United States

Liquid immersion is used in laser machining to cool and remove debris from samples.&amp;nbsp; The use of fluid media such as water, oils, or air for machining is well researched; however, the application of low-vapor-pressure fluids, such as ionic liquids, for laser micromachining has not been researched in depth.&amp;nbsp; Ionic liquids are electrically neutral salts that are liquid at room temperature.&amp;nbsp; These liquids tend to have low vapor pressure, which impacts a fluid’s ability to vaporize and cavitate during laser machining.&amp;nbsp; Fluids with high vapor pressures, on the other hand, can be considered volatile and easily vaporize with minimal added energy.&amp;nbsp; This paper discusses the effects of various immersion fluids on laser micromachining.&amp;nbsp; &amp;nbsp;Different immersion media were tested on a borosilicate glass cover slip using ultrafast (300 fs) laser pulses focused by a high numerical aperture (NA 1.2) immersion lens.&amp;nbsp; For each immersion fluid, rectangular arrays of holes were machined with single, well-separated pulses, where the cover slip was translated laterally and stepped axially to change the focal position and bracketing the glass surface with positions both above (in the fluid) and below it (in the glass).&amp;nbsp; The machined samples were then replicated with polymer imprint lithography, and both the substrate and replica were imaged using optical and scanning electron microscopies.&amp;nbsp; The results suggest that fluids with lower vapor pressures, like ionic liquids, can expand the usable machining focal range, while also reducing the diameter of the holes and perhaps even mitigating collateral surface damage.

ICALEO 2023

Fluid-Immersion Laser Micromachining Using Ionic Liquids

Liquid immersion is used in laser machining to cool and remove debris from samples.&nbsp; The use of fluid media such as water, oils, or air for machining is well researched; however, the application of low-vapor-pressure fluids, such as ionic liquids, for laser micromachining has not been researched in depth.&nbsp; Ionic liquids are electrically neutral salts that are liquid at room temperature.&nbsp; These liquids tend to have low vapor pressure, which impacts a fluid’s ability to vaporize and cavitate during laser machining.&nbsp; Fluids with high vapor pressures, on the other hand, can be considered volatile and easily vaporize with minimal added energy.&nbsp; This paper discusses the effects of various immersion fluids on laser micromachining.&nbsp; &nbsp;Different immersion media were tested on a borosilicate glass cover slip using ultrafast (300 fs) laser pulses focused by a high numerical aperture (NA 1.2) immersion lens.&nbsp; For each immersion fluid, rectangular arrays of holes were machined with single, well-separated pulses, where the cover slip was translated laterally and stepped axially to change the focal position and bracketing the glass surface with positions both above (in the fluid) and below it (in the glass).&nbsp; The machined samples were then replicated with polymer imprint lithography, and both the substrate and replica were imaged using optical and scanning electron microscopies.&nbsp; The results suggest that fluids with lower vapor pressures, like ionic liquids, can expand the usable machining focal range, while also reducing the diameter of the holes and perhaps even mitigating collateral surface damage.

technical paper

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
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ICALEO® brings together the brightest minds, seasoned professionals, and pioneering researchers from around the world. With a laser-focused agenda, ICALEO offers an exceptional platform to delve into the cutting-edge advancements, diverse applications, and transformative research that define lasers and electro-optics.

In this study, laser doping of n-type 4H-SiC with boron for mid-wave infrared (MWIR) optical properties is reported. A novel doping technique based on a pulsed Nd:YAG laser (1064 nm) for doping silicon carbide (SiC) was developed. An aqueous boric acid (H 3 BO 3 ) solution was utilized as a precursor to facilitate p-type doping in 4H-SiC. FTIR spectrometry was used to characterize the optical properties of the as-received and boron-doped samples within the MWIR range. The boron dopant created an acceptor energy level at (E v + 0.29 eV) within the bandgap of 4H-SiC, corresponding to 4.3 µm wavelength. Upon absorption of photons of this wavelength by the boron-doped 4H-SiC, the electron densities in valence and acceptor energy levels were altered, resulting in a change in the refraction index of the 4H-SiC substrate. A theoretical heat transfer model was devised to establish a correlation between laser processing parameters and various other factors that impact the effectiveness of the laser doping process. These factors include the laser-substrate interaction time, minimum dopant diffusion time, dopant concentration in precursor solution and dopant concentration diffused into the substrate. To ensure a thorough analysis, a comprehensive investigation was carried out on the as-received and boron-doped samples, encompassing a careful examination of their optical properties for the MWIR wavelength range. The laser doping process resulted in a notable decrease in the refraction index from 2.873 to 2.517. Our experimental results aligned with the theoretical model developed for selecting the laser processing parameters, affirming its success.

