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Understanding Cleaning of Vias Fabricated Using Micro Mechanical Punching in Liquid Crystal Polymer (LCP) Substrate
Keywords: Wet Chemical Etching of Micro Vias, Oxygen Plasma Cleaning, Via Wall Roughness and Micro Texturing
The objective of the research is to understand the effect of wet chemical etching treatment time and plasma cleaning conditions on via wall roughness and adhesion of the electroplated through via fabricated using micro mechanical punching (MMP) process on liquid crystal polymer (LCP) substrate. One drawback of micro mechanical punching process is the generation of LCP and copper burr and needs post-cleaning. Sequential wet chemical etching and oxygen plasma cleaning technique are being explored as effective hybrid tool for removal of burr residues. The wet chemical process includes oxidation, etching, and acid neutralization treatment steps on the LCP substrate. These treatments have an effect on the surface roughness of the through via wall. To understand this further, time dependent experiments were designed for the wet chemical etching. Sequential oxidation, etching, and acid neutralization processes were performed, where only one process time was varied for 05, 10, and 15 minutes keeping the other process time constant for 05 minutes. After examining three different experimental matrices to study the via wall roughness using scanning electron microscope (SEM) it was observed that the longer etching time generates higher surface roughness. On the other hand, oxygen plasma cleaning tends to initiate precipitation of LCP burr at the edge. For plasma cleaning, experimental conditions were designed to examine the effect of chamber pressure and power delivered for plasma cleaning without any LCP precipitation. It was observed that there is a significant correlation between plasma pressure and power on the rate and nature of carbonated and precipitated LCP burr removal. In addition, oxygen plasma creates micro and sub-micro texture on via wall, which have an effect on the adhesion between the electroplated via and the wall. The effect of texturing on adhesion between the plated via and the wall was tested using MIL standard thermo-cycling and thermo-shock. The micro texture formation has a direct correlation on improvement of adhesion property between the electroplated via and via wall.
Mohammad K. Chowdhury, Ph.D. Student
University of Arkansas
Fayetteville, AR

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