Non-Inverting Ring Oscillators for Separate NMOS and PMOS Delay Measurement
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Solution Overview
Problem
Conventional silicon test structures cannot separately measure the delays of NMOS and PMOS transistors, leading to inaccurate identification of process corners and transistor performance, which is crucial for device design and performance optimization.
Innovation Solution
The development of silicon test structures using non-inverting stages to construct multi-stage ring oscillators, where each stage is sensitive to either NMOS or PMOS transistors, allowing for separate measurement of transistor delays by propagating primary transitions and generating opposing transitions through a series of stages, thus isolating the frequency determination to one type of transistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional inverting stages are used in ring oscillators, then both rising and falling transitions are measured, but separate measurement of NMOS and PMOS delays is not achieved
Solution Approach 1:
The ring oscillator is segmented into multiple non-inverting stages, where each stage is configured to be sensitive to only one transistor type (NMOS or PMOS). This segmentation allows the oscillator to separately measure delays for each transistor type by propagating specific transition types through designated stages, thereby preventing mixing of delay information from both transistor types.
Solution Approach 2:
Instead of using conventional inverting stages that naturally alternate between rising and falling transitions, the patent employs non-inverting stages that can maintain and propagate specific transition types. This inversion of the conventional approach allows primary transitions (either rising or falling) to be preserved through the oscillator loop, enabling separate measurement of transistor delays based on transition type.
2Reliability
If conventional ring oscillators measure both rising and falling transitions, then overall frequency is determined, but process corner identification accuracy is reduced
Solution Approach 1:
Different stages of the ring oscillator are assigned different local qualities by configuring them with specific transistor types (NMOS or PMOS). Each stage has a specialized sensitivity to particular transition types, allowing the system to locally measure delays for specific transistor types while maintaining overall oscillator functionality. This local specialization improves process corner identification accuracy.
Solution Approach 2:
The patent changes the parameter of transition type sensitivity by configuring non-inverting stages to respond differently to rising versus falling transitions. By controlling which transitions are propagated and which are reset, the system can selectively measure delays for NMOS or PMOS transistors, thereby changing the measurement parameter to match the specific process corner being evaluated.
3Measurement precision
If non-inverting stages are used to propagate primary transitions, then separate transistor delay measurement is enabled, but device complexity increases
Solution Approach 1:
The non-inverting stages are designed with multi-functionality to perform both signal propagation and transition type control. Each stage can function as either a primary transition propagation path or a reset path depending on the transition type it receives, reducing the need for completely separate circuit paths for measuring NMOS and PMOS delays. This universal design reduces overall device complexity while maintaining measurement separation.
Solution Approach 2:
The ring oscillator circuit uses self-service mechanisms where the propagation of primary transitions through non-inverting stages automatically generates the necessary reset pulses for opposing transitions. The circuit leverages its own internal signal flow to create reset conditions without requiring external control logic, thereby reducing complexity while enabling separate transistor delay measurements.
Data Source
AI summary
Silicon test structures are described that enable separate measurement of n-channel metal-oxide semiconductor (NMOS) and p-channel metal-oxide semiconductor (PMOS) transistor delays. NMOS and PMOS specific non-inverting stages may be used to construct a multi-stage ring oscillator. Each of the non-inverting stages generates either a rising or falling primary transition that is determined by either NMOS or PMOS transistors, respectively. The opposing transition for a particular non-inverting stage is triggered by propagation of the primary transition to a subsequent non-inverting stage (producing a “reset” pulse). A frequency of the ring oscillator is determined by the primary transition and one transistor type (NMOS or PMOS). Specifically, the frequency is determined by the propagation delay of the primary transition through the entire ring oscillator.