Trends in Integrated Circuits Technology - Stanford … · Trends in Integrated Circuits Technology...

EE 311 Notes/Prof Saraswat Handout # 2

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Trends in Integrated Circuits Technology

Semiconductors have become increasingly more important part of worldeconomy

Silicon CMOS has become the pervasive technology


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Year

Tra

nsi

sto

rs o

r b

its

per

ch

ip Min

imu

m lith

og

raph

ic featu

re size ( µm

)

Ref: A. I. Kingon et al., Nature 406, 1032 (2000).

Year 1997 1999 2003 2006 2009 2012Technology node(DRAM half pitch)

250 nm 180 nm 130 nm 100 nm 70 nm 50 nm

Minimum FeatureSize

180 nm 120 nm 70 nm 60 nm 40 30

DRAM Bits/Chip 256M 1G 4G 16G 64G 256GDRAM Chip Size

(mm2)280 400 560 790 1120 1580

MicroprocessorTransistors/chip

11M 21M 76M 200M 520M 1.40B

Maximum WiringLevels

6 6-7 7 7-8 8-9 9

Minimum MaskCount

22 22/24 24 24/26 26/28 28

Minimum SupplyVoltage (volts)

1.8-2.5 1.5-1.8 1.2-1.5 0.9-1.2 0.6-0.9 0.5-0.6

Future projections for silicon technology taken from the SIA ITRS 1999


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Ref. H. komiya IEEE ISSCC 1993Device structures are becoming increasingly more complex

The scaling trends for Intel microprocessors.


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MOS Device Scaling

N a

P

N+N+

Lxox Xj

ol

Why do we scale MOS transistors?1. Increase device packing density

2. Improve frequency response (transit time) α1L

3. Improve current drive (transconductance gm)

€

gm =∂ID∂VG VD = const

≈WL

µnKo x

to xVD for VD <VDSAT

, linear region

≈WL

µnKo x

to xVG − VT( ) for VD >VDSAT

, saturation region

Decreasing the channel length and gate oxide thickness increases gm, i.e., the current driveof the transistor. Much of the scaling is therefore driven by decrease in L and tox. However ifonly these two parameters are scaled many problems are encountered, e.g., increasedelectric field.

The most widely used scaling rule is to maintain the electric field in the device constant

Device/Circuit Parameter Constant Field Scaling FactorDimension : xox, L, W, Xj, 1/KSubstrate doping : Na KSupply voltage : V 1/KSupply current : I 1/KGate Capacitance : W L/xox 1/KGate delay : C V / I 1/KPower dissipation : C V2 / delay 1/K2Delay power product : 1/K3

P

N+N+S

GD

Scaled MOS Transistor


5

Speed increases as a result of scaling

So Does the Cost of a Factory

1965 1975 1985 1995104

105

106

107

108

109

4004

8080

80286

386

486

Pentium

Intel Microprocessors

Number ofInstructionsper Second

speed doubles each3-yr generation

the effect of bettermicroprocessorarchitectures

1960 1970 1980 1990 20001

10

100

1000

10,000

cost of a modernwafer fab($ million)

2x every 4 years

1.47x every 2 years2x every 3 years


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In reality constant field scaling has not been observed strictly. Since the transistorcurrent is proportional to the gate overdrive (VG-VT), high performance demands havedictated the use of higher supply voltage. However, higher supply voltage impliesincreased power dissipation (CV2f). In the recent past low power applications havebecome important and have required a scaling scenario with lower supply voltage.

Ref: Davri, et al. Proc. IEEE, April 1995

In general the device scaling methodology does not take into account many other chipperformance and reliability issues, e.g., interconnects, contacts, isolation, etc. Thesefactors are now becoming an obstacle in the evolution of integrated circuits.

How far can we continue to scale?

