Method and system for calibrating a charged particle spectrometer

Abstract

The present invention provides a method for producing a monochromatic incident charged particle beam (FP) having a first energy E1; B. generating an incident laser beam (FL) having a second energy E2; C. irradiating a surface (SE) of a sample (Ech) with the incident laser beam (FL) to generate an evanescent electromagnetic field (EV) in a region (R) near the surface; and D. spatially and temporally overlapping the incident laser beam and the incident charged particle beam within the region to couple via the evanescent electromagnetic field, thereby producing a plurality of incident charged particle beams spectrally separated by a value equal to the second energy E2. A method for calibrating a charged particle spectrometer (SM) includes the steps of: generating a charged particle beam, referred to as an output beam (FS), having a spectrum with distinct energy peaks; E. measuring all or part of the spectrum of the output beam using the spectrometer, and then determining an energy change ΔE relative to the first energy E1 of at least two of the distinct energy peaks; and F. determining from the energy change ΔE a value for the scale factor S and a value for the offset O specific to the measurement of the spectrum of the output beam by the spectrometer.

Description

[Technical Field]

[0001] The present invention relates to the field of charged particle spectrometers, and more particularly to the calibration of charged particle spectrometers. A "charged particle spectrometer" refers to a spectrometer designed to measure the energy spectrum of charged molecules, electrons or ions. [Background technology]

[0002] Figure 1A shows a typical electron energy loss spectrum (or EELS) when electrons interact with light. The central zero-energy peak is known as the zero-loss peak (ZLP). Two peaks are visible on either side: the first-order stimulated EELS peak (right) and the stimulated electron energy gain spectrum (EEGS) peak (left).

[0003] As is well known, measurements made with charged particle spectrometers, and more particularly electron spectrometers, are subject to several types of calibration errors.

[0004] Figure 1B shows the effect of linear spectrometer calibration errors on the measurement of the electron energy spectrum of the spectrum shown in Figure 1A. For example, Figure 1B shows a symmetric shift of the peaks relative to zero toward energies with larger absolute values ​​than those observed in Figure 1A. More generally, this is the effect of scaling errors.

[0005] Figure 1C shows the effect of spectrometer offset type errors on the measurement of electron energy spectra with the spectrum shown in Figure 1A. Here, it can be seen that the peaks are asymmetrically shifted by the same value O to higher energies than those observed in Figure 1A. More generally, the effect of offset errors is to add a value equal to the offset O to the energy values ​​of the EEGS and EELS peaks. This value O can be negative.

[0006] Figure 1D shows the effect of nonlinear spectrometer error on the measurement of an electron energy spectrum with the spectrum shown in Figure 1A. It can be seen that the peaks are shifted asymmetrically relative to zero, and there is a difference between the EEGS and EELS peaks relative to Figure 1A.

[0007] In the best electron spectrometers, typically used in electron microscopes for EELS spectroscopy, the linear precision and accuracy are limited to about 1% (10 eV for 1000 eV or 20 meV for 2 eV). Here, "linear precision and accuracy" refers to the precision of the determination of S and O, respectively. In practice, there are typically two known methods for calibrating the scale factor S and offset O of charged particle spectra:

[0008] The first method involves using a standard sample through which a charged particle beam passes and which has known spectral characteristics (absorption, gain) at known energies. These spectral characteristics can be determined by prior calculation or experiment. The scale factor S and offset O can then be determined by measuring the spectrum of the charged particle without passing it through the standard sample and then measuring the spectrum of the charged particle with the standard sample. However, this method has one major drawback. In fact, uncertainty regarding the transition energy in solids leads to uncertainty in the determination of the scale factor S and offset O. For example, the Ni L absorption edge in NiO can be determined with an accuracy of 0.1 eV, but the exact energy position of the L absorption edge depends on the actual oxidation state of Ni, so the accuracy is low.

[0009] The second method is to change the energy of the emitted beam particles by a known amount, typically by applying a magnetic field or potential. Then, by measuring the charged particle spectrum twice at two different energies, the scale factor S and offset O can be determined.

