Diffusive transport in supercritical CO2 drying of MEMS structures

Supercritical CO2 drying has been shown to be an effective method for drying complex MEMS structures with little or no stiction. This process typically involves transferring released parts from ultrapure water into a solvent, such as methanol, that is miscible with liquid CO2, and subsequently displacing the methanol with liquid CO2. During this process sequence, transport of methanol into and out of the tortuous pathways of the MEMS device is dominated by diffusion. The non-steady state diffusion equation (Fick's second law) with length scales relevant to MEMS structures can be applied to understand the speed at which diffusion occurs. This analysis indicates that diffusion of methanol into the pathways of a MEMS device occurs very rapidly, typically on the order of minutes. Experimental data are consistent with the rapid diffusion hypothesis.