(Source: J. Plummer)


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Effect of Reducing Channel Length: Drain Induced Barrier Lowering

Q depletedby source

Q depletedby drain

B B

N+ source N+ drain

Gate

P-Si

Depletion region

L’

L

rj

In devices with long channel lengths, the gate is completely responsible for depletingthe semiconductor (QB). In very short channel devices, part of the depletion is

accomplished by the drain and source bias. Since less gate voltage is required todeplete QB, the barrier for electron injection from source to drain decreases

Potential variation along the channel for MOS transistors with 2.5and 0.5 µm channel lengths. The 0.5 µm device shows DIBL effect.

The reduction in the barrier is known as “drain induced barrier lowering (DIBL)”. DIBLresults in an increase in drain current at a given VG. Therefore VT↓ as L↓. Similarly, as


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VD ↑, more QB is depleted by the drain bias, and hence ID↑ and VT↓. This is

An approximate relation for threshold voltage due to DIBL is:

VT = VFB − 2 ⋅φF −QB

Cox⋅ 1− 1 +

2 ⋅Wrj

−1

⋅rjL

To minimize the effect of DIBL:

• Cox should be increased, i.e., decrease gate oxide thickness.This results in increased control of the gate.

• Decrease junction depth (rj)

Scaling of gate oxide thickness and junction depth causes many other problems.

Hot Carrier Effects

For a reverse biased p-n junction discussion we remember that the maximum electricfield intensity is near the junction itself and it increases with the reverse bias.

ξmax =2qNa (φ i − VD )

εox

In the case of MOS transistor the potential drop along the channel is not uniform withmost of it across the reverse biased drain-substrate junction. Therefore the electricfield intensity is also non-uniform with the maximum occurring near the drain junction.As the channel length is reduced the electric field intensity in the channel near thedrain increases more rapidly in comparison to the long channel case, even if VD isscaled, as φi does not scale.


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The free carriers passing through the high-field can gain sufficient energy to causeseveral hot-carrier effects. This can cause many serious problems for the deviceoperation.

Hot carriers can have sufficient energy to overcome the oxide-Si barrier. They areinjected from channel to the gate oxide (process 1) and cause gate current to flow.Trapping of some of this charge can change VT permanently. Avalanching can takeplace producing electron-hole pairs (process 2). The holes produced by avalanchingdrift into the substrate and are collected by the substrate contact (process 3) causingIsub IR drop due to Isub(process 4) can cause substrate-source junction to beforward biased causing electrons to be injected from source into substrate (process5). Some of the injected electrons are collected by the reversed biased drain andcause a parasitic bipolar action (process 5).


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Scaling of MOS Gate Dielectric

(Ref: S. Asai, Microelectronics Engg., Sept. 1996)

By the end of this decade the MOS gate dielectric thickness will be well below 10 Å.

• How far can we push MOS gate dielectric thickness?• How will we grow such a thin layer uniformly?• How long will such a thin dielectric live under electrical stress?• Can we improve the endurance of the dielectric by changing its structure?

Problems in scaling gate oxide

Defects andnonuniformity of filmDielectric breakdown

Reliability due tocharge injection

Si substrate

Polysilicon gate electrode

Dopantpenetration

gate oxide

Leakage current

ID ∝ gm ∝K

thickness


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Gate Dielectric Degradation and Breakdown

Under high field electrons are injected in the SiO2 conduction band because ofreduction in barrier height and thickness. Some electrons gain excess energy in theconduction band of the oxide. At the anode they lose kinetic and potential energycausing physical damage leading to traps generation. Further trapping of electronsand holes causes dielectric degradation.

eCathode

Oxide

Anode

h

(1)

(2)

(4)

(5)(6)

Hydrogen

e

e

h

(1) Electron injection(2) Energy released by hot electron(3) Bond breaking at the interface - trap generation(4) Hot hole generation by impact ionization and injection(5) Energy released by hot hole - trap generation(6) Hydrogen release - trap generation

(3)

(3)

Ref: Apte & Saraswat IEEE Trans. Electron Dev., Sept 1994

We can improve the endurance of the dielectric by optimizing the process technologyand changing its structure. For example incorporating nitrogen or fluorine instead ofhydrogen strengthens the Si/SiO2 interface and increases the gate dielectric lifetimebecause Si-F and Si-N bonds are stronger than Si-H bonds.

