Correlation of surface roughness with edge roughness in PMMA resist

The edge roughness of lithographically defined resist features is an important aspect of the microfabrication of semiconductor devices. Previous work has demonstrated the dependence of the surface roughness of PMMA resist on developer composition and conditions; from which it was concluded that the origin of surface roughness was polymer phase separation during the development step. In this work, the same AFM microscopy technique is used to investigate edge roughness. For very steep edges, the AFM technique is unable to image the resist roughness close to the substrate due to shadowing by the higher parts of the resist. However, the change in dose at the feature edge can be lithographically controlled, which allows the edge roughness of steep features to be determined by extrapolation from AFM measurements on shallow exposure gradients. The effects of resist polymer re-deposition, due to phase separation at the rinse step, are clearly seen.