Magnetization switching in high-density magnetic nanodots by a fine-tune sputtering process on a large-area diblock copolymer mask.

Ordered magnetic nanodot arrays with extremely high density provide unique properties to the growing field of nanotechnology. To overcome the size limitations of conventional lithography, a fine-tuned sputtering deposition process on mesoporous polymeric template fabricated by diblock copolymer self-assembly is herein proposed to fabricate uniform and densely spaced nanometer-scale magnetic dot arrays. This process was successfully exploited to pattern, over a large area, sputtered Ni80Fe20 and Co thin films with thicknesses of 10 and 13 nm, respectively. Carefully tuned sputter-etching at a suitable glancing angle was performed to selectively remove the magnetic material deposited on top of the polymeric template, producing nanodot arrays (dot diameter about 17 nm). A detailed study of magnetization reversal at room temperature as a function of sputter-etching time, together with morphology investigations, was performed to confirm the synthesis of long-range ordered arrays displaying functional magnetic properties. Magnetic hysteresis loops of the obtained nanodot arrays were measured at different temperatures and interpreted via micromagnetic simulations to explore the role of dipole-dipole magnetostatic interactions between dots and the effect of magnetocrystalline anisotropy. The agreement between measurements and numerical modelling results indicates the use of the proposed synthesis technique as an innovative process in the design of large-area nanoscale arrays of functional magnetic elements.

Influence of block copolymer feature size on reactive ion etching pattern transfer into silicon.

A successful realisation of sub-20 nm features on silicon (Si) is becoming the focus of many technological studies, strongly influencing the future performance of modern integrated circuits. Although reactive ion etching (RIE), at both micrometric and nanometric scale has already been the target of many studies, a better understanding of the different mechanisms involved at sub-20 nm size etching is still required. In this work, we investigated the influence of the feature size on the etch rate of Si, performed by a cryogenic RIE process through cylinder-forming polystyrene-block-polymethylmethacrylate (PS-b-PMMA) diblock copolymer (DBC) masks with diameter ranging between 19-13 nm. A sensible decrease of the etch depth and etch rate was observed in the mask with the smallest feature size. For all the DBCs under investigation, we determined the process window useful for the correct transfer of the nanometric cylindrical pattern into a Si substrate. A structural and physicochemical investigation of the resulting nanostructured Si is reported in order to delineate the influence of various RIE pattern effects. Feature-size-dependent etch, or RIE-lag, is proved to significantly affect the obtained results.

Neutral wetting brush layers for block copolymer thin films using homopolymer blends processed at high temperatures.

Binary homopolymer blends of two hydroxyl-terminated polystyrene (PS-OH) and polymethylmethacrylate (PMMA-OH) homopolymers (Mn âˆ¼ 16000 g mol(-1)) were grafted on SiO2 substrates by high-temperature (T > 150 °C), short-time (t < 600 s) thermal treatments. The resulting brush layer was tested to screen preferential interactions of the SiO2 substrate with the different symmetric and asymmetric PS-b-PMMA block copolymers deposited on top of the grafted molecules. By properly adjusting the blend composition and the processing parameters, an efficient surface neutralization path was identified, enabling the formation, in the block copolymer film, of homogeneous textures of lamellae or cylinders perpendicularly oriented with respect to the substrate. A critical interplay between the phase segregation of the homopolymer blends and their grafting process on the SiO2 was observed. In fact, the polar SiO2 is preferential for the PMMA-rich phase that forms a homogeneous layer on the substrate, while the PS-rich phase is located at the polymer-air interface. During the thermal treatment, phase segregation and grafting proceed simultaneously. Complete wetting of the PS rich phase on the PMMA rich phase leads to the formation of a PS/PMMA bilayer. In this case, the progressive diffusion of PS chains toward the polymer-SiO2 interface during the thermal treatment allows tuning of the brush layer composition.

High aspect ratio PS-b-PMMA block copolymer masks for lithographic applications.

