Laser-induced Forward Transfer Research Articles

Laser‐Induced Forward Transfer of High Viscous, Non‐Newtonian Silver Nanoparticle Inks: Jet Dynamics and Temporal Evolution of the Printed Droplet Study

Current technological trends in the field of microelectronics highlight the requirement to use cost‐effective techniques for precise deposition of highly resolved features. Laser‐induced forward transfer (LIFT) meets these requirements and is already applied for direct printing of electronic components. However, to improve the process' reproducibility and printing resolution, further research has to be conducted, regarding the rheological characteristics of the printable fluids and their jetting dynamics. Herein, a high‐speed imaging setup is used to investigate the liquid jet's propagation during the printing process. Different Ag nanoparticle inks are studied and compared, over a wide range of viscosities and two different values of surface tension. The main focus of this investigation is the influence of the ink's rheological properties, both on the jet propagation and on the spatial and temporal evolution of the printed droplet during the wetting phase on three different receiver substrates (glass, SU‐8, and gate dielectric). The results indicate that both the surface tension and the wetting properties of the receiver determine the shape of the printed droplet, whereas the inks' viscosity and laser fluence determine the printed volume.

Jetting regimes of double-pulse laser-induced forward transfer

We use the double-pulse laser-induced forward transfer (DP-LIFT) process, combining a quasi-continuous wave (QCW) and a femtosecond (fs) laser pulse to achieve jetting from a 1-µm thick copper film. The influence of the fs laser fluence on the dynamics of the liquid copper jetting is experimentally investigated by time-resolved shadowgraphy and theoretically analyzed with a simple energy balance model. Different jetting regimes are identified when varying the fs laser fluence. We demonstrate that the adjustment of this latter parameter while keeping all the others constant, allows accurate control of the diameter of the printed droplets from 1.9 µm to 6.0 µm. This leads us to a demonstration in which we print debris-free micro-pillars with an aspect ratio of 19 onto a silicon receiver substrate set as far as 60 µm away from the donor film.

Local Isolation of High-Voltage Photovoltaic Cells Using Buried Layers of Oxidized Porous Silicon

In this paper, we have developed a simple method to isolate epitaxially grown thin silicon film using micrometer thick layers of buried porous silicon. The process is based on formation of trenches within epitaxial p-type Si layer that was grown on top of a p+ -type Si (100) wafer. Either electrochemical or galvanic etching in hydro-fluoric solutions procedures were employed to etch the p+ -type silicon under and around the trenches, at the interface of the substrate and the epi-layer, and to transform the etched material into buried PSi. Electrical characteristics of the formed isolation, called “local isolation by buried oxidized PSi”, have been measured. The isolation resistance of the subsequently oxidized PSi film was found to increase by 3-6 orders of magnitude up to the level of few GΩ (GigaOhms). Finally, this procedure has been exploited to demonstrate a miniature photovoltaic solar array where two photovoltaic cells were connected in series using the laser-induced forward transfer metallization process, as a model for high voltage photovoltaic solar cell.

Non-contact Laser Printing of Ag Nanowire-based Electrode with Photodegradable Polymers

The roll-to-roll process is synonymous with newspaper production. If a similar high-throughput process is developed to fabricate electronics over large areas, it would revolutionize the printed electronics manufacturing process. Rapid fabrication of electrode, including patterning and nanoscale welding, is a necessary integration technique to reduce the duration of the process, but faces difficulties in being realized using conventional methods. This paper discusses material factors that affect printability, in the context of developing a promising fabrication technique called laser induced forward transfer (LIFT); LIFT is non-contact printing technique applied previously to realize simultaneous pattern deposition and nanowelding of Ag nanowire (AgNW)-based electrodes. A photodegradable polymer, which is a key component in the printing process to render droplet acceleration, is investigated with regards to its mechanical and optical properties. Furthermore, the printing process of the AgNW-based electrode is visualized, resulting in deeper understanding of LIFT. Knowledge of these factors will contribute to rapid and precise patterning of AgNW-based electrodes with high stretchability and transparency toward flexible optoelectronics devices.

Bioactive micropatterning of apatite immobilizing cell adhesion protein by laser-induced forward transfer with a shock absorber

The additive patterning of apatite with good biocompatibility and osteoconductivity is a useful technique for the production and surface functionalization of biomaterials. We developed this technique through our laser-induced forward transfer (LIFT) process using a laser-absorbing sacrificial layer in combination with a shock-absorbing polydimethylsiloxane (PDMS) receiver. With the PDMS shock-absorbing function, even the brittle apatite and that immobilizing the cell adhesion protein fibronectin (Fn-apatite) were successfully transferred and micropatterned while maintaining their dense, filmy state. The laser pulse energy effect was investigated, leading to the optimum energy range just above the transfer threshold. The apatite and Fn-apatite micropatterns exhibited superior cytocompatibility compared to the PDMS surface, and could potentially be used for cellular micromanipulation.

