Quantitative access to phase effects in high numerical aperture extreme-ultraviolet photomasks using aerial image metrology

BackgroundThe upcoming introduction of high-numerical aperture extreme ultraviolet (EUV) lithography is accompanied by ongoing activities to mitigate the imaging issues combined with the 3D photomask absorber. In addition to suitable exposure and patterning techniques, the absorber material has a large impact on image contrast and the usable depth of focus.AimThe study of solutions to the imaging downsides combined with the EUV reflective mask typically is based on rigorous imaging simulations. We show that aerial image metrology using Aerial Image Measurement System (AIMS)® EUV is a supplementary way to verify the impact of mask 3D effects in lithography.ApproachAn AIMS® EUV study of the impact of mask 3D effects on the through-focus image formation in a three-beam interference regime is presented.ResultsExperiments on a 20-nm vertical equal lines and spaces clip using dedicated monopole illumination demonstrate a prominent, focus-dependent line-shape behavior for each of the pole positions. Applying an analytical model for three-beam-imaging in the presence of defocus and mask 3D effects, we obtain excellent agreement between measurements and theory that allows us to derive the mutual phase shifts of the single interference cross-terms involved.ConclusionsUsing AIMS® EUV, an evaluation of the imaging impact of various absorber materials can be done experimentally without relying on wafer exposures.