Crack-free High-Aspect-Ratio Micro-holes in Glass Via Top-down Percussion Drilling with Infrared Femtosecond Laser GHz-Bursts

01 Introduction

Femtosecond laser GHz-burst processing has shown significant advantages in efficient ablation of metals and semiconductors, but research on dielectric materials like glass remains limited. Traditional laser drilling of glass is prone to cracks, large heat-affected zones (HAZ), and limited aspect ratios. GHz-bursts can combine nonlinear absorption with thermal accumulation effects, offering new possibilities for crack-free high-aspect-ratio glass micro-drilling. This study investigates top-down percussion drilling in soda-lime glass and fused silica using an infrared femtosecond laser with GHz-bursts to overcome conventional processing bottlenecks.

02 Overview

In this study, a 1030 nm femtosecond laser was used to construct GHz-bursts (50 pulses per burst, 1 GHz intra-burst repetition rate). An experimental platform equipped with a side-imaging system was established to systematically study the effects of burst count and energy fluence on the depth, diameter, aspect ratio, and sidewall morphology of holes in soda-lime glass and fused silica. The three-stage physical mechanism of glass micro-hole formation was elucidated. Ultimately, crack-free micro-holes with aspect ratios exceeding 30 in soda-lime glass and 73 in fused silica were achieved. The core reasons for the quality differences between the two types of glass were identified, providing an experimental basis for high-efficiency, crack-free fabrication of glass micro-devices.

03 Figure Analysis

Figure 1 shows the experimental setup for glass drilling with femtosecond laser GHz-bursts. The core is a Ytterbium-doped femtosecond laser system, equipped with a microscopic objective to focus the beam and an XYZ motorized stage to control sample and focal positions. It integrates both top-monitoring and side-imaging systems for real-time observation of the drilling process. The granite base ensures stability, providing hardware support for precise parameter tuning and in-situ observation.

Figure 1. Schematic of the experimental setup with a side-imaging system.

Figure 2 displays microscopic images of holes in soda-lime glass and fused silica at different drilling times. Under fixed inter-burst frequency, pulse count, and fluence, the holes in both materials are cylindrical with depths increasing linearly with time, showing no significant HAZ. Fused silica requires higher fluence due to its larger bandgap. This confirms that GHz-bursts can achieve uniform, low-thermal-damage micro-drilling in both glass types.

Figure 2. Microscopic images of holes at 1 kHz inter-burst frequency (50 pulses per burst at 1 GHz): (a) soda-lime glass at 52 J cm⁻² and (b) fused silica at 136 J cm⁻² for drilling times of 20–100 ms.

Figure 3 presents the curves of hole depth versus burst count at different fluences. For both soda-lime glass and fused silica, the depth initially rises rapidly and then reaches saturation, with the saturation depth increasing at higher fluences. It also confirms that GHz-bursts enable effective drilling below the single-pulse ablation threshold, demonstrating the thermal accumulation characteristics of burst processing.

Figure 3. Hole depth versus burst count for fluences of 136–339 J cm⁻² in (a) soda-lime glass and (b) fused silica.

Figure 4 illustrates the three-stage mechanism of glass micro-hole formation. Stage 1 is surface ablation, where the ablation plume diffuses freely with a high drilling rate. Stage 2 is deep-hole ablation, where the plume is confined by the sidewalls, leading to reduced efficiency and a lower drilling rate. Stage 3 is drilling termination, where the energy at the hole bottom falls below the ablation threshold, and depth stops increasing. This mechanism explains the drilling dynamics and depth limits.

Figure 4. Three-stage schematic of micro-hole formation: (1) surface ablation with free plume expansion; (2) deep ablation with sidewall confinement; (3) drilling termination when bottom fluence falls below threshold.

Figure 5 compares the sidewall quality of soda-lime glass and fused silica. Soda-lime glass shows structured features such as bumps, ripples, and grooves, while fused silica maintains smooth sidewalls throughout. This difference stems from the distinct thermal and mechanical properties of the two materials. The smooth sidewalls in fused silica reduce laser reflection losses, enabling higher aspect ratios.

Figure 5. (a) Sidewall evolution in soda-lime glass at 270 J cm⁻² (800/900/1000 ms); (b)(c) comparison of (b) soda-lime glass and (c) fused silica at 7 seconds, showing microscopic images and enlarged entry views.

04 Conclusion

This study successfully utilized infrared femtosecond laser GHz-bursts for top-down percussion drilling in soda-lime glass and fused silica, achieving crack-free, high-aspect-ratio results (up to 37 in soda-lime glass and 73 in fused silica). The research reveals a three-stage dynamic mechanism for micro-hole formation and clarifies the relationship between material properties, sidewall morphology, and aspect ratio. This method requires no complex post-processing and eliminates thermal damage and cracks, providing a new, high-quality technical solution for glass micro-via fabrication in photonic and microelectronic systems.

Original link: https://doi.org/10.1088/2631-7990/acaa14