Directed Self-assembly Research Articles

Resist-Free Directed Self-Assembly Chemo-Epitaxy Approach for Line/Space Patterning

This work reports a novel, simple, and resist-free chemo-epitaxy process permitting the directed self-assembly (DSA) of lamella polystyrene-block-polymethylmethacrylate (PS-b-PMMA) block copolymers (BCPs) on a 300 mm wafer. 193i lithography is used to manufacture topographical guiding silicon oxide line/space patterns. The critical dimension (CD) of the silicon oxide line obtained can be easily trimmed by means of wet or dry etching: it allows a good control of the CD that permits finely tuning the guideline and the background dimensions. The chemical pattern that permits the DSA of the BCP is formed by a polystyrene (PS) guide and brush layers obtained with the grafting of the neutral layer polystyrene-random-polymethylmethacrylate (PS-r-PMMA). Moreover, data regarding the line edge roughness (LER) and line width roughness (LWR) are discussed with reference to the literature and to the stringent requirements of semiconductor technology.

Novel Guiding Template and Mask Assignment for DSA-MP Hybrid Lithography Using Multiple BCP Materials

Directed self-assembly (DSA) is one of the leading candidates for extending the resolution of optical lithography to sub-7 nm and beyond. By incorporating DSA in multiple patterning lithography (DSA-MP), the flexibility and resolution of contact/via patterning can be further enhanced by using multiple block copolymer (BCP) materials. Prior work faces the dilemma between solution quality and efficiency and is unable to handle 2-D templates. In this article, we capture the essence of template and mask assignment in DSA-MP by a new graph model and a new problem reduction: Our graph model explicitly represents spacing conflict edges and template hyperedges; thus, extra enumeration and manipulation of incompatible via grouping edges can be avoided, and arbitrary 1-D/2-D templates can be natively handled. We further reduce the assignment problem to exact cover, which is encoded by a sparse matrix. Our concise integer linear programming (ILP) formulation and fast backtracking heuristic achieve substantially superior solution quality and efficiency to the state-of-the-art work. Moreover, our problem and graph modelling is flexible and extensible to utilize dummy vias to improve manufacturability. To narrow down the search region of dummy vias, we devise a conflict core finding technique, which is general and applicable to conflict core analysis of exact cover and other multiple patterning layout decomposition problems. Experimental results show the effectiveness and efficiency of our approach.

Self-Directed Self-Assembly of 3D Tailored Block Copolymer Nanostructures.

Directed self-assembly (DSA) of block copolymers (BCPs) provides a powerful tool to fabricate various 2D nanostructures. However, it still remains a challenge to extend DSA to make uniform and complex 3D nanostructures through BCP self-assembly. In this paper, we introduce a method to fabricate various nanostructures in 3D and test it using simulations. In particular, we employ dissipative particle dynamics (DPD) simulation to demonstrate that uniform multilayer nanostructures can be achieved by alternating the stacking of two "orthogonal" BCPs films, AB copolymer film and AC copolymer film, without the need to cross-link or etch any of the components. The assembly of a new layer occurs on top of the previous bottom layer, and thus the structural information from the substrate is propagated upward in the film, a process we refer to as self-directed self-assembly (SDSA). If this process is repeated many times, one can have tailored multilayer nanostructures. Furthermore, the natural (bulk) phases of the block copolymers in each layer do not need to be the same, so one can achieve complex 3D assemblies that are not possible with a single-phase 3D system. This method in conjunction with grapho (or chemo) epitaxy is able to evolve a surface pattern into a 3D nanostructure. Here we show several examples of nanostructures fabricated by this process, which include aligned cylinders, spheres on top of cylinders, and orthogonal nanomeshes. Our work should be useful for creating complex 3D nanostructures using self-assembly.

