Optimized design of block copolymers with covarying properties for nanolithography

The ability to impart multiple covarying properties into a single material represents a grand challenge in manufacturing. In the design of block copolymers (BCPs) for directed self-assembly and nanolithography, materials often balance orthogonal properties to meet constraints related to processing, structure and defectivity. Although iterative synthesis strategies deliver BCPs with attractive properties, identifying materials with all the required attributes has been difficult. Here we report a high-throughput synthesis and characterization platform for the discovery and optimization of BCPs with A-block-(B-random-C) architectures for lithographic patterning in semiconductor manufacturing. Starting from a parent BCP and using thiol-epoxy 'click' chemistry, we synthesize a library of BCPs that cover a large and complex parameter space. This allows us to readily identify feature-size-dependent BCP chemistries for 8-20-nm-pitch patterns. These blocks have similar surface energies for directed self-assembly, and control over the segregation strength to optimize the structure (favoured at higher segregation strengths) and defectivity (favoured at lower segregation strengths).

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

Kinetic Pathways of Block Copolymer Directed Self-Assembly: Insights from Efficient Continuum Modeling

We introduce a computationally efficient continuum technique to simulate the complex kinetic pathways of block copolymer self-assembly. Subdiffusive chain dynamics is taken into account via nonlocal Onsager coefficients. An application to directed self-assembly of thin films of diblock copolymers on patterned substrates reveals the conditions under which experimentally observed metastable structures intervene in the desired thin-film morphology. The approach generalizes easily to multiblock copolymers and more complex guiding patterns on the substrate, and its efficiency allows for the systematic optimization of guiding patterns and process conditions.

Engineering the Kinetics of Directed Self-Assembly of Block Copolymers toward Fast and Defect-Free Assembly

Directed self-assembly (DSA) of block copolymers (BCPs) can achieve perfectly aligned structures at thermodynamic equilibrium, but the self-assembling morphology can become kinetically trapped in defective states. Understanding and optimizing the kinetic pathway toward domain alignment is crucial for enhancing process throughput and lowering defectivity to levels required for semiconductor manufacturing, but there is a dearth of experimental, three-dimensional studies of the kinetic pathways in DSA. Here, we combined arrested annealing and TEM tomography to probe the kinetics and structural evolution in the chemoepitaxy DSA of PS- b-PMMA with density multiplication. During the initial stages of annealing, BCP domains developed independently at first, with aligned structures at the template interface and randomly oriented domains at the top surface. As the grains coarsened, the assembly became cooperative throughout the film thickness, and a metastable stitch morphology was formed, representing a kinetic barrier. The stitch morphology had a three-dimensional structure consisting of both perpendicular and parallel lamellae. On the basis of the mechanistic information, we studied the effect of key design parameters on the kinetics and evolution of structures in DSA. Three types of structural evolutions were observed at different film thicknesses: (1) immediate alignment and fast assembly when thickness < L₀ ( L₀ = BCP natural periodicity); (2) formation of stitch morphology for 1.25-1.45 L₀; (3) fingerprint formation when thickness >1.64 L₀. We found that the DSA kinetics can be significantly improved by avoiding the formation of the metastable stitch morphology. Increasing template topography also enhanced the kinetics by increasing the PMMA guiding surface area. A combination of 0.75 L₀ BCP thickness and 0.50 L₀ template topography achieved perfect alignment over 100 times faster than the baseline process. This research demonstrates that an improved understanding of the evolution of structures during DSA can significantly improve the DSA process.

Well-ordered self-assembled nanostructures of block copolymer films via synergistic integration of chemoepitaxy and zone annealing

The integrated chemical template/zone annealing method has the capability to rapidly fabricate well-aligned and well-oriented nanostructures over a macroscopic area.

Real-Time Atomic Force Microscopy Imaging of Block Copolymer Directed Self Assembly

The kinetics of directed self-assembly of symmetric PS-b-PMMA diblock copolymer on chemically patterned templates were measured during in situ thermal annealing. Although these chemical guide patterns lead to well-aligned, defect-free lamellar patterns at thermodynamic equilibrium, in practice, challenges remain in understanding and optimizing the kinetic evolution for technological applications. High-speed, environmentally controlled atomic force microscopy imaging was used to track pattern evolution on the time scale of individual microdomain connections in real space and time, allowing the direct visualization of defect healing mechanisms. When we apply this highly general technique to films on chemically patterned substrates, we find that pattern alignment is mediated by a metastable nonbulk morphology unique to these samples, referred to as the "stitch" morphology. We observe diverse and anisotropic mechanisms for the conversion from this morphology to equilibrium lamellar stripes. Directed self-assembly on chemical templates is observed to follow exponential kinetics with an apparent energetic barrier of 360 ± 80 kJ/mol from 210-230 °C, a significant enhancement when compared with ordering rates on unpatterned substrates. Ultimately, from local imaging, we find that the presence of a chemical guiding field causes morphological ordering and lamellar alignment to occur irreversibly.

