Anisotropic and selective reactive ion etching of polysilicon using SF6

Experiments performed in a reactive ion etching system are discussed with the purpose of displaying the influence of reactive gas nature, ion energy, and the nature of the cathode on etching anisotropy and selectivity. Using SF6 etching, isotropy or anisotropy is believed to be related to the enthalpy of product formation reaction. Cathodes consisting of elements that form volatile fluorides lead to perfect anisotropic etching and low selective etching of polysilicon over Si02; whereas cathode material which does not react leads to higher selectivity but undercutting appears with an overetch. The possibility of achieving anisotropic and selective etching of polysilicon over thermal Si02 (etch rate ratio = 10:1) is demonstrated for NMOS technology. Examples of polysilicon gates of submicron (0.6 μ) MOST are given.