Deposition-on-contact regime and the effect of donor-acceptor distance during laser-induced forward transfer of viscoelastic liquids

Optimizing the direct-writing of viscoelastic liquids requires an understanding of the governing physics during jet and droplet formation. In this article, we study the effect of the distance between the donor and acceptor surfaces and identify a unique deposition-on-contact regime associated with viscoelasticity. For a given laser pulse energy, depending on the liquid film thickness, rheological properties, and the distance between the liquid film and the acceptor surface, the resulting jet can result in either a rapid deposition of a small volume or the formation of a liquid bridge that delays the breakup of the liquid filament and can result in deposition of multiple droplets. By adjusting the gap distance between the donor and acceptor surfaces, we show that it is possible to obtain single drop depositions via deposition-on-contact from viscoelastic liquids. Using dimensionless parameters, we present criteria that can be used to predict the different regimes observed in the experiments.

Surface morphology formation of Ti films in laser-induced forward transfer

Titanium is frequently adopted in laser-induced forward transfer (LIFT) due to its unique industrial advantages (low density, high specific strength and good corrosion resistance). To understand the LIFT process assisted with Ti films, topographies of on Ti films with a thickness irradiated by ultraviolet nanosecond laser pulses are investigated. Five different topographies (the pit, blister, bump, spray, and strip) are observed and classified, among which the pit, with a basin-like profile, is first addressed in this work. The probability distribution of these types with respect to laser fluence on 435 nm and 1250 nm Ti films is provided and an analytical model is developed to explain the driving mechanism of the ablation threshold and the formation of different topographies.

Destructive mechanisms in laser induced forward transfer

Laser Induced Forward Transfer (LIFT) is an additive direct-writing technique, in which a piece of material (ink) is transferred from a donor to a receiver surface, utilizing a laser impulse. In practice, the process of jet formation can suffer from irreproducibility. We identify two possible destructive mechanisms due to multiple optical breakdowns (originating from imperfections of the optical system) and rarefaction waves (originating from impurities), both with harmful consequences caused by cavitation. Based on experiments in a model system that allows for visualization and numerical simulations employing the boundary integral method, we reveal the underlying fluid dynamics of both mechanisms. Finally, to overcome the irreproducibility, we provide recommendations for the industrial use of LIFT.

Femtosecond lasers: the ultimate tool for high-precision 3D manufacturing

Abstract The ever-growing trend of device multifunctionality and miniaturization puts enormous burden on existing manufacturing technologies. The requirements for precision, throughput, and cost become increasingly harder to achieve with minimal room for compromises. Femtosecond lasers, which saw immense development throughout the last few decades, have been proven time and time again to be a superb tool capable of standing up to the challenges posed by modern science and the industry for ultrahigh-precision material processing. Thus, this paper is dedicated to provide an outlook on how femtosecond pulses are revolutionizing modern manufacturing. We will show how they are exploited for various kinds of material processing, including subtractive (ablation, cutting, and etching), additive (lithography and laser-induced forward transfer), or hybrid subtractive-additive cases. The advantages of using femtosecond lasers in such applications, with main focus on how they enable the most precise kinds of material processing, will be highlighted. Future prospects concerning emerging industrial applications and the future of the technology itself will be discussed.

An investigation of the influence of thermal process on the electrical conductivity of LIFT printed Cu structures

The electrical properties of copper tracks printed by laser-induced-forward-transfer (LIFT) are typically inferior to the bulk values. Several limiting factors have been indicated such as oxidation, droplet boundaries and grain boundaries. However, the relative contribution of each of these factors has not been determined. To improve the electrical properties of LIFT printed structures, an analysis of various factors is essential. Here, we concentrate on the effect of post-printing thermal treatment on LIFT printed copper structures. A reduction in electrical resistivity by a factor of ~3 is obtained resulting in a value lower than 3 times the bulk copper value (1.68 µΩ · cm). Real time resistance measurements indicate that an efficient thermal annealing can be achieved at 200 °C–300 °C within a couple of minutes. The morphological changes associated with such thermal treatment were analyzed using HR-SEM and XRD and highlight the role of stress relief, grain growth and formation of new grains primarily at the inter-droplet interfaces.

Blister-Actuated LIFT Printing for Multiparametric Functionalization of Paper-Like Biosensors.

