1

Oxidation

Reading Assignments:

Plummer, Chap 6.1~6.4, 6.5.1, 6.5.3, 6.5.4, 6.5.5, 6.5.13

2

Why SiO2 ? The extraordinary properties of SiO2 are the basis of

the success of MOS-technology

Non-crystalline insulator Very high energy gap Easy to grow on Si Easy to integrate in a process Excellent interface between Si substrate Stable and insensitive to following process-steps Excellent scaling possibilities no real candidates for replacement

EV

E0

EM

3.0eVM

S

si

ox

4.9eV

9.0eV

1.0eV

EC

3

Properties of SiO2 Si-Si : 3.12 , Si-O: 1.62

, O-O: 2.27 10 nm : 40-50 atomic

layers 1.2 nm : only about 4

atomic layers Bonding angle : 110~180

( 144 ) Dielectric constant: 3.9 Energy gap : 9 eV Density : 2.20 g/cm3 Refractive Index : ~1.462 Dielectric strength 10-15

MV/cm

Bridging Oxygen

OxygenSilicon

Tetrahedra structure

4

Application of Thermal Oxides

5

Transistor Process Flow (1970s)

Clean

Oxide Etch

Poly Dep.

P+ Ion Implant

Field Oxidation p-Si

Gate Oxidation p-Si

Poly Etch

Annealing

p-Si

n+ npolyp-Si

+

p-Si

polyp-Si

polyp-Si

polyp-Si

n+ n+poly

PSG

AlSiSiN

p-Si

nMOSCross-section

6

Masking Oxide

Much lower B and P diffusion rates in SiO2than that in Si

SiO2 can be used as diffusion mask

Si

Dopant

SiO2 SiO2

7

Pad Oxide

Silicon nitride

Silicon Substrate

Pad Oxide

Relieve strong tensile stress of the nitride Prevent stress induced silicon defects

8

SiliconWafer Clean

Silicon

Field OxidationActivation Area

Silicon

Field Oxide

Oxide Etch

Silicon Dioxide

Blanket Field Oxide Isolation

9

Gate Oxide

Poly Si

Si Substrate

n+

Gate

Thin oxide

Source Drainp-Si

n+

VD > 0

Electrons

VG

Refer to Dr. Hong Xiao

Source: Dr. PT Liu

Thickness of oxide (Tox) must closely match the specification of the MOSFET design Tox must be sufficiently uniform across the entire wafer, and from wafer to wafer, and from

run-to-run Extremely low Qf and Dit (Good Si/SiO2 interfacial properties) EBD > 8MV/cm, pinhole free and negligible defects Sufficient long lifetime under normal operating High resistance to hot-carrier damage Resistance to boron penetration

10

Growth MechanismNative oxide: Si surface has a high affinity for oxygen formed in the air or chemical cleaning process quality is bad and should be eliminated 10-20

Thermal oxidation:

2 2( ) ( )Si solid O SiO solid+

2 2 2( ) ( ) 2Si solid H O SiO solid H+ +

Dry Oxidation

Wet Oxidation(steam oxide)

11

Silicon Dioxide

(SiO2)

Silicon wafer

(Si)O2 O2

O2 O2O2O2

O2

O2

O2 O2

Original Silicon Surface

45%55%

O2O2

O2

O2

O2

Dry Oxidation

Thickness of silicon = 0.45 x (thickness of SiO2)

Si + O2 SiO21000oC

Source: Dr. PT Liu Oxidizing species diffuse through SiO2 to Si/SiO2

1

1

1.3

11

1

1.2

1

1

1

1.3

1.3

Si substrate Si substrate

12

LOCOS Process

Silicon nitride

P-type substrate

P-type substrate

Silicon nitride

p+p+ p+Isolation Doping

P-type substrate p+p+ p+Isolation Doping

SiO2

Pad Oxide

Pad oxidation, nitride deposition and patterning

LOCOS oxidation

Nitride and pad oxide strip

Birds Beak

SiO2

13

Typical LOCOS and Birds Beak

SiO2

Deposited Polysilicon

Si Substrate

Original Si SurfaceVolume Expansion

Location of Si3N4 Mask

14

LOCOS

Compare with blanket field oxide Better isolation Lower step height Less steep sidewall

