[ Paper review ]

13.  Improving Quality and Machining Efficiency of Hole During AlN Trepanning with Nanosecond Pulse Laser 

Lingzhi Wang, Wanqin Zhao, Xuesong Mei, Zixuan Yang, Xiaowei Shen, Haodong Liu 

This study investigated how different trepanning parameters affect the quality and machining efficiency of holes produced in AlN ceramic using a nanosecond laser. A 355 nm laser with a pulse duration of 10–23 ns was used to process 0.38 mm thick AlN substrates, and the effects of laser beam jump direction, scanning mode, number of scans, and filling circle interval were examined. The jump direction had little effect on the entry and exit hole diameters, but it strongly influenced the hole-wall morphology. The inside-to-outside direction produced smoother hole walls, whereas the outside-to-inside direction resulted in more debris and transverse structures along the wall. The scanning mode also showed a clear effect on hole quality. Cyclical scanning produced a larger exit diameter and a smaller hole taper than sequential scanning, while sequential scanning caused local wall detachment and cracking due to repeated melting and solidification of the material. The authors also found that the filling circle interval controlled both the energy overlap and the required machining time. As the interval decreased, the laser overlap increased and the through-hole could be formed with fewer scans, although an excessively small interval caused more material accumulation and deformation at the hole bottom. Considering both machining quality and efficiency, the best range of the filling interval-to-beam waist ratio, S/2ω₀, was found to be between 2/9 and 1/3. Within this range, relatively smooth hole walls and clean entry and exit surfaces could be obtained while maintaining a short machining time. Overall, the study shows that trepanning path design and scanning strategy are important parameters for improving the quality and efficiency of nanosecond laser drilling of AlN ceramics. 

12.  Optimization of Trepanning Patterns for Holes Ablated Using Nanosecond Pulse Laser in Al₂O₃ Ceramics Substrate 

Wanqin Zhao, Xuesong Mei 

This study investigated the effect of different trepanning strategies on the quality and processing efficiency of laser-drilled holes in Al₂O₃ ceramic substrates. Two scanning patterns, filled spiral trepanning and multiple rings trepanning, were compared using a 355 nm nanosecond laser with a pulse duration of 11 ns. Al₂O₃ substrates with thicknesses ranging from 0.12 to 0.5 mm were processed with target hole diameters between 100 and 1500 μm. Under the optimized conditions of 120 μJ pulse energy, 50 kHz repetition rate, and 300 mm/s scanning speed, both methods produced holes with little spatter, cracking, or heat-affected zone, while the recast layer was limited to about 2–3 μm. The hole taper decreased as the processing time increased and eventually reached a saturated value, while the outlet diameter and circularity gradually increased before reaching saturation. For a 0.38 mm thick substrate, multiple rings trepanning produced a slightly smaller saturated taper of 4.338°, compared with 4.716° for filled spiral trepanning, and also showed better outlet circularity. The difference mainly came from the material removal mechanism: filled spiral trepanning removes material across the entire hole area, whereas multiple rings trepanning mainly cuts around the hole boundary and allows the central material to separate after the ring-shaped region is removed. Multiple rings trepanning was particularly effective for relatively large hole diameters, providing both lower taper and shorter processing time. Depending on the substrate thickness and hole diameter, the improvement in processing efficiency ranged from about 32% to 157% compared with filled spiral trepanning. Overall, the results show that selecting an appropriate trepanning pattern according to the required hole size and substrate thickness can improve both hole quality and machining efficiency in nanosecond laser processing of Al₂O₃ ceramics. 

