Metal Deposition for Microelectronics Using CO2 as a Solvent

MiCell Technologies, Inc., proposes a new process for the deposition of metallic thin films of copper, ruthenium, titanium, and other metals used as barrier layers, seed layers, and interconnects. This process would replace the current electroplating approach used in filling deep trenches and forming thin films in microelectronic circuit manufacturing. The electroplating process generates large quantities of aqueous wastes with copper ions and other dangerous chemicals that must be treated in place. The process being proposed utilizes liquid or supercritical carbon dioxide as the solvent. In addition to being environmentally benign, this process also will provide additional control of the metal deposition processes to create high-quality films and electrical interconnects. This project is part of an overall strategy to replace all aqueous and organic solvents in microelectronics fabrication. The proposed fluid displacement deposition process utilizes a two-step approach to the formation of the deposited metallic layers. In the first step, organometallic precursors will be dissolved in either liquid or supercritical carbon dioxide. The wafer to be coated will be immersed in either the liquid or supercritical solvent. This solution will be displaced either with carbon dioxide itself or by a second fluid, such as helium, in the supercritical state. This displacement step will cause the formation of a thin film that will result in the deposition of the organometallic precursor on the wafer surface. Because of the low surface tension and viscosity of the carbon dioxide phase, the precursor will penetrate uniformly into the narrow gaps on the surface of the circuit. After this film displacement step, the system can be heated and a reducing agent, such as hydrogen, can be introduced to remove the organic ligands bound to the metal atoms. After the reduction step, a solid metallic layer will remain on the surface, which will form the desired interconnect or thin layer structures. Phase I will determine the more important operating variables in both the liquid and supercritical carbon dioxide surface deposition processes. Phase II will involve the design of a metallization tool that will meet the operating requirements of industrial microelectronics fabrication. Because of the demand for faster, more sophisticated structures in modern electronic products, copper interconnects and metallic barrier and seed layers will play an increasing role in device fabrication. This environmentally benign process will have a preferred place in the marketplace.