Filimon Zacharatos

Despite the remarkable progress of graphene and other 2D materials (2DM) over the past 10 years, the wide application of these materials in electronic and photonic integrated circuits is still hampered by hurdles related to their transfer and wafer‐scale integration processes. We report on the single‐step patterning and transfer of single‐layer graphene (SLG) and hexagonal boron nitride (hBN) SLG/hBN heterostructures, as well as graphene multilayer stacks onto patterns widely applied in photonic and electronic components using Laser‐Induced Forward Transfer (LIFT). By optimizing laser fluence, beam shaping, and alignment conditions, graphene and graphene/hBN heterostrucures with lateral dimensions down to 15 μm were successfully deposited across surface topographies with step‐heights up to 100 nm. Raman spectroscopy and electrical characterization confirm the preservation of the material’s structural and electronic integrity, with FET devices exhibiting characteristic ambipolar transport. These results demonstrate LIFT as a scalable, digital, and residue‐free paradigm for incorporating 2D active elements into complex SiN‐based optoelectronic platforms and photodetectors.

One of the key challenges in technological applications based on two-dimensional (2D) materials is to reliably and precisely transfer them onto target substrates with a digital, high resolution and high throughput method. In this regard, here we demonstrate the unique advantages that laser-induced transfer can offer as a solution for the simultaneous printing and patterning of individual hBN (hexagonal boron nitride) monolayers, as well as of graphene/hBN bilayers. By employing a single, precisely shaped laser pulse, we exert fine control over the shape, size, and stacking of the transferred pixels, enabling the fabrication of heterostructures. Raman Spectroscopy and Scanning Electron Microscopy (SEM) confirm the high structural quality and uniformity of the transferred materials, with minimal defects. Furthermore, ab initio molecular dynamics simulations corroborate the experimental findings, demonstrating the feasibility of transferring graphene/hBN heterostructures from the donor to the receiver substrate. Overall, this approach offers a single-step strategy for integrating 2D materials into electronic and optoelectronic devices, significantly reducing the complexity of the fabrication process.

The laser induced forward transfer and sintering of metal nanoparticle inks has been proven a key enabling technology for flexible electronics. Nevertheless, many challenges concerning the conformal processing of non-planar substrates incorporating thermally sensitive layers are yet to be addressed. In this work, we study the behaviour of conformal laser printing of silver nanoparticle inks on patterned samples comprising sensitive underlying structures, by correlating the laser sintering powers employed to the undesired effects on the adjacent interfaces. The latter include demanding surface topographies with periodic patterns and micro-components exhibiting aspect ratio in the nano to 100-micron scale. We investigate the contribution of crucial processing parameters, such as the per pulse energy, repetition rate and the pulse to pulse spatial and temporal overlap to the overall result. The demonstrated results validate the versatility of laser processing which can offer application specific solutions on different use cases involving multilayered and multimaterial electronics.

Current challenges in printed circuit board (PCB) assembly require high-resolution deposition of ultra-fine pitch components (<0.3 mm and <60 μm respectively), high throughput and compatibility with flexible substrates, which are poorly met by the conventional deposition techniques (e.g., stencil printing). Laser-Induced Forward Transfer (LIFT) constitutes an excellent alternative for assembly of electronic components: it is fully compatible with lead-free soldering materials and offers high-resolution printing of solder paste bumps (<60 μm) and throughput (up to 10,000 pads/s). In this work, the laser-process conditions which allow control over the transfer of solder paste bumps and arrays, with form factors in line with the features of fine pitch PCBs, are investigated. The study of solder paste as a function of donor/receiver gap confirmed that controllable printing of bumps containing many microparticles is feasible for a gap < 100 μm from a donor layer thickness set at 100 and 150 μm. The transfer of solder bumps with resolution < 100 μm and solder micropatterns on different substrates, including PCB and silver pads, have been achieved. Finally, the successful operation of a LED interconnected to a pin connector bonded to a laser-printed solder micro-pattern was demonstrated.

