Xuanxuan Chen

Three-Tone Chemical Patterns for Block Copolymer Directed Self-Assembly

Chemical patterns for directed self-assembly (DSA) of lamellae-forming block copolymers (BCP) with density multiplication can be fabricated by patterning resist on a cross-linked polystyrene layer, etching to create guide stripes, and depositing end-grafted brushes in between the stripes as background. To date, two-tone chemical patterns have been targeted with the guide stripes preferentially wet by one block of the copolymer and the background chemistry weakly preferentially wet by the other block. In the course of fabricating chemical patterns in an all-track process using 300 mm wafers, it was discovered that the etching process followed by brush grafting could produce a three-tone pattern. We characterized the three regions of the chemical patterns with a combination of SEM, grazing-incidence small-angle X-ray scattering (GISAXS), and assessment of BCP-wetting behavior, and evaluated the DSA behavior on patterns over a range of guide stripe widths. In its best form, the three-tone pattern consists of guide stripes preferentially wet by one block of the copolymer, each flanked by two additional stripes that wet the other block of the copolymer, with a third chemistry as the background. Three-tone patterns guide three times as many BCP domains as two-tone patterns and thus have the potential to provide a larger driving force for the system to assemble into the desired architecture with fewer defects in shorter time and over a larger process window.

Directed Self-Assembly of Polystyrene-b-poly(propylene carbonate) on Chemical Patterns via Thermal Annealing for Next-Generation Lithography.

Directed self-assembly (DSA) of block copolymers (BCPs) combines advantages of conventional photolithography and polymeric materials and shows competence in semiconductors and data storage applications. Driven by the more integrated, much smaller and higher performance of the electronics, however, the industry standard polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) in DSA strategy cannot meet the rapid development of lithography technology because its intrinsic limited Flory-Huggins interaction parameter (χ). Despite hundreds of block copolymers have been developed, these BCPs systems are usually subject to a trade-off between high χ and thermal treatment, resulting in incompatibility with the current nanomanufacturing fab processes. Here we discover that polystyrene-b-poly(propylene carbonate) (PS-b-PPC) is well qualified to fill key positions on DSA strategy for the next-generation lithography. The estimated χ-value for PS-b-PPC is 0.079, that is, two times greater than PS-b-PMMA (χ = 0.029 at 150 °C), while processing the ability to form perpendicular sub-10 nm morphologies (cylinder and lamellae) via the industry preferred thermal-treatment. DSA of lamellae forming PS-b-PPC on chemoepitaxial density multiplication demonstrates successful sub-10 nm long-range order features on large-area patterning for nanofabrication. Pattern transfer to the silicon substrate through industrial sequential infiltration synthesis is also implemented successfully. Compared with the previously reported methods to orientation control BCPs with high χ-value (including solvent annealing, neutral top-coats, and chemical modification), the easy preparation, high χ value, and etch selectivity while enduring thermal treatment demonstrates PS-b-PPC as a rare and valuable candidate for advancing the field of nanolithography.

Characterization of the shape and line-edge roughness of polymer gratings with grazing incidence small-angle X-ray scattering and atomic force microscopy

Grazing-incidence small-angle X-ray scattering (GISAXS) is increasingly used for the metrology of substrate-supported nanoscale features and nanostructured films. In the case of line gratings, where long objects are arranged with a nanoscale periodicity perpendicular to the beam, a series of characteristic spots of high-intensity (grating truncation rods, GTRs) are recorded on a two-dimensional detector. The intensity of the GTRs is modulated by the three-dimensional shape and arrangement of the lines. Previous studies aimed to extract an average cross-sectional profile of the gratings, attributing intensity loss at GTRs to sample imperfections. Such imperfections are just as important as the average shape when employing soft polymer gratings which display significant line-edge roughness. Herein are reported a series of GISAXS measurements of polymer line gratings over a range of incident angles. Both an average shape and fluctuations contributing to the intensity in between the GTRs are extracted. The results are critically compared with atomic force microscopy (AFM) measurements, and it is found that the two methods are in good agreement if appropriate corrections for scattering from the substrate (GISAXS) and contributions from the probe shape (AFM) are accounted for.

Grazing-incidence small angle x-ray scattering studies of nanoscale polymer gratings

Grazing-Incidence Small Angle X-ray Scattering (GISAXS) offers the ability to probe large sample areas, providing three-dimensional structural information at high detail in a thin film geometry. In this study we exploit the application of GISAXS to structures formed at one step of the LiNe (Liu-Nealey) flow using chemical patterns for directed self-assembly of block copolymer films. Experiments conducted at the Argonne National Laboratory provided scattering patterns probing film characteristics at both parallel and normal directions to the surface. We demonstrate the application of new computational methods to construct models based on scattering measured. Such analysis allows for extraction of structural characteristics at unprecedented detail.

