Internal Strain
Mechanical tension remaining in the attachment points of a micro-machined structure after the fabrication process is complete defines the state. Residual anchor strain occurs when the material properties of the sensor die and the substrate do not match. Mechanical stresses can cause the proof mass to tilt or shift.
Bias Instability
Constant pressure on the suspension members leads to an unpredictable offset in the output signal. As the strain relaxes over time, the zero-point of the sensor drifts, which degrades the accuracy of the measurement. High-precision applications require the use of stress-relief features to minimize this effect.
Thermal Hysteresis
Changes in temperature cause the anchor to expand or contract at a different rate than the surrounding package. If the strain does not return to its original state after a thermal cycle, the sensor exhibits hysteresis. Calibration for varying environmental conditions is difficult when the structural state is not repeatable.
Stress Decoupling
Selecting an adhesive with a low youngs modulus helps to decouple the sensor from the external housing. The choice of die-attach material is a critical step in the assembly process to ensure long-term stability. Engineers must balance the need for a secure mount with the requirement for mechanical isolation.
Testing the assembly across the full temperature range confirms that the mounting stress remains within acceptable limits for the intended use. Proper management of the interface between the silicon and the package is necessary for maintaining the integrity of the signal. Characterization of the strain state is performed using micro-raman spectroscopy or optical profilometry.
