C Thermal Oxidation

7/29/2019 C Thermal Oxidation

1/43

Oxidation -

National Chiao-Tung UniversityDepartment of Electronics Engineering &

Institute of electronics

Thermal Oxidation

Oxidation method wet oxidation

dry oxidation

rapid thermal oxidation

high pressure oxidation plasma enhanced oxidation

Oxide hardness technology post oxidation anneal

nitrogen passivation

N2O or NO oxidation, N2O or NO anneal, nitrogen implantation fluorine passivation

HF dip, VHF clean, NF3 additive, fluorine implantation


2/43

Oxidation -



Atomic Structures

SiO2 Density=2.26(2.4) g/cm3


3/43

Oxidation -



Thermal Oxidation

Dry Oxidation Si (s) + O2 (g) SiO2 (s)

Wet Oxidation 2H2 (g) + O2 (g) H2O (g)

Si (s) + H2O (g) SiO2 (s) +2H2 (g)

0.44d Si to 1d SiO2


4/43

Oxidation -



Deal-Grove Model-1

Oxidants must be transported from the bulk of the gas tothe oxide surface.

Cg : oxidant concentration in bulk of gas

Cs

: oxidant concentration right next to the oxide surface

hg : gas phase mass-transfer coefficient

Henrys law In equilibrium, the concentration of a species within a solid is proportional

to the partial pressure of that species in the surrounding gas.

C=Hp, where H is the Henrys law constant and p is the gas pressure

C* = H pg(equilibrium concentration in bulk SiO2)

Co= H ps(equilibrium concentration at bulk gas/SiO2interface)

)(1 sgg CChF

HkThhCChF

p/kTC

go /where,)(

gasidealFor

*

1


5/43Oxidation -



Deal-Grove Model-2

Oxidants must diffuse across the oxide layer already

present.

D is the diffusitivity of oxidant in bulk oxide

Ci is the oxidant concentration in bulk oxide

at the oxide/silicon interface

xo is the thickness of oxide layer already present

Oxidants must react at the

oxide/silicon interface

ks is the chemical surface-reaction rate constant

o

io

x

CCDF

2

isCkF 3


6/43Oxidation -



Deal-Grove Model-3

Steady state : F=F1=F2=F3

h

k

CCCk

CCCD

D

xk

h

k

CD

xk

C

D

xk

h

k

CC

soi

oi

oss

os

ooss

i

1

0As

and00As

1

1

and

1

*

s

*

*

*

Diffusion control

Reaction control


7/43Oxidation -



Deal-Grove Model-4

N : The number of oxidant molecules incorporated into a

unit volume of oxide. O2 oxidation : N=2.2x10

22 molecules/cm3

H2O oxidation : N=4.4x1022 molecules/cm3

B

Axx

N

DCB

hkDA

tBAxx

xx

D

xk

h

kCkF

dtdxN

ii

s

oo

iooss

so

2

*

2

*

2

112

0conditioninitial,

1


8/43Oxidation -



Deal-Grove Model-5

constantratelineartheis

iprelationshlinearcalledist

4tAs

constantrateparabolictheis

iprelationshparaboliccalledis

4As

1

4

1

2

From

*

2

2

2

2

2

N

C

hk

hk

A

BA

Bx

BA

B

Btx

BAt

BA

t

A

x

tBAxx

s

s

o

o

o

oo

Reaction control Diffusion control


9/43Oxidation -



Initial Oxidation

Massouds experimental model:

Apply to either (111) or (100) oriented Si.

The first term is the Deal-Grove Model.

The second term represents an additional oxidation mechanism.

The actual mechanism is still not clear.

nmLeVEhrmC

kT

E

CC

L

xC

Ax

B

dt

dx

Ao

A

o

o

o

o

7and,35.2,/106.3

exp

exp2

8


10/43Oxidation -



1

Oxidation Equipments

Furnace oxidation

(Hot wall)

Rapid thermal oxidation

(Cold wall)


11/43Oxidation -



1

Oxidation Furnaces


12/43Oxidation -



1

Oxidation Recipe


13/43

Oxidation -



1

Measurement of Oxide

Color chart

Physical methods Scanning Electron Microscope (SEM), Transmission Electron Microscope

(TEM), Surface Profiler, Atomic Force Microscope (AFM)

Detect : thickness, interface structure, surface roughness

Optical methods Ellipsometer

Detect : thickness (T), refraction index (n), absorption index (k)

Electrical methods Capacitance-Voltage curve, electrical strength, interface state density, etc.


