Technological break-through

Ultrafast laser welding is a powerful technique for joining workpieces. The fast, clean, and contactless aspects of this technique make it unique and attractive for industrial applications. It relies on the propagation of ultrashort laser pulses through a first workpiece, and the energy delivery at the interface with a second workpiece. This leads to local melting in both parts, which mix, and resolidify so that the pieces are joint permanently. This technique is applicable to numerous configurations where the first workpiece is transparent (for instance, glass, polymer). The second workpiece can be either transparent, or opaque (for instance, metal, semiconductor). However, until recently, joining semiconductors to metals with ultrashort laser pulses was impossible.

Researchers from the IAP have found that, during their propagation in silicon, infrared ultrashort laser pulses exhibit a “sausage-chain-like” behavior called filamentation. This is due to a mutual feedback between light and matter: light tells matter how to react, and retroactively, matter tells light how to propagate. One major consequence of such a propagation is the delocalization of the energy delivery far away from the desired position. Precise characterization of the propagation inside silicon, and relocation of the brightest point at the interface with the metal, have allowed us to demonstrate silicon–copper ultrafast laser welding for the first time. As these materials are the backbone of microelectronics and photovoltaics, this achievement opens up new opportunities for device manufacturing.

Concerning the genesis of the idea, Dr. Maxime Chambonneau, postdoc in the group Ultrafast Optics led by Prof. Dr. Stefan Nolte, says: “This achievement is the perfect illustration of how powerful teamwork is for pushing scientific and technological boundaries. My colleague Markus Blothe and I were characterizing propagation in silicon, while, in the same lab, our colleague Dr. Qingfeng Li was working on glass–metal ultrafast laser welding. The idea to bridge our respective activities came naturally among us, and it actually worked pretty well. Further collaborations with Leibniz-IPHT for fine laser metrology, as well as Texas A&M University at Qatar for theoretical calculations, allowed us to improve the fundamental understanding and devise original applications.”