Laboratory Setup for Scanning-Free Grazing Emission X-ray

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Laboratory Setup for Scanning-Free Grazing Emission X-Ray Fluorescence Jonas Baumann, Christian Herzog, Malte Spanier, Daniel Groetzsch, Lars Lühl, Katharina Witte, Adrian Jonas, Sabrina Günther, Frank Förste, Robert Hartmann, Martin Huth, David Kalok, Daniel Steigenhöfer, Markus Krämer, Thomas Holz, Reiner Dietsch, Lothar Strüder, Birgit Kanngiesser, and Ioanna Mantouvalou Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04449 • Publication Date (Web): 24 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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Analytical Chemistry

Laboratory Setup for Scanning-Free Grazing Emission X-Ray Fluorescence J. Baumann,

A. Jonas,

†

∗, †, ‡

C. Herzog,

S. Günther,

D. Steigenhöfer,

¶

†, ⊥

†

M. Spanier,

F. Förste,

M. Krämer,

B. Kanngieÿer,

§ †

†

†

D. Grötzsch,

R. Hartmann,

T. Holz,

§

¶

†

L. Lühl,

M. Huth,

R. Dietsch,

and I. Mantouvalou

§

¶

†

K. Witte,

D. Kalok,

L. Strüder,

†

¶

¶,k

†

†Technical University of Berlin, Institute of Optics and Atomic Physics, Hardenbergstr. 36,

D-10587 Berlin, Germany ‡Humboldt University of Berlin, School of Analytical Sciences Adlershof (IRIS-Building),

Unter den Linden 6, D-10099 Berlin, Germany ¶PNSensor GmbH, Otto-Hahn-Ring 6, D-81739 München, Germany §AXO DRESDEN GmbH, Gasanstaltstr. 8b, D-01237 Dresden, Germany kUniversity of Siegen, Department of Physics, Walter-Flex-Straÿe 3, D-57068 Siegen,

Germany ⊥Current address: IFG GmbH, Rudower Chaussee 29/31, 12489 Berlin, Germany E-mail: [email protected]

Abstract Grazing incidence and grazing emission x-ray uorescence spectroscopy (GI/GEXRF) are techniques that enable non-destructive, quantitative analysis of elemental depth proles with a resolution in the nanometer regime. A laboratory setup for soft X-ray GEXRF measurements is presented. Resonable measurement times could be 1

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achieved by combining a highly brilliant laser produced plasma (LPP) source with a scanning-free GEXRF setup, providing a large solid angle of detection. The detector, a pnCCD, was operated in a single photon counting mode in order to utilize its energy dispersive properties. GEXRF proles of the Ni-Lα,β line of a nickel-carbon multilayer sample, which displays a lateral (bi-)layer thickness gradient, were recorded at several positions. Simulations of theoretical proles predicted a prominent intensity minimum at grazing emission angles between 5◦ -12◦ , depending strongly on the bi-layer thickness of the sample. This information was used to retrieve the bi-layer thickness gradient. The results are in good agreement with values obtained by X-ray reectometry, conventional X-ray uorescence and transmission electron microscopy measurements and serve as proof of principle for the realized GEXRF setup. The presented work demonstrates the potential of nanometer resolved elemental depth proling in the soft X-ray range with a laboratory source, opening e.g. the possibility of in-line or even in-situ process control in semiconductor industry.

Introduction Considering the fabrication of ever smaller electronic devices, eects due to diusion processes, chemical processes or roughness at (buried) interfaces become crucial to device performance. This is, for example, of importance in thin lm technology, which is applied in the development of solar cells, thermoelectric devices, transistors or gas sensors. Invasive methods like secondary ion mass-spectrometry (SIMS), transmission electron microscopy (TEM) or glow-discharge optical emission spectrometry (GD-OES) are frequently used to investigate the elemental composition of layered structures, buried interfaces or dopant implants. 13 A comparison between the mentioned and various other depth proling techniques in terms of their analytical capabilities for the investigation of Cu(In,Ga)Se 2 solar cell absorber layers is given in the work of Abou-Ras et al. 4 The authors conclude, that no single technique is "suitable for an unambiguous and quantitative elemental distribution analysis

