Programming For Three-Dimensional Nand Memory
Patent No. US11887671 (titled "Programming For Three-Dimensional Nand Memory") was filed by Yangtze Memory Technologies Co Ltd on Nov 4, 2021.
What is this patent about?
’671 is related to the field of 3D NAND flash memory , specifically addressing the challenge of program disturb and pass disturb in vertically stacked memory cells. As memory devices shrink to increase storage density, 3D architectures are employed to overcome limitations in planar memory cells. However, during programming, unselected memory cells can experience unwanted threshold voltage shifts, leading to reduced performance and reliability. This patent aims to mitigate these disturbances by optimizing the programming process.
The underlying idea behind ’671 is to apply different pass voltages to different groups of unselected word lines during the programming of a 3D NAND memory. Instead of using a single pass voltage for all unselected word lines, the invention proposes dividing the unselected word lines into groups and applying a specifically tailored pass voltage to each group. This allows for a more precise control over the voltage stress experienced by unselected memory cells, reducing both program and pass disturb.
The claims of ’671 focus on a method, a 3D memory device, and a storage system that implement the concept of applying different pass voltages to different groups of unselected word lines. The independent claims cover the steps of applying a program voltage to a selected word line, applying a first pass voltage to a first group of unselected word lines, and applying a second, different pass voltage to a second group of unselected word lines. Crucially, the claims also include determining a maximum pass voltage based on threshold voltage shifts in unselected memory cells.
In practice, the invention involves a peripheral circuit within the 3D memory device that controls the word line driver. This driver, guided by a control signal, applies the program voltage to the selected word line and the optimized pass voltages to the different groups of unselected word lines. The grouping of word lines can be based on their physical location within the 3D stack, as memory cells closer to the substrate may exhibit different characteristics due to process variations during manufacturing.
The differentiation from prior approaches lies in the dynamic adjustment of pass voltages based on real-time monitoring of threshold voltage shifts. By measuring the threshold voltage shifts of unselected memory cells after applying a certain number of programming pulses, the system can determine the maximum allowable pass voltage for each group of word lines. This feedback mechanism allows for a more robust and reliable programming process, minimizing the impact of program and pass disturb and ultimately improving the endurance and data retention of the 3D NAND flash memory.
How does this patent fit in bigger picture?
Technical landscape at the time
In the early 2020s when ’671 was filed, 3D NAND flash memory was widely adopted to increase storage density. At a time when memory cells were typically stacked vertically to form memory strings, challenges arose due to program disturb and pass disturb affecting unselected memory cells. When hardware or software constraints made it non-trivial to suppress both program disturb and pass disturb, there was a need for methods to improve the performance and reliability of 3D NAND memory.
Novelty and Inventive Step
The examiner allowed the claims because the prior art did not disclose applying a second pass voltage on a second group of unselected word lines, where the second pass voltage is different from the first pass voltage. The prior art also did not disclose determining a first maximum pass voltage when shifts of threshold voltages of a first set of unselected memory cells connected to the first group of unselected word lines are less than a maximum allowed shift.
Claims
This patent contains 18 claims, with independent claims numbered 1, 9, and 15. The independent claims are directed to a method for programming a 3D memory device, a 3D memory device, and a storage system incorporating such a device, respectively, all focusing on applying different pass voltages to groups of unselected word lines during programming. The dependent claims generally elaborate on and refine the specifics of the independent claims, such as how maximum pass voltages are determined and applied to suppress pass disturb.
Key Claim Terms New
Definitions of key terms used in the patent claims.
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Date
Description
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US11887671
- Application Number
- US17518783
- Filing Date
- Nov 4, 2021
- Status
- Granted
- Expiry Date
- Jul 15, 2041
- External Links
- Slate, USPTO, Google Patents