1: Introduction Slide 1CMOS VLSI Design

Microelettronica

Anno Accademico 2006-2007

Prof. Adelio SalsanoPresentazione e programma del

corso

1: Introduction Slide 2CMOS VLSI Design

OBIETTIVO

Tecnologie, blocchi elementari e architetture per l’analisi e la sintesi di circuiti e sistemi microelettronici

ALTERNATIVECircuiti non programmabili

Circuiti programmabiliCircuiti dedicatiSoluzioni miste

Il corso fornisce le competenze necessarie per la valutazione delle prestazioni di circuiti e sistemi elettronici come

prerequisito per la sintesi

1: Introduction Slide 3CMOS VLSI Design

Notizie sul corsoOrarioMercoledì 11,30 Aula 9Giovedì 11,30 Aula 7Venerdì 11,30 Aula 12

Materiale didattico Diapositive lezioni N.H.E. Weste, D. Harris “Principles of CMOS VLSI Design”,

Addison Wesley R. L. Geiger, P.E. Allen, N.R. Strader VLSIdesign techniques

for analog and digital Circuits, Mac Graw Hill Int. Ed.

1: Introduction Slide 4CMOS VLSI Design

PROGRAMMA DEL CORSO Introduzione Considerazioni generali Aspetti tecnici ed economici Richiami circuitali:

– Inverter, NAND, NOR– Pass transistor, transmission gate– Latch, flip flop– Regole di progetto

Il transistor MOS– Caratteristiche I-V– Caratteristiche C-V– Modelli delle capacità G,S,D– Effetti non ideali

1: Introduction Slide 5CMOS VLSI Design

(segue Programma) Inverter CMOS

– Caratteristiche in DC– Beta, rapporto dei beta, margini di rumore– Inverter dipendenti dal rapporto dei beta

Inverter a pass transistor e tristate Modelli RC di ritardo Tecnologie CMOS:

– Litografia,formazione del canale, ossidazione, contatti e metallizzazione

– Regole di progetto Elementi circuitali: transistor, caoacità, resistenze,

transistor bipolari, memorie

1: Introduction Slide 6CMOS VLSI Design

(segue Programma)

Stima delle prestazioni– Ritardi dei circuiti elementari: sforzo logico– Dissipazione di potenza– nterconnessioni– Margini progettuali

Affidabilità e diagnostica dei circuiti integrati: – elettromigrazione, riscaldamento, latchup– guasti transitori e permanenti – testing on line e off line– modelli di guasto– design for testability

1: Introduction Slide 7CMOS VLSI Design

(segue Programma) La simulazione circuitale: SPICE Logica a pass transistor Circuiti BICMOS Confronto tra le famiglie Logica statica e dinamica Sistemi digitali complessi: latch, flip flop, sincronizzazione

– microprocessori, memorie, logica programmabile Circuiti e sistemi analogici:

– Interruttori e resistenze attive– specchio di corrente– riferimenti di corrente e tensione – amplificatori invertenti e differenziali – amplificatore operazionale

1: Introduction Slide 8CMOS VLSI Design

Brief HistoryTill 1970 I.C. bipolar, afterwards MOSFET

SSI 1-100 MOS

MSI 100-1000 MOS

LSI 1000-100.000 MOS

VLSI 100.000 -106 MOS

ULSI > 106 MOS

1: Introduction Slide 9CMOS VLSI Design

Brief History (cont.) 1958: First integrated circuit

– Flip-flop using two transistors– Built by Jack Kilby at Texas Instruments

2003– Intel Pentium 4 processor (55 million transistors)– 512 Mbit DRAM (> 0.5 billion transistors)

53% compound annual growth rate over 45 years– No other technology has grown so fast so long

Driven by miniaturization of transistors– Smaller is cheaper, faster, lower in power!– Revolutionary effects on society

1: Introduction Slide 10CMOS VLSI Design

Vantaggi della tecnologia integrata

Dimensioni: Fette di silicio (2003) fino a 12 pollici Velocità Consumo di potenza Dimensioni del sistema Costo del sistema

