United States

There has been strong interest in laser direct lithography(LDL) due to flexibility of patterning based on software. However, the conventional LDL has a problem that is too slow compared to mask lithography because it depends on the scanning velocity of laser beam or stages, which causes a decrease in productivity, making it difficult to apply to manufacturing. To overcome these drawbacks, we implemented a resonant mirror based LDL, which is different from the conventional laser scanning method based on a stepper motor. The laser source of the instrument is a laser diode(LD) which has 405 nm wavelength, and the optics are designed to have a beam spot size of less than 20um in the focal plane. We applied this feature to laser scan system and have set 800Hz frequency. It can create a pattern in 1.25us for a line of about 10 mm. Compared to the conventional LDL system using the laser scanning method, the resonant mirror based LDL system has the advantages of being over 80,000 times faster in processing time. The designed LDL system capable of high-speed scanning with uniform physical characteristics is expected to meet the demand for small quantity production of diverse items as well as low operation costs.

ICALEO 2023

Resonant Mirror Based Laser Direct Lithography

resonant mirror

laser direct lithography

poster

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
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**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

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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.

Laser welding using high-power laser forms a keyhole inside a molten pool to obtain a deep penetration. However, a spatter which is the scattering of a portion of the molten pool occurs causing welding defects. Therefore, it is necessary to elucidate the mechanism of spatter generation. In our previous study, it has been found the spatter generation is due to the variability of the keyhole. However, it is difficult to understand the spatter generation from keyhole observation only. In addition to the keyhole variability, real-time observation of molten pool dynamics would be understood the spatter generation mechanism. 
In this study, the longitudinal cross-section of the molten pool was directly observed using the glass transmission method to measure the temporal variation of the molten pool area in keyhole welding. A bead-on-plate welding test was conducted for the specimen with a 16 kW disk laser when the SS304 specimen butted against a glass plate was set in a vacuum chamber. The vacuum chamber was evacuated at the pressure of 10 Pa and then filled with Ar gas to the desired pressure. As the results, the average molten pool area was 17 mm2&nbsp;at 105 Pa, which generate a large amount of spatter, while, it was 29 mm2 at 10 Pa, where the spatter generation was small. Furthermore, the standard deviation of the molten pool area was 0.9 mm2 at 105&nbsp;Pa, and 0.36 mm2 at 10 Pa, it was indicated a large molten pool time variation was influenced to generate the spatter.

Elucidation of Spatter Generation Mechanism in Keyhole Welding of Stainless Steel With 16 kW Disk Laser by In-Situ Observation of Welding Dynamics

Besides applications like layering functional surfaces, laser cladding can be used for additive manufacturing and repair welding purposes called directed energy deposition (DED). DED is a more and more used technology in industry and research today. It fills the gap between high-resolution powder bed selective laser melting and high output wired arc additive manufacturing processes. 
The temperature-time-regime is important to be managed in the whole process. With increasing melt pool sizes according to heat accumulation, distortions and residual stresses get out of control. Hence, the part could be cooled between each layer. The temperature could be measured in order to derive the cooling time. However, if the part geometry is known and repeated in an industrial process, the cooling time will always remain the same. Modelling the process with a Finite Element Analysis (FEA) can also help to predict the correct cooling time. In order to achieve suitable results of the FEA Model, the Model has to be calibrated. Moreover, calculation times of FEA can be high according to the complexity and dimensions. 
In powder based additive manufacturing process FEA it is quite common to work with replacement heat sources. These Approaches cannot be transferred to DED because of the process conditions. However, some simplification in the model might lead to sufficient results. In this study we have investigated a process with 15 thermocouples and compared the results with a FEA model. The influence of the process efficiency, heat radiation and thermo-physical-properties in the FEA result is shown.

Finite Element Analysis and Calibration of a Directed Energy Deposition Process by Thermocouples

Cobalt based tungsten carbide composite material [WC-Co] is industrial applied for coating on tip of cutting tools and molds due to its high hardness and wear resistance. However, it is difficult to coat composite materials while maintaining the properties of the powder because of the different melting points of the metals. Thus, we pay attention to a Laser metal deposition (LMD) method. The LMD is one of additive manufacturing technologies in which the material powder is fed, and laser irradiated simultaneously at the processing point to melt and solidify the material powder to form layers, which can add new functions to material surface of any shape. However, conventional LMD has some problems to heat the powder ununiformly in flight, which a molten pool is required on the substrate to weld with powder and substrate, it was caused significantly thermal distortion and dilution. Thus, we have developed a multi-beam LMD system with two high intensity 200W blue diode lasers. The multi beam LMD can heat uniformly the powder in flight resulting in lower thermal distortion and dilution. Especially, using a blue laser with a shorter wavelength than the IR laser used in conventional LMD can improve the light absorption rate for WC-Co power and the steel substrate, to save an energy compared with the conventional LMD. In this study, we try to form a WC-Co coating on a steel substrate surface using the multi-beam LMD system equipped with two blue diode lasers.

