Patterning Of Thin Films Research Articles

Geometric model of crack-templated networks for transparent conductive films

Crack-templated networks, metallic frameworks fabricated from crack patterns in sacrificial thin films, can exhibit high optical transmittance, high electric conductivity, and a host of other properties attractive for applications. Despite advances in preparing, characterizing, and analyzing optoelectronic performance of cracked template networks, limited efforts have focused on predicting how their disordered structures help to determine their electrical and optical properties and explain their interrelationships. We introduce a geometric modeling approach for crack-templated networks and use simulation to compute their wavelength- and incident angle-dependent optical transmittance and sheet resistivity. We explore how these properties relate to one another and to those of metallic meshes with periodically ordered aperture arrays. We consider implications of the results for optoelectronic applications, compare figure-of-merit predictions to experimental data, and highlight an opportunity to extend the modeling approach using inverse methods.

Controlled Wrinkling Analysis of Buckled Thin Films on Gradient Elastic Substrate System: A Numerical Study

Continuous change in wrinkle patterns process of thin films to a gradient substrate is the most challenging problem regarding applying reliable and robust numerical methods for postbuckling analysis of the film/substrate system. For example, in the finite element method, the postbuckling simulation suffers from the convergence issue, while, in spectral methods, it is very difficult to capture the localized behavior in soft matters when the boundary conditions are complex. When a thin film is compressed, it can form a wrinkle of a certain amount of wavelength when the compression exceeds a critical value. The compressed compliant substrate system translates to sinusoidal wrinkles and then to period-doubling wrinkling after further compression. In this work, we investigate the mathematical model arising from the changing nature of wrinkle patterns of postbuckled thin films using a robust and efficient numerical algorithm based on the spectral method to evolve the wrinkle patterns. We consider the gradient substrates of three typical variations in the modulus, namely, the symmetry, exponential, and power-law model. It has been observed that the stable equilibrium path has two bifurcation points. At the first bifurcation point, the buckling instability of wrinkling occurs, while the period-doubling buckling instability occurs at the second bifurcation point. For the substrates of material gradients of various types, the amplitude and wavelength are obtained. This study may help in better understanding of wrinkle patterns formation which could be very useful for the designing of stretchable and flexible electronic devices of most substrate systems and to avoid resonance in the noise environment.

Thermocapillary patterning of non-Newtonian thin films

Deformation of thin viscous liquid films exposed to a transverse thermal gradient results in Bénard–Marangoni instability, which would lead to the formation of micro- and nano-sized features. Linear and nonlinear analyses are performed to investigate the thermally induced pattern formation in shear thinning and shear thickening liquid films. The so-called thin film (TF) equation is re-derived to include viscosity variations using the power-law (PL) model. The characteristic wavelength for the growth of instabilities is found using a linear stability analysis of the PL-TF equation. A finite-difference-based discretization scheme and adaptive time step solver are used to solve the PL-TF equation for the nonlinear numerical model. The results show that the rheological property affects the timescale of the process and the size and final shape of the formed features. The fastest growth pillar reaching the top substrate in a shear thickening fluid is shorter than both the shear thinning and the Newtonian fluid cases. Moreover, morphological changes between patterns of shear thinning and shear thickening fluids are correlated with local viscosity variations. The number of formed pillars considerably increases with the increasing flow behavior index. The existing model also predicts the formation of pillars and bicontinuous structures at very low and high filling ratios.

Photochemically assisted patterning: An interfacial hydrodynamic model perspective

Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.

Nanosecond pulse fiber laser patterning of bilayer molybdenum thin film on 2-sq inch soda-lime glass substrate for CIGS thin film solar cell applications

Nanosecond pulse fiber laser patterning of bilayer molybdenum thin film on 2-sq inch soda-lime glass substrate for CIGS thin film solar cell applications

