CEA-Leti, Intel develop self assembly die-to-wafer technique

Technology News |
By Nick Flaherty

French research lab CEA-Leti and Intel have optimized a hybrid direct-bonding, self-assembly process that could boost the use of die-to-wafer (D2W) bonding.

The technique can increase the alignment accuracy as well as boost the fabrication throughput by several thousand dies per hour by using a water droplet to align dies on a target wafer.

D2W hybrid bonding process is seen as essential for combining memory, HPC and photonic chiplets on a wafer substrate, but 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,” said Bourjot.

A paper at the 2022 Electronic Components and Technology Conference (ECTC) this week describes the technique that uses capillary forces that 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.

CEA-Leti developed a D2W system inhouse that showed a mean misalignment under 150 nm for 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). This compares to 1µm alignment for a pick-and-place tool post bonding, and the best case of 700nm, while a self-alignment process offers an alignment below 500nm and even less than 200nm, post bonding.

“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,” said CEA-Leti.

The aim of the paper is to encourage assembly equipment makers to adopt the technology. “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,” said the researchers.;

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