Legge di MOORE: raddoppio ogni anno e mezzo del numero di componenti per chip

CHIP

1: Introduction Slide 11CMOS VLSI Design

MERCATO DEI CIRCUITI INTEGRATI

1: Introduction Slide 12CMOS VLSI Design

COMPLESSITA’ MICRO INTEL

1: Introduction Slide 13CMOS VLSI Design

FREQUENZE MICRO INTEL

1: Introduction Slide 14CMOS VLSI Design

Silicon Lattice Transistors are built on a silicon substrate Silicon is a Group IV material Forms crystal lattice with bonds to four neighbors

Si SiSi

Si SiSi

Si SiSi

1: Introduction Slide 15CMOS VLSI Design

Dopants Silicon is a semiconductor Pure silicon has no free carriers and conducts poorly Adding dopants increases the conductivity Group V: extra electron (n-type) Group III: missing electron, called hole (p-type)

As SiSi

Si SiSi

Si SiSi

B SiSi

Si SiSi

Si SiSi

-

+

+

-

1: Introduction Slide 16CMOS VLSI Design

p-n Junctions A junction between p-type and n-type semiconductor

forms a diode. Current flows only in one direction

p-type n-type

anode cathode

1: Introduction Slide 17CMOS VLSI Design

nMOS Transistor Four terminals: gate, source, drain, body Gate – oxide – body stack looks like a capacitor

– Gate and body are conductors– SiO2 (oxide) is a very good insulator– Called metal – oxide – semiconductor (MOS)

capacitor– Even though gate is

no longer made of metaln+

p

GateSource Drain

bulk Si

SiO2

Polysilicon

n+

1: Introduction Slide 18CMOS VLSI Design

nMOS Operation Body is commonly tied to ground (0 V) When the gate is at a low voltage:

– P-type body is at low voltage– Source-body and drain-body diodes are OFF– No current flows, transistor is OFF

n+

p

GateSource Drain

bulk Si

SiO2

Polysilicon

n+D

0

S

1: Introduction Slide 19CMOS VLSI Design

nMOS Operation Cont. When the gate is at a high voltage:

– Positive charge on gate of MOS capacitor– Negative charge attracted to body– Inverts a channel under gate to n-type– Now current can flow through n-type silicon from

source through channel to drain, transistor is ON

n+

p

GateSource Drain

bulk Si

SiO2

Polysilicon

n+D

1

S

1: Introduction Slide 20CMOS VLSI Design

pMOS Transistor Similar, but doping and voltages reversed

– Body tied to high voltage (VDD)– Gate low: transistor ON– Gate high: transistor OFF– Bubble indicates inverted behavior

SiO2

n

GateSource Drain

bulk Si

Polysilicon

p+ p+

1: Introduction Slide 21CMOS VLSI Design

Power Supply Voltage GND = 0 V In 1980’s, VDD = 5V VDD has decreased in modern processes

– High VDD would damage modern tiny transistors

– Lower VDD saves power VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, …

