Micro-XRF excitation in an SEM

Transcription

1 X-RAY SPECTROMETRY X-Ray Spectrom. 2007; 36: Published online 8 May 2007 in Wiley InterScience ( Micro-XRF excitation in an SEM M. Haschke, 1 F. Eggert 2 andw.t.elam 3 1 IfG Institute for Scientific Instruments GmbH, Berlin, Germany 2 IAP Institut für Angewandte Photonik e.v., Berlin, Germany 3 EDAX Inc, Mahwah, New Jersey, USA Received 29 September 2006; Revised 22 February 2007; Accepted 22 February 2007 Electron microscopes are often used for position-sensitive elemental analysis of non-homogeneous material by electron-probe-micro-analysis (). Due to the high spectral background, this method is limited in sensitivity. The availability of x-ray optics allows the generation of focussed x-ray beams with spot sizes in the micrometer range. The x-ray excited spectra have a better peak-to-background ratio and, therefore, a higher sensitivity. Further, the excitation efficiency for both electrons and photons varies with the atomic number. Therefore, light elements can be analysed better with electron excitation. The combination of analytical results from electron and x-ray excitation, therefore, should improve the accuracy of quantification. The paper presents a m-xrf excitation unit for scanning electron microscopes (SEMs) and describes the quantification model that is prepared for this device. The quantification is performed for few samples. These results show that the standardless quantification give a good accuracy. Especially in case of presence of light elements like C, O or N in the matrix, the XRF results can be improved by consideration of results from. Copyright 2007 John Wiley & Sons, Ltd. INTRODUCTION The requirement for spatially resolved analysis of chemical composition is increasing because more and more inhomogeneous materials are used in modern technologies. Several analytical methods with high spatial resolution are available, but all of them have certain drawbacks they are destructive (such as LA-ICP-MS); they need an extensive experimental set-up (such as PIXE or SIMS); or they have insufficient sensitivity for trace elements (such as electron probe microanalysis ()). But synthesis of the information from all of these methods allows good characterization of a material. A very common tool for spatially resolved analysis is the scanning electron microscope (SEM) that not only gives topological information via surface images but also analytical information via. Unfortunately, the sensitivity of is limited due to the overlap of fluorescence lines with the bremsstrahlung background from decelerated electrons. X-ray fluorescence in general, has better sensitivity for elements with higher atomic number, but this method has typically no spatial resolution. The availability of x-ray optics opens up new analytical possibilities. They can concentrate x-rays to a small sample area and allow micro-x-ray fluorescence with spatial resolution in the range of a few tens of micrometers. In comparison to, the spatial resolution is poorer but the sensitivity for trace elements is higher by factors of The higher sensitivity is a result of a reduced spectral background excitation with photons does not generate bremsstrahlung. For high-energy Ł Correspondence to: M. Haschke, IfG Institute for Scientific Instruments GmbH, Berlin, Germany. haschke@ifg.adlershof.de Paper presented as part of a special issue of papers from the 2006 European X-ray Spectrometry Conference, Paris, France, June, Part 4. fluorescence lines there is an additional reason for improved sensitivity the excitation efficiencies are higher with x-ray excitation. X-ray capillary optics has been used in several instruments for the excitation of small sample areas. 1 3 These were mostly specialised instruments dedicated to -XRF. There were also several ideas proposed for excitation of small sample areas in an SEM with x-rays, for example, by having a film as a transmission target between the electron beam and the sample. But in this case the spatial resolution is limited due to the isotropic emission pattern of the x-rays. It is possible to generate a micro-x-ray beam using capillary optics in an SEM and complement its capabilities by a more sensitive analytical method. This is realised with the imoxs. 4 In this case, a micro-focus x-ray tube in combination with a polycapillary lens is adapted to an SEM. The focussed x-ray beam has to be adjusted to match the location where the electron beam hits the sample. In that case, it is possible to view the x-ray analysis region with the SEM. Because the x-ray cannot be controlled electronically like the electron beam it is necessary to change the sample position with the stage. But this is typically not a problem with modern SEMs, as they have high-precision motorised stages. -XRF is an excellent extension to and adds some other advantages. First, the sample preparation for x-ray excitation can be much simpler samples need not be electrically conductive and the surface preparation requirements are reduced due to the higher penetration depth for x-rays. Second, the sensitivity for heavy element trace analysis is enhanced due to the improved peak-to-background ratios and the higher excitation efficiencies for heavy elements. On the other hand, can deliver information about elements that cannot be detected with XRF light elements such as B, C, N, O or F. As a third benefit, it should be possible to use the information from about these light elements to improve Copyright 2007 John Wiley & Sons, Ltd.

