MEMS Micro Mirror Vertical Combdrive Actuation
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Solution Overview
Problem
MEMS micro mirrors with electrostatic actuation face challenges such as optical component alignment, hermetical packaging difficulties, limited controllable tilting angle range, non-linear actuation, electrical charging, and high actuation voltage requirements, along with complex and costly fabrication processes for both vertical combdrive and parallel plate actuators.
Innovation Solution
A micro-electro-mechanical system (MEMS) micro mirror design utilizing a vertical combdrive actuator with different upper and lower comb finger designs to achieve direction control, reducing actuation voltage and eliminating tilting angle drifting and air damping, fabricated using Silicon On Insulator (SOI) wafers with photolithography and partial silicon etching, and bonding to a carrier wafer to reduce air damping and enhance mechanical stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If vertical combdrive actuator with same layer material is used for taller and shorter fingers, then fabrication is simplified, but initial overlapping areas create opposite electrical field contributions that affect actuation
Solution Approach 1:
The patent applies different materials to different parts of the combdrive structure. Specifically, the taller fingers are made of a first material while the shorter fingers are made of a second material, allowing each region to have optimized properties for its specific function while eliminating the overlapping field problem
2Volume of moving object
If micro mirror thickness is reduced below 20 microns, then device size is reduced, but radius of curvature increases and mechanical strength decreases
Solution Approach 1:
The patent uses composite material structures where the micro mirror is supported by a combination of materials with different mechanical properties. The support structure includes elements made of materials optimized for both mechanical strength and stress management, allowing thin mirror designs to maintain adequate strength and flatness
3Strength
If 20 microns of thick material is used for taller and shorter fingers, then mechanical strength is maintained, but photolithography quality deteriorates due to higher topography
Solution Approach 1:
The patent segments the combdrive structure into taller and shorter fingers made of different materials, allowing each segment to have optimized thickness. This segmentation enables the shorter fingers to have reduced thickness for better photolithography while the taller fingers maintain sufficient thickness for mechanical support
4Device complexity
If single metal coating is used for both reflective film and bonding pads, then fabrication steps are reduced, but either reflectivity or wiring bonding quality deteriorates
Solution Approach 1:
The patent applies different metal coatings to different locations: a first metal coating with optimized properties for optical reflectivity is applied to the mirror surface, while a second metal coating with optimized properties for electrical conductivity and wire bonding is applied to the bonding pads
5Ease of manufacture
If wet structure release processing is used, then device release is achieved, but stiction occurs causing defective devices
Solution Approach 1:
The patent replaces the wet chemical release process with a mechanical or thermal release mechanism. The structure is designed to be released through controlled mechanical means or thermal expansion/contraction, avoiding the stiction problems associated with wet chemical processing
6Ease of manufacture
If parallel plate electrostatic actuator is used, then fabrication is simplified, but pull-in effect limits controllable tilting angle range to one third of gap
Solution Approach 1:
The patent employs a combdrive actuator mechanism where the electrostatic force is distributed along the length of the comb fingers. This dynamic structure allows for larger controllable tilting angles by progressively engaging different portions of the comb fingers, extending the linear actuation range beyond the limitations of parallel plate actuators
7Adaptability or versatility
If higher actuation voltage is applied to increase tilting angle range, then controllable range is expanded, but electrical charging and tilting angle drifting occur
Solution Approach 1:
The patent optimizes the combdrive geometry parameters including finger length, width, spacing, and number of fingers to achieve the desired tilting angle range at reduced actuation voltages. By carefully controlling these geometric parameters, the system achieves larger angular ranges without the electrical charging and drifting problems associated with high voltage operation
8Adaptability or versatility
If increased gap between fixed electrode and mirror is used, then larger tilting angle is achieved, but actuation voltage increases causing electrical charging
Solution Approach 1:
The patent transitions from a planar parallel plate geometry to a three-dimensional combdrive structure. This dimensional change allows the electrostatic force to be distributed along the vertical extent of the comb fingers, achieving larger tilting moments without requiring proportionally higher voltages, thus avoiding electrical charging effects
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 achieves higher actuation force density, better linearity, and reduced actuation voltage, while eliminating pull-in effects and air damping, resulting in improved performance and stability of the micro mirror with reduced fabrication complexity and cost.
Implementation Method 1
These micro mirrors use electrostatic actuation. The electrostatic actuation is favored for micro mirror due to its low power consumption and relative small footprint.
Data Source
AI summary
A micro-electro-mechanical system (MEMS) micro mirror and a method of making the same. The micro mirror includes a body having a mirror support, opposed anchor s and flexible hinges which connect the mirror support to the anchor s. The mirror support has opposed comb edges with comb fingers. Electrodes, which have comb fingers to interact with the comb fingers of the mirror support, are spaced from the comb edges. The comb fingers along each of the comb edges of the mirror support surface are positioned on different horizontal planes from and the comb fingers on the electrodes so as to maximize electrostatic actuation.