Selecting Bumping Materials
Our expertise in thin film materials and polymers will provide you with a more robust process or help you choose the wafer bumping supplier best suited to your application. Development of bumping or WLP starts with a thorough evaluation of the potential materials, their impact on the process and implications on long term reliability. Take advantage of our years of experience with thin film materials, bypass years of experimentation, and achieve your goals ahead of the competition.
Many polymers are available and competing for potential high volumes in wafer level packaging. For several years, some of these polymers have been used as passivation layer over semiconductor wafers. Choosing or comparing polymers typically involves reviewing their physical, mechanical, chemical and a number of other properties that may favor or prevent their use in packaging. The selection criteria must be rigorous and must consider that in WLP, the thin polymer layer spun on the wafer will become the only protection of the chip against the environment.
Many of the so-called low-k dielectrics intended for wafer inner-layer-dielectric (ILD) are not suitable for packaging because of their inability to form layers thicker than one micron without cracking. For instance poly-silsesquioxane has a favorable dielectric constant but cracks under its own intrinsic stress when building layers sufficiently thick for packaging. SiO2 or Si3N4 have similar problems or have low deposition rates rendering them impractical for packaging.
Dielectric selection is not a well-defined science and it involves making numerous compromises between competing and often contradictory requirements. For instance, outstanding material properties may conflict with processability or adhesion to the substrate. Adhesion of a polymer to metals, to itself or to other dielectrics can be a source of difficulties.
We have experience building multi-layer metallization devices with non-photo sensitized, photo imageable, spun, or CVD polymers that will be an asset to your organization.
We have successfully used aluminum, copper and gold based interconnect metallization for years. Choosing between sputtering, evaporation, electro-plating or electroless plating is a function of the purpose of your project and of the equipment available to you. For instance, operation at very high speed or low level signals may require low interconnect noise and losses that would be tolerable in digital work, therefore favoring thick copper interconnect over thinner aluminum.
Several other factors are important when choosing an interconnect metal. An adhesion or a diffusion barrier metal may be necessary. The selectivity of etchants and the potential damage of some chemistry to polymers, including photoresist, needs to be taken in consideration. Etching recipes out of handbook are of little value because of they do not take in account the electro-chemistry prevailing when several dissimilar metals are in contact.
The choice of RDL metallurgy is dictated by the desired long-term reliability. Several reliable systems have been validated over the years. For instance, Nickel or Cobalt alloys or their pseudo-alloys can be equally a good choice from a reliability aspect, the final choice resting with other factors, such as etching or deposition method.
Almost any desirable polymer for WLP is useable with any soft solder. The temperatures, including temperatures for 95-5 Pb, are not an issue. The choice of solder is dictated by the end use and not by bumping fabrication issues.