Trench MOSFET Sidewall Doping for Lower On-Resistance
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Solution Overview
Problem
Current trench MOSFET devices face challenges in achieving a smaller pitch or cell size, leading to higher on-resistance per unit area, and require improvements in source region formation for enhanced performance.
Innovation Solution
The introduction of source dopant through the sidewall of the trench from doped polysilicon, allowing for a more precise control of dopant concentration in the body region away from the channel region, and the use of a trench-gated MOSFET structure with a doped glass and polysilicon layer configuration to facilitate lower on-resistance and higher density vertical current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If source dopant is introduced through conventional methods, then the basic device structure is formed, but the pitch and cell size cannot be reduced further and on-resistance remains high
Solution Approach 1:
The dopant introduction method transitions from planar surface diffusion to three-dimensional sidewall diffusion. The doped polysilicon layer is formed on the trench sidewalls, allowing dopant to be introduced from the vertical sidewall surface rather than only from the horizontal top surface, effectively utilizing the third dimension to achieve higher dopant concentration in the source region while maintaining precise control
Solution Approach 2:
The invention creates non-uniform dopant concentration distribution by forming doped polysilicon specifically on the trench sidewalls. This localized dopant source ensures high dopant concentration where needed (in the source region near the channel) while allowing different doping levels in different spatial zones, enabling precise control of electrical characteristics without affecting the entire device uniformly
2Reliability
If dopant concentration in the body region is increased to reduce on-resistance, then on-resistance decreases, but breakdown voltage control becomes difficult
Solution Approach 1:
The dopant introduction is segmented into distinct spatial zones: the doped polysilicon on the sidewalls provides dopant primarily to the source region, while the body region receives dopant through a separate controlled diffusion process. This segmentation allows independent optimization of source region doping (for low on-resistance) and body region doping (for breakdown voltage control), resolving the trade-off between these two parameters
3Productivity
If trench dimensions are reduced to achieve smaller pitch, then device density increases, but manufacturing precision and dopant control become more difficult
Solution Approach 1:
The doped polysilicon layer is formed on the trench sidewalls before the final dopant diffusion process. This preliminary formation of the dopant source layer on the sidewalls ensures that when diffusion occurs, the dopant is already positioned optimally, making the subsequent diffusion process more controllable and less sensitive to variations in trench dimensions, thereby facilitating manufacturing of smaller pitch devices
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 approach enables the fabrication of DMOS transistors with reduced on-resistance per unit area and higher density, improving the breakdown voltage and overall performance by optimizing the source and body diffusion regions.
Implementation Method 1
The introduction of source dopant through the sidewall of the trench from doped polysilicon
Data Source
AI summary
A vertical-current-flow device includes a trench which includes an insulated gate and which extends down into first-conductivity-type semiconductor material. A phosphosilicate glass layer is positioned above the insulated gate and a polysilicon layer is positioned above the polysilicate glass layer. Source and body diffusions of opposite conductivity types are positioned adjacent to a sidewall of the trench. A drift region is positioned to receive majority carriers which have been injected by the source, and which have passed through the body diffusion. A drain region is positioned to receive majority carriers which have passed through the drift region. The gate is capacitively coupled to control inversion of a portion of the body region. As an alternative, a dielectric layer may be used in place of the doped glass where permanent charge is positioned in the dielectric layer.