Direct bonding of semiconductors by through-transmission laser welding

International audience ; Ultrafast laser welding is an advanced manufacturing technology capable of bonding materials by selectively irradiating the interface between them. In this process, the beam is focused at the interface through the transparent top material, resulting in melting and subsequent bonding after resolidification (Fig. 1a). This has allowed to weld several materials in different configurations including through-glass, polymers or ceramics. However, it is striking to note that this process remains difficult to apply on silicon (Si) or other semiconductors, which would most certainly benefit the microelectronics industry.The intrinsic properties of Si (high refractive index and narrow bandgap) impede producing internal modifications by common laser processing configurations since nonlinear propagation effects and prefocal plasma avoid reaching high enough intensities at the focal spot to induce modification. To circumvent these effects, recently the first demonstration of laser welding of silicon on copper was made with picosecond pulses, obtaining relatively modest ~2 MPa bonding strengths [1]. After examining pulse durations from 200 fs up to ~20 ps, we confirm important physical limitations with intense pulses. In view of this, and inspired by stealth dicing in which the thermal properties of nanosecond pulses are used to create below-surface modification for subsequent dicing, we propose to use a similar approach to melt the interface of two Si pieces and join them.In this work, by disruptively moving to longer pulses of 5 ns at 1550 nm, we present the first demonstration of laser welding of Si. First, by single-spot irradiations (Fig. 1b), we identify that small imperfections in the contact constitute a Fabry-Perot cavity that leads to no modification of the bottom piece. Relying on regions in optical contact, we later vary the irradiations conditions (focus position and scan speed) to determine the most material mixture between wafers without the appearance of detrimental cracks, cavities or ...