DE102018222792B4 - Laser-induced emission spectrometry for quick structural analysis - Google Patents

Abstract

Method for fast, spatially resolved element distribution analysis of a sample surface (2) of a sample (1) of a material, whereby in a method step for evacuation and filling with plasma gas a sample chamber (7) is evacuated and filled with a plasma gas, in a first method step in the sample chamber (7) a volume of the sample surface (2) is vaporized at a measuring point on the sample surface (2) by a focused laser pulse and electronically excited in a gas plasma (4 '), in a second process step a line spectrum of a light (3) emitted by the gas plasma (4 ') is emitted, is spectrally broken down and registered by a spectrometer, a new measuring point is selected in a third process step, the first process step, the second process step and the third process step being run through cyclically, with when the first process step is cyclically run through, the second process step and the third th process step, the measuring points are selected so that a selected measuring area is scanned on the sample surface (2), the sample chamber (7) being evacuated by a first vacuum pump (8) and at least one second vacuum pump (9) during the process step for evacuation, whereby for evacuating the sample chamber (7) above a switching pressure in the sample chamber (7) the first vacuum pump (8) and the at least one second vacuum pump (9) are connected to the sample chamber (7) for parallel pumping and the first when the switching point is reached Vacuum pump (8) and the at least one second vacuum pump (9) are connected in series for pumping.

Description

State of the art

The present invention is based on methods for analyzing the local distribution of elements in metal production and processing.

In the manufacture and processing of steel and flat steel products, a fast, spatially resolved determination of structural components, such as segregations, residues from casting aids, non-metallic inclusions and precipitations, as well as internal cracks, metallic and non-metallic coatings and conversion layers on surface-refined flat steel products or surface defects or surface finishes, is essential Not possible on areas of up to 10 dm ^2.

Such a rapid analysis is of great interest on larger sample areas, which for example correspond to a quarter of a slab cross section or a fifth of the width of a strip section.

The Baumann print is known from the prior art. This can be taken from the surface of slab sections after milling and grinding the surface using sulfuric acid and photo paper. The Baumann print can be used to detect sulfur segregation, but only if there is a sufficient amount of sulfur and sufficiently large sulfur gradients in the steel.

The local distribution of carbon, nitrogen, sulfur, phosphorus, aluminum, silicon, manganese, chromium or copper can be determined on very small samples of the size of a few square millimeters on polished sample surfaces in very time-consuming electron microscopic methods such as EDX, BSE, AES, WDX or Measure EELS. All of these processes are associated with immense effort, for example with EELS the sample has to be thinned to a few nanometers and only capture a fraction of the interesting elements with sufficient sensitivity. A combination of the methods mentioned is ruled out due to the amount of work and time required.

Scanning laser-induced optical emission spectrometry (LOES) is also able to determine light elements such as boron, carbon, nitrogen, oxygen, phosphorus, sulfur, chlorine or fluorine. The use of LOES in materials science is described, for example, in the publications “Developement of inclusion reference materials and simultaneous determination of metals and non-metallic inclusions by rapid LIBS analysis in steel samples” (Boue-Bigne, et al.) Or “Fast laser-based analysis of metallic and non-metallic inclusions in steel ”(Sturm and Noll).

Also disclosed JP 2010 038560 A a device and a method for the determination of element concentrations of samples, including steel samples, by means of LIBS / L-OES.

WO 2001 033202 A1 discloses a device for optical emission spectroscopy with which the homogeneity or mixing of pellets of up to 10 μm in size made of plutonia and uranium oxide can be determined by scanning the sample using an XY table.

The pamphlet DE10 2005 000840 A1 discloses a method and apparatus for element analysis using laser emission spectroscopy. The laser beam is split into several parts by a lens array in order to simultaneously cover a larger measuring area.

Furthermore, from the publication DE 199 32 069 A1 a device for laser-induced emission spectrometry is known, in which an improvement in the size of the investigable areas on the sample is achieved by a special arrangement of the optical elements. Further devices for laser-induced breakdown spectroscopy (LIBS), including on steel, are, for example, from the publications US 2006/0 173 4321 A1 , US 2016/0 069 745 A1 and DE 10 2007 016 612 A1 known.

Disclosure of the invention

It is therefore an object of the present invention to provide a method and a device which can be used for the quantitative determination of the elements B, C, N, P, O, S, Al, Si, Na, K, Mg, Ca, Sr, Ba, Ti, Ta, Zr, Cr, Mo, W, Mn, Co, Ni, V, Nb, Cu, Ag, Zn, Sn, Pb, Bi and rare earth metals (e.g. Ce) as well as internal cracks and their local distribution with a detection limit (please refer DIN 32645: 2008-11 ) of <100 µg / g and the elements Cl and F with a detection limit of <1 mg / g at a high examination speed on areas of eg 10 dm ^{2 is} suitable.

The object is achieved by a method according to claim 1. In order to be able to examine larger areas than previously at a higher rate than previously quantifying in a particularly advantageous manner, the first method step, the second method step and the third method step at a rate of 400 to 5000 per Second (Hertz, Hz), preferably at 800 to 2000 Hz, in particular at 800 to 1200 Hz.

