FR2960975A1 - DEVICE FOR TOPOGRAPHIC CHARACTERIZATION AND CHEMICAL CARTOGRAPHY OF SURFACES - Google Patents

Abstract

Surface characterization device characterized in that it comprises: a capillary (C) called imaging for guiding a near or mid-infrared, visible, ultraviolet or X-ray electromagnetic radiation from or towards a surface (SE) to be characterized, said capillary having a so-called front end (EA) facing said surface and having an aperture of micrometer or sub-micrometer diameter; and a micromechanical oscillator (DP) for inducing an oscillatory movement of said capillary with respect to said surface. Use of such a device for simultaneously acquiring topographic information and spatially resolved spectroscopic information on a surface to be characterized.

Description

The invention relates to a device for topographic characterization and chemical mapping of surfaces. The device of the invention makes it possible simultaneously to acquire local information of a spectroscopic (fluorescence, and especially X-ray fluorescence) and topographic nature with a high spatial resolution, typically micrometric or sub-micrometric. More particularly, the device of the invention makes it possible to acquire, with the indicated spatial resolution, fluorescence spectra induced by X, ultraviolet or visible excitation radiation, or even in the near or intermediate infrared. This spectral information is also coupled with topographic information (surface relief) with comparable spatial-lateral resolution.

In the case where the radiation used is of type X, its large depth of penetration also makes it possible to characterize layers and buried interfaces. "X-ray radiation" or "X-rays" means electromagnetic radiation having a wavelength of between 0.01 nm and 10 nm or, in an approximately equivalent manner, an energy of between 120 eV (elettronvolts) and 120 keV . By "ultraviolet radiation" or "UV" is meant electromagnetic radiation having a wavelength of between 10 nm and 400 nm (approximately between 3 and 120 eV). By "visible radiation" is meant, conventionally, electromagnetic radiation having a wavelength between 400 nm and 780 nm (approximately between 1.5 and 3 eV). By "near or medium infrared" is meant electromagnetic radiation having a wavelength of between 780 nm and 3 μm (approximately between 0.4 and 1.5 eV).

X-ray spectroscopy is a very powerful technique for the analysis of solids, which makes it possible to know the chemical composition of a sample and even the interatomic distances in this sample.

2 sample. The principle of a conventional X-ray fluorescence (XRF) analysis is shown in Figure 6. An X-ray excitation beam (FEX reference), generated by a synchrotron or a rotating anode-type laboratory source, focused or not, is incident on the surface SE of a sample E to be analyzed. The incident beam is often focused using KB (Kirkpatrick-Baez) and / or Fresnel lens systems (not shown). The exciter beam size is typically of the order of 5 pm 2 in the case of a synchrotron beam, and 100 pm 2 with a laboratory source. This limits the lateral resolution that can be achieved.

A detection system D is placed in the far field, at a distance r (typically a few centimeters) from the irradiated zone to detect the electromagnetic fluorescence radiation emitted by said irradiated zone. This fluorescence radiation can be X-ray, ultraviolet, visible or even in the near or medium infrared. The fluorescence, intensity lo, is emitted on 4n steradians. Therefore, the collected signal is proportional to IoSD / 4nr2, where SD is the surface of the detection system. The distance r between the surface and the detection system can not be arbitrarily reduced for reasons of steric hindrance; therefore, to improve the signal-to-noise ratio it is necessary to increase l0 and thus to work at high incident flux. The maximum flux, however, is limited by the brightness of the excitation radiation source, as well as by the threshold of damage to the surface. The focusing of the excitation beam makes it possible to improve the lateral resolution of the measurement (within the limits indicated above), but not to improve the signal-to-noise ratio, since the irradiated surface decreases in the same ratio as the increase in the incident power density. According to the signal that it is desired to collect (X-ray, UV, visible or even infrared) the detection system D can be: - either an energy dispersive X-ray detection system ("EDX"), which allows spectral analysis of X-ray fluorescence radiation; either a point detection system, such as a photodiode or a CCD camera, coupled to a dispersive system or a monochromator. This technique does not make it possible to acquire information of a topographic nature at the same time as fluorescence spectra. Of course, it is possible to acquire an atomic force microscopy (AFM) image before or after the chemical analysis, but it is difficult to correlate the data acquired separately. The article by Y. Hosokawe et al. "An X-Ray Tube Guide and a Desk-Top Scanning Analytical Microscope," X-Ray Spectrometry, Vol. 26, pp. 280-387 (1997) discloses an X-ray analytical microscope using a capillary to direct a collimated X-ray beam onto a predetermined region of a sample to be characterized. The transmitted x-rays, as well as the secondary X-rays emitted by fluorescence are detected. An optical microscope acquires an image of the region of the sample illuminated by X-rays. The experimental data presented by this article show a spatial resolution of several micrometers, therefore of the same order of magnitude as in conventional XRF analysis. In addition, as in the conventional technique, no topographic information is acquired at the same time as the transmitted and secondary optical images and X-ray spectra. The article by D. Pailharey et al. "Nanoscale x-ray absorption spectroscopy using XEOL-SNOM mode detection," Functional Material and Technologies (FM & NT 2007); Journal of Physics: Conference Series 93 (2007), 012038 describes a surface characterization technique coupling X-ray-excited optical luminescence (XEOL) and near-field optical microscopy. An X-ray beam from a synchrotron is focused on a surface to be characterized. The tip of a near-field optical microscope scans the surface and collects the visible (light) radiation emitted by the irradiated surface. Thus, spectroscopic and topographic data are acquired simultaneously (the latter are obtained by the servo mechanism which maintains the tip of the microscope

