Method for operating a particle beam microscope and particle beam microscope

Abstract

Method for operating a particle beam microscope (1), wherein the method comprises generating an image of a sample (9) to be examined and reducing a charge on the sample generated during image generation; wherein a sequence of the following actions is performed several times successively: (a) directing a particle beam (5) at the sample (9) and detecting particles emerging from the sample (9) during a first period (T1) for generating the image of the sample (9); (b1) generating a first beam (31) of electrons having a first distribution (p1) of their kinetic energies and directing these electrons at the sample (9) during a second period (T2) for reducing the charge on the sample (9) generated by directing the particle beam (5) at the sample (9);and (b2) generating a second beam (31') of electrons having a second distribution (p2) of their kinetic energies, and directing these electrons towards the sample (9) during a third period (T3) for further reducing the charge of the sample (9) generated by directing the particle beam (5) towards the sample (9); wherein an energy average of the first distribution (p1) of the kinetic energies is greater than an energy average of the second distribution (p2) of the kinetic energies.;

Description

The invention relates to a method for operating a particle beam microscope and to a particle beam system.In particular, the invention relates to a method for generating an image of a sample to be examined by means of a particle beam generated by a particle beam microscope, wherein a charge of the sample, which is generated when the particle beam is directed onto the sample, is reduced.In taking an image of a sample to be examined by means of a particle beam microscope, a primary particle beam generated by the particle beam microscope is directed to predetermined sites of the sample to be examined, and secondary particles such as secondary electrons dissolved therefrom are detected by interaction of the primary particle beam with the sample to obtain information regarding the site of the sample to which the particle beam is directed, respectively. The intensities of secondary particles detected at a plurality of locations on the sample to which the particle beam is directed represent an image of the sample to be examined. In this case, the primary particle beam striking the sample and the secondary particles dissolved from the sample change the state of charge of the sample, in particular at its surface, so that a positive or negative charge of the sample can arise. This charging of the sample changes, on the one hand, the landing energy of the primary particle beam and the rate at which the secondary particles can reach a detector for their detection. Thus, the intensity of detected secondary particles varies depending not only on the location where the primary particle beam is directed to the sample, but also on the time during which the study continues.Conventionally, therefore, particle beam microscopes comprise devices for reducing a charge of the sample generated during the examination. Such a device is, for example, an electron beam source which directs an electron beam, different from the primary particle beam, with an adjustable kinetic energy onto the sample. Depending on this kinetic energy, positive or negative electric charge can be applied to the sample, since the electrons striking the sample generate secondary electrons there, which leave the sample, and the generation rate of the secondary electrons is energy-dependent. The publication US 2010 / 0 258 722 A1 discloses such a particle beam microscope and particle beam microscopy method.It has been found that the conventional method for reducing charging of the sample by means of electron beams often fails to achieve a satisfactory reduction in the charging of the sample.Accordingly, it is an object of the present invention to propose a method for operating a particle beam microscope, with which images of the sample to be examined are generated with the aid of a particle beam and charging of the sample generated by the particle beam directed onto the sample is reduced better. It is a further object of the present invention to propose a particle beam microscope with which a better reduction in the charge of the sample can be achieved.Embodiments include a method for operating a particle beam microscope according to claims 1 and 5 and a particle beam system according to claim 28, in which an image of a sample to be examined is generated and a charge of the sample generated when the image is recorded is reduced. For this purpose, a sequence of measures is carried out a plurality of times in succession, wherein the sequence comprises directing a particle beam onto the sample and detecting particles emerging from the sample, such as secondary electrons, for the purpose of generating the image, and generating and directing electrons onto the sample for the purpose of reducing the charge of the sample generated by the directing of the particle beam onto the sample. During the directing of the particle beam onto the sample, the latter can be directed in each case onto a predetermined location of the sample, wherein image information relating to the respective location is obtained by detecting the secondary electrons. This image information can also be referred to as a pixel. During the directing of the particle beam onto the sample, the latter can be directed successively onto a plurality of different locations of the sample in order to acquire image information relating to a plurality of locations or a plurality of pixels by the detection. These multiple locations can be a subset of all predetermined locations at which image information is obtained in order to generate the image of the sample to be examined. For example, if the locations or pixels of the image are arranged as a rectangular field, which are scanned line by line with the particle beam, a subset of the scanned locations can comprise, for example, a part of a line or an entire line. When executing the sequence a plurality of times, the directing of the particle beam onto the sample and the detecting of the particles are likewise executed a plurality of times, wherein the subsets of the locations onto which the particle beam is directed during the directing and detecting can each be different from one another and in particular disjunct. For example, it is possible here for there to be locations on the sample to which the particle beam is directed twice or more times, wherein these locations are contained in different subsets of locations to which the particle beam is directed in each case during the directing and detecting when the sequence is executed more than once.The particles detected for generating the image can be dissolved out of the sample by the particle beam impinging on the sample. When an electron beam or an ion beam is used as the particle beam, the particles dissolved out of the sample can be secondary electrons, for example. The particles detected for generating the image can, however, also be particles of the particle beam impinging on the sample itself, which have acted upon by the sample and emerge from the sample again. If an electron beam is used as the particle beam, these particles emerging from the sample can be, for example, back-scattered electrons which emerge from the sample in the direction from which the particle beam also impinges on the sample, but they can also be transmission electrons which pass through the sample.If an ion beam is used as the particle beam, these particles emerging from the sample can be, for example, backscattered ions which emerge from the sample in the direction from which the particle beam impinges on the sample, but they can also be ions which pass through the sample.According to exemplary embodiments, directing the particle beam and detecting the particles within the sequence occurs during a first time period that is shorter than the entire time period of the sequence. The generation and directing of electrons onto the sample for the purpose of reducing the charge of the sample can then take place during a period which does not overlap with the first period within the sequence, so that when the sequence is carried out a plurality