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of a thin lm with unknown compositional in-depth distribution". 4 Nevertheless, for the investigation of samples with unknown elemental depth distributions, the use of X-ray uorescence (XRF) methods might be advantageous due to the quantitative, non-preparative and non-destructive properties and, thus, the possibility of complementary investigations of the same sample. Furthermore, if sucient a priori knowledge about a sample is at hand, in-line process control and analysis are applicable. By measuring the angular dependent XRF signal, elemental depth resolution in the nanometer range can be achieved. 5,6 For example, Streeck and co-workers were able to quantify the double linear concentration prole of gallium in a 3 µm thick Cu(In1−x ,Gax )Se2 absorber layer of thin lm solar cells. 7 Also, Caby et al. determined layer properties like thickness, density and roughness of transparent conductive nanometer scaled multilayer systems which can be used as electrodes in e.g. photovoltaic devices. 8 In order to analyze light elements from carbon (Z=6) to the 3d transition metals (Z=30) and gain increased sensitivity for structures below 100 nm, excitation with soft X-rays (250 eV - several keV) is advantageous. This was demonstrated in the investigation of ultra-shallow junctions 911 or transistor gate stacks 12 using synchrotron radiation. However, the access to synchrotron radiation facilities is limited because of beamtime restrictions, rising the demand for laboratory-based approaches. For these laboratory-based approaches, stable and reliable soft X-ray sources and optics are needed, as well as an ecient spectrometer concept. After shortly describing the principles of grazing incidence and grazing emission X-ray uorescence (GIXRF and GEXRF) spectroscopy, we will present our scanning-free GEXRF setup with a laser produced plasma (LPP) source and a 2-dimensional, energy dispersive detector. Image and data processing, needed to extract the GEXRF proles, will be explained in detail. First measurements performed with a multilayer test sample indicate the performance and possibilities of the method and serve as proof-of-principle for the suggested spectrometer concept. The investigated multilayer sample is well suited for a proof-of-principle experiment as

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the geometry is well dened and it is characterized by other techniques such as XRR. In addition to that, it also represents a whole class of approachable stratied technical samples, e.g. photovoltaic devices, 7 transistor gate stacks, 12 gas sensors 13,14 or (periodic) multilayers for X-ray 15 to soft gamma-ray optics. 16 In all these devices, properties are strongly coupled to the device structure, making control and monitoring of elemental depth distributions and diusion processes in the nm range necessary. With this in mind, laboratory based GEXRF can play an important role for complementary measurements to XRR or even overcome limits of XRR when dealing with elemental distributions and lateral inhomogeneities.

Principles of GI- and GEXRF By using shallow incident angles (with respect to the sample surface) of the exciting radiation (GIXRF) or shallow detection angles (GEXRF), XRF experiments can give access to elemental depth distributions in the nm range. The shallow angles lead to an enlarged path of the X-rays in the sample and to increased absorption of the incoming or outgoing X-rays. Therefore, the information depth is reduced compared to excitation and detection with rather steep angles. Thus, by varying the incident or detection angle, the information depth can be adjusted, leading to depth resolving capabilities. In addition, a at sample surface or interface (with a root mean square roughness in the order of the X-ray wavelength) can give rise to interference eects of the incoming X-rays at angles close to the critical angle of total external reection, known as the X-ray standing wave (xsw) eld. The nodes and anti-nodes of the xsw eld, i.e. high and low intensity areas of the exciting X-rays, can be shifted perpendicular to the sample surface by varying the incident angle, eectively acting as depth sensor and increasing depth sensitivity drastically. Interestingly, a similar eect can be observed for the GEXRF geometry, too. 17 In this case, the uorescence radiation of a single atom, which can take dierent paths to the detector due to non-negligible reection and refraction at various interfaces of a sample, can lead to interference eects at the detec-

