Boron-Rich Cap Layer for SiGe Contact Stability
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Solution Overview
Problem
Metal contacts to silicon germanium source/drain regions in transistors face issues of thermal instability and increased parasitic resistance due to germanium diffusion and inadvertent etching, which affect transistor performance and reliability.
Innovation Solution
The use of a thin boron-rich cap layer on top of the silicon germanium source/drain regions to prevent germanium diffusion and etching, maintaining low contact resistance and thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal contacts are formed directly to silicon germanium source/drain regions, then electrical connection is established, but parasitic contact resistance increases due to germanium diffusion during annealing
Solution Approach 1:
A boron-rich cap layer is introduced as an intermediary between the metal contact and the silicon germanium source/drain region. This cap layer prevents germanium diffusion into the metal contact during annealing processes, thereby maintaining low and stable contact resistance. The cap layer acts as a diffusion barrier while allowing electrical conduction to pass through.
Solution Approach 2:
The contact structure uses a composite material approach by combining metal contact material with a boron-rich cap layer. The cap layer has specific properties (boron enrichment) that differ from the underlying silicon germanium, creating a composite structure that leverages the advantages of both materials: the metal provides electrical conduction while the boron-rich layer provides diffusion protection.
2Ease of manufacture
If silicon germanium source/drain regions are exposed during processing, then manufacturing is simplified, but inadvertent etching increases parasitic resistance
Solution Approach 1:
The boron-rich cap layer is formed preliminarily on the silicon germanium source/drain regions before subsequent processing steps. This preliminary action provides etch protection during manufacturing processes, preventing inadvertent etching of the silicon germanium while allowing other processing operations to proceed. The cap layer serves as a protective barrier throughout the manufacturing sequence.
Solution Approach 2:
The cap layer acts as an intermediary protective layer between the silicon germanium source/drain region and the etching environment. It mediates the interaction by providing chemical resistance to etchants, thereby protecting the underlying silicon germanium from inadvertent etching while allowing the manufacturing process to continue.
3Reliability
If high germanium concentration is used in silicon germanium source/drain regions, then contact resistance is reduced, but etch resistance decreases
Solution Approach 1:
The solution applies local quality by creating a boron-rich cap layer with specific local properties at the surface of the silicon germanium source/drain region. The underlying silicon germanium maintains its high germanium concentration for low contact resistance, while the cap layer provides the etch resistance that high-germanium material lacks. Each layer has optimized properties for its specific function.
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 boron-rich cap layer effectively reduces parasitic contact resistance and enhances the reliability and performance of metal contacts by preventing germanium diffusion and etching, thereby improving transistor performance.
Implementation Method 1
The cap layer can comprise boron and can have a boron concentration in the cap layer can be in the range of about 10-100%. The presence of the cap layer can reduce (or prevent) the diffusion of germanium from the silicon germanium layer into the contact metal
Implementation Method 2
Metal contacts to silicon germanium can be thermally unstable in that their resistance (contact resistance) increases after being subjected to annealing and other high-temperature processing steps. The creation of voids in a silicon germanium layer at the contact metal-silicon germanium interface due to diffusion of germanium from the silicon germanium layer into the contact layer
Data Source
AI summary
Cap layers are formed on silicon germanium (SiGe) source/drain regions to provide etch resistance to processing steps that can occur in a semiconductor manufacturing process between formation of the SiGe source/drain regions and metal contact formation. The cap layers comprise boron and are thin (e.g., 2 nm or less) to provide for a low metal contact resistance. The atomic concentration of boron in the second layer is in a range of about 0.2-20%. In addition to providing etch resistance, the cap layer provides for a thermally stable contact resistance as the cap layer can prevent or limit the creation of voids in the SiGe layer by preventing or limiting the diffusion of germanium from the SiGe layer into the metal in subsequent annealing and other high-temperature processing steps.