Abstract:
A transmission electron microscope comprises a high-voltage source for outputting a high voltage at two high-voltage outputs and outputting a control signal at a controller output; a focusing lens for focusing a beam; a monochromator which allows only those particles of the particle beam to pass whose kinetic energy is within an adjustable energy interval; an energy-dispersive component which deflects particles of different kinetic energies differently; a detector; and a controller connected to the controller output, which controls a beam deflector, arranged between the energy-dispersive component and the detector, the monochromator, or the energy-dispersive component in dependence on the control signal, or superposes plural of intensity distributions detected by the detector with an offset relative to one another, which offset is set in dependence on the control signal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims priority of Patent Application No. 10 2010 056 337.4, filed Dec. 27, 2010 in Germany, entitled “PARTICLE BEAM SYSTEM AND SPECTROSCOPY METHOD”, the content of which is hereby incorporated by reference in its entirety. 
     FIELD 
     The invention relates to transmission electron microscopes (TEMs) and to methods of operating transmission electron microscopes. The invention in particular relates to transmission electron microscopes and methods of operating transmission electron microscopes for performing spectroscopic measurements of a sample, such as recording energy loss spectra that are representative of energy losses experienced by electrons while interacting with the sample. 
     BACKGROUND 
     Energy loss spectra are suitable for detecting elementary excitations of a sample, such as excitation of surface plasmons in the sample, for example. To this end, for example a monochromatic electron beam can be directed onto the sample, resulting in excitations of the sample and a reduction of the kinetic energy of electrons of the beam by an amount corresponding to the energy of the excitation. If an energy spectrum of the electron beam subsequent to its interaction with the sample is recorded, the recorded energy spectrum contains information relating to the excitation energies. The articles J. Nelayah, L. Gu, W. Sigle, C. T. Koch, I. Pastoriza-Santos, L. M. Liz-Marzán, and P. A. van Aken, “Direct imaging of surface plasmon resonances on single triangular silver nanoprisms at optical wavelength using low-loss EFTEM imaging”, Opt. Lett. 34, 1003-1005 (2009) and Wilfried Sigle, Jaysen Nelayah, Christoph T. Koch, and Peter A. van Aken, “Electron energy losses in Ag nanoholes—from localized surface plasmon resonances to rings of fire,” Opt. Lett. 34, 2150-2152 (2009) describe applications of this method. 
     SUMMARY 
     It is an object of the present invention to propose a particle beam system and a spectroscopy method which can be used to record energy loss spectra with a higher degree of accuracy. 
     According to embodiments, a particle beam system comprises a high-voltage source for providing a high voltage at a high-voltage output of the high-voltage source and providing a control signal, which is indicative of a deviation of the provided high voltage from a reference voltage or of a temporal change or fluctuation of the high voltage, at a controller output of the high-voltage source, an acceleration electrode, which is electrically connected to the high-voltage output, for accelerating the particles in a particle beam to a kinetic energy that corresponds to the high voltage, a focusing lens, which is arranged in the beam path of the particle beam downstream of the acceleration electrode, for focusing the beam onto a location of a sample, an energy-dispersive component, which is arranged in the beam path of the particle beam downstream of the sample and is configured to deflect particles of different kinetic energies differently, a detector, which is arranged in the beam path downstream of the energy-dispersive component, and a controller, which is connected to the controller output of the high-voltage source and is configured to effect, in dependence on the control signal provided by the high-voltage source, changes in operating parameters of the energy-dispersive component or of other particle-optical components arranged in the beam path of the particle beam or to correct intensities detected by the spatially resolving detector with respect to the energy. 
     The inventor has recognized that instabilities in a high-voltage source providing an acceleration voltage for a particle beam that is used for the measurement are the cause of a limit on the resolution that can be achieved when measuring energy loss spectra. It is therefore conceivable to better stabilize high-voltage sources used for providing the acceleration voltage. In conventional high-voltage sources for particle beam systems, however, considerable measures for stabilizing said high-voltage sources are employed already, which incur substantial costs and cannot satisfactorily prevent still remaining instabilities, in particular drifting and low-frequency noise. 
     In order to achieve the abovementioned object it is proposed therefore to accept a certain instability of the high-voltage source and to detect changes in the high voltage or deviations of the provided high voltage from a reference voltage and to undertake correction measures at another point in the beam path of the particle beam before or after interaction with the sample. 