1: Introduction Slide 22CMOS VLSI Design

Transistors as Switches We can view MOS transistors as electrically

controlled switches Voltage at gate controls path from source to drain

gs

d

g = 0

s

d

g = 1

s

d

gs

d

s

d

s

d

nMOS

pMOS

OFF ON

ON OFF

1: Introduction Slide 23CMOS VLSI Design

CMOS InverterA Y

0

1

VDD

A Y

GNDA Y

1: Introduction Slide 24CMOS VLSI Design

CMOS InverterA Y

0

1 0

VDD

A=1 Y=0

GND

ON

OFF

A Y

1: Introduction Slide 25CMOS VLSI Design

CMOS InverterA Y

0 1

1 0

VDD

A=0 Y=1

GND

OFF

ON

A Y

1: Introduction Slide 26CMOS VLSI Design

CMOS NAND GateA B Y

0 0

0 1

1 0

1 1A

B

Y

1: Introduction Slide 27CMOS VLSI Design

CMOS NAND GateA B Y

0 0 1

0 1

1 0

1 1

A=0

B=0

Y=1OFF

ON ON

OFF

1: Introduction Slide 28CMOS VLSI Design

CMOS NAND GateA B Y

0 0 1

0 1 1

1 0

1 1

A=0

B=1

Y=1OFF

OFF ON

ON

1: Introduction Slide 29CMOS VLSI Design

CMOS NAND GateA B Y

0 0 1

0 1 1

1 0 1

1 1

A=1

B=0

Y=1ON

ON OFF

OFF

1: Introduction Slide 30CMOS VLSI Design

CMOS NAND GateA B Y

0 0 1

0 1 1

1 0 1

1 1 0

A=1

B=1

Y=0ON

OFF OFF

ON

1: Introduction Slide 31CMOS VLSI Design

CMOS NOR GateA B Y

0 0 1

0 1 0

1 0 0

1 1 0

A

BY

1: Introduction Slide 32CMOS VLSI Design

3-input NAND Gate Y pulls low if ALL inputs are 1 Y pulls high if ANY input is 0

1: Introduction Slide 33CMOS VLSI Design

3-input NAND Gate Y pulls low if ALL inputs are 1 Y pulls high if ANY input is 0

A

B

Y

C

1: Introduction Slide 34CMOS VLSI Design

CMOS Fabrication CMOS transistors are fabricated on silicon wafer Lithography process similar to printing press On each step, different materials are deposited or

etched Easiest to understand by viewing both top and

cross-section of wafer in a simplified manufacturing process

1: Introduction Slide 35CMOS VLSI Design

Inverter Cross-section Typically use p-type substrate for nMOS transistors Requires n-well for body of pMOS transistors

n+

p substrate

p+

n well

A

YGND VDD

n+ p+

SiO2

n+ diffusion

p+ diffusion

polysilicon

metal1

nMOS transistor pMOS transistor

1: Introduction Slide 36CMOS VLSI Design

Well and Substrate Taps Substrate must be tied to GND and n-well to VDD

Metal to lightly-doped semiconductor forms poor connection called Schottky Diode

Use heavily doped well and substrate contacts / taps

n+

p substrate

p+

n well

A

YGND VDD

n+p+

substrate tap well tap

n+ p+

1: Introduction Slide 37CMOS VLSI Design

Inverter Mask Set Transistors and wires are defined by masks Cross-section taken along dashed line

GND VDD

Y

A

substrate tap well tapnMOS transistor pMOS transistor

1: Introduction Slide 38CMOS VLSI Design

Detailed Mask Views Six masks

– n-well– Polysilicon– n+ diffusion– p+ diffusion– Contact– Metal

Metal

Polysilicon

Contact

n+ Diffusion

p+ Diffusion

n well

1: Introduction Slide 39CMOS VLSI Design

Fabrication Steps Start with blank wafer Build inverter from the bottom up First step will be to form the n-well

– Cover wafer with protective layer of SiO2 (oxide)– Remove layer where n-well should be built– Implant or diffuse n dopants into exposed wafer– Strip off SiO2

p substrate

1: Introduction Slide 40CMOS VLSI Design

Oxidation Grow SiO2 on top of Si wafer

– 900 – 1200 C with H2O or O2 in oxidation furnace

p substrate

SiO2

1: Introduction Slide 41CMOS VLSI Design

Photoresist Spin on photoresist

– Photoresist is a light-sensitive organic polymer– Softens where exposed to light

p substrate

SiO2

Photoresist

1: Introduction Slide 42CMOS VLSI Design

Lithography Expose photoresist through n-well mask Strip off exposed photoresist

p substrate

SiO2

Photoresist

1: Introduction Slide 43CMOS VLSI Design

Etch Etch oxide with hydrofluoric acid (HF)

