Enhanced reliability and precision for internal laser
processing of semiconductors using temporal domain
strategies

Pol Sopeña, Andong Wang, Niladri Ganguly, Alexandros Mouskeftaras, David Grojo

► To cite this version:

Pol Sopeña, Andong Wang, Niladri Ganguly, Alexandros Mouskeftaras, David Grojo. Enhanced reliability and precision for internal laser processing of semiconductors using temporal domain strategies. Proceedings of SPIE, the International Society for Optical Engineering, 2025, Nanoscale and Quantum Materials: From Synthesis and Laser Processing to Applications 2025, 13352, pp.1335205. 10.1117/12.3049658 . hal-05029009

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# Enhanced reliability and precision for internal laser processing of semiconductors using temporal domain strategies

Pol Sopeña*<sup>a</sup>, Andong Wang<sup>b</sup>, Niladri Ganguly<sup>a</sup>, Alexandros Mouskeftaras<sup>a</sup>, David Grojo<sup>a</sup>

<sup>a</sup> Aix-Marseille Université, CNRS, LP3 UMR 7341, 163 Av. Luminy, 13009 Marseille, France;

<sup>b</sup> BIT, SME, Laser Micro/Nano-Fabrication Laboratory, 100081 Beijing, China

## ABSTRACT

Ultrashort laser processing allows writing three-dimensional patterns in the bulk of most transparent materials. However, for semiconductors like silicon or gallium arsenide this is extremely challenging due to the high refractive index and the detrimental nonlinear propagation and plasma effects in most cases. Nonetheless, by temporally tailoring the laser pulses, we demonstrate solutions for high-performance in-volume laser processing. Notably, by using nanosecond and picosecond pulses or THz-repetition rate bursts of ultrashort pulses we can write micro-sized structures in different semiconductors. With this, we demonstrate laser welding of dissimilar semiconductors, high-aspect ratio optical channels, and stealth dicing of wafers. These demonstrations must open the door for new solutions for monolithic optical devices and quantum technologies in the semiconductor industry.

**Keywords:** Laser processing, semiconductor, silicon, femtosecond, infrared

*pol.sopena-martinez@univ-amu.fr

## 1. INTRODUCTION

The extremely high intensities of ultrafast laser sources allow triggering multiphoton absorption processes when tightly focused in the volume of transparent materials. Consequently, when crossing the fluence modification threshold, the local structural and/or chemical properties of the material are modified, leading to the writing of micrometric structures on a deterministic manner. This has led to astonishing demonstrations such as the fabrication of gigabyte 5D memories in glasses [1] or miniaturized three-dimensional (3D) polymeric structures by two-photon polymerization [2]. Nonetheless, when translating these demonstrations from dielectrics to semiconductors (i.e. silicon Si or gallium arsenide GaAs), femtosecond pulses and conventional focusing configurations do not permit material modifications [3]. In fact, their intrinsic properties, high refractive index (~3.5 for Si) and narrow bandgap (~1.2 eV for Si), limit the effective numerical aperture (NA) and lead to propagations nonlinear effects and prefocal plasma screening, respectively. Thus, efficient energy delivery at the focal point cannot be achieved, which impedes the ability to modify below the surface of semiconductors.

In a quest to compensate for all these detrimental effects, over the past decade extensive efforts have been made to systematically achieve precise and controllable below-surface modifications in semiconductors. The first demonstration was performed by strongly focusing 60-fs 1300-nm pulses with a 0.85NA lens at the center of a Si sphere [4]. This allowed to avoid interface refraction and reach an effective NA of ~3, crossing this way the modification threshold and inducing permanent modification. However, this result was limited in terms of practical capabilities in view of commercial flat-wafer configurations. In this regard, efforts were put in other directions, mostly focusing on temporal strategies to reduce competing nonlinear effects and prefocal plasma formation. In this manuscript, we will review different temporal regimes and strategies going from longer pulses in the nanosecond (ns) to THz-repetition-rate train of femtosecond (fs) pulses. By this, we have been able to produce in-volume structural modification in different semiconductors, targeting different applications such as wafer bonding or stealth dicing.

