6.1. thermal oxidation 1,2.micro tech,2013

Microelectronic Technology

Thermal Oxidation

Thermal Oxidation

• A Process to grow high quality silicon di oxide film by thermally oxidizing silicon in oxygen containing ambient.

Basic Concepts

Basic Concepts

10 nm

0.1 µm

1 µm

1 nm

Masking Oxides

Gate OxidesTunneling Oxides

Field Oxides

Pad Oxides

Chemical Oxides from CleaningNative Oxides

Thermally Grown OxidesOxide

Thickness

Deposited Oxides

Backend InsulatorsBetween Metal Layers

Masking Oxides

Basic Concepts

• SiO2 and the Si/SiO2 interface are the principal reasons for silicon’s dominance in the IC industry.

SiO2 Properties

• Easily selectively etched using lithography.

• Masks most common impurities (B, P, As, Sb).

• Excellent insulator ( ).

• High breakdown field ( )

• Excellent junction passivation.

• Stable bulk electrical properties.

• Stable and reproducible interface with Si.

• No other known semiconductor/insulator combination has properties that approach the Si/SiO2 interface.

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ρ>1016 Ωcm, Eg >9 eV

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107 Vcm-1

Future Projections for Silicon Technology (SIA NTRS)

Year of 1st DRAM Shipment

1997 1999 2003 2006 2009 2012

Minimum Feature Size (nm) 250 180 130 100 70 50

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

Minimum Supply Voltage (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

Gate Oxide Tox Equivalent (nm)

4-5 3-4 2-3 1.5-2 <1.5 <1.0

Thickness Control (% 3s ) ± 4 ± 4 ± 4-6 ± 4-8 ± 4-8 ± 4-8

Equivalent Maximum E-field (MV cm-1)

4-5 5 5 >5 >5 >5

Gate Oxide Leakage (DRAM) (pA m m-2)

<0.01 <0.01 <0.01 <0.01 <0.01 <0.01

Tunnel Oxide (nm) 8.5 8 7.5 7 6.5 6

Maximum Wiring Levels 6 6-7 7 7-8 8-9 9

Dielectric Constant, K forIntermetal Insulator

3.0-4.1 2.5-3.0 1.5-2.0 1.5-2.0 <1.5 <1.5

Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani

Basic Oxidation Process

Basic Concepts

• Oxidation involves a volume expansion (≈ 2.2X).• Oxide layers grown on silicon has compressive stress.

1

1

1.3

11

1

1.2

1

1

1

1.3

1.3

Si substrate Si substrate

(a) Actual Silicon

(b) Converted Silicon into Silicon Dioxide

(c) Readjustment of Silicon Dioxide to Original Geometry

(a) (b) (C)

Basic Concepts

SiO2

Deposited Polysilicon

Si Substrate

Original Si SurfaceVolume Expansion

Location of Si3N4 Mask

• Oxidation Involves volume expansion ( 2.2X)• Especially in 2D &3D structures,stress effects play a dominant role

SEM Cross Sectional View of LOCOS


TEM Image of Si/SiO2 interface

Grown Oxide

Chlorine Oxidation Process

Si (Solid) + HCl+ O2 --- SiO2 (Solid) + H2 + Cl2 (3)

Other Chlorine bearing species like

TCE ( C2HCl3) , Trichloroethane (C2H3Cl3)

Oxides are Grown by:

Dry Oxidation Process

Si (solid) + O2 SiO2 (solid) at high temperatures (1)

Wet Oxidation Process

Si (solid) + 2H2O SiO2 (solid) + 2H2 ↑ at high temperature (2)


Basic Concepts

• Oxidation systems are conceptually very simple.

• In practice today, vertical furnaces, RTO systems and fast ramp furnaces all find use.

QuartzTube

Wafers

Quartz Carrier

Resistance Heating

H2O2

Oxide

C I

C G

C O

C S

C I

x O

Gas

0.01 - 1 µm 500 µm

Silicon

F 1 F 2 F 3

SiO2 Growth KineticsDeal -Grove Model

Where, (CG - CS) is oxidant concentration difference between main gas flow and oxide surface. CO and CI are the oxidant concentration at just inside the oxide surface and SiO2/Si interface respectively.

