X-ray Reflectivity Study on the Density and the Roughness of

Silicon Oxide Thin Films under Various Fabrication Conditions Kazumasa KAWASE, Advanced Technology R&D Center, Mitsubishi Electric Co.

Kawase.Kazumasa@wrc.melco.co.jp

CVD-SiO2 films are technologically very important to

semiconductor devices; however, one problem is that leak current of the

films is higher than that of thermal oxidized SiO2 films. We have

found that radical oxidation of CVD-SiO2 films reduce leak current of

the films. In this study, we estimated the surface roughness and

density of radical oxidized CVD-SiO2 films using X-ray reflectivity

study and investigated the relationship with the electric properties.

SiO2 films of about 3 nm thick were grown on Si (100) wafers with

CVD method. Some samples were then radical oxidized in Ar/O2

plasma. The X-ray reflectivity measurements of the films were

performed at BL16B2 using monochromatized X-rays of 0.124 nm. Leak current of the radical oxidized CVD-SiO2 film was lower than

that of the as-deposited sample. Figure 1 shows x-ray reflection

curves of the two samples. The oscillation amplitude of the radical

oxidized sample was smaller than that of the as-deposited sample.

The oscillation amplitude is related to the difference of density between the film and the substrate, and the results above indicate

that the density of CVD-SiO2 film is raised by radical oxidation. It is considered that the leak current would be reduced by the

increase of the film density.

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0 1 2 3 4

Reflectivity

2θ(deg)

with radical oxidation

without radical oxidation

Fig. 1. X-ray reflection curves of CVD-SiO2films with and without radical oxidation.

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0 1 2 3 4

Reflectivity

2θ(deg)

with radical oxidation

without radical oxidation

Fig. 1. X-ray reflection curves of CVD-SiO2films with and without radical oxidation.

X-ray reflectivity study on the density and the roughness of silicon oxide thin films under various fabrication conditions

Advanced Technology R&D Center, Mitsubishi Electric Co.

Kazumasa Kawase

1. Introduction2. Theory3. Experiments4. Results5. Discussion6. Conclusion

1. Introduction

Merits of CVD silicon oxide・low temperature process・fabrication on any layer・avoiding effect of substrate defect

Advantages of O2 radical treatment・high oxidizability・low temperature proces ・affecting only surface

(no oxidation of under layer by small diffusion constant)

X-ray reflectivity mesurements were performed to the CVD silicon oxide film treated by O2 radical, and density and roughness were investigated.

Demerits of CVD silicon oxide・high leak current ← The density would be low.

Conventional methods to estimate density ・XPS ： variance of bonding angle・TDS ： H2O desorption (adsorption)

← indirect and non quantitative

I / I0 = R (ρSiO2, ρSi, σsurface, σinterface, t SiO2, λ, θ)

I0 I

ρSiO2

ρSi

σsurface

σinterface

λ

t SiO2

θ

2. Theory

surface roughnessdensity

These parameter is able to be obtained by curve fitting.

（L. G. Parratt, Phys. Rev. 95, 359(1954).）

ρSiO2 ： density of SiO2 filmρSi ： density of Si substrateσsurface ： roughness of SiO2 surfaceσinterface ： roughness of SiO2/Si interfacet SiO2 ： thickness of SiO2 filmλ ： wavelength of X-rayθ ： incident angle

Case of a silicon oxide layer on a silicon substrate.

Si substrate

SiO2 film

X-ray reflection curve is described as

interface roughness

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0 1 2 3 4 5 6 7

σsurface =0.29σinterface=0.10σsurface =0.29σinterface=0.29

2θ(deg)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0 1 2 3 4 5 6 7

σsurface =0.29σinterface=0.10

σsurface =0.10σinterface=0.10

2θ(deg)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0 1 2 3 4 5 6 7

ρ(SiO2)=2.33

ρ(SiO2)=2.12

refl

ecti

vity

2θ(deg)

Fig. 1. The shape variation of X-ray reflection curves versus density (density difference between film and substrate), surface roughness and interface roughness.

low density

high density

calculation

small surface roughness

large surface roughness

calculation

small interface roughness

large interface roughness

calculation

Density is high → Oscillation amplitude is smallSurface roughness is small → Decay is smallInterface roughness is small → Spectrum shape is not change