11.  Femtosecond laser drilling of alumina ceramic substrates

X.C. Wang, H.Y. Zheng, P.L. Chu, J.L. Tan, K.M. Teh, T. Liu, Bryden C.Y. Ang, G.H. Tay 

This study investigated femtosecond laser drilling of alumina ceramic substrates to produce high-quality microholes for electronic applications. Alumina substrates with thicknesses of 250, 381, and 625 μm were processed using 150 fs laser pulses at a wavelength of 775 nm and a repetition rate of 1 kHz. The effects of focal position, traverse speed, drilling pattern, and pausing time on hole geometry and machining defects were examined. For the 381 μm-thick substrate, positioning the focus approximately 0.35 mm below the surface minimized the hole taper to approximately 2.5°. Higher traverse speeds suppressed entrance cracking but increased exit ellipticity, whereas combining high-speed and low-speed passes improved exit circularity without inducing entrance cracks. For holes larger than 250 μm in the 625 μm-thick substrate, a multiple-circle drilling pattern enabled penetration where single-circle trepanning failed. Introducing 30 s pauses between scanning sequences further reduced heat accumulation and thermal cracking. Under optimized conditions, holes exhibited clean surfaces without observable cracks or recast layers, and Ti/Au-coated substrates were drilled without coating delamination. X-ray micro-computed tomography further confirmed the absence of detectable cracks. The results demonstrate that controlling energy deposition and heat dissipation is essential for high-quality femtosecond laser drilling of alumina substrates. 

10.  High Throughput Manufacturing of Micro-Structured Electrodes by Femtosecond Laser in MHz and GHz Burst Regimes  

Aurélien Sikora, Girolamo Mincuzzi, Marc Faucon, Laura Gemini 

This study investigated the use of MHz and GHz burst-mode femtosecond lasers to improve the processing speed of lithium-ion battery electrode structuring. High-nickel NMC cathodes (NMC84, NMC811) and Si-graphite composite anodes were processed using 360 fs laser pulses with burst lengths ranging from 32 to 3552 sub-pulses. The results showed that burst processing significantly increased drilling and grooving efficiency compared with single-pulse machining, while maintaining comparable machining quality under optimized conditions. For drilling, the maximum processing speed improvements reached approximately 4× and 5× for NMC84 in MHz and GHz burst modes, respectively. Si-graphite electrodes exhibited speed improvements up to 2.3× (MHz) and 3× (GHz), while NMC811 showed gains of approximately 2.5–2.7×. The enhancement strongly depended on burst duration and material type rather than solely on the intra-burst repetition rate. Under optimized burst conditions, SEM observations confirmed that machining quality remained comparable to conventional single-pulse processing. The study demonstrates that burst-mode femtosecond laser processing is a promising strategy for achieving industrial-scale, high-throughput electrode structuring while preserving electrode integrity and electrochemical functionality. 

9.  Analysis of the Hole Shape Evolution in fs-Pulse Percussion Drilling with Bursts  

H. Kämmer, F. Dreisow, A. Tünnermann, Stefan Nolte 

This study investigated the influence of femtosecond burst-mode processing on deep-hole percussion drilling. Silicon was used as a model material and drilled using 200 fs laser pulses at 1030 nm, where each burst consisted of multiple sub-pulses separated by delays ranging from 1 ps to 4 ns. The results showed that burst parameters strongly affected drilling depth, hole shape, quality, and reproducibility. Short pulse separations (1–8 ps) produced deeper holes and larger ablated volumes than conventional single-pulse drilling, but often resulted in poor reproducibility and increased hole deformation such as branching and bending. In contrast, longer pulse separations (510 ps–4 ns) significantly improved hole quality and reproducibility while maintaining equal or greater drilling depths compared with conventional processing. The authors concluded that burst-mode femtosecond drilling can enhance deep-hole formation, but the pulse-to-pulse separation time is a critical parameter governing the balance between machining efficiency and hole quality. 