4 near-field optics at a constant distance from the surface). The technique only detects radiation emitted by the surface in the visible region of the electromagnetic spectrum (or, at most, between the near infrared and the near ultraviolet). The experimental data presented in this paper show that the spatial (lateral) resolution of the spectroscopic data is greater than one micrometer. In addition, the method is quite complex to implement. US Pat. No. 7,095,822 discloses a micro-probe for near-field X-ray fluorescence. This micro-probe is based on a conventional atomic force microscope tip of the recessed lever type. This built-in lever has a region forming a micro-anode X-ray generator. An electron source directs an electron beam on this micro-anode, which generates an (non-collimated) beam of X-rays. Because of the close proximity between the micro-anode and the surface, only a small region of the latter is irradiated by the generated X-rays, and emits fluorescence radiation captured by a far-field detection system. It is obvious that this technique is very complex to implement, because the presence of an electron beam requires working under ultra-high vacuum. For the same reason, it is not suitable for the characterization of biological samples. The invention aims to remedy, in whole or in part, the disadvantages of the prior art. More specifically, the object of the invention is to provide a device for the topographic and spectroscopic characterization of surfaces which is simple in its structure and in its use, and which can be used with minimal modifications in several spectral ranges (both with respect to excitation radiation and detected secondary radiation) and having a high lateral resolution. According to the invention, this object is achieved by a surface characterization device comprising: a so-called imaging capillary for guiding ultraviolet or X electromagnetic radiation from or towards a surface to be characterized, said capillary having a so-called front end, oriented towards said surface and having an aperture of micrometer or sub-micrometer diameter; and a micromechanical oscillator for inducing an oscillatory movement of said capillary with respect to said surface.

In the device of the invention, the imaging capillary simultaneously performs two functions: it serves as a waveguide for electromagnetic radiation (X, UV, visible or near / medium infrared, continuous or pulsed) excitation, and / or for fluorescence radiation (which can also be X / UV or visible or even infrared); and "feeler" to determine the topography of the analyzed surface. The use of capillaries as waveguides in the X-ray domain is known from the prior art; see for example the following articles: - D. H. Bilderback, "Review of Capillary Optics Meeting Capillary X-ray Optics from the International Capillary Optics Meeting", X-Ray Spectrom. 2003: 32: 195 - 207; and - D. H. Bilderback et al. "Monocapillary Optics Develpments and Applications", Advances in X-ray Analysis, Volume 46, 2002, pages 320 - 325. A capillary is generally defined as a tube having a small inside diameter (typically, less than 1 mm); by extension, it may be a bundle of such tubes, parallel to each other; we then distinguish between "monocapillary" and "polycapillary". In the X-ray field, the refractive index changes little from one material to another; consequently, capillaries made of metal, silicon, glass, silica, etc. can be used. On the other hand, it is necessary for the inner surface to be smooth at a nanoscale. Capillaries used in X-ray optics may have a curvature and are not necessarily cylindrical; on the contrary, a curved and tapered inner shape (e.g., with an elliptical or parabolic profile) is often required to focus X-rays.

