Combining scanning probe microscopy and x-ray spectroscopy
© Fauquet et al; licensee Springer. 2011
Received: 3 November 2010
Accepted: 7 April 2011
Published: 7 April 2011
A new versatile tool, combining Shear Force Microscopy and X-Ray Spectroscopy was designed and constructed to obtain simultaneously surface topography and chemical mapping. Using a sharp optical fiber as microscope probe, it is possible to collect locally the visible luminescence of the sample. Results of tests on ZnO and on ZnWO4 thin layers are in perfect agreement with that obtained with other conventional techniques. Twin images obtained by simultaneous acquisition in near field of surface topography and of local visible light emitted by the sample under X-Ray irradiation in synchrotron environment are shown. Replacing the optical fibre by an X-ray capillary, it is possible to collect local X-ray fluorescence of the sample. Preliminary results on Co-Ti sample analysis are presented.
Non destructive tools providing elemental and chemical analysis at high lateral resolution are needed for life and physical sciences. For example electronics or glass industries need sub-100 nm resolution tools for material processing and control (RRAM, FeRAM, smart materials, solar cells) . During the last ten years, numbers of characterization tools were thus developed to obtain with the same apparatus sample imaging and chemical mapping. For example TEM (Transmission Electron Microscopy) is combined with EELS (Electron Energy Loss Spectroscopy) techniques to study oxidation states in transition metal oxides . Near Field Microscopes are powerful tools for surface topography and analysis at nanometric lateral resolution. These equipments allow various in-situ spectroscopies, to probe surface local magnetic properties , electronic states  or even to identify and localize specific chemical group on very small features . Combination of equipments can give further insights in sample analysis as, e.g. a combination of PEEM with STM . However, those techniques are not simultaneously performed, so that authors had to mark the surface to recover the PEEM analysis localization for STM imaging at the same place.
Conventional X-Ray Absorption Spectroscopies are fine analysis techniques providing chemical and structural properties of a material, based on the spectroscopy of the emitted photons or photoelectrons. They require a high brightness X-Ray excitation source, usually a synchrotron beam, to irradiate the sample. Emergent high resolution microscopies take advantage of X-ray analysis to perform chemical mapping on samples . For example, STXM (Scanning Transmission X-Ray Microscope) in transmission mode  and XPEEM (X-ray Photoemission Electron Microscopy) enable to obtain a sample chemical contrast and electronic structure from individual nanostructures [8, 9].
Coupling X-Ray Spectroscopy and Scanning Probe Microscopy allows collecting with the microscope probe, the sample emission (electron, photons) under X-ray excitation, leading to surface topography and chemical mapping at high resolution at the same place. This concept is now widely investigated in synchrotron environment [10–13].
In this work, we present a versatile Shear Force Microscope head, which can be coupled to an X-ray beam illuminating the sample just at the level of microscope probe apex. This microscope has been fitted to a synchrotron beam line, to simultaneously perform XAFS-XEOL (X Ray Absorption Fine Structure - X Ray Excited Optical Luminescence) spectroscopy, and surface topography. A sharp optical fiber is used as microscope tip for sample topography and for local sample visible luminescence collection. Spectra exhibit the variation of the visible light intensity as a function of incident primary beam energy. As an absorption threshold, characteristic of an emitting element present in the material is crossed, the intensity of the visible light drastically increases and is followed by oscillations linked to the atomic environment and structure of this element . Chemical mapping was achieved on ZnO and ZnWO4 - ZnO samples. μ-XRF (micro X-Ray Fluorescence) analysis was successfully carried out on Co-Ti samples, replacing the optical fibre, microscope probe, by a thin X-ray capillary and using a rotating anode (Cu Kα) as excitation source.
This apparatus also enables the XRF signal local collection of the excited sample, replacing the device tuning fork-optical fibre by a fixed X-ray cylindrical capillary (internal diameter 10 μm, length 50 mm). The sample is excited by a rotating anode (excitation at constant energy, Cu Kα at 8 keV, power 40 kV × 40 mA) while the fluorescence signal is analyzed by EDX (Energy Dispersive X-ray). The excitation beam is focused on the sample by a capillary lens (spot diameter 20 μm) provided by IFG GmbH. The XRF technique is particularly suitable for analysis of heavy elements, typically heavier than sodium.
Collecting the XEOL signal in near field significantly increases the lateral resolution of this technique, which is now only limited by the aperture of the optical fibre. In fact, the resolution of the apparatus is limited by the tip curvature for topography (~100 nm) and by the optical aperture for the light collection (~50 nm).
In-lab μ-XRF analysis
Where θc is the critical angle of the capillary material (in our case fused silica with θc of about 5 mrad at the X-ray energy considered in this paper ), D is the detector-sample distance and d is the capillary diameter. G is about 3.103 (resp. 3.105) for a 50 mm long and 10 μm (resp. 1 μm) diameter cylindrical capillary approached at 5 mm from the sample surface. Moreover, the use of elliptical instead of cylindrical capillary would further increase the signal level by a factor 20 [20, 21]. Our experience shows that we can combine X-ray capillary optics for both excitation and detection to substantially increase the resolution of in-lab XRF technique which can be better than 1μm keeping a significant signal to noise ratio and remaining in satisfactory acquisition times .
Conclusion and perspectives
We have constructed a new Shear-Force Microscopy head that is able to simultaneously record the topography and the light emitted by a sample. We have demonstrated in synchrotron environment the possibility of simultaneous XEOL mapping and surface topography with a resolution of 50 nm. The instrument is thus able to image the surface and to localize a peculiar object that can be further chemically analyzed by XEOL analysis. Thanks to the recent development of new X-Ray capillary lens, we now equip our home-made Shear Force Microscope with a tightly focused laboratory X-ray source for on-table simultaneous Luminescence-Topography measurements. The sensitivity of the technique, limited by the signal to noise ratio, will be evaluated in the future.
We have demonstrated the concept feasibility of XRF analysis at micrometer scale. In fact, replacing the optical fibre of our microscope head by a 10 μm diameter cylindrical capillary, we succeeded in local collection of sample XRF under X-ray illumination using an in-lab source. The signal level obtained in this work enables to estimate that the lateral resolution of the technique can still be improved. Consequently sub-1 μm resolution can be reached in lab, whereas, using brighter excitation sources (synchrotron), sub-100 nm resolution is expected, limited today by capillary technology. The final idea is to use an elliptic capillary as shear force probe to simultaneously obtain topography and the XRF mapping of the sample.
The authors thank European Community for financial support through FP6-XTIP strep project and FP7-Eureka-Eurostars LUMIX contracts.
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