This preview shows page 1. Sign up to view the full content.
Unformatted text preview: ses is the well separation distance. In addition,
each well needs at least one well tap to ensure that the well is at the proper electrical potential.
To minimize the area consumption by meeting these requirements, wells in subcells are often
extended to the cell boundary if an adjacent cell contains a similarly placed well (sharing wells
across cell boundaries). Well taps are then added at the top level for the shared well. All realworld processes have a design rule for the area of the well or substrate that can be covered by a
given tap. As the distance to the tap increases, the substrate/well resistance causes the bulk
voltage to deviate farther from the ideal value. In an extreme case, the result can be the forward
biasing of the diode that exists between the source and the substrate/well resulting in latch-up or
other undesired effects. 11 It is necessary to strap transistor source and drain junctions with as many contacts as possible.
Place one contact as close as design rules allow to each end of the source and drain and spread as
many additional contacts as possible between the ones on either end (Fig.2.17 (a)). This ensures
that the entire width of the device is useful in drain current conduction. (a) (b) (c) (d) Fig.2.17 Contact layers: (a) Good, (b) bad, (c) bad, and (d) bad 12 References
[SNT] C. Saint and J. Saint, IC Layout Basics: A Practical Guide, McGraw-Hill, 2001.
[KAL] S.M. Kang and Y. Leblebici, CMOS Digital Integrated Circuits: Analysis and Design,
McGraw-Hill, Second Edition, 1999.
[WAE] N.H.E. Weste and K. Eshraghian, Principles of CMOS VLSI Design, Addison-Wesley
Publishing Company, Second Edition, 1993.
[RAB] J.M. Rabaey, Digital Integrated Circuits, Prentice Hall, 1996.
[MJS] Michael John Sebastian and John Smith, Application-Specific Integrated Circuits,
Wesley Publishing Company, 1997.
[CLN] Dan Clein, CMOS IC Layout, Newnes Publications. 2000.
[BTT] J.A. Butts (2001, December), Layout Tips and Techniques. Available:
View Full Document
This document was uploaded on 01/14/2014.
- Spring '13