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National Institute for Materials Science,Tsukuba, Japan

- A method for evaluating layer thickness of thin films and multilayers 1. Introduction Why Layer Thickness? How to Measure? 2. How XR Experiments are Performed 3. Interpretation of XR Data 4. Examples of XR Analysis 5. Some Remarks in Data Analysis 6. Summary

What's XR?

X-Ray Reflectometry

Kenji SAKURAI

1

Because function of devices depends on the thickness

IC (Semiconductors) solar cell (photovoltaic materials)

Why Layer Thickness?

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Display (liquid crystals) Data storage (magnetic multilayer)

?

LED (Electroluminescence)

Biomaterial (protein, lipid,etc)

coating & plating (Optical materials, etc.)

2

Because function of devices depends on the thickness

1pm

Stylus AFM Multi-beam interferometry Confocal laser microscopy SEM TEM XR Ellipsometry XRF AES SIMS QCO Resistance Inductance

How to Measure Layer Thickness?

1nm 1µm 1mm

2/34

Distance

thickness

Mass

Properties

3

The method uses optical reflection at surface and interefaces

Refractive index

n = 1 - - i

4.13 6.54

2 e = 2c 2 e = 2c ( Z j +f j' )

j

What's X-Ray Reflectometry (XR) ?

3/34

Critical angle

c = 2

for 8 keV (0.155nm) Si 3.92 SiO2 3.80 Al 4.13 Ti 5.23 Cr 6.54 Fe 6.72 Ni 7.01 Cu 7.02 Ag 7.70 W 9.61 Au 9.70 Pt 10.5 1 mrad = 0.06 deg

(f j" )

j

c (Al) c (Al) c (Cr) c (Cr)

X-Ray(8keV) air f period of oscillation i Al (300Å ) film 1 layer thickness Cr (700Å ) film 2 Si: substrate

critical angle density decay roughness, diffusion

"X R Spectrometry: Recent Technological Advances" (John ay K. Sakurai

W 2004) Chapter 5.1 iley,

4

What's X-Ray Reflectometry (XR) ?

1923 Total reflection of X rays 1931 Interference fringes 1954 Multilayer model

H. Keissig, Ann. Phys. 10,769 (1931)

It has a long history

4/34

A.H.Compton, Phil. Mag. 45, 1121 (1923)

1988 Theory of diffuse scattering

S. K. Sinha, Phys.Rev. B38, 2297 (1988)

1989 Grazing incidence small angle scattering

J. R. Levine, J. B. Cohen, J. Appl. Cryst. 22, 528 (1989) A.Naudon, J. Appl. Cryst. 24, 501 (1991)

(GISAXS)

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

Manual frequency analysis

N. Wainfan, L.G Parratt J. Appl. Phys. 30, 1604 (1959)

1992 Fourier analysis

1963 Yoneda wing (diffuse scattering)

A. Segmuller, Thin Solid Films 18, 287 (1973)

K.Sakurai, A.Iida, Jpn. J.Appl.Phys. 31, L113 (1992)

Y.Yoneda, Phys. Rev. 131, 2010 (1963)

rd 1994 3 generation SR

1973 Application to artificial nano structures

Whole pattern fitting

1998 Use of anomalous dispersion 2000 Wavelet transform

ESRF (Grenoble, 1994) APS (Argonne, 1996) SPring-8 (Harima, 1997)

A. Segmuller, AIP Conf Proc.53, 78 (1979)

T. Hirano, K.Usami, Synchrotron Rad. 5, 969 (1998). E. Simigel, A. Crnet, J. Phys. D 33, 1757 (2000) I.R.Prudnikov, R.J.Matyl, R.D.Deslattes J. Appl. Phys. 90, 3338 (2001)

1980 Evaluation of roughness

L. Nevot, P. Croce, Rev Phys Appl. 11, 113 (1976) Rev Phys Appl. 15, 761 (1980) Rev Phys Appl. 23, 1675 (1988)

5

How XR Experiments are Performed ? /2 scan at very small angle region

cto

5/34

Measurement of reflected intensity during /2 scan Monochromatic X-rays

i f

r

de te

Typical conditions

Source - ay X r energy Beam size Photon number Range /2 scan range Cu tube 8.04 keV (CuK 1) 0.04mm x 2.5mm 0.6 1.2 x 107 counts/sec 7.5 decades 0 ~ 50 mrad (0~3°) 0.05 ~ 0.1 mrad 0.5 ~ 10 sec /point 15 ~20 min

Sample

i= f

2 (cos i - cos f ) 2 qz = (sin i + sin f ) qx =

Angular step Counting time Total measuring time

6

Typical X-ray reflectometer based 2-axes goniometer

How XR Experiments are Performed ?

