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A Presentation on

Growth kinetics for thermal oxidation of silicon, thespecial case for VLSI/MEMS technology

ByPrachi Sharama

Dept. of ECE, Banasthali University, Banasthali.

4/11/2011 1

Compiled by:-

1. Kulwant Singh2. Soney Varghese3. Uvais V N4. Prachi Sharama5. Ashish Kumar


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Overview

SiO 2 and the Si/SiO 2 interface are the principal reasons for silicons dominancein the IC industry.

10 nm

0.1 m

1 m

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

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T hickness & uses of SiO 2 in Si Technology


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A dvantages/ applications SiO 2:-

Easily selectively etched usinglithography. Masks most common impurities(B, P, A s, Sb).

Excellent insulator ( ).

High breakdown field( ) Excellent junction passivation. Stable bulk electrical properties. Stable and reproducible interface

with Si.

V "1016 ; cm, E g " 9 eV

10 7 Vcm -1

No other known semiconductor/insulator combination has properties thatapproach the Si/SiO 2 interface.

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Future Projection Si TechnologyYear of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018

Technology Node (half pitch) 250 nm 180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm

MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm

DRAM Bits/Chip (Sampling) 256M 512M 1G 4G 16G 32G 64G 128G 128G

MPU Transistors/Chip (x10 6) 550 1100 2200 4400 8800 14,000

Gate Oxide T ox Equivalent (nm)

MPU

1.2 0.9 0.7 0.6 0.5 0.5

Gate Oxide T ox Equivalent (nm)

Low Operating Power

1.5 1.2 0.9 0.8 0.7 0.7

Gate Dielectric Leakage

(nA/m @ 100 C) MPU

170 230 330 1000 1670 1670

Thickness Control (% 3 W) < 4 < 4 < 4 < 4 < 4 < 4

Min Supply Voltage (volts) 1.8-2.5 1.5-1.8 1.2-1.5 0.9-1.2 0.8-1.1 0.7-1-0 06-0.9 0.5-0.8 0.5-0.7

Adapted from NTRS SIA, 1997

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Manufacturing Methods & Equipment

Qu artz

ub eWafers

Qu artz

arrier

esista ce Heati g

H2

2Conceptual Silicon Oxidation System

Oxidation systems are conceptuallyvery simple.

In practice today, vertical furnaces,RT O systems and fast ramp furnacesall find use.

T hermal oxidation potentially being used in many places in chipfabrication. In practice, deposited SiO 2 layers are increasingly being used

in case of back-end process.

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Si O 2 p SiO 2

Si 2H 2O p SiO 2 2H 2


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Charge Associated with the SiO 2/Si

System

++++ +xxxxxx

+++ -

--

K +

TransitionRegion

Na +SiO 2

Q m

Q ot

Q f

Q itSilicon

Four charges are associated withinsulators and

insulator/semiconductor interfaces.

Q f - fixed oxide charge Q it - interface trapped charge Q m - mobile oxide charge Q ot - oxide trapped charge

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Measurement Methods

Physical measurement Scanning tunneling microscope(ST M)

Atomic Force microscope (A FM) SEM etc..

Optical Measurement Color chart by Pliskin and Conrad

Electrical measurement- The C MOS Capacitor

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A . Planar growth kinetics

Ox i e

I

O

C S

C I

x O

Gas

0.01 - 1 m - 500 m

Silico

1 2 3

T he basic model for oxidationwas developed in 1960 by Dealand Grove.

Si O 2 SiO 2

Si 2H 2O S iO 2 2H 2

(1)

(2)

F 1 ! G C G C S

F2 !

xxx

! D C O C Ix O

F 3 ! k S C I

(3)

(4)

(5)4/11/2011 9


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Under steady state conditions, F 1 = F 2 = F 3 so

C I !C *

1k Sh

k S x OD

$ C*

1k S xO

D

(6)

C O

C * 1k S x O

D

1k S

h

k S x O

D

C * (7)

Note that the simplifications are made by neglecting F 1 which is a very goodapproximation.

Combining (6) and (7), we have

d xd t !