The sample to be examined is located in the sample chamber with the sample surface to be examined facing the laser beam. The sample surface is preferably milled or ground flat on the surface to be examined, so that the arithmetic mean roughness value R _a (see DIN EN 10049: 2014-03 ) is less than 10 μm, preferably less than 5 μm, particularly preferably a maximum of 3 μm. The waviness value W _a is preferably (see DIN EN ISO 4287: 2010-07 ) less than 60 μm, preferably less than 40 μm, particularly preferably a maximum of 20 μm. The laser pump modules, laser head and emission spectrometer are preferably thermostated above room temperature in order to avoid condensation of air humidity and to ensure spectrometric stability both during the examination of a sample and between two calibrations. This guarantees measurement results with stable quality over a long measurement of a sample as well as several samples. It is conceivable that a detection of local defects is carried out by previous light or electron optical analysis, if necessary with previous treatment with acidic pickling solution, preferably with nitric, hydrochloric and / or sulfuric acid or alkaline pickling solution. This advantageously makes it possible to concentrate on clearly critical areas during the LOES.

It is preferably provided that the sample is fastened on a sample table that can be moved in the main plane of extent. The sample table is preferably moved perpendicular to the beam of the laser so that the focus of the laser beam lies in the measuring point. The minimum travel distance of the sample table is preferably less than 50 μm, particularly preferably less than 30 μm, in particular less than 10 μm. The unidirectional or bidirectional repeat accuracy of the sample table is preferably less than 2 μm or +/- 10 μm, particularly preferably less than 1 μm or +/- 5 μm, in particular less than 0.5 μm or +/- 2.5 μm. The maximum positioning speed is preferably more than 50 mm / s, particularly preferably more than 100 mm / s. The acceleration or deceleration of the sample table is preferably less than 150 mm / s ² , particularly preferably less than 100 mm / s ² . The sample table is preferably suitable for taking samples of a size of up to 210 mm x 210 mm, a thickness of up to 30 mm and a weight of up to 11 kg, particularly preferably of a size of up to 410 mm x 270 mm, a thickness of up to 50 mm and a weight of up to 45 kg. Provision is preferably made for the sample table to receive a plurality of samples, preferably up to ten samples, the overall dimensions of which do not exceed the dimensions that the sample table is suitable for receiving. This makes it possible to examine samples in the desired sizes.

Furthermore, it is preferably provided that the sample chamber has a light source and / or a camera. A measuring position can thus be approached in an advantageous manner.

In order to minimize aberrations, the laser beam is preferably focused by means of an aspherical convex lens or one of several spherical convex and concave lenses. The laser beam illuminates a measuring point on the sample surface. A volume of the sample surface is vaporized by laser ablation and electronically excited in a plasma and sometimes also ionized. A diode-pumped Q-switched solid-state laser is preferably used as the laser, the wavelength of which is between 400 nm and 2200 nm, preferably between 1000 nm and 1100 nm, particularly preferably about 1030 nm or about 1064 nm and the pulse duration of which is between 0.1 ns and 20 ns half-width, preferably between 5 ns and 10 ns half width, its pulse energy between 0.1 mJ and 10 mJ, preferably between 1 mJ and 5 mJ, in particular around 2 mJ and its beam profile quality M ² preferably between 1.0 and 1.2, but at least is below 1.3 (with TEMoo). Preferably, it is provided that coaxial double or multiple pulses with between 1 microseconds and 20 microseconds variable, preferably 0.5 microseconds precisely adjustable, particularly preferably 0.1 microseconds precisely adjustable, time interval between the partial pulses are output in which the pulse energy of the partial pulses can be distributed equally or also in a defined unequal manner over all partial pulses. A focused laser beam is preferably used which is suitable for ablating sample material with a lateral resolution of preferably less than 40 µm, particularly preferably less than 15 µm (both based on an Fe matrix) and generating plasmas with sufficiently high emission intensity. Furthermore, it is provided that the intensity of the laser can be attenuated in a reproducible manner with an iris diaphragm and / or a pair of polarization filters that can be rotated relative to one another and / or a rotatable semi-permeable mirror. The expansion of the laser beam is preferably reduced with the aid of optical components, particularly preferably with a telescope structure. This advantageously enables the depth of field of the focused laser beam to be increased in order to reduce negative influences of the sample topography on the scattering of the emission intensity of the plasma radiation. It is conceivable that the wavelength, pulse energy, time sequence of the partial pulses, beam profile quality and opening angle of the laser beam are selected so that the best possible spatial resolution and the necessary detection sensitivity are achieved for the given topography of the area to be examined.

