The pursuit of miniaturization and enhanced performance in microelectronics is placing unprecedented demand on the next generation of manufacturing technologies. As devices shrink and complexity grows, reliable, high-density interconnections are increasingly important to component and device design. Through-glass vias (TGVs), the microscopic pathways that enable electrical connections between layers in advanced semiconductor packages, are among the most critical components in microelectronics. Glass is steadily replacing silicon and organic substrates as the preferred substrate material, due to its advantageous thermal properties, lower costs, and superior performance for next-generation devices. However, this shift also heightens certain manufacturing challenges as conventional and established processes struggle to optimize efficiency and sustainability when they are implemented with/for glass. Courtesy of Fluence.technology. For glass, ultrafast laser processing stands out as the essential enabling technology, offering high speeds and precise control over via diameter and pitch while preserving the structural integrity of the glass wafer. Femtosecond lasers are rapidly gaining popularity in this context. These lasers offer ablation efficiency and quality benefits compared with nanosecond and picosecond laser sources. Current femtosecond sources are available with exceptional beam quality, parameter stability, and extended operational lifetimes as a result of recent advancements in ultrashort-pulse generation in fibers. Robust all-fiber modules limit the use of traditional free-space and degradable components, significantly enhancing laser system reliability and reducing maintenance requirements. A pivotal breakthrough relates to recent advancements in percussion drilling of high-aspect-ratio TGVs. Using ultrashort pulses with precisely controlled energy and timing, femtosecond laser technology is enabling process engineers and manufacturers to achieve what was once impossible: faster, cleaner, and more precise micromachining from a single step in support of next-generation electronics. A shifting landscape: From silicon to glass The transition from organic substrates and silicon interposers to glass represents one of the most significant material shifts in the history of microelectronics manufacturing. This change is driven by several factors, spanning both intrinsic material properties and broader trends shaped by the evolving trajectory of Moore’s law. Through-glass vias (TGVs) in borosilicate glass fabricated using a femtosecond laser bottom-up process. TGV diameters: 60 µm, 100 µm, and 250 µm. Courtesy of Fluence.technology. As it compares to silicon, glass offers numerous cost benefits, particularly for applications that require larger substrates or higher production volumes. This cost difference becomes increasingly important as devices become more complex. Additionally, glass exhibits superior thermal stability over silicon. Thermal management and tolerance are essential parameters for maintaining dimensional stability across operating temperatures. At the same time, glass exhibits a coefficient of thermal expansion that is similar to silicon die, making this combination of materials a perfect match for achieving high performance. On its own, glass allows more chips to be placed on larger substrates since the material is more rigid, flat, and stable than organic substrates. Glass also maintains structural integrity across wider temperature ranges than organic substrates. This expanded operating envelope enables devices to function reliably in challenging and extreme environments. And it offers a lower dielectric constant than many commonly used substrates. This translates to faster signal propagation and improved performance at high frequencies. Additionally, glass enables co-packaged optics and photonics integration on a chip. This important quality opens possibilities for advanced designs where a reduction of energy consumption is critical. Specific applications include high-performance compute, 5G communication, and Internet of Things and/or AI accelerators. Percussion drilling of 0.5-mm-thick Eagle XG glass. Taper-free holes are fabricated by depositing only 800 single femtosecond pulses/hole with varying pulse duration between 250 fs and 9 ps. Taper does not appear until a hole depth of ~450 µm is reached. Courtesy of Fluence.technology. Another advantage for microelectronics manufacturing exists: Glass allows for smaller hole diameters and lower spacing than organic substrates, thereby enabling denser connection patterns without compromising performance. This is vital while chip designs are growing ever more complex, and interconnection density is increasing in lockstep. The combination of these advantages makes glass an attractive option for use in next-generation microelectronics. But the material presents its own set of challenges for micromachining processes. When it comes to creating densely packed, high-aspect-ratio holes, these challenges are particularly dynamic. Creating high-aspect-ratio TGVs Creating microscopic holes in glass with high-aspect ratios requires fabricators and process engineers to overcome significant technical barriers. Traditional mechanical drilling methods lack necessary precision, while conventional laser approaches often produce tapered holes, micro-cracks, or other unwanted defects. The ideal TGV must meet several criteria. It must feature a small diameter, often 10:1. The fabricated TGV must also exhibit minimal, or, in some cases, zero taper; an absence of micro-cracks and/or heat-affected zones; and clean, precisely engineered edges. Critically, the TGV must also have a consistent, repeatable geometry. Current widely adopted methods for achieving these characteristics can necessitate complicated multistep processes. Processes such as selective laser etching, for example, involve ultrafast laser modification followed by chemical etching. Though these methods are suitable for dense hole array manufacturing, they can raise environmental concerns. The benefits of advanced femtosecond laser technology are the most noticeable in this sequence of operations. Even as laser modification and chemical etching remain valuable and widely used, an alternative approach using direct drilling introduces an option that can be advantageous for specific manufacturing scenarios. This approach expands the toolbox of precision microfabrication