be improved when the original figure is treated by compensating the distance between the numerical prediction and the initial objective.

Concept of round non-flat thin film solar cells and their power conversion efficiency calculation

Thin-film solar cells that are considered as the second generation of solar cells are known for their low cost and acceptable efficiency. In this technology, semiconductor layers with a thickness of micrometer are deposited on thick enough substrates to maintain physical consistency. The relatively low processing temperature helps use substrates of different materials. Compared with crystalline solar cells, which are mainly made up of rigid flat plates, it is also possible to make thin-film cells on flexible or non-flat substrates. In this study, a method was first proposed to calculate the efficiency of such cells without the need for 3D simulation, and then it is investigated using non-flat conical and paraboloid substrates as a novel method to enhance the light trapping. As a result, a significant increase in the efficiency of the studied non-flat cells was observed and reported in comparison with the flat cells. In addition, the paraboloid shape shows a better performance than that of the conical, to use as the cell's substrate.

Electron beam lithography for contacting single nanowires on non-flat suspended substrates

A methodology based on the use of Electron Beam Lithography for contacting individual nanowires on top of non-flat micromembranes and microhotplates has been implemented, and the practical details have been exhaustively described. The different fabrication steps have been adapted to the substrate’s topology, requiring specific holders and conditions. The methodology is demonstrated on individual SnO2 nanowires, which, after fabrication, have been characterized as functional resistive gas nanosensors towards NH3 and benchmarked against similar devices fabricated using more conventional Dual Beam Focused Ion Beam techniques, demonstrating the superior properties of the here presented methodology, which can be further extended to other non-conventional suspended substrates and nanomaterials.

A self-assembled covalent nanoglue

Nano-glues rely on surface chemistry to intimately bond two surfaces together. The bond can be removable, which means it has to weaken, usually with temperature, or it can be a permanent bond, valued for strength, with a covalent bond yielding the highest strength. A practical covalent nanoglue based upon self-assembled monolayers (SAM) with amine and carboxyl termination is demonstrated, and applicable to any surface that bonds these SAM layers. For bonding flat or deformable layers, the SAM layers on each surface bond directly to each other with a peptide or nylon-like bond. For a removable bond or longer covalent structure, nylon chains are grown between the layers to bridge the gap for non-flat and non-flexible substrates. Bond strength and reliability are measured for several preparation schemes for the intermediate layer. A crystallization process is developed to pre-align the intermediate layer precursors and drive off the solvent to improve bond reliability and insure covalent bonding from wafer to wafer (W2W). Both covalent and removable bonds are created. Temperature dependence of the removable bond strength is measured, while the covalent bonds are stronger than our measurement process. The nanoglue does not bond until activated by (modest) heating, so alignment is enabled, and it is directly compatible with a wafer level fluid-self-alignment (FSA) process described elsewhere.

Flexible Strain Sensors Fabricated by Meniscus-Guided Printing of Carbon Nanotube-Polymer Composites.

Printed strain sensors have promising potential as a human-machine interface (HMI) for health-monitoring systems, human-friendly wearable interactive systems, and smart robotics. Herein, flexible strain sensors based on carbon nanotube (CNT)-polymer composites were fabricated by meniscus-guided printing using a CNT ink formulated from multiwall nanotubes (MWNTs) and polyvinylpyrrolidone (PVP); the ink was suitable for micropatterning on nonflat (or curved) substrates and even three-dimensional structures. The printed strain sensors exhibit a reproducible response to applied tensile and compressive strains, having gauge factors of 13.07 under tensile strain and 12.87 under compressive strain; they also exhibit high stability during ∼1500 bending cycles. Applied strains induce a contact rearrangement of the MWNTs and a change in the tunneling distance between them, resulting in a change in the resistance (Δ R/ R0) of the sensor. Printed MWNT-PVP sensors were used in gloves for finger movement detection; these can be applied to human motion detection and remote control of robotic equipment. Our results demonstrate that meniscus-guided printing using CNT inks can produce highly flexible, sensitive, and inexpensive HMI devices.

Effect of curved surfaces on the impacting nano-droplets and their shape control: A molecular dynamics simulation study

Considerable non-flat solid substrates are widely-existed where liquid drops inevitably impact on their surfaces, precise control of shape evolution of these impacting drops on such surfaces is desired in nature and some advanced technologies. Herein, a common but less-concerned substrate with curvature is used to investigate the impact dynamic of liquid metals by performing molecular dynamics (MD) simulations, which exhibits anisotropic impact behavior with distinct spreading and retraction in the two perpendicular directions and results in an elongated strip-like shape during shape evolution. The effects of curvature diameter, liquid properties (drop size and surface tension), and impact velocity are deeply demonstrated by quantitatively comparing the value of length ratio that is defined to describe the drop asymmetrical impact behaviors. This work helps advance our understanding of how the liquid metal evolves after impact on the curved surface and provides some feasible strategies to control its deposition and shape for some potential applications, such as 3D printing techniques, special coating, as well as drop-casting process.

Directing the diffusive motion of fullerene-based nanocars using nonplanar gold surfaces.

