Unit and method for filtering and / or correcting a charged particle beam, device comprising such a unit

Abstract

In summary, the invention relates to a unit for use in a charged particle beam device The unit comprises an input and an output, which are arranged spaced apart on a center line. A series of electrostatic deflectors is arranged between the input and output aperture. The series of electrostatic deflectors comprises a first deflector for deflecting a beam of charged particles away from the center line, a second deflector for bending the deflected beam of charged particles back to the center line, and a third deflector for bending at least part the beam of charged particles back along the center line. The unit can be configured to provide an energy filter or a chromatic aberration corrector. When configured as an energy filter, the unit further comprises a spatial filter arranged on the center line after the third deflector, for selecting charged particles that pass through according to their energy.

Description

[0001] Unit and method for filtering and / or correcting a charged particle beam, device comprising such a unit

[0002] BACKGROUND

[0003] The invention relates to a unit and method for filtering and / or correcting a charged particle beam . In particular, the invention relates to a unit that can be used as an energy filter for a charged particle beam. In addition the same unit can also be used for a chromatic aberration correction of a charged particle beam. The invention further relates to a charged particle device , such as , but not limited to , an electron microscope, comprising such a unit .

[0004] Charged particle beam devices are commonly provided with a charged particle source configured for emitting a beam of charged particles . This beam of charged particles can be used for imaging a sample , as for example in a Scanning Electron Microscope ( SEM) or Transmission Electron Microscope ( TEM) , or for materials processing, as for example in a Focused Ion Beam ( FIB ) system .

[0005] The charged particle beam commonly comprises charged particles with various energies , wherein these energies provide a certain energy distribution . These dif ferent energies of the charged particles in the charged particle beam yield chromatic aberrations in the charged particle device . Chromatic aberration is the result of charged particles of different energies being focused at dif ferent locations by lenses in the charged particle device . By reducing the energy spread of the charged particle beam, the chromatic aberrations can be reduced . The energy spread of a charged particle beam can be reduced by using an energy filter, also referred to as a monochromator . Such an energy filter can also be used to filter the charged particle beam after interaction with a sample to spectrally measure intensities and is then referred to as an analyzer .

[0006] One type of energy filter is a so-called 'chicane ' dual-bend energy filter, as for example described in EP 2592642 . In such an energy filter, the charged particle beam is j ogged off-axis and then back on-axis , usually by four dipole deflectors in series down a charged particle column . Between the second and third deflectors of the four deflector chicane , there is usually a round aperture to block the passage of charged particles outside a nominal energy range or a knife-edge to block the passage of charged particles with an energy below or above a nominal energy .

[0007] According to EP 2592642 , prior art chicane filters are negatively impacted by both space charge effects and aperture damage, the latter speci fically when using such an energy filter in a FIB system. EP 2592642 solves these problems by the addition of electrostatic quadrupole and hexapole excitations in the chicane energy filter, which are configured to form a charged particle beam into a line focus at an aperture positioned between the second and third elements of the four element chicane . In addition, the addition further improves the energy resolution by making the line focus both sharper and straighter at the aperture plane .

[0008] In addition US 2010 / 0327180 Al discloses an example of an electrostatic filter that includes four sets of deflectors ; a first deflector is configured for deflecting a beam of charged particles away from an optical axis , a second deflector is configured for deflecting charged particles with a specific energy back onto a traj ectory that is essentially parallel to the original traj ectory, a third deflector that deflects the charged particles back towards a fourth deflector that deflects the charged particles back onto a traj ectory that is essentially parallel to the traj ectory of the charged particles before they entered the electrostatic filter . Between the second and third deflector a barrier is arranged, wherein the barrier is configured to block the passage of charged particles with an energy lower than the specific energy . Accordingly the electrostatic filter disclosed by US 2010 / 0327180 Al is a "high-pass" filter . The electrostatic filter as described in US 2010 / 0327180 Al causes astigmatism of the outgoing charged particle beam, which may be corrected using known techniques , such as an octupole lens .

[0009] SUMMARY OF THE INVENTION

[0010] A disadvantage of the known energy filters for charged particle devices is , that the addition of electrostatic quadrupole , hexapole or octupole excitations makes the energy filter relatively complicated to manufacture and to use .

[0011] It is an obj ect of the present invention to provide an alternative energy filter that also allows to provide an improved energy resolution and / or to reduce aberrations .

[0012] According to a first aspect , the present invention pertains to a unit for filtering and / or correcting a charged particle beam, wherein the unit comprises : an input and an output which are arranged spaced apart on a center line of the unit, a series of electrostatic deflectors arranged in between the input and output , wherein the series of electrostatic deflectors comprises a first deflector which configured for deflecting a beam of charged particles away from the center line, a second deflector which is configured for bending the deflected beam of charged particles back in the direction of the center line, and a third deflector which is configured for bending at least part of the beam of charged particles back along the center line, a ring-shaped electrode between the input and the first deflector, between the first and second deflector, between the second and third deflector, and between the third deflector and the output , wherein the ring-shaped electrodes are connectable to one or more voltage sources .

[0013] In an embodiment , in particular where the unit is employed as an energy filter, the unit preferably comprises a spatial filter arranged on the center line at the output or between the third deflector and the output , wherein the spatial filter is configured for, in use , a selecting charged particles that pass through the spatial filter according to their energy .

[0014] The ring-shaped electrodes are provided with a through opening or aperture inside the ring-shaped electrode to provide passage of the charged particle beam, and are configured for, in use , provide an electrostatic lens . It is noted that , in use as an energy filter, the ring-shaped electrodes , in particular the ones adj acent to the second deflector, are configured to preferably provide : a retardation of charged particles of the charged particle beam, which corresponds to a lowering of the charged particle beam energy, preferably before the charged particle beam enters the second deflector, and an acceleration of the charged particles of the charged particle beam, which corresponds to a raising of the charged particle beam energy, preferably after the charged particle beam leaves the second deflector .

[0015] The first deflector provides an amount of dispersion, as also described in the energy filter of the prior art . However, because in use the charged particle beam energy is briefly lowered, preferably to a small fraction of the input beam energy, a dispersion caused by the second deflector is enhanced and preferably surpasses the dispersion of the first deflector, and most of the angular dispersion of the energy filter of the invention is created in or in the vicinity of the deflector plates of the second deflector .

[0016] Accordingly, when the unit is employed as an energy filter, it is particularly advantageous that the spatial filter is arranged at the output aperture or between the third deflector and the output of the energy filter, because then the full length of the energy filter of the present invention is used to obtain the net- dispersion .

[0017] In addition or alternatively, aberrations can be reduced when compared to prior art energy filters with the same deflector configuration, in particular because the energy filter of the present invention can be operated using lower field strengths . Accordingly, the present invention provides a less complicated solution to provide an improved energy filter, substantially without requiring electrostatic quadrupole , hexapole or octupole deflectors as presented in the prior art document .

