Engineered Substrates for High-Mobility MOSFETs Nathan Cheung

FLCC Seminar 11/8/04 1

Engineered Substrates for High-Mobility Engineered Substrates for High-Mobility MOSFETsMOSFETs

Nathan Cheung Dept of EECS, UC-Berkeley

[email protected]

GSR: Eric Liu and Vorrada Loryuenyong


OUTLINEOUTLINE

•Motivations for SOI, SSOI, and GeOI substratesMotivations for SOI, SSOI, and GeOI substrates

•Layer Transfer TechnologiesLayer Transfer Technologies

- Epitaxial Growth and Implantation- Epitaxial Growth and Implantation - Plasma Activated Bonding- Plasma Activated Bonding

- Delamination- Delamination- Post-Delamination Surface Smoothing- Post-Delamination Surface Smoothing

•FLCC ResearchFLCC Research - GeOI layer transfer- GeOI layer transfer - Transfer Thickness Mechanisms- Transfer Thickness Mechanisms - Thermal-Mechanical Stress Analysis- Thermal-Mechanical Stress Analysis


Various mobility enhancement device structures

•Strain Si•Ge


Layer Transfer Approaches

Wafer bonding Thermal exfoliation > 400°C

Splitting By Internal Force

Handle wafer

Donor waferH peak

H+

Splitting By External Force

Mechanically weakened

Layer

Handle wafer

Wafer bonding

Donor wafer

Edge initiated crack propagation

M.K. Bruel, Electron. Lett. 31, 1201 (1995).

SiO2

SiO2

En et al, SOI Conference Proc, 163 (1998)Yonehara et al, APL, 64, 2104 (1994)


Direct Wafer Bonding

Chemical Cleaning:HF, H2SO4, H2O2

Annealing

Plasma exposure

Room temperature bonding

IR Transmission ImageThrough a Bonded Pair

Complete bonding over 4 inch diameter


Delamination Methods

(2) Cleavage along implant damage region (gas jet) [Sigen]

(3) Mechanical rupture of Porous Si (water jet) [Canon]

(1) Exfoliation of implanted hydrogen [ SOITEC, Amberwave]

Si donor

Transferred Si overlayer

Handle wafer


Layer Transfer Theory: Layer Transfer Theory: Bonding strength > Cutting layer strengthBonding strength > Cutting layer strength

Cho et al, J. Phys. Lett., 92, 5980 (2003)

0 50 100 150 200 250 3000.0

0.5

1.0

1.5

2.0

2.5

3.0

bond

cut

Temperature (°C)

(J

/m2 )

cut > bond cut < bond

Strengths of Bonding and Cutting Layers

cut > bond cut ~ bond cut < bond

SiO2

1 cm

Donor Si Donor SiDonor Si

Transferred Si

Receptor Si Receptor Si Receptor Si

SiO2

Separation Modes

No transfer Partial transfer Full transfer

Full transfer requires a full strength at the bonding interface layer

Non-uniform bonding induces partial transfer

i = surface energy of interface i


Advantages of Layer Transfer ApproachAdvantages of Layer Transfer Approach

•Donor wafer can be recycledDonor wafer can be recycled

•Transferred thickness and buried oxide thickness Transferred thickness and buried oxide thickness are are independentlyindependently controlled controlled

•(100), (110), and (111) Epi layers can be transferred(100), (110), and (111) Epi layers can be transferred

•Multi-stack structures can be achieved with various epiMulti-stack structures can be achieved with various epiand transfer combinationsand transfer combinations


Some state-of-the-art results Some state-of-the-art results GeOIGeOISSOISSOISOI

CanonCanon AmberwaveAmberwave SigenSigen

SOITECSOITEC

300mm SOI300mm SOI

50nm Si50nm Sirange=1.2nmrange=1.2nm


Why transfer of Epi Donor Wafers ?Why transfer of Epi Donor Wafers ?

