Block-copolymer self assembly for nanotechnology applications

Resume : Directed self-assembly (DSA) of block copolymer (BCP) patterns is a possible option for future semiconductor device patterning,1 but pattern transfer of most BCP masks is somewhat hindered by the inherently low etch contrast between blocks. Here, we demonstrate a simple process flow for forming well-registered and aligned silicon (Si) nanofins. Aligned nanofins were enabled via pattern transfer of robust metal oxide nanowire masks through the DSA of BCPs. A cylindrical forming poly(styrene)-block-poly(4-vinyl-pyridine) (PS-b-P4VP) BCP was employed producing ‘fingerprint’ line patterns over macroscopic areas. The DSA of PS-b-P4VP line patterns was achieved by electron-beam lithographically defined hydrogen silsequioxane (HSQ) prepatterns. We developed metal oxide nanowire features through infiltration of P4VP microdomains which facilitated high quality pattern transfer to the underlying Si substrate. This work highlights the precision at which long range ordered 10 nm Si nanofin features with 32 nm pitch can be defined using a cylindrical BCP system for nanolithography application. The results show promise for future nanocircuitry fabrication to access sub-16 nm critical dimensions using ‘high χ’ cylindrical systems as surface interfaces are easier to tailor than lamellar systems. 1. Morris, M. A., Directed self-assembly of block copolymers for nanocircuitry fabrication. Microelectronic Engineering 2015, 132, 207-217.

Resume : Block copolymers (BCs) thin films have become the subject of an intense research activity because of their ability to form well periodic structures via self-assembly (SA) at nanometric scale. Among the various materials, PS-b-PDMS BCs are gaining even more interest because they form high resolution nanostructures with dimensions below 10 nm. The most common approach to induce the SA involve the use of complex solvo-thermal setup. No reports are present in literature on the use of simple and easy to be integrated thermal annealing approach. However, it would be highly desirable to develop a thermal treatment also for the PS-b-PDMS able to achieve the SA in a process time compatible with their exploitation as lithographic mask for semiconductor industry. In this work, we report on the SA of cylinder-forming PS-b-PDMS, having molar mass of 22 Kg/mol (d = 9 nm, L0 = 20 nm), by means of a Rapid Thermal Processing (RTP) machine on flat surfaces. By properly adjusting the heating rate, final temperature and processing time, we demonstrate the possibility to promote the SA of cylindrical PDMS nanostructures parallel to the substrate with timescale well below 900s. In addition, we investigated the trend of the correlation length of the cylindrical nanostructures deposited on silicon surfaces grafted with different functional polymers (PS, PMMA, PS-r-PMMA, PS-r-PDMS). Correlation lengths greater than 1000 nm were achieved in homogeneous thin films covering the entire substrate area.

Resume : The performance of many high performance electronic devices including solar cells, batteries and rely on the detailed control over the assembly of individual components. These assemblies are complex, comprising an array of organic and inorganic components. While semiconductor technology excels in miniaturizing device architectures down to 10 nm, these “top-down” methods are expensive and not scalable. Nanoarchitectures created by “bottom-up” self-assembly are, on the other hand, typically limited to organic materials. In my talk I will introduce several strategies how materials for solar cell and batteries can be structured by means of polymer self-assembly and how these morphologies control the performance of devices, including solar cells, super capacitors and optical meta-materials.

Resume : Conjugated polyelectrolytes (CPEs) combine organic semiconductor properties and the charge-mediated behaviour of polyelectrolytes in a single functional material and show great promise potential as optical and charge-transport components in organic photovoltaic (OPV) cells. Simultaneous control of morphology and electronic properties of CPE thin films is critical for this application and will be influenced by the polymer organisation in the processing solution. Here we report the solvent- and surfactant-mediated self-assembly of a series of amphiphilic rod–rod diblock copolymers, containing a hydrophobic poly(3-hexylthiophene) (P3HT) block and a hydrophilic cationic P3HT block bearing different terminal ionic groups. The optical properties and aggregate structures were studied in methanol, water and water/methanol mixtures using photoluminescence (PL), scattering and microscopic techniques. Small-angle neutron scattering (SANS) studies showed that these CPEs assemble into either core-shell cylindrical or disc-like aggregates in solution, depending on solvent. Atomic force microscopy showed that the aggregate structure formed in solution could be successfully transferred to thin films maintaining their distinctive topographies. Notably, while the solvent induces just minor changes to the optical properties, complexation with an anionic surfactant results in a series of surfactochromic transitions in the absorption and PL spectra, which were correlated to adaptive long-range supramolecular structures using SANS. Solvent- and surfactant mediated self-assembly of all-conjugated block CPEs may therefore provide a simple, inexpensive route to nanostructured active interfaces suitable for OPV cells.

