Reliability of ptychography on periodic structures

Structural analysis of periodic samples with lensless imaging techniques in the extreme ultraviolet spectral range is an important and versatile tool for inspecting nanoscale structures including lithographic masks and quasi-crystals. More specifically ptychography is the method of choice for imaging such structures with high lateral resolution and an arbitrarily large field-of-view. This work reports on the reconstruction of highly periodic samples that occasionally contain isolated defects. For a realistic scenario scan map distortions, i.e. the actual scanning position does not exactly coincide with the position used as input for the reconstruction code, were taken into account. For such conditions it is well known, that the reconstruction will not provide always a physical reliable image. It will be shown, that the reconstructed illumination function is very sensitive to map distortions and small defects in otherwise periodic objects allowing to decide whether the reconstructed object is reliable or not. Applying this criterion the minimum detectable size of local defects in otherwise periodic structures will be compared to both the period length and the magnitude of the scan map distortion. As both quantities are usually much larger than the resolution given by the wavelength and numerical aperture of the imaging setup, they determine the resolution limit.