Direct-Write Assembly of Three-Dimensional Photonic Crystals: Conversion of Polymer Scaffolds to Silicon Hollow-Woodpile Structures

Impressive developments in silicon microfabrication are enabling new applications in photonics, microelectromechanical systems (MEMS), and biotechnology. Yet conventional Si microfabrication techniques require expensive masks and time-consuming procedures, including multiple planarization or bonding steps, to generate three-dimensional (3D) structures. In contrast, direct-write approaches, such as laser scanning and ink deposition, provide rapid, flexible routes for fabricating 3D microperiodic structures. However, these approaches are currently limited to polymeric structures that lack the high refractive index contrast and mechanical integrity required for many applications. To take full advantage of these rapid, flexible assembly techniques, one must develop a replication (or templating) scheme that enables their structural conversion within the temperature constraints imposed by both the organic and inorganic components of the system. Here, we present a novel route for creating 3D Si hollow-woodpile structures that couples direct-write assembly of concentrated polyelectrolyte inks with a sequential silica/Si chemical vapor deposition (CVD) process. The optical properties of the 3D microperiodic woodpiles are characterized after each fabrication step. These interconnected, hollow structures may find potential application as photonic materials, low-cost MEMS, microfluidic networks for heat dissipation, and biological devices. Since the concept of a photonic bandgap (PBG) was first introduced, there has been intense interest in generating 3D microperiodic structures composed of alternating highand low-refractive-index materials. Si is an ideal material for photonic crystals, because it has a high refractive index (n ∼ 3.45) and is optically transparent in the infrared. The woodpile structure, which consists of a 3D array of orthogonally stacked rods, is particularly well suited to microfabrication and direct-write assembly techniques. Engineered defects that add functionality to photonic crystals can also be readily incorporated into woodpile structures. For example, waveguides containing 90° bends can be formed by simply removing two orthogonal filaments in adjacent layers. In contrast, it is inherently difficult to introduce controlled defects into inverse face-centered cubic structures produced by colloidal self-assembly routes. Additional processing steps, such as two-photon polymerization, are necessary to impart the desired functionality to these structures. 3D microperiodic polymer scaffolds are fabricated in a woodpile architecture via direct-write assembly of a concentrated polyelectrolyte ink in a layer-by-layer build sequence (Fig. 1). 8and 16-layer woodpile structures ranging in lateral dimensions from 250 lm × 250 lm to 500 lm × 500 lm were formed with in-plane center-to-center rod spacings (d) of 2.8 lm and 4.0 lm, and a rod diameter of 1 lm. These dimensions do not represent the limit of the direct-writing process, where the maximum lateral dimensions can exceed 1 cm × 1 cm, rod diameters can be as low as 600 nm, and rods can be close packed on a pitch that equals the rod dimension. Our ink design utilizes concentrated polyelectrolyte complexes composed of a non-stoichiometric mixture of C O M M U N IC A IO N S