It is also possible to use, as X-ray capillaries, hollow-core photonic crystal fibers. These fibers are also suitable for guiding infrared, visible or ultraviolet radiation. In general, they are better adapted to the capture of fluorescence radiation than to the supply of excitation radiation. By "micrometer diameter" is meant a diameter of between about 1 pm and 10 pm; "sub-micrometer diameter" means a diameter less than 1 micron, and in particular between 10 nm and 1 micron.

According to various features of the invention, taken separately or in combination: said imaging capillary may be a mono-capillary, its front end preferably having an opening with a diameter of between 10 nm and 1 μm. - The device may also include an actuating mechanism for causing a relative displacement of the capillary and the surface in three directions of space, a servo-control being provided to maintain a constant distance between the front end of the capillary and the surface. using a feedback signal depending on the amplitude and / or frequency of said oscillatory movement. Preferably, said constant distance may be between 1 and 100 nm, and in particular be of the order of 10 nm. Preferably, the relative displacement in the plane of the surface can be obtained by moving the surface of the sample under the capillary; the displacement perpendicular to the surface, controlled by said servocontrol, can be achieved by the surface or by the capillary. - The device may also include an ammeter capable of being connected to the surface to be characterized for measuring an electric current induced by ultraviolet radiation or X directed to a region of said surface by said imaging capillary. The device may also comprise a second capillary (mono- or, preferably, polycapillary) having a so-called front end having a diameter greater than that of the imaging capillary and oriented towards a region of the surface to be characterized located directly below. the front opening of said imaging capillary. In this case, the device may also comprise a spectral analysis system of an infrared electromagnetic radiation, visible, ultraviolet or X, continuous or pulsed, coupled to said second capillary; and / or an ultraviolet or X-ray source coupled to said imaging capillary. Alternatively, the device may also comprise a spectral analysis system of infrared, visible ultraviolet or X radiation coupled to said imaging capillary and / or a source of radiation in the near or intermediate infrared, visible ultraviolet or X coupled to said auditory second capillary. In a variant, the device may also comprise a separating plate disposed near a rear end of said imaging capillary, opposite said front end, said separating plate being adapted to separate a first beam of electromagnetic radiation directed towards the rear end of said imaging capillary and having at least a first wavelength, a second electromagnetic radiation beam from the rear end of said imaging capillary and having at least a second wavelength different from the first one. In this case, the device may also comprise a source of near-infrared radiation, visible, ultraviolet or X, continuous or pulsed, to generate said first beam of electromagnetic radiation, and / or a spectral analysis system of said second beam of radiation. electromagnetic radiation. In particular, said separator blade may be a monocrystalline blade adapted to pass said first beam and reflect said second beam by Bragg diffraction, or conversely, said first and second beams being X-ray beams. More particularly, said splitter blade may be adapted to reflect a spectral component of said second beam by Bragg diffraction to a radiation detection system, said blade actuating means and said detection system being provided for selecting said spectral component so as to analyze said second beam . Alternatively, said separator blade may be a monocrystalline blade having a metal coating, adapted to let said first beam, X-ray, and reflect said second beam, near / mid-infrared, visible or ultraviolet.

Another object of the invention is the use of a device as described above to simultaneously acquire topographic information and spectroscopic information resolved spatially on a surface to be characterized. Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended drawings given by way of example and which represent, respectively: FIG. 1, a diagram of a device according to a first embodiment of the invention; FIG. 2 is a diagram of a device according to a second embodiment of the invention; FIG. 3 is a diagram of a device according to a first variant of said third embodiment of the invention; - Figure 4, a diagram of a device according to a second variant of said third embodiment of the invention; FIG. 5 is a diagram of a device according to a fourth embodiment of the invention; and Fig. 6 is a diagram of a conventional X-ray fluorescence analyzer. In a first embodiment of the invention, illustrated in FIG. 1, an excitation beam FEX (X or UV rays), generated by an SRE radiation source (synchrotron, laboratory source X, laser, etc.). ) is directed on the surface SE of the sample E to be characterized by a polycapillary C2 having a diameter of several μm (micrometers), so as to irradiate a region RI of said surface having a diameter typically of less than 100 μm, for example from about 1 to 50 μm. The polycapillary C2 is inclined relative to the surface so as not to prevent approaching the monocapillary imaging CI, used

