A Review of Wafer Coating Methods for 3D Packaging

Abstract

For many years, Front End of Line (FEOL) wafer fabrication, has used spin coating as the method of choice for photoresist application. Today, 3D packaging requires coatings applied to the non-active side of the wafer. Popular applications for back side coatings are, temporary adhesives to hold wafers to carriers during Through Silicon Via (TSV) fabrication, applying die / wafer bonding materials, and dielectric layers. Spin coating of wafers can be used to apply a number of materials for backside coatings. Although this application method is quite fast, it has a number of drawbacks which include high material waste and its limitation to low viscosity materials. Wafer Backside Coating (WBC) adhesives applied prior to dicing poses some unique challenges due to their filler loadings, and therefore the formulations can be limited by the spinning process. Today, most companies using spin coating to apply WBC materials have to lower the viscosity of the fluid with solvents, which creates other problems. Screen or stencil printing to apply WBC adhesives is a proven method and is currently in production at manufacturing sites. Screen printing gives good print uniformity, but requires a formulation that self-levels after printing. Stencil printing can be used with almost any formulation or filler loading, but is highly dependent on the tooling accuracy to maintain the coating thickness uniformity. Both methods are limited to coating thicknesses in the 25–100 micron range. There exists a need for a wafer coating methodology that can overcome the aforementioned restrictions. The ability to apply materials of varying rheologies, with low material waste and yet provide high quality, uniform coatings is required. This paper will review the current methods of applying WBC coatings. The authors will also present results from a newly developed spray coating process developed to process high viscosity fluids such as WBC coatings.