     According to an exemplary embodiment, the particle beam system comprises a beam deflector, which is arranged in the beam path between the energy-dispersive component and the spatially resolving detector. The beam deflector can provide an adjustable electric or magnetic field that has a deflecting effect on the particle beam or adjustably deflect the particle beam in another way. The beam deflector is controlled by the controller in dependence on the control signal provided by the high-voltage source. In particular, the deflection angle for the particle beam produced by the beam deflector is increased or decreased here if the deviation of the high voltage provided by the high-voltage source from the setpoint voltage increases or decreases. 
     With this measure it is possible to improve the quality of the energy loss spectrum detected by the spatially resolving detector. Without the controller actuating the beam deflector, a relative increase in the high voltage provided by the high-voltage source would, for example, result in a relative increase in the kinetic energy of the particles in the beam. However, independently of their kinetic energy, the particles experience equal energy losses at the sample that corresponds to the energies of the excitations in the sample. An energy spectrum that is recorded using the spatially resolving detector downstream of the energy-dispersive component is thus shifted to higher energies owing to the relative increase in the high voltage, without changing its relative form that is defined by the unchanging energy losses. A high voltage that varies while the spectrum is recorded using the spatially resolving detector thus results in smearing of the recorded spectrum. Owing to the beam deflector, which is arranged between the energy-dispersive component and the spatially resolving detector, being actuated as explained above, it is possible, after appropriate calibration, to entirely avoid the spectrum shifting to higher or lower energies, with the result that a stable energy loss spectrum can be recorded. 
     According to a further exemplary embodiment, the particle beam system comprises an actuator, which is configured to displace the detector in a direction transverse to the beam path or an incidence direction of the particles on the detector. The actuator is controlled by the controller in dependence on the control signal provided by the high-voltage source. 
     Similar to the case where the previously described beam deflector is used, it is possible, by actuating the actuator to displace the detector, to compensate for shifts, caused by changes in the high voltage, in the energy spectrum, which is recorded using the detector, such that the same energy loss spectra can be recorded for different high voltages. 
     According to a still further exemplary embodiment, the particle beam system comprises a high-voltage source for providing a high voltage at a high-voltage output and for providing a control signal, which is indicative of a deviation of the provided high voltage from a reference voltage, at a controller output, an acceleration electrode, which is electrically connected to the high-voltage output, for accelerating the particles in a particle beam to a kinetic energy that corresponds to the high voltage, a focusing lens, which is arranged in the beam path of the particle beam downstream of the acceleration electrode, for focusing the beam onto a sample, a monochromator, which is arranged in the beam path of the particle beam upstream of the focusing lens and is configured to allow only those particles of the particle beam to pass whose kinetic energy is within an adjustable energy interval, a detector, which is arranged in the beam path of the particle beam system downstream of the sample, for detecting particle beam intensities, and a controller, which is connected to the controller output of the high-voltage source and is configured to control the monochromator such that a central energy of the energy interval changes in dependence on the control signal provided by the high-voltage source. 
     According to a still further exemplary embodiment, the particle beam system comprises a high-voltage source for providing a high voltage at a high-voltage output and providing a control signal, which is indicative of a deviation of the provided high voltage from a reference voltage, at a controller output, an acceleration electrode, which is electrically connected to the high-voltage output, for accelerating the particles in a particle beam to a kinetic energy that corresponds to the high voltage, a focusing lens, which is arranged in a beam path of the particle beam system downstream of the acceleration electrode, for focusing the beam onto a sample, an energy-dispersive component, which is arranged in the beam path downstream of the sample and is configured to deflect particles of different kinetic energies differently, a detector, which is arranged in the beam path downstream of the energy-dispersive component, and a controller, which is connected to the controller output of the high-voltage source and is configured to control the energy-dispersive component such that a dispersion by the energy-dispersive component changes in dependence on the control signal provided by the high-voltage source. 
     Similar to the case where the previously described beam deflector, which is arranged in the beam path of the particle beam system downstream of the energy-dispersive component, is used, it is possible, by controlling the energy-dispersive component itself, to compensate for changes, caused by changes in the high voltage, in an energy spectrum of the particle beams, which is recorded with the aid of the detector, such that the same energy loss spectra can be recorded for different high voltages. 
