XPS Applications in Thin Films Research

Shujiang Geng 1, Sam Zhang 1 and H. Onishi 2

1. School of Mechanical & Production Engineering, Nanyang Technological University, 50

Nanyang Avenue 639798 Singapore

2. Shimadzu (Asia Pacific) Pte Ltd, 16 Science Park Drive, Singapore 118227

Abstract: XPS (X-ray Photoelectron Spectroscopy) or ESCA (Electron Spectroscopy for

Chemical Analysis) as a surface analytical technique has been developed over years and has been

used successfully in characterization of a variety of thin films. Information such as composition,

chemical states, depth profile, imaging and thickness of thin films is especially useful in thin film

research. This paper aims at reviewing these applications of XPS in thin film area.

Keywords: XPS, ESCA, applications, thin films

Introduction

XPS (X-ray Photoelectron Spectroscopy) or ESCA (Electron Spectroscopy for Chemical

Analysis) as a surface analysis technique has been most widely used due to its high information

content, flexibility in addressing a wide variety of samples and sound theoretical basis since K.

Siegbahn and his research group developed the instrumentation and theory of it in the mid 1960s 1. Over the past few decades XPS has been developed into the key surface characterization

method which combines surface sensitivity with the ability to quantitatively obtain both

elemental and chemical state information 2. Data interpretation has been established within a

number of published databases 3, 4 and in the modern times on-line services have become

available on the World Wide Web 5. As we know, nowadays XPS is a very important analytical

tool in the area of thin films 6. XPS analysis gives rise to useful information such as composition,

chemical state, and thickness etc. of thin films. This paper reviewed these applications with

emphasis on thin film composition, imaging and thickness measurements.

1. Composition and Chemical state

The principle of XPS is based on the photoelectric effect outlined by Einstein in 1905 where

the concept of the photon was used to describe the ejection of electrons from a surface when

photons impinge upon it. This process can be expressed by the following equation 1:

BE = νh − KE − φ (1)

Where BE is the binding energy of the electron in the atom (a function of the type of atom and its

environment), νh is the photon energy of X-ray source, KE is the kinetic energy of the emitted

electron that is measured in the XPS spectrometer and φ is the spectrometer work function. For

XPS, Al Kα (1486.6eV) or Mg Kα (1253.6eV) is often the photon energies of choice. Other X-

ray lines can also be chosen such as Ti Kα (2040eV). The XPS technique is highly surface

specific due to the short range of the photoelectrons that are excited from the solid. The energy

of the photoelectrons leaving the sample is determined using an analyzer and this gives a

spectrum with a series of photoelectron peaks. The binding energy of the peaks are characteristic

of each element. The peak areas can be used (with appropriate sensitivity factors) to determine

the composition of the materials surface. The shape of each peak and the binding energy can be

slightly altered by the chemical state of the emitting atom. Hence XPS can provide chemical

bonding information as well. XPS is not sensitive to hydrogen or helium, but can detect all other

elements. Normally, in the outmost 10nm of thin films surface XPS can identify all elements

(present at concentration >0.1 atomic %) except H and He 1. The analysis of composition has

been widely applied in the thin film research fields. We can obtain the composition of thin films

from XPS spectra. The information of all elements in thin film can be gained from the survey

scan spectrum of XPS. The detailed information of each element in the thin film can be obtained

from the narrow scan spectrum of XPS.

1.1 Survey scan spectra of thin films

The information of elements in thin films and contamination adsorbed on the thin films can be

given from the wide scan spectra of XPS. Fig. 1 shows the survey spectra (pass energy of 160eV

was applied) for typical undoped and doped TiO2 films on silicon wafer 7. C KLL, Ti LMM and

O KLL Auger peaks and XPS spectra peaks of Ti, O and C were recorded for both the undoped

and doped TiO2 film. The peaks of Nb3d and Nb3p detected from the doped TiO2 film were very

weak (See Fig.1 b).

Fig.1 XPS survey spectra for the surfaces of (a) undoped and (b) doped TiO2 film on silicon wafer 7.

1.2 Narrow scan spectra of thin films

Narrow scan gives detailed information of each element in thin films. Fig.2 through Fig.5

show the narrow scan spectra (pass energy of 20 eV was used) of C1s, Ti2p, O1s and Nb3d for

both undoped and doped films 7. The chemical composition of both film surfaces can be

calculated from these spectra. The calculation is based on the integrated area under the assigned

element peak and the sensitivity factor for the element 8. Furthermore, Fig.3 and Fig.5 indicated

that the peak position of Ti2p3/2 corresponds to that of the Ti4+ oxidation state and the center of

the Nb3d3/2 peak corresponds to that of Nb5+ oxidation state. One point should be noted, the

spectra must be calibrated with the standard sample such as Au 4f7/2 and Ag 3d5/2. For AXIS

Ultra type XPS apparatus, the bending energy of Au 4f7/2 is 83.89 eV (Al Kα ) or 84.00 eV (Mg

Kα ) and that of Ag 3d5/2 is 368.26 eV (Al Kα ) or 368.27 eV (Mg Kα ). Normally, the simple

calibration is done by C1s (approximately 285 eV) spectrum of adventitious carbon that exists on

all samples.

