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Micro/Nano Lithography

Improved lithography by directed self-assembly of ultra-high-density patterns

Hexagonally close-packed arrays of polymeric spheres self-assembled on a chemically patterned template offer one possible approach to the challenges of next-generation lithography.
7 February 2013, SPIE Newsroom. DOI: 10.1117/2.1201301.004546

Innovative lithographic processes are critical to continue shrinking semiconductor devices beyond the 22nm node, and to enable new devices with nanometer-scale features. Although photolithography is the industry standard, resolution requirements have reached beyond the wavelength of light. Consequently, it is becoming increasingly complicated and expensive to further minimize feature dimensions. Among the available lithographic alternatives, self-assembling processes have received much attention.

Directed self-assembly (DSA) of block copolymers on chemically patterned templates is one promising route to obtaining patterns with sub-photolithographic resolution. This material consists of immiscible polymers that are joined end to end. They exhibit a wide variety of structures (morphologies) by virtue of microphase separation, including ordered arrays of lamellar (layered), cylindrical, and spherical microdomains in the equilibrium configuration. The morphologies depend on the relative volume fraction of the constituent polymers and on temperature, whereas the sizes of the structures depend on their molecular weight. The typical size of the microdomains is a few tens of nanometers, which makes that system typically attractive for lithographic application.1 A large body of work has been done in this field using poly(styrene-block-methylmethacrylate) (PS-b-PMMA) as a phase-segregating material.2–7 However, because low molecular weight PS-b-PMMA is a weakly segregating system, the minimum dimension achievable is about 20–25nm full pitch. Although various higher-segregating block copolymer systems have been proposed to overcome this size limitation,8–10 multiple challenges still exist in fabricating high-density patterns with sufficient uniformity and placement accuracy. Here, we report DSA of hexagonally arranged spheres over an area of around 6Tera-dots/inch2 with improved fidelity using strongly segregating polyhedral oligomeric silsesquioxane (POSS) containing block copolymer.

Figure 1. Chemical structure of poly(methyl methacrylate-block-methacrylate polyhedral oligomeric silsesquioxane) (PMMA-b-PMAPOSS). Si: Silicon. m, n: Degree of polymerization.

We synthesized poly(methyl methacrylate-block-methacrylate POSS)—PMMA-b-PMAPOSS (see Figure 1)—by so-called living anionic polymerization.11, 12 PMMA-b-PMAPOSS self-assembles into an hcp (hexagonal close-packed) array of PMMA spheres in a PMAPOSS matrix with (10) lattice plane spacing d0=12.4 and 9.7nm. PMMA-b-PMAPOSS thin films were allowed to self-assemble on sparse chemically prepatterned templates following the process illustrated in Figure 2. We prepared the chemical templates by electron beam lithography on a polystyrene layer grafted to a silicon wafer. We spin-coated PMMA-b-PMAPOSS thin films on the templates with (10) lattice plane spacing, dsub=26.0 and 29.0nm. We allowed the samples to self-assemble by thermal annealing at 150°C or by solvent annealing in a neutral solvent atmosphere, namely, a vapor of carbon disulfide/acetone, 9/1 (v/v).12, 13

Figure 2. Schematic illustration of directed self-assembly with density multiplication process using a chemically prepatterned template. E-beam: Electron beam. RIE: Reactive ion etching. d0, dsub, d: (10) Lattice plane spacing of PMMA-b-PMAPOSS self-assembled on a nonpatterned Si substrate, (10) lattice plane spacing of the template, and (10) lattice plane spacing of PMMA-b-PMAPOSS self-assembled on the template, respectively.

Obtaining well-defined patterns by chemical epitaxy requires adapting the suitable annealing condition to the individual block copolymer sample. For a block copolymer composed of strongly segregating and relatively viscous blocks, we often found solvent annealing—which promotes molecular diffusion by swelling—to be superior to thermal annealing. Figure 3shows the DSA of PMMA-b-PMAPOSS with d0=12.4nm on solvent annealing. Figure 3(a) shows a scanning electron microscopy (SEM) image of an e-beam resist mask used to prepare the chemical template with dsub=26.0nm, which is about two times d0. Figure 3(b) shows a PMMA-b-PMAPOSS pattern assembled on the template. On measurement, the lattice plane spacing of the assembled PMMA-b-PMAPOSS was d=13.0nm, which is half of dsub, demonstrating that the areal density of the pattern was a factor of four greater than that of the underlying chemical template. Further comparison of the SEM images revealed that the larger size variation in e-beam resist was successfully rectified by the self-assembly.