     According to a still further exemplary embodiment, the particle beam system comprises a high-voltage source for providing a high voltage at a high-voltage output and providing a control signal, which is indicative of a deviation of the provided high voltage from a reference voltage, at a controller output, an acceleration electrode, which is electrically connected to the high-voltage output, for accelerating the particles in a particle beam to a kinetic energy that corresponds to the high voltage, a focusing lens, which is arranged in a beam path of the particle beam system downstream of the acceleration electrode, for focusing the beam onto an object plane, a detector, which is arranged in the beam path downstream of the object plane, and a controller, which is configured to record, with the aid of the detector, energy spectra of the particles contained in the particle beam downstream of the object plane. A plurality of energy spectra are successively recorded and manipulated here by shifting each of the energy spectra in the direction of the energy by an amount that is determined in dependence on the control signal provided by the high-voltage source. The spectra thus manipulated are then added up to a total spectrum. It is possible in this manner to compensate for changes, caused by changes in the high voltage, in the spectra measured with respect to the energy thereof such that the individual measured spectra are in each case representative of identical energy loss spectra which can be superposed in order to increase a statistical significance of the spectra. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The forgoing as well as other advantageous features of the invention will be more apparent from the following detailed description of exemplary embodiments of the invention with reference to the accompanying drawings. It is noted that not all possible embodiments of the present invention necessarily exhibit each and every, or any, of the advantages identified herein. 
         FIG. 1  is a schematic illustration of a particle beam system; 
         FIG. 2  is a graph of two energy loss spectra, recorded with the particle beam system illustrated in  FIG. 1 , for various values of a high voltage without compensation; and 
         FIG. 3  is a schematic illustration of a high-voltage source which can be used in the particle beam system illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In the exemplary embodiments described below, components that are alike in function and structure are designated as far as possible by alike reference numerals. Therefore, to understand the features of the individual components of a specific embodiment, the descriptions of other embodiments and of the summary of the invention should be referred to. 
       FIG. 1  schematically shows a particle beam system  1  suitable for recording energy loss spectra. The particle beam system  1  is a transmission electron microscope (TEM) comprising a particle beam source  3  having an emitter  5 , an extractor electrode  6  and a plurality of other electrodes  7  for generating an electron beam  9 . The electron beam source can be, for example, a Schottky field emission source, which for example generates a beam current of 100 pA with an energy width of 0.7 eV. 
     The particle beam system  1  may comprise a monochromator  11 , which is configured to allow only those electrons of the particle beam to pass whose kinetic energy is within an adjustable energy interval. To this end, the monochromator comprises a plurality of electrodes (not shown in  FIG. 1 ) in order to guide the electron beam  9  in the illustrated Q-shaped path. An aperture plate  12  is arranged in the beam path, wherein only particles of the beam whose kinetic energy is within the set energy interval may traverse an aperture of the plate  12 . One example of a suitable monochromator is described in the patent specification U.S. Pat. No. 6,770,878 B2, the disclosure of which is incorporated in its entirety in the present application. With a suitable monochromator it is possible to limit the width of the energy interval for example to 50 meV. 
     The monochromator  11  is merely optional in the particle beam system and can be omitted if an energy width of the electron beam provided by the electron source  3  is already less than or equal to an energy width desired for the examination to be carried out. 
     A plurality of acceleration electrodes  13  are arranged in the beam path of the particle beam system  1  downstream of the monochromator for accelerating the electrons of the electron beam  9  to a kinetic energy of, for example, 200 keV, while their energies before traversing the acceleration electrodes  13  was, for example, 4 keV. In the illustrated example, the monochromator  11  is arranged between the electron source  3  and the acceleration electrodes  13 , with the result that comparatively small deflection fields are necessary to guide the electron beam on a curved path required for monochromatization. However, it is also possible to arrange a monochromator in the beam path downstream of the acceleration electrodes  13  and to monochromatize the electron beam of high kinetic energy. 
     After acceleration, the electron beam  9  traverses a condenser system  15  and is then focused at an object plane  19  by a single-field condenser objective lens  17 . A sample to be examined is arranged in the object plane  19  which is traversed by the particle beam  9 . A beam deflector  16  may be provided in the beam path upstream of the object plane  19  in order to scan the beam over the object plane  19  and direct it onto selected locations of a sample. Depending on the design of the system and in particular of the condenser lens, the electron beam in the sample plane can have diameters that are smaller than 0.2 nm, for example. 