The decay of electronically excited states in the plasma emits light. The emitted light contains characteristic element-specific line spectra of the plasma gas or plasma gases and the atomized sample vapor contained therein. The light from the spectrometer is broken down into these and the intensity of the lines of defined wavelengths, which can be assigned to the analyte or reference elements, is determined. A spectrometer is preferably used which is suitable for reading out spectra and / or the intensity of individual emission lines with a repetition rate of up to 3 kHz. It is also provided that emissions resulting from molecular vibrations are measured with the spectrometer or a second spectrometer. This advantageously enables conclusions to be drawn about the form of binding of the substances examined. The signal intensities of the emission lines of the analyte elements of the sample are preferably based on the intensity of the emission lines of the main matrix element of the sample (e.g. in the case of steel Fe (I) 273.0734 nm and / or Fe (I) 371.993 nm) or other lines that are not in saturation of the main matrix element and / or non-saturation emission lines of the plasma gas (e.g. in the case of argon Ar (II) 434.806 nm and / or Ar (II) 460.957 nm, or in the case of neon or helium corresponding lines) and / or the of the beam reflected in the zeroth order is referenced. The raw signal intensities or the referenced intensities are calibrated using standard materials of known chemical composition. The composition can then be determined on the basis of the raw or referenced signals of samples of previously unknown composition.

The sample is scanned by the focused and pulsed laser beam. For this purpose, the sample is moved step by step from the sample table in such a way that the focused laser beam hits the sample surface at a new measuring point within the grid to be recorded after each movement step. The sample is moved one step size in each movement step. The step size of all movement steps is preferably the same. A measurement is made at each position of the grid, i.e. at the respective measuring point. Even with a laser pulse length of less than 10 ns, the distance between two raster points of less than 100 µm and a repetition rate of 3000 Hz, it is virtually irrelevant for the geometry of the laser ablation crater (round / oval) whether the sample is stationary during the ablation step or is moved with, for example, 300 mm / s (3 · 10 ^-3 µm in 10 ns) and is only decelerated at the ends of a raster line and re-accelerated in the opposite direction offset by one raster line. The totality of the measuring points is the measuring range. With 1000 examined measuring points per second and a step size of 20 µm, for example, an area of 200 mm x 200 mm is measured within around 34 hours and the same area with a step size of 100 µm within 3.5 hours, with the acceleration and the Delay of the sample at the beginnings and ends of lines outside the area to be examined have already been taken into account.

However, it is also conceivable that a first measurement area is initially scanned in a first measurement step with a larger first step size and second measurement areas in the first measurement area, the analysis of which could be of great interest, are identified from the data obtained. This could be local defects, for example. Then, in a second measuring step, the second measuring areas are scanned with a smaller step size. This advantageously enables the method to be carried out, on the one hand, in a time-saving manner and, on the other hand, with a high spatial resolution of the areas of interest.

Furthermore, it is preferably provided to thermostate the laser at least partially and / or the spectrometer above room temperature in order to avoid condensation of air humidity and to ensure the spectrometric stability both during the examination of a sample and between two calibrations.

It is also conceivable that the optical components, such as, for example, the optics of the laser and / or the spectrometer with baffles, are protected from ablated material.

Advantageous configurations and developments of the invention can be found in the subclaims and the description with reference to the drawings.

According to a preferred embodiment of the present invention it is provided that a gas or gas mixture comprising helium, neon and / or argon is used as the plasma gas in the first method step.

According to the invention, it is provided that during the process step for evacuation, the sample chamber is evacuated by a first vacuum pump and at least one second vacuum pump, the first vacuum pump and the at least one second vacuum pump for pumping in parallel with the sample chamber to evacuate the sample chamber above a switching pressure in the sample chamber are connected and when the switchover point is reached, the first vacuum pump and the at least one second vacuum pump are connected in series for pumping. This makes it possible to pump down the sample chamber to a good vacuum in a very short time. The vacuum to be achieved is the target vacuum

Oil-free vacuum pumps such as scroll, screw and / or Roots pumps are preferably used.

According to a further preferred embodiment of the present invention, it is provided that before the method step for evacuation and filling with plasma gas, an evacuated buffer volume that is pushed off towards the sample chamber is opened to the sample chamber. As a result of the pressure equalization between the buffer volume and the sample chamber, the air pressure in the sample chamber drops. When the pressure equalization is complete, the buffer volume is pushed off again. This also makes it possible to shorten the time it takes for the target vacuum to be reached. Volumes present in the measurement setup are preferably used as buffer volumes. These can be internal volumes of racks, optical tables, frames or holders, for example. It is conceivable to pump out the buffer volume with the first vacuum pump and / or the second vacuum pump and / or to pump out the combination of claim 2. It is also conceivable to evacuate the buffer volume as soon as the sample chamber is not evacuated, for example while a new sample is being inserted or the measuring process. The measuring chamber is then filled with the pure noble gas argon, neon or helium or a mixture of these gases as plasma gas to a suitable target pressure. The best detection sensitivity is achieved for different analyte elements at different target pressures. The speed at which the sample aerosol is discharged from the area of the laser focus is also dependent on the pressure of the plasma gas. Since this discharge is necessary before the next laser pulse, the maximum repetition rate with which the scanning examination can be carried out is also dependent on the target pressure of the plasma gas.

