AES - Auger Electron Spectrosopy

Transcription

1 Advanced Materials - Lab Intermediate Physics Ulm University Institute of Solid State Physics AES - Auger Electron Spectrosopy Sebastian Schnurr March 13, 2013

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3 Safety Precautions MAKE SURE THAT YOU UNDERSTAND THIS HANDOUT BEFORE YOU ATTEND THE EXPERIMENT! I The AES-system is complex and cannot be operated without detailed understanding of its principles. DO NOT operate the system without your advisor.

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5 1 1 Introduction The aim of this experiment is to show how surface sensitive measurements are performed. For this purpose the students will measure the Auger electron spectra (AES) of an Aluminium thin film and of a Gold thin film. Those films are prepared in advance by the tutor. The characteristics of the spectra are to be discussed, especially the difference between the two elements according to the excited surface plasmons. In the second part of the experiment a double layered film made from both Aluminum and Gold will be investigated. With the use of a ion-sputtering technique the film will be removed gradually while at the same time Auger spectra are recorded. Here information about the individual layer thicknesses, interface intermixing and oxidation will be obtained. 2 Introduction to Auger Electron Spectroscopy The underlaying effect for AES is the Auger effect which is based on electric transitions within the atom. This effect was found 1926 by Pierre Auger [1] and independently by Lise Meitner 1922 [2] so that sometimes this effect is also called Auger-Meitner-Effect. When an atom is hit by an electron of a sufficient energy, there is a chance to repell an inner shell electron. Excited in this manner, the atom can relax from the high energy state by emitting a X-Ray photon or outer shell electrons filling up the emerged hole in the inner shell (mostly K-shell). Both processes are sketched in Fig. 2. In the second case, by filling up the inner shell hole, the electrons release a portion of energy dependend on the initial shell they start from and the shell they fall into. This energy can now be transfered to another electron (either from the same shell or others), which now can escape the atom. This electron is called the Auger electron. Now one can measure the energy of the escaped electrons and therefore distinguish between each single transition. For this purpose usually the counted number of electrons or current intensity is plotted as a function of energy. In such a plot, each diffrent transition will result in a single peak. Those are labeled with the initial hole shell as first letter, second and third letters name the shell from where electrons fill up this hole. (e.g. Al-KLL: Peak in Aluminium, initial hole in K-Shell, electron falling from L-shell and escaping from L-shell). For better assignment of the single lines one can also measure the first deriviative dn/de of this curve (see also Fig. right hand side). The here explained technique has two major limitations. Because of the excited transitions, the energy of Auger electrons ranges from 20eV to 2000eV. For this reason the corresponding mean free path of the Auger electrons ranges from 2 Å to 20 Å in the material (compare to the universal curve of mean free path)[5]. Therefore only from this depth electrons can escape the sample and contribute to the measurement. The second limitation is the fact that light elements tend to have

6 2 2 INTRODUCTION TO AUGER ELECTRON SPECTROSCOPY Figure 1: Sketch of an Auger process. If excited by external irradiation a inner shell electron can be knocked of from the atom. It can now release the received energy either by emitting a X-Ray photon (right hand side) or outer electrons fill the inner core hole. In this case an Auger electron is emitted [3]. Figure 2: Examples of Auger spectra obtained from a Nickel sample [4]. On the left hand side the measured signal is directly plotted as a function of kinetic energy. In comparision on the right hand side the first derivative dn/de is plotted. One can observe a Nickel and an Oxygen Peak.

