Laser-induced Etching Research Articles

Viscous flow through microfabricated axisymmetric contraction/expansion geometries

We employ a state-of-the-art microfabrication technique (selective laser-induced etching) to fabricate a set of axisymmetric microfluidic geometries featuring a 4:1 contraction followed by a 1:4 downstream expansion in the radial dimension. Three devices are fabricated: the first has a sudden contraction followed by a sudden expansion, the second features hyperbolic contraction and expansion profiles, and the third has a numerically optimized contraction/expansion profile intended to provide a constant extensional/compressional rate along the axis. We use micro-particle image velocimetry to study the creeping flow of a Newtonian fluid through the three devices and we compare the obtained velocity profiles with finite-volume numerical predictions, with good agreement. This work demonstrates the capability of this new microfabrication technique for producing accurate non-planar microfluidic geometries with complex shapes and with sufficient clarity for optical probes. The axisymmetric microfluidic geometries examined have potential to be used for the study of the extensional properties and non-linear dynamics of viscoelastic flows, and to investigate the transport and deformation dynamics of bubbles, drops, cells, and fibers.Graphic abstract

Tunable wetting surfaces with interacting cavities via femtosecond laser patterning and wet etching

This paper presents the production of bioinspired slippery glass surfaces with interacting cavities via wet-etching-assisted femtosecond laser fabrication. A femtosecond laser irradiates a glass surface to fabricate microvoid arrays inside the substrate. Then, wet etching is performed to induce microcavities on the sample surface. With laser-induced selective etching, the region below the substrate surface is etched faster, thereby developing microcavities. The microvoid separation distance is found to be important for controlling the contact angle (CA) of the liquid wetting the surface. By choosing an adequate microvoid separation distance and etching time, interacting/interconnected cavities can be successfully fabricated. CAs are expected to be tuned from almost 0° to 137° ± 2.5° based on the cavity separation distance and the processes used (laser patterning, etching, and silanization). These interconnected structures fabricated with small separation distances (e.g., 10 μm) can lock in an infused lubricating liquid and form a stable, inert, slippery interface, known as a slippery liquid-infused porous surface, which acts as a smooth cushion for liquid repellence. Moreover, the infused liquid can significantly increase the transmittance owing to the index matching effect. Such slippery surfaces could be used in several self-cleaning, optical-sensing, and biomedical applications.

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs.

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a result, these platforms have not been fully exploited for direct observation of high-pressure transport in fractures. This work introduces microfluidic platforms that enable direct observation of multiphase flow in devices featuring surrogate permeable media and fractured systems. Such platforms provide a pathway to address important and timely questions such as those related to CO2 capture, utilization and storage. This work provides a detailed description of the fabrication techniques and an experimental setup that may serve to analyze the behavior of supercritical CO2 (scCO2) foam, its structure and stability. Such studies provide important insights regarding enhanced oil recovery processes and the role of hydraulic fractures in resource recovery from unconventional reservoirs. This work presents a comparative study of microfluidic devices developed using two different techniques: photolithography/wet-etching/thermal-bonding versus Selective Laser-induced Etching. Both techniques result in devices that are chemically and physically resistant and tolerant of high pressure and temperature conditions that correspond to subsurface systems of interest. Both techniques provide pathways to high-precision etched microchannels and capable lab-on-chip devices. Photolithography/wet-etching, however, enables fabrication of complex channel networks with complex geometries, which would be a challenging task for laser etching techniques. This work summarizes a step-by-step photolithography, wet-etching and glass thermal-bonding protocol and, presents representative observations of foam transport with relevance to oil recovery from unconventional tight and shale formations. Finally, this work describes the use of a high-resolution monochromatic sensor to observe scCO2 foam behavior where the entirety of the permeable medium is observed simultaneously while preserving the resolution needed to resolve features as small as 10 µm.

Laser nanoablation of a diamond surface in air and vacuum

The nanoablation (cold laser-induced etching) of a diamond surface has been experimentally investigated at different ambient air pressures ranging from atmospheric to high vacuum (10-7 Torr). The exposure of a diamond monocrystal sample was performed by using the second harmonic (λ=400 nm) of a Ti-sapphire laser (τ=120 fs). The pressure dependence of the surface etching rate was obtained and corroborates a model that regards nanoablation as a result of multiphoton photochemical oxidation of the diamond. The features of the diamond oxidation at various environmental pressures are discussed as well as the effect of a surface water layer on the etching rate of the diamond.

