ESCALAB 250: High Performance Imaging XPS

Transcription

1 Application Note: ESCALAB 250: High Performance Imaging XPS Key Words Surface Analysis High Resolution High Sensitivity Multitechnique Parallel Imaging Introduction The Thermo Scientific ESCALAB 250, Figure 1, is a high-performance, imaging XPS system, capable of multitechnique analysis and which, optionally, has an extensive and versatile range of optional sample preparation facilities. X-ray Sources ESCALAB 250 is equipped with a monochromatic X-ray source. An optional twin anode source is also available. The twin-crystal, microfocusing monochromator has a 500 mm Rowland circle and uses an Al anode. The user is able to select any size of X-ray spot at the sample over a range of 120 µm to 650 µm. Some of the advantages of a microfocusing monochromator are: Small area XPS measurements can be made without reducing the sensitivity of the spectrometer. Consequently, the sensitivity is improved and the analysis time is reduced. Only that part of the sample being analyzed is exposed to X-rays. Sensitive samples do not become damaged in areas remote from the analysis position. In angle-resolved measurements, the whole of the X-ray spot is within the analysis area over the range of sample angles. With use, the aluminum coating on the monochromator anode will wear, affecting the intensity of the X-rays and therefore the sensitivity of the instrument. When this happens on instruments from other suppliers, the anode has to be replaced. On ESCALAB 250, the anode can be moved to expose a fresh area to the electron beam, increasing the lifetime of the anode significantly. The total anode movement of 25 mm can be accomplished without breaking the vacuum. Figure 1: ESCALAB 250 Description Its key features are: High sensitivity spectroscopy Small area XPS Image resolution <3 µm Depth profiling capability Angle resolved XPS Microfocusing monochromator Multi-technique analytical versatility Multiple sample preparation options Automated, unattended analysis Multiple sample analysis Avantage, Windows -based data system. Lens/Analyzer/Detectors It is the lens/analyzer/detector combination on ESCALAB 250 that makes the instrument unique for both imaging and small area XPS. The system consists of A set of input lenses, including a magnetic immersion lens A 180 hemispherical energy analyzer An array of 6 channel electron multipliers (which are the detectors for spectroscopy measurements, see Figure 2) An output lens A pair of channel plate detectors for imaging XPS

2 Figure 2: Schematic diagram of ESCALAB 250 operating in its spectroscopy mode The input lens system is equipped with two iris mechanisms, one controls the field of view and the other controls the angular acceptance of the lens. When using ESCALAB 250 for imaging, photoelectrons from the whole of the field of view are detected simultaneously (see Figure 3). Electrons of a given kinetic energy are focused on the channel plate detector to produce a direct image of the sample without scanning. This is known as parallel imaging. Figure 3: Schematic diagram of ESCALAB 250 operating in its imaging mode With some other instruments, an XPS map of the sample is produced pixel by pixel by scanning the X-ray beam, the acceptance area defined by the lens or the sample stage. Parallel imaging on ESCALAB 250 provides XPS images of the highest spatial resolution and is generally faster than other methods. There are many advantages of parallel imaging over scanned or serial acquisition methods but the following are the most important: Speed. Electrons are collected from the whole of the field of the imaged area and not point by point. This improves the signal to noise ratio and allows a given image to be acquired in a much shorter time (if a 256 x 256 pixel image is required then there are effectively 65,536 parallel collection channels). Interactivity. The operator can see the image forming from the whole field of view as it is being acquired. The time for the experiment can therefore be extended or reduced during the acquisition. Using serial acquisition, the experiment duration can only be multiples of complete frame times. This can further increase the time for an experiment. Resolution. Using the serial acquisition, the best spatial resolution is determined by the minimum field of view of the lens or the minimum spot size of the X-ray source. Therefore, field of view and spatial resolution are strongly coupled. Using parallel imaging, these are independently controllable. Thus, ESCALAB 250 can produce images whose resolution is in the region of 1 µm whereas the best that can be achieved from other instruments is ~10 µm. Physical Imaging. The parallel imaging facility allows real-time physical imaging. This is achieved by using a flood electron source and operating the spectrometer in its imaging mode. This produces a physical or topographic image of the sample. The detection of the scattered electrons uses the same optics and detector as the XPS analysis, and so the image formed by the scattered electrons is from precisely the same area as the XPS image and with the same spatial resolution. Thus, the area from which the XPS image is to be acquired is located rapidly and with great precision. Small Area XPS. Both the size and the position of a small area XPS analysis can be set rapidly and precisely using the physical image. The Magnetic Immersion Lens ESCALAB 250 is fitted with a magnetic immersion lens, which increases the sensitivity of the instrument by increasing its angular acceptance. Alternatively, for a given signal intensity, the spatial resolution is improved. An instrument equipped with this type of lens has a wide range of fields of view. Using only the electrostatic lens system, the field of view can be as large as 8 mm but, by using the combination of the electrostatic and magnetic lenses, spectra can be obtained with a spatial resolution below 20 µm. ESCALAB 250 can be operated with the magnetic lens switched off. This is important, especially when analyzing some magnetic samples or when using the instrument for Auger electron spectroscopy.