Experimental Evaluation of WC-Co Alloy Layer Formation Process by Multi Beam Type Laser Metal Deposition With Blue Diode Lasers

Controlling the wetting characteristics of fluids (oil and water) on surfaces is highly desirable for a variety of industrial processes, such as painting, bonding, and lubrication applications. This can be achieved by altering the surface chemistry and/or topography of a material, which can be accomplished through a variety of techniques, including plasma treatment, chemical modification, surface coatings as well as microprocessing methods such as direct laser writing and direct laser interference patterning. 
This study summarizes recent developments in the field of functionalization of polymer surfaces, including polyethylene terephthalate (PET), polycarbonate (PC) and polytetrafluoroethylene (PTFE) using hot-embossing methods as well as direct laser fabrication approaches. 
The influence of different hot embossing parameters on the resulting texture topography was investigated. In case of the oil wettability tests (on PET), a method to characterize its spreading behavior was developed, allowing a comparison between the produced surface textures. In particular, a superoleophilicity state was reached, in which the contact angle approached zero. Furthermore, hierarchical structured surfaces showed fast wetting rates up to 1.4 mm²/s. 
Regarding water-wettability, the microstructured PET surfaces showed an increased hydrophobic state rising from 76° on the flat reference up to 120°. In case of PC, similar results are obtained, reaching 140°, whereas on PTFE surfaces a super-hydrophobic state was achieved, characterized by water contact angles over 150° and a very small hysteresis (&lt; 5°).

Water and Oil Wettability Customization by Tailoring Micro-Structured Polymers

Globally, the supply of electric vehicles is rapidly increasing due to the regulation of CO2 emission. Laser welding is essential for core parts of electric vehicles, and welding of copper (Cu) material accounts for a large proportion. Unstable welding quality can cause fire and threaten the life of the operator. Quality is urgently needed. In-depth research on Cu sheet laser welding is needed. Laser welding for electric vehicle battery uses a lot of copper. However, IR laser is difficult to achieve stable melting and quality control due to the high reflectivity of copper, and the beam penetrates the battery, causing cell damage and fire. In this study, core parts such as cylindrical batteries of electric vehicles are welded using a green laser for stable fusion and quality. In this study, a 2 kW green laser welding system and a 6-axis industrial robot were used. Green laser welding process technology is applied using nickel-coated 0.5 mm copper on the top and 0.5 mm mild steel on the bottom. Optical microscope analysis was used to examine the penetration after green laser welding.&nbsp; The laser welding microstructural modification was investigated using scanning electron microscope(SEM) and energy disperse X-ray spectrometer(EDS). After green laser welding, the result is measured using shear stress test. Moerever, Using Laser welding monitoring system(LWM) the causes of welding defects were identified and the process conditions for welding quality deterioration were matched.

A Study on Green Laser Welding Characteristics of Core Materials for Electric Vehicles

Enhancing the durability of molds, jigs, and tools is crucial for the industry, and one approach to achieve this is by forming a metallic layer with high hardness on their surfaces. Metallic layers with high hardness can be formed through laser metal deposition (LMD), which is one of the additive manufacturing processes, using cemented carbide powder. However, crack initiation typically occurs inside cemented carbide layers formed by the LMD. Therefore, achieving a cladding process for cemented carbide layers without cracks is desired for practical applications. In this study, effects of WC ratios in WC-Co cemented carbide granulated powder on formed bead size and crack initiation during the LMD processing were investigated. The number of cracks generated during the LMD processing was evaluated using an acoustic emission (AE) technique. The number of burst-type AE signals generated was counted as the number of cracks. Seven types of WC-Co cemented carbide granulated powders with WC ratios ranging from 30.5mass% to 92mass% were prepared. Beads were formed using each powder through the LMD, with AE signals being measured. In the case of a WC ratio of 42.9mass% or less, no crack was observed. On the other hand, cracks were observed when the WC ratio was 53.9mass% or greater, and the number of cracks increased with an increase in the WC ratio.

