Processing of Transparent Materials (LiM 2021)

Processing and wetting behavior of hierarchically microstructured polymer foils
Felix Bouchard, Marcos Soldera, Robert Baumann, Andrés Fabián Lasagni

The demand for surface functionalized plastics is constantly rising. Therefore, industrial-scalable methods capable to provide surfaces with new functions are necessary. In this study, we demonstrate a strategy to apply hierarchical microstructures to transparent polyethylene terephthalate (PET) foils by plate-to-plate hot embossing. To that end, a stainless-steel stamp was patterned using two laser-based processes, namely Direct Laser Writing (DLW) and Direct Laser Interference Patterning (DLIP). Several single scale and multi scale structures with feature sizes in the range 3 μm up to 50 μm and depths between 0.1 μm and 10 μm were processed on the metal master and transferred to the PET surface. The topography characterization by confocal microscopy and scanning electron microscopy revealed a satisfactory replication of the microstructures from the stamp to the polymer, even for the smallest features with lateral sizes of ~100 nm. The patterned surfaces showed an increased hydrophobic behavior characterized by static water contact angles up to 105°.

Keywords: direct laser writing; direct laser interference patterning; hot embossing; hierarchical structures; stainless steel; polymer; water contact angle; polyethylene terephthalate


Optical coherence tomography for 3D weld seam localization in absorber-free laser transmission welding
Frederik Maiwald, Clemens Roider, Michael Schmidt, Stefan Hierl

Thulium fiber lasers emit in the intrinsic absorption spectrum of polymers and enable the welding of transparent parts without absorbent additives. Focusing with high NA provides large intensity gradients inside the workpiece, enabling selective fusing of the joining zone without affecting the surface. Therefore, absorber-free laser transmission welding is well suited to fulfil the high demands on quality and reliability in the manufacturing of optical and medical devices. However, monitoring the welding process is required, since seam size and position are crucial for quality.
The aim of this work is the volumetric acquisition of the weld seam’s location and size using optical coherence tomography. Due to the change of the optical properties during melting, the seam can be distinguished from the base material. The results coincide with microscopic images of microtome sections and demonstrate that weld seam localization in polyamide 6 is possible with an accuracy better than a tenth of a millimeter.

Keywords: laser transmission welding; transparent polymers; optical coherence tomography; process monitoring;


Ultrashort pulse laser cutting of clear polyimide and hard coat film stacks for flexible OLED displays
Jim Bovatsek

New mobile devices include flexible and foldable OLED displays which require new protective cover materials. Options for this include ultrathin glass (UTG) as well as a new type of clear polyimide (clear PI), which is transparent at visible wavelengths. The clear PI is combined with a scratch resistant hard coat (HC) layer and a final PET (polyethylene terephthalate) protective film. Here we present results for cutting thick clear PI/HC/PET layered stacks using high power UV ultrashort pulse lasers. Ablation thresholds were found to vary by almost a factor of ten, and a layer-optimized cutting approach was used. The optimized cutting process is of high quality and throughput, with heat-affect zone (HAZ) of <10 μm and cutting speed of >400 mm/s. These results are comparable to that for individual sheets of polymers used in OLED display manufacturing.

Keywords: OLED display; foldable phone; polymer film; picosecond UV laser; ablation threshold


Ultra-high quality through-transmission micro-welding and cutting of glass with ultrashort pulse lasers
Terence Hollister, Jim Bovatsek

In recent years, glass has seen a renaissance of sorts, expanding into a wide variety of thicknesses, shapes, compositions and uses. Novel forms of glass are now widely used in microelectronics packaging, mobile device, automotive and bio-medical applications. Technologies to process glass have evolved as well, with ultrashort pulse (USP) laser technology becoming an important capability. Ultrashort pulse widths offer precision processing with controlled heat input that can minimize or even eliminate chipping and cracking. Combined with Bessel beam optics, high quality cutting of ultrathin glass (UTG) down to 100 μm thick can be realized. Operating at high pulse repetition frequencies, controlled thermal phenomena allows micro-welding of glass-to-glass and other materials. In this work, we demonstrate UTG cutting with roughness in the 10s of nm and ~1 m/s throughput as well as glass-glass and glass-aluminum micro-welding, with throughput approaching 500 mm/s and line widths of 10s of μm.

Keywords: Ultrashort pulse lasers; micro welding; glass materials


Laser-manufactured glass microfluidic devices with embedded sensors
Krystian L. Wlodarczyk, William N. MacPherson, Duncan P. Hand, M. Mercedes Maroto-Valer

We describe a laser-based process that allows the rapid manufacturing of custom microfluidic devices from transparent borosilicate glass slides, as well as an inexpensive method that enables the integration of commercially-available fiber optic pH and pressure sensors with microfluidic devices. For this purpose, we fabricated a microfluidic device with bespoke ports in the inlet and outlet channels that were deliberately designed to embed the sensors. The microfluidic device was manufactured using an ultrashort pulsed picosecond laser (TruMicro 5x50, Trumpf), which was used to: (a) generate a microfluidic pattern on the glass surface by ablating the material; (b) drill an inlet, outlet and sensor ports in a second glass plate; and (c) close the microfluidic pattern from the top with a second glass plate by creating weld seams at the glass-glass interface and permanently bonding the two glass slides together. The fiber optic sensors were attached to the microfluidic device using custom connectors that were manufactured from transparent UV-curable resin using a desktop, stereolithography 3D printer (Form 2, Formlabs). The pH sensors (“pH SensorPlugs”, manufactured by PreSens Precision Sensing GmgH) were tested with pH calibration buffers, while the pressure sensors (FOP-MIV, manufactured by FISO Technologies Inc.) were used to measure pressure directly in the ports during the flow of water through the microfluidic pattern, providing quantitative information on the dynamic events occurring in the microfluidic channels.

