Metrology – Computational Imaging past

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 Computational hybrid imaging reconstruction (Waller group)

Metrology – Computational Imaging

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Past Research Achievements

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A. EUV Microscope Characterization Using Photomask Surface Roughness

In a recent work, the Waller group developed a method for characterizing the aberrations of a full-field imaging system that does not require hardware modifications or the fabrication of test objects.1 While the method was applied to the SHARP High-NA Actinic Reticle Review Project (an EUV microscope that operates near 13.5 nm wavelength, Figure 1), the team found that it is suitable for any full-field imaging system that has coherent, steerable illumination. To implement, speckle images of a suitable object at multiple angles of plane-wave illumination were acquired. Importantly, the object being imaged does not need to be precisely fabricated; it only needs to have a pseudo-random surface, weak phase and sufficient power-spectral density extending to the imaging system’s resolution limit. For example, the statistical uniformity of the inherent, atomic-scale roughness of readily available photomask blanks enables a self-calibrating computational procedure using images acquired under standard operation. Across a 30-mm field-of-view, a minimum aberration magnitude of smaller than lambda/21 rms averaged over the center 5-mm area was demonstrated, with a measurement accuracy better than lambda/180 rms. This method has several advantages over other approaches that entail imaging test objects. First, no special fabrication is required, as suitable objects can be found opportunistically. Second, unlike periodic objects, uncorrelated surface roughness provides isotropic sampling of frequency space. Third, no registration or alignment of the test object is required, as the statistics of the roughness should not change across the object. Finally, the method is data-efficient; in the current implementation, 10 speckle images were used to recover all field-varying aberrations of up to order 5.

Figure 1. SHARP EUV microscope imaging configuration. A mirror conjugated with the object plane (which contains a blank EUV photomask) allows control over illumination angle. The objective lens (an off-axis Fresnel zone plate) images the beam scattered by the mask blank onto the sensor. The system suffers from field-dependent aberrations, primarily due to Petzval curvature.

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References

  1. G. Gunjala, A. Wojdyla, S. Sherwin, A. Shanker, M. P. Benk, K. A. Goldberg, P. P. Naulleau, and L. Waller, “Extreme ultraviolet microscope characterization using photomask surface roughness,” Scientific Reports, vol. 10, pp. 11673, Jul 2020.