Composite Hard Mask Etching for Thick SOI Buried Oxide
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Solution Overview
Problem
Current technologies are unable to etch a thick buried oxide layer on a silicon wafer while protecting the surface of the silicon wafer, which is necessary for a BCD process compatible with SOI high-voltage integrated circuits.
Innovation Solution
A variable selective etching technology is employed, using a composite hard mask with a variable selection ratio, comprising multiple layers etched with different methods to selectively remove top silicon and buried oxide layers without damaging the silicon wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a traditional hard mask with invariable selection ratio is used, then the etching process is simple, but the silicon wafer surface cannot be protected while etching a thick buried oxide layer
Solution Approach 1:
The patent divides the hard mask into a multi-layer composite structure with different materials (oxide layers, silicon nitride layer, polysilicon layer) having different etching rates and properties. This segmentation allows each layer to serve specific functions: protecting the silicon surface while enabling selective etching of the buried oxide layer, thereby resolving the contradiction between etching precision and surface protection.
Solution Approach 2:
The patent employs a composite hard mask consisting of multiple material layers with distinct etching characteristics. The combination of oxide, silicon nitride, and polysilicon layers creates a mask system with variable selection ratio that can selectively remove buried oxide while preserving the silicon wafer surface, thus achieving both high etching precision and surface protection.
2Object-affected harmful factors
If a thick hard mask is used to protect the silicon surface, then the silicon wafer is protected, but the mask stress increases and process complexity increases
Solution Approach 1:
Instead of using a single thick mask layer that would create high stress and complexity, the patent segments the protection function across multiple thin layers with different materials. Each layer contributes to the overall protection while maintaining manageable thickness and stress levels, reducing process complexity compared to a single thick mask.
Solution Approach 2:
The patent changes the material composition parameters of the hard mask system, using layers with different etching rates and mechanical properties. This allows optimization of both protection capability and stress distribution, achieving surface protection without requiring an excessively thick single-layer mask that would increase complexity.
3Manufacturing precision
If a single etching method is used, then the process is simple, but selective removal of top silicon and buried oxide layer cannot be achieved
Solution Approach 1:
The patent changes the etching parameters by employing different etching methods (reactive ion etching with different gas compositions, plasma conditions) for different layers. This allows selective removal of top silicon versus buried oxide layer by controlling etching rate and selectivity through parameter adjustment, achieving high selective etching precision.
Solution Approach 2:
The patent applies different etching conditions locally to different regions and layers. The composite hard mask structure enables local quality control where specific etching methods are applied to remove specific layers (top silicon or buried oxide) while leaving others intact, thereby achieving selective removal with controlled process complexity.
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 avoids damage to the side wall of the deep trench and reduces the need for an excessively thick hard mask, lowering costs and improving process stability.
Implementation Method 1
the first oxide layer is obtained through thermal oxide growth
Implementation Method 2
the silicon nitride layer is obtained through low pressure chemical vapor deposition
Implementation Method 3
the first etching method for etching the composite hard mask is: reactive ion etching is adopted
Data Source
AI summary
A new variable selective etching technology for thick SOI devices. An SOI material is etched by the following steps: (1) providing an SOI wafer; (2) depositing a composite hard mask with a variable selection ratio to replace a traditional hard mask with an invariable selection ratio; (3) applying a photoresist; (4) mask making, namely defining a to-be-etched region by using a photoetching plate; (5) etching the photoresist in the defined region; (6) etching the composite hard mask; (7) removing the photoresist; (8) etching top silicon by using a second etching method at a first selection ratio; and (9) etching a buried oxide layer by using a third etching method at a second selection ratio. The new variable selective etching technology avoids the damage to a side wall of a deep trench when the buried oxide layer is etched, and does not need to use an excessive thick hard mask.