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VLSI Techniques

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VLSI DesignWhat is VLSI?

“Very Large Scale Integration”Defines integration level1980s hold-over from outdated taxonomy for integration

levels Obviously influenced from frequency bands, i.e. HF, VHF, UHF

Sources disagree on what is measured (gates or transistors?)

SSI – Small-Scale Integration (0-102)MSI – Medium-Scale Integration (102-103)LSI – Large-Scale Integration (103-105)VLSI – Very Large-Scale Integration (105-107)ULSI – Ultra Large-Scale Integration (>=107)

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Moore’s LawIn 1960 Gordon Moore predicted “the

number of components that can be integrated on a single chip would increase at such a rapid rate that it will become twice in every 18 months”.

So by using Moore’s law we get an approximate integration level trend at any time.

But now moore’s law has reached its physical limit.

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Integration Level Trends

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Integrated Circuits/MEMsToday, VLSI refers to systems implementation with integrated

circuitsIntegrated circuit refers mostly to general manufacturing

technique micro/nano-scale devices on a semiconductor (crystalline) substrate Formed using chemical/lithography processing

What kind of devices / structures?transistors (bipolar, MOSFET)wires (interconnects and passives)diodes (junction, LEDs, VCSELs, MSM, photoconductor, PiN)MEMs (piezoelectric integration, accelerometers, gyroscopes,

pressure sensors, micro-mirrors)

For CMOS digital design, we only use MOSFET transistors (used as switches) and wires

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ChipsIntegrated circuits consist of:

A small square or rectangular “die”, < 1mm thick Small die: 1.5 mm x 1.5 mm => 2.25 mm2

Large die: 15 mm x 15 mm => 225 mm2

Larger die sizes mean: More logic, memory Less volume Less yield

Dies are made from silicon (substrate) Substrate provides mechanical support and electrical common point

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CMOS technique of IC fabricationCommon metal oxide semiconductor for

constructing FET on wafer chipN-well technique of fabrication on doped

silicon, poly silicon, metal oxide and silicon oxide layer is implemented.

On this pattern of various layers Optical lithography followed by photo resisting and etching is done.

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CMOS fabrication

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Chip Design stylesDesign styles

Full custom Semi custom

Array based

Pre diffused like gate arrays, sea of

gates etc

Pre wired like anti fuse based memory

based

Cell based

Macro cell like PLA gate matrix etc

Standard cell, hierarchical cell

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Phases of creating microelectronic chipsDesign : Circuit representation is converted

into geometric representationFabrication : involves method of deposition

and diffusion on waferTesting : circuit is tested to meet design

specificationsPackaging : each circuit is packaged by

establishing interconnections.

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DESIGN

Modeling

Synthesis and

optimization

Validation

FABRICATION

Mask fabrica

tion

Wafer fabrica

tion

TESTING

Tester 10000

110011 111000

Wafer

PACKAGING

Slicing

Packaging

Validation

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Concept of VLSI designPolygons represent layers deposited on the substrate

More of an art than science

One 2-input NAND gate with 4 transistorsTypical microprocessor contains 50 – 200 million

transistors (10-50 million gates)

Scale: approximately 10 um x 10 um

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Need of computerized design tools Manual layout of complex large scale design is obviously not

practical Design complexity:

Manually drawing layout for a billion transistors would take too long Even if we could… there are many problems like…

How to verify (test) designs for functionality, speed, power, etc.? Complexity scales faster than actual design

How to reuse designs? How to create human-readable designs? How to speed-up design process?

These problems form a great deal of work Electronic Design Automation (EDA) a.k.a. CAD

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VLSI CAD Various software like synopsys , cadence etc.

are used by designers to synthesize highly efficient VLSI chips.

Hardware description for IC is written in Verilog or VHDL.

It describes the hardware ,interconnection of circuit blocks and functionality.

VHDL(very high speed IC hardware design language) is the C of VLSI technology.

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VLSI applicationsBasically three areas of application exist

today for VLSIAnalog : Small transistor count precision

circuits such as Amplifiers, Data converters, filters, Phase Locked Loops, Sensors etc.

ASIC: application specific IC a microchip to perform and execute a particular task like digital signal processing, image compression etc.

SoC: systems on a chip are highly complex mixed signal processors like a network chip or a wireless radio chip.

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Challenges to VLSI technologyAs integration increases VLSI chips

somewhat suffer from the challenges such asPower dissipation due to increasing

componentsNoise delays due to capacitive or inductive

couplingDecrease in clock frequency by skin effect on

VLSI chipImproper scaling of wires for increasing

components.

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Future of VLSI

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Parameter 1979 1999 2019

Gate length(um) 5 0.2 0.008

Gate delay(ps) 3000 150 7.5

Clock cycle(ns) 200 2.5 0.08

Wire pitch(mm) 15 1 0.07

Grid/chip 2 x 10^5 3 x 10^8 3 x 10^11

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Future of VLSITechnology is evolving everyday and VLSI is

the most progressing one it is moving to ULSI.

It has been predicted that VLSI will develop more in the coming decade.

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