2 Micro-XRF excitation in an SEM 255 the compensation for matrix effects in XRF. The combination of some sensitivity to light elements (that are very often in high concentrations and can be measured with ) and the high sensitivity for trace amounts of heavy elements (via XRF) should give much better quantification results than using either method alone. This paper presents a description of the experimental setup for micro x-ray fluorescence in an SEM, discusses a model for standardless quantification in XRF with consideration of the influence of the x-ray optic on the excitation spectrum, and gives examples for the achievable accuracy for quantification. Both were samples where all elements can be measured with XRF, and those requiring a combination of and XRF quantification. EXPERIMENTAL The micro-beam source for SEMs starts with a micro-focus x-ray tube from rtw Warrikhoff. 5 The target spot size is about 50 ð 50 µm. The target material is selected according to the analytical requirements. Rh or Mo are preferred due to their low overlap with the lines of typical analyte elements. The tube radiation is captured by a polycapillary lens with a large acceptance angle and focussed onto the sample surface. These lenses must be individually adapted to the sample chamber of each SEM. They are typically quite long due to the large dimensions of the SEM sample chamber. A length of 300 mm with a focus-to-focus distance of approximately 400 mm is typical. For these long lenses the smallest focal spots on the sample are not achievable but they are in the range of a few tens of micrometres. The lens must be aligned to the x-ray tube spot to get high excitation intensity. The tube with x-ray optics pre-aligned is adapted to the SEM via a special flange that allows adjustment of the x-ray spot to the position of the unsteered electron beam. This is important in order to locate the XRF excitation region accurately in the SEM image. This adjustment is achieved by tilting the tube together with the optics with respect to the SEM port. The best analytical performance occurs when the excitation unit has an azimuth angle of approximately 90 relative to the x-ray detector of the SEM. In this case, the scatter of excitation radiation in the direction of the detector is reduced. Excitation parameters for the x-ray tube can be up to 50 kv and 30 W in steps of 1 kv and 10 µa. The stability of both parameters is better than 0.2%. A pressure sensor interlock attached to the tube flange fulfills the requirement of door contacts for radiation protection. The complete unit is displayed in Fig. 1. QUANTIFICATION Quantification for x-ray fluorescence is strongly influenced by matrix interactions. These interaction processes are well understood and can be accurately described by physical models. These models can be augmented by calibration curves for special matrices and limited concentration ranges with the help of reference samples. This yields very high accuracy and is practical for quality control or any application where samples of similar composition have to be analysed Figure 1. The imoxs Source with HV-generator. repeatedly. But for a wide range of sample compositions this method is not very efficient because many sets of reference samples are necessary. Particularly for inhomogeneous samples, where the sample composition can change with every measurement position, this method is not practical for fast and efficient analysis. Therefore, an approach was used which is based on the Sherman equation 6 (1) that describes the interaction of an incident x-ray beam in the sample, the generation of fluorescence photons, and their absorption on the way to the sample surface. 7 This method is often referred to as the fundamental parameters method. n i D G ð E with: n i intensity of the element i G geometrical factor (S 1)/S jump ratio p i transition probability ω i fluorescence yield (E) attenuation coefficient µ(e) mass absorption coefficient 1,2 incident/take-off angle n o (E) excitation intensity c i concentration of the element i S 1 S c i Ð p i Ð ω i Ð i E E C E Ð n 0 E Ð de 1 i sin 1 sin 2 In this formula, only first order interactions are considered. But in the calculations, second order interactions have also been taken into account. Third order effects can be neglected. Quantification is performed by an iterative solution of the Sherman equation. The concentrations are changed until the predicted and measured intensities have a difference of less than 0.1%. For accurate analytical results it is necessary to use a good data base for fundamental parameters. 8 First tests of the functionality of this approach show good accuracy for both alloys and oxides. 7 For oxides, the content of oxygen is included in proportion to the stoichiometric relation of oxygen to the observed elements.