Improvement of Etching Anisotropy in Fused Silica by Double-Pulse Fabrication.

Femtosecond laser-induced selective etching (FLISE) is a promising technology for fabrication of a wide range of optical, mechanical and microfluidic devices. Various etching conditions, together with significant process optimisations, have already been demonstrated. However, the FLISE technology still faces severe limitations for a wide range of applications due to limited processing speed and polarization-dependent etching. In this article, we report our novel results on the double-pulse processing approach on the improvement of chemical etching anisotropy and >30% faster processing speed in fused silica. The effects of pulse delay and pulse duration were investigated for further understanding of the relations between nanograting formation and etching. The internal sub-surface modifications were recorded with double cross-polarised pulses of a femtosecond laser, and a new nanograting morphology (grid-like) was demonstrated by precisely adjusting the processing parameters in a narrow processing window. It was suggested that this grid-like morphology impacts the etching anisotropy, which could be improved by varying the delay between two orthogonally polarized laser pulses.

A Microfluidic Mixer of High Throughput Fabricated in Glass Using Femtosecond Laser Micromachining Combined with Glass Bonding.

We demonstrate a microfluidic mixer of high mixing efficiency in fused silica substrate using femtosecond laser-induced wet etching and hydroxide-catalysis bonding method. The micromixer has a three-dimensional geometry, enabling efficient mixing based on Baker’s transformation principle. The cross-sectional area of the fabricated micromixer was 0.5 × 0.5 mm2, enabling significantly promotion of the throughput of the micromixer. The performance of the fabricated micromixers was evaluated by mixing up blue and yellow ink solutions with a flow rate as high as 6 mL/min.

Three-Dimensional Laser Printing of Macro-Scale Glass Objects at a Micro-Scale Resolution.

Three-dimensional (3D) printing has allowed for the production of geometrically complex 3D objects with extreme flexibility, which is currently undergoing rapid expansion in terms of materials, functionalities, as well as areas of application. When attempting to print 3D microstructures in glass, femtosecond laser-induced chemical etching (FLICE)—which is a subtractive 3D printing technique—has proved itself a powerful approach. Here, we demonstrate the fabrication of macro-scale 3D glass objects of large heights up to ~3.8 cm with an identical lateral and longitudinal feature size of ~20 μm. The remarkable accomplishment is achieved by revealing an unexplored regime in the interaction of ultrafast laser pulses with fused silica, which results in depth-insensitive focusing of the laser pulses inside fused silica.

Nanosecond Nd:YAG laser induced backside wet etching of NaCl with eutectic gallium-indium alloy as absorbing liquid

Laser-induced back-side etching of NaCl (100) with eutectic gallium-indium (EGaIn, Ga 78% / In 22% weight percent) as the liquid absorber is demonstrated using nanosecond pulsed Nd:YAG laser with pulse width τp = 7 ns. The influences of the laser fluence and the pulse number on the etch rate are studied. The etch rate rises linearly with the laser fluence and is characterized by a high etch rate e (between 13.2 µm/pulse to 0.25 µm/pulse)in the laser fluence range of Φ = 15 J/cm2 to 0.6 J/cm2.Meanwhile, the etch rate is constant up to 6 pulses and a low threshold fluence for etching of Φth = 0.27 J/cm2 is found. According to the SEM image, cleavage cracks appear at the NaCl surface after laser radiation. The thermal stress of NaCl is estimated and compared with the rupture modulus to explain the mechanisms of the cleavage cracks of NaCl. In order to explain the high etch rate of NaCl, the maximum molten layer thickness during laser heating process is calculated to estimate the etch rate, which is significantly lower than the experimental value. Here, the thermal effects like cleavage cracks, fracture of material and defect production are most likely the reason for the high etch rate of NaCl. However, further experiments need to be completed for a detailed understanding of the etching mechanisms.

