September 30, 2011 — Engineers at the NIST Center for Nanoscale Science and Technology (CNST) NanoFab have developed a plasma etch/argon process that improves etch rate, mask selectivity, and the sidewall profile of silicon structures.
The technique optimizes the addition of argon to the process flow, creating fluorinated plasma chemistry that is inherently isotropic. Alternative etch and argon-only steps enables users to fabricate small, high aspect ratio silicon structures more quickly and with less difficulty. In contrast, directly adding argon to a typical SF6/C4F8 plasma causes dilution and reduces the etch rate.
In a deep silicon etch, mixing argon with the etchant gases creates little or no etch rate improvement. C4F8 is used to protect the Si sidewalls and SF6 is used to etch. The researchers found tha alternating argon surface bombardment with chemical etch resulted in a four-fold increase in the silicon etch rate. The silicon structures maintained vertical sidewalls. The silicon etch rate increases with the argon step time, independent of the SF6 step time, and the argon bombardment step is rate-determining. It influences the etch rate, as well as the selectivity and etching profile.
Argon surface bombardment renders the top atomic layers of the silicon amorphous, and then gas phase fluorine can react with and remove the silicon, the researchers theorized.
Results are published in Microelectronic Engineering: Effect of alternating Ar and SF6/C4F8 gas flow in Si nano-structure plasma etching, L. Chen, V. Luciani, and H. Miao, Microelectronic Engineering 88, 2470-2473 (2011). Access it here: http://www.sciencedirect.com/science/article/pii/S0167931710005708
The National Institute of Standards and Technology (NIST) is an agency of the U.S. Department of Commerce. Learn more at http://www.nist.gov/cnst/.