According to a further preferred embodiment of the present invention it is provided that a gas flow of the plasma gas is passed into the sample chamber during the first method step or during the first method step and the second method step or during the first method step and the second method step and the third method step, preferably During the process step for evacuation and filling with plasma gas, the plasma gas is passed diagonally through the sample chamber, for which purpose the connections for the plasma gas supply and the first vacuum pump and / or the second vacuum pump are ideally at a maximum distance from one another. The gas flow is preferably laminar and is directed along the sample surface, particularly preferably along the sample surface and along the optical components. By using neon and / or helium (1st ionization potential at 21.6 eV (Ne) or 24.6 eV (He)) and / or their mixtures with argon ( 1 . Ionization potential at 15.8 eV) as a plasma gas, it is possible to electronically excite and thus detect fluorine, whose intense emission transitions require comparatively high excitation energies. By guiding the gas flow along the sample surface, ablated sample material is also advantageously removed from the measurement area and thus ensures that the following laser pulse does not hit the aerosol cloud or condensate on the sample surface from the previous pulse, even at high repetition rates in the kHz range.

It is also conceivable that, during the method step for evacuating the sample chamber, a laminar plasma gas stream flows through the sample chamber diagonally. This advantageously has the effect that the sample chamber is flushed free of the room air by the plasma gas in a significantly shorter time.

According to a further preferred embodiment of the present invention it is provided that the plasma gas is circulated and filtered. This advantageously makes it possible to use less plasma gas and to avoid contamination of the sample surface as well as the optical and mechanical components in the measuring chamber and the resulting damage. The filters are preferably easily accessible for regular replacement.

According to a further preferred embodiment of the present invention, it is provided that a range of preferably more than 10 mm × 10 mm, particularly preferably more than 210 mm × 210 mm, very particularly preferably more than 270 mm × 410 mm is selected as the measuring range . This enables samples to be measured in a range that is interesting for industrial applications.

According to a further preferred embodiment of the present invention it is provided that all components necessary for performing the method are placed on the sample surface in order to close the sample chamber and a wall of the sample chamber is formed by the sample surface. This makes it possible to carry out the process in a mobile manner and instead of installing the sample in the sample chamber, the entire measuring instrument can be placed on the sample. It is conceivable that the sample is not moved to scan the measurement area, but rather the attached components are moved. This could be done with an industrial robot, for example. This means that there are even larger areas than those mentioned Simplified sampling and preparation and thus to examine at a very high speed.

According to a further preferred embodiment of the present invention it is provided that, in preparation for the measurement, a reagent layer, preferably graphite, gold, silver, zinc oxide, porphyrins, phthalocyanine, azo compounds, benzoic acid, its esters or salts, which reduces the reflectivity of the sample surface and / or the excitation properties of the sample plasma is increased, is applied to the sample surface. In this way, an optimization of the detection strength is achieved in an advantageous manner. It is conceivable, for example, to apply a graphite film or a layer of metal or metal oxide particles with a suitable particle size as a layer to reduce the reflectivity, the elements contained in these particles having to be different from the analyte elements. Particularly advantageous here are gold and / or silver nanoparticles (independent of most analytical issues) and ZnO particles in the case of steel as the sample matrix. It is also conceivable to increase the excitation properties of the sample plasma, for example, by means of organic, optionally aromatic, compounds whose binding energies are matched to the laser wavelength. Derivatives of aromatic compounds with low vapor pressure such as benzoic acid and its esters and salts, porphyrins and phthalocyanine and their metal complexes, but also azo compounds such as sodium 4- [4- (dimethylamino) phenylazo] benzenesulfonic acid and its esters and salts are advantageous. Parameters for optimizing the excitation properties of the sample plasma could be, for example, the electron density or the gas temperature. However, it is also conceivable to apply a plasmonic structure whose electron gas density configuration leads to local field increases and thus to improved properties of the sample plasma and / or to a significantly increased detectability of molecular vibrations.

According to a further preferred embodiment of the present invention, it is provided that before the cyclical execution of the first method step, the second method step and the third method step, a height profile is measured perpendicular to the main plane of extent of the sample surface, with fluctuations in the measurement results caused by the height profile being compensated for using the height profile . It is conceivable that the height profile is determined by means of a line laser scanner. A calibration matrix is preferably determined from the height profile, with which the measured intensities are normalized and computationally adapted. However, it is also conceivable that the laser focus for each measuring point is set on the sample surface on the basis of the ascertained height profile. It is also conceivable that the sample surface is moved into the laser focus for each measuring point on the basis of the ascertained height profile with the sample table. It is also conceivable that the focus of the laser for each measuring point is adapted to the sample surface on the basis of the determined height profile in such a way that the focus of the laser lies on the sample surface. This makes it possible to measure samples with simple sample preparation and / or to increase the measurement accuracy.

According to a further preferred embodiment of the present invention it is provided that the aperture of the focused laser pulse is set by means of a telescope.

