In-situ optical monitoring of single-crystal silicon membrane etching

We present a simple yet efficient technique to monitor membrane thickness during etching of silicon in anisotropic etching bath. This technique uses a mechanical holder to protect the front side of the wafer and the measurement of light absorption to obtain remotely the thickness of a reference zone in the etched wafer. The original feature in our set-up is that we measure the absorption in two different bands of wavelength, one where the silicon is strongly absorbing and the other where it is not, improving the robustness of the measurement. Actually, this principle allows for effectively compensating the fluctuation in the optical path, and after calibration provides real-time information on the membrane thickness, which proves to be particularly useful for fabricating membranes below 40 mu m.