Modeling and analysis of sulfur hexafluoride plasma etching for silicon microcavity resonators

Silicon microcavity resonators are an important component in modern photonics. In order to optimize their performance, it is fundamental to control their final shape, in particular with respect to the involved CMOS compatible, two-step sulfur hexafluoride plasma etching fabrication process. To that end, we use a ray-tracing based pseudo-particle model to enhance a level-set topography simulator enabling us to effectively capture the characteristics of sulfur hexafluoride plasma etching in the low-voltage-bias regime. By introducing a novel and robust calibration procedure and by applying it to experimental data of a reference two-step etching process, we are able to optimize the etch times and photoresist geometry without costly reactor-scale simulations and simultaneously explore beyond conventional statistical process modeling. Through defining objective design criteria by way of Gaussian beam analysis, we analyze the plasma etching process and provide new insights into alternative processing guidelines which impact shape measurements such as cavity opening and parabolic form. By means of scale analysis, we propose that the radius of curvature of the microcavity is optimized with a reduction of the photoresist opening diameter. After simulated fabrication runs, we surmise that the cavity quality parameters are improved by increasing the duration of the first etch step by a factor of 2 and by decreasing the second etch step duration by up to $50\%$ in comparison to the reference two-step etching process. This results in an overall reduction of etch time of at least $35\%$, allowing to significantly optimize the overall fabrication process.