Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

Anti-reflective coatings: A critical, in-depth review

Nanoimprint is an emerging lithographic technology that promises high-throughput patterning of nanostructures. Based on the mechanical embossing principle, nanoimprint technique can achieve pattern resolutions beyond the limitations set by the light diffractions or beam scatterings in other conventional techniques. This article reviews the basic principles of nanoimprint technology and some of the recent progress in this field. It also explores a few alternative approaches that are related to nanoimprint as well as additive approaches for patterning polymer structures. Nanoimprint technology can not only create resist patterns as in lithography but can also imprint functional device structures in polymers. This property is exploited in several non-traditional microelectronic applications in the areas of photonics and biotechnology.

https://deepblue.lib.umich.edu/bitstream/handle/2027.42/48920/d4_11_r01.pdf?sequence=2

Recent progress in nanoimprint technology and its applications

Standard solar cells heat up under sunlight. The resulting increased temperature of the solar cell has adverse consequences on both its efficiency and its reliability. We introduce a general approach to radiatively lower the operating temperature of a solar cell through sky access, while maintaining its solar absorption. We first present an ideal scheme for the radiative cooling of solar cells. For an example case of a bare crystalline silicon solar cell, we show that the ideal scheme can passively lower its operating temperature by 18.3 K. We then demonstrate a microphotonic design based on real material properties that approaches the performance of the ideal scheme. We also show that the radiative cooling effect is substantial, even in the presence of significant convection and conduction and parasitic solar absorption in the cooling layer, provided that we design the cooling layer to be sufficiently thin.

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Radiative cooling of solar cells

Broadband antireflection schemes for silicon surfaces based on the moth-eye principle and comprising arrays of subwavelength-scale pillars are applicable to solar cells, photodetectors, and stealth technologies and can exhibit very low reflectances. We show that rigorous coupled wave analysis can be used to accurately model the intricate reflectance behavior of these surfaces and so can be used to explore the effects of variations in pillar height, period, and shape. Low reflectance regions are identified, the extent of which are determined by the shape of the pillars. The wavelengths over which these low reflectance regions operate can be shifted by altering the period of the array. Thus the subtle features of the reflectance spectrum of a moth-eye array can be tailored for optimum performance for the input spectrum of a specific application.

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Tunable reflection minima of nanostructured antireflective surfaces

Antireflective surfaces based on biomimetic nanopillared arrays

An antireflection surface with sub-wavelength structure has been successfully fabricated on a fused silica substrate. The fabricated antireflection structured surface consists of a microcone array of fused silica with a period shorter than the wavelengths of visible light. The microcone array is made by a reactive ion etching method using fluorocarbon plasma. A microdisk array of chromium thin film, formed by an electron-beam lithography and lift-off process, is used as the etching mask. Since an electric field induced near the substrate was focused on the edges of the metal disks, these disks gradually shrank. Consequently, a conical shape was formed. The fabricated cone array has a period of 250 nm and a height of 750 nm. Measured reflectivity of the antireflection structured surface is less than 0.5% in the wavelength range of 400–800 nm for normal incidence.

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern : Optics and Quantum Electronics

An antireflection surface with sub-wavelength structure has been successfully fabricated on a fused silica substrate The fabricated antireflection structured surface consists of a microcone array of fused silica with a period shorter than the wavelengths of visible light The microcone array is made by a reactive ion etching method using fluorocarbon plasma A microdisk array of chromium thin film, formed by an electron-beam lithography and lift-off process, is used as the etching mask Since an electric field induced near the substrate was focused on the edges of the metal disks, these disks gradually shrank Consequently, a conical shape was formed The fabricated cone array has a period of 250 nm and a height of 750 nm Measured reflectivity of the antireflection structured surface is less than 05% in the wavelength range of 400–800 nm for normal incidence

https://iopscience.iop.org/article/10.1143/JJAP.40.L747/pdf

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern

Fabrication of patterns with curved cross-sectional profiles for diffractive optical elements are demonstrated by imprint lithography using replicated Ni mold. The master mold pattern is fabricated by an electron beam lithography, where dosage distribution is automatically optimized by a computer aided design (CAD) system taking the proximity effects and resist development process into account. Utilizing the CAD system, a modulated pitched resist pattern with polynomial shaped cross-sectional profiles are successfully obtained by poly (methylmethacrylate) (PMMA) on Si substrate. Using the PMMA resist as a master pattern, replicated mold is fabricated by Ni electroforming followed by Ni electroless plating. Fine and curved cross-sectional patterns are successfully transferred to a resist on Si substrate by imprint lithography using the Ni replicated mold. In the same way, fabrication of sine waved and circular curved structures are demonstrated. This method is exceedingly useful for fabrication of integrat...

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Imprint lithography for curved cross-sectional structure using replicated Ni mold

A compact spectroscopic imaging device consisting of a planar reflection grating lens, a probe fiber array, and a two-dimensional image sensor was proposed and discussed. Reflected or luminescent lights from a subject are coupled to the probe fibers, guided to fiber output ends, radiated into the air, diffracted by the grating lens with wavelength-dependent angle, and focused onto lines on the image sensor. Two-dimensional intensity distribution on the image sensor can give one-dimensional spectrum distribution along a specified direction. A grating lens was designed with a fiber array and a CCD image sensor for 100-nm wavelength range and 10-mm fiber array width. A spectral resolution of 5 nm and a spatial resolution of 0.25 mm were experimentally confirmed.

Planar reflection grating lens for compact spectroscopic imaging system

We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 µm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.

Masato Okano

Papers

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern : Optics and Quantum Electronics

Fabrication of Microcone Array for Antireflection Structured Surface Using Metal Dotted Pattern

Imprint lithography for curved cross-sectional structure using replicated Ni mold

Planar reflection grating lens for compact spectroscopic imaging system

Optimization of diffraction grating profiles in fabrication by electron-beam lithography.