Packaging Architecture
Semiconductor fabrication technology integrates protective caps over micro-machined device wafers before slicing individual sensor dies. Implementing wafer-level vacuum encapsulation seals MEMS resonators and capacitive accelerometers inside hermetic vacuum cavities at the wafer stage. The process protects delicate moving structures from mechanical damage and environmental contamination during downstream assembly.
Seal integrity boundaries require maintaining internal cavity pressure below sensor-specific threshold levels throughout product lifespans. Encapsulated wafers proceed directly to standard dice-and-attach packaging lines without requiring cleanroom handling.
Hermetic Sealing
Silicon or glass cap wafers bond to device wafers using eutectic or direct fusion bonding processes. High temperature bonding cycles require careful thermal expansion matching to prevent residual thermal stress in sensing structures. Micro-machined getters deposited inside cap cavities absorb outgassed hydrogen and water vapor during bonding steps.
Hermetic seals block moisture ingress that would alter internal cavity pressure or cause stiction in moving parts.
Cavity Pressure
Internal vacuum quality dictates mechanical damping levels for high Q-factor resonant MEMS sensors. Lower cavity pressures reduce viscous air damping, maximizing quality factors and frequency sensitivity. Outgassing from cavity walls slowly elevates pressure over time, damping structural resonance and shifting calibration baselines.
Quality control monitors resonant frequency shifts across thermal aging cycles to detect vacuum degradation.
Industrial Verification
Zero-leak rate testing employs specialized fine-leak detection routines to confirm seal hermeticity. Resonant frequency and quality factor measurements on test structures serve as non-destructive indicators of internal cavity pressure. High volume automated wafer probers test encapsulated devices before dicing, discarding defective dies early in production.
Quality assurance standards require accelerated thermal aging tests to guarantee vacuum stability over ten year field lifetimes.