A new method for guiding the motion of fullerene and fullerene-based nanocars is introduced in this paper. The effects of non-flat substrates on the motion of C60, a nanocar and a nanotruck are investigated at different conditions and temperatures. Their behavior is studied using two different approaches: analyzing the variation in potential energy and conducting all-atom classical molecular dynamics simulations. This paper proposes that the use of a stepped substrate will make their motion more predictable and controllable. The results of the simulations show that C60 stays on the top side of the step and cannot jump over the step at temperatures of 400 K and lower. However, at temperatures of 500 K and higher, C60 has sufficient energy to travel to the down side of the step. C60 attaches to the edge and moves just alongside of the edge when it is on the down side of the step. The edge also restricts the motion of C60 alongside the edge and reduces its range of motion. By considering the motion of C60, the general behavior of the nanocar and nanotruck is predictable. The nanocar stays on the top side of the step at temperatures of 400 K and less; at 500 K and higher temperatures, its wheels jump off the edge, and its range of motion is restricted. The relatively rigid chassis of the nanotruck does not allow the free individual motion of the wheels. As a result, the entire nanotruck stays on the top side of the step, even at 600 K. A pathway with the desired route can be fabricated for the motion of C60 and nanocars using the method presented in this paper. This represents a step towards the directional motion of C60 and nanocars.

Thin-film growth dynamics with shadowing effects by a phase-field approach

Shadowing effects during the growth of nano- and microstructures are crucial for the realization of several technological applications. They are given by the shielding of the incoming material flux provided by the growing structures themselves. Their features have been deeply investigated by theoretical approaches, revealing important information to support experimental activities. However, comprehensive investigations able to follow every stage of the growth processes as a whole, particularly useful to design and understand targeted experiments, are still challenging. In this work, we study the thin-film growth dynamics by means of a diffuse interface approach accounting for both deposition with shadowing effects and surface diffusion driven by the minimization of the surface energy. In particular, we introduce the coupling between a phase-field model and the detailed calculation of the incoming material flux at the surface deposited from vacuum or vapor phase in the ballistic regime. This allows us to finely reproduce the realistic morphological evolution during the growth on nonflat substrates, also accounting for different flux distributions. A general assessment of the method, focusing on two-dimensional profiles, is provided thanks to the comparison with a sharp-interface approach for the evolution of the early stages. Then, the long-time-scale dynamics is shown in two and three dimensions, providing a general overview of the features observed during deposition on corrugated surfaces involving flattening, increasing of surface roughness with the growth of columnar structures, and voids formation.

Layer-by-Layer Assembly of Fluorine-Free Polyelectrolyte-Surfactant Complexes for the Fabrication of Self-Healing Superhydrophobic Films.

Fluorine-free self-healing superhydrophobic films are of significance for practical applications because of their extended service life and cost-effective and eco-friendly preparation process. In this study, we report the fabrication of fluorine-free self-healing superhydrophobic films by layer-by-layer (LbL) assembly of poly(sodium 4-styrenesulfonate) (PSS)-1-octadecylamine (ODA) complexes (PSS-ODA) and poly(allylamine hydrochloride) (PAH)-sodium dodecyl sulfonate (SDS) (PAH-SDS) complexes. The wettability of the LbL-assembled PSS-ODA/PAH-SDS films depends on the film structure and can be tailored by changing the NaCl concentration in aqueous dispersions of PSS-ODA complexes and the number of film deposition cycles. The freshly prepared PSS-ODA/PAH-SDS film with micro- and nanoscaled hierarchical structures is hydrophilic and gradually changes to superhydrophobic in air because the polyelectrolyte-complexed ODA and SDS surfactants tend to migrate to the film surface to cover the film with hydrophobic alkyl chains to lower its surface energy. The large amount of ODA and SDS surfactants loaded in the superhydrophobic PSS-ODA/PAH-SDS films and the autonomic migration of these surfactants to the film surface endow the resultant superhydrophobic films with an excellent self-healing ability to restore the damaged superhydrophobicity. The self-healing superhydrophobic PSS-ODA/PAH-SDS films are mechanically robust and can be deposited on various flat and nonflat substrates. The LbL assembly of oppositely charged polyelectrolyte-surfactant complexes provides a new way for the fabrication of fluorine-free self-healing superhydrophobic films with satisfactory mechanical stability, enhanced reliability, and extended service life.

Atmospheric spatial atomic layer deposition of Zn(O,S) buffer layer for Cu(In,Ga)Se2 solar cells

Zinc oxysulfide has been grown by spatial atomic layer deposition (S-ALD) and successfully applied as buffer layer in Cu(In, Ga)Se2 (CIGS) solar cells. S-ALD combines high deposition rates (up to nm/s) with the advantages of conventional ALD, i.e. excellent control of film composition and superior uniformity over large-area and even non-flat substrates. Diethylzinc, water and hydrogen sulfite (H2S) have been used as zinc, oxygen and sulfur precursor, respectively. The S/(S+O) ratio in the film is accurately controlled by exposing the substrate simultaneously to both H2O and H2S precursors, which are pre-mixed and co-injected in the same deposition zone. The optoelectronic and morphological properties of Zn(O,S) are characterized as a function of the S/(S+O) ratio. Zn(O,S) buffer layers with different values of S/(S+O) ratio are applied in CIGS solar cells. An optimum value of S/(S+O) ratio of about 0.4 is found for which both the short circuit current density (Jsc=34.2mA/cm2) and cell efficiency (η=15.9%) increase, as compared to reference cells with CdS buffer layer (Jsc=32.1mA/cm2, η=15.5%).