Interface Engineering and Microstructure Control in Thermocompression Bonding for 3D ICs

Thermocompression bonding is utilized in situations where robust mechanical and electrical connections are necessary, such as in the vertical 3D integration of dies (chips), direct chip attachment to substrates or packages, or in MEMS devices, photonics and optoelectronics. The TCB process involves several steps, including heating and cooling, pressure application, solder melting, diffusion, and the formation of intermetallic phases between solder bumps and landing pads. These procedures are typically completed in few seconds. A thorough understanding of both the bonding process and the surfaces involved is important for achieving reliable bonds.

Research scope

Surface cleaning to remove native oxides and contaminants.

Achieving effective bonding requires diffusion across the interface, which can be impeded by surface oxides or contamination on solders or pads. Oxides at the interface may result in highly resistive, non-yielding, and non-wetting cold joints. Plasma treatments can help reduce oxides without leaving foreign chemical residues. To prevent solder re-oxidation, performing bonding in an inert atmosphere with in-situ cleaning is optimal. A thorough understanding of the effects of plasma gases, process temperature, and plasma parameters on surface chemistry is essential.

Bond profile and reflow optimizations for improved joint reliability (solder & intermetallic compound microstructure)

Having oxide free solders, there is still a challenging part to optimize the intermetallic compound (IMC) formation which is both necessary and problematic at the same time. IMCs are needed for metallurgical bonding, and they are thermally more stable than the pure solder material which help with multiple vertical stackings and overall reliability. However, they are in general brittle and prone to cracking especially under thermal cycling or mechanical stress. They can form voids, change phase depending on thermal history, can over-grow with solid state diffusion with time and lead to resistive and brittle joints. Control of their thickness, phase, morphology and distribution of grains at the joint interface is critical to achieve and maintain reliable bonding. As diffusion is a key parameter in IMC formation; bumping process which controls the incoming solder grain size, grain orientation is critical.

Additionally, the TCB process consists of rapid heating/cooling/pressure application which re-creates the joint microstructure. This can be a powerful tool in joint reliability improvements. Sn, as an example to solder material, has body-centered cubic crystal structure and thus has anisotropic diffusivities, thermal expansion coefficient and mechanical properties. The self-diffusion and diffusion of other elements along its [001] direction is known to be at least two orders of magnitude higher than other directions. These anisotropies affect the IMC formations, EM failures, creep and fatigue or overall failure of joints. Certain textures (alignments) of solder grains have been shown to clearly relate to the early failure of solder joints. Anisotropic properties become increasingly important as pitches and joint dimensions decrease. By controlling the heating/cooling/pressure profiles during bonding the final joint microstructure and texture can be optimized for reduced electron migration, slower IMC growth and better mechanical reliability of the joint.

Methods and Outcomes

This PhD project aims to enhance bond reliability of Sn and In solders, through evaluation of advanced surface cleaning techniques, including atmospheric plasma, and by refining joint microstructure through the control of bond profiles. Effects can be assessed by electrical tests, bond shear tests and microscopy examinations. Characterization techniques such as electron back scattered diffraction, energy-dispersive X-ray spectroscopy, X-ray diffraction techniques can be used to study the final texture, phases and compositions in the joint.

TC bond profile variations can be used to control the solidification structures, thus controlling grain size and texture to improve mechanical, electrical performance and reduce electron migration failures. To support the experiments, finite element method (FEM) modelling can be used to understand the thermal behaviour at both joint-level and the die-level. Combined with characterization results, the modelling can help in the fundamental understanding of how the process works for a dedicated solder material. Additionally, there is a possibility of extending the study by working multi-disciplinary and across different imec teams, such as collaboration with electroplating team to investigate different incoming solder bump microstructures, chemistries and textures.