MEMS Spring-Mass System with Variable Thickness Torsion Springs
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Solution Overview
Problem
Current methods for producing MEMS structures with piezoelectric actuators face challenges in achieving precise and varying layer thicknesses, particularly for torsion springs and actuators, leading to suboptimal resonance frequency and stability due to limitations in manufacturing processes such as thermal oxidation and etching inaccuracies.
Innovation Solution
A MEMS structure is formed using a stack with a substrate and two semiconductor layers separated by etching stop layers, allowing for varying thicknesses by selectively removing layers, with the use of polysilicon layers and oxide etching stop layers to create different thickness regions for actuators, torsion springs, and a micromirror, enabling precise control over resonance frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the layer thickness of actuators is reduced to enable high deflections, then deflection increases, but resonance frequency and structural stability deteriorate
Solution Approach 1:
The patent applies local quality by creating different layer thicknesses in different regions of the device. The actuator region has reduced thickness for high deflection, while the mirror support region maintains greater thickness for stability. This is achieved through selective etching that removes material only from specific areas, allowing each region to have optimized thickness for its specific function.
2Reliability
If the layer thickness of torsion springs is increased to achieve high resonance frequency, then resonance frequency improves, but material stress and risk of premature failure increase
Solution Approach 1:
The torsion springs are designed with non-uniform thickness through selective etching. The thickness is optimized locally - thicker at the anchor points for strength and thinner in the active deformation zones for higher resonance frequency. This local differentiation allows the spring to achieve high resonance frequency without excessive material stress throughout the entire structure.
3Manufacturing precision
If thermal oxidation is used to limit maximum layer thickness to a few micrometers, then manufacturing precision is improved, but resonance frequency and stability of actuators deteriorate
Solution Approach 1:
The patent uses selective etching to create local thickness variations that allow different regions to have different thicknesses. This enables the actuator region to have sufficient thickness for stability while other regions maintain thin profiles for precision control. The thermal oxidation provides precise thickness control where needed, while selective etching adds thickness where structural stability is required.
4Adaptability or versatility
If conventional etching methods are used to create varying layer thicknesses, then manufacturing flexibility is improved, but etching accuracy and homogeneity deteriorate
Solution Approach 1:
The patent introduces an intermediary layer (such as a sacrificial layer or mask layer) that enables precise thickness control during etching. This intermediary layer protects certain regions from etching while allowing other regions to be etched to the desired thickness. The intermediary layer is then removed, leaving the precisely etched structure. This approach maintains etching homogeneity while achieving the required thickness variations.
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 method allows for the production of MEMS structures with customized layer thicknesses, enhancing resonance frequency and stability by allowing precise adjustment of thicknesses in mechanically active areas, thereby improving the dynamic properties and producibility of the MEMS structure.
Implementation Method 1
the micromirror is driven by actuators which consist of a piezoelectric layer enclosed by two electrodes and an underlying carrier layer and function in the sense of bending beams
Implementation Method 2
A MEMS structure is formed using a stack with a substrate and two semiconductor layers separated by etching stop layers, allowing for varying thicknesses by selectively removing layers
Data Source
Figure 1A
Figure 1B
Figure 2~3A
AI summary
A MEMS structure comprises a stack consisting of a substrate, a first etch stop layer, a first semiconductor layer, a second etch stop layer, and a second semiconductor layer, arranged in that order. The MEMS structure includes a spring-mass system formed within the stack, with at least one spring and an element suspended from the spring. The thickness of the spring-mass system varies by removing, starting from the back side of the substrate opposite the first etch stop layer, the substrate at a first location, leaving the first semiconductor layer intact; and at a second location laterally different from the first location, the substrate, the first etch stop layer, and the first semiconductor layer are removed.The at least one spring varies in thickness to form a stiffening spring in a first section of the spring and a mechanically active area of the spring in a second section of the spring, the first section being thicker than the second section, so that the mechanically active area has a lower stiffness compared to the stiffening spring.