An understanding of the aging and oxidation of the (0001) surface of Bi2Se3 is critical to a comprehensive physical picture of its topologically protected surface states. Here, the authors contribute new experimental observations about the aging and oxidation process. The authors find that surface aging in ambient conditions occurs in two major steps. Within 2 h of exfoliation, a series of ∼3.2 Å high islands are observed by atomic force microscopy over approximately 10% of the surface. Subsequently, patch growth stops, and oxidation begins after the 2 h and continues until one quintuple layer has been oxidized. X-ray photoelectron spectroscopy shows no sign of oxidation before ∼120 min of exposure to air, and the oxygen 1 s peak, as well as oxidized Se 3d and Bi 4d peaks, are clearly present after ∼190 min of ambient exposure. Variable angle spectroscopic ellipsometry indicates that the oxidation of a full quintuple layer occurs on the time scale of days. These results are in good agreement with the time dependent changes observed in the surface crystal structure by second harmonic generation. In addition to providing the ability to nondestructively measure oxide on the surface of Bi2Se3 crystals, ellipsometry can be used to identify the thickness of Bi2Se3 flakes. With these methods, the authors have constructed a consistent, experimentally based model of aging process at the surface of Bi2Se3.

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Abstract. Measurement and control of line edge roughness (LER) is one of the most challenging issues facing patterning technology. As the critical dimensions (CDs) of patterned structures decrease, an LER of only a few nanometers negatively impacts device performance. Here, Mueller matrix (MM) spectroscopic ellipsometrybased scatterometry is used to characterize LER in periodic line-space structures in 28-nm pitch Si fin samples fabricated by directed self-assembly patterning. The optical response of the MM elements is influenced by structural parameters like pitch, CDs, height, and side-wall angle, as well as the optical properties of the materials. Evaluation and decoupling MM element response to LER from other structural parameters requires sensitivity analysis using scatterometry models that include LER. Here, an approach is developed that can be used to characterize LER in Si fins by comparing the optical responses generated by systematically varying the grating shape and measurement conditions. Finally, the validity of this approach is established by comparing the results obtained from power spectral density analysis of top down scanning electron microscope images and crosssectional transmission electron microscope image of the 28-nm pitch Si fins. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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In this paper, we propose a cross-grating structure to enhance the critical dimension sensitivity of one dimensional nanometer scale metal gratings. Making use of the interaction between slight changes in refractive index and localized plasmons, we demonstrate sub-angstrom scale sensitivity in this structure. Compared to unaltered infinite metal gratings and truncated finite gratings, this cross-grating structure shows robust spectra dependent mostly on the dimension of the smaller line width and pitch. While typical scatterometry simulations show angstrom resolution at best, this structure has demonstrated picometer resolution. Due to the wide range of acceptable specifications, we expect experimental confirmation of such structures to soon follow.

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