Aerial image formation in synchrotron-radiation-based x-ray lithography: The whole picture

Abstract

The problem of the image formation in a proximity x-ray lithography (XRL) system has been addressed several times in the past, while a full description of the system has never been implemented for the case of a synchrotron radiation source. It is then necessary to (a) define exactly the properties of source, optical system and mask on the image and (b) correlate the different terms into a common ground. We first discuss the degree of coherence of the synchrotron source in term of the the broadband soft x-ray spectrum and of the partial degree of coherence of the source (finite size) on the linewidth control and the processing latitude (broadening). The effect of the optical system is fully taken into account. We propose a new image calculation method based on fast Fourier transform. On the basis of the result of the aerial image calculation, we discuss the possibility of using a uniform bias on the mask pattern to improve linewidth control in the submicrometer feature range. The result shows that for printing 0.25-μm features, the synchrotron-radiation-based XRL is an ideal tool when the gap is set correctly and that typical wafer topography can be handled without any problem.