Directed self-assembly of PS-b-PMMA with ionic liquid addition

Directed self-assembly of block copolymers is a promising candidate to address grand challenges towards new generations of low-cost, high-resolution nanopatterning technology. Over the past decade, poly(styrene-b-methyl methacrylate) (PS-b-PMMA) has been the most popular block copolymer applied in this area. However, further scaling towards pitches below 20 nm is hindered by its relatively low segregation strength between constituent blocks, characterized by a low Flory-Huggins interaction parameter, χ (~ 0.038 at r.t). To reach sub-10 nm feature dimensions, many high- χ block copolymer materials and processes are currently being studied. Here we investigate the DSA of PSb- PMMA with blended ionic liquid (IL) on chemically-patterned substrates via thermal annealing with a free surface. In this materials system, by adding low volume fraction of IL, a substantially higher χ than the pure block copolymer is achieved with manageable change in surface and interfacial properties so that poly(styrene-random-methyl methacrylate) brushes may be used to control substrate wetting behavior, and the blend could be assembled using thermal annealing with a free surface. In other words, PS-b-PMMA/IL may serve as a high- χ drop-in replacement for PS-b-PMMA. In this work, we provide key DSA results to determine if PS-b-PMMA/IL blends would offer a solution for sub-10 nm lithography.

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.

Directed self-assembly of triblock copolymers for sub-10 nm nanofabrication using polymeric additives

Directed self-assembly (DSA) of block copolymers (BCPs) is one of the most promising techniques to tackle the everincreasing demand for sub-lithographic 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 in sub-10 nm regime. 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 polymeric additive with poly(2-vinylpyridine)- block-polystyrene-block-poly(2-vinylpyridine) (P2VP-b-PS-b-P2VP) and successfully achieved perpendicular orientation control of the triblock copolymer. The polymeric additive has lower surface energy than both PS and P2VP blocks, and it selectively interacts with high surface energy P2VP blocks via hydrogen bonding. As a result, the surface energies of PS and P2VP blocks are balanced and perpendicular orientation forms upon thermal annealing. Using this approach, we demonstrate 5X density multiplication DSA with a half pitch of 8.5 nm via chemo-epitaxy. This material system is also amenable to sequential infiltration synthesis (SIS) without the need to remove the additive, revealing its pattern transfer potential. We believe that this integration-friendly DSA approach using simple thermal annealing holds the promise of bringing high-χ BCPs to advanced nanopatterning applications.

Ionic Liquids as Additives to Polystyrene-Block-Poly(Methyl Methacrylate) Enabling Directed Self-Assembly of Patterns with Sub-10 nm Features

Polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) is one of the prototypical block copolymers in directed self-assembly (DSA) research and development, with standardized protocols in place for processing on industrially relevant 300 mm wafers. Scaling of DSA patterns to pitches below 20 nm using PS- b-PMMA, however, is hindered by the relatively low Flory-Huggins interaction parameter, χ. Here, we investigate the approach of adding small amounts of ionic liquids (ILs) into PS- b-PMMA, which selectively segregates into the PMMA domain and effectively increases the χ parameter and thus the pattern resolution. The amount of IL additive is small enough to result in limited changes in PS- b-PMMAs surface and interfacial properties, thus maintaining industry-friendly processing by thermal annealing with a free surface. Three different ILs are studied comparatively regarding their compositional process window, capability of increasing χ, and thermal stability. By adding ∼3.1 vol % of the champion IL into a low-molecular-weight PS- b-PMMA ( Mn = 10.3k- b-9.5k), we demonstrated DSA on chemically patterned substrates of lamellar structures with feature sizes <8.5 nm. Compatibility of the PS- b-PMMMA/IL blends with the standardized processes that have been previously developed suggests that such blend materials could provide a drop-in solution for sub-10 nm lithography with the processing advantages of PS- b-PMMA.

Evaluating structure in thin block copolymer films with soft x-rays (Conference Presentation)

The semiconductor industry is evaluating a variety of approaches for the cost efficient production of future processing and memory generations. Amongst the technologies being explored are multiple patterning steps, extreme ultraviolet (EUV) lithography, multiple-beam electron beam lithography and the directed self-assembly (DSA) of block copolymers (BCPs). BCP DSA utilizes a chemical or topographical template to induce long range order in a thin film of BCP which enhances the resolution of the original pattern. The characterization of buried structure within a DSA BCP film is challenging due to the lack of contrast between the organic materials. Critical-dimension small angle x-ray scattering (CDSAXS) measurements were performed on DSA BCP films, using soft X-rays to tune the contrast, in order to understand the relationship between template structure and film morphology.1 The results of these measurements show that as the width of the guiding stripe widens the arrangement of the BCP on the guiding stripe inverts, shifting from the A block being centered on the guiding stripe to the B block being centered on the guiding stripe. The initial results of integration of mean field simulations into the analysis of scattering data will also be discussed. In addition to examining the BCP structure with CDSAXS, soft X-ray reflectivity2 measurements were performed on BCP to better understand the relationship between interface width for systems with alternative architectures (triblocks) and additives (polymers/ionic liquids). The addition of a selectively associating additive increases the interaction parameter between the two blocks, resulting in the reduction of the interface width and access to smaller pitches. The use of soft X-ray reflectivity allows the evaluation of the distribution of the additive. (1) Sunday, D. F.; Hammond, M. R.; Wang, C.; Wu, W.; Delongchamp, D. M.; Tjio, M.; Cheng, J. Y.; Kline, R. J.; Pitera, J. W. Determination of the Internal Morphology of Nanostructures Patterned by Directed Self Assembly. ACS Nano 2014, 8, 8426–8437. (2) Sunday, D. F.; Kline, R. J. Reducing Block Copolymer Interfacial Widths through Polymer Additives. Macromolecules 2015, 48, 679–686.