Laser-induced Etching Research Articles

Influence of flexible multibeam intensity distributions on selective laser-induced etching process regimes

The precision and versatility of selective laser-induced etching in fused silica render it indispensable for many applications in medical engineering, the semiconductor industry, and quantum technology. However, its process-limited productivity renders it unsuitable for a wider set of applications, particularly those that require cost-effective solutions. As the process is largely concerned with microprocessing applications, a promising method to enhance process efficiency is through parallelization using multiple beams with high packing density. This study examines the dynamics of flexible multibeam intensity distributions relative to the number of beamlets and spacing. The study identifies stable process regimes, examines the increased absorption from interactions among parallel modified tracks, and develops a strategy for optimized processing using flexible multibeams. The final demonstration illustrates the generation of multibeam fabricated through glass vias with varying degrees of complexity.

Effect of heat treatment processes on the Cu-electrodeposited through glass vias (TGV) plate

Based on laser-induced deep etching technology and through-hole filling techniques, TGV interposers were successfully fabricated. The residual stress in the Cu overburden film directly affects the yield improvement after subsequent grinding of the interposer. Therefore, we studied the evolution of residual stress, microstructural changes, and micro-nano mechanical properties within the Cu overburden film after heat treatment at different temperatures. Residual stress analysis using the sin2Ψ method with an X-ray diffractometer (XRD) indicates that during the process of increasing temperature from 100°C to 200°C, the internal residual stress in the Cu overburden film changes from compressive stress to tensile stress. Furthermore, the maximum absolute value of the residual stress, 70.09 MPa, appears at room temperature, and the minimum absolute value of the residual stress, 15.59 MPa, occurs after the heat treatment process at 200°C. Electron backscatter diffraction (EBSD) microstructural analysis shows that the application of heat treatment can effectively accelerate static recrystallization (SRX), promote a higher proportion of high-angle grain boundaries (HAGB), and refine grain size. Additionally, a (001) preferred orientation at high residual stress states and a (111) preferred orientation at low residual stress states are observed. Nanoindentation test results indicate that the Cu overburden film treated at 100°C has the most potential for removal during the grinding process.

Study on nanosecond laser-induced iron plasma reactive etching of single-crystal CVD diamond

Single-crystal diamond has great potential applications in national defense, military, and aerospace fields. However, the high hardness, chemical inertness, and anisotropy bring great challenges to machine microstructures on the diamond. Here, iron plasma, generated by infrared nanosecond laser ablation of a Fe target, was utilized to etch single-crystal diamond. The EDS, Raman, and XPS analysis revealed the formation of iron carbide within the sputtered layer surrounding the diamond microgrooves, thereby confirming that the iron plasma reacts chemically with the diamond. It was observed that employing a reactive metal, such as iron, for etching diamond microgrooves yields improved morphological characteristics and enhanced etching efficiency compared to the use of a non-reactive metal, such as copper. Moreover, the process of material removal through laser-induced iron plasma etching of diamond was elucidated. Subsequently, a systematic investigation was conducted on the impact of processing parameters on the laser-induced iron plasma etching of diamonds. Various periodic microstructures with regular edges and no chipping and cracks were successfully processed on the diamond using suitable laser parameters. This study furnishes a novel approach for the efficient fabrication of high-quality microstructures on single-crystal diamonds.

Local analysis of micro mixing for the Villermaux-Dushman protocol by using the imaging UV-Vis spectroscopy

The Villermaux-Dushman protocol with UV-Vis analytics is an established tool to characterize the global micro mixing performance in process equipment. The local mixing process is described by a micro mixing model (incorporation model), originally designed for turbulent flows. The novel imaging UV-Vis spectroscopy used in this work uncovers locally resolved micro mixing phenomena in a laminar split-and-recombine (SAR) mixer unit manufactured from selective laser-induced etching (SLE). The local absorbance is recorded with a high spatial resolution camera through a telecentric lens and post-processed into local concentration fields of components. The method unveils discrepancies of the micro mixing time determined from the conventional incorporation model in laminar flow and the locally recorded mixing process. The micro mixing time rather needs to be seen as a mean micro mixing time instead. Furthermore, the spatial information obtained by the imaging UV-Vis spectroscopy gives insight into local micro mixing and selectivity, yielding new approaches to equipment optimization.