High-fidelity patterning of AlN and ScAlN thin films with wet chemical etching

We report on the anisotropic wet etching of sputtered AlN and Sc0.2Al0.8N thin films. With tetramethyl ammonium hydroxide at 80 °C, the etch rates along the c-axis were 330 and 30 nm/s for AlN and Sc0.2Al0.8N, respectively. Although the etching was anisotropic, significant lateral etching below the mask occurred, perpendicular to the c-axis. With a 1 µm Sc0.2Al0.8N film, it could be up to 1800 nm. We studied the lateral etching with molybdenum, SiO2, SiNx and TiO2 masks, and found the leading cause for the lateral etching to be modification of the AlN or Sc0.2Al0.8N surface caused by ion bombardment and surface oxidation by ambient air. The lateral etching was reduced by optimizing the mask deposition and with thermal annealing. With Sc0.2Al0.8N, the lateral etching was reduced down to 35–220 nm depending on the mask, while with AlN, it was reduced to negligible. These results can be used for developing optimised mask deposition processes for better etch characteristics and for microfabrication of AlN and ScxAl1‑xN thin-film structures.

Strain ratio and thickness effects on plasticity and crack patterns of Nickel thin films

The influence of the strain ratio (equibiaxial or uniaxial) and the strain path (equibiaxial or sequenced uniaxial) on the cracking of metallic thin films is studied. We used 20, 100 and 600 nm Nickel films deposited onto flexible substrates, analysed with in-situ X-ray diffraction and digital image correlation, and post-mortem optical microscopy. It is shown that the thickness and the loading ratio effects are interweaved. In particular, 600 nm films show a small (1.9%) von Mises fracture strain and straight cracks when strained equibiaxially, but a larger (2.7%) fracture strain and more tortuous cracks when strained uniaxially. On the contrary, 20 and 100 nm thin films show different von Mises ultimate tensile strengths, though their von Mises fracture strain and theirs cracks tortuosity hardly depend on the test.

Near-Infrared Light-Driven Shape-Programmable Hydrogel Actuators Loaded with Metal-Organic Frameworks.

Shape-programmable hydrogel-based soft actuators that can adaptively respond to external stimuli are of paramount significance for the development of bioinspired aquatic smart soft robots. Herein, we report the design and synthesis of near-infrared (NIR) light-driven hydrogel actuators through in situ photopolymerization of poly(N-isopropylacrylamide) (PNIPAM) hydrogels loaded with metal-organic frameworks (MOFs) onto the surface of the poly(dimethylsiloxane) (PDMS) thin film. The MOFs can not only function as an excellent photothermal nanotransducer but also accelerate the adsorption/desorption of water due to their porous nanostructure, which speeds up the response rate of the actuators. Shape-programmable hydrogel actuators are fabricated by tailoring the patterning of PDMS thin film, and thus different shape-morphing modes such as directional bending and chiral twisting are observed under the NIR light irradiations. As the proof-of-concept demonstrations, an artificial hand, biomimetic mimosa, and flower are conceptualized with light-driven MOF-containing hydrogel actuators. Interestingly, we are able to achieve an octopus-inspired light-driven soft swimmer upon cyclic NIR illumination due to the fast photoresponsiveness of as-prepared hydrogel actuators. This work can offer insights for fabricating programmable and reconfigurable smart aquatic soft actuators, thus shining a light into their potential applications in emerging fields including soft robots, biomedical devices, and beyond.

Programming fracture patterns of thin films.

Controlled fracture presents opportunities for the advanced fabrication of thin films. However, programmability analogous to that of Chinese paper cutting is still challenging, where fracture patterns can be created as required without preformed cracks for guidance. Here, we establish a design framework for tearing adhesive thin films from foldable substrates with such programmability. Our analytical model captures the observed crack behavior, demonstrating that the deflection of crack paths can exceed 90^{∘}. Besides, for thick foldable substrates with multiple ridges, we additionally propose a robust method of directional fracture where the cracks are forced to extend along the ridges.

