AC Stress Test Circuit for HCI Degradation Evaluation
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Solution Overview
Problem
The traditional methods for evaluating hot carrier injection (HCI) degradation in semiconductor devices lack accuracy, particularly in determining the AC/DC conversion factor, which is crucial for predicting device reliability as transistor sizes shrink, leading to increased electric fields and device degradation.
Innovation Solution
An AC stress test circuit with a ring oscillator circuit, isolating switches, and probing pads is used to directly measure AC Idsat degradation during ring oscillator stressing, avoiding simulation and maintaining accurate AC stress waveforms, while a separate DC stress test circuit evaluates DC-induced degradation, thereby enhancing the accuracy of HCI degradation evaluation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional AC/DC conversion factor method using inverter ring oscillator is used, then device degradation rate can be evaluated, but measurement precision is insufficient due to waveform distortion and simulation dependency
Solution Approach 1:
The patent introduces isolating switches as intermediary components between the ring oscillator circuit and probing pads. These switches act as mediators that prevent direct connection during AC stress application, thereby avoiding waveform distortion while still enabling measurement capability. The isolating switches are controlled to be in high-impedance state during AC stress, effectively decoupling the measurement system from the stressed circuit.
Solution Approach 2:
The patent employs dynamic switching control where the isolating switches change their state based on the operating mode (AC stress mode vs. measurement mode). During AC stress, switches are open/high-impedance to avoid distortion; during measurement, switches are closed/low-impedance to enable probing. This dynamic adaptation allows the system to optimize performance for each specific operation phase.
2Measurement precision
If higher stress-voltages are forced in ring oscillator to get AC frequency degradation, then AC stress effect can be observed, but stress waveform distortion occurs reducing measurement accuracy
Solution Approach 1:
The isolating switches serve as intermediaries that block the direct path between probing pads and ring oscillator during AC stress application. This prevents the probing equipment from loading the circuit and distorting the stress waveform, while still allowing voltage to be applied and frequency degradation to be measured through the isolated connection.
Solution Approach 2:
The patent segments the measurement system from the stress application system using isolating switches. The AC stress generation path is separated from the measurement path, allowing independent optimization of each function without mutual interference. This segmentation enables high-voltage stress application without compromising measurement signal integrity.
3Reliability
If traditional simulation-based AC Idsat degradation method is used, then correlation between AC frequency and DC Idsat degradation can be obtained, but accuracy is insufficient compared to direct measurement
Solution Approach 1:
The ring oscillator circuit serves dual purposes: it generates the AC stress waveform and simultaneously provides the measurement signal for frequency degradation detection. The same circuit that applies stress also reports its health status through frequency changes, eliminating the need for separate simulation models and reducing system complexity.
Solution Approach 2:
The patent merges the AC stress generation function and the degradation measurement function into a single integrated ring oscillator circuit. By combining these functions, the system eliminates the need for separate simulation-based prediction methods, directly obtaining accurate degradation data from the oscillator's own frequency responses under stress conditions.
Data Source
AI summary
An AC stress test circuit for HCI degradation evaluation in semiconductor devices includes a ring oscillator circuit, first and second pads, and first and second isolating switches. The ring oscillator circuit has a plurality of stages connected in series to form a loop. Each of the stages comprises a first node and a second node. The first and second isolating switches respectively connect the first and second pads to the first and second nodes of a designated stage and both are switched-off during ring oscillator stressing of the designated stage. The present invention also provides a method of evaluating AC stress induced HCI degradation, and a test structure.