Inverse problem solving and optical index determination of resist films by ellipsometry

Spectroscopic ellipsometry (SE) is known to be a technique of great sensitivity in thickness determination of thin layers. The sensitivity is said to be close to some angstrom when optical indexes of materials are perfectly known. However, for resist films, those optical indexes are unknown and can vary from one process to another. Optical indexes and film thicknesses are determined by using Fresnel laws in order to calculate theoretical ellipsometry signatures and by solving the inverse problem. This article presents two strategies developed by LTM in order to accurately determine optical indexes and the thickness of resist films.