Laser Doping of N-Type 4H-SiC With Boron Using Solution Precursor for MWIR Optical Properties

As emerging of IoT devices, the ICs should be fast. To fulfill the fast performance, 3D ICs has been adopted. A 3D IC has relatively shorter connection electrode than conventional 2D ICs. And it is achieved by interposer. A interposer has via holes which are filled with conducting materials. In previous studies, various researchers have been suggested glass interposer, so called through glass via (TGV). Since glass has low electric permittivity, thus it is suitable for interposer. Also, glass can match its thermal expansion coefficient with silicon and it can suppress a warpage of wafer. Therefore, we have been studying of TGV for last few years. There are many methods which can generate via holes in the glass. Among them, recently, selective laser etching is emerging as a good candidate for mass production. It consists with two steps for generating TGV. Which is laser local modification and chemical etching. With previous study, we could enhance a process speed with Bessel beam shaping and picosecond interval pulses. In this study, we are focused on chemical etching process. In order to enhance the etching rate, 4 different etchants are tested. Which is HF, KOH, NaOH, and NH4F. Each etchant has its own optimal etching conditions, such as temperature and molar concentration. Here, we compare their etching results. The main criteria of comparison is selectivity. In our study, NH4F shows the best selectivity. Thus, we claim that NH4F can be a good candidate etchant for mass production of Glass interposer.

Study of Selective Laser Etching Process With Various Etchants

The amount of absorbed energy in the keyhole and its spatial and temporal distribution is essential to model the laser beam welding process. The recoil pressure, which develops as a consequence of the evaporation process induced by the absorbed laser energy at the keyhole wall, is a key determining factor for the macroscopic flow of the molten metal in the weld pool during high-power laser beam welding. Consequently, a realistic implementation of the effect of the laser radiation on the weld metal is crucial to obtain reliable and accurate simulation results. 
In this paper, we discuss manyfold different improvements on the laser-material interaction, namely the ray-tracing method, in the numerical simulation of the laser beam welding process. The first improvement relates to locating the exact reflection points in the ray tracing method using a so-called cosine-condition in the determination algorithm for the intersection of the reflected rays and the keyhole surface. A second correction refers to the numerical treatment of the Gaussian distribution of the laser beam, whose beam width is defined by a decay of the laser intensity by a factor of 1/e2 thus ignoring around 14 % of the total laser beam energy. In a third step, the changes in the laser radiation distribution in vertical direction were adapted by using different approximations for converging and diverging regions of the laser beam thus mimicing the beam caustics. Finally, a virtual mesh refinement was adopted in the ray tracing routine. The obtained numerical results were validated with experimental measurements.

Challenges in Dynamic Heat Source Modeling in High-Power Laser Beam Welding

Through experimental observation and auxiliary numerical simulation, this investigation studies the different types of grain refinement of 5754 aluminum alloy laser beam welding by applying a transverse oscillating magnetic field. Scanning electron microscope results have proved that the application of a magnetic field can reduce the average crystal branch width and increase its number. The interaction between the induced eddy current generated by the Seebeck effect and the applied external magnetic field produces a Lorentz force, which is important for the increase in the number of crystal branches. Based on the theory of dendrite fragmentation and the magnetic field-induced branches increment, the grain size reduction caused by the magnetic field is studied. Furthermore, the effects of the magnetic field are analyzed by combining a phase field method model and simulations of nucleation and grain growth. The grain distribution and average grain size after welding verify the reliability of the model. In addition, the introduction of a magnetic field can increase the number of periodic three-dimensional solidification patterns. In the intersection of two periods of solidification patterns, the metal can be re-melted and then re-solidified, which prevents the grains, that have been solidified and formed previously, from further growth and generates some small cellular grains in the new fusion line. The magnetic field increases the building frequency of these solidification structures and thus promotes this kind of grain refinement.