Resume : Directed self-assembly (DSA) of block co-polymers (BCP) by chemical surface modification is driven by interfacial and surface interactions. In this communication, we show that by properly tuning the strength of these interactions, chemical patterns formed by wide backgrounds and guiding stripes can properly direct the alignment of BCP, relaxing the requirements on the lithography process in terms of resolution. We show the formation of directed self-assembled (DSA) patterns (lines and spaces) by chemical surface modification using wide (2.5 + 3.5) L0 guiding stripes. Guiding patterns are created on PS-OH brush layers by electron beam lithography and oxygen plasma exposure, which drive the orientation of lamella forming PS-b-PMMA block co-polymers of L0=28 nm, achieving a density multiplication factor x6. We have determined the optimal conditions (annealing process, block co-polymer thickness and oxygen plasma exposure) for the formation of the DSA patterns with an accurate control of the chemical interactions between the blocks of the co-polymer and the surface. The behavior is rationalized by the balance between surface free energy and grain boundary free energy according to the results of self-consistent field theory and simulations.

Resume : Self-assembling (SA) block copolymers (BCPs) generate nanostructured patterns that could be useful for advanced lithographic applications. In this work, we studied incidence of the film thickness on the orientation and the morphology of nano-domains confined inside densely packed and nanometer wide trenches. The SA process of cylinder-forming PS-b-PMMA BCPs having different molecular weight (between 54 and 102 kg/mol) have been promoted by varying the annealing parameters (temperature and time) inside a Rapid Thermal Processing (RTP) machine. With this method, we were able to fabricate hexagonally packed cylindrical structures with diameters ranging between 13 and 22 nm and periodicity L0 from 28 to 47 nm. When the BCP film is heated up over the glass transition temperature, the mobility of the polymeric film increases and the capillarity forces allow the macromolecules to flow from the mesa into the trenches. As a consequence, the resultant film thickness is not the same inside all the trenches, but increases symmetrically when moving from the central trenches toward the lateral trenches. Moreover, the ordering dynamic of the BCP within the trenches showed some peculiar behaviors such as a flipping of the cylindrical nanodomains from parallel to perpendicular orientation with respect to the surface and a untimely degradation of the ordering at high annealing temperatures when compared with the kinetics of the BCP self assembly process on flat substrates.

Resume : Directed self-assembly of block copolymers is a route to obtain tailored 3D patterns on the 10nm scale that have a high degree of order. These patterns are promising for multiple applications, including lithography and functional materials. In this talk, we will present our computational work on directed self-assembly of diblock copolymers templated by graphoepitaxial methods. In particular, we will show how inverse self-assembly techniques, in which the input is a given target pattern and the algorithm provides an optimal template solutions for such pattern, can be an important tool for the development of complex patterns. In the second part we will discuss about how one can then “dope” the system with other materials as could be nanoparticles and realize well ordered functional heterogeneous systems. A perspective on future challenges and opportunities in this area will be presented at the end.

Resume : Tailoring surface energies is the key factor to control the orientation of nanoscopic structures in block copolymer (BCP) thin films. Perpendicular orientation of the BCP features can be achieved with non preferential interactions at both the bottom and top interfaces. A common approach to the formation of a neutral surface consists in grafting of end-functional random copolymers. As the random copolymer chemical composition can be precisely controlled, a fine-tuning of the surface characteristics is possible. In this work, a series of hydroxyl terminated poly(styrene-r-methylmethacrylate) random copolymers was prepared with different molar masses, ranging from 1500 to 100000 g/mol. The Rapid Thermal Processing (RTP) technology was employed to perform flash grafting reactions of the random copolymers to the activated silicon wafer surface at different temperatures for time periods ranging from few seconds to several minutes. The characteristics of the grafted layer were delineated as a function of the molar mass of the random copolymers. The subsequent ordering propensity of a symmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA) diblock copolymer was found to be dependent on both the grafting temperature and grafting density of the random copolymer. Efficient neutral wetting brush layers with thickness lower than 2 nm were obtained. These results shed new light on the nature of the surface neutralization.

