Effect of Surface Roughness of Polymer Dielectric Materials on Resolution of Fine Line Features

High density interconnects is a key factor for achieving high bandwidth which is required for AI and high performance compute applications. The high density interconnects can be achieved by, 1) fine lines and spaces in a metal layer, or 2) a stack-up consisting of multiple redistribution layers with not so fine lines and spaces. Fabrication of fine lines and spaces is conventionally achieved by advanced lithography and photoresists. However, in order to attain fine lines and spaces formation, the resists and the following processes should lead to a smooth surface which will yield fine lines and spaces. Demonstration of lines and spaces (L/S) has been a method to evaluate new dielectrics, photoresists and lithography processes. Adhesion between copper and polymer is an important factor determining the stability of these copper lines on the polymer as the substrate goes through different processes such as photoresist stripping and seed layer etching. Several methods have been employed to improve the adhesion between copper and polymer, the most important among them being plasma treatment. This method relies on mechanical roughening of the surface which results in interlocking between the metal and polymer to improve adhesion. However, there have also been reports indicating that excessive roughness resulting in the detachment of copper traces from the substrate. Therefore, it is important to study how the roughness of the dielectric influences the resolution of fine line features. This paper presents results on the effect of surface roughness of the laminated dielectrics on the resolution of lines and spaces. Ultra-low-k (ULK) dielectric materials having dielectric constants = 3 are selected for this study in addition to an industry standard dielectric material for comparison. These ULK materials were considered owing to their significance as build-up layers in 2.5D interposers with multiple redistribution layers (RDL). However, with their decreasing dielectric constants, they are increasingly inert, resulting in lower adhesion with copper. This leads to a question on whether these inert dielectric materials have higher roughness values compared to the standard dielectric material. To answer this question, the roughness values of these materials were measured using two different techniques., over different dimensions. While the 3D surface profiler was used to measure the roughness over larger areas, Atomic Force Microscope (AFM) was used for smaller areas. To correlate the surface roughness with resolution, fine line features are fabricated by the semi-additive processing (SAP) and the results of this study are reported in this paper.