Submicron patterning of 1 in. diameter curved surfaces with a 46 mm radius of curvature has been demonstrated with step and flash imprint lithography ͑SFIL͒ using templates patterned by ion beam proximity printing ͑IBP͒. Concave and convex spherical quartz templates were coated with 700-nm-thick poly͑methylmethacrylate͒ ͑PMMA͒ and patterned by step-and-repeat IBP. The developed resist features were etched into the quartz template and the remaining PMMA stripped. During SFIL, a low viscosity, photopolymerizable formulation containing organosilicon precursors was introduced into the gap between the etched template and a substrate coated with an organic transfer layer and exposed to ultraviolet illumination. The smallest features on the templates were faithfully replicated in the silylated layer.

Dose rate measurements from Mars Science Laboratory-radiation assessment detector (MSL-RAD) for 300 sols on Mars are compared to simulation results using the Badhwar-O'Neill 2011 galactic cosmic ray (GCR) environment model and the high-charge and energy transport (HZETRN) code. For the nuclear interactions of primary GCR through Mars atmosphere and Curiosity rover, the quantum multiple scattering theory of nuclear fragmentation is used. Daily atmospheric pressure is measured at Gale Crater by the MSL Rover Environmental Monitoring Station. Particles impinging on top of the Martian atmosphere reach RAD after traversing varying depths of atmosphere that depend on the slant angles, and the model accounts for shielding of the RAD "E" detector (used for dosimetry) by the rest of the instrument. Simulation of average dose rate is in good agreement with RAD measurements for the first 200 sols and reproduces the observed variation of surface dose rate with changing heliospheric conditions and atmospheric pressure. Model results agree less well between sols 200 and 300 due to subtleties in the changing heliospheric conditions. It also suggests that the average contributions of albedo particles (charge number Z < 3) from Martian regolith comprise about 10% and 42% of the average daily point dose and dose equivalent, respectively. Neutron contributions to tissue-averaged effective doses will be reduced compared to point dose equivalent estimates because a large portion of the neutron point dose is due to low-energy neutrons with energies <1 MeV, which do not penetrate efficiently to deep-seated tissues. However the exposures from neutrons to humans on Mars should become an important consideration in radiobiology research and risk assessment.

Articles you may be interested inCombined helium ion beam and nanoimprint lithography attains 4 nm half-pitch dense patterns J. Vac. Sci. Technol. B 30, 06F304 (2012); 10.1116/1.4758768 Amplified nanopatterning by self-organized shadow mask ion lithography Appl. Phys. Lett. 97, 053102 (2010); 10.1063/1.3473770Scanning-helium-ion-beam lithography with hydrogen silsesquioxane resist The authors present neutral particle proximity lithography, a high resolution, parallel exposure technique where a broad beam of energetic neutral particles floods a stencil mask and transmitted beamlets transfer the mask pattern to resist on a substrate. It preserves the advantages of nanoscale penumbra, diffraction, and resist scattering of ion beam lithography ͑IBL͒ yet is intrinsically immune to charge accumulation on the mask and substrate. In a series of direct comparisons, involving insulating substrates, large proximity gaps, and ultrasmall features, the authors show that the use of neutral particles provides a simple method for completely eliminating the charging artifacts of IBL. They demonstrate the ability to print 8 nm mask features with 5 nm pattern fidelity. Exposure times are about 200 s in poly͑methyl methacrylate͒ resist.

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Electron beam and optical proximity effect reduction for nanolithography: New results J.One solution to stencil mask density limitations in particle lithography is to print dense patterns using multiple offset exposures of a lower density mask. The goal of this article is to demonstrate the ability to make high accuracy stepped exposures in proximity lithography with energetic neutral atoms where magnetic or electrostatic deflection is impossible. The authors' approach is to clamp the mask to the wafer, setting the proximity gap with a suitable spacer, and to mechanically incline the mask/wafer stack relative to the beam. This approach is remarkably insensitive to vibration and thermal drift; nanometer scale image offsets have been obtained with ±2 nm placement accuracy for experiments lasting over 1 h. The reported error may include significant contributions from shot noise, resist roughness, and metrology errors, not related to the stepping process itself. The authors also show that the standard deviation of Gaussian image blur is 4.4± 1.4 nm, about three times smaller than the secondary electron range limitation in electron beam lithography. Thus, the technology has the potential to form high density, periodic patterns with ϳ10 nm resolution.