NMOS Source-Drain Formation with Carbon and Phosphorus
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Solution Overview
Problem
The challenge in forming n-channel MOS transistors is the high diffusivity of dopants like phosphorus and boron, which leads to increased off-state leakage current and series resistance, and the use of carbon to inhibit diffusion results in gated diode leakage. Additionally, arsenic implantation causes silicon lattice damage and pipe defects in SRAM bits.
Innovation Solution
The method involves forming NMOS transistors without carbon in the n-type lightly doped drain (NLDD) and using a pre-amorphization implant (PAI), phosphorus, and carbon implants in the n-type source/drain (NSD), with arsenic doses less than 1×10^14 cm^-2, to reduce diffusion and enhance transistor performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If carbon is implanted into the NLDD to reduce diffusion of P and B atoms during anneal, then dopant diffusion is reduced, but gated diode leakage current increases
Solution Approach 1:
The invention extracts carbon from the NLDD region while retaining it in the NSD region. By removing carbon from the NLDD, the harmful gated diode leakage is eliminated, while carbon remains in the NSD to continue providing diffusion barrier functionality during anneal processes.
Solution Approach 2:
The invention applies different carbon concentrations to different regions: zero carbon in the NLDD region to prevent GDL, and high carbon in the NSD region to inhibit dopant diffusion. This local differentiation resolves the contradiction by optimizing each region's carbon content for its specific function.
2Quantity of substance
If arsenic is ion implanted into NLDD and NSD regions, then n-type doping is achieved, but silicon crystal lattice damage occurs causing pipe defects and SRAM failures
Solution Approach 1:
The invention extracts arsenic from the implantation process entirely, replacing it with phosphorus-based doping. This eliminates the harmful pipe defects and SRAM failures caused by arsenic while maintaining the necessary n-type doping concentration through alternative phosphorus implantation schemes.
Solution Approach 2:
The invention changes the dopant type parameter from arsenic to phosphorus, and adjusts implantation parameters (dose, energy, multiple implants) to achieve the required doping levels without the harmful effects of arsenic. This parameter substitution resolves the contradiction between achieving sufficient doping and avoiding lattice damage.
3Quantity of substance
If phosphorus is used for NLDD and NSD implantation, then n-type doping is achieved, but high diffusivity causes increased off-state leakage current
Solution Approach 1:
The invention introduces carbon as an intermediary substance in the NSD region that acts as a diffusion barrier. The carbon atoms physically block phosphorus atoms from diffusing into the channel region during anneal, thereby preventing off-state leakage while allowing the phosphorus to remain in the NSD for proper doping.
Solution Approach 2:
The invention performs carbon implantation into the NSD region before the final anneal process. This preliminary carbon placement creates a diffusion barrier in advance, preventing phosphorus diffusion during subsequent thermal processing steps.
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 reduces off-state leakage current, series resistance, and pipe defects while maintaining high on-state drive current, by effectively controlling dopant distribution and minimizing the adverse effects of arsenic and boron diffusion.
Implementation Method 1
a pre-amorphizing implant (PAI), phosphorus implant and a carbon species implant
Implementation Method 2
NLDD and NSD sub-elements are formed by ion implanting n-type dopants into a p-type region
Implementation Method 3
Phosphorus has a disadvantage for use in the NLDD and NSD, which arises from the high diffusivity of P atoms in silicon
Implementation Method 4
After the n-type dopants are ion implanted into the NLDD or NSD, the wafer is annealed at high temperature to repair the silicon crystal lattice damage done by the ion implantation process
Data Source
AI summary
Pipe defects in n-type lightly doped drain (NLDD) regions and n-type source/drain (NDS) regions are associated with arsenic implants, while excess diffusion in NLDD and NSD regions is mainly due to phosphorus interstitial movement. Carbon implantation is commonly used to reduce phosphorus diffusion in the NLDD, but contributes to gated diode leakage (GDL). In high threshold NMOS transistors GDL is commonly a dominant off-state leakage mechanism. This invention provides a method of forming an NMOS transistor in which no carbon is implanted into the NLDD, and the NSD is formed by a pre-amorphizing implant (PAI), a phosphorus implant and a carbon species implant. Use of carbon in the NDS allows a higher concentration of phosphorus, resulting in reduced series resistance and reduced pipe defects. An NMOS transistor with less than 1·1014 cm−2 arsenic in the NSD and a high threshold NMOS transistor formed with the inventive method are also disclosed.