[0018] Furthermore , the unit of the present invention does not require a pre- focusing of the charged particle beam by an external lens , or a complex focusing by the quadrupole excitations as in the prior art energy filter, to focus the charged particle beam on the plane of the spatial filter .

[0019] The inventors further found that the same unit can also be used as a chromatic aberration corrector . When in use as chromatic aberration corrector, the ring-shaped electrodes , are configured to preferably provide : a retardation of charged particles of the charged particle beam, which corresponds to a lowering of the charged particle beam energy, preferably before the charged particle beam enters the first deflector, an acceleration of the charged particles of the charged particle beam, which corresponds to a raising of the charged particle beam energy, preferably before the charged particle beam enters the second deflector, a retardation of charged particles of the charged particle beam, which corresponds to a lowering of the charged particle beam energy, preferably before the charged particle beam enters the third deflector, and an acceleration of the charged particles of the charged particle beam, which corresponds to a raising of the charged particle beam energy, preferably before the charged particle beam leaves the output .

[0020] In particular when applying suitable voltages to the components of the unit , the dispersion of the charged particle beam in the unit can be arranged to provide a negative chromatic dispersion . This negative dispersion can be used to cancel the ef fect of positive chromatic aberrations of other components in a charged particle beam device that comprises the unit of the invention . It is noted that the negative chromatic dispersion occurs in the direction of the deflection provided by the deflectors in the unit . Accordingly, in order to provide a chromatic aberration correction in a surface perpendicular to the center line , an assembly of two units of the invention is required, wherein the two units are arranged in series , one behind the other, along the center line , wherein the deflection provided by the deflectors of one of the two units is arranged substantially perpendicular to the deflection provided by the deflectors of the other one of the two units . Accordingly, the assembly can provide a negative chromatic aberration in two perpendicular directions in a plane perpendicular to the center line .

[0021] It is noted that within the context of this document , the term 'dispersion' relates to an energydependent deflection of charged particles in an electrostatic deflector, or in other words ; the amount of deflection of a charged particle in an electrostatic deflector is dependent on the energy of the charged particle .

[0022] It is further noted that the second deflector in the unit of the invention may also comprise two electrostatic deflectors in series ; one for bending the deflected beam of charged particles back in the direction substantially parallel to the center line ( at least for one specific charged particle energy) , and a subsequent one for bending the beam of charged particles that is substantially parallel to the center line back in the direction of the center line, as is also the case in the energy filter of the prior art .

[0023] In some applications of the unit of the present invention, when used as an energy filter, the spatial filter can be arranged at or provided by the input aperture of a charged particle detector . In addition, the unit of the present invention, when used as an energy filter, can be provided with multiple spatial filters or multiple charged particle detectors with a corresponding input aperture , which are arranged on a line, or plane that extends in a direction substantially perpendicular to the center line . This allows to use the unit of the present invention as a spectrometer or spectrograph .

[0024] In another application of the unit of the present invention, the input of the unit is arranged at or adj acent to an charged particle source of a charged particle beam device . In this case the input of the unit can provide an electrode that can be used to function as an extractor for the charged particle source .

[0025] In an embodiment, the first deflector comprises two or more spaced apart deflector plates , wherein an area between the deflector plates of the first deflector comprises a first center point , wherein a distance between the first center point and the deflector plates of the first deflector is larger than a distance between the first center point and the center line . Accordingly, the center line traverses between the deflector plates of the first deflector .

[0026] It is noted that a center point as defined above , is a point in a center between the deflector plates of the deflector . Thus the center point is at a position with equal distances to the deflector plates of the deflector .

[0027] In an embodiment , the first center point is spaced apart from the center line . In other words , the first deflector is arranged of f-axis with respect to the center line of the unit . The inventors have found that by arranging the first center point at a distance from the center line, coma or comatic aberration in the unit of the invention can be reduced . As coma is a limiting factor for the energy resolution of prior art energy filters , reducing coma is very advantageous and enhances the energy resolution of the unit of the present invention when used as an energy filter .

[0028] In an embodiment, the second deflector comprises two or more spaced apart deflector plates , wherein an area between the deflector plates of the second deflector comprises a second center point, wherein said second center point is arranged spaced apart from the center line .

[0029] In an embodiment, the deflector plates of the second deflector are arranged at opposite sides of the center line . By arranging the deflector plates of the second deflector at opposite sides of the center line, the center line passed between the two or more spaced apart deflector plates , and a beam path for undeflected charged particles is created which traverses the first, second and third deflector in between the corresponding deflector plates . Accordingly, the charged particle beam can traverse the unit of the present invention substantially undisturbed when substantially no voltage dif ferences are provided to the deflector plates of the first, second and third deflector and no deflecting electrostatic fields are present in the unit of the present invention .

[0030] In an embodiment, a distance between the second center point and the deflector plates is larger than a distance between the second center point and the center line . In other words , the second deflector is arranged of f- axis with respect to the center line of the unit . The inventors have found that by arranging the second center point at a distance from the center line, coma or comatic aberration in the unit of the invention can be reduced, which is very advantageous for enhancing the energy resolution of the unit of the present invention when used as an energy filter .

[0031] It is noted that in the embodiment where the first and / or second deflectors are arranged off-axis with respect to the center line, the asymmetry of the design requires a very small but non- zero voltage dif ference across the deflector plates to provide a close to zero dispersion at the image plane . In this way the fields are tailored such that the charged particle beam substantially does not deflect away from the center line .

[0032] In an embodiment, the third deflector comprises two or more spaced apart deflector plates , wherein an area between the deflector plates of the third deflector comprises a third center point, wherein a distance between the third center point and the deflector plates of the third deflector is larger than a distance between the third center point and the center line . Accordingly, the center line traverses between the deflector plates of the third deflector .

[0033] In an embodiment , the third center point is spaced apart from the center line . In other words , the third deflector is arranged of f-axis with respect to the center line of the unit . The inventors have found that by arranging the third center point at a distance from the center line, coma or comatic aberration in the unit of the invention can be reduced, which is very advantageous for enhancing the energy resolution of the unit of the present invention when used as an energy filter .

[0034] It is noted that , the briefly lowering of the energy of the charged particle beam at the second deflector also provides that a shi ft of the second center point with respect to the center line is very ef fective in reducing the comatic aberration . Accordingly, the contribution of shi fting the second center point away from the center line is much larger than the contribution of shi fting the first and the third center point away from the center line , and the shi fting of the first and third center point can be omitted in an embodiment .

[0035] In a preferred embodiment, the first, second and third deflectors are all arranged of f-axis , preferably off- axis in a same direction that is perpendicular to the center line .

[0036] In an embodiment , the ring-shaped electrode arranged between the first deflector and the second deflector is further referred to as a first ring-shaped electrode , and the ring-shaped electrode between the second deflector and the third deflector is further referred to as a second ring-shaped electrode , wherein the first and second ring-shaped electrodes are connectable to a voltage source , preferably wherein the first and second ring-shaped electrodes are connectable to the same voltage source .