[0010] However, currently both methods suffer from uncertainties due to deviations in the electron trajectory caused by various effects (e.g., stray fields of charged particle optics, aberrations, etc.) These effects lead to uncertainties in the determination of the scale factor S and offset O.

[0011] The present invention aims to overcome certain problems of the prior art. To this end, the present invention provides a method (and associated system) for calibrating a charged particle spectrometer, comprising the step of creating coupling between a laser beam and a charged particle beam via an evanescent electromagnetic field. This coupling produces a charged particle beam with a spectrum having a plurality of distinct energy peaks spectrally separated by an energy equal to the energy of the laser beam. The charged particle spectrometer is used to determine the energy variation of at least two distinct energy peaks relative to the energy of the charged particle beam. From the energy variation, values ​​for a scale factor S and an offset value O specific to the spectrometer measurement of the output beam spectrum can then be determined.

[0012] The method of the present invention has the advantage that it can determine the scale factor S and offset O with an accuracy limited only by the accuracy of the laser beam energy. The method provides an order of magnitude improvement in measuring the scale factor S and offset O compared to other methods known in the art. [Prior art documents] [Non-patent literature]

[0013] [Non-Patent Document 1] Barwick, B., Flannigan, DJ, & Zewail, AH (2009). Photon-induced near-field electron microscopy. Nature, 462(7275), 902-906 [Non-patent document 2] F. Javier Garcia de Abajo et al., Nano Letters, 10, 1859 (2010) Summary of the Invention

[0014] To this end, an object of the invention is a method for calibrating a charged particle spectrometer, comprising the following steps: A. Producing a monoenergetic incident charged particle beam having a first energy E1. B. Producing an incident laser beam having a second energy E2. C. Illuminating the surface of the sample with an incident laser beam to generate an evanescent electromagnetic field in a region near said surface. D. Spatially and temporally overlapping the incident laser beam and the incident charged particle beam within said region to couple via said evanescent electromagnetic field, producing a charged particle beam referred to as an output beam having a spectrum having a plurality of distinct energy peaks spectrally separated by a value equal to a second energy E2. E. Using a spectrometer, measuring all or a portion of the spectrum of the output beam and then determining the energy change ΔE relative to the first energy E1 of at least two different energy peaks. F. From the energy change ΔE, determining a value of a scale factor S and an offset O specific to the measurement of the spectrum of the output beam by said spectrometer.

[0015] Preferably, the energy change ΔE of each of the at least two different energy peaks is equal to ΔE=±p×E2 (where p is a positive or zero integer equal to the number of inter-peak spacings separating the different energy peak from the spectral peak of the first energy E1), and the energy change ΔE of each of the at least two different energy peaks is determined by the spectrometer in step E according to the following relationship, referred to as the first equation:

[0016]

number

[0017] According to a first embodiment, the nonlinearity experienced by the charged particle is determined to be weak or zero, and in step E, the energy change ΔE of two different energy peaks numbered with indexes 1 and 2, respectively, is determined via a first equation to obtain the following first system of equations (S1):

[0018]

number

[0019] Preferably, in the first embodiment, the integer m>1 inter-peak spacing separating the two peaks for which the energy change ΔE is determined is such that the error in determining the scale factor value is 1% or less of the second energy.

[0020] According to the second embodiment, it is determined that the nonlinearity experienced by the charged particle is not weak or non-zero, and in step E, the energy change ΔE of N>2 different energy peaks, each numbered by index i∈[1;N], is determined via the first equation to obtain the following second system of equations (S2):

[0021]

number

[0022] Preferably, the method of the second embodiment includes a step following step F of iteratively minimizing the value of the NL function by repeating steps A to F multiple times and varying the parameters of the path and detection of the charged particles during each iteration.

[0023] According to an embodiment, step D further comprises the sub-step of measuring the spectrum of the incident laser beam simultaneously with the generation of the output beam, and the determination of the value of the scale factor S and the value of the offset O is made from the measurement of the spectrum of the incident laser beam.