Another object of the present invention for solving the problem set out at the beginning is a device according to claim 11 for the rapid, spatially resolved element distribution analysis of a sample surface of a sample of materials, comprising a laser, a spectrometer, a gas-tight sample chamber, a first vacuum pump, the sample surface inside the Sample chamber is arranged,
The laser is suitable for evaporating a small volume of the sample from the sample surface, at least partially atomizing it and electronically stimulating it in a gas plasma, the spectrometer being suitable for generating a line spectrum of a sample-specific light produced when the gas plasma decays or individual spectral lines therefrom, To register reference lines as well as molecular bands.
The sample to be examined is located in the sample chamber with the sample surface to be examined facing the focused beam of the laser. The sample surface is preferably milled or ground flat on the surface to be examined, so that the arithmetic mean roughness value R _a (see DIN EN 10049: 2014-03 ) is less than 10 μm, preferably less than 5 μm, particularly preferably a maximum of 3 μm. The waviness value W _a is preferably (see DIN EN ISO 4287: 2010-07 ) less than 60 μm, preferably less than 40 μm, particularly preferably a maximum of 20 μm. The laser preferably has a laser head and laser pump modules, the laser head and / or the laser pump modules and / or the spectrometer being particularly preferably thermostated above room temperature. This guarantees measurement results with stable quality over an investigation that is extended over a long period of time.

It is preferably provided that the sample is attached to a sample table that can be moved in the main extension plane. The sample table is preferably perpendicular to the laser beam relative to the The focus of the laser beam can be moved so that the focus of the laser beam is in the measuring point on the sample surface. The minimum travel distance of the sample table is preferably less than 50 μm, particularly preferably less than 30 μm, in particular less than 15 μm. The unidirectional or bidirectional repeat accuracy of the sample table is preferably less than 2 μm or +/- 10 μm, particularly preferably less than 1 μm or +/- 5 μm, in particular less than 0.5 μm or +/- 2.5 μm. The maximum positioning speed is preferably more than 50 mm / s, particularly preferably more than 100 mm / s. The acceleration or deceleration of the sample table is preferably less than 150 mm / s ² , particularly preferably less than 100 mm / s ² . The sample table is preferably suitable for taking samples of a size of up to 210 mm x 210 mm, a thickness of up to 30 mm and a weight of up to 11 kg, particularly preferably of a size of up to 410 mm x 270 mm, a thickness of up to 50 mm and a weight of up to 45 kg. Provision is preferably made for the sample table to receive a plurality of samples, preferably up to ten samples, the overall dimensions of which do not exceed the dimensions which the sample table is suitable for receiving. This makes it possible to examine samples in the desired sizes.

Furthermore, it is preferably provided that the sample chamber has a light source and / or a camera. A measurement position can thus be approached in an advantageous manner.

The focused laser beam illuminates a measuring point on the sample surface. A volume of the sample surface is vaporized by laser ablation and electronically excited in the gas plasma. The laser is preferably a diode-pumped Q-switched solid-state laser, whose wavelength is between 400 nm and 2200 nm, preferably between 1000 nm and 1100 nm, particularly preferably about 1030 nm or about 1064 nm and whose pulse duration is between 0.1 ns and 20 ns half-width between 5 ns and 10 ns, its pulse energy between 0.1 mJ and 10 mJ, preferably between 1 mJ and 5 mJ, in particular at 2 mJ and its beam profile quality M ² preferably between 1.0 and 1.2, but at least below 1 , 3 (at TEMoo). It is preferably provided that the laser is suitable for outputting coaxial double or multiple pulses with a time interval between the partial pulses variable between 1 μs and 20 μs, preferably 0.5 μs precisely adjustable, particularly preferably 0.1 μs precisely adjustable, between the partial pulses the pulse energy of the partial pulses can be distributed equally or also in a defined unequal manner over all partial pulses. The laser is preferably suitable for ablating sample material with a lateral resolution of preferably less than 40 µm, particularly preferably less than 20 µm (both based on an Fe matrix) and generating sample plasmas with sufficiently high emission intensity. The laser preferably has a repetition rate of 400 Hz to 5000 Hz, particularly preferably from 1000 Hz to 2000 Hz, very particularly preferably from 800 Hz to 1200 Hz. The wavelength of the laser is preferably between 200 nm and 2000 nm, particularly preferably between 1000 nm and 1100 nm, in particular at 1030 nm or 1064 nm +/- 5 nm. The pulse energy of the laser is preferably between 0.1 mJ and 10 mJ, particularly preferably between 1 mJ and 4 mJ, in particular at 2 mJ +/- 0, 5 mJ. Furthermore, it is provided that the intensity of the laser can be attenuated in a reproducible manner with an iris diaphragm and / or a pair of polarization filters that can be rotated relative to one another and / or a rotatable semitransparent mirror. The expansion of the laser beam can preferably be reduced with the aid of optical components, particularly preferably with a telescope structure. This advantageously enables the depth of field of the focused laser beam to be increased and thus reduces the dependence of the emission intensity of the plasma on the sample topography.

In order to minimize aberrations, the laser beam is preferably focused by means of an aspherical convex lens or one of several spherical convex and concave lenses. The laser beam illuminates a measuring point on the sample surface. Due to the disintegration of excited states of the sample material in the gas plasma, specific light is emitted for the sample composition. The emitted light contains characteristic element-specific line spectra. The light is split into these by the spectrometer and the wavelengths of the lines and their intensities are determined. The spectrometer is preferably suitable for recording and reading out spectra or discrete element lines with a repetition rate of up to 5 kHz. It is also provided that the spectrometer or a second spectrometer can register emissions that result from molecular vibrations. This advantageously enables conclusions to be drawn about the form of binding of the substances examined.