Disadvantage rough surface topography Birds beak

Replacing by shallow trench isolation (STI)

15

Stress

Mismatch between different materialsTwo kinds of stresses, intrinsic and extrinsic Intrinsic stress develops during the film nucleation and growth process. The extrinsic stress results from differences in the coefficients of thermal expansion Tensile stress: cracking film if too highCompressive stress: hillock if too strong

16

Bare Wafer After Thin Film Deposition

Compressive Stress (tend to expand)

Negative curvature

Tensile Stress (tend to contract)Positive curvature

Substrate Substrate Substrate

Film Stress

17

Illustration of Thermal Stress

SiO2Si

L

SiO2Si

At 400 C

At Room Temperature

L

L = T LUnder compressive stress

18

Coefficients of Thermal Expansion

(SiO2) = 0.5106 C1

(Si) = 2.5106 C1

(Si3N4) = 2.8106 C1

(W) = 4.5106 C1

(Al) = 23.2106 C1

19

Interfacial Structure

Si

Si Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Dangling bondSiO2

surface

Si

Si Si

Si Si

Si

Si

Si

Si

Si

Si

Si

Si Si

Interfacialtrap

impurity Stretched bondOxygenvacancy

20

total Oxygen count

bulk-oxide count

sub-oxide count

tox

TEM boundary

D. Muller, et. al, Nature, 399, 758(1999)

Interfacial Structure

21

Oxide Traps and Defects

K+Na+

Mobile Ionic Charge

Oxide Trapped Charge+

+

+

+ + + + + +

Fixed Oxide Charge

Interface Trapped Charge

(Deal, 1980)

Fixed Charge (Qf, Nf) (Positive): structural defects due to incomplete oxidation

and stress, within 2.5 nm from Si/SiO2 not electrically communicate with interface

states Mobile Oxide Charge (Qm, Nm): Na+,

Li+,K+,H+

Oxide Trapped Charge (Qot,Not): positive or negative due to hole or electron

traps in borken Si-O bonds radiation, charge injection in high field

Interface Trapped Charge (Qit,Nit, Dit): structural defects due to incomplete oxidation

at Si/SiO2 donor or acceptor-like surface-potential dependent

22

SiO2/Si InterfaceInterface states: imperfect bonds

Electrically interacted with channel carriers Assuming each dangling bond give rise to one interface state

Impact on device characteristics threshold voltage ( Vt ) carrier mobility ( Gm ) reliability, oxide integrity and HCI( hot-carrier-injection)

degradation Hydrogen annealing at 300-500oC is effective to passivate

Qit at the very end step of process.

23

Deal-Grove Triangle Qf decreases with

increasing oxidation temperature

Post oxidation annealing in N2 or Argon ambient is needed to minimize Qf

However, annealing should be kept within specific time period without causing increase of Qf 1200600 900

Fixe

d C

harg

e, Q

f Dry O2

Dry N2 or Argon

24

Oxide Growth: Deal-Grove ModelO

xide

Thi

ckne

ss

Oxidation Time

Linear Growth (Reaction-Limited) Regime

BA

X = t

Diffusion-limited Regime

X = B t

Silicon

tox

Top oxide surface

Originalsiliconsurface

Oxide-siliconsurface

tSi 0.44 tox

Deal-Grove Relation (linear-parabolic growth law):

25

Silicon Dry Oxidation

2 4 6 8 10 12 14 16 18 20Oxidation Time (hours)

0.2

0

0.4

0.6

0.8

1.0

1.2

Oxi

de T

hick

ness

(mic

ron)

1200 C

1150 C

1100 C

1050 C

1000 C

950 C

900 C

Silicon Dry Oxidation

26

Deal-Grove Model-1Oxidants must be transported from the bulk of the gas to the 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 =

HkThhCChFp/kTC

go / where,)( gas idealFor

*1 ==

=

Oxide SiliconGasCG Cs

Co

Ci

x

d

F1 F2 F3

27

Deal-Grove Model-2Oxidants must diffuse across the oxide layer already present.