3D ultrasound directed self-assembly of high aspect ratio particles: On the relationship between the number of transducers and their spatial arrangement

Ultrasound directed self-assembly (DSA) enables noninvasively aligning high aspect ratio particles in three-dimensional (3D) user-specified orientations, which finds application in a myriad of engineering applications, including manufacturing engineered materials. However, the number of ultrasound transducers and their spatial arrangement limit the accuracy of the particle alignment with any 3D user-specified orientation. We define a set of 3D user-specified orientations and use numerical simulations to quantitatively evaluate the effect of the number of ultrasound transducers, their spatial arrangement including a sphere, cube, and two parallel plates, and the size of the spatial arrangement on the orientation error of a high aspect ratio particle in a standing ultrasound wave field. We demonstrate that a spatial arrangement of ultrasound transducers with more than two unique wave propagating directions is required to orient a high aspect ratio particle in 3D, and we determine that the orientation error decreases with the increasing number of unique wave propagation directions. Furthermore, we show that in a spherical arrangement of ultrasound transducers, the orientation error is independent of the size of the arrangement of transducers. This knowledge facilitates using ultrasound DSA as a fabrication method for engineered composite materials that derive their function from the location and orientation of particle inclusions.

Simulation study of the pH sensitive directed self-assembly of rheins for sustained drug release hydrogel

The hydrogel formed by the directed self-assembled rhein molecules at an appropriate pH value for sustained drug release has been reported recently. Although the application in drug therapy has been experimentally verified, the research on the mechanism of the self-assembly by rhein is still incomplete. In this study, we provide a new insight of the pH-induced self-assembly mechanism employing the dissipative particle dynamics (DPD) as well as multiscale molecular simulations. It comes to the conclusion that protonated rheins incline to aggregate, so spherical, columnar, and membrane-shaped aggregated micelles can be observed with the mounting drug concentrations. However, the thermodynamic solubility of these structures is relatively weak, which ultimately causes the system to precipitate. While, driven by the better hydrophilicity and charge effect of the deprotonated rheins, the self-assembly conformation of deprotonated rheins grows through a sphere, wormlike, long wormlike manner, resulting in a host of discontinuous and highly dispersive morphologies. Those structures, having good thermodynamic solubility though, cannot provide the required mechanical properties, thus the rhein system can only exist in the aqueous solution state. Only when the value of pH is moderate (the corresponding degree of deprotonation is 36 %–60 %), could a continuous network self-assembled structure be obtained, leading to the formation of hydrogel to meet the clinical requirements.

Poly(2-vinylpyridine)-b-poly(fluorinated methacrylate) Block Copolymers Forming 5 nm Domains Containing Metallocene

Directed self-assembly (DSA) of thin films of a block copolymer (BCP) is a promising approach for sub-10 nm lithographic patterning. In this paper, a series of poly(2-vinylpyridine)-block-poly(pent...

Etching of sub-10 nm half-pitch high chi block copolymers for directed self-assembly (DSA) application

Directed Self-Assembly of block copolymers is a lithographic technique being developed to reach sub-10 nm technological nodes. Recently, high chi block copolymers have been developed to achieve higher resolution. In this paper, the high chi system investigated is a modified polystyrene-b-poly(methyl methacrylate)(PS-b-PMMA) presenting a pitch of 18 nm. One critical step for its integration is the PMMA removal selectively to the PS. Two approaches to remove the PMMA phase are presented, highlighting the challenges encountered due to its smaller dimensions. The first one is a full dry approach based on a CH4/N2 chemistry, which presents some bridge formation due to intensive sidewalls passivation and to a species confinement effect accentuated by the small 9-nm critical dimension of the lines. Therefore, the main high chi PS-b-PMMA dry etching difficulties come from the trade-off between obtaining high selectivity and avoiding bridge formation. The second approach is a mixed wet and dry PMMA removal process based on UV exposure followed by solvent rinse. An Ar/O2 dry brush layer opening was developed and the pattern transfer into the SiO2 and Si underlayers is demonstrated for the high chi PS-b-PMMA.