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tor. This is described by Urbach and de Bokx 18 in a forward calculation approach. Another way of calculating the GEXRF proles is to apply the inverse theorem to the algorithm of de Boer for xsw calculations, 6 as is also shown by de Bokx and Urbach. 19 This demonstrates the physical equality of GEXRF and GIXRF. Conventionally, GEXRF proles are recorded by scanning the emission angle and detecting the uorescence radiation of a sample. To meet the demands of high angular resolution, the solid angle of detection is typically small. By applying wavelength dispersive detectors, this drawback can be compensated in terms of superior energy resolution compared to energy dispersive detectors. 20,21 However, Kayser et al. showed that a scanning-free approach can be realized, using a semi-energy dispersive PILATUS 100 K area detector for angle discrimination and well-chosen excitation wavelengths of synchrotron radiation. 22 This is preferable in terms of detection eciency, solid angle of detection and stability, making the concept suitable for applications with a laboratory source.

Experimental Setup

Figure 1: Schematic setup of the laboratory setup for grazing emission X-ray uorescence measurements. Our laboratory setup for scanning-free grazing emission X-ray uorescence (GEXRF) measurements in the soft X-ray range uses a high brilliance laser produced plasma (LPP) 5



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source 23 with refocusing Kirkpatrick-Baez mirror optics and a pnCCD 24 (see FIG. 1). The LPP source, designed for stable and long-term operation, is driven by an infrared Yb:YAG thin disk laser (pulse energies: 200 mJ, pulse duration: 1.2 ns, repetition rate: 100 Hz) and uses a rotating copper cylinder as target. From the total polychromatic emission spectrum of the hot dense plasma, the intense line emission at 1078 eV, originating from transitions in the 20-fold ionized copper, is used for uorescence excitation (see Mantouvalou et al. 25 ). The divergent radiation is monochromatized and refocused onto the 1090 mm distant sample plane in the sample manipulation chamber by means of a Kirkpatrick-Baez multilayer mirror system. The optics, especially designed for the LPP source, is irradiated at its Bragg angle in the mirror center of 7.8 ◦ and exhibits a photon density gain factor of about 2500 for 1078 eV. This leads to a photon ux of roughly 10 8 photons/s on the sample within a focus size of 100 x 500 µm2 . A 7-axis goniometer 26 is used for sample alignment and for setting the sample surface perpendicular to the incoming X-ray beam during the measurements. The detector is situated in a 90 ◦ geometry with respect to the sample and the incident X-ray beam. The whole X-ray setup is evacuated with pressures down to 10−7 mbar. The distance of the pnCCD chip to the pivotal point of the goniometer can be adjusted and determines the detected angular range and resolution. The measurements presented here were recorded at a detector distance of 65.7 mm, resulting in an angular range of about 10◦ and a pixel resolution of better than 0.05 ◦ for the 264 x 264 pixels of the pnCCD with a pixel size of 48 x 48 µm2 . For a similar GEXRF prole taken with a series of sequential measurements for every detector angle, the recording time would be more than 200 times longer. Indeed, only due to the scanning-free approach, GEXRF measurements with recording times in the range of hours can be achieved with the presented laboratory setup. In order to prove the feasibility and performance of the method, a multilayer mirror structure for the X-ray range was investigated. The sample was fabricated by Dual Ion

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Beam Deposition (DIBD) at AXO DRESDEN GmbH and consists of 15 bi-layers of carbon and nickel, with layer thickness changing in one lateral direction over the range of 40 mm from about 5 nm to 6 nm. In depth, the bi-layer thickness variation can be expected to be less than a few per mill and the interfaces need to be sharp to achieve the desired reectivity at grazing angles when used as an X-ray mirror. Therefore, the sample is well dened, which makes it suitable to validate the rst proof-of-principle measurements presented in this paper. We analyzed 7 measurement points (labeled as "P1", "P2", "P3", "P4", "P5", "P6" and "Q1", the latter taken at a separately aligned sample fraction) at dierent lateral positions on the multilayer. To gain satisfactory statistics, measuring times for each point were between 1 and 2.5 hours, i.e. 3.6 · 105 to 1 · 106 frames were recorded for each measurement series.

