Development and Application of Gas-Assisted Soft Mold Embossing Technology for Fabrication SU-8 Ridge Optical Waveguides

Abstract

Polymer optical waveguides are important components for Short-haul optical communication. Several methods of fabricating polymer waveguides have been reported, including photolithography, polymer injection molding and hot embossing, etc. Photolithography is complicated and needs expensive facility. Polymer injection molding and hot embossing involve high temperature and high pressure. They are time-consuming batch-wise processes.
In the thesis, we report a simple and effective technique for rapid fabrication of polymeric waveguides based on gas-assisted UV embossing with soft mold. In this method, the soft mold with waveguides cavity is made by casting a pre-polymer of PDMS against a silicon master, which is prepared using photolithography and deep reactive ion etching. During the process operation, a seal film, a soft mold with waveguides cavity and a silicon wafer coated with SU-8 resist layer are placed in a closed chamber to form a stack. The stack is supported by a solid holder to avoid distortion of patterns on the soft mold during the embossing stage. The nitrogen gas is then introduced into the chamber to pressurize the stack for a period time. Sequentially, the SU-8 resist is cured by UV-irradiation at room temperature. After the soft mold is removed, the polymer waveguides on the silicon wafer are obtained.
In this study, a gas-assisted UV embossing facility with UV exposure capacity has been designed, constructed and tested. The effects of processing parameters on the quality of fabricated polymeric waveguides are also investigated. Under proper processing conditions, the multimode polymeric waveguides, with a line-width of 94μm, a height of 69μm and a length of 4cm has been successfully fabricated. The measured surface roughness on the surface of the waveguide component is 5.591 nm and the average propagation loss of those waveguides is 0.88dB/cm measured at a 1310 nm wavelength. These results show the great potential of using gas-assisted UV embossing for rapid fabrication of polymeric waveguides with high productivity and low cost.
In addition, we propose an innovative CNP technique that combines the advantages of nanoimprint lithography (NIL) and photolithography. In the experiment, a hybrid mask mold with a light-blocking layer placed on top of the mold protrusions is used. The hybrid mask mold is fabricated by PDMS casting. The light-blocking layer on the PDMS mold is made from carbon-powders or metal material. A large area micro-pattern without residual resist layer can be formed in the resist in a single imprint lithography step. Future work will study the capability of the CNP technique and investigate new improvements to fabricate polymer waveguide and other optical devices.