Resume : Preparation of long-range ordered nanostructures is motivated by the interest to design new functional devices. Among the research areas of interest are fundamental issues relating to the nanofabrication of the materials to achieve designed molecular assembly.1 Here we explores the fundamentals of confined crystallization in micromolded polystyrene-block-poly(epsilon-caprolactone) (PS-b-PCL) diblock copolymer thin films through a combination of grazing incidence X-ray diffraction and nanobeam grazing incidence X-ray scattering.2,3 It is aiming to unravel the designed hierarchical confinement regulate the molecular assembly. The well-arranged PCL crystal unit, namely the crystallographic b-axis parallel to the long axis of grating, is induced by micromolding the PS-b-PCL thin film. Moreover, we report for the first time that the degree of crystallinity increases with the decrease of the grating size. (1) Hu, Z.; Tian, M.; Nysten, B.; Jonas, A. Nat. Mater. 2009, 8, 62. (2) Zhang, P.; Huang, H.-y.; Chen, Y.; Yu, S.; Krywka, C.; Vayalil, S.; Roth, S.; He, T.-b. Chin. J. Polym. Sci. 2014, 32, 1188. (3) Zhang, P.; Huang, H.; He, T.; Hu, Z. ACS Macro Letters 2012, 1007.

Resume : On-chip etch masks formed via block copolymer thin films have garnered increased attention in recent years for patterning future sub-10 nm device features. In this work, we demonstrate ways to develop highly oriented and ordered Poly(styrene)-block-poly(d,l)lactide (PS-b-PLA) lamellar patterns with 34 nm periodicity through ‘solvo-microwave’[1] annealing and solvent vapor annealing. Various methods of PLA removal were studied in both thin (50 nm) and thick films (200 nm) to act as on-chip etch masks. Grazing incident small angle X-ray scattering (GISAXS) measurements were carried out to elucidate the interior orientation of the PS-b-PLA domains following both self-assembly and PLA etching. Wet etches based on alkaline and enzymatic solutions were studied in depth. Comparison to a dry etch procedure using a reactive ion etch (RIE) technique was also made.[2] Furthermore, directed self-assembly of thin PS-b-PLA BCP films via topographically patterned silicon nitride is demonstrated with alignment over macroscopic areas. Interestingly, we observed lamellar patterns oriented parallel as well as perpendicular to the sidewalls. [1] P. Mokarian-Tabari, C. Cummins, S. Rasappa, C. Simao, C. M. Sotomayor Torres, J. D. Holmes, M. A. Morris, Langmuir 2014, 30, 10728. [2] C. Cummins, P. Mokarian-Tabari, J. D. Holmes, M. A. Morris, Journal of Applied Polymer Science 2014, 131.

Resume : With the aim of finding an interpretation for the isomerization reaction of decapentaene by quantum methods, we have studied a series of three molecules giving the following results:  The studied segments (C10H12, C10H6Br6, C10H6I6) are very stable. This stability is justified by the HOMO-LUMO found energy gap. However, examination of the stability of several conformations shows that the trans conformer is more stable than the cis conformer in the general assembly.  According to the study of different reaction profiles, we noticed that the size and nature of the dopant plays a very important role on the evolution of the activation energy.  From the obtained values of the activation energy, we find that the speed constants of the isomerization reaction are in the order: kC10H12 >>kC10H6Br6>> kC10H6I6  The search for intermediate products during the transition Cis-Trans shows that the geometric parameters (angles and dihedral angles) are the most varied settings, this remark has been observed in the case of substituted and non-substituted PA.  The methods of calculations performed in this work are the Ab-initio and DFT methods, with the bases (6-31G, 3-21G **). All these calculations are performed with the Hyperchem software, where parameters obtained are in a closer order to those obtained with the Gaussian 03W software  Examination of different molecules obtained during the Cis-Trans isomerization reaction shows that the total energy of the resulting intermediate product is of the order of -10487.05 eV, corresponding to a 0.87 eV activation energy (23.67 kcal / mol).  With the same HF method (6-31G and 3-21 G**), a close geometry was obtained for the intermediate product in the isomerization reaction with a total energy of 0.93 eV (25.30 kcal/mole), which shows that the different values of the activation energy obtained by the HF and DFT methods at the 6-31G level can be compared to those obtained by Ito, Montaner and Bernier. Keywords: Ab-initio; DFT; kinetics; isomerisation; substituted decapentaene

