Epitaxial Silicon Access Channels for Low-Leakage 3D Memory
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Solution Overview
Problem
In vertical three-dimensional (3D) memory devices, the use of polycrystalline silicon in access devices leads to leakage issues due to its leaky nature, while single crystal silicon cannot grow on common amorphous dielectric materials used in transistors, limiting the effectiveness of memory cells.
Innovation Solution
The formation of multiple alternating layers of epitaxially grown silicon germanium (SiGe) and silicon (Si) is employed to create horizontal access devices, where a silicon wafer is used as a substrate for high-temperature processes, allowing single crystal silicon growth on silicon germanium, forming a superlattice structure that mitigates defects and strain through selective etching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If polycrystalline silicon is used for access devices in DRAM arrays, then manufacturing is easier, but current leakage occurs through the polycrystalline structure
Solution Approach 1:
The patent changes the material parameter from polycrystalline silicon to single crystal silicon, fundamentally altering the crystalline structure to eliminate grain boundaries. This parameter change resolves the contradiction by maintaining ease of manufacture through epitaxial growth while eliminating current leakage through the single crystal structure.
Solution Approach 2:
The patent employs a composite structure with multiple alternating layers of silicon germanium (SiGe) and silicon (Si). This composite material approach allows selective etching of SiGe layers to create suspended single crystal silicon channels, achieving both manufacturability and reduced leakage simultaneously.
2Reliability
If epitaxial structures are used to reduce leakage, then reliability improves, but strain-induced dislocations occur in the epitaxial structures
Solution Approach 1:
The patent introduces silicon germanium (SiGe) layers as intermediary sacrificial layers between the single crystal silicon channels. These SiGe layers serve as mediators that can be selectively etched away, allowing the formation of suspended single crystal silicon structures that maintain high reliability while avoiding strain-induced dislocations through the controlled removal process.
Solution Approach 2:
The patent extracts the harmful SiGe sacrificial layers from the final device structure through selective etching. By removing these intermediary layers, the patent eliminates the source of strain-induced dislocations while preserving the beneficial single crystal silicon channels, thus resolving the contradiction between reliability improvement and dislocation prevention.
3Productivity
If design rules are shrunk to increase memory capacity, then productivity improves, but semiconductor space for fabricating access devices becomes limited
Solution Approach 1:
The patent transitions from planar access devices to three-dimensional vertically suspended single crystal silicon channels. By utilizing the vertical dimension and creating channels that extend through multiple layers, the patent increases memory capacity per unit area without compromising access device functionality, thus resolving the contradiction between productivity and available semiconductor space.
Solution Approach 2:
The patent segments the access device structure into multiple alternating layers of SiGe and Si, with the single crystal silicon channels suspended vertically. This segmentation allows for efficient use of three-dimensional space, enabling higher density memory fabrication while maintaining adequate space for access device operation through the vertically distributed architecture.
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 results in improved performance by reducing leakage current, enhancing I-on and I-off characteristics, and providing better electrostatic control, with three orders of magnitude lower off-current compared to traditional silicon-based access devices, while maintaining a cost-effective process.
Implementation Method 1
Multiple, alternating epitaxially grown silicon germanium (SiGe) and silicon (Si)
Data Source
AI summary
Systems, methods and apparatus are provided for an array of vertically stacked memory cells having horizontally oriented access devices and storage nodes. And, more particularly, to multiple, alternating epitaxial silicon, e.g., in horizontal access devices in vertical three dimensional (3D) memory. The horizontally oriented access devices can have a first source/drain regions and a second source drain regions separated by epitaxially grown, single crystalline silicon (Si) channel regions. Horizontally oriented access lines can connect to gates opposing the channel regions formed fully around every surface of the channel region as gate all around (GAA) structures. Vertical digit lines coupled to the first source/drain regions.