of times, the particle beam is alternately directed onto the sample for obtaining image information, wherein the sample is charged, and in a subsequent step the charge of the sample is reduced again. However, it is also possible that electrons are generated during the directing of the particle beam onto the sample and detecting the particles emerging from the sample in order to obtain the image information and are directed onto the sample in order to reduce the charging of the sample, so that the first period of time in which image information is obtained and the period of time in which the charging of the sample is reduced overlap within the sequence.According to exemplary embodiments, generating and directing electrons onto the sample for the purpose of reducing the charge of the sample comprises generating electrons with higher kinetic energies and directing these electrons onto the sample during a second period of time and generating electrons with lower kinetic energies and directing these electrons onto the sample during a third period of time, wherein the third period of time is temporally after the second period of time.The inventor has recognized that reducing charging of the sample with the aid of two types of electrons is particularly effective, wherein the two types of electrons differ in terms of kinetic energy and the electrons of the lower kinetic energy are still directed to the sample within the sequence in a period in which the electrons of the higher kinetic energy are no longer directed to the sample. The electrons of the higher kinetic energy lead to a rapid reduction in the charge of the sample due to a comparatively high achievable beam current and due to the comparatively large number of secondary electrons they generate, although usually a charge of the sample, although a smaller charge, remains. This remaining charge of the sample is then reduced even further by the electrons of the lower kinetic energy and in the ideal case completely compensated. Without directing the electrons of the higher kinetic energy onto the sample, the extensive reduction of the charge of the sample would certainly also be possible in some cases, wherein this would take a comparatively longer time on account of the comparatively smaller achievable beam intensity of the electrons of lower kinetic energy. However, the charges of the sample often generate interference fields which influence the impingement of the electrons of low kinetic energy on the sample and prevent uniform irradiation of the sample with electrons of low kinetic energy, so that, in the case of a present strong charge of the sample, the irradiation with electrons of low kinetic energy alone does not lead to a satisfactory reduction in the charge. Furthermore, components of the particle beam microscope, such as detectors or particle-optical lenses, generate electric or magnetic interference fields which can deflect the electrons of low kinetic energy on their way to the sample and impair their effectiveness with regard to reducing the charge of the sample. However, the electrons of the higher kinetic energy are less influenced by the interference fields generated by strong charges of the sample or the fields generated by other components of the particle beam microscope and accordingly lead to an effective reduction of the charge and the generated interference fields of the sample, so that subsequently the electrons of the lower kinetic energy can effectively achieve a still further reduction of the charge.During the third period, in which the electrons of the lower kinetic energy reach the extensive reduction of the charge of the sample, the electrons of the higher kinetic energy are not directed, or are directed only to a small extent, onto the sample. However, the electrons of the lower kinetic energy may be directed to the sample along with the electrons of the higher kinetic energy during the second period. Likewise, even in the third period of time, electrons of the higher kinetic energies can still be directed onto the sample, as long as the proportion of these electrons is significantly smaller than the proportion of the electrons with the lower kinetic energy.According to exemplary embodiments, during the first period of time, in which the particle beam is directed onto the sample for the purpose of generating the image and the particles generated by the particle beam and emerging from the sample are detected, no electrons are directed onto the sample for the purpose of reducing the charge of the sample, since these electrons can likewise generate particles, in particular secondary electrons, which could be detected and would interfere with the image information. However, it is possible that during the generation and directing of electrons onto the sample for the purpose of reducing the charge of the sample, the particle beam is still directed onto the sample. Particles can also be detected during this time. However, the method may then be carried out such that particles which are detected while electrons are generated and directed to the sample for the purpose of reducing the charge of the sample do not contribute directly to the image information, i.e. directly determine the individual intensities of pixels of the image. However, the information obtained by detecting particles within periods of time during which electrons are directed thereto for the purpose of reducing the charge of the sample may be quite useful information by means of which, for example, an extent of the charge of the sample itself can be determined, the execution of the method can be varied and / or a quality of the image can be improved, for example by making corrections to the image on the basis of this information.As explained above, the two types of electrons used to reduce the charge of the sample differ in terms of their kinetic energy they reach after leaving a corresponding electron source and before striking the sample. However, this kinetic energy is not exactly the landing energy with which the electrons impinge on the sample, since electric fields in front of the sample, which are generated, for example, by the charge of the sample itself or by an electric potential applied to the sample, can still accelerate or retard the electrons before they impinge on the sample. Thus, the kinetic energy with respect to which the two types of electrons differ is the kinetic energy with which the electrons exit from the electron source toward the sample. In this case, the electron source can have a complex structure, such that the generated electrons, which are accelerated in the source and are shaped into a beam, can be accelerated within this source, for example, first to a higher kinetic energy in order to be decelerated to a desired kinetic energy immediately before exiting from the source.According to exemplary embodiments herein, the electrons used to reduce the charge of the sample have kinetic energies, each of which is included in a broader energy range. Such an energy range has a maximum energy and a minimum energy, wherein the minimum energy denotes the smallest energy of electrons occurring in the energy range and the maximum energy denotes the largest energy of the electrons occurring in the energy range. Furthermore, the electrons used to reduce the charge of the sample have a distribution of their kinetic energies, so that different proportions or groups of the electrons have different energies. For these distributions of the kinetic energies, there is in each case an energy mean value, that is to say a value of the energy which the kinetic energies of the electrons used to reduce the charge of the sample have on average.The above-mentioned two types of electrons then differ in the distribution of the kinetic energies of the generated electrons directed onto the sample.According to exemplary embodiments, the electrons generated during the second period have kinetic energies from a first energy range and the electrons generated during the third period have kinetic energies from a second energy range, wherein a minimum energy of the first energy range is greater than a maximum energy of the second energy range and wherein during the third period electrons with kinetic energies greater than the maximum energy of the second energy range are not generated and