United States

We discuss laser micromachining as a means of fabricating dielectric-based electrospray thrusters that use ionic liquid propellants for nanosatellite propulsion applications. Because of their small size, nanosats typically lack means of propulsion, which limits their lifetimes and mission scopes and is problematic for deorbit at end of life. Electrospray thrusters, however, can be very compact, using electric potentials to accelerate and eject ionic matter, thereby achieving high specific impulse. Traditional fabrication methods involve numerous steps, including lithography and/or chemical etching, resulting in microscale emitter structures that protrude from a substrate and can easily be damaged during assembly and/or launch. This format also requires a separate, detached grid electrode that serves as an extraction grid needed to induce ionic emission of propellant from the emitters. Our fabrication approach uses Bessel-beam laser micromachining to form embedded, micron-diameter capillaries up to several hundred microns long through a transparent glass substrate, without chemical etching. One surface of the glass is coated with a transparent conductive layer that serves as an integrated extraction electrode and which needs no alignment. Further laser machining with Gaussian-beam, water-assisted cavitation is used to excavate cylindrical electrospray cavities in this surface around each capillary, to catch errant propellant and prevent shorting while optimizing the capillary emitter-extractor separation distance. The resulting array of capillary emitters and cavities with integrated extractor comprises an electrospray emitter chip that forms the core of an electrospray thruster, and provides the potential to achieve significantly lower onset voltages than traditional electrospray thruster designs.

ICALEO 2023

Laser Microfabrication for Ionic Propulsion Systems

electrospray thruster

laser micromachining

beam shaping

ionic liquid

technical paper

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
We were excited to host the world’s leading international conference on Lasers and Electro-Optics in Chicago. The Laser has been shown to have the potential to revolutionize solutions for major global challenges like energy usage and supply, global warming, and CO2 footprint. Even the fusion energy breakthrough is based on laser technology. The conference, with peer-reviewed scientific and technical presentations, focused on the newest results in Micro Applications, Macro Applications, Additive Manufacturing, Beam Shaping and Frontiers in Laser Applications. Additionally, the Battery-Manufacturing Track gave an overview of the process combined with the newest advances in laser application results. 

The Business Session brought the business aspect and environment of the laser to the audience and the possibility to discuss with seasoned industry leaders from around the world. 

Whether this is the first time you are hearing about ICALEO or you are a longtime attendee, you will find something of interest, new ideas, and an excellent opportunity to network with other laser specialists worldwide. 

**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

**What is ICALEO?**

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.

Participants will be immersed in a dynamic environment that fosters collaborative learning, networking, and idea exchange. Through engaging plenary presentations, technical sessions, an industry business panel, and networking events, attendees gain insights into the forefront of laser technology. From industrial applications to medical breakthroughs, additive manufacturing to materials processing, ICALEO covers the entire spectrum of laser-related disciplines. Moreover, the event serves as a launching pad for synergistic partnerships, paving the way for professionals to forge connections that shape the future of our industry.

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Industrial leaders seeking to stay ahead of the curve in their fields, engineers working on laser applications, and manufacturing experts interested in additive manufacturing and materials processing will find ICALEO to be an invaluable platform. Researchers and scientists exploring the forefront of laser technology, from fundamental principles to groundbreaking applications, will also discover a wealth of knowledge and networking opportunities.

Medical professionals investigating laser-based medical advancements, entrepreneurs seeking to innovate with lasers, educators aiming to stay updated with the latest in laser education, and policy makers shaping the industry's landscape are all essential attendees. Whether you're an established expert or a rising star, the 42nd annual ICALEO promises to offer a diverse ecosystem of expertise, connections, and knowledge sharing that caters to the laser and electro-optics communities.

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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.

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.

Fluid-Immersion Laser Micromachining Using Ionic Liquids

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.

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Laser Microfabrication for Ionic Propulsion Systems

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Fluid-Immersion Laser Micromachining Using Ionic Liquids