D is the diffusivity 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 theoxide/silicon interface

ks is the chemical surface-reaction rate constant

o

io

xCCDF =2

isCkF =3

CgO2 F1

SiO2 Si

C0Ci

O2 F2

C* F3

O2+Si

SiO2

X0O2

Concentration

Gas phase

28

Deal-Grove Model-3

Steady state : F=F1=F2=F3* *

*

*

and 1 1

1

1

As 0

As 0

is s o s o

s o

os s o

s oi o

s oi

C CC k k x k xh D Dk x CDC Ck k x

h Dk x C CD

k x CD

= + + +

+ =

+ +

Reaction control

Diffusion control

Cg

O2 F1

SiO2 Si

C0 Ci

O2 F2

C* F3

O2+Si

SiO2

X0O2

Concentration

Gas phase

CgO2 F1

SiO2 Si

C0Ci

O2 F2

C* F3

O2+Si

SiO2

X0O2

Concentration

Gas phase

29

Deal-Grove Model-4

*

3 21o s o

Is s o o

dx k C dx BN F k k xdt dt x Ah D

= = =++ +

There are 2.21022 SiO2 atoms in one cubic centimeter: NI

Applying the boundary condition: xo=xi at t=0, the solutionof above equation is as expressed in the next slice

* 21 1 22 [ ] i is I

DC x AxA D Bk h N B

+= + = =

A, B : temperature, ambient composition, pressure and crystalline orientation is related to the initial oxide thickness

30

Growth Mechanism

2oxT Bt=

2 ( )ox oxT AT B t + = +

Deal-Grove Relation (linear-parabolic growth law):

for t >> , t >> A2/4B

for (t+ )

31

Effect of Temperature

VLSI Technology S.M. Sze

exponentially

near to Si-Si bond breaking ~ 1.83eV Oxygen diffusivity ~1.17eVWater diffusivity ~0.80eV

32

Effect of PressureThe concentration of oxidant just inside the oxide at the gas/SiO2interface C* is proportional to pg, then both B and B/A are proportional to pg.

33

Effect of Crystal OrientationEffect of crystal orientation is explained by the differences in the surface density of silicon atoms on the various crystal faces.

34

Orientation Dependence

Orien-tation

Area of unit cell (cm2)

Si atoms in area

Si bonds in area

Bonds available

Bonds1014cm-2

Available bonds1014cm-2

N relativeto

2a2 4 8 4 19.18 9.59 1.000

1/2 3a2 2 4 3 15.68 11.76 1.227

a2 2 4 2 13.55 6.77 0.707

35

Thin Oxide Growth

Massouds empirical 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.

nmLeVEhrmCkTECC

LxC

AxB

dtdx

Ao

Ao

o

o

o

7 and ,35.2 ,/106.3

exp

exp2

8

=

+

+=

36

Conventional Furnace Equipment

37

Thermal Process Hardware

Control System Gas Delivery System Loading System Exhaust System Process Tube

38

Furnace SystemMFC

MFCMFC

MFC

Scrubber

Exhaust

Control Valve

Regulator

Process Tube

Control System,,HCl or TCA (trichloroethane),,, (Interface state charge) !!

