ULTRATHIN LAYER DEPOSITIONS A NEW TYPE OF REFERENCE SAMPLES FOR HIGH PERFORMANCE XRF ANALYSIS

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1 ULTRATHIN LAYER DEPOSITIONS A NEW TYPE OF REFERENCE SAMPLES FOR HIGH PERFORMANCE XRF ANALYSIS M. Krämer 1), R. Dietsch 1), Th. Holz 1), D. Weißbach 1), G. Falkenberg 2), R. Simon 3), U. Fittschen 4)5), T. Krugmann 4), M. Kolbe 6), M. Müller 6), B. Beckhoff 6) 1) AXO DRESDEN GmbH, Winterbergstr. 28, D Dresden, Germany; phone: ; contact@axo-dresden.de 2) HASYLAB at DESY, Notkestr. 85, D Hamburg, Germany; 3) Institute for Synchrotron Radiation, FZ Karlsruhe, Hermann-von-Helmholtz-Platz 1, D Eggenstein, Germany; 4) Institute for Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D Hamburg, Germany 5) Chemistry Division, Los Alamos National Laboratory, P.O. Box 1663 Mail Stop K484, Los Alamos, NM 87545, USA 6) Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, Berlin, Germany ABSTRACT Detection limits and performance of X-ray based spectrometric methods such as micro X-ray fluorescence analysis (µ-xrf) and total reflection X-ray fluorescence analysis (TXRF) have been improved constantly in the last decades. Quantification in these methods depends on suitable, well-known reference samples. However, in many cases those samples are not available commercially or only in non-optimal composition. For this reason, we developed dedicated reference samples suitable for TXRF and related techniques like micro-xrf by applying deposition techniques such as magnetron sputter deposition (MSD) and pulse laser deposition (PLD) that are typical in the production of multilayers. First test samples with nickel on silicon showed layer-like deposition down to atoms/cm², which is in the range of contamination critical in semiconductor production and accessible by TXRF. Similar samples with higher mass depositions of various elements were made for micro-xrf. It could be shown that high precision deposition as applied in the multilayer production is a promising tool to produce reference samples for challenges in modern TXRF and micro-xrf analyses.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 INTRODUCTION Developments in the various X-ray fluorescence (XRF) analysis techniques have led to constant improvement of sensitivity. The lower limit of detection for total reflection X-ray fluorescence analysis (TXRF) is even significantly lower than for conventional XRF and reaches regions of atoms/cm² (Klockenkämper, 2006). The technological relevance of contaminations in the same order of magnitude is important in high technology as well, especially in semiconductor industry. Quantification in TXRF as in XRF depends on suitable reference samples. A typical calibration of TXRF relies on dried droplets of a well known solution. Other calibration techniques include spin-coated wafers, alkaline coating or picoliter droplets (Sparks et al.; 2010). Hoever, studies have shown that these droplets do not dry homogeneously (Beckhoff et al.; 2007; Nutsch et al., 2009; Horntrich et al., 2010) which may lead to errors in quantification. In addition, a reference sample should be very similar to the sample to be characterized, both in terms of elemental composition and lateral structure (layer type, clusters or islands). However, controlled preparation of such small material amounts is very challenging. High precision deposition techniques such as magnetron sputter deposition (MSD) or pulse laser deposition (PLD) permit manufacturing of well defined, laterally very homogeneous layers of arbitrary elements. In order to assess this approach sub-monolayer and multielement reference samples have been developed. The sub-monolayer references were characterized using TXRF as described in the following sections. THEORETICAL BACKGROUND Any X-ray fluorescence related method, such as TXRF, is based on the excitation of element specific fluorescence radiation emitted from an atom that is exposed to radiation with energy high enough to cause an inner shell electron to leave the atomic hull. The electron vacancy is then filled by an outer shell electron with the difference in energy emitted as a fluorescence photon. In case of large grazing incidence angles, the intensity of fluorescence radiation is only slightly dependent on the angle of incidence of the exciting radiation. However, for small grazing incidence angles, interference occurs between the incident and reflected beam, leading to the intensity I being dependent on the grazing incidence angle and the height z above the reflector surface (Klockenkämper, 1996) sin z I (, z) I 0 1 R( ) 2 R( ) cos 4 ( ) with I 0 being the intensity of the incident beam, the photon wavelength and the angle dependent phase shift occurring in the reflection on the surface. This leads to the formation of so-called X-ray standing waves (XSW) on and in the sample (Krämer et al., 2006a; Krämer et al., 2006b) with the intensity varied dependent on the position above the sample. Typically, in TXRF one assumes that the lateral inhomogeneity of the XSW field does not need to be considered in evaluation as it is averaged in the measured signal. Furthermore, the sample structure as well as the reference sample structure is postulated to be homogeneous. If this is not the case for the sample and/or the reference sample, errors in the quantification can occur. Figure 1 shows the calculated reflectivity vs. grazing incidence angle for a sample of 1 nm Ni on Si based on different models of the sample structure.

