SiC Trench Bottom Oxide Formation with Self-Aligned Sidewall Protection
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Solution Overview
Problem
The production of thick bottom oxides in silicon carbide (SiC) trench structures is challenging due to higher oxidation rates on the trench sidewalls, which limits the thickness difference between the bottom and sidewall oxides in existing methods.
Innovation Solution
A self-aligned method is employed using a polysilicon layer, followed by oxide and nitride depositions, anisotropic dry etching, thermal oxidation, and isotropic wet etching to form a thick oxide layer on the trench bottom while preventing oxidation on the sidewalls, thereby achieving a controlled thick bottom oxide.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If thermal oxidation is performed on SiC trench structures to form gate oxide, then oxide layer is formed on the substrate, but the oxidation rate on trench sidewalls is higher than on the bottom, limiting the thickness difference between bottom and sidewall oxides
Solution Approach 1:
A polysilicon layer is deposited on the trench structure before oxidation. This preliminary layer serves as a sacrificial material that oxidizes preferentially, allowing controlled formation of thick bottom oxide while protecting sidewalls. The polysilicon is deposited conformally across the entire trench structure, creating a uniform starting point for subsequent selective oxidation processes.
Solution Approach 2:
An oxide layer is deposited between the polysilicon layer and the nitride mask layer. This intermediate oxide layer serves multiple functions: it provides a buffer zone during etching processes, protects the polysilicon from direct exposure to etchants, and enables precise control over the final oxide thickness profile by acting as a sacrificial layer during the etch-back process.
2Manufacturing precision
If LOCOS method with Si3N4 mask is used to form thick bottom oxide, then thick oxide can be formed on trench bottom, but the method is difficult to apply to SiC trench structures due to higher sidewall oxidation rates
Solution Approach 1:
The patent applies different materials and processes to different regions of the trench structure. The polysilicon layer is present on both sidewalls and bottom, but the oxidation process selectively affects different regions. The nitride mask is strategically positioned to protect specific areas, creating local variations in oxide thickness that match the desired final profile for SiC trench MOSFETs.
Solution Approach 2:
The patent changes the oxidation parameters by using polysilicon as the oxidizing material instead of directly oxidizing SiC. Polysilicon has different oxidation characteristics compared to SiC, allowing for more controlled and uniform oxidation. The oxidation temperature, time, and atmosphere are optimized for polysilicon oxidation rather than SiC oxidation, enabling better control over the oxide thickness profile.
3Manufacturing precision
If oxide deposition followed by etch back is used to form TBO, then thick bottom oxide can be formed, but sufficient selectivity is required to prevent etching into the SiC substrate
Solution Approach 1:
A nitride mask layer is deposited and patterned before the etch-back process. This preliminary masking action defines the regions that will be etched away and protects the SiC substrate from etchant exposure. The mask layer is strategically positioned to cover areas where etching should not occur, preventing accidental etching into the substrate while allowing controlled removal of excess oxide and polysilicon.
Solution Approach 2:
The structure consists of multiple material layers (polysilicon, oxide, nitride) with distinct etching characteristics. This composite structure allows selective removal of materials based on their different chemical properties. The etchant is chosen to selectively remove oxide and polysilicon while leaving the nitride mask and SiC substrate intact, enabling precise control over the etching process without compromising substrate integrity.
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 allows for the production of a thick bottom oxide with improved quality and manufacturability, enabling enhanced channel mobility and reduced chip area in SiC trench MOSFETs, while simplifying the lithography process.
Implementation Method 1
thermal oxidation of the polysilicon of the polysilicon layer to produce a thick oxide layer locally
Implementation Method 2
anisotropic dry etching of the nitride layer
Implementation Method 3
wet etching
Data Source
AI summary
A method for producing a (thick) bottom oxide in a trench structure, having the following steps: providing a substrate having at least one trench structure comprising a bottom and sidewalls; depositing a polysilicon layer on a surface of the substrate, the bottom and the sidewalls; depositing an oxide layer on the polysilicon layer; depositing a nitride layer on the oxide layer; anisotropic dry etching of the nitride layer; thermal oxidation of the polysilicon of the polysilicon layer to produce a thick oxide layer locally; and wet etching; depositing a further oxide layer on the thick oxide layer along at least the bottom and the surface or on the residual oxide along the bottom and the surface and on the sidewalls.