Wetting Properties of Black Silicon Layers Fabricated by Different Techniques

The wettability of black silicon (BSi) layers fabricated by reactive ion etching (RIE), metal‐assisted chemical etching (MACE), and laser‐induced etching (LIE) techniques is studied. The contact angles of wetting on the samples with deionized water and methylammonium iodide‐based perovskite solutions are determined. It is found that the element composition and the enlargement area factor of BSi layers have a significant effect on their wettability. When tested with water, the RIE and MACE BSi layers exhibit hydrophobic properties, while the LIE BSi layer demonstrates hydrophilic properties due to the SiOx‐rich surface structures. It is also shown that aging leads to a decrease in the water contact angle. Upon exposure to perovskite solution droplets, BSi layers become highly lyophilic. Based on the Wenzel and Cassie–Baxter models, the mechanisms responsible for the wetting states of the fabricated samples are identified. In the results obtained, valuable insights are provided into the potential of using these layers in tandem perovskite/silicon solar cells.

Controllable Si micro-structuring by temporally modulated single-shot femtosecond pulse lithography

Micro-structured silicon surface plays a significant role in the electronics industry. Crystalline and amorphous phases of silicon, which have significantly different physical properties, can be transformed into each other using femtosecond laser, and assisted chemical etching enables the easy fabrication of various micro-structures. However, efficient and controllable fabrication of micro-cylinders, micro-rings, and micro-ring grooves on silicon surfaces still remains a challenge. Here, we propose a temporally modulated single-shot femtosecond pulse lithography technology, combining laser-induced silicon modification and chemical etching. In this technology, the occurrence of recrystallization allows for the flexible manipulation of the shape of the modified area, transitioning from circular to annular shapes by adjusting the laser fluence. This corresponds to the formation of micro-cylinders and micro-rings after etching. In the case of temporally shaped pulses, with an increasing pulse delay, the micro-rings formed after etching gradually transform into micro-ring grooves, as the recrystallization area created by the first sub-pulse is reacted by the second sub-pulse. Due to the characteristics of the single shot, the technology, when combined with the flying punch method, can be used for the high-efficiency fabrication of large-area silicon surface micro-structure arrays.

Multi-color transparent display based on perovskite quantum dots fabricated by laser-induced plasma etching.

Display technology is being revolutionized by cutting-edge transparent displays that can provide visual information on the screen while allowing the surrounding environment to be visible. In this report, a new method is proposed for patterning displays based on perovskite quantum dots (PQDs) on glass surfaces. A glass substrate with a polyvinylidene fluoride (PVDF) constraint layer is patterned using laser-induced plasma etching, and then a PQDs film is spin-coated on the etched sample. The PQDs pattern on the glass substrate is obtained after peeling off the PVDF constraint layer. The thickness of the film is obtained by carrying out simulations. The plasma output from different metal targets is recorded and analyzed to select the most suitable parameters and materials for improvement of the patterning accuracy. The transparent pattern display of PQDs is realized with an accuracy of 10-20 µm and a burial depth of about 1 µm. This method allows PQDs to be encapsulated under the substrate surface, which decreases the susceptibility of environmental impact. Additionally, encapsulation prevents the quantum dots from leaking out and causing environmental pollution. The proposed method has potential in the design of transparent displays and anti-counterfeiting applications.

Effect of in-plane and out-of-plane bifurcated microfluidic channels on the flow of aggregating red blood cells.

The blood flow through our microvascular system is a renowned difficult process to understand because the complex flow behavior of blood is intertwined with the complex geometry it has to flow through. Conventional 2D microfluidics has provided important insights, but progress is hampered by the limitation of 2-D confinement. Here we use selective laser-induced etching to excavate non-planar 3-D microfluidic channels in glass that consist of two generations of bifurcations, heading towards more physiological geometries. We identify a cross-talk between the first and second bifurcation only when both bifurcations are in the same plane, as observed in 2D microfluidics. Contrarily, the flow in the branch where the second bifurcation is perpendicular to the first is hardly affected by the initial distortion. This difference in flow behavior is only observed when red blood cells are aggregated, due to the presence of dextran, and disappears by increasing the distance between both generations of bifurcations. Thus, 3-D structures scramble in-plane flow distortions, exemplifying the importance of experimenting with truly 3D microfluidic designs in order to understand complex physiological flow behavior.