The focused and pulsed laser beam scans the sample. For this purpose, the sample can be moved step by step from the sample table in such a way that the laser beam hits the sample surface at a new measuring point after each movement step.

Furthermore, it is preferably provided to at least partially thermostate the laser and / or the spectrometer. The target temperature is ideally above the ambient temperature in order to avoid the formation of condensation.

According to the invention it is provided that the device has at least one second vacuum pump, the arrangement of the first vacuum pump and the at least one second vacuum pump being switchable between parallel arrangement and sequential arrangement of the first vacuum pump and the at least one second vacuum pump. The vacuum pumps are preferably oil-free.

According to a further preferred embodiment of the present invention it is provided that the device has a buffer volume which can be pushed off towards the sample chamber for buffering a vacuum. The buffer volume can be evacuated when it is shut off. If the evacuated buffer volume that has been pushed off is opened to the sample chamber, the pressure in the sample chamber drops due to a pressure equalization between the sample chamber and the buffer volume. When the pressure equalization is complete, the buffer volume is pushed off again. This also makes it possible to shorten the time it takes for the target vacuum to be reached. The buffer volume preferably consists of volumes present in the measurement setup. These can be internal volumes of racks, optical tables, frames or holders, for example. It is conceivable to pump out the buffer volume with the first vacuum pump and / or the second vacuum pump and / or to make it evacuable according to the combination of claim 2. Due to the optical transparency in the relevant wavelength range between 120 nm and 800 nm and the first ionization potential necessary for the electronic excitation of the above-mentioned analyte elements, including the particularly difficult to excite fluorine, the noble gases argon, neon and helium and, for reasons of cost, mixtures thereof are advantageous.

According to a further preferred embodiment of the present invention it is provided that the device has a device for generating a gas flow of the plasma gas in the sample chamber. The gas flow is preferably designed so that it does not affect the position of the laser focus.

According to a further preferred embodiment of the present invention it is provided that the device has a device for circulation and a filter for cleaning the plasma gas. It is thus advantageously possible to reduce gas consumption and to minimize contamination of the sample surface as well as the optical and mechanical components and to avoid damage to the device.

According to a further preferred embodiment of the present invention it is provided that the sample is a wall of the sample chamber. This makes it possible to use the device as a mobile device. The device can thus advantageously be exposed to a sample. It is conceivable that the parts of the device which are attached in the sample chamber and the parts of the sample chamber which are not the sample are designed as a measuring head. It is also conceivable that the measuring head for examining a measuring area is designed to be movable over the sample. It is also conceivable that the measuring head is attached to an industrial robot for this purpose.

According to a further preferred embodiment of the present invention, it is provided that the sample surface relative to the position of the focus of the laser beam around an area along the main extension plane of the sample surface of preferably more than 10 mm x 10 mm, particularly preferably more than 210 mm x 210 mm, is very particularly preferably movable by more than 270 mm × 410 mm, the minimum step size of the movement along the main plane of extent preferably being less than 20 μm, particularly preferably less than 15 μm. This enables the investigation of measuring ranges in orders of magnitude, which are interesting for industrial applications with high spatial resolution.

According to a further preferred embodiment of the present invention it is provided that the device has a device for measuring a height profile of the sample surface perpendicular to the main extension plane of the sample surface. It is conceivable that the device for measuring the height profile has a line laser scanner.

Further details, features and advantages of the invention emerge from the drawing and from the following description of preferred embodiments with reference to the drawing. The drawing only illustrates exemplary embodiments of the invention, which do not restrict the essential inventive concept.

Figure list

• 1 shows a schematic illustration of the generation of the sample plasma and the resulting light according to a preferred embodiment of the invention.
• 2 shows a schematic illustration of the generation of the sample plasma according to an exemplary embodiment of the present invention.
• 3 shows a schematic representation of the interconnection of the first vacuum pump and the second vacuum pump in a preferred embodiment of the invention.
• 4th shows a schematic representation of the interconnection of the first vacuum pump, the second vacuum pump and the buffer volume in a preferred embodiment of the invention.

Embodiments of the invention

In the various figures, the same parts are always provided with the same reference numerals and are therefore usually only named or mentioned once.

In 1 is a schematic of the basic principle of laser ablation of sample material and generation of the gas plasma 4 ' and the resulting light 3 shown in accordance with an exemplary embodiment of the present invention. A thermostated diode-pumped Q-switched laser (not shown) emits a laser beam 4th with 1064 nm wavelength with double pulses with a pulse energy of 2 mJ. The laser beam 4th is applied with optical components (not shown) to a measuring point on the sample surface 2 the sample 1 steered and largely collimated. The resulting Gaussian profile of the laser beam 4th thus has a great Rayleigh length. A pair of polarization filters (not shown) that are twisted against each other regulate the intensity of the laser beam 4th down to be able to reduce the size of the ablation crater for optimal lateral resolution.

