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Effect of angstrom-scale surface roughness on the self-assembly of polystyrene-polydimethylsiloxane block copolymer

Self-assembly of block copolymers has been identified as a potential candidate for high density fabrication of nanostructures. However, the factors affecting its reliability and reproducibility as a patterning technique on various kinds of surfaces are not well-established. Studies pertaining to block copolymer self-assembly have been confined to ultra-flat substrates without taking into consideration the effect of surface roughness. Here, we show that a slight change in the angstrom-scale roughness arising from the surface of a material creates a profound effect on the self-assembly of polystyrene-polydimethylsiloxane block copolymer. Its self-assembly was found to be dependent on both the root mean square roughness (Rrms) of the surface and the type of solvent annealing system used. It was observed that surface with Rrms< 5.0 Å showed self-assembly. Above this value, the kinetic hindrance posed by the surface roughness on the block copolymer leads to its conforming to the surface without observable phase separation.


S1. Characterization of magnetic media samples
In both continuous and granular CoCrPt-SiO 2 magnetic media, a Ru underlayer was deposited at low argon pressure (3 mTorr) and room temperature to achieve the hcp (00.2) orientation. It provided a (00.2) crystallographic texture to the CoCrPt-SiO 2 recording layer necessary for out-of-plane coercivity. The granular CoCrPt-SiO 2 media comprised of a second Ru layer deposited at high pressure (60 mTorr) to attain grain segregation in the recording layer. FePt-C-Cu magnetic media, deposited at 400 °C, was unable to achieve the L1 0 order. Therefore, no (001) peak of the FePt layer was observed. The FePt (002) peak is asymmetrical towards the lower theta values. It is due to the presence of (200) peak of FePt which shows the presence of FCC phase in the FePt-C-Cu layer. In the case of FePt-C magnetic media, deposited at 600 °C, the (001) and (002)  followed during the film growth. The XRD of the samples are provided in Figure S1.
The deposition conditions ensured that the magneto-crystalline anisotropy was oriented perpendicular to the surface leading to out-of-plane coercivity essential for perpendicular magnetic recording. FePt-C showed higher out-of-plane coercivity when compared to the inplane coercivity. On the other hand, the in-plane and out-of-plane coercivities of FePt-C-Cu were approximately equal indicating the absence of high perpendicular magneto-crystalline anisotropy. The hysteresis loops obtained using polar magneto optical Kerr effect (polar-MOKE) and alternating gradient force magnetometer (AGFM) are shown in Figure S2.

S2. Hansen Solubility Parameters of the solvents and individual polymer chains in the block copolymer
The solubility parameters of the blocks polystyrene (PS) and polydimethylsiloxane (PDMS) as well as the solvents used for annealing -toluene, heptane and tetrahydrofuran (THF) are tabulated in Table SI. The solubility parameters of THF and toluene match that of PS leading to increased solubility of the block in these solvents and hence enhanced chain mobility.
Similar is the case with PDMS and heptane.
Annealing in THF-heptane mixture did not yield acceptable results. From Table SI, we find that the solubility parameter of THF and heptane matches that of PS and PDMS, respectively.
Since heptane is non-polar and THF is polar aprotic, it can be speculated that the polarity difference between the two solvents in the mixture may result in a heterogeneity in the vapor phase. THF was then replaced by non-polar toluene. When a 6:1 toluene-heptane mixture was employed, robust PDMS dots were obtained.       The bulk morphology provided by the block copolymer system used in this study is spherical.

S5. Effect of surface roughness on the lamellar structures
However, heptane used in the solvent mixture caused the PDMS block to swell and increased its effective volume fraction. As a result the bulk morphology changed from spherical to lamellar (from the block copolymer phase diagram). 1 Dots and lamellar structures were seen to co-exist together on a smoother surface, i.e., continuous CoCrPt-SiO 2 magnetic media. It was observed that as the surface roughness increased, the lamellar structures become less abundant ( Figure S4). It can be surmised that as roughness rises, the increase in the effective volume fraction of PDMS block by heptane may not be enough for it to assemble into lamellae.

S6. Self-assembly of PS-b-PDMS on Si wafers
R rms = 1.5 Å (a) R rms = 1.3 Å (b) Figure S5: AFM images of (a) silicon and (b) silicon with TranSpin coating. Figures S5 (a) and (b) show the AFM images of silicon with and without a layer of TranSpin, respectively. It clearly shows that silicon substrates have an ultra-flat surface. As a result, the surface roughness hardly changed even after being coated with a layer of TranSpin. Figure S6 shows SEM images of self-assembly of PS-b-PDMS on Si wafers with and without the TranSpin underlayer, solvent annealed in THF and the 6:1 toluene-heptane mixture. A trend similar to that of continuous CoCrPt-SiO 2 magnetic media was observed. Si wafers not coated with TranSpin showed an incomplete phase separation when solvent annealed in THF. The self-assembly improved when the 6:1 toluene-heptane mixture was employed for solvent annealing. However, completely isolated dots were observed when the Si samples were coated with TranSpin prior to solvent annealing in the two solvent systems. TranSpin enhances the chain mobility of the blocks by providing them a softer platform, thus enabling them to self-assemble.