A perpendicular orientation of cylindrical microdomains in diblock copolymer thin films is achieved by control over polymer-surface interactions. The block which forms cylindrical microdomains is removed by UV exposure and a chemical rinse to yield a nanoporous polymer film. The porous film can be used as a template for electrodeposition of metal nanodots or as a mask for reactive ion etching.
Contributors: T.P. Russell, Craig J. Hawker
Lab: NSF Center for Hierarchical Manufacturing
Manufactured Nanomaterial or Structure: Polystyrene film with hexagonal array of nanopores|
Chemical composition: Polystyrene
Physical Form: Nanoporous thin film
Properties: 35 nm thick film containing a hexagonal array of 18 nm diameter pores with a period of 34 nm. Pores are hydrophobic.
|1.||Purchase or synthesize random copolymers P(S-r-MMA) with benzyl alcohol as an end group or P(S-r-BCB-r-MMA) with a molecular weight of 35,000 daltons and a PS/PMMA volume ratio of 0.58/0.42 or a PS/BCB/PMMA ratio of 56/2/42. These two random copolymers have been approved to have an equal interfacial interaction with PS and PMMA block.|
|2.||Synthesize or purchase diblock copolymer P(S-b-MMA), consisting of PS and PMMA, with a molecular weight 88,000 daltons and a PS volume fraction of 0.72. This polymer will self-assemble into hexagonally packed array of PMMA cylinders.|
|3.||Clean a silicon wafer with piranha solution to activate hydroxyl groups at the substrate surface. (This step is only required for using P(S-r-MMA) copolymers. For P(S-r-BCB-r-MMA), any kind of substrates, such as metal, metal oxide, semiconductor or polymer surfaces can be used with gentle cleaning.)|
|4.||Spin coat a 1 wt% solution of P(S-r-MMA) or 0.3wt% solution of P(S-r-BCB-r-MMA) in toluene onto the fresh cleaned substrate to make a random copolymer film.|
|5.||Anneal P(S-r-MMA) copolymer film at 170 °C under vacuum for 72 hrs or bake P(S-r-BCB-r-MMA) copolymer film at 250 °C under N2 for 20 mins to anchor the copolymers to the substrate.|
|6.||Rinse off the excess, unanchored random copolymers with toluene, leaving a ~ 7 nm thick film, which behaves as a neutral layer to balance the surface interactions between PS and PMMA block.|
|7.||Spin coat a 1 wt% solution of P(S-b-MMA) copolymers in toluene onto the modified substrate to make a block copolymer film with film thickness of 35 nm.|
|8.||Anneal the film at 170 °C under vacuum for 48 hrs and then quench the sample to room temperature.|
|9.||To cross-link PS and degrade PMMA, expose film to 254 nm ultraviolet light (25 J/cm2 dosage) for 35 mins. The sample should be under vacuum for this step to avoid ozone degradation.|
|10.||Remove degraded PMMA by soaking polymer film in an acetic acid bath at room temperature for 20 minutes and then rinsing by deionized water.|
|11.||The remaining structure will be a PS film with hexagonally ordered pores with 18 nm in diameter and 34 nm in spacing.|
Notes: This nanoporous thin film can be used as a scaffold or template for various applications. The nanoporous thick film can be made by an electric field aligning BCP process:
Fabrication of a nanoporous template from a diblock copolymer film - electric field alignment
A long-range lateral ordered nanoporous thin film can be made by a solvent annealing process, which is another related self-alignment process:
Fabrication of a nanoporous template from a diblock copolymer film - solvent annealing
- Polystyrene-block-poly (methyl methacrylate) (denoted as P(S-b-MMA) copolymers
- Polystyrene-random-poly (methyl methacrylate) (denoted as P(S-r-MMA) copolymers or Polystyrene-random-benzocyclobutene-random-poly (methyl methacrylate) copolymers (denoted as P(S-r-BCB-r-MMA))
- Toluene, solvent
- Acetic acid, rinse
- piranha solution
- clean silicon wafer
- Cleanroom environment (recommended), room temperature, annealing at 170 °C or bake at 250 °C.
Login to add your comment.