Chemical Etching and Patterning of Copper, Silver, and Gold Films at Low Temperatures

Copper, silver, and gold share unique properties that offer numerous application possibilities. For instance, low resistivity and high electromigration resistance make them attractive as interconnect layers in integrated circuits and microelectronic devices, while the optical properties of their nano-scale structures allow fabrication of plasmonic devices. Etching or patterning techniques are required in order to fabricate nano-structures, devices, and circuits. Although liquid or vapor phase techniques can be applied, plasma or glow discharge methods are typically invoked due to the need to generate anisotropic nanometer pattern sizes. Group 11 metals (Cu, Ag, and Au) form few volatile compounds at temperatures below 150°C, which limits the approaches that can be used to perform etching/patterning. Halogenated plasmas are widely used to etch metal layers; however, the low volatility of Cu, Ag, and Au halides precludes low temperature processes. Group 11 metals form hydrides readily in plasmas containing hydrogen species. Despite the thermodynamic instability of these metal hydrides, they appear to form at low (below room) temperature and can be desorbed from the etching surface by ion- or photon-assisted processes. Similarly, methylated metal etch products can be generated with hydrocarbon etching plasmas. As a result, etch rates above 12 ± 1 nm/min can be achieved, even when polymer forming plasmas (e.g., methane or other hydrocarbons) are used for patterning. These simple subtractive plasma etching approaches offer significant advantages relative to other vapor phase or liquid techniques in the manufacture of nano-scale devices and circuits.