Fig.2 C1s spectra: (a) doped and (b) undoped films 7

Fig.3 Ti 2p spectra: (a) doped and (b) undoped films 7

Fig.4 O1s spectra: (a) doped and (b) undoped films 7

Fig.5 Nb3d spectra for doped films 7

It is very important to deconvolute these spectra in thin film elemental analysis. Extra care

must be exercised in deconvoluting a spectrum since quite a degree of human judgement is

involved the process. For example, the XPS spectra of O1s in Fig.4 show a two-band structure.

The dominant peak at 529.7 eV is characteristic of metallic oxides, which is in agreement with

O1s electron binding energy for TiO2 and also is assigned to the niobium for doped samples 9.

Another O1s band at 531.1 eV may be attributed to different oxidized hydrocarbon

(contamination).

2. Depth analysis

In XPS, two methods can be used for composition variation with depth of thin film, the non-

destructive analysis and the destructive analysis.

2.1 Non-destructive depth profiling

The escape depth of a photoelectron increases with its kinetic energy in the energy range

higher than 50 eV. One of the non-destructive analysis is analyzing XPS spectra at varying

photoelectron take-off angles 10, 11. Another method is using a high-energy and continuously

variable-energy source in order to observe the dependence of the signal intensity on the kinetic

energy of the photoelectron and therefore on the electron escape depth at a given take-off angle 12.

Fig.6 shows that varying the collection angleθ will change the sensitivity to the surface 13. At

very low take-off angles (90º−θ ), most of the collected electrons originate from atoms near the

surface, but at higher take-off angles more electrons will be collected from atoms deeper in the

sample.

Fig.6 Schematic drawing showing the variation of sampling depth as a function of take-off angle in X-ray photoemission experiments. (a) At normal incidence (θ = 0º), the escape depth is equal to the attenuation length λ . This geometry probes the deepest into the sample. (b) At shallow take-off angles (large θ), the escape depth (which equals λcosθ) is less than the attenuation

length, and only photoelectrons close to the surface (within a distance λ cosθ from the surface) are detected. Thus, photoelectrons from atom in plane 2 and below have to travel a distance greater

than the attenuation length and are less likely to be detected, unlike those from plane 1 and above13.

Fig.7 gives an example of how the photoemission features corresponding to the near surface

region of a thin silicon dioxide film on silicon are enhanced relative to the bulk Si signal on

going to low (i.e., shallow) take-off angles 13. A comparison of region A and B for spectra

recorded at 25º and 75º clearly shows an increase in the relative intensity of the oxidized Si

features at the lower take-off angle.

Thus, if one is interested in characterizing the surface layer of thin films, one should perform

photoemission measurement at low take-off angles to emphasize the surface layer features.

However, the depth resolution of this method deteriorates with decreasing take-off angle.

Furthermore, surface roughness and contamination of the specimen also affect the depth

resolution.

Fig.7 X-ray photoemission spectra of the Si 2p region, recorded at different take-off angles 13

Another method of non-destructive analysis for deeper surface region is the use of a high-

energy and continuously variable-energy source, such as synchrotron radiation (SR) 12. In this

case, relative intensities measured at a given take-off angle are essentially independent of the

surface toughness. Fig.8 shows the photon energy dependence of the Si 1s XPS spectra for the

Si(100) oxidized at 700°C for 30min. Peaks A and B can be assigned as elemental Si and SiO2,

respectively. The relative intensity of peak A increases gradually with the photon energy, in

contrast with peak B. This result indicates that the Si substrate is being observed since the depth

of photoelectrons becomes larger with an increasing kinetic energy.

Fig.8 Si 1s XPS spectra for the oxidized Si (700°C, 30min), taken at various photon energies. The spectra were obtained at increments of about 200 eV between 2001.7 eV and 5380.7 eV.

The photon energies used for each spectrum are shown on the right-hand axis 12.

2.2 Destructive depth profiling

The destructive depth profiling is realized by bombardment of the film surface with Ar+ ions

at controlled power and timing. After certain time of this “bombardment erosion”, the

composition is analyzed before next bombardment starts. Thus a composition depth profile is

obtained. Fig.9 shows the XPS depth profile of Ti0.4Al0.6N thin film annealed at 700°C for

10min in pure oxygen 14.

Fig.9 XPS depth profile of Ti0.4Al0.6N thin film on Si substrate 14

3. Imaging

In 1995, the first chemical XPS images of thin film grain boundaries was acquired 15. The

elemental distribution of thin films can be revealed by XPS imaging. Fig.10 is an example of the

best resolution IXPS currently available (with a spatial resolution up to 10nm) 6. A thin silicon

nitrate film was grown on a silicon wafer. Composition imaging provides a valuable way to

examine the compositional uniformity or segregation of elements in thin films. Fig.11 shows the

lateral distribution of a laser ablated hole on the poly sample (20 pulses with 0.55J/cm2; ø =

300µm) of carbon (C1s) and oxygen (O1s) recorded with XPS imaging 16. The concentration of

C is lower within the hole than that outside the hole, while the O1s concentration within the hole

is higher than that outside the hole.

