Directed self-assembly (DSA) has emerged as one of the most promising nanolithography techniques for sub-10 nm patterning. The key material in DSA is block copolymer (BCP), which consists of two or more covalently bonded polymers of different properties. BCPs can self-assemble into periodic nanostructures with different morphologies. In DSA, a guiding pattern is usually needed to guide BCPs to form large-scale, well-registered nanopatterns with desired orientation. The guiding pattern can be created by chemoepitaxy or grapho-epitaxy. The pitch size of the nanostructure of BCP is determined by its Flory-Huggins interaction parameter (χ) and the degree of polymerization (N) (L₀ =χ^1/6N^2/3) and is fixed after BCPs are produced. The advancement of DSA is highly related to the properties of BCP materials, especially the pitch size of BCP, that determines the patterning resolution of DSA. During last two decades, various new BCPs have been developed, with ever decreasing pitch size to enhance the patterning resolution of DSA.

One of the most commonly used BCP materials is polystyrene-b-polymethylmethacrylate (PS-b-PMMA)^[1]. PS-b-PMMA has a pitch size ranging from 20-28 nm and a negligible difference in the surface energy of its constituent blocks. It can easily form different nanostructures, such as lamellae and cylinder on a neutral substrate under thermal or solvent annealing.

However, because of its low Flory-Huggins interaction parameter(χ), PS-b-PMMA cannot achieve sub-10 nm feature sizes. Thus, BCPs of higher χ and smaller pitch sizes are needed for sub-10 nm patterning. Achieving defect-free block copolymer nanopatterns with a long-ranged orientation over a large area remains a persistent challenge, impeding the successful and widespread application of self-assembly^[2]. Recently, interest in the patterning of self-assembly of block copolymers has attracted considerable attention because of their wide and promising applications in many fields of nanoscience and nanofabrication, including nanochannels for nanofluidic fabrication^[3], nanostructure fabrication^[4], biomolecule detection^[5], advanced IC devices^[6], and as a nanoimprint template to create nanopillars^[7]. The potential applications have driven extensive efforts in both academia and industry to apply the technique to achieve precisely controlled geometric size, highly aligned and ordered arrays to meet the stringent requirements for a large amount of emerging applications, especially in semiconductor IC manufacturing for functional electronic devices. While great progress has been made to implement block copolymer lithography with features in the range of 10-20 nm, patterning solutions below 10 nm are still not mature. Many BCP systems self-assemble at this length scale, challenges remain in simultaneously tuning the interfacial energy atop the film to control the orientation of BCP domains, designing material, templates, and processes for ultra-high-density DSA, and establishing a robust pattern transfer strategy^[8].

Among the various high χ BCPs, polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) emerge as one of the promising candidates for high resolution patterning. PS-b-P2VP can achieve through-film perpendicular structure on a neutral substrate via solvent annealing without the need of top-coat materials^[9-10]. By varying the solvent annealing conditions in a sealed chamber, such as the solvent type and vapor pressure, the nanostructure of PS-b-P2VP can be precisely controlled and most of the defects in BCPs can be healed. Even though a BCP like PS-b-P2VP can meet the stringent requirement of high-resolution patterning, its pitch size, like PS-b-PMMA and other BCPs, are determined by χ and N and thus fixed upon synthetization. Changing the parameters during the annealing process can only change the pitch size of BCP in a negligible range. BCPs of different molecular weights are needed for different patterning resolution. To reduce the efforts of synthesizing new BCPs and further extend the use of existing BCPs, blending BCPs of different molecular weights have been proposed as a method to tune the pitch size of a specific BCP^[11-13].