Self-Registered Self-Assembly of Block Copolymers

Directed self-assembly (DSA) of block copolymer (BCP) thin films, especially with density multiplication, is one of the most promising options for further improving resolution and throughput in nanolithography. However, controlling defect density has been one of the major hurdles for many DSA applications. Both thermodynamically and kinetically, defect-free patterns favor the use of low density multiplication factors and thinner films, which undermine the promise of enhanced resolution and the formation of robust masks for pattern transfer. Here, we demonstrate a self-registered self-assembly method to enable nearly perfect DSA on loosely defined chemical patterns with high density multiplication factor. Self-registered self-assembly involves two DSA steps. In the first step, an ultrathin BCP blend film is used to obtain vanishingly low defect densities. Concurrently as the film is annealed, preloaded chemical markers separate into the different polymer blocks and graft to the substrate locking in a new chemical contrast pattern with 1:1 feature registration. After thorough removal of the blend film, the remaining self-registered chemical pattern can establish defect-free DSA of thick BCP films.

Rapid ordering of block copolymer thin films

Block-copolymers self-assemble into diverse morphologies, where nanoscale order can be finely tuned via block architecture and processing conditions. However, the ultimate usage of these materials in real-world applications may be hampered by the extremely long thermal annealing times-hours or days-required to achieve good order. Here, we provide an overview of the fundamentals of block-copolymer self-assembly kinetics, and review the techniques that have been demonstrated to influence, and enhance, these ordering kinetics. We discuss the inherent tradeoffs between oven annealing, solvent annealing, microwave annealing, zone annealing, and other directed self-assembly methods; including an assessment of spatial and temporal characteristics. We also review both real-space and reciprocal-space analysis techniques for quantifying order in these systems.

Morphological evolution of lamellar forming polystyrene-block-poly(4-vinylpyridine) copolymers under solvent annealing

In this work, we are reporting a very simple and efficient method to form lamellar structures of symmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymer thin films with vertically (to the surface plane) orientated lamellae using a solvent annealing approach. The methodology does not require any brush chemistry to engineer a neutral surface and it is the block neutral nature of the film-solvent vapour interface that defines the orientation of the lamellae. The microphase separated structure of two different molecular weight lamellar forming PS-block-P4VP copolymers formed under solvent vapour annealing was monitored using atomic force microscopy (AFM) so as to understand the morphological changes of the films upon different solvent exposure. In particular, the morphology changes from micellar structures to well-defined microphase separated arrangements. The choice of solvent/s (single and dual solvent exposure) and the solvent annealing conditions (temperature, time etc.) has important effects on structural transitions of the films and it was found that a block neutral solvent was required to realize vertically aligned P4VP lamellae. The results of the structural variation of the phase separated nanostructured films through the exposure to ethanol are also described.

Directed self-assembly of solvent-vapor-induced non-bulk block copolymer morphologies on nanopatterned substrates

We report a study on directed self-assembly (DSA) with solvent annealing to induce the formation of non-bulk block copolymer microdomains on chemical patterns. Ultrathin films of symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) display morphologies of PMMA dots, stripes, and PS hexagons with increasing exposure time to acetone vapor, a PMMA-selective solvent. All three nanostructures form long-range-ordered and registered arrays on striped chemical patterns with periods (LS) commensurate to the solvated PS-b-PMMA microdomain period (L0,s). Solvent annealing is shown to facilitate DSA on non-regular chemical patterns, on which the local periods are incommensurate to L0,s. DSA with feature density multiplication, via solvent annealing, is also demonstrated.

Alignment of Copolymer Morphology by Planar Step Elongation during Spinodal Self-Assembly

Using simulation and numerical self-consistent field theory of an unentangled diblock copolymer melt, we study the interplay between relaxation of molecular conformations from a highly stretched, nonequilibrium state and structure formation of the local, conserved density during self-assembly from a disordered state. We observe that the planar elongation of molecular conformations in the initial, disordered state results in an alignment of lamella normals perpendicular to the stretch direction during the subsequent self-assembly. Although thermodynamically the parallel orientation is favored, the alignment of the lamella normal perpendicular to the stretch direction is characterized by the larger growth rate of composition fluctuations during the spinodal ordering process.

