Dino Deligiannis | INTLVAC THIN FILM: Why is depositing indium for micro-display interconnects so difficult, especially below a 10-micron pitch?

How do you tame indium's wild growth habits to create perfect, one-micron bumps for microLED displays?

The key to eliminating defects like spits and dendritic growth lies in a multi-pronged, patented approach, starting with in-situ super-purification of the indium source material. Standard "five nines" (99.999%) pure indium is found to be insufficient for this demanding process. Further purification is performed within the deposition tool itself to remove residual impurities that act as nucleation sites for spitting and uncontrolled crystallization.

A second critical parameter is a high deposition rate, which can be run as high as 70 angstroms per second. This rapid deposition kinetically suppresses the natural tendency for indium to form undesirable dendritic structures. By depositing the material faster than it can organize into large, irregular crystals, the process forces a more uniform, columnar growth, which is ideal for filling high-aspect-ratio features.

The final piece of the puzzle is aggressive substrate cooling, using a system with up to 2 kilowatts of cooling power capable of reaching -100°C. Counter-intuitively, cooling the substrate prevents the indium atoms from having enough surface mobility to form "snowflakes" or dendritic crystals. This combination of extreme purity, high rate, and deep cooling is essential for producing the dense, uniform, and defect-free indium structures required for one-micron bumps.

In this short video, you can learn:
* The necessity of in-situ "super-purification" beyond standard 5N indium.
* How high deposition rates (up to 70 Å/s) kinetically suppress dendritic growth.
* The counter-intuitive role of aggressive substrate cooling in preventing "snowflake-like" crystal formation.
📋 **Clip Abstract** Deligiannis reveals the patented, three-part strategy for creating high-quality, sub-4-micron indium bumps. The process combines in-situ super-purification, high deposition rates, and aggressive substrate cooling to overcome indium's natural tendency for spitting and dendritic growth.
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Your indium bumps are perfect, but will they actually stick to the wafer?

Achieving robust adhesion for indium bumps is critical for device reliability, and it begins with meticulous surface preparation. A dedicated cleaning station is integrated into the system's load lock, which is kept separate from the main deposition chamber where indium partial pressures are high. This allows the wafer surface to be prepared in a clean environment immediately before being transferred for coating.

The primary cleaning method is a low-energy argon pre-clean. This gentle plasma process effectively sputters away surface contaminants and, most importantly, the native oxide layer that forms on the bonding pads. Using a low-energy source is crucial to avoid causing physical damage to the sensitive underlying device structures on the wafer.

For more demanding applications or more resilient native oxides (such as those on titanium oxynitride), the process can be enhanced by adding reactive gases like hydrogen to the argon plasma. This in-situ chemical and physical cleaning capability ensures a pristine interface between the wafer and the deposited indium. The result is an exceptionally strong bond that meets the stringent Mil-Spec "scotch tape" adhesion test, a critical requirement for high-reliability applications.

In this short video, you can learn:
* The importance of an in-situ, load-lock-based pre-clean for adhesion.
* How a low-energy argon clean removes native oxides without damaging the substrate.
* The option to enhance cleaning with hydrogen for robust, Mil-Spec compliant adhesion.
📋 **Clip Abstract** Dino Deligiannis details the critical in-situ pre-cleaning process used to ensure robust adhesion of indium bumps. By using a low-energy argon plasma clean (with optional hydrogen) in the load lock, native oxides are removed just before deposition, resulting in interconnects that meet Mil-Spec adhesion standards.
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