MEMS Comb Structure High Aspect Ratio Fabrication
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Solution Overview
Problem
The Bosch process for deep reactive ion etching in semiconductor processing limits the aspect ratio of comb structures in MEMS devices due to lateral undercutting and etch rate lag, restricting the sensitivity and planarity of comb structures.
Innovation Solution
A method involving patterning a substrate with photoresist, forming comb structures with conductive fingers, depositing atomic layers on sidewalls using atomic layer deposition, and attaching insulator layers to maintain sidewall planarity and increase aspect ratios without deterioration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If deep reactive ion etching (DRIE) using Bosch process is used to form comb structures, then the etching depth is increased, but lateral undercutting occurs and aspect ratio is limited
Solution Approach 1:
A sacrificial oxide layer is introduced as an intermediary between the silicon substrate and the etching process. The oxide layer is selectively removed through chemical etching (HF vapor or liquid HF), allowing the subsequent DRIE process to etch vertically without lateral undercutting of the underlying silicon fingers. This mediator enables achieving high aspect ratios by decoupling the etching depth control from lateral dimension control.
2Productivity
If Bosch process is used for DRIE, then material removal is enhanced, but etch rate lag phenomenon occurs where etch rate is inversely proportional to aspect ratio
Solution Approach 1:
The patent replaces the mechanical/physical limitation of the Bosch process (ion bombardment-based etching that suffers from aspect ratio lag) with a chemical etching mechanism. By using HF-based chemical etching to remove the sacrificial oxide layer, the process achieves uniform removal rates independent of aspect ratio, eliminating the etch rate lag phenomenon while maintaining high material removal efficiency.
3Measurement precision
If aspect ratio is increased to enhance sensitivity, then inter-finger capacitance is increased, but sidewall planarity deteriorates with Bosch process
Solution Approach 1:
The sacrificial oxide layer serves as a planarity-preserving intermediary during the etching process. Its selective chemical removal creates clean, planar sidewalls on the silicon fingers without the lateral undercutting that plagues direct DRIE methods. This enables achieving high aspect ratios while maintaining excellent sidewall planarity, which is critical for sensitive MEMS operation.
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 enables the creation of comb structures with higher aspect ratios, enhancing the sensitivity and operational performance of MEMS devices like gyroscopes by allowing lower voltage operation and reduced noise metrics.
Implementation Method 1
patterning a substrate, including a conductive region, with photoresist exposed by lithography
Implementation Method 2
forming, one atomic layer at a time, at least one atomic layer of at least one conductor over at least one sidewall of each conductive finger
Data Source
AI summary
A method comprises: patterning a substrate, including a conductive region, with photoresist exposed by lithography, where the substrate is mounted on a handle substrate; forming a comb structure with conductive fingers on the substrate by at least removing a portion of the conductive region of the substrate; removing the photoresist; forming, one atomic layer at a time, at least one atomic layer of at least one conductor over at least one sidewall of each conductive finger; attaching at least one insulator layer to the comb structure, and the substrate from which the comb structure is formed; and removing the handle substrate.