Very-large-scale integration

"VLSI" redirects here. For the former company, seeVLSI Technology.

Not to be confused withVery High Speed Integrated Circuit.

Very-large-scale integration(VLSI) is the process of creating anintegrated circuit(IC) by combining millions orbillionsofMOS transistorsonto a single chip. VLSI began in the 1970s whenMOS integrated circuit(metal oxide semiconductor) chips were developed and then widely adopted, enabling complexsemiconductorandtelecommunicationstechnologies. Themicroprocessorandmemory chipsare VLSI devices.

Before the introduction of VLSI technology, most ICs had a limited set of functions they could perform. Anelectronic circuitmight consist of aCPU,ROM,RAMand otherglue logic.VLSI enables IC designers to add all of theseinto one chip.

A VLSI integrated-circuitdie

History

Background

Thehistory of the transistordates to the 1920s when several inventors attempted devices that were intended to control current in solid-state diodes and convert them into triodes. Success came after World War II, when the use of silicon and germanium crystals as radar detectors led to improvements in fabrication and theory. Scientists who had worked on radar returned to solid-state device development. With the invention of the firsttransistoratBell Labsin 1947, the field of electronics shifted from vacuum tubes tosolid-state devices.

With the small transistor at their hands, electrical engineers of the 1950s saw the possibilities of constructing far more advanced circuits. However, as the complexity of circuits grew, problems arose.One problem was the size of the circuit. A complex circuit like a computer was dependent on speed. If the components were large, the wires interconnecting them must be long. The electric signals took time to go through the circuit, thus slowing the computer.

Theinvention of the integrated circuitbyJack KilbyandRobert Noycesolved this problem by making all the components and the chip out of the same block (monolith) of semiconductor material.The circuits could be made smaller, and the manufacturing process could be automated. This led to the idea of integrating all components on a single-crystal silicon wafer, which led to small-scale integration (SSI) in the early 1960s, and then medium-scale integration (MSI) in the late 1960s.

VLSI

See also:MOS integrated circuit

General Microelectronicsintroduced the first commercialMOSintegrated circuitin 1964.In the early 1970s, MOS integrated circuit technology allowed the integration of more than 10,000 transistors in a single chip.This paved the way for VLSI in the 1970s and 1980s, with tens of thousands of MOS transistors on a single chip (later hundreds of thousands, then millions, and now billions).

The first semiconductor chips held two transistors each. Subsequent advances added 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 tendiodes,transistors,resistorsandcapacitors,making it possible to fabricate one or morelogic gateson a single device. Now known retrospectively assmall-scale integration(SSI), improvements in technique led to devices with hundreds of logic gates, known asmedium-scale integration(MSI). Further improvements led tolarge-scale integration(LSI), i.e. systems with at least a thousand logic gates. Current technology has moved far past this mark and today'smicroprocessorshave 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 likeultra-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.

In 2008, billion-transistor processors became commercially available. This became more commonplace as semiconductor fabrication advanced from the then-current generation of65 nmprocessors. Current designs, unlike the earliest devices, use extensivedesign automationand automatedlogic synthesistolay outthe transistors, enabling higher levels of complexity in the resulting logic functionality. Certain high-performance logic blocks, like the SRAM (static random-access memory) cell, are still designed by hand to ensure the highest efficiency.

Structured design

Structured VLSI design is a modular methodology originated byCarver MeadandLynn Conwayfor saving microchip area by minimizing the interconnect fabric area. This is obtained by repetitive arrangement of rectangular macro blocks which can be interconnected usingwiring 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 laterbecause of the advent ofplacement and routingtools wasting a lot of area byrouting,which is tolerated because of the progress ofMoore's law.When introducing thehardware description languageKARL in the mid-1970s,Reiner Hartensteincoined the term "structured VLSI design" (originally as "structured LSI design" ), echoingEdsger Dijkstra'sstructured programmingapproach by procedure nesting to avoid chaoticspaghetti-structuredprograms.

Difficulties

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

• Process variation– Asphotolithographytechniques get closer to the fundamental laws of optics, achieving high accuracy indopingconcentrations and etched wires is becoming more difficult and prone to errors due to variation. Designers now must simulate across multiple fabricationprocess cornersbefore a chip is certified ready for production, or use system-level techniques for dealing with effects of variation.
• Stricter design rules– Due to lithography and etch issues with scaling,design rule checkingforlayouthas become increasingly stringent. Designers must keep in mind an ever increasing list of rules when laying out custom circuits. The overhead for custom design is now reaching a tipping point, with many design houses opting to switch toelectronic design automation(EDA) tools to automate their design process.
• Timing/design closure– Asclock frequenciestend to scale up, designers are finding it more difficult to distribute and maintain lowclock skewbetween these high frequency clocks across the entire chip. This has led to a rising interest inmulticoreandmultiprocessorarchitectures, since anoverall speedupcan be obtained even with lower clock frequency by using the computational power of all the cores.
• First-pass success– Asdiesizes shrink (due to scaling), andwafersizes go up (due to lower manufacturing costs), the number of dies per wafer increases, and the complexity of making suitablephotomasksgoes up rapidly. Amask setfor 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), andDesign for X.
• Electromigration

See also

• Application-specific integrated circuit
• Caltech Cosmic Cube
• Interface logic model
• List of semiconductor fabrication plants
• Mead–Conway VLSI chip design revolution
• Neuromorphic engineering
• Polycrystalline silicon
• System on a chip(SoC)

Further reading

• Baker, R. Jacob (2010).CMOS: Circuit Design, Layout, and Simulation, Third Edition.Wiley-IEEE. p. 1174.ISBN978-0-470-88132-3.
• Weste, Neil H. E. & Harris, David M. (2010).CMOS VLSI Design: A Circuits and Systems Perspective, Fourth Edition.Boston: Pearson/Addison-Wesley. p. 840.ISBN978-0-321-54774-3.
• Chen, Wai-Kai (2007).The VLSI handbook.Boca Raton, FL: CRC/Taylor & Francis.ISBN978-1-4200-0596-7.OCLC83977431.
• Mead, Carver A.;Conway, Lynn(1980).Introduction to VLSI systems.Boston: Addison-Wesley.ISBN0-201-04358-0.

External links

• Lectures on Design and Implementation of VLSI Systems at Brown University
• Design of VLSI Systems