9 to collect the RSF fluorescence secondary radiation and lead it to a detection system D, preferably adapted to perform a spectral analysis of this radiation (EDX detector, monochromator ...). The capillary CI is attached to an arm of a piezoelectric tuning fork DP, the axis of the capillary being parallel to that of said arm and perpendicular to the surface SE of the sample to be characterized. Thus, the capillary CI oscillates in a direction parallel to the surface, like a microscope tip with shear force; the amplitude of the oscillation is at the nanoscale (1 - 100 nm, and typically of the order of 30 nm), and preferably less than the diameter of the opening of the front end EACi of the capillary. As in a microscope of this type, the capillary sweeps a predetermined region of the SE surface, which coincides with - or is included in - the region irradiated by the excitation beam brought by C2. An actuating mechanism is provided for this purpose. In the embodiments of Figures 1 and 2, this mechanism moves the sample in the plane of its surface, while the capillaries are held fixed, not to lose their reciprocal alignment. The actuating mechanism is also provided to move the sample in a vertical direction (perpendicular to the surface and / or parallel to the axis of the capillary CI); this movement is controlled by a servo system which keeps the amplitude and / or the oscillation frequency of the tuning fork constant. In a known manner, this amounts to ensuring a constant distance (from a few nanometers to a few tens of nanometers, for example between 5 and 50 nm) between said front end of the capillary and the surface; we then speak of a "near field" or "mechanical close field" to distinguish this condition from the "optical near field", a notion that depends on the wavelength of the radiation considered.

The recording of the vertical displacements of the sample makes it possible to obtain a profile of the surface SE. The lateral resolution of measurement depends essentially on the opening of the front end EAc1 of the capillary.

The diameter of this aperture is preferably less than or equal to 1 μm, and may be as small as 10-20 nm. The current lower limit is both technological and related to the need to collect a sufficient secondary radiation flux.

To improve the lateral resolution of the topographic characterization of the surface, it is possible to have at the front end of the capillary C1 a thin tip of organic material (polymer), transparent to X-rays. The technology for manufacturing such a tip is described by the documents WO03012504 and FR2734914, as well as in the following articles: - R. Bachelot et al. "Integration of Micrometer-sized Polymers at the End of Optical Fibers by Free-radical Photopolymerization", Applied Optics, Vol. 40, No. 32, pages 5860-5871, November 10, 2001. - M. Hochine et al. The process has been developed for conventional optical fibers, but can be transferred to hollow heart or other types of capillaries. The actuating mechanism and its servocontrol are shown schematically in FIG. 2 (reference MA); they may be those of a conventional near-field optical microscope. Concretely, it is possible to obtain a three-dimensional mapping of the surface with a lateral resolution less than or equal to 100 nm. Alternatively, the actuating mechanism can horizontally move the sample (that is to say in the plane of its surface) and vertically the capillary CI. Indeed, this vertical movement does not affect, or slightly, the alignment between the capillaries. In the "self-aligned" embodiments (FIGS. 3 and 4) it is even conceivable to move the single capillary in the three directions of space and hold the sample steady. The front opening, tapered and curved, of the capillary CI allows to collect in the near field (in the mechanical sense) the secondary radiation emitted by the portion of the surface SE which is directly below said capillary, to bring it to a spectrometer or remote monochromator. The small size of the IC capillary minimizes steric hindrance problems, and allows the collection aperture to be brought closer to the surface. This results in high collection efficiency and high spatial resolution. Depending on the brightness of the radiation source coupled to the capillary C2, the spatial resolution in spectroscopy is of the order of the diameter of the front opening of the capillary CI. If a high gloss excitation source is used, it is possible to reduce the size of the aperture of the IC capillary to obtain side resolutions of less than 100 nm. In the UV domain, such a source can be obtained, for example, by focusing a femtosecond laser beam on a solid surface; see for example G. Kazutaka al, "Picosecond time-resolved X-ray diffraction from laser-shocked semiconductors" Laser and Particle Beams (2004), 22: 3: 285-288. Femtosecond or picosecond lasers, in the visible, near UV or even near or medium infrared, can also be used directly as a source of excitation, including markers in applications to biology. In this case, the use of a capillary is advantageous, compared to that of an optical fiber, because it makes it possible to avoid non-linear effects such as phase auto-modulation, as well as the extension of the pulses to because of the dispersion of the material of the fiber. By way of non-limiting example, the excitation beam may have an energy of between 20 and 25 keV, and the detection spectrometer may then be sensitive in the 2-20 keV range. The exciter beam may be narrowband, especially if it is a beam X, but this is not necessary. Depending on the nature of the sample, the energy of the excitation photons and the brightness of the source, the penetration depth of said photons can vary from about 100 nm to several tens of micrometers (with a synchrotron source ). In certain embodiments, the device of the invention thus makes it possible to chemically characterize the subsurface of the sample (buried layers and interfaces), in addition to the surface in the proper sense.

