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.
In the fabrication of MEMS devices, what has come to be known as "release stiction" occurs when the device is removed from the liquid phase into the ambient air. One widely used method for dealing with stiction is to deposit a hydrophobic coating on the surface of the device before it is removed from the liquid phase. This method can produce coatings with inconsistent morphology and device yield. This is to be compared with a new coating deposition scheme developed at Sandia National Labs, termed VSAMS (vapor-deposited self-assembled monolayers) that employs supercritical CO2 drying and chemical vapor deposition to address many of the concerns associated with release stiction. VSAMS is attractive due to its process benefits, which include increased throughput, reduced waste, and most importantly, it can be easily scaled to full wafer production. It is also attractive because films produced by this method are uniform and very hydrophobic. The deposition step makes use of a class of compound that is particularly suited for vapor phase reactions, amino-functionalized silanes. The yield of microengine test devices coated with films made from amino-functionalized silanes was examined over an extended period. Their function was determined before and after the application of VSAMS. The advantage of using amino-functionalized silane precursors for VSAMS is related to the strength of the bond between the film and the polysilicon surface as evidenced by the fact that films made with these precursors are stable across the entire humidity scale.
This work describes full wafer encapsulation of released, self-assembled monolayer (SAM) coated, multi-level polysilicon surface micromachines using the anodic bonding technique. This process has been utilized to protect fragile surface micromachines from damage due to particles, moisture contamination, and post-release die handling. The anodic bonding process was optimized to ensure strong glass-to-wafer bonds, while maintaining the effectiveness of liquid-phase and vapor-phase deposited SAM coatings. The temperature, time, and voltage effects on each SAM coating was analyzed. Glass-to-silicon and glass-to-SAM coated silicon had shear strengths of approximately 18 MPa. Glass-to-polysilicon bonds had lower shear strengths of approximately 10 MPa. Bonds were hermetic to 5 X 10-8 atm-cm3/s.
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