     The kinetic energy of the electrons of the beam  9  that are traversing the sample plane  19  is defined by a voltage difference between an area surrounding the sample plane  19  and the emitter  5  of the electron source. Typically the sample arranged in the object plane  19  is kept at ground potential and the emitter  5  at high-voltage potential. However, it is also possible to deviate from this. In any case a high-voltage source  21  is necessary in order to provide the necessary high potential difference between the emitter  5  and the sample. In the exemplary embodiment illustrated, a high-voltage output  23  of the high-voltage source  21  is electrically connected to the emitter  5  so as to keep the latter at a high electric potential with respect to earth. 
     The high-voltage source  21  further comprises a control input  25  in order to be supplied with a control signal which is indicative of a reference voltage for the high voltage to be provided at the high-voltage output  23 . This control signal is generated by and supplied from a controller  27  via an output  29  of the controller  27 . 
     The controller  27  further comprises an output  31  for controlling the monochromator  11  and for setting a central energy of the interval of kinetic energies which are allowed to traverse the monochromator  11 . 
     Here, the electric potentials to be applied to the electrodes  6 ,  7 , the components of the monochromator  11  and the acceleration electrodes  13  are high-voltage potentials which can be generated by separate high-voltage sources or using the high-voltage source  21  together with auxiliary circuits such as voltage dividers and/or low-voltage sources. The monochromator  11  in that case is for example not connected directly to the output  31  of the controller  27  but to a suitable auxiliary circuit, such as a voltage divider, that is connected to the high-voltage source, for generating the voltages suitable for controlling the monochromator  11 . 
     The electrons of the electron beam  9 , which are directed onto the sample, interact with the sample and can excite said sample, in which case individual electrons lose an amount of energy that corresponds to the excitation energy of an excitation of the sample. Therefore it is of interest to determine the energy spectrum of the electrons of the electron beam after interaction with the sample to obtain information relating to the possible excitations of the sample. 
     To this end, the particle beam system  1  comprises a projection system  35 , which comprises one or more electron-optical lenses and is arranged in the beam path downstream of the objective lens  17 . Arranged in the beam path downstream of the projective system  35  is an energy-dispersive electron-optical component  37 , which provides different deflection angles for particles of different kinetic energies, such that a spatially resolved energy spectrum of the beam  9  is produced in a spectrum plane  39  in the beam path downstream of the energy-dispersive component  37 . The energy-dispersive component  37  can be configured as is described, for example, in U.S. Pat. Nos. 4,740,704 and 6,384,412, wherein the disclosure of these documents is incorporated in its entirety in the present application. 
     The spectrum plane  39  is imaged onto a spatially resolving detector  43  using a further projection system  41 , which may comprise one or more electron-optical lenses. The spatially resolving detector  43  can be configured, for example, as a line detector which is oriented such that particles of different energies are incident on the detector at different locations due to the dispersion by the energy-dispersive component  37 . By spatially resolved detection of intensities of the incident particles it is thus possible to detect the intensities in an energy-resolved manner, i.e. to measure the energy spectrum of the electron beam  9  subsequent to its interaction with the sample. 
     Line  47  in  FIG. 2  schematically illustrates such an energy spectrum as can be recorded by the detector  43  if the high voltage provided by the high-voltage source  21  corresponds exactly to the reference voltage and a given sample is arranged in the object plane  19 . Line  49  in  FIG. 2  schematically represents a spectrum, recorded using the detector  43 , as is formed for the same sample where however the high voltage provided by the high-voltage source  21  is greater than the reference voltage. This results in a higher kinetic energy of the electrons travelling through the sample and, owing to the dispersion by the energy-dispersive component  37 , in a shift of the spectrum detected using the detector  43  to the left in the illustration of  FIG. 2 . 
     If the high-voltage source  21  is not sufficiently stable and the high voltage provided is subject to noise, this results in the spectrum in the illustration of  FIG. 2  to being persistently displaced to and fro during the measurement using the detector  43  due to the dispersion by the energy-dispersive component  37 . The spectrum finally obtained by integration is correspondingly smeared and thus prevents obtaining of certain information contained in the energy distribution of the electrons. 
