Richard A. Farrell

Chemical Interactions and Their Role in the Microphase Separation of Block Copolymer Thin Films

The thermodynamics of self-assembling systems are discussed in terms of the chemical interactions and the intermolecular forces between species. It is clear that there are both theoretical and practical limitations on the dimensions and the structural regularity of these systems. These considerations are made with reference to the microphase separation that occurs in block copolymer (BCP) systems. BCP systems self-assemble via a thermodynamic driven process where chemical dis-affinity between the blocks driving them part is balanced by a restorative force deriving from the chemical bond between the blocks. These systems are attracting much interest because of their possible role in nanoelectronic fabrication. This form of self-assembly can obtain highly regular nanopatterns in certain circumstances where the orientation and alignment of chemically distinct blocks can be guided through molecular interactions between the polymer and the surrounding interfaces. However, for this to be possible, great care must be taken to properly engineer the interactions between the surfaces and the polymer blocks. The optimum methods of structure directing are chemical pre-patterning (defining regions on the substrate of different chemistry) and graphoepitaxy (topographical alignment) but both centre on generating alignment through favourable chemical interactions. As in all self-assembling systems, the problems of defect formation must be considered and the origin of defects in these systems is explored. It is argued that in these nanostructures equilibrium defects are relatively few and largely originate from kinetic effects arising during film growth. Many defects also arise from the confinement of the systems when they are ‘directed’ by topography. The potential applications of these materials in electronics are discussed.

Self-assembled templates for the generation of arrays of 1-dimensional nanostructures: From molecules to devices

Self-assembled nanoscale porous architectures, such as mesoporous silica (MPS) films, block copolymer films (BCP) and porous anodic aluminas (PAAs), are ideal hosts for templating one dimensional (1D) nano-entities for a wide range of electronic, photonic, magnetic and environmental applications. All three of these templates can provide scalable and tunable pore diameters below 20 nm [1-3]. Recently, research has progressed towards controlling the pore direction, orientation and long-range order of these nanostructures through so-called directed self-assembly (DSA). Significantly, the introduction of a wide range of top-down chemically and physically pre-patterning substrates has facilitated the DSA of nanostructures into functional device arrays. The following review begins with an overview of the fundamental aspects of self-assembly and ordering processes during the formation of PAAs, BCPs and MPS films. Special attention is given to the different ways of directing self-assembly, concentrating on properties such as uni-directional alignment, precision placement and registry of the self-assembled structures to hierarchal or top-down architectures. Finally, to distinguish this review from other articles we focus on research where nanostructures have been utilised in part to fabricate arrays of functioning devices below the sub 50 nm threshold, by subtractive transfer and additive methods. Where possible, we attempt to compare and contrast the different templating approaches and highlight the strengths and/or limitations that will be important for their potential integration into downstream processes.

Study on the combined effects of solvent evaporation and polymer flow upon block copolymer self-assembly and alignment on topographic patterns.

Microphase separation of a polystyrene-block-polyisoprene-block-polystyrene triblock copolymer thin film under confined conditions (i.e., graphoepitaxy) results in ordered periodic arrays of polystyrene cylinders aligned parallel to the channel side-wall and base in a polyisoprene matrix. Polymer orientation and translational ordering with respect to the topographic substrate were elucidated by atomic force microscopy (AFM) while film thickness and polymer profile within the channel were monitored by cross-sectional transmission electron microscopy (TEM) as a function of time over a 6 h annealing period at 120 degrees C. Upon thermal annealing, the polymer film simultaneously undergoes three processes: microphase separation, evaporation of trapped solvent, and mass transport of polymer from the mesas into the channels. A significant volume of solvent is trapped within the polymer film upon spin coating arising from the increased polymer/substrate interfacial area due to the topographic pattern. Mass transport of polymer during this process results in nonuniform films, where subtle changes in the film thickness within the channel have profound effects on the microphase separation process. The initially disordered structure within the film underwent an orientation transition via an intermediate formation of perpendicular cylinders (nonequilibrium) to a parallel (equilibrium) orientation with respect to the channel base. Herein, we present a time-resolved study of the cylinder reorientation process detailing how changing film thickness during the annealing process dramatically affects both the local and lateral orientation of the observed structure. Finally, a brief mathematical model is provided to evaluate spin coating over a complex topography following a classical asymptotic approximation of the Navier-Stokes equations for the as-deposited films.

Measurements of the lattice constant of ceria when doped with lanthana and praseodymia - the possibility of local defect ordering and the observation of extensive phase separation

Conventionally, the addition of sesquioxide cation dopants to ceria has been thought of as a class of almost model systems. The most important defect mechanism involves simple anion vacancy charge compensation with those vacancy defects associating themselves with the trivalent cation and being distributed randomly through the lattice. However, this simple model has been significantly challenged in recent years and it seems possible that these associated defects might cluster in ordered arrangements. Whilst evidence has been provided by theoretical work, only limited experimental data are available. This letter reports the first observation of local ordering in these systems as observed by careful powder x-ray diffraction studies. In detail, it is shown that measurements of the lattice parameter do not vary monotonically with dopant concentration. It is also shown that far from being ideal systems with very high dopant solubilities and true solid-state solutions, these systems have complex solubility.

