MEMS Sensor Contact Detection for Electrostatic Stiction Control
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Solution Overview
Problem
MEMS sensors, particularly accelerometers, suffer from stiction phenomena due to adhesion forces between the movable mass and stoppers, leading to malfunction and reduced sensor stability, with existing solutions either increasing noise or complexity and cost.
Innovation Solution
A MEMS sensor design with a contact sensing structure and control circuit that modifies the voltage difference between the movable mass and stator electrodes to reduce electrostatic force, using a deformable region to prevent stiction by detecting contact and adjusting the electrostatic force, thereby maintaining sensor functionality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the coupling elastic elements are designed to have high stiffness to exert a high elastic return force, then the risk of stiction phenomenon is decreased, but higher sensing noise and lower sensor stability occur
Solution Approach 1:
The patent replaces the purely mechanical high-stiffness spring system with an electrostatic force system. The electrostatic actuator generates an electrostatic force that acts on the movable mass to prevent stiction, substituting the mechanical elastic return force with an electrical field-based force. This allows for softer mechanical springs (reducing noise) while maintaining stiction prevention through controlled electrostatic forces.
2Reliability
If an anti-stiction layer is provided on the stoppers to reduce the adhesion force, then the stiction phenomenon is reduced, but manufacturing complexity and manufacturing costs increase
Solution Approach 1:
The patent replaces the need for anti-stiction coating layers with an electrostatic force-based solution. Instead of modifying the surface properties of the stoppers through additional manufacturing steps, the system uses an electrostatic actuator to generate a force that prevents the movable mass from adhering to the stoppers, thereby eliminating the need for complex anti-stiction layer deposition processes.
3Reliability
If dedicated electrodes are configured to exert an electrostatic force on the movable mass to detach it from the stoppers, then the stiction is reduced, but the movable mass may bump with further stoppers on the opposite side, causing further stiction
Solution Approach 1:
The patent applies preliminary action by continuously maintaining an electrostatic force on the movable mass through the electrostatic actuator, rather than waiting for stiction to occur and then attempting to resolve it. This preventive electrostatic force ensures the movable mass never fully contacts the stoppers, eliminating both primary and secondary stiction issues by preventing contact before it happens.
Solution Approach 2:
The patent incorporates a control circuit that monitors the position of the movable mass and adjusts the electrostatic force accordingly. When the movable mass approaches the stoppers, the control circuit increases the electrostatic force to prevent contact. This feedback mechanism ensures that the electrostatic force is precisely controlled to maintain separation without causing the movable mass to overshoot and contact stoppers on the opposite side.
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
The design enhances sensor robustness against stiction, ensuring high reliability and stability by minimizing adhesion forces, allowing the sensor to return to operational state effectively.
Implementation Method 1
The movable mass is coupled to the substrate through coupling elastic elements... the return force exerted by the coupling elastic elements
Implementation Method 2
In response to the contact between the movable mass and the stoppers, an attractive adhesion force may establish between the movable mass and the stoppers
Implementation Method 3
the sensor may have dedicated electrodes configured to exert an electrostatic force on the movable mass, in the opposite direction to that of the adhesion force
Data Source
AI summary
A MEMS sensor (1) is configured to measure a physical quantity and has a substrate (10) and a movable mass (12) suspended at a distance from the substrate along a direction (Z), wherein the movable mass is coupled to the substrate so as to undergo a movement (M) along a sensing direction (S), with respect to the substrate, as a function of the physical quantity to be measured. The MEMS sensor also has a contact sensing structure (30) coupled to the substrate and which extends, at rest, at a distance (gSW) from the movable mass along the sensing direction; and at least one stator electrode (18A, 18B) coupled to the substrate and configured to form with the movable mass at least one capacitor having a capacitance variable as a function of the movement of the movable mass. A control circuit (5) is configured to: induce a voltage difference between the movable mass and the stator electrode, for sensing a capacitance variation between the movable mass and the stator electrode; sense a contact between movable mass and contact sensing structure; and in response to sensing the contact between movable mass and contact sensing structure, modify the induced voltage difference, so as to reduce an electrostatic force exerted by the at least one stator electrode on the movable mass.