Microfabrication of membrane-based devices by HARSE and combined HARSE wet etching

Deep-reactive ion etching (DRIE) of silicon, also known as high-aspect-ratio silicon etching (HARSE), is distinguished by fast etch rates (similar to 3 mu m/min), crystal orientation independence, anisotropy, vertical sidewall profiles and CMOS compatibility. By using through-wafer HARSE and stopping on a dielectric Nm placed on the opposite side of the wafer, freestanding dielectric membranes were produced. Dielectric membrane-based sensors and actuators fabricated in this way include microhotplates, flow sensors, valves and magnetically-actuated flexural plate wave (FPW) devices. Unfortunately, low-stress silicon nitride, a common membrane material, has an appreciable DRI etch rate. To overcome this problem HARSE can be followed by a brief wet chemical etch. This approach has been demonstrated using KOH or HF/Nitric/Acetic etchants, both of which have significantly smaller etch rates on silicon nitride than does DRIE. Composite membranes consisting of silicon dioxide and silicon nitride layers are also under evaluation due to the higher DRIE selectivity to silicon dioxide.