Highly Ordered 3D Interconnected Structures in a Block Copolymer by Directed Self-Assembly

Block copolymers (BCP) thin films designed to have specific microdomain geometries such as interconnected cylinders could enable the fabrication of three-dimensional devices for photovoltaics and microelectronics. In our preliminary work, we made inverted-T structures consisting of a layer of in-plane cylinders at the bottom of the film connected to vertical cylinders spanning the remainder of the film thickness. These structures were made from a polystyrene-<i>b</i>-poly(2-vinylpyridine) (PS-<i>b</i>-P2VP) BCP which in bulk forms close-packed P2VP cylinders within a matrix of PS. The inverted-T structure is favored by selection of film thickness and solvent annealing conditions. However, on a bare silicon wafer the bottom cylinders formed a fingerprint pattern, lacking long range order of in-plane orientation and hence limiting potential device applications.<br/>We show here how the 3D structure can be ordered using substrate templating. We use electron-beam lithography (EBL) on a resist-coated oxidized silicon substrate to generate grating patterns with widths from 200 nm to 500 nm and then transfer the pattern into the silica by reactive ion etching (RIE). The trenches have a trapezoidal cross section with the base width slightly narrower than the top width of the trench. The BCP is spin-coated over the substrate and solvent-annealed to induce microphase separation, which is revealed by loading the P2VP microdomains with Pt from an acid solution of a Pt salt and then etching in oxygen to remove the PS. By selection of trench geometry, BCP thickness and annealing conditions, we achieve highly ordered structures in the trenches, in which the out-of-plane cylinders are hexagonally packed and the underlying connected in-plane cylinders are parallel to the trenches. The parallel orientation of the bottom layer of cylinders depends on the bottom trench width (<i>w_bottom</i>). It is crucial to match <i>w</i>_bottom with the periodicity of the BCP (<i>L₀</i>= 49 nm), <i>i.e.</i>, <i>w</i>_bottom close to an integer multiple of <i>L₀</i>(n<i>L</i>₀), so the parallel orientation is preferred. The structure of the top layer is determined by <i>w</i>_bottomand film thickness (<i>t</i>). Due to the tapered trench wall of 7° originating from the RIE process, <i>w</i> increases towards the top of the trench. Deeper trenches (depth = <i>d</i>) therefore have a larger difference between the widths of the top and bottom surfaces of the BCP within the trench. If the width at the top of film during annealing (<i>w</i>_top) is close to <i>w_bottom</i> = n<i>L</i>₀, highly ordered inverted-T structures are generated with each in-plane cylinder connected to one row of vertical cylinders. If <i>w</i>_topis closer to (n+1)<i> L</i>₀, the mismatch of period results in a split in one of the rows of vertical cylinders, yielding Y-shaped structures. These highly ordered templated 3D structures fabricated by directed self-assembly widen the opportunities for BCP-derived nanofabrication.