Refractory Metal Barrier for Poly Gate Resistivity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In CMOS technologies, the incorporation of boron difluoride (BF2) in poly gates for P-channel field effect transistors leads to increased sheet resistance and resistivity variations in titanium silicide due to fluorine diffusion, which conventional approaches like using boron or forming a silicon oxide layer fail to adequately address without affecting transistor performance or increasing process complexity.
Innovation Solution
A method involving the use of a refractory metal like molybdenum implanted in the poly gate to retard fluorine diffusion, thereby reducing the formation of voids and bubbles in titanium silicide, resulting in lower sheet resistance and reduced resistivity variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If BF2 is implanted to form P type polysilicon gates, then shallow junction depth and improved radiation hardness are achieved, but fluorine diffusion causes increased sheet resistance and resistivity variations in titanium silicide
Solution Approach 1:
A refractory metal layer (such as tungsten, molybdenum, or tantalum) is introduced as an intermediary between the BF2-implanted polysilicon gate and the titanium silicide contact. This refractory metal layer acts as a diffusion barrier that prevents fluorine from migrating into the titanium silicide, thereby eliminating void formation and sheet resistance variations while allowing the BF2 implantation to proceed for its radiation hardness benefits
Solution Approach 2:
The gate structure is transformed into a composite material system consisting of multiple layers: the BF2-implanted polysilicon gate layer, the refractory metal barrier layer, and the titanium silicide contact layer. This composite structure combines the radiation hardness benefits of BF2 implantation with the electrical stability of fluorine-blocked titanium silicide
2Manufacturing precision
If boron is used instead of BF2 for implantation, then titanium silicide sheet resistance problems are avoided, but faster diffusion rate adversely affects transistor performance
Solution Approach 1:
The refractory metal layer serves as a selective barrier that blocks fluorine diffusion while being impermeable to boron. This allows the use of BF2 implantation (which provides shallower junctions and better radiation hardness) without suffering from the fluorine-induced sheet resistance problems, effectively decoupling the two issues
3Manufacturing precision
If a silicon oxide layer is formed on BF2 implanted poly gate before titanium silicide formation, then fluorine diffusion is reduced, but transistor design is affected and process complexity increases
Solution Approach 1:
The refractory metal layer is deposited using physical vapor deposition (PVD) or atomic layer deposition (ALD) processes that operate under different parameter conditions than chemical vapor deposition. This parameter change enables the formation of a diffusion barrier layer without requiring the high-temperature processing and complex chemistry of silicon oxide formation, thereby simplifying the overall process
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 method significantly reduces the sheet resistance of titanium silicide by approximately 50% and minimizes variations, leading to a poly gate with reduced resistivity without affecting the transistor design or process complexity.
Implementation Method 1
a refractory metal, such as molybdenum, is implanted in a poly gate to retard fluorine diffusion in the poly gate
Implementation Method 2
fluorine that dissociates from the implanted BF2 can form voids and/or bubbles in the titanium silicide... the refractory metal, such as molybdenum, is implanted in a poly gate to retard fluorine diffusion
Data Source
AI summary
According to an exemplary embodiment, a method for forming a reduced resistivity poly gate includes a step of implanting a refractory metal, such as molybdenum, in an N type poly layer in a PFET region of a semiconductor substrate. The method further includes a step of implanting a boron-fluoride compound, such as boron difluoride, in an N type gate in the PFET region, where the N type gate comprises a portion of the N type poly layer. The method further includes a step of forming a titanium silicide segment in the N type gate. The refractory metal reduces a resistivity of the titanium silicide segment, thereby forming the reduced resistivity poly gate.