InterNano - a project of the National Nanomanufacturing Network - Fabrication of a nanoporous template from a diblock copolymer film

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3. Fabrication of a nanoporous template from a diblock copolymer film - neutral brush

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.

Process:

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

Raw Materials

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

Environmental Variables: Cleanroom environment (recommended), room temperature, annealing at 170 °C or bake at 250 °C.