Memory Sensing Using Ramped Voltage for Multilevel Cell Verification
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Solution Overview
Problem
Current flash memory devices face challenges in efficiently sensing memory cells during programming and read operations, particularly in maintaining high performance and reliability as processor speeds increase and memory densities grow, without becoming a bottleneck in data transfers.
Innovation Solution
The implementation of a time varying access line voltage architecture using a counter and digital-to-analog converter to generate a ramped voltage, applied to selected word lines in NAND memory arrays, allows for efficient sensing operations by comparing count values to target threshold voltages, enabling effective program verify and read operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If memory density is increased by increasing the quantity of bits storable in each memory cell, then storage capacity is improved, but the time required for sensing operations increases
Solution Approach 1:
The sensing operation is divided into multiple parallel sensing paths, where each path handles a portion of the memory cells. This segmentation allows simultaneous sensing of multiple cells, reducing the total sensing time while maintaining high storage capacity through multilevel cells.
Solution Approach 2:
A time-varying ramp voltage is applied periodically to the word line during sensing operations. The voltage ramps over time to systematically scan through different threshold voltage ranges, enabling efficient detection of multilevel cell states without extending the overall sensing duration.
2Productivity
If the quantity of data to be transferred and programmed is increased to match processor performance, then data throughput is improved, but the sensing and programming time increases
Solution Approach 1:
Target threshold voltage values are pre-calculated and stored in latches before the actual programming operation begins. During programming, a counter generates ramp voltages that are compared against these pre-stored values, enabling rapid verification without recalculating thresholds during the time-critical programming phase.
Solution Approach 2:
The target threshold voltage information is copied into data latches in advance, creating a reference copy that can be quickly accessed during sensing and verification operations. This eliminates the need to retrieve or recalculate threshold values during the actual programming process, reducing latency.
3Quantity of substance
If multilevel cells are used to store multiple bits per cell, then storage density is improved, but the complexity of sensing operations increases
Solution Approach 1:
The sensing system uses dynamic ramp voltages that change over time rather than static voltage levels. The counter-generated ramp signal automatically adapts to scan through multiple threshold voltage ranges, dynamically identifying which voltage threshold the memory cell crosses, thereby simplifying the detection of multilevel states.
Solution Approach 2:
The sensing circuit incorporates feedback through comparison logic that monitors when the ramp voltage matches the stored target threshold voltage. When a match occurs (indicating the cell threshold has been reached), the system receives feedback to stop the ramp and record the result, enabling automatic detection of multilevel cell states.
Data Source
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AI summary
Methods for sensing, method for programming, memory devices, and memory systems are disclosed. In one such method for sensing, a counting circuit generates a count output and a translated count output. The count output is converted into a time varying voltage that biases a word line coupled to memory cells being sensed. Target data for each memory cell is stored in a data cache associated with that particular memory cell. When it is detected that a memory cell has turned on, the translated count output associated with the count output that is indicative of the voltage level that turned on the memory cell is compared to the target data. The comparison determines the state of the memory cell.