3 256 M. Haschke, F. Eggert and W. T. Elam x-ray fluorescence. These are elements with an atomic number less than 11 (Na). One possibility is the inclusion of oxygen by use of the stoichiometric relation to the observed elements. But if the sample is not an oxide or more than one light element is present in the sample, another procedure must be used. It is possible to consider the light elements that are not measured with XRF in the Sherman relation if their concentrations are known and entered separately. In that case the matrix interaction is calculated correctly and these elements are used for the normalisation of concentrations. This should improve the reliability of quantitative results. TESTS OF THE QUANTIFICATION PROCEDURE Figure 2. Scattered tube spectra (PMMA) with different x-ray optics. For micro-fluorescence it is necessary to consider that the capillary optic will change the excitation spectrum. This change depends on energy as seen in Fig. 2, which displays the scattered spectra of different optics a collimator (as an example of a spectrum without the influence of an optic) and the spectra of a monocapillary and a polycapillary lens. These changes can be described by a transmission function of the optic that depends on energy and takes into account for n o in Eqn (1). A special problem arises from the fact that the transmission function is dependent on the adjustment of the optics relative to the x-ray tube spot. Therefore, it is necessary to include a procedure for determining the transmission function in the instrument itself. This can be done by measurements of the scatter spectra of both a collimator for the non-influenced spectrum and with the capillary optics. 9 Using this transmission function, it is possible to calculate the real excitation spectrum for any excitation conditions from a calculated tube spectrum. The tube spectrum is calculated via the algorithms of Ebel 10 and of Finkelnstein 11 for different tube voltages. The described quantification procedure also allows the inclusion of both reference samples as standards and known elements that are not measured with x-ray fluorescence. Quantification with standards If a suitable reference material is available it is possible to measure the intensities from this material and to calculate the expected intensities for the known concentrations. The comparison between calculated and measured intensities can be used to determine correction factors for every element that take into account changes in geometry (for example, due to different penetration depths), errors in the fundamental parameters, and inadequacies of the theoretical model. These correction factors can be used to improve the predicted intensities for unknown samples before comparison with measured intensities. It will be shown that this improves the accuracy by a factor of approximately 2. Light elements Very often it is necessary to analyse samples that contain elements that cannot be measured with energy dispersive Comprehensive tests of the described quantification procedure were performed by measurements on a series of reference samples. For that reason, a special software program for the imoxs was produced, the imoxs-quant. This program has the following functions: ž Input filter for different spectrum formats (of different EDS detectors, in particular the EMSA-format) ž Correction of spectrum artefacts like escape, shelf, and tail ž Peak identification and peak area calculation by both Bayes deconvolution 12 and peak fitting ž Quantification for both and XRF spectra Tests were performed for qualities of different alloys, i.e. for samples where all elements could be measured. The target of these tests was to determine if the correction for the influence of the capillary optics to the excitation spectrum was adequate. Another series of measurements were done using a combination of and XRF results with inclusion of the results into the calculation of matrix interactions for XRF. Results for alloyed samples Approximately 30 different Cu- and Fe- (steel) alloy references were analysed. Measuring conditions were 40 kv, approximately 600 µa and a measuring time of at least 100 s. A polycapillary lens with a length of approximately 300 mm was used to focus the excitation beam. In sum, approximately 100 single element determinations were performed for minor and major components in these tests. The results are shown in Fig. 3. It shows that the relative deviations of the analytical results from the given concentrations are small for high concentrations but can be higher for minor components. This is to be expected. Further it is seen that using a single standard improves the accuracy already. These results are summarised in Table 1, which gives the average relative deviation for minor and major elements with and without a standard. The average deviation for minors and majors are less than 10% and are improved by a factor of approximately 3 by using a standard. If only the majors are considered the situation is better i.e. the average deviations are less than 2.5% and the improvement by a standard is a factor of 1.5 for the examined reference samples. The average statistical error of these measurements is approximately 1% for minors and majors and 0.5%