directed onto the sample. In this case, it is nevertheless possible to generate further electrons from an energy range during the second time period and to direct them onto the sample, the minimum energy of which is smaller than the minimum energy of the first energy range.According to further exemplary embodiments, the electrons generated during the second period have a first distribution of their kinetic energies, and the electrons generated during the third period have a second distribution of their kinetic energies, wherein an energy mean value of the first distribution of the kinetic energies is greater than an energy mean value of the second distribution of the kinetic energies.According to exemplary embodiments, the energy averages of the first and second kinetic energy distributions for a given sample are chosen such that the energy average of the first kinetic energy distribution is greater than the energy value of the sample's lower neutral point and the energy average of the second kinetic energy distribution is less than the energy value of the sample's lower neutral point. A value of the kinetic energy of impinging electrons is assumed here as a neutral point of the sample, in which, statistically, exactly one secondary electron or back-scattered electron leaves the sample for each electron impinging on the sample, so that no charge is applied to the sample. The exact values of the neutral points are dependent on the material of the sample used. Typically, materials have two neutral points, a lower one at a lower kinetic energy and an upper one at a higher kinetic energy. If the electrons have a kinetic energy between the two neutral points, more electrons emerge from the sample than are incident, which has the result that the sample is positively charged. If the electrons have kinetic energies that are less than the lower neutral point or greater than the upper neutral point, fewer electrons emerge from the sample than are incident, which results in the sample being charged negatively. However, since the emission of electrons as a result of incident electrons is a statistical effect, the energy of a single electron is not important-or at least not strictly important. Therefore, very low energy electrons can be contained in the electron beam even in the second period of time, as long as a sufficiently large proportion of higher energy electrons is also contained in the beam, which bring about rapid charge compensation. Likewise, a certain proportion of electrons with higher energy may be contained in the third period as long as their proportion is sufficiently small so that they do not cancel out the charge compensation effected by the low energy electrons.According to exemplary embodiments, the maximum energy of the second energy range and / or the energy mean value of the second distribution of the kinetic energies is less than 500 eV, less than 300 eV, less than 100 eV or less than 50 eV.According to further exemplary embodiments, the minimum energy of the first energy range and / or the energy mean value of the first distribution of the kinetic energies is greater than 200 eV, greater than 300 eV, greater than 400 eV or greater than 500 eV.According to further exemplary embodiments, the maximum energy of the first energy range and / or the mean energy value of the first distribution of the kinetic energies is less than 2000 eV or less than 1500 eV.According to a further embodiment, a method for operating a particle beam microscope comprises generating an image of a first region of a sample to be examined to be imaged and reducing a charge of the sample generated during the generation of the image, wherein a sequence is carried out several times in succession which comprises directing a particle beam onto the first region of the sample and detecting particles emerging from the sample, such as secondary electrons and back-scattered electrons, during a first period for the purpose of generating the image of the sample and, for the purpose of reducing the charge of the sample, directing electrons during a third period onto a second region of the sample which is arranged next to the first region of the sample, wherein during the third period the electrons are not directed onto the first region of the sample.The electrons directed onto the sample for the purpose of reducing the charge of the sample in the first region to be imaged are thus directed onto a second region which is arranged next to the first region to be imaged. The inventor has recognized that electrons directed alongside the region to be imaged also generate secondary electrons of lower energy, which reach the first region to be imaged and lead there to an effective reduction in the charge of the sample, as has already been explained above.However, since existing strong charges of the sample in the first region to be imaged and fields generated by other components of the particle beam microscope can prevent the effective reduction of the charge with the aid of electrons of low kinetic energy, as has already been explained above, the method according to a further exemplary embodiment can further comprise directing electrons onto the first region of the sample during a second time period which is temporally before the third time period. Thus, electrons of higher kinetic energy are directed directly onto the region to be imaged in order to initially reduce strong charges present there, so that subsequently a further reduction of the charge can be carried out, which takes place by electrons of lower kinetic energy, which are generated by directing electrons of higher kinetic energy onto the second region arranged next to the first region to be imaged. The electrons of lower kinetic energy are generated in the immediate vicinity of the region to be imaged, so that electric or magnetic fields generated by other components of the particle beam microscope, such as detectors or electron-optical lenses, hardly influence the incidence of the electrons of lower kinetic energy in the region to be imaged.Thus, the reduction of the charge of the sample in the region to be imaged is in turn effected by two types of electrons which differ in terms of their kinetic energy with which they actually strike the sample and in terms of the period in which they strike the region to be imaged. However, it is possible here to first generate electrons with identical kinetic energies by an electron source for both types of electrons, since these are directed to different regions of the sample in the second and the third period of time.The electrons directed onto the first region of the sample to be imaged or onto the second region of the sample can have kinetic energies from an energy range which has a minimum energy which is greater than 200 eV, greater than 300 eV, greater than 400 eV or greater than 500 eV after they have been generated by a suitable electron beam source.According to exemplary embodiments, a maximum energy of this energy range is less than 2000 eV or less than 1500 eV.According to exemplary embodiments, the first region of the sample to be imaged has an area of more than 100 x 100 nm 2, in particular more than 100 μm 2, and in particular more than 10,000 μm 2. Furthermore, the first region to be imaged can have an area which is smaller than 1.5 mm 2 and in particular smaller than 0.1 mm 2.According to exemplary embodiments, the first region to be imaged has a rectangular shape, which can also be square in particular.According to exemplary embodiments, the second region arranged next to the first region can completely surround the first region and in this case can have in particular an annular shape, wherein, for example, when the region to be imaged has a rectangular shape, the second region can have the shape of a rectangular ring.The second region can have an area of more than 0.1 mm 2, more than 1 mm 2 and in particular more than 100 mm 2.In all of the above-described embodiments, the detected particles may be secondary electrons or backscattered electrons, and an electron detector may be used to detect the secondary electrons or backscattered electrons during the first period, and secondary electrons or backscattered electrons may be further prevented from striking the electron detector during the second and / or third period. This can prevent