6/34

ANSTO Bragg Institute, Australia

http://www.ansto.gov.au/ansto/bragg/xray/xrayreflect.html

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Reflectomter at NIMS, Tsukuba, JAPAN (1995)

How XR Experiments are Performed ?

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YAP:Ce detector Goniometer Sample Monochromator

Si(Li)detector X-Ray source(40kV-80mA)

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Combination of angular and energy dispersive system

How XR Experiments are Performed ?

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"Novel Versatile X Ray Reflectometer for Angle and Energy Dsipersive Characterization of Liquid and Solid surfaces and Interfaces",

T. H. Metzger, C. Luidl, U. Pietsch and U. Vierl, NIM A350, 398 (1994).

9

How XR Experiments are Performed ?

Combination with ellipsometry

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"A New Versatile Instrument for Characterization of Thin Films and Multilayers Using Spectroscopic Ellipsometry and Grazing X R Reflectance", ay

P. Boher and J. L. Stehle; , Phys.Stat.Sol.(a)., 170, 211 (1998).

10

How XR Experiments are Performed ?

Towards in-situ measurements

4 qz = 2

10/34

(sin i + sin f ) =

sin i

Ge Detector

Mirror SR Curved Monochro.

White X-rays Sample

Y.Nakano, T.Fukamachi, K.Hayakawa;JJAP17,329331,Suppl.2,1978.

Sample

Fast CCD Knife edge Monochro.

A.Naudon, J. Chihab, P.Goudeau, J.Mimault, J. Appl.Cryst.,22, 460, 1989.

Line forcus

Sample PSPC

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Interpretation of XR Data

Manual frequency analysis

11/34

im2-c2=m2 2t

( )

2

BN Si

"High Resolution X- R Scattering from ThinFilms and Multilayers", Springer, 1999 ay

V.Holy, U. Pietsch, T. Baumbach;

12

Whole pattern fitting using Parratt's layered model

1

1 1 2 2 j-1 j-1 jj j+1 j+1 Substrate air

Interpretation of XR Data

R = R 1, 2

R

j-1, j

12/34

2

X-Ray Reflectivity

= a

4 j-1, j

R R

j, j+ 1 j, j+ 1

+ F j-1, j

F j-1 , j + 1

10

-2

10

-4

Rj-1,j : Reflection coefficient at j-1,j interface aj-1,j : The amplitude of the electric vector at the center of the j th layer Fj-1,j : Fresnel coefficient at j-1,j interface

C/Si calculation model

"Surface Studies of solids by Total Reflection of X R ays"

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

10

Layer No., -6 material 1 Carbon 2 C-50%Si 3 Si

Thickness Roughness Density [nm] [nm] [g/cm3] 50.0 1.8 1.70 3.0 1.2 1.73 0.5 2.34

5

10

15

20

25

30

Glancing Angle [mrad]

13

Interpretation of XR Data

Fourier analysis

3 layered model 1 air 2 3

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film substrate

d "Fourier analysis of interference

structure in X ray specular reflection from thin films"

K.Sakurai, A.Iida, Jpn. J.Appl.Phys. 31, L113 (1992).

R = R 1, 2 = =

2 2

when R << 1

R=

2

R 2,3 + F1, 2 R 2,3 F1, 2 + 1

F122 + F22,3 - 2F1, 2 F2,3 cos , 1 - F122 - F22,3 + F122 F22,3 , ,

q' = 4

2 2 - C

exp(-i )F2,3 + F1, 2 exp(-i )F2,3 F1, 2 + 1

= 4d ( 2 - c 2 / )

R A j cos 4d j

j

2 - c 2 j

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Interpretation of XR Data

Fourier analysis

log R

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R vs.

c determined

For each layer

R vs.