Laser-based fabrication chain enabling high quality mini aspheres

In recent years, 3D machining in glass for micro-components has seen a notable boost, notably with the development of a novel optical fabrication chain. This approach uses lasers for shaping and polishing, enabling the creation of complex mini-optics efficiently. The process begins with selective laser-induced etching (SLE) to define the optics’ outer shape and surface figure. Subsequent polishing, using a “one-shot laser polishing” technique, removes imperfections and reduces roughness in a single step, achieving optical-grade smoothness. This fabrication chain is applicable to mini-aspheres as well, enhancing their production efficiency. Additionally, it allows for wafer-level production, where multiple mini-optics are interconnected on a single glass substrate.

Universal selective transfer printing via micro-vacuum force

Transfer printing of inorganic thin-film semiconductors has attracted considerable attention to realize high-performance soft electronics on unusual substrates. However, conventional transfer technologies including elastomeric transfer printing, laser-assisted transfer, and electrostatic transfer still have challenging issues such as stamp reusability, additional adhesives, and device damage. Here, a micro-vacuum assisted selective transfer is reported to assemble micro-sized inorganic semiconductors onto unconventional substrates. 20 μm-sized micro-hole arrays are formed via laser-induced etching technology on a glass substrate. The vacuum controllable module, consisting of a laser-drilled glass and hard-polydimethylsiloxane micro-channels, enables selective modulation of micro-vacuum suction force on microchip arrays. Ultrahigh adhesion switchability of 3.364 × 106, accomplished by pressure control during the micro-vacuum transfer procedure, facilitates the pick-up and release of thin-film semiconductors without additional adhesives and chip damage. Heterogeneous integration of III-V materials and silicon is demonstrated by assembling microchips with diverse shapes and sizes from different mother wafers on the same plane. Multiple selective transfers are implemented by independent pressure control of two separate vacuum channels with a high transfer yield of 98.06%. Finally, flexible micro light-emitting diodes and transistors with uniform electrical/optical properties are fabricated via micro-vacuum assisted selective transfer.

Oriented design of wafer-level stacking structures to enhance reliability of laser debonding of thin devices

The laser debonding process in advanced packaging has become a key technology for the debonding of ultra-thin wafers. However, improving the reliability of debonding and efficient removal of adhesive residues from thin and brittle devices remains challenging. Here, we fabricated bonding pairs formed by inorganic responsive layer, thermal buffer layer, and adhesive layer based on a structural design-oriented strategy. Experimental results showed that the thinned device wafers did not show any damage after laser debonding, and the residual adhesive film could be easily torn off. The response layer (a-Si:H) with high temperature resistance and corrosion resistance has strong photosensitivity under the action of ultraviolet laser with a wavelength of 355 nm, and rapidly decomposes to release H2 and polysilicon nanoparticles. The thermal buffer layer is primarily intended to block the heat flow and compensate internal stress. The adhesive layer acts both as an adhesive during temporary bonding and as a barrier to laser-induced gas shock waves and plasma etching of device layers during debonding. This rational structural design contributes to improves the reliability of the laser debonding process and shows great potential for application in the next generation of thin device debonding.

Pulse duration dependent laser-induced plasma etching of polyimide using a high repetition rate laser

Although laser ablation of polymers is well investigated the achievable surface quality of this process is limited due to laser-induced structural and morphological defects, arising surface roughness and redeposition of debris. Because high quality machining with laser radiation is highly requested, laser-induced plasma etching (LIPE) of polyimide film is studied for the development of ultraprecise surface machining (UPSM) techniques for various applications. Thus, the key parameters such as laser power, etching time, plasma distance of the LIPE are studied experimentally to explore the impact to the etching rate and the surface morphology. LIPE rates as low as 1 pm per laser pulse were measured that fulfil UPSM requirements. The footprint of the LIPE is circular shaped and enable therefore the development of areal processing strategies for surface correction, aspherization and freeform processing. In addition, regular nanostructures that feature a highly regular hexagonal pattern with a period of 117 nm to 125 nm were found at particular conditions.