Evaluation and Analysis

Image Processing On each frame, which was recorded by the pnCCD in a measurement series, single photon events have to be distinguished and discriminated concerning their energy and uorescence emission angle. This is possible due to the low noise level of the detector, the fast readout times and by synchronizing the readout with the laser pulses of the LPP source. In a rst step, background correction is performed by subtraction of a dark image and cosmic particle rejection by means of a software band pass lter. In addition, correction of gain and charge transfer eciency is implemented. 27,28 Afterwards, in every frame, the photon events consisting of up to four adjacent pixels 1 are reconstructed providing information about the intensity of the event and its position on the pnCCD chip.

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Figure 2: Cumulative soft X-ray uorescence spectrum for measurement series "P2". The deconvolution with gaussian functions and background was used for the estimation of LLDs (see section Results) and determination of energy resolution. The ROIs were dened for the creation of GEXRF proles.

XRF Spectra In order to obtain an energy calibration of the pnCCD and information about the energy resolution, a cumulative XRF spectrum is calculated out of all detected photon events of one measurement series. The spectrum is created by plotting the number of detected events with respect to their intensity, not considering the photon position in this step. FIG. 2 shows exemplarily the XRF spectrum of measurement series "P2", with uorescence lines that can be assigned to C-K α , Ni-Ll,n and Ni-Lα,β originating from the multilayer. O-K α is presumably coming from partial oxidation or contamination. The energy resolution of the pnCCD is 113 eV at the Ni-L α line. This makes it possible to analyze the GEXRF proles of dierent uorescence lines separately. For the evaluation process, not only the Ni-L α,β 1 The

charge cloud is smaller than one pixel. Therefore, 4 is the maximum number of pixels collecting a part of the charge cloud of a single photon event, if this event hit the center of the 2x2 pixels.

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GEXRF prole was used, but also the Ni-L l,n lines were needed for the angular calibration (see section Angular Calibration). To discriminate between these two uorescence signals, either an angle-wise deconvolution of the XRF spectra can be applied or regions of interest (ROI) can be dened, which are used for the whole angular range. The signicantly decreased statistics in the angle-wise spectra, if the resolution of 0.05 ◦ is to be preserved, can lead to artifacts in the deconvolution process. Therefore, we dened ROIs for the further GEXRF evaluation process. The ROIs for Ni-L α,β and Ni-Ll,n were chosen in a way that the fraction of the respective uorescence line was dominant, while also maximizing the absolute intensity. The compromise led to 94.1 % signal in the Ni-L α,β ROI (780 eV-1000 eV) coming actually from the respective lines and 97.7 % in the Ni-L l,n ROI (600 eV-700 eV) in the cumulative spectrum.

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GEXRF Proles In every frame of the recorded pnCCD images of a GEXRF measurement series, on average 5 photon events were detected. These events are distributed over the whole CCD chip and therefore correspond to various uorescence emission angles. This demonstrates that the detected GEXRF interference origins from the wave function of every single photon, since interference between dierent photons is excluded. The procedure for the assignment of emission angle and solid angle of detection to each pixel position is described below (section Angular Calibration). The measured GEXRF proles are now obtained, by rstly taking all photon events of all frames into account that belonging to a ROI (i.e. uorescence energy) as dened in section XRF Spectra. Then, all photon events belonging to the same angle increment (schematically indicated as red line in gure 4) in every CCD image are counted and normalized to the solid angle of detection of this angle increment. A plot of the normalized photon numbers over the respective emission angles yields the GEXRF proles. 2 It

has to be noted, that the intensity ratios of the Ni-L l,n and Ni-Lα,β lines change with the emission angle according to the grazing emission X-ray uorescence proles. Therefore, also the given numbers for the actual photon ratios in each of the ROIs vary slightly, depending on the emission angle. This eect was found not to be crucial for the further evaluation.

