Photon Factory Activity Report 2017 #35 (2018)
BL-3B/2016G094

Study on the behavior of the photoelectrons with lower energies
in the ultra thin film of silicon oxide
Motoyasu Imamura ∗11 AIST, 1-1-1 Higashi, Tsukuba 305-8565, Japan

1 Introduction
X-ray photoelectron spectroscopy (XPS) has beenused for the quantitative and qualitative analysis
methods in the developments of functional materials
because of its high surface sensitivity, and it is useful
to determine the thickness of the thin film. For determination of the the thickness using XPS, the attenuation length (AL) of the photoelectron is an importantparameter. For a long time, the inelastic mean freepath (IMFP) has been used as an alternative of AL
because the values of IMFP can be calculate easily
using the predictive equation TPP-2M [1–3]. However the effect of the elastic scattering is not considered in the evaluation of IMFPs. Recently effective attenuation length (EAL) [1, 4, 5] is proposed and used
for the practical purposes. The author has been experimentally measured the EAL of the photoelectronin the SiO2 thin films with the energy range of 100–
1000 eV [6] at old BL13C station [7, 8].
For experimental determination of EAL in SiO2 ,the SiO2 thin film and bulk samples of Si and SiO2
are necessary. The thickness of the thin film must be
certified by another method. As shown Fig. 1, photoelectron intensity from each sample was measured.
From the bulk samples of the silicon and silicon oxide,Ro (hν, θ ) =∞
ISi∞
ISiO
2(1)

∞ and I ∞is caluculated where ISi
SiO2 are the photoelectron intensities from the Si substrate and the substrate of oxide only, respectively.

Rd (hν, θ ) =ISi
ISiO2(2)is calculated where ISi and ISiO2 are the photoelectron
intensities from the Si substrate and SiO2 thin film,respectively.
The EAL is calculated from the equation below:
λSiO2 (hν, θ ) =1d·cos θ ln{[ Ro (hν, θ )/Rd (hν, θ )] + 1}
(3)

For determination of the EAL in the SiO2 thin film, it
is important to measure the Ro precisely.
2 Experiment
The measurements of XPS spectra were carried atBL3B using the apparatus built for the angle-resolved
x-ray photoemission spectroscopy (ARPES II). In thisapparatus, the hemispherical electron analyzers witha 50 mm mean radius (VSW HA54) are fixed on thegoniometer, which enable the analyzer to be rotate
in the vacuum chamber as shown in Fig. 2. The energy step was 0.05 eV in the measurements of theXPS spectra. The pass energy was set as 10 eV. The
acceptance angle was ±1◦ . The angle between theincident x-ray and the surface normal of the sample
was fixed as 55◦ . The emission angle of the photoelectrons from the surface normal was adjusted to0◦ , 15◦ , 30◦ , 45◦ , 60◦ , and 75◦ by rotating the electron
analyzer. The energy of the incident x-ray was set as
130 eV, 150 eV, 180 eV, 200 eV.
As the bulk samples, Si(100) wafer with a thicknessof 0.5 mm and SiO2 /Si thin film with a thickness of
100 nm prepared by a thermal process were used.
The thickness 100 nm of the SiO2 thin film is thickenough to avoid the photoelectron signal from thesubstrate and thin enough to suppress the charging
by the irradiation of x-ray. Both samples were cut asAnalyzer

SRSample

Fig. 1: Samples for EAL detemination
Fig. 2: Alignment of sample and analyzer

Photon Factory Activity Report 2017 #35 (2018)the size of 5 mm × 3 mm using a diamond cutter andfixed on the same stub for the measurement using
carbon tape as shown in Fig. 3, because two samples
must be used for measurement by turns in the same
condition to suppress the time dependent influence.

Si3 Results and Discussion
Fig. 4 and 5 shows the Si 2p spectra of SiO2 and Si,
respectively which measured with the emission angleof surface normal excited by x-ray with the energyof 150 eV. Si peak and Si oxide peak are recorded
clearly. To estimate the photoelectron intensities inthe spectra, curve fitting procedure was carried out
using Shirley function for the background and Gauss
function for the peak. The Ro is calculated using
Eq.(1) for each energy and emission angle, which ismentioned in the experimental section. Some of theRo data were plotted as functions of the emission angle in Fig. 6.

SiO2

Acknowledgement
I would like to thank Prof. Ozawa of Tokyo Techand Prof. Mase of PF for their help in my experiments at BL3B station.
Fig. 3: Samples on the stub

Photoelectron Intensity

SiO2 substrate

36404448
Kinetic Energy (eV)52

Fig. 4: XPS spectra of SiO2References[1] ISO 18115:2001-surface chemical analysis - vocabulary.
[2] S. Tanuma et al., Surf. Interface Anal., 17 (1991)
927; 20 (1993) 77; 21 (1993) 165.
[3] C. J. Powell and A. Jablonski. NIST Electron
Inelastic-Mean-Free-Path Database: Version 1.1. National Institute of Standards and Technology(2000).
[4] C. J. Powell and A. Jablonski. NIST Electron
Effective-Attenuation-Length Database - Version 1.0.
National Institute of Standards and Technology(2001).
[5] A. Jablonski and C. Powell, Surf. Sci. Rep. 47(2002) 33.
[6] M. Imamura et al., Meas. Sci. Technol. 22 (2011)024007.
[7] N. Matsubayashi et al., Rev. Sci. Instrum. 63 (1992)
1363.
[8] H. Shimada et al., Rev. Sci. Instrum. 66 (1995) 1780.
∗ motoyasu@ni.aist.go.jp

2.5

Si substrate

Photoelectron Intensity

80eV
100eV

Ro (ISi/ISiO2)

2.0
1.5
1.00.50.036404448
Kinetic Energy (eV)Fig. 5: XPS spectra of Si

52

01530

4560

75

90

Emission Angle (degree)

Fig. 6: Ro of several emission angles