SRAM Process Corner Sensing With Ring Oscillator Compensation
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Solution Overview
Problem
Existing process sensors are ill-suited for static random access memory (SRAM) device process corner sensing, as SRAM bitcells do not use standard core devices and require modification of the SRAM array, making them ineffective in detecting device skew.
Innovation Solution
A sensor compensation system that includes ring oscillators for evaluating P-devices and N-devices, allowing for accurate characterization of process corners without modifying the SRAM array, using control logic to activate the appropriate oscillator and generate compensation signals for read and write assist strategies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing process sensors are used for SRAM device sensing, then standard core devices can be monitored, but SRAM device process corners cannot be accurately detected because SRAM bitcells do not use standard core devices
Solution Approach 1:
The patent creates sensor copies of SRAM bitcells that replicate the exact transistor structure and layout of production bitcells. These sensor bitcells are placed within the SRAM array and mirror the electrical characteristics of actual SRAM devices, enabling accurate process corner detection without requiring modification to production bitcells.
Solution Approach 2:
The patent segments the SRAM array by dedicating specific bitcells as sensor elements while maintaining the majority as functional storage elements. This segmentation allows process monitoring without impacting the functionality of production bitcells, as sensors are spatially separated but electrically representative.
2Measurement precision
If SRAM array is modified to include process sensors, then SRAM device process corners can be detected, but the SRAM array configuration is altered and device skew detection capability is lost
Solution Approach 1:
The patent designs sensor bitcells that serve dual purposes: they function as process sensors for corner detection and simultaneously maintain compatibility with standard SRAM array operations. The sensors can be selectively activated for process monitoring while remaining electrically integrated with the SRAM structure, avoiding the need for separate sensor arrays.
Solution Approach 2:
The patent incorporates sensor bitcells into the SRAM array during the initial fabrication process, before the array is deployed for production use. This preliminary integration ensures that sensors are already positioned and configured to detect process variations without requiring subsequent modifications or disruptions to the SRAM array structure.
3Measurement precision
If ring oscillators are used to evaluate P-devices and N-devices, then accurate process corner characterization can be achieved, but additional control logic and oscillator circuits are required
Solution Approach 1:
The patent merges the ring oscillator evaluation circuits directly with the sensor bitcells, combining process sensing and oscillation functions into a unified structure. The sensor bitcells form part of the oscillator loop, eliminating the need for separate sensing and oscillation circuits, and reducing overall system complexity while maintaining measurement accuracy.
Data Source
AI summary
A system to enable detection of the process corner of each of the P and N devices of an SRAM array (of bitcells) and of peripheral devices. This permits more focused (optimized and compensated) design of non-SRAM peripheral circuits and of read and write assist circuits for better handling of process distribution impact on circuits improving functionality and yield.