Atomic force microscopy studies of LAPONITE® directed self-assembly of single-walled carbon nanotubes in electronic nanonetworks

Many commercial applications of carbon nanotubes (CNTs) have not yet been realized owing to their limited water processability that leads to poor electrical performance. We report the syntheses of oxidized single-walled carbon nanotubes (SWCNTs) with improved percolating network connectivity and conductivity in thin films processed from aqueous colloids mixed with dispersing agents including LAPONITE® nanoparticles and surfactants. The influences of clay loading and acid functionalization on surface morphology and the self-assembly of interconnected networks in SWCNT films were studied. Conductive atomic force microscopy (c-AFM) analyses indicated that the most favorable nanonetwork for charge conduction was achieved at a low loading of LAPONITE® nanoparticles. In addition, hydrazine reduction treatments after acid functionalization led to augmented electrical conductivities in the CNT films regardless of the extent of clay loading. These studies provide important insight on tuning the heterointerfaces that control CNT self-assembly and polydispersity and may prove insightful to realize percolating networks for green energy applications.

Block copolymer directed self-assembly defect modes induced by localized errors in chemoepitaxial guiding underlayers: A molecular simulation study

Block copolymer (BCP) directed self-assembly (DSA) has been presented as a potential economically attractive enhancement to extend the capabilities of optical lithography for semiconductor manufacturing. One concern in DSA is the level of defectivity that can be achieved in such a process. Although entropic effects will always lead to some degree of defectivity, highly ordered structures with a low theoretical equilibrium defect density can be produced by guiding the ordering and placement of the BCP domains using a patterned underlayer. Recent experimental studies have shown that while DSA processes can significantly reduce the observed defect density, defectivity levels are generally still higher than allowable for high-volume manufacturing and higher than what would be anticipated from free energy estimates of the observed defect modes. In particular, bridge defects are one of the most commonly observed defect modes in experimental DSA studies. A number of hypotheses have been proposed to explain the origins of these defects. One hypothesis is that so-called affinity defects present in the underlayer can spawn bridge defects in the overlying BCP film. The goal of the work reported here was to investigate the extent to which bridge defects can be generated or further reinforced in lamellae-forming block copolymer films due to affinity defects in the underlayer pattern. Coarse-grained molecular dynamics simulations were used to simulate the chemoepitaxial DSA of monodisperse block copolymer films atop underlayers with varying affinity defect sizes. Affinity defects were simulated by creating circular regions of a single polymer block type (which is the opposite block type of that used to pattern the underlayer guiding stripes) in the nominally neutral background region of the underlayer. These affinity defects were positioned in regions of the underlayer where they were the incorrect type to match the overlying block copolymer pattern. It was observed that the presence of an affinity defect in the neutral region of the underlayer caused the energetically preferential polymer block to wet the affinity defect, thus creating the nucleus of what could potentially become a bridge defect—even when the affinity defects were very small. As the radius of the underlayer affinity defect (RoD) increased, the amount of block copolymer of incorrect type (with respect to a perfectly assembled copolymer pattern) that assembled above the affinity defect increased; but, in general, the thickness of the wetting layer in contact with the affinity defect was only roughly one polymer chain thick. These data suggest that an affinity defect in the underlayer alone is unlikely to be noticeably enhanced by significant bridge defect formation in a monodisperse block copolymer film.

Spatial Control of the Self-assembled Block Copolymer Domain Orientation and Alignment on Photopatterned Surfaces.

Polarity-switching photopatternable guidelines can be directly used to both orient and direct the self-assembly of block copolymers. We report the orientation and alignment of poly(styrene-block-4-trimethylsilylstyrene) (PS-b-PTMSS) with a domain periodicity, L0, of 44 nm on thin photopatternable grafting surface treatments (pGSTs) and cross-linkable surface treatments (pXSTs), containing acid-labile 4-tert-butoxystyrene monomer units. The surface treatment was exposed using electron beam lithography to create well-defined linear arrays of neutral and preferential regions. Directed self-assembly (DSA) of PS-b-PTMSS with much lower defectivity was observed on pXST than on pGST guidelines. The study of the effect of film thickness on photoacid diffusion by Fourier transform infrared spectroscopy and near-edge X-ray absorption fine structure spectroscopy suggested slower diffusion in thinner films, potentially enabling production of guidelines with sharper interfaces between the unexposed and exposed lines, and thus, the DSA of PS-b-PTMSS on thinner pXST guidelines resulted in better alignment control.