[0024] Another object of the invention is a system for calibrating a charged particle spectrometer, said system comprising: a charged particle source adapted to generate a monoenergetic incident charged particle beam having a first energy E1; a laser source adapted to generate an incident laser beam having a second energy E2. -optical and charged particle transport assembly adapted for: irradiating an incident laser beam onto a surface of a sample to generate an evanescent electromagnetic field in a region near said surface; Spatially and temporally overlapping the incident laser beam and the incident charged particle beam within the region to couple via the evanescent electromagnetic field, thereby producing a charged particle beam, referred to as an output beam, having a spectrum with a plurality of distinct energy peaks spectrally separated by a value equal to a second energy E2. The spectrometer is adapted to measure all or a portion of the spectrum of the output beam, and the system further includes a processor coupled to the spectrometer and adapted to: determining the energy change ΔE relative to the first energy E1 of at least two different energy peaks; From the energy change ΔE, determining the value of the scale factor S and the value of the offset O specific to the measurement of the spectrum of the output beam by said spectrometer.

[0025] Preferably, the incident laser beam is a continuous beam or a pulsed beam with a spectral width of less than 40 meV.

[0026] According to one embodiment, the system includes an additional optical spectrometer adapted to measure the spectrum of the incident laser beam simultaneously with generating the output beam, the processor also being connected to the additional spectrometer, and the determination of the value of the scale factor S and the value of the offset O being made from the measurement of the spectrum of the incident laser beam.

[0027] Preferably, the laser source and transport assembly is adapted to provide an incident laser beam intensity of 10 at said region. 8 W / cm 2 It is adapted to be more than.

[0028] Preferably, the laser source and transport assembly are adapted to polarize the incident laser beam in a direction compatible with the geometry and symmetry of the sample so as to locally maximize the evanescent field intensity in said region.

[0029] Preferably, the transport assembly includes an off-axis paraboloid adapted to focus an incident laser beam onto a surface of the sample, the paraboloid having an aperture through which the incident charged particle beam passes, so that after the incident laser beam reflects off the off-axis paraboloid, the incident charged particle beam co-propagates with the incident laser beam towards the region.

[0030] Preferably, the system includes a resonant optical cavity for an incident laser beam in which a sample is disposed, said transport assembly and said optical cavity being further adapted to cause the incident laser beam to undergo multiple reflections within the optical cavity as it passes through said region. [Brief explanation of the drawings]

[0031] Further features, details and advantages of the invention will become apparent from reading the description given with reference to the accompanying drawings, given by way of example and showing respectively the following:

[0032] [Figure 1A] FIG. 1 shows a typical electron energy loss spectrum (EELS) when an interaction occurs between electrons and light. [Figure 1B] FIG. 1B illustrates the effect of spectrometer offset-type errors on the measurement of an electron energy spectrum having the spectrum of FIG. 1A. [Figure 1C] FIG. 1B illustrates the effects of scaling, offset, and nonlinear errors on the measurement of an electron energy spectrum having the spectrum of FIG. 1A. [Figure 1D] FIG. 1B illustrates the effect of nonlinear errors on the measurement of an electron energy spectrum having the spectrum of FIG. 1A. [Figure 2] FIG. 1 is an illustration of a method of the present invention for calibrating a charged particle spectrometer. [Figure 3] 1 is a schematic illustration of a system of the present invention for calibrating a charged particle spectrometer; [Figure 4A] FIG. 2 is an explanatory diagram of the evanescent field intensity generated in a region. [Figure 4B] FIG. 1 is an energy diagram illustrating the charged particle-photon interaction of step D of the method of the present invention. [Figure 4C] FIG. 1 shows an example of an energy loss spectrum with three EELS peaks, three EEGS peaks, and a ZLP peak. [Figure 5]FIG. 1 is a schematic illustration of a particular embodiment of the present invention, wherein the system of the present invention includes an additional optical spectrometer adapted to measure the spectrum of the incident laser beam simultaneously with the generation of the output beam. [Figure 6] 1 is a schematic illustration of a particular embodiment of the invention in which the system of the invention includes an optical cavity in which the sample is placed. [Figure 7] FIG. 4 is an illustration of a method according to a second embodiment of the present invention.