8.  Improving Quality and Machining Efficiency of Hole During AlN Trepanning with Nanosecond Pulse Laser  

Lingzhi Wang, Wanqin Zhao, Xuesong Mei, Zixuan Yang, Xiaowei Shen, Haodong Liu 

This study systematically investigated the effects of trepanning parameters on hole quality and machining efficiency in AlN ceramics using a 355 nm nanosecond pulse laser. The influence of laser beam jump direction, scanning mode, and filling circle interval on hole dimensions, wall morphology, taper, and machining time was analyzed. The results showed that the inside/out jump direction produced smoother hole walls than the outside/in direction by reducing debris accumulation along the sidewall. Cyclical scanning (CS) generated higher-quality holes with less wall damage and larger exit diameters compared with sequential scanning (SS), which often caused oxide layer detachment and wall defects. The filling circle interval strongly affected both machining efficiency and hole geometry; smaller intervals increased laser energy overlap and enlarged hole diameters, whereas excessively small intervals reduced circularity. The optimum condition was obtained when the ratio between filling circle interval and laser beam diameter satisfied 2/9 < S/2ω₀ < 1/3, providing a balance between machining efficiency and hole quality. The study concluded that appropriate optimization of trepanning parameters can significantly improve hole wall quality, reduce defects, and enhance drilling efficiency in AlN ceramic substrates.  

7.  Ablation Characteristics of Aluminum Oxide and Nitride Ceramics During Femtosecond Laser Micromachining  

Sung Hoon Kim, Ik-Bu Sohn, Sungho Jeong

This study compared the femtosecond laser ablation characteristics of Al₂O₃ and AlN ceramics using a Ti:sapphire laser (785 nm, 184 fs, 1 kHz). The single-pulse ablation thresholds were found to be similar, with values of 5.62 J/cm² for Al₂O₃ and 5.90 J/cm² for AlN. Both materials exhibited a strong incubation effect during multi-pulse irradiation, although the effect was more pronounced in AlN. While the crater diameter, depth, and ablation rate showed similar trends for both ceramics, their surface morphologies differed significantly. Al₂O₃ primarily exhibited cracking and exfoliation with little evidence of melting, whereas AlN showed clear melting and resolidification along the crater walls. In the high-fluence region, AlN demonstrated a steeper increase in ablation rate than Al₂O₃, indicating more efficient material removal. XPS analysis further confirmed that the chemical composition of both ceramics remained essentially unchanged after femtosecond laser processing. These results suggest that material properties such as thermal conductivity and thermal expansion coefficient strongly influence the ablation mechanism and resulting surface morphology during femtosecond laser drilling. 

5.  Experimental and Theoretical Investigation of the Drilling of Alumina Ceramic using Nd:YAG Pulsed Laser  

M.M. Hanon, E. Akman, B. Genc Oztoprak, M. Gunes, Z.A. Taha, K.I. Hajim, E. Kacar, O. Gundogdu, A. Demir 

This study investigated laser drilling of 5 mm and 10.5 mm thick alumina ceramics using a millisecond pulsed Nd:YAG laser and compared experimental results with ANSYS Fluent simulations. The results showed that increasing peak power and pulse duration increased both hole diameter and penetration depth, while increasing the number of pulses improved drilling depth but also promoted recast layer formation and resolidified material around the hole entrance. The focal plane position strongly affected hole geometry and circularity, with a focus position located below the surface producing more favorable hole characteristics. Higher repetition rates reduced penetration depth and increased spatter formation due to plasma shielding and insufficient melt expulsion. Microstructural analysis revealed the formation of a thin resolidified layer, recast layer, and thermal cracks along the hole wall caused by cooling stresses. The study concluded that precise control of laser peak power, pulse duration, repetition rate, and focal position is essential for achieving high-quality ceramic drilling while minimizing thermal damage and recast layer formation.  

4.  Millisecond Laser Oblique Hole Processing of Alumina Ceramics  

Yuyang Chen, Xianshi Jia, Zhou Li, Chuan Guo, Ranfei Guo, Kai Li, Cong Wang, Wenda Cui, Changqing Song, Kai Han, Ji’an Duan 

This study investigated the formation mechanism and processing characteristics of oblique holes in alumina ceramics using a millisecond fiber laser. Numerical simulations, high-speed shadow imaging, and drilling experiments were combined to analyze temperature evolution, molten material behavior, and hole formation during laser processing. The results showed that material removal occurs through three stages: preheating, intense ablation, and stable ablation. Increasing single-pulse energy improved material removal efficiency and promoted deeper hole formation, while excessive energy caused recast layer formation and hole closure. Repetition rate, laser power, ablation time, and duty cycle significantly affected hole diameter, taper, and drilling efficiency. Lower repetition rates provided higher material removal rates because of longer intense ablation periods, whereas higher repetition rates reduced thermal effects but lowered drilling efficiency. The study concluded that optimizing pulse energy and repetition rate is essential for achieving high-speed drilling with reduced taper and improved hole quality in alumina ceramic substrates. 