Molecular pathways for defect annihilation in directed self-assembly

Over the last few years, the directed self-assembly of block copolymers by surface patterns has transitioned from academic curiosity to viable contender for commercial fabrication of next-generation nanocircuits by lithography. Recently, it has become apparent that kinetics, and not only thermodynamics, plays a key role for the ability of a polymeric material to self-assemble into a perfect, defect-free ordered state. Perfection, in this context, implies not more than one defect, with characteristic dimensions on the order of 5 nm, over a sample area as large as 100 cm(2). In this work, we identify the key pathways and the corresponding free energy barriers for eliminating defects, and we demonstrate that an extraordinarily large thermodynamic driving force is not necessarily sufficient for their removal. By adopting a concerted computational and experimental approach, we explain the molecular origins of these barriers and how they depend on material characteristics, and we propose strategies designed to overcome them. The validity of our conclusions for industrially relevant patterning processes is established by relying on instruments and assembly lines that are only available at state-of-the-art fabrication facilities, and, through this confluence of fundamental and applied research, we are able to discern the evolution of morphology at the smallest relevant length scales-a handful of nanometers-and present a view of defect annihilation in directed self-assembly at an unprecedented level of detail.

Defects in the Self-Assembly of Block Copolymers and Their Relevance for Directed Self-Assembly

Block copolymer self-assembly provides a platform for fabricating dense, ordered nanostructures by encoding information in the chemical architecture of multicomponent macromolecules. Depending on the volume fraction of the components and chain topology, these macromolecules form a variety of spatially periodic microphases in thermodynamic equilibrium. The kinetics of self-assembly, however, often results in initial morphologies with defects, and the subsequent ordering is protracted. Different strategies have been devised to direct the self-assembly of copolymer materials by external fields to align and perfect the self-assembled nanostructures. Understanding and controlling the thermodynamics of defects, their response to external fields, and their dynamics is important because applications in microelectronics either require extremely low defect densities or aim at generating specific defects at predetermined locations to fabricate irregular device-oriented structures for integrated circuits. In this review, we discuss defect morphologies of block copolymers in the bulk and thin films, highlighting (a) analogies to and differences from defects in other crystalline materials, (b) the stability of defects and their dynamics, and (c) the influence of external fields.

Defect removal in the course of directed self-assembly is facilitated in the vicinity of the order-disorder transition

The stability of prototypical defect morphologies in thin films of symmetric diblock copolymers on chemically patterned substrates is investigated by self-consistent field theory. The excess free energy of defects and barriers of defect-removal mechanisms are obtained by computing the minimum free-energy path. Distinct defect-removal mechanisms are illustrated demonstrating that (i) defects will become unstable at a characteristic value of incompatibility χN* above the order-disorder transition and (ii) the kinetics is accelerated at weak segregation. Numerical findings are placed in the context of physical mechanisms, and implications for directed self-assembly are discussed.

Determination of the internal morphology of nanostructures patterned by directed self assembly

The directed self-assembly (DSA) of block copolymers (BCP) is an emerging resolution enhancement tool that can multiply or subdivide the pitch of a lithographically defined chemical or topological pattern and is a resolution enhancement candidate to augment conventional lithography for patterning sub-20 nm features. Continuing the development of this technology will require an improved understanding of the polymer physics involved as well as experimental confirmation of the simulations used to guide the design process. Both of these endeavors would be greatly facilitated by a metrology, which is capable of probing the internal morphology of a DSA film. We have developed a new measurement technique, resonant critical-dimension small-angle X-ray scattering (res-CDSAXS), to evaluate the 3D buried features inside the film. This is an X-ray scattering measurement where the sample angle is varied to probe the 3D structure of the film, while resonant soft X-rays are used to enhance the scattering contrast. By measuring the same sample with both res-CDSAXS and traditional CDSAXS (with hard X-rays), we are able to demonstrate the dramatic improvement in scattering obtained through the use of resonant soft X-rays. Analysis of the reciprocal space map constructed from the res-CDSAXS measurements allowed us to reconstruct the complex buried features in DSA BCP films. We studied a series of DSA BCP films with varying template widths, and the internal morphologies for these samples were compared to the results of single chain in mean-field simulations. The measurements revealed a range of morphologies that occur with changing template width, including results that suggest the presence of mixed morphologies composed of both whole and necking lamella. The development of res-CDSAXS will enable a better understanding of the fundamental physics behind the formation of buried features in DSA BCP films.

