A Label-Free Measurement Method for Plane Stress States in Optical Isotropic Films with Spectroscopic Ellipsometry

Background

Stress measurement for thin films is crucial in a variety of fields such as in semiconductor manufacturing, the optoelectronics industry, and biomedical science, among others. However, most measurement methods require surface treatment of the thin film.

Objective

A label-free measurement method for plane stress states in optical isotropic thin films based on spectroscopic ellipsometry analysis is proposed and verified in this paper.

Methods

The proposed method is based on the modulation of the stress-optic effect on reflected spectroscopic ellipsometry. A theoretical model is established to describe the relation between all components of the plane-stress state and the classic ellipsometric parameters (Ψ, Δ). An algorithm is developed to determine all components of a plane-stress state by fitting the model to the experiment data.

Results

In the verification experiment, we determined the plane stress state of a Cu film coated on a PI (polyimide) substrate. The results show a reasonable agreement between the experimental measurements from spectroscopic ellipsometry and the theoretical analysis based on the applied loading.

Conclusion

The results prove that our method can effectively measure the plane stress state of optical isotropic thin films.