Anisotropic plasma etching of polysilicon using SF6 and CFCl3

Abstract

Undercutting of polysilicon overetched in fluorine-based plasmas has been limited to 0.1 μ per edge by adding small amounts of CFCl3 to SF6. This 0.1 μ undercut occurs during the initial etching stage and does not increase with overetch. To test possible mechanisms, these experiments were repeated with CHF3 and Cl2 used in place of CFCl3. It was found that Cl and photoresist are responsible for passivating the sidewalls of the polysilcon lines. This passivation is relatively insensitive to exact amount of CFCl3 added. Etch rate measurements were made for polysilicon, SiO2, and AZ 1470 photoresist and found to be essentially indistinguishable from those obtained in pure SF6 etching, thus the fast polysilicon etch rate and good selectivity characteristics of SF6 etching are preserved. The ability to etch polysilicon without sacrificing linewidth, selectivity, or etch rate makes CFCl3/SF6 plasma etching very useful for VLSI processing.