The invention relates to methods and systems for improving integrated circuit performance through stress-engineering of the layout, and articles manufactured thereby.
It has long been known that semiconductor materials such as silicon and germanium exhibit the piezoelectric effect (mechanical stress-induced changes in electrical resistance). See for example C. S. Smith, “Piezoresistance effect in germanium and silicon”, Phys. Rev., vol. 94, pp. 42-49 (1954), incorporated by reference herein. The piezoelectric effect has formed the basis for certain kinds of pressure sensors and strain gauges, but only recently has it received attention in the manufacture of integrated circuits. In integrated circuit fabrication, one of the major sources of mechanical stress is the differential expansion and contraction of the different materials used. For example, typical fabrication technologies involve electrically isolating the active regions of groups of one or more transistors by surrounding them with shallow trench isolation (STI) regions which are etched into the silicon and then filled with an insulator, such as an oxide. The filling is performed at an elevated temperature. During the subsequent wafer cooling, oxides tend to shrink less than the surrounding silicon, and therefore develop a state of compressive stress laterally on the silicon regions of the device. Of significance is the stress exerted by the STI regions on the silicon forming a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) channel, because the piezoelectric impact of such stress can affect carrier mobility, and therefore current flow through the channel (Ion). In general, the higher the electron mobility in the channel, the faster the transistor switching speed.
The stress exerted on a region of silicon decays rapidly as a function of distance from the stress-causing interfaces. In the past, therefore, while process technologies could not produce today's extremely narrow channel widths, the stress-induced impact on performance could be ignored because only the edges of the diffusion region (adjacent to the STI regions) were affected. The channel regions were too far away from the STI regions to exhibit any significant effect. As process technologies have continued to shrink, however, the piezoelectric effect on transistor performance is no longer negligible.
Methods have been developed to model the impact of stress on the behavior of integrated circuit devices at the level of individual transistors. These methods include, for example, full-scale analysis with a Technology Computer Aided Design (TCAD) system; and a method known as the “Length-of-Diffusion” (LOD) method described in R. A. Bianchi et al., “Accurate Modeling of Trench Isolation Induced Mechanical Stress Effects on MOSFET Electrical Performance,” IEEE IEDM Tech. Digest, pp. 117 120 (December 2002), in U.S. Patent Publication No. 2002/0173588 (2003), and in Xuemei (Jane) Xi, et al., “BSIM4.3.0 Model, Enhancements and Improvements Relative to BSIM4.2.1”, University of California at Berkeley (2003), available at http://www device.eecs.berkeley.edu/, all incorporated herein by reference.
Behaviors characterized by the various methods for analyzing stress impact at the level of individual transistors can be used to derive circuit level parameters (e.g. SPICE parameters) of the device for subsequent analysis of the circuit at macroscopic levels. Such analysis can help predict whether the circuit will operate as intended, and with what margins, or whether the design or layout needs to be revised. If revision is necessary, it typically involves applying certain general rules-of-thumb, such as increasing the size of any transistor that, according to the stress analysis, turns out to be weaker than expected. But increasing the transistor size can degrade other performance measures, such as power consumption, so a compromise becomes necessary. In addition, the impact of stress on transistor performance is layout sensitive. Since typical irregularities in an integrated circuit layout result in different amount of impact on the performance of different transistors across the layout, these kinds of compromises typically must be made manually on a transistor-by-transistor basis. Still further, if automated place-and-route software is then used to re-layout the revised circuit design, the revised layout will differ from the original and show different stress effects than the original, often completely upsetting the circuit modifications that were made to accommodate the stress impact of the original layout.