Anisotropic Ta2O5 photonic crystal waveguide etching using inductively coupled plasma etching

Achieving smooth and vertical sidewall profile in the 2D photonic crystals (PhCs) is crucial for the photonic crystal waveguides (PCWs) in integrated optical devices, since “volcano” shaped PhCs lead to light leakage and the drift of photonic bandgap. This paper optimizes the inductively coupled plasma reactive ion etching (ICP-RIE) process for improving the sidewall profile of Ta2O5 PCW with minimum PhC dimension of 300 nm pitch and 185 nm diameter. A set of experiments was generated by statistical software (minitab) to efficiently investigate the region of interest in the process space by varying RF platen power, ICP power, %C4F8 in gas mixture (C4F8 and O2), and chamber pressure. Subsequently, the most suitable experimental conditions were identified and used as the central run for analyzing the influence of individual parameters on ICP etching of Ta2O5. Key findings include the importance of maintaining a 50% C4F8 in total gas flow and trade-offs related to ICP and RF power adjustments. Pressure has a negligible impact on sidewall angle but exerts a more pronounced influence on etch rate and micromasking. A delicate balance among these parameters is crucial for optimal etching results. The recommended recipe (50 W RF platen power, 2000 W ICP power, 10 mTorr pressure, 50% C4F8 in total gas flow) achieves an etch rate of 113.325 nm/min with a smooth and vertical sidewall in PhCs. The entire etching exploration process was conducted on a Ta2O5 layer deposited on a Si substrate. Additionally, this recipe was applied to etch Ta2O5 with a 2 μm SiO2 layer interposed between the Ta2O5 layer and the Si substrate, yielding favorable results and preserving the integrity of the PhC sidewall.