unnecessary irradiation of the detector with secondary electrons in the narrow sense or back-scattered electrons which should not contribute directly to image formation. The incidence of secondary electrons or back-scattered electrons on the electron detector can be effected, for example, by applying a negative potential to the detector relative to the sample or to an electrode arranged between the detector and the sample. The incidence of backscattered electrons on the electron detector can furthermore be reduced, for example, by a suitable positioning of the electron source, the sample and the detector relative to one another.According to example embodiments, the second time period and / or the third time period is shorter than the first time period. The second period and / or the third period may be less than 50 microseconds, for example.According to exemplary embodiments, the particle beam directed onto the sample for the purpose of generating the image thereof is a beam of positive ions. These positive ions lead to a positive charge of the sample, which can be reduced particularly well by the method described above.According to exemplary embodiments, a first electrical potential is applied to a sample holder holding the sample during the first period of time, and a second electrical potential is applied to the sample during the second and / or third period of time, wherein the second electrical potential is greater than the first electrical potential. This measure can be used to suppress the emission of secondary electrons from the sample, so that a positive charge present in the sample is reduced more quickly and efficiently.According to exemplary embodiments, a beam current of the positive ion beam directed onto the sample during the first period is greater than 0.1 pA.In all of the embodiments explained above, a current of the electrons directed onto the sample during the second and / or third period of time can be greater than 100 μA.According to exemplary embodiments, a current of the electrons directed to the sample during the second period is greater than a current of the electrons directed to the sample during the third period.According to exemplary embodiments, a particle beam system comprises a particle beam microscope configured to direct a particle beam onto a first region of a sample to be examined to be imaged and to detect particles emerging from the sample, such as secondary electrons, for example, and an electron beam system configured to direct an electron beam onto the sample, which has an annular beam cross section, such that electrons are directed onto a second region of the sample, which is arranged outside the first region of the sample but not onto the first region.With such a particle beam system, the above-described embodiments of the method for operating a particle beam microscope can be carried out.The invention is explained in more detail below with reference to figures. The following shows: FIG. 1 shows a schematic representation of an embodiment of a particle beam microscope; FIG. 2 shows a schematic illustration for explaining an embodiment of a method for operating the particle beam microscope of FIG. 1 ; FIG. 3 shows a schematic illustration of an embodiment of a particle beam microscope which can likewise be operated with the method explained with reference to FIG. 2 ; FIG. 4 shows a schematic illustration of an embodiment of a particle beam microscope which can likewise be operated with the method explained with reference to FIG. 2 ; FIGS. 5 ato 5 c are schematic representations of regions of a sample surface which result during operation of the particle beam microscope shown in FIG. 4 ; and FIG. 6 shows a graph with spectra of kinetic energies of electrons in the particle beam microscope of FIG. 3.FIG. 1 is a schematic illustration of a particle beam microscope. The particle beam microscope 1 comprises a first particle beam column 3 for generating a particle beam 5, which can be directed onto a surface 7 of a sample 9 to be examined. For this purpose, the particle beam column 3 comprises a source 11 for particles, an electrode 13 for extracting particles forming the beam 5 from the source 11 and for accelerating these particles, an electrode or coil arrangement 15 in order to focus the beam 5 of particles passing through the electrode 13 onto the surface 7 of the sample 9. The particle beam column 3 further comprises deflection electrodes 17 which are connected via a connection 18 to a controller 19 of the particle beam microscope. The controller 19 is configured to excite the deflection electrodes 17 such that the electric or magnetic field generated thereby deflects the beam 5 from an axis of symmetry 21 of the particle beam column 3 in order to direct it onto a location on the surface 7 of the sample 9 within an image region 23 predefined by the controller 19. The image area 23 can have a rectangular shape on the surface 7, for example. Dashed lines 5' in Figure 1 represent the beam 5 when directed to the left and right edges of the region 23, respectively, by operation of the deflector 17.The particles of the particle beam 5 striking the sample 9 again leave (secondary) particles, such as secondary electrons, from the sample 9. A detector 25 is provided to detect particles which emerge from the sample 9 on the side thereof on which the particle beam 5 also impinges. The detector 25 is connected to the controller 19 via a connection 26 so that the controller 19 can detect and record an intensity of the particles dissolved out of the sample 9 by the particle beam 5 at each point in time at which the particle beam 5 is directed onto the sample 9. The information about the intensity of the detected particles recorded depending on the location within the region 23 to which the particle beam 5 is directed represents a particle beam microscopic image of the region 23 of the surface 7 of the sample 9.The particle beam 5 may be a beam of electrons, so that the beam 5 forms a primary electron beam and the particles detected by the detector 25 include backscattered electrons and secondary electrons in a narrow sense. The backscattered electrons extracted from the sample have a kinetic energy which is equal to or slightly lower than the kinetic energy of the electron beam 5 impinging on the sample. Secondary electrons extracted from the sample in the narrower sense are electrons from the sample which have gained so much energy by an impingement of the electron beam 5 that they can leave the sample. These typically have substantially lower kinetic energies than the electrons of the beam 5.However, the beam 5 can also be a beam of positively charged ions, such as He +- ions. For this purpose, the electron beam column 3 can be configured, for example, as described in WO 2007 / 067 328 A2. The positively charged ions impinging on the sample 9 can likewise release secondary electrons from the sample, which secondary electrons can be detected by the detector 25.A further detector 65 is provided in order to detect such particles which emerge from the sample 9 on the side thereof which is opposite the side on which the particle beam 5 impinges. The detector 65 is connected to the controller 19 via a connection 67 so that the controller 19 can detect and record an intensity of the particles dissolved out from the sample 9 by the particle beam 5 at each point in time at which the particle beam 5 is directed onto the sample 9. The information about the intensity of these detected particles recorded depending on the location within the region 23 to which the particle beam 5 is directed likewise represents a particle beam microscopic image of the region 23 of the surface 7 of the sample 9.When the particle beam 5 is a beam of electrons, the particles detected by the detector 65 are transmission electrons. If beam 5 is a beam of ions, such as He +- ions, the particles detected by detector 65 are transmissions, i.e., the ions of the incident beam that have passed through the sample. However, these can have a different charge than the impinging ions due to interactions with the sample.In the case of positive ions, the impinging positive ions positively charge the surface of the sample and the secondary electrons leaving the sample result in a further positive charge of the sample. Positive charging of