4 Micro-XRF excitation in an SEM 257 Figure 3. Relative deviation of quantification results from given concentrations in dependence of concentration. Table 1. Average relative deviation Major &MinorC>5% Major C >30% Without standard With standard for majors only. This is still significantly lower than the average deviations i.e. the accuracy is not limited by the measuring conditions but by other errors (for example, sample preparation, incorrect description of matrix interaction, errors of fundamental parameters, and possibly others). The influence of the capillary optics on the excitation spectrum seems to be correctly considered. This is also demonstrated by another measurement. Some Cu-alloys were measured with different tube voltages. 30, 40 and 50 kv were used to excitate the same samples. For Cu and Zn, the results are displayed for several samples in Table 2. These results show again the very good accuracy of quantification and also that the model works very well for different tube voltages. Results using results for XRF matrix interaction To demonstrate the possibility of improving calculations of matrix interactions using results in a first step, several samples of different quality were measured a glass sample (to test the accuracy of oxygen determination in the sample), a low-alloyed steel with a relatively high carbon content (to test obtaining one element from ) a mineral that contains a high content of oxygen and some traces of both light and heavy elements, and a plastic sample with some heavy element contaminants. The spectra of the glass sample are displayed in Fig. 4. The measurement conditions were 20 kv and 2 na for electron excitation, and 40 kv and 350 µa for x-ray excitation. These spectra show very clearly the differences between electron and x-ray excitation. The electron-excited spectrum has a higher background, in particular for the low-energy part of the spectrum, and the fluorescence peaks in that part are stronger. For the energy range near 4 kev, the intensity of the fluorescence lines in both spectra is comparable the measurements were performed with similar dead time for both excitations and elements with higher atomic number but low concentrations can only detected with x-ray excitation. The quantification results for this sample are summarised in Table 3. It can be seen that the results for the main components are quite acceptable, but the low concentrations of heavy elements cannot be detected. On the other hand, XRF can detect these elements, but because oxygen cannot be measured, the quantification gives incorrect results, mainly due to the normalisation to 100% (imoxs-wt% column). If Figure 4. Spectra of a glass reference excited with electrons and x-rays. Table 2. Concentrations and deviations from given value for different excitation voltages Concentration Deviation Sample 30kV 40kV 50kV 30kV 40kV 50kV 368 Cu Zn Cu Zn Cu Zn Cu Zn

5 258 M. Haschke, F. Eggert and W. T. Elam Table 3. Quantification results for a glass reference in wt% imoxs wt% wt% stat. error wt% stat. error Wt.% O by stoich. C XRF O nd Mg Al Si P K Ca Ti Cr nd Fe Cu nd Ga nd Sr nd Zr nd Ba nd nd not detected oxygen is considered by the stoichiometric relation to every element and these concentrations are then converted back to pure element concentrations (wt% O by stoich. column) the results look better but still not satisfying, since the oxygen content is still too small. But if the quantification results for light elements that have a lower statistical error (bold figures in Table 3) are used from to improve the matrix interaction in XRF, the final result shown in the last column is achieved ( C XRF) which shows an excellent agreement with the given values in consideration of a standardless procedure. For a low-alloyed steel the results are displayed in Table 4. This steel has a relatively high content of approximately 2% of carbon. This element cannot be detected by XRF, but can quantify carbon. Again, some smaller concentrations of heavy elements are not detected with. Therefore, the Fe concentration is not correct due to the normalisation of all concentrations to 100% both for and XRF. Also, in using a standard for the XRF quantification, only the results for the traces are improved. But the Fe content is still significantly in error due to the lack of carbon. That means, quantifications using only or XRF give a Fe concentration that deviates from the given value Table 4. Quantification results for a low-alloyed steel XRF w/o stand XRF w stand C XRF C nd nd 1.51 Cr 0.15 nd Mn 0.13 nd Fe Cu 0.28 nd W 0.97 nd by at least 2 wt%. But the use of the result for carbon for the matrix interaction in XRF gives highly acceptable results for all elements also in case of a standardless quantification. Another sample quality considered is a mineral that has a complex composition