Deposition and characterization of self-cleaning TiO2 thin films for photovoltaic application

TiO2 is a self-cleaning material generally employed in engineering today because of its excellent physical and chemical characteristics. However, its self-cleaning behavior on photovoltaic panels has not been sufficiently studied and reported in the literature. This study synthesized, deposited and, characterized titanium dioxide (TiO2) thin film for self-cleaning photovoltaic application. The TiO2 was synthesized using the sol–gel method and spin coating was used for the deposition on glass substrate at optimized parameters. The characterization was done to ascertain the morphology, structural, chemical bond and absorption spectrum, optical properties, photocatalytic, and thin-film durability. The surface morphology shows a snowflakes-like shape and at higher magnification shows they are clustered and pockets of a linked chain. The X-ray diffraction patterns of the TiO2 thin films deposited and annealed at 600 °C displayed mainly anatase phase in its crystal structure with an average particle size of 15.82 nm. The FTIR analysis revealed prominent bands at 2921.4, 1445.5, and 463 cm−1, with high anatase Titania peaking at 463–349 cm−1. The band for TiOTi stretching vibration is observed at 489 cm−1. Water contact angle measurement proved that the thin film has a super hydrophilic tendency. The TiO2 thin film calcined at 600 °C had a thickness of 63 nm and absorbed light in the UV–visible region starting from a wavelength of 500 nm. The obtained surface morphology for the calcined film encourages photocatalytic and self-cleaning activities on the photovoltaic application as oppose to the non-calcined TiO2 thin film. The snowflake shapes bring out well-exposed surfaces that promote the valuable spectrum of sunlight adsorption on the surface per unit area.

Security labeling and optical information encryption enabled by laser-printed silicon Mie resonators.

Fighting against the falsification of valuable items remains a crucial social-threatening challenge stimulating a never-ending search for novel anti-counterfeiting strategies. The demanding security labels must simultaneously address multiple requirements (high density of the recorded information, high protection degree, etc.) and be realized via scalable and inexpensive technologies. Here, the direct reproducible femtosecond-laser patterning of thin glass-supported amorphous (α-)Si films is proposed for optical information encryption and the scalable and highly reproducible fabrication of security labels composed of Raman-active hemispherical Si nanoparticles (NPs). Laser printing conditions allow the precise control of the diameter of the formed NPs ensuring translation of their dipolar Mie resonance position within the entire visible spectral range. Two-temperature molecular dynamics simulations clarify the origin of α-Si NP formation by rupture of the molten Si layer driven by a negative GPa-range pressure near the liquid-solid interface. Arrangement of the laser-printed Mie-resonant NP allows the creation of hidden security labels offering several easy-to-realize information encryption strategies (for example, local laser-induced post-crystallization or mixing Mie-resonant and non-resonant NPs), additional protection modalities, facile Raman mapping readout and dense information recording (up to 60 000 dots per inch) close to the optical diffraction limit. The developed fabrication strategy is simple, inexpensive, and scalable and can be realized based on cheap Earth-abundant materials and commercially-available equipment justifying its practical applicability and attractiveness for anti-counterfeit and security applications.

Patterning of WOx, VOx, and MoOx thin-films with picosecond and nanosecond laser sources

Transition metal oxide (TMOs) layers have interesting properties as selective contacts, i.e., hole or electron transport layers for novel semiconductor devices. Especially, oxides of molybdenum (MoO3), vanadium (V2O5), and tungsten (WO3) show good bahaviour acting as front hole-selective contacts for n-type crystalline-silicon heterojunction solar cells. Laser scribing has been widely used for thin-film ablation and seems the appropriate technology for device manufacturing with such non-conventional materials. In this work, we study the laser scribing of non-stoichiometric evaporated WOx, VOx, and MoOx films with three different wavelengths (1064, 532, and 355 nm) with pulse duration in the nanosecond and picosecond regimes. The selection of the proper laser source allows a wide parametric window, with complete removal of the TMO films and no alteration of the silicon substrate. The results on the isolation of diodes and their electrical characteristics show the quality of the laser scribing processes.