     Since even conventional high-voltage sources of complex design are not sufficiently stable for high-resolution energy loss measurements, the particle beam system  1  has various options for compensating for fluctuations of the high-voltage source  21 . 
     A temporal fluctuation of the high voltage provided at the output  23  of the high-voltage source  21  is determined in said high-voltage source  21  itself and provided, as an electric signal, at the control output  51  of the high-voltage source  21 . The control output  51  is connected to an input  53  of the controller  27 . The controller  27  can thus, via the input  53 , read a signal that is indicative of the fluctuation in the high voltage provided or the deviation of the high voltage provided at the high-voltage output  23  from the reference voltage. 
     A first possibility of compensating for the fluctuations in the high voltage during the recording of energy loss spectra uses a beam deflector  55 , which is arranged in the beam path downstream of the energy-dispersive component  37  and upstream of the detector  43 . The beam deflector  55  can generate a magnetic and/or an electric field that deflects the electron beam  9 . The beam deflector  55  is controlled by the controller  27 , which provides a control signal at its output  57  for the deflector  55  which is determined by the controller  27  in dependence on the control signal of the high voltage. For example, the control signal can be determined such that a change in the deflection angle provided by the deflector  55  for the beam is proportional to a deviation of the provided high voltage from the reference voltage. The controller  27  here produces, in dependence on the control signal of the high-voltage source, a change in the deflection signal such that the deflection produced by the deflector  55  compensates for that shift of the electron spectrum on the detector which corresponds to the energy deviation of the electrons from the reference energy, caused by the deviation of the high voltage from the setpoint voltage, and to the shift of the electron spectrum on the detector, caused therefrom due to the dispersion by the energy-dispersive component  37 . 
     By controlling the deflector  55  in dependence on the deviation of the provided high voltage from the reference voltage it is possible to largely avoid the shift of the spectra due to the deviation of the high voltage, explained previously in conjunction with  FIG. 2 . 
     A second possibility of avoiding such shifts, which can be used alternatively to or complementary with the previously described first possibility, is controlling of the monochromator  11  in dependence on the deviation of the provided high voltage from the reference voltage. Here the controller  27  controls, via its output  31 , the monochromator  11  such that the central energy of the interval of kinetic energies of the electrons, which are allowed to pass through the monochromator  11 , is increased or decreased if the high voltage provided decreases or increases, respectively, due to the determined temporal fluctuations. 
     A third possibility for avoiding the deterioration of the detected energy spectra due to the deviation of the provided high voltage from the reference voltage, which can be used alternatively or in addition to one or both of the possibilities described previously, involves the energy-dispersive component  37  being controlled by the controller  27  via an output  61  thereof in dependence on the fluctuation of the high voltage provided or in dependence on the deviation of the provided high voltage from the reference voltage. This may achieve an effect similar to that of the deflector  55  and results in a shift of a spectrum of electrons impacting the detector  43  due to changes in the high voltage does not take place, or takes place only to a very limited extent. 
     A fourth possibility of avoiding the smearing of the measured spectrum due to fluctuations of the high voltage, which can be used alternatively to or in combination with one or more of the possibilities described previously, involves the controller  27  reading in quick succession a plurality of spatially resolved intensity spectra from the detector  43  via a data line  42 . To this end, the detector can be a CMOS sensor, for example, which allows a particularly quick reading operation. 
     The controller reads the intensity values, which were detected in a spatially resolved manner by the detector  43 , together with a value of the control signal, which is representative of the current deviation of the provided high voltage from the reference voltage. In dependence on this deviation, each spectrum that is read from the detector  43  is corrected by shifting the respective measured values towards higher or lower energies, wherein the value of the shift is determined in dependence on the respective deviation of the provided high voltage from the setpoint voltage. Thus each of the read spectra is corrected with respect to the deviation of the provided high voltage from the reference voltage such that in the end the plurality of spectra can be added up to obtain a total spectrum of high statistical significance that is not smeared due to the fluctuations of the provided high voltage. 
     A fifth possibility for avoiding the smearing of the measured spectrum due to fluctuations of the high voltage, which can be used alternatively to or in combination with one or more of the possibilities described previously, involves an actuator  60 , which may for example include an electric motor or piezo actuator, being actuated by the controller  27  via an output  58  thereof, wherein the actuator  60  is configured to displace the spatially resolving detector  43  transversely to an incidence direction of the beam  9  on the detector in a lateral direction illustrated by an arrow  62 . This can achieve an effect similar to that of the previously described deflector  55  and results in a shift of the spectrum of electrons incident on the detector  43  due to changes in the high voltage taking place to a comparatively limited extent. 
