Plasma-free Hydrophilic Bonding for Wafer to Wafer and Die to Wafer 3D and Photonic Applications

F. Fournel, V. Larrey, H. Hijazi, K. Abadie, L. Sanchez, C. Licitra, P. Montmeat, C. Morales

Univ. Grenoble Alpes, CEA, LETI, F-38000 Grenoble

Among the various direct bonding types, molecular hydrophilic bonding is the most common. It is a spontaneous bonding process that uses hydrophilic surfaces, such as freshly cleaned silicon dioxide surfaces. Direct bonding technology hinges on two important energies. The first is adhesion energy, which is the energy required to enable the bonding and its most famous manifestation is the bonding wave. The second is adherence energy, which is the energy needed to separate the bonded surfaces, typically after a post-bonding annealing (PBA). For hydrophilic bonding, adhesion energy ranges from 50 to 100 mJ/m² [1]. The adherence energy, also known as bonding energy, increases with annealing temperature, typically from around 150 mJ/m² to 5 J/m², which is considered to be the fracture energy of silicon bulk material.

To achieve the highest adherence at the lowest annealing temperature, many surface preparations have been developed over the past three decades. One commonly used method is plasma treatment just before bonding, which allows very high adherence around 4 to 5 J/m² after PBA at only 200 or 300 °C. However, plasma activation can be detrimental or unfavorable for some applications.

For CFET elaboration with direct bonding integration, the large amount of water introduced by the plasma prevents the reduction of the bonding dielectric layer thickness. For hybrid bonding in 3D applications, as revealed by H. Hahn from Samsung [2], plasma activation on copper pads can cause sputtering of copper onto the dielectric surface. This poses a risk of short circuits, especially with the minimization of pitches. In die-to-wafer processes, when a tape is used during die surface preparation, plasma can damage the tape and increase organic contamination on the bonded surface. This limits the choice of tapes to minimize these effects, which is even more critical for self-assembly die-to-wafer technology, where thermal release tapes are often used. The self-heating effect during plasma treatment may also influence the critical thermal debonding step in this last technology.

For several years now, we have discovered at LETI that specific molecules can  significantly enhance bonding energy of SiO₂ hydrophilic bonding, much like plasma treatment [3-4]. Amino-alcohol molecules like ethanolamine or strong bases like NaOH or KOH can have a catalytic effect at the bonding interface. We are not referring to the use of these molecules during surface cleaning to enhance hydrophilicity or cleaning efficiency [5]. Instead, these molecules are applied to the surface just before bonding, without any deionized water rinse, and they become active during the annealing process. 

A detailed description of these bonding energy chemical boosters will be presented, along with their advantages for various applications. These include wafer-to-wafer CFET elaboration, low-temperature hybrid bonding (wafer-to-wafer, standard die-to-wafer, and self-assembly), and photonic applications using our collective die-to-wafer bonding process.    

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