Smear residue from the build-up dielectric material is left at the bottom of the microvia after laser drill process which, if not cleaned, poses risk to the electrical functionality of the device. Thus, microvia cleanliness is the key to a reliable and electrically functional device. Currently, industry employs a wet process to clean the etch residue that results in significant chemical waste. Here, we evaluated an alternative, but effective Photodesmear method that provides a low cost of ownership and almost negligible environmental impact. We have demonstrated in IMAPS 2013 that this process can achieve residue- and silica filler free via bottoms by a two-step process: i) illuminating 172 nm vacuum ultraviolet light (VUV) on the panels, resulting in a photochemical ashing, and ii) a water clean. This process does not reduce the surface energy of the build-up material, thus not impacting the downstream processes.
The main technical challenge in developing Photodesmear technology will be panel level uniformity in cleaning all the microvias within the same process step. We have demonstrated that our process can achieve a highly uniform treatment over 510 mm wide panels. The process was optimized to clean microvias with a range of aspect ratios on insulating film (material N) drilled by CO2 laser. The microvia bottoms were also found to be clean when the vias were drilled by UV laser to test the desmear capability.
The quality of the Photodesmear was tested by measuring the peel strength between electrolytically plated Cu and dielectric surface, and by performing the quick via pull (QVP) to verify the failing interface. We found high peel strength of 0.7 kgf/cm when sputtered Cu seed layer was used. QVP experiments confirmed that the via residue is cleaned effectively since the interface between the plated Cu and the underlying Cu pad did not fail. This study shows that Photodesmear process is capable to produce clean vias along with acceptable peel strength. Future issues are to research the reliability, productivity, and cost of the Photodesmear process to compare with the existing process.
Commentary: Problems and Prospects in the Utilization of Cytogenetics to Estimate Exposure at Toxic Chemical Waste Dumps