the sample can on the one hand reduce the intensity of the secondary electrons dissolved from the sample and on the other hand can also reduce the landing energy of the positively charged ions of the beam 5 on the surface of the sample 7. The charge of the sample, which increases with the time during which the particle beam 5 is directed onto the sample, thus distorts the intensity of the detected secondary electrons.In the case where the particle beam 5 is an electron beam, charging of the sample is avoided if exactly one secondary electron leaves the sample for each of the electrons of the electrode beam 5 striking the sample. The conditions required for this purpose can, however, only be reached rarely, so that the sample is usually charged positively or negatively in a time-dependent manner and likewise influences the detected intensity of secondary electrons.The particle beam microscope 1 comprises a possibility for reducing the electrical charge of the sample 9. This possibility can be used if, after a certain period of time during which the particle beam 5 is directed onto the sample 9 and secondary electrons are detected, the charge of the sample is too strong to detect the intensity of the secondary electrons in an uncorrupted manner.The possibility of reducing the charge of the sample 9 uses two kinds of electrons directed to the surface 7 of the sample 9. The two types of electrons are directed in the example described here as two separate beams 31 and 31' onto the sample 9, which are generated by separate electron beam columns 33 and 33'. The two electron beam columns 33 and 33' can have the same or a similar structure and each comprise an electron source 35, 35', an electrode 37, 37' for extracting electrons from the source 35, 35' and accelerating them, so that an electron beam 31, 31' is formed in each case. This can be directed as an extended electron beam onto the surface 7 of the sample 9 so that a beam cross section of the beam 31, 31' in the plane of the surface 7 of the sample 9 has a substantially constant distribution of the beam intensity and includes the region 23 to be imaged which is scanned by the particle beam 5. However, it is also possible for the electron beam 31, 31' to strike the surface 7 of the sample 9 as a focused beam, wherein a diameter of the focused beam at the surface 7 is substantially smaller than the lateral extent of the region 23 to be imaged.The electron beam columns 33, 33' further comprise an accelerating electrode 39, 39' for adjusting the kinetic energy of the electrons of the beam 31, 31' leaving the electron beam columns 33, 33'. For this purpose, an electrical potential can be applied to the acceleration electrode 39, 39' via a terminal 40, 40' which is controlled by the controller 19. As a result, the controller 19 can adjust the kinetic energy of the electrons generated by the electron beam column 33, 33' and incident on the sample 9 as the electron beam 31, 31'. The two types of electrons directed as separate beams 31, 31' to the sample differ in their kinetic energies, which are adjusted by the controller 19 via the application of different electrical potentials to the terminals 40, 40' of the electrodes 39, 39'.In the exemplary embodiment explained here, the electrons of the beam 31 have a higher kinetic energy than the electrons of the beam 31'. For example, the kinetic energies of the beam 31 are in a range between 200 eV to 2000 eV, and the kinetic energies of the beam 31' are in a range from 20 eV to 500 eV.All the particle beam columns 3, 33, 33' are controlled by the controller 19. For this purpose, the particle jet columns each also comprise a possibility, controlled by the controller 19, of switching on and off the respectively generated particle jet 5, 31, 31'. This possibility can comprise, for example, a beam deflector and a pinhole, wherein the controller controls the beam deflector such that the beam passes through the pinhole when the beam is switched on and impinges on the pinhole when the beam is switched off at the location of the sample.FIG. 2 is a time chart for explaining a method of operating the particle beam microscope 1 shown in FIG. 1.FIG. 2 shows a plurality of time diagrams 41, 42, 43 and 44, each of the diagrams representing measures as a function of time t under the control of the controller 19.The operation of the particle beam microscope takes place in sequences S 1, S 2,... which are carried out successively in time. Each sequence S can be divided into three time periods T 1, T 2 and T 3, which differ within a sequence S with regard to the measures carried out in the time periods, but which repeat from sequence S to sequence S.The diagram 42 contains hatched rectangles 51 which represent the directing of the particle beam 5 onto the sample and the detection of the secondary electrons dissolved by the sample during the first time period T 1. Accordingly, during the first time periods T 1, particles of the beam 5 are respectively directed onto the sample and secondary electrons are detected, while outside the time periods T 1, particles are not simultaneously directed onto the sample and secondary electrons are detected in order to directly obtain image information of the region 23 to be imaged. Outside the periods T 1, however, the particle beam 5 may be directed onto the sample 9 as long as secondary electrons are not detected or used for the purpose of forming the image. This can be done, for example, by secondary electrons being detected by the detector 25 and the corresponding detection signals not being read out or read out by the controller 19, but not being directly assigned to a location on the surface of the sample or a pixel of the image.Further, the detection of secondary electrons by the detector 25 can be prevented by preventing secondary electrons released from the sample by the particles of the beam 5 from striking the detector 25. This can be effected, for example, by a grid electrode 47, which is arranged between the region 23 to be imaged of the sample 9 and the detector 25, and to which an adjustable electrical potential can be applied by the controller 19 via a terminal 48. A time dependence of this electrical potential applied to the electrode 47 is shown as a line 56 in the graph 44. From this, it can be seen that the electric potential applied to the electrode 47 is positive within the periods T 1, so that secondary electrons released from the sample are attracted and accelerated by the grid electrode, pass through it, and finally strike the detector 25, so that it can detect the secondary electrons, and the controller can use the detected intensities as image information. Outside the time periods T1 and in particular within second time periods T2 and third time periods T3, which are explained in more detail below, the electrical potential applied to the electrode 47 by the controller 19 is negative, so that secondary electrons, which should be generated by the particle beam 5 and / or one or both of the electron beams 31, 31', are repelled by the grid electrode 47, do not pass through it and thus do not pass to the detector 25. Secondary electrons dissolved from the sample 9 thus cannot contribute to the generation of the image.The graph 41 is an energy-time graph explaining the kinetic energies E and the time dependence of the electron beams 31 and 31' directed to the sample 9. The diagram 41 shows two ranges of kinetic energies, namely a first energy range ΔE1of higher kinetic energy with a minimum energy E1 min and a maximum energy E1 max and a second energy range ΔE2of lower kinetic energy with a minimum energy E2 min and a maximum energy E2 max. The minimum energy E 1 min of the first energy range ΔE 1 is greater than the maximum energy E 2 max of the second energy range ΔE 2.Hatched rectangles 53 represent times at which the electrons of the beam 31 with higher kinetic energies ΔE1 are directed onto the sample, and hatched rectangles 55 represent the directing of the electrons of the beam 31' with lower kinetic energies ΔE2 onto the sample 9. Furthermore, periods T 3 during which the electrons 55 of lower kinetic energy are directed onto the sample lie temporally after the periods T 2. The periods T 1, T 2 and T 3 do not overlap in time within the sequences S. However, it is not excluded here that