Dr. Ramsankar Senthamaraikannan Profile

Dr. Ramsankar Senthamaraikannan

at Univ College Cork

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Publications (4)

Proceedings Article | 20 August 2020 Poster

Block co-polymers for etch mask definition to create anti-reflective surfaces

Brian Jennings, Riley Gatensby, Elsa Giraud, Andrew Selkirk, Sajjad Mir, Ramsankar Senthamaraikannan, Parvaneh Mokarian-Tabari

Proceedings Volume 11467, 1146727 (2020) https://doi.org/10.1117/12.2568487

KEYWORDS: Etching, Glasses, Metals, Nickel, Oxides, Optical components, Optical lithography, Reflectivity, Refractive index, Visible radiation

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Proceedings Article | 26 July 2016 Presentation

Nanopatterning by large block copolymers for application in photonic devices (Conference Presentation)

Parvaneh Mokarian-Tabari, Ramsankar Senthamaraikannan, Timothy Collins, Colm Glynn, Colm O'Dwyer, Michael Morris

Proceedings Volume 9885, 98850B (2016) https://doi.org/10.1117/12.2227766

KEYWORDS: Silicon, Nanolithography, Nanostructures, Photonic devices, Reflectivity, Photonic crystals, Manufacturing, Polymers, Infrared cameras, Molecular self-assembly

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The extensive benefits of the new generation of nanostructured surfaces is very promising for enhancing light absorption efficiency in photonic devices. However, the low throughput and the high cost of available technologies such as lithography for fabrication of nanostructures has proved to be a difficult technological hurdle for advanced manufacturing. In this research we present a solution based process based on high molecular weight block copolymer (BCP) nanolithography for fabrication of periodic structures on large areas of optical surfaces. Block copolymer self- assembly technique is a solution based process that offers an alternative route to produce highly ordered photonic crystal structures. BCPs forms nanodomains (5-10 nm) due to microphase separation of incompatible constitute blocks. The size and shape of the nanostructure can be customised by the molecular weight and volume fraction of the polymer blocks. However, the major challenge is BCPs do not phase separate into their signature ordered pattern above 100 nm, whereas for nanofeatures to be used as photonic gratings, they must be greater than 100 nm (typically ¼ wavelength). This is due to significant kinetic penalty arising from higher entanglement in high molecular weight polymers. In this work we present the results of exploiting commercially available block copolymers to phase separate into periodic domains greater than 100 nm. The process do not include any blending with homopolymers, or adding colloidal particles, and to our best knowledge, has not been yet achieved or reported in the literatures. We have pattern transferred the BCP mask to silicon substrate by reactive ion etch (ICP-RIE). The final product is black silicon, consists of hexagonally packed conic Si nanofeatures with diameter above 100nm and periodicity of 200 nm. The height of the Si nanopillars varies from 100 nm to 1 micron. We have characterized the angle dependent optical reflectance properties of the black silicon. The antireflective properties of the Si nanofeatures were probed in the 400 nm – 2500 nm wavelength range and compared to an Au reflectance standard. As the subwavelength grating is made from the same material as the substrate (Si), the index matching at the substrate interfaces has lead to highly improved antireflecting performance. The reflectivity of the silicon substrate shows one order of magnitude reduction in a broad range of wavelength from NIR to UV-visible, below 1%. The simplicity of the solution based large block copolymer nanolithography and the capability of integration to existing fabrication process, makes this novel technique a very attractive alternative for manufacturing photonic crystals on large, arbitrary shaped and curved objects such as photovoltaics and IR camera lenses for medical imaging.

Proceedings Article | 19 March 2015 Paper

The development and advantages of helium ion microscopy for the study of block copolymer nanopatterns

Alan Bell, Ramsankar Senthamaraikannan, Tandra Ghoshal, Atul Chaudhari, Michael Leeson, Mick Morris

Proceedings Volume 9424, 94241R (2015) https://doi.org/10.1117/12.2087494

KEYWORDS: Scanning electron microscopy, Atomic force microscopy, Polymers, Imaging systems, Ions, Picosecond phenomena, Scanning helium ion microscopy, Electrons, Photomasks, Neodymium

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Helium ion microscopy (HIM) has been used to study nanopatterns formed in block copolymer (BCP) thin films. Owing to its’ small spot size, minimal forward scattering of the incident ion and reduced velocity compared to electrons of comparable energy, HIM has considerable advantages and provides pattern information and resolution not attainable with other commercial microscopic techniques.

In order to realize the full potential of BCP nanolithography in producing high density ultra-small features, the dimensions and geometry of these BCP materials will need to be accurately characterized through pattern formation, development and pattern transfer processes. The preferred BCP pattern inspection techniques (to date) are principally atomic force microscopy (AFM) and secondary electron microscopy (SEM) but suffer disadvantages in poor lateral resolution (AFM) and the ability to discriminate individual polymer domains (SEM). SEM suffers from reduced resolution when a more surface sensitive low accelerating voltage is used and low surface signal when a high accelerating voltage is used. In addition to these drawbacks, SEM can require the use of a conductive coating on these insulating materials and this reduces surface detail as well as increasing the dimensions of coated features. AFM is limited by the dimensions of the probe tip and a skewing of lateral dimension results. This can be eliminated through basic geometry for large sparse features, but when dense small features need to be characterized AFM lacks reliability. With this in mind, BCP inspection by HIM can offer greater insight into block ordering, critical dimensions and, critically, line edge roughness (LER) a critical parameter whose measurement is well suited to HIM because of its’ enhanced edge contrast.

In this work we demonstrate the resolution capabilities of HIM using various BCP systems (lamellar and cylinder structures). Imaging of BCP patterns of low molecular weight (MW)/low feature size which challenges the resolution of HIM technique. Further, studies of BCP patterns with domains of similar chemistry will be presented demonstrating the superior chemical contrast compared to SEM. From the data, HIM excels as a BCP inspection tool in four distinct areas. Firstly, HIM offers higher resolution at standard imaging conditions than SEM. Secondly, the signal generated from He⁺ is more surface sensitive and enables visualization of features that cannot be resolved using SEM. Thirdly; superior chemical contrast enables the imaging of un etched samples with almost identical chemical composition. Finally, dimensional measurement accuracy is high and consistent with requirements for advanced lithographic masks.

Proceedings Article | 27 March 2014 Paper

An in-situ hard mask block copolymer approach for the fabrication of ordered, large scale, horizontally aligned, Si nanowire arrays on Si substrate

Tandra Ghoshal, Ramsankar Senthamaraikannan, Matthew Shaw, Justin Holmes, Michael Morris

Proceedings Volume 9051, 90510J (2014) https://doi.org/10.1117/12.2045754

KEYWORDS: Silicon, Oxides, Iron, Etching, Annealing, Transmission electron microscopy, Polymers, Scanning electron microscopy, Bioalcohols, Nanowires

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