2 - c 2 /

2 - c 2

Normalization

|FT|

Background removal Fourier transform

d

15

Wavelet tranform to extend Fourier analysis

Wavelet transform

W ( a , b) =

C ( z) =

Interpretation of XR Data

3.5x10-4 3.0x10-4 2.5x10-4 2.0x10-4 1.5x10-4 1.0x10-4 5.0x10-5 0.0 0 100

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1

K 4 | RN |2 exp(iKz )dK

W(b), arb.units

z -b C ( z) g dz a a -

+

Extract roughness for specific interfaces

a: scaling parameter b: position parameter W(a,b)

a =10Å

100 layer 1

z2 2 g ( z ) = 1 - z exp - 2 Mexican Hat mother wavelet

(

-

)

250 layer2

350 layers 1+2

100 10-1 10-2

Specular X r reflectivity - ay

C, 250Å, =8Å Cr, 100Å, =5Å Si, =3Å

200

300

400

500

b, Å

X-ray reflectivity

R1

W(a), arb.units

1=3Å, 2=5Å, 3=8Å

1.5x10-4

10-3 10-4 10-5 10-6 10-7

R

1.0x10-4

b =100Å b =250Å b =350Å

5.0x10-5

0

2

4

6

8

"Determination of interface roughness of Gd films deposited on Si surface using a, Å improved wavelet transform of X ray reflectivity data", O.Starykov and K.Sakurai; Appl. Surf. Sci. 244, 235 (2005)

2 , deg

0.0 0

Fitting of W(a,b=zji)

10 20

16

Interpretation of XR Data

Other model-free analysis

kinematical approximation

16/34

phase recovery

R(q ) = RF (q )

d ( z ) exp(iqz )dz dz -

2

R

q

"Model independent inversion of z ray or neutron reflectivity data", x

E. Bengu,M. Salud, L. D. Marks; Phys. Rev.B, 63, 195414 (2001).

17

Use of anomolous dispersion effects to enhance prescision

Interpretation of XR Data

17/34

Slide Courtesy of Dr. T. Hirano (Hitachi)

0

Anal yzed t ckness ( ) hi nm

Experimental Calculated

1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0

Ta

( 3)

l ( efectvt og r l iiy)

-2 -4 -6 -8

20) Cr nPt ( M 1. u-CoFe1 ( 5) Ru ( =0 0. t 8) 2. u-CoFe2 ( 0) Cu ( 3) 2. 1) d-CoFe ( Ni Fe ( 5) Ta ( 5) Sisub. ()i nm :n

Co-K Cu-K Cu-K

- 10 0.1 0.2 0.3

0.4

0.5

GM R

"Layered structure analysis of GMR multilayers by x ray reflectometry using the anomalous dispersion effect", T. Hirano, K. Usami, K. Ueda, and H. Hoshiya, J Synchrotron Rad 5, 969-971 (1998).

Hitachi Research Laboratory (HRL)

18

Interpretation of XR Data

Software for data analysis

18/34

19

Interpretation of XR Data

Software for data analysis

19/34

20

Examples of XR Analysis

1. GaAs/AlAs superlattice

20/34

GaAs 7.6nm

AlAs 5.12nm

2.8Å (2ML) change can be easily detected.

(designed as 5nm)

(designed as 7.5nm)

(6 periods)

GaAs (100)

"Smooth and coherent layers of GaAs and AlAs grown by molecular beam epitaxy",

L. L. Chang, A. Segmuller, L. Esaki; , Appl. Phys. Lett., 28, 39 (1976).

21

2. Pt/Co multilayer for magnetic device

q Magnetic multilayers Substrate Si 15× Layer PtO Pt Co Pt Ag Co GaAs to(Å) 0 18 2 18 200 10 t(Å) 3.8 18.9 2.1 19.8 215.0 14.8

N

Examples of XR Analysis

21/34

(Å) 2.2 6.7 4.1 3.4 4.9 2.4 22.0 R(%) 3.09

0.24 0.88 1.32 0.88 0.78 0.68 1.0

"Characterization of single and multiple layer films by X ray reflectometery", T. C. Huang and W. Parrish, Adv. in X-ray Anal., 35A, 137 (1992).

22

3. Fe/U multilayer for 5f-3d novel magnetic materials

Exp.