by offering process engineers greater flexibility based on project needs. A multiparameter breakthrough Three parameters — pulse energy, pulse duration, and pulse timing — are most essential to enable single-step TGV fabrication while maintaining quality and efficiency. Of course, this is not to discount or diminish the significance of other parameters, including fluence/spot diameter, numerical aperture, and focal plane — all of which play important roles. Another important consideration is peak intensity, resulting from beam and pulse quality. Modern industrial femtosecond lasers with pulse energies >200 µJ provide sufficient energy to efficiently drill through 0.5-mm-thick glass substrates. This high energy, when optimally deployed, enables higher aspect ratios and faster processing without sacrificing precision. At the same time, pulse durations And precise control over pulse delivery, which advanced pulse-on-demand (POD) capabilities enable, ensures exact positioning even during high-speed scanning operations. This results in consistent, repeatable hole patterns with high throughput. A through-glass via (TGV) is fabricated by repetitive single femtosecond-pulsed percussion drilling in Eagle XG glass. Zoomed-in hole entrance and inner wall (middle). Shiny inner walls, crack-free and without post-processing (right). The substrate is 500 μm thick. Courtesy of Fluence.technology. Femtosecond lasing enables two distinct single-step approaches to TGV fabrication: bottom-up drilling and percussion drilling. Each presents unique advantages depending on variables such as process parameters and production/scaling needs. Precision without compromise Bottom-up drilling, or rear-side ablation, represents a desirable alternative approach for TGV fabrication. Unlike conventional methods, bottom-up drilling commences at the bottom surface of the glass and works upward using nonlinear absorption characteristics. A femtosecond laser beam focuses through the glass substrate, positioning the focal point at the bottom surface. The high intensity at this point then triggers nonlinear absorption, removing material only at this specific location without affecting the material above. The progressive upward motion of the focal point creates a straight, non-tapered hole. In addition to non-tapered holes with consistent diameters throughout, bottom-up drilling ensures angle flexibility; the holes are angled with inclinations of up to 50°. Further, the diameters of the drilled holes can range from These qualities make bottom-up drilling an accurate approach for prototyping devices with different diameters and hole profiles. Laboratory tests showed holes with diameters as small as 46 μm in 1.1-mm-thick BK7 optical glass, achieving aspect ratios of 24:11. Even more impressive, holes with diameters of 32 μm have been created with aspect ratios reaching 34:1 using specialty focusing optics. Moreover, the ablation process itself is fast enough to support many applications, typically requiring Speed without sacrifice Bottom-up drilling excels in geometric control. For applications for which processing time is a critical consideration, the method of percussion drilling offers much higher speed. Percussion drilling is simple and fast. This technique delivers multiple laser pulses to the same location, progressively deepening the hole without moving the focal plane. However, with ultrashort pulses, this method faces common challenges, such as conical shape formation (taper), drilling saturation, low aspect ratios, wing-shaped in-volume defects at the hole’s entrance, heat-affected zones, and cracking. As a result, drilling through >0.5-mm-thick substrates is extremely challenging, especially in wide-bandgap dielectrics such as glass. The current class of advanced femtosecond laser systems overcomes common percussion drilling limitations, such as nonrepeatable hole geometry and unwanted thermal defects, through precise pulse parameter control and optimized focusing. Pulse energies >200 µJ and optimized focusing conditions allow modern lasers to penetrate 0.5-mm-thick Eagle XG glass and 1-mm-thick fused silica, for example, in just 20 ms/hole with repetitive single pulses (aspect ratio 1:50). Research reflected that pulse duration significantly affects drilling efficiency, and tests that compared durations from 250 fs to 9 ps demonstrated that shorter pulses create substantially deeper holes with the same number of pulses. The 250-fs pulses created holes 1.7× deeper than those created with pulses >3 ps. This result, achieved with repetitive single pulses, was remarkable given the high quality. It showed shiny walls and minimal defects near the hole entrance. The key factor in this case is high peak intensity. This is a product of energy and pulse duration, as well as laser beam quality (M2) and temporal pulse shape, where the quality, among other factors, is defined by Strehl ratio. High laser intensity enables characteristic effects within the deep structure, particularly at the air-glass interface, where multiple internal reflections contribute to further deepen the microhole, resulting in taper-free >400-µm structures1. The narrow hole opening is a major challenge for both repetitive single pulses and burst modes. This effect appears at the onset of plasma evacuation, at the rear side of the substrate causing tapered hole exit. This will remain a bottleneck and must be addressed to enable and sustain future developments. However, single femtosecond pulses already show promise in achieving smaller hole diameters without melt ejection at the surface, enhancing precision and reducing the need for postprocessing. Pulses always on time For industrial applications requiring thousands or even millions of TGVs, advanced POD enables on-the-fly drilling during continuous scanner movement. An advanced laser triggering scheme, with nanosecond-level timing jitter, enables distributed drilling by providing exact positioning of each laser pulse with submicron precision. Achieving a mode of such laser operation requires the laser to produce the pulses with the same energy, despite constantly changing conditions in amplification stages, due to varying pulse repetition rate. As such, this operation can be difficult to obtain. In a conventional approach, the beam must stop at each hole location to deliver pulses, limiting throughput. Advanced POD allows scanners to move continuously at speeds of tens of