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F1 =hG CG −CS( ) (3)

Deal Grove Model

The flux representing the transport of oxidant in the gas phase to the oxide surface,

Where, hG is mass transfer coefficient

Oxidant conc. just inside oxide surface CO is given as by Henry’s Law

CO = HPS Where, PS is the partial pressure of species at the solid surface but not known

experimental parameter so,

The oxidant concentration in the oxide, C* is given as

C* = HPG Where, PG is the bulk gas pressure

(4)

(5)

Deal Grove ModelFrom the ideal gas law,

CG = PG / kT and CS = PS / kT

By substituting equations 4, 5 and 6 in equation 3, we get

F1 = h (C* - CO)Where, h = hG / HkT

(6)

(7)

The flux representing the diffusion of oxidant through the oxide to the SiO2/Si interface is given by Fick’s law,

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F2 =D∂N∂x

=DCO −CI

xO

(8)

Where, D is the oxidant diffusivity in the oxide

The flux representing the reaction at SiO2/Si interface is given as

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F3 =kSCI (9)Where, kS is the interface reaction rate constant

• Under steady state conditions, F1 = F2 = F3 so combining eqs.( 7-9)

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CI = C*

1 +kS

h+kSxO

D

≅ C*

1 +kSxO

D

(10)

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CO =C* 1 +kSxO

D

1 +kS

h+kSxO

D

≅C* (11)

• Note that the simplifications are made by considering h very large as compared to kS which is a very good approximation.

Deal Grove Model

• Combining (9) and (10), we have the growth rate,

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dxdt

= FN1

= kSC*

N1 1 +kS

h+kSxO

D

(12)

• Integrating this equation, results in the linear parabolic model.

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xO2 −x i

2

B+xO −x i

B/ A=t (13)

where (parabolic rate constant)

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B = 2DC*

N1

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BA

= C*

N11

kS

+ 1h

≅ C*kS

N1

(linear rate constant) and

(14)

(15)

Deal Grove Model

Where, N1 is the number of oxidant molecules incorporated per unit volume of oxide grown.

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τ=x i2 +Ax i

B

τ+=+ tAB

x

B

x OO

/

2

It is sometimes convenient to rewrite the linear parabolic law as follows:

Where,

Two Limiting Forms

Xo= B/A (t+ г) or x20=B( t+ г )

• (13) can also be written with oxide thickness as a function of time.

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xO =A2

1 + t +τA2 / 4B

−1

where

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τ = x i2 +Ax i

B

(16)

(17)

• The rate constants B and B/A have physical meaning (oxidant diffusion and interface reaction rate respectively).These can be described by Arrhenius expressions as

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B=C1 exp −E1 / kT( )

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BA

=C2 exp −E2 / kT( )(18)

(19)

Deal Grove Model

Extraction of Rate Constants

Ambient B B/A

Dry O2 C1 = 7.72 x 102 µ2 hr-1

E1 = 1.23 eVC2 = 6.23 x 106 µ hr-1

E2 = 2.0 eV

Wet O2 C1 = 2.14 x 102 µ2 hr-1

E1 = 0.71 eVC2 = 8.95 x 107 µ hr-1

E2 = 2.05 eV

H2O C1 = 3.86 x 102 µ2 hr-1

E1 = 0.78 eVC2 = 1.63 x 108 µ hr-1

E2 = 2.05 eV

• Numbers are for (111) silicon, for (100) divide C2 by 1.68.

Deal Grove Model

Activation Energy• Diffusivity of oxygen in fused silica: 1.17ev• Diffusivity of water in fused silica:0.80ev

.

0.0001

0.001

0.01

0.1

1

10

100

0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

B µ

m2hr

- 1

B/A

µm

hr

- 1

1000/T (Kelvin)

800900100011001200T (ÞC)

B/A H2O

B/A Dry O2

B Dry O2

B H2O

• Plots of B, B/A using the values in the Table.

Deal Grove Model

Parabolic rate constant

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B=2DC*

N1

B/A =C*Ks/N1

Linear Rate constant

Deal Grove Model

Calculated oxidation ratesDry O2 Wet O2

0

0.5

1

1.5

2

0 1 2 3 4 5 6 7 8 9 10 Time - hours

1100 oC

700 oC

1000 oC

900oC

800 oC O

xid

e T

hic

knes

s µ

m)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 Time - hours

1200 oC

1100 oC

1000 oC

900 oC 800 oC

Oxi

de

Th

ickn

ess

(µm

)

Typical values of oxidant C*= 5x1016cm-3 Dry O2

Solubility C*=3x1019cm-3 Wet O2

Thin Oxide Growth Kinetics

• A major problem with the Deal Grove model was recognized when it was first proposed - it does not correctly model thin O2 growth kinetics.

• Experimentally O2 oxides grow much faster for ≈ 20 nm than Deal Grove predicts.

• MANY models have been suggested in the literature.