Fig.10 XPS images. (a) The N1s and (b) the O1s signal distribution of oxidized silicon pattern in

a silicon-nitrate matrix, (c) overlay image 6.

Fig.11 Carbon (C1s) and oxygen (O1s) distribution of a laser ablated hole measured by XPS imaging. Brighter color indicates higher concentration 16.

4. Thickness

XPS can offer precise thickness measurements for ultra thin films. The SiO2 thickness oxd was

determined using the following equation 17, 18:

oxd = oxλ sinθ ln

+ 1

Si

ox

II

β (2)

where oxλ is the attenuation length of the Si 2p photoelectrons in SiO 2, θ is a photoelectron take-

off angle, i.e. the angle between the sample surface plane and the incident beam, β = ∞oxI / ∞

SiI

(where ∞oxI and ∞

SiI are the Si2p intensities of “infinitely thick” (greater than 30nm) SiO2 and Si,

respectively) and oxI / SiI is the ratio of intensities from the unknown film. Unfortunately, there is

considerable disagreement in the XPS literature in the two sample-dependent constants: oxλ and

β . Reported oxλ values vary from 2.4-3.8nm, while β varies from 0.6-1.01 18, 19, 20, 21, 22. It has

been suggested that much of the discrepancy in the reported oxλ values stems from the use of

poorly calibrated ellipsometry ‘standards’ that tend to overestimate the thickness of ultra thin

SiO2 films 18, 23, 24, 25, 26. For this reason, it was decided to use an attenuation length of

2.7± 0.2nm, averaged from four different TEM/XPS publications 18, 19, 20. The value of β was

determined by analyzing two different standard samples: a 5%HF etched Si (100) surface and a

thick thermal SiO2 film which is thicker than 30nm. Both standards were lightly sputtered with 3

keV Ar+ to remove absorbed organic species and the native oxide and fluoride (on the HF etched

Si sample). Normally, the β value is about between 0.75 and 0.85 20, 27.

For ultra-thin films, XPS gives very precise thickness. Fig.12 shows the ability of XPS to

measure very small difference in ultra-thin oxides of SiO2 27. Although TEM can offer the

precise thickness measurement, analysis on films thinner than 1nm is extremely difficult 28.

Furthermore TEM sample preparation is also an arduous endeavor. In order to check the

precision of thickness measurement by XPS, the same sample of SiO2 film was measured by

both XPS and TEM. According to Fig. 13 20 and Eq (2), the calculated thickness of SiO2 film is

6.4nm which is amazingly the same as the TEM result (Fig.14) 20. XPS is so sensitive to small

changes in oxide thickness for ultra-thin films (<2nm, the precision is ± 0.1nm). However, the

inherent limitation is its insensitivity in change in oxide thickness of thicker films (>10nm)

because the Si relative intensity decays exponentially with the increase of oxide thickness ( SiI ∝

θλ sin/de− ). This can be seen from Fig.15 27.

Fig.12 Si 2p spectra of ultra-thin SiO2 oxides showing difference of as small as 0.1nm 27

Fig.13 XPS Si 2p spectrum of a 6.4 nm SiO2 film on Si (bulk) (100) 20

Fig.14 TEM micrograph showing the thickness of SiO 2 film on Si(100) 20

Fig.15 Exponential decay of Si relative intensity of samples 27

Mitsutoshi Hoshino 29 measured the thickness of lubricant layer on magnetic recording disks

by using the XPS O1s signal ratio of lubricant and silicon dioxide according to this equation:

d = 16ln[7.2( 1I / 2I ) + 1] (3)

Where d is the thickness of lubricant layer, 1I / 2I is the ratio of the intensities of O1s

photoelectrons coming from lubricant layer (perfluoropolyether) and from silicon dioxide. It is

obvious that the thickness of lubricant layer can be calculated from the ratio of 1I / 2I that can be

obtained from the XPS spectra as demonstrated in Fig.16 for different concentration polyether.

Fig.16 XPS O1s signal of different concentration perluoropolyether, O1s from SiO2 (534.6eV), O1s from perluoropolyether (537.2eV) 29

5. Summary

XPS is an indispensable tool on advanced thin film research. Useful information such as

composition, chemical state, depth profile, compositional distribution and thickness can be

measured by using XPS analysis.

The peak areas are used to determine the composition. The alteration of shape of each peak

and the binding energy reveal information about chemical state of the ejected atom. The XPS

compositional depth profiling can be obtained through destructive or non-destructive method.

XPS imaging for the elemental distribution of thin films offers information on uniformity of thin

films and segregation of elements in the thin films. For precise determination of thickness in

ultra-thin films, the precision is ± 0.1nm for films thinner than 2nm. For thicker films (thicker

than 10nm), however, XPS thickness measurement is not accurate.

References

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