## 2. RESULTS AND DISCUSSION

### Laser welding of similar and dissimilar semiconductors

Common approaches for Si and general semiconductor wafer bonding rely on intermolecular bonds that appear when two wafers are appropriately prepared and placed in the most intimate contact. Moving away from complex surface preparation methods and adhesives, ultrashort pulses have been routinely used for through-glass laser welding by focusing the beam at the interface between two pieces and bonding them upon resolidification (Fig. 1a). Translating this to the semiconductor
industry, direct bonding could not be achieved and it was only demonstrated by using an intermediate sacrificial gold layer at the interface between two Si pieces where absorption was promoted and resulted in bonding [5]. In order to reduce the important physical limitations found with intense pulses in Si and the use of sacrificial layers, we propose in this work a turning point in the approach and employing nanosecond pulses as two-photon absorption has been demonstrated in the stealth dicing technology of semiconductors despite reduced intensities [6].

<figure><img src="image_2.png" /><figcaption>Figure 1. (a) Scheme of the laser welding procedure. (b) Lateral view by IR transmission microscopy of the Si-Si interface. Imperfect contact leads to faulty welding. (c) IR transmission image of a welded serpentine in the Si-GaAs configuration. (d) Image of the laser source used in the experiments (~3 kg) hanging from a Si wafer welded on GaAs.</figcaption></figure>

Using longer pulses of 5 ns at 1550 nm, we present the first demonstration of direct through laser welding of Si [7]. First, by single-spot irradiations (Fig. 1b), we identify that small imperfections in the Si-piece contact can form a Fabry-Perot cavity that prevents modification through the interface. By considering optimum optical contact conditions, we later vary the irradiations conditions (focus position and pulse overlap) to determine the most material mixture between wafers without the appearance of detrimental cracks, cavities, or porosities. We then raster-scan the optimized beam to produce a large welded area (Fig. 1c) and by shear force measurements, we demonstrate a joining strength of 32±10 MPa for the Si-Si configuration. We extend this systematic study to GaAs and test different configurations alongside with Si finding shear joining strengths >10 MPa in all processed cases, one order of magnitude larger than those obtained employing a sacrificial gold layer [5].

This provides not only the first demonstration of laser welding of similar semiconductors but also dissimilar ones (Si and GaAs). The bonding strengths are similar to those obtained by molecular wafer bonding and ultrafast laser welding of dielectrics, leading to unparalleled process efficiency, given the use of a very compact fiber laser source. To illustrate this, we produce a few millimeters Si-GaAs weld (3.5×7 mm²), allowing to hang the employed laser equipment (Fig. 1d). We anticipate that the process can be advantageously scaled up to large wafers with the use of industrial high-power sources, holding great potential for a unique additive manufacturing solution applicable to semiconductors.

## Fabrication of ultra-high aspect ratio microchannels through Si

The formation of long high aspect ratio of vertical structures is essential in Si photonics and electronics. By means of ultrafast laser processing, these architectures have been routinely achieved in dielectrics either by scanning a tightly focused Gaussian beam or by generating Bessel beams. However, these strategies have simply failed [8] or not reached the expected performances [9] when translated to Si. With the idea to mitigate propagation limitations, we propose here moving to the picosecond (ps) regime [10], which combined with an axicon-lens doublet away from 4-f relay configurations, should allow fabricating the desired high aspect ratio structures.

<figure><img src="image_3.png" /><figcaption>Figure 2. Reconstructed beam-profiles of the pseudo micro-Bessel beam generated by an axicon-lens doublet configuration (a) Longitudinal intensity maps, (b) transverse peak-intensity profile (FWHM~1.5 µm), and (c) a typical high-aspect-ratio micro-structure formed inside Si under repeated 50-ps laser irradiations.</figcaption></figure>
The implementation of an axicon-lens doublet allows us to identify a novel and efficient beam-shaping solution for laser-material processing of elongated structures [11]. We produce a high-angle pseudo micro-Bessel beam by directly placing a 2° axicon at the entrance pupil of a 0.45 NA objective lens, resulting on the doublet configuration. Using the tailored beam shown in Fig. 2a-b, and by appropriately stretching 1550-nm femtosecond pulses to 50 ps, we show that multi-shot laser irradiations give rise to a percussion writing modality inducing permanent microchannel modifications spontaneously growing from a primarily initiated front-surface precursor towards the rear-surface of Si samples, obtained without any beam motion during irradiation process (Fig. 2c). Although direct micro-explosion and hollow hole drilling remain inaccessible, our work evidences the formation of uniform through-structures inside 1-mm thick Si wafers with an aspect ratio around 700. The structures exhibit a high positive index change of ~10⁻² making them potential waveguides for vertical optical interconnects through chips. Additionally, time-resolved analysis of the excited micro-plasma unveils near-luminal plasma-front propagation with a local increase of the conductivity up to ~10³ Ω⁻¹cm⁻¹. This supports the vision of vertical electrical interconnections optically controllable at GHz switching rates. Overall, our findings open the door to unique monolithic solutions for both optically and electrically accessible through-Si-vias, which could be potentially the key elements for ultrafast vertical interconnections in next-generation 3D chips.

