Directed Self-assembly Lithography Research Articles

Directed self-assembly of poly(styrene-b-vinyl acetate) block copolymers on chemical patterns for sub-10 nm nanopatterning via thermal annealing

Directed self-assembly (DSA) of block copolymers (BCPs) is an effective approach for the fabrication of sub-10 nm features for semiconductor devices. Although many high-χ BCPs have been developed for DSA, the majority of them has a significant difference in surface energies (γs) between two blocks. Here we report the synthesis and self-assembly of poly (styrene- b -vinyl acetate) (PS- b -PVAc). PS- b -PVAc has a higher Flory-Huggins interaction parameter (χ) than poly (styrene- b -methyl methacrylate) (PS- b -PMMA) and the smallest achievable period ( L O ) is 18.2 nm. PS and PVAc have nearly equal γs and a perpendicular orientation of domains is obtained on random copolymer brushes under thermal annealing. DSA of PS- b -PVAc (SV-1 and SV-2) is successfully demonstrated in the thin films on chemical patterns, and the feature size down to 9.5 nm is obtained in DSA of SV-4 with 2 × density multiplication. These results demonstrate that PS- b -PVAc could be a candidate BCP for next-generation DSA lithography. • PS- b -PVAc has a higher χ than PS- b -PMMA. • Perpendicularly orientated domains of PS- b -PVAc are obtained under thermal annealing. • Feature size down to 9.5 nm is obtained in DSA with 2x density multiplication.

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.

On Coloring Rectangular and Diagonal Grid Graphs for Multipatterning and DSA Lithography

Rectangular grid graph (RGG) and diagonal grid graph (DGG) are induced subgraphs of a rectangular or diagonal grid, respectively. Their $k$ -coloring problem has direct applications in printing contact/via layouts by multipatterning lithography (MPL). However, the problem in general is computationally difficult for $k>2$ , while it remains an open question on grid graphs due to their regularity and sparsity. On the other hand, directed self-assembly (DSA) technique can work with MPL to optimize the graph by grouping neighboring vertices such that $k$ can be reduced, but the problem of deploying the grouping for coloring is even more intractable. In this paper, we study both of the $k$ -coloring problems, with and without DSA grouping, on RGG and DGG. Without grouping, a complete $k$ -coloring analysis is conducted and particularly the NP-completeness of 3-coloring on a diagonal grid is proved. When considering grouping, we present a 3-coloring solution and prove the NP-completeness to solve the problem of $k=2$ .

STOMA: Simultaneous Template Optimization and Mask Assignment for Directed Self-Assembly Lithography With Multiple Patterning

Block copolymer directed self-assembly (DSA) is a promising technique to print contacts/vias for the 10 nm technology node and beyond. By using hybrid lithography that incorporates DSA with multiple patterning, multiple masks are used to print the DSA templates and then the templates can be used to guide the self-assembly of the block copolymer. In this paper, we propose approaches to solve the simultaneous template optimization and mask assignment problem for DSA with multiple patterning. We verified in experiments that our approaches remarkably outperform the state-of-the-art work in reducing the manufacturing cost.

Fast Verification of Guide-Patterns for Directed Self-Assembly Lithography

Guide-patterns (GPs) are critical to the construction of contacts and vias in directed self-assembly (DSA) lithography. Simulations can be used to verify GPs, but runtime is excessive. Instead, we categorize the shapes of GPs using a small number of geometric parameters. Then a verification function is built to predict whether a GP will produce the required contacts, as follows: a vector in parameter space is constructed to represent each GP in a test set; the acceptability of each GP is then assessed by DSA simulation, and each vector is tagged “good” or “bad” accordingly; next, the parameter space is deformed to convert a radial distribution into one in which the good and bad vectors can be separated by a hyper-plane, which finally becomes the verification function. We also show how to reduce the dimensionality of the parameter space by principal component analysis, and how to generalize the geometric description of GPs to allow different types of GP to be verified in a uniform fashion. The proposed GP verification is demonstrated in 10 nm technology.

Directed Self-Assembly on Photo-Crosslinked Polystyrene Sub-Layers: Nanopattern Uniformity and Orientation.

