Systems and methods for improving radiation tolerance of three-dimensional flash memory
Inventors
Ray, Biswajit • Milenkovic, Aleksandar
Assignees
University of Alabama in Huntsville • University of Alabama at Birmingham UAB
Publication Number
US-11728004-B1
Publication Date
2023-08-15
Expiration Date
2041-08-04
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Abstract
A system for improving radiation tolerance of memory senses an amount of radiation exposure and, based on the sensed amount of radiation exposure, determines whether to perform one or more techniques for mitigating the effects of the radiation exposure. As an example, the system may perform a data refresh operation by re-writing data that has been corrupted by radiation, or the system may adjust the reference voltage used to read memory cells. In another example, the system may perform a fault repair operation by re-programming cells that have erroneously transitioned from a program state to an erase state. The system may selectively perform different radiation-mitigation techniques in a tiered approach based on the sensed amount of radiation in order to limit the adverse effects of the more invasive techniques.
Core Innovation
The invention pertains to systems and methods for enhancing the radiation tolerance of three-dimensional (3D) flash memory structures. A key aspect of the system is a memory controller that senses the amount of radiation exposure—either by means of a dedicated dosimeter or by using sections of flash memory (radiation sensing memory, RSM) to detect bit errors caused by radiation. Based on the sensed radiation level, the controller applies one or more mitigation techniques, including: data refresh operations, adjusting read reference voltage, fault repair operations, and, specifically for 3D structures, region- or cell-dependent assignments of error correction code (ECC) strength.
The problem addressed is the susceptibility of flash memory—particularly 3D NAND structures—to errors and charge leakage caused by ionizing radiation. Such issues are especially pronounced in high-radiation environments (e.g., satellites or space vehicles), reduce the operational reliability of flash memory, and restrict its use without bulky shielding. Additionally, the impact of radiation exposure is not uniform within 3D NAND structures: cells near the top and bottom (outer cells) are more affected than those in the center (inner cells).
The patent discloses a tiered approach to mitigation: for low radiation exposure, the controller adjusts the read voltage; for higher radiation levels, it conducts fault repair (reprogramming flipped bits); for still higher exposures, it refreshes the data by rewriting it. Furthermore, ECC strength is varied depending on memory cell location, with stronger ECC applied to outer cells and weaker ECC to inner cells, optimizing resource allocation. Data priority can also dictate storage location, assigning higher-priority data to less radiation-susceptible inner cells. These approaches enable flexible, dynamic, and resource-efficient improvement of memory reliability in radiation-prone environments.
Claims Coverage
The patent contains four main independent claim sets, each focusing on innovative allocation of error correction and data placement in 3D flash memory based on radiation susceptibility and data priority.
Adaptive ECC bit allocation based on memory cell location within 3D structure
A memory controller identifies whether data is to be stored in outer or inner memory cells within a 3D flash structure and assigns a greater number of error correction code (ECC) bits to data stored in outer memory cells than to data stored in inner cells. - Outer memory cells are defined as being closer to the top or bottom of the 3D flash structure, and are more affected by radiation. - The number of ECC bits is selected based on the selected memory location, with a higher number for outer cells. - The system comprises a 3D flash structure and a memory controller configured for this ECC selection.
Adaptive ECC algorithm strength based on memory cell location
A memory controller chooses between ECC algorithms of different error correction capacities based on whether data is stored in outer or inner memory cells of a 3D flash structure. - Outer memory cells use an ECC algorithm with higher error correction capacity. - Inner memory cells use an ECC algorithm of lower error correction capacity. - The system, including a 3D flash memory structure and the adaptive memory controller, thus customizes ECC according to radiation susceptibility.
Data storage location selection based on data priority
A memory controller receives data with an associated priority value, and allocates storage in either outer or inner memory cells of a 3D flash structure based on the priority. - Data with a first priority is stored in outer cells. - Data with a second (different) priority is stored in inner cells. - The controller uses the priority value to determine and execute the physical data placement.
Method for dynamic ECC allocation and data placement in 3D flash structures
A method including: 1. Receiving data for 3D flash memory storage. 2. Selecting a storage location (outer or inner memory cells) within the 3D structure. 3. Associating ECC bits with the data, where the number of ECC bits depends on the selected location (more bits for outer cells, fewer for inner). 4. Storing the data at the selected location and using the associated ECC for error correction.
The inventive features encompass dynamic allocation of ECC strength and data storage location within 3D flash memory structures, tailored by memory cell location and/or data priority, in order to improve radiation resilience and optimize resource use.
Stated Advantages
Efficiently increases the radiation tolerance of 3D flash memory without significantly raising cost or requiring bulky shielding.
Allocates stronger ECC only to memory regions most susceptible to radiation, optimizing memory usage and resource allocation.
Allows tiered and selective application of radiation mitigation techniques based on actual measured radiation exposure, limiting memory degradation from more invasive operations.
Enables higher-priority data to be preferentially stored in more radiation-resilient inner cells, enhancing data protection for critical information.
Extends the useful life of memory by reducing the frequency of operations, such as data refresh, that can degrade memory cells.
Documented Applications
Use in memory systems for devices exposed to high-radiation environments, such as vehicles or satellites in outer space.
Implementation in smartphones, mobile devices, tablets, personal computers, game consoles, memory cards, USB flash drives, solid-state drives, and other electronic devices deploying 3D flash memory.
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