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Figure 3: Exemplary measured and simulated GEXRF proles of measurement sets P1 and P2 for photons belonging to the Ni-L α,β -ROI. In the inset, the simulation was tted to the data by variation of the atomic scattering factors f 1 and f2 of nickel at 849 eV (Ni-L α1 ). The GEXRF measurements are accompanied by estimations of measuring times and simulations of the expected proles by using in-house software based on Sherman's equation. 29,30 The software calculates X-ray uorescence intensities for a given measurement setup, taking into account the eects of the optical properties of a model sample. For the shape of the GEXRF proles, the reciprocal theorem was applied, suggesting to use a virtual X-ray standing wave eld for the intensity calculations. 17,19,31 In gure 3, measured and simulated GEXRF proles of two dierent positions on the multilayer sample ("P1" and "P2") are shown. In the simulation, the Ni-L α,β intensity increases strongly with increasing angles up to 2 ◦ . In this regime, the shape of the curve is dominated by the reectance of the uorescence radiation at the interfaces in the sample, preventing it from reaching the detector. The further increase of the Ni-L α,β intensity with higher angles originates from reduced absorption in the sample due to shorter paths through the sample. However, at angles around 8 ◦ , close to the Bragg's angle of the multilayer for the Ni-Lα,β uorescence photons, the simulations show a sudden increase of the uorescence intensity, followed by an intensity minimum. This can be explained by the used xsw eld 10


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in the calculations, which displays nodes in the Ni layers and antinodes in the C layers for angles at the intensity maximum and vice versa for angles at the intensity minimum of the GEXRF prole. The position of the maximum and minimum intensity is strongly dependent on the layer thickness, while simulations with moderate variations of Debye-Waller factors (modeling interlayer roughness 32,33 ), of layer densities, or even of atomic scattering factors (aecting refraction, reection and absorption) show only a small inuence. Therefore, the maximum and minimum positions can be used to derive the bi-layer thickness gradient. The general shape of the measured proles is comparable to the simulated proles. However, the strong intensity increase at low angles and the intensity maximum predicted by the simulations is strongly damped. A variation of the atomic scattering factors f1 and f2 from the values of Chantler et al. 34 by a factor 0.6 and 3.2 can explain the measured curve (gure 3, inset) and is justied by the proximity of the Ni-L α line (849 eV) and the Ni-L3 absorption edge (853 eV). Small uncertainties of a few eV on the energy axis of the tabulated data in this energy region can easily change the optical constants at the Ni-L α line by an order of magnitude. The distortion of the shape of the curve and of the interference pattern close to the Bragg's angle, which originates from the uncertainties of the atomic scattering factors, renders the analysis of the multilayer structure in terms of roughness and densities unreliable. These analytical questions can be addressed, if the atomic scattering factors for the uorescent energies are accurately determined in advance, e.g. by synchrotron radiation X-ray reectometry measurements. However, the position of the intensity minimum is almost unaected by the atomic scattering factors (the shift in position was smaller than 0.05◦ when using the tted atomic scattering factors) and is used for further evaluation of the bi-layer thickness.

Angular Calibration For the calculation of the GEXRF proles, it is necessary to know the uorescence emission angle corresponding to each pixel on the CCD chip as well as their solid angles of detection. 11



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Figure 4: Geometric parameters, determining the emission angle and solid angle assignment for each pixel on the pnCCD chip. The red parameters are the crucial ones for the angular calibration. The close distance of the CCD chip to the excitation footprint (65.0 mm) greatly increases the total solid angle of detection, but complicates the pixel-to-angle assignment, since the equi-angle lines are bent curves on the detector chip, whose positions and shapes strongly depend on the exact geometry of the setup. With known setup geometry, the emission angles and solid angles of detection can be calculated for every pixel. While the former can be calculated straight forward, the latter is computed according to Asvestas et al. 35 The setup was designed to be exible, with the pnCCD positioned via a exible bellows. This scheme allowed for an adjustable detector distance, but also reduced the knowledge of the exact camera position. In order to dene the exact geometry of the setup, parameters like the distance of the camera to the excitation footprint ( dCCD ), its horizontal and vertical shift (lCCD and hCCD ), as well as rotational angles of the pnCCD ( φCCD , ωCCD , and θCCD ) and of the sample (ωsample ) have to be known (gure 4). For the presented proof-of-principle setup, some of the geometric parameters were measured, but dCCD , lCCD and φCCD had to be determined with a tting algorithm described in the supplementary material. 36 The algorithm