The control of the self-assembly (SA) process and nanostructure orientation in diblock copolymer (DBC) thick films is a crucial technological issue. Perpendicular orientation of the nanostructures in symmetric and asymmetric poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer films obtained by means of simple thermal treatments was demonstrated to occur in well-defined thickness windows featuring modest maximum values, thus resulting in low aspect ratio (h/d < 2) of the final lithographic mask. In this manuscript, the thickness window corresponding to the perpendicular orientation of the cylindrical structures in asymmetric DBC is investigated at high temperatures (190 °C ≤ T ≤ 310 °C) using a rapid thermal processing machine. A systematic study of the annealing conditions (temperature and time) of asymmetric PS-b-PMMA (Mn = 67.1, polydispersity index = 1.09) films, with thicknesses ranging from 10 to 400 nm, allowed ordered patterns, with a maximum value of orientational correlation length of 350 nm, to be obtained for film thicknesses up to 200 nm. The complete propagation of the cylindrical structures through the whole film thickness in a high aspect ratio PS template (h/d ≈ 7) is probed by lift-off process. Si nanopillars are obtained having the same lateral ordering and characteristic dimensions of the DBC lithographic mask as further confirmed by grazing-incidence small-angle X-ray scattering experiments.

Solid-state dewetting of ultra-thin Au films on SiO2 and HfO2.

Ultra-thin Au films with thickness (h) ranging from 0.5 to 6.0 nm were deposited at room temperature (RT) by means of e-beam evaporation on SiO2 and HfO2. Due to the natural solid-state dewetting (SSD) of the as-deposited films, Au nanoparticles (NPs) were formed on the substrates. By properly adjusting the h value, the size and the density of the Au NPs can be finely tuned. For h = 0.5 nm, spherical-like Au NPs with diameter below 5 nm and density in the order of 10(12) Au NPs cm(-2) were obtained without any additional thermal treatment independently from the substrate. The dependence of the Au NPs characteristics on the substrate starts to be effective for h ≥ 1.0 nm where the Au NPs diameter is in the 5-10 nm range and the density is around 10(11) Au NPs cm(-2). The effect of a subsequent high temperature (400-800 °C) annealing in N2 atmosphere on the Au NPs was investigated as well. For h ≤ 1.0 nm, the Au NPs characteristics evidenced an excellent thermal stability. Whereas the thermal treatment affects the cristallinity of the Au NPs. For the thicker films (2.0 ≤ h ≤ 6.0 nm), the thermal treatment becomes effective to induce the SSD. The proposed methodology can be exploited for the synthesis of Au NPs with diameter below 10 nm on different substrates at RT.

Rapid thermal processing of self-assembling block copolymer thin films.

Self-assembling block copolymers generate nanostructured patterns which are useful for a wide range of applications. In this paper we demonstrate the capability to control the morphology of the self-assembling process of PS-b-PMMA diblock copolymer thin films on unpatterned surfaces by means of fast thermal treatment performed in a rapid thermal processing machine. The methodology involves the use of radiation sources in order to rapidly drive the polymeric film above the glass transition temperature. Highly ordered patterns were obtained for perpendicular-oriented cylindrical and lamellar PS-b-PMMA block copolymers in less than 60 s. This approach offers the unprecedented opportunity to investigate in detail the kinetics of the block copolymer self-assembly during the early stages of the process, providing a much deeper understanding of the chemical and physical phenomena governing these processes.

Collective behavior of block copolymer thin films within periodic topographical structures.

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.

In-plane organization of silicon nanocrystals embedded in SiO2 thin films.

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.

The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography and atomic layer deposition.

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.

The effect of random copolymer on the characteristic dimensions of cylinder-forming PS-b-PMMA thin films.

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.

Si nanocrystal synthesis in HfO2/SiO/HfO2 multilayer structures.

The synthesis of two-dimensional arrays of Si nanocrystals in an HfO2 matrix has been achieved by deposition of HfO2/SiO/HfO2 multilayer structures followed by high temperature (1100 degrees C) thermal treatment in nitrogen atmosphere. Silicon out-diffusion from the SiO layer through the HfO2 films has been shown to be the limiting factor in the formation of the Si nanocrystals. Suitable strategies have been identified in order to overcome this limitation. Si nanocrystal formation has been achieved by properly adjusting the thickness of the SiO layer.