3 Small Area XPS In the range down to 120 µm, the area for analysis may be defined by the size of the X-ray spot (i.e. it is source-defined small area XPS). The relative merits of this approach are described in the application note AN31033 Below 120 µm the analysis area can be defined by the irises in the lens (lens-defined small area analysis). This extends the lateral resolution for spectroscopy down to 20 µm. The Flood Electron Source An electron source co-axial with the analyzer input lens, is provided with the instrument. This is used for charge compensation when analyzing non-conducting samples using the monochromatic X-ray source. The co-axial geometry of this source is ideal for XPS charge compensation because shadowing effects are eliminated (any feature visible to the transfer lens is also visible to the electron source). This geometry is essential when the magnetic lens is being used. Ion Gun In common with the other Thermo Scientific surface analysis instruments ESCALAB 250 is fitted with the EX05 ion gun. This is described in more detail in application note AN Digital Control All of the major analytical functions of ESCALAB 250 are controlled from the Windows-based Avantage data system. The Sample Sample Size ESCALAB 250 instruments have a fully motorized sample manipulator with 4 axes of movement. A fifth axis, rotation, is available as an option. The range of motion is 50 mm in X, 20 mm in Y and 12 mm in Z, each with 5 µm resolution. If fitted, the motorized rotation is continuous. Some of the sample holders available for ESCALAB 250 are shown in Figure 4. These include sample holders for heating and cooling the sample. Sample Alignment All axes of movement on the sample stage are controlled by the Avantage data system. As can be seen in Figure 1, there is a zoom microscope fitted to the instrument and aligned with the analysis position. The field of view from this optical system is in the range of 500 µm to 3 mm. The analysis position is accurately aligned with the cross hairs at the center of the graticule when the image is in focus. To align a feature for analysis it must be visible at the center of the graticule. This can be done by moving the sample using the tracker ball or pointing to a position on the optical image using the mouse pointer. Alternatively, the fast parallel imaging on ESCALAB 250 can be used to align the sample in real time, either using an XPS peak or using electrons elastically backscattered from an electron gun. Vacuum System The analysis chamber is constructed from 5 mm thick mumetal to maximize the efficiency of the magnetic shielding. The analysis chamber on all Thermo Scientific XPS instruments are constructed this way. The use of internal or external shields, the shielding method used by other XPS manufacturers, is not so effective. The chamber is pumped using both a turbomolecular pump and a titanium sublimation pump. This arrangement allows the analysis chamber to achieve a vacuum better than 5 x mbar. As well as ports for the energy analyzer and X-ray sources, the chamber is equipped with ports for Auger electron spectroscopy, UV lamp, ion guns, etc. This allows the system to be used as a true multi-technique facility. The entry lock is pumped using a turbo molecular pump backed by a rotary pump. This system is also used to provide differential pumping for the ion gun. The whole vacuum system can be baked in order to achieve the ultimate base pressure. Multi-technique Capability ESCALAB 250 is designed to accommodate other analytical techniques without compromise to the XPS performance. The power supplies for the lenses and analyzer are reversible and so, provided the standard ion gun is present, ISS (ion scattering spectroscopy) is always available. In addition, the following techniques are options on ESCALAB 250: XPS with non-monochromatic X-rays AES (Auger electron spectroscopy) UPS (Ultra-violet photoelectron spectroscopy) Figure 4: A selection of the sample holders available for use with ESCALAB 250 Preparation Chambers The base system is fitted with a simple air lock, which does not allow for any preparation facilities (see Figure 1). If preparation facilities are required then one of a range of PREPLOC chambers can be fitted. Figure 5, shows two examples of PREPLOC chambers.

