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VUV Birefringence Meter

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. Based on years of experience in polarization measurements, the BRV-100 was developed by JASCO to measure the birefringence of optical materials. 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.

System diagram


CaF2 retardation

The measured values agree with the value of 11.2 nm for the NIST standard. The precision of 0.05 nm/cm is much less than the acceptable birefringence value of 1 nm/cm for 157 nm optical lithography required by the ITRS.


Specifications

Wavelength range 140 to 320 nm
Sensitivity at 157 nm 0.05 nm
Stability at 157 nm 1/1000 per hour
Baseline at 157 nm 0.17 nm


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