Jer-Haur Chang

Direct imprinting using soft mold and gas pressure for large area and curved surfaces

In this paper we report a simple and effective method that renders direct imprinting of sub-micron structures onto PMMA resist coated on large area and curved substrates using the PDMS mold on a closed chamber. Nitrogen gas was employed to generate a uniform pressure. The patterns of the soft mold could be replicated with high quality over an entire 12in. resist-coated area. The process was further successfully applied to the imprinting of a curved substrate.

Fabrication of concave gratings by curved surface UV-nanoimprint lithography

The authors report a new method for fabricating submicron gratings on curved substrates by combining thermoforming and nanoimprint technologies. A preshaping film was used to provide uniform pressure distribution throughout the whole concave substrate with the diameter of 60mm and the radius of curvature of 92.5mm. The concave glass could be resin coated uniformly by a conventional spin coater through the support of a soft holder. Either the preshaping film or the soft holder could be a buffer layer to prevent the curved glass from crumbling, which may result from high gas pressure during the imprint process. As for demonstration, a Rowland circle type concave grating with a period of 1.2μm was made, and the measured first order reflectance was about 20% at various wavelengths.

Stitching periodic submicron fringes by utilizing step-and-align interference lithography

The authors develop a step-and-align interference lithography system to fabricate large-area periodic submicron structures by stitching the unit exposure area step-by-step. A metal mask with a square transparent window in the center is used to intercept the quasi-flat-top region of the expanded Gaussian beam, and thus it serves as a beamshaper to approximate the ideal unit beam that has uniform intensity and spatial coherence. Two-dimensional precision dual-actuator motion stages could provide travel distance for full wafer exposure with 2nm high precision positioning capability for stitching the submicron patterns. The gratings with period of 700nm are successfully stitched along two directions on 100mm diameter wafers.

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, indicating the fidelity of replication. If the ghost image were eliminated, the AR effect would be significantly enhanced for both cases.

Packaged symmetric/asymmetric corrugated long period fiber gratings for refractive index sensing applications

We demonstrate a new method that could make possible the mass production of symmetric and asymmetric corrugated long period fiber gratings (C-LPFG) by utilizing hot embossing and imprint lithography on polycarbonate (PC). The poly-dimethylsiloxane (PDMS) is imprinted on PC to create the periodic revealed region of fiber for hydrofluoric acid (HF) etching. The asymmetric C-LPFGs show some unique optical characteristics of two separate dips due to their asymmetric structures. After the torsion, the second dip would be gone and the first one would begin to shift toward shorter wavelength. It is assumed that the asymmetric C-LPFG behaves similar to the symmetric one with the torsion of a full circle. A C-LPFG was well packaged by fixing itself in the PC and having some open area close to the fiber to receive the liquid under test. The corrugated structures close to the fiber core were covered by the liquid that the packaged refractive index sensor has the sensitivity of 26.7 nm/RIU.

A novel micro-scale recombining technique using lateral joining for a large-area molding with small features

A novel micro-scale recombining technique using a lateral joining of silicon crystal planes is first presented. The new concept of the technique is realized by fabricating a large silicon molding plate beyond a wafer size with small features. The replication of the joint plate using the hot embossing technique has been characterized in surface profiles, pictures, and optical performance. Both experiment and simulation results reached good agreement in diffractive optical intensity. Furthermore, the scalability of the technique was extensively demonstrated with three silicon wafers. As a result, the novel technique provides a potentially low-cost approach to fill the technology gap between the conventional precision machining and photolithography-based micromachining for a beyond-wafer-size silicon plate with small features.

An insertion loss insensitive and mean wavelength stable polarized superfluorescent fiber source

In conclusion, we have developed a linear polarized superfluorescent fibre source (SFS) in double pass backward (DPB) configuration by inserting a polarizer between the rear WDM and the fiber mirror. Compared with the conventional unpolarized SFS, the current polarized SFS can provide nearly 1.7 times power when pumped at the same level. In addition, our polarized DPB SFS shows less sensitivity to the insertion loss and conserves the mean wavelength stability over a large pump power region as the conventional DPB SFS.