Fabrication of micrometer-scale, patterned polyhedra by self-assembly

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Original Entry: Stephen Fleming, AP 225, Fall 2012

Figure 1, reproduced from [1]. Photolithographic techniques are used to pattern a metal layer and metal is evaporated in that pattern on top of a sacrificial layer. The metal template is then dip-cast in solder. Finally, the sacrificial layer is removed and the metal template with solder is heated, and it self-assembles into the minimal free-energy polyhedron.

General Information

Authors: DH Gracias, V Kavthekar, JC Love, KE Paul, and GM Whitesides

Publication: DH Gracias, V Kavthekar, JC Love, KE Paul, and GM Whitesides. (2002) "Fabrication of micrometer-scale, patterned polyhedra by self-assembly." Advanced Materials 14 (3), pp. 235-238.

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Keywords: Thin film, Surface Forces, Surface tension

Summary

Fabricating three dimensional structures on the micron scale is difficult, due to the fact that photolithographic techniques usually only allow for two dimensional projection of a pattern onto a surface. However, using this technique, patterned 2D metal structures can be made to fold and self-assemble into 3D structures on the micron length scale. The technique makes use of surface forces and surface tension.

Explanation of Experimental Methods

The technique used to fabricate micron sized metallic polyhedra relies on a self-assembling process called "auto-folding." The experimental method has four steps:

  1. The polyhedra are first designed in a 2D shape that will be able to fold correctly into the desired 3D shape.
  2. The 2D shapes are fabricated in metal on a sacrificial polymer using traditional photolithography and metal evaporation.
  3. The 2D faces of the metal templates are covered with a thin film of liquid solder by dip-coating.
  4. The sacrificial layer is dissolved and the 2D template structures are released into solution. Folding into a 3D structure automatically takes place when the surface tension of the molten solder is allowed to pull the faces of the polyhedra together.

See Figure 1 for a schematic diagram of the process.

When the 2D structures are dipped in solder, the solder bridges the faces of the unfolded structure and forms a continuous layer. When the solder-coated 2D structures are heated above the melting point of solder, the surface tension tries to minimize its surface area by pulling the faces together to form a 3D polyhedron. The finished polyhedron is filled with solder, and so the amount of solder used should be equal to the volume of the 3D polyhedron.


Results

Figure 2, reproduced from [1]. (a) Optical pictures of 2D metal template structures. (b) Pictures of the metal template structures with solder on top. (c) SEM images of assembled polyhedra.

The results of the self-assembly process driven by surface tension are shown in Figure 2. SEM images show that the 2D precursor metal structures can spontaneously fold into 3D polyhedra due to the surface tension of molten solder.


Discussion

Because this whole process of self-assembled auto-folding is dependent on surface tension, it is important to control surface tension carefully. The surface tension of liquid solder decreases approximately linearly with increasing temperatures [1], and so this can be used to tune the amount of surface tension in the liquid solder.

When the liquid solder is initially deposited on the 2D metal precursors, the thickness can be controlled by elevating the temperature to decrease the surface tension and make the solder film thinner.

The interfacial energy between the metal structure and the solder was also found to play an important role. Conditions were optimized to increase the interfacial energy by doing extensive cleaning of the metal structures to remove organic debris and oxide layers.


Reference

[1] DH Gracias, V Kavthekar, JC Love, KE Paul, and GM Whitesides. (2002) "Fabrication of micrometer-scale, patterned polyhedra by self-assembly." Advanced Materials 14 (3), pp. 235-238.