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Total reflection X-ray fluorescence: A technique for trace element analysis in materials
Progress Crystal
in Growth
and Characterization Progress
in Crystal Growth and Characterization of Materials of Materials (2002) 65-74 http://www.elsevier.com/locate/pcrysgrow
PERGAMON
TOTAL
REFLECTION
Fuel Chemistry
X-HAY FLUORESCENCE: A TECHNIOUE ELEMENT ANALYSIS IN MATERIALS -
Division,
FOR TRACE
N.L.Misra and K.D.Singh Mudher Bhabha Atomic Research Centre, Mumbai 400 08.5 India ABSTRACT
X-ray Total Reflection X-ray Fluorescence (TXRF) is a variant of Energy dispersive Fluorescence (EDXRF). It is a comparatively new method of trace element analysis and finds its application in various research areas of material development and processing. The versatility of TXRF is due to (i.) requirement of very less amount of sample (ii.) its capability to analyse very low concentrations and (iii.) capability of analysing surface and shallow layers up to a depth of few nanometers profiling up to few nanometers depth in materials. Presence of impurities in semiconductor wafers affects the quality of the wafer significantly. Analysis of thin wafers is one of the several applications of TXRF where no other technique can compete with it. In the present paper the principle, advantages and some applications of this technique are briefly summarised.
Key Words: X-Ray Fluorescence,
Trace analysis, Nondestructive
and nanomers
1. INTRODUCTION X-ray fluorescence (XRF) is a non-destructive method for the elemental analysis. The method has broad dynamic range and can be used for the analysis of wide range of concentration of all elements beyond beryllium. XRF methods are being routinely used for quality control and development of process parameters in steel, cement, petroleum, chemical and nuclear industry etc [l]. XRF is an instrumental method of qualitative and quantitative analysis for chemical elements based on the measurement of wavelengths and intensities of their spectral lines emitted. Mosley established the basis for qualitative and quantitative XRF analysis, which lies in the relationship between wavelength (h) of the characteristic radiation emitted and the atomic number (Z) of the element, namely l/A = k (Z-s 12, in which ‘k’ and ‘s’ are constants that depend upon the spectral series of the emission line. Characteristic X-ray spectra are excited when a specimen is irradiated with a sufficiently short wavelength X-radiation. Intensities of these Xrays are proportional to the concentration of the analyte. Though XRF is having several analytical merits e.g. nondestruction and versatility of sample, capability to analyse multielements, less time consuming etc., it has comparatively high detection limits due to high background produced by the absorption and scattering of X-ray beam by the sample and matrix. Further due to matrix effects, different calibration plots are required for different matrices. These deficiencies have been taken care in Total reflection X-ray Fluorescence (TXRF) spectrometry. TXRF makes use of the fact that exciting X-rays fall at smooth flat surface containing small amount of sample, at a glancing angle less than the critical angle of the material and are totally reflected. This reduces the background and increases the analyte line intensity, thereby improving the detection limits of the elements up to picogram 0960.8974/02/$ - see front matter 0 2002 Published PII: SO960-8974(02)00029-3
by Elsevier
Science
Ltd.
66
N.L. Misra,
K.D. Singh Mudher
level. In addition, since the samples matrix effects are negligible.
/ Prog. Crysfal
Growth and Characr.
are taken in very small amount
45 (2002)
65-74
in the form of thin film,
TXRF is a comparatively new and specialised technique for the trace and sub-trace level analysis of elemental concentration and has been found to be very useful in material development and characterisation. The concept of TXRF remained unexploited for chemical analysis until the year 1071, when Yoneda and Horiuchi [2] reported the potential of this technique for chemical analysis. This was further strengthened by the work of Aiginger and Wobrauscheck [3]. First TXRF spectrometer was built in 1977. From the year 1986, onwards this technique is being regularly used and a good number of professional manufacturers are now making TXRF spectrometers. Due to continuous improvements in TXRF instrumentation and methodology, the technique is finding wide-ranging applications covering different areas like analysis of surface and near surface layers of semiconductors and advanced materials, environmental studies, mineralogical investigations, chemical oceanography, medicine and high purity chemicals. 2. INSTRUMENTATION TXRF is a variant of EDXRF. TXRF spectrometers are designed and operated on the basis of classical dispersion law and Fresnell’s equation according to which the total reflection is valid for a perfectly flat and smooth surface. The schematic of the instrument is shown in Fig. 1. The X-ray beam from the X-ray tube after passing through a filter is still polychromatic. After passing through a slit this beam impinges on first reflector at a glancing angle equal to the critical angle of x-ray wavelength required for excitation of the sample. The totally reflected portion of the beam is almost monochromatic and is used for sample excitation. This monochromatic beam in form of a paper strip passes on a flat reflecting surface holding the sample at an angle less than the critical angle. The characteristic Xrays thus emitted enter a solid-state detector and their intensity is measured with the help of an amplifier and multichannel analyser. The detector axis is perpendicular to the sample support surface in contrast to EDXRF set up where it makes an angle of about 45 degrees. This particular set up is used for trace elemental analysis. For the analysis of layers of semiconductors, provision of accurate change of angle is also made. For analysis of residues, a small quartz block is sufficient whereas for thin layer or surface profiling multilayer monochromators are used for the monochromatization. In some of the instruments, a strong monochromatic X-ray beam produced from synchrotron light source is used for obtaining a tuneable X-ray beam with very high brilliance and thereby giving a very good detection limits [lo]. 3. ANALYTICAL
STRATEGY
TXRF is a microanalysis method [4-S]. Normally samples are analysed as received but in few cases, a pre-treatment is required as per the nature of the sample. Samples should be converted to solutions, suspensions, fine powders or thin films for the analysis. The various steps involved in analysis are preparation of samples, presentation of specimen, calibration of the instrument, recording and interpretation of spectrum or peak identification, intensity calculation and finally quantifying the elemental concentration [4]. The sample after dissolution or homogenisation is mixed with internal standard. Small amount (5-100 ~1) of solution is placed on a sample carrier. Total mass of the solution should be about l200 pg. The samples are pipetted out on a clean sample support. This solution volume is dried to
N.L. Misra,
K.D. Singh Mudher
/ Prog. Crystal
Growth and Charact.