XR PNR thickness [Å] U Fe roughness [Å] U Fe 17.0 15.0

Examples of XR Analysis

22/34

magnetic moment[µB] U Fe

23.7 31.4 6.8 25.0 29.1 2.5

0.017 0.850

Calc.

oxide U 3nm Fe 4nm low density

30BL

glass substrate

"Magnetism of U/Fe Multilayers", M.F.Thomas et al., J.Alloys and Compounds, 369, 14-17 (2004).

23

Examples of XR Analysis

4. L-B membrane

packing of 11 ester molecules

23/34

H2P1: tricosylamine: methyl eicosanoate=1:4:12

29BL

fused silica substrate total thickness(29BL)=967Å 25° from UV and IR H2P1:mesobilayer dichromism periodicity=66.4Å tetra(trimethylsilylethynyl)-tetra(carboxymethyl)porphyrin measurements "Organization of a New Tetraalkyl Porphyrin by the Langmuir B - lodgett Technique",

F.D.Cruz, F.Armand, P.-A. Albouy, A. Nierlich, A. Ruaudel-Teixier.Langmuir,15,3653-3660 (1999).

tricosylamine methyl eicosanoate

24

5. Zirconia coating for optical applications

ZrO2 thin transparent film prepared by ion beam assisted deposition Particle size 15~40 nm

Examples of XR Analysis

24/34

surface density 5.4±0.25g/cm3 surface thickness 15nm Total ZrO2 thickness 66 nm Si

"Structure of ZrO2 Optical Thin Films Prepared by Dual Ion beam Reactive Sputter Deposition",

J. P. Riviere, S. Harel, P. Guerin, A. Strabori; Surface and Coating Technology, 84, 470-475 (1996).

25

6. Contact angle for polymer thin film

Examples of XR Analysis

25/34

large thickness + small roughness

17F: CH2=CHCOO(CH2)2(CF2)8F

large contact angle Poly(fluorolkyl acrylate)

polymer film

17FM: CH2=C(CH3)COO(CH2)2(CF2)8F 3FM: CH2=C(CH3)COO(CH2)CF2

Poly(fluoroalkyl methacrylate)

Si(100)

dipping method (CF2CLCFCL2 solution 1wt%)

"Surface Structure Analysis of Polyacrylate Thin films"

Heat treatment: 130°C 1min air cooling water cooling

T.Katsurgawa, E. Chiba, K. Okada, K.Tani and H. Tomono; Jpn J. Appl. Phys., 34, 649 (1995).

26

7. Pore characterization of low-k materials

HSQ (Hydrogensilsesquioxane)

Examples of XR Analysis

26/34

Low-k film Substrate Si

MSQ (Methylsilsesquioxane)

"Pore Characterization in Low kDielectric Films Using X ray Reflectivity: X ray Porosimetry", Christopher L. Soles, Hae-Jeong Lee, Eric K. Lin, and Wen-li Wu, NIST Special Publication 960-13 (2004).

27

8. GaAs/MnZn ferrite for magnetic field sensor

Examples of XR Analysis

27/34

1.4nm 39.8nm 27.0nm GaAs MnAs MnZn ferrite 1.1nm 0.2nm

Buffer

"Characterization of Heterointerfaced in GaAs/MnAs/MnZn Ferrite",

S Ito, H.Fujioka, M. Oshima; J.Cryst.Growth., 260, 384-387 (2004).

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Examples of XR Analysis

9. Liquid Sn surface

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modified model for anomalous intensity Flat & abrupt surface peak by surface layering

roughened by capillary wave

simple capillary wave

O. G. shpyrko, A. Yu, C. Steimer, P. S. Pershan, B. Lin, M. Meron, T.Graber, J. Gerbhardt, B. Ocko, M. deutsch; Phys.Rev., B70, 224206 (2004).