Efficiency of laser-induced backside wet microstructuring of sapphire increases with pressure

The study is directed to the development of the complex machining of hard transparent materials such as quartz, sapphire, and diamond using pulsed laser light (5 ns pulse duration, 532 nm wavelength, 1 kHz repetition rate). The deepening rate of the laser-induced backside etching of sapphire in a water solution of silver nitrate was studied as a function of liquid pressure in the range of 0.1–25 MPa. Its linear increase of about 1.5–2 times with pressure was demonstrated, which associated with the stronger microbubbles cavitation impact on hard material and chemical etching by sub- and super-critical water. The increase in the etching efficiency of sapphire at a higher pressure for this particular case can be generalized to other types of laser-induced backside wet etching and other materials.

Laser-induced frontside etching of silicon using 1550-nm nanosecond laser pulses

High-quality, low-defect laser machining of dielectric materials remains challenging due to the high energy densities required for inducing the physical processes of laser ablation. Therefore, experiments were performed to demonstrate the laser-induced frontside etching (LIFE) of silicon by nanosecond laser pulses at a wavelength of 1550 nm, utilizing a thin chromium layer as the absorber. Etching was found at a fluence range from 200 to 700 mJ/cm², where the etched surfaces were smooth in the center (5 to 80 nm rms). The etching process was accompanied by incubation as well as saturation effects. At a fluence of 280 mJ/cm², an averaged etching rate of about 0.05 nm/pulse was determined for a high number of laser pulses (N > 1000). At the edges where the fluence was below the etching threshold of about 200 mJ/cm², chromium film melting and the formation of different kinds of laser-induced periodic surface structures (LIPSS) were observed.The LIFE was accompanied by different surface modifications. The optical absorption of silicon likely increased due to the incorporation of chromium into the silicon and the formed defects near the surface. Further, LIPSS were observed at fluences near the threshold of laser etching. In particular, the formation of regular linear ripples with a period of approximately 1.52 µm was observed. The morphological characteristics of the ripples suggest that the mechanism was related to surface scattering at surface features or small ripples and the hydrodynamic instabilities of the molten silicon. In addition, outside the etching track, periodic but nonlinear LIPSS were observed in the chromium film. These hexagonal arranged dot-like patterns had a period of around 440 nm. It is suggested that the hexagonal dot-like patterns resulted from the multi-pulse irradiation and the sequential stress relaxation in the chromium film below the melting point of chromium during spot movement.

Machining of optical micro-mold by laser-induced chemical etching

Triangular grating has an important application in modern optical field. The large-scale production of such components can be realized by injection molding process, among which the fabrication of optical molds is the most important technology. This paper proposed to adopt femtosecond laser-induced chemical etching to make a micro-mold based on triangular diffraction gratings. Based on theoretical analysis and processing experiments, we studied the feasibility applied to manufacture the microstructure of optical mold.

Through Glass Vias for hermetically sealed High Frequency Application

In this work we demonstrate the fabrication and use of a glass interposer platform with Through Glass Vias (TGVs) for high frequency applications. The platform is part of a versatile hermetically sealed glass package including passive and active devices (radar sensor, pressure sensors, infrared sensors). Glass has ideal properties for such applications, like excellent chemical resistance, mechanical strength and low costs. The hermetic sealing allows the seamless operation in hazardous environments. The TGVs may be deployed, for instance, in miniaturized sensor systems as vertical DC and RF interconnections with reduced parasitics for improved performance of the sealed device, while maintaining the hermetic sealing. TGVs provide a superior RF performance compared to bonding wires for frequencies >20 GHz and will provide larger bandwidths. TGVs improve the performance drastically and will give us an easy way to provide a hermitically sealed connection to the inner circuitries of the package. We investigate the whole process chain of the fabrication of the metallized TGVs. Initially 200 mm glass wafer with a thickness of 500 μm are used. Vias are created by laser-induced deep etching (LIDE). Subsequently, selective metallization of wafer surface areas and TGV sidewalls is carried out. Vias with diameters ranging from 50 μm to 1mm are created. The wafer thickness is reduced to 465 μm during the via production. Thus vias with an aspect ratio of 1:9 to 2:1 are created on one wafer. The vias with the high-aspect ratio are filled by sputtering with an appropriate seed layer for electroplating. Depending on the via geometry and the aspect ratio, normal DC sputtering is not viable. Thus, High Power Impulse Magnetron Sputtering (HIPIMS) is utilized to deposit a thin film of titanium and copper. Titanium served as the adhesion layer and copper is used as the conductive seed layer for the following electroplating step. Then, electroplating is carried out to line the different sized TGVs and create a conducting path through the wafer. Afterwards, the metallization in the vias has to be exposed by removing only the plated copper and sputtered titanium on top of the wafer by a combination of etching and chemical mechanical polishing. In the following steps, the front and backside are sputtered and coated with a resist to allow electroplating of the RDL on both backside and front side. Finally, the wafer is diced and measured. The LIDE process shows excellent repeatability with respect to the position and shape of the TGVs on all axes. The repeatability of the shape is critical for the following TGV metallization and the matching of the simulated RF behaviour compared to the behaviour of the fabricated structures. The metallization process of the TGVs shows very promising results regarding the repeatability of the formation of the TGVs and the metallization inside the vias. Different shapes and dimensions of vias could be metallized with the same process, which makes it stable against deviations in the structures to be fabricated on the wafer The measured RF properties of the realized structures are very promising and showing that the usage of glass and TGVs improves the performance in the millimetre wave regime significantly, compared to conventional bonding wires.

