System Description

CaF₂ is the most suitable material to be used for lenses in 157 nm optical lithography applications. However, CaF₂ is an anisotropic material subject to birefringence, distorting the focal point of the laser light used in optical lithography. Although the birefringence errors can be corrected, the CaF₂ lenses and optical systems require accurate characterization to ensure a proper focal point of the illuminating laser. Characterization of the CaF₂ birefringence requires a metrological measurement with extremely high accuracy.

A highly sensitive birefringence instrument using a PEM (photoelastic modulator) for polarization modulation, the transmission wavelength is extended to the vacuum ultraviolet region by purging the optical path with nitrogen to reduce absorption by oxygen and water vapor.

System Features

• High sensitivity

The phase difference angle is calculated from an arc-sine function providing a highly sensitive measurement of birefringence below 1 nm/cm. The birefringence (F-signal) is calculated as f = -sin(D), with accurate detection of signals around zero.

• High stability

The LiF PEM is controlled by an optical servo loop, providing measurements with highly stable ordinate values. A reference beam monitors the retardation amplitude of the PEM to observe an achromatic modulation even in the vacuum ultraviolet region. The optical servo provides a stable measurement of below 0.1% in the purged instrument environment.