[0033] In the drawings, elements are not drawn to scale unless otherwise indicated. DETAILED DESCRIPTION OF THE INVENTION

[0034] 2 is an illustration of a method of the present invention for calibrating a charged particle spectrometer SM. As previously mentioned, the method of the present invention aims to calculate a scale factor S and an offset O associated with the measurements of the spectrometer SM.

[0035] Figure 3 shows a schematic representation of a system 1 according to the invention for calibrating a charged particle spectrometer SM. The system 1 is particularly adapted for carrying out the method of the invention shown in Figure 2. The system 1 comprises, inter alia, a charged particle source SP, a laser source SL, an optical and charged particle transport assembly SO, a sample Ech, and a spectrometer SM.

[0036] In step A of the method of the present invention, a charged particle source SP generates an incident charged particle beam FP. The charged particle source SP is adapted so that the beam FP is monochromatic and has a first energy E1, where "monochromatic" means that the beam FP has the first energy E1±0.1%.

[0037] In step B, the laser source SL emits a second energy

[0038]

number

[0039] The laser sources SL and SP are configured such that the beams FL and FP are directed towards the transport assembly SO.

[0040] In step C, the transport assembly SO directs an incident laser beam FL to illuminate the surface SF of the sample Ech and generate an evanescent electromagnetic field EV in a region R near the surface SF.

[0041] FIG. 4A shows, for illustrative purposes only, an illustration of the evanescent field strength EV in a region R based on the distance d from a surface SF.

[0042] The sample of the present invention can take any form known to those skilled in the art that allows for the generation of an evanescent field EV by laser irradiation. By way of non-limiting example, the sample can be a metal surface, a metal nanotube, a metal nanowire, a nanosphere, an optical fiber, a ring fiber cavity, a waveguide, or more generally an optical cavity.

[0043] In step D, the transport assembly SO spatially and temporally overlaps the incident laser beam FL and the incident charged particle beam FP within the region R. This overlapping allows coupling between the beams FL and FP via an evanescent field EV. This coupling allows the generation of a charged particle beam, referred to as the output beam FS, consisting of the absorption or emission of photons from the laser beam FL by the charged particles and having a spectrum with multiple distinct energy peaks spectrally separated by a value equal to the second energy E2.

[0044] This coupling mechanism is known to those skilled in the art and is described in detail in the paper "Photon / electron interactions" by Friedrich Schwarz, "Electron-photon interactions," IEEE Transactions on Photonics, Vol. This phenomenon is a nonlinear mechanism in which a charged particle in beam FP with initial energy E1 absorbs or emits n ≥ 1 photons, gaining or losing energy equal to a multiple of the energy E2 of a photon in laser beam FL. After this interaction, the energy of the charged particle is E'1 = E1 ± n × E2, where E'1 is the energy of the charged particle in output beam FS. Figure 4B shows an energy diagram illustrating this interaction.

[0045] As a result of this photon / charged particle interaction, the output beam FS therefore has an energy spectrum with multiple distinct energy peaks at energies E'1 = E1 ± n × E2 (n ≥ 1) and energy E1 (the so-called ZLP peak). Hereinafter, for ease of notation, the output beam FS will be referred to as having an energy spectrum including multiple distinct energy peaks at energies E'1 = E1 ± p × E2 (p is an integer greater than or equal to 0).

[0046] In step E, the output beam is directed to a spectrometer SM, which measures all or part of the spectrum of the output beam FS. More precisely, the spectrometer measures the energy loss spectrum of the output beam FS. For the purposes of the present invention, it is essential that the measured part of the output beam FS spectrum contains at least two distinct energy peaks so that the scale factor S and the offset O can be determined (see below).

[0047] FIG. 4C shows an example of an energy loss spectrum obtained in step E, including three EELS peaks, three EEGS peaks, and a ZLP peak at zero energy change (i.e., a peak corresponding to a charged particle of energy E1). As a non-limiting example, in FIG. 4C, the laser source SL emits a beam FL of 519 nm wavelength, i.e., 2.4 eV. Thus, the peaks in the spectrum of FIG. 4C are separated by 2.4 eV. Note that the number p of photons absorbed / emitted by a charged particle of energy E'1 = E1 ± p × E2 is equal to the number of inter-peak spacings separating different energy peaks from the ZLP peak.

