MEMS Stopper Structure for Z-Axis Shock Protection
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Solution Overview
Problem
Microelectromechanical devices, such as z-axis MEMS devices, face issues with undesirable displacements along the z-axis due to external shocks, leading to potential damage or breaking of suspension elements, and electrostatic interactions between the mobile mass and stopper structures cause malfunctioning, including zero-level displacement and non-ratiometric output signals.
Innovation Solution
The stopper structure is designed with downward projections from the cap that minimize electrostatic interaction by positioning stopper elements to balance twisting moments and reduce electrostatic forces, ensuring minimal interference with the mobile mass's operation, using a configuration that balances the twisting moments generated on the mobile mass's portions and extends beyond the mobile mass's plane to reduce misalignment effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If stopper structures are provided as downward projections from the package cap to limit mobile mass movement, then the risk of damage from shock is reduced, but electrostatic interaction between the stopper elements and mobile mass causes malfunctioning including zero-level displacement
Solution Approach 1:
The harmful electrostatic interaction is extracted and isolated by introducing a third element (insulating layer or conductive connection) that separates the stopper elements from the mobile mass, allowing the stopper structure to be removed from close proximity to the mobile mass while maintaining its protective function
Solution Approach 2:
An insulating layer or conductive connection is introduced as an intermediary between the stopper elements and the mobile mass. This intermediary element allows the stopper structure to limit movement while preventing direct electrostatic interaction, thereby maintaining both protection and operational correctness
2Length of moving object
If stopper elements are positioned close to the mobile mass to effectively limit movement, then the range of motion is restricted, but electrostatic forces cause undesirable displacements and non-ratiometric output
Solution Approach 1:
The electrostatic field source is extracted from the stopper structure by introducing an insulating layer or conductive connection, allowing the stopper elements to be positioned close to the mobile mass for effective movement limitation while eliminating the source of electrostatic interference with measurement precision
Solution Approach 2:
An insulating layer or conductive connection serves as an intermediary that allows close positioning of stopper elements for movement control while preventing electrostatic interference with the mobile mass, thereby maintaining measurement precision and output signal ratiometricity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively limits undesirable movements, reduces the risk of damage, and minimizes electrostatic interactions, resulting in a ratiometric and robust z-axis microelectromechanical device that is insensitive to electrostatic charges and misalignments, ensuring reliable operation.
Implementation Method 1
projections directed towards the mobile mass extend from an internal face of the package facing a top face of the mobile mass. These projecting portions of the package reduce the range of movement along the axis z for the mobile mass; in particular, the mobile mass stops, abutting against the projections, prior to onset of damage.
Implementation Method 2
if the mobile mass and package are kept at different potentials, an electrostatic interaction of a non-negligible amount is generated. This interaction can cause undesirable displacements of the mobile mass and in general malfunctioning of the microelectromechanical device
Data Source
Figure 1a~2b
Figure 3a~4b
Figure 5a~6a
AI summary
In a microelectromechanical device (1), a mobile mass (2) is suspended above a substrate (3) via elastic suspension elements (5), and is rotatable about said elastic suspension elements (5), a cover structure (10) is set above the mobile mass (2) and has an internal surface (10a) facing the mobile mass (2), and a stopper structure (12, 14) is arranged at the internal surface (10a) of the cover structure (10) and extends towards the mobile mass (2) in order to stop a movement of the mobile mass (10) away from the substrate (3) along an axis (z) transverse to the substrate (3). The stopper structure (12, 14) is arranged with respect to the mobile mass (2) so as to reduce an effect of reciprocal electrostatic interaction, in particular so as to minimize a resultant twisting moment of the mobile mass (2) about the elastic suspension elements (5).