Extensional rheometry of mobile fluids. Part I: OUBER, an optimized uniaxial and biaxial extensional rheometer

Numerical optimization of a “six-arm cross-slot” device yields several three-dimensional shapes of fluidic channels that impose close approximations to an ideal uniaxial (biaxial) stagnation point extensional flow under the constraints of having four inlets and two outlets (two inlets and four outlets) and for Newtonian creeping flow. One of the numerically designed geometries is considered suitable for fabrication at the microscale, and numerical simulations with the Oldroyd-B and Phan-Thien and Tanner models confirm that the optimal flow fields are observed in the geometry for both constant viscosity and shear thinning viscoelastic fluids. The geometry, named the optimized uniaxial and biaxial extensional rheometer (OUBER), is microfabricated with high precision by selective laser-induced etching of a fused-silica substrate. Employing a refractive index-matched viscous Newtonian fluid, microtomographic-particle image velocimetry enables the measurement of the flow field in a substantial volume around the stagnation point. The flow velocimetry, performed at low Reynolds number (<0.1), confirms the accurate imposition of the desired and predicted flows, with a pure extensional flow at an essentially uniform deformation rate being applied over a wide region around the stagnation point. In Part II of this paper [Haward et al., J. Rheol. 67, 1011–1030 (2023)], pressure drop measurements in the OUBER geometry are used to assess the uniaxial and biaxial extensional rheometry of dilute polymeric solutions, in comparison to measurements made in planar extension using an optimized-shape cross-slot extensional rheometer [OSCER, Haward et al., Phys. Rev. Lett. 109, 128301 (2012)].

MMIC-to-Dielectric Waveguide Transitions for Glass Packages Above 150 GHz

In this work, novel concepts of an electromagnetically coupled transition and a galvanically coupled transition in glass technology are presented. This enables efficient coupling of signals above 150 GHz into a dielectric waveguide (DWG). Laser-induced deep etching (LIDE) technology provides fabrication of glass holes, cavities, and cutouts with a precision suitable for use in highly integrated mm-wave modules. Based on the package concept presented by Galler et al. (2022), this article introduces an ultracompact galvanically isolated transition from monolithic microwave integrated circuit (MMIC) to a mechanically flexible DWG, offering a minimum insertion loss of 2.95 dB. The simple fabrication provides a cost-effective variant for modern system-in-package solutions. In addition, a galvanic through-plating of the glass cover with through-glass vias (TGVs) and a ring slot structure for the excitation of a DWG are presented. Thereby, the integration density and performance can be further increased. A minimum insertion loss of 2.62 dB within a large 1-dB bandwidth of 18 GHz around the operating frequency of 166 GHz is verified by measurement. Both the transitions cover a high integration density, hermetic sealing, and modular use suitable for requirements of today’s modern radar and communication packages.

Femtosecond laser-assisted fabrication of piezoelectrically actuated crystalline quartz-based MEMS resonators

A novel technology for the precise fabrication of quartz resonators for MEMS applications is introduced. This approach is based on the laser-induced chemical etching of quartz. The main processing steps include femtosecond UV laser treatment of a Cr-Au-coated Z-cut alpha quartz wafer, followed by wet etching. The laser-patterned Cr-Au coating serves as an etch mask and is used to form electrodes for piezoelectric actuation. This fabrication approach does not alter the quartz’s crystalline structure or its piezo-electric properties. The formation of defects, which is common in laser micromachined quartz, is prevented by optimized process parameters and by controlling the temporal behavior of the laser-matter interactions. The process does not involve any lithography and allows for high geometric design flexibility. Several configurations of piezoelectrically actuated beam-type resonators were fabricated using relatively mild wet etching conditions, and their functionality was experimentally demonstrated. The devices are distinguished from prior efforts by the reduced surface roughness and improved wall profiles of the fabricated quartz structures.

Repair of Fused Silica Damage Using Selective Femtosecond Laser-Induced Etching

Timely repair of fused silica damage ensures the stable operation of high-power laser systems. In the traditional repair process, the material nearby the damaged area is gradually ablated with CO2 or femtosecond laser. Subsequently, homogenization and residual stress removal are required because of the microcracks and thermal accumulation generated with the ablation. As a result, the repair efficiency is greatly restricted. In this paper, a new method using in-volume, selective femtosecond laser-induced etching to repair the damage of fused silica is proposed. The region irradiated by femtosecond laser becomes more susceptible to the etching solution due to its constitutive characteristics having undergone chemical restructuring. In this way, material nearby the damaged area transparent to the laser radiation is modified locally inside the volume. A femtosecond laser is used to scan the damaged area with a 3D hollow trajectory. The applicable modification of fused silica occurs when the single pulse energy is approximately 2 μJ to 5 μJ, the repetition frequency is approximately 200 kHz to 500 kHz, and the scanning speed is approximately 10 mm/s. Then, the etching solution reacts quickly along the 3D profile of the modified path, and the damaged area is removed as a whole piece. This method can greatly reduce the workload of repair, and the etching process of fused silica is carried out synchronously. So, the etching efficiency is not affected by the number of damage points. In addition, the weak reaction between the etching solution and the substrate could homogenize the interface. It provides an efficient way to repair the surface damage of fused silica.

