Koster G., Rijnders G. (Eds.) In situ Characterization of Thin Film Growth

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92 In situ characterization of thin ﬁlm growth

hydroxylation at a relative humidity of ~0.01%, accompanied by a reduction in

oxide coverage. Water adsorption increases notably at this relative humidity,

too. A detailed analysis of the data leads to a model for the reaction of water

vapor with MgO(100) under ambient relative humidity, depicted in Fig. 4.15.

At a critical relative humidity (~0.01%), water molecules dissociate on the

MgO(100) surface, leading to the hydroxylation of the oxygen atoms in

the topmost MgO layer, and to the addition of a monolayer of OH groups

to the surface. The validity of this reaction mechanism is borne out in the

reduction of the total MgO(100) lm thickness and the increase in the total

O 1s surface signal. This underscores the utility of the application of thin

lms for the study of surface reactions, as depicted in Fig. 4.11.

4.4 In situ measurements of buried interfaces using

high kinetic energy XPS (HAXPES)

In the nal part of this chapter we would like to illuminate another promising

XPS technique for the in situ study of thin lm properties, namely high

1.0

0.5

0.0

1.5

1.0

0.5

0.0

MLML

nmnm

0.3

0.2

0.1

0.6

0.4

0.2

1.0

0.8

0.6

0.4

0.2

0.0

–4

–3

–2

–1

1 10

RH (%)

4.14 Integrated O 1s peak intensities from ambient pressure XPS

0.15 torr isobar measurements on 4 ML MgO(100)/Ag(100). Peak

assignations are as in Fig. 4.13. The ﬁlm thicknesses are plotted

in units of monolayers (left axis) and nanometers (right axis). 1

ML of Ox, OH, and water are deﬁned as 0.21, 0.48 and 0.31 nm,

respectively. Reprinted from Newberg et al.

permission from Elsevier.



93X-ray photoelectron spectroscopy (XPS)

kinetic energy XPS. This technique has been pioneered by Lindau et al.

who demonstrated in 1974 the use of high-energy photons to measure the

intrinsic line width of Au 4f core levels.

The attraction of high kinetic

energy XPS lies in the ability to measure buried interfaces at much larger

depths than in conventional soft X-ray XPS (close to 10 nm at 10 keV in the

case of Cu, as opposed to close to 1 nm at 1 keV – see Fig. 4.2). However,

the detrimental aspects of this technique are the low photoemission cross-

sections at high kinetic energies (several orders of magnitude lower than

at low kinetic energies) as well as the need for high voltages on the lens

elements for sufcient deceleration of electrons for high energy resolution

measurements. With the recent availability of high-brightness synchrotron

sources the low cross-sections can be compensated for. Figure 4.16 shows

recent results for buried SiO

layers on a Si substrate, which are covered

by a NiGe overlayer. Using incident photons with an energy of 7935 eV

the authors recorded Si 1s spectra (binding energy ~1840 eV) with electron

kinetic energies of over 6000 eV, which allowed probing at a depth of more

4.15 Illustration of the reaction of a MgO(100)/Ag(100) ﬁlm with water

vapor. (a) Without and (b) with water vapor. In the presence of water

vapor at a relative humidity larger than 0.01%, water molecules

at the surface dissociate to form two hydroxyl ion moieties: those

sitting above the initial MgO interface near Mg

surface sites, and

those that convert the top layer of MgO oxygen ions to OH

–

ions.

Assuming the entire MgO interface is passivated with OH, the net

effect reduces the MgO ﬁlm from 4 to 3 ML. Reprinted from Newberg

et al.

= H

= Mg

= O

= Ag

Film under vapor

Initial dry ﬁlm

1 ML

Water

1 ML

3 ML

Oxide

4 ML

Oxide

(a) (b)



94 In situ characterization of thin ﬁlm growth

than 15 nm.

In another case study the smoothness of the solid/solid interface

of buried Ni/Cu interfaces was probed using high kinetic energy XPS.

Figure 4.17 (upper panel) shows the layout of the sample. Alternate layers

of nickel and copper (5 ML each) were deposited on a MgO(100) substrate

that was covered by intermediate layers of Fe, Pt, Cu and Ni. This structure

was covered by a 1 nm thick Pt cap layer. High kinetic energy (~1200 eV)

XPS spectra with a resolution of ~0.25 eV were taken of the Cu 2p core

level (bottom panel of Fig. 4.17). Upon heating of the sample from room

temperature to 300 °C a marked shift in the Cu 3p

3/2

peak position to lower

binding energy was observed, which was interpreted as a signature for the

smoothing of the Cu/Ni interface.