Poly-Si Gate

Si substrate

Oxide N or F


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Problems caused by conduction in ultrathin gate oxide

As we decrease the gate dielectric thickness, the conduction through the dielectricfilm becomes appreciable. This may increase power dissipation and cause problemsfor circuit stability. Increased leakage due to direct tunneling through the gatedielectric may make dynamic and static circuits unstable.

OxideSiSi

Thin OxideDirect Tunneling

Thick OxideFowler-Nordheim Tunneling

Gate Leakage Current Density Versus GateVoltage for Various Oxide Thicknesses

(Ref: From Y. Taur et al., Proc. IEEE, April 1997.)


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0.00001

0.001

0.1

10

1000

1801301007050

Technology Generation (nm)

Cu

rren

t (µ

A/ µ

m)

Igate

Ioff

Ion

Source G. Bersuker, et al. Sematech

Rather than scaling thickness of SiO2 perhaps we can scale the dielectric constant (K)to improve the performance. Alternatively for the same performance we can increasethe dielectric thickness by increasing its K.

20 Å

100 ÅSi3N4 K ≈ 8

40 ÅToday Near future Long term

SiO2 K ≈ 4

high K > 20

Near term and long term approaches for scaling the MOS gate dielectric.

However, replacing SiO2 by another dielectric is a very difficult task as it isone of the best dielectrics and is one of the main reasons of the successof Si technology.

ID ∝ gm ∝K

thickness


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Scaling of Ohmic Contacts and Junctions

source

Rch

Silicide

Rc

Rsdrain

RdRd’Rs’

metal

Xj

Poly-Si

• Device scaling dictates shallow junctions.

• How will we form such shallow junctions?

• How will we make low resistance contacts to them?

• What will be the impact of the resistance of the contacts and junctions?

ρCRc

Source: Jasonn Woo, UCLA

Year 1997 1999 2003 2006 2009 2012Min Feature Size 0.18µ 0.12µ 0.07µ 0.06µ 0.04µ 0.03µContact xj (nm) 100-200 70-140 50-100 40-80 15-30 10-20

xj at Channel (nm) 50-100 36-72 26-52 20-40 15-30 10-20


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Specific contact resistivity

ρc =ρ co exp2φBqh

εsm*

N

ohm − cm2

where φB is the barrier height and N is the doping density in thesemiconductor.

Problem in scaling:• Contact resistance is a strong function of doping density at the

metal/silicon interface• Sheet resistance of a junction is a strong function of doping density in

the junction• However, the maximum doping density is limited by solid solubility and

it does not scale !


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Solutions to Shallow Junction Problem

Shallow extension implants to minimize (DIBL)

Elevated source/ drain to minimize (DIBL)

Silicidation to junction minimize resistance


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Scaling of Device Isolation Technologies

Isolation pitch as a function of minimum dimension

0.0 0.2 0.4 0.6 0.8 1.0Minimum dimension [µm]

0.0

0.5

1.0

1.5

2.0

2.5

16M

64M

1G

P. Fazan, Micron, IEDM-93LOCOS based isolation technologies have serious problems in loss of area due tobird’s beak.

NitridePad oxide

Fully recessed LOCOS

Nitride Pad oxide

Semi-recessed LOCOSNitride

Field oxide

After field oxidation

After field oxidation

Large stresses can build up in LOCOS based isolation technologies.

F4

F1

F2

F3


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Deep trench isolation

N-wellP-substrate

Shallow trench isolation

Trench isolation can minimize area loss, however, large stresses can build up intrench structures resulting in bandgap reduction and eventually if the stress is toomuch it can cause crystal defects leading to increase in leakage and yield loss.

0 500 1000 1500Compressive stress [MPa]

10-14

10-13

10-12

10-11

Distance [µm]

N+ P

The stress is a function of process temperatures as at higher temperatures SiO2 hasviscous flow which can relieve the stress. However, thermal budget demands lowtemperature processing.