[0037] In an embodiment , the ring-shaped electrode between the input and the first deflector is further referred to as an input ring-shaped electrode , and the ring-shaped electrode between the third deflector and the output is further referred to as the output ring-shaped electrode , wherein the input and output ring-shaped electrodes are connectable to a voltage source , preferably wherein the input and output ring-shaped electrodes are connectable to the same voltage source .

[0038] In an embodiment , the input ring-shaped electrode , the first ring-shaped electrode , the second ring-shaped electrode , and the output ring-shaped electrode are configured for, in use , providing an Einzel lens .

[0039] Preferably, the ring-shaped electrodes in the energy filter of the present invention are configured for, in use , imaging the plane of the input , also denoted as input plane , on the plane of the output , also denoted as output plane , with a magni fication of ± 1 . This means that the unit of the present invention can easily be incorporated in any existing charged particle column without requiring extensive alterations . In addition, this allows to stack two or more of the units of the present invention in series one behind the other to ampli fy the accuracy or energy resolution, or to provide chromatic aberration correction in di f ferent , preferably orthogonal , directions .

[0040] It is noted that a magni fication of - 1 means that the absolute value of the magni fication is 1 , but that the image on the output aperture is inverted with respect to the source at the input aperture .

[0041] It is further noted that the input plane and the output plane extend substantially perpendicular to the center line of the unit .

[0042] In an embodiment , the first and second ringshaped electrodes are arranged symmetrically around the center line , preferably also the input and output ringshaped electrodes are arranged symmetrically around the center line . Thi s symmetrical arrangement is provided by arranging the center point of the circular through opening or aperture inside the ring-shaped electrodes on the center line of the unit .

[0043] In an embodiment , the unit comprises an input aperture arranged at the input and comprising an aperture that is substantially centered on the center line , and / or wherein the unit comprises an output aperture arranged at the output and comprising an aperture that is substantially centered on the center line , preferably wherein the output aperture is connectable to a voltage source . In a preferred embodiment , the input aperture and / or the output aperture is connectable to a voltage source , It is noted that preferably the center line of the unit of the present invention is defined by the positions of the input and output apertures ; the center line traverses through the center of both the input and the output apertures . The further components of the unit of the present invention are not necessarily arranged symmetric with respect to this center line .

[0044] It is further noted that , when the spatial filter is arranged at the output , the spatial filter can be or can provide the output aperture .

[0045] In an embodiment , the unit comprises a symmetry plane which is substantially perpendicular to the center line , wherein the components of the unit between the symmetry plane and the input , and the components of the unit between the symmetry plane and the output are mirror symmetric with respect to each other . Accordingly, the unit of the present invention is relatively easy to manufacture .

[0046] In an embodiment , the unit comprises a stigmator which is arranged between the input and the output , preferably wherein the stigmator is arranged at or near a position where , in use , the deflected beam of charged particle beam is bending back in the direction of the center line . In an embodiment , the stigmator is arranged between the deflector plates of the second deflector, preferably the stigmator is arranged in or near the symmetry plane . The stigmator allows to provide a stigmatic correction in order to correct for astigmatism and thereby can further reduce aberrations . It is noted that, in use, the stigmator adds a small quadrupole excitation to the second deflector; for example the voltage on the stigmator is preferably less than 20 % of the deflection voltage of the second deflector . Accordingly, the quadrupole excitation of the stigmator in the unit of the present invention is only used for making small corrections to reduce astigmatism and / or to the alignment of the charged particle traj ectories , but not for the main deflection in the second deflector .

[0047] In an embodiment , the stigmator is arranged spaced apart from the center line . The center line does not pass through the stigmator, in particular the center line does not pass through any electrodes of the stigmator . Accordingly, the charged particle beam can traverse the unit of the present invention substantially undisturbed when substantially no voltage dif ferences are provided to the deflector plates of the first, second and third deflector and no deflecting electrostatic fields are present in the unit of the present invention, wherein the charged particle beam does not travel through the stigmator, but travels past the stigmator .

[0048] In an embodiment, the unit comprises a stack of silicon wafer parts , wherein each silicon wafer part comprises one electrostatic deflector of the series of electrostatic deflectors or one ring-shaped electrode of the one or more ring-shaped electrodes . This provides an easy and low cost way to accurately manufacture the unit of the present invention, by using, preferably MEMS production techniques .

[0049] In an embodiment , the input aperture comprises a silicon wafer part with a membrane deposited on top of the wafer part , wherein the membrane comprises an aperture opening . The input aperture is preferably configured to rej ect external ( oscillating) fields , clean up the beam, assist in aligning the beam while completing the symmetry along the optical axis , and / or capping the residual field emitted by the ring-shaped electrode adj acent to the input aperture .

[0050] In an embodiment , the output aperture comprises a silicon wafer part with a membrane deposited on top of the wafer part , or comprises a membrane without a silicon wafer part , wherein the membrane comprises an aperture opening . In addition or alternatively, the membrane for the output aperture can be grown in a conducting wafer part that is directly glued to the ring-shaped electrode adj acent to the output aperture , because in a preferred embodiment , the membrane of the output aperture and the adj acent ring-shaped electrode are configured to be at substantially the same potential value .

[0051] In an embodiment, the unit comprises a spacer plate which is arranged in between two adj acent silicon wafer parts of said stack of silicon wafer parts , wherein the spacer plate is made from material that is substantially electrically isolating, preferably the unit comprises a spacer plate in between each two adj acent silicon wafer parts of said stack of silicon wafer parts . The spacer plates are provided with through openings to allow the charged particle beam to traverse the spacer plate at the appropriate positions .

[0052] In an embodiment, the spacer plate is a glass plate . Accordingly, the unit according to this embodiment comprises a glass plate which is arranged in between two adj acent silicon wafer parts of said stack of silicon wafer parts , preferably wherein the unit comprises a glass plate in between each two adj acent silicon wafer parts of said stack of silicon wafer parts . The glass plates are preferably provided with through openings to allow the charged particle beam to traverse the glass plate at the appropriate positions . The glass plate or glass plates serve as a spacer and / or isolator between the silicon wafer parts of the unit of the present invention .

[0053] In an embodiment , the silicon wafer parts and the spacer plate are connected to each other by means of a glue connection . This provides a compact energy filter, monochromator or chromatic aberration corrector which can easily be incorporated into an existing charged particle device .

[0054] According to a second aspect, the present invention pertains to a charged particle beam device comprising a unit or an embodiment thereof as described above, wherein the charged particle beam device comprises a charged particle source configured for emitting a charged particle beam along a beam path in the charged particle device , wherein the unit is arranged in said beam path . In an embodiment, the unit is arranged in a substantially linear part of the beam path, preferably wherein the centerline substantially coincides with said substantially linear part of the beam path .