Resume : Our study aims to gather the maximum amount of information on the changes produced at the structural level (rotation, torsion, conversion) during the Cis-Trans transition (isomerization reaction) on one hand, and to examine the influence of different dopants or substituents on these kinetic parameters on the other hand. The results obtained through the optimization of molecules gave us the different distances and angles according to the methods and bases applied with a C2v symmetry. We were able to determine the total energies, the energy gap ΔE (HOMO-LUMO) of the two conformers Cis and Trans (the HF and DFT at 6-31G and 3-21G** levels) and finally a comprehensive analysis on the topological charges. The analysis of the results show that for the six molecules, the Trans conformer is energetically very stable compared to the Cis one, this stability is confirmed by the obtained values for the total energy ETOTAL. The increase in the stability energy leads to a less important HOMO-LUMO energy gap. The analysis of the optimized geometrical parameters of the six molecules using the AM1 and PM6 methods, are in agreement with the experimental structures characterized by X-ray diffraction. Finally, we were able to determine the reaction profiles of the Cis-Trans isomerization reactions of the polyacetylene in the gas phase, and to calculate the activation energy (Ea), as well as the diagrams of energies E (eV) based on the coordinates of the isomerization reaction of its molecules. All calculations performed in this study are carried out using the Hyperchem software. Keywords: substituted polyacetylene , semi empirical, HF (Ab-initio), DFT (B3LYP)

Resume : Atomic force microscopy (AFM) is widely used to image surface topography at the nanometric scale or to map the qualitative differences of local surface properties such as friction, adhesion or elastic modulus. Using PeakForce quantitative nanomechanical mapping (QNM) is possible to reliably measure Young’s modulus of materials with high spatial resolution and surface sensitivity. Exploiting such technique we could map and identify the different phases of PS and PMMA homopolymer blends after dewetting and also probe block co-polymers (BCP) thin films oriented by lithographic patterns. Thin films of block copolymers can self-assemble into ordered periodic structures at the molecular scale (5 to 50 nm) with a rich variety of nanophase-separated structures (lamellar, pillars, etc.). AFM nano-indentation brings important information regarding the changes in mechanical properties of the different oriented domains, being able to overcome typical limitations of surface analysis (as in the case of friction maps) by probing few nm underneath the surface. In terms of lateral resolution, probing nanometric length structures like the ones created by the self assembly of block co-polymers (BCP), brings the technique to its spatial limits, the use of ultra sharp tips (indenters) in order to minimize the indentation depth could eventually enhance the resolution capabilities of the technique.

Resume : Compared with extreme UV, block copolymer (BCP) lithography is a highly desirable method for production of sub-10nm features on silicon, boasting rapid reactions time, low cost and small feature sizes. Our research looks at real-time plasma diagnostics for analysis of BCP etching. Our initial work focused on the development of a novel solvent annealing chamber, which provides an environment in which BCPs can be annealed with a both controlled gas flow and temperature. Using this state-of-the-art chamber, dewetting of samples is reduced and defects caused by impurities during annealing are eliminated. The second phase of our work focused on the preparation, characterisation and etching of spin-coated high χ monomers and BCP samples, consisting of poly(methyl methacrylate), polystyrene, and poly(4-vinylpyridine). These surfaces were treated with plasmas from both an Oxford Intruments Plasmalab 100 system and a closed loop process controlled microwave electron cyclo resonance plasma etch reactor. Physical and chemical characterisation of films using AFM, SEM, ellipsometry and profilometry, prior to and post plasma treatment was performed. Alongside characterisation techniques, in-situ plasma diagnostics and sensors to monitor and control plasma radical densities were used. The effects of altering plasma conditions such as the densities and energies of reactive species in the plasma, as well as substrate conditions such as temperature, pressure and gases were investigated.

Resume : Block copolymer self-assembly is a fascinating approach to fabricate functional nanostructured materials. Highly ordered nanostructures are achieved by micro-phase separation of the block copolymer. Incorporation of these nanostructures in devices typically requires a thin film geometry, which gives rise to morphological changes induced by the interface interactions. As a consequence, complex inner film morphologies can result, which may significantly deviate from equilibrium bulk morphologies. To fully characterize these nanostructures is an inherent challenge. With advanced scattering techniques the nanostructures of block copolymer films are probed in a non-destructive manner and with high statistical relevance. In addition to surface structures commonly probed with imaging techniques, inner film structures are accessible, which is of high relevance for many applications such as in energy conversion and storage. In-situ measurements give further insights into the morphology development during film preparation as well as in morphology transformations induced by changes of external control parameters.