Mesoporous bismuth ferrite with amplified magnetoelectric coupling and electric field-induced ferrimagnetism

Coupled ferromagnetic and ferroelectric materials, known as multiferroics, are an important class of materials that allow magnetism to be manipulated through the application of electric fields. Bismuth ferrite, BiFeO3, is the most-studied intrinsic magnetoelectric multiferroic because it maintains both ferroelectric and magnetic ordering to well above room temperature. Here we report the use of epitaxy-free wet chemical methods to create strained nanoporous BiFeO3. We find that the strained material shows large changes in saturation magnetization on application of an electric field, changing from 0.04 to 0.84 μb per Fe. For comparison, non-porous films produced using analogous methods change from just 0.002 to 0.01 μb per Fe on application of the same electric field. The results indicate that nanoscale architecture can complement strain-layer epitaxy as a tool to strain engineer magnetoelectric materials.

Surface-directed dewetting of a block copolymer for fabricating highly uniform nanostructured microdroplets and concentric nanorings.

Through a combination of nanoimprint lithography and block copolymer self-assembly, a highly regular dewetting process of a symmetric diblock copolymer occurs whereby the hierarchal formation of microdroplets and concentric nanorings emerges. The process is driven by the unique chemical properties and geometrical layout of the underlying patterned silsesquioxane micrometer-sized templates. Given the presence of nonpreferential substrate-polymer interactions, directed dewetting was utilized to produce uniform arrays of microsized droplets of microphase separated polystyrene-block-poly(methyl methylacrylate) (PS-b-PMMA), following thermal annealing at 180 °C. Microdroplets with diameters greater than 400 nm exhibited a hexagonal close-packed arrangement of nanodots on the surface with polydomain ordering. At the droplet periphery, the polydomain ordering was severely disrupted because of a higher in-plane radius of curvature. By reducing the droplet size, the in-plane radius of curvature of the microdroplet becomes significant and the PMMA cylinders adopt parallel structures in this confined geometry. Continuous scaling of the droplet results in the generation of isolated, freestanding, self-aligned, and self-supported oblique nanorings (long axis ∼250-350 nm), which form as interstitial droplets between the larger microdroplets. Optical and magnetic-based nanostructures may benefit from such hierarchal organization and self-supporting/aligned nanoring templates by combining more than one lithography technique with different resolution capabilities.

Lattice Constant Dependence on Particle Size for Ceria prepared from a Citrate Sol-Gel

High surface area ceria nanoparticles have been prepared using a citrate solgel precipitation method. Changes to the particle size have been made by calcining the ceria powders at different temperatures, and X-ray methods used to determine their lattice parameters. The particle sizes have been assessed using transmission electron microscopy (TEM) and the lattice parameter found to fall with decreasing particle size. The results are discussed in the light of the role played by surface tension effects.

Graphoepitaxial assembly of asymmetric ternary blends of block copolymers and homopolymers

Ternary blends of cylinder-forming polystyrene-block-poly(methyl methacrylate) block copolymers and polystyrene and poly(methyl methacrylate) homopolymers were assembled in trench features of constant width. Increasing the fraction of homopolymer in the blend increased the spacing and size of block copolymer domains, which were oriented perpendicular to the substrate to form a hexagonal lattice within the trench. The number of rows of cylinders within the trench was controlled by the blend composition. Depending on the domain size and spacing, the hexagonal lattice was stretched or compressed perpendicular to the trench walls but not perturbed parallel to the walls, indicating a decoupling of the perturbation in the perpendicular and parallel directions. The row spacing was uniform across the trench as a function of position from the trench wall. The results are compared with an analytical model and with Monte Carlo simulations.

Pore directionality and correlation lengths of mesoporous silica channels aligned by physical epitaxy.

Herein we report on the alignment of mesoporous silica, a potential host for sub-10 nm nanostructures, by controlling its deposition within patterned substrates. In-depth characterization of the correlation lengths (length of a linear porous channel), defects of the porous network (delamination), and how the silica mesopores register to the micrometer-sized substrate pattern was achieved by means of novel focused ion beam (FIB) sectioning and in situ SEM imaging, which to our knowledge has not previously been reported for such a system. Our findings establish that, under confinement, directed deposition of the sol within channeled substrates, where the cross-sectional aspect ratio of the channels approaches unity, induces alignment of the mesopores along the length of the channels. The pore correlation length was found to extend beyond the micrometer scale, with high pore uniformity from channel to channel observed with infrequent delamination defects. Such information on pore correlation lengths and defect densities is critical for subsequent nanowire growth within the mesoporous channels, contact layout (electrode deposition etc.), and possible device architectures.