[0048] The system further includes a processor UT connected to the spectrometer SM and adapted to determine energy changes ΔE of at least two different energy peaks relative to a first energy E1. That is, the processor selects at least two peaks from the measured spectrum, each having an energy E'1 = E1 ± p × E2, and calculates ΔE = E'1 - E1 = ± p × E2. The peaks selected for calculating the energy change ΔE are not necessarily different from the spectral peaks at energy E1. Thus, the processor can select a ZLP peak and a peak having a specific energy E'1 = E1 ± n × E2 (n ≥ 1).

[0049] Let m denote the number of inter-peak intervals separating two selected peaks, and E denote the energy separating them. m It is written as follows.

[0050] In a final step F, the processor determines from these energy changes ΔE the value of the scale factor S and the offset value O specific to the spectrometer SM. In practice, as is well known, the energy change ΔE is determined in step E by the spectrometer and the processor via the relationship referred to as the first equation below:

[0051]

number

[0052] The term "spectroscope matrix detector channel" is used herein to refer to a pixel row or sub-row (or column or sub-column depending on the detector orientation) where different energy peaks are detected.

[0053] The present invention includes two different embodiments: a first embodiment that determines only the value of the scale factor S and the offset value O (linear error), and a second embodiment that determines the value of the scale factor S and the offset value O and extrapolates the NL function (non-linear error) (see FIG. 7 and related discussion below).

[0054] According to a first embodiment of the present invention, a processor determines that the charged particles are subject to weak or zero nonlinearity when they are moved and detected. For example, the processor may determine that the value

[0055]

number

[0056]

number

[0057]

number

[0058] This results in a first system of two equations (S1) with two unknowns. Step F of the first embodiment then consists of solving the first system of equations (S1) to determine the values ​​of the scale factor S and the offset O.

[0059] The number of peak-to-peak intervals equal to m is equal to the energy E m Consider two different energy peaks separated by m × d channels on the matrix detector of the spectrometer. The number of channels separating the two adjacent different energy peaks is d. By "adjacent different energy peaks," we mean energy peaks separated by an energy equal to E2. The two selected peaks are separated by m × d channels on the matrix detector.

[0060] In this case, the following equation holds:

[0061]

number

[0062] The energy value E that separates two different energy peaks m The accuracy of ΔE m The accuracy of determining the number of channels that separates two different energy peaks on the matrix detector is Δd m Accuracy ΔE m is equal to the precision of the photon energy of the laser beam FL emitted by the laser source SL. This precision is the spectral width LS of the laser beam FL.

[0063] Furthermore, the accuracy Δd m is considered to be equal to the pixel pitch Δd of the detector.

[0064] Then, the accuracy of determining S is as follows:

[0065]

number

[0066] ΔS m =ΔS / m, we obtain the following equation:

[0067]

number

[0068] The value of the determination accuracy of S is inversely proportional to the number m of peak-to-peak intervals, and ΔE m =LS and Δd m = Δd, so ΔS m =ΔS / m.

[0069] Two important conclusions can be drawn from these calculations. A. The method of the present invention according to the first embodiment of the present invention allows the values ​​of the scale factor S and the offset value O to be determined with an error limited by the spectral width LS of the laser beam. To improve the accuracy of the BS determination, it is necessary to measure peaks with a larger number m of inter-peak spacings.

[0070] The calibration method of the present invention allows the determination of the scale factor S with an accuracy of ΔS on the order of 0.01% or less of the energy E2 of the laser beam FL, even when measuring the energy change ΔE at two different energy peaks with the peak-to-peak spacing m equal to 1. For example, using a commercially available laser and optical spectrometer, a ΔS accuracy of 30 μeV for an energy E2 = 2 eV (i.e., a wavelength of 620 nm) can be achieved by using a laser source SL with a spectral width of 10 pm. The accuracy of ΔS can be improved by using a laser with better spectral resolution. Currently, even an accuracy of 0.001% of the energy E2 for the scale factor S and offset O is two orders of magnitude higher than the spectral resolution of state-of-the-art electron spectrometers.

