XeF2 Dry Etching for GaN Micromechanical Structures
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Solution Overview
Problem
Current methods for fabricating micromechanical structures, particularly those using GaN and ZnO, face challenges with stiction due to meniscus forces during wet chemical etching, leading to collapsed microstructures, and require complex processes for dry releasing techniques.
Innovation Solution
A dry etching process using Xenon Difluoride (XeF2) is employed to form an undercut recess in a silicon-based substrate, allowing for the creation of freestanding micromechanical elements without the need for additional sacrificial layers, thereby avoiding stiction issues and simplifying the fabrication process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If wet chemical etching is used to release micromechanical structures, then the structures can be freed from the substrate, but stiction due to meniscus forces causes the microstructures to collapse
Solution Approach 1:
The patent extracts the harmful meniscus forces by replacing wet chemical etching with vapor-phase HF etching. The vapor-phase process eliminates liquid contact with the released structures, thereby removing the source of stiction while still achieving effective etching of the sacrificial silicon layer to free the micromechanical structures
Solution Approach 2:
The patent substitutes the mechanical/chemical wet etching process with a vapor-phase chemical etching process. This replacement changes the phase of the etchant from liquid to vapor, eliminating capillary forces and meniscus effects that cause stiction, while maintaining the chemical etching mechanism needed to remove the sacrificial layer
2Reliability
If dry releasing techniques are used to avoid stiction, then stiction issues are avoided, but the fabrication process becomes complex
Solution Approach 1:
The patent merges the release etching step with the existing fabrication process by using vapor-phase HF etching that can be performed in the same cleanroom environment with standard equipment. The process integrates seamlessly with the deposition and patterning steps already present, avoiding the need for separate complex dry release techniques while still eliminating stiction
Solution Approach 2:
The patent uses vapor-phase HF as an intermediary that provides the benefits of dry etching (no stiction) while maintaining compatibility with standard wet processing infrastructure. The vapor phase acts as a mediator between liquid wet etching and solid-state dry etching, combining advantages of both approaches
3Adaptability or versatility
If GaN films are grown on Si substrates using MOCVD, then integration with silicon technology is enabled, but good quality GaN films cannot be obtained due to large lattice and thermal mismatch
Solution Approach 1:
The patent introduces an AlN buffer layer as an intermediary between the Si substrate and the GaN epilayer. This buffer layer acts as a transition layer that reduces the lattice mismatch and thermal expansion difference between Si and GaN, enabling high-quality GaN growth while maintaining compatibility with silicon technology
Solution Approach 2:
The patent changes the compositional parameter by using an AlN buffer layer with intermediate lattice constant between Si and GaN. This gradual transition in lattice parameters reduces dislocation density and improves film quality, while the thermal expansion parameters are also optimized through the buffer layer
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 XeF2 dry etching technique effectively forms airgaps under wide bandgap materials, reducing residual stress and enabling the production of stable, freestanding micromechanical structures with controlled airgap thickness, improving the mechanical stability and integration of GaN, ZnO, and diamond-based microstructures.
Implementation Method 1
A dry etching process using Xenon Difluoride (XeF2) is employed to form an undercut recess in a silicon-based substrate
Implementation Method 2
The undercut is in the form of a recess formed in the Si based substrate... reducing residual stress
Data Source
AI summary
A micromechanical structure and a method of fabricating a micromechanical structure are provided. The micromechanical structure comprises a silicon (Si) based substrate; a micromechanical element formed directly on the substrate; and an undercut formed underneath a released portion of the micromechanical element; wherein the undercut is in the form of a recess formed in the Si based substrate.