4 Figure 1: Simulated angle scans for 1 nm Ni on a Si substrate. Depending on the sample structure model applied, the fluorescence yield obtained varies significantly. If the Ni forms very irregular clusters (figure 1, upper left), interference cannot occur and the XSW field is destroyed, leading to double intensity below the critical angle of total reflection of the substrate. For regular sample distributions, an XSW field occurs with a typical peak structure near the critical angle. However, the peak shape is different in the three cases of an undisturbed XSW field (lower left), where the Ni layer or islands are invisible for the formation of the XSW field, or an XSW field modified by a continuous layer (upper right) or regular islands of constant height (lower right). It can be seen that the curve has a completely different shape for an unordered structure compared to a layer or regular island structure. However, even though the curve shape is quite similar for the three cases of islands/layers, still errors in quantification can occur, especially if the measurement is performed at a fixed grazing angle like TXRF. At a value of 0.10 for example, the fluorescence yield differs by a factor of two between the XSW non disturbed and the XSW modified cases which in consequence could lead to a comparable quantification error as well. In general, it can be said that most reliable quantification can be achieved if the reference sample structure is quite similar to the sample to be analyzed. Thus, dried solution droplets appear not to be ideal for quantification of layer-like samples. On the other hand, high precision deposition techniques as applied in multilayer production should be optimal for the preparation of layer-type reference samples. SAMPLE PRODUCTION Pulse laser deposition (PLD) is a technique applied in multilayer production permitting a deposition of a layer material with homogeneity better than 10 pm for a 1 nm thick layer over a sample diameter of several centimeters. The reproducibility of a coating run is better than 99.5%. This is necessary to achieve the precision needed in multilayers consisting of several hundred bi-layers of two different materials (Dietsch et al., 2010). A typical single layer in a multilayer (1-3 nm) consists of several atomic monolayers. Since the mass densities of the TXRF samples in this study are markedly in the sub-monolayer range, the target mass deposition has to be scaled down by two orders of magnitude or more. As the dependence of the layer growth process on the typical PLD parameters (pulse length, pulse energy, power density) is not linear anymore in that low material amount range, the pulse laser deposition method has to be very well understood and the machine tuned exactly

5 to maintain the precise lateral thickness homogeneity also for shorter and less intense laser pulses. Ultraclean polished silicon wafers were used as sample substrates and nickel was selected as a target material. The fluorescence K line of Ni has a low detection limit in TXRF and Ni is usually not present in remarkable amounts in the substrates or instrumentation. Furthermore, strict precautions have to be taken regarding the prevention of sample contamination as any type of external modification of the sample from the laboratory environment, during sample handling, packing and shipping can cause larger contaminations than the deposited material amount of interest. However, external contamination would probably be of a particle type that may be distinguishable from the layer deposition structure of the Ni. The deposition cannot be analyzed in-situ but has to be quantified with dedicated external laboratory or synchrotron instrumentation. CHARACTERIZATION USING TXRF Due to the extremely low mass deposition, no comparable reference samples were available for the quantification of the Ni mass deposited on the Si wafers. However, absolutely calibrated instrumentation available at the PTB laboratories in combination with fundamental parameters based reference-free TXRF (Beckhoff et al., 2007) measurements permitted to determine the Ni deposition to be as low as atoms/cm² and atoms/cm² in the two samples that were observed. Further, angle dependent X-ray fluorescence measurements, also referred to as grazing incidence X-ray standing waves measurements (Krämer et al., 2006a; Krämer et al., 2006b) both in the soft (1.06 kev) and hard X-ray (10 kev) range showed a significant intensity peak near the critical angle of silicon as shown in figure 2. This suggests that nickel was present in a layer-like distribution rather than unordered clusters. Thus, the demanded sub-monolayer was achieved; in the case of the lowest deposition of atoms/cm² the distribution even corresponds to less than 1 of an atomic monolayer. Figure 2: Angular scans of the TXRF reference samples coated with ~10 12 (upper curve, measured at 1.06 kev photon energy) and ~10 14 Ni atoms/cm² (middle curve, measured at 10 kev photon energy). The silicon substrate signal (lower curve) for 10 kev is shown as well. The angular values for the 1.06 kev measurement are scaled by a factor of 1.06/10, the R scale is adjusted for each curve. The measurements were performed by PTB at BESSY II.

