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.
Single-Anchor Mounting
Attachment Strategy
Mechanical designs that secure a sensitive component to its housing at only one point provide a way to minimize the transfer of external strain and thermal stress. Using a single-anchor mounting ensures that the sensor can expand and contract freely in all directions relative to the support structure. This approach is widely used in high precision MEMS and optical systems to prevent the deformation of the housing from affecting the alignment of the internal elements.
By decoupling the active area from the mounting frame, the design reduces the risk of calibration drift caused by the mechanical torque of the installation. The single point of contact acts as a pivot that allows for the relief of internal tension.
Thermal Expansion
Mismatch between the coefficients of expansion of the sensor and the package can lead to large forces if the device is constrained at multiple points. In a single-anchor mounting configuration, the sensor is allowed to change its dimensions without being subjected to compressive or tensile loads. This freedom is essential for maintaining the accuracy of pressure and force sensors across a wide temperature range.
Without this relief, the thermal stress would manifest as an offset or a change in the sensitivity of the measurement. The location of the anchor is often chosen to be at a point of symmetry to further reduce the impact of any residual stress. Engineers must select bonding materials that are stable and have low creep to maintain the position of the anchor over time.
Rotational Stability
Maintenance of the orientation of the component is more challenging when only one support point is used to hold the mass. To ensure the success of a single-anchor mounting, the anchor itself must be rigid enough to resist the forces of vibration and mechanical shock. This is often achieved by using a central pedestal or a wide bond pad that provides a stable base.
If the anchor is too flexible, the sensor may tilt or rotate, leading to errors in the measurement of directional variables like acceleration. High aspect ratio etching is used to create anchors that are stiff in the vertical direction but allow for lateral movement. The center of mass of the isolated component is typically aligned with the anchor to minimize the moment of inertia.
Packaging Constraint
Design of the surrounding enclosure must provide enough space for the isolated component to move without coming into contact with the walls. When implementing a single-anchor mounting, the clearances must be large enough to accommodate the maximum expected thermal expansion and the displacement from mechanical shock. This can lead to a larger package size compared to traditional mounting methods.
Protective features like travel stops are often included to limit the movement during extreme events and prevent the fracture of the anchor. The assembly process requires high precision to ensure that the sensor is correctly positioned within the cavity. Final inspection involves verifying the integrity of the bond and checking for any unintended points of contact.