Visible RGB(450,520,638 nm) Si3N4 Photonic Switch with MEMS Actuator

Abstract

본 논문은 가시광 RGB 파장 대역에서 동작 가능한 MEMS 집적 Si₃N₄ 광 스위치의 설계, 시뮬레이션 및 fabrication process 개발 결과를 제시한다. 제안하는 소자는 200 nm 두께의 LPCVD Si₃N₄ waveguide platform을 기반으로 하며, 450 nm, 520 nm, 638 nm의 blue, green, red 파장에서 동작하도록 설계되었다. 광 스위칭은 방향성 결합기(directional coupler)의 결합 간격을 MEMS 구동을 통해 기계적으로 조절하는 방식으로 구현되며, 이를 통해 thermo-optic 또는 carrier 기반 굴절률 변조에 의존하지 않는 broadband optical switching을 목표로 한다. 소자의 광학 설계는 effective-index cutoff simulation과 directional coupler simulation을 기반으로 수행하였다. 먼저 450 nm, 520 nm, 638 nm에서 higher-order mode를 억제하기 위해 200 nm 두께의 Si₃N₄ platform에서 waveguide width를 분석하였고, 최종적으로 250 nm width의 Si₃N₄ waveguide를 기본 설계값으로 선정하였다. 이후 coupling gap에 따른 directional coupler simulation을 수행하였으며, 100 nm gap 조건에서 약 13.63 μm의 coupling length를 통해 높은 coupling efficiency를 얻을 수 있음을 확인하였다. 또한 RGB 전 파장 대역에서 25 dB 이상의 simulated extinction ratio를 확보하여, 제안한 directional coupler 구조가 가시광 RGB optical switching에 적합함을 확인하였다. MEMS actuator 설계에서는 방향성 결합기의 coupling gap을 기계적으로 조절하기 위한 electrostatic actuator 구조를 검토하였다. Cantilever, etching hole을 포함한 cantilever, vertical comb-drive actuator 구조를 비교하였으며, operating voltage, footprint, fabrication feasibility, charging effect를 함께 고려하였다. 특히 vertical comb-drive 구조는 electrostatic force가 comb finger 사이에서 주로 발생하므로 Si₃N₄ film 내부의 electric-field concentration을 줄일 수 있고, 향후 non-conductive 또는 transparent substrate 기반 visible photonic system으로 확장 가능하다는 장점이 있다. Fabrication 측면에서는 Si₃N₄ optical waveguide와 Cr/Au actuation electrode를 동일한 chip 위에 집적하기 위한 photonic-MEMS-compatible fabrication flow를 개발하였다. 해당 flow는 Si₃N₄ patterning, ICP/RIE etching, Cr/Au electrode formation, Cr/Au dry etching, MEMS release 단계로 구성된다. 본 연구에서는 Si₃N₄ etching process와 Cr/Au metal lift-off process를 실험적으로 구축하였으며, 이를 통해 visible Si₃N₄ waveguide와 MEMS actuation electrode 제작을 위한 unit process 기반을 마련하였다. 다만 최종 MEMS release와 opto-electrical characterization은 본 연구에서 완료되지 못하였다. 주요 fabrication bottleneck은 Cr/Au dry etching 단계에서 발생하였으며, plasma-induced damage로 인한 photoresist loss와 partial pattern collapse가 관찰되었다. 이러한 문제는 Ar ion bombardment와 Cl₂/O₂ 기반 chemical reaction에 의한 mask damage와 관련이 있으며, vertical comb-drive와 같은 fine pattern 구조에서 특히 치명적이다. 향후 연구에서는 cooling-cycle 또는 pulsed dry etching, bias power optimization, mask protection 개선을 통해 Cr/Au dry etching process를 안정화하고, 이후 MEMS release 및 optical/electrical characterization을 수행할 계획이다. 본 연구는 최종 소자 측정까지 완료하지는 못했지만, 가시광 RGB Si₃N₄ MEMS photonic switch 구현을 위한 optical design, MEMS actuator concept, 그리고 fabrication-process foundation을 확립했다는 점에서 의의가 있다. 제안한 플랫폼은 향후 AR/VR display, sensing, optical routing, quantum photonic system 등 가시광 기반 programmable photonic integrated circuit 구현을 위한 기반 기술로 활용될 수 있다. |Visible-wavelength silicon nitride (Si₃N₄) photonic integrated circuits are promising for emerging applications such as augmented and virtual reality displays, biosensing, microscopy, quantum optics, and visible optical routing. However, practical visible photonic systems require compact and reconfigurable optical switching elements that can operate across multiple visible wavelengths while maintaining low optical loss and fabrication compatibility. In particular, Si₃N₄ provides low absorption in the visible spectrum, but its weak material-based refractive-index modulation makes conventional active switching approaches challenging. This thesis presents the design, simulation, and fabrication-process development of a visible RGB Si₃N₄ photonic switch integrated with a micro-electro-mechanical systems (MEMS) actuator. The proposed device is based on a MEMS-actuated directional coupler that mechanically controls the coupling gap between adjacent Si₃N₄ waveguides. By physically modulating the waveguide gap rather than relying on thermo-optic or carrier- based refractive-index modulation, the proposed approach aims to provide broadband and low-static-power optical switching in the visible wavelength range. The optical design was performed on a 200-nm-thick LPCVD Si₃N₄ platform for red, green, and blue wavelengths of 638 nm, 520 nm, and 450 nm, respectively. Effective-index cutoff simulation was used to determine a 250-nm-wide Si₃N₄ waveguide for visible-wavelength operation. Directional coupler simulations were then performed by sweeping the coupling gap and coupling length. At a coupling gap of 100 nm, a coupling length of approximately 13.63 μm was selected as a practical broadband design length for RGB operation. The simulated extinction ratio exceeded 25 dB across the target RGB wavelength range, confirming that the proposed directional coupler geometry can provide sufficient optical switching contrast. MEMS actuator structures were also investigated to mechanically tune the coupling gap. Several electrostatic actuator concepts, including a cantilever, a cantilever with etching holes, and a vertical comb-drive structure, were compared in terms of operating voltage, footprint, and charging effects. Among these structures, the vertical comb-drive actuator was selected as a promising candidate because it can reduce electric-field concentration inside the Si₃N₄ film and can potentially operate on non-conductive substrates. COMSOL simulations were used to evaluate the relationship between applied voltage and mechanical displacement, with the target operating voltage set below 32 V. A photonic-MEMS-compatible fabrication flow was developed to integrate Si₃N₄ waveguides and Cr/Au actuation electrodes on the same platform. The unit processes included Si₃N₄ waveguide patterning, ICP/RIE etching, Cr/Au metal lift-off, and Cr/Au dry etching for MEMS structure definition. Si₃N₄ etching and Cr/Au lift-off were experimentally established. However, during the Cr/Au dry etching step, severe photoresist loss and partial pattern collapse were observed due to plasma-induced damage and limited mask selectivity. Therefore, further process optimization, including cooling-cycle-based dry etching and improved mask protection, is required before final MEMS release and opto-electrical characterization. Overall, this work establishes the optical design, MEMS actuator concept, and fabrication-process basis for a visible RGB Si₃N₄ MEMS photonic switch. The proposed platform provides a foundation for future low- loss, low-power, and scalable visible photonic switching systems. Keywords: Visible-wavelength photonics, silicon nitride, Si₃N₄ waveguide, MEMS actuator, directional coupler, optical switch, fabrication process, RGB wavelength