45 (2002)
65-74
67
form a smooth bright spot residue of diameter about l-5 mm. There are different methods of preparation of specimen of pigments, air dust or aerosols, individual particles. Solid bulk sample can be taken in specimen form by method of ablation or even by rubbing the sample on sample support. Wafers not profiled can directly be applied as flat disks. Total wafer area can be analysed by a displacement device. Sample support should have certain properties like easy machinability to a perfectly smooth and flat surface, should be immune to aggressive chemical and mechanical stresses, free from its own fluorescence lines and contamination, hydrophobic etc.
Fig. 1: Instrumental set-up for TXRF For recording the spectra, the sample holders are pushed in the sample position of the instrument. The spectra are recorded and stored in a computer. For quantification of the multielements in a sample, the instrument is calibrated using multielement standards (MES), containing all the elements to be analysed but having no interfering characteristic X-ray lines. The TXRF spectra of these MES are recorded in similar manner and in same instrumental condition as for the actual samples. Intensities of characteristic X-ray lines of the elements present are determined with the computer program. These integrated intensities are divided by the concentration of the respective elements present. This gives the absolute sensitivity for each element. The absolute sensitivities when divided by absolute sensitivity of a particular element give the relative sensitivities of the corresponding elements. The quantification is made by the following relation: N. i
c,=~c..
i ‘“s,, where N is net intensity, S the relative sensitivity and C the concentration of either analyte (x) or internal standard (is). The reliability of determinations can be judged by precision, which is below 5% and accuracy slightly poorer.
4. CONDITIONS
AND LIMITATIONS
As the main advantage of TXRF is absence of matrix effects and high background, there is limit on the size of sample. Considering the height of the beam lo-30 pm, limited counting capability of detector and X-ray absorption effects, thickness of the sample should be restricted to lo-15
68
N.L. Misra, K.D. Singh Mudher / Prog. Crysial Growth and Charact. 45 (2002) 65-74
pm. Complete non-destructive analysis is impossible. Non-volatile samples also cannot be analysed. Flat and polished Wafer disks e.g. silicon and germanium are fully suitable for detection of surface contamination and characterisation of surface layers and coatings. However, for trace element analysis of various layers addition of internal standard is not possible. Model calculations are carried out to fit the measured data for quantification. Entire instrument is kept in flow box to avoid contamination from air. Mechanical sensors for the sample positioning should be avoided as it may introduce errors. 5. APPLICATIONS TXRF is used in multiple scientific areas due to capability, better lower limit of detection (LLD) to matrix’ effects, easy calibration, fast analysis suitable for unlimited applications [4-191. Some below:
its three main advantages namely multielement and small sample volume. Further insensitivity and low costs of analysis make the technique of the areas of applications of TXRF are given
1. Characterisation and analysis of thin layer materials and Wafers: Thin layer materials with a layer thickness in range of nanometers are used as high tech materials in integrated circuit (IC) technology and glass industry. Trace element analysis of these layers is the real breakthrough in the application of TXRF [4-6, 15-161. TXRF has been successfully used in different stages of semiconductor material manufacturing e.g. Crystal pulling, Crystal slicing etching, wafer polishing and cleaning, wafer backside analysis, wafer transport and handling, Hg Probe resistivity measurements, wafer packaging, haze and clean room particle sources identification. It has been also used for the semiconductor material processing e.g. wafer quality audit, recleaning, implantation, oxidation, diffusion, deposition, dry processing, subsurface contamination, silicide etching, metal particles in gases etc. [ll]. The TXRF technique has been used in manufacturing and quality control of semiconductor materials e.g. Si, GaAs, GaP, InP etc. 2.
Industrial applications: Various industrial applications of TXRF are: a) Quality Control: It has been used for the quality control of ultra pure acids, bases solvents, high purity metal e.g. Al, Fe or Si. Minerals, synthetic oils, crude oils, lubricating oils, motor oils etc. [4-51. b) Forensic and archaeology: Because of very small amount of sample, required TXRF is well suited for forensic applications. Another application is to differentiate between the fake and true art objects [4, 71. c) Nuclear: In nuclear industry, the application of TXRF is still in the early stage. It has been used to determine the low concentration of Nb in irradiated steel samples e.g. core components of a nuclear power plant and trace elements in materials required for construction of future fusion power plants [8], determination of chemical composition of spent nuclear fuel, U and Th concentration in monazite processing, U concentration in human urine etc. [8, 10, 13-14, 181 d) In addition, it has been used for the analysis of inorganic materials, pigments, textile fibres etc. 3. Environmental application: TXRF has been used for the analysis of rainwater, tap water, river water, seawater, wastewater, plants and geological samples. Recently, studies have been
N.L. Misra,
K.D. Singh Mudher
/ Prog.
