Leak Diagnosis
Measurement of hermetic package integrity involves monitoring the microscopic deflection of a silicon or glass cap over a sealed vacuum cavity. When executing an optical interferometric leak test, the system tracks the changing interference patterns of laser light reflected from the cap surface. This method identifies the entry of atmospheric gas by measuring the gradual swelling of the thin cap as internal pressure rises.
The high resolution of the laser measurements allows the detection of extremely small leak rates that would escape standard liquid bubble tests. This technique is particularly useful for assessing wafer-level hermetic seals.
Measurement Mechanism
Laser light split into two paths creates a phase-dependent fringe pattern that reflects the physical position of the membrane. As gas leaks into the cavity and reduces the pressure differential, the membrane relaxes and shifts the reflection angle of the laser beam. This optical technique detects sub-nanometer displacement changes without making physical contact with the delicate microstructures.
Sensitivity Boundary
The rate of pressure rise that can be detected depends on the testing duration and the thickness of the sensor cap. Thick caps deflect very little under pressure, which requires longer monitoring periods to resolve slow leaks. To overcome this limitation, tests are often performed under high-pressure helium environments to accelerate the gas entry through any structural defects.
Production Screening
Screening of completed sensor wafers utilizes automated optical platforms to scan hundreds of individual cavities sequentially. This automated inspection identifies failed packages before the wafer is diced into separate chips, saving time and assembly costs. The scanned data establishes a precise leakage rate for each sensor to guarantee the long-term reliability of the shipped products.
