1

1

Ch5 Oxidation and Diffusion()

Introduction to Semiconductor Processing

2

Overall View

2

3

Ch5 -1 Oxidation

Si Si

SiO2

4

Horizontal Furnace

0.5 C1000 C 0.05%!

3

5

Vertical Furnace

Dummy wafers 1 Test wafer Production wafers 1 Test wafer Production wafers 1 Test wafer Dummy wafers

Why vertical furnace: Smaller footprint Better contamination control Easier wafer handling

6

Furnace Material Considerations

High purity quartzPros

stable at high TCleanliness

ConsFragileContain metallic ionsNot a sodium barrierSmall flakes at T>1200 C (glassy substrates crystalline solids)

SiCPros

Better thermal stabilityBetter barrier

ConsHeavyExpensive

Only for critical parts, but not the chamber body.

4

7

Oxygen Sources

(Si+ O2 SiO2) (Si+ 2H2O SiO2 + 2H2

Flash systems, H2 + O2 H2O

HCl (TCE, ), (TCA)

8

Si

():

5

9Si

SiO2 SiO2

(field oxide;)

():

10

Special Topic: Isolation Techniques

Goal:Wafer

Device 1 Device 1ElectricalInsulation

Field Oxide(Early 80s) Si Si

SiO2Wet

oxidation Etching

SiSiO2 SiO2

Thickness:

Problems:1.2.

6

11

Isolation Techniques (LOCOS)

LOCOS (Local Oxidation of Silicon), ~ mid 90s

12

Isolation Techniques (STI)

STI (Shallow Trench Isolation), mid 90s~

7

13

RCA ()

Kern Puotinen1960 RCAICSC-1-- NH4OH:H2O2:H2O1:1:51:2:7 7080 C SC-2-- HCl:H2O2:H2O1:1:61:2:87080 C

HFHF

14

Cleaning

Cleaning before oxidation, removeParticulates, organic and inorganic residues, and native oxide

Sequence of not well-cleaning:____________, ____________

DI water: ultra pure water (UPW), ultra high purity, high resistivity > 18M-cm Total organic content < 5g/LOxygen content

8

15

Silicon Oxidation Model

CG

CS

CO

CiXOX

Oxide Silicon

CG: concentration of O2 in the gas phase, =PG/RT

CS: Concentration of O2 at the gas-solid interface

C0: O2 solubility in SiO2(S); C0=H*CS; H: Henrys Constant

Ci: O2 concentration at SiO2-Si interface

r1=hG(CG-CS),

if define CA=H*CGr1=(hG/H)(CA-C0)=h(CA-C0), where h=hG/H

r2=D*(C0-CX)/XOXr3=ks*CiAt steady state, r1=r2=r3

DXkhkCC

OXSS

Ai //1 ++=

16

Silicon Oxide Model Oxide Growth Rate

DXkhkCkCk

dtdXN

OXSS

Asis

OXi //1 ++

==

Define N1= number of oxygen atoms per unit volume of SiO2 (4.6*1022 cm-3)

At t=0, XOX=Xi (initial oxide thickness)

++=

+=

=

+=

+=+

14/

12

/)(

/2

)11(2

)(

2/1

2

21

2

BAtAX

BAXX

NDCBhk

DAwhere

tBAXX

OX

ii

A

s

OXOX

9

17

Oxidation Model Limiting cases

++= 1

4/1

2

2/1

2 BAtAXOX

Case 1: For short time, t+

Linear growth regionLinear growth rate constant:_______

Parabolic growth regionParabolic growth rate constant:_______

18

B/A: Linear Rate Constant

Source: Microelectronics: processing and device design, R. A. Colclaser, J. W. & S. 1980

10

19

B: Parabolic Rate Constant

Source: Microelectronics: processing and device design, R. A. Colclaser, J. W. & S. 1980

20

2 4 6 8 10 12 14 16 18 20

0.2

0

0.4

0.6

0.8

1.0

1.2

()

(

)

1200 C

1150 C

1100 C

1050 C

1000 C

950 C

900 C

11

21

Example: Silicon Oxidation Rates

22

2 4 6 8 10 12 14 16 18 20

0.5

0

1.0

1.5

2.0

2.5

3.0

()

(

)

1150 C

1100 C

1050 C

1000 C

950 C

900 C

12

23

Oxidation vs. Diffusivity of O2 and H2O

24

13

25

Si-SiO2

SiO2

Si

Si-SiO2

SiO2

Si

P N

26

Conventional oxidation process is a multi-wafer batch processhard to precisely control the temperature at every points

Rapid Thermal Oxidation, RTOProcess features and applications:_____________________________________________________________________Integrate with HF native oxide removalO2 + HCl

14

27

n1, k1, t1

n2, k2

sp

Film thickness measurements:

28

http://www.ee.byu.edu/cleanroom/color_chart.parts/sio2_chart.jpg

SiO2 Titanium

But, DONOT always trust your eyes!!

Empirical Thickness Estimation

15

29

C-V

Test for:

1:

2:

30

Ch5 -2 Diffusion

N-Silicon

Masking Oxide

N-Siliconp+ p+

Masking Oxide

16

31

(Pre-deposition): B2H6 + 2 O2 B2O3 + 3 H2O2 B2O3 + 3 Si 3 SiO2 + 4 B

Thermal oxidation: remove un-reacted gas, oxide formation that covers the dopant. (Drive-in)

N-Siliconp+ p+

Masking Oxide

Major concern:

________________________________

Materials disperses from high concn. to low concn. SiO2 as the diffusion mask Was widely used for doping

Replaced by ion implantation for better control from mid 70s Still being used in drive-in for ____________.

32

Si

SiO2

Si

SiO2Drive In

Si Substrate

SiO2Cap oxidation

Pre-deposition

Major Steps

Determines:

______________

Determines:

______________

17

33

, xj

34

MFC

MFC

MFC

O2

POC

l 3

MFC

Regulator ():

MFC():

Control valves():

:

:

18

35

thermal budget)

D = D0 exp (EA/kT)

S/D

36

1 m

Thermal Budget

1

10

100

1000

104/T (K)7 8 9 10

10001100 900 800

2 m

0.5 m

0.25 m

Ther

mal

Bud

get (

sec)

T (C)

Source: Chang and Sze, ULSI Technology

19

37

Diffusion Model

Diffusion1-D diffusion problem without bulk flow

*Same governing equation for pre-deposition and drive-in steps

2

2

xND

tN

=

1. Pre-deposition: Constant (Saturated) surface concentration

38

Diffusion Model - Conti.

2. Drive In: Constant (Saturated) surface concentration

20

39

Diffusion: Spatial Profile

40

REF 1

21

4141

REF 2

Source: VLSI

4242

REF 3

22

43

REF 4

44

REF 5

1000

23

45

Special Topic: USJ Formation

Diffusion for Boron USJ formationSmall devices needs ultra shallow junctionBoron is small and light, implanter energy could be too high for it goes too deepControlled thermal diffusion is used in R&D for shallow junction formation

46

Surface Clean

Si SubstrateSTI STI

Silicide

Sidewall Spacer Sidewall Spacer

24

47

BSG CVD

Si SubstrateSTI STI

Boro-Silicate Glass

Silicide

Sidewall Spacer Sidewall Spacer

48

RTP Dopant Drive-in

Si SubstrateSTI

Boro-Silicate Glass

STI

SilicidePolysilicon

Gate Oxide

25

49

Strip BSG

Si SubstrateSTISTI

SilicidePolysiliconGate Oxide

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org) /PDFXTrapped /Unknown

/Description >>> setdistillerparams> setpagedevice