Structural Deformation
The physical displacement of a thin silicon or glass membrane over an evacuated chamber occurs when external atmospheric pressure exceeds the pressure inside. Monitoring this vacuum cavity deflection provides a direct method for evaluating the pressure differential and the hermetic sealing quality of microelectronic packages. The magnitude of this displacement depends on the thickness of the membrane and the mechanical properties of the material.
Thick membranes show minimal bending and are less sensitive to small pressure changes, while thin structures deflect significantly but are more susceptible to mechanical failure. This trade-off dictates the optimal membrane geometry for different sensor applications.
Deflection Measurement
Optical profilers measure the surface contour of the sensor cap to determine the maximum displacement at the center of the membrane. A perfectly sealed cavity maintains a consistent concave deflection, which demonstrates that the interior vacuum has not been compromised by leaks. This measurement technique is non-destructive and can be applied at the wafer scale to evaluate multiple cavities simultaneously.
Stress Effect
High mechanical deflection generates localized stresses at the edges of the silicon or glass membrane. These structural stresses can induce drift in the active sensor elements if they are positioned too close to the anchored borders of the cap. Engineers must balance the thickness of the cap to ensure sufficient deflection for leak testing while avoiding excessive mechanical stress.
Leak Detection
Slow pressure leaks within the cavity cause a gradual reduction in membrane displacement as the pressure differential approaches zero. Regular measurements of the displacement over a storage period allow the calculation of the leak rate and the estimation of the sensor lifetime. This testing ensures that only packages with high hermetic integrity are selected for final device assembly.
