ABSTRACT
Metalenses, ultra-thin optical elements that focus light using subwavelength structures, have been the subject of a number of recent investigations. Compared to their refractive counterparts, metalenses offer reduced size and weight, and new functionality such as polarization control. However, metalenses that correct chromatic aberration also suffer from markedly reduced focusing efficiency. Here we introduce a Hybrid Achromatic Metalens (HAML) that overcomes this trade-off and offers improved focusing efficiency over a broad wavelength range from 1000-1800 nm. HAMLs can be designed by combining recursive ray-tracing and simulated phase libraries rather than computationally intensive global search algorithms. Moreover, HAMLs can be fabricated in low-refractive index materials using multi-photon lithography for customization or using molding for mass production. HAMLs demonstrated diffraction limited performance for numerical apertures of 0.27, 0.11, and 0.06, with average focusing efficiencies greater than 60% and maximum efficiencies up to 80%. A more complex design, the air-spaced HAML, introduces a gap between elements to enable even larger diameters and numerical apertures.
ABSTRACT
Nickel nanostructures have found widespread application as both functional components, e.g. in magnetic systems, and as part of the lithographic pattern transfer process as etch masks, EUV mask absorbers, and imprint templates. Electron-beam induced etching of nickel is highly desirable for the repair and editing of masks and templates with high resolution and without substrate damage. However, there are no known gas-phase reactants that produce volatile nickel products under e-beam irradiation. Here we report the successful local etching of nickel by a focused electron beam in an environmental scanning electron microscope using a liquid reactant, aqueous sulfuric acid. Sulfuric acid did not spontaneously etch nickel under ESEM conditions, but nickel was etched in areas exposed to the electron beam. Etching parameters such as dose, refresh time, and addition of a surfactant were investigated. The extent of the etch increases with dose before terminating at sub-micron feature sizes. The etch resolution improves with the addition of surfactant. This approach enables local nickel patterning with complete film removal but without damaging underlying layers. With further refinement, the process may enable nickel absorber repair and editing and remove a significant obstacle to the use of nickel in EUV lithography.
ABSTRACT
Well-controlled, focused electron-beam induced etching of copper thin films has been successfully conducted on bulk substrates in an environmental scanning electron microscope by controlling liquid-film thickness with an in situ correlative interferometry system. Knowledge of the liquid-film thickness enables a hybrid Monte Carlo/continuum model of the radiation chemistry to accurately predict the copper etch rate using only electron scattering cross-sections, radical yields, and reaction rates from previous studies. Etch rates depended strongly on the thickness of the liquid film and simulations confirmed that this was a result of increased oxidizing radical generation. Etch rates also depended strongly, but non-linearly, on electron beam current, and simulations showed that this effect arises through the dose-rate dependence of reactions of radical species.
ABSTRACT
Variable-pressure electron-beam lithography (VP-EBL) employs an ambient gas at subatmospheric pressure to reduce charging of insulating films and substrates during electron exposure. In this work, VP-EBL proves to be an efficient method for patterning a widely used, but challenging to process, fluoropolymer, Teflon AF. However, rather than solely mitigating charging, the ambient gas is found to alter the radiation chemistry of the exposure process. Specifically, irradiating Teflon AF under water vapor increases the dissolution rate of the exposed regions in non-fluorinated solvents and enables complete patterning in a positive tone process. When compared to conventional e-beam resists, the contrast (≈4), clearing dose (<700 µC cm-2), and resolution (≈40 nm half-pitch) of Teflon AF are adequate. However, these figures of merit are quite remarkable when the process is considered as a means for directly patterning a functional material with extremely low surface energy, dielectric constant, and refractive index. Intriguingly, VP-EBL of Teflon AF under water vapor also exhibits non-reciprocity, through dose-rate dependence, and exhibits anomalous proximity effects. Thus, the influence of the ambient gas on radiation chemistry must be considered for VP-EBL, and some of the resulting effects may offer significant benefits for patterning both functional and lithographic materials.