These results demonstrate that XPS is

not only a valuable tool for the study of the sample surface or near-surface

region, but also for investigation of solid/solid interfaces up to a depth of

several nanometers.

4.5 Conclusions

In summary, X-ray photoelectron spectroscopy is an excellent tool for the

investigation of in situ lm growth, reaction, as well as the study of buried

interfaces. Recent developments of in situ XPS spectrometers as well as

high-brilliance synchrotron facilities have expanded the application of XPS

to higher pressures as well as larger sampling depths. The challenge for the

future lies in the adaption of XPS instruments to realistic environments of

thin lm growth as well as gas/solid interactions.

Hard X-ray

hn = 7935 eV

NiGe

7 nm

12 nm

15 nm

SiO

Si sub.

Photoelectrons

Intensity (arb. units)

1850 1846 1842 1838 1834

Binding energy (eV)

Si 1s

TOA = 80°

Si–O

Si–Si

4.16 (a) Experimental parameters of the sample under investigation.

(b) Si 1s XPS spectra of the sample shown in (a). The incident

photon energy is 7935 eV. Spectral signatures from the SiO

ﬁlm and

the Si substrate can be observed even at a NiGe overlayer thickness

of 15 nm. Reprinted from Kobayashi.

permission from Elsevier.

(a) (b)



95X-ray photoelectron spectroscopy (XPS)

4.17 (a) Left: schematic view of an idealized (top) Ni/Cu interface,

where the interfacial roughness is negligible, as compared to a

realistic interface (bottom). The right panel shows the conﬁguration

of the sample under investigation. (b) Cu 2p

3/2

spectra, taken at

an incident photon energy of 2010 eV. The shift in the core level

binding energy at temperatures above 250 °C indicates a change in

the roughness of the Ni/Cu interface. Reprinted from Gorgoi et al.

4.6 Acknowledgments

The contributions in particular of D. E. Starr, J. T. Newberg, E. R. Mysak,

and K. R. Wilson are gratefully acknowledged. This research, the ALS,

A perfect interface Multilayer structure

A real interface is never perfect

Repeated unit:

x = 2, 4, 5

~20 repetitions

Pt 10 Å

5 ML Ni

x ML Cu

Ni 5 ML

Cu 45 Å

Pt 50 Å

Fe 5 Å

MgO(001) substrate

5 ML Cu

Cu 2p

3/2

Normalized intensity (arb. units)

Bulk Cu

20 °C

150 °C

200 °C

250 °C

300 °C

934 933 932 931

Binding energy (eV)

(b)

(a)

{



96 In situ characterization of thin ﬁlm growth

and the ALS-MES beamline 11.0.2 are supported by the Director, Ofce of

Science, Ofce of Basic Energy Sciences, Division of Chemical Sciences,

Geosciences of the US Department of Energy at the Lawrence Berkeley

National Laboratory under Contract No. DE-AC02-05CH11231.

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11 J.J. Yeh, I. Lindau, Atomic subshell photoionization cross sections and asymmetry

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17 M.A. Kelly, M.L. Shek, P. Pianetta, T.M. Gür, M.R. Beasley, In situ X-ray

photoelectron spectroscopy for thin lm synthesis monitoring, J. Vac. Sci. Technol.

A 19, 2127 (2001).

18 B. Shin, J.B. Clemens, M.A. Kelly, A.C. Kummel, P.C. McIntyre, Arsenic decapping

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97X-ray photoelectron spectroscopy (XPS)

19 E.J. Kim, E. Chagarov, J. Cagnon, Y. Yuan, A.C. Kummel, P.M. Asbeck, S. Stemmer,

K.C. Saraswat, P.C. McIntyre, Atomically abrupt and unpinned Al

/In

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interfaces: experiment and simulation, J. Appl. Phys. 106, 124508 (2009).

20 S. Swaminathan, Y. Oshima, M.A. Kelly, P.C. McIntyre, Oxidant prepulsing of

Ge(100) prior to atomic layer deposition of Al

: in situ surface characterization,

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21 E.R. Mysak, J.D. Smith, J.T, Newberg, P.A. Ashby, K.R. Wilson, H. Bluhm,

Competitive reaction pathways for functionalization and volatilization in the

heterogeneous oxidation of coronene thin lms by hydroxyl radicals and ozone,

Phys. Chem. Chem. Phys. 13, 7554 (2011).