[0071] According to a preferred embodiment of the present invention, the incident laser beam is a continuous beam or a pulsed beam with a spectral width of less than 40 meV, and thus by adapting the detection of the spectrometer SM, it is possible to determine the value of the scale factor S with an accuracy ΔS of 0.01% or better of the energy E2.

[0072] In one embodiment, the laser source SL is a femtosecond laser with a spectral width LS=40 meV, and the matrix detector of the spectrometer SM has 1000 channels with a dispersion of 20 meV / channel. By measuring the energy change ΔE at two different energy peaks with the number m of peak-to-peak spacings equal to 5, the value of the scale factor S is

[0073]

number

[0074] More generally, according to a preferred embodiment of the invention, for a constant spectral width LS, the spectrometer SM is adapted such that the number m of inter-peak intervals separating two peaks from which the energy change ΔE is determined is such that ΔS is less than or equal to 1% of the second energy E2, preferably less than 0.01% of the second energy E2.

[0075] It has been shown that the probability of photon / charged particle interactions giving rise to different energy peaks is proportional to the strength of the evanescent field EV (Non-Patent Document 2). In addition, in a known manner, the detection sensitivity of the spectrometer SM is limited by background noise generated by a number of factors. Therefore, according to a preferred embodiment of the present invention, the laser source SL and the transport assembly SO are arranged such that the incident laser beam is 10 8 W / cm 2 The incident laser beam is adapted to have an intensity equal to or greater than 1000 kJ / s. This intensity ensures that the rate of photon / charged particle interactions is high enough to generate an evanescent field EV that is strong enough to produce multiple energy peaks in the output beam FS that are strong enough to allow detection by the spectrometer SM. Thus, increasing the intensity of the incident laser beam in the R region allows the detection of a larger number of different peaks in the spectrum of the output beam, and the selection of the number m of inter-peak spacings in step E improves the accuracy of the determination of the scaling factor S and the offset O.

[0076] In the embodiment shown in Figure 3, the transport assembly SO includes an off-axis paraboloid that allows the incident laser beam FL to be focused onto a region R. The paraboloid has an aperture through which the incident charged particle beam passes, so that after the incident laser beam is reflected from the off-axis paraboloid, the incident charged particle beam co-propagates with the incident laser beam towards the region R. This embodiment has the advantage that it is easier to implement.

[0077] Although this arrangement is preferred, it is understood that one skilled in the art may form the transport assembly SO using other transport devices for the beams FL and FP. Thus, according to another embodiment, the transport assembly SO includes one or more mirrors and / or one or more lenses and / or fiber couplings.

[0078] To more easily control the temporal overlap of the beams FL and FP, according to one embodiment of the present invention, the system 1 includes a delay line arranged in the optical path of the laser beam FL.

[0079] Preferably, the laser source and transport assembly are adapted to polarize the incident laser beam in a direction that substantially matches the geometry and symmetry of the sample to locally maximize the evanescent field EV intensity in region R. For example, if the sample is a nanotube or nanowire, polarization in a direction substantially parallel to the longitudinal axis of the nanotube or nanowire maximizes the evanescent field EV intensity in region R.

[0080] FIG. 5 shows a schematic representation of a specific embodiment of the invention in which the system 1 includes an additional spectrometer SMA adapted to measure the spectrum of the incident laser beam simultaneously with the generation of the output beam. In this manner, the additional spectrometer SMA can measure the second energy E2(t) and the spectral width LS of the beam FL in real time or at a predetermined frequency. Furthermore, the processor UT is connected to the additional spectrometer SMA, and the determination of the value of the scale factor S and the offset value O is performed from the measurement of the second energy E2(t) measured by the additional spectrometer SMA. This embodiment reduces inaccuracies in the values ​​of the scale factor S and the offset value O due to fluctuations in the value of the second energy over time. Furthermore, measuring the spectral width LS allows for a more accurate determination of the value of the precision ΔS.