Experimental and Numerical Study on Grain Refinement in Electromagnetic Assisted Laser Beam Welding of 5754 Al Alloy

Laser welding in transmission manufacturing opens up completely new kinds of product solutions with excellent properties in terms of wear, corrosion resistance and service life. Current welding designs are characterized in particular by difficult-to-weld material combinations (e.g., steel vs. cast iron) and a high component stiffness for load transfer, that is typically correlated with high residual welding stresses. The major challenge for these welded structural components remains both their crack-free weldability and their complex cyclic load capacity. 
In this frame, the subject of this contribution is the presentation of a simulation-based strategy for process- and load-compatible laser welded transmission components. Specifically, a systematic study is being conducted to understand and qualitatively evaluate effective methods for reducing residual weld stresses in circumferential welds. The recommendations developed as part of this study take particular account of the influence of process modifications, material conditions and geometric aspects on weldability and component distortion. In this context, the residual stresses can be reduced by up to 30 % using suitable modifications. In particular, structural welding simulations are performed and verified by experimental welding trials including metallographic examinations. To ensure the required component fatigue strength, a practical concept for determining Wöhler curves is presented. 
The simulation-based development methods enable the production of&nbsp;load-transmitting components in the fields of e-mobility, aerospace and industrial engineering.&nbsp;In particular, time-consuming and cost-intensive iterations of laser welding tests can be significantly reduced. Furthermore, the systematic investigations provide effective recommendations for phenomenological understanding and solving typical welding challenges in practice.

New Possibilities for Laser Welding of Highly Loaded Transmission Components by Strategic Use of Simulation Methods

Effective monitoring technologies are crucial for optimizing the efficiency of laser processes and manufactured parts. The integration of a monitoring system is especially important in industrial environments where the production of micro- and submicrometer features requires high levels of precision and reproducibility to enhance the surface functionalities. 
In this frame, to ensure the quality of the machined surfaces, various characterization methods such as scanning electron microscopy (SEM), atomic force microscopy (AFM), or confocal microscopy (CM) are commonly used to analyze the micro- and nanostructured surfaces ex situ. However, it is not possible to integrate these characterization methods as real-time monitoring systems. 
A possible solution is to use optical monitoring systems as well as infrared or other camera systems. In addition, technologies based on acoustic emission can also provide various insights into the laser process by analyzing the noise produced by the laser treatment. 
This presentation summarizes different approaches used recently for monitoring laser-based microstructuring processes, including LIPSS and DLIP surface treatments. A diffractive-based approached is used to determine in real-time topographical information (height and period) about produced LIPSS and DLIP features using ps-laser sources. In addition, infrared cameras are also implemented, allowing correlations between the detected signal with topographical features of the produced microstructures. In addition, general information regarding the homogeneity of surface features can be retrieved. Finally, the implementation of acoustical microphones is discussed, allowing to determining relevant information about the process.

Sensing Sound and Light for Monitoring Laser Microfabrication Processes

Ablation using ultrafast lasers is a commonly used method for creating structures and geometries in various materials, ranging in size from a few micrometers to millimeters. The most widely used system architecture involves the combination of a galvanometer scanner with ultrafast lasers that have pulse durations ranging from a few picoseconds down to a few hundred femtoseconds. In principal, almost any known material can be processed using a laser in this pulse duration regime. However, it is crucial to understand how to machine a specific material to achieve the desired quality, particularly when key parameters, such as the initial surface geometry or material composition, are unknown or may change during the machining process. One way to simplify the process planning and minimize the required knowledge is to use a sensor that can directly measure the surface topography. In recent years, the use of optical coherence tomography for laser ablation has gained significant attention in the field of laser material processing. This technique allows for precise measurement of surface topography and can help with a smart adjustment of the processing parameters to improve quality and to reduced processing time. This paper describes recent advancements in the use of optical coherence tomography to control laser ablation when machining difficult to process parts and materials from additive manufacturing or materials like carbon fiber-reinforced polymer. To achieve this, the OCT was superimposed with the processing laser beam, enabling real-time measurement and closed-loop adjustment of laser and scan parameters with developed software and special hardware.