Crystal
Growth and Charact.
45 (2002)
65-74
69
made to use the TXRF for trace element analysis of ice-cores. These types of analyses give information about past environmental condition. Detection limits have been found in ppm to ppb level depending on the sensitivities and concentrations of analytes and can even be reached in ppt level in suitable cases [9]. Another recent environmental application of TXRF is its use for characterisation of aerosol particles [12]. Aerosol particles deposited on filter papers can be dissolved in acids and analysed. Alternatively, they can be deposited directly on sample reflector and analysed after adding a known amount of internal standard. Individual components of plant can be analysed after suitable sample preparation. Similarly, it has been used for the analysis of vegetable oils with a detection limit of 320 ppb. 4. Medical Applications: Either depletion or accumulation of trace elements affects biological functions of human beings. Because of this trace element determination in medical field has found extensive interest. It has been used for the analysis of whole blood, blood serum, organ tissue, hair and dental plaque etc [4]. It is beyond the scope of this paper to discuss all the applications of the specific applications are being highlighted below:
in detail. However,
a few
5. Development of semiconductor wafer materials Analysis of thin layers in wafer industry is one of the important applications of TXRF where no other technique can compete with it [4]. Presence of impurities in wafers affects its quality significantly. Impurities can be present on surface of a wafer as a particulate, thin film or bulk type. In case of granular residues, intensity of the X-ray line above the critical angle will remain almost constant, as the sample is completely excited by incident 4r
/I
I =i s g2v) 5 E
/
1 .,
__-------
_.......-.--
. . . . . .!c,.
.:. O_~-~
:
:
_.
______----
.:.
1
.-
(b)
.:. (a
(a)
0
2acrn
4 crit Anqle
of Incidence
a
Fig. 2: Variation of intensities of X-ray lines depending on type of impurities (a = bulk, b = granular and c = thin film) beam. At critical angle, it will almost become double of the previous value as incident as well as reflecting beams excites the sample. In case of thin films the intensity will increase with decrease
70
N.L. Misra, K.D. Singh Mudher / Prog. Crystal Growth and Charact.
45 (2002) 65-74
in glancing angle reaching maximum at the critical angle after which it will decrease with the decrease of glancing angle and will become zero finally. For the bulk type of sample, it will be maximum at angles above critical angle, then decrease suddenly to almost zero at the critical angle and remain so until the grazing angle reaches zero. These types of graphs showing
Table 1: Results of quantitative analysis from the fitting procedure of TXRF for a Si wafer structure Structure Upper layer Lower Layer Substrate
Cr 46.7 -
Element Fe 29.5 -
composition (%) Ni Pd 23.8 100 -
Si 100
Thickness (nm) 5.9 257 Q
Density g/cm’ 7.0 11.2 2.3
variation of XRF lines with the grazing angle are used for the identification and quantification of impurities (Fig. 2). For the quantitative estimation, theoretical models of the plot are made for different composition, which are compared with the experimental plot. The composition corresponding to best-fit plot is assumed as result of the analysis [4]. The quantitative results of analysis of such a sample containing granular, thin film and bulk type of specimen is shown Table 1. 6. Elemental analysis of ice-cores As mentioned earlier, recently TXRF has been utilised for trace element analysis of ice samples collected from Colle Gnifeti ice fields (Switzerland) 3545 m as1 in Jungfraujoch mountains in Alps. These ice cores are one of the best archives of past environmental pollution. Dating of these ice cores and subsequent analysis gives information about past atmospheric temperature (from isotopic ratios e.g. ‘HI’H or ‘sO/‘6O), radiative forcing (from the concentration of trapped greenhouse gases e.g. C&, CO2 and NzO), volcanic eruptions (from SOae2, No3-, and metallic impurities), storms (from dust, sea salt, Al Ca etc), nuclear weapon testing (radioactivity and fission products) etc. Pollution of anthropogenic and natural origin can also be differentiated. To decipher these ice cores a
N.L. Misra, K.D. Singh Mudher / Prog. Crystal Growth and Characr. 45 (2002) 65-74
71
1500y=O.90 x + 18.96
0
500 1000 1500 Ca cow. (ppb) by ICP-MS
Fig. 3. Intercomparision
0 2 4 6 8 101214 160 Mn cont. (ppb) by ICP-MS
of TXRF and ICP-MS results for Ca and Mn in ice samples
large number of ice samples are required to be analysed for a wide range of elements present in very small concentration (ppt or ppb level). Because of its various merits as discussed before, TXRF is a suitable technique for its analysis. TXRF was assessed for the analysis of ice samples for the first time. Prconcentration and TXRF analysis methods of ice samples were standardised and elements Si, P, S, K, Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Se, Rb, Sr, Zr, Nb and MO were analysed using W tube and Quartz/Plexiglas sample supports in multi element standards and ice samples with different precision and accuracy depending on the sensitivities of elements and their concentration. Some elements were earlier analysed by ICP-MS in these ice samples and a comparative study was made [9]. An intercomparision plot of ICP-MS and TXRF results reported for Ca and Mn are shown in Fig. 3. TXRF method has an advantage that it can analyse solutions as well as suspensions alike, samples on carriers can be stored for analysis of other set of elements and non-uniformity of the samples due to presence of any insoluble species in ice cores can be ascertained. 7. Medical applications As an example of application in medical field, Klockenkaemper and von Bohlen [7,17] have investigated the microtome sections of lung tissues. In Fig. 4, TXRF spectrums of lung tissue of a painter and foundry worker are shown. It is clear that in lung tissue of painter components of paint and varnishes e.g. Ti and Pb are present whereas the lung tissue of foundry worker contains excessive amount of iron.
