Dry Etching Method Suppressing Notch Formation in Silicon MEMS
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Solution Overview
Problem
Existing dry etching methods for silicon layers in MEMS devices often result in undesired erosion and notch formation at the boundary between the silicon layer and the etching stopper layer, leading to suboptimal performance in devices like optical paths due to positive ion deviation and uneven etching.
Innovation Solution
A dry etching method involving anisotropic etching with a gas mixture containing sulfur hexafluoride, oxygen, and hydrogen bromide, followed by alternating organic film formation and etching processes, where the exposure of the etching stopper layer is detected through etching product monitoring to prevent positive charging and notch formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If continuous isotropic etching with radicals is performed over the entire thickness of the silicon layer, then the etching process is simple and fast, but the recess greatly extends not only in the thicknesswise direction but also in the direction intersecting the thicknesswise direction, resulting in poor anisotropy
Solution Approach 1:
The etching process is segmented into multiple stages: first performing isotropic etching to quickly create initial recesses, then switching to anisotropic etching to precisely control the shape and extend the recess in the thicknesswise direction. This segmentation allows each process to optimize its function without compromising the other.
Solution Approach 2:
The patent employs periodic alternation between different etching modes (isotropic and anisotropic) and repeated cycles of etching followed by protective film formation. This periodic action enables controlled progression of the recess while maintaining desired shape characteristics at each stage.
2Manufacturing precision
If the etching step and protective film forming step are alternately repeated to form a through hole, then the side surface is protected from erosion, but the bottom surface is further etched due to faster protective film removal, causing positive ion deviation and notch formation at the boundary
Solution Approach 1:
The patent applies different etching conditions to different regions: using protective films selectively on side surfaces while allowing controlled etching at the bottom surface. The protective film formation and removal rates are locally optimized to prevent erosion on sides while managing bottom surface etching to avoid notch formation.
Solution Approach 2:
The patent changes etching parameters (gas composition, power, pressure) between different etching steps to control the relative removal rates of protective films at different locations. By adjusting these parameters, the process prevents excessive bottom surface etching that would cause positive ion deviation and notch formation.
3Productivity
If the silicon oxide layer is exposed to positive ions during final etching step, then the etching process continues, but the positive ions cause positive charging of the silicon oxide layer, leading to deviation of advancing direction toward the side surface and increased erosion
Solution Approach 1:
The patent performs preliminary protective film formation on the silicon oxide layer before the final etching step where positive ions would cause harmful effects. This preliminary action prevents positive charging and subsequent ion deviation, allowing the through hole to be completed without excessive erosion.
Solution Approach 2:
The protective film acts as an intermediary layer between the positive ions and the silicon oxide layer during the final etching step. This intermediary prevents direct interaction that would cause positive charging and ion deviation, while still allowing the etching process to proceed to complete the through hole.
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 effectively suppresses notch generation and ensures precise, anisotropic etching of silicon layers, maintaining the desired shape and functionality of MEMS devices by controlling the etching process and preventing positive ion-induced erosion.
Implementation Method 1
plasma using sulfur hexafluoride (SF6) gas, that is, an etchant 54 containing fluorine radicals (F*) and various types of positive ions is generated in a vacuum container
Implementation Method 2
the fluorine radicals contacting the surface of the substrate S advances the etching reaction in the etched region 52a
Implementation Method 3
the positive ions drawn into the substrate S by bias voltage applied to the substrate S advance the etching reaction
Implementation Method 4
hydrocarbon trifuloride (CHF3) gas 56 is sent into the vacuum container to form a protective film 57 of polytetrafluoroethylene ((C2F2)n) over the entire surface of the substrate S
Implementation Method 5
form a protective film 57 of polytetrafluoroethylene ((C2F2)n) over the entire surface of the substrate S
Implementation Method 6
a gas mixture, which includes an oxidation gas that oxidizes the silicon layer and a fluorine containing gas
Data Source
AI summary
A dry etching method includes a first step and a second step. The first step includes generating a first plasma from a gas mixture, which includes an oxidation gas and a fluorine containing gas, and performing anisotropic etching with the first plasma on a silicon layer to form a recess in the silicon layer. The second step includes alternately repeating an organic film forming process whereby an organic film is deposited on the inner surface of the recess with a second plasma, and an etching process whereby the recess covered with the organic film is anisotropically etched with the first plasma. When an etching stopper layer is exposed from a part of the bottom surface of the recess formed in the first step, the first step is switched to the second step.