Investigation of chemical de-burring and subsequent plasma cleaning of mechanically punched micro via array fabricated in LCP substrate

The purpose of this paper is to present new findings in process variability for wet chemical etching and subsequent plasma cleaning of mechanically punched micro vias fabricated in liquid crystal polymer (LCP) substrate. It was observed that the micro-mechanical punching process tends to form a LCP burr inside the through via, and form a copper burr on the bottom copper layer. The bottom copper layer is pre-laminated in ULTRALAM 3850, supplied by Rogers Corporation. An experimental procedure was designed to remove LCP and copper burr by chemical etching method. It was found that the conventional method of etching polymeric materials by strong base like KOH or NaOH does not work for LCP due to its high chemical resistance. Hence, the LCP surface had to be functionalized by using a strong oxidizer (NaKMnO₄) before starting conventional chemical etching by a strong base solution. A systematic approach has been developed to etch out the LCP and copper burr formation during the punching process. After examining three different experimental matrices it was found that a sequential treatment by oxidizer (NaKMnO₄), etchant (NaOH), and neutralizer (3%H₂SO₄ + 3%H₂O₂) gave the best results during the etching process. Considerable improvements were made on LCP and copper burr removal process using the wet chemical treatments including the development of an oxygen plasma treatment to clean the carbonated LCP debris produced during chemical etching. It was found that a subsequent oxygen plasma treatment after the wet chemical processes provided the cleanest through vias for interconnection on LCP substrates.