Fabrication of Three‐Dimensional Photonic Crystals Using Multibeam Interference Lithography and Electrodeposition

2009 WILEY-VCH Verlag Gmb Considerable research has been carried out on photonic crystals since the concept was introduced in 1987. However, fabrication of high-quality 3D photonic crystals still remains a challenge. Among a wide range of proposed techniques, creation of large-area, defect-free 3D polymeric templates through multibeam interference lithography, direct laser writing, or direct ink writing, and subsequent infiltration of a high-refractive-index material into the templates are among the most promising. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been used for infiltration of Si, Ge, and TiO2. [15] These conformal coating methods, however, cannot fill the 3D templates completely, reducing or eliminating the photonic bandgap in many structures including diamond, gyroid, and inverse woodpile photonic crystals. In this communication, we demonstrate the fabrication of a high-quality 3D photonic crystal through electrodeposition into a polymer template created by multibeam interference lithography, followed by removal of the template. Complete infilling of the template is achieved through bottom-up electrodeposition. Interference lithography is attractive because of its versatility in creating photonic crystals of many different symmetries over large areas. The resulting photonic crystals are expected to exhibit excellent optical properties due to their defect-free nature. Electrodeposition is capable of growing a variety of metals and semiconductors into complex 3D geometries, as demonstrated by successful electrodepostion of inverse opal structures. The combination of interference lithography and electrodeposition, therefore, has a great potential to produce a variety of photonic crystals with unique properties. Recently, a TiO2 replica of an interference-lithographically defined template was fabricated using electrodeposition of titania sol–gel. However, as is the case in conventional sol–gel processes, this technique requires calcination of infilling precursor to form TiO2, resulting in volume shrinkage and a loss of long-range order of the structure. Here, we selected Cu2O as an infillingmaterial because of its high index of 2.6 and transparency at wavelengths greater than 600 nm. Since crystalline Cu2O is directly electrodeposited from an aqueous solution, high-temperature processes are not required, helping to maintain structural integrity. The 3D polymer template was fabricated on a conductive, transparent indium tin oxide (ITO)-glass substrate by exposing a negative-tone photoresist, SU-8, to superimposed interference beams over a spot size of 3mm. The template is required to stay in contact with the substrate for reproducible infiltration. However, swelling of SU-8 in developer solution (propylene glycol methyl ether acetate, PGMEA) often induces delamination from the substrate. In addition, the high surface tensions of the aqueous solution and electrodeposited Cu2O cause the template film to lift off during electrodeposition. To avoid these problems, the ITO substrate was treated by O2 reactive-ion etching (RIE) before coating with SU-8. Additionally, the developed template was hard-baked at 90 8C. These treatments improved the adhesion between the template and ITO, and prevented delamination during processing. Figure 1 presents scanning electron microscopy (SEM) images of the polymer template formed via the interference lithography. The 3D pattern has an fcc-like symmetry with shrinkage in the h111i direction, which is orientated perpendicular to the substrate. The distance between nearest neighbors in a (111) plane was measured by SEM to be 900 nm and the spacing of (111) planes to be 520 nm, corresponding to 30% shrinkage in the h111i direction from the true fcc lattice. The thickness of the 3D film was 7mm. The cathodic electrodeposition of Cu2O [31] was potentiostatically performed at 65 8C using the ITO substrate with the polymer template as a working electrode. The electrolyte was an aqueous solution containing copper sulfate and lactic acid at pH 9. Although Cu2O could be electrodeposited over a pH range between 9 and 12, the smoothest films were deposited at pH 9. A cross-sectional SEM image of the sample after electrodeposition (Fig. 2) shows that Cu2O grew inside the template starting from