Silicon Substrate Thermal Expansion Mechanics in Inertial Sensors
Substrate thermal expansion creates packaging shear stress that warps MEMS proof masses, demanding central single-anchor isolation and polynomial offset calibration.
Mechanical Isolation Flexure
Structural Decoupling
Flexible mounting elements designed to protect sensitive components from external strain ensure that the primary measurement remains unaffected by the deformation of the housing. A mechanical isolation flexure acts as a buffer that absorbs the stresses caused by the mounting of the sensor or the thermal expansion of the surrounding frame. By providing a compliant path between the sensing element and the rigid body of the instrument, these structures prevent the transfer of unwanted forces.
This is particularly important for high precision MEMS devices where even a few nanometers of unintended movement can cause a significant shift in the output. Designers carefully calculate the geometry of these parts to maximize the isolation while maintaining the structural integrity of the assembly.
Strain Mitigation
Attachment of a sensor to a surface with a different coefficient of thermal expansion can lead to large mechanical stresses as the temperature changes. Using a mechanical isolation flexure allows the two components to expand and contract independently, reducing the risk of fracture or calibration drift. The flexure is typically shaped as a thin beam or a folded spring that has low stiffness in the directions of the expected strain.
This compliance ensures that the energy of the expansion is dissipated in the flexure rather than being transmitted to the active sensing area. In pressure transducers, this mechanism prevents the mounting torque from altering the zero point of the bridge circuit. Engineers use finite element modeling to optimize the shape and thickness of these features.
Resonance Control
Vibrational energy from the environment can excite the natural frequencies of the internal components and lead to measurement noise or mechanical failure. A mechanical isolation flexure can be tuned to act as a mechanical low pass filter, attenuating high frequency vibrations before they reach the sensor. The mass of the isolated part and the stiffness of the flexure determine the cutoff frequency of this filter.
If the frequency of the external vibration matches the resonance of the system, the displacement can become large enough to damage the device. Damping materials are sometimes added to the flexure to reduce the amplitude of the resonance peak. Maintaining a stable mechanical environment is a requirement for the operation of high performance inertial sensors and optical components.
Mounting Accuracy
Alignment of the sensing element relative to the external package must be maintained even when the isolation features are present. While a mechanical isolation flexure must be compliant to absorb strain, it must also be stiff enough to keep the sensor in its intended orientation. This trade off is managed by designing the flexure to be soft in the plane of the expansion but rigid in the other axes.
Silicon is often used for these features because of its excellent elastic properties and the ability to be etched into complex shapes with high precision. After the assembly is complete, the effectiveness of the isolation is verified by subjecting the unit to thermal cycling and mechanical shock. A successful design shows minimal change in the sensor output despite the presence of significant external loads.