The focused laser beam 4th ablates parts of the sample with the first laser pulse of the double pulse 1 on the sample surface 2 at the measuring point. The ablated parts of the specimen 1 are from the laser beam 4th with the second laser pulse of the double pulse to the gas plasma 4 ' electronically stimulated. When the excited states in the gas plasma decay 4 ' emits this light 3 with a line and band spectrum, which is characteristic of the elements and element connections present in the ablated material. Intensity and wavelengths of the spectral lines and bands of the spectrum are recorded by a thermostatted spectrometer (not shown). The spectrometer references the measurement in the case of steel as sample material on the emission lines Fe (I) 273.0734 nm and / or Fe (I) 371.993 nm and in the case of argon as plasma gas on the lines Ar (II) 434.806 nm and Ar ( II) 460.957 nm. In particular, the spectrometer also measures molecular bands.

The sample surface 2 is milled flat and with a reagent layer to reduce the reflectivity of the sample surface and a layer to enhance the excitation properties of the gas plasma 4 ' Mistake.

The sample 1 is mounted on a sample table (not shown). The sample table carries the sample up to 410 mm x 270 mm in size, 50 mm in thickness and 45 kg in weight 1 . Around a measurement area (not shown) on the sample surface 2 to examine, the sample table moves the sample 1 along its main extension plane, measuring point by measuring point, meandering over the measuring range previously defined by the user with a step size of 20 µm. A laser pulse is used to generate a plasma at each measuring point and a spectrum or parts of it are recorded.

In order to obtain particularly meaningful results, a height profile of the sample surface is created before starting the measurement of the measuring range 2 recorded. With the help of this height profile, the data obtained are normalized with regard to the recorded intensities for each measuring point within the rasterized area.

In 2 is a schematic of the generation of the sample aerosol and the gas plasma 4 ' shown in accordance with an exemplary embodiment of the present invention. Before the measurement, the sample chamber 7th evacuated by air. In order to reduce the disruptive influence of oxygen, nitrogen and carbon dioxide, the sample chamber is flushed diagonally with the plasma gas, which is a mixture of argon and neon, and filled to a target pressure during evacuation. During the measurement there is a laminar gas flow 5 from plasma gas along the sample surface 2 guided. This gas flow 5 takes ablated material with it and thus prevents contamination of the areas still to be examined and falsification of the measurement data as well as damage to the mechanical and optical components in the measurement chamber. The gas flow 5 is circulated and in a filter 6th filtered. To reduce the measuring time, the measuring range is first roughly measured with a large step size. An analysis of the data obtained shows interesting areas in the measuring range. These areas are then examined again, now with a small step size, that is to say with a high spatial resolution.

In 3 is a schematic of the circuitry of the first vacuum pump 8th and the second vacuum pump 9 with the sample chamber 7th illustrated in accordance with an exemplary embodiment of the present invention. For evacuation at high pressures in the sample chamber 7th are the first vacuum pump 8th and the second vacuum pump 9 with the first valves 10 wired so that these parallel the sample chamber 7th evacuate. This results in faster evacuation. When the pressure in the sample chamber 7th has fallen below a certain value, the first valves 10 wired so that the second vacuum pump 9 behind the first vacuum pump 8th is switched. This enables a better target vacuum.

In 4th is a schematic of the circuitry of the first vacuum pump 8th and the second vacuum pump 9 with the sample chamber 7th and the buffer volume 11th illustrated in accordance with an exemplary embodiment of the present invention. During a previous measurement, the buffer volume 11th , which is located inside the device frame (not shown) of the device, from the first vacuum pump 8th and the second vacuum pump 9 evacuated. This is done first by having the first vacuum pump 8th and the second vacuum pump 9 operated in parallel. The pressure in the buffer volume falls 11th below a certain value, it will be the first vacuum pump 8th and the second vacuum pump 9 with the first valves 10 and the second valves 10 ' wired so that the second vacuum pump 9 is connected behind the first vacuum pump. Is the previous measurement finished and a new sample in the sample chamber 7th inserted, the first vacuum pump 8th and the second vacuum pump 9 with the first valves 10 and the second valves 10 ' wired in such a way that this is the sample chamber 7th evacuate. At the same time the buffer volume 11th via a third valve (not shown) to the sample chamber 7th open to. There is a pressure equalization between the sample chamber 7th and the buffer volume 11th instead, reducing the pressure in the sample chamber 7th sinks. Once the pressure equalization has been completed, the third valve is closed and the sample chamber is closed 7th further pumped out. Like the buffer volume 11th also becomes the sample chamber 7th initially with a parallel connection of the first vacuum pump 8th and the second vacuum pump 9 evacuated. If the pressure inside the sample chamber falls below 7th a certain value, so with the help of the first valves 10 and the second valves 10 ' the first vacuum pump 8th and the second vacuum pump 9 wired so that the second vacuum pump 9 behind the first vacuum pump 8th is switched.

