Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
A New Method: Thickness Determination of Thin Films by Energy Dispersive X-ray
Spectroscopy
Sedat Canli 1,2* , Mustafa Kulakci 3 , Urcan Guler 3 , Rasit Turan 2,3
1 Micro and Nanotechnology Department, Middle East Technical University, 06531 Ankara, Turkey
2 Central Laboratory, Middle East Technical University, 06531 Ankara, Turkey
3 Department of Physics, Middle East Technical University, 06531 Ankara, Turkey
Abstract- EDS is a tool for quantitative and qualitative analysis of the materials. In electron microscopy, the energy of the
electrons determines the depth of the region where the x-rays come from. By varying the energy of the electrons, the depth of
the region where x-rays come from can be changed. Different quantitative ratios of the elements for different electron
energies can be obtained using a thin film. The thickness of a specific film on a specific substrate corresponds to a unique
energy-ratio diagram. In this study, it is shown that thickness of a thin film can be obtained by an appropriate analysis of the
energy-ratio diagram of the EDS data obtained from the film.
Scanning Electron Microscopes (SEM) and Energy
Dispersive X-ray Spectroscopy (EDS) methods are
being widely used in materials and nanotechnology
researches. Thin films are very important for several
industries, such as electronic semiconductor industry,
optical coating industry or photovoltaic cells. In the
current work, EDS was investigated as a potential tool
to be used as a relatively easy methodology for
measuring the thickness of thin films.
Silicon, the main material of semiconductor
technology was used as the substrate in this study.
Gold, germanium and aluminum were coated on
substrates by thermal evaporation method and SiO 2 was
grown by the wet oxidation method. Corroborative
thickness measurements were obtained by profilometer
and ellipsometry.
It is known that the electron beam generated by an
SEM hits perpendicularly onto the surface of the films
and the interaction volume changes with the applied
electron energy. In the present study, energy values in
the range of 3keV to 30keV with steps of 1keV were
used.
Figure 1: Monte Carlo electron trajectory simulations of the
interaction volume in Fe at (a) 10keV. (b) 20keV. (c) 30keV [1].
At atomic scale, focused electrons hitting the sample
excite inner electrons of the atoms. During the
relaxation of the atoms, each atom radiates
characteristic x-rays from K α , K β , L α , etc shells [2]. The
detector collects and counts x-rays coming from the
sample and constructs the spectrum. The EDS system
normalizes and corrects the data using ZAF
coefficients, and finally gives ratios of the elements.
Atomic percentage ratios of the coated elements of
varying thicknesses were collected. The gold ratio
obtained from this sample set can be seen in Figure 2.
Gold Ratio (%)
100
80
60
40
20
0
Voltage & Atomic Gold Ratio
25 nm
50 nm
75 nm
100 nm
125 nm
150 nm
175 nm
SEM Voltage (keV)
0 5 10 15 20 25 30
Figure 2: Atomic percent ratios obtained on gold films with different
thicknesses.
Similar plots were separately obtained for each
element and it was observed that different atomic ratio
curves were obtained for the same thicknesses of
varying coatings.
For a specific gold ratio (i.e. %50) we draw a
horizontal line and found corresponding SEM Voltage,
for each film thickness. The collection of all data
obtained from all films at each thickness was used to
interpolate the data and construct a new graph of
voltage vs. thickness. The reference graph was used as
a tool to figure out the unknown thicknesses of these
films. These results were further correlated using the
Monte Carlo simulation software called Casino.
In summary, it was shown that specific thickness of
thin films on a substrate give unique atomic percent
voltage-ratio curves and these data can be used as a
tool to construct reference data for further thickness
determination. It is shown that this methodology can
be exploited for all elements of interest that were used
as a coating on thin films at nanoscale, which is made
available by the Monte Carlo simulations developed
for this particular study.
* Corresponding author: canli@metu.edu.tr
[1] Goldstein J.I. Scanning Electron Microscopy and X-ray
Microanalysis, New York 1992
[2] Reimer L. Scanning Electron Microscopy, Springer-Verlag,
Berlin 1998
6th Nanoscience and Nanotechnology Conference, zmir, 2010 660