Trench MOSFET Body Doping for Gate Oxide Reliability
Solution Overview
Problem
Trench MOSFETs face challenges in achieving uniform dopant concentration and reliable gate oxide performance due to difficulties in P-type doping of polysilicon gate electrodes, which require high thermal budgets and can lead to contamination and poor conductivity.
Innovation Solution
The method involves multiple ion implantation steps at different energies to achieve uniform dopant concentration in the body region, allowing for deeper trench etching and in-situ N-type doping of polysilicon, which reduces the need for nitridization and enhances gate oxide reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If P type dopant ions are implanted into polysilicon gate electrode by ion implantation followed by high temperature diffusion, then the work function is reduced and threshold voltage is lowered, but the gate oxide becomes contaminated and reliability deteriorates
Solution Approach 1:
The patent performs nitridization of the gate oxide before P-type ion implantation to create a protective nitrided layer at the gate oxide interface. This preliminary action prevents boron diffusion into the gate oxide during subsequent high-temperature processing, eliminating contamination while allowing the P-type doping to proceed effectively
Solution Approach 2:
The nitrided gate oxide layer acts as an intermediary barrier between the polysilicon gate electrode and the gate oxide. This intermediate nitrided region allows P-type dopant diffusion into the polysilicon while blocking boron from penetrating into the gate oxide, thus protecting the gate oxide from contamination
2Quantity of substance
If high temperature diffusion is used to diffuse boron into polysilicon gate electrode, then dopant concentration is increased, but manufacturing complexity and cost increase due to precise temperature control requirements
Solution Approach 1:
The patent performs preliminary nitridization of the gate oxide before ion implantation, which simplifies the overall process by eliminating the need for precise high-temperature diffusion control. The nitrided gate oxide allows for simpler, more robust processing conditions while achieving the desired dopant concentration in the polysilicon gate electrode
3Manufacturing precision
If single energy ion implantation is used to dope body region, then manufacturing process is simplified, but uniform dopant concentration cannot be achieved adjacent to gate oxide
Solution Approach 1:
The patent divides the body region doping into multiple ion implantation steps, each with different energies and target depths. The first implantation step targets deeper regions with higher energy, while the second step targets regions adjacent to the gate oxide with lower energy, achieving uniform dopant concentration through segmented processing
4Strength
If trench depth is increased to shift electric field peak deeper, then breakdown voltage is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies local quality by creating different dopant concentrations in different regions of the body. By uniformly doping the body region adjacent to the gate oxide through multiple ion implantation steps, the electric field distribution is optimized locally, allowing the trench to achieve higher breakdown voltage without requiring excessive depth or ultra-precise etching control
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 results in a trench MOSFET with improved performance and reliable electrical characteristics, including reduced electric field and lower threshold voltage, while avoiding the drawbacks of high-temperature diffusion and contamination.
Implementation Method 1
P type dopant ions, for example boron, are implanted into the polysilicon gate electrode 43
Implementation Method 2
a diffusion step is performed to diffuse the boron into the deep portions of the trench. This diffusion step requires heating the device to very high temperatures
Data Source
AI summary
A method for forming a trench MOSFET includes doping a body region of the trench MOSFET in multiple ion implantation steps each having different ion implantation energy. The method further comprises etching the trench to a depth of about 1.7 μm.