Robust Circuit Design: Challenges and Solutions

Scaling with Moore’s law is taking us to feature sizes of 32nm and smaller. At these technology nodes designers are faced with an explosion in design complexity at all levels. In this tutorial we discuss three somewhat novel and particularly confounding dimensions of this complexity: • Electrical complexity: Digital circuit designers have particularly benefited from abstractions of the underlying MOS devices while designing circuits. For them, a transistor is an ideal switch and a wire is a perfect short between two nodes. This simplified abstraction is the driving force behind our capability to design and verify chips with about a billion transistors today. However, with aggressive device scaling, the properties of the devices that are being manufactured today are moving further away from the abstractions that we have been using to verify our designs. In this section of the tutorial, we look at some of the recent trends along these lines and some of the techniques that designers use to extract ideal functionality from non-ideal devices. We use design examples, both from analog/mixed-signal (PLL, ADC design) and digital domain (clock tree, power network, static timing, etc.), as illustrative cases studies. • Manufacturing complexity: Minimum feature sizes at 45nm are already a quarter of the wavelength of light used for lithography. Consequently, imperfections in manufacturing are unavoidable and large enough to significantly change the intended design, resulting in dreaded yield loss. Any design today has to satisfy stringent manufacturability and yield requirements. At the same time, the complexity of critical variation mechanisms renders any simplified methods, like corner analysis, ineffective. Design methods and tools are being changed at all levels of the design flow to improve yield prediction and increase manufacturing robustness. In this vein, this tutorial will cover a broad spectrum of topics: 1) relevant state-of-the-art manufacturing process steps at 45 nm (193 nm lithography, ion implantation, etc.) and the physical mechanisms resulting in electrical performance variations, 2) recently proposed design techniques for mitigating the electrical variability, and 3) recently proposed design tools for increasing robustness and predicting the yield impact of this variability. We will look at various design phases from circuit architecture down to post-layout, and at several applications from SRAMs to ASICs to analog. • Reliability complexity: With nearly three decades of continued CMOS scaling, the devices have now been pushed to their physical and reliability limits. Transistors on the latest chips in 45nm technology are so small that some of their parts are just a few atoms apart. Designs manufactured correctly may become unreliable over time because of mechanisms like NBTI, gate oxide breakdown and soft errors. The impact of unreliability manifests as time-dependent variability where the electrical characteristics of the devices vary statistically in a temporal manner, directly translating into design uncertainty in manufactured chips. Scaling to sub45nm technology nodes changes the nature of reliability effects from abrupt functional problems to progressive degradation of the performance characteristics. The material presented in this section of the tutorial is intended for designers to form a thorough