Process Description #1

1. Fabrication of a nanoporous template from a diblock copolymer film - electric field alignment

An applied electric field aligns a cylindrical-phase diblock copolymer perpendicular to a substrate. One polymer block 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 nanowires or as a mask for reactive ion etching.

Contributors: T.P. Russell, Mark Tuominen
Lab: NSF Center for Hierarchical Manufacturing

Manufactured Nanomaterial: Polystyrene film with hexagonal array of nanopores
Chemical composition: Polystyrene
Physical Form: Nanoporous thin film
Properties: 1000 nm thick film containing a hexagonal array of 14 nm diameter pores with a period of 24 nm. Pores are hydrophobic.

Process:

1.Clean a silicon wafer (or alternate substrate) and coat with a smooth continuous Au/Cr film by vacuum deposition.
2.Synthesize or purchase diblock copolymer P(S-b-MMA), consisting of PS and PMMA, with a molecular weight of 39,600 daltons and a PS volume fraction of 0.71. As described below this polymer will phase separate and self-assemble into hexagonally packed array of PMMA cylinders 14 nm in diameter.
3.In a 10% solution with toluene the P(S-b-MMA) is spin cast onto a conducting substrate into a film on order of 1 µm thick.
4.Apply a DC field (~30-40 V/µm) across the bottom substrate and a piece of aluminized Kapton film placed on top of the P(S-b-MMA) film. See Figure 1.
Figure 1. Magnify
Figure 1.

Figure 1.

5.Anneal the film under this E-field for 14 hours at 180°C in a vacuum oven, a temperature above the glass transition temperature, but below the order-to-disorder transition.
6.Cool sample to room temperature, maintaining the presence of the E-field.
7.Remove E-field and peel upper (aluminized Kapton) away from film.
8.In order to cross-link the PS and degrade the PMMA, expose film to 254 nm ultraviolet light (25 J/cm2 dosage) for 35 minutes. The sample should be under vacuum for this step to avoid ozone degradation.
9.Remove degraded PMMA by soaking polymer film in an acetic acid bath at room temperature for 20 minutes and then rinsing by deionized water.
10.The remaining structure will be a PS film with hexagonally ordered pores with 14 nm diameter and 24 nm spacing.

Notes: This nanoporous film can be used as an electrodeposition template as was demonstrated in the original Science (2000) article. Since the film is hydrophobic, it is important to use a surfactant in an aqueous electroplating bath to enable wetting down the pore. Thinner versions of the templates are useful for reactive ion etching. This process is a relative of a closely related self-alignment process:
Fabrication of a nanoporous template from a diblock copolymer film - neutral brush

Raw Materials Expand

  1. Polystyrene denoted as PS
  2. Polymethylmethacrylate denoted as PMMA
  3. Toluene, solvent
  4. Conducting substrate (e.g. silicon, gold-plated silicon, aluminized Kapton, etc.)
  5. Aluminized Kapton film (electrode used in process)
  6. Acetic acid, rinse
  7. Methanol, surfactant, needed in some preparations

Environmental Variables: Cleanroom environment (recommended), room temperature, annealing at 175±5°C under vacuum, low humidity

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