Preclinical-grade microendoscope for optical coherence tomography and angiography inside the bladder

We present a preclinical-grade, forward-viewing endomicroscope for in-contact optical coherence tomography (OCT) and optical coherence angiography (OCA) imaging through the working channel of a conventional cystoscope. Beam scanning is achieved with a fiber scanner driven by a tubular piezoelectric actuator. A focusing lens at the fiber tip helps engineering of the operation frequency within a compact probe length to avoid lateral undersampling. Microstructuring of fused silica through selective laser-induced etching was used for manufacturing a self-aligning housing for the probe head. The entire micro-optical system is assembled and encapsulated within a custom-developed sterilizable packaging with 4.5 mm outer diameter. The presented design and fabrication strategy can be used for any forward-viewing probe, independent of its imaging modalities. We demonstrate OCT imaging within a 2.1-mm diameter field of view at a transverse resolution of 19 μm and microvasculature visualization through OCA. The presented probe’s mechanical characteristics and optical performance make it particularly attractive for outpatient care use in the detection of tissue pathology inside the bladder. The presented fabrication methodology provides a reliable strategy for enabling preclinical trials with endoscopic imaging probes.

Ab initio simulations of ultrashort laser pulse interaction with Cl-Si(100): implications for atomic layer etching.

Due to the remarkable resistance of SiCl against photo-induced desorption, achieving atomic layer etching (ALE) of Cl-Si(100) through a laser-based method has remained a formidable challenge. In this study, we investigate the interaction between ultrashort laser pulses and the Cl-Si(100) surface via ab initio simulations that combine real-time time-dependent density functional theory and molecular dynamics. Our results demonstrate the direct desorption of the stubborn SiCl layer through the application of appropriate femtosecond laser pulses. Notably, the desorption process is enhanced by pulses with higher laser intensity, shorter wavelength, and longer pulse duration. There is a threshold intensity beyond which the SiCl can be directly desorbed under laser pulses with a wavelength of 488 nm and a pulse duration of 40 ℏ eV-1 (26.3 fs). Analysis of electron localization function reveals a critical bond breaking length of 2.98 Å between Si-Si, connecting SiCl to the bulk material. The time evolution of bond lengths and forces reveals that the desorption of SiCl is primarily driven by repulsive forces generated within the Si-Si bond. Furthermore, electron density difference analysis and Keldysh factor calculations indicate that these repulsive forces arise from multiphoton ionization. This study provides crucial atomic-level insights into the interaction of ultrashort laser pulses with Cl-Si(100), thereby propelling the advancement of laser-induced atomic layer etching techniques.

GaN on Si (111) nanostructure for solar cells application

The fabrications of nanostructured GaN by electrochemical etching and laser-induced etching have been attempted. Laser processing has the advantages of not causing damage or contamination, as well as of special selectivity with high resolution and high efficiency. The studies on the fundamental properties of these nanostructures are very important due to their unique structural and optical properties relative to the bulk form of the corresponding material. To the best of our knowledge, this has never been reported before.

Analysis and characterization of GaN nanostructure by electrochemical etching

Nanostructured GaN layers have been fabricated by electrochemical and laser-induced etching (LIE) processes based on n-type GaN thin films grown on the Si (111) substrate with AlN buffer layers. The effect of varying current and laser power density on the morphology of the GaN layers is investigated. The etched samples exhibited a dramatic increase in photoluminescence intensity as compared to the as grown samples. The average diameter of the GaN crystallites was about 7–10 nm, as determined from the PL data The Raman spectra also displayed stronger intensity peaks, which were shifted and broadened as a function of etching parameters. A strong band at 522 cm−1 is from the Si (111) substrate, and a small band at 301 cm−1, due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm−1 and 568 cm−1due to E2 (low) and E2 (high) respectively.