In a variant of the embodiment of FIG. 1, the second capillary C2 could be omitted. In this case, the excitation beam would be a focused beam in free propagation. The piezoelectric tuning fork DP may be replaced by any other micromechanical, and in particular microelectromechanical oscillator, suitable. This is true for all embodiments of the invention. The embodiment of FIG. 2 differs from that of FIG. 1 in that the imaging capillary CI is used to bring the excitation beam (excitation in the near field, at least in the mechanical sense) onto the sample. , and the second capillary C2 is used to collect the signal (far-field detection). The main advantage of this alternative embodiment is that, simultaneously with the chemical and topographic analysis, it is possible to measure the electrical current IXB, c of the sample under irradiation X ("XBIC", for "X-Beam Induced Current ", ie current induced by beam X). This provides additional information on the flow of charges near defects below the surface or on the electrical behavior of nanoparticles embedded in a matrix. Since the current is generated only by the very small irradiated region RI ', an ammeter AM connected to the surface makes it possible to perform an XBIC measurement with sub-micrometric spatial resolution. In a variant of the embodiment of FIG. 2, the second capillary C2 could be omitted. In this case, the detection of the fluorescence signal would be as in the case of FIG. 6. The first and the second embodiment of the invention pose the problem of the alignment between the excitation and the detection capillary. . The third embodiment, of which two variants are the subject of FIGS. 3 and 4, solves this problem by using the single IC imaging capillary both as excitation source and as secondary radiation collection probe (as well as in as topography probe of the sample surface). This allows a self-alignment of the device. This embodiment requires a splitter plate to separate the FEX excitation beam, from the source and to be injected into the capillary CI through its open back end, and the RSF fluorescence secondary radiation that exits said rear end towards the source. of the detection system. If the excitation radiation is of the ultraviolet type, it is possible to use a separating plate of conventional type capable of operating in this spectral range; in this case, the excitation beam may pass through the slide and the fluorescence radiation be reflected, or vice versa. In the case of X-type excitation radiation and X-ray fluorescence detection, it is preferable to use a monocrystalline LM plate capable of producing a Bragg diffraction of said excitation radiation or fluorescence radiation. For example, at 20 keV one can use a HOPG (Highly Oriented Pyrolytic Graphite) plate, which is substantially transparent at this wavelength. As shown in Figure 3, the FEX excitation beam passes through the LM blade. The fluorescence beam RSF is deflected by the blade only if the condition of Bragg is respected, ie if the plate LM forms with the surface SE an angle 0 such that 2dsin (0) = na, where d is the distance between the atomic planes of the monocrystalline plate, is the wavelength of the fluorescence radiation (or rather of one of its spectral components) and n is an integer different from zero. It is understood that the LM blade is not only a separating plate, but also a spectral selection element. If it is motorized to rotate at an angular velocity w, and the detection system D rotates at an angular velocity 2 w, a monochromator is produced in the 0-20 configuration. It is therefore no longer necessary to use an EDX detector a simple PIN junction is enough.

If one is interested in the detection of an infrared, visible or ultraviolet fluorescence, it is sufficient to deposit a thin CM metal layer (approximately 200 nm thick) on the LM plate, functioning as a mirror (FIG. 4) to reflect this infrared, visible or UV fluorescence radiation to an optical spectrometer SM. If one wishes to return to an X-ray fluorescence analysis, as the X-rays pass through the thin metallic layer, it suffices to change the detection system - by replacing the spectrometer by simple PIN junction - and motorize the blade (and the system of detection) to find the configuration of Figure 3.

