Nano-moulding of integrated optical devices
Creators: name: Han, Ting
Publisher: name: The Australian National University
Year: 2011
Subjects: subject: TK7874.843.H36 2011
subject: Nanoimprint lithography
subject: Nanolithography Materials.

description: Planar waveguide devices made by conventional semiconductor methods are typically very costly as a result of expensive tooling, high cost of raw materials, and many slow processing steps. To address the many mass market opportunities that exist today in fields such as consumer devices, sensing, medicine, etc, much lower cost methods are needed. Nanoimprint lithography was introduced to the semiconductor industry in recent years and was regarded as a serious candidate for next generation lithography. The reported resolution was down to molecular scale (2nm), and yet it offers high throughput and low cost. At this point, Nanoimprint lithography has been added to the International Technology Roadmap for Semiconductors (ITRS) for the 32 and 22nm nodes. In this thesis, Nanoimprint lithography was used for the fabrication of integrated optical devices on two types of material platforms. Firstly, the fabrication of Polysiloxane waveguide devices usmg UV-Nanoimprint Lithography (UV-NIL) is demonstrated. With an innovative minimum displacement method, low loss channel waveguides were fabricated in a 4 step process that showed no excess loss from 750 to 1700nm. Using the same imprint technique, more complicated waveguide Bragg gratings were realized. For the first time, the transmission spectrum showed a polarization independent response and almost perfect agreement with the theoretically expected spectrum. Next, the challenge was to extend this technique to directional couplers and multimode interference couplers. However it is well known that planar directional couplers are generally very sensitive to fabrication errors (waveguide dimension varies randomly in the lithography and etching processes), and true low excess loss multimode interference couplers remain to be demonstrated. Here we proposed a design mechanism to solve the problem of sensitivity to waveguide width and the full vector beam propagation modeling of the optimized directional coupler design showed superior fabrication tolerance to all other designs. For the first time it was identified that a single point in the design space exists where the design is very tolerant to variations in the waveguide width for both TE and TM polarizations. By putting the theory into test, we demonstrated such optimized directional couplers could be fabricated using the same imprint process and the performance was indeed better than other random designs. It was also shown that MMIs can be designed with very low excess losses and good imbalance simultaneously. However, the practical realization of this was thwarted by issues in the imprint stage that require more sophisticated tools than available at the time to resolve the issue. The other material system investigated was based on the chalcogenide glasses. Thermal Nanoimprint Lithography (T-NIL) is a technique which offers freedom from the constraints and process complexities imposed by lithographic/plasma etching processes in chalcogenide waveguides. It was used to directly mould single mode rib waveguides on chalcogenide thin films thermally evaporated on 4 inch silica wafers. Using a soft stamp made from PDMS, the moulding of As{u2082}{u2084}Se{u2083}{u2086}S{u2083}{u2086} was accomplished at 190{u00B0}C and cutback measurements revealed a loss down to O.26dB/cm which was more than 10 times lower than previously published results and the first time a soft stamp has been used to perform hot embossing of chalcogenides. The moulding of the more robust glass composition AS{u2082}S{u2083} was also demonstrated and the measured waveguide nonlinearity was shown to be 13.5W{u207B}{u00B9}m{u207B}{u00B9} on a {u223C}2x1{u00B5}m waveguide.