[0055] According to a third aspect , the present invention pertains to a method for filtering and / or correcting a charged particle beam in a charged particle beam device , wherein the charged particle beam device comprises a charged particle source that emits a charged particle beam along a beam path in the charged particle device , wherein the method comprises the steps of : providing a unit or an embodiment thereof as described above , arranged in said beam path such that the centerline substantially coincides with said beam path, providing a first voltage dif ference over deflector plates of the first deflector to deflect the charged particle beam away from the center line, providing a second voltage dif ference over deflector plates of the second deflector to bend the deflected charged particle beam back in the direction of the center line, and providing a third voltage di fference over deflector plates of the third deflector to bend at least part of the charged particle beam back along the center line, providing voltages to one or more of the ringshaped electrodes to provide one or more electrostatic lenses , and wherein the voltages provide a dispersion of the charged particle beam in the unit to arranged charged particles with dif ferent energy next to each other along a line substantially parallel to a direction of deflection in the deflectors .

[0056] It is noted that in use the deflection of the charged particle beam in the unit according to the invention is preferably provided by a combination of the voltages over the deflector plates of the various deflectors , and an additional contribution to the deflection may be provided by the voltages provided on the ring-shaped electrodes . Accordingly, in a preferred working of the unit of the invention, the deflections of the charged particle beam is preferably provided by a combination of the electrostatic fields created by the deflectors and the lenses in the unit .

[0057] In an embodiment, the voltages provide a negative chromatic aberration at the output of the unit . Accordingly, this method allows to introduce a negative chromatic aberration, which can be used to correct a chromatic aberration in other components in a charged particle beam device that comprises a unit of the present invention .

[0058] In an embodiment where the unit is used as an energy filter, the method further comprises the step of using a spatial filter to block the passage of charged particles according to their energy . By adj usting the voltage di f ferences over the deflector plates of the first , second and / or third deflector, a nominal energy can be selected, wherein the unit then provides passage of charged particles with an energy in a specific range or bandwidth around the nominal energy . Preferably, also the voltages applied to the ring-shaped electrodes are adj usted to bring the charged particles with a dif ferent nominal energy into focus at the spatial filter . Accordingly, in an embodiment, all voltage di fferences between any given element in the energy filter on the one hand, and the input / output aperture on the other hand, are scaled by substantially the same ratio .

[0059] In an embodiment, the one or more ring-shaped electrodes comprises an input ring-shaped electrode between the input and the first deflector, a first ring-shaped electrode between the first deflector and the second deflector, a second ring-shaped electrode between the second deflector and the third deflector, and an output ring-shaped electrode between the third deflector and the output , wherein the method comprises the step of : providing voltages to the input , first , second and output ring-shaped electrodes to provide an Einzel lens , preferably for imaging a plane at the input on a plane at the output .

[0060] The various aspects and features described and shown in the specification can be applied, individually, wherever possible . These individual aspects , in particular the aspects and features described in the attached dependent claims , can be made subj ect of divisional patent applications .

[0061] BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The invention will be elucidated on the basis of an exemplary embodiment shown in the attached drawings , in which :

[0063] Figure 1 shows a schematic cross-section view of a first example of a unit of the present invention;

[0064] Figure 2A shows a schematic cross-section view of a second example of a unit of the present invention;

[0065] Figure 2B shows a schematic top-view at the symmetry plane of figure 2A;

[0066] Figure 3 shows a schematic cross-section view of the second example of the unit for use as a spectrometer ;

[0067] Figure 4 shows a first example of a charged particle beam device comprises a unit of the present invention, and

[0068] Figure 5 shows a second example of a charged particle beam device comprising an assembly of two units of the present invention .

[0069] DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a schematic cross-section view of a first example of a unit 1 according to the present invention . 1 . The unit 1 comprises an input 2 and an output 6 with an output aperture 61 which are arranged spaced apart on a center line CL of the energy filter 1 . In between the input and output 2 , 6, a series of electrostatic deflectors is arranged . The series of electrostatic deflectors comprises a first deflector 3 which configured for deflecting a charged particle beam CPB away from the center line CL, a second deflector 4 which is configured for bending the deflected charged particle beam CPB back in the direction of the center line CL, and a third deflector 5 which is configured for bending at least part of the charged particle beam CPB back along the center line CL . The first deflector 3 and the third deflector 5 comprises two or more spaced apart deflector plates which are arranged at opposite sides of the center line CL .

[0070] In addition, as shown in figure 1 , also the second deflector 4 comprises two or more spaced apart deflector plates which are arranged at opposite sides of the center line CL . By arranging the second deflector 4 such that the center line CL passed between the two or more spaced apart deflector plates , a beam path for undeflected charged particles is created along the center line CL, which traverses the first deflector 3 , second deflector 4 and third deflector 5 in between the corresponding deflector plates . Accordingly, a charged particle beam can traverse the unit 1 substantially undisturbed when no voltage di fferences are provided to the deflector plates of the first deflector 3 , second deflector 4 and third deflector 5 and no deflecting electrostatic fields are present in the unit 1 .

[0071] The center point CP2 between the two or more spaced apart deflector plates of the second deflector 4 , is spaced apart from the center line CP, in the same direction as the deflection of the charged particle beam CPB by the first and third electrostatic deflectors 3 , 5, for reducing comatic aberration .

[0072] In addition, the unit 1 comprises an input ringshaped electrode 7 between the input 2 and the first deflector 3 , a first ring-shaped electrode 8 between the first deflector 3 and the second deflector 4 , a second ring-shaped electrode 9 between the second deflector 4 and the third deflector 5, and an output ring-shaped electrode 10 between the third deflector 5 and the output 6. The ring-shaped electrodes 7 , 8 , 9 , 10 , and the input 2 and output aperture 61 , are connectable to one or more voltage sources for, in use , providing an electrostatic lens , preferably an Einzel lens , which is preferably configured for imaging the input 2 onto the output aperture 61 , preferably with a magni fication of approximately - 1 .

[0073] It is noted that in case the unit is configured to be used as an energy filter, the ring-shaped electrodes , in particular the first ring-shaped electrode 8 and the second ring-shaped electrode 9 , also provide a retardation of charged particles of the charged particle beam CPB, which corresponds to a lowering of the charged particle beam energy, before the charged particle beam enters the second deflector 4 , and an acceleration of the charged particles of the charged particle beam CPB, which corresponds to a raising of the charged particle beam energy, after the charged particle beam leaves the second deflector 4 . Because the charged particle beam energy is briefly lowered, preferably to a small fraction of the input beam energy, a dispersion caused by the second deflector 4 is enhanced, and most of the angular dispersion of the unit 1 of this embodiment is created in or in the vicinity of the deflector plates of the second deflector 4 . In addition, this briefly lowering of energy of the charged particle beam CPB at the second deflector 4 also provides that a shi ft of the second center point SP2 with respect to the center line CL is very ef fective in reducing comatic aberration .

[0074] In the unit 1 in the example of figure 1 , the first deflector 3 , the second deflector 4 , and the third deflector 5 all provide or contribute to a net-dispersion of the charged particle beam. The dispersion provides that charged particles with di fferent energies arrive at the output aperture 61 at di fferent positions in a plane perpendicular to the center line CL . Accordingly, the unit can operate as an energy filter and the output aperture 61 also functions as a spatial filter that is arranged on the center line CL . Accordingly, the output aperture 61 , acting as the spatial filter, is configured for, in use, selecting charged particles that pass through the output aperture 61 according to their energy .