Effects of WC Ratios on Bead Size and Crack Initiation in Forming WC-Co Cemented Carbide by Laser Metal Deposition

Inconel has been used in many industrial components such as a blade of gas turbine because of having its high creep strength and high corrosion resistance. The gas turbine for next generation requires more complex shape to improve the performance. A selective laser melting (SLM) method is one of additive manufacturing (AM) technologies, which is suitable for making metal objects with complex shape rapidly. The AM for metals is a fabrication method that enables direct modeling of three-dimensional (3D) products from a metal powder based on 3D-CAD data. The AM method using a laser as a heat source is classified as selective laser melting (SLM) technology. In SLM, the powder absorbs the laser energy to heat. &nbsp;Then the powder melts and solidifies to form a single layer. Generally, IR lasers with a wavelength of around 1 um are employed in SLM process. Recently, a high-power blue diode laser has been developed.&nbsp; Blue and IR lasers have different light absorption rates for Inconel, which may affect samples fabricated with SLM. 
&nbsp;In this study, 3D fabrication of Inconel was demonstrated by the blue and IR laser, respectively to compare with the process and mechanical property. As a result, high efficiency and fine object was obtained with blue laser.

Comparison with Blue and IR Laser for Inconel Fabrication in Selective Laser Melting

This study reports a laser annealing technique to activate dopants in Si for semiconductor application. In this study, a CW laser of 532 nm was used to anneal the phosphorus (P) doped Si wafer, and the effects of laser power and scan speed on electrical, microstructural and chemical characteristics of the P-doped Si were investigated. It was found that the amorphized Si during the dopant implantation process was successfully recrystallized to either polycrystalline Si or single crystal Si by laser annealing. The sheet resistance decreased with increasing laser power or decreasing scan speed due to the improved crystallinty of Si. The full epitaxial growth of Si was achieved by inducing the complete melting of the amorphized Si, and under the this condition, a minimum sheet resistance was obtained. According to the dopant concentration analysis, it was found that dopant diffusion to the depth direction was very limited. The redistribution of dopants was only generated near the shallow surface region. As a result, relatively uniform concentration of the dopant was achieved, which is advantageous for the formation of the sallow junction. The results of this study show that the potential benefits of the laser annealing technique for improving the properties of semiconductor materials. Further research to explore various characteristics of the laser annealed Si is in progress.

Dopant Activation Annealing of Si Using CW Laser of 532 NM

The processability of pure Inconel X750 and Inconel X750 mixed with 15% vol. of titanium carbide particulate through laser Directed Energy Deposition (l-DED) was evaluated. The powders used had a particle size in a range unusual to l-DED processing (0.18 µm to 24.05 µm), this case study presents the difficulties in processing a thin quadri-modal powder and describes possible measures to mitigate them, while also reporting, likely for the first time, on the l-DED processing of Inconel X750 and such related MMC. The choice in reinforcement particle size and composition aimed for a reduction in material density and insertion of additional reinforcement mechanisms.&nbsp; Both powders used were analysed in an FT4 rheometer and compared to a reference Inconel 625 powder. l-DED was made viable, but results show that the powders tested here represent a lower limit for the rheological properties accepted by usual l-DED systems. A methodology to quantify the stability of a given processing condition is presented and validated, indicating also that low powder flows are recommended when processing powders of this sort. Inconel X750 demonstrated sensibility to oxidation during processing as depletion of Nb and Ti was detected in the deposits. Nor the MMC, nor the pure material cracked or showed excessive porosity. Microstructural characterization of non-heat-treated materials also showed microsegregation of Nb and Ti and a slight degradation of the added particulate.

Processability of Thin-Powdered Inconel X750 and TiC Metal Matrix Composite by Laser-Directed Energy Deposition

Dicing is one of the main processes during the packaging of semiconductors and the fabrication of electronic devices. As the design of semiconductor chips and electronic devices has become more complex over the years, conventional fabrication methods like saw-dicing present limitations regarding precision, flexibility, and quality. In this study, we present an ultrafast laser based dicing technique for cutting epoxy molding compound with fused silica filler of two different sizes of 32 microns and 55 microns. A three-dimensional two-temperature model (3D-TTM) was used to explore a wide range of process parameters of the dicing process with validation from the experiments using a femtosecond laser. The effect of the process parameters on the efficiency and ablation profile was systematically studied. The effect of the fused silica filler size on the heat transfer and ablation behavior was also discussed in detail. One of the key novelties of the presented work is that the 3D-TTM can be used to find the optimal cutting conditions beyond the capability of the experimental setup. In this study, the 3D-TTM was also used to predict the optimal parameters for a picosecond laser of a different beam wavelength. The study shows that finding the optimal parameters using the 3D-TTM is more effective and lower cost than experimental methods. The study also illustrates the unique advantage of the ultrafast laser based dicing method, making it a technique with huge potential in the fabrication of next-generation semiconductor chips and electronic devices.

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Resonant Mirror Based Laser Direct Lithography

Next from ICALEO 2023

Elucidation of Spatter Generation Mechanism in Keyhole Welding of Stainless Steel With 16 kW Disk Laser by In-Situ Observation of Welding Dynamics