Absolute distance interferometry for wafer-stage applications

Abstract

Future industrial lithography machines will use interferometers for wafer-stage position metrology. Interferometers suffer from phase wrapping, which is why in this thesis an absolute phase detection system has been developed. The absolute phase detection system will use two lasers, with the second laser allowing for limited changes in the wavelength. An efficient implementation of the excess fractions method has been developed to perform the absolute phase capture. To improve the longevity of the system the second laser should ideally be allowed to turn of when not used for absolute phase capture, which requires a wavelength estimation algorithm. Such a wavelength estimation algorithm has been implemented, and it can also provide an initial estimate for the OPD of the interferometer to be used in the excess fraction method. Both the excess fractions method and the wavelength estimation method have been experimentally verified. The results were satisfactory, thus providing all the building block for an absolute phase capture system.