MEMS Moveable Feature Release via Dry Etch and Cavity Formation
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Solution Overview
Problem
The existing methods for forming moveable features in microelectromechanical systems (MEMS) on composite substrates face issues with stiction between the released feature and the handle wafer due to liquid etchant meniscus forces, which complicates the manufacturing process and limits design flexibility and material choices for additional structures.
Innovation Solution
The method involves forming release holes in the device layer and etching cavities beneath them in the sacrificial layer, allowing for dry etching of the moveable feature outline, which avoids stiction and enables the formation of additional structures without exposing them to the liquid etchant, thereby enhancing design latitude and process flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If liquid etchant is used to remove the sacrificial layer, then the moveable feature is released from the substrate, but stiction occurs between the released feature and handle wafer due to meniscus forces
Solution Approach 1:
The patent applies preliminary action by forming cavities in the sacrificial layer before defining the moveable feature. This allows the feature to be released during or after definition without exposure to liquid etchant, preventing stiction while ensuring effective release. The cavities are prepared in advance to receive the moveable feature in a released state.
Solution Approach 2:
The patent extracts the harmful liquid etching step from the feature definition process. By pre-forming cavities and using dry etching to define the feature outline, the liquid etchant is removed from the process entirely, eliminating the source of meniscus forces and stiction while maintaining release effectiveness.
2Adaptability or versatility
If additional structures are deposited on the moveable feature, then device functionality is enhanced, but the structures must have high selectivity to liquid etchant to survive the release etch process
Solution Approach 1:
The patent removes the liquid etching step from the fabrication process by using dry etching to define the moveable feature outline after cavities are pre-formed. This extraction eliminates the selectivity constraint on additional structures, allowing use of any material without requiring high etch selectivity, while simplifying the manufacturing process.
Solution Approach 2:
The patent performs preliminary cavity formation before depositing additional structures on the moveable feature. This sequence allows additional structures to be added without needing to survive liquid etching, expanding material choices while reducing fabrication complexity since no special etch selectivity requirements need to be met.
3Loss of time
If the moveable feature is made small or many release holes are incorporated, then release etch time is minimized, but the attack on previously patterned additional structures increases
Solution Approach 1:
The patent extracts the liquid etching step entirely and replaces it with dry etching for feature definition. This eliminates the trade-off between release time and structure attack, as dry etching provides precise control and stops at defined boundaries without the aggressive lateral attack characteristic of liquid etchants.
Solution Approach 2:
The patent pre-forms cavities in the sacrificial layer before defining the moveable feature. This preliminary action allows the feature to be released during the definition process itself, eliminating the need for separate release etching steps and avoiding exposure of additional structures to aggressive etchants, thereby preserving structure integrity while maintaining efficient release.
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 approach prevents stiction issues and allows for the use of a wider range of materials and longer etch times, reducing complexity and cost while ensuring the moveable features are freely suspended without additional release steps.
Implementation Method 1
The feature is then released from the underlying oxide layer by etching the oxide layer with, for example, a hydrofluoric (HF) acid etch.
Implementation Method 2
Upon drying, the residual liquid may have substantial meniscus forces, such that the feature remains adhered to the handle wafer even after the liquid etchant has been substantially removed
Data Source
AI summary
A method for forming moveable features suspended over a substrate is described, wherein a cavity beneath the moveable feature is first formed using a liquid etchant applied through one or more release holes. After formation of the cavity, the outline of the moveable feature is formed using a dry etch process. Since the moveable feature is free to move upon its formation using the dry etch process, no stiction issues arise using the systems and methods described here.