electrons whose energy is greater than the maximum energy E 2 max of the second energy range ΔE 2 are additionally also directed onto the sample in periods outside the period T 2.In the exemplary embodiment shown here, electrons 53 with kinetic energies that are greater than the maximum energy E 2 max of the second energy range ΔE 2 are not directed onto the sample during the periods T 3, which is represented by crosses 57 in the illustration of FIG. 2.In addition to the measures shown in FIG. 2, it is also possible for electrons with low kinetic energy to be directed onto the sample even outside the periods T 3, such as, for example, within the periods T 1 and T 2.The diagram 43 shows a line 59 which represents a potential U which can be applied to the sample 9 by the controller 19 via a terminal 60. This potential U is negative within the periods T 1 and positive outside the periods T 1 and in particular within the periods T 2 and T 3. This ensures that the particles of the beam 5, if these are positively charged ions, can reach the sample well even when the latter is slightly negatively charged. Moreover, regardless of whether the particles of the beam 5 have a positive or negative charge, this leads to secondary electrons being able to easily separate from the sample and be detected. During the periods T 2 and T 3 during which image information is not to be obtained, the positive potential U on the sample 9 has the effect that secondary electrons, which are generated by the impingement of the electrons 53 of higher kinetic energies or the electrons 55 of lower kinetic energies, may not easily separate from the sample and the intensity thereof is thus reduced.The sequences S 1, S 2,... thus comprise time periods T 1 which serve to generate the image of the sample and time periods T 2 and T 3 which serve to reduce the charge of the sample. During the periods T 2, electrons with higher kinetic energies ΔE 1 contribute to rapidly reducing, in particular, strong positive charges of the sample. During periods T 3, electrons with lower kinetic energies substantially contribute to further reducing charge of the sample remaining after periods T 2.In the example explained above, the minimum energy E 1 min of the first energy range ΔE 1 is greater than the maximum energy E 2 max of the second energy range ΔE 2, and in the periods T 3, the electrons 53 having kinetic energies greater than the maximum energy E 2 max of the second energy range ΔE 2 are not directed to the sample.Since the first energy range ΔE 1 and the second energy range ΔE 2 do not overlap, it is ensured that the energy mean value of the distribution of the kinetic energies of the first energy range ΔE 1 is greater than the energy mean value of the distribution of the kinetic energies of the second energy range ΔE 2.However, the first energy range ΔE 1 and the second energy range ΔE 2 may also overlap, such that the minimum energy E 1 min of the first energy range ΔE 1 is less than the maximum energy E 2 max of the second energy range ΔE 2. Then, however, the condition should be further satisfied that the energy mean value of the distribution of the kinetic energies of the electrons directed to the sample in the time periods T 2 is greater than the energy mean value of the distribution of the kinetic energies of the electrons directed to the sample in the time periods T 3.Further, even in the periods T 3, electrons 53 having kinetic energies greater than the maximum energy E 2 max of the second energy range ΔE 2 may be directed to the sample as long as the energy average value of the distribution of the kinetic energies of the electrons directed to the sample in the periods T 1 is greater than the energy average value of the distribution of the kinetic energies of the electrons directed to the sample in the periods T 2.The energy average of a distribution of kinetic energies can be calculated by the integral, where E represents the kinetic energy, E represents the energy average and p(E) represents the energy spectrum of the generated electrons.Further embodiments are explained below with reference to figures, in which components which correspond to one another in terms of structure and function are provided with the same reference numerals as in FIGS. 1 and 2 and additionally with a letter for differentiation purposes. Reference is made to the entire preceding and subsequent description.FIG. 3 shows a further embodiment of a particle beam microscope which is similar to the particle beam microscope explained with reference to FIG. 1 and differs therefrom with regard to the generation of two types of electrons which are directed onto the sample in order to reduce charging of the sample with different kinetic energies.The particle beam microscope 1 ashown in FIG. 3 comprises a particle beam column 3 afor generating a particle beam 5 awhich is directed onto a surface 7 aof a sample 9 awithin a region 23 ato be imaged in order to extract secondary electrons therefrom, which secondary electrons can be detected by a detector 25 a, as has already been described in connection with FIG. 1.The particle beam microscope 1 acan also be operated according to the method explained with reference to FIG. 2. Therein, after directing the particle beam 5a to the sample and detecting secondary electrons for the purpose of image formation, during first periods T1 in second and third periods T2 and T3, electrons having different kinetic energies are generated and directed to the sample to reduce the charge of the sample. In the embodiment of Fig. 1, two different electron columns 33, 33' are provided for generating the electrons having the different kinetic energies. In the example explained here with reference to FIG. 3, a single electron beam column 33 ais provided, which comprises an electron source 35 a, an electrode 37 afor extracting electrons from the source 35 aand accelerating and shaping an electron beam. Furthermore, the electron beam column 33 aincludes an acceleration electrode 39 awhich is connected to the controller 19 avia a terminal 40 ato adjust the kinetic energy of the electrons leaving the electron beam column 33 a.Within the electron beam column 33a, a beam deflector 73 is also arranged, which is controlled by the controller 19a via a connection 74. The controller 19a may selectively adjust the deflector 73 so that the electrons generated by the electron beam column 33a are incident as a beam 31a directly on the surface of the sample 7a or directed as a beam 77 against a converter element 79 at which the incident electrons generate secondary electrons which ultimately are incident partly as a beam 31' on the surface 7a of the sample 9a. The controller 19 adjusts the electrical potential applied to the accelerating electrode 39a so as to generate electrons of higher kinetic energy for the beam 31a, which then strike the sample during the periods T2. During the periods T3, the deflector 73 is controlled by the controller 19a in such a way that the electron beam generated by the electron beam column 33a impinges on the converter element 79, so that the secondary electrons generated there impinge on the surface of the sample 9a with substantially lower kinetic energy. The controller 19 acan apply the same electrical potential to the acceleration electrode 39 ain the time periods T 3 as in the time periods T 2 in order to largely not change the setting of the electron beam column 33 aand in particular a setting of the focusing thereof during operation. The change of kinetic energy for generating the two kinds of electrons, namely the higher kinetic energy electrons directed to the sample during the periods T2 and the lower kinetic energy electrons directed to the sample during the periods T3, is made by changing the deflection by the beam deflector 73 and converting the higher energy electrons incident to the converter element 79 into lower kinetic energy secondary electrons leaving the converter element 79 and incident to the surface 7a of the sample 9a.FIG. 6 shows a graph in which intensity I is plotted against energy E and in which a line p 1( E) represents the normalized spectrum of the kinetic energies of the electrons of higher kinetic energy and a line p 2( E) represents the normalized spectrum of the kinetic energies of the electrons of lower kinetic