MEMS Proof Mass Structure for Consistent Acceleration Sensing
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Solution Overview
Problem
Process variations in the isotropic etch process lead to variations in the mass of the proof mass element in MEMS devices, resulting in inaccuracies in acceleration measurements.
Innovation Solution
A semiconductor matrix material layer is patterned to form a recess cavity with a diffusion barrier spacer, followed by oxidation to create semiconductor oxide plates, and comb trenches are formed to define a cavity for a movable structure, which is then etched to form a precise proof mass with uniform thickness, bonded to a handle substrate, and integrated with comb structures for capacitive sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If an isotropic etch process is used to form the proof mass element, then the manufacturing process is simple and efficient, but process variations lead to mass variations that reduce measurement precision
Solution Approach 1:
The patent applies preliminary action by forming a sacrificial layer and defining a cavity structure before etching the proof mass element. This preliminary structuring constrains the etch process to remove material only from intended areas, preventing over-etching and ensuring consistent proof mass dimensions despite variations in etch process parameters. The cavity structure acts as a physical boundary that limits the etch depth and lateral spread.
Solution Approach 2:
The patent changes the physical and chemical parameters of the etch process by using a two-stage approach: first forming the cavity with specific etch conditions, then forming the proof mass element with controlled etch parameters. By carefully controlling etch time, temperature, and chemistry in each stage, the process achieves consistent proof mass dimensions. The use of different etch selectivities for different material layers further enables precise dimensional control.
2Measurement precision
If the proof mass element mass is increased to improve sensing capability, then the sensitivity of acceleration detection is enhanced, but the mass variations from etch process inconsistencies become more significant
Solution Approach 1:
The patent applies local quality by creating a defined cavity structure with specific geometric characteristics that locally constrain the proof mass element formation. The cavity acts as a localized template that ensures consistent mass distribution and dimensions. By controlling the cavity geometry (depth, lateral dimensions, shape), the process achieves uniform proof mass properties even when overall device size increases for enhanced sensitivity.
Solution Approach 2:
The patent uses the cavity structure and sacrificial layer as intermediary elements that mediate between the etch process and the final proof mass element. These intermediaries buffer the effects of etch process variations, allowing the formation of consistent proof mass elements. The sacrificial layer serves as a temporary intermediary that defines the proof mass boundaries during fabrication and is subsequently removed, leaving precisely defined proof mass structures.
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 method ensures a high precision proof mass with reduced mass variations, enhancing the accuracy of acceleration measurements in MEMS devices.
Implementation Method 1
oxidation to create semiconductor oxide plates
Data Source
AI summary
A semiconductor oxide plate is formed on a recessed surface in a semiconductor matrix material layer. Comb structures are formed in the semiconductor matrix material layer. The comb structures include a pair of inner comb structures spaced apart by a first semiconductor portion. A second semiconductor portion that laterally surrounds the first semiconductor portion is removed selective to the comb structures using an isotropic etch process. The first semiconductor portion is protected from an etchant of the isotropic etch process by the semiconductor oxide plate, the pair of inner comb structures, and a patterned etch mask layer that covers the comb structures. A movable structure for a MEMS device is formed, which includes a combination of the first portion of the semiconductor matrix material layer and the pair of inner comb structures.