A photo-crosslinked polystyrene (PS) thin film is investigated as a potential guiding sub-layer for polystyrene-block-poly (methyl methacrylate) block copolymer (BCP) cylindrical nanopattern formation via topographic directed self-assembly (DSA). When compared to a non-crosslinked PS brush sub-layer, the photo-crosslinked PS sub-layer provided longer correlation lengths of the BCP nanostructure, resulting in a highly uniform DSA nanopattern with a low number of BCP dislocation defects. Depending on the thickness of the sub-layer used, parallel or orthogonal orientations of DSA nanopattern arrays were obtained that covered the entire surface of patterned Si substrates, including both trench and mesa regions. The design of DSA sub-layers and guide patterns, such as hardening the sub-layer by photo-crosslinking, nano-structuring on mesas, the relation between trench/mesa width, and BCP equilibrium period, were explored with a view to developing defect-reduced DSA lithography technology.

Dry development for a directed self-assembly lithography hole-shrink process using CO / H 2 plasma

A dry development, directed self-assembly lithography (DSAL) hole-shrink process was studied, with a focus on the selectivity of the etching of poly(methyl methacrylate) (PMMA) over polystyrene (PS), and the suppression of etch stop. The highly selective etching of PMMA over PS was achieved using CO gas chemistry. However, it was found that the PMMA etching did not proceed beyond a certain depth. Scanning transmission electron microscopy and x-ray photoelectron spectroscopy analysis indicated that a deposition layer formed not only on the PS but also on the PMMA. The addition of H2 to the CO plasma was effective in controlling the thickness of the deposition layer and in suppressing the etch stop. The CO/H2 plasma process was combined with ion energy control to achieve a dry development for hole shrinkage. The dry development DSAL hole-shrink process was successfully realized by tailoring the etching gas chemistry and controlling the ion energy.

(Invited) Supercritical Drying: A Sustainable Solution to Pattern Collapse of High-Aspect-Ratio and Low-Mechanical-Strength Device Structures

