Flash Memory Programming via Hot Electron Injection
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Solution Overview
Problem
The existing programming methods for MLC flash memory cells, such as FN tunneling, CHE injection, and SSI injection, face challenges in achieving high-speed, precise, and efficient programming due to variations in threshold voltage levels and power limitations, leading to non-uniform programming characteristics and high current requirements.
Innovation Solution
A method that applies a high voltage to the control gate, a low voltage to the select gate, and specific voltages to the drain and source regions to generate and direct hot electrons consistently to the floating gates, while using a capacitor and constant current sources or current mirrors to maintain constant charge flow and reduce variations in channel current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If Fowler-Nordheim tunneling is used for programming MLC flash memory cells, then parallel programming capability is improved, but programming time increases to at least 50 μs
Solution Approach 1:
The patent changes the fundamental programming mechanism from Fowler-Nordheim tunneling to channel-hot-electron injection, altering the physical parameters of electron transport. This enables faster programming by generating hot electrons through high electric fields in the channel that can be injected into the floating gate more rapidly than tunneling processes
Solution Approach 2:
The patent employs pulsed voltage applications to the control gate and drain, using periodic high-voltage bursts to generate hot electrons. This pulsed approach maintains high programming speed while enabling parallel operation across multiple cells by synchronizing the pulse timing
2Speed
If channel-hot-electron injection is used for programming, then programming speed is improved, but current requirements increase to about or more than 100 μA per cell
Solution Approach 1:
The patent concentrates the high electric field and hot electron generation in specific localized regions of the transistor channel through selective voltage application. By focusing the energy injection to specific locations rather than requiring uniform high current across the entire cell structure, the method achieves fast programming with reduced overall current consumption
Solution Approach 2:
The patent applies excessive voltage to the control gate (e.g., 20V or higher pulses) to generate sufficient hot electrons for programming, but limits the duration and spatial extent of this high-voltage application. This partial application of excessive action achieves the necessary electron injection without requiring sustained high current flow through the entire cell
3Productivity
If SSI injection is used with split gate structure, then programming parallelism is improved, but programming precision deteriorates due to exponential variation of channel current with threshold voltage
Solution Approach 1:
The patent introduces feedback mechanisms through sense amplifiers that read the threshold voltage of the select gate transistor and adjust the control gate voltage accordingly. This feedback loop compensates for the exponential sensitivity of channel current to threshold voltage variations, maintaining programming precision across parallel operations
Solution Approach 2:
The patent introduces an intermediate compensation mechanism that decouples the select gate transistor's threshold voltage variations from the programming current. By using the sense amplifier as an intermediary to measure and compensate for threshold variations, the system maintains precise control over the actual programming current despite variations in the select gate characteristics
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 stabilizes the programming speed, reduces current requirements, and improves the precision and efficiency of programming by maintaining constant charge injection into the floating gates, addressing the limitations of SSI injection and enhancing the overall performance of MLC flash memory cells and arrays.
Implementation Method 1
The FN tunneling is achieved by forcing electrons to or from the floating gate, which is achieved by applying a voltage between a control gate and the substrate of the MLC flash memory cell
Implementation Method 2
The CHE injection is achieved by generating hot electrons and is applied on simple stacked-gate devices. In the method, an MLC flash memory cell is switched on with a high voltage level at the control gate, and an intermediate voltage level at the drain region. Therefore, with the aid of a large electric field formed on a drain junction, electrons are accelerated by the electric field, and the hot electrons are thus generated
Implementation Method 3
The SSI injection acquires a fast cell programming and a good programming parallelism. Note that a split gate structure is further applied in the SSI injection, and an additional select gate, which may be a sidewall gate of an MLC flash memory cell, is thus applied. For applying the SSI injection, the select gate has to be operated in a sub-threshold region of the MLC flash memory cell, and a pinch-off point appears at the boundary between the select gate and the floating gate. Since the select gate is utilized for enhancing generated hot electrons, a vertical electric field at the pinch-off point help inject hot electrons into the floating gate
Data Source
AI summary
A programming method for programming stored bits in floating gates of a flash memory cell or selected flash memory cells of a flash memory array is utilized for applying SSI injection on said flash memory cell or said selected flash memory cells of a flash memory array is disclosed. Constant charges at the drain regions of said flash memory cell or said selected flash memory cells of the flash memory array is implemented with a capacitor and a related switch for suppressing variant injected-charges-related properties in applying the SSI injection. A constant biasing current, which may be implemented with a constant current source or a current mirror equipped with a constant current source, is applied on source regions of said flash memory cell or said selected flash memory cells of the flash memory array for enhancing the suppression of said variant biasing properties.