VLSI Computers

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Nov 26, 2013 (3 years and 10 months ago)

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VLSI Computers

COT 4810

Ken Pritchard

30 Sep 04

Overview


Introduction

History

Physical Description

Design Process

Fabrication Process

Introduction


VLSI


V
ery
L
arge
S
cale
I
ntegration


As soon as the transistor was invented, people
started trying to combine lots of them together
on a single chip.


VLSI is a physical model of integrating many
discrete components on a single chip.


VLSI is a design methodology for manufacturing
integrated circuits with as many components as
possible.

History


First generation computers were
assembled with many discrete
components, including relays, capacitors,
and vacuum tubes.


Second generation computers replaced
tubes with transistors.


Third generation computers put many
transistors on an integrated circuit or
microchip.


Integration History


1959


Single Transistor


1960


Unit Logic


1964


Small Scale Integration with 20 logic
units per chip


1967


Medium Scale Integration with 200 logic
units per chip


1972


Large Scale Integration with 2,000 logic
units per chip


1978


Very Large Scale Integration with 29,000
logic units per chip


2004


410,000,000 transistors on a chip

Intel Integration Timeline

Processor

Year

Transistors

4004

1971

2,250

8008

1972

2,500

8008

1974

5,000

8086

1978

29,000

286

1982

120,000

Intel386
TM

Processor

1985

275,000

Intel486
TM

Processor

1989

1,180,000

Intel® Pentium® Processor

1993

3,100,000

Intel® Pentium® II Processor

1997

7,500,000

Intel® Pentium® III Processor

1999

24,000,000

Intel® Pentium® 4 Processor

2000

42,000,000

Intel® Itanium® Processor

2002

220,000,000

Intel® Itanium® 2 Processor

2003

410,000,000

Intel Integration Chart


Physical Aspects


VLSI computers use metal
-
oxide
-
semiconductor (MOS)
technology.


A MOS transistor has three terminals, with the gate
terminal controlling flow between the source and the
drain.


There are two kinds of transistors classified by whether
they conduct when a high or low voltage is placed on
their gate.


The nMOS transistor conducts when a logic 1 is placed
on its gate.


The pMOS transistor conducts when a logic 0 is placed
on its gate.

A layer of metal sits on top of a

layer of silicon dioxide which acts

like an insulator. It is on top of a

layer of the semiconductor silicon.

A photographic
-
like etching process

removes the unnecessary metal.

The silicon is doped by diffusing

with a special element. The area

between the diffusion areas is

called the channel. Here it is

length
L
.

L

Source

Drain

Gate

nMOS Transistor

Source

Drain

Gate

pMOS Transistor

The metal gate sits on top

of a layer of silicon dioxide

which sits on top of the

silicon substrate. The two

green areas are diffusion

areas with an excess of

electrons. When a charge

is placed on the gate, the

electrons in the silicon are

pulled up into the channel

and a connection is made

between the two diffusion

areas.

Diagram of nMOS Transistor

Details


The metal layer has been replaced by polycrystalline
silicon, or polysilicon, but the name MOS survives.


Diffusing the silicon introduces impurities that alter the
behavior of the material.


Diffusing with arsenic and phosphorus leaves extra
electrons, while boron removes electrons to form “holes”
of positive charge.


The length of the channel in an nMOS transistor is
inversely proportional to the switching speed.


An nMOS transistor is good at passing logic 0 and not so
good at passing logic 1. The opposite is true for a pMOS
transistor. Combining the best of both, they are
connected to form CMOS, or complimentary metal
-
oxide
-
semiconductor, units.



GND

+5 V

C

~C

Creating a VLSI Circuit


Logic design and layout


Modeling


Simulation


Testing


Fabrication


Not a strict ordering. May have to go from
one back to another.

Design

The design problem is organized into distinct hierarchical
layers that can be optimized.

1.
Partitioning


Divides the circuit into smaller circuits that are
more manageable.

2.
Floor Planning


Determines the approximate location of each
module in a chip area.

3.
Placement


Decides exactly where modules will be placed
and determines wire lengths.