72
N.L. Misra, K.D. Singh Mudher / Prog. Crystal Growth and Charact. 45 (2002) 65-74
Fig. 4. TXRF spectra obtained from lung tissue of a painter (Top) and a foundry worker (Bottom) 8. TRENDS IN TXRF ANALYSIS TXRF has been used in various research areas. Still it is being assessed for its suitability in other new areas with improvement in instrumentation, application in newer areas and sample preparation methodology [10,19] as discussed below: 1. Analysis of low Z elements has been always a challenging job in XRF because of the X-ray
2. 3. 4. 5. 6.
7.
absorption edge of low Z elements and X-ray energy of generally used X-ray sources have a large differences. This results in very low excitation efficiency with most of the exciting anodes. Situation becomes more difficult because of low fluorescence yield, high background due to scattering by low Z elements etc [lo]. TXRF due to its low background producing property and efficient excitation is very much suitable for these elements. Improvements are being made to analyse low Z elements in various matrices [lo]. Lateral and vertical resolution for micro distribution analysis can be performed using TXRF. Combination of XRD, X-ray absorption and X-ray emission techniques with the TXRF can give very good information about crystal structure and chemical state [5-61. TXRF and X-ray reflectometry (XRR) can be combined to work in tandem for layer analysis. For the characterisation of near surface layers. Synchrotron radiation source is very much helpful for trace and ultra trace analysis of a wide range of elements as a tuneable X-ray beam of order of 3-5 times intensity of conventional X-ray tubes is available. Background is further reduced because of beam being polarised. It is very much helpful for the lower Z element analysis and to increase the detection power. Detection limit up to fg level can be achieved. However wider usage will require easier access to Synchrotron Light sources which are very few in the world. Elements to be analysed can be deposited on glassy carbon carriers by electrochemical/chemical reaction and can be analysed. Alternatively, solids can be decomposed electrochemically and diffused to electrolyte, which can be analysed.
73
N.L. Misra, K.D. Singh Mudher / Prog. Crystal Growth and Charact. 45 (2002) 65-74
8.
Wavelength dispersive deposit total reflection interest of scientists.
total reflection X-ray fluorescence (WD-TXRF) and Vapour phase X-ray Fluorescence (VPD-TXRF) are other area which is attracting
9. CONCLUSION Thus TXRF is a technique of multielemental analysis which has unlimited applications in different scientific research and industrial areas. Its main application at present is in the field of wafer analysis due to its non-destructive capability of layer profiling. By the use of synchrotron light source, the detection limits can be as low as fg level. It is comparable with other instrumental method of analysis in terms of detectable element range, stability, accuracy, precision and simple quantification. Further improvements in the excitation, detection and measurement methodologies will reduce the detection limits and make the technique suitable for many other applications in newer scientific areas. Acknowledgement: Authors express their sincere thanks to Dr.V.Venugopal, Head Chemistry Division, and Shri R.Prasad Head Fuel Development Chemistry Section, Chemistry Division BARC, for their encouragement.
Fuel Fuel
10. REFERENCES 1. Principles and Practice of Press, New York 1984.
X-ray Spectrometric
and T. Horiuchi,
Analysis,
2.
Y.Yoneda
3.
P. Wobrauscheck
4.
RKlockenkaemper, Total Reflection X-ray Fluorescence 140, John Wiley & Sons, New York (1996).
5.
Handbook of X-ray spectrometry, Inc. (1993).
6.
K.N.Stoev
and H.Aiginger,
Anal. Chem. 47 (1975) 852.
and K Sakurai, Spectrochimica
Chemical
Advances
, Advances
Analysis
Vol.
Marcel and Dekker
Acta 54-B (1999) 41. in X-ray analysis 35 (1992) 899. M.Victoria,
P.Winkler
and P.Wobrauschek,
L.Tobler and M. Schwikowski, Paul Scherrer Institute of Radio and Environmental Chemistry, Ann. Rep. (2001) 31.
10. C. Streli, X-ray Spectrom. 11. R.S.Hockett
Analysis,
R.E. Van Griken and A. Markowicz,
8. J.Van Aarle, R Abela, F.Hegedues, C.Streli, Spectrochimica Acta 54-B (1999) 1443. N.L.Misra, Laboratory
E.P. Bertin, Plenum
Rev. Sci. Instr. 42 (1971) 1069.
