The Tousimis Supercritical “Automegasamdri” Model 815B Series C Critical Point Drier (CPD) is a system for drying typically MEMS devices after sacrificial layer etch or samples with high aspect ratio photoresist features. If the HF etchant was rinsed with water and dried in atmosphere, the surface tension of the water could pull structures such as cantilevers into contact with other parts resulting in bonding which would not allow movement of the MEMS part. The CPD process uses liquid CO2 to displace the transfer fluid (usually alcohol that replaced the HF etchant), and then takes the pure CO2 fluid to high temperature and high pressure to circumvent the critical point on the phase diagram and then returns to normal atmospheric conditions. This technique avoids any capillary forces that could damage delicate features on the substrate.
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