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makes use of calculated and measured GEXRF proles of Ni-L α,β and Ni-Ll,n at measurement series "P2". For the simulations, a bi-layer thickness at measurement position "P2" must be assumed. Of the three complementary measurements shown in section Results, the XRR measurements have the smallest uncertainties. Furthermore, the XRR results depend on the reection and refraction of X-rays in the multilayer, which is also the case for the position of the intensity minimum in the GEXRF proles. For these reasons, a bi-layer thickness of 5.63 nm, retrieved from the interpolated X-ray reectometry measurements, was chosen as reference in the GEXRF simulations. Note, that due to the angular calibration at "P2", the bi-layer thickness results of the XRR and GEXRF measurements at this point are identical. The total uncertainty of the position of the intensity minimum due to the angular calibration procedure is estimated to be 0.05 ◦ . The angular calibration is performed once and holds as long as the camera position is xed with respect to the excitation spot. This is accurate for dierent measurement points on the same sample ("P1" to "P6"), which is shifted through the pivotal point of the goniometer. If a separate sample is analyzed ("Q1"), uncertainties in the angular axis increase due to uncertainties in the sample alignment procedure. If complete knowledge about the measurement geometry is at hand, a calibration sample would not be needed. Indeed, rst estimations of uncertainties and tests in the laboratory with an optical laser showed, that the accuracy of the determination of all geometrical parameters is sucient and thus an absolute angular calibration feasible for soft X-ray GEXRF.

Results The cumulative spectrum of measurement series "P2" (FIG. 2) can also be used to estimate lower limits of detection (LLD) for carbon and nickel. The bi-layer thickness of the multilayer at "P2" was determined by XRR to be d =5.63 nm, with a thickness ratio of

Γ = dNi /dbi-layer = 0.45. Assuming bulk densities ρ for the 15 carbon and nickel layers yield mass deposition values for both elements m ˆ Ni = 15 · dNi · 0.45 · ρNi and m ˆ C = 15 · dC · 0.55 · ρC .

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Figure 5: Comparison of the bi-layer thickness results of XRR, TEM, conventional XRF and GEXRF measurements. The angular calibration of the GEXRF measurements was performed at "P2", using the interpolated bi-layer thickness of the XRR measurements. Furthermore, if Poisson noise is considered to be the main noise contribution, the LLDs are calculated by LLD = m ˆ 3√NN , with N being the number of detected photons of the corresponding uorescence line. In this estimation, the LLDs for carbon and nickel for the cumulative spectrum are 300 ng/cm 2 and 100 ng/cm2 , respectively. The minimum position in the GEXRF proles was evaluated using a Savitzky-Golay lter and choosing the angle with the lowest intensity in the local intensity minimum. Comparison of the minimum position with the simulated positions for dierent bi-layer thicknesses leads to the bi-layer thickness results. The uncertainties for the bi-layer thickness of this rst demonstration measurement are only roughly estimated. They originate from the angular calibration and the determination of the minimum, resulting in angle uncertainties of twice the pixel resolution for "P1" to "P6". "Q1" was mounted and aligned separately, leading to a further contribution to the uncertainties originating from the sample alignment. Furthermore, the low statistics for "Q1" increase the uncertainties for the determination of the minimum position directly. 14


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In gure 5 the results of the bi-layer thicknesses of the multilayer sample obtained with the GEXRF approach are compared to X-ray reectometry (XRR), TEM and conventional XRF measurements. The XRR measurements were performed with a laboratory twin mirror arrangement (TMA) using Cu-K α -radiation. They provide the most reliable results in the set of complementary methods because of the non-preparative approach and high precision. TEM micrographs of cross-sections were recorded with a FEI Tecnai G 2 20 S-TWIN of the Zentraleinrichtung Elektronenmikroskopie (ZELMI) at the Technical University of Berlin. The uncertainties for the bi-layer thickness obtained with the TEM result from statistical deviations of up to ten measurements at both lateral multilayer positions. Clearly, the results of the TEM measurements dier from the XRR measurements, which could be explained by artefacts induced by the preparation process of the TEM lamella. During the preparation of the cross-sections the sample was kept at 130 ◦ C for 30 minutes for the hardening of the glue, which might have induced oxidation and layer broadening. For the conventional XRF measurements, a Fischerscope X-Ray XDV-SDD (HELMUT FISCHER GmbH) was used. The device measured the total mass deposition of Ni, neglecting the absorption in the carbon layers. The value was then converted to a bi-layer thickness via division by the number of bi-layers, the thickness ratio Γ of 0.45 and an assumed reduced layer density by 7.5 % compared to bulk material. The latter two are in accordance with XRR measurements, but the layer density changes with layer thickness, which might explain the indicated deviations. In general, the obtained results with the applied methods illustrate the diculties when quantitatively analyzing thin layer samples.