Density Surface roughness interface roughness

3. Experiments

・I-V （leak currents）

・X-ray reflectivity （denisty, surface roughness）Beam line ： SPring-8 BL16B2Energy ： 10 keV, Al filter

(1) Sample perparation

(2) Meausrements

○ HF etching○ CVD silicon oxide ○ O2 radical treatment

reference： pyrogenic, without O2 radical treatment

・XPS （chemical bonding state, Si 2p, O 1s → density ）

・AFM（surface roughness）・TDS （H2O desorption, m/e=18 (H2O) → density ）

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0 1 2 3 4

The oscillation amplitude decreased with O2 radical treatment. Fig. 2. X-ray reflection curves of various silicon oxide films.

2θ(deg)

Ref

lect

ivity

X-ray reflection spectra4. Results

CVDCVD with O2 radicalpyrogenic

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

-16-14-12-10-8-6-4-20

Eox(MV/cm)

Jg(A

/cm

2 )

Fig. 3. I-V curves of silicon oxide gate insulator.

The leak current decreased with O2 radical treatment.

Leak currents

n+ poly-Si gate/p-Si(100)6 nm, 0.01 mm2

MOS Cap.

Si substrate

Isolation

Gate poly-SiGate negative bias

SiO2

CVDCVD with O2 radicalpyrogenic

CV

D

CV

D w

ithO

2ra

dica

l

pyro

geni

c

1.8

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

dens

ity o

f SiO

2fi

lm (g

/cm

3 )

Fig. 4. The oxide film density obtained by X-ray reflection.

The silicon oxide film density increased with O2 radical treatment.

Density

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

▲ O 1s● Si 2p 3/2

4+

FWH

M (e

V)

Fig. 5(a) FWHM of XPS peaks.

The variance of bonding angle decreased. → The density would increase.

variance of bonding angle

Fig. 5(b) XPS spectra.

9798991001011021031041051061070

2000

4000

6000

8000

10000

12000030523p030.spe

Binding Energy (eV)

c/s

5285295305315325335345355365375380

1

2

3

4

5

6x 10

4 030523p030.spe

Binding Energy (eV)

c/s

O 1s

Si 2p 3/24+

CV

D

CV

D w

ithO

2ra

dica

l

pyro

geni

c

0

5 10-10

1 10-9

1.5 10-9

2 10-9H

2O d

esor

ptio

n (a

rb. u

nits

)

Fig. 6(a) The amounts of H2O desorption with TDS.

The amounts of H2O desorption decreased. → The density would increase.

desorption of H2O gas

CV

D

CV

D w

ithO

2ra

dica

l

pyro

geni

c

0

1 10-11

2 10-11

3 10-11

4 10-11

5 10-11

0 200 400 600 800

Temperature (℃)

inte

nsity

(arb

. uni

ts) m/e=18 (H2O)

Fig. 6(b) TDS spectra.

0

0.1

0.2

0.3

0.4

0.5R

ough

ness

of S

iO2

surf

ace,

RM

S (n

m)

Fig. 7. The surface roughness of oxide film obtained by X-ray reflection.

The surface roughness was not change with O2 radical treatment.

surface roughness with XRR

CV

D

CV

D w

ithO

2ra

dica

l

pyro

geni

c0

0.1

0.2

0.3

0.4

0.5

Rou

ghne

ss o

f SiO

2su

rfac

e by

AFM

, RM

S (n

m)

Fig. 8. The surface roughness of oxide film obtained by AFM.

surface roughness with AFM

CV

D

CV

D w

ithO

2ra

dica

l

pyro

geni

c

5. discussion

Si Si

Si

O

Si

SiSi

OO

O

O

O

Si

Si

O

O

Si

Si

OH

Si

O

Si

Si

OO

O

O

Si

Si

O

O

Si

Si

The Si-O network would be compact and defects would be erased by O2 radical treatments. Therefore leak current would be reduced.

H2O

H

CVD CVD with O2 radical

O2 radical treatment cause CVD silicon oxide film that

density increases.variance of bonding angle decreases.amounts of H2O desorption decreases.leak current decreases.

6. conclusion

X-ray reflectivity study is very powerful tool to estimate film density and surface roughness of silicon oxide thin films.

○

O2 radical treatment is very effective method to improve the insulation characteristics of CVD silicon oxide films.

○