Study of the perpendicular self-assembly of a novel high- χ block copolymer without any neutral layer on a silicon substrate

Abstract

A novel type of high-χ block copolymer, polystyrene-block-polycarbonate (PS-b-PC), which contains an active -NH- group on the polymer backbone between the PS block and the PC block, has been successfully synthesized. Vertical micro-phase separation can be successfully achieved on Si substrates with neutral-layer-free materials with a pitch of 16.8 nm. Water contact angle experiments indicate that PS and PC have approximate surface energy values on Si substrates. A hydrogen bond mechanism has been proposed for the formation of a periodic and lamella-forming phase separation structure, with the domains oriented perpendicular to the substrate. A combination of both theory and experimental verification proves that the hydrogen bonding plays a dominant role as a real driving force to promote vertical micro-phase separation in the absence of a neutral layer. Subsequently, the study of a novel block copolymer on four different types of substrate without any neutral layer further confirms that the newly synthesized material enables greater flexibility and potential applications for the fabrication of various nanostructures and functional electronic devices in a simple, cost-effective and efficient way, which is of considerable importance to contemporary and emerging technology applications.

Fig. 1. Different characterization results of the new BCP (PS-b-PC) based on (a) GPC, (b) NMR and (c) TGA.

Fig. 2. Chemical structure of PS-b-PC.

Fig. 3. (a) The PS diiodomethane contact angle is 15.8°. (b) The PS water contact angle is 88.6°. (c) The PC diiodomethane contact angle is 17.2°. (d) The PC water contact angle is 86.8°.

Fig. 4. (a) Cross-sectional SEM image of PS-b-PC before annealing. (b) Cross-sectional and (c) top-down SEM images of PS-b-PC with no neutral layer. (d) Cross-sectional SEM image of PS-b-PC after annealing.

Fig. 5. (a) GISAXS pattern of PS-b-PC. (b) GISAXS pattern of a bulk sample of PS-b-PC. A vertical layered microstructure of the sample is obtained and the second peak of q_y is 0.03718 Å⁻¹ for the thin film, and its phase period is 16.8 nm.

Fig. 6. (a) Si–OH bonds are formed on the surface of the silicon substrate by appropriate wet cleaning treatment. (b) Molecular structure of the novel BCP (PS-b-PC). (c) An illustration of vertical micro-phase separation structure formation by hydrogen bonding. (d) A schematic diagram of the formation mechanism of hydrogen bonds.

Fig. 7. SEM images of PS-b-PC mixed with n-butyl acrylate glycidyl ether after annealing and with corresponding mixing ratios of (a) 3 : 1, (b) 2 : 1, (c) 1 : 1, (d) 1 : 2, (e) 1 : 3 and (f) 1 : 5.

Fig. 8. A schematic diagram of the “V” model forming a periodic and lamella-forming phase separation structure perpendicular to the substrate without a neutral layer.

Fig. 9. SEM images of PS-b-PC on four different substrates after annealing at 160 °C and 170 °C.