Ring Oscillator Stages for PFET-NFET Process Corner Sensing
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Solution Overview
Problem
Integrated circuits (ICs) face challenges in accurately assessing process variation and aging effects on n-channel and p-channel field effect transistors (NFETs and PFETs) due to variations in switching speeds, which affect the performance and reliability of mass-produced ICs, particularly in identifying process corners and compensating for temperature-related instabilities.
Innovation Solution
The implementation of ring oscillators (ROs) with cascaded stages and diode-connected field effect transistors (FETs) to generate oscillating signals whose frequencies are dependent on the switching speeds of NFETs and PFETs, allowing for the characterization of process performance and aging effects, enabling more precise process monitoring and compensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ring oscillators are used to assess process variation, then measurement capability is provided, but sensitivity to process performance variations and aging effects is insufficient
Solution Approach 1:
The ring oscillator is divided into multiple cascaded stages, each containing separate PFET and NFET logic inverters. This segmentation allows independent measurement of PFET and NFET performance characteristics, enabling more precise assessment of process variations and aging effects on each transistor type.
Solution Approach 2:
Diode-connected FETs are strategically placed within specific stages to create localized current mirrors that reference the switching speeds of individual transistor types. This local quality enhancement improves the sensitivity of measurements for specific process corners while maintaining overall circuit functionality.
2Productivity
If mass production of ICs is pursued, then productivity increases, but process variation affecting transistor switching performance becomes more significant
Solution Approach 1:
The ring oscillator circuit provides inherent feedback through its cascaded stage structure, where the output of each stage feeds back to influence the switching behavior of subsequent stages. This feedback mechanism amplifies small process variations, making them measurable and enabling compensation strategies to maintain manufacturing precision across mass production.
Solution Approach 2:
The circuit uses its own oscillating signal to characterize its performance parameters. By measuring the frequency and duty cycle of its self-generated signal, the ring oscillator automatically provides process monitoring data without requiring external test equipment, enabling self-service quality control in mass production.
3Reliability
If temperature-related instabilities are present, then reliability decreases, but compensation mechanisms are needed that do not add excessive complexity
Solution Approach 1:
The circuit employs asymmetric configurations of PFET and NFET logic inverters with different numbers of series/parallel transistor connections. This asymmetry creates differential responses to temperature changes, enabling temperature compensation through comparison of the oscillating signals from PFET-dominated and NFET-dominated stages without requiring additional compensation circuitry.
Data Source
AI summary
An integrated circuit (IC) including a first ring oscillator (RO) including a first set of cascaded stages, wherein each of the first set of cascaded stages comprises a first logic inverter, including: a first set of field effect transistors (FETs) coupled in parallel between a first voltage rail and a first intermediate node, wherein the first set of FETs include a set of gates coupled to an input of the first logic inverter; and a second set of FETs coupled in series between the first intermediate node and a second voltage rail, wherein the second set of FETs includes at least a first FET including a gate coupled to the input of the first logic inverter, and at least a second FET that is diode-connected in accordance with a first mode of operation.