BEAM SHAPING


3D BEAM SHAPING FOR VOLUMETRIC LASER PROCESSING


JAKOB GUEYE, HERIOT-WATT UNIVERSITY


asymmetric intensity profile along the axial direction (see Figure 1(b)), which results in variations in the produced feature shape. Imperfections in the manufacturing of the optic can also cause interference due to tip rounding errors [5]. Therefore, a method to shape and modify the intensity profile along the full focal depth is required.


3D BEAM SHAPING


Figure 1: Diagram of beam propagation through a standard convex lens (a), compared to the formation of a Bessel beam through an axicon (b). Illustrating the difference in the on-axis intensity profile and extended depth of focus. Blue line in (b) also shows effects of tip rounding on the resultant intensity profile.


As microelectronics continues to scale toward smaller dimensions with higher performance, conventional silicon-based manufacturing and packaging technologies are approaching their physical limits. In response, alternative interconnect technologies, such as through- glass vias (TGVs) are beginning to emerge as a promising solution for next generation chipset integration [1]. Glass interposers offer several advantages including lower electrical losses, higher electrical resistivity, and the potential for a higher density of interconnections at reduced manufacturing costs. However, fabricating high aspect ratio uniform vias in glass quickly remains challenging, with conventional laser- based methods producing tapered and non-uniform features limiting throughput and overall productivity.


ULTRASHORT LASER SELECTIVE ETCHING


While there has been recent work investigating direct drilling of TGVs [2], these are typically fabricated using a two-step process that combines ultrashort laser inscription, and selective chemical etching. During laser inscription, the glass substrate is translated through the laser focus, inscribing localised modification tracks. These modification regions are then preferentially etched at a faster rate


22 | LASER USER 119 MARCH 2026


than the pristine glass, enabling the formation of well-defined microstructures.


Current methods generally rely on Gaussian beam focussing, where the focal spot size and focal depth are inherently coupled, restricting achievable aspect ratios. To generate high-aspect ratio vias the focal volume must be translated throughout the full sample, since small focal volumes are required to achieve non-linear absorption. This necessitates multiple pulses to be delivered at different locations to mark larger volumes, hence increasing processing time. Due to the modification shape following the axial intensity profile of the beam, TGVs often taper along their length producing a distinct hourglass feature limiting via density (Figure 1a).


As a result, the use of non-Gaussian beam profiles such as quasi-Bessel beams (QBBs) in laser material processing is increasing, due to their ability to generate extended focal volumes with an invariant spot size along the axis of propagation, where the spot size and focal depth are also decoupled [3]. They can exist in the order of centimetres with micron- sized focal spots, unlike the much shorter focal depths of gaussian beams in the order of ~1×10-5


m for


comparable spot sizes [4]. However, standard Bessel beams display an


Conventional beam shaping methods such as diffractive optical elements (DOEs) or spatial light modulators (SLMs) are effective at generating an arbitrary beam shape in the transverse direction but offer limited control over the axial intensity, where away from the focal plane the beam quality rapidly degrades, making these methods unsuitable for volumetric laser processing.


Here we propose a three- dimensional beam shaping solution based on QBBs (Figure 2). Since the resultant on-axis intensity of the QBB is determined by the incident intensity profile and radius dependant refraction angle; shaping the incident wavefront gives a method to manipulate the axial intensity profile. This is achieved using a pair of custom refractive optics: the first shapes the input Gaussian intensity to a desired target transverse profile, whilst the second compensates for any phase aberrations, producing a collimated output. By redistributing the energy of the beam into a ring- shaped profile, contributions from rays interacting with the axicon tip are suppressed, mitigating unwanted interference effects due to tip rounding.


EXPERIMENTAL DEMONSTRATION


To demonstrate the feasibility of this method for laser processing, we generated single-pulse laser modifications in bulk fused silica for a range of pulse energies in the region of 100s µJ in the picosecond regime.


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