Back Bias for Non-Volatile Storage Verify Accuracy
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Solution Overview
Problem
In flash memory programming, verifying intermediate states is challenging due to low verify reference voltage levels, which can lead to difficulties in accurately determining the programming state, especially as process technology scales and cell-to-cell interference increases, causing shifts in threshold voltage distributions.
Innovation Solution
Applying a back bias during the verification of intermediate states, while using no or reduced back bias during the verification of final states, helps in testing the verify reference levels effectively and reduces sense amplifier headroom issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If verify reference voltage levels are kept low during intermediate state verification, then power consumption is reduced, but measurement precision deteriorates making it difficult to accurately determine programming state
Solution Approach 1:
The patent applies back bias voltage to the substrate during verification operations, which dynamically changes the electrical parameters of the memory cell. This back bias adjustment modifies the threshold voltage characteristics, enabling accurate verification at low reference voltage levels by compensating for voltage drops and maintaining sufficient signal margins for accurate state determination.
2Measurement precision
If back bias is applied during intermediate state verification, then measurement precision improves, but device complexity increases due to additional bias control circuitry
Solution Approach 1:
The back bias control circuitry is designed to serve multiple functions: it provides substrate bias during verification operations, controls threshold voltage shifts during programming, and maintains compatibility with both intermediate and final state verification. This multi-functionality reduces the need for separate dedicated circuits for each operation type.
3Measurement precision
If verify reference levels are increased to improve measurement precision, then programming state determination accuracy improves, but sense amplifier headroom is reduced
Solution Approach 1:
Rather than increasing verify reference voltage levels, the patent changes the substrate bias parameter during verification. This back bias adjustment effectively shifts the memory cell's threshold voltage, allowing verification to be performed at low reference voltage levels while maintaining accurate state determination, thus preserving sense amplifier headroom.
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 allows for more accurate verification of intermediate states with lower verify reference levels without reducing sense amplifier headroom, effectively managing threshold voltage shifts and improving programming accuracy across different states.
Implementation Method 1
a back bias (also referred to as body bias) is applied during verify of an intermediate state
Data Source
AI summary
Different back bias (or body bias) conditions are applied to a non-volatile storage system during different program verify operations of a programming operation. A back bias may be applied during verify of an intermediate state (e.g., a lower page, middle page). The intermediate state is a state that exists during a program operation, but is not one of the final states. A lower back bias or no back bias is applied during verify of a final state (e.g., an upper page). Thus, a different back bias may be used when verifying an intermediate state than the back bias used when verifying a final state. Using the back bias makes it easier to verify a low VTH, such as a negative VTH. Also, using the back bias is effective at dealing with sense amplifier headroom issues.