SiGe Pillar-Based Strain-Relaxed Buffer Layers
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Solution Overview
Problem
The growth of high germanium content Si1-xGex layers on silicon substrates is hindered by lattice mismatch, leading to defect formation and strain issues, making it challenging to achieve smooth, defect-free, and 100% relaxed strain-relief-buffer layers necessary for high mobility semiconductor devices.
Innovation Solution
The eMESA technique involves forming epitaxial Si1-xGex layers on silicon substrates, etching them to create mesa pillars with high aspect ratios, and growing a suspended layer over air gaps, allowing for elastic relaxation and reducing threading dislocations, thereby achieving a 100% relaxed and defect-free buffer layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high germanium content Si1-xGex layer is grown via heteroepitaxy on a silicon substrate, then the hole mobility of p-MOS devices is boosted, but a large number of threading dislocation defects are formed due to lattice mismatch
Solution Approach 1:
The patent segments the SiGe layer into multiple thinner sub-layers with graded germanium composition. Each sub-layer has a thickness below the critical thickness for dislocation formation, allowing strain accumulation without defect generation. The germanium content is gradually increased across layers, creating a composition gradient that reduces lattice mismatch stress incrementally.
Solution Approach 2:
The patent introduces a vertical dimension to the composition profile by creating a graded structure where germanium content varies with depth. This vertical composition gradient allows the system to accommodate lattice mismatch through gradual adaptation rather than abrupt transitions, preventing dislocation formation while achieving high Ge content in the final layer.
2Manufacturing precision
If a Si1-yGy buffer layer is used as a virtual substrate to grow high germanium Si1-xGex layers, then the lattice mismatch is reduced, but meeting stringent fabrication needs requires the buffer layer to be smooth, defect-free, and 100% relaxed simultaneously which is difficult to achieve
Solution Approach 1:
The patent systematically changes multiple parameters including layer thickness, germanium composition, and growth conditions to achieve the desired buffer layer properties. By controlling the germanium content gradient and maintaining each layer below critical thickness, the method achieves 100% relaxation with smooth, defect-free surfaces suitable for high-precision fabrication.
3Reliability
If the SiGe buffer layer thickness is increased to achieve 100% relaxation, then strain relief is improved, but threading dislocations form to release the strain
Solution Approach 1:
The patent divides the buffer layer into multiple thin sub-layers, each with controlled thickness below the critical thickness for dislocation formation. This segmentation allows cumulative strain relaxation across layers without triggering defect formation in any individual layer.
Solution Approach 2:
The patent creates local variation in germanium composition across different layers and regions of the buffer structure. Each local region has optimized composition and thickness to achieve strain relief without exceeding the critical thickness threshold that would trigger dislocation formation.
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 effectively reduces threading dislocation density to values below 10^4/cm2, enabling the formation of high-quality, strain-relaxed SiGe layers without generating defects, which is crucial for advanced semiconductor fabrication and device performance.
Implementation Method 1
allowing for elastic relaxation and reducing threading dislocations
Implementation Method 2
epitaxially growing a suspended layer of Si1-xGex from upper portions of the Si1-xGex pillars
Data Source
AI summary
Methods of forming a layer of silicon germanium include forming an epitaxial layer of Si1-xGex on a silicon substrate, wherein the epitaxial layer of Si1-xGex has a thickness that is less than a critical thickness, hc, at which threading dislocations form in Si1-xGex on silicon; etching the epitaxial layer of Si1-xGex to form Si1-xGex pillars that define a trench in the epitaxial layer of Si1-xGex, wherein the trench has a height and a width, wherein the trench has an aspect ratio of height to width of at least 1.5; and epitaxially growing a suspended layer of Si1-xGex from upper portions of the Si1-xGex pillars, wherein the suspended layer defines an air gap in the trench beneath the suspended layer of Si1-xGex.