Characterization of plasma-treated surfaces by X-ray Photoelectron Spectroscopy (XPS)

Transcription

1 Characterization of plasma-treated surfaces by X-ray Photoelectron Spectroscopy (XPS) Teresa de los Arcos September 13, 2010 X-Ray photoelectron spectroscopy (XPS) is a technique used to determine the elemental composition and chemical state (among other things) of surfaces. The goal of this practical course is to use XPS in order to gain information about the changes induced in a surface due to dierent plasma treatments. Due to time constrictions, the samples necessary for the execution of this experiment must have been previously prepared in other plasma-related experiments. At the moment, this experiment can be combined with the following f-praktika: Oberächen Modikation mit Mikroplasmen (Dr. Volker Schulz von der Gathen) 402 Anwendungsorientierte Plasmaphysik (Dr. Marc Böke) 1

2 1 Theory X-ray photoelectron spectroscopy (XPS) is an extremely powerful technique for the analysis of the chemical composition and electronic characteristics of surfaces. The technique is based in the photoelectric eect, where electrons are emitted by a material after irradiation with photons. The kinetic energy of the emitted electrons can be related to the energy of the photons by the following formula: hν = E kin + E b + Φ (1) Where hν is the energy of the incoming photon; E kin is the kinetic energy of the emitted electron, which we measure; Φ is the work function of the sample (the extra energy that the electron needs in order to physically leave the solid); and E b is the binding energy of the electron in the material, which is what we want to determine. Therefore, if we irradiate the sample with photons of known energy, we can determine the original binding energy of the electron by simply measuring the kinetic energy. Since each atom has its own distribution of core energy levels, it is possible to use XPS to determine the elemental composition of a sample surface (Si, C, O, etc). Furthermore, the precise binding energy position is also strongly inuenced by the chemical environment of the atom, and therefore, XPS can be used also to determine quantitatively the precise chemical composition of each element (Si, SiO 2, etc.) Photoelectron spectroscopy is a Ultra-High-Vacuum (UHV) methode, and we will operate within a pressure range of Pa. The low pressure is neccesary in order to: avoid scattering of the photoelectrons between the sample surface and the detector, avoid arcing between the high voltage components of the electronic, keep the sample surface clean. The mean free path of the photoelectrons within the sample is relatively short. This means that, although we will produce photoelectrons at several microns depth within the sample, only those electrons originating from the top few atomic layers will be able to actually leave the sample without being reabsorbed. This fact makes XPS extremely surface sensitive, and we will only be able to investigate the chemical composition of approximately the rst 5 nm of the surface. This means that if the sample surface is covered with a contaminant layer of adsorbed species (for example, from contact with the air), we will always measure this contaminant layer. If this layer is thicker than 5 nm we will not be able to see the surface of interest. XPS is therefore a unique surface sensitive technique for chemical analysis. Determination of a binding energy reference: We have seen that according to equation 1, we need to know the value of the work function of the sample in order to determine the binding energy of the electrons. However, from an experimental point of 2

3 view, it is very dicult to determine simultaneously a binding energy reference and the work function of the sample. The solution employed in modern spectrometers is to use as a reference, not the work function of the sample under study, but the work function of the spectrometer. What we do is to put the sample in good electric contact with the spectrometer, so that the two Fermi levels will be aligned. Our value for binding energy zero will be the position of the Fermi Level of both sample and spectrometer. The problem of the determination of the surface work function is also solved, since we can now use in our formula the known constant value of the work function of the spectrometer. The computer will perform this calculation automatically. We will obtain in our computer screen already the converted binding energy values of the photoelectrons. You can see an illustration if this in Figure 1. E kin Ekin Efot Vac Level sample spect Fermi Level Ebin Figure 1: Illustration of Fermi-level alignment of a metallic sample and the spectrometer. In certain cases it is not possible to have a good electric contact between the sample and the spectrometer. This is going to be the case when the samples are not good conductors. In this case there are two eects that we need to take into account: sample charging and a oating Fermi Level for the sample with respect to the spectrometer. Sample charging: A non conducting sample does not have sucient delocalized conduction band electrons available in order to neutralize the positive holes left behind during the photoeect. As a result, a positive potential builds near the sample surface, which retards the outgoing electrons, making them lose some of their original kinetic energy. This 3