•For SOI, Epi Si has less COP defects than bulk Si

•For GeOI, no 300mm Ge bulk wafers yet

•For s-SOI and SGOI , layer formed epitaxially on SiGe buffer layers


Plasma Activated Bonding

Surface damage 1-2 nm

O

H

O

H

O

H

O

H

O

H

O

H

O

H

O

H

Si rich surface

Defect layer High mobility of H2O

High coverage of OH

PLASMA


Si/SiO2 Bonding Energy vs. Temperature

Hydrophobic Si

Hydrophilic Si

3000

2500

2000

1500

1000

500

0100 200 300 400 500 600 700 800 900

Bon

din

g E

ner

gy (

mJ/

m2 )

Annealing Temperature (oC)

O2 plasma

Si (100)Si (100)Fracture Fracture StrengthStrength

Cho et al, UCB, 2000.


Requirements for Direct Bonding

Surface micro-roughness ~ nm

No macroscopic wafer warpage

Minimal particle density and size - “soft “ particle size < 0.2 um

*Deposited films will need CMP*Deposited films will need CMP


As-split surfaceAs-split surface

After-anneal surfaceAfter-anneal surface

Surface Smoothing by Hydrogen annealSurface Smoothing by Hydrogen anneal

FLCC Seminar 11/8/04 16Current et al, European Semiconductor, Feb 2000

NanoCleave rms ~ 0.8nm

Hydrogen Induced Thermal Separationrms ~ 8.5 nm


Ultra-Thin (<1KÅ tSOI) Non-Uniformity

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700 800

Ultra-Thin SOI Layer Thickness

Ra

ng

e (M

ax

-Min

) (

Å)

Device Layer Thickness (Å)

10%

Typical Range <25Å

Early 2002 Late 2002

Range Now Independent of Layer Thickness


Size: 1x1 cm2

Ge/Si3N4/Si and Ge/SiO2/Si substrates by ion-cut

Fabrication Method

Processing Temp

(ºC)

Transfer thickness

(nm)

Mobility (as-cut) cm2/V-sec

Bulk =300 cm2/V-sec

Ge/Si3N4/Si

Mechanical

Ion Cut205 439 240

Thermal Ion Cut

~250 450 280

Ge/SiO2/Si Thermal Ion Cut

~270 410 252

Ge donor wafer

Si Substrate

ImplantedHydrogen

Ge/Si3N4/Si

GeOI


Ge/Si3N4/Si surface roughness by AFM

(Size of AFM images: 5x5µm2; H ion dose: 6x1016 /cm2.)

(a)GeOI by mechanical cut;Tanneal=205°C, RMS: 17.5nm

(b)GeOI by thermal cut; Tcut=360°C, RMS: 20.5nm

•GeOI transfer surface roughness > SOI transfer surface roughness (RMS<7 nm) •Post-transfer smoothing is required


0 100 200 300 400 500 600 700

0

100

200

300

400Transferred Ge

SiO2Thi

ckne

ss,t,

(nm

)

Scan distance (µm)

400nm Ge

30nm dry ox

Si substrate

Fabrication processes:•Oxygen plasma activationfor 15sec;•Direct bonding;•Post-bonding annealing:130°C for 20h;220 °C for 10h;•Thermal-cut at T>270 °C ;

Ge/SiO2/Si by thermal ion-cut

Transferred Ge

SiO2


Ge/SiO2 (or Si3N4)/Si system by ion-cut shows that the cutting depth is deeper than the implantation zone !

2000 40000.0000

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

Vac

ancy

Dis

trib

utio

n

Hyd

roge

n D

istr

ibut

ion

Depth (Ǻ)

Observed cutting depth

SRIM2000

Si

Ge

Ideal crack propagation

Mixed-mode crack propagation

Pgas

Thermal Cut

Data is obtained in courtesy of ZhengXin Liu

1

2

Pgas


29 nm19 nm

10 nm

Höchbauer et al, J. Appl. Phys, 89, 5980 (2001)

SOI: H+ 175 keV 5.0 x 1016 cm-2 600C

• Ion-cut take place at shallower depth than the center of the hydrogen platelet distribution

• The ion-cut location is found to occur at the depth of maximum damage.