[0081] 6 illustrates a specific embodiment of the present invention in which system 1 includes an optical cavity CO in which a sample is placed. This cavity CO is resonant with an incident laser beam FL, and its geometry and structure are adapted to cause multiple reflections within the optical cavity as the incident laser beam FL passes through region R, thereby increasing the intensity of the beam FL in region R. This increase in intensity increases the intensity of the evanescent field EV, thereby generating a greater number of distinct energy peaks in the output beam FS than would be obtained in the absence of the optical cavity. As discussed above, detecting a greater number of distinct peaks in the output beam spectrum improves the accuracy of determining the scale factor S and offset O by selecting the number m of inter-peak spacings in step E.

[0082] Preferably, the cavity CO is a micrometer cavity in order to improve the compactness of the system 1. Further preferably, the transport assembly SO comprises an optical fiber adapted to couple the beam FL into the cavity CO.

[0083] 7 shows a schematic diagram of a method according to a second embodiment of the present invention. In this second embodiment, the processor UT determines that the nonlinearities experienced by the charged particles as they are moved and detected are not weak or non-zero. For example, the processor may determine that the value

[0084]

number

[0085]

number

[0086] In a second embodiment, step F solves a second system of equations (S2) to find NL(c i ) (i∈[1;N]), and the values ​​of the scale factor S and the offset value O. Furthermore, step F determines the N values ​​NL(c i )(i∈[1;N]) to characterize the nonlinearities experienced by the charged particle during its path and detection. Following step F, once N distinct peaks of sufficiently high energy have been detected, the method of the present invention allows the characterization of the nonlinearities experienced by the charged particle as it passes through the system 1.

[0087] Preferably, the method of the second embodiment of the present invention includes a step following step F of iteratively minimizing the value of the NL function by repeating steps A to F multiple times and varying the parameters of the path and detection of the charged particles during each iteration. In this way, the method minimizes nonlinear effects experienced by the charged particles in the system 1.

[0088] For example, these parameters may be the alignment of the optical components of the system 1.

[0089] Alternatively, the minimization of the NL function value is performed using the processor UT by implementing the following substeps: A. Generating a correction table for pixel energy values ​​of the matrix detector of the spectrometer SM. B. Correcting subsequent measurements of the charged particle beam spectrum by the spectrometer SM using the correction table to obtain energy measurements that are unaltered or only slightly altered by nonlinear effects.

Claims

1. A. First energy E １ generating a monoenergetic incident charged particle beam (FP) having B. Second energy E ２ generating an incident laser beam (FL) having C. Irradiating the incident laser beam (FL) onto a surface (SE) of a sample (Ech) to generate an evanescent electromagnetic field (EV) in a region (R) near the surface; D. Spatially and temporally overlapping the incident laser beam and the incident charged particle beam within the region to couple via the evanescent electromagnetic field, and generating the second energy E ２ generating a charged particle beam, referred to as an output beam (FS), having a spectrum with a plurality of distinct energy peaks spectrally separated by a value equal to E. Using the spectrometer, measure all or part of the spectrum of the output beam, and then measure the first energy E of at least two of the different energy peaks. １ determining an energy change ΔE for F. determining, from the energy change ΔE, a value of the scale factor S and a value of the offset O specific to the measurement of the spectrum of the output beam by the spectrometer; A method for calibrating a charged particle spectrometer (SM), comprising:

2. The energy change ΔE of each of the at least two different energy peaks is ΔE=±p×E ２ where, in the formula for the energy change ΔE, p is the distance between the different energy peak and the first energy E １ is a positive or zero integer equal to the number of peak-to-peak intervals separating said peaks in said spectrum The energy change ΔE of each of the at least two different energy peaks is calculated in step E by the spectrometer using the following relationship, referred to as the first equation: [Equation 1] is determined by In the first formula, c is the channel of the matrix detector of the spectrometer at which the different energy peaks are detected; NL(c) is a function of the channel c that represents the nonlinearity experienced by the charged particle during its path and detection, the so-called NL function. The method of claim 1.