14/43

Oxidation -



1

Color Chart


15/43

Oxidation -



1

Scanning Electron Microscopy

Principle : image contrast obtained by electron from different material or different

topography

Signal to Form Images : Secondary Electron (SE)

Ee


16/43

Oxidation -



1

Transmission Electron Microscopy

Principle : image contrast obtained by electron transmitted through thin materials

Signal to Form Images : bright field image

direct transmitted beam to provide micro-structure and morphology

dark field image diffracted beam image to obtain crystallographic information

lattice image (high resolution TEM, HREM) a combination of diffracted and direct beams to yield images with atomic resolution

diffraction diffraction yields crystallographic and orientation information

x-ray spectroscopy (EDX/WDX)

electron energy loss spectroscopy (EELS)

energy analysis of transmitted electron to analyze low-Z element


17/43

Oxidation -



1

TEM Sample Preparation

Plane-View Sample circular sample cut to diameter 3mm

chemical or mechanical thinning to 100 um

chemical or ion-milling to form ultra-thin area

Cross-Sectional Sample rectangular sample cut to 2mm x 3 mm

stack samples

mechanical polish to 20 um

fixed by Cu-ring

ion-milling to form ultra thin area

Precision Sample FIB cut

3 mm

ultra thin area

3 mm

2 mm

E-beam


18/43

Oxidation -



1

SiO2/Si Interface

Si Lattice

Image

SiO2


19/43

Oxidation -



1

Atomic Force Microscopy

Principle : a sharp tip is vibrated perpendicular to the sample surface at the resonant

frequency.

Van der Waals force between probe tip and sample surface modify the

resonant frequency.

The change of resonance provides a corresponding displacement.

Application : surface morphology measurement

sensitivity ~ atomic level

three dimensional topography measurement accuracy +-1 nm, repeatability +-10 nm


20/43

Oxidation -



2

Ellipsometer

layeriththeofthicknessthet

layeriththeofindexabsorptionthek

layeriththeofindexrefractionthen

airofindexrefractionthen

1allwith),,,,,,,

1allwith),,,,,,,

.measurableare

)3600(and)900(anglesricellipsometThe

)tan(ratioreflectioncomplexThe

)(

)(;

(

)(

i

i

i

o

ikntknnf(

ikntknnf(

eR

R

IncidentE

relectedER

incidentE

reflectedER

ssiiio

ssiiio

j

s

p

s

ss

p

p

p


21/43

Oxidation -



2

Spectrum Ellipsometer


22/43

Oxidation -



2

Effect of Oxidation Ambient

Wet oxidation rate is much higher than dry oxidation rate

because the oxidant solubility in SiO2 (C*) is much higher

for H2O than for O2.


23/43

Oxidation -



2

Effect of Crystal OrientationDry O2/1000C/1hr

Effect of crystal orientation is explained by the differences

in the surface density of silicon atoms on the various

crystal faces.


24/43

Oxidation -



2

Effect of Pressure

The concentration of oxidant just inside the oxide at thegas/SiO2 interface C

* is proportional to pg, then both B and

B/A are proportional to pg.


25/43

Oxidation -



2

Boron-doped

Effect of Impurity Concentration

The mechanism associates with the faster oxidation at theSi/SiO2 interface due to higher vacancy concentration, i.e.

B/A is affected. The differences of oxidation time is more pronounced for n+ Si than for p+

Si and is more pronounced for low temperature oxidation compared to

high-temperature oxidations.

Phosphorus-doped

N i l Chi T U i i


26/43

Oxidation -



2

Chlorine Incorporation

O2 + HCl or TCA or DCE Effective for dry oxidation only

Lower interface state density

Lower defect density

Gettering of metal ions in oxides

Enhanced oxidation rate due to

Rough interface at high Cl concentration (TCA>8%)

OHClOHCl 222 24

N ti l Chi T U i it


27/43

Oxidation -



2

Fluorine Incorporation

Methods HF surface immersion

F2 surface treatment

Oxidation with NF3 additive

F+ implantation into Si or oxide

Fluorine effects Enhanced oxidation rate

Lower dielectric constant

Better oxide quality

Enhanced boron penetration Poorer dielectric reliability with excess fluorine

National Chiao Tung University


28/43

Oxidation -



2

Nitrogen Incorporation

Higher low field mobility Lower high field mobility

Lower defect density

Higher reliability Stronger radiation immunity

Slower boron penetration

National Chiao-Tung University


29/43

Oxidation -



2

Nitrogen Incorporation



30/43

Oxidation -

National Chiao Tung UniversityDepartment of Electronics Engineering &


3

Oxide Charges

Flat-band voltage (Vfb) and threshold voltage (Vth)

F

ox

Sifbth

T

m

oxoxox

f

ox

it

ox

otmsfb

C

QVV

dxxT

x

CC

Q

C

Q

C

QV

ox

2

1

0



31/43

Oxidation -

g yDepartment of Electronics Engineering &


3

Oxide Charges

Interface charge (Qit) Magnitude depends on Si orientation : (111)>(110)>(100)

Magnitude depends on surface impurity and interface bonding

Magnitude can be reduced by annealing at 400-500oC in H2 ambient

Fixed charge (Qf)

Immobile and independent of surface potential Depends on Si orientation and oxidation condition



32/43

Oxidation -

g yDepartment of Electronics Engineering &


3

Oxide Charges

Mobile ion (Qm) Comes from metal contamination

Drift rate is thermally activated and is field dependent

Trapped charge (Qot)

Maybe positive or negative Maybe caused by ionization radiation or carrier injection

Trap site comes from defect in oxide layer

Low temperature annealing can not remove trap site but cause

neutralization or compensation of trapped charge.