Ion-cut location

[100]

[011]

What controls Transfer Layer thickness ? What controls Transfer Layer thickness ?


Transferred layer of implanted Si is thicker than non-implanted Si

Transfer thickness of Ion-Cut is different Transfer thickness of Ion-Cut is different with substrate stresswith substrate stress

-0.04

0

0.04

0.08

0.12

0 50 100 150 200 250

SiNon-implanted Si (100) thickness

Implanted Si (100) Thickness

Distance (m)Top views of transferred layers

Small Area Transfer Large Area Transfer

Implanted Si (100) H+ 8 1016 cm-2 28

keVSi (100)

Donor Si

Glass

Donor Si

Glass

Thic

kn

ess

(m

)Transferred Si

Transferred Si

H Peak


Transferred thickness is a function of ion implantation dose Compressive stress induced by implantation was determined by measuring wafer curvature Compressive stress leads to additional shear forces at the crack tip

Thickness Measurement Data

Transfer Thickness versus Implantation DoseTransfer Thickness versus Implantation Dose

Com

pres

sive

Str

ess

(Mpa

)

Stress Measurement Data

Tra

nsfe

rred

Thi

ckne

ss (

nm)

2 4 6000 1 2 3 4 5 6

0100

200

300H Peak

Damage Peak

100

200

300

Implantation dose 1016 cm-2

Implantation at 40 keV

Implantation dose 1016 cm-2

0 2 4 6-800

-700

-600

-500

-400

-300

-200

-100

0

Si

SU-8 σo


Layer Transfer Without Hydrogen ImplantationLayer Transfer Without Hydrogen Implantation

Crack tends to propagate into brittle substrate

Crack propagation driving force is inversely proportional to fracture toughness of materials. Si(100)1: 0.91 MPam1/2

Si(111)1: 0.82 MPam1/2

SiO22: 0.7-0.8 MPam1/2

Non-uniform thermal stress result in non-uniformity of the transferred surface

Original donor wafer

SiO2 on

SU-8SU-8

1 cm

SU-8

Transferred Si

SiO2

Donor Si

Glass

Full Transfer

Partial Transfer

SiO2 Si (100)

Transfer

Si (111)

Donor Si Donor Si

Glass Glass

Top views of transferred layers 1Chen et al., American Ceramic Society Bulletin, 59, 469 (1980)2Lucas et al., Scripta Metallurgica et Materialia, 32, 743 (1995)

SU-8


Layer Transfer – Layer Transfer – A Mechanical Fracture PerspectiveA Mechanical Fracture Perspective

• Stress intensity factors

• Effect of KII on KI crack propagation loading

Opening mode, KI

Out-of-plane shear mode, KIII

Shear mode, KII

KII = 0 KII > 0 KII < 0

Desired condition for uniform layer transferDesired condition for uniform layer transfer


0.001 0.010 0.100 1.0000.0

0.5

1.0

1.5

2.0

2.5

3.0

Σ = Efilm/Esubstrate

λ

KII = 0

Substrate

λh

Experimental Data vs. Analytical Data

Model by Drory et al, Acta Metall., 36, 2019 (1988)

Substrate

FilmM

Pdh

h

0.000

0.005

0.010

0.015

0.020

0.025

250Analytical

SU-8/Si (111) SU-8/Si (100)SU-8/SiO2

0.0050

1/Ecleaved materialT

rans

ferr

ed T

hick

ness

(n

m)

0.020

Analytical Model

Transferred ThicknessTransferred Thickness

))(

)1(217.0558.0(

1

))(

)1(279.0434.0(

1

0

0

Ih

K

Ih

K

II

I

Neutral plane

Ki – Stress intensity factor (mode i)

o – Thermal stress

I – Moment of inertia of the transferred beam


Derailing Mechanism of Mixed-Mode Crack PropagationDerailing Mechanism of Mixed-Mode Crack Propagation

• Interfacial delamination

• Partial substrate cracking

• Steady state substrate cracking

Thouless et al. (1991)