[0075] As schematically shown in figure 1 , the unit 1 comprises a stack of silicon wafer parts 13 - 20 , wherein each silicon wafer part comprises one electrostatic deflector of the series of electrostatic deflectors or one ring-shaped electrode of the one or more ring-shaped electrodes . The input 2 is provided at the input ringshaped electrode 7 , a first silicon wafer part 17 comprises the input ring-shaped electrode 7 , a second silicon wafer part 13 comprises the first deflector 3 , a third silicon wafer part 18 comprises the first ring-shaped electrode 8 , a fourth silicon wafer part 14 comprises the second deflector 4 , a fifth silicon wafer part 19 comprises the second ring-shaped electrode 9 , a sixth silicon wafer part 15 comprises the third deflector 5, a seventh silicon wafer part 20 comprises the output ring-shaped electrode 10 , and an eighth silicon wafer part 16 comprises the output aperture 61 . In between two adj acent silicon wafer parts of said stack of silicon wafer parts , a glass plate 11 is arranged . The glass plate or glass plates 11 serve as a spacer and / or isolator between the silicon wafer parts of the unit 1 of the present invention . The glass plates 11 are provided with through openings 21 to allow the charged particle beam CPB to traverse the glass plate at the appropriate positions . The silicon wafer parts and the glass plates are connected to each other by means of a glue connection . This provides an easy and low cost way to accurately manufacture the unit 1 of the present invention, by using, preferably MEMS production techniques . Furthermore, this provides a compact unit 1 which can easily be incorporated into an existing charged particle beam device .

[0076] It is noted that the unit 1 of figure 1 comprises a symmetry plane SP which is substantially perpendicular to the center line CL, wherein the components of the unit 1 between the symmetry plane SP and the input ring-shaped electrode 7 , and the components of the unit 1 between the symmetry plane SP and the output ring-shaped electrode 10 are substantially mirror symmetric with respect to each other .

[0077] Figure 2A shows a schematic cross-section view of a second example of a unit 100 according to the present invention . The unit 100 comprises substantially the same components as the unit 1 of the first example in figure 1 , with some additions and changes .

[0078] The unit 100 comprises an input 102 with an input aperture 142 , an input ring-shaped electrode 107 , a first electrostatic deflector 103 , a first ring-shaped electrode 108 , a second electrostatic deflector 104 , a second ringshaped electrode 109, a third electrostatic deflector 105 , an output ring-shaped electrode 110 , and an output 106 with an output aperture 146, which are arranged in series and spaced apart for each other along a center line CL of the energy filter 100 .

[0079] The input aperture 142 is arranged at the input 102 and comprises an aperture that is substantially centered on the center line CL . The input aperture 142 is connectable to a voltage source , and / or is preferably configured to rej ect external ( oscillating) fields , clean up the charged particle beam CPB, assist in aligning the charged particle beam CPB while completing the symmetry along the optical axis , and / or capping residual electrostatic fields emitted by the input ring-shaped electrode 107 .

[0080] The first deflector 103 comprises two or more spaced apart deflector plates which are arranged at opposite sides of the center line CL, wherein an area between the deflector plates comprises a first center point CPI , wherein said first center point CPI is positioned spaced apart from the center line CL .

[0081] Also the third deflector 105 comprises two or more spaced apart deflector plates which are arranged at opposite side of the center line CL, wherein an area between the deflector plates comprises a third center point CP3 , wherein said third center point CP3 is spaced apart from the center line CL, preferably in a direction in which the charged particle beam CPB, in use , is being deflected . This off-axis arrangement of the first deflector 103 and the third deflector 105 is used to reduce comatic aberrations in the unit 100 when in use .

[0082] As is also the case in the first example , the center point CP2 between the two or more spaced apart deflector plates of the second deflector 104 , is also spaced apart from the center line CP, in the same direction as the deflection of the charged particle beam CPB by the first and third electrostatic deflectors 103 , 105, for reducing the comatic aberration 1 .

[0083] Furthermore , the unit 100 comprises a stigmator 111 which is arranged at or near a position where, in use , the deflected charged particle beam CPB is bended back in the direction of the center line CL . As schematically shown in figure 2A, the stigmator 111 is arranged in between the two deflector plates of the second deflector 104 . As schematically shown in figure 2B, the stigmator electrodes 1111 , 1112 extend substantially parallel to the two deflector plates of the second deflector 104 , wherein the tips of the stigmator electrodes 1111 , 1112 are spaced apart to allow passage of the charged particle beam between said stigmator electrodes 1111 , 1112 . The stigmator 111 allows to provide a stigmatic correction in order to correct for astigmatism.

[0084] As schematically shown in figure 2A, the stigmator 111 is arranged spaced apart from the center line CL, and as schematically shown in figure 2B the center line CL does not pass through the stigmator electrodes 1111 , 1112 . Accordingly, the charged particle beam can traverse the unit 100 of this second example substantially undisturbed when no voltage di fferences are provided to the deflector plates of the first deflector 103 , second deflector 104 , third deflector 105, and the stigmator 111 and no deflecting electrostatic fields are present in the unit 100 .

[0085] It is noted that the stigmator 111 , in use, will introduce a minor quadrupole field that is configured to provide a stigmatic correction . Accordingly, the quadrupole field generated by the stigmator 111 in the unit 100 of this example is configured to reduce aberrations and substantially not for deflecting the charged particle beam CPB back towards the center line CL .

[0086] In addition, the unit 100 comprises an input ring-shaped electrode 107 between the input aperture 142 and the first deflector 103 , a first ring-shaped electrode 108 between the first deflector 103 and the second deflector 104 , a second ring-shaped electrode 109 between the second deflector 104 and the third deflector 105, and an output ring-shaped electrode 110 between the third deflector 105 and the output aperture 146. The ring-shaped electrodes 107 , 108 , 109, 110 , and the input aperture 142 and output aperture 146 , each are arranged symmetrical around the center line CL and are connectable to one or more voltage sources for, in use , providing an electrostatic lens , preferably an Einzel lens , which is configured for imaging the input aperture 142 onto the output aperture 146 , preferably with a magni fication of - 1 .

[0087] It is noted that also in this example , when the unit is configured to be used as an energy filter, the ring-shaped electrodes , in particular the first ringshaped electrode 108 and the second ring-shaped electrode 109 , also provide a retardation of charged particles of the charged particle beam CPB, which corresponds to a lowering of the charged particle beam energy, before the charged particle beam enters the second deflector 104 and the stigmator 111 , and an acceleration of the charged particles of the charged particle beam CPB, which corresponds to a raising of the charged particle beam energy, after the charged particle beam leaves the second deflector 104 and the stigmator 111 . Because the charged particle beam energy is briefly lowered, preferably to a small fraction of the input beam energy, a dispersion caused by the second deflector 104 is enhanced, and most of the angular dispersion of the unit 100 of this embodiment is created in or in the vicinity of the deflector plates of the second deflector 104 . In addition, this briefly lowering of energy of the charged particle beam CPB at the second deflector 104 also provides that a shi ft of the second center point SP2 with respect to the center line CL is very ef fective in reducing the comatic aberration .