Resume : Block copolymers (BCPs) are widely used for bottom-up nanofabrication processes, exploiting their capability of self-assembling (SA) into small size and high density features of different shape. These characteristics suggest the possibility to use BCPs films as 3D nano structured lateral standard for AFM calibration in the length scale ranging from 10 nm to 100 nm. To address this issue, in this work we systematically studied the variations of the geometrical parameters (i.e. center-to-center distance L0 and diameter d) of cylinder forming PS-b-PMMA BCP patterns when confined within periodic trenches of different width (ranging between 50 and 600 nm), obtained by electron beam lithography. The SA process of the BCP, having molecular weight of 54 kg/mol and 30/70 PMMA/PS composition, was achieved through a Rapid Thermal Processing (RTP) machine at 190 °C for 900 s. All the geometrical parameters of the confined cylinders were measured by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis and compared with the results on the flat surface. When the BCP film is confined within trenches, the cylinder periodicity changed with respect to the unpatterned surface. The variations of BCP confined film parameters, L0 and d, will be discussed as a function of the lateral trench size W.

Resume : The self-assembly of block copolymers (BCPs) into a nanometric ordered structure and the chemical similarity with the photoresist materials employed in standard photolithography have triggered the application of BCPs as template masks. Asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA) can form perpendicularly oriented nanodomains since the weakly unbalanced surface interactions are easily neutralized by grafting the appropriate P(S-r-MMA) random copolymer (RCP) to the surface. The enlargement of the pore diameters during the selective etching of the cylinder-building component and the remove of the RCP at the bottom can be reduced by using a thin RCP layer. In this work, the self-assembling of an asymmetric PS-b-PMMA BCP with a molecular weight of 54 kg mol-1 over a 2 nm thick P(S-r-MMA) grafted layer was accomplished through a fine tuning of the annealing temperature (Ta=140-260 °C) and time (ta=1-900 s) in a rapid thermal processing machine. Cylindrical PMMA structures perpendicularly oriented with respect to the substrate with high lateral order were obtained. The competition between self-assembling and polymer degradation processes is investigated through the quantification of the lateral ordering. This systematic analysis indicates the two processes occur on different time scale. This finding allows improving the lateral ordering increasing Ta while decreasing ta. After PMMA and RCP removal, nanoporous templates with pore diameters close to 10 nm were obtained.

Resume : Numerous technologies and fields under active development are fundamentally limited by access to novel nanoscale materials. From metamaterials to batteries and solar cells, the ability to derive new nanoscale features in classical materials often enables new capabilities. Block copolymers provide a natural solution allowing the encoding of a desired 3D architecture into the chemical building blocks and assembly conditions. This powerful, yet nuanced, design platform has led to tunable morphologies having predictable feature sizes and connectivity. Towards this end we describe the development of a new structure directing agent that we have applied to study the supercapacitor performance of niobium oxide. Although niobium oxide recently emerged as an exciting lithium intercalation material, little is yet known on its feature-size dependent performance. The resulting inorganic morphologies are interpreted through our recently developed assembly guidelines for transitioning from solution-type morphologies to those classically of the condensed state.

Resume : Block copolymer (BCP) self-assembly in thin films can create lithographic nanotemplates with sub-10-nm scale pattern dimension. Unfortunately, BCP self-assembled nanopatterning has been regarded as an intrinsic two-dimensional patterning method specifically useful for hard and flat inorganic substrates. The well-established processing steps including the formation of uniform thickness, ultrathin BCP film via spin casting and subsequent thermal/solvent annealing are generally considered incompatible to three-dimensional geometries or conventional flexible polymer substrates with low chemical/thermal stability and surface roughness typically larger than nanoscale. I will present BCP nanopatterning for three-dimensional, flexible and complex geometry exploiting chemically modified graphene. Mechanically robust but compliant chemically modified graphene (CMG) film will be introduced as a transferrable and disposable substrate for the self-assembled nanopatterning of nonplanar, flexible, and even multi-stack device oriented structures. Taking advantage of the high chemical/thermal stability, genuine atomic scale flatness, and mechanical robustness with compliance, graphene based materials can be excellent substrates for nanopatterning. While pristine graphene has a low surface energy, CMG prepared via graphene oxide, may have an improved surface energy to promote wettability to a BCP thin film. Additionally, CMG is obtainable from natural graphite in a cost effective manner. BCP nanopatterns stably formed at CMG film can be readily transferred onto three-dimensional or flexible substrates.

Resume : Closing Remarks