HC

l

O2

N2

39

Furnace Configuration

CenterZone

Gas Flow

FlatZone

Quartz Tube

T

Distance

Heating Coils

Tower

Heaters

Horizontal Tube Vertical Tube

0.5

several hundred wafers

1/sec

40

Rapid Thermal Process (RTP)

IR Pyrometer

External Chamber

Process Gas

Tungsten-Halogen LampQuartz Chamber

Tem

pera

ture

Time

RampUp

RampDown

>100oC/sec >50oC/sec

41

RTP ToolBottom Lamps

Wafer

Top Lamps

42

Dry Oxide Process Sequence Idle with purge N2 flow Idle with process N2 flow Wafer boat push in with process N2 / O2 flow Temperature ramp-up with process N2 / O2 flow Temperature stabilization with process N2 / O2 flow Oxidation with O2, HCl; stop N2 flow Oxide annealing; stop O2; start process N2 flow Temperature cool-down with process N2 flow Wafer boat pull out with process N2 flow Idle with process N2 flow Repeat process with next boat

43

Oxidation Recipe

44

Faster, higher throughput (H2O, HO species) Thick oxide, such as LOCOS Dry oxide has better quality

Process Temperature Thickness Oxidation Time

Dry oxidation 1000 C 1000 ~ 2 hr

Wet oxidation 1000 C 1000 ~ 12 min

Wet Oxidation

Source: Dr. PT Liu

45

Effect of Oxidation Ambient

Wet oxidation rate is much higher than dry oxidation rate because HO- or H2O diffuses much faster than O2 in SiO2.

46

Pyrogenic Steam System

H2

O2

Thermal Couple

To Exhaust

Hydrogen Flame, 2 H2 + O2 2 H2O

Process Tube Wafer BoatPaddle

Typical H2:O2 ratio is between 1.8:1 to 1.9:1.

47

Outside Torch System (OTS)

48

Pyrogenic Wet Oxidation System

MFC

MFC

MFC

Control Valves

Regulator

Proc

ess N

2

Purg

e N

2

O2

H2

MFC

Scrubbier

Exhaust

Process Tube

Wafers Burn Box

49

Pyrogenic Oxide Process SequenceIdle with purge N2 flowIdle with process N2 flowRamp O2 with process N2Wafer boat push in with O2 and process N2 flowsTemperature ramp-up with O2 and process N2 flowsTemperature stabilization with O2 and process N2 flowsRamp O2 turn off N2 flowStabilize O2 flow Turn on H2 flow, ignition, and H2 flow stabilization

50

Pyrogenic Oxide Process Sequence (Cont.)

Steam oxidation with O2 and H2 flowsHydrogen termination; turn off H2 while keeping O2 flowOxygen termination; turn off O2 , start process N2 flowTemperature ramp-down with process N2 flowWafer boat pull out with process N2 flowIdle with process N2 flowRepeat process with next boatIdle with purge N2 flow

51

Oxide Measurement

ThicknessUniformity SEM, TEM, Profilermeter Color chart Spectrophotometry

(Reflectometry) Ellipsometry C-V

I-V, breakdown voltageC-V, oxide charge

52

t

21

Substrate

Dielectric film, n( )

Incident light

Human eye orphotodetector

Spectrophotometry (Reflectometry)

Interference

53

n2 > n1 > n0=1

180o phase change

0

1= 0/n1

n0sin= n1sin

0 1 00 0

1

2 cos2 cos

1,2,3 : constructive interference1/ 2,3 / 2,5 / 2 : distructive interference

n xx mn m

mm

= =

==

Interference in Thin Films

54

Color Chart

55

Substrate

UV lamp

Detectors

Film

Spectroreflectometry System

1 2 3

358 417 476 535 594 653 712 771

Ref

lect

ance

(%)

05

101520253035404550

Wavelength (nm)

Constructive interferenceDestructive interference

56

Capacitance Measurement of MOS-CSmall-signal capacitance

MOS capacitance is defined as small signal capacitance and is measured by applying a small ac voltage on the top of a dc bias

Impedance is measured by an precision impedance meter The imaginary part of the measured impedance is converted to capacitance.

