Global Interconnects in the Presence of Uncertainty

Global interconnect reliability is becoming a bigger issue as we scale down further into the submicron regime. As transistor dimensions get smaller, variations in the manufacturing process, and temperature variations may cause undesired behavior, and as a result, compromise performance. This work makes an effort to characterize the effects of such variations, to provide designers with a guideline for making designs tolerant to these variations while benefiting from tighter design margins. Since interconnects contribute to most of the delay and power on a chip, interconnect performance becomes a primary issue in design. One of the main concerns when considering physical transistor dimension variations is the effect on delay. Due to smaller transistor dimensions, the photolithographic process may produce transistors with significant variations from the ideal physical dimensions. Such variations cause delay uncertainty which can lead to over or underestimation in the design phase. This work examines interconnects to establish a guideline of the effect that process variations have on delay. A repeated interconnect is analyzed and the effects of physical device variations on delay are observed. Given the delay distribution in the presence of Leff variation, a supply voltage assignment technique is proposed to correct the observed deviation from the nominal delay on a long, repeated interconnect. This technique results in a significant reduction of the delay distribution, with a negligible power overhead. After looking at static variation effects on interconnect performance, this thesis addresses thermal variations on global signals, which cause delay degradation and may lead to timing failures. Given the presence of a large thermal gradient along a clock signal in a data path clocked by two leaves of an H-tree, several thermal scenarios which can compromise timing are discussed. A buffer-based skew compensation technique is proposed to correct the effect of thermal and manufacturing variations on this system. Having characterized repeated interconnect performance under process variations, the bandwidth of the line can be more effectively utilized by using a technique called phase coding. Phase coded interconnects are introduced in the context of using them once an interconnect has been adequately modeled in the presence of variations. With guidelines quantifying the effects of process variations on interconnect techniques and careful characterization, designers can factor these considerations into their design process, reducing the variation from the nominal expected behavior and allowing for smaller design margins. This will lead to more reliable products as we advance into future technologies and transistor dimensions get smaller