High Suspension
Micro-mechanical spring elements with a height substantially greater than their width define the structural class. High aspect ratio flexures provide a high degree of stiffness in the vertical plane while remaining flexible in the horizontal direction. Geometry is used to minimize cross-axis sensitivity in silicon sensors.
Cross-Axis Sensitivity
Designing a suspension that resists motion in the vertical axis while allowing movement in the horizontal axis requires deep, narrow beams. If the beams are too thin, they may buckle under load or break during the manufacturing process. The ratio of height to width often exceeds ten to one in modern micro-machined designs.
Fabrication Limit
Creating these tall structures requires deep reactive ion etching to achieve vertical sidewalls with minimal taper. Any deviation from a perfect ninety-degree angle changes the stiffness of the spring and affects the sensor calibration. Process control is essential for maintaining uniform performance across the entire wafer.
Energy Dissipation
Damping of the proof mass is influenced by the surface area of the flexures and the viscosity of the surrounding gas. Narrow gaps between the beams can lead to squeeze-film damping, which limits the bandwidth of the device. Engineers model these interactions to ensure the sensor responds correctly to high-frequency vibrations.
Final testing involves measuring the quality factor of the resonator in a vacuum to isolate the mechanical losses from the air resistance. Data is used to refine the design for future iterations of the product. Optimization of the beam profile reduces the accumulation of internal stress at the anchor points.