Collapse of nanostructures with high aspect ratio (STI, 3D NAND, capacitor, FinFET, etc.) and/or low mechanical strength (ultra low-k trench, EUV resist pattern, etc.) during drying step in wet process has become a critical issue in semiconductor industry. Even current workable drying practices are facing steeply rising challenges from rapid device scaling advancements. Imbalanced capillary pressure induced at the liquid-gas interface contributes to line distortion during conventional drying process. Commencement of pattern collapse or stiction depends not only on structure aspect ratio, but also on material strength. Figure 1 depicts non-stiction limit of conventional liquid drying with respect to feature critical dimensions and device material strength. Regular 2x STI trench structures start leaning at aspect ratios 17 or higher, while pattern collapse can be observed on ultra low-k features (k ~2.55) with aspect ratio as low as 5 due to porous nature of the material. The objective of this work, following evaluation of multiple advanced drying technique in previous publication (H.-W. Chen et al., ECS Trans. 2013 58(6): 205-211), is to develop a sustainable non-stiction drying solution incorporating supercritical fluids. Supercritical fluids exhibit both liquid-like (transport/ dissolution) and gas-like (low viscosity/ negligible surface tension) properties, leading to effective extraction of wet clean chemicals from nanostructures with extremely low capillary force for stiction-free drying. As an example, the principle of supercritical drying is illustrated in Figure 2 with Carbon Dioxide, which has been preferably employed for commercial and industrial practices due to its high stability, insignificant toxicity, and low environmental impact. The process started with liquefied CO2 to mix and displace chemicals from the normal wet process. Subsequently, both temperature and pressure are elevated to establish supercritical state for CO2 (30.8°C, 73 bar) in the system. Isothermal depressurization then follows to transition the CO2 from supercritical state back to gaseous phase for drying without surface tension. A comparison of our approach with other drying schemes showed that the integrated supercritical drying method is superior in non-collapse performance to all conventional (e.g. spinning, solvent-assisted, etc.) and advanced (self-assembled monolayer formation, sublimation, etc.) practices. Figure 3 presents the study results of various drying technique containing sublimation drying (~90% stiction), SAM (self-assembled monolayer) for surface tension modification (~60% stiction), solvent-assisted Marangoni drying (~40% stiction), and supercritical CO2 drying (no stiction). Leaning-free performance have been consistently demonstrated with our supercritical drying sequence on 2x NAND STI (trench aspect ratio ~20) and low-k trench (aspect ratio ~6.0) structures, where pattern collapses were observed under conventional solvent-assisted drying or even with advanced SAM approach. Figure 4 exhibits non-stiction results of complete integration of standard aqueous chemical clean (HF-SC1-SC2), DI water rinse, and supercritical drying. We also demonstrated de-stiction as shown in Figure 4(b), where light leaning of lines prior to the process flow was totally released. Collapse-free drying on 2x NAND STI structures was further verified on 300mm wafers with 36-spot inspection (Figure 5). Extendibility to ultra low-k (k~ 2.55) features was successfully achieved as displayed in Figure 6, where supercritical drying clearly presented stiction-free performance over simple N2 blow dry. Particle adder performance was also evaluated in this work. We found that cleanliness of process chemicals and hardware, suppression of flow turbulence, and control of depressurization sequence are critical to particle reduction. Figure 7 sketches particle adder data of the integrated supercritical drying process. With background particles subtracted, in average < 50 adders @0.09μm was attained from a mini-marathon fulfilled. Metallic contamination analysis with TXRF also demonstrated all metal components interested were below detection limit (Table 1). The significance of supercritical drying lies in its unrivalled extendibility to future technology nodes due to the negligible surface tension. This unique drying scheme not only sustains continuous device scaling, but also enables respective material innovation, e.g., ultra ultra low-k (k ≤ 2.2) features, EUV resist patterning, DSAL (directed self-assembly lithography) wet development, etc. Supercritical drying has lately attracted great attention due to the eager demand for pattern leaning suppression in the semiconductor industry. As a result, this emerging technology is expected to play a prominent role as the ultimate solution to device structure collapse problem in the near future. Figure 1

A Simulation Study on Defectivity in Directed Self-assembly Lithography

We have investigated the generation mechanisms of pattern defects in directed self-assembly (DSA) lithography using a simulation method based on self-consistent field theory (SCFT). The SCFT simulation results could reproduce grid defects, which are considered to be a kind of hexagonally perforated lamellar (HPL) metastable phase, as one of the characteristic pattern defects in the coordinated line epitaxy (COOL) DSA lithography process using polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA). We found that the grid defects were susceptible to be generated when a neutral layer was weakly attractive to PMMA block. Investigation of the PMMA segment node density profiles in the grid defects revealed that the terminal segments of the PMMA blocks indicated higher node density immediately above the neutral layer. These simulation results imply that the generation mechanism of the grid defects is strongly related with the interaction between the neutral layer bottom wafer and the terminal segment of the PMMA block.

Defect Precursor and Metal Reduction from DSAL Resist using Filtration and Ion Exchange

Gel particle is a primal defect in block copolymer (BCP) layer in directed self assembly lithography(DSAL). We studied gel removal performance for various lithography process filters and found that nylon 6,6 membrane outperformed other membrane materials exerting adsorption performance similarly to the conventional chemical amplified resist (CAR) filtration. Contrary to the CAR, BCP is synthesized using metal polymerization initiator and the metal in the resist layer should be removed to prevent potential impact to the device integrity failure or the yield loss. We also studied metal reduction from the BCP solution using ion exchange filter and particle removal filters. As a result, ion exchange filter effectively reduced metals from BCP solution as expected. In addition, unexpectedly, nylon 6,6 membrane best reduced some metal species in the BCP solution.