Plasmonic nanocavities fabricated by directed self-assembly lithography and nanotransfer printing and used as surface-enhanced Raman scattering substrates

In this study, plasmonic nanocavity arrays were fabricated as surface-enhanced Raman scattering (SERS) substrates through directed self-assembly (DSA) of block copolymer and nanotransfer printing (NTP). NTP is a simple and efficient process of replicating plasmonic nanofeatures from a silicon template. The template was fabricated with the combination of nanoimprint and DSA lithographies and can be reused repeatedly. Utilizing DSA lithography allows us to cost-effectively produce nanofeatures. Gold nanocavities with 10 nm gap size, 30 nm height, and 40 nm period were fabricated. Gold nanoslits with the same dimensions were also fabricated for comparison. The field enhancement properties of both structures were investigated numerically and experimentally. The localized field enhancement in the nanocavity was considerably stronger than that in the nanoslit. The SERS sensing result greatly agreed with the simulation result. The nanocavity substrate enhanced the SERS signals six times higher than the nanoslit substrate. This approach is a promising candidate for SERS substrate fabrication, thus eliminating the trade-off between throughput and reproducibility.

Lamellar Orientation of a Block Copolymer via an Electron-Beam Induced Polarity Switch in a Nitrophenyl Self-Assembled Monolayer or Si Etching Treatments

Directed self-assembly (DSA) was investigated on self-assembled monolayers (SAMs) chemically modified by electron beam (EB) irradiation, which is composed of 6-(4-nitrophenoxy) hexane-1-thiol (NPHT). Irradiating a NPHT by EB could successfully induce the orientation and selective patterning of block copolymer domains. We clarified that spatially-selective lamellar orientations of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) could be achieved by a change of an underlying SAM. The change of an underlying SAM is composed of the transition of an NO2 group to an NH2 group, which is induced by EB. The modification in the polarity of different regions of the SAM with EB lithography controlled the lamellar orientation of PS-b-PMMA. The reduction of the NPHT SAM plays an important role in the orientation of block copolymer. This method might significantly simplify block copolymer DSA processes when it is compared to the conventional DSA process. By investigating the lamellae orientation with EB, it is clarified that only suitable annealing temperatures and irradiation doses lead to the vertical orientation. We also fabricated pre-patterned Si substrates by EB lithographic patterning and reactive ion etching (RIE). DSA onto such pre-patterned Si substrates was proven to be successful for subdivision of the lithographic patterns into line and space patterns.

Directed Self‐Assembly of Patchy Microgels into Anisotropic Nanostructures

Multi-geometry nanostructures with high-order, complex, and controllable geometries have attracted extensive attention in the development of functional nanomaterials. A simple and versatile strategy is proposed to construct various anisotropic nanostructures through the directed self-assembly (DSA) of patchy microgels. A general criterion for interaction parameters is developed by the variance analysis method to achieve the formation of 1D nanorods by the single directional DSA process, and 2D or 3D polymorphs including V/T/h/cross shapes, multiple arms, multi-directional bending, single/multiple rings, nanocages, etc., by the multi-directional DSA process of binary microgel blends. At the optimum interaction parameters, the nanorods exhibit the quickest formation process and the most thermodynamically stable geometry, while the various 2D or 3D assemblies exhibit controlled jointing behaviors for versatile assembly geometries. The number of recognition sites on the patchy microgel surface guides the aggregation modes of microgels during the DSA process. These assemblies can bear large curvature variance with the increase of shear rates due to the high flexibility and the ability of adjusting orientation spontaneously. The DSA behavior of patchy microgels differs from the traditional self-assembly process of block copolymers, which may open a new route for guiding the formation of controllable nanoparticle architectures.

Strategies for Increasing the Rate of Defect Annihilation in the Directed Self-Assembly of High-χ Block Copolymers.

