Image contrast metrology for EUV lithography

Image contrast is an important measure of image quality, especially for EUV lithography where high contrast can mitigate stochastic process fluctuations(1) such as Line-Edge Roughness (LER), Line Width Roughness (LWR), Local CD Uniformity (LCDU) and stochastic defects. In this paper, several aspects of image contrast for EUV lithography are discussed. We will look at some of the fundamental mechanisms which degrade image contrast, including resist blur from acid diffusion. We assume the imaging of dense line/space images with k(1)<0.5. This assumption guarantees that the image is a simple sinusoid, and implies that the exposure latitude is proportional to the image contrast. We then consider various methods to experimentally measure image contrast. The classic measure would be to measure the exposure latitude from a Focus/Exposure Matrix (FEM) wafer. The exposure latitude is generally giving a contrast measure which is averaged over some portion of the wafer. In this paper, we propose the use of MEEF (Mask Error Enhancement Factor) targets to track contrast at specific locations on the wafer. In principle, one could measure many points across the image field (or even across the whole wafer) to map out the spatial variation of contrast, i.e. a contrast map. The paper includes experimental contrast measurements that relate to stage fading, source shape and resist blur from different processes. We will also briefly look at fading from the Pole-to-Pole image shift, and how to mitigate this with a novel exposure method called Dual Monopole.