Laser induced forward transfer (LIFT) is a flexible digital printing process for maskless, selective pattern transfer, which uses single laser pulses focused through a transparent carrier substrate onto a donor layer to eject a tiny volume of the donor material towards a receiver substrate. Here, we present an advanced method for the high-resolution micro printing of bio-active detection chemicals diluted in a viscous buffer solution by transferring droplets with precisely controllable volumes using blister-actuated LIFT (BA-LIFT). This variant of the LIFT process makes use of an intermediate polyimide layer partially ablated by the laser pulses. The expanding gaseous ablation products lead to blisters in the polyimide and ejection of droplets from the subjacent viscous solution layer. A relative movement of donor and receiver substrates for the transfer of partially overlapping pixels is realized with a custom-made positioning system. Using a specially developed donor ink containing bio-active components presented method allows to transfer droplets with well controllable volumes between 20 fL and 6 pL, which is far more precise than other methods like inkjet or contact printing. The usefulness of the process is demonstrated by locally functionalizing laser-structured nitrocellulose paper-like membranes to form a multiparametric lateral flow test. The recognition zones localized within parallel micro channels exhibit a well-defined and homogeneous color change free of coffee-ring patterns, which is of utmost importance for reliable optical readout in miniature multiparametric test systems.

Spatial Modes of Laser-Induced Mass Transfer in Micro-Gaps

We have observed the concentric deposition patterns of small molecules transferred by means of laser-induced forward transfer (LIFT). The patterns comprised different parts whose presence changed with the experimental constraints in a mode-like fashion. In experiments, we studied this previously unknown phenomenon and derived model assumptions for its emergence. We identified aerosol micro-flow and geometric confinement as the mechanism behind the mass transfer and the cause of the concentric patterns. We validated our model using a simulation.

Extrusion bioprinting of soft materials: An emerging technique for biological model fabrication

Bioprinting has attracted increasing attention in the tissue engineering field and has been touted to potentially become the leading technology to fabricate, and regenerate, tissues and organs. Bioprinting is derived from well-known additive manufacturing (AM) technology, which features layered deposition of materials into complex three-dimensional geometries that are difficult to fabricate using conventional manufacturing methods. Unlike the conventional thermoplastics used in desktop, AM bioprinting uses cell-laden hydrogel materials, also known as bioinks, to construct complex living biological model systems. Inkjet, stereolithography, laser-induced forward transfer, and extrusion are the four main methods in bioprinting, with extrusion being the most commonly used. In extrusion-based bioprinting, soft materials are loaded into the cartridges and extruded from the nozzle via pneumatic or mechanical actuation. Multiple materials can be printed into the same structure resulting in heterogeneous models. In this focused review, we first review the different methods to describe the physical mechanisms of the extrusion process, followed by the commonly employed bioprintable soft materials with their mechanical and biochemical properties and finally reviewing the up-to-date heterogeneous in vitro models afforded via bioprinting.

Transparent and conductive silver nanowires networks printed by laser-induced forward transfer

Networks of silver nanowires (Ag-NWs) can be electrically conductive and optically transparent at the same time. Thus, Ag-NWs are promising candidates for substituting transparent and conductive oxides like indium-tin-oxide. Direct-write methods for printing patterns are suitable in order to reduce the amount of material used with respect to actual deposition methods on large areas that require post-processing steps.In this work, we study the laser-induced forward transfer of Ag-NWs with the aim of printing conductive patterns that appear invisible at naked eye. A Nd:YAG laser system delivering 150 ns pulses at 1064 nm wavelength was coupled with a scan head for printing the Ag-NWs at different pulse energies (0.20–0.45 mJ).It has been found that the area coverage of Ag-NWs, which is directly related with the optical an electrical properties of the patterns, increases as the laser pulse energy increases. A sheet resistance of 140 Ω/sq. is reached when printing at the highest pulse energy tested. As a proof-of-concept, we printed simple circuits with a pair of invisible electrodes connecting an LED on glass with a transmittance of 98.8%, a haze of 0.5%, a reflectance below 0.1% and a sheet resistance of 340 Ω/sq.

Printing of single-wall carbon nanotubes via blister-based laser-induced forward transfer

The blister-based laser-induced forward transfer (BB-LIFT) of single-walled carbon nanotubes (SWCNTs), both raw and dispersed in carboxymethyl cellulose films, is demonstrated. Under optimized laser fluences, ejection of the SWCNTs from the donor substrate was driven by fast blistering of an underlying aluminum film that was not accompanied by its rupture. To transfer the ‘polymer/nanotube’ composite, the basic BB-LIFT technique was modified by adding preliminary cutting of the donor layer into square pixels due to total ablation of both the metal and composite in the areas between the pixels. Micro-Raman investigations have proved that SWCNTs are transferred without significant degradation in both cases.