To access a full pitch of BCP lower than 20 nm in nanoimprint master, either a higher χ material or a double patterning technique has to be used. While multiple block copolymers with higher χ values have been proposed in the literature^[14], full integration with a strategy that includes a SA to DSA scheme and a robust pattern transfer has remained elusive. As a matter of fact, among the various block copolymers with χ values higher than that of PS-b-PMMA, PS-b-P2VP stands as a promising candidate thanks to the following three properties. Firstly, it is relatively simple to achieve a perpendicular orientation, with respect to the substrate, of lamellar domains of PS-b-P2VP with solvent annealing without the need of top-coat materials^[15-16]. Second, the use of common polymer mats and brushes can be readily extended to PS-b-P2VP to achieve high quality from SA to DSA. Finally, a high etch contrast for pattern transfer can be obtained by means of a selective infiltration synthesis from organometallic precursors that bind with the pyridine group in the P2VP block^[17]. Furthermore, solvent annealing in the annealing chamber that was designed specifically to control vapor pressure enables SA to occur at desired swelling ratios, thereby simultaneously achieving microphase separation in the solvated state and high block copolymer mobility for SA to DSA in our experiment. Under normal conditions, solvent annealing works well for block copolymers that degrade at elevated temperature during thermal annealing, and for diblock, triblock copolymers that need extended thermal annealing for both diblock and triblock chains to assemble.

Here in this paper, we report tuning the pitch size of triblock copolymer P2VP-b-PS-b-P2VP (VSV) by blending VSV with different volume fraction of another VSV or diblock PS-b-P2VP (SV). The self-assembly of as-blended VSV/SV and VSV/VSV films are realized under solvent annealing process. The lamellar structures of annealed VSV/SV and VSV/VSV films were visualized using Sequential Infiltration Synthesis (SIS) method to selectively deposit aluminum oxide (AlO_x) in P2VP domain, followed with characterization by scanning electron microscopy (SEM) and the calculation of the resulting pitch sizes. It is observed by adding different amount of SV and VSV into VSV, the pitch size of VSV can be tuned continuously without changing its original morphology.

As a promising high χ BCP for sub-20 nm patterning, PS-b-P2VP can easily form through-film lamellae nanostructure on silicon substrate by solvent annealing. PS-b-P2VP different molecular weights has different pitch sizes. By blending PS-b-P2VP polymers of different molecular weights, it is possible to tune the pitch size of a specific PS-b-P2VP continuously within a certain range.

The lamellar PS-b-P2VP thin film can be converted into a metal oxide hard mask through SIS process, in which the metal oxide (e.g AlO_x) will selectively deposit in P2VP domain. This metal oxide mask can be used to visualize the lamellae pattern under SEM.

In our experiments, triblock copolymer VSV 12k-24k-12k (PDI=1.25), VSV 6.7k-13k-6.7k (PDI=1.26) and diblock copolymer SV 8.2k-8.3k (PDI=1.09) were used for blending. A poly(styrene-rand-2-vinylpridine-rand-hydroxyethyl methacrylate) (P(S-r-2VP-r-HEMA)) random copolymer with 60% styrene contain was used as the neutral brush. The VSV (12k-24k-12k) was blended with VSV (6.7k-13k-6.7k) or SV (8.2k-8.3k) at different volume ratios (10:0.5, 10:1, 10:1.5, 10:2). The self-assembly process of the as-blended VSV/SV and VSV/VSV films was as follows. First, the neutral brush was directly spin-coated on the silicon substrate, annealed at 200 °C for 1 hr. The role of the neutral brush is to help VSV form lamellar structure perpendicular to the substrate. The blended VSV/SV or VSV/VSV film was then coated on the neutral brush and solvent annealed in acetone. Acetone was chosen as the annealing agent because it was a neutral solvent to both PS and P2VP blocks. The blended films were kept in the annealing chamber with controlled amount of acetone vapor (with nitrogen as carrier gas, 5-100 sccm) at room temperature (25°C). To characterize the as-annealed films, sequential infiltration synthesis (SIS) process was employed to selectively deposit AlO_x into P2VP domain. After SIS, the polymer was removed by reactive ion etching (RIE) and the remaining AlO_x lines represented the self-assembled fingerprint pattern of the blended films. The images of the VSV films after different processes (SIS and etching) were taken by scanning electron microscopy (SEM).

Figure 1 shows the SEM images of solvent annealing VSV (12k-24k-12k), VSV (6.7k-13k-6.7k) and SV (8.2k-8.3k) films after SIS process and RIE etching. It is found that VSV (12k-24k-12k) form very clear fingerprint pattern after solvent annealing, with a pitch size of 20.9 nm. However, for VSV (6.7k-13k-6.7k) and SV (8.2k-8.3k), no clear fingerprint patterns were formed after same processing conditions.