4 retardation appears in the spectrum as a positive shift towards higher binding energies. This eect does not occur in good conducting samples or in narrow gap semiconductors, because at room temperature these systems have enough conduction electrons to neutralize the positive charge at the surface. The solution to this problem is to neutralize the positive charge externally. In order to do this, we will irradiate the sample simultaneously with a shower of low energy electrons (to neutralize the positive charge) and low energy ions (to improve the homogeneity of the neutralization). Failure to compensate the charge in this way results not only in extreme shifts in binding energy positions, but also in deformation of the peak shape. Fermi Level alignment: A serious problem is the establishment of a Fermi Level position, or zero reference position for the energy scala. This problem is inherent to the way in which modern spectrometers perform the measurement, and cannot be solved by the neutralization procedure described above. As we can see in the Figure 1, in the case of metallic samples the establishment of a reference energy level is a problem because the Fermi levels of sample and spectrometer align themselves automatically. In the case of a non-conducting sample this alignment does not take place, becuase the sample and the spectrometer are not in electrical contact. In these cases, the best procedure is to establish an arbitrary reference that will serve to x the scala. In the case of polymer samples, we will do this by xing the C1s core level of C-C bonding at a binding energy of exactly ev. In the general case, we will use the C1s signal corresponding to adventitious carbon (an undened carbon-containing layer that appears on the surface on every sample exposed to the air due to adsorption of dierent molecules), which we will x at an arbitrary value of ev. 1.1 The spectrometer The spectrometer that we will use for this experiment is a Versaprobe model from Physical Electronics, whose schema we can see in the Figure 2. It is basically divided in three parts: X-ray source, spherical analyzer and detector. X-ray source: An electron beam with an energy of 15 kv impinges onto an Al anode. There is production of X-ray radiation due to the excitation and radiative relaxation of internal Al core levels. This X-ray radiation passes through a monochromator. Finally we obtain a monochromatized beam of photons with energy of ev, corresponding to the Al K α transition. The diameter of the X-ray beam is 100 µm. Spherical Analyzer: The core of the spectrometer is the spherical analyzer, which performs electron separation according to their kinetic energy. The analyzer is composed of two concentric half spheres, separated by a thin space. The entrance slit is at one side, and the detector is located at the other side. The electrons emitted from the sample have to travel the thin space between the hemispheres in order to reach the detector. By applying a potential dierence between the two hemispheres we create a radial electric 4

5 Energy Analizer Electron Source #1 15kV Monochromator Al K x-rays eV Multi-channel detector Photoelectrons Al Anode Sample Figure 2: Schema of the spectrometer. eld that changes the initial straight path of the electrons into a curved one. Only electrons with a very specic kinetic energy will be able to reach the detector without either colliding with the outer hemisphere (if they are too fast) or the inner (if they are too slow). This particular energy which allows the electrons to reach the detector is called the Pass Energy (E 0 ). The relationship between energy resolution ( E) and pass energy for our spectrometer is given by: E = 0.015E 0. A higher pass energy value corresponds to higher amounts of electrons going through, which means higher intensity in our spectrum. High intensity, however, comes at the cost of spectral resolution. We will use high pass energy values when we want to have an overview of all possible elements present in the sample; we will use low pass energy values when it is necessary to perform detailed analysis of particular core level peaks. Detector: The electrons are nally gathered in a multi-channel detector. It is composed by two parallel plates through which there are 16 channels. Each channel acts as an electron multiplier. Between the two plates we establish a voltage of 2 kv that accelerates the electrons going through the rst plate into the second plate. 2 Procedure In the case of a follow-up of the f-praktikum Oberächen Modikation mit Mikroplasmen (Dr. Volker Schulz von der Gathen) you will investigate the changes induced by the 5

6 plasma treatment on polymers, or on metal surfaces. In this case, we will investigate one un-treated sample and one treated sample in order to establish the chemical changes induced in the surface due to the plasma treatment. In the case of a follow-up to the f-praktikum 402 Anwendungsorientierte Plasmaphysik (Dr. Marc Böke) you will perform a comparative analysis of the bare silicon surface and of the deposited lm. The experimental procedure will be as follows: 1. Sample preparation for the measurement. 2. Low resolution measurement (survey spectrum). 3. High resolution measurements of the relevant core levels. 4. Mathematical analysis of the data and discussion. 2.1 Introduction of the sample in the spectrometer It is important that the samples are never handled with bare hands, because of two reasons: to avoid covering the surface of interest with a fat-layer, and to avoid the degassing of fat molecules in the UHV environment. After cutting the sample to an appropriate size, you will mount it on a sample holder and introduce the sample holder in the interchange lock. You need to use the computer in order to vent this interchange lock. The hardware of the spectrometer is controlled by a single program: PHI SUMMIT. When you click into this icon for the rst time, three windows will open: Watcher, Acquisition settings, and Image. Watcher will allow you to control the transfer of the sample and the venting and pumping down procedures. Image will allow you to control the hardware necessary for the measurement: sample position, neutralizer, ion gun, X-ray power, etc. Acquisition setting will allow you to dene your measurement conditions. How to introduce the sample: First you will introduce your sample in the interchange lock (labelled intro in the window Watcher): 1. Open the N 2 bottle. In the window Watcher, press backll intro. This will vent the small chamber on the side of the spectrometer. CAUTION: The top cover of this chamber is not xed. It can be blown away if the venting pressure is set too high! As a precaution, always vent with your hand on top of this cover. 2. After mounting the sample in the fork of the manipulator, cover the chamber again and press pump intro. You will have to wait ve minutes for the pressure to reach a good value before transferring the sample to the UHV chamber. 6

