(Henry B. Linford Award for Distinguished Teaching Address) Low Temperature Plasma Etching of Copper, Silver, and Gold Films
Authors: Hess Dennis W. 
Published: February 2020

Copper, silver and gold films have garnered considerable interest due to the numerous applications possible as a result of their unique electrical and optical properties. Specifically, low resistivity and good electromigration resistance make these films attractive as interconnect layers in integrated circuits and microelectronic devices, while nano structures of these metals can be used to form plasmonic devices.

Fabrication of structures, devices and circuits with these metals generally requires etching or patterning methods. However, the Group 11 metals are inherently difficult to reactively plasma etch at reasonable (<100 oC) temperatures because of the limited volatility of compounds that can be formed with these metals. As a result, additive processes (e.g., lift-off or electroplating followed by polishing) have been the preferred approach to generating patterns/structures. In contrast, pattern formation by subtractive etching offers simplicity, consistency with the patterning processes used for other films in device manufacture, and in some cases may alleviate limitations (size effect in Cu) in the fabrication of future generation devices.

In this presentation, etching of Cu, Ag, and Au in hydrogen-based plasmas at temperatures at or below room temperature is described. Group 11 metal hydrides form readily in an H2 plasma, suggesting that these compounds are the likely etch products, despite their limited thermodynamic stability. Due to the low ion flux and momentum of hydrogen ions under the conditions used in the inductively-coupled plasma reactor system, sputtering alone cannot account for the etch rates observed (13, 33, and 26 nm/min respectively for Cu, Ag, and Au at 20 mTorr and 10 oC in H2). In these cases, desorption of metal hydrides is promoted by ion, and likely photon, bombardment. Exposure of Cu, Ag, and Au to Ar and He plasmas demonstrates that Ag and Au etch rates increase with increasing atomic mass, while Cu etch rates decrease with ion mass, suggesting that chemical reactions may be more important than physical processes for Cu etching.

A limitation to the use of H2 plasmas for patterning of Cu, Ag, and Au is that photoresist layers are etched rapidly under the conditions used, thereby requiring hard masks such as SiO2. Furthermore, the feature slope generated with pure H2 plasmas is ~80o. Plasma etching with methane (CH4) plasmas under the same plasma conditions as those for H2 plasmas allows the use of a photoresist mask due to the deposition of a carbon layer during the etch process. An added benefit for Cu etching is that the etch rate increases to 17 nm/min relative to that of H2 (13 nm/min) with no change in wall slope. These enhancements relative to H2 plasma etching appear to be due to the formation of etch products that have improved thermodynamic stability compared to copper hydrides (e.g., CH3Cu and CH3CuH-). Preliminary etch studies of Ag and Au films using CH4 plasmas demonstrate that chemical aspects of the etch process are important, since etch rates are not reduced even with hydrocarbon deposition during the etch process.