4.
Global Routing


Decomposes overall routing into separate
detailed routing problems.

5.
Detailed Routing


Decides actual wire routing taking into
consideration horizontal and vertical paths.

6.
Layout Optimization


After layout is finished, see if it can be
improved.

7.
Layout Verification


Make sure that it can be built.

Design Continued


There is usually a cooperation between the
silicon foundry, the IC design team, and the CAD
tool provider during the design and build
process.


CAD tools are used throughout the process for
design and verification.


Makes use of pre
-
optimized SSI or MSI cell
libraries.


There are various algorithmic techniques
employed to optimize the layout process.

Layout Algorithms


Exhaustive Search


Try every possibility and pick the
best one. Not realistic when working with n =
100,000,000.


Greedy


Greedy algorithms are often used with
optimizations, not always successfully.


Dynamic


The problem is broken into sub
-
problems
recursively and then combined from the bottom up. Used
when the sub
-
problems are related.


Hierarchical


Break the problem into unrelated sub
-
problems and solve each one.


Genetic Algorithms


Grow a population of pre
-
optimized
cells into the final design.


GA seems to be getting a lot of attention.

Design Styles


Field Programmable Gate Array


All programming is done by the user and turn around time is fast. Not all
of the switches can be used. Programming is valid as long as the chip is
powered on or until it is programmed again.


Gate Array


Similar to FPGA except that programming is done in manufacturing with
masks. Again, not all the switches can be used. Programming is
persistent.


Standard
-
Cells


Custom design built with logic cells stored in standard cell libraries.
Requires CAD tools to accomplish the layout and many custom masks
for fabrication. All switches are employed.


Full Custom


The entire design is done with CAD tools and custom masks without
pre
-
designed cell libraries. This is becoming very expensive, so there
are processes being developed to reuse parts of custom designs in later
projects.

Fabrication


Integrated circuits are composed of layers


Each layer is essentially completed before the next is added.


The patterns on the layers are created by a process called
lithography which uses masks.


Each layer uses its own mask.


As each layer is built on the previous, a coating of photo
-
resistant
material is applied. The masking process covers the part of the
material that is to be kept so that it is not exposed to UV light.


The part of the photo
-
resistant material that was exposed can now
be removed with a solvent.


The exposed layer can be etched with a chemical process.


The remaining photo
-
resistant material can now be removed with
another solvent.

Relationship to Performance


The channel length
L

is inversely proportional to
the switching speed of a transistor.


The smallest possible channel length will
produce the fastest speed.


Generally the channel length
L

is the same as
the smallest wire that the fabrication process can
produce.


Intel is currently using 90nm technology.


Intel is planning on 65nm technology in 2005.

Summary


Originally, computers were composed of a large number of discrete parts
that consumed a lot of power and took up a lot of space.


VLSI computers use a material that is not a conductor or an insulator called
a semiconductor. The properties of the semiconductor are modified so that
it forms part of an electronically controlled switch. This is called metal
-
oxide
-
semiconductor technology.


The MOS components are combined to form logic units and then wired
together. Then, as a unit, they perform the functions that had been
performed by many discrete parts.


VLSI designs are composed of different layers that are etched or diffused by
using masks.


VLSI design has led to a very high density of logic components that use
comparatively little power.


The number of transistors on a logic chip is roughly doubling every two
years.


Almost any device can be fitted with a VLSI logic and memory device.


Advances in VLSI lead to faster systems which lead to advances in VLSI.


References

Katz, Randy A.
Contemporary Logic Design
. Redwood City: The
Benjamin/Cummings Publishing Company, Inc, 1994


Vai, M. Michael
VLSI Design
. Boca Raton: CRC Press LLC, 2001


Sarrafzadeh, M. and C. K. Wong
An Introduction to VLSI Physical Design
.
Portland: McGraw
-
Hill, 1996


Dewdney, A. K.
The New Turing Omnibus
. New York: Henry Holt and
Company, 2001


http://developer.intel.com/


http://encyclopedia.thefreedictionary.com/VLSI


http://lsiwww.epfl.ch/LSI2001/teaching/webcourse/