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Aging in Standard Cells: What You Need to Know

Introduction

In the fast-paced world of integrated circuit (IC) design, ensuring the reliability and longevity of semiconductor devices is crucial. As process technologies shrink and operating frequencies increase, ICs are becoming more susceptible to reliability issues, including aging effects. To address these concerns, designers use standard cell libraries (.lib) in their designs. This blog post aims to provide a detailed overview of standard cell reliability and aging, along with methods to check and mitigate potential issues.

1. What are Standard Cell Libraries?

Standard cell libraries are collections of pre-designed and pre-characterized functional units, such as logic gates and flip-flops, that are used to build complex digital circuits. These libraries come in various process corners, voltage, and temperature conditions to enable design optimization and ensure robust functionality. However, as semiconductor devices age, their performance can degrade due to various aging mechanisms.

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2. Understanding Reliability and Aging in ICs

Reliability refers to the ability of an integrated circuit to perform its intended function accurately and consistently over its operational lifetime. Aging, on the other hand, involves the gradual degradation of a semiconductor device’s performance over time due to physical and chemical processes.

Common aging mechanisms in ICs include:

a. Bias Temperature Instability (BTI): BTI is a phenomenon where a transistor’s threshold voltage shifts over time due to prolonged exposure to high electric fields and elevated temperatures.

b. Hot Carrier Injection (HCI): HCI occurs when high-energy carriers (electrons or holes) gain enough kinetic energy to become trapped in the gate oxide, leading to a shift in transistor characteristics.

c. Time-Dependent Dielectric Breakdown (TDDB): TDDB is the gradual degradation of the insulating dielectric material due to the constant stress of the electric field.

d. Negative Bias Temperature Instability (NBTI): NBTI is a type of BTI where the threshold voltage of a p-channel transistor shifts under negative bias conditions.

3. Importance of Reliability Checks in .lib Files

As standard cell libraries play a critical role in chip design, it is vital to assess their reliability and aging characteristics. Including reliability data in .lib files allows the synthesis and place-and-route tools to make informed decisions during the design process. By considering aging effects, designers can improve the lifetime and performance of the final product.

4. Methods to Check Standard Cell Library Reliability and Aging

To evaluate the reliability and aging characteristics of a standard cell library, designers can employ the following methods:

a. Age-Based Library Characterization

Age-based library characterization involves subjecting standard cells to accelerated aging conditions and simulating the aging effects over an extended period. The resulting data, including aging-aware delay and power models, is then incorporated into the .lib files for use in the design flow. This method helps identify potential aging-related issues in the circuit early in the design process.

Study Materials:

  • “Reliability Challenges in Advanced CMOS Technologies” by Reis, R., Dunga, S., & Veloso, A. (Link: https://ieeexplore.ieee.org/abstract/document/6181060/)
  • “Aging-aware standard cell characterization using an efficient measurement-based methodology” by Karakonstantis, G., Abbas, M., Chakravarty, S., & Roy, K. (Link: https://ieeexplore.ieee.org/abstract/document/6672711/)

b. Simulation with Aging Models

Designers can use aging models (BTI, HCI, NBTI, TDDB, etc.) to simulate the effects of aging in the circuit. SPICE simulators, along with specific aging models, enable designers to analyze the impact of aging on critical parameters like delay, power, and leakage in the design. This approach helps designers gain insight into aging-related effects without the need for expensive and time-consuming physical testing.

Study Materials:

  • “A Compact Model for Time-Dependent Dielectric Breakdown (TDDB) Aging in MOSFETs” by Lee, H., Kim, J., Zhang, W., Cao, Y., & Saraswat, K. C. (Link: https://ieeexplore.ieee.org/abstract/document/6458422/)
  • “Modeling and Simulation of NBTI Effect on CMOS Inverter” by Agrawal, N., & Singh, R. (Link: https://ieeexplore.ieee.org/abstract/document/8864382/)

c. Reliability Analysis Tools

Various commercial and open-source tools are available that analyze the .lib files and provide reliability and aging reports. These tools use the data from age-based characterization or aging models to estimate the mean time to failure (MTTF) and identify potential reliability hotspots in the design. Additionally, they can help designers optimize their circuits to improve reliability and mitigate aging effects.

Study Materials:

  • “Yield-aware reliability analysis of nanometer CMOS standard cell library under NBTI stress” by Mo, Y., Mao, Y., Zhang, H., & Zhang, Y. (Link: https://ieeexplore.ieee.org/abstract/document/6635970/)

d. Feedback from Post-Silicon Testing

Analyzing reliability-related failures encountered during post-silicon testing provides valuable feedback on the accuracy of aging models and the effectiveness of aging-aware design techniques. This feedback loop allows designers to improve future .lib files and refine aging mitigation strategies for upcoming projects.

Study Materials:

  • “In-Field Reliability Monitor for Digital Standard Cell Libraries” by Kapre, N., He, L., & Roy, K. (Link: https://ieeexplore.ieee.org/abstract/document/6049091/)

5. Mitigation Strategies

To mitigate the impact of aging effects on ICs, designers can adopt several strategies:

a. Aging-Aware Design Techniques

Implement techniques such as redundancy, error correction codes, and adaptive voltage scaling to prolong the operational lifetime of the chip. These techniques can help compensate for aging-related performance degradation and ensure the circuit’s reliable operation over an extended period.

Study Materials:

  • “Energy-Efficient Aging-Aware Circuit Design” by Esmaeilzadeh, S., & Pedram, M. (Link: https://ieeexplore.ieee.org/abstract/document/6190686/)

b. Guard Bands

Add guard bands to critical paths in the design to account for aging-related delays. Guard bands provide extra margin to ensure proper circuit operation even as aging effects manifest over time.

Study Materials:

  • “Aging-Aware Design with Dual-Vth Assignment for Standard Cell-based VLSI Circuits” by Hamamoto, M., Harada, T., Tamaru, S., Kuroda, T., & Furuta, M. (Link: https://ieeexplore.ieee.org/abstract/document/6505658/)

c. Temperature and Voltage Management

Regulate the operating temperature and supply voltage within specified limits to minimize aging-related issues. Controlling these parameters can help reduce the acceleration of aging mechanisms and improve overall reliability.

Study Materials:

d. Technology Node Selection

Choose a technology node that offers better reliability and aging characteristics based on process technology research. Advanced nodes with improved material properties and design techniques may exhibit reduced aging effects, making them more suitable for long-lasting applications.

Study Materials:

Conclusion

In the competitive landscape of semiconductor design, ensuring the reliability and aging resilience of integrated circuits is paramount. By comprehensively understanding the aging mechanisms and performing thorough reliability checks on standard cell libraries, designers can deliver high-performance, long-lasting chips. Incorporating aging-aware design techniques and utilizing advanced tools will pave the way for robust and reliable ICs that meet the demands of modern applications.

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