Process Evaluation of Directed Self-assembly Lithography using Simulation Method based on Self-consistent Field Theory

Process Evaluation of Directed Self-assembly Lithography using Simulation Method based on Self-consistent Field Theory

Contact hole shrink process using graphoepitaxial directed self-assembly lithography

A contact hole shrink process using directed self-assembly lithography (DSAL) for sub-30 nm contact hole patterning is reported on. DSAL using graphoepitaxy and poly (styrene-block-methyl methacrylate) (PS-b -PMMA) a block copolymer (BCP) was demonstrated and characteristics of our process are spin-on-carbon prepattern and wet development. Feasibility of DSAL for semiconductor device manufacturing was investigated in terms of DSAL process window. Wet development process was optimized first; then critical dimension (CD) tolerance of prepattern was evaluated from three different aspects, which are DSA hole CD, contact edge roughness (CER), and hole open yield. Within 70+/−5 nm hole prepattern CD, 99.3% hole open yield was obtained and CD tolerance was 10 nm. Matching between polymer size and prepattern size is critical, because thick PS residual layer appears at the hole bottom when the prepattern holes are too small or too large and results in missing holes after pattern transfer. We verified the DSAL process on a 300-mm wafer at target prepattern CD and succeeded in patterning sub-30 nm holes on center, middle, and edge of wafer. Average prepattern CD of 72 nm could be shrunk uniformly to DSA hole pattern of 28.5 nm. By the DSAL process, CD uniformity was greatly improved from 7.6 to 1.4 nm, and CER was also improved from 3.9 to 0.73 nm. Those values represent typical DSAL rectification characteristics and are significant for semiconductor manufacturing. It is clearly demonstrated that the contact hole shrink using DSAL is a promising patterning method for next-generation lithography.

Sub-30nm via interconnects fabricated using directed self-assembly

In this study, sub-30nm via interconnects were fabricated and fully integrated on a 300mm wafer using directed self-assembly lithography (DSAL). They were tested electrically and initial test results are reported. DSAL was applied on the via layer, which is connecting between the lower metal layer and the upper metal layer. A trilayer resist process was utilized for preparing a guiding pattern of graphoepitaxy. Exposure dose of 193nm immersion lithography was centered so that via size in spin-on-carbon (SOC) was just 70nm in diameter. Poly (styrene-block-methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) solution was applied on the SOC pre-pattern, and then annealed in N2 atmosphere to induce micro-phase separation. PMMA domain was finally transferred into a dTEOS oxide film as a via hole. Via interconnects were fabricated using tungsten deposition followed by CMP. Intra-wafer variation of via resistance was measured and the correlation between the via resistance and the via dimension was discussed.

Application of Directed Self-Assembly Lithography to Semiconductor Device Manufacturing Process

Directed self-assembly (DSA) lithography are recently getting in the spotlight as one of the most promising new generation lithography (NGL) candidates, which have a potential to fabricate semiconductor device patterns down to sub-30nm. In this report, latest experimental and simulation results of contact hole shrink process using the DSA are demonstrated.

Nanopatterning of diblock copolymer directed self-assembly lithography with wet development

A method for using wet development in a directed self-assembly lithography (DSAL) application is reported. For the typical diblock copolymer poly(styrene-block-methyl methacrylate) (PS-b-PMMA), the PMMA area is removed by an oxygen plasma. However, the oxygen plasma has poor selectivity for the PS portion of the block polymer and etches it simultaneously. As a result, the thickness of the residual PS pattern is thinner than desired and creates a challenge for subsequent pattern transfer. A wet development technique is discussed which offers higher selectivity between the PMMA and PS blocks in the assembled pattern. Specifically, a method using a low pressure mercury lamp and conventional tetramethylammonium hydroxide (TMAH, 2.38%) developer is proposed. Using this method, DSA pattern formation is completed in a single track having coating, baking, exposure, and development modules.

Directed Self-Assembly Lithography and Its Application Based on Simulation Approach

The directed self-assembly (DSA) technology of block copolymers is a candidate for next-generation lithography. The computation model of this lithography can assist in overcoming its challenges. In this paper, a template-assisted self-assembly (DSA graphoepitaxy) is modeled and simulated in a molecular scale to reduce the process complexity. For a full simulation, the template fabrication and DSA process are modeled and simulated to evaluate the impact of the process parameters on the pattern profile. Simulation results similar to the experimental results allow the prediction of the self-assembled patterns of the confined block polymers.