7. A. Prange and H. Schwenke,
9.
2”d edition,
29 (2000) 203. in X-ray Analysis
37 (1994) 565.
and
Univ.
Bern,
N.L. Misra, K.D. Singh Mudher / Prog. Crystal Growth and Charact. 45 (2002) 65-74
74 12. N.
Streit, E. Weingartner, C. Zellweger, M.Schwikowski, H.Gaeggeler and U.Baltensperger, Intern. J. Environ. Anal. Chem. 76 -1 (1999) 1.
13. G.Pepponi, P.Wobrauschek, C.Streli, N.Zoeger and F.Hegadues, X-ray Spectrom. 30 (2001) 267. 14. Y.A. El-Nadi, J.A.Daund, H.F.Aly and PKregsamer, Conference on Total Reflection X-ray Fluorescence Analysis and related methods, Vienna (Austria) 25-29 September 2000. 15. Y.Iijima and K.Miyoshi, X-ray Spectrom. 28 (1999) 427. 16. C. Weiss, JKnoth, H. Schwenke, H.Geisler, J. Lerche, R.Sculz and H. Ullrich, Mikrochim. Acta 133 (2000) 65. 17. A.von Bohlen, RKlockenkaemper, H.Otto, G.Tolg and B. Wiecken, Qualitative Survey Analysis of Thin Layers of Tissue Samples- Heavy Metal Traces in Human Lung Tissues, In. Arch. Occup. Environ. Health 59 (1987) 403. 18. Ch. Zarkadas, A.G.Karydas 19. RKlockenkaemper
and T.Paradellis Spectrochimica Acta 56B-12 (2001) 2505.
and A. von Bohlen Spectrochimica Acta 56B-ll(2001)
2005
in Growth
and Characterization Progress
in Crystal Growth and Characterization of Materials of Materials (2002) 65-74 http://www.elsevier.com/locate/pcrysgrow
PERGAMON
TOTAL
REFLECTION
Fuel Chemistry
X-HAY FLUORESCENCE: A TECHNIOUE ELEMENT ANALYSIS IN MATERIALS -
Division,
FOR TRACE
N.L.Misra and K.D.Singh Mudher Bhabha Atomic Research Centre, Mumbai 400 08.5 India ABSTRACT
X-ray Total Reflection X-ray Fluorescence (TXRF) is a variant of Energy dispersive Fluorescence (EDXRF). It is a comparatively new method of trace element analysis and finds its application in various research areas of material development and processing. The versatility of TXRF is due to (i.) requirement of very less amount of sample (ii.) its capability to analyse very low concentrations and (iii.) capability of analysing surface and shallow layers up to a depth of few nanometers profiling up to few nanometers depth in materials. Presence of impurities in semiconductor wafers affects the quality of the wafer significantly. Analysis of thin wafers is one of the several applications of TXRF where no other technique can compete with it. In the present paper the principle, advantages and some applications of this technique are briefly summarised.
Key Words: X-Ray Fluorescence,
Trace analysis, Nondestructive
and nanomers
1. INTRODUCTION X-ray fluorescence (XRF) is a non-destructive method for the elemental analysis. The method has broad dynamic range and can be used for the analysis of wide range of concentration of all elements beyond beryllium. XRF methods are being routinely used for quality control and development of process parameters in steel, cement, petroleum, chemical and nuclear industry etc [l]. XRF is an instrumental method of qualitative and quantitative analysis for chemical elements based on the measurement of wavelengths and intensities of their spectral lines emitted. Mosley established the basis for qualitative and quantitative XRF analysis, which lies in the relationship between wavelength (h) of the characteristic radiation emitted and the atomic number (Z) of the element, namely l/A = k (Z-s 12, in which ‘k’ and ‘s’ are constants that depend upon the spectral series of the emission line. Characteristic X-ray spectra are excited when a specimen is irradiated with a sufficiently short wavelength X-radiation. Intensities of these Xrays are proportional to the concentration of the analyte. Though XRF is having several analytical merits e.g. nondestruction and versatility of sample, capability to analyse multielements, less time consuming etc., it has comparatively high detection limits due to high background produced by the absorption and scattering of X-ray beam by the sample and matrix. Further due to matrix effects, different calibration plots are required for different matrices. These deficiencies have been taken care in Total reflection X-ray Fluorescence (TXRF) spectrometry. TXRF makes use of the fact that exciting X-rays fall at smooth flat surface containing small amount of sample, at a glancing angle less than the critical angle of the material and are totally reflected. This reduces the background and increases the analyte line intensity, thereby improving the detection limits of the elements up to picogram 0960.8974/02/$ - see front matter 0 2002 Published PII: SO960-8974(02)00029-3
by Elsevier
Science
Ltd.
66
N.L. Misra,
K.D. Singh Mudher
level. In addition, since the samples matrix effects are negligible.