7 3. In window Image, go to Stage and press the buttom: intro. This will move the sample stage holder (in the UHV chamber) to the appropriate position for transferring the sample. 4. In window Watcher, press transfer sample. Follow the instructions in the screen. Once the sample holder is secured in the stage within the UHV chamber, remove carefully the manipulator until the end (when you reach the end position, valve V1 will close automatically.) 2.2 Spectrometer start-up SECURITY CONSIDERATIONS. For your security: Be careful. You are going to operate a machine which produces X-ray radiation by means of accelerating electron beams with high voltage (15kV). The electron detector operates also with a voltage of 2kV. For the security of the machine: The XPS spectrometer operates in ultra high vacuum (UHV). It is important that you always monitor the pressure before starting any procedure that will switch on the X-ray tube. If the pressure is higher than Pa do not proceed with the measurement. Once your sample is within the UHV chamber, you need to nd the right position for the measurement. 1. In window Image, go to Stage and select a value for the z-position of 16.5 mm. Press move. This will move the sample to a good starting position. Now you have to ne-tune the z-position. 2. By pressing the arrow-buttons in the small box in the screen, move the sample holder and select with help of the camera the position where you want to measure. 3a. If you are measuring conducting samples: In window Image press auto-z. The software will nd the right height for the best measurement. Then go to Acquisition Settings. 3b. If you are measuring polymers or other non-conducting samples (such as an oxide layer): you will need to use neutralizing hardware. This implies preparing a source of low energy electrons (with the neutralizer) and a source of low energy Ar ions (with the ion gun): 1. First, we prepare the neutralizer. In window Image, go to Neutralizer. If the pressure is in the range of 10 7 Pa, press Standby. 7

8 2. Now we prepare the ion gun. (Note that this is also the procedure you have to follow if you want to use the ion gun in order to sputter the sample). In window Watcher press Di vlv Open. This will open Valve V4 and close valve V3. VERY IMPORTANT! Never open the valve that supplies Ar to the ion gun until you are sure that valve V4 is open. The software will not warn you if you make this mistake! If valve V4 is closed when you put Ar to the ion gun, the Ar will enter into the UHV chamber and the pressure will increase without control until eventually the security measures of the machine will kick in and all the electronics will be shut o. 3. In window Image, go to Ion gun. Press Standby. The program will ask you if Valve V4 is open. If you have opened it in the previous step, say Ok (if not, open it now). Now select the extractor pressure box. 4. Open slowly the manual valve (green color) of the ion Gun, controlling simultaneously the main pressure and the extractor pressure. Open the valve until the extractor pressure shows a value of 30 mpa. At the same time, the pressure in the chamber should not be higher than Pa. If the pressure in the main chamber increases over this value without stabilizing, interrupt the procedure immediately and close the valve. 5. Switch on the valve controller RVG050C (in the rack) with the on/o black lever. Now put the white lever limit/set point in the set point position. Wait until the extractor pressure stabilizes at 10 mpa. You can now decide if you want to use the ion gun in the sputtering mode or in the neutralize mode. Deselect now extractor pressure (the continuous measurement of this pressure is not really necessary and it takes too many resources from the computer). 6. After preparing the neutralizing software, go to window Image, SXI/Auto-Z. Select for Mode [AutoZ] the boxes Neutralizer and Ion Gun Neut.. Go to the upper right side of the window Image and press auto-z. The software will nd the right height for the best measurement. Now you can go to Acquisition Settings and program your measurement. 2.3 Programming the measurement The programming of the experiment is performed in the window Acquisition Setting. First, select the folder where your measurement will be saved, and give a name to your measurement. Following measurements will add a number automatically. Survey spectrum. Perform always rst a low resolution measurement with broad energy range. Use the following parameters: Range -2, 1200 ev. 8