Facile and Controlled Synthesis of Ultra-Thin Low Dielectric Constant Meso/Microporous Silica Films

Sustained and aggressive scaling within the microelectronics industry has necessitated reducing the permittivity of the back end dielectric, namely silicon dioxide, to help reduce the resistance–capacitance delay. One approach currently under consideration involves the incorporation of pores within the dielectric itself. This is achieved predominantly via the self-assembly of block copolymers and a silicon source, using sol–gel techniques—a scheme which may potentially minimise the RC delay. Self-assembled mesoporous films with defined nanostructures have received considerable attention over the last decade owing to their excellent insulating properties, well-defined porosity, reliable processing and ability to form very thin and continuous films. In recent times, focus has also expanded towards investigating spin-on zeolite (microporous) thin films as they offer increased thermal stability (i.e. no pore collapse or unidirectional shrinkage), defined micropore size and porous (intraparticle) architecture, low dielectric constants of 1.8 and significant interparticle mechanical strengths. Unfortunately zeolite films suffer from a variety of problems such as poor cohesive strengths and unwanted large diameter, non-ordered mesoporosity caused by irregular nanoparticle shapes. Therefore, these films present an enormous integration challenge for high-volume manufacturing. Furthermore the zeolite films are essentially agglomerations of nanoparticles deposited across a silicon substrate and, as a result, have high surface roughness that potentially can cause two major concerns: 1) poor gap fill and 2) poor patterning (etching) properties. Non-uniformities within films results in poor etch rates/ variations, difficulties at chemical mechanical planarization (CMP) steps and interconnect misalignment problems. The addition of a dense tetraethoxyorthosilane (TEOS) binder material enhances the adhesion of the film to the silicon substrate and removes the large mesostructural porosity, but concurrently results in an increase of the dielectric constant. To offset this relationship, work has shifted towards investigating binder materials with low permittivities in an attempt to negate the increasing dielectric constant. Addition of polymer binder material was previously demonstrated by Li and co-workers, whereby 5–15 wt.% of c-cyclodextrin was added to promote adhesion between the nanoparticles. The authors reported little loss in mechanical properties whilst maintaining a low dielectric constant. Larlus and co-workers have also reported the use of silicalite-1 zeolite films covered in an acryl latex layer to enhance mechanical properties and reduce surface roughness. A dynamic dielectric constant ranging from 2.1 to 2.4 was reported using spectroscopic ellipsometry. To reduce the dielectric constant further the binder substance may be exchanged for a mesoporous material, fabricated via evaporation-induced self-assembly (EISA) routes. Previously, zeolite-beta and silicalite-1 nanoparticles were mixed with mesoporous films and demonstrated high degrees of bimodal ordering and reasonably good film-forming properties. Unfortunately the mesoporous structure-directing agent was an ammonium-based cationic surfactant, which generates pore sizes close to 2 nm and consequently reduces the overall porosity of the films. Herein we examine the insulating properties of zeolite/mesoporous films, whereby the binder material is templated by an anionic block copolymer with special emphasis on utilizing them for advanced semiconductor dielectrics. The mesophase and the orientational alignment of the mesoporous system and silicalite nanoparticles is confirmed by grazing-incident small-angle X-ray scattering (GISAXS) and wide-angle X-ray diffraction. The mesophase of a thin film deposited from a coating solution containing 50–50 volume percentage of silicalite-1 nanoparticles and silica/P123 sol–gel is presented in Figure 1a. The mesophase is compressed hexagonally, with channels oriented parallel to the substrate surface. The silicalite-1 nanocrystals within the films show preferred orientation with their a or b axis parallel to the substrate surface, evidenced by the appearance of the (h00) and (0k0) reflections only (Supporting Information, Figure 1). The silicalite-1 nanoparticles within the films exhibit orientations with either sinusoidal or straight channels perpendicular to the substrate surface. This orientation is possible because the well-developed crystalline shapes have coffin-like morphologies (Figure 1b), that allow for orientational alignment when in contact with a substrate during deposition of very thin films. Similar results are obtained for pure silicalite-1 thin films deposited by spin coating (Supporting Information, Figure 2). Further details on [a] Dr. R. A. Farrell, Dr. N. Petkov, Dr. J. D. Holmes, Prof. M. A. Morris Department of Chemistry and Tyndall National Institute University College Cork, Cork (Ireland) Fax: (+353)2144274197 E-mail : m.morris@ucc.ie [b] Dr. J. D. Holmes, Prof. M. A. Morris Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) Trinity College Dublin, Dublin 2 (Ireland) [c] Dr. K. Cherkaoui, Dr. P. K. Hurley Tyndall National Institute, University College, Cork, Cork (Ireland) [d] Dr. H. Amenitsch Institute of Biophysics and X-ray S Steyrergasse 17/VI, 8010 Graz (Austria.) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800158.