22 H. Grönbeck, Mechanism for NO

charging on metal supported MgO, J. Phys. Chem.

B 110, 11977 (2006).

23 H. Siegbahn, K. Siegbahn, ESCA applied to liquids, J. Electron Spectrosc. Relat.

Phenom. 2, 319 (1973).

24 H. Fellner-Feldegg, H. Siegbahn, L. Asplund, P. Kelfve, K. Siegbahn, ESCA

applied to liquids IV. A wire system for ESCA measurements on liquids, J. Electron

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25 R.W. Joyner, M.W. Roberts, K. Yates, A ‘high-pressure’ electron spectrometer for

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26 H.J Ruppender, M. Grunze, C.W. Kong, M. Wilmers, In situ X-ray photoelectron

spectroscopy of surfaces at pressures up to 1 mbar, Surf. Interf. Anal. 15, 245

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27 J. Pantförder, S. Pöllmann, J.F. Zhu, D. Borgmann, R. Denecke, H.-P. Steinrück,

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76, 014102 (2005).

28 D.F. Ogletree, H. Bluhm, G. Lebedev, C.S. Fadley, Z. Hussain, M. Salmeron, A

differentially pumped electrostatic lens system for photoemission studies in the

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29 A. Knop-Gericke, E. Kleimenov, M. Hävecker, R. Blume, D. Teschner, S. Zafeiratos,

R. Schlögl, V.I. Bukhtiyarov, V.V. Kaichev, I.V. Prosvirin, A.I. Nizovskii, H.

Bluhm, A. Barinov, P. Dudin, M. Kiskinova, High-pressure X-ray photoelectron

spectroscopy: a tool to investigate heterogeneous catalytic processes, in: B.C. Gates,

H. Knözinger, Eds: Advances in Catalysis, Vol. 52, Academic Press, Burlington,

2009, pp. 213–272.

30 M.E. Grass, P.G. Karlsson, F. Aksoy, B.W.M. Lundqvist, B.S. Mun, Z. Hussain,

Z. Liu, New Ambient Pressure Photoemission Endstation at ALS Beamline 9.3.2,

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31 D.F. Ogletree, H. Bluhm, E.L.D. Hebenstreit, M. Salmeron, Photoelectron spectroscopy

under ambient pressure and temperature conditions, Nuclear Instr. Methods A 601,

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for surface science and nanotechnology, Surf. Sci. Rep. 63, 169 (2008).

33 H. Bluhm, M. Hävecker, A. Knop-Gericke, M. Kiskinova, R. Schlögl, M. Salmeron,

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34 H. Bluhm, Photoemission spectroscopy under humid conditions, J. Electron Spectrosc.

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98 In situ characterization of thin ﬁlm growth

36 J. Wollschläger, J. Viernow, C. Tegenkamp, D. Erdos, K. M. Schroder, H. Pfnur,

Stoichiometry and morphology of MgO lms grown reactively on Ag(100), Appl.

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hydroxyl and water layers on MgO lms studied with ambient pressure XPS, Surf.

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

In situ spectroscopic ellipsometry (SE) for

characterization of thin film growth

J. N. Hilfiker, J.A. Woollam Co., inc., USA

Abstract: In situ spectroscopic ellipsometry is a versatile optical

measurement technique for characterizing thin lms. Ellipsometric

measurements commonly determine thin lm thickness, growth and etch

rates, optical constants, surface and interface quality, composition, and

other related material properties. This chapter covers the principles of in

situ spectroscopic ellipsometry, considerations for in situ integration, and

common applications.

Key words: in situ spectroscopic ellipsometry, thin lm characterization, real-

time process monitoring and control, optical monitoring, growth/etch rates.

5.1 Introduction

Thin lms are commonplace in our everyday lives – from coatings on contact

lenses and solar-efcient windows to cell phone components and at-panel

displays. The expansion of thin lm use into a long list of technologies has

placed critical demands on precise lm measurements, and both research

laboratories and commercial manufacturers have come to rely on optical

characterization methods. These methods enable engineers to measure lm

thickness, optical constants and other related material properties rapidly and

non-destructively. Optical characterization refers to a number of techniques

that use light to study materials and thin lms (samples).

