In a breakthrough for the future of die-to-wafer (D2W) bonding, CEA-Leti and Intel have optimized a hybrid direct-bonding, self-assembly process that has the potential to increase the alignment accuracy as well as fabrication throughput by several thousand dies per hour. The approach uses capillary forces of a water droplet to align dies on a target wafer.
The results were presented in a paper, “Collective Die-to-Wafer Self-Assembly for High Alignment Accuracy and High Throughput 3D Integration”, at the 2022 Electronic Components and Technology Conference (ECTC). While leading microelectronics companies see the D2W hybrid bonding process as being essential for the success of future memory, HPC and photonic devices, it is much more complex than wafer-to-wafer bonding, with lower alignment accuracy and lower die-assembly throughput. CEA-Leti has been developing a self-assembly method for several years, with the goal of substantially increasing throughput and placement accuracy.
“Commercial scale throughput with D2W self-assembly presents two main challenges related to die handling,” said Emilie Bourjot, CEA-Leti’s 3D integration project manager. “If the self-assembly process is combined with a pick-and-place tool, the throughput can be increased by reducing the time of alignment, since the fine alignment is performed by the droplet. When self-assembly is combined with a collective die-handing solution, the throughput is increased by the fact that all dies are bonded together at the same time without any high precision placement at any time along the process flow.”
Process optimization is also an important part of this work for increasing process maturity and targeting industrial requirements. “With such alignment and throughput performances, it is definitely a promising step allying the magic of physics and a simple drop of water,” Bourjot said.
Water Is ‘Excellent Candidate for Self-assembly Process’
The paper noted that “capillary forces arise from the principle of surface minimization and are exerted through surface tension in the case of a liquid. From a macroscopic point of view, the liquid tends to minimize its liquid/air interface to reach an equilibrium state with minimized energy. This mechanism allows the self-alignment of the die on its bonding site. The liquid chosen as the realignment vector must present a high surface tension and has to be compatible with direct bonding. Most of the liquids have a surface tension between 20 and 50 mN/m, except water that exhibits a surface tension of 72.1 mN/m, which makes it an excellent candidate for self-assembly process using hydrophilic bonding in which water is already a key mechanism parameter.”
“The water dispense technique and the surface preparation to tune the surface hydrophilicity appeared as critical for the proper conduct of the self-assembly process,” according to the paper. “Thus, excellent alignment performance on a homemade collective self-assembly bonding bench was achieved. It resulted in a mean misalignment inferior to 150 nm with a 3σ inferior to 500 nm. Finally, the compatibility of the self-assembly process with a wide range of die dimensions (8×8 mm², 2.7×2.7 mm2, 1.3×11.8 mm2 and 2.2×11.8 mm2) was demonstrated.”
By comparison, state of the art alignment is 1µm with a pick-and-place tool post bonding, and the best case is 700nm, while a self-alignment process offers an alignment below 500nm and even less than 200nm, post bonding.
Explaining the “homemade collective self-assembly bonding bench”, CEA-Leti said: “As no industrial tools for the self-assembly approach exist, the team fabricated its own lab bench enabling a collective self-assembly. The low-reproducibility, manual process control none-the-less achieved alignment of 500nm and below, which strongly suggests that an industrial tool dedicated to this process would deliver higher reproducibility, robustness and precision.”
The paper’s conclusion emphasized this point: Despite these breakthroughs, “many aspects of the self-assembly still need to be explored and great improvements will only be possible when tool suppliers will develop (an) adapted tool to automate this process.”
For this collaboration CEA-Leti designed the process flow, and performed wafer processing and self-assembly bondings with its expertise in bonding physics, processes and process integration. It also performed characterizations like nano-topography, scanning acoustic microscopy and alignment. Intel’s participation included providing specifications, modeling and pre-and-post bonding process integration expertise to make the self-assembly process foundry compatible.