Mixed Substrate Strain Engineering for CMOS
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Solution Overview
Problem
Current methods for manufacturing substrates with tensile-stressed silicon-on-insulator and compressive-stressed germanium-on-insulator portions face limitations such as lattice mismatch, stress relaxation, and defect formation, which hinder the production of high-performance CMOS circuits with balanced N-MOS and P-MOS transistors.
Innovation Solution
A method involving a base substrate of strained silicon on insulator, where one zone is masked and subjected to germanium enrichment through localized oxidation heat treatment to create a compressively strained germanium-on-insulator portion, while maintaining tensile stress on the silicon-on-insulator portion, thereby minimizing defects and ensuring mechanical constraint conservation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If germanium enrichment is performed on silicon-on-insulator substrate to create germanium-on-insulator portion, then the P-MOS transistor characteristics are improved, but lattice mismatch and stress relaxation occur causing defect formation
Solution Approach 1:
The patent applies preliminary action by pre-straining the silicon substrate in tension before germanium enrichment. This pre-straining creates a compressive stress state that counteracts the tensile stress generated during germanium enrichment, preventing stress relaxation and dislocation formation throughout the entire enrichment process
Solution Approach 2:
The patent changes the stress state parameter of the silicon substrate from relaxed or compressive to tensile-strained before enrichment. By modifying this physical parameter, the substrate can accommodate the lattice mismatch during germanium enrichment without relaxing, thereby preventing defect formation
2Reliability
If high germanium concentration is achieved in the enriched layer, then the mobility of holes is improved, but the critical plastic relaxation thickness decreases leading to stress relaxation
Solution Approach 1:
The patent uses counterweight by applying a pre-compressive stress to the silicon substrate that acts as a counterforce to the tensile stress generated during germanium enrichment. This counter-stress prevents the enriched layer from exceeding its critical plastic relaxation thickness, allowing high germanium concentration to be achieved while maintaining stress stability
3Reliability
If localized germanium enrichment is performed to create mixed substrate, then N-MOS and P-MOS transistors are balanced in current level and time constants, but the process complexity increases
Solution Approach 1:
The patent applies local quality by performing localized germanium enrichment only in specific regions of the substrate where P-MOS transistors will be fabricated. This allows different regions of the same substrate to have different material compositions and stress states, enabling optimization of both N-MOS and P-MOS transistor characteristics simultaneously
Solution Approach 2:
The patent simplifies the localized enrichment process by applying a global pre-straining step to the entire substrate before localized enrichment. This preliminary action creates a uniform stress field that facilitates subsequent localized enrichment while maintaining stress control, reducing the overall process complexity
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 allows for the production of substrates with minimal defects, maintaining mechanical stress in both silicon and germanium layers, enabling optimal characteristics for CMOS circuits with balanced N-MOS and P-MOS components and reducing short channel effects.
Implementation Method 1
Stress relaxation of the enriched layer will not be observed until the enriched layer exceeds a certain thickness, called the critical plastic relaxation thickness
Implementation Method 2
germanium enrichment treatment of the second tensile strained silicon zone of the base substrate, up to to obtain said portion of germanium on insulator strained in compression
Data Source
Figure 1~4
Figure 5~7
AI summary
The process for manufacturing a mixed substrate (1), comprising a portion (2) of silicon on a tensile insulator and a portion (3) of germanium on a compression-strained insulator, includes a first step of producing a basic silicon on a tension-strained insulator substrate, comprising first (7) and second zones of silicon under tension. After producing the basic substrate, the process includes the successive steps of masking the first zone (7) of silicon under tension, forming the portion (2) of silicon on a tension-strained insulator of the substrate (1), of enriching the second zone of silicon under tension of the basic substrate with germanium until a layer (13) of germanium under compression is obtained, forming said portion (3) of germanium on a compression-strained insulator of the substrate (1), and of removing the masking.