Robustness analysis of non-linear phase retrieval from single intensity measurement

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.