Directing the self-assembly of block copolymers into a metastable complex network phase via a deep and rapid quench

The free-energy landscape of self-assembling block copolymer systems is characterized by a multitude of metastable minima. Using particle-based simulations of a soft, coarse-grained model, we explore opportunities to reproducibly direct the spontaneous ordering of these self-assembling systems into a metastable complex network morphology--specifically, Schoen's I-WP periodic minimal surface--starting from a highly unstable state that is generated by a rapid expansion. This process-directed self-assembly provides an alternative to fine-tuning molecular architecture or blending for fabricating complex network structures. Comparing our particle-based simulation results to recently developed free-energy techniques, we critically assess their ability to predict spontaneous formation and highlight the importance of nonequilibrium molecular conformations in the starting state and the local conservation of density.

Geometry-controlled interface localization-delocalization transition in block copolymers

Lamellar copolymers confined into a film of thickness D by two stripe-patterned surfaces, which are rotated against each other by a twist angle α, form lamellar domains that register and align with the respective chemical surface patterns. The two domains of thickness x and D-x are separated by an interface that resembles a twist grain boundary. At small twist angles α or strong selectivity of the surface patterns, this interface fluctuates around the middle of the film, x≈D/2, while the interface is localized at one of the surfaces, x≈0 or x≈D, in the opposite limit. These two morphologies are separated by an interface localization-delocalization transition (ILDT) that can be controlled by the twist angle α. For thin films, we find a second-order ILDT while the ILDT is first-order for large D values. A phenomenological interface Hamiltonian is used to relate the findings to the ILDT of symmetric mixtures, and the predictions are confirmed by molecular simulation.

Directed assembly of non-equilibrium ABA triblock copolymer morphologies on nanopatterned substrates

The majority of past work on directed assembly of block copolymers on chemically nanopatterned surfaces (or chemical patterns) has focused on AB diblock copolymers, and the resulting morphologies have generally corresponded to equilibrium states. Here we report a study on directed assembly of ABA triblock copolymers. Directed assembly of thin films of symmetric poly(methyl methacrylate-b-styrene-b-methyl methacrylate) (PMMA-b-PS-b-PMMA) triblock copolymers is shown to be capable of achieving a high degree of perfection, registration, and accuracy on striped patterns having periods, L(s), commensurate with the bulk period of the copolymer, L(o). When L(s) is incommensurate with L(o), the triblock copolymer domains can reach dimensions up to 55% larger or 13% smaller than L(o). The range over which triblock copolymers tolerate departures from a commensurate L(s) is significantly larger than that accessible with the corresponding diblock copolymer material on analogous directed assembly systems. The assembly kinetics of the triblock copolymer is approximately 3 orders of magnitude slower than observed in the diblock system. Theoretically informed simulations are used to interpret our experimental observations; a thermodynamic analysis reveals that triblocks can form highly ordered, non-equilibrium metastable structures that do not arise in the diblock.

Functional polymers in protein detection platforms: optical, electrochemical, electrical, mass-sensitive, and magnetic biosensors

The rapidly growing field of proteomics and related applied sectors in the life sciences demands convenient methodologies for detecting and measuring the levels of specific proteins as well as for screening and analyzing for interacting protein systems. Materials utilized for such protein detection and measurement platforms should meet particular specifications which include ease-of-mass manufacture, biological stability, chemical functionality, cost effectiveness, and portability. Polymers can satisfy many of these requirements and are often considered as choice materials in various biological detection platforms. Therefore, tremendous research efforts have been made for developing new polymers both in macroscopic and nanoscopic length scales as well as applying existing polymeric materials for protein measurements. In this review article, both conventional and alternative techniques for protein detection are overviewed while focusing on the use of various polymeric materials in different protein sensing technologies. Among many available detection mechanisms, most common approaches such as optical, electrochemical, electrical, mass-sensitive, and magnetic methods are comprehensively discussed in this article. Desired properties of polymers exploited for each type of protein detection approach are summarized. Current challenges associated with the application of polymeric materials are examined in each protein detection category. Difficulties facing both quantitative and qualitative protein measurements are also identified. The latest efforts on the development and evaluation of nanoscale polymeric systems for improved protein detection are also discussed from the standpoint of quantitative and qualitative measurements. Finally, future research directions towards further advancements in the field are considered.