Numerical study of structural and magnetic properties of thin films obliquely deposited on rippled substrates

Structural modulation in thin films plays a substantial role in the formation of their magnetic properties. By producing topographic patterns in thin films, it is possible to engineer their magnetic response. Here, we report on the numerical study of the relationship between structural and static magnetic properties of thin films obliquely deposited on substrates with the sinusoidal surface. 3D Monte Carlo film growth simulations show that, under certain deposition conditions, an inhomogeneous columnar morphology can form in the films caused by the shadowing effect and the rippled substrate. Calculations of the demagnetizing tensors for these films demonstrate that their columnar structure is the source of the shape-induced uniaxial magnetic anisotropy that varies nonmonotonically with the deposition angle. Micromagnetic simulations of the generated films confirm the uniaxial character of the shape-induced anisotropy, and also show that magnetization reversal occurs via an incoherent rotation of magnetic moments.

Self-Aligned Thin-Film Patterning by Area-Selective Etching of Polymers

Patterning of thin films with lithography techniques for making semiconductor devices has been facing increasing difficulties with feature sizes shrinking to the sub-10 nm range, and alternatives have been actively sought from area-selective thin film deposition processes. Here, an entirely new method is introduced to self-aligned thin-film patterning: area-selective gas-phase etching of polymers. The etching reactions are selective to the materials underneath the polymers. Either O2 or H2 can be used as an etchant gas. After diffusing through the polymer film to the catalytic surfaces, the etchant gas molecules are dissociated into their respective atoms, which then readily react with the polymer, etching it away. On noncatalytic surfaces, the polymer film remains. For example, polyimide and poly(methyl methacrylate) (PMMA) were selectively oxidatively removed at 300 °C from Pt and Ru, while on SiO2 they stayed. CeO2 also showed a clear catalytic effect for the oxidative removal of PMMA. In H2, the most active surfaces catalysing the hydrogenolysis of PMMA were Cu and Ti. The area-selective etching of polyimide from Pt was followed by area-selective atomic layer deposition of iridium using the patterned polymer as a growth-inhibiting layer on SiO2, eventually resulting in dual side-by-side self-aligned formation of metal-on-metal and insulator (polymer)-on-insulator. This demonstrates that when innovatively combined with area-selective thin film deposition and, for example, lift-off patterning processes, self-aligned etching processes will open entirely new possibilities for the fabrication of the most advanced and challenging semiconductor devices.

Ink-Lithography for Property Engineering and Patterning of Nanocrystal Thin Films.

Next-generation devices and systems require the development and integration of advanced materials, the realization of which inevitably requires two separate processes: property engineering and patterning. Here, we report a one-step, ink-lithography technique to pattern and engineer the properties of thin films of colloidal nanocrystals that exploits their chemically addressable surface. Colloidal nanocrystals are deposited by solution-based methods to form thin films and a local chemical treatment is applied using an ink-printing technique to simultaneously modify (i) the chemical nature of the nanocrystal surface to allow thin-film patterning and (ii) the physical electronic, optical, thermal, and mechanical properties of the nanocrystal thin films. The ink-lithography technique is applied to the library of colloidal nanocrystals to engineer thin films of metals, semiconductors, and insulators on both rigid and flexible substrates and demonstrate their application in high-resolution image replications, anticounterfeit devices, multicolor filters, thin-film transistors and circuits, photoconductors, and wearable multisensors.

Sb2Se3 solar cells fabricated via close-space sublimation

Antimony selenide (Sb2Se3) is regarded as an excellent photovoltaic absorber material due to its suitable bandgap, large light absorption coefficient, abundant raw material reserves, and environmental friendliness. However, the commonly used rapid thermal evaporation strategy for deposition of Sb2Se3 films results in low film quality, which is undesirable from the perspective of photovoltaic performance. Herein, we fabricate highly efficient and stable Sb2Se3 solar cells via a close-space sublimation (CSS) process, which allows separate control of the source and substrate temperatures, leading to high-quality thin films and better solar cell performance. Four growth patterns of Sb2Se3 thin films are optimized by controlling the source temperature of CSS. It is found that the Sb2Se3 thin film prepared at 475 °C has the best crystallinity, smoothest surface, and best density. Moreover, solar cells based on ZnO/Sb2Se3 thin films can achieve maximum efficiency with VOC of 0.312 V, JSC of 27.91 mA/cm2, fill fact of 41.35%, and power conversion efficiency of 3.61%. The performance of the devices was not adversely affected by the air environment, and thus, they were shown to exhibit appropriate stability.