Map of Failure MechanismMixed-Mode Crack Propagation

Normalized substrate toughness, S

2/o2h

Interfacial delamination

Partial cracking

20

Nor

mal

ized

inte

rfac

ial

toug

hnes

s,

i2/

o2h

0.5

1.0

No cracking

1

Steady state

cracking

Substrate cracking

Substrate

σoFilm h d

λh

5020

2 .h/i

83020

2 .h/s

34020

2 .h/s

Stress intensity factor of the kink crack inclined at to the main crack

IIIII

IIII

kckcK

kckcK

43

21

Where kI and kII are the stress intensity factors acting on the main crack and,

)/cos/(cosc

)/sin/(sinc

)/sin/(sinc

)/cos/cos(c

23324

1

2324

1

2324

3

23234

1

4

3

2

1


Thermal Stress Simulation of Ge-SiO2-Si systems by finite element analysis

(800 °C) Annealing

• Ge layer has a tendency to buckle due to the compressive direct stress.

• The interfacial stress may exceed the fracture tensile strength of SiO2

(approximately 110MPa)300 400 500 600 700 800

-240

-180

-120

-60

0

60

120

SiO2

Ge

Annealing T (ºC)

Dir

ect

Str

ess

σx

x (

MP

a)

Si (500 μm)

Ge (100 nm)

SiO2 (100 nm)

y

xSi

Ge

SiO2


Thermal Stress Simulation of Patterned Ge-SiO2-Si systems by finite element analysis

0 100 200 300 400 500

80

120

160

200

240

300ºC annealing

500ºC annealing

800ºC annealing

w (nm)

Max

Dir

ect

Str

ess

σx

x (

MP

a)

Si (500 μm)

Ge (50 nm)

SiO2 (100 nm)

50 nm50nm

Compressive Tensile

SiO2

Si

Ge

Direct Stress σxx distribution after annealing

y

x

500ºC


Impact of Fin Orientation

ElectronMobility

HoleMobility

Source: Professor T-J. King (UCB)

(110)

(100)

(110)

(110)

(110)

S D

S

D

(110)

PMOS

S

D

SD

(100)

(100)

NMOS

(110)

(100)

(110)

(110)

(110)

(100)

(110)

(110)

(100)

(110)

(110)

(110)

S D

S

D

(110)

PMOS

(110)

S D

S

D

(110)

PMOS

S

D

SD

(100)

(100)

NMOS

S

D

SD

(100)

(100)

NMOS

(110)

(110)

(110)

(110)

S D

S

D

(110)

PMOS

(110)

(110)

(110)

(110)

(110)

(110)

(110)

(110)

(100)

(110)

S D

S

D NMOS

(110)

S D

S

D

(100)

PMOS


Manufacturing Equipment IssuesManufacturing Equipment Issues

•High-throughput, low-cost Epi ReactorsHigh-throughput, low-cost Epi Reactors•CMP or smoothing of SiGe and s-Si CMP or smoothing of SiGe and s-Si •High Current Hydrogen implantersHigh Current Hydrogen implanters•Plasma Activated BondersPlasma Activated Bonders•Mechanical Delamination MachinesMechanical Delamination Machines

Plasma BonderPlasma Bonder Gas Jet DelaminationGas Jet Delamination H+ Plasma ImplanterH+ Plasma Implanter


SummarySummary Ultra-thin (<10nm) SOI, GeOI, and s-SOI pose new Ultra-thin (<10nm) SOI, GeOI, and s-SOI pose new challenges to meet stringent uniformity and challenges to meet stringent uniformity and roughness specificationsroughness specifications

Mechanical stress distribution (bonding-induced Mechanical stress distribution (bonding-induced and implantation-induced) are key factors in transfer and implantation-induced) are key factors in transfer thickness control.thickness control.

Improved process recipes are needed to ensure Improved process recipes are needed to ensure thermal stability of sSOI and GeOI structuresthermal stability of sSOI and GeOI structures Challenges for process control , metrology, and Challenges for process control , metrology, and low-cost manufacturabilitylow-cost manufacturability

Engineered Substrates for High-Mobility MOSFETs Nathan Cheung

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