meters/s while delivering precisely timed pulses that consistently hit the same locations on each pass. Advanced POD offers two main advantages in operation: distributed heat load and maximized laser use. Pulses are spread across many locations, minimizing heat accumulation at any single point. And the full laser repetition rate can be used across multiple hole locations. Through-glass vias (TGVs) fabricated in 1-mm-thick glass. The hole diameter is 65 µm, taper-free, and has an aspect ratio of 15:1. The use of the femtosecond source enables a single-step process that bypasses wet etching. Courtesy of Fluence.technology. Tests demonstrated significant productivity improvements with this approach; when drilling a 5- × 10-mm matrix of holes with 70-μm spacing, for example, the advanced POD feature enabled a user to achieve 69 holes/s, compared with 17 holes/s using conventional sequential drilling. These results marked a 4× improvement with a 30-W laser source1. This capability stems from a 100× improvement in positioning accuracy compared with standard triggering methods. Even after thousands of passes at high speeds, the holes appeared to be identical to those made with a stationary beam, but without heat-related issues. Environmental and efficiency benefits Beyond technical advantages, approaches based on direct femtosecond laser approaches offer significant environmental and efficiency benefits by eliminating the use of chemicals. Single-step laser processes offer flexibility in prototyping and testing while simultaneously offering the potential for future single-step mass-scale fabrication methods. A side view of optimized percussion drilling with 250-fs pulses provides high-aspect-ratio holes of up to 800 µm in 1.1-mm-thick borosilicate glass. The number of pulses/hole is 2000, with a pulse repetition rate between 2 and 36 KHz. Courtesy of Fluence.technology. Additioal advantages span energy efficiency, conservation of materials, and downstream process simplification. By concentrating energy precisely where it is needed with minimal heat losses, the minimization of material waste and reduced scrap rates conserves resources and improves manufacturing economics. And a decrease in required process steps leads to a reduction in handling requirements and mitigates errors and/or contamination. Beyond microelectronics Microelectronics presents the primary market for TGV technology, though the capabilities of advanced femtosecond laser machining extend to additional industries. The medical device industry increasingly requires microscopic features in biocompatible glass for applications including opto- and microfluidic devices for diagnostics; implantable sensors and electrodes; drug delivery systems; and lab-on-chip technology. The prospect of creating precise, high-aspect-ratio holes without chemicals is particularly valuable for medical applications, such as these, where material purity is critical. Aerospace applications also benefit from glass’ durability, optical properties, and thermal stability. Applications in this sector range from sensor systems that require high-precision optical and electronic integration to display technologies for cockpit instrumentation. Likewise, communication components that require radio frequency transparency, and the hermetic packaging for sensitive electronics in harsh environments, also benefit from the capabilities of sophisticated femtosecond industrial lasing. In telecommunications, advanced cellular communications systems require complex glass substrates with precise micro-features. These include antenna arrays with integrated components; radio frequency filters and waveguides; and high-frequency circuit boards. Industry adoption Leading semiconductor packaging companies are integrating femtosecond laser processes into their manufacturing lines. Major glass manufacturers, including Corning, Schott, Nippon Electric Glass, and AGC, meanwhile, have developed specialized glass formulations that are optimized for electronic devices. Equipment manufacturers offering femtosecond laser solutions span established industry leaders to innovative startups. Companies around the world are offering systems specifically designed for high-volume TGV production. The flourishing ecosystem surrounding this technology includes glass substrate suppliers, laser system manufacturers, process development specialists, and end-product manufacturers. This ever-growing range of participants indicates a maturing market with competition driving innovation. Future outlook As industry continues its evolution toward greater miniaturization and functionality, several trends are emerging. The first is achieving even higher aspect ratios, as research gets closer to exceeding 100:1. In addition, ongoing advancements in laser technology are poised to reduce processing times even further, with the potential of reaching Combining femtosecond laser processing with complementary technologies is also poised to lead to breakthroughs. The combination of advanced femtosecond processing and plasma monitoring, for example, could enable even greater precision and quality control. These opportunities stem from the shift from silicon to glass substrates, driven by compelling technical and economic advantages, and requiring innovative manufacturing approaches that overcome the distinct challenges posed by glass. By enabling single-step processing with greater precision, speed, and quality, advanced femtosecond laser technologies are helping manufacturers to meet next-generation application demands while reducing environmental impact and improving efficiency. The demonstrated capabilities — such as sub-50-μm-diameter holes with aspect ratios >24:1; processing speeds as fast as 20 ms/hole; and angled holes — open possibilities for advanced package designs and applications. Further, the development of direct-laser TGV fabrication using advanced spatial and temporal shaping of femtosecond pulses could enable the manufacture of repeatable single-micron vias with extreme aspect ratios. Meet the author Bogusz Stepak, Ph.D., is the R&D director of laser microprocessing at Fluence’s Ultrafast Laser Application Laboratory; email: bstepak@fluence.technology. Reference 1. B. Stepak (January 2025). Single-step fabrication of high-aspect-ratio through-glass vias using ultrafast fiber laser. Proc SPIE Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXX, Vol. 13350, San Francisco, California.