How can polymer structures be shielded from thermal or chemical post-treatment?

To prevent the polymer templates from melting and unintended chemical reactions during post-processing, a thin stabilizing dielectric layer can be coated on top of the polymer surface. Of course, this layer has to be applied by using a compatible 3D process which does not alter or deteriorate the original structure. We found pulsed layer deposition (PLD) of silica (SiO₂) [1] and atomic layer deposition (ALD) of titania (TiO₂) [2] to be useful for many purposes.

Pulsed Layer Deposition of Silica:

The deposition of silica can be performed at room temperature and atmospheric pressure in a gas-tight glass reactor which is connected to reservoirs of silicon tetrachloride (SiCl₄) and water (H₂O) via computer-controlled valves. To induce the chemical reaction [3]

SiCl₄ + 2 H₂O → SiO₂ + 4 HCl

(a) H₂O is introduced to the reaction chamber in gaseous phase by an inert carrier gas, e.g., nitrogen (N₂). (b) After a thin layer of H₂O is adsorbed on the template surface, (c) gaseous SiCl₄ is passed into the chamber which locally reacts with the already present H₂O. (d) As a consequence, a thin silica layer (typically 3 nm) is created on top of the surface. The resulting HCl is carried out of the chamber by using a carrier gas.

A cyclic repetition of this procedure results in closed and robust silica films. In analogy to conventional glass, the deposited silica has a refractive index of n_silica=1.45 at 1 μm wavelength.

Atomic layer deposition of titania:

The deposition of titania protection layers works quite similar to silica. SiCl₄ just has to be replaced by titanium tetrachloride TiCl₄. Additional heating is required during step during the deposition process to induce the chemical reaction [4,5]

TiCl₄ + 2 H₂O → TiO₂ + 4 HCl

The process is performed under high vacuum at a temperature of 110°C. Notably, the polymer SU-8 is mechanically stable up to about 120°C. Since the deposited titania is amorphous, the refractive index can be taken as n_titania=2.05 at 2 μm wavelength. Besides TiO₂, also other dielectric and metallic materials can be coated on top of polymer templates, of course.

A commercially available ALD reactor is provided, e.g., by Cambridge NanoTech.

The main difference between both coating processes lies in the control of thickness and the required equipment. While in the case of titania an atomically thin layer is created during each cycle, the silica PLD deposits several nanometers at once (... actually, that is why both techniques have different names!). Thus, by using an ALD process, one can adjust the target thickness in a very controlled fashion. Moreover, from our experience, the titania surfaces are advantageous compared to silica for the growth of smooth thin metal films which can be accredited to its higher surface energy. However, the titania ALD needs more advanced vacuum and heating equipment.

References:

1. M. Hermatschweiler, A. Ledermann, G. A. Ozin, M. Wegener, and G. von Freymann, “Fabrication of silicon inverse woodpile photonic crystals,” Adv. Funct. Mater. 17, 2273–2277 (2007).
2. M. S. Rill, Three-Dimensional Photonic Metamaterials by Direct Laser Writing and Advanced Metallization Techniques, EVA Star: urn:nbn:de:swb:90-186141, ISBN: 978-383 811 8888.
3. D. J. Ehrlich and J. Melngailis, “Fast room-temperature growth of SiO₂-films by molecular-layer dosing,” Appl. Phys. Lett. 58, 2675–2677 (1991).
4. M. Ritala, M. Leskelä, E. Nykänen, P. Soininen, and L. Niinistö, “Growth of titaniumdioxide thin-films by atomic layer epitaxy,” Thin Solid Films 225, 288–295 (1993).
5. S. Haukka, E. L. Lakomaa, and A. Root, “An IR and NMR-study of the chemisorption of TiCl₄ on silica,” J. Phys. Chem. 97, 5085–5094 (1993).

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