A non-linear phase retrieval algorithm is used to characterise the aberrations in an Extreme Ultraviolet (EUV) stepper. The retrieval is based on intensity measurement in a single plane in the focal volume. A statistical-based characterisation of the robustness of the algorithm with respect to the out-of-focus distance has been carried out. This allows to identify a measurement plane that is optimal for the phase retrieval and reduces the computation time and the complexity of the problem. Experimental results obtained in the visible spectrum range confirm the predictions of the simulations and are in a good agreement with an independent wavefront measurement. The phase retrieval method can be used to implement a dedicated adaptive optics system in a EUV stepper.
Measurement techniques to determine the aberration of an optical system, by obtaining through-focus intensity
images that are produced when the object is a point source at infinity, are shown. The analysis of the aberrations
is made using the extended version of the Nijboer-Zernike diffraction theory. This theory provides a semi
analytical solution of the Debye diffraction integral and thus a direct relation between the intensity distribution
of the field at the focal region and the exit pupil of the optical system.
A Hartmann Wavefront Sensor (HWS) is used as a tool to measure phase aberration at the EUV wavelength.
Nevertheless, a conventional HWS measures only the wavefront slope in each sub-aperture and is not able to
measure the phase structure inside it. This leads to an accuracy loss in the aberration reconstruction. In this
work a phase retrieval algorithm is applied to the intensity pattern data in order to reconstruct the phase feature
inside the sub-aperture and hence improve the accuracy. Experimental data confirms our simulations making
this technique feasible regarding both the achieved accuracy and computational time. The phase information
can be used to develop an adaptive optics system dedicated to a EUV stepper.
Accurate wavefront aberration measurement are essential for next-generation Extreme Ultraviolet (EUV) Lithography.
During the past years several accurate interferometric techniques have been developed, but these techniques
have limitation. In this work we discuss a different technique based on the Hartmann Wavefront Sensor
that requires no interferometry. We present a mathematical model of this system and describe our experimental
setup which demonstrates the feasibility and advantages in terms of dynamic range and accuracy compared to
interferometric techniques.
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