– Seeps through skin and eats bone; nasty stuff!!! Only attacks oxide where resist has been exposed

p substrate

SiO2

Photoresist

1: Introduction Slide 44CMOS VLSI Design

Strip Photoresist Strip off remaining photoresist

– Use mixture of acids called piranah etch Necessary so resist doesn’t melt in next step

p substrate

SiO2

1: Introduction Slide 45CMOS VLSI Design

n-well n-well is formed with diffusion or ion implantation Diffusion

– Place wafer in furnace with arsenic gas– Heat until As atoms diffuse into exposed Si

Ion Implanatation– Blast wafer with beam of As ions– Ions blocked by SiO2, only enter exposed Si

n well

SiO2

1: Introduction Slide 46CMOS VLSI Design

Strip Oxide Strip off the remaining oxide using HF Back to bare wafer with n-well Subsequent steps involve similar series of steps

p substraten well

1: Introduction Slide 47CMOS VLSI Design

Polysilicon Deposit very thin layer of gate oxide

– < 20 Å (6-7 atomic layers) Chemical Vapor Deposition (CVD) of silicon layer

– Place wafer in furnace with Silane gas (SiH4)– Forms many small crystals called polysilicon– Heavily doped to be good conductor

Thin gate oxidePolysilicon

p substraten well

1: Introduction Slide 48CMOS VLSI Design

Polysilicon Patterning Use same lithography process to pattern polysilicon

Polysilicon

p substrate

Thin gate oxidePolysilicon

n well

1: Introduction Slide 49CMOS VLSI Design

Self-Aligned Process Use oxide and masking to expose where n+ dopants

should be diffused or implanted N-diffusion forms nMOS source, drain, and n-well

contact

p substraten well

1: Introduction Slide 50CMOS VLSI Design

N-diffusion Pattern oxide and form n+ regions Self-aligned process where gate blocks diffusion Polysilicon is better than metal for self-aligned gates

because it doesn’t melt during later processing

p substraten well

n+ Diffusion

1: Introduction Slide 51CMOS VLSI Design

N-diffusion cont. Historically dopants were diffused Usually ion implantation today But regions are still called diffusion

n wellp substrate

n+n+ n+

1: Introduction Slide 52CMOS VLSI Design

N-diffusion cont. Strip off oxide to complete patterning step

n wellp substrate

n+n+ n+

1: Introduction Slide 53CMOS VLSI Design

P-Diffusion Similar set of steps form p+ diffusion regions for

pMOS source and drain and substrate contact

p+ Diffusion

p substraten well

n+n+ n+p+p+p+

1: Introduction Slide 54CMOS VLSI Design

Contacts Now we need to wire together the devices Cover chip with thick field oxide Etch oxide where contact cuts are needed

p substrate

Thick field oxide

n well

n+n+ n+p+p+p+

Contact

1: Introduction Slide 55CMOS VLSI Design

Metalization Sputter on aluminum over whole wafer Pattern to remove excess metal, leaving wires

p substrate

Metal

Thick field oxide

n well

n+n+ n+p+p+p+

Metal

1: Introduction Slide 56CMOS VLSI Design

Layout Chips are specified with set of masks Minimum dimensions of masks determine transistor

size (and hence speed, cost, and power) Feature size f = distance between source and drain

– Set by minimum width of polysilicon Feature size improves 30% every 3 years or so Normalize for feature size when describing design

rules Express rules in terms of = f/2

– E.g. = 0.3 m in 0.6 m process

1: Introduction Slide 57CMOS VLSI Design

Simplified Design Rules Conservative rules to get you started

1: Introduction Slide 58CMOS VLSI Design

Inverter Layout Transistor dimensions specified as Width / Length

– Minimum size is 4 / 2sometimes called 1 unit– In f = 0.6 m process, this is 1.2 m wide, 0.6 m

long

1: Introduction Slide 59CMOS VLSI Design

Summary MOS Transistors are stack of gate, oxide, silicon Can be viewed as electrically controlled switches Build logic gates out of switches Draw masks to specify layout of transistors

Now you know everything necessary to start designing schematics and layout for a simple chip!