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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.


T.P. Russell, Mark Tuominen


NSF Center for Hierarchical Manufacturing

Manufactured Material or Structure: 
Polystyrene film with hexagonal array of nanopores
Chemical Composition: 
Physical Form: 
Nanoporous thin film
Material Properties: 
1000 nm thick film containing a hexagonal array of 14 nm diameter pores with a period of 24 nm. Pores are hydrophobic.
Step 1:

Clean a silicon wafer (or alternate substrate) and coat with a smooth continuous Au/Cr film by vacuum deposition.

Step 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.

Step 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.

Step 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
Figure 1
Step 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.

Step 6:

Cool sample to room temperature, maintaining the presence of the E-field.

Step 7:

Remove E-field and peel upper (aluminized Kapton) away from film.

Step 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.

Step 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.

Step 10:

The remaining structure will be a PS film with hexagonally ordered pores with 14 nm diameter and 24 nm spacing.

Process 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: 
  • Polystyrene denoted as PS
  • Polymethylmethacrylate denoted as PMMA
  • Toluene, solvent
  • Conducting substrate (e.g. silicon, gold-plated silicon, aluminized Kapton, etc.)
  • Aluminized Kapton film (electrode used in process)
  • Acetic acid, rinse
  • Methanol, surfactant, needed in some preparations
Environmental Variables: 

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

T. Thurn-Albrecht, J. Schotter, G.A. Kästle, N. Emley, T. Shibauchi and L. Krusin-Elbaum et al., Science 290 (2000), p. 2126. doi:10.1126/science.290.5499.2126