4 Figure 7: A UHV Carousel Preparation chamber is available on ESCALAB 250. This chamber allows a number of radial preparation stages to be attached providing the basis for a very large range of in-vacuum preparation and analysis techniques Performance Figure 5: Two examples of PREPLOC chambers (combined preparation and entry lock chambers). The upper example is fitted with a parking stage and has ports for other accessories. The lower example is fitted with a heating stage, a gas cell and an ion gun. If a UHV preparation facility is required, an additional chamber can be supplied so that the preparation chamber is not exposed to the atmosphere when samples are introduced, Figure 6. Specifications The ESCALAB 250 specifications are defined under typical operating conditions. The microfocus monochromator is operated at the maximum power appropriate for each spot size. Count rate scaling is therefore not necessary. The sample is placed on the standard sample holder and at an angle such that its surface normal is parallel to the input lens axis. Sensitivity and spot size specifications are both achieved with the sample in this orientation. When comparing specifications from different instruments, it is important to ensure that the conditions under which the data are collected are the same. Spectroscopy The combination of an efficient transfer lens, a magnetic immersion lens, multi-channel detector and high intensity X-ray source ensures high sensitivity in all XPS applications. The detector is a multi-channeltron array providing maximum linear dynamic range Sensitivity Extremely high sensitivity and resolution are key features of ESCALAB 250. Figure 8 shows a survey spectrum of barium oxide, acquired in only 10 seconds. Note both the count rate and the extremely good signal-to-noise ratio. Figure 6: This ESCALAB 250 is equipped with both a PREPLOC and a UHV preparation chamber For more sophisticated sample preparation or analysis requirements, a UHV carousel preparation chamber is available, Figure 7. This consists of a large, disc-shaped chamber containing a series of radial ports. These ports can have further chambers added which can be designed to accommodate a wider variety of techniques. This chamber also has a fast entry air lock so that the preparation chamber is not exposed to atmospheric pressure during sample loading. Figure 8: XPS survey spectrum of barium oxide acquired in only 10 seconds using the monochromated X-ray source. Valence band and Fermi edge spectra may be acquired in just a few minutes.

5 Small Area XPS Small area XPS analysis, to < 20 µm, is fast and precise. Figure 9 shows the image from the zoom microscope of a set of aluminum bond pads with one pad centrally located in the analysis position. Figure 10 shows the Al 2p region of the XPS spectrum, acquired using the microfocus monochromator at a spot size of 120 µm. Figure 11: C 1s spectrum of polypropylene and flame treated polypropylene, the latter also showing the results of peak fitting Figure 9: Image of a set of aluminum bond pads on silicon, as viewed through the zoom microscope fitted to ESCALAB 250 A 60-point, 20 mm line scan was performed automatically under computer control across a defect area of an unmasked, polyester coated, paper sample. Spectra typical of the polyester coating are visible at most points, while spectra typical of the uncoated paper are seen from the defect area. This application illustrates the stability and effectiveness of the charge compensation system in unattended operation over a large analysis area. Figure 10: Al 2p spectrum from one of the bond pads in Figure 9. This was acquired using the monochromator set to 120 µm spot size. Note also, the excellent energy resolution in the elemental Al peaks which were fitted using asymmetric peak fitting. This spectrum was acquired using a low pass energy to obtain excellent energy resolution in the spectrum. The full width at half maximum (FWHM) of the elemental aluminum is < 0.3 ev. To obtain a good fit to the experimental data, asymmetric peak fitting was used. Asymmetric peak fitting is a feature of the Avantage data system. Insulating Samples Insulating samples are easily analyzed using the electron flood source for charge compensation. High-resolution polymer spectra enable C 1s envelopes to be fitted with confidence and precision, Figure 11. Figure 12: A C 1s spectrum line scan over a defect on a coated paper and a concentration profile of the two components present Energy Resolution For the Ag 3d 5/2 peak from clean silver, the guaranteed resolution is <0.45 ev, FWHM. The instrument can demonstrate better resolution on peaks whose natural line width is smaller. For example, Figure 13 shows a FWHM value of 0.29 ev (see application note AN31068). Figure 13: Part of the XPS spectrum from WSe 2 showing a FWHM of 0.29 ev

