Hermetic Sealing
Hermetic encapsulation denotes a thin film fabrication methodology that encloses micro-electromechanical systems within a permanent internal low-pressure environment. Wafer level vacuum packaging creates this enclosure by bonding a cover substrate directly to the sensor wafer before the individual components undergo dicing. The integrity of the internal cavity depends on the permeability of the seal material and the surface quality of the interface.
Cavity Integrity
Thermal expansion coefficients between the device wafer and the cap substrate dictate the mechanical stress experienced at the bond interface during cycle testing. Wafer level vacuum packaging requires precise control of bond temperature to prevent gas desorption from surfaces inside the cavity which would otherwise degrade the pressure level over the service life. Standardized leak rate measurements quantify the performance of the seal by monitoring tracer gas ingress under accelerated environmental conditions.
Performance Drift
Pressure changes inside the sealed enclosure shift the resonant frequency or damping characteristics of the internal sensing elements. Wafer level vacuum packaging introduces a calibration offset since the getter material inside the cavity must actively neutralize outgassing contaminants to maintain the initial pressure specification. Any variation in the getter capacity or the seal conductance directly impacts the long term stability of the measurement output.
Measurement Variance
Absolute pressure verification relies on the correlation between the output signal and the thermal conduction across the cavity gap. Wafer level vacuum packaging limits the convective heat transfer between the sensor and the package walls which forces designers to account for conductive losses through the support structures. A higher base pressure increases the noise floor of inertial sensors by augmenting the air damping on the moving parts.
