Memory Read Accuracy via Dual Sense Times and Word Line Grounding
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Solution Overview
Problem
Memory devices face challenges in accurately determining the read situation of memory cells due to variations in threshold voltage (Vth) caused by coupling up of word lines, leading to potential read errors and data state misinterpretation.
Innovation Solution
Implementing techniques such as using two sense times per read voltage, grounding word line voltages, applying a weak pulldown to dissipate coupling up voltages, and periodically applying a dummy word line voltage to maintain cells in specific read situations, allowing for optimal data retrieval and error reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If word lines are allowed to float after programming, then power consumption is reduced, but coupling up of word lines causes threshold voltage variations leading to read errors
Solution Approach 1:
The patent applies preliminary grounding of word lines after programming operations to prevent coupling up from occurring in the first place. By establishing a known reference state (grounded) immediately after programming, the system eliminates the subsequent voltage drift that would otherwise cause read errors, while still allowing power savings during idle periods.
Solution Approach 2:
The patent dynamically changes the voltage parameter of word lines between two states: grounded (0V) after programming to prevent coupling, and floating during idle periods to save power. This parameter switching based on operational state resolves the contradiction between power consumption and read accuracy.
2Speed
If a single sense time is used per read voltage, then read operation speed is improved, but accuracy decreases due to inability to distinguish between different read situations
Solution Approach 1:
The patent segments the read operation into multiple sense times (first sense time and second sense time) to capture voltage states at different moments. This segmentation allows the system to distinguish between first read situations (immediate post-programming) and second read situations (after coupling up), thereby improving detection accuracy without significantly impacting overall read speed.
Solution Approach 2:
The patent uses feedback from multiple sense time measurements to determine the appropriate read situation. By comparing voltage readings taken at different times, the system can identify whether coupling up has occurred and adjust its interpretation accordingly, improving measurement precision through iterative verification.
3Measurement precision
If dummy read operations are performed to determine read situation, then read accuracy is improved, but additional time is consumed
Solution Approach 1:
The patent merges the dummy read operation with the actual data read operation by performing both sensing activities in sequence rather than as separate operations. The first sense time serves dual purposes: it acts as a dummy read to determine read situation and simultaneously begins the data retrieval process, thereby reducing total time loss.
Solution Approach 2:
The patent performs a partial read operation (sense only, without full data retrieval) to determine read situation, then uses this information to optimize the subsequent full read operation. This partial action approach minimizes time consumption while still achieving accurate read situation detection.
Data Source
AI summary
Techniques are provided for improving the accuracy of read operations of memory cells, where the threshold voltage of a memory cell can shift depending on when the read operation occurs. A memory cell is sensed by discharging a sense node into a bit line and detecting an amount of discharge at two sense times relative to a trip voltage. A bit of data is stored in first and second latches based on the two sense times, to provide first and second pages of data. The pages are evaluated using parity check equations and one of the pages which satisfies the most equations is selected. In another option, word line voltages are grounded and then floated to prevent coupling up of the word line. A weak pulldown to ground can gradually discharge a coupled up voltage of the word lines.