List of reference symbols

1

sample

2

Sample surface

3

light

4th

laser beam

4 '

Gas plasma with electronically excited sample material in it

5

Gas flow

6th

filter

7th

Sample chamber

8th

First vacuum pump

9

Second vacuum pump

10

First valves

10 '

Second valves

11

Buffer volume

Claims (17)

Method for fast, spatially resolved element distribution analysis of a sample surface (2) of a sample (1) of a material, wherein In a process step for evacuating and filling with plasma gas, a sample chamber (7) is evacuated and filled with a plasma gas, in a first process step in the sample chamber (7) a volume of the sample surface (2) is vaporized at a measuring point on the sample surface (2) by a focused laser pulse and electronically excited in a gas plasma (4 '), In a second process step, a line spectrum of a light (3), which is emitted by the gas plasma (4 '), is spectrally broken down and registered by a spectrometer, a new measuring point is selected in a third process step, the first process step, the second process step and the third process step being run through cyclically, wherein when the first method step, the second method step and the third method step are cyclically run through, the measuring points are selected in such a way that a selected measuring area is scanned on the sample surface (2), wherein during the process step for evacuation, the sample chamber (7) is evacuated by a first vacuum pump (8) and at least one second vacuum pump (9), whereby for evacuating the sample chamber (7) above a switching pressure in the sample chamber (7) the first vacuum pump (8) and the at least one second vacuum pump (9) are connected to the sample chamber (7) for parallel pumping and the first when the switching point is reached Vacuum pump (8) and the at least one second vacuum pump (9) are connected in series for pumping. Procedure according to Claim 1 , wherein in the first process step a gas or gas mixture comprising helium, neon and / or argon is used as the plasma gas. Method according to one of the preceding claims, wherein before the method step for evacuating and filling with plasma gas, an evacuated buffer volume (11) pushed off towards the sample chamber (7) is opened towards the sample chamber (7). The method according to any one of the preceding claims, wherein during the first process step or during the first process step and the second process step or during the first process step and the second process step and the third process step a gas stream (5) of the plasma gas is passed into the sample chamber (7), the plasma gas preferably already during the process step for evacuation diagonally through the sample chamber ( 7) is directed. Procedure according to Claim 4 , whereby the plasma gas is circulated and filtered. Method according to one of the preceding claims, wherein a range of preferably more than 10 mm x 10 mm, particularly preferably more than 210 mm x 210 mm, very particularly preferably more than 270 mm x 410 mm is selected as the measuring range. Method according to one of the preceding claims, wherein all components necessary for carrying out the method are placed on the sample surface (2) for closing the sample chamber (7) and a wall of the sample chamber (7) is formed by the sample surface (2). Method according to one of the preceding claims, wherein in preparation for the measurement a reagent layer, preferably graphite, gold, silver, zinc oxide, porphyrins, phthalocyanine, azo compounds, benzoic acid, its esters or salts, which reduces the reflectivity of the sample surface (7) and / or the excitation properties of the sample plasma (4 ') increased, is applied to the sample surface (7). Method according to one of the preceding claims, wherein before the cyclical execution of the first method step, the second method step and the third method step, a height profile is measured perpendicular to the main extension plane of the sample surface (7), fluctuations in the measurement results caused by the height profile being compensated for using the height profile. Method according to one of the preceding claims, an aperture of the focused laser pulse being set by means of a telescope. Device for fast, spatially resolved element distribution analysis of a sample surface (7) of a sample (1) of materials, having a laser, a spectrometer, a gas-tight sample chamber (7), a first vacuum pump (8) and at least one second vacuum pump (9), the first vacuum pump (8) and the at least one second vacuum pump (9) for parallel pumping or for Pumps can be connected in series with the sample chamber (7), wherein the first vacuum pump (8) and the at least second vacuum pump (9) are configured to pump in parallel to evacuate the sample chamber (7) above a switching pressure in the sample chamber (7) and to pump in series when a switching point is reached, wherein the sample surface (2) is arranged inside the sample chamber (7), wherein the laser is suitable for evaporating a small volume of the sample (1) from the sample surface (2) and exciting it in a gas plasma (4 '), wherein the spectrometer is suitable for registering a line spectrum of a sample-specific light (3) or individual spectral lines, reference lines and / or molecular bands arising therefrom when the gas plasma (4 ') decays. Device according to one of the preceding claims, wherein the device has a buffer volume (11) which can be pushed off towards the sample chamber (7) for buffering a vacuum. Device according to one of the preceding claims, wherein the device has a device for generating a gas flow (5) of a plasma gas in the sample chamber (7). Device according to one of the preceding claims, wherein the device has a device for circulation and a filter (6) for cleaning the plasma gas. Device according to one of the preceding claims, wherein the sample (1) is a wall of the sample chamber (7). Device according to one of the preceding claims, wherein the sample surface (2) relative to the position of the focus of the laser around an area along the main extension plane of the sample surface (2) of preferably more than 10 mm x 10 mm, particularly preferably more than 210 mm x 210 mm, very particularly preferably more than 270 mm × 410 mm, the minimum step size of the movement along the main plane of extension being preferably less than 20 μm, particularly preferably less than 15 μm. Device according to one of the preceding claims, wherein the device has a device for measuring a height profile of the sample surface (2) perpendicular to the main extension plane of the sample surface (2).

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