The lecture will be given by Pol Sopeña, from Aix-Marseille Université, CNRS, LP3. Marseille, France, in the center’s conference room (c/ Serrano, 121).
Overcoming the challenges of ultrafast laser processing of semiconductors
P. Sopeña, A. Wang, A. Mouskeftaras, D. Grojo
Aix-Marseille Université, CNRS, LP3, UMR7341, 13009 Marseille, France
Ultrafast laser processing has been widely studied and developed as a powerful method to transform the structural and chemical properties of materials. The inherently high intensities of femtosecond pulses opened the door to not only modifying the surface of opaque materials but also the interior of transparent ones through nonlinear absorption mechanisms. In this context, ultrafast laser bulk modification of semiconductors -i.e. silicon (Si) or gallium arsenide (GaAs)- has always represented a great challenge due to their high refractive index and narrow bandgap.
Whenever a beam is focused below their surface, nonlinear phenomena and prefocal plasma appear, which avoid crossing the modification threshold at the focal point. To circumvent these limitations several methods have been proposed, such as hyper-focusing conditions, or temporally tailored irradiations to limit the nonlinear propagation effects. Based on the last strategy, in this presentation, we will explore two different approaches with immediate application in the semiconductors industry.
In the first approach, we propose splitting a 1.55 μm femtosecond pulse into a terahertz repetition-rate train of pulses . By introducing a set of six birefringent crystals, the original pulse is transformed into a train of 64 equally delayed pulses. We first study the impact of the train duration (pulse number) by analyzing the energy distribution and prefocal plasma formation in GaAs. The superior performance of the burst-mode irradiation is confirmed by a comparative study conducted with infrared luminescence microscopy which indicates a reduction of the plasma density in the prefocal region and stronger achievable excitation at the focus. This allows successfully crossing the writing threshold inside semiconductors providing a solution for ultrafast laser processing, including a proof-of-concept in stealth dicing.
In the second approach, we disruptively move to longer pulses of 5 ns at 1.55 μm. By focusing the beam at the interface between two Si workpieces in intimate contact, we present the first demonstration of laser welding of semiconductors . After an optimization process, we identify that optical contact is required in order to avoid having contact issues that might result in a Fabry-Perot cavity. By processing large areas, we measure the shear force, demonstrating a joining strength of 32±10 MPa for the Si-Si configuration. Later, we extend this to GaAs and test different configurations alongside Si, finding shear joining strengths >10 MPa in all cases. This provides not only the first demonstration of laser welding of similar semiconductors but also dissimilar ones (Si and GaAs), obtaining strengths similar to those obtained by molecular wafer bonding and ultrafast laser welding of dielectrics.
On this basis, we show how these methods allow reducing undesired nonlinear propagation effects and changing the structural properties of semiconductors. This opens new possibilities for 3D laser processing solutions as, for instance, the generation of defects for photonics applications, phase changes for waveguide writing, or the production of porous structures.
Acknowledgment: This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement Nº 101034324.
 A. Wang, P. Sopeña, D. Grojo, Burst mode enabled ultrafast laser inscription inside gallium arsenide. Int. J. Extrem. Manuf. 4, 045001 (2022).
 P. Sopeña, A. Wang, A. Mouskeftaras, D. Grojo, Transmission laser welding of similar and dissimilar semiconductor materials, Laser Photon. Rev. 16, 2200208 (2022).