Equipment for critical point drying of MEMS structures to avoid stiction

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Additional Restrictions

Critical point drying apparatus is typically used after HF-release of a micro electro mechanical systems (MEMS) device to prevent stiction in the device. Drying MEMS in air or under vacuum can drastically alter their structures or even destroy them completely. They must therefore be dried by a gentler method. One well-known method is "Critical Point Drying". The surface tension of the water, isopropanol or acetone in a micro device at the point at which it changes from the liquid phase to the gaseous phase can destroy the device through capillary forces. By increasing the pressure and temperature of the substrate it is possible to dry it without crossing a phase boundary. This is possible because once the critical point has been passed, the density of the "liquid" and the density of the "gas" are the same. The critical point for water is 228.5 bar and 374°C. However, this high pressure and extreme temperature would normally destroy a substrate. For this reason the substrate must be treated in a suitable transitional fluid such as CO2 whose critical point of 73.8 bar and 31°C is considerably more advantageous.


Drying of MEMS devices or high aspect ratio structures without stiction or feature collapse