[0088] In the unit 100 in the example of figure 2A, the first deflector 103 , the second deflector 104 , and the third deflector 105 all provide or contribute to a net- dispersion of the charged particle beam . The dispersion provides that charged particles with different energies arrive at the output aperture 146 at dif ferent positions in a plane perpendicular to the center line CL . Accordingly, when the unit is used as an energy filter, the output aperture 146 , acting as the spatial filter, is configured for, in use , selecting charged particles that pass through the aperture opening of the output aperture 146 according to their energy .

[0089] As schematically shown in figure 2A, most of the components of the unit 100 are arranged in their own silicon wafer part . A first silicon wafer part 112 comprises the input aperture 142 , a second silicon wafer part 117 comprises the input ring-shaped electrode 107 , a third silicon wafer 113 comprises the first deflector 103 , a fourth silicon wafer 118 comprises the first ring-shaped electrode 108 , a fi fth silicon wafer part 114 comprises the second deflector 104 , a sixth silicon wafer part 119 comprises the second ring-shaped electrode 109 , a seventh silicon wafer part 115 comprises the third deflector 105, an eighth silicon wafer part 120 comprises the output ringshaped electrode 110 , and a ninth silicon wafer part 116 comprises the output aperture 146 . The silicon wafer parts are coated with a conducting layer 123 , which conducting layer ( s ) can also be used for providing one or move voltages to the particular component of a particular silicon wafer part .

[0090] In between two adj acent silicon wafer parts of said stack of silicon wafer parts , a glass plate 122 is arranged, which serve as a spacer and / or isolator between the silicon wafer parts of the unit 100 . The glass plates 122 are provided with through openings 121 to allow the charged particle beam CPB to traverse the glass plate at the appropriate positions . The silicon wafer parts and the glass plates are connected to each other by means of a glue connection . This provides an easy and low cost way to accurately manufacture the unit 100 of the present invention, by using, preferably MEMS production techniques . Furthermore , this provides a compact unit 100 which can easily be incorporated into an existing charged particle device .

[0091] In this example , the output aperture 146 is not arranged on a silicon wafer part , but comprises a membrane that is preferably less than 1 micrometer thick . Alternatively, the membrane of the output aperture 146 is grown on a conducting wafer that can be directly glued to the output ring-shaped electrode 110 , thus without the glass plate 122 between the output ring-shaped electrode 110 and the output aperture 146 , in particular in case the energy filter 100 is configured such that the output aperture 146 and the output ring-shaped electrode 110 are , at least in use, at the same potential value .

[0092] It is noted that by arranging the output aperture 146 directly on to the output ring-shaped electrode 110 , and / or by not using an input aperture 142 and the glass plate 122 between the input aperture 142 and the first ring-shaped electrode 107 ( as is the case in the first example of figure 1 ) , the unit 100 of the present invention can become even more compact and allows to mount further components of charged particle beam devices as close as possible to the unit 100 .

[0093] Further below a more specific example is presented of the dimensions of unit according to the invention . It is noted that these dimensions are in principle scalable (using appropriate scaling procedures ) , but this example uses appropriate waf er-thicknesses and is well balanced in view of the Boersch effect . This more specific example is described with reference to the elements of the unit in figure 2 .

[0094] In this more speci fic example , the first silicon wafer part 112 , the second silicon wafer part 117 , the fourth silicon wafer part 118 , the sixth silicon wafer part 119 , and the eighth silicon wafer part 120 , are all about 300 micrometers thick . In addition, also the glass plates 122 are all about 300 micrometers thick .

[0095] The third silicon wafer part 113 , the fifth silicon wafer part 114 , and the seventh silicon wafer part 115 , all comprising an electrostatic deflector, are all about 500 micrometers thick .

[0096] The output aperture 146 is not arranged in a silicon wafer part , but is manufactured by means of a membrane that is less than 1 micrometer thick, which is provided with the output aperture 146.

[0097] Accordingly, the complete stack of the energy filter 100 of this more specific example is a little over 5 millimeters thick in the direction of the charged particle beam in a charged particle beam device , and provides a very compact energy filter that can easily be incorporated into a charged particle beam device , such as for example an electron microscope .

[0098] The diameter of the input aperture 142 is equal to or larger than the diameter of the output aperture 146.

[0099] The input and output ring-shaped electrodes 107 , 110 have a radius of about 200 micrometers and are centered on the center line CL .

[0100] The first and third electrostatic deflectors 103 , 105 comprises two deflector plates with a distance of about 600 micrometers between said two deflector plates . The center points CPI , CP3 are spaced apart from the center line CP over a distance of about 50 micrometers .

[0101] The first and second ring-shaped electrodes 108 , 109 have a radius of about 600 micrometers and are centered on the center line CL .

[0102] The second electrostatic deflector 104 comprises two deflector plates with a distance of about 1000 micrometers between said two deflector plates . The center point CP2 , is spaced apart from the center line CP over a distance of about 200 micrometers , in the same direction as the shi ft of the first and third electrostatic deflectors 103 , 105 .

[0103] The stigmator 111 is centered around the center point CP2 of the second electrostatic deflector 104 . The stigmator 111 comprises stigmator electrodes 1111 , 1112 , with substantially round tips , which are spaced apart over a distance of about 2000 micrometers . The thickness of the stigmator electrodes 1111 , 1112 in a direction substantially perpendicular to the deflector plates of the second electrostatic deflector 104 is substantially equal to one-third of the distance between the two deflector plates of the second electrostatic deflector 104

[0104] It is noted that the voltages on the various components of the unit of the present invention are, inter alia, dependent on the specific application of the unit and / or the average beam energy of the charged particle beam.

[0105] However, in a preferred embodiment, the voltage on the input aperture 142 , the input ring-shaped electrode 107 , the output ring-shaped electrode 110 and the output aperture 146 are substantially the same . Furthermore, the voltage dif ference on the deflector plates of the first deflector 103 and the third deflector 105 are substantially the same . Also the voltage on the first ring-shaped electrode 108 and the second ring-shaped electrode 109 are substantially the same . Accordingly, in this embodiment it is not necessary to provide each component with its own voltage control , and the number of dif ferent voltages needed to control the unit of the present embodiment can be reduced, which makes the control of the energy filter much easier .