Current trends in microelectronics chip bonding and assembly encompass ultra-fine pitch components with flexible form factors and advanced die-attach materials, which can sustain conformal strain without compromise in adhesion and reliability. These trends have highlighted the need for advanced microfrabrication technologies, which allow the on-demand digital fabrication of interconnections and die-attach bumps on chip carriers and interposers with challenging topographies. Laser printing, based on the laser induced forward transfer (LIFT) technique, has addressed such micromanufacturing challenges over the past decade. Several previous demonstrations of highly resolved and conformal interconnections for the bonding and packaging of micro and opto-electronic components are summarized in this paper. Latest advances in the laser printing and laser sintering of highly viscous nanopastes for achieving planar or three-dimensional (3D) structures consisting of Ag paste are demonstrated. The latter is achieved by stacking of arrays or bumps in a digital manner. The reported results verify that LIFT offers control over both the lateral and vertical dimensions. This control can facilitate the soldering process of heterogeneous components, whose pads have significant height and pitch dissimilarities. The applicability of LIFT in the fields of packaging and assembly of flexible and heterogeneous integration schemes, is further validated.

Graphene and related 2D semiconductors, hold great promise as active materials in flexible electronics applications, owing to their exquisite optoelectronic and mechanical properties. Here, we report on the use of Laser Induced Forward Transfer for the precise and micro-scale printing of graphene pixels on conventional and flexible substrates in a single step process. Complementary to the experimental studies, Density Functional Theory and ab initio Molecular Dynamics are employed to investigate the interactions between graphene and metallic substrates and to shed light on the atomic-scale processes that enable transfer from a donor to a receiver, for applications in flexible touch sensors.

The increasing development of flexible and printed electronics has fueled substantial advancements in selective laser sintering, which has been attracting interest over the past decade. Laser sintering of metal nanoparticle dispersions in particular (from low viscous inks to high viscous pastes) offers significant advantages with respect to more conventional thermal sintering or curing techniques. Apart from the obvious lateral selectivity, the use of short-pulsed and high repetition rate lasers minimizes the heat affected zone and offers unparalleled control over a digital process, enabling the processing of stacked and pre-structured layers on very sensitive polymeric substrates. In this work, the authors have conducted a systematic investigation of the laser sintering of micro-patterns comprising Ag nanoparticle high viscous inks: The effect of laser pulse width within the range of 20–200 nanoseconds (ns), a regime which many commercially available, high repetition rate lasers operate in, has been thoroughly investigated experimentally in order to define the optimal processing parameters for the fabrication of highly conductive Ag patterns on polymeric substrates. The in-depth temperature profiles resulting from the effect of laser pulses of varying pulse widths have been calculated using a numerical model relying on the finite element method, which has been fed with physical parameters extracted from optical and structural characterization. Electrical characterization of the resulting sintered micro-patterns has been benchmarked against the calculated temperature profiles, so that the resistivity can be associated with the maximal temperature value. This quantitative correlation offers the possibility to predict the optimal process window in future laser sintering experiments. The reported computational and experimental findings will foster the wider adoption of laser micro-sintering technology for laboratory and industrial use.

Ag nanowire (NW) networks have exquisite optical and electrical properties which make them ideal candidate materials for flexible transparent conductive electrodes. Despite the compatibility of Ag NW networks with laser processing, few demonstrations of laser fabricated Ag NW based components currently exist. In this work, we report on a novel single step laser transferring and laser curing process of micrometer sized pixels of Ag NW networks on flexible substrates. This process relies on the selective laser heating of the Ag NWs induced by the laser pulse energy and the subsequent localized melting of the polymeric substrate. We demonstrate that a single laser pulse can induce both transfer and curing of the Ag NW network. The feasibility of the process is confirmed experimentally and validated by Finite Element Analysis simulations, which indicate that selective heating is carried out within a submicron-sized heat affected zone. The resulting structures can be utilized as fully functional flexible transparent electrodes with figures of merit even higher than 100. Low sheet resistance (90%) make the reported process highly desirable for a variety of applications, including selective heating or annealing of nanocomposite materials and laser processing of nanostructured materials on a large variety of optically transparent substrates, such as Polydimethylsiloxane (PDMS).