Photonic crystals in biology - NanoTR-VI

Poster Session, Thursday, June 17Theme F686 - N1123A New Method: Thickness Determination of Thin Films by Energy Dispersive X-raySpectroscopySedat Canli 1,2* , Mustafa Kulakci 3 , Urcan Guler 3 , Rasit Turan 2,31 Micro and Nanotechnology Department, Middle East Technical University, 06531 Ankara, Turkey2 Central Laboratory, Middle East Technical University, 06531 Ankara, Turkey3 Department of Physics, Middle East Technical University, 06531 Ankara, TurkeyAbstract- EDS is a tool for quantitative and qualitative analysis of the materials. In electron microscopy, the energy of theelectrons determines the depth of the region where the x-rays come from. By varying the energy of the electrons, the depth ofthe region where x-rays come from can be changed. Different quantitative ratios of the elements for different electronenergies can be obtained using a thin film. The thickness of a specific film on a specific substrate corresponds to a uniqueenergy-ratio diagram. In this study, it is shown that thickness of a thin film can be obtained by an appropriate analysis of theenergy-ratio diagram of the EDS data obtained from the film.Scanning Electron Microscopes (SEM) and EnergyDispersive X-ray Spectroscopy (EDS) methods arebeing widely used in materials and nanotechnologyresearches. Thin films are very important for severalindustries, such as electronic semiconductor industry,optical coating industry or photovoltaic cells. In thecurrent work, EDS was investigated as a potential toolto be used as a relatively easy methodology formeasuring the thickness of thin films.Silicon, the main material of semiconductortechnology was used as the substrate in this study.Gold, germanium and aluminum were coated onsubstrates by thermal evaporation method and SiO 2 wasgrown by the wet oxidation method. Corroborativethickness measurements were obtained by profilometerand ellipsometry.It is known that the electron beam generated by anSEM hits perpendicularly onto the surface of the filmsand the interaction volume changes with the appliedelectron energy. In the present study, energy values inthe range of 3keV to 30keV with steps of 1keV wereused.Figure 1: Monte Carlo electron trajectory simulations of theinteraction volume in Fe at (a) 10keV. (b) 20keV. (c) 30keV [1].At atomic scale, focused electrons hitting the sampleexcite inner electrons of the atoms. During therelaxation of the atoms, each atom radiatescharacteristic x-rays from K α , K β , L α , etc shells [2]. Thedetector collects and counts x-rays coming from thesample and constructs the spectrum. The EDS systemnormalizes and corrects the data using ZAFcoefficients, and finally gives ratios of the elements.Atomic percentage ratios of the coated elements ofvarying thicknesses were collected. The gold ratioobtained from this sample set can be seen in Figure 2.Gold Ratio (%)100806040200Voltage & Atomic Gold Ratio25 nm50 nm75 nm100 nm125 nm150 nm175 nmSEM Voltage (keV)0 5 10 15 20 25 30Figure 2: Atomic percent ratios obtained on gold films with differentthicknesses.Similar plots were separately obtained for eachelement and it was observed that different atomic ratiocurves were obtained for the same thicknesses ofvarying coatings.For a specific gold ratio (i.e. %50) we draw ahorizontal line and found corresponding SEM Voltage,for each film thickness. The collection of all dataobtained from all films at each thickness was used tointerpolate the data and construct a new graph ofvoltage vs. thickness. The reference graph was used asa tool to figure out the unknown thicknesses of thesefilms. These results were further correlated using theMonte Carlo simulation software called Casino.In summary, it was shown that specific thickness ofthin films on a substrate give unique atomic percentvoltage-ratio curves and these data can be used as atool to construct reference data for further thicknessdetermination. It is shown that this methodology canbe exploited for all elements of interest that were usedas a coating on thin films at nanoscale, which is madeavailable by the Monte Carlo simulations developedfor this particular study.* Corresponding author: canli@metu.edu.tr[1] Goldstein J.I. Scanning Electron Microscopy and X-rayMicroanalysis, New York 1992[2] Reimer L. Scanning Electron Microscopy, Springer-Verlag,Berlin 19986th Nanoscience and Nanotechnology Conference, zmir, 2010 660