Perpendicular orientation of cylindrical domains upon solvent annealing thin films of polystyrene-b-polylactide

Abstract

Polystyrene-b-polylactide (PS-PLA) was employed as a precursor to nanoporous thin films containing perpendicular cylindrical channels. Cylinder-forming PS-PLA was spin coated onto Si substrate and solvent annealed using acetone, chlorobenzene and tetrahydrofuran (THF) for different durations. By atomic force microscopy, three types of final morphology were observed at the free surface of the films (PLA surface layer, perpendicular cylinders and parallel cylinders) depending on the type of solvent and annealing time. Well-organized perpendicular domains were obtained by annealing in THF. From this oriented PS-PLA annealed thin films, a mild hydrolysis led to a highly ordered array of perpendicularly-oriented cylindrical nanopores arranged on a hexagonal lattice, rendering the resulting nanoporous mask useful for nanopattern transfer processes. The weak resistance of the film/substrate interface during PLA etching was overcome by UV light exposure prior hydrolysis.

Introduction

Block polymers have well-established utility as self-organizing soft materials for nanolithographic applications [1]. Cylinder-forming diblock copolymers are particularly useful in this regard if the cylinders can be oriented perpendicularly to the substrate interface and then subsequently removed by a selective etching process to leave a nanoporous film [2], [3], [4], [5], [6]. Such thin film templates have been used as masks for the deposition of metallic nanodots [7], [8], [9] and for nanopattern transfer to the underlying substrate [10]. There are three key requirements for the practical use of cylinder-forming block copolymers in nanolithography: (i) the cylindrical domains should readily adopt a perpendicular orientation; (ii) the cylinder-forming component should be easily and selectively etched without compromising the integrity of the thin film; and (iii) the resultant nanopores should traverse the entire film thickness so that the substrate can be readily accessed. In addition, long-range order in block copolymer thin films is often also a desirable feature [11], [12]. Solvent annealing of cylinder-forming block copolymer thin films has emerged as a very useful technique for generating both perpendicular orientation and highly organized arrays [13], [14], [15]. Particularly relevant is the recent work of Russell and coworkers who have shown that poly(ethylene oxide)-containing block copolymers can form well-ordered arrays upon prolonged solvent annealing in benzene [16].

Polylactide (PLA)-containing block copolymers are particularly useful for the formation of nanoporous bulk materials and thin films due to the ease of PLA removal by hydrolysis [17], [18], [19]. Thin films of PLA-containing block copolymers with perpendicularly-oriented cylindrical domains have been effectively utilized in nanolithographic applications [20]. In the case of both polystyrene-b-polylactide (PS-PLA) diblock copolymers and polystyrene-b-polyisoprene-b-polylactide (PI-PLA) triblock terpolymers, the cylindrical domains can be oriented perpendicularly to various substrates upon thermal annealing [21]. Thermal annealing is however subject to experimental limitations since degradation of the PLA phase can occur [21] and facile reorganisation is limited in the case of high molecular weights due to the slow dynamics in the molten state. Other attempts to improve the degree of order and the orientation of the PLA during spin coating, without subsequent thermal annealing, have been thus reported. Following this idea, Ho et al. [22] have studied the influence of the type of solvent used in the preparation of the block copolymer solution in the case of polystyrene-b-poly(l-lactide) (PLLA). They showed that the degree of order and the orientation of the PLLA cylinders were highly dependent on the selectivity and evaporation rate of the solvent used for the preparation of the spin coated block copolymer solution. In a more recent work perpendicular orientation was achieved directly upon spin coating polystyrene-b-polyisoprene-b-polylactide (PS-PI-PLA) terpolymers from chlorobenzene [23]. In this case, the presence of the PI block improves the organisation of the PLA domains in the as-spun systems. Using a rather different approach, Cavicchi et al. studied the influence of an additional step of solvent annealing in PI-PLA thin films and showed that highly oriented and organized arrays of perpendicular PLA cylinders can be generated at specific film thicknesses and solvent concentrations [24]. In the particular case of PS-PLA thin films, the influence of an additional step of solvent annealing after the preparation of the film by spin coating has not been examined. In this paper we report the influence of solvent vapour exposure, under various annealing conditions (type of solvent and annealing time), on the orientation of cylindrical domains in thin films of PS-PLA block copolymers. We demonstrate that high-organized perpendicular domains can be achieved by judicious choice of the solvent annealing conditions.

Another important requirement for the conversion of the thin film into a nanoporous template is that the integrity of the film is preserved during the selective etching of the minor component. In the case of PLA based block copolymers, the formation of a PLA wetting layer at the substrate/film interface can be a limitation, due to the delamination of the film upon hydrolysis. To overcome this problem, the formation of the PLA wetting layer has been prevented either by using surface treatment of the substrate in the case of PI-PLA [25] or by performing spin coating at elevated temperature in the case of PS-PLLA [22]. We report a facile alternative method for PS-PLA that is based on a simple UV light treatment that immobilizes the thin film layer to enhance stability during the etching step.