/ Prog. Crysfal
Growth and Characr.
are taken in very small amount
45 (2002)
65-74
in the form of thin film,
TXRF is a comparatively new and specialised technique for the trace and sub-trace level analysis of elemental concentration and has been found to be very useful in material development and characterisation. The concept of TXRF remained unexploited for chemical analysis until the year 1071, when Yoneda and Horiuchi [2] reported the potential of this technique for chemical analysis. This was further strengthened by the work of Aiginger and Wobrauscheck [3]. First TXRF spectrometer was built in 1977. From the year 1986, onwards this technique is being regularly used and a good number of professional manufacturers are now making TXRF spectrometers. Due to continuous improvements in TXRF instrumentation and methodology, the technique is finding wide-ranging applications covering different areas like analysis of surface and near surface layers of semiconductors and advanced materials, environmental studies, mineralogical investigations, chemical oceanography, medicine and high purity chemicals. 2. INSTRUMENTATION TXRF is a variant of EDXRF. TXRF spectrometers are designed and operated on the basis of classical dispersion law and Fresnell’s equation according to which the total reflection is valid for a perfectly flat and smooth surface. The schematic of the instrument is shown in Fig. 1. The X-ray beam from the X-ray tube after passing through a filter is still polychromatic. After passing through a slit this beam impinges on first reflector at a glancing angle equal to the critical angle of x-ray wavelength required for excitation of the sample. The totally reflected portion of the beam is almost monochromatic and is used for sample excitation. This monochromatic beam in form of a paper strip passes on a flat reflecting surface holding the sample at an angle less than the critical angle. The characteristic Xrays thus emitted enter a solid-state detector and their intensity is measured with the help of an amplifier and multichannel analyser. The detector axis is perpendicular to the sample support surface in contrast to EDXRF set up where it makes an angle of about 45 degrees. This particular set up is used for trace elemental analysis. For the analysis of layers of semiconductors, provision of accurate change of angle is also made. For analysis of residues, a small quartz block is sufficient whereas for thin layer or surface profiling multilayer monochromators are used for the monochromatization. In some of the instruments, a strong monochromatic X-ray beam produced from synchrotron light source is used for obtaining a tuneable X-ray beam with very high brilliance and thereby giving a very good detection limits [lo]. 3. ANALYTICAL
STRATEGY
TXRF is a microanalysis method [4-S]. Normally samples are analysed as received but in few cases, a pre-treatment is required as per the nature of the sample. Samples should be converted to solutions, suspensions, fine powders or thin films for the analysis. The various steps involved in analysis are preparation of samples, presentation of specimen, calibration of the instrument, recording and interpretation of spectrum or peak identification, intensity calculation and finally quantifying the elemental concentration [4]. The sample after dissolution or homogenisation is mixed with internal standard. Small amount (5-100 ~1) of solution is placed on a sample carrier. Total mass of the solution should be about l200 pg. The samples are pipetted out on a clean sample support. This solution volume is dried to
N.L. Misra,
K.D. Singh Mudher
/ Prog. Crystal
Growth and Charact.
45 (2002)
65-74
67
form a smooth bright spot residue of diameter about l-5 mm. There are different methods of preparation of specimen of pigments, air dust or aerosols, individual particles. Solid bulk sample can be taken in specimen form by method of ablation or even by rubbing the sample on sample support. Wafers not profiled can directly be applied as flat disks. Total wafer area can be analysed by a displacement device. Sample support should have certain properties like easy machinability to a perfectly smooth and flat surface, should be immune to aggressive chemical and mechanical stresses, free from its own fluorescence lines and contamination, hydrophobic etc.
Fig. 1: Instrumental set-up for TXRF For recording the spectra, the sample holders are pushed in the sample position of the instrument. The spectra are recorded and stored in a computer. For quantification of the multielements in a sample, the instrument is calibrated using multielement standards (MES), containing all the elements to be analysed but having no interfering characteristic X-ray lines. The TXRF spectra of these MES are recorded in similar manner and in same instrumental condition as for the actual samples. Intensities of characteristic X-ray lines of the elements present are determined with the computer program. These integrated intensities are divided by the concentration of the respective elements present. This gives the absolute sensitivity for each element. The absolute sensitivities when divided by absolute sensitivity of a particular element give the relative sensitivities of the corresponding elements. The quantification is made by the following relation: N. i
c,=~c..
i ‘“s,, where N is net intensity, S the relative sensitivity and C the concentration of either analyte (x) or internal standard (is). The reliability of determinations can be judged by precision, which is below 5% and accuracy slightly poorer.