Figure 3. Directed self-assembly with 4×density multiplication of a hexagonally closed-packed (hcp) pattern with (10) lattice spacing d=13.0nm corresponding to 3.3Tera-dots/inch2. (a) Scanning electron microscopy (SEM) image of the resist mask with dsub=26.0nm prepared by e-beam lithography, which was used in the chemically prepatterned template. (b) SEM image of a PMMA25-b-PMAPOSS13 thin film solvent-annealed on the chemical template. Solvent annealing was carried out by exposing the sample to vapor of carbon disulfide/acetone=9/1 v/v mixture for 2h at degree of swelling=140%. Details of the solvent annealing process are available elsewhere.12 Because PMAPOSS contains Si atoms, it discharges more secondary electrons than PMMA. As a result, the dark and bright sections of the SEM images correspond to PMMA spheres and PMAPOSS matrix, respectively.

Further minimizing the pattern dimensions requires lowering the molecular weight (Mw) of the block copolymer. However, when we decreased Mw to assemble sub-10nm features, we found that thermal annealing resulted in better ordering than solvent annealing. Figure 4 compares thermally versus solvent-annealed thin films of PMMA-b-PMAPOSS with d0=9.7nm on both an unpatterned silicon substrate and a prepatterned chemical template. For films on the silicon substrate, the thermally annealed sample—see Figure 4(a)—resulted in well-defined hcp crystallites with a relatively long lateral correlation length, whereas the solvent-annealed sample—see Figure 4(b)—showed a much shorter correlation length with a more disordered pattern. These results suggest that thermal annealing might be more appropriate for PMMA-b-PMAPOSS with lower Mw than solvent annealing. Similarly, for DSA on chemical contrast patterns, the thermally annealed film—see Figure 4(c)—showed a well-defined single-crystal structure with d=9.7nm, which corresponds to 5.9Tera-dots/inch2 in areal density, achieving a 9× density multiplication factor with respect to the underlying chemical template with dsub=29.0nm. On the other hand, the solvent-annealed sample—see Figure 4(d)—resulted in an imperfect pattern, which suggests that the chemical contrast pattern provided insufficient influence to control the self-assembly of PMMA-b-PMAPOSS.

Figure 4. Directed self-assembly with 9× density multiplication of an hcp pattern with (10) lattice spacing d=9.7nm correspond to 5.9Tera-dots/inch2. SEM and corresponding 2D fast Fourier transform images of PMMA17-b-PMAPOSS9 thin films self-assembled on unpatterned Si substrates (a, b) and on chemically prepatterned templates with dsub=29.0nm (c, d). (a, c): Thermally annealed at 150°C for 48h. (b, d): Solvent-annealed under vapor of carbon disulfide/acetone=9/1 v/v mixture for 2h at degree of swelling=135%. Details of the solvent annealing process are available elsewhere.12 Because PMAPOSS contains Si atoms, it discharges more secondary electrons than PMMA. As a result, the dark and bright sections of the SEM images correspond to PMMA spheres and PMAPOSS matrix, respectively.

As shown in Figure 3, as well as in our previous studies, we achieved successful DSA by solvent annealing for larger Mw PMMA-b-PMAPOSS (areal density ≤4.6Tera-dots/inch2).12, 13 Solvent annealing not only increases molecular diffusion, it also depresses the effective interaction parameter. In the case of larger Mw, where the thermal diffusion of the BCP is limited and the segregation strength is large, solvent annealing is appropriate in helping with molecular diffusion provided that segregation strength remains above the order-disorder transition. On the other hand, in the case of lower Mw block copolymers, swelling of the block copolymer films with the solvent may depress the segregation strength below or near the order-disorder transition, which results in patterns of poor quality. Under these conditions, thermal annealing is more advantageous in forming well-defined patterns at higher densities. Our results show the importance of adopting the appropriate annealing process for successful directed self-assembly, depending on the segregation strength and molecular weight of the BCP.

In summary, the needs of the data storage and microelectronics industries are fueling demand for feature dimensions in single-digit nanometers. DSA of block copolymers is one promising candidate for meeting both the size and economic requirements of next-generation lithography. The results we have described provide further insight into the DSA process for achieving ultra-high-density patterns for practical application. Our current work focuses on further improving pattern quality to meet the lithography requirements and developing a robust pattern transfer process.

We gratefully acknowledge Ricardo Ruiz and Elizabeth Dobisz at the San Jose Research Center of HGST, a Western Digital Company, for their support in e-beam lithography and fruitful discussions. Part of this work was funded in Japan by the New Energy and Industrial Technology Development Organization storage memory project ‘Development of Nanobit Technology for Ultra-high Density Magnetic Recording (Green IT project).’

Hiroshi Yoshida, Yasuhiko Tada
Hitachi Research Laboratory
Hitachi Limited
Hitachi, Japan
Yoshihito Ishida, Teruaki Hayakawa
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
Tokyo, Japan
Mikihito Takenaka, Hirokazu Hasegawa
Department of Polymer Chemistry
Kyoto University
Kyoto, Japan

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