High-density objects, such as metal prostheses or surgical clips, generate streak-like artifacts in CT images. We designed a radial adaptive filter, which directly operates on the corrupted reconstructed image, to effectively and efficiently reduce such artifacts. The filter adapts to the severity of local artifacts to preserve spatial resolution as much as possible. The widths and direction of the filter are derived from the local structure tensor. Visual inspection shows that this novel radial adaptive filter is superior with respect to existing methods in the case of mildly distorted images. In the presence of strong artifacts we propose a hybrid approach. An image corrected with a standard method, which performs well on images with regions of severe artifacts, is fused with an adaptively filtered clone to combine the strengths of both methods.
Multilayer interference coatings on reflective elements in extreme ultraviolet (EUV) projection lithography systems introduce phase and amplitude variations at reflection, which have important implications for imaging properties, e.g., resolution, depth of focus, and tolerances. We discuss the numerical results of the optical effects of multilayers (MLs) and the inclusion of these effects in optical design. This numerical study demonstrates the advantages of spatially varying (graded) MLs compared to multilayers with constant layer thicknesses. We present a new method to calculate the optimum grading of multilayers. Using this new method, we are able to fully optimize the wavefronts emerging from the projection system toward the image plane.
Extreme UltraViolet (EUV) projection systems consist of reflective optics, as the 13.4 nm illuminating radiation is highly absorbed in all materials. The reflectors are multilayers, which typically consist of alternating layers of molybdenum and silicon. The multilayers entail important consequences for the imaging properties, such as resolution, depth of focus and tolerances. To incorporate the influence of multilayers in optical design software an approach using the effective reflective depth is proposed. A new method to calculate the spatially varying optimum thickness of multilayers ('grading') is presented.
The absence of obstructions is an essential requirement for reflective ring-field projection systems to be used in EUV lithography. However, due to the large number of variables and constraints involved, choosing unobstructed starting configurations for subsequent optimization is a nontrivial issue in EUV system design. For this purpose we have developed a systematic method based on paraxial obstruction analysis. Despite the fact that in the parameter space of all possible system configurations unobstructed domains are very small, we can identify the unobstructed regions very effectively. The paraxial results are validated through comparison with ray tracing. We can convert even most of the paraxial solutions that lead to ray failure into ray-traceable starting points for optimization. We also introduce a new classification method based on the relative arrangement of the mirrors in a EUV system - a feature which in typical situations is unaffected by optimization. We present some new possible design forms for EUV imaging systems belonging to different classes and show the remarkable flexibility of the obscuration borders.