Mechanical stress and circuit aging aware VLSI CAD

Date

2010-12

Authors

Chakraborty, Ashutosh

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Abstract

With the gradual advance of the state-of-the-art VLSI manufacturing technology into the sub-45nm regime, engineering a reliable, high performance VLSI chip with economically attractive yield in accordance with Moore's law of scaling and integration has become extremely difficult. Some of the most serious challenges that make this task difficult are: a) the delay of a transistor is strongly dependent on process induced mechanical stress around it, b) the reliability of devices is affected by several aging mechanisms like Negative Bias Temperature Instability (NBTI), hot carrier injection (HCI), etc and c) the delay and reliability of any device are strongly related to lithographically drawn geometry of various features on wafer. These three challenges are the main focus of this dissertation.

High performance fabrication processes routinely use embedded silicon-germanium (eSiGe) technology that imparts compressive mechanical stress to PMOS devices. In this work, cell level timing models considering flexibility to modulate active area to change mechanical stress, were proposed and exploited to perform timing optimization during circuit placement phase. Analysis of key physical synthesis optimization steps such as gate sizing and repeater insertion was done to understand and exploit mechanical stress to significantly improve delay of interconnect and device dominated circuits.

Regarding circuit reliability, the proposed work is focused on reducing the clock skew degradation due to NBTI effect specially due to the use of clock gating technique for achieving low power operation. In addition, we also target the detrimental impact of burn-in testing on NBTI. The problem is identified and a runtime technique to reduce clock skew increase was proposed. For designs with predictable clock gating activities, a zero overhead design time technique was proposed to reduce clock skew increase over time. The concept of using minimum degradation input vector during static burn-in testing is proposed to reduce the impact of burn-in testing on parametric yield.

Delay and reliability strongly depend on dimension of various features on the wafer such as gate oxide thickness, channel length and contact position. Increased variability of these dimensions can severely restrict ability to analyze or optimize a design considering mechanical stress and circuit reliability. One key technique to control physical variability is to move towards regular fabrics. However, to make implementation on regular fabrics attractive, high quality physical design tools need to be developed. This dissertation proposes a new circuit placement algorithm to place a design on a structured ASIC platform with strict site and clock constraints and excellent overall wirelength. An algorithm for reducing the clock and leakage power dissipation of a structured ASIC by reducing spine usage is then proposed to allow lower power dissipation of designs implemented using structured ASICs.

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