In the BCP blending experiment, the triblock copolymer VSV (12k-24k-12k) was blended with diblock copolymer SV (8.2k-8.3k) or VSV (6.7k-13k-6.7k) at different volume ratios (10:0.5, 10:1, 10:1.5, 10:2) and annealed in acetone. From the SEM images shown in Figure 2, the VSV/SV films of all blending ratios self-assemble into lamellar fingerprint patterns, with a pitch size varying from 19.7 nm to 20.8 nm. After RIE etching, the AlO_x lines clearly reproduce the fingerprint patterns, with correlation length varying from 138 nm to 40.5 nm. In the case of VSV/VSV blending, again it was observed that VSV/VSV films successfully form lamellar fingerprint patterns at different blending ratios. The pitch size of blended VSV/VSV changed from 20 nm at 10:2 to 20.7 nm at 10:0.5 (Figure 3). The correlation length of the blended VSV/VSV films lied in the range of 68 nm to 173 nm.

The pitch tuning capability of blending different BCPs is based on the molecular interactions between different BCP blocks. The blending process reconstruct the constituent blocks of the as-blended BCP, and therefore change the effective domain size of the as-blended BCP. By adding different polymers, the pitch size can be tuned in different range, as demonstrated in Figure 2 and Figure 3. Compare the two blending experiments, it is observed that blending VSV with SV results in larger pitch tuning range compared with VSV/VSV blending due to the stronger interaction between VSV and SV molecules.

From the BCP blending experiments, the triblock copolymer VSV (12k-24k-12k) was blended with diblock copolymer SV (8.2k-8.3k) or VSV (6.7k-13k-6.7k) at different volume ratios (10:0.5, 10:1, 10:1.5, 10:2) and annealed in acetone. The pitch size versus the different volume ratios of VSV/ SV or VSV/VSV blending is shown in Figure 4. As the volume fraction of SV (8.2k-8.3k)or VSV (6.7k-13k-6.7k) increases, the pitch size of VSV (12k-24k-12k) after blending aslo decreases. Simultaneously, we can further explain from the chain mechanism of block copolymers why the pitch size decreases after blengding (VSV/ SV or VSV/VSV). The schematic diagram of VSV/ SV or VSV/VSV blending is shown in Figure 5 which is including “Half pitch 3 < Half pitch 2< Half pitch 1” in different volume fraction.

To explore the extreme blending conditions, as is shown in Figure 6. VSV (12k-24k-12k) was mixed with VSV (6.7k-13k-6.7k) and SV (8.2k-8.3k) respectively at 1:1 volume ratio. After same solvent annealing process, it was observed that VSV/SV (1:1) can still form good quality fingerprint pattern with pitch size of 18.3 nm (correlation length 164 nm). The pitch size plotting of quantitative effect of blending is shown in Figure 7. However, in case of VSV/SV at 1:1 blending ratio, the film was nearly in disordered status after annealing. Even though VSV/SV blending can achieve larger pitch tuning range, VSV/VSV blending has much larger processing window, with effective blending ratio range from 10:0.5 to 1:1.

In this work, we demonstrated tuning the pitch size of P2VP-b-PS-b-P2VP by blending it with PS-b-P2VP and P2VP-b-PS-b-P2VP of lower molecular weights. It is observed that by adding small amount (<20%) of lower molecular weight PS-b-P2VP into P2VP-b-PS-b-P2VP, its pitch size can be fine-tuned with 0.1 nm precision. We assume that when the ABA symmetric triblock copolymer blended either with diblock or triblock copolymer, the loop structure is favored to avoid the entropy penalty. Meanwhile, the segmentation strength between domains determines the maximum blending ratio (regarding to lower molecular weight BCP) that leads to the formation of well-defined lamellar fingerprint pattern. The blending process shown in this work can be easily applied on other types of BCPs. The pitch tuning capability of blending BCPs of different molecular weight provides an easy and effective method of tuning the patterning resolution in DSA process. It also greatly extends the functionality of available BCPs and reduces the efforts of synthesizing new BCPs for practical applications.