Development of seed layer for electrodeposition of copper on carbon nanotube bundles

Abstract

The reliability of metal filled through silicon vias (TSVs) will determine their suitability in the semiconductor industry. The adoption of this technology will depend strongly on the materials used for filling vias that function as vertical interconnects. Conventional materials for filling TSVs include copper and tungsten. Carbon nanotubes (CNTs) are an excellent candidate to replace or be used in conjunction with these materials due to their superior electrical and mechanical properties. Metallic CNT bundles can allow nearly unhampered transport of electrons, resulting in a low resistance interconnect capable of withstanding high current densities. The growth of CNTs with ethylene and methane precursors by thermal chemical vapor deposition has been investigated using thin films of aluminum oxide and iron as catalysts. Nickel and cobalt were investigated as well with limited success. CNT growth is followed by electrochemical deposition (ECD) of copper. Traditionally, sputtered or evaporated Cu is used as a seed layer for ECD; however, the high temperatures necessary for CNT growth made Cu an unsuitable seed layer due to migration. The authors investigated combinations of ECD seed layers and CNT catalysts on the surface of a silicon wafer to qualify the best materials for the formation of a Cu/CNT matrix. ECD of Cu was studied on as-grown and densified CNTs with a tungsten seed layer. Once the process for covering CNT bundles with copper is optimized, future work will focus on placing the materials system inside cylindrical vias formed using an inductively coupled plasma etch process.