Laser-Induced Forward Transfer Nano-Printing Process - Multiscale Modelling, Experimental Validation and Optimization

LIFT is a direct-write microfabrication and micro/nano printing technique that has received much attention in the research communities and industries in recent years. It offers significant advantages over other competing printing methodologies and has potential applications in many high-tech high-value industries. However, questions remain regarding how to select a small set of experimentally controllable parameters to produce the finest, the most uniform, the most desirable single printed feature and print arrays. Despite the extensive and expensive experiments carried out by the applicants and other groups around the world, fundamental understanding of the phenomena involved in LIFT is lacking. This is attributed to the limited spatial and temporal resolutions in experiments and to the fact that many quantities/properties are not directly measurable especially at nanoscales. Crucially, the causal relationships among the various parameters are difficult to establish without an exhaustive number of expensive experiments. Therefore, it is highly desirable to develop theoretical and/or numerical models to capture the essential physics in LIFT so that trends can be predicted more easily and LIFT design more grounded on fundamental physics. Success here will revolutionise key industries that have photonics, plasmonics and microelectronics as their cornerstone.Conventional macroscopic modelling methods do not directly lend the solution to the LIFT problem, due to the truly multiscale and multiphysics features of LIFT. The most promising approach for LIFT is the LBM, which can be viewed as a coarse-grained molecular dynamics approach, albeit with very different numerical algorithms and affordable computational expenses for real-world problems. LBM preserves the microscopic kinetic principles while recovering the full Navier-Stokes equations at the macroscales. Therefore, LBM bridges the microscales and macroscales, which makes it a valuable method for multiscale problems like LIFT. Here, we propose the very first multiscale modelling study of LIFT, supported by existing and further experimental measurements conducted at the state-of-the-art FASTlab facilities in Southampton. This is built upon the recent successes of ours and other researchers in simulating some isolated sub-processes relevant to LIFT using LBM. The novelty and significance of the proposed multiscale LBM approach is its ability to simulate the complete LIFT process including donor material melting, molten droplet formation, droplet growth, transfer, and deposition processes. The model development will proceed in a systematic manner in order of increasing sophistication. First, an isothermal multiphase LBM model will be employed to isolate the multiphase flow dynamics effects from the thermal effects. Then a thermal multiphase LBM will be tested for LIFT processes to determine the capabilities and limitations of the current (pure) LBM methodologies. The focus, however, is to develop a new multiscale LBM approach to study laser heating, donor material melting, heat conduction, thermal expansion and re-solidification. Such a multiscale approach couples LBM seamlessly with a macroscopic Navier-Stokes solver, taking advantage of each method's scale-resolving capability and numerical efficiency in different ranges of the Reynolds and Knudsen numbers. Finally, Marangoni effects will be investigated by incorporating temperature-dependent surface tension into the LBM modelling. The Marangoni effects are believed to affect the final morphology of the printed features but have not been studied in detail before. Throughout the project, the modelling and experimental teams as well as our academic and industrial partners will work closely with each other to ensure timely exchange of ideas, data and information. The final phase is to create the finest optimized features of a single printed dot and print arrays following first principles and modelling guidance.

LIFT is a direct-write microfabrication and microprinting technique that can offer significant advantages in terms of precision, resolution, speed, simplicity and flexibility over other competing printing methodologies. It uses a laser pulse to transfer a portion of the donor material over a short distance into a receiver substrate, resulting in controlled and high precision deposition. By varying the laser fluence and/or pulse duration, the deposition can be larger, equal or smaller than the laser spot size. Our recent experimental work has demonstrated the capability to deposit nanoscale features using femtosecond laser pulses. Other studies have shown that, in addition to metals, the donor materials can be superconductors, oxides, inorganic materials, biomolecules, DNA, living cells and microorganisms. Therefore, LIFT not only constitutes an attractive alternative to more conventional printing techniques, but also has a potentially wide range of applications in nanotechnology in general, microfabrication, micropatterning, microelectronics, lab-on-a-chip devices, biosensors, tissue engineering and scaffolding, and so on. The proposed research addresses the EPSRC's priority research areas of Nanoscience through Engineering and Digital Economy directly and Next Generation Healthcare and Energy indirectly. For the first time to our knowledge, a laser-based printing technique will benefit from rigorous modelling guidance. Given the high cost of doing experiments on LIFT and the empirical nature of tuning test conditions, the modelling tool based on sound physical principles would cut cost and shorten the time for designing and optimising LIFT technologies. Both investigators have extensive contacts with industries, and spin-outs from the University. Under a recently awarded EU programme titled e-LIFT, we are working with four SMEs and one larger company which manufacture RFID tags for security, product labelling and tracking. The final phase is to specify and demonstrate a laser LIFT printer, which involves a laser machining company in the UK. All these companies have as a fundamental interest the ability to print materials using LIFT to extreme precision, and the importance of the modelling, validation and further refinements that we are offering via this proposed programme is hard to overstate. We have had recent discussions and secured direct funding from TNO, Holland, for injection of micron-scale metal interconnects between stackable Si integrated circuit chips. If we can perfect this, with the help of the detailed modelling in this proposal, then the benefit to microelectronics will be extreme. The world is waiting for, and the manufacturing roadmap dictates that further decrease in feature size, and higher packing density is paramount for the continued growth in semiconductor chip manufacture. Success here will certainly lead to both technological and financial benefits to the UK and the EU. Indeed, if our results turn out to be as expected, we would consider the route of spin-out. Within the ORC in particular, there is a long history of spin-outs with more than 10 companies emerging as a direct consequence of ORC research activities. Laser printing via LIFT might well be a suitable candidate for the next successful spin-out. The SES has a dedicated technology transfer unit, Research Institute for Industry (RIfI), who routinely works with a network of over 80 companies. Within the University of Southampton, the Research and Innovation Services (RIS) provide dedicated support on a broad range of research and enterprise activities, from contractual negotiation through to commercialisation of IP. Once we are at the stage of technology transfer and/or spin-out, we will use these established services. And finally, there will be ample opportunities for dissemination of results to wider audiences via local colloquia, open days, outreach activities and a dedicated website.