Nonvolatile Memory Bit Line Voltage Control for Dense Threshold Distribution
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Solution Overview
Problem
Current nonvolatile memory devices face challenges in programming methods that result in inefficient data retention and integration, particularly in flash memory devices, due to the limitations of threshold voltage distribution and programming voltages.
Innovation Solution
A program method for nonvolatile memory devices that includes a pre-program verify step to set the bit line voltage based on the threshold voltage of the selected memory cell, followed by a programming operation and a post-program verify step to ensure accurate programming, utilizing a data input/output circuit to precharge and discharge bit lines according to the threshold voltage for dense threshold voltage distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a ground voltage is applied to the bit line and high voltage to the word line for programming, then the memory cell can be programmed through F-N tunneling, but the threshold voltage distribution becomes sparse resulting in poor data retention
Solution Approach 1:
The patent changes the bit line voltage parameter from a fixed ground voltage (0V) to a variable voltage that is adjusted according to the threshold voltage of the memory cell. This is achieved by measuring the memory cell threshold voltage and setting the bit line voltage to a specific value (e.g., Vbl = Vcc - Vth) based on the measured threshold voltage. This dynamic parameter adjustment enables dense threshold voltage distribution and improves data retention reliability.
2Manufacturing precision
If conventional programming method is used with fixed bit line voltage, then the programming process is simple, but the programming precision and threshold voltage control are insufficient
Solution Approach 1:
The patent applies preliminary action by performing a verification operation before the actual programming to determine the memory cell threshold voltage. Based on this preliminary measurement, the bit line voltage is pre-adjusted to the optimal value before programming begins. This preliminary characterization enables precise threshold voltage control during programming while managing complexity through a structured two-stage process.
Solution Approach 2:
The patent implements feedback control by measuring the memory cell threshold voltage and using this information to adjust the bit line voltage setting. The verification operation provides feedback about the memory cell characteristics, which then feeds into the programming voltage selection. This closed-loop feedback mechanism ensures precise programming while the systematic approach keeps the process manageable.
3Productivity
If memory cells are connected in series to one bit line for high integration, then the degree of integration improves, but the random access time increases
Solution Approach 1:
The patent applies local quality by tailoring the bit line voltage to each individual memory cell's threshold voltage characteristics. Even though memory cells are connected in series for high integration, each cell receives a customized voltage setting based on its specific threshold voltage. This localized optimization enables precise control of each cell's programming process, improving overall system performance despite the series connection 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 method improves the reliability and integration of nonvolatile memory devices by ensuring precise programming and dense threshold voltage distributions, enhancing data retention and programming efficiency.
Implementation Method 1
In order to program a memory cell of a flash memory device, a ground voltage (that is, 0V) is applied to the bit line which is connected to the memory cell, and a high voltage is applied to the word line which is connected to the memory cell. As these voltages are applied to the bit line and the word line of the memory cell to be programmed, the memory cell is programmed through F-N (Fowler-Nordheim) tunneling.
Data Source
AI summary
A program method of a nonvolatile memory device includes a pre-program verify step for verifying a threshold voltage of a selected memory cell; a step of setting a bit line voltage of the selected memory cell according to the threshold voltage of the selected memory cell which is determined through the pre-program verify step; a step of applying a program voltage to the selected memory cell set with the bit line voltage; and a post-program verify step for verifying a programmed state of the selected memory cell applied with the program voltage.