       FIG. 3  is a schematic diagram of an exemplary embodiment of a high-voltage source  21 . 
     The high-voltage source  21  provides a high voltage of for example 200 kV between the connections  23  and  24 . Here, the connection  24  may be connected to an internal ground of the high-voltage source  21 , which in turn is connected to a ground of the particle beam system  1 . The high-voltage source  21  further comprises the control input  25  for setting the reference value of the high voltage to be provided between the connections  23  and  24 , wherein a signal, which is representative of the instantaneous deviation of the high voltage provided between the connections  23  and  24  from their reference value, is output at the control output  51 . 
     The high-voltage source  21  comprises an alternating voltage generator  81 , which generates an alternating voltage with a given voltage amplitude. This alternating voltage is transformed in a transformer  83  into an alternating voltage with a higher voltage amplitude. On the basis of this alternating voltage with higher voltage amplitude, a high-voltage converter  85 , such as a Cockroft-Walton generator for example, generates a rectified high voltage, which is smoothed by one or more filter resistors  87  and filter capacitors  89 , such that the smoothed high voltage is available at the connection  23 . 
     The magnitude of the high voltage is determined via an amplitude controller  91 , which controls the voltage amplitude of the alternating voltage generator  81 . To this end, a voltage signal is supplied to the amplitude controller  91  via a measurement resistor  93 , which voltage signal is indicative of the magnitude of the high voltage provided between the connections  23  and  24 , wherein, in the amplitude controller  91 , this supplied voltage is compared with the voltage supplied to the high-voltage source  21  via the connection  25  and, in dependence on this comparison, the voltage amplitude of the alternating voltage generator is increased or decreased. The controller  27  can thus control the magnitude of the high voltage provided between the connections  23  and  24  via the connection  25 . 
     A load current measurement instrument  95  is provided for measuring the current flowing between the connections  23  and  24 . 
     A capacitive voltage divider having capacitors  97  and  98  is provided between the connectors  23  and  24 , wherein an amplifier  99  amplifies the voltage present between the capacitors  97  and  98  and provides a signal corresponding to the amplified voltage at the output  51  such that a signal is output via the output  51  to the controller  27  of the particle beam system  1 , which signal corresponds to the instantaneous temporal change in the high voltage provided between the connections  23  and  24  and thus to the deviation thereof from its reference value. 
     In the previously explained exemplary embodiments, the detector  43  is a linear detector detecting an energy spectrum of the electron beam which is incident on it. However, it is likewise possible to detect an energy spectrum with a non-spatially resolving detector, by moving this detector or an aperture associated therewith, which is arranged in the beam path downstream of the energy-dispersive component  37 , transversely to the beam direction, and the intensities detected by the detector are recorded in dependence on the position of the aperture or the detector, respectively. It is furthermore possible for a two-dimensionally resolving detector to be used to detect the energy spectrum. 
     In the previously explained exemplary embodiments, the particle beam system is a transmission electron microscope. 
     However, the present disclosure is not limited thereto. In further possible exemplary embodiments, the particle beam system can be a transmission ion microscope. One example of this is a gas field ion microscope, in which an ion beam is generated by ionizing gas atoms in an electrostatic field of an emission peak. The object is then irradiated with an ion beam, wherein ions transmitted through the object can lose energy, with the result that their energy loss spectrum, too, can be detected. If the particle beam apparatus is an ion microscope, the objective lens can be a magnetic lens, an electrostatic lens or a combination of a magnetic lens and an electrostatic lens. 
     In the previously explained exemplary embodiments, the particle beam system has a monochromator arranged in the beam path upstream of the objective lens. This does not necessarily have to be the case. In other exemplary embodiments, no monochromator is provided between the particle source and the object. In particular, a monochromator is not necessary where an energy width of the particle beam produced by the particle source is sufficiently small for desired examination. This can be the case in particular for cooled ion sources or electron sources. 
     While the invention has been described with respect to certain exemplary embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention set forth herein are intended to be illustrative and not limiting in any way. Various changes may be made without departing from the spirit and scope of the present invention as defined in the following claims.