The block copolymer self-assembly approach has received great attention in recent years as a possible way to overcome the limits of conventional lithography and to fabricate sub-22 nm structures. At this level, precise nanometric control is crucial for technological applications and the search for a flexible and reproducible protocol is a great challenge. The polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) system, with a styrene fraction of 0.71, spontaneously separates into a periodic array of hexagonally packed PMMA cylinders embedded in a matrix of PS and, under suitable processing conditions, this is perpendicularly oriented with respect to the underlying substrate. The selective removal of the PMMA allows us to obtain a nanoporous PS matrix with well-defined pore dimensions. Perpendicular orientation of the PMMA cylinders requires surface neutralization by means of a suitable PS-r-PMMA random copolymer. The choice of the random copolymer is not trivial, because different PS-r-PMMA copolymers strongly affect the characteristics of the PS-b-PMMA film deposited on it. In this paper the effects of the selected PS-r-PMMA on the arrangement as well as on the peculiar dimensions (pore diameter, pore to pore distance) of the final nanoporous PS thin film are studied. Reliable protocols for the fabrication of a disposable polymeric mask are proposed in view of its application in advanced lithographic processes.
Patterned nanoscale materials with controllable characteristic feature sizes and periodicity are of considerable interest in a wide range of fields, with various possible applications ranging from biomedical to nanoelectronic devices. Block-copolymer (BC)-based lithography is a powerful tool for the fabrication of uniform, densely spaced nanometer-scale features over large areas. Following this bottom-up approach, nanoporous polymeric films can be deposited on any type of substrate. The nanoporous periodic template can be transferred to the underlying substrate by dry anisotropic etching. Nevertheless the physical sizes of the polymeric mask represent an important limitation in the implementation of suitable lithographic protocols based on BC technology, since the diameter and the center-to-center distance of the pores cannot be varied independently in this class of materials. This problem could be overcome by combining block copolymer technology with atomic layer deposition (ALD): by means of BC-based lithography a nanoporous SiO2 template, with well-reproducible characteristic dimensions, can be fabricated and subsequently used as a backbone for the growth of perfectly conformal thin oxide films by ALD. In this work polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) BC and reactive ion etching are used to fabricate hexagonally packed 23 nm wide nanopores in a 50 nm thick SiO2 matrix. By ALD deposition of Al2O3 thin films onto the nanoporous SiO2 templates, nanostructured Al2O3 surfaces are obtained. By properly adjusting the thickness of the Al2O3 film the dimension of the pores in the oxide films is progressively reduced, with nanometer precision, from the original size down to complete filling of the pores, thus providing a simple and fast strategy for the fabrication of nanoporous Al2O3 surfaces with well-controllable feature size.
Ultra low energy ion implantation is a promising technique for the wafer-scale fabrication of Silver nanoparticle planar arrays embedded in thermal silica on silicon substrate. The stability versus time of these nanoparticles is studied at ambient conditions on a time scale of months. The plasmonic signature of Ag NPs vanishes several months after implantation for as-implanted samples, while samples annealed at intermediate temperature under N2 remain stable. XPS and HREM analysis evidence the presence of Silver oxide nanoparticles on aged samples and pure Silver nanoparticles on the annealed ones. This thermal treatment does not modify the size-distribution or position of the particles but is very efficient in stabilizing the metallic particles and to prevent any form of oxidation.
We perform a systematic study of the effect of adjacent nanostructures on the confinement of block copolymers (BCP) within pre-patterned trenches in 100 nm thick SiO2 films. Asymmetric PS-b-PMMA BCP with a styrene fraction of 0.71, Mn = 67100 are used. When deposited in the form of thin film, these BCP naturally self-organize upon annealing and form a PS matrix with hexagonally packed PMMA cylinders perpendicularly oriented with respect to the substrate. An accurate study of the confinement of this BCP thin film within isolated trenches is performed as a function of their width (80-260 nm). In this specific configuration the confinement of the BCP thin film within the pre-patterned structures has only been partially achieved. The effect of adjacent trenches on the arrangement of the BCP thin film is investigated using parallel trenches periodically distributed on the surface. The effective confinement of the BCP film is strongly modified by the periodicity of the pre-patterned structures.
Nanofabrication of buried structures with dimensions below 5 nm and with controlled 3D-positioning at the nanoscale was attempted to open new routes to future nanodevices where single nanostructures could be systematically interfaced. A typical example is ultralow-energy ion beam synthesis where already the depth positioning of embedded arrays of silicon nanocrystals can be finely controlled with nanometric precision. In this study, we investigated for the first time the control of the in-plane organization of the nanocrystals using a legitimate patterning option for microelectronic industries, self-assembled block-copolymer. The compatibility with the ultralow-energy ion beam synthesis process of polymeric nanoporous films used as mask was demonstrated together with the capability to control in 3D the organization of Si nanocrystals. The resulting nano-organization consists in a hexagonal array of 20 nm wide nanovolumes containing on average 8 nanocrystals embedded at a controlled depth within a silica matrix.
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