F

N1 !

k S C*

N1 1

k Sh

k S xOD

Integrating this equation, results in the linear parabolic model.

(8)

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x O2 x i

2 xO x i/

! t (9)

where (parabolic rate constant) (10)

(11)

B

A! C

*

N1

1k

S

1h

$ C*k S

N1

(linear rate constant)and

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

x O !A

21

t XA 2 / 4 B

1

(12)

where X x i

2 Ax iB

(13)

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B ! 2DC*

N 1


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T he rate constants B and B/ A have physical meaning (oxidant diffusion andinterface reaction rate respectively).

B ! C 1 exp E1 / k

B

A! C 2 exp E2 / k

(14)

(15)

A m b ie t B B /A

Dry O 2 C 1 = 7.72 x 102 2 h r -1

E 1 = 1.23 eC 2 = 6.23 x 10

6 h r -1

E 2 = 2.0 e

Wet O 2 C 1 = 2.14 x 102 2 h r -1

E 1 = 0.71 eC 2 = 8.95 x 10

7 h r -1

E 2 = 2.05 e

H 2O C 1 = 3.86 x 102 2 h r -1

E 1 = 0.78 eC 2 = 1.63 x 10

8 h r -1

E 2 = 2.05 e

Plots of B, B/ A using thevalues in the above Table.

Numbers are for (111) silicon,for (100) divide C 2 by 1.68.

.

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

m2

hr- 1

B/A m hr

- 1

1000/ (Kelvi )

800900100011001200

( C )

B /A H2O

B /A Dry O 2B Dry O2

B H 2O

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10

Oxide hickess - micros

ime - hou rs

1200 C

1100 C

1000 C

900 C800 C

0

0.5

1

1.5

2

0 1 2 3 4 5 6 7 8 9 10

Oxide hickess - micros

ime - hou rs

1100 C

700 C

1000 C

900 C

800 C

Calculated (100) silicon dry O 2oxidation rates using Deal Grove.

Calculated (100) silicon H 2Ooxidation rates using Deal Grove.

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B. T hin Oxide Growth KineticsA major problem with the Deal Grove model was recognized when it was firstproposed - it does not correctly model thin O 2 growth kinetics. Experimentally O

2oxides grow much faster for 20 nm than Deal Grove predicts

M A NY models have been suggested in the literature.1. Reisman et. al. Model

x O ! a t t i b or xO ! a t x ia

1b

b

(16)

Power law fits the data for all oxide thicknesses. a and b are experimentally extracted parameters. Physically - interface reaction controlled, volume expansion and viscous flow

of SiO 2 control growth.2. Han and Helms Model

dx Odt

B12x O A 1

B22x O A 2

(17)

Second parallel reaction added - fits the data for all oxide thicknesses.T hree parameters (one of the A values is 0).

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3. Massoud et. al. Model

d x Od t

!B

2x O AC exp

x OL

(18)

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, @ used in process simulators. Data agrees with the Reisman, Han and Massoud models. (800C dry O 2 model

comparison below.)

.

0

0.005

0.01

0.015

0.02

0.025

0.03

0 2 4 6 8 10

Oxide Thickness - microns

an &

elms Model

Reisman et. al. Model

Time - hours

Deal Grove Model ( X 0)

Deal Grove Model ( X 8 hrs)

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1

1

1.3

11

1

1.2

1

1

1

1.3

1.3

Si substrate Si substrate

Si 2

e site si i

Si Substrate

ri i a Si Sur a eu e

a si

a t i S i 3 a s

Oxidation involves a volumeexpansion

( 2.2X). Especially in 2D and 3Dstructures, stresseffects play a dominant role.

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C. 2D SiO 2 Growth Kinetics


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Etched Si Ring

Si Substrate

Side Views Top Views

Polysilicon

SiO 2

Si

a)

b)

c)

d)

T hese effects were investigated indetail experimentally by Kao et. al.about 15 years ago.

Typical experimental results below.

(Kao et.al)4/11/2011 17


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.

