Flash Memory Channel Boosting via Dynamic Pass Voltage
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Solution Overview
Problem
In NAND flash memory devices, controlling the channel boosting level is challenging due to interference between memory cells, especially as cell size decreases, leading to program disturbance issues, particularly with Hot Carrier Injection (HCI) and Gate Induced Drain Leakage (GIDL), which complicates maintaining the threshold voltage distribution and read margin.
Innovation Solution
The method involves varying the pass voltage applied to unselected memory cells during programming to maintain a predetermined gap between the channel voltage and word line voltage, adjusting the pass voltage based on the program voltage level and the number of program loops, thereby controlling channel boosting and reducing program disturbance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If cell size is decreased to increase memory density, then storage capacity is improved, but channel boosting is reduced leading to program disturbance
Solution Approach 1:
The patent changes the pass voltage parameter dynamically during the programming process. Initially, a first pass voltage is applied to unselected word lines, and then a second, higher pass voltage is applied in later programming stages. This parameter change compensates for the reduced channel boosting in smaller cells, maintaining sufficient electric field to prevent program disturbance while allowing high-density cell scaling.
2Reliability
If channel boosting is increased to prevent program disturbance, then reliability is improved, but Hot Carrier Injection (HCI) is generated in the first word line
Solution Approach 1:
The patent employs periodic action by applying different pass voltages at different stages of the programming process. In early stages, a lower first pass voltage is applied to minimize HCI. In later stages when program disturbance becomes more critical, a higher second pass voltage is applied to strengthen channel boosting. This time-dependent voltage adjustment balances both concerns.
Solution Approach 2:
The pass voltage is made dynamic rather than static. The system transitions from a first pass voltage to a second pass voltage based on the programming progress and cell state. This dynamic adjustment allows the system to optimize channel boosting at each stage while minimizing harmful HCI effects, adapting to the changing electrical conditions during programming.
3Manufacturing precision
If program voltage is increased step by step using ISPP method, then threshold voltage distribution control is improved, but program time is extended
Solution Approach 1:
The patent incorporates feedback mechanisms by monitoring the programming progress and cell state to dynamically adjust the pass voltage. The system uses verification results and programming stage information to determine when to switch from the first pass voltage to the second pass voltage, optimizing the balance between threshold voltage control precision and programming speed through closed-loop control.
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 effectively reduces program disturbance by maintaining a suitable bias difference between the program voltage and channel voltage, enhancing the threshold voltage distribution and overall device performance by adjusting the pass voltage in response to program voltage levels and loop counts.
Implementation Method 1
The NAND flash memory device is erased and programmed using Fowler-Nordheim Tunneling. When the program voltage is applied to the word line of the selected memory cell and the ground voltage is applied to the bit line, a high electric field is formed between a floating gate and a channel of the memory cell. By this electric field, a tunneling, in which electrons of the channel pass through a tunnel oxide layer between the floating gate and the channel, is generated.
Data Source
AI summary
A method of programming a flash memory device controls a channel boosting level to ensure device properties. The flash memory device is programmed in an Incremental Step Pulse Program (ISPP) manner by applying a program voltage to a selected memory cell and a pass voltage to unselected memory cells. The programming is performed by varying the pass voltage so that a gap of a predetermined range is maintained between a channel voltage and a word line voltage of the unselected memory cell.