MOS Well Dopant Migration Control for Threshold Voltage Matching
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In PMOS transistors, high-energy phosphorus implants for well and channel stop implants tend to diffuse to the surface upon anneal, smearing out the retrograde arsenic or antimony threshold voltage implant concentration profile, leading to increased threshold voltage mismatch and performance degradation in matched transistors.
Innovation Solution
An argon implant is used at a specific processing step to suppress phosphorus diffusion, creating a retrograde-type arsenic/antimony/phosphorus concentration profile in the channel of a MOSFET without undesirable side effects, either as a blanket implant or through the gate, positioned away from the substrate surface to inhibit phosphorus migration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-energy phosphorus implants are used for well and channel stop implants, then the channel stop function is achieved, but the phosphorus diffuses to the surface upon anneal, smearing out the retrograde arsenic or antimony threshold voltage implant concentration profile
Solution Approach 1:
A nitrogen implant is introduced as an intermediary layer between the phosphorus channel stop implant and the arsenic/antimony threshold voltage implant. The nitrogen forms a diffusion barrier that prevents phosphorus from migrating to the surface during annealing, thereby protecting the retrograde dopant profile from smearing while maintaining the channel stop function
Solution Approach 2:
The invention changes the physical and chemical parameters of the implant structure by introducing nitrogen at a specific depth and concentration. This creates a diffusion barrier with different properties than the surrounding silicon lattice, controlling phosphorus migration behavior during thermal processing without affecting the electrical characteristics of the channel stop region
2Manufacturing precision
If phosphorus diffusion is suppressed to maintain retrograde profile, then threshold voltage mismatch is reduced, but junction leakage may increase
Solution Approach 1:
The nitrogen implant is applied locally at specific depth ranges (shallower than the channel stop peak and deeper than the threshold voltage peak) rather than uniformly throughout the structure. This creates a localized diffusion barrier that selectively blocks phosphorus migration in the critical region while allowing proper dopant distribution in other areas, thus preventing both profile smearing and junction leakage
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
The argon implant effectively reduces threshold voltage mismatch by maintaining a retrograde dopant profile, improving transistor matching without adding masks to the process flow and avoiding increased junction leakage.
Implementation Method 1
high-energy phosphorus implants for well and channel stop implants tend to diffuse to the surface upon anneal
Implementation Method 2
An argon implant is used at a specific processing step to suppress phosphorus diffusion
Data Source
AI summary
A channel stop and well dopant migration control implant (e.g., of argon) can be used in the fabrication of a transistor (e.g., PMOS), either around the time of threshold voltage adjust and well implants prior to gate formation, or as a through-gate implant around the time of source/drain extension implants. With its implant depth targeted about at or less than the peak of the concentration of the dopant used for well and channel stop implants (e.g., phosphorus) and away from the substrate surface, the migration control implant suppresses the diffusion of the well and channel stop dopant to the surface region, a more retrograde concentration profile is achieved, and inter-transistor threshold voltage mismatch is improved without other side effects. A compensating through-gate threshold voltage adjust implant (e.g., of arsenic) or a threshold voltage adjust implant of increased dose can increase the magnitude of the threshold voltage to a desired level.