High quality terahertz glass wave plates.

In this paper we present four λ/2-wave plates made out of fused silica glass for operation in the terahertz frequency range. The design of the wave plates is based on form birefringence. They were fabricated by selective laser-induced etching resulting in a series of glass bars separated by air grooves. Wave plates operating at single, two and several frequencies were designed, fabricated and characterized.

Chemical etching mechanisms and crater morphologies pre-irradiated by temporally decreasing pulse trains of femtosecond laser

We report the influence of temporally decreasing pulse trains on femtosecond laser-induced chemical etching (FLICE) of fused silica. A systematic comparison of the unshaped pulse and decreasing pulse trains of femtosecond laser for FLICE was conducted, and the differences were interpreted using a plasma model. The results revealed that the decreasing pulse trains not only affected the etching efficiency but also affected the morphology of the etched crater. When an etched crater was pre-irradiated by decreasing pulse trains, it presented a funnel-like shape at the early stage of the etching process, which contrasted with the one pre-irradiated by unshaped pulse. At the later stage of the etching process, the funnel-like shape gradually disappeared, and the crater increased in size. Compared with the unshaped pulse under the same processing conditions, the decreasing pulse trains enhanced the etched crater volume by approximately 18 times. Theoretical calculations based on the plasma model indicated that the free-electron density generated using the unshaped pulse was much higher than that generated by the decreasing pulse trains in skin layer of the sample during the first few hundred femtoseconds. The high free-electron density increased the reflectivity in skin layer of the sample; thus, the tail part of the incident pulse was strongly reflected. Consequently, the laser energy deposition into the fused silica sample decreased, eventually led to a low etching efficiency.

Printing of complex free-standing microstructures via laser-induced forward transfer (LIFT) of pure metal thin films

A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach.

Steady viscoelastic flow around high-aspect-ratio, low-blockage-ratio microfluidic cylinders

We employ a state-of-the-art microfabrication technique (selective laser-induced etching, SLE) to produce microfluidic cylinder geometries that explore new geometrical regimes. Using SLE, two microchannels are fabricated in monolithic fused silica substrate with height H=2 mm and width W=0.4 mm (aspect ratio α=H/W=5) containing cylinders of radius r=0.02 mm (blockage ratio β=2r/W=0.1), centered at the channel mid-width, W/2. An ‘sc’ channel contains a single cylinder, while a ‘dc’ channel contains two axially-aligned cylinders separated by a distance L=1 mm (L=50r). Compared with cylinder geometries fabricated by soft lithography (which typically have α ≪ 1 and β ≳ 0.5), these rigid glass devices provide a quasi-two-dimensional flow along the direction of the cylinder axis and also more clearly reveal the effects of the strong extensional wake regions located at the leading and trailing stagnation points. Using flow velocimetry and quantitative birefringence measurement techniques, we study the behaviour of a well-characterized viscoelastic polymer solution in flow around the cylinders. The small cylinder radii result in low inertia and very high elasticity numbers El ≈ 2400. For the sc device, we report strong flow modification effects around the cylinder as the flow rate is incremented. This is associated with the deformation of polymer molecules primarily in the upstream wake region, leading to the onset of a purely elastic flow asymmetry upstream of the cylinder. Stretched polymer molecules are advected around the cylinder and relax downstream of the cylinder, resulting in an extremely long elastic wake extending for > 300r downstream. In the dc channel, at lower flow rates, similar flow modification effects are observed to develop around, and downstream of, both cylinders. However, at higher flow rates the wake of the first cylinder extends > 50r downstream, and begins to interact with the second cylinder. The second cylinder becomes encapsulated by the wake of the first and is effectively obviated from the flow field. The results will be of relevance to understanding practical applications of viscoelastic fluids, for example in particle suspension and porous media flows, and also for benchmarking against numerical simulations using viscoelastic constitutive models.