"Anomalous Layering at the Liquid Sn Surface",

29

Comparison with Other Analytical Methods

Purpose:Thickness of AlOx using 2-wave XRR

0 -2 log(reflectivity) -4 -6 -8 measurement calculation

Determination of AlOx layer thickness in Hitachi TMR

OL TaOx Ta NiFe CoFe AlOx CoFe Ru CoFe MnPt NiFe RL Ta IF Si

(x10 )

-4

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Ta (5) NiFe (4) CoFe (0.5) AlOx (1) CoFe (2.5) Ru (0.8) CoFe (2) MnPt (15)

(nm)

tD

(nm)

tA

(nm)

2.13 14.82 26.02 5.96 (15.36) (22.41) (15.36) 22.37 (15.80) 20.70 26.02 11.13 (5.02)

5.0

1.11 3.04 4.31 4.05 1.07 2.37 1.07 2.01 15.04 2.41 0.84 3.17 (1.00) -

0.30 0.44 0.34 0.93 0.42 0.42 0.38 0.59 0.46 0.30 0.22 0.31 0.19 (1.50)

Cu-K1

(15.80) 4.5 2.5 0.8 2.0 15.0 3.0 3.0

NiFe (3) Ta (3) Si sub.

Co-K

-10 0.0 0.2 0.4 0.6 q (4 sin / ) 0.8

TMR

We can determine the AlOx thickness of 1.07 nm

Slide Courtesy of Dr. T. Hirano (Hitachi)

Hitachi Research Laboratory (HRL)

30

Comparison with Other Analytical Methods

-Analysis results of the thickness of AlOx by HRTEM, EELS, EDS and XRRTaOx Ta 5 NiFe 4

Determination of AlOx layer thickness in Hitachi TMR

30/34

Method XRR HRTEM

thickness (nm) 1.07 ± 0.05 1.08 ± 0.08 1.12 ± 0.03 1.10 ± 0.03 1.4 ± 0.1 1.4 ± 0.2

CoFe 0.5 Al 0.5 oxidation CoFe 2.5 Ru 0.8 CoFe 2

MnPt 15 NiFe 3 Ta 3 Si wafer :in nm

Z-contrast STEM Zero-loss image

20 nm

2D-EELS EDS line profile

Slide Courtesy of Dr. T. Hirano (Hitachi)

Hitachi Research Laboratory (HRL)

31

Comparison with Other Analytical Methods Conclusion

Determination of AlOx layer thickness in Hitachi TMR

31/34

AlOx thickness of ~1 nm analyzed by HRTEM, Z-contrast STEM, zero-loss image was in good agreement with that by X-ray Reflectometry. EELS-TEM and EDS-STEM gave a larger AlOx thickness mainly due to chromatic aberration for EELS, and both probe size and multiple scattering for EDS. X-ray reflectometry method is very useful for thickness calibration.

Slide Courtesy of Dr. T. Hirano (Hitachi)

Hitachi Research Laboratory (HRL)

32

Density grading at surface and interfaces

simple grading

Some Remarks in Data Analysis

32/34

Si

high-density layer with large interface roughness

"X-Ray Reflectivity for Graded Surfaces: Calculations",

M.Mizusawa and K. Sakurai, Adv. X-Ray Chem Anal. Jpn, 33, 175 (2002). (in Japanese) .

33

Non-uniform layer and/or particles on substrate

Total reflection Total reflection region region Critical angle Critical angle for Cu for Cu ( 7.02 mrad ) ( 7.02 mrad )

Some Remarks in Data Analysis

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C (3.3nm)

10

6

Intensity (counts)

10 10 10

5

4

A B C D E F G

Critical angle Critical angle for Si for Si ( 3.92 mrad ) ( 3.92 mrad )

1.4 nm 2.4 nm 3.3 nm 4.5 nm 5.3 nm 20 nm Si wafer

E (5.3nm)

3

A (1.4nm)

F (20nm)

10

2

5

/ 2 ( mrad )

10

15

34

Feasible for determination of layer thickness

XR gives info on layer thickness, density and roughness Advantages of XR:

interference fringes

Summary

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1. Non-destuctive 2. Good statistics average info over cm2 (irradiation area) 3. High resolution ~0.01 nm 4. Free from limitations in measurement environment 5. Easy to apply any type of samples Crystals/Non-Crystals Organic/Inorganic Constructing proper models for the analysis is crucial. The parameters obtained by fitting should be carefully examined.

Future research with advanced X-ray sources

- Small area analysis (10~100 nm) - Quick measurements (1~100 msec) - Further info on surface and interfaces (use of coherence etc.)

35

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