Directed self-assembly (DSA) of high-χ block copolymer thin films is a promising approach for nanofabrication of features with length scale below 10 nm. Recent work has highlighted that kinetics are of crucial importance in determining whether a block copolymer film can self-assemble into a defect-free ordered state. In this work, different strategies for improving the rate of defect annihilation in the DSA of a silicon-containing, high-χ block copolymer film were explored. Chemo-epitaxial DSA of poly(4-methoxystyrene-block-4-trimethylsilylstyrene) with 5× density multiplication was implemented on 300 mm wafers by using production level nanofabrication tools, and the influence of different processes and material parameters on dislocation defect density was studied. It was observed that only at sufficiently low χN can the block copolymer assemble into well-aligned patterns within a practical time frame. In addition, there is a clear correlation between the rate of the lamellar grain coarsening in unguided self-assembly and the rate of dislocation annihilation in DSA. For a fixed chemical pattern, the density of kinetically trapped dislocation defects can be predicted by measuring the correlation length of the unguided self-assembly under the same process conditions. This learning enables more efficient screening of block copolymers and annealing conditions by rapid analysis of block copolymer films that were allowed to self-assemble into unguided (commonly termed fingerprint) patterns.

Kinetic approach to defect reduction in directed self-assembly

As a potential solution to next-generation nanolithography, directed self-assembly (DSA) of block copolymers (BCPs) is still restrained in high-volume manufacturing primarily due to its defectivity issue. Though defects possess greater free energies than aligned morphologies and are highly energetically unfavorable, they can be kinetically trapped by the energy barriers and persist for a long time during annealing. Therefore, understanding the kinetics of defect annihilation is crucial in revealing the mechanism of defect formation and in further reducing defectivity in DSA. We focus on two types of predominant defects in DSA—dislocation and bridge. A kinetic model of each defect type is developed through statistical analysis of experimental data, providing insight into possible approaches of further defect reduction. We also investigate the impact of annealing temperature and film thickness on annihilation kinetics and discuss the reasons behind the observed results. By simply optimizing annealing conditions and film thickness, we have successfully reduced the total defect density by 1 order of magnitude. Though these findings are based on polystyrene- b -poly(methyl methacrylate) (PS- b -PMMA), we anticipate they could be readily applied to other BCP platforms as well.

Defect Annihilation in the Directed Self-Assembly of Block Copolymers in Films with Increasing Thickness

We investigate directed self-assembly (DSA), with 3× density multiplication, of symmetric polystyrene-block-poly(methyl methacrylate) (L0 = 28 nm) with increasing film thicknesses and reveal a thickness limit above which the defect annihilation mechanism is different than in thinner films. Block copolymer films of increasing thickness underwent DSA on a macroscopic chemical pattern under optimum geometrical and chemical conditions. Scanning electron microscopy and grazing-incidence small-angle X-ray scattering (GISAXS) were used to characterize the long-range ordering and buried structures as a function of film thickness and thermal annealing time. Rotational GISAXS measurements revealed a critical thickness of ∼3L0, below which defect annihilation is cooperative across the film depth and above which it is faster at the free surface than in the film’s interior. In the latter case, defects can persist in the film’s interior despite perfect assembly and registration at the free surface of the film.

DSA guiding template assignment with multiple redundant via and dummy via insertion

As an emerging manufacture technology, block copolymer directed self-assembly (DSA) is promising for via layer fabrication. Meanwhile, redundant via insertion is considered as an essential step for yield improvement. For better reliability and manufacturability, in this paper, we first concurrently consider DSA guiding template cost assignment with multiple redundant via and dummy via insertion. Firstly, by analyzing the structure property of guiding templates, we propose a building-block based solution expression to discard redundant solutions. Then, honoring the compact solution expression, we construct a conflict graph with dummy via insertion, and then formulate the problem to an integer linear programming (ILP). In addition, to optimize the guiding template cost, we incorporate it into the objective of ILP by introducing vertex weight and edge weight in conflict graph. To make a good trade-off between solution quality and runtime, we relax the ILP to an unconstrained nonlinear programming (UNP). Finally, a line search optimization algorithm is proposed to solve the UNP. Experimental results verify the effectiveness of our new solution expression and the efficiency of our proposed algorithm. Specifically, our guiding template cost optimization method can save 18% total guiding template cost.