3. It is determined that the nonlinearity experienced by the charged particle is weak or zero, and in step E, the energy change ΔE of the two different energy peaks numbered with the indexes 1 and 2, respectively, is determined via the first equation to obtain the following first equation system (S1): [Equation 2] A method for obtaining 3. The method of claim 1 or 2, wherein step F solves the first system of equations (S1) to determine the value of the scale factor S and the value of the offset.

4. 4. The method of claim 1, wherein an integer m>1 of the inter-peak spacing separating the two peaks for which the energy change ΔE is determined results in an error in the determination of the scale factor value of the second energy of 1% or less.

5. It is determined that the nonlinearity experienced by the charged particle is not weak or non-zero, and in step E, the energy change ΔE of N>2 different energy peaks, each numbered by an index i∈[1;N], is determined via the first equation to obtain the following second system of equations (S2): [Equation 3] A method for obtaining: Step F is Solving the second system of equations (S2) to obtain the value of the scale factor S, the value of the offset O, and the N values ​​NL(c i ) (i∈[1;N]); The N values ​​NL(c i ) (i∈[1;N]) to characterize the nonlinearity experienced by the charged particle during its path and detection.

6. 6. The method of claim 1, further comprising a step following step F of iteratively minimizing the value of the NL function by repeating steps A to F multiple times and varying parameters of the charged particle path and detection during each iteration.

7. 7. The method of claim 1, wherein step D further comprises the sub-step of measuring a spectrum of the incident laser beam simultaneously with the generation of the output beam, and wherein the determination of the value of the scale factor S and the value of the offset O is made from the measurement of the spectrum of the incident laser beam.

8. A system (1) for calibrating a charged particle spectrometer (SM), comprising: First energy E １ a charged particle source (SP) adapted to generate a monoenergetic incident charged particle beam (FP) having Second energy E ２ a laser source (SL) adapted to generate an incident laser beam (FL) having 1. An optical and charged particle transport assembly (SO) adapted for: irradiating the surface of a sample (Ech) with the incident laser beam (FL) to generate an evanescent electromagnetic field (EV) in a region (R) near the surface; The incident laser beam and the incident charged particle beam are spatially and temporally overlapped in the region to couple via the evanescent electromagnetic field, thereby generating the second energy E ２ and generating a charged particle beam, referred to as the output beam, having a spectrum having a plurality of distinct energy peaks spectrally separated by a value equal to the spectrometer (SM) is adapted to measure all or part of the spectrum of the output beam, the system further comprising: a spectrometer connected to the spectrometer; determining an energy change ΔE relative to the first energy E1 of at least two of the different energy peaks; and determining from the energy change ΔE a value of the scale factor S and a value of the offset O specific to the measurement of the spectrum of the output beam by the spectrometer.

9. 9. The system of claim 8, wherein the incident laser beam is a continuous beam or a pulsed beam having a spectral width of less than 40 meV.

10. 10. The system of claim 8 or claim 9, further comprising an additional spectrometer (SMA) adapted to measure the spectrum of the incident laser beam simultaneously with the generation of the output beam, the processor (UT) also being connected to the additional spectrometer (SMA), and the determination of the value of the scale factor S and the value of the offset O being made from the measurement of the spectrum of the incident laser beam (FL).

11. The laser source and transport assembly is configured such that the intensity of the incident laser beam in the region is 10 ８ W / cm ２ 11. A system according to any one of claims 8 to 10, adapted to:

12. 12. The system of claim 8, wherein the laser source and transport assembly is adapted to polarize the incident laser beam in a direction compatible with the geometry and symmetry of the sample so as to locally maximize the intensity of the evanescent field (EV) in the region (R).

13. 13. The system of claim 8, wherein the transport assembly (SO) comprises an off-axis paraboloid adapted to focus the incident laser beam onto the surface of the specimen, the paraboloid having an aperture through which the incident charged particle beam passes, such that after the incident laser beam reflects off the off-axis paraboloid, the incident charged particle beam co-propagates with the incident laser beam towards the region.

14. 14. The system of claim 8, further comprising a resonant optical cavity (CO) for the incident laser beam (FL) in which the sample is disposed, the transport assembly (SO) and the optical cavity being further adapted to cause the incident laser beam to reflect multiple times within the optical cavity as it passes through the region.

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