4. CONDITIONS
AND LIMITATIONS
As the main advantage of TXRF is absence of matrix effects and high background, there is limit on the size of sample. Considering the height of the beam lo-30 pm, limited counting capability of detector and X-ray absorption effects, thickness of the sample should be restricted to lo-15
68
N.L. Misra, K.D. Singh Mudher / Prog. Crysial Growth and Charact. 45 (2002) 65-74
pm. Complete non-destructive analysis is impossible. Non-volatile samples also cannot be analysed. Flat and polished Wafer disks e.g. silicon and germanium are fully suitable for detection of surface contamination and characterisation of surface layers and coatings. However, for trace element analysis of various layers addition of internal standard is not possible. Model calculations are carried out to fit the measured data for quantification. Entire instrument is kept in flow box to avoid contamination from air. Mechanical sensors for the sample positioning should be avoided as it may introduce errors. 5. APPLICATIONS TXRF is used in multiple scientific areas due to capability, better lower limit of detection (LLD) to matrix’ effects, easy calibration, fast analysis suitable for unlimited applications [4-191. Some below:
its three main advantages namely multielement and small sample volume. Further insensitivity and low costs of analysis make the technique of the areas of applications of TXRF are given
1. Characterisation and analysis of thin layer materials and Wafers: Thin layer materials with a layer thickness in range of nanometers are used as high tech materials in integrated circuit (IC) technology and glass industry. Trace element analysis of these layers is the real breakthrough in the application of TXRF [4-6, 15-161. TXRF has been successfully used in different stages of semiconductor material manufacturing e.g. Crystal pulling, Crystal slicing etching, wafer polishing and cleaning, wafer backside analysis, wafer transport and handling, Hg Probe resistivity measurements, wafer packaging, haze and clean room particle sources identification. It has been also used for the semiconductor material processing e.g. wafer quality audit, recleaning, implantation, oxidation, diffusion, deposition, dry processing, subsurface contamination, silicide etching, metal particles in gases etc. [ll]. The TXRF technique has been used in manufacturing and quality control of semiconductor materials e.g. Si, GaAs, GaP, InP etc. 2.
Industrial applications: Various industrial applications of TXRF are: a) Quality Control: It has been used for the quality control of ultra pure acids, bases solvents, high purity metal e.g. Al, Fe or Si. Minerals, synthetic oils, crude oils, lubricating oils, motor oils etc. [4-51. b) Forensic and archaeology: Because of very small amount of sample, required TXRF is well suited for forensic applications. Another application is to differentiate between the fake and true art objects [4, 71. c) Nuclear: In nuclear industry, the application of TXRF is still in the early stage. It has been used to determine the low concentration of Nb in irradiated steel samples e.g. core components of a nuclear power plant and trace elements in materials required for construction of future fusion power plants [8], determination of chemical composition of spent nuclear fuel, U and Th concentration in monazite processing, U concentration in human urine etc. [8, 10, 13-14, 181 d) In addition, it has been used for the analysis of inorganic materials, pigments, textile fibres etc. 3. Environmental application: TXRF has been used for the analysis of rainwater, tap water, river water, seawater, wastewater, plants and geological samples. Recently, studies have been
N.L. Misra,
K.D. Singh Mudher
/ Prog.
Crystal
Growth and Charact.
45 (2002)
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69
made to use the TXRF for trace element analysis of ice-cores. These types of analyses give information about past environmental condition. Detection limits have been found in ppm to ppb level depending on the sensitivities and concentrations of analytes and can even be reached in ppt level in suitable cases [9]. Another recent environmental application of TXRF is its use for characterisation of aerosol particles [12]. Aerosol particles deposited on filter papers can be dissolved in acids and analysed. Alternatively, they can be deposited directly on sample reflector and analysed after adding a known amount of internal standard. Individual components of plant can be analysed after suitable sample preparation. Similarly, it has been used for the analysis of vegetable oils with a detection limit of 320 ppb. 4. Medical Applications: Either depletion or accumulation of trace elements affects biological functions of human beings. Because of this trace element determination in medical field has found extensive interest. It has been used for the analysis of whole blood, blood serum, organ tissue, hair and dental plaque etc [4]. It is beyond the scope of this paper to discuss all the applications of the specific applications are being highlighted below:
in detail. However,
a few
5. Development of semiconductor wafer materials Analysis of thin layers in wafer industry is one of the important applications of TXRF where no other technique can compete with it [4]. Presence of impurities in wafers affects its quality significantly. Impurities can be present on surface of a wafer as a particulate, thin film or bulk type. In case of granular residues, intensity of the X-ray line above the critical angle will remain almost constant, as the sample is completely excited by incident 4r
/I
I =i s g2v) 5 E
/
1 .,
__-------
_.......-.--
. . . . . .!c,.
.:. O_~-~
:
:
_.
______----
.:.
