Deformation Behavior
Reduction of mechanical stress in a material under constant strain over time occurs when the material is subjected to elevated temperatures. In composite sensor structures, thermal stress relaxation occurs as the plastic or metallic layers deform micro-plastically to relieve the stresses generated by thermal expansion mismatches. This behavior can cause zero-point drift in sensors that are subjected to thermal cycles.
Material Selection
Selecting materials with high thermal stability and low viscoelastic behavior minimizes unwanted deformation. When a sensor uses materials that undergo rapid thermal stress relaxation, the output signal becomes unstable after thermal cycling, making accurate measurements difficult. Ceramic and silicon structures are preferred because they exhibit very little relaxation at typical operating temperatures.
Testing Procedure
Measurement of relaxation behavior involves heating the sensor structure to its maximum operating temperature and monitoring the output drift. This test reveals the rate at which stress relaxation occurs in the material stack.
Mitigation Strategy
To prevent drift during operation, sensor assemblies undergo a thermal burn-in or pre-aging process before calibration. This thermal treatment accelerates the initial thermal stress relaxation, allowing the materials to stabilize before the final calibration measurements are made. Wafers and sensors that have undergone this treatment show much higher long-term stability in the field.
