Real-time etch-depth measurements of MEMS devices

An in situ, real-time process control tool was developed for MEMS deep reactive-ion etch (DRIE) fabrication. DRIE processes are used to manufacture high-aspect-ratio silicon structures up to several hundred microns thick, which would be difficult or impossible to produce by other methods. DRIE MEMS technologies promise to deliver new devices with increased performance and functionality at lower cost. A major difficulty with DRIE is the control of etch depth. Our research shows that it is possible to monitor the etch depth of various MEMS structures (holes, pillars, trenches, etc.) through measurement and analysis of the infrared reflectance spectrum. Depths as large as 150 μm have been measured. Excellent correlation is found between the etch depths determined by analysis of these measurements and those measured with an SEM. In addition to etch depth, other parameters such as the photoresist thickness (e.g., mask erosion) can be simultaneously extracted. Based on these results, an infrared-reflectance etch monitor was integrated onto a reactive ion etcher at the Berkeley Sensor and Actuator Center for real-time monitoring and end-point determination. The integrated optical metrology system demonstrated accurate real-time monitoring of the etch depth and photoresist mask erosion