9 Pass energy: ev. Energy step 0.5 ev. Repeats: 1. Total Cycles: 5. In the case of conducting samples: Neutralize OFF. If the case of non-conducting samples: Neutralize ON. When everything is ready, press Acquire. Core level lines. After identication of the dierent elements present in the sample, you will measure one core level peak of each element (choose the most intense according to the Handbook of Photoelectron Spectroscopy [1]). Measurement parameters: Range: depends on the region. Select the appropriate range with help of the Element Table in the window Acquisition Setting. Pass energy: 23.5 ev. Energy step 0.05 ev. Repeats: Will depend on the intensity of the chosen peaks. For the most intense signals in the spectrum, 10 scans are usually ok. Total Cycles: As many as are necessary. Note: you can measure several regions in the same measurement. In each cycle, the program will measure each region as many times as you determine with repeats. If you have conducting samples: Neutralize OFF. If you have non-conducting samples: Neutralize ON. When everything is ready, press Acquire. 2.4 Data interpretation In general, XPS spectra are featured as a plot of intensity vs decreasing binding energy (BE). A typical spectrum consists of well dened narrow lines (core level lines) over a electronic background that increases with increasing BE. The background is formed by electrons that have suered inelastic scattering processes after their emission from their original atom. The background is continuous because the energy loss processes are random and multiple. 9

10 2.4.1 Types of lines Several types of lines are observed in a XPS spectra. Some appear always, some depend on the particular physical and chemical characteristics of the sample, and some are instrumental in nature. Here we show a selection of the eects relevant to this experiment: Photoelectron lines These are the narrowest lines observed in the spectra. They are generally symmetric, although pure metals can exhibit considerable asymmetry due to interaction of photoelectrons with conduction electrons. Auger lines The relling of the inner shell vacancy left by the emission of an electron can be done by radiative (uorescence) or non radiative(auger) processes. However, in the energy range where we will work ( 1 kev), Auger will be the predominant lling mechanism. Radiative processes compete with Auger only for transition energies of 10 kev. Therefore, in addition to photoelectrons emitted through the photoeect, Auger electrons will be emitted because of the relaxation of the excited ions remaining after photoemission. In an Auger process (which occurs roughly seconds after the photoelectric event), an outer electron falls into the inner orbital vacancy, and a second electron is simultaneously emitted, carrying o the excess energy. The kinetic energy of the Auger electron is equal to the dierence between the energy of the initial ion and the doubly charged ion, and it is independent of the mode of the initial ionization. There are four main Auger series observable in XPS. They are the KLL, LMM, MNN and NOO series. Spin-orbit splitting Due to spin-orbit interactions (j=l±s), the p,d, and f levels split during ionization, leading to vacancies in the p 1/2 and p 3/2 (l=1), d 3/2 and d 5/2 (l=2) orbitals and so on. The relative intensities of the doublet components depend on the multiplicity of the states, given by the available amount of dierent magnetic states (dierent possible values of the quantum number m z : 2j + 1 dierent possible numbers). Therefore, the spin-orbit splitting ratio is 1:2 for p levels, 2:3 for d levels and 3:4 for f levels. Energy loss lines With some materials, there is the possibility of losing certain amount of energy due to interaction between the photoelectron and other electrons in the surface region of the sample. In insulators these lines are not very intense, and can be rather broad. However, in metals the eect is much more dramatic. Energy loss to the conduction electrons occurs in well dened quanta characteristic of each metal. These plasmons arise from group oscillations of the conduction electrons. The photoelectron line is mirrored at intervals of higher BE with reduced intensity. The energy interval between the main peak and the loss peak is called the plasmon energy. The so-called bulk plasmons are the more prominent of these lines. A second series, the surface plasmons, exists at energy intervals determined approximately by dividing the energy of the bulk plasmon by 2. For insulators, usually only the rst plasmon peak is seen. 10