[0106] Preferably the voltages used in the unit are relative with respect to a bias voltage, which may be chosen such that the voltage on the input 102 is substantially equal to the average charged particle beam energy divided by the elementary charge . In the unit with the dimensions of the above described more specific example and with a bias voltage of 528 V with respect to the charged particle source in case of an electron beam system, the voltages on the components of the unit 100 , when used as an energy filter may be as follows :

[0107] - Input 102 » 528 V

[0108] - Input ring-shaped electrode 107 » 528 V

[0109] - Attracting electrode of the first deflector 103 » 339 V

[0110] - Repelling electrode of the first deflector 103 » 239 V

[0111] - First ring-shaped electrode 108 « 73 V

[0112] - Repelling electrode of the second deflector 104 « 31 V

[0113] - Attracting electrode of the second deflector 104 » 177 V

[0114] - Stigmator electrodes 1111 , 1112 « 80 V

[0115] - Second ring-shaped electrode 108 « 73 V - Attracting electrode of the third deflector 105 » 339 V

[0116] - Repelling electrode of the third deflector 105 » 239 V

[0117] - Output ring-shaped electrode 110 » 528 V

[0118] - Output 106 » 528 V

[0119] Such an energy filter 100 can, for example, be used to filter the charged particle beam from a charged particle source 124 by arranging the input 102 at the charged particle source 124 and to provide a charged particle beam out of the output 106 with a reduced energy spread with respect to the charged particle beam that is emitted by the charged particle source 124 .

[0120] In the unit with the dimensions of the above described more speci fic example and with a bias voltage of 528 V with respect to the charged particle source in case of an electron beam system, the voltages on the components of the unit 100 , when used as a chromatic aberration corrector, may be as follows :

[0121] - Input 102 » 528 V

[0122] - Input ring-shaped electrode 107 » 528 V

[0123] - Attracting electrode of the first deflector 103 » 1042 V

[0124] - Repelling electrode of the first deflector 103 » 613 V

[0125] - First ring-shaped electrode 108 « - 78 V

[0126] - Repelling electrode of the second deflector 104 » 423 V

[0127] - Attracting electrode of the second deflector 104 » 716 V

[0128] - Stigmator electrodes 1111 , 1112 » 525 V

[0129] - Second ring-shaped electrode 108 « - 78 V

[0130] - Attracting electrode of the third deflector 105 » 1042 V

[0131] - Repelling electrode of the third deflector 105 » 613 V

[0132] - Output ring-shaped electrode 110 » 528 V

[0133] - Output 106 » 528 V

[0134] Clearly the applied voltages when the unit of the invention is used as a chromatic aberration corrector, are completely different from the applied voltages when the unit of the invention is used as an energy filter . It is noted that in the example above where the unit is used as a chromatic aberration corrector, the ring-shaped electrodes , are configured to preferably provide : a retardation of charged particles of the charged particle beam, which corresponds to a lowering of the charged particle beam energy, preferably before the charged particle beam enters the first deflector 103 , an acceleration of the charged particles of the charged particle beam, which corresponds to a raising of the charged particle beam energy, preferably before the charged particle beam enters the second deflector 104 , a retardation of charged particles of the charged particle beam, which corresponds to a lowering of the charged particle beam energy, preferably before the charged particle beam enters the third deflector 105 , and an acceleration of the charged particles of the charged particle beam, which corresponds to a raising of the charged particle beam energy, preferably before the charged particle beam leaves the output 106 .

[0135] In addition, in this example , an additional focus of the charged particle beam is provided in the second deflector 104 . Whereas when the unit is used as an energy filter, the unit does not comprise a charged particle beam focus or cross-over inside the unit .

[0136] Figure 3 shows a schematic cross-section view of the second example of the unit of figure 2A as used as an energy filter 100 ' and as described in detail above, wherein the spatial filter is replaced by a series of spatial filters 146' . Each one of the spatial filters of the series of spatial filters 146 ' comprises an input aperture of a charged particle detector, and accordingly the example as shown in figure 3 is provided with an array of charged particle detectors 300 , which is arranged at the output 106 of the energy filter 100 ' . The array of charged particle detectors 300 provides an example of a multichannel detector system. Other types of multi-channel detector systems suitable to detect charged particles , may also be used instead of the array of charged particle detectors 300 . Alternatively, a plate of a scintillator material may be arranged at the output 106 , which converts impinging charged particles into photons , and subsequently an optical multi-channel detector system, such as a CCD of photodiode-array, can be arranged at a side of the plate of the scintillator material that faces away from the energy filter 100 ' to detect the light from the plate of scintillator material .

[0137] As schematically shown in figure 3 , the array of charged particle detectors 300 is arranged on a line , or plane, that extends in a direction substantially perpendicular to the center line CL and substantially parallel to the direction of deflection provided by the first, second and third deflectors 103 , 104 , 105 . Due to the dispersion in the energy filter 100 ' , charged particles with a dif ferent energy arrive at di fferent positions along the array of charged particle detectors 300 , traverse dif ferent input apertures of dif ferent detectors of said array of charged particle detectors 300 , and are subsequently detected by di fferent detectors of said array of charged particle detectors 300 . In use the array of charged particle detectors 300 is connected to a control unit for recording charged particles by their energy . This allows to use the energy filter 100 ' of the present invention as a spectrometer or spectrograph to obtain an energy spectrum of the charged particles in a charged particle beam CPB that enters the input 102 .

[0138] Figure 4 schematically a first example of a charged particle beam device 200 comprises a unit 220 of the present invention configured to provide an energy filter . The charged particle beam device 200 , in particular a Scanning Electron Microscope ( SEM) , comprises components which are arranged along a centre axis 230 , also denoted as 'optical axis ' . The SEM comprises an electron source 201 , also denoted as 'electron gun' which in use emits a diverging beam of electrons towards the gun lens 202 , also denoted as source lens .

[0139] It is noted that the components of the SEM are arranged in a vacuum chamber 204 , inside which, at least in use , a high vacuum is created using suitable vacuum pumps as known by the person skilled in the art . However at least the electron source 201 , is commonly arranged in its separate ultra-high vacuum chamber 203 . Between the ultra- high vacuum chamber 203 and the vacuum chamber 204 , a vacuum seal 205 is arranged .

[0140] From the gun lens 202 , a substantially parallel electron beam is directed to the unit 220 according to the invention configured as an energy filter . Preferably, the unit 220 is suitably arranged such that the centre line CL ( see figures 1 and 2 ) is in line with the centre axis 230 . In addition, as shown in figure 4 , the unit 220 is preferably arranged outside the ultra-high vacuum chamber 203 , at a side of the vacuum seal 205 that is facing towards the sample holder 213 . The arrow shown in the unit 220 schematically represents the direction of deflection of the charged particle beam inside the unit 220 .

[0141] From the unit 220 , the filtered electron beam is directed towards the sample holder 213 and is focused on top of a sample on the sample holder 213 by means of the obj ective lens 207 , 208 . It is noted that in between the energy filter 220 and the obj ective lens 207 , 208 further components may be present such as an aberration correcting device and / or an electron accelerator, for example .

[0142] The obj ective lens 207 , 208 comprises a set of scanning deflectors 209, 210 , which are provided for scanning the electron beam over the surface of the sample on the sample holder 213 .

