Perpendicular orientation of cylindrical domains upon solvent annealing thin films of polystyrene-b-polylactide
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.
Section snippets
Materials
The asymmetric PS-PLA used in this study has a total molecular weight of 90 kg mol− 1 (Mn(PS) = 56 kg mol− 1, Mn(PLA) = 34 kg mol− 1) and polydispersity index of 1.07 [17]. This block copolymer adopts a hexagonally packed cylindrical morphology and exhibits a principal domain spacing of 57 nm. Using the volume fraction of PLA (34%) gives a cylinder center-to-center distance of 65 nm and cylinder diameter of 42 nm.
Si wafer substrates were (100)-oriented, p-type/boron doped, kindly provided by STMicroelectronics
Results and discussion
An AFM image of a representative as-spun PS-PLA thin film is shown in Fig. 1A. We used a tapping ratio (ratio between the set point amplitude and the amplitude of free oscillation) between 0.6 and 0.8 which induces a contrast in both the height and phase images [26]. Under these conditions, this height surface contrast is not only the result of topography but also results from a difference of oscillation amplitude damping between PS and PLA (PS domains are brighter). There is clear evidence of
Discussion
As-spun thin films of cylinder-forming block copolymers typically adopt a non-equilibrium state resulting from the solvent evaporation that occurs during the film preparation.
The final state of a solvent annealed and dried block copolymer films depends on:
- (i.)
phenomena in the solvent-swollen state: the solvent imparts mobility to the block copolymer, allowing morphological reorganization. Furthermore, the enthalpically unfavourable segment–segment interactions are typically mediated by the solvent
Summary
We have systematically explored the impact of solvent annealing on the behaviour of PS-PLA thin films as a function of solvent quality and exposure times. We discovered that well-ordered perpendicularly-oriented cylindrical domains of PLA can be obtained by room temperature solvent annealing thin PS-PLA films with THF for 4 h. We suggest that self-adjustment of the film thickness is limited under these conditions, thereby preventing the ready adoption of a thickness that is commensurate with the
Acknowledgements
MAH thanks Université d'Orléans for supporting a visiting professorship and the National Science Foundation (DMR-0605880) for support of this work. We also thank Andrew Zalusky and Marc Rodwogin for preparation and characterization of the PS-PLA used in this work, Majorie Roulet for samples preparation and Annie Richard for SEM imaging.
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