Metal-Ferroelectric-Metal Characterization for Optical Modulator

Thermal annealing was performed on fused silica substrate for metal-ferroelectric-metal (MFM) at 900°C for 2 hours by sol-gel coated of BaTiO3 as ferroelectric material and e-beam evaporation of ITO as electrode. Standard optical and physical characterization were performed such as XRD, UV-VIS Spectrophotometer, AFM and Hall measurement for electrical properties. XRD patterns of thin films shows that the samples is crystalline in a cubic structure. Optical analysis showed high transparency at 400-800nm which is >80% transmission while the low resistivity for ITO ∼ 10-3.

Organic Semiconductors: Solutal‐Marangoni‐Flow‐Mediated Growth of Patterned Highly Crystalline Organic Semiconductor Thin Film Via Gap‐Controlled Bar Coating (Adv. Funct. Mater. 28/2021)

In article number 2100196, Boseok Kang, Kilwon Cho, and co-workers report a novel bar-coating approach using a solutal-Marangoni flow to form an array of single-crystal-like organic semiconductor thin-film patterns at a high rate in a consecutive manner. The use of an appropriate solvent additive enhances molecular mass transport and induces vertical phase separation between a small-molecule semiconductor and a polymeric semiconductor.

(Invited) Plasma Electrochemistry: Plasmas Are so Much More Fun for Electrochemistry Than Liquids!

As electrochemists, we are interested in electron attachment and detachment processes. Traditionally, we control the availability of electrons via an electrically conducting solid and measure electron transfer across the solid/liquid interface. Of course, there are exceptions to this picture, e.g. liquid/liquid interfaces, but often liquids are involved to provide an electrolyte medium to support the chemical species. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments.The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1, 2. We describe results supporting both modes.As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.4 A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system5, we may measure unique voltammograms for a series of small organic molecules and amino acids.6 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater. References Rumbach, P., Bartels, D.M., Sankaran, R.M. & Go, D.B. The solvation of electrons by an atmospheric-pressure plasma (vol 6, pg 7248, 2015). Nature Communications 7 (2016).Elahi, A., Fowowe, T. & Caruana, D.J. Dynamic Electrochemistry in Flame Plasma Electrolyte. Angewandte Chemie-International Edition 51, 6350-6355 (2012).Sener, M.E., Palgrave, R., Quesada Cabrera, R., Caruana, D.J.*, Patterning of Metal Oxide thin Films using H2/He Atmospheric Pressure Plasma Jet. Green Chemistry,22, 1406-1413 (2020).Sener, M.E. & Caruana, D.J. Modulation of copper(I) oxide reduction/oxidation in atmospheric pressure plasma jet. Electrochemistry Communications 95, 38-42 (2018).Fowowe, T., Hadzifejzovic, E., Hu, J.P., Foord, J.S. & Caruana, D.J. Plasma Electrochemistry: Development of a Reference Electrode Material for High Temperature Plasma. Advanced Materials 24, 6305-6309 (2012).Calleja, M., Elahi, A. & Caruana, D.J. Gas phase electrochemical analysis of amino acids and their fragments. Communications Chemistry 1, 48 (2018).

Curvature-controlled delamination patterns of thin films on spherical substrates

SummaryPeriodic delamination patterns in multilayer structures have exhibited extensive applications in microelectronics and optics devices. However, delamination behaviors of a closed thin shell on spherical substrates are still elusive. Herein, a unique instability mechanism of buckle delamination in a closed thin film weakly bonded to spherical substrates is studied by experiments, simulations, and theoretical analyses. The system of an Al film depositing on polystyrene spheres subjected to thermal mismatch strain is used for demonstration. Unlike traditional phenomena of wrinkling and wrinkle-induced delamination under increasing misfit strain, the weak adhesion between the core and shell results in a periodic pattern of delaminated hexagonal dimples that emerges directly from the smooth sphere configuration, before which no wrinkling occurs. Both substrate curvature and interfacial adhesion are revealed to control the dimple size and delamination width. These findings open a new venue for manifesting new controllable features for surface microfabrication.