3.  Rapid micro hole laser drilling in ceramic substrates using single mode fiber laser  

B. Adelmann, R. Hellmann 

This study investigated rapid micro-hole drilling of alumina (Al₂O₃) and aluminum nitride (AlN) substrates using a 500 W single-mode fiber laser. The results showed that the laser focus position had the greatest influence on hole diameter, enabling the fabrication of sub-50 μm holes with very low taper. For 0.63 mm-thick substrates, hole diameters of approximately 50 μm were achieved with a taper angle of only 1.57° and good circularity (≤1.07). Drilling times were extremely short, reaching 0.1 ms for 0.25 mm-thick alumina, while AlN required longer processing times due to its higher thermal conductivity. The study also demonstrated that splatter generated during drilling could be effectively removed by applying a water-soluble pigment before processing and subsequently cleaning the substrate in an ultrasonic bath. Overall, the work showed that single-mode fiber lasers can provide high-speed, high-quality micro-hole drilling in ceramic substrates suitable for electronic packaging applications. 

2.  A review on laser drilling optimization technique: parameters, methods, and physical-field assistance 

Tao Wei, Shufeng Sun, Fengyun Zhang, Xi Wang, Pingping Wang, Xunhuan Liu, Qinyang Wang 

This review summarizes optimization strategies for laser drilling, focusing on laser parameters, processing methods, and physical-field-assisted techniques. The paper explains that micro-hole quality is mainly evaluated by taper, roundness, heat-affected zone, recast layer, micro-cracks, spatter, and processing efficiency. Laser wavelength, pulse width, power, repetition rate, and defocus strongly affect hole geometry and thermal damage. In particular, ultrashort pulse lasers can reduce HAZ, recast layers, and cracks, but they often show lower processing efficiency. The review also shows that optimized scanning paths and secondary repair processes can improve hole shape and reduce heat accumulation. In addition, ultrasonic and magnetic-field-assisted laser drilling can enhance molten material removal, reduce plasma shielding, improve sidewall quality, and increase drilling efficiency. Overall, this paper suggests that multi-physics-assisted laser drilling is an effective direction for producing high-quality micro-holes in advanced manufacturing. 

1. Deep ultraviolet excimer laser processing for the micro via hole on semiconductor package

Yasufumi Kawasuji, Junichi Fujimoto, Masakazu Kobayashi, Akira Suwa, Akira Mizutani, Masaki Arakawa, Takashi Onose, and Hakaru Mizoguchi

Moore’s law has almost reached the limit of resolution on semiconductor die and, therefore, multidie packaging is one of the alternative solutions. Substrate materials currently use organic build-up films and silicon substrates [through silicon via (TSV)] in applications. But recently, organic films, too, have reached the resolution limit, and TSV is expensive. In this situation, nonalkali glass (glass) and fused silica (SiO2) substrates are expected to be good alternatives in high-frequency signal transfer applications like 5G telecommunication. But the via holes are hard to process with less defects (tips and cracks) on the glass and SiO2 substrates. Deep ultraviolet (DUV) excimer laser ablation is expected to have a finer (<10 μm) resolution with a shorter wavelength (248–193 nm) and also hard material processing with a higher photon energy (5–6.4 eV). Therefore, the authors have explored the application of the DUV excimer laser ablation process for a via hole on hard materials like glass and SiO2 substrates. In this study, they have investigated the via hole quality through the DUV excimer laser ablation process. The results show the possibilities of micromachining on both glass and SiO2 substrates. The authors have succeeded by achieving a value of <50 μm through the via hole grid aspect ratio of 6 on the glass substrate without any significant defects. As the ablation rate is quite an affordable value, DUV excimer lasers are expected to play a crucial role in the next-generation manufacturing process for semiconductor packages. The authors also investigate the SiO2 substrate with DUV excimer lasers.