1. Reisman et. al. Model

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xO =a t +t i( )b or xO =a t + x i

a

1b

b

• Power law “fits the data” for all oxide thicknesses.

• a and b are constants & experimentally extracted parameters and ti time corresponding to growth of any existing oxide thickness (xi)

• Physically - interface reaction controlled, volume expansion and viscous flow of SiO2 control growth.

2. Han and Helms Model

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dxO

dt= B1

2xO +A1

+ B2

2xO +A2

• Two terms representing parallel reaction - “fits the data” for all oxide thicknesses. (O2&O or oxygen vacancies)

• Three parameters ( A1 values is 0).

• Second process may be out-diffusion of OV and reaction at the gas/SiO2 interface.

3. Massoud et. al. Model

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dxO

dt= B

2xO +A+Cexp −xO

L

• Second term added to Deal Grove model - higher dx/dt during initial growth.

• L ≈ 7 nm, second term disappears for thicker oxides.

• Easy to implement along with the DG model; therefore, used in process simulators.

• Data agrees with the Reisman, Han and Massoud models. (800˚C dry O2 model comparison below.)

Thin Oxide Growth Kinetics

Oxi

de

Th

ickn

ess

(µm

)

8 910 20 30 40 50 60 708090100 200 300 400

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

Dry Oxidation of Silicon - Orientation Dependency

<100>

<111> Orientation

Oxi

de T

hick

nes

s (M

icro

n)

Oxidation Time (min)


Orientation Dependence

• In most oxidation conditions (Dry , Wet) (111) > (100)

• Difference in the surface density of silicon atoms on the various crystal faces.

• Density of Si - Si bonds

• For very thin oxides grown at very low pressures in dry O2 ( Rapid Oxidation Phase)

• High pressures at low temperatures in wet oxidation (150 Atm) (High Stress)

(110) >(111) > (100)

100111

68.1

=

A

B

A

B

Oxidation Simulation: Orientation Effects

Parameters: 30Min. At 900C in H2O

High Pressure Oxidation

The DG model predicts that the oxide growth rate should be directly proportional to oxidant pressure. Valid for wet oxidation

Dopant Segregation

• m= concentration of dopant in Si/concentration in oxide.

• m<1, oxide accepts dopant. m>1, oxide rejects dopants

• Boron depletes and phosphorus accumulates at the oxide/Si interface

• Segregation is much worse when oxidation takes place at lower temperatures

Slow & fast diffusors

Dopant DependenceBoron-doped, Wet oxidation

B segregates into oxide, weakens bond structure, increases D

Dopant Dependence (continued)

Phosphorus-doped

Wet Oxidation Dry Oxidation

Oxidation of Heavily Doped Regions

Phosphorus Ion Implant: 20KeV, 5x1015 Cm-2

30Min. In H2O at 800C 5Min.in H2O at 1000C

x5 x2

Silicon-Nitride: Oxidation Kinetics

Si3N4 + 6H2O -- 3SiO2 + 6H2 + 2N2

-- 3SiO2 + 4NH3

Kooi Effect

Two Step: - Si2N2O - SiO2

LOCOS Structure

2D Effects in Thermal Oxidation

• Variety of shaped structures including cylinder.

•The oxide is thinner on both concave and convex corners than in flat regions

39

.

0 1 2 3 4 5 6 7 8 1/r µm-1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Nor

mal

ized

Oxi

de T

hick

ness

Convex Radii

Concave Radii

1200 ÞC

1100 ÞC

1000 ÞC

900 ÞC

800 ÞC

1100 ÞC

1000 ÞC900 ÞC

1 µm 0.2 µm 0.125 µm

• Several physical mechanisms seem to be important: • Crystal orientation • 2D oxidant diffusion • Stress due to volume expansion • Oxide Viscosity • To model the stress effects, Kao et. al. suggested modifying the Deal Grove parameters.

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kS (stress)=kS exp −σnVR

kT

exp −σtVT

kT

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D(stress)=Dexp −P()VD( )

kT

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C*(stress)=C* exp −P()VS( )

kT

(20)

(22)

(21)

where and are the normal and tangential stresses at the interface. VR, VT and VS are reaction volumes and are fitting parameters.

(Kao et.al)

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σ n

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σ t

8 910 20 30 40 50 60 708090100 200 300 400

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Dry Process

Wet Process

Temp: 1000oC

Comparison Between Dry and Wet Oxidation ProcessesO

xide

Thi

ckne

ss (

Mic

ron)

Oxidation Time (min)


6.1. thermal oxidation 1,2.micro tech,2013

Engineering

Transcript of 6.1. thermal oxidation 1,2.micro tech,2013