11 Valence bands These signals are produced by photoelectron emission from molecular orbitals and from solid state energy bands. Dierences between insulators and conductors are specially noted by the absence of presence of electron from conduction bands at the Fermi level Chemical Shift The core electron of an element has a unique binding energy, like a ngerprint. Thus almost all elements except for hydrogen and helium can be identied by measuring the binding energy of its core electron. Furthermore, the binding energy of core electrons is sensitive to the chemical environment of element. The inuence of the chemical environment can be seen as a shift of the binding energy of the corresponding XPS peak, ranging from 0.1 ev to 10 ev. This eect is called chemical shift. The chemical shift is therefore the dierence between the energy of a photoelectron line for an element in a specic compound and the corresponding energy for the element in its pure state. The chemical shift is frequently reported in publications and handbooks. As an example you can see in Figure 3 the C1s core level of a PET (Polyethylene terephthalate) sample. The carbon atoms of the polymer see a dierent chemical environment depending on their position within the polymer chain. Dierent chemical environments result in dierent chemical shifts, which we can analyze also quantitatively. This fact is the essence of the utility of XPS for chemical analysis, and the reason why it is also known as electron spectroscopy for chemical analysis (ESCA). Note: you might nd apparent shifts in binding energy positions that are not due to a chemical shift! This refers to the two eects discussed previously: oating Fermi energy level of the sample, and sample charging (see Section 1). 2.5 Mathematical treatment of the data: Fitting procedure and chemical state identication The identication of chemical states depends primarily on the accurate determination of line energies. Once we have eectively determined the line positions, we can start digging into the chemical composition determination problem by looking at the provided literature. In order to get a precise value for the binding energy position of each peak, we will t the experimental data with a mathematical model. The peak shape for XPS spectra is best described by a convolution of the natural line width, the width of the x-ray line which created the photoelectron line, and the instrumental contribution. Additionally, in some cases we have to consider an asymmetry factor (in the case of metallic components.). With the Multipak program, we will however simulate the line prole with a product of a Gaussian and a Lorentzian lines. The Gaussian component accounts for the instrumental broadening, chemical disorder, charging, etc; the Lorentzian function accounts for the nite lifetime of the core-hole in the photoionization process. The mathematical analysis 11

12 Figure 3: 1s core level peak of a PET (Polyethylene terephthalate) polymer sample. with Multipak will give you the following information (necessary for the discussion of the data) BE position, FWHM, percentage of Lorentz value the relative intensities of the peaks you have tted (with Anotate). Now you can try to determine the chemical composition of the sample, by comparing the BE energies that you have determined, with BE found in literature (use handbook and if necessary, additional literature). Compare the situation that you see before and after plasma treatment. 12

13 Quantitative analysis Since the number of photoelectron of an element is dependent upon the atomic concentration of that element in the sample, XPS is used to not only identify the elements but also quantify the chemical composition. In many cases it is important to be able to quantify the relative concentrations of the various constituents of the sample surface. In order to do this we will use the peak areas together with tabulated sensitivity factors. The program Multipak will perform this quantitative analysis automatically, when you press the % symbol on the top right area of the window. For detailed information on the procedure behind this simple mouse click, you can read the rest of this section. For a sample homogeneous in the analysis volume, the number I of photoelectrons per second in a specic spectra peak is given by: I = nfσθyλat n: Number of atoms of the element per cm 3 of the sample. f: X-ray ux in photons/cm 2 s σ: Photoelectric cross section for the atomic orbital of interest in cm 2. θ: Angular eciency factor for the instrumental arrangement based on the angle between the photon path and the detected electron. y: Eciency in the photoelectric process for formation of photoelectrons. λ: Mean free path of the photoelectrons in the sample A: Area of the sample from which photoelectrons are detected T: Detection eciency for the electrons emitted from the sample We are interested in n, which we can write as: I n = fσθyλat I S. The factors contained in S are either a characteristic of the spectrometer, or have a weak dependence from sample to sample, so that, if we have an homogeneous sample, we can establish the following relationships between peaks from dierent elements: n 1 n 2 = I 1S 1 I 2 S 2 where, for each spectrometer, it is possible to determine the values of S for all elements. Note that these concrete values will be applicable only to a particular spectrometer, since they also need to be corrected by a transmission function typical of each machine. 13

14 So now, a general expression for determining the atom fraction of any constituent in a sample C x can be written simply as: 2.6 Report C x = n x ni = I x/s x Ix /S x For the report, discuss the following points about your measurements: Identify all the features observed in the survey spectra. Using the core level spectra, discuss quantitatively the chemical composition of the sample before and after the plasma treatment. Apart from the discussion relevant to the samples that you have investigated, you can use the following set of questions to help you shape your report: What is the photoeect? What is the Auger-Eect? What kind of information can we get from XPS? What features do we expect to see in a XPS spectrum? What is the origin of these structures? What is the approximate range of detectable BEs of all detectable lines by Al radiation? Why does the background level increase abruptly after each line? What is the so-called chemical shift in XPS? What is the origin of charging problems in XPS measurements? What can we do against it? What are the diculties involved in the determination of a binding energy reference in the case of non conducting samples? References [1] J F Moulder, W F Stickle, P E Sobol, K D Bomben. Handbook of X-ray Photoelectron Spectroscopy. Published by ULVAC-PHI,