Non-invasive in-situ monitoring of deep etching processes using terahertz metasurfaces

This study presents an in-situ and non-invasive process control and monitoring (PCM) method for deep silicon etching, leveraging terahertz metasurfaces. The technique addresses the challenges for monitoring deep and high aspect ratio etching processes, which are prevalent in semiconductor microfabrication. By incorporating metasurfaces with identical geometric shapes and sizes as crucial components of targeted devices, the method enables accurate monitoring of the etching depth in the process. Continuous shifts of terahertz reflection spectra provide information on etching depth, while abrupt change in the curves highlights the etching endpoint, preventing over-etching. For the commonly used comb-finger structure, numerical simulations demonstrate a strong linear relationship between etching depth and terahertz resonant wavelengths (nonlinearity < 1%) and an abrupt resonant frequency change (> 0.6 THz) at the endpoint. Experimental validations confirm the accuracy of the PCM method, with an etching depth estimation error below 2 µm. This approach enhances the precision of PCM in microfabrication, offering the potential for widespread applications in the production of micromechanical sensors, actuators, and other microelectronic devices.