AVS 45th International Symposium
    Plasma Science and Technology Division Monday Sessions
       Session PS-MoM

Paper PS-MoM5
Notch Formation by Stress Induced Etching of Polysilicon

Monday, November 2, 1998, 9:40 am, Room 318/319/320

Session: Feature Evolution
Presenter: H. Sawin, Massachusetts Institute of Technology
Authors: J.P. Chang, Massachusetts Institute of Technology
H. Sawin, Massachusetts Institute of Technology
Correspondent: Click to Email
We have demonstrated that notch formation during over-etching of polysilicon is in part caused by stress enhanced spontaneous etching and is not solely a result of feature charging. Mechanical stresses applied to patterned polysilicon samples were shown to be effective in enhancing or reducing the extent of notching formation. The tensile stress at the polysilicon-oxide interface enhances spontaneous etching of polysilicon by reactive neutrals and leads to the notch formation. Non-uniform stress fields are observed within the patterned lines and large localized stresses are induced at the interface, especially the corner adjacent to the substrate. Stress concentration at the polysilicon-oxide interface can enhance the surface kinetics of etching, thereby causing notching. Notching has been attributed to the electric-field-induced ion trajectory distortion and the subsequent etching of polysilicon by these ions near the polysilicon-oxide interface. The large local charging potential at the silicon dioxide surface is caused by the difference in the directionality of ions and electrons; i.e. the isotropically directed electrons charge the photoresist sidewalls negatively and the directional ions charge the underlying oxide positively during overetch. For submicron features, the potential necessary to deflect low energy ions (<45eV) to form notches is ~500V/µm (5V/100Å) on an oxide surface. However, the magnitude of this field is within a factor of two to the breakdown voltage for bulk oxide, ~1000V/µm. Such a large field is likely to cause leakage along the surface, reducing the field and thereby preventing the deflection of ions to the extent needed to form notches. Even through bulk oxide, leakage is significant for 100Å thick oxide above applied voltages of 5V.