Integrated optical switching using titanium nitride micro electromechanical systems

Abstract

This thesis reports an integrated optical wavelength specific switching device for applications in optical integrated circuits (OICs) based on micro electromechanical systems (MEMS). The device consists of a ring resonator add-drop filter and a conductive MEMS bridge which is actuated by electrostatic force. Introducing conductive material into the electromagnetic evanescent field of the ring waveguide results in loss in the propagating light within, disabling the resonance and the filtering capabilities of the ring resonator. Therefore, by actuating the MEMS bridge in and out of the waveguide's evanescent field, the filter can be toggled between the on and off states. One large problem that must be faced when fabricating and actuating a MEMS cantilever or bridge structure for this type of device is the residual stress that may deflect the structure in an undesired way. This is because the vertical displacement of the structure is crucial. In order to solve this problem, this thesis is based on the use of titanium nitride (TiN) as structural material for the bridge. Titanium nitride has very attractive mechanical properties as well as good conductivity, which makes it an ideal structural material for electrostatically actuated devices.

(cont.) Moreover, the residual stress within the material can be relieved by proper control of deposition conditions and/or post processing. This thesis focuses on the post process annealing of titanium nitride in order to eliminate the residual stress in the structure and obtain a fiat bridge profile. Titanium nitride MEMS bridge structures were fabricated and tested. Their deflection from a flat state and stress was measured and characterized, and a structure with minimal residual stress was successfully fabricated. The actuation of the MEMS bridge is also demonstrated, and its characteristics are analyzed. Also discussed is the possibility of extending the design of the MEMS switch to implement the three-electrode ultra-fast strain-induced switching and MEMS wavelength tuning of an integrated optical filter. A realistic design of these devices is proposed in context with the requirements imposed by the optical telecommunication industry, and fabrication methods are considered. Simulations have been conducted using finite element analysis and mode solving to establish the feasibility of these designs.