Photomasks are expected to have phase effects near edges due to their 3D topography, which can be modeled as imaginary boundary layers in thin mask simulations. We apply a modified transport of intensity (TIE) phase imaging technique to through-focus aerial images of photomasks in order to recover polarization-dependent edge effects. We use AIMS measurements with 193nm light to study the dependence of recovered phase on mask type and geometry. The TIE is an intensity conservation equation that quantitatively relates phase in the wafer plane to intensity through-focus. Here, we develop a modified version of the TIE for strongly absorbing objects, and apply it to recover wafer plane phase of attenuating masks. The projection printer blurs the fields at the wafer plane by its point spread function, hence an effective deconvolution is used to predict the boundary layers at the mask that best approximate the measured thick mask edge effects. Computation required for the inverse problem is fast and independent of mask geometry, unlike FDTD computations.

We report theoretical and experimental results for imaging of electromagnetic phase edge effects in lithography photomasks. Our method starts from the transport of intensity equation (TIE), which solves for phase from through-focus intensity images. Traditional TIE algorithms make an implicit assumption that the underlying in-plane power flow is curl-free. Motivated by our current study, we describe a practical situation in which this assumption breaks down. Strong absorption gradients in mask features interact with phase edges to contribute a curl to the in-plane Poynting vector, causing severe artifacts in the phase recovered. We derive how curl effects are coupled into intensity measurements and propose an iterative algorithm that not only corrects the artifacts, but also recovers missing curl components.

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Chromeless Phase Lithography (CPL) is discussed as interesting option for the 65nm node and beyond offering high resolution and small Mask Error Enhancement Factor. However, it was shown recently that at high NA CPL masks can exhibit large polarization and also phase effects. A well known phase effect occurring for CPL semi dense lines are through focus Bossung tilts. However, another manifestation of phase effects for dense lines and spaces is a reduced contrast for a symmetrical offaxis illumination due to phase errors between 0 th and 1 st diffraction order. In this paper it is shown that these phase effects can lead to a significant contrast loss for dense features smaller than 60nm half pitch. While also present for trench structures, the contrast reduction is more pronounced for mesa style structures. It is shown that for mesa structures an adjustment of etch depth can not recover an effective pi-phase shift. Furthermore, significant polarization effects are observed. As an example, the optimum mesa structure for TE polarization is shifted to small lines. For an experimental validation, a CPL mask containing dense lines and spaces was fabricated. Their imaging performance was characterized with an AIMS 45i offering NA's greater than 1 and linearly polarized illumination as well as by wafer printing. Gratings with pitches down to 100 nm with varying duty cycles were measured with TE, TM and unpolarized dipole illumination. Very good agreement between measurement and simulation results confirmed the validity of theoretical predictions.