Optimization of the absorber stack is becoming a critical issue in extreme ultraviolet (EUV) lithography. The attenuated phase shift mask (att-PSM) can be a long-term solution for continued technological improvements. The designed structure is based on the Fabry-Perot structure that consists of a TaN attenuator, an Al 2 O 3 spacer, and a Mo phase shifter. The total thickness of the absorber stack is 52 nm. The characteristics of the designed att-PSM are an EUV reflectivity (9.5% at 13.5 nm) and phase shift effect (180 at 13.5 nm) that maximize the image contrast in EUV exposure, and a low deep ultraviolet (DUV) reflectivity (1.72% at 257 nm) for high efficiency DUV inspection. All the simulations were carried out with the EM-Suite simulation tool. As a result, we determined that the att-PSM shows a higher image contrast, a larger process window, and a lower horizontal-vertical (H-V) CD bias than a binary image mask.

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The optimization of capping structure for extreme ultraviolet lithography was conducted to minimize the mask shadowing effect. The capping structure used in this study consisted of a phase shift layer and a capping layer on the 40 pairs of Mo–Si multilayer. A phase shift layer was added below tantalum nitride (TaN) absorber pattern. The authors analyzed the effect of the following capping materials such as ruthenium (Ru), silicon (Si), and molybdenum (Mo) with thickness variation. TaN absorber thickness for out-of phase condition shifted with the capping layer thickness change. The variation in capping materials showed less of an effect compared to thickness change. The addition of phase shift layer below absorber pattern showed a significant effect on the phase difference. Ru phase shift layer with a higher δ value than TaN absorber, shifted phase difference to the positive direction, whereas Si phase shift layer with a lower δ value than TaN absorber, shifted phase difference to the negative direction. However, phase difference was not shifted by the addition of Mo phase shift layer which has a similar δ value with TaN absorber. We also calculated the horizontal-vertical overlapping process window according to Mo phase shift layer thickness using 22nm 1:1 line and space pattern. As Mo phase shift layer thickness increased, the overlapping zone in the exposure latitude of the focus-exposure plots between the horizontal and vertical features increased.

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