Ultrathin initiated chemical vapor deposition polymer interfacial energy control for directed self-assembly hole-shrink applications

Integrated circuit layouts consist of patterned lines and holes, where holes define the electrical contacts between adjacent layers. Block copolymer directed self-assembly (DSA) successfully shrinks the critical dimension (CD) of these contacts beyond the resolution of conventional lithography. DSA also radically improves the CD uniformity. One particularly difficult step of the DSA hole-shrink process involves establishing the correct interfacial energy throughout a lithographically templated hole to ensure good assembly. Initiated chemical vapor deposition (iCVD) is a uniform, ultrathin, ultraconformal, all-organic deposition technique that allows for precise control of the interfacial energy. In this work, the authors use iCVD of polydivinylbenzene at film thicknesses below 5 nm to blend the interfacial energy of the coated film with that of the silicon/spin-on carbon template. They fully characterize the iCVD surface by means of two liquid surface energy measurements. They further identify the interfacial energies presented by these functionalized templates through qualitative hole-island tests as well as quantitative harmonic mean estimations. In parallel, the authors run theoretically informed coarse grained simulations with the determined interaction parameters and DSA experiments and find good agreement across the range of chemistries created. Through careful control of iCVD conditions, especially filament temperature, they achieve a strong polystyrene-preferential sidewall with a nonpreferential bottom which they then demonstrate, both in the simulation and in the experiment, allows for a successful hole-shrink process across a wide range of template hole diameters.

Aligning high-aspect ratio particles in user-specified orientations with ultrasound directed self-assembly

We use ultrasound directed self-assembly (DSA) to create two-dimensional patterns of high aspect ratio particles with user-specified orientation. We theoretically derive a method to determine the operating parameters of any arrangement of ultrasound transducers, required to align high aspect ratio particles in any user-specified orientation. The method finds the ultrasound wave field that maximizes the curvature of the acoustic radiation potential orthogonal to the user-specified particle orientation and in user-specified locations. We simulate the theoretical solution resulting from this method and experimentally validate it using carbon microfibers in water, and we quantify the position and orientation error. The method enables controlling the location and orientation of high aspect ratio particles, including simultaneously orienting multiple high aspect ratio particles in different directions. This work finds application in the biomedical field and in using ultrasound DSA as a processing or manufacturing method for engineered materials.

Orientation control of high-χ triblock copolymer for sub-10 nm patterning using fluorine-containing polymeric additives

Directed self-assembly (DSA) of block copolymers (BCPs) is one of the most promising techniques to tackle the ever-increasing demand for sublithographic features in semiconductor industries. BCPs with high Flory–Huggins parameter (χ) are of particular interest due to their ability to self-assemble at the length scale of sub-10 nm. However, such high-χ BCPs typically have imbalanced surface energies between respective blocks, making it a challenge to achieve desired perpendicular orientation. To address this challenge, we mixed a fluorine-containing polymeric additive with poly(2-vinylpyridine)-<italic>block</italic>-polystyrene-<italic>block</italic>-poly(2-vinylpyridine) (P2VP-<italic>b</italic>-PS-<italic>b</italic>-P2VP) and successfully controlled the orientation of the high-χ triblock copolymer. The additive selectively mixes with P2VP block through hydrogen bonding and can reduce the dissimilarity of surface energies between PS and P2VP blocks. After optimizing additive dose and annealing conditions, desired perpendicular orientation formed upon simple thermal annealing. We further demonstrated DSA of this material system with five times density multiplication and a half-pitch as small as 8.5 nm. This material system is also amenable to sequential infiltration synthesis treatment to selectively grow metal oxide in P2VP domains, which can facilitate the subsequent pattern transfer. We believe that this integration-friendly DSA platform using simple thermal annealing holds the great potential for sub-10 nm nanopatterning applications.