Keywords: Microfludiic devices; Ultrafast lasers; Laser abalation; Microwelding; Glass; Fibre optic sensors;


Fully reflective bessel beam generation with constant energy distribution over the propagation axis for complex glass cutting
Siddharth Sivankutty, Antonin Billaud, Gwenn Pallier, Pu Jian, Olivier Pinel, Guillaume Labroille

Glass cutting with femtosecond lasers is spreading led by the touch panel displays development. Bessel beams are very efficient and precise way to process glass thanks to their extended depth of focus 100 times longer than a standard Gaussian beam and their central beam which can be smaller than the diffraction limit. High quality glass cutting with a reflective axicon has already been demonstrated with no oscillations leading to cleaner cuts and faster processes. The beam is able to propagate through a galvo-scanner and a F-theta lens. The reflective design is compatible with extreme high peak and average power. Here we describe the generation of a complex Bessel beam profile flatter over the propagation axis based on a reflective design. The tail of this profile is five times sharper compared to standard Bessel beams paving the way to complex glass cutting such as multi-layer glasses.

Keywords: Glass processing ; Ultra-Short Pulse laser ; femtosecond ; beam shaping ; Bessel beam


Solvent evaporation and annealing of solution-processedorganic materials by laser irradiation on flexible substrates
Kiel, Frederik, Esen, Cemal; Ostendorf, Andreas, Wirth, Volker

The market for flexible, printed and organic electronics is rapidly growing. The production is often based on wet-chemical processes and the residual solvent needs to be removed. Conventional methods for solvent evaporation or annealing of functional layers in roll-to-roll processing are circulating-air- or infrared-dryers. These methods require a relatively large drying section and entail a long exposure time at elevated temperatures for the flexible substrates. By utilizing a diodelaser as heating source the installation space and especially the exposure time at elevated temperatures could beminimized. A LIMO line laser system (450 W, 980 nm) was used to investigate the substitution of conventional drying/ annealingmethods on a lab scale (laser line FWHM: 12,22x0,06 mm²) for organic photovoltaic cells resulting in comparable powerconversion efficiencies to conventional methods at processing speeds of 1m/min. These results are expected to betransferred from the lab scale to a R2R-system.

Keywords: laser drying; laser annealing; organic electronics; transparent materials; roll-to-roll processing


Micromachining of transparent biocompatible polymers used as vision implants with bursts of femtosecond laser pulses
Evaldas Kažukauskas, Simas Butkus, Vytautas Jukna, Domas Paipulas, Martynas Barkauskas and Valdas Sirutkaitis

Biocompatible polymers are used for many different purposes (catheters, artificial heart components, dentistry products, etc.). One important field where biocompatible polymers are utilized is in production of vision implants known as intraocular lenses or custom-shape contact lenses. Typically, curved surfaces are manufactured by mechanical means such as milling, turning or lathe cutting. The 2.5 D objects/surfaces can also be manufactured by means of laser micromachining, however due the nature of light-matter interaction, it is difficult to produce a surface finish with a surface roughness better than ~ 1 μm Ra. Therefore, laser micromachining alone can’t produce the final parts with optical-grade quality. Laser machined surfaces may be polished via mechanical methods; however, the process may take up to several days, which makes it economically challenging. The aim of this study is the investigation of the polishing capabilities of rough (~ 1 μm Ra) hydrophilic acrylic surfaces using bursts of femtosecond laser pulses. By changing different laser parameters, it was possible to find a regime where the surface roughness can be minimized to 18 nm Ra, while the polishing of the entire part takes a matter of seconds. The produced surface demonstrates a transparent appearance and shows great promise towards commercial fabrication of low surface roughness custom-shape optics.

Keywords: femtosecond micromachining, burst processing, intraocular lens, hydrophilic acrylic, surface roughness, polishing;


Femtosecond laser-assisted mould fabrication for metal casting at the micro-scale
Enrico Casamenti, Luciano Borasi, Sacha Pollonghini, Adeline Durand, Samuel Rey, Raphaël Charvet, Cyril Dénéréaz, Andreas Mortensen, and Yves Bellouard

Existing technologies for the production of metallic 3D micro-components are, in their current state, either too slow to be economically viable, restricted to augmented two-dimensional shapes, or produce structures of porous metal. Here we overcome those barriers by bringing metal casting into the realm of microfabrication. We present a process that enables the production of freeform, dense, 3D metal architectures at printing speeds well above alternative approaches for metal 3D printing at the same resolution. Using femtosecond laser micro-machining combined with chemical etching, arbitrarily shaped 3D-cavities are carved out of fused quartz substrates and subsequently pressure-infiltrated with high melting point metals, such as pure silver, copper or gold, and their alloys. The resulting glass/metal combinations contain interconnected dense metal 3D structures that are shaped freely with micrometric resolution to enable new types of micro-devices and composite structures.

Keywords: micro-manufacturing; freeform fabrication; microcasting; femtosecond laser