6 The Fermi edge data in Figure 14 was acquired on ESCALAB 250 using a microfocused monochromator with the sample at room temperature. The Fermi edge width is defined as the energy difference between 20% and 80% of the intensity measured below the Fermi edge. The relationship between the Fermi edge resolution and the intensity of XPS signals is discussed in detail in application note AN31007, which compares core level and valence band performance on silver. Figure 14: Fermi edge and Ag 4d region of a silver XPS spectrum measured at a pass energy of 3 ev. The measured Fermi edge width is <0.19 ev using the 20%-80% definition Parallel Imaging Parallel imaging produces rapid, high-resolution XPS images. ESCALAB 250 is the only instrument to use the same input lens and analyzer for both parallel imaging and spectroscopy. This eliminates the need to align two sets of electron optics and provides unambiguous location of features to be analyzed. The electron flood gun facilitates physical imaging of the sample, permitting rapid sample alignment and identification of the features to be analyzed. < 3 µm imaging resolution Variable field of view (120 µm - 8 mm) Imaging of large and small features Figure 15: Au 4f line scan over the image (above) showing a resolution of 2.8 µm Elemental Imaging In Figure 16 and Figure 17, two samples of metal brazing were analyzed. The overlays of the silver (green) and copper (red) show the differences between a good and poor quality braze. Image Resolution ESCALAB 250 produces an image resolution of < 3 µm. Figure 15 shows parallel images from a gold feature on glass which can be used to define small areas from which spectra can be acquired. The resolution of 2.8 µm is measured from the line scan. Figure 16: Image of a poor quality braze Figure 17: Image of a good quality braze

7 In the good quality braze, the silver more effectively wets the copper forming a strong joint. Where the wetting is less effective, the silver forms a single layer between the copper surfaces. Chemical State Imaging Figure 18 shows a set of images taken from the edge of a razor blade coated with a fluorocarbon lubricant. These C 1s images show the distribution of C-C and C-F on the blade edge and show that the lubricant distribution is not uniform and does not reach the edge of the blade. Indicating that the blade quality is poor. Azimuthal rotation of the sample during sputtering (optional on ESCALAB 250) minimizes the development of sputter induced topography. Figure 19 shows an example of a profile from ESCALAB 250. This is from a Ta/Si multi-layer on silicon. and shows excellent depth resolution (at a depth of 450 nm, the interface width is less than 6 nm). Figure 19: Depth profile from a Ta/Si multi-layer Figure 18: C 1s images from the edge of a razor blade. Top image is C-C, the middle image is C-F and the lower image is an overlay of the other two. Depth Profiling ESCALAB 250 provides concentration depth profiles from a small area with high sensitivity. Small sputtered areas ensure high etch rates and short acquisition times. Many features of ESCALAB 250 have been developed for optimum sputter profiling performance: The EX05 ion gun may be operated at high current for maximum profiling speed and at low energy (down to 100 ev) for optimum depth resolution Multi-sample sputter profile acquisition allows unattended operation for maximum sample throughput The data system includes Target Factor Analysis (TFA) and Linear and Non-Linear Least Squares Fitting (LLSF and NLLSF). These functions are fully integrated and ensure that the maximum possible chemical information is extracted from every profile. Angle Resolved XPS For very thin surface layers, ARXPS is the technique of choice. ESCALAB 250 features a variable aperture allowing the analyst to optimize the angular acceptance of the analyzer for each sample. Using the microfocused monochromator, photoelectrons from the whole X-ray spot are analyzed throughout the tilt range. This means: Peak resolution remains constant Peak position remains constant. Figure 20 shows the results of an ARXPS analysis of hafnium oxide on silicon dioxide on silicon. Two oxygen species are present, one at high concentration near to the surface, associated with Hf, and one in the bulk material, associated with silicon. Figure 20: Results of an ARXPS measurement on HfO 2 on SiO 2 on Si

8 Using the multi-overlayer thickness calculator, which is an integral part of the Avantage data system, the HfO 2 layer in this sample was found to be 1.6 nm thick and the SiO 2 was 0.58 nm. ISS All of the appropriate power supplies on ESCALAB 250 have reversible polarity. This allows the instrument to be used for ion scattering spectroscopy. The ISS technique is sensitive only to the top atomic layer of a sample. ISS measurements are made using the EX05 ion gun (fitted as standard) as the primary ion source. The spectrum, Figure 21, illustrates the high resolution that can be achieved using this technique on ESCALAB 250. Figure 22: Twin anode X-ray source (yellow) fitted to an ESCALAB 250. Figure 23: Comparison of copper XPS spectra acquired using an Al anode and a Mg anode Auger Electron Spectroscopy The optional Thermo Scientific FEG1000 field emission electron gun allows AES spectra and images to be acquired with a spatial resolution of <95 nm. The Avantage data system controls all aspects of the FEG 1000 operation. The gun is mounted in the optimum position on the chamber, such that its axis is orthogonal to the sample tilt axis, Figure 24. Figure 21: Part of the ISS spectrum from a phosphor bronze surface showing the separation of the two copper isotopes. This spectrum was acquired using argon ions. Multi-technique Capability In addition to ISS, which is present as standard on ESCALAB 250, the instrument was designed to accommodate a number of optional analytical facilities. Non-Monochromated XPS The sensitivity and resolution of the monochromator make it the first choice X-ray source for routine analysis. However, a non-monochromatic, twin anode source can be used when there are interferences with Auger transitions or when large areas need to be analyzed. This optional source is usually supplied with aluminum and magnesium anodes but other materials are available if required. Figure 22 shows the position of the X-ray source on the instrument while Figure 23 compares the XPS spectrum of copper acquired using the Al anode with that using the Mg anode. The shift of the Auger peaks on the binding energy scale is clear in this Figure. Figure 24: FEG1000 field emission electron gun (yellow) fitted to an ESCALAB 250 Figure 25 shows an SEM image of part of the surface of a copper nickel alloy. The SEM revealed the presence of surface corrosion pits and was used to define the points for the subsequent Auger analysis (P1 and P2 on Figure 25). Figure 25: SEM image of the surface of a copper-nickel alloy showing the presence of corrosion pits. The SEM was acquired using the FEG1000 field emission electron gun. Auger analysis, again using FEG1000, showed chlorine to be associated with these pits.

