Investigation and Optimization of Rowland ghosts in high efficiency spectrometer gratings fabricated by e-beam lithography

Any violation of the periodicity of a perfect grating will result in diffuse scattering. In the particular case of a periodic violation the generated stray light shows deterministic, also periodic features that arise as distinct peaks in the stray light spectra, especially so-called Rowland ghosts. In this paper the characteristics of the spurious Rowland ghosts in binary spectrometer gratings are investigated and the potential of a randomization technique in order to suppress the Rowland ghosts is analyzed. Especially in sequential fabrication technologies, e.g. electron beam lithography, the Rowland ghosts originate in a segmentation process that is necessary in order to write large scale gratings. Hence several subareas are sub sequentially exposed and stitched together leading to the final full size grating. Due to this stitching approach the subareas induce secondary periodic structures and thus generate the spurious Rowland ghosts in the order of magnitude of < 10(-4) compared to the useful diffraction order. A randomization of this segmentation process is investigated both theoretically in rigorous simulations and experimentally by fabricating a purposely designed optical grating. As approach for the randomization in experiment we applied a special multi-pass-exposure. Here the sample is exposed multiple times with an accordingly shifted and dose-reduced subarea in each pass. The achieved simulation results show that a randomization of the subareas effectively reduces the Rowland ghosts. Furthermore the applied randomization technique during grating fabrication was able to suppress one kind of Rowland ghosts completely.