Analysis of Propagation Delay Deviation under Process Induced Threshold Voltage Variation

Process variation has become a major concern in the design of many nanometer circuits, including interconnect pipelines. The primary sources of manufacturing variation include Deposition, Chemical Mechanical Planarization (CMP), Etching, Resolution Enhancement Technology (RET). Process variations manifest themselves as the uncertainties of circuit performance, such as delay, noise and power consumption. The performance of VLSI/ULSI chip is becoming less predictable as device dimensions shrinks below the sub-100nm scale. The reduced predictability can be attributed to poor control of the physical features of devices and interconnects during the manufacturing process. Variations in these quantities maps to variations in the electrical behavior of circuits. Threshold voltage of a MOSFET varies due to changes in oxide thickness; substrate, polysilicon and implant impurity level; and surface charge. This paper provides a comprehensive analysis of the effect of threshold variation on the propagation delay through driver-interconnect-load (DIL) system. The impact of process induced threshold variations on circuit delay is discussed for three different technologies i.e 130nm, 70nm and 45nm. The comparison of results between these three technologies shows that as device size shrinks, the process variation issues becomes dominant during design cycle and subsequently increases the uncertainty of the delays.