energy. The electrons of the higher kinetic energy have an energy mean value E 1 and the electrons of the lower kinetic energy have an energy mean value E 2. In the spectrum p 2( E) of the kinetic energies of the electrons of lower kinetic energy, a peak 101 is caused by the secondary electrons which are generated in or at the converter element 79 from the electrons striking the converter element 79 with the spectrum p 1( E).A peak 103 in the spectrum p 2( E) of the kinetic energies of the electrons of lower kinetic energy is caused by electrons which strike the converter element 79 with the spectrum p 1( E) and are scattered there without energy loss or with only a small energy loss. The electrons with the larger mean energy value E 1 are directed to the sample during periods T2 and the electrons with the smaller mean energy value E 2 are directed to the sample during periods T3.A further embodiment of a particle beam microscope which can be operated according to the method explained with reference to FIG. 2 is explained below with reference to FIGS. 4 and 5, wherein FIG. 4 shows a schematic representation of the particle beam microscope 1 band FIGS. 5 ato 5 c show surface regions of a surface 7 bof a sample 9 bto be examined.The particle beam microscope 1 bincludes a particle beam column 3 bfor generating a particle beam 5 bwhich can be directed onto selectable locations within a region 23 bto be imaged on the surface 7 bof the sample 9 b, as has already been explained above in connection with FIGS. 1 and 3.The particle beam microscope 1 bfurther comprises a detector for detecting secondary electrons which are released from the sample 9 bby the particle beam 5 b, wherein this detector is not illustrated in FIG. 4 and can have a configuration as has already been explained with reference to FIGS. 1 to 3.FIG. 5 ashows the region 23 bto be imaged on the surface 7 bof the sample 9 bin plan view. The region to be imaged is rectangular and in particular square. Within the region 23 bto be imaged, a plurality of predetermined locations are arranged, onto which the beam 5 bis successively directed. At each of the predetermined locations, image information is obtained by the controller 19b by detecting the secondary electrons dissolved from the sample. The image information corresponding to a location on the sample is also referred to as a pixel. The pixels of the image correspond to regions on the sample in which one of the predetermined locations is respectively arranged, onto which the particle beam 5 bis directed, and which are illustrated in FIG. 5 a as a rectangular grid of pixels 85. This rectangular grid is scanned by the particle beam 5 b, for example, line by line, wherein the beam remains at each of the predetermined locations or pixels for a time, referred to as "threshold time", of 100 ns, for example, and then proceeds to the respective next pixel in the line. Once the end of a line has been reached, the process continues with the next line.The directing of the beam and the corresponding detection of secondary electrons for all pixels 85 of the region 23 bto be imaged would take a time such that the charge of the sample generated by the particle beam assumes an extent that, in any case, the image information obtained with respect to the pixels 85 last scanned is excessively adulterated by the charge of the sample.Therefore, individual sequences are carried out several times in succession, wherein image information relating to only a subset of pixels, such as a single line or a half line or two lines, is obtained in each sequence and the charge on the surface of the sample is subsequently reduced. The subsets of the pixels for which image information is obtained within a sequence differ from sequence to sequence in such a way that after carrying out all the sequences for each of the pixels 85 of the region 23 bto be imaged, image information is obtained at least once by directing the particle beam onto the location on the sample corresponding to the pixel and detecting secondary electrons dissolved from the sample.In this respect, the method explained above with reference to FIG. 5a corresponds to the method explained with reference to FIGS. 1 to 3. In the method carried out with the particle beam microscope 1 b, electrons of different energies are likewise used to reduce the charge of the sample, electrons of higher kinetic energy contributing to the reduction of the charge of the sample in the time periods T 2 following the irradiation with the particle beam 5 band electrons of comparatively lower kinetic energy contributing to the still further reduction of the charge of the sample in the time periods T 2 following the time periods T 3.For this purpose, a beam of electrons of higher kinetic energy is directed onto the surface of the sample in the periods T 2 such that its beam cross section 87 contains the region 23 bto be imaged. The region on the surface 7 bof the sample 9 b, on which electrons of higher kinetic energy thus strike during the periods T 2, is illustrated in FIG. 5 b with cross-hatching.In the periods T 3, electrons with low energy are intended to contribute to reducing the charge of the sample in the region 23 bto be imaged. However, these low energy electrons are not generated outside the sample and directed to the region 23 bto be imaged, as is the case with the embodiments of FIGS. 1 and 3. In the particle beam microscope 1 b, electrons of higher kinetic energy are likewise directed onto the surface of the sample in the third time periods T 3, but in this case do not strike directly the region 23 bto be imaged but rather a region 89 arranged next to the region 23 bto be imaged, as is illustrated in FIG. 5 c. The region 89 on which the electrons of the higher kinetic energy strike during the periods T 3 is marked by cross-hatching in FIG. 5 c and has an annular shape surrounding the region 23 bto be imaged and arranged outside thereof. The electrons of the higher kinetic energy striking the region 89 generate there secondary electrons with comparatively much lower kinetic energy, which partially strike the surface 7 bof the sample 9 bup again. In addition to an annular region 91 which surrounds the region 89 and is marked with simple hatching in FIG. 5 c, the secondary electrons with the low energy also strike a circular region 93 which is arranged within the annular region 89 and contains the region 23 bto be imaged. The circular region 93 is also simply shaded in FIG. 5c.If the higher energy electrons are directed onto the ring-shaped region 89 in the periods T 3, secondary electrons of lower kinetic energy are generated there, which contribute to reducing the charge of the surface in the regions 91 and 93 and thus in particular also in the region 23 bto be imaged. Such low energy secondary electrons reducing the charge of the sample also strike a ring region 95 surrounding the region 87 in FIG. 5 bduring the second time periods T 2, but have no significance there for reducing the charge of the sample in the region 23 bto be imaged, just like the secondary electrons striking the region 91 of FIG. 5 cduring the third time period T 3.The particle beam system 1 bincludes a diaphragm arrangement 101 for switching the beam 31 bdirected onto the surface of the sample from the beam cross section shown in FIG. 5, in which the beam cross section in the plane of the surface 7 bof the sample 9 ahas a circular shape 87, into the beam cross section shown in FIG. 5 c, in which it has the shape of the ring 89. The diaphragm arrangement 101 comprises a diaphragm plate 103 having an annular recess 105 and a circular recess 107 arranged next to it. The diaphragm plate 103 can be moved back and forth between two positions by an actuator 109 which is controlled by the controller 19 bvia a connection 111, as is symbolized by a double arrow 113. In one position, the electron beam generated by the electron beam column 3 bappears the region of the aperture plate 103 in which the circular cutout 107 is arranged, so that the circular region 87 of FIG. 5 bis illuminated on the surface 7 bof the sample 9 b, while in the other position, the beam generated by the electron beam column 3 bis incident on the annular cutout 105, so that the annular region 89 shown in FIG. 5 cis illuminated on the surface 7 bof the sample 9 b.