Free Energy of Defects in Ordered Assemblies of Block Copolymer Domains

We investigate commonly occurring defects in block copolymer thin films assembled on chemically nanopatterned substrates and predict their probability of occurrence by computing their free energies. A theoretically informed 3D coarse grain model is used to describe the system. These defects become increasingly unstable as the strength of interactions between the copolymer and the patterned substrate increases and when partial defects occur close to the top surface of the film. The results presented here reveal an extraordinarily large thermodynamic driving force for the elimination of defects. When the characteristics of the substrate are commensurate with the morphology of the block copolymer, the probability of creating a defect is extremely small and well below the specifications of the semiconductor industry for fabrication of features having characteristic dimensions on the scale of tens of nanometers. We also investigate how the occurrence of defect changes when imperfections arise in the underlying patterns and find that, while defects continue to be remarkably unstable, stretched patterns are more permissive than compressed patterns.

Controlling Order in Block Copolymer Thin Films for Nanopatterning Applications

An attractive “unconventional” lithographic technique to pattern periodic, sub-100 nm features uses self-assembled block copolymer thin films as etch masks. Unfortunately, as-cast films lack the orientational and positional order of the microphase-separated domains that are necessary for many desired applications. Reviewed herein are techniques developed to guide the self-assembly process in thin films, which permit varying degrees of control over the patterns formed by the microdomains. Techniques that can control the out-of-plane order of the microdomains are first summarized. Then, techniques that control the lateral ordering are reviewed, beginning with those that generate large defect-free grains, then those that impart orientational order to the microdomains, and finally those that can control both the orientation and position of individual microdomains. Each technique is summarized with experimental examples and discussions regarding the mechanism of the guided self-assembly process.

Phase-field modeling of the formation of lamellar nanostructures in diblock copolymer thin films under inplanar electric fields

Recent experiments show that external inplanar electric field can be employed to guide the molecular self-assembly in diblock copolymer (BCP) thin films to form lamellar nanostructures with potential applications in nanotechnology. We study this self-assembly process through a detailed coarse-grained phase-separation modeling. During the process, the free energy of the BCP films is modeled as the Ginzburg-Landau free energy with nonlocal interaction and electrostatic coupling. The resulting Cahn-Hilliard (CH) equation is solved using an efficient semi-implicit Fourier-spectral algorithm. Numerical results show that the morphology of order parameter formed in either symmetric or asymmetric BCP thin films is strongly influenced by the electric field. For symmetrical BCPs, highly ordered lamellar nanostructures evolved along the direction of the electric field. Phase nucleation and dislocation climbing in the BCP films predicted by the numerical simulation are in a good agreement with those observed in recent BCP electronanolithography. For asymmetrical BCPs, numerical simulation shows that nanodots are guided to align to the electric field. Furthermore, in the case of high electric field, nanodots formed in asymmetrical BCPs may further convert into highly ordered lamellar nanostructures (sphere-to-cylinder transition) parallel to the electric field. Effects of the magnitude of electric field, BCP asymmetry, and molecular interaction of BCPs on the self-assembly process are examined in detail using the numerical scheme developed in this study. The present study can be used for the prediction of the formation of nanostructures in BCP thin films and the quality control of BCP-based nanomanufacturing through optimizing the external electric fields.

Substrate wetting and topographically induced ordering of amorphous PI-b-PFS block-copolymer domains

The substrate wetting of an amorphous, low-glass-transition-temperature spherical poly(isoprene-block-ferrocenylsilane) (PI-b-PFS) block copolymer and the alignment of the microdomains in grooves of various geometry are studied. Compositional analysis by time-of-flight secondary ion mass spectrometry depth profiling (TOF-SIMS) indicates the presence of both PI and PFS directly at the film-substrate interface on silicon and silica substrates. The TOF-SIMS depth-profiling study indicates a transition in the packing of the domains between the two-dimensional (2D) monolayer and 3D, thicker layers. In a monolayer of domains, a hexagonal packing is adopted. In films of two or three layers, the hexagonal packing reorganizes towards a body-centered cubic (bcc) packing by the extension of the copolymer chains in the direction normal to the substrate, as indicated by an increase in spacing between PFS layers and an increase in domain size. For thicker layers, a bcc morphology with the (110) plane parallel to the substrate is found to extend from the free surface downwards. Films of one monolayer of domains of the copolymer exhibit long-range lateral ordering on the micrometer scale on flat substrates without high-temperature annealing. On topographically patterned silicon substrates the position of the domains of the minority PFS phase directly near the side walls is fixed by the neutral wetting condition. Successful positioning of the block-copolymer spheres in linear and hexagonal grooves is achieved in grooves up to 1.3 microm wide, whereby the hexagonal grooves demonstrate complete 2D alignment. In circular pits, this graphoepitaxial effect is absent.