Simulation of the dynamics of heat accumulation during in-volume modification with ultrashort laser pulses

Working with ultrashort pulsed (USP) laser systems with high repetition rates ≥ 100 kHz, heat accumulation effects evolve and subsequently the absorption becomes linear. As a consequence, the process dynamics change significantly. Structuring glasses with those USP Lasers leads to melted lines that are used for waveguides, welding or selective laser-induced etching whose resulting qualities are strongly depending of the structuring results and given smoothness. We perform simulations during in-volume modification processes, by recalculating the absorption behavior in between the iteration steps of heat accumulation. The simulation results provide explanations for smoothness or non-smoothness of modifications comparable to experiments in glass.

Surface smoothing of indium tin oxide film by laser-induced photochemical etching

Surface smoothing of indium tin oxide (ITO) film by laser irradiation was demonstrated. The ITO surface was etched by choline radicals, which were activated by laser irradiation at a wavelength of 532 nm. The RMS surface roughness was improved from 5.6 to 4.6 nm after 10 min of laser irradiation. We also showed the changes in the surface morphology of the ITO film with various irradiation powers and times.

Studies of the confinement at laser-induced backside dry etching using infrared nanosecond laser pulses

In the present study, laser-induced backside etching of SiO2 at an interface to an organic material using laser pulses with a wavelength of λ=1064nm and a pulse length of τ=7ns have been performed in order to investigate selected processes involved in etching of the SiO2 at confined ablation conditions with wavelengths well below the band gap of SiO2. Therefore, in between the utilized metallic absorber layer and the SiO2 surface, a polymer interlayer with a thickness between 20nm to 150nm was placed with the aim, to separate the laser absorption process in the metallic absorber layer from the etching process of the SiO2 surface due to the provided organic interlayer. The influence of the confinement of the backside etching process was analyzed by the deposition of different thick polymer layers on top of the metallic absorber layer. In particular, it was found that the SiO2 etching depth decreases with higher polymer interlayer thickness. However, the etching depth increases with increasing the confinement layer thickness. SEM images of the laser processed areas show that the absorber and confinement layers are ruptured from the sample surface without showing melting, and suggesting a lift off process of these films. The driving force for the layers lift off and the etching of the SiO2 is probably the generated laser-induce plasma from the confined ablation that provides the pressure for lift off, the high temperatures and reactive organic species that can chemically attack the SiO2 surface at these conditions.

Closed-loop quality control system for laser chemical machining in metal micro-production

Small geometrical features in the scale of 10 to 400 μm can be produced on metallic workpieces with the laser-induced chemical etching process at low costs. However, the interactions among the subsystems of the laser chemical etching process, the mechanical positioning unit, the etchant pump, the laser source, and among the removal paths hinder the precise determination of the required process parameters for producing the desired geometry. For this reason, a closed-loop quality control is designed to compensate the deviations of quality features of a single removal and also of a workpiece produced by a sequence of removals. The developed control approach is based on inverse process models, which are used for a feed-forward control. Finally, a closed-loop control is achieved by designing an adaptive controller for the non-linear multi-inputs-multi-outputs process. The closed-loop control is realized as a production-discrete control by using a post-production measurement, and applied for producing a micro forming tool in the shape of a rectangular die. As a result, the laser chemical etching process is stabilized and, thus, the desired geometrical quality features of the workpiece are obtained with a reduced shape deviation of 2.4 μm.