Silicon Substrate Thermal Diffusion Gradients and MEMS Stress Variations
Substrate thermal gradients induce anisotropic strain fields that degrade MEMS accuracy through piezoresistive offset shifts and capacitive gap distortion.
Parasitic Substrate Strain
Stress Coupling
Mechanical deformation of the underlying silicon wafer or circuit board transmits unwanted forces to the micro-electromechanical structures anchored to its surface. In high-precision inertial sensors, parasitic substrate strain changes the shape of the resonant beams, resulting in frequency shifts and bias drift. This strain originates from external sources such as mounting screw torque, solder joint shrinkage and PCB thermal expansion.
The phenomenon is decoupled when the sensing element is mounted on a compliant suspension system.
Sensing Deviation
Mounting the sensor package onto a system board involves temperature profiles that can deform the sensor substrate. This deformation results in parasitic substrate strain that warps the active MEMS structures. The warping alters the capacitive gaps in the sensor, which leads to an erroneous output signal.
This drift is particularly problematic in applications with wide temperature swings.
Structural Minimization
Engineers can reduce this strain by using specialized package substrates, such as ceramic leadless chip carriers. These packages have a coefficient of thermal expansion that closely matches that of the silicon sensor die. This match minimizes the parasitic substrate strain that is generated during thermal cycling.
The package layout is optimized to provide maximum isolation to the active die.
Validation Assessment
Evaluating the sensor’s sensitivity to external strain involves applying mechanical bend tests to the evaluation board. Sensors with high strain sensitivity are redesigned to improve their mechanical decoupling. This testing is crucial for sensors deployed in harsh industrial environments.