SRAM Ring Oscillator Circuit for Process Variation Evaluation
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Solution Overview
Problem
Existing SRAM simulation methods fail to accurately predict operating performance due to process variations, and direct hardware measurements of SRAM cells do not provide comprehensive process information.
Innovation Solution
An evaluation circuit utilizing a ring-oscillation control circuit and multiple SRAM instances connected in series forms an SRAM ring oscillator, allowing for the extraction of instance-level AC and DC information, as well as process information of memory cell current and metal RC loading, by comparing actual oscillation signals with simulation waveforms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If simulation methods are used to evaluate SRAM operating performance, then evaluation can be performed without direct hardware measurement, but simulation results cannot accurately predict SRAM operating performance due to process variations
Solution Approach 1:
The patent introduces an intermediary evaluation circuit that includes a ring-oscillation control circuit and multiple SRAM instances connected in series. This intermediary structure bridges the gap between simulation and actual hardware by capturing real process information through internal signals while maintaining a controllable test structure that can be evaluated against simulation data.
Solution Approach 2:
The patent creates a simplified copy of the SRAM structure (the evaluation circuit with ring oscillator) that replicates the essential characteristics of the full SRAM device but in a more measurable and controllable form. This copy allows direct measurement of process information while maintaining relevance to the full device's performance characteristics.
2Measurement precision
If direct hardware measurement of SRAM cell characteristics is performed, then actual current and operating performance can be measured, but other operating performance and related process information cannot be known
Solution Approach 1:
The patent segments the SRAM evaluation into multiple independent SRAM instances connected in series, where each instance can be independently measured and evaluated. This segmentation allows comprehensive measurement of different parameters (current, voltage, timing) across multiple instances while maintaining the ability to extract specific process information from each segment's internal signals.
Solution Approach 2:
The patent adds a new dimension to measurement by introducing internal signal output terminals that provide access to intermediate signals within the SRAM instances. This additional measurement dimension enables extraction of process information (AC and DC characteristics) that cannot be obtained from external cell characteristics alone, while maintaining the ability to measure actual operating performance.
3Loss of information
If multiple SRAM instances are connected in series to form an SRAM string with ring-oscillation control, then comprehensive process information can be extracted, but the device complexity increases
Solution Approach 1:
The patent designs the evaluation circuit so that the same SRAM instances serve multiple functions: they act as both the device under test and as part of the measurement structure. The ring-oscillation control circuit uses the same SRAM instances to generate test signals and to be measured, eliminating the need for separate test structures and reducing overall device complexity despite the series connection.
Solution Approach 2:
The patent merges the test structure with the device under test by using the same SRAM instances for both functionality and measurement. The ring-oscillation control circuit is integrated into the SRAM string rather than being a separate external circuit, combining the device operations with the measurement capability in a single unified structure.
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
Enables accurate tracking of process variations and performance calibration of SRAM instances, enhancing the prediction of SRAM operating characteristics and silicon-to-simulation alignment.
Implementation Method 1
The ring-oscillation control circuit is coupled to the SRAM string to form a ring oscillator circuit
Data Source
AI summary
The disclosure provides an evaluation circuit for SRAM operations. The evaluation circuit includes a ring-oscillation control circuit and a plurality of SRAM instances connected in series with each other into an SRAM string. A WLE signal input terminal of an intermediate SRAM instance in the SRAM string is coupled to an internal signal output terminal of a previous-stage SRAM instance, and an internal signal output terminal of the intermediate SRAM instance is coupled to a WLE signal input terminal of a next-stage SRAM instance. An input terminal of the ring-oscillation control circuit is coupled to an internal signal output terminal of a last-stage SRAM instance in the SRAM string, and an output terminal of the ring-oscillation control circuit is coupled to a WLE signal input terminal of a first-stage SRAM instance in the SRAM string. Therefore, the ring-oscillation control circuit and the SRAM string form a ring oscillator circuit.