Discussion In the cumulative spectrum, signals from the low-Z elements carbon and oxygen, as well as Ni were clearly detectable with detection limits in the range of a few hundred ng/cm 2 . In principle, the sensitivity can be increased further by choosing glancing incidence angles or

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evaluating only the uorescence intensity at shallow angles, which would both reduce the background signal. However, while the latter would also decrease count rates drastically, a shallow incident angle in combination with a steep angle of the CCD would lead to an ecient, but not depth sensitive, spectrometer in the soft X-ray range. The GEXRF results describe very well the bi-layer thickness gradient measured by XRR, even though the angular calibration was only performed at "P2". Uncertainties are already satisfactory, but could be decreased further, if the optical constants used in the simulation were better known and the whole GEXRF prole could be used for a tting routine. Indeed, the evaluation performed here is limited to the determination of the bi-layer thickness only, while more information, e.g. about roughness and intermixing, can be derived from the GEXRF proles of multilayer samples. 37 So far, the evaluation process depends on a calibration sample, which also increases uncertainties originating from sample alignment. Therefore, precise knowledge about the measurement geometry and an attempt towards absolute angular calibration is preferable and aspired for this setup. The comparison of layer thicknesses derived with complementary methods shows, that depth proling is a challenging task, even with a rather simple sample structure like a multilayer. When analyzing elemental depth proles, rough samples or contamination in a sample, results from most of the presented complementary methods will be limited. For such sample systems, the non-destructive and quantitative GEXRF approach might be a helpful tool, especially if it becomes a readily available laboratory based method in the future.

Conclusions and Outlook This study demonstrates the analytical potentials of XRF in general and GEXRF in particular in the soft X-ray range with a laboratory setup. By making use of a prominent, characteristic intensity minimum in the GEXRF prole, it was possible to resolve thickness variations of C-Ni bi-layers in a multilayer on the sub-nanometer scale. In contrast to

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GIXRF, where a large footprint on the sample is inevitable, the GEXRF geometry allowed for a lateral resolution of 100 x 500 µm. The scanning-free approach makes it possible to achieve recording times of about one hour for the presented setup and enables to measure the whole GEXRF prole simultaneously. The latter might be, for example, benecial for future non-destructive, in-line process control in the semiconductor industry. Future eort will focus on the improvement of measurement strategy, e.g. by using an optical laser for the precise determination of the measurement geometry. This will make the angular calibration independent of a calibration sample and could reduce uncertainties. Also, after this successful proof-of-principle, a spectrometer could be designed, which is specialized in scanning-free GEXRF measurements. Here, a xed geometry and a reference sample holder could simplify the angular calibration procedure. Concerning the excitation channel, elliptical or toroidal optics are advantageous compared to the utilized KirkpatrickBaez optics with respect to eciency and will further reduce the necessary recording times. With these improvements, non-destructive and spatially resolved analysis of elemental depth proles, e.g. in solar cell material, will be pursued.

Acknowledgement The authors thank the Zentraleinrichtung Elektronenmikroskopie (ZELMI) of the Technical University of Berlin for their support with the TEM analysis and the Excellence Initiative of the Deutsche Forschungsgemeinschaft for nancial support.

Supporting Information Available • Supplementary.pdf: Details of the procedure used for the angular calibration of the GEXRF measurements are given. This material is available free of charge via the Internet at http://pubs.acs.org/ . 17



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For Table of Contents Only 69x44mm (300 x 300 DPI)


Laboratory Setup for Scanning-Free Grazing Emission X-ray

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