Claims

A method for operating a particle beam microscope (1), the method comprising generating an image of a sample (9) to be examined and reducing a charge of the sample generated during the generation of the image; wherein a sequence of the following measures is carried out several times in succession: (a) directing a particle beam (5) onto the sample (9) and detecting particles emerging from the sample (9) during a first time period (T1) for generating the image of the sample (9); (b1) generating a first beam (31) of electrons which have a first distribution (p1) of their kinetic energies, and directing these electrons onto the sample (9) during a second time period (T2) for reducing the charge of the sample (9) generated by the directing of the particle beam (5) onto the sample (9); (b2) generating a second beam (31') of electrons having a second distribution (p2) of their kinetic energies, and directing these electrons onto the sample (9) during a third period of time (T3) for further reducing the charge of the sample (9) generated by directing the particle beam (5) onto the sample (9); wherein an energy mean value of the first distribution (p1) of the kinetic energies is greater than an energy mean value of the second distribution (p2) of the kinetic energies.Method according to claim 1, wherein the energy mean value of the second distribution (p2) of kinetic energies is less than 500 eV or less than 300 eV or less than 100 eV or less than 50 eV.Method according to claim 1 or 2, wherein the energy mean value of the first distribution (p1) of kinetic energies is greater than 100 eV or greater than 200 eV or greater than 300 eV or greater than 400 eV or greater than 500 eV.Method according to any one of claims 1 to 3, wherein the mean energy value of the first kinetic energy distribution (p1) is less than 2000 eV or less than 1500 eV.Method for operating a particle beam microscope (1b), wherein the method comprises generating an image of a first region of a sample (9b) to be imaged and reducing a charge of the sample (9b) generated during the generation of the image; wherein a sequence of the following measures is carried out a plurality of times in succession: (a) directing a particle beam (5b) onto a first region (23b) of the sample (9b) and detecting particles emerging from the sample (9b) during a first time period (T1) for generating the image of the sample (9b); (b2) directing a first beam (31b) of electrons onto a second region (89) of the sample (9b) arranged next to the first region (23b) of the sample (9b) during a third period (T3) for reducing the charge of the sample (9b) in the first region (23b) generated by the directing of the particle beam (5b) onto the sample (9b); wherein during the third period (T3) electrons are not directed onto the first region (23b) of the sample (9b).The method of claim 5, further comprising (b1) directing a second beam (31b) of electrons onto the first region (23b) of the sample during a second time period (T2) for reducing the charge of the sample (9b) generated by the directing of the particle beam (5b) onto the sample (9b) in the first region; wherein a beginning of the third time period (T3) is later than an end of the second time period (T2).Method according to claim 5 or 6, wherein the first region (23b) has an area of more than 100 × 100 nm 2, in particular of more than 100 μm 2, and in particular of more than 10,000 μm 2.Method according to any one of claims 5 to 7, wherein the first region (23b) has an area of less than 1.5 mm 2, and in particular of less than 0.1 mm 2.The method according to any one of claims 5 to 8, wherein the first region (23b) has a rectangular shape.The method of any one of claims 5 to 9, wherein the second region (89) completely surrounds the first region (23b).The method of any one of claims 5 to 10, wherein the second region (89) has an annular shape.Method according to one of Claims 5 to 11, wherein the second region (89) has an area of more than 0.1 mm 2, in particular of more than 1 mm 2, and in particular of more than 100 mm 2.The method of any one of claims 5 to 12, wherein directing the electrons onto the sample (9b) comprises generating electrons having kinetic energies from an energy range having a minimum energy greater than 200 eV or greater than 300 eV or greater than 400 eV or greater than 500 eV.The method of claim 13, wherein a maximum energy of the energy range is less than 2000 eV or less than 1500 eV.Method according to one of Claims 1 to 4 or 6 to 14, wherein a start of the second time period (T2) lies temporally after an end of the first time period (T1).The method according to any one of claims 1 to 15, wherein the detected particles are secondary electrons, and wherein the method further comprises: using an electron detector (25) to detect the secondary electrons during the first period (T1), and preventing secondary electrons from striking the electron detector (25) during the second (T2) and / or third (T3) period.Method according to one of Claims 1 to 16, wherein the second time period (T2) and / or the third time period (T3) is shorter than the first time period (T1).Method according to one of Claims 1 to 17, wherein the second time period (T2) and / or the third time period (T3) lasts less than 50 μs.The method according to any one of claims 1 to 18, wherein the particle beam (5) is a beam of positively charged ions.The method of claim 19, further comprising: applying a first electrical potential (56) to the sample (9) during the first time period (T1) and applying a second electrical potential (59) to the sample during the second (T2) and / or third (T3) time periods; wherein the second electrical potential is greater than the first electrical potential.Method according to claim 19 or 20, wherein a beam current of the particle beam (5) directed onto the sample (9) during the first time period (T1) is greater than 0.1 pA.The method according to any one of claims 1 to 21, wherein a current of the electrons directed to the sample (7) during the second (T2) and / or third (T3) time period is greater than 100 μA.The method according to any one of claims 1 to 22, wherein a current of the electrons directed to the sample (9) during the second period (T2) is greater than a current of the electrons directed to the sample (9) during the third period (T3).The method according to any one of claims 1 to 23, wherein the particle beam (5) is directed to a plurality of predetermined locations on the sample (9).The method of claim 24, wherein the particle beam (5) is directed to only a subset of the predetermined locations on the sample (9) during the first time period (T1) of each sequence.The method of claim 25, wherein the subset comprises only a single one of the predetermined locations at a time.The method according to any one of claims 1 to 26, wherein the particles emerging from the sample (9) are secondary electrons.A particle beam system comprising: a particle beam microscope (3b) configured to direct a particle beam (5b) onto a first region (23b) to be imaged of a sample (9b) to be examined and to detect particles emerging from the sample (9b), in particular secondary electrons; and an electron beam system (33b) configured to direct an electron beam (31b) onto the sample (9b) which has an annular beam cross section such that electrons are directed onto a second region (89) of the sample which is arranged outside the first region (23b) of the sample (9b) but not onto the first region (23b).The particle beam system according to claim 28, wherein the particle beam (5b) is a positively charged ion beam.The particle beam system of any one of claims 28 to 29, wherein the particle beam system is configured to perform the method of any one of claims 1 to 27.

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