0 1 2 3 4 5 6 7 8 1/r m -1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Normalized Oxide Thickness

Convex Radii

Concave Radii

1200 C

1100 C

1000 C

900 C

800 C

1100 C

1000 C900 C

1 m 0.2 m 0.125 m

Several physical mechanisms seem to beimportant: Crystal orientation 2D oxidant diffusion Stress due to volume expansion

To model the stress effects, Kao et. al.suggested modifying the Deal Groveparameters.

k S (stress ) ! k S exp Wn VR k

exp Wt V T

k T

D (stress ) ! D expP VD

k T

C *(stress ) ! C * exp P VS k T

(19)

(21)

(20)

where and are the normal andtangential stresses at the interface.VR, VT and V S are reaction volumes andare fitting parameters.

(Kao et.al)

Wn

Wt

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In addition, the flow properties of the SiO 2 need to be described by a stressdependent viscosity

L(stress ) ! L(T) WS VC / 2k Tsinh W

SV

C/ 2k T

(22)

Where is the shear stress in the oxide and V C is again a fitting parameter.

WS

arameter alue0.0125 m 3 0.0065 m 3

S, 00.3 m 3 @ 850

0.72 m 3 @ 1050 ( ) - Si 2 3.13 10 10 e (2.19 e /k ) oise( ) - Si 3N4 4.77 10 10 e (1.12 e /k ) oise

T hese models have been implemented in modern process simulators and allowthem to predict shapes and stress levels for VLSI structures (above right).

AT HEN A simulation: Left - no stress dependent parameters, Right - includingstress dependence.

.

Silico n

SiO 2Si 3 N 4

0

0.2

0.4

0.6

0.8

-0.2

-0.4

icro ns

icrons

0 0.4 0.8-0.4-0.8 icro ns

0 0.4 0.8-0.4-0.8

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D. Point Defect Based ModelsT he oxidation models we have considered to this point are macroscopic models(diffusion coefficients, chemical reactions etc.).

*

*

O 2

H 2O

D iff u sion

Ox id e Silico n

II

V

1 2K Si Si 2O I 2FV m SiO 2 2KI stress

T here is also an atomistic picture of oxidation that has emerged in recentyears.

Most of these ideas are driven by thevolume expansion occurring duringoxidation and the need for free volume.

In Chapter 3 we described internal oxidation in the following way:(23)

Surface oxidation can be thought of in the same way.

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T he connection between oxidation and other processes can then be modeled asshown below.

B u rie d Dopan t arker Layer

G R

B u lk R ecom b inatio n

Su rfaceR ecom b inatio n

*

OEDInertDiff u sion

O2

I

.

Inert Diff u sion

OED

Microns

0

0.5

1.0

1.5

M icro ns0 1 2-1-2

Si 3 N 4SiO 2

Example - AT HEN A simulation of OED.

Oxidation injects interstitials to create free volume for the oxidation process. Oxidation can also consume vacancies for the same reason.T hese processes increase I concentrations and decrease V concentrations innearby silicon regions.

A ny process (diffusion etc) which occurs via I and V will be affected.

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E. Complete Process Simulation of Oxidation

Many of these models (and others in Chapter 6), have been implemented inprograms like SUPREM.

.

0

0.4

0.8

Microns

0

0.4

0.8

Microns

Microns1-1 0

Simulation of an advancedisolation structure (the SW A MIprocess originally developed byHewlett-Packard), usingSSUPREM IV. T he structureprior to oxidation is on the topleft. A 450 min H 2O oxidation at1000 C is then performed whichresults in the structure on thetop right. A n experimental

structure fabricated with asimilar process flow is shownon the bottom right. T he stresslevels in the growing SiO 2 areshown at the end of theoxidation on the bottom left.

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Summary of Key Ideas

T hermal oxidation has been a key element of silicon technology since itsinception.

T hermally, chemically, mechanically and electrically stable SiO 2 layers on silicondistinguish silicon from other possible semiconductors.

T he basic growth kinetics of SiO 2 on silicon are controlled by oxidant diffusionand Si/SiO 2 interface chemical reaction.

T his simple Deal-Grove model has been extended to include 2D effects, highdopant concentrations, mixed ambients and thin oxides.

Process simulators today include all the physical effects (and more) and arequite powerful in predicting oxidation geometry and properties.

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