激光诱导海藻酸钠溶液转移的研究

激光诱导海藻酸钠溶液转移的研究

Laser printing-enabled direct creation of cellular heterogeneity in lab-on-a-chip devices.

Lab-on-a-chip devices, capable of culturing living cells in continuously perfused, micrometer-sized channels, have been intensively investigated to model physiological microenvironments for cell-related testing and evaluation applications. Various chemical, physical, and/or biological culture cues are usually expected in a designed chip to mimic the in vivo environment with defined spatial heterogeneity of cells and biomaterials. To create such heterogeneity within a given chip, typical methods rely heavily on sophisticated fabrication and cell seeding processes, and chips fabricated with these methods are difficult to readily adapt for other applications. In this study, laser-induced forward transfer (LIFT)-based printing has been implemented to create heterogeneous cellular patterns in a lab-on-a-chip device to achieve the efficiency in creating heterogeneous cellular patterns as well as the flexibility in adapting different evaluation configurations in lab-on-a-chip devices. Two applications, parallel evaluation of cellular behavior and targeted drug delivery to cancer cells, have been implemented as proof-of-concept demonstrations of the proposed fabrication method. For the first application, the morphology of cells in different extracellular matrix (ECM) materials cultured under varying conditions has been investigated. It is found that less stiff ECM and dynamic culturing are preferred for spreading of fibroblasts. For the second application, different drug carriers have been utilized for targeted delivery of anticancer drugs to breast cancer cells. It is found that targeted drug delivery is important to realize effective chemotherapy and drug release rate from drug carriers affects the chemotherapy effect. Consequently, the proposed laser printing-based method enables direct creation of heterogeneous cellular patterns within lab-on-a-chip devices which improves the efficiency and versatility of cell-related sensing and evaluation using lab-on-a-chip devices.

3D printing methods for micro- and nanostructures

The physical and physicochemical fundamentals of three-dimensional (3D) micro- and nanoprinting are presented. 3D printing (or additive manufacturing technology) is a process which fabricates structures and devices by depositing material (usually layer by layer) according to a 3D digital model. The methods and results reviewed here are limited to those from micro- and nanoscale fields, which are in demand in the fields of electronics, photonics, and bionics. Special attention is given to methods for fabricating sub-100-nm structures, including single- and two-photon polymerization stereolithography, electrohydrodynamic inkjet printing, and laser-induced forward transfer. The advantages and disadvantages of 3D printing methods are discussed, together with prospects for their development and application.

Direct and opposite droplet ejections from metal films induced by nanosecond laser pulses: experimental observation and lattice Boltzmann modeling

Laser-induced forward transfer is a versatile method for the fabrication of surface microstructures. In this paper, the material ejection from copper films induced by nanosecond laser pulses was studied. Two reversely oriented ejections were induced simultaneously by a single laser shot. The opposite ejected droplets were significantly smaller than the direct ejected ones, and were separated from the remnant donor material clearly, which might be utilized to fabricate microstructures on the donor substrates. Three distinct ejection regimes: no ejection, stable ejection and sputtering were characterized with respect to the laser intensity. The thermal field and the phase transition within the donor film were analyzed with a finite element model. Correlations between the phase transitions and the ejection regimes were noticed, suggesting that the material ejection was mainly caused by the cavitation effect. The dynamics of the laser-induced cavitation bump and the formation of the jets were simulated with a vapor–liquid lattice Boltzmann model. The formation of the bump and the material ejections were clearly demonstrated. It was concluded that the jets were mainly induced by the collision of the cumulating flows, which were generated within the bump shell by the asymmetrical cavitation of the bump. The dynamics of the vapor also plays an important role in the development of the jets. The simulation results agree well with the present experimental observations.

Femtosecond laser-assisted fabrication of chalcopyrite micro-concentrator photovoltaics.

Micro-concentrator solar cells offer an attractive way to further enhance the efficiency of planar-cell technologies while saving absorber material. Here, two laser-based bottom-up processes for the fabrication of regular arrays of CuInSe2 and Cu(In,Ga)Se2 microabsorber islands are presented, namely one approach based on nucleation and one based on laser-induced forward transfer. Additionally, a procedure for processing these microabsorbers to functioning micro solar cells connected in parallel is demonstrated. The resulting cells show up to 2.9% efficiency and a significant efficiency enhancement under concentrated illumination.