An Empirical Study of Boss/Seat Materials and Geometries for Ultra Low-Leakage MEMS Micro-Valves

We report work on the testing and characterization of the sealing properties of various micro-valve seat/boss interfaces. Using a custom test set-up, we have measured helium leak rates for a variety of boss materials and seat geometries. The seat geometries are micro-machined in silicon, and an orifice is DRIE etched through the chip. The test fixture allows for leak-tight edge sealing of seat chips against a viton o-ring, independent of the force used to seal the boss against the seat. Bosses are sealed against the various seat chips with forces up to 400 mN by using a precision micrometer to deflect a small spring that is coupled to the boss chip. Soft metals, such as copper and gold, and polymers such as polydimethylsiloxane (PDMS) and parylene-c, coated on silicon boss chips have been tested on hard silicon seats. In all cases, leak rates were determined as a function of sealing pressure. Seat geometries include a concentric o-ring configuration, and a silicon knife-edge. Both seats have orifice diameters varying from 60 to 110 μm. Experimental results indicate that practical MEMS-scale forces (up to several hundred mN) are sufficient to cause deformation of the soft materials coating the bosses given the small loading area, which can improve sealing capacity but not repeatability. However, uneven loading of the boss prevented a tight seal across the entire seat, which is reflected in the leak rates detected. Soft boss-materials, like PDMS, however, have shown promising results for obtaining ultra-low leak rates. Leak rates as low as 1 × 10⁻⁴ atm·cc/sec were obtained on knife-edge seats with 110 μm diameter orifices.