Very-large-scale integration

Very-large-scale integration

Very-large-scale integration (VLSI) is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.

A VLSI integrated-circuit die

The first semiconductor chips held two transistors each. Subsequent advances added more and more transistors, and, as a consequence, more individual functions or systems were integrated over time. The first integrated circuits held only a few devices, perhaps as many as ten diodes, transistors, resistors and capacitors, making it possible to fabricate one or more logic gates on a single device. Now known retrospectively as small-scale integration (SSI), improvements in technique led to devices with hundreds of logic gates, known as medium-scale integration (MSI). Further improvements led to large-scale integration (LSI), i.e. systems with at least a thousand logic gates. Current technology has moved far past this mark and today's microprocessors have many millions of gates and billions of individual transistors.

At one time, there was an effort to name and calibrate various levels of large-scale integration above VLSI. Terms like ultra-large-scale integration (ULSI) were used. But the huge number of gates and transistors available on common devices has rendered such fine distinctions moot. Terms suggesting greater than VLSI levels of integration are no longer in widespread use.

As of early 2008, billion-transistor processors are commercially available. This is expected to become more commonplace as semiconductor fabrication moves from the current generation of 65 nm processes to the next 45 nm generations (while experiencing new challenges such as increased variation across process corners). A notable example is Nvidia's 280 series GPU. This GPU is unique in the fact that almost all of its 1.4 billion transistors are used for logic, in contrast to the Itanium, whose large transistor count is largely due to its 24 MB L3 cache. Current designs, as opposed to the earliest devices, use extensive design automation and automated logic synthesis to lay out the transistors, enabling higher levels of complexity in the resulting logic functionality. Certain high-performance logic blocks like the SRAM (Static Random Access Memory) cell, however, are still designed by hand to ensure the highest efficiency (sometimes by bending or breaking established design rules to obtain the last bit of performance by trading stability)[citation needed].


Structured design

Structured VLSI design is a modular methodology originated by Carver Mead and Lynn Conway for saving microchip area by minimizing the interconnect fabrics area. This is obtained by repetitive arrangement of rectangular macro blocks which can be interconnected using wiring by abutment. An example is partitioning the layout of an adder into a row of equal bit slices cells. In complex designs this structuring may be achieved by hierarchical nesting.

Structured VLSI design had been popular in the early 1980s, but lost its popularity later because of the advent of placement and routing tools wasting a lot of area by routing, which is tolerated because of the progress of Moore's Law. When introducing the hardware description language KARL in the mid' 1970s, Reiner Hartenstein coined the term "structured VLSI design" (originally as "structured LSI design"), echoing Edsger Dijkstra's structured programming approach by procedure nesting to avoid chaotic spaghetti-structured programs.


As microprocessors become more complex due to technology scaling, microprocessor designers have encountered several challenges which force them to think beyond the design plane, and look ahead to post-silicon:

  • Power usage/Heat dissipation – As threshold voltages have ceased to scale with advancing process technology, dynamic power dissipation has not scaled proportionally. Maintaining logic complexity when scaling the design down only means that the power dissipation per area will go up. This has given rise to techniques such as dynamic voltage and frequency scaling (DVFS) to minimize overall power.
  • Process variation – As photolithography techniques tend closer to the fundamental laws of optics, achieving high accuracy in doping concentrations and etched wires is becoming more difficult and prone to errors due to variation. Designers now must simulate across multiple fabrication process corners before a chip is certified ready for production.
  • Stricter design rules – Due to lithography and etch issues with scaling, design rules for layout have become increasingly stringent. Designers must keep ever more of these rules in mind while laying out custom circuits. The overhead for custom design is now reaching a tipping point, with many design houses opting to switch to electronic design automation (EDA) tools to automate their design process.
  • Timing/design closure – As clock frequencies tend to scale up, designers are finding it more difficult to distribute and maintain low clock skew between these high frequency clocks across the entire chip. This has led to a rising interest in multicore and multiprocessor architectures, since an overall speedup can be obtained by lowering the clock frequency and distributing processing.
  • First-pass success – As die sizes shrink (due to scaling), and wafer sizes go up (to lower manufacturing costs), the number of dies per wafer increases, and the complexity of making suitable photomasks goes up rapidly. A mask set for a modern technology can cost several million dollars. This non-recurring expense deters the old iterative philosophy involving several "spin-cycles" to find errors in silicon, and encourages first-pass silicon success. Several design philosophies have been developed to aid this new design flow, including design for manufacturing (DFM), design for test (DFT), and Design for X.


See also

Further reading

  • Baker, R. Jacob (2010). CMOS: Circuit Design, Layout, and Simulation, Third Edition. Wiley-IEEE. pp. 1174. ISBN 978-0-470-88132-3.
  • Weste, Neil H. E. and Harris, David M. (2010). CMOS VLSI Design: A Circuits and Systems Perspective, Fourth Edition. Boston: Pearson/Addison-Wesley. pp. 840. ISBN 978-0-321-54774-3.
  • Chen, Wai-Kai (ed) (2006). The VLSI Handbook, Second Edition (Electrical Engineering Handbook). Boca Raton: CRC. ISBN 0-8493-4199-X. 
  • Mead, Carver A. and Conway, Lynn (1980). Introduction to VLSI systems. Boston: Addison-Wesley. ISBN 0-201-04358-0. 


External links

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