324466 Patterning Nano-Square Arrays Using Shear-Aligned Block Copolymer Thin Films
Block copolymers can microphase-separate, at sufficiently high values of the interaction parameter and degree of polymerization (N), and show different morphologies depending on the volume fraction of each block. Dimensions are easily controllable over the range of 10-100 nm by varying N. In thin films, due to their ability to create dense periodic patterns, block copolymers have been highlighted in nanolithographic applications expected to provide fine resolution and scalability [1, 2]. While many techniques have been introduced to impart long-range order to arrays of block copolymer microdomains, such as thermal annealing, solvent annealing, graphoepitaxy, surface chemical patterning, and shear stress , only a few symmetries have been demonstrated in such arrays [3, 4]: hexagonally packed arrays of dots can be formed from spherical or cylindrical phase block copolymers, while periodic stripes can be created from cylindrical or lamellar phase block copolymers. There is increasing interest in creating square or rectangular patterns since these patterns are compatible with semiconductor integrated circuit design standards . Nevertheless, creating square or rectangular arrays-which do not naturally form by spontaneous self-assembly of simple diblock copolymers- is more challenging . A few attempts have been made to create rectangular patterns via directed assembly of block copolymers via electron-beam lithography or by fabricating arrays of posts on the substrate that guide the block copolymer microdomains into a square array; however, these methods are often time consuming and expensive [6, 7].
In this study, we demonstrate the generation of nanoscale rectangular patterns over large areas, using cylinder-forming block copolymers aligned in a cross-pattern. The cross-pattern is created from two monolayers of cylinder-forming block copolymers, where each layer is spin-coated and shear-aligned sequentially; shear stress can rapidly produce alignment over centimeter-scale areas . After the first block copolymer layer is shear-aligned, the film structure is fixed with UV irradiation; the second block copolymer layer is then spin-coated and shear-aligned in an orthogonal direction. Reactive ion etching (RIE) is used to selectively remove the matrix polymer surrounding the cylinders, yielding the double-layer cross-pattern which serves as a template for square arrays.
Diblocks of polystyrene-b-poly(ferrocenylisopropylmethylsilane) (PS-PFS), forming iron-containing PFS cylinders, and polystyrene-b-polyhexylmethacrylate (PS-PHMA), forming PS cylinders, were employed to create patterns in double-layer thin films. A spin-coated monolayer film was first aligned by shearing a 1×1 cm2area with a cured polydimethylsiloxane pad at 150 ºC, well above the glass transition temperatures of all the blocks. In the PS-PFS case, after shear alignment, the majority PS block was cross-linked using UV irradiation. In the PS-PHMA case, UV irradiation both cross-linked the minority PS block and partially degraded the PHMA matrix; degraded polymer residues were removed by cleaning with toluene briefly, reducing the total film thickness by 2-5 nm. Thus, in both cases, UV irradiation fixed the shear-aligned cylinder structure of the monolayer film. Then, a second uniform monolayer of block copolymer was spin-coated on top of the first layer. A second shear stress was then applied to the film perpendicular to the first shear direction. While the second shear produced alignment of the top layer perpendicular to the first alignment direction, the cylinders in the first layer retained their orientation due to the cross-linked PS blocks.
After the second alignment, RIE was employed to selectively remove the PS or PHMA blocks in PS-PFS and PS-PHMA, respectively, so that only the cylinder-forming minor blocks remained. For PS-PFS, oxygen was used to remove the PS block; PFS is strongly resistant to oxygen RIE due to the iron and silicon it contains, while the all-organic PS blocks are completely removed by this process . After RIE, the stacked PFS cylinders yielded square patterns; the pitch of the nano-square grid was about 35 nm, set by the intercylinder spacing of the PS-PFS (50k-16.5k block molecular weights). The lines constituting the grid each had a width of approximately12 nm. The strong etch resistance of PFS allows the pattern to be transferred into other substrates, creating nano-square wells. In one example, after oxygen RIE of the double-layer film, additional Bosch etching  was used to create a regular array of nano-square wells in the silicon wafer substrate. The depth of the wells could be increased up to 60 nm, or an aspect ratio of approximately 2, depending on the etch time. For pattern transfer from PS-PHMA diblocks, CF4 was used to preferentially remove PHMA blocks, as CF4RIE etches PHMA twice as fast as PS. When the etch time was carefully controlled, most of the PHMA matrix could be removed while most of the PS in both layers remained, yielding square grids from the stacked PS cylinders. The pitch of the array was about 40 nm (from PS-PHMA 33k-126k), while the width of each line was again about 12 nm. We synthesized PS-PHMA diblocks with various molecular weights so that the pitch of the grid could be varied from 25 nm to 40 nm. If block copolymers of different molecular weights are used for the two layers, rectangular-shaped (vs. square) patterns could be generated. Also, since the second alignment direction can be arbitrarily selected, a parallelogram-shaped pattern with an arbitrary angle can also be created.
To conclude, we present a simple way to create nano-square arrays by building up a double-layer film of a cylinder-forming diblock, where each layer is sequentially deposited, shear-aligned, and cross-linked. The pitch of the nano-square arrays is tunable by varying polymer molecular weight, and this method capitalizes on the fact that shear alignment is a simple and cost-effective approach to generate ordered arrays over macroscopic areas.
This work was supported by the National Science Foundation (MRSEC Program) through the Princeton Center for Complex Materials (DMR-0819860).
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