EUV mask absorber induced best focus shifts

We investigate the induced best focus shifts by the mask absorber. The effect of n, k, bias and target size on BF shifts is studied. We consider lines and spaces with pitch = 5× target size. We present a correlation between the BF shifts and Zernike phase offset and the fourth-order Zernike coefficient that represents defocus. When no mitigation strategies are applied, low n absorber materials can show stronger BF shifts and stronger phase variation versus target size. The knowledge gained from this study will help to identify combinations of absorber properties (n, k, thickness) and biasing strategies, which provide high NILS, and threshold to size and enable proper focus control.