9 UPS An optional ultra-violet source fitted to ESCALAB 250 can be operated either as a source of He I or He II radiation. The position of the high-intensity UV lamp on the ESCALAB 250 chamber is shown in Figure 28. Figure 26: Spectra from the points labeled P1 and P2 on Figure 25 This was confirmed by the Auger maps shown in Figure 27. Figure 28: The high intensity UV lamp (yellow) fitted to an ESCALAB 250 The He I valence band spectrum shown below was acquired from a silver sample. Figure 27: Scanning Auger maps of Cl (red) and Cu (green) confirming the presence of chlorine in the corrosion pits Figure 29: He I spectrum of silver Figure 30: The silver Fermi edge spectrum acquired from silver at room temperature showing a resolution of 98 mev

10 Configuration ESCALAB 250 comprises a UHV mu-metal analysis chamber fitted with: Electron Analyzer Double-focusing full 180 spherical sector analyzer Magnetic and multi-element electrostatic input lenses for spectroscopy Multi-channel spectroscopic detector High resolution 2-D image detector Microfocused Monochromated XPS Facility 0.5 meter Rowland circle monochromator Digitally controlled microfocused electron gun and multi-position aluminum anode for extended life Two toroidal quartz crystals Flood Gun Charge compensation Electron imaging Digital control Ion Source Digital control Depth profiling Sample cleaning Secondary electron imaging for rapid and accurate alignment ISS Automated 4-axis Sample Manipulator Interfaced to the data system Avantage Data System Package Instrument control Data acquisition and processing Multi-sample, multi-point data acquisition Unattended data acquisition Options Twin Anode Non-monochromated XPS Facility Dual anode (MgKα /AlKα) X-ray source with linear motion drive Digital control Other anode coatings available UV Photoelectron Spectroscopy (UPS) Facility High intensity UV lamp with two-stage differential pumping Helium gas admission system with high precision leak valve Field Emission 95 nm Electron Gun for AES/SEM/SAM FEG1000 electron gun (Schottky type field emission source) Digital control Source ion pump SEM detector System vibration isolation Sample Rotation 5-axes sample manipulator including motorized azimuthal rotation for enhanced depth profiling resolution Full computer control of all 5 axes Sample Heating Sample holder with integral heater and thermocouple Sample Cooling Liquid nitrogen/chilled gas circuit PREPLOC Combined Fast Entry and Preparation Chamber Ports available for options including sample heating and cooling probe, sputter cleaning ion gun and sample scraper UHV Preparation Chamber with Separate Fast Entry Chamber Maintain preparation chamber at UHV during sample introduction In addition to these offices, Thermo Fisher Scientific maintains a network of representative organizations throughout the world. Africa Australia Austria Belgium Canada China Denmark Europe-Other France Germany India Italy Japan Latin America Middle East Netherlands South Africa Spain Sweden/Norway/ Finland Switzerland UK USA Thermo Fisher Scientific Inc. All rights reserved. Windows is a registered trademark of Microsoft Corporation. All trademarks are the property of Thermo Fisher Scientific Inc. and its subsidiaries. Specifications, terms and pricing are subject to change. Not all products are available in all countries. Please consult your local sales representative for details. VG Systems Ltd. Trading as Thermo Fisher Scientific, East Grinstead, UK is ISO Certified. AN31063_E 04/08M Part of Thermo Fisher Scientific