Photometric measurements determine the change in reected or transmitted

light intensity caused by a sample (referred herein as ‘R/T’ to describe either

reectance or transmittance measurements). Standard R/T measurements are

relatively simple experiments, but often inadequate to measure increasingly

thin layers and complex multilayer structures found in modern thin lm-

based devices. Spectroscopic ellipsometry (SE) measurements consider

the electric eld properties (polarization) of the measured light once it is

reected or transmitted by the sample. SE is increasingly popular for thin

lm characterization because it is signicantly more sensitive to very thin

lms (including sub-nanometer thickness), provides excellent precision, and

yields enhanced information about the sample.

In situ SE offers the combined benets of in-process measurement and

high precision. This versatile characterization technique is routinely used



100 In situ characterization of thin ﬁlm growth

to monitor growth or etch from a wide variety of processes. Measurements

can be collected from samples in ultra-high vacuum (UHV), exposed to

air, submerged within a liquid, or held in controlled gas environment. This

versatility is primarily due to the non-invasive nature of optical measurements,

where only the light beam travels through the process chamber. Common in

situ Se measurements include:

∑ surface quality before processing;

∑ growth or etch rate, even with changes during processing;

∑ thin lm optical constant variations with process conditions;

∑ fast development of optical constant libraries that consider variations in

process conditions or material properties;

∑ ability to study many layers during a single process run;

∑ temperature and composition of compound semiconductors;

∑ real-time process control of thickness (end-point detection), growth rate,

or material properties.

Furthermore, SE measurement speeds allow real-time measurements for all

but the fastest processes. Data can be collected in a fraction of a second

with hundreds of simultaneous wavelengths.

This chapter will focus on in situ Se measurements, analysis, and

applications. Section 5.2 covers the principles of ellipsometry along with

primary benets of in situ Se. Section 5.3 introduces common measurements

and analysis procedures. Section 5.4 discusses the considerations specic

to in situ SE measurements; including mechanical, optical, and software

integration. Section 5.5 explores a few important materials and application

areas involving in situ SE; including compound semiconductors, optical

coatings, photovoltaics, biological lms, and nanotechnology. The chapter

concludes with the outlook for in situ Se.

5.2 Principles of ellipsometry

Ellipsometry measurements describe the polarization change caused by

interaction of a light beam with a sample. The polarization change itself is

not generally the property of interest, but can be used to determine material

properties such as thin lm thickness, optical constants, and microstructure.

This section introduces the fundamentals of ellipsometry measurements, basic

instrumentation, data analysis procedures, and typical thin lm characterization

capabilities. We further consider in situ ellipsometry and the benets of SE

measurements during lm growth or processing.

5.2.1 Spectroscopic ellipsometry (SE)

Ellipsometry measures the change in polarization that occurs when a light

beam is reected from or transmitted through a sample. For reected light,



101In situ spectroscopic ellipsometry (SE)

this change in polarization, r, is often described with two values, Y and D

(Fujiwara, 2007, p. 347):

= tan

()

–

()



[5.1]

where Y is the ratio of re ected amplitudes and D is the phase difference

produced upon re ection. Polarization changes arise from the re ectivity

difference between electric  eld components oriented parallel (p-) and

perpendicular (s-) to the plane of incidence, as shown in Figure 5.1.

Measurements generally occur at an oblique angle of incidence (f), de ned

as the angle between the incident light beam and sample surface normal. In

Fig. 5.1, the incident light is linearly polarized with electric- eld components

in both p- and s-planes. In general, light re ection produces a change in

amplitude ratio and phase difference between the p- and s-components.

SE measures this ‘change’ in polarization to help determine the sample

properties.

Basic ellipsometry measurements produce two quantities (typically expressed

as Y and D) at each wavelength and angle of incidence. SE data are often

taken at many wavelengths, leading to thousands of measured data sets.

The amount of collected data can further be increased by varying the angle

of incidence, but not all measured information is equally useful. Although

certain wavelengths and angles are more sensitive to material properties,

multiple measurements can contain equivalent information (Hil ker et al.,

2008b).

p-plane

s-plane

Sample

Plane of incidence

Angle of

incidence

Sample

normal

5.1 When linearly polarized incident light, consisting of p- and s-

orthogonal polarization components, is reﬂ ected from a surface at

oblique angle of incidence (f) the result is often elliptical polarization.

Ellipsometry measurements determine the change in polarization

that occurs when light interacts with the sample.