Polarization charges (Qp) Comes from electric field induced dipole



33/43

Oxidation -

Department of Electronics Engineering &


3

Effect of Mobile Ions

SiO2

HfO2

EOT 1

Q(x)

EOT

EOT 1

EOT 2

0

X

HfO2

SiO2

P-type

v

EOT

2

2

2 2

,

2

,

1[ 1* *

1 11 * ( 1 )* ]

2 2

it S iOit High

Hig

FB ms

ox

S iOh

V EOT EOT

EOT EOT EOT

Q

Q

Q

Q

0

1 1( )dFB

o

V xQ x dxC d

- 12 -



34/43

Oxidation -



3

Application of Thermal Oxides

National Chiao-Tung UniversityD t t f El t i E i i &


35/43

Oxidation -



3

ITRS Roadmap (2003)



36/43

Oxidation -



3

Carrier Injection

Tunneling Mechanism Schottky Emission

Frenkel-Pool Emission

Fowler-Nordheim Tunnelingor Field Emission

Ionic Conduction

Space Charge Limited

Ohmic

Direct Tunneling

kT

q

T

VaT

kT

qEqTAJ BIB

exp~

4/exp

22*

kT

q

T

VaV

kT

qEqEJ BIB

2exp~

/exp

V

b

VEq

qm

EJB

exp~3

24

exp2

2/3*2

T

c

T

V

kT

E

T

EJ a exp~exp

T

cV

kT

EEJ a exp~exp

23

23

23

22

1

2

exp

1 s

oxss

oxsoxs

ox qV

E

C

qV

BEJ

2

3

2

~4

VL

VJ

S.E.

F-P

F-N

D.T.

Ionic



37/43

Oxidation -



3

Issues of Thin Oxide

Direct tunneling current

Maximum tolerable Igate

2

321

2

3

23

23

23

2

21

2

2

3

4

16

exp

1

sox

s

s

oxss

ox

s

oxs

oxDT

q

mC

qB

qV

E

C

qV

BEJ

2

gate

/1

m0.1//1

m0.1LAs

/1

cmA

mnAI

mnAII

gate

offgate

1um

0.1 um



38/43

Oxidation -



3


Poly-Si depletion

Influence Constant field & constant Npoly

=> constant Vpoly

An 0.3-0.5nm thick EOT loss is

inevitable.

polySi

gox

accoxinvox

poly

Sipoly

polyoxSifbg

Nq

V

TT

N

QV

VVVVV

22

,

2

,

2

2

meas

oox

C

AEOT



39/43

Oxidation -

p g g


3


Quantum effect Because of the conduction band triangular like well at the surface,electron states are a series of discrete levels above the edge of the

conduction band.

The surface potential is larger than classical predicted.

Charge is located 0.5-1.0 nm away from surface than classical predicted.

Consequences A 0.3 nm increase of effective oxide thickness

An increased depletion layer charge density



40/43

Oxidation -


4


Boron penetration Boron may diffuse through gate oxide

from p+ poly-Si gate of PMOSFET. Positive flat-band voltage shift

Degradation of oxide wearout properties

Thinner oxides result in severer boronpenetration effect.

Fluorine and hydrogen enhance boron

diffusion.

Crystallized a-Si retards boron diffusion.

Nitrided oxides reduce boron diffusitivity.

Trade-off between boron

penetration and poly depletion

Tox=3.3nm

IEDM, 1995, p.85

VLSI-TSA, 1997, p.167



41/43

Oxidation -


4

Advantages of High K Dielectric

Thicker thickness

Lower EOT

Lower tunneling current

ox

ox

HKHK TT


I tit t f l t i


42/43

Oxidation -


4

Requirement of High K Dielectric

Criteria Requirement

EOT scalability < 1 nm Dielectric constant > 15

Negligible FIBL effect Dielectric constant < 60

Leakage current < 1A/cm2 Band gap > 5eV

Barrier height > 1eV

Thermal stability No silicidation and reduction

Hysteresis < 20mV

Dispersion 10 years




43/43

Oxidation -


4

Band Alignment Consideration

Energy gap of dielectric depends on dielectric constant.

Oxide Dielectric

Constant

SiO2 3.9

Si3N

4 7-9

HfO2 ~30

ZrO2 ~25

Al2O

3 9-13

Y2O

3 11-17Ta

2O

5 25-45

La2O

3 21

C Thermal Oxidation

Documents

Transcript of C Thermal Oxidation