## Inscription in GaAs by THz repetition rate ultrashort bursts

Previously mentioned experiments where performed at kHz repetition rates, where no benefit was observed from pulse-to-pulse accumulation effects. Considering the typical free carrier excitation times in Si (~3 ns) and the heat dissipation time from a micrometer-sized heated volume (~10 ns), we anticipate that repetition rates exceeding GHz are required to meet the demand of accumulation effect [12]. Based on this, we anticipate that working at THz repetition rate, we can have the benefit from accumulation that leads a gradual excitation of the material while avoiding nonlinear propagation and excessive plasma formation in pre-focal regions.

<figure><img src="image_4.png" /><figcaption>Figure 3. a) Luminescence images obtained from irradiations at total burst energy of 208 nJ and different burst pulse number (indicated on top). b) Schematic representation of the laser writing configuration used for the GaAs stealth dicing demonstration and IR lateral images of modified lines by dual scan (two depths).</figcaption></figure>

We propose here the use of THz repetition rate trains of 1550-nm 180-fs pulses to translate the benefits observed in Si towards below-surface writing of GaAs [13]. By introducing a set of six birefringent crystals (YVO₄) of evenly increasing thicknesses, the original pulse is transformed into a train of 64 equally delayed pulses of up to 11 ps for burst-mode irradiation. We first study the impact of the train duration (number of pulses in the train) by analyzing the induced luminescence in GaAs when focusing with a 0.85 NA objective lens (Fig. 3a). This indicates a reduction of the plasma density in the prefocal region and stronger achievable excitation at the focus that increases with the number of pulses in the burst. The same method is applied to identify optimum irradiation conditions considering asymmetric trains of 64 pulses. We observe a higher energy delivery when the ratio between the front and rear of the train is 1:2, indicating that a gradually increasing in intensity in the applied trains is an optimum.

Setting the best irradiation conditions, we successfully manage to cross the writing threshold inside GaAs, providing a solution for ultrafast laser processing. However, only 80% modification probability is obtained by employing the maximum available energy of ~208 nJ in these experiments. Despite this limitation, we show that the writing process can be stabilized by continuous scanning of the sample as any seed modification allows (with appropriate irradiation overlap) creating modifications following the focus path anywhere in 3D inside GaAs (Fig. 3b). As a proof-of-concept, we show that the method can be directly applied for stealth dicing applications. More generally, this opens a wide variety of possible applications for fabricating undersurface structures inside GaAs, which holds strong potential for microelectronics and photonics.
### 3. CONCLUSION

The high intensities of tightly focused ultrafast lasers allow locally transforming the structural properties of most transparent materials by multiphoton absorption. However, when trying to focus within semiconductors, the appearance of nonlinear propagation and pre-focal plasma, limit the energy delivery at the focus and, in turn, modification. In this context, by temporal shaping and deviating from single femtosecond pulses, we can reduce the pulse intensity and cross the modification threshold. In the nanosecond regime, despite lower intensities, we induce enough multiphoton absorption for 3D writing and laser welding of dissimilar semiconductors. We induce internal structural changes with picosecond pulses, being able to fabricate high aspect ratio micro-sized modifications in Si. With THz-repetition rate bursts of ultrashort pulses, we can inscribe lines in GaAs for stealth dicing applications. In summary, we show how we can surpass the bulk modification threshold in semiconductors using non-conventional configurations, opening a new range of applications for 3D laser processing technologies.

### ACKNOWLEDGEMENTS

This work was conducted using LaMP facilities at LP3. The project received funding from the French National Research Agency (ANR-22-CE92-0057-0, KiSS project) and the European Union's Horizon 2020 research and innovation program under grant agreements No. 101034324 (MSCA-COFUND) and No. 724480 (ERC).

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