Superimposed AC signal

57

W

Wdm

Wdm

Accumulation

ac charge response

dc charge response

Depletion

LFCV HFCV

Wdm

Deep Depletion

0

11 1 1

2

oxdepl

ox

ox si si ox

sis

A

CC WC C t

WqN

= =+ +

=

The DC VG is swept very fast (

58

MOS CV: LFCV, HFCV and DD

MetalOxide

SiVG

vss

Csi

VG (quiescent point)

Cox VG

C/Cox

1

HFCV

LFCV

deep depletion

Inverted minority carrier can follow VG but not vss

Minority carrier can follow VGand vss

Minority carrier cannot follow either VG or Vss

Flat band will have a little smaller Cdepending on the size of vss

Vth

Acc.Dep.

Inv.

ssi

s

dQCd

=

59

N-type substrate (PMOS):

P-type substrate (NMOS):

0depl.invers. accumul.

0 depl. invers.accumul.

VG

VG

VG

C/Cox

VG

p-type bulk (NMOS)

VG

C/Cox

VG

n-type bulk (PMOS)

Substrate Type

60

Oxide Thickness DeterminationOxide Thickness ( Tox )

Cox is high frequency capacitance with the device biased in strong accumulation

VG

CH

p-sub

High-Frequency

strongaccumulation

o xo x

o x

AtC

=

where A=gate area ( cm2 )ox = permittivity of oxide material ( F/cm )Cox = oxide capacitance ( pF )

Tox estimated from the CV method may be slightly larger than the thickness measured by the optical method because the additonal QM correction in the accumulation layer.

61

Effect of Oxide ChargesFlat-band voltage (Vfb) and threshold voltage (Vth)

( )

Fox

Sifbth

T

moxoxox

f

ox

it

ox

otmsfb

CQVV

dxxTx

CCQ

CQ

CQV ox

2

10

+=

=

K+Na+

Oxide Trapped Charge+

+

+

+ + + + + +

Fixed Oxide Charge

Mobile Ionic Charge

Interface Trapped Charge

C/CoxPositive oxide charge

Vfb Definition: The gate voltage at which the energy band in Si substrate is flat, i.e. zero field in Si.

62

EV

ECEF

pMOS

We will first examine an ideal case: a midgap trap with donor-like behavior: when the Fermi level is above the trap level, the trap is filled and exhibits no charge (trap filled with electron); when the Fermi level is below the trap level, the trap is empty and exhibits a positive charge state (trap empty).

0filled EV

ECEF0

partially filled EV

ECEF

+empty

+

accumulation depletion inversionC/CoxpMOS

midgap depletion point

ideal HFCV

HFCV distorted by midgapdonor-like trap

Interface Traps at One Energy Level

63

EV

ECEF

pMOS

If we have instead a distribution of trap levels across the band gap, then the resulting influence of HFCV will be a distortion of the ideal CV curve. Notice that how different trap types can distort CV in different ways.

C/Coxaffected by donor-like traps

ideal HFCV

donor-like traps

acceptor-like traps

affected by acceptor-like traps

Passivation of Interface Traps

64

High-Low Frequency CV

HF must be high enough so that the charge/discharge of traps cannot follow.HF must be low enough so that the charge/discharge of traps can follow.

Application of Thermal OxidesGrowth MechanismLOCOS ProcessLOCOSStressIllustration of Thermal StressCoefficients of Thermal Expansion Oxide Growth: Deal-Grove ModelDeal-Grove Model-1Deal-Grove Model-2Deal-Grove Model-3Deal-Grove Model-4Growth MechanismEffect of PressureEffect of Crystal OrientationOrientation Dependence Thin Oxide GrowthOxidation RecipeEffect of Oxidation AmbientPyrogenic Oxide Process SequencePyrogenic Oxide Process Sequence (Cont.)Oxide MeasurementColor ChartMOS CV: LFCV, HFCV and DDEffect of Oxide Charges