Fabrication of antireflection structures on flat and curved substrates by interference lithography and nanoimprint technique

Summary form only given. Surface-relief gratings with periods smaller than the wavelength of light, named sub-wavelength grating (SWG), may behave as antireflection (AR) surfaces. The fabrication methods have been studied such as e-beam writing with scattering (Y. Kanamori et al., 1999), holograplic lithography (P. Lallane and G.M. Morris, 1997), spin-coating replication (Y.Kanamori and et al., 2005), etc. However, almost in each method the structures were fabricated on flat substrates such as Si or transparent materials. We propose an effective, simple way to fabricate AR structures on flat and curved substrates, by combining two-beam interference lithography and nanoimprint technique. We divided our work into to parts. One is AR SWG on a flat Si substrate; the other is on curved glass with SU-8 resist pattern. First, the grating pattern was obtained by interference lithography with a period of 380 nm. Then by employing lift-off process and ICP etching, we transferred the SWG to the Si substrate with a high aspect ratio. The AR structure we fabricated was 850 nm in height as shown in Fig. 1. The reflectance spectrum was shown in Fig. 4. The AR ability was undervalued because of the ghost image in interference lithography. The other work is AR SWG on a curved substrate. We utilized interference pattern as the process of AR on a flat Si, with grating period 380 nm. Then we chose PDMS material to cast. After that, we spin-coated SU-8 photoresist on a curved glass substrate, and used the PDMS elastomeric stamp to transfer the AR pattern to SU-8 photoresist by employing nanoimprint technique as shown in Fig. 2. The replicated SWG resist pattern on the glass had a period of 380 nm. The sample was coated with gold as shown in Fig. 3 to facilitate to observe the AR effect. Also the reflectance spectrum was shown in Fig. 5. Besides SWG, the unwanted interference pattern caused by ghost image from the beam splitter was transferred completely by nanoimprint technique, indicat- ing the fidelity of replication. If the ghost image were eliminated, the AR effect would be significantly enhanced for both cases.