1
.-
(b)
.:. (a
(a)
0
2acrn
4 crit Anqle
of Incidence
a
Fig. 2: Variation of intensities of X-ray lines depending on type of impurities (a = bulk, b = granular and c = thin film) beam. At critical angle, it will almost become double of the previous value as incident as well as reflecting beams excites the sample. In case of thin films the intensity will increase with decrease
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in glancing angle reaching maximum at the critical angle after which it will decrease with the decrease of glancing angle and will become zero finally. For the bulk type of sample, it will be maximum at angles above critical angle, then decrease suddenly to almost zero at the critical angle and remain so until the grazing angle reaches zero. These types of graphs showing
Table 1: Results of quantitative analysis from the fitting procedure of TXRF for a Si wafer structure Structure Upper layer Lower Layer Substrate
Cr 46.7 -
Element Fe 29.5 -
composition (%) Ni Pd 23.8 100 -
Si 100
Thickness (nm) 5.9 257 Q
Density g/cm’ 7.0 11.2 2.3
variation of XRF lines with the grazing angle are used for the identification and quantification of impurities (Fig. 2). For the quantitative estimation, theoretical models of the plot are made for different composition, which are compared with the experimental plot. The composition corresponding to best-fit plot is assumed as result of the analysis [4]. The quantitative results of analysis of such a sample containing granular, thin film and bulk type of specimen is shown Table 1. 6. Elemental analysis of ice-cores As mentioned earlier, recently TXRF has been utilised for trace element analysis of ice samples collected from Colle Gnifeti ice fields (Switzerland) 3545 m as1 in Jungfraujoch mountains in Alps. These ice cores are one of the best archives of past environmental pollution. Dating of these ice cores and subsequent analysis gives information about past atmospheric temperature (from isotopic ratios e.g. ‘HI’H or ‘sO/‘6O), radiative forcing (from the concentration of trapped greenhouse gases e.g. C&, CO2 and NzO), volcanic eruptions (from SOae2, No3-, and metallic impurities), storms (from dust, sea salt, Al Ca etc), nuclear weapon testing (radioactivity and fission products) etc. Pollution of anthropogenic and natural origin can also be differentiated. To decipher these ice cores a
N.L. Misra, K.D. Singh Mudher / Prog. Crystal Growth and Characr. 45 (2002) 65-74
71
1500y=O.90 x + 18.96
0
500 1000 1500 Ca cow. (ppb) by ICP-MS
Fig. 3. Intercomparision
0 2 4 6 8 101214 160 Mn cont. (ppb) by ICP-MS
of TXRF and ICP-MS results for Ca and Mn in ice samples
large number of ice samples are required to be analysed for a wide range of elements present in very small concentration (ppt or ppb level). Because of its various merits as discussed before, TXRF is a suitable technique for its analysis. TXRF was assessed for the analysis of ice samples for the first time. Prconcentration and TXRF analysis methods of ice samples were standardised and elements Si, P, S, K, Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Se, Rb, Sr, Zr, Nb and MO were analysed using W tube and Quartz/Plexiglas sample supports in multi element standards and ice samples with different precision and accuracy depending on the sensitivities of elements and their concentration. Some elements were earlier analysed by ICP-MS in these ice samples and a comparative study was made [9]. An intercomparision plot of ICP-MS and TXRF results reported for Ca and Mn are shown in Fig. 3. TXRF method has an advantage that it can analyse solutions as well as suspensions alike, samples on carriers can be stored for analysis of other set of elements and non-uniformity of the samples due to presence of any insoluble species in ice cores can be ascertained. 7. Medical applications As an example of application in medical field, Klockenkaemper and von Bohlen [7,17] have investigated the microtome sections of lung tissues. In Fig. 4, TXRF spectrums of lung tissue of a painter and foundry worker are shown. It is clear that in lung tissue of painter components of paint and varnishes e.g. Ti and Pb are present whereas the lung tissue of foundry worker contains excessive amount of iron.
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Fig. 4. TXRF spectra obtained from lung tissue of a painter (Top) and a foundry worker (Bottom) 8. TRENDS IN TXRF ANALYSIS TXRF has been used in various research areas. Still it is being assessed for its suitability in other new areas with improvement in instrumentation, application in newer areas and sample preparation methodology [10,19] as discussed below: 1. Analysis of low Z elements has been always a challenging job in XRF because of the X-ray
2. 3. 4. 5. 6.
7.
absorption edge of low Z elements and X-ray energy of generally used X-ray sources have a large differences. This results in very low excitation efficiency with most of the exciting anodes. Situation becomes more difficult because of low fluorescence yield, high background due to scattering by low Z elements etc [lo]. TXRF due to its low background producing property and efficient excitation is very much suitable for these elements. Improvements are being made to analyse low Z elements in various matrices [lo]. Lateral and vertical resolution for micro distribution analysis can be performed using TXRF. Combination of XRD, X-ray absorption and X-ray emission techniques with the TXRF can give very good information about crystal structure and chemical state [5-61. TXRF and X-ray reflectometry (XRR) can be combined to work in tandem for layer analysis. For the characterisation of near surface layers. Synchrotron radiation source is very much helpful for trace and ultra trace analysis of a wide range of elements as a tuneable X-ray beam of order of 3-5 times intensity of conventional X-ray tubes is available. Background is further reduced because of beam being polarised. It is very much helpful for the lower Z element analysis and to increase the detection power. Detection limit up to fg level can be achieved. However wider usage will require easier access to Synchrotron Light sources which are very few in the world. Elements to be analysed can be deposited on glassy carbon carriers by electrochemical/chemical reaction and can be analysed. Alternatively, solids can be decomposed electrochemically and diffused to electrolyte, which can be analysed.
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8.
Wavelength dispersive deposit total reflection interest of scientists.
total reflection X-ray fluorescence (WD-TXRF) and Vapour phase X-ray Fluorescence (VPD-TXRF) are other area which is attracting
9. CONCLUSION Thus TXRF is a technique of multielemental analysis which has unlimited applications in different scientific research and industrial areas. Its main application at present is in the field of wafer analysis due to its non-destructive capability of layer profiling. By the use of synchrotron light source, the detection limits can be as low as fg level. It is comparable with other instrumental method of analysis in terms of detectable element range, stability, accuracy, precision and simple quantification. Further improvements in the excitation, detection and measurement methodologies will reduce the detection limits and make the technique suitable for many other applications in newer scientific areas. Acknowledgement: Authors express their sincere thanks to Dr.V.Venugopal, Head Chemistry Division, and Shri R.Prasad Head Fuel Development Chemistry Section, Chemistry Division BARC, for their encouragement.
Fuel Fuel
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