Release Mechanism
Gas generation during electrostatic hermetic sealing of glass to silicon substrates occurs when high voltage and elevated temperatures drive electrochemical reactions at the bonding interface. During the sealing of microelectromechanical devices, anodic bonding outgassing releases oxygen and sodium reduction products into the sealed cavity. This release establishes a baseline pressure that is often orders of magnitude higher than the process chamber vacuum level, necessitating a post-process evaluation of the hermetic seal.
The electrochemical breakdown of glass is the primary source of these gaseous byproducts.
Hermetic Consequence
Sealed cavity pressure degrades the performance of resonant or inertial microstructures by introducing viscous damping. High residual gas pressure within a silicon glass sensor cavity shortens the lifetime of the sensor by accelerating the damping of mechanical oscillations. This deterioration occurs because oxygen molecules generated at the bonding boundary cannot escape once the seal completes.
Mitigation Strategy
Mechanical extraction or active chemical absorption of the released species prevents the decay of the inner vacuum. Integrating a microscopic channel or a thin-film getter on the silicon wafer captures these gases before they can accumulate. High temperature bakes prior to sealing also reduce the concentration of absorbed water on the glass surface.
Measurement Protocol
Verification of the pressure rise involves measuring the deflection of a thin glass membrane over the sealed cavity after bonding has finished. This diagnostic approach translates physical deflection into a pressure value using interferometric techniques to track changes over time. Restricting the temperature of the bonding process minimizes the reaction rate and prevents excess gas release.
