MEMS Proof Mass Patterning to Reduce Stiction
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Solution Overview
Problem
MEMS sensors, such as accelerometers, face the challenge of stiction forces between the proof mass and bump stops, which can prevent the proof mass from returning to its original position after extreme forces are applied, potentially leading to damage.
Innovation Solution
The solution involves patterning the surface of the proof mass to reduce the contact area with the bump stop, thereby minimizing stiction forces. This is achieved by creating patterns such as through-holes, partial through-holes, or pillars on the surface of the proof mass during the fabrication process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the bump stop area is reduced to minimize stiction force, then the adhesion force between proof mass and bump stop is reduced, but the bump stop becomes prone to chipping or damage
Solution Approach 1:
The bump stop is segmented into a first portion and a second portion with different material properties. The first portion (contacting the proof mass) is made of a softer material to reduce stiction and prevent chipping, while the second portion is made of a harder material to provide structural support and protection against damage.
Solution Approach 2:
Different regions of the bump stop are assigned different material qualities. The region in contact with the proof mass uses a softer material (e.g., polymeric material) to minimize adhesion forces, while other regions use a harder material (e.g., silicon) to maintain structural integrity and resist mechanical damage.
2Reliability
If the contact area between proof mass and bump stop is reduced, then stiction force is minimized, but the bump stop structure becomes more vulnerable to damage
Solution Approach 1:
The bump stop is divided into functional segments: a first portion optimized for minimal contact (softer material, smaller effective contact area) to reduce stiction, and a second portion optimized for structural strength (harder material, larger support area) to prevent damage.
Solution Approach 2:
The bump stop is constructed as a composite structure combining materials with different properties - typically a softer polymeric material for the contact portion and a harder silicon or ceramic material for the support portion, creating a multi-material component that simultaneously reduces stiction and prevents damage.
Data Source
AI summary
A proof mass of a MEMS sensor is located above one or more bump stops that extend in the direction of the proof mass from a base substrate, and that are intended to prevent high-impact collisions between the proof mass and base substrate such as when the sensor is dropped or experiences other substantial external forces. A portion of the proof mass located above the bump stop is patterned at the same time that the functional features of the MEMS layer such as springs and masses are fabricated. The patterning reduces stiction between the proof mass and the bump stop, allowing the MEMS sensor to resume operation promptly after an event that results in contact between the proof mass and the bump stop.