7 3 Figure 3: Scetch of the Dual Beam System. The AES measurement will be done in the UHV analysis chamber depicted on the right hand side. The sample is already mounted on the transporting device on the left hand side. The middle chamber is used for film preparation by ion-sputtering [9]. Figure 4: Sketch of the sample. A Gold film is deposited on a Si substrate. Afterwards one half of the sample is masked and an Aluminium film is deposited. In this way it is possible to obtain the AES of Au and Al with one sample. a higher Auger yield in comparison to heavier atoms. Heavy atoms otherwise have a higher X-Ray yield. Therefore, AES is a ideal technique to examine the surface of thin films made from light elements. For more detailed information please read in the standart textbooks of your choice or consult the library. Here you can find even more specified sources (e.g. [6, 7, 8] ) 3 Experimental setup A sketch ot the experimental setup is illustrated as below in Fig. : The Auger electron spectra are measured in the analysis chamber connected to the Dual Beam System. It contains an electron gun and an energy analyser to obtain the spactra. Furthermore, an ion-source is mounted to perform a depth sensitive AES measurement. The sample measured will be a thin Gold film on a Silicon single crystal substrate. Afterwards one half of the sample is covered with a mask and the sample is then coated with another layer of Aluminium. So one half of the sample shows a Au surface and the other half forms a double layer, a side view of the sample can be seen in Fig.. The measurement takes place in the analysis chamber as described above. A schematic of the sample position in respect to the electron and ion beam is shown in Fig.. The electron beam to excite Auger electrons is directed to the sample from above

8 4 4 EXPERIMENTAL PROCEDURE Figure 5: Schematic of the sample relative to electron and Ion beam in the Analysis chamber. whereas the Ion beam for the second experiment originates from the side. The generated Auger electrons are detected by a energy detector also located at the top of the chamber. 4 Experimental procedure Before performing the measurement, the sample must be put into the measurement position. This is done by moving the sample holder with the help of a motor through the system. The exact sample position can be monitored either from the transportation system outside or directly by looking into the UHV chamber through a window. To perform the AES experiment it is neccesary to set the electron beam and the detection system to the active measurement mode. This will be done under guidance of the tutor. The obtained data will be captured and stored via PC. The first measurements obtained are the pure Auger spectra of the Au and the Al film. Therefore the sample position is fine-adjusted relative to the electron beam visible on the sample. First the electron will be positioned on the side of the sample which is only covered by the gold film. Then the parameters for the desired measure scan have to be adjusted according to the literature data availiable in the lab. The same procedure is performed on the Aluminium-covered side of the sample. For this sake the sample has to be repostioned. For a better significance of the date not only one but multiple scans are taken and stored in a data file. After taking both spectra, the last aim of the experiment is to obtain a Auger depth profile, which means taking Auger spectra while sputtering the film slowly. Therefore the Argon ion-beam is used and first has to be switched on. All those actions are guided and overseen by the advisor alike before. The experimental challange with this technique is to match the position of the ion beam and the electron beam. For this purpose the sample holder is first moved so both beams hit a empty Al 2 O 3 single crystal. On this sample the two beams are alined in respect to each other as also shown on Fig. on the right hand side. Then the Gold/Aluminum sample is put into the measurement postion. Again the measurement system must be prepared before. Unlike the first part now several spectra are measured one after another, while the ion beam slowly removes the film.

9 5 5 Report and data treatment Below you find some details of data analysis and questions that should be addressed in the report. Prepare your report in accordance to the guidelines for lab reports! 1. Treat the data into a useful form. The single scans are recorded one after another. For this purpose best use a program like Matlab e.g. Afterwards plot the obtained data in a sensful manner. 2. Identify the Auger electron peaks according to literature (see e.g. [4]), discuss the position of surface plasmons. Compare here the both films (Au and Al) in this respect. 3. The depth profiling measurement you also have to plot in a sensful matter. Discuss the individual film peaks in the beginning, during the sputtering and at the end. Try to put a focus on the layer interface. References [1] Pierre Auger. Sur les rayons β secondaires produits dans un gaz par des rayons. Comptes Rendus, 180:25, [2] Lise Meitner. Über dieβ-strahl-spektra und ihren Zusammenhang mit der γ- Strahlung. Zeitschrift der Physik, 11(1):35 54, [3] Young Yun. Auger Electron Spectroscopy, [4] ThermoFisher. XPS, AES, UPS and ESCA. [5] Stefan Hüfner. Photoelectron Spectroscopy. Springer, [6] Siegfried Hofmann. Auger- and X-Ray Photoelectron Spectroscopy in Materials Science : a User-Oriented Guide. Springer, Berlin, [7] Ian F. Ferguson. Auger microprobe analysis. Bristol. [8] David Briggs. Practical surface analysis. Wiley, 2 edition, [9] Ellen Rieger. Strukturelle und magnetische Eigenschaften sputterdeponierter metallischer Multilagen. PhD thesis, Ulm, 2011.