The microelectromechanical oscillator coupled to the capillary CI has been omitted from FIGS. 3 and 4 for the sake of clarity. So far only the case of an imaging capillary CI mounted to oscillate in a direction parallel to the surface SE has been considered. This is not a limitation of the invention, which can also operate by oscillating the imaging capillary in a direction perpendicular to the surface, such as an atomic force microscope (AFM) tip. Figure 6 shows such a configuration, in which the capillary IC is very short, and in fact constituted by a pierced AFM tip.

Like the embodiment of FIG. 2, those of FIGS. 3 to 5 allow measurement of the IxBic current.

Claims (16)

REVENDICATIONS1. Surface characterization device characterized in that it comprises: a capillary (CI) called imaging for guiding a near or mid-infrared, visible ultraviolet or X-ray electromagnetic radiation from or towards a surface (SE) to be characterized, said capillary having a so-called front end (EAc1) oriented towards said surface and having an opening of micrometric or sub-micrometric diameter; and a micromechanical oscillator (DP) for inducing an oscillatory movement of said capillary with respect to said surface. 2. Device according to claim 1 wherein said imaging capillary is a mono-capillary and its front end has an opening diameter of between 10 nm and 1 pm. 3. Device according to one of the preceding claims also comprising an actuating mechanism (MA) for causing a relative movement of the capillary and the surface in three directions of space, a servo-control being provided to maintain a constant distance between the front end of the capillary and the surface by exploiting a feedback signal depending on the amplitude and / or the frequency of said oscillatory movement. 4. Device according to one of the preceding claims also comprising an ammeter (AM) capable of being connected to the surface to be characterized for measuring an electric current (IxB, c) induced by a near or mid-infrared radiation, visible, ultraviolet or X directed to a region of said surface by said imaging capillary. 5. Device according to one of the preceding claims also comprising a second capillary (C2) having a so-called end having a diameter greater than that of the imaging capillary and oriented to a region of the surface to be characterized located directly below the front opening of said imaging capillary 6. Device according to claim 5 also comprising a system (D) for spectral analysis of a near or mid-infrared electromagnetic radiation, visible, ultraviolet or X coupled to said second capillary. 7. Device according to claim 6 also comprising a source (SRE) of near or mid-infrared radiation, visible, ultraviolet or X coupled to said imaging capillary. 8. Device according to claim 5 also comprising a spectral analysis system of near or medium infrared radiation, visible, ultraviolet or X coupled to said imaging capillary. 9. Device according to claim 8 also comprising a near or medium infrared radiation source, visible, ultraviolet or X coupled to said second capillary. 10. Device according to one of claims 1 to 4 also comprising a separator blade (LM, CM) disposed near a said rear end of said imaging capillary, opposite said front end, said separator blade being adapted to separate a first electromagnetic radiation beam, said excitation beam, directed towards the rear end of said imaging capillary and having at least a first wavelength, of a second electromagnetic radiation beam, said secondary beam, emitted by said surface and emerging from the rear end of said imaging capillary and having at least a second wavelength different from the first. 11. Device according to claim 10 also comprising a source of near or mid-infrared radiation, visible, ultraviolet or X to generate said first beam of electromagnetic radiation. 12. Device according to one of claims 10 or 11 also comprising a spectral analysis system of said second beam of electromagnetic radiation. 13. Device according to one of claims 10 to 12 wherein said separating plate is a monocrystalline blade (LM) adapted to pass said first beam and reflect said second beam by 17 Bragg diffraction, or conversely, said first and second beams. being X-ray beams. 14. Device according to claims 12 and 13 wherein said splitter plate is adapted to reflect a spectral component of said second beam by Bragg diffraction to a radiation detection system, said blade actuating means and said detection system being provided to select said spectral component so as to analyze said second beam. 15. Device according to one of claims 10 to 12 wherein said separating plate is a monocrystalline blade having a metal coating (CM), adapted to let said first beam X-ray, and to reflect said second beam, close or medium infrared, visible or ultraviolet. 16. Use of a device according to one of the preceding claims for simultaneously acquiring topographic information and spectroscopic information resolved spatially on a surface (SE) to be characterized.