By Dr. Zhihong Liu, Executive Chairman, ProPlus Design Solutions, Inc.
SPICE Models Play a Critical Role in Both Modeling and Design Communities
Circuit designers work with foundry libraries to evaluate a foundry process before they run real circuit designs. It, therefore, becomes necessary to understand the models and use them properly. Complexities of modern libraries have made it inefficient or almost impossible to understand them by browsing into the files.
A library can easily contain many different sections besides core models in a macro (sub-circuit) format, such as multiple corner model sections, statistical model sections, mismatch model components, models for layout dependent effects and reliability models. Without a good understanding of those details, simulations by combining those model sections may lead to inaccurate results.
Second, foundry models often are not built for specific applications. Design companies are investing in SPICE models by doing model validations, customizing models for specific needs or even building their own models. High-end systems-on-chip (SoCs) are now integrating more functionalities and may have different operation modes, evaluated by performance, power, area, lifetime, cost (yield) and time to market.
Design specifications are tougher, but the room to maneuver has shrunk. One set of generic models can’t meet the requirements for all different applications. Thus, it’s worthwhile for design companies to identify the real needs of their applications, then work with foundries or third parties or build their own capabilities to make model libraries more application specific and provide more value for their designs.
Third, the key motivation for a circuit designer to understand foundry model libraries is the impact of process variations on circuit performance and yield. Although process engineers have tried different ways to mitigate variation sources during manufacturing, some remain in a design that are fundamental and must be managed during different design stages, including global and local random variations or LDE.
Designers can only cross their fingers if they do not know the possible results before tapeout. Modeling engineers have figured out ways to model those systematic and random variation effects. The next step is to apply that information and analyze the impact to a design.
Strain engineering improves device performance, but leads to the strong layout dependence of device characteristics. Designers then need to consider the impact of LDE during pre-layout design, layout design, LVS extraction and post-layout verifications. Understanding the LDE based on the models would help designers better optimize area versus performance, and reduce differences between pre- and post-layout designs to shorten design time.
Increasing random variations, especially the local mismatch for paired transistors, affect the final chip yield and performance. Traditional PVT analysis and selective Monte Carlo analysis give limited information that can help achieve chip’s functionalities, but not the possible yield or performance distributions.
A reliable and practical design for yield (DFY) flow with fast and accurate statistical simulation engine is required. Moreover, before using DFY tools for yield analysis targeting yield and performance trade-off, designers need to know how corner models and statistical models are defined. Otherwise analysis results, based on improper use of the variation models, will offer the wrong direction for design optimizations.
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