Application of Closed-Loop Controlled Laser Ablation for High Quality Processing

Laser chemical machining (LCM) is a method of laser processing based on a gentle material removal by means of thermal induced chemical dissolution. Since LCM depends predominantly on the surface temperature of the workpiece, the process window is restricted by the appearance of gas bubbles at higher laser powers and their associated shielding effect. In order to extend the process understanding, the influence of the laser power modulation on the removal behavior is investigated in the present work. The experiments were conducted on titanium grade 1 and with phosphoric acid. Based on the response time in experiments with a single step function of the laser power, a threshold special frequency was determined above which a constant removal depth could be expected. To proof this hypothesis, the laser power was modulated rectangularly in time resulting in combination with the process velocity in different spatial frequencies varying from 1 mm-1 to 20 mm-1 . The investigations showed that the removal cavity exhibited sin-shaped variation in depth along the line with a spatial frequency corresponding to the spatial frequency of the laser power. When the spatial frequency exceeded the determined threshold frequency, the cavity depths is constant. This established the basis for generating complex removal profiles by varying the power in the range below the threshold frequency.

Material Removal in Laser Chemical Processing With Modulated Laser Power

Ultrashort pulsed lasers have been proven to foster micro-manufacturing processes such as drilling, cutting, structuring and even welding, respectively. Using ultrashort pulsed laser, these processes are associated to a significantly reduced heat-affected zone, high precision and the possibility to process various materials via single or multi-photon absorption. However, in typical laser machine setups using galvanometer scanners or conventional processing optics, the processing area is limited to a 2 - 2.5 D micromachining with limited workpiece dimension. To expand the potential of ultrashort pulsed lasers for micromachining to a more flexible and large-area 3D micro processing tool, we report for the first time on the design, realization and application of an ultrashort pulsed laser equipped to a 6-axis robot. In addition, we show and discuss a sophisticated beam guidance and focus positioning concept, that are required for such a combination of laser-robot. For the fundamental characterization of the laser robot system, the dynamic laser beam position is evaluated under movement of the robot by CMOS cameras. As a particular result, for the compensation of occurring position deviations of the laser beam during and after the movement of the robot axes, the development of an innovative and high dynamic beam stabilization concept is necessary. In addition, differently pronounced influences of individual axes must be considered on beam positioning. Moreover, first laser cutting and structuring applications are demonstrated to highlight the enormous potential of the laser robot system for a flexible and large 2D and 3D laser micromachining using ultrashort laser pulses.

Realization of an Ultrashort Pulsed Laser Robot System: Characterization, Limits and Application

In laser metal deposition (LMD), the powder is fed into the laser-induced melt pool using different powder nozzles for the purpose of additive manufacturing and the generation of wear and corrosion protection coatings. So far, there are no industrially established in-process monitoring systems for the powder stream, but mainly measuring systems that examine the powder stream propagation offline and without the processing laser. A challenge in implementing an image-based in-process monitoring system is the process illumination, for the segmentation of the powder particles from the background radiation caused by the processing laser and the melt pool. To overcome this challenge, filtering is needed to attenuate the process emissions and simultaneously brighten the powder stream. Therefore, this work focuses on generating a continuous high contrast between the powder and the background. The powder particles are illuminated by a light source mounted laterally to the powder stream in the horizontal plane below the nozzle opening to make the reflecting powder particles visible to the camera. The optical process emissions were characterized during LMD with respect to the influence of an increasing laser power which was presented in correlation to the increasing process emissions. The evaluation of the spectrograms has made it possible, due to the adapted illumination and filtering, to ensure a constantly high contrast between the process emissions and the powder, so that online monitoring of the powder stream was implemented successfully during the LMD process despite the active processing laser.

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Fluid-Immersion Laser Micromachining Using Ionic Liquids

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Laser Doping of N-Type 4H-SiC With Boron Using Solution Precursor for MWIR Optical Properties