1 10 100Active area pitch [µm]

100

101

102

103

900 ˚C

1000 ˚C

1100 ˚C


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Source: J. D. Meindl, , “Integration Limits on XXI Century Gigascale Integration”, IEEEInterconnect Technology Conf. Short Course, San Francisco, CA, May 31, 1998

1 10 100 1000

1E-4

1E-2

1E0

1E2

1E4

1E6

1E8

Interconnect Length, l (gate pitches)

2D

3D

Local GlobalSemiglobal

LSemi-global

LLocal

• Number of interconnects are increasing• Most of the interconnects are very short but a few are very long


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Impact of Scaling of Interconnections on Circuit Performance

• Chip area is increasing => length of the longest wire is increasing

• Cross sectional dimensions of the interconnects are decreasing resulting in anincrease in resistance and capacitance

• Increased R and C results in higher signal propagation delay

•

Space Width (µm) [=Line Width]0.0 0.5 1.0 1.5 2.0 2.5 3.0

Ca

pa

cita

nc

e (fF

/mm

)

0

100

200

300

400

500

Line To GroundLine To LineTotal

CAPACITANCE

Higher Packing Density⇓

Decreased Space BetweenInterconnects

⇓Higher RC-Delay


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• Parasitic resistance and capacitance associated with interconnections and contacts arenow begining to influence circuit performance and will be the primary factors in theevolutions of submicron ULSI technology.

• Lower resistivity metals and lower dielectric constant insulators will reduce the R and C

80 100 120 140

0.1

0.01

Technology Generation (nm)

Typical Gate Delay

Interconnect Delay1.0

160 180

Del

ay (

ns)

60

.

Copper 6

Copper 1

TungstenLocal Interconnect

Copper 3

Copper 5

Copper 4

Copper 2

Current Cu technology (Courtesy of IBM)


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global

semiglobal

local

By increasing the levels of wires interconnect problem can be minimized. Shorter(local) wires can be placed in thinner interconnects and longer (global) wires can be

made with larger cross section to minimize R and C.

123456789

1011121314

Al & SiO2 (κ = 4)

Cu & SiO2 (κ = 4)

Al & low-κ (κ = 2)

Cu & lo w- κ ( κ = 2)0.092007

0.132004

0.182001

0.251998

0.351995

Tec hnology Generation

µmYear

Reduced resistivity and dielectric constant results in reduction in number of metal layersas more wires can by placed in lower levels of metal layers.


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Problems in Scaling of Interconnections

Cu diffuses in most dielectrics readily and acts as a recombination centerin Si. Hence a barrier is generally needed to enclose Cu line to avoid itsdiffusion in the dielectric. Barriers are generally highly resistive.

As λ decreases• Resistivity increases as grain size decreases• Resistivity increases as main conductor size decreases but not the

surrounding film size

Pure Metal Interconnect

Layered Interconnect

Surrounded Interconnect

Al

Al

Cu

ρav

Minimum Feature Size (λ)

Barrier

Barrier

Cu


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Thermal Behavior• Energy dissipated (CV2f) is increasing as performance improves• Thermal conductivity of low-k insulators is poor• Average chip temperature is rising

3550 70 130 1801000

1

2

3

4

00.20.4

0.60.811.2

Technology Node [nm]

Die

lect

ric

Co

nst

ant

Th

ermal C

on

du

ctivity[ W

/ mK

]

35 50 70 100 130 18005

1015

20

25

30

0

1

2

3

4

5

Po

wer

Den

sity

[ W

/ cm

2 ]

Technology Node [nm]

Jm

ax [ MA

/ cm2 ]


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Electromigration

Electromigration due to electron wind induced diffusion of Al through grain boundaries

SEM of hillock and voids formation due to electromigration in an Al(Cu,Si) line

Mean time to failure due to electromigration is given by

MTF =A

rmJnexp Ea

kT


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Summary: Technology Progression

MOS Transistor in 2010Gate oxide thickness 10 ÅChannel Length < 500 ÅJunction depth < 500 ÅSize of an atom ~ 5 Å

A Circuit in 2010In integrated system

> 1 billion components> 10 interconnect layers

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