6 MULTI-ELEMENT REFERENCE SAMPLES FOR µ-xrf Taking advantage of the high precision deposition machinery used in multilayer production multi-element reference samples were fabricated as well. Due to the higher mass amounts detectable by µ-xrf these samples were designed with layer thicknesses of around 1-2 monolayers for each element. Pb, La, Pd, Mo, Cu, Fe and Ca were sputtered on thin (100 nm to 200 nm) silicon nitride membranes by magnetron sputter deposition due to the higher flexibility of the magnetron in comparison to the PLD machine. These samples were then analyzed with µ-xrf set-ups at different synchrotron radiation facilities (HASYLAB, Hamburg and ANKA, Karlsruhe). The lateral heterogeneity could be verified to be below 1% by µ-xrf mapping scans. Numerous fluorescence peaks of comparable intensity could be observed distributed over a wide energy range in the spectra recorded by an energy dispersive detector (figure 3). Figure 3: Energy spectrum of a 7-element reference sample recorded at HASYLAB (HPGe Detector, XIA Electronics). The energy range from ~2 kev to ~40 kev is covered with fluorescence lines of comparable intensity. SUMMARY Based on the requirements of modern analytical instruments capabilities regarding lower limits of detection, thin layer type reference samples have been developed, manufactured and tested for TXRF analysis. Exemplary TXRF reference samples of very low mass deposition (~ Ni atoms/cm²) distributed as a layer on a substrate surface could be manufactured. Reference-free TXRF analysis was applied for the quantification while angle dependent XRF scans showed the layer-type structure even at this extremely low mass deposition. A multi-element layer stack sample with a content of few ng/mm² of seven different elements was fabricated as well. XRF measurements showed an energy spectrum with numerous fluorescence peaks of comparable height while µ-xrf mappings confirmed the lateral homogeneity of the elemental distribution over the entire sample surface. It could be shown that the high precision deposition techniques applied in multilayer production are feasible and advantageous tools for the development of new and customized reference samples in state-of-the-art X-ray analysis.

7 REFERENCES Beckhoff, B., Fliegauf, R., Kolbe, M., Müller, M., Weser, J., Ulm, G. (2007). Reference-Free Total Reflection X-ray Fluorescence Analysis of Semiconductor Surfaces with Synchrotron Radiation, Anal. Chem. 79, Dietsch, R., Holz, T., Krämer, M., Weißbach, D. (2010). High precision deposition of single and multilayer X-ray optics and their application in X-ray analysis, Proc. of SPIE - Thin Film Physics and Applications, 79951U U-6 Horntrich, C., Smolek, S., Maderitsch, A., Simon, R., Kregsamer, P., Streli, C. (2010). Investigation of element distribution and homogeneity of TXRF samples using SR-micro-XRF to validate the use of an internal standard and improve external standard quantification, Anal. Bioanal. Chem. DOI: /s Klockenkämper, R. (1996). Total-Reflection X-ray Fluorescence Analysis (Wiley, New York). Klockenkämper, R. (2006). Challenges of total reflection X-ray fluorescence for surface- and thin-layer analysis, Spectrochim. Acta B 61, Krämer, M., von Bohlen, A., Sternemann, C., Paulus, M., Hergenröder, R. (2006). X-ray standing waves: a method for thin layered systems, J. Anal. At. Spectrom. 21, Krämer, M., von Bohlen, A., Sternemann, C., Hergenröder, R. (2006). Synchrotron radiation induced X-ray standing waves analysis of layered structures, Appl. Surf. Sci. 253, Nutsch, A., Beckhoff, B., Altmann, R., Polignano, M. L., Cazzini, E., Codegoni, D., Borionetti, G., Kolbe, M., Müller, M., Mantler, C., Streli, C., Kregsamer, P. (2009). Comparability of TXRF Systems at Different Laboratories, ECS Transactions 25, Sparks, C., Fittschen, U. E. A., Havrilla, G. J. (2010). Picoliter solution deposition for total reflection X-ray fluorescence analysis of semiconductor samples, Spectrochim. Acta B 65,