Flash Memory Programming Method for Narrowing Threshold Voltage Distribution
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Solution Overview
Problem
Flash memory devices experience increased threshold voltage distribution and read errors due to random telegraph noise (RTN), which worsens with technology scaling, especially in multi-bit cells with reduced margins.
Innovation Solution
A method and device for programming flash memory cells that includes a write circuit, a read circuit, and a controller to manage programming and verify operations, detecting and addressing RTN bits by adjusting threshold voltages to ensure all cells meet the verify read voltage, thereby reducing the number of program inhibit cells and narrowing the threshold voltage distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If technology scaling is applied to shrink cell size, then storage density is improved, but threshold voltage distribution widens and read errors increase
Solution Approach 1:
The patent applies preliminary action by performing multiple verify read operations during the programming process to detect and correct threshold voltage shifts before they cause read errors. The controller repeatedly reads the programmed cells and adjusts programming parameters based on the read results, ensuring all cells meet the verify read voltage threshold before considering programming complete.
Solution Approach 2:
The patent implements feedback through the verify read operation mechanism, where the controller continuously monitors the threshold voltage of programmed cells by applying verify read voltage and detecting current flow. Based on the feedback from these readings, the controller adjusts the programming process to ensure all cells achieve the required threshold voltage, thereby reducing read errors.
2Reliability
If multiple verify read operations are performed to reduce read errors, then reliability is improved, but programming time increases
Solution Approach 1:
The patent applies partial action by performing verify read operations selectively and a limited number of times rather than indefinitely. The controller determines when programming is sufficient by stopping verify reads once all cells consistently meet the verify read voltage threshold, avoiding excessive verification time while ensuring reliability.
3Reliability
If threshold voltage is increased to ensure data retention, then reliability is improved, but manufacturing precision requirements become more stringent
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting programming voltage and current parameters during the programming process based on real-time threshold voltage measurements. The controller modifies these parameters to ensure all cells achieve and maintain the required threshold voltage, accommodating manufacturing variations without requiring extremely tight manufacturing tolerances.
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
The solution effectively reduces threshold voltage distribution and minimizes read errors by ensuring all programmed cells have a threshold voltage higher than the verify read voltage, maintaining data integrity and device reliability.
Implementation Method 1
A NOR flash cell is programmed by applying a program voltage to a control gate and a high voltage of, for example, 5V to 6V to a drain, with a source being grounded. According to this bias condition, current flows from the drain to the source. This programming manner is called hot-electron injection.
Implementation Method 2
The floating gate is placed between the control gate and a substrate (or bulk). Since the floating gate is insulated by an insulating film, electrons on the floating gate are trapped, thus storing information.
Implementation Method 3
In order to erase the NOR flash cell, a large voltage difference is forced between a control gate and a substrate (or bulk), which enables electrons to be discharged from a floating gate through F-N tunneling.
Data Source
AI summary
A method for programming a flash memory device which includes a plurality of memory cells arranged in rows and columns. The method includes programming selected memory cells from among the plurality of memory cells according to loaded data bits. Data bits are read from the programmed selected memory cells. It is determined whether each of the programmed memory cells has been successfully programmed based on the results of the reading step. The programming of memory cells that have been determined to have been successfully programmed are inhibited. The programming, reading, determining and inhibiting steps are repeated until each of the selected memory cells has been determined to have been successfully programmed. A memory cell that has been previously determined to have been successfully programmed and inhibited is uninhibited and subsequently re-programmed when it is determined that the previously inhibited memory cell is no longer successfully programmed.