Circuit Reliability Simulation
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Chenming Hu University of California Dept. of Electrical Engineering and Computer Science 231 Cory Hall, #1770, Berkeley, CA 94720-1770 ABSTRACT In designing a complex circuit, designers make a large number of circuit simulations, design changes and optimizations and can predict the circuit's performance reasonably accurately before committing it to silicon. It would be unthinkable to bypass detailed circuit simulation and optimization and rely on simple design rules and the testing of finished IC's to discover errors or to find out if the performance of the circuit meet specifications. Yet, this is basically the way IC reliability is treated today. A logical alternative is to predict circuit reliability at the circuit design stage through reliability simulation. Reliability simulator BERT is used to illustrate the physical models and approaches used to simulate the hot electron effect, oxide time-dependent breakdown, electromigration, bipolar transistor gain degradation, and radiation effects. The goal is to make circuit reliability simulation a part of the IC design process. INTRODUCTION Circuit designers use circuit simulators to predict and optimize circuit performance before committing the design to silicon, saving in development costs and providing the opportunity for interactive feedback during the design process. As computer aided design grows in sophistication, more and more aspects of the IC development process will be modeled. In today's high performance VLSI chips, designs have been pushed to a limit imposed by reliability concerns. There is obviously a place for circuit reliability simulators in the CAD tool library. This paper describes one such IC reliability simulator, Berkeley Reliability Tools (BERT). BERT simulates the circuit degradation (drift) due to hot-electron degradation in MOSFET's and bipolar transistors and predicts circuit failure rates due to oxide breakdown and electromigration in CMOS, bipolar, and BiCMOS circuits. With the increasing importance of reliability in today's and future technology, a reliability simulator such as this is expected to serve as the engine of circuit "design-for-reliability" in a "building-in-reliability paradigm. BERT works in conjunction with a circuit or timing simulator in order to simulate reliability of actual circuits, and acts as an interactive tool for design. The user may use BERT repetitively to understand how specific design choices affect reliability as well as to quantify the reliability of the chosen design as shown in Fig. 1. After a reliable circuit has been designed with the CAD tools, fabrication commences. During the production process, BERT can further help by determining the optimal stress-screen and bumin conditions for each product.
3 Mat. Res. Soc. Symp. Proc. Vol. 391 0 1995 Materials Research Society
Use of BERT in an IC Development Process
Figure 1.
The process of designing for reliability with BERT.
HOT-CARRIER DEGRADATION OF MOSFET'S AND BIPOLAR TRANSISTORS For hot-carrier effect simulations, the fresh circuit is first sim
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