Deep Trench Etching Directionality Control in Semiconductor Wafers
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Solution Overview
Problem
Existing methods for digging deep trenches in semiconductor wafers face challenges in maintaining directionality and precision as depth increases, leading to widening trenches and thinning diaphragms, which can collapse, especially when forming multiple trenches close together, such as in SOI wafer fabrication.
Innovation Solution
A process involving alternating steps of directional etching and passivation, followed by a polymerizing plasma etch, controls the trench width and directionality by gradually forming tapered portions and using controlled plasma conditions to maintain the angle of inclination, ensuring consistent trench dimensions and preventing excessive thinning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If alternating etching and passivation steps are used to maintain directionality, then trench directionality is improved, but manufacturing complexity increases
Solution Approach 1:
The etching process is segmented into multiple alternating steps: directional etching steps followed by passivation steps. This segmentation allows the process to maintain directionality by periodically protecting trench walls, preventing excessive widening while achieving the required depth through cumulative etching.
Solution Approach 2:
The process employs periodic alternation between aggressive directional etching and passivation steps. This periodic action creates a rhythm of etching-protection that maintains trench integrity throughout the deep etching process, balancing material removal with wall protection.
2Productivity
If etching speed is increased to improve productivity, then trench formation speed is improved, but trench directionality deteriorates
Solution Approach 1:
The process dynamically adjusts between two operational modes: high-speed aggressive etching phases and protective passivation phases. This dynamic alternation allows the system to achieve high overall productivity while maintaining precision through periodic correction of directionality issues.
Solution Approach 2:
By periodically interrupting high-speed etching with passivation steps, the process maintains trench directionality without significantly reducing overall productivity. The passivation steps are brief enough to allow rapid resumption of etching while providing sufficient protection to prevent excessive widening.
3Manufacturing precision
If passivating layer deposition is used to protect trench walls, then trench directionality is improved, but etching speed deteriorates
Solution Approach 1:
The passivation layer is deposited to a thickness that provides sufficient protection for the next etching step, but not so thick as to require excessively long etching times to penetrate. This partial action approach balances wall protection with maintaining reasonable etching speed.
Solution Approach 2:
The alternation between passivation and etching creates a periodic process where the cumulative effect of multiple cycles achieves deep trench formation with controlled directionality, while the overall time remains acceptable due to the efficiency of each individual cycle.
4Adaptability or versatility
If deep trenches are formed with thin diaphragms, then device integration is improved, but diaphragm stability deteriorates
Solution Approach 1:
The periodic passivation steps provide beforehand protection to the trench walls and diaphragms during the etching process. This prior cushioning prevents excessive thinning and structural damage before it occurs, maintaining diaphragm stability even when forming deep trenches with thin separating structures.
Solution Approach 2:
The passivation layer provides localized protection precisely where needed - on the trench walls and diaphragm surfaces exposed during etching. This local quality enhancement protects vulnerable thin diaphragms without affecting the overall etching progress or requiring changes to the diaphragm design.
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 process enables the formation of deep trenches with a substantially constant width, preventing deviations and ensuring the stability of thin diaphragms, crucial for SOI wafer fabrication and subsequent steps like epitaxial growth and thermal oxidation.
Implementation Method 1
In order to favor polymerization and hence formation of the passivating layer, the process must be performed at temperatures that are significantly lower than the ones required for normal etching processes
Implementation Method 2
The wafer is then immersed in an etching fluid, for example SF6, which, after eliminating the passivating layer, etches the underlying structures
Data Source
AI summary
A process for digging deep trenches in a body of semiconductor material includes forming a mask having an opening, above a surface of a semiconductor body. A passivating layer is conformally formed on the mask and on the semiconductor body within the opening. A directional etch is extended to first remove the passivating layer from on top of the semiconductor body and then etch the semiconductor body through the opening. Forming the passivating layer and executing the directional etch are carried out repeatedly in sequence so as to form a trench through the opening. A tapered portion of the trench is formed, which has a transverse dimension decreasing as a distance from the surface of the semiconductor body increases.