and contains both light elements and traces of heavy elements. Table 5 presents the results for a coal ash sample. Again gives the first results which are shown in the corresponding column. They are acceptable for the light elements but traces of heavy elements cannot be detected (nd). The characterisation of the material is already better if the XRF spectrum is evaluated and oxygen is considered by stoichiometric relation to every other detected element. This allows a more exact description of matrix interaction. But the concentrations of Na and Mg are so small for that sample that the sensitivity of XRF is not sufficient, and therefore, also here, a complete analysis is not possible, as can be seen in column (XRF O by stoich.). Just the consideration of results for the light elements, in particular O, Na and Mg (bold figures in column & XRF) allow both the detection of all elements in the sample and also a good description of matrix interaction for XRF. These results displayed in the last column show both, a complete characterisation of the composition, and also a very good accuracy for a standardless quantification. Another sample considered was a plastic with some contamination by heavy elements that could be for a RoHS application. In this case, both carbon and oxygen are present in the sample, but only Ca and Zn are specified all other elements are non-specified (ns). The quantification results are presented in Table 6. can quantify the light elements and also some other main constituents. But the concentrations, especially for Zn, deviate significantly from the certified values. For XRF excitation, the light elements cannot be detected, and due to their high concentrations, the

6 Micro-XRF excitation in an SEM 259 Table 5. Quantification results for a mineral XRF O by stoich. C XRF LOI 2.5 O Na nd 0.73 Mg nd 0.49 Al Si P S K Ca Ti V 0.03 nd Cr 0.02 nd Mn 0.02 nd Fe Ni 0.02 nd Cu 0.01 nd Zn 0.02 nd As 0.01 nd Rb 0.01 nd Sr 0.08 nd Ba 0.15 nd Pb 0.01 nd nd not detected Table 6. Quantification results for a plastic sample XRF C XRF C ns nd O ns nd Cl ns Ca Fe ns Cu ns Zn results for Ca and Zn are too high. If the results of are used again to correct for matrix interaction in XRF, the results for the specified elements are very close to the given concentrations. CONCLUSION X-ray excitation in an SEM can be an interesting complement to for elemental analysis. The sensitivity for heavy trace elements can be enhanced by factors in the range of in comparison to electron excitation. This allows a much better characterisation of the material. Also, the deeper penetration of the exciting radiation into the sample can be advantageous, giving a more representative analysed volume for bulk material, and the possibility of coating thickness measurements for thicker layers and multiple layer systems. A quantification model is presented that performs an iterative calculation of element intensities based on the Sherman equation. This model takes into account the influence of the polycapillary concentrator on the excitation spectrum. Further, it can use single standards for an improvement of the quantification results. Several tests on Cu- and Fe-alloy reference samples show that the accuracy with this model is acceptable for a completely standardless quantification and even better with the use of single standards. Another interesting feature of the model is the possibility to include concentrations of given elements into the calculation to improve the matrix correction. A special benefit of the excitation with photons in a SEM is the possibility of using the different detection sensitivities of both methods for different elements. Determining the concentrations of light elements, in particular, elements like C, O, N or F with, opens the possibility of using this information for an improvement of the matrix correction in XRF because these elements cannot currently be measured with ED-XRF. The few examples examined here show that this combination for selected sample compositions can give a significant improvement of the quantification results. It will be necessary to validate this observation for more samples of different sample quality and with complex compositions. Acknowledgement This research was supported partly by the Senate of Berlin under the ProFIT Program. REFERENCES 1. Carpenter DA, Taylor MA, Holcombe CE. Adv. X-ray Anal. 1989; 32: Hosokawa Y, Ozawa S, Nakazawa H, Nakayama Y. X-ray Spectr. 1997; 26: HaschkeM,ScholzW,TheisU,NicolosiJ,ScruggsB,HerczegL. J. Phys. IV 2002; 12: Bjeoumikhov A, Arkadiev V, Eggert F, Hodoroaba VD, Langhoff N, Procop M, Rabe J, Wedell R. X-ray Spectr. 2005; 34: (Accessed 2005). 6. Sherman J. Spectrochim. Acta 1955; 7: Elam WT, Shen RB, Scruggs B, Nicolosi JA. Adv. X-ray Anal. 2004; 47: Elam WT, Ravel BD, Sieber JR. Radiat. Phys. Chem. 2002; 63: Elam WT, Nicolosi JA, Shen RB, US Patent No. 6,845,147, granted on Jan. 18, Ebel H. X-ray Spectr. 1999; 28: Finkelshtein AL, Pavlova TO. X-ray Spectr. 1999; 28: Eggert F, Scholz W. Phys. Status Solidi A 1986; 97: K9.