[0143] The SEM further comprises one or more detectors 211 , 212 which are configured for detecting charged particles , such an elastically scattered electrons or secondary electrons , or photons from the sample on the sample holder 213 , which are created in said sample upon incidence of the primary electron beam coming from the electron source 201 , after passing through the energy filter 220 .

[0144] It is noted that , the charged particle beam device 200 may comprise a further energy filter 220 ' according to the present invention after the unit 220 , in particular to substantially cancel or nullify the dispersion of a charged particle beam exiting the unit 220 . As schematically shown in figure 4 , the direction of deflection in the further energy filter 220 ' can be the same as in the unit 220 , as each of the unit 220 and further energy filter 220 ' are configured such that the input plane is imaged on the output plane with a magnification of -1 .

[0145] It is further noted that in a similar way, the energy filter of the present invention can also be arranged in any other type of charged particle device , such as , but not limited to, a Transmission Electron Microscope ( TEM) or a Focused Ion Beam ( FIB) device .

[0146] Figure 5 schematically a second example of a charged particle beam device 200 ' also comprises a unit 220 of the present invention, which in this example is configured to provide a chromatic aberration corrector . The charged particle beam device 200 ' , in particular a Scanning Electron Microscope ( SEM) , comprises components which are arranged along a centre axis 230 , also denoted as 'optical axis ' . The SEM comprises an electron source 201 , also denoted as 'electron gun' which in use emits a diverging beam of electrons towards the gun lens 202 , also denoted as source lens .

[0147] It is noted that the components of the SEM are arranged in a vacuum chamber 204 , inside which, at least in use , a high vacuum is created using suitable vacuum pumps as known by the person skilled in the art . However at least the electron source 201 , is commonly arranged in its separate ultra-high vacuum chamber 203 . Between the ultra- high vacuum chamber 203 and the vacuum chamber 204 , a vacuum seal 205 is arranged .

[0148] From the gun lens 202 , a substantially parallel electron beam is directed to the unit 220 according to the invention configured as a chromatic aberration corrector . Preferably, the unit 220 is suitably arranged such that the centre line CL ( see figures 1 and 2 ) is in line with the centre axis 230 . In addition, as shown in figure 4 , the unit 220 is preferably arranged outside the ultra-high vacuum chamber 203 , at a side of the vacuum seal 205 that is facing towards the sample holder 213 . The arrow shown in the unit 220 schematically represents the direction of deflection of the charged particle beam inside the unit 220 , which is also the direction in which the negative chromatic aberration is provided .

[0149] From the unit 220 , the electron beam enters a second unit 220" according to the invention, which is also configured as a chromatic aberration corrector . However, the second unit 220" is rotated over 90 degrees around the centre axis 230 with respect to the unit 220 , and which in use provide a deflection in a plane perpendicular to the plane of figure 5 . Accordingly, the direction in which the negative chromatic aberration is provided by the second unit 220" is perpendicular to the direction in which the negative chromatic aberration is provided in the unit 220 .

[0150] In use the magnitude of the negative chromatic aberration provided by the unit 220 and the second unit 220" is set so as to at least partially cancel the ef fect of positive chromatic aberrations of the other components in the SEM, such as for example the gun lens 202 and the obj ective lens 207 , 208 .

[0151] The other components of the SEM according to this second example are substantially the same as in the first example as described above .

[0152] It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention . From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the scope of the present invention .

[0153] In summary, the invention relates to a unit for use in a charged particle beam device . The unit comprises an input and an output, which are arranged spaced apart on a center line . A series of electrostatic deflectors is arranged between the input and output aperture . The series of electrostatic deflectors comprises a first deflector for deflecting a beam of charged particles away from the center line, a second deflector for bending the deflected beam of charged particles back to the center line, and a third deflector for bending at least part the beam of charged particles back along the center line . The unit can be configured to provide an energy filter or a chromatic aberration corrector . When configured as an energy filter, the unit further comprises a spatial filter arranged on the center line after the third deflector, for selecting charged particles that pass through according to their energy .

Claims

C L A I M S1 . A unit for filtering and / or correcting a charged particle beam, wherein the unit comprises : an input and an output which are arranged spaced apart on a center line of the unit, a series of electrostatic deflectors arranged in between the input and output , wherein the series of electrostatic deflectors comprises a first deflector which configured for deflecting a beam of charged particles away from the center line, a second deflector which is configured for bending the deflected beam of charged particles back in the direction of the center line, and a third deflector which is configured for bending at least part of the beam of charged particles back along the center line, a ring-shaped electrode between the input and the first deflector, between the first and second deflector, between the second and third deflector, and between the third deflector and the output , wherein the ring-shaped electrodes are connectable to one or more voltage sources .2 . The unit according to claim 1 , wherein the unit further comprises a spatial filter arranged on the center line at the output or between the third deflector and the output, wherein the spatial filter is configured for, in use , a selecting charged particles that pass through the spatial filter according to their energy .3 . The unit according to claim 1 or 2 , wherein the first deflector comprises two or more spaced apart deflector plates , wherein an area between the deflector plates of the first deflector comprises a first center point, wherein a distance between the first center point and the deflector plates of the first deflector is largerthan a distance between the first center point and the center line .4 . The unit according to claim 3 , wherein the first center point is position spaced apart from the center line .

5. The unit according to any one of the claims 1- 4 , wherein the second deflector comprises two or more spaced apart deflector plates , wherein an area between the deflector plates of the second deflector comprises a second center point , wherein said second center point is arranged spaced apart from the center line .

6. The unit according to claim 5 , wherein the deflector plates of the second deflector are arranged at opposite sides of the center line, or wherein a distance between the second center point and the deflector plates is larger than a distance between the second center point and the center line .7 . The unit according to any one of the claims 1- 6, wherein the third deflector comprises two or more spaced apart deflector plates , wherein an area between the deflector plates of the third deflector comprises a third center point , wherein a distance between the third center point and the deflector plates of the third deflector is larger than a distance between the third center point and the center line .8 . The unit according to claim 7 , wherein the third center point is position spaced apart from the center line .9 . The unit according to any one of the claims 1- 8 the ring-shaped electrode arranged between the first deflector and the second deflector is a first ring-shapedelectrode , and the ring-shaped electrode between the second deflector and the third deflector is a second ringshaped electrode , wherein the first and second ring-shaped electrodes are connectable to a voltage source , preferably wherein the first and second ring-shaped electrodes are connectable to the same voltage source .10 . The unit according to claim 9 , wherein the ring-shaped electrode between the input and the first deflector is an input ring-shaped electrode , and the ringshaped electrode between the third deflector and the output is the output ring-shaped electrode , wherein the input and output ring-shaped electrodes are connectable to a voltage source , preferably wherein the input and output ring-shaped electrodes are connectable to the same voltage source .11 . The unit according to any one of the claims 1 - 10 , wherein the one or more ring-shaped electrodes are arranged symmetrically around the center line .12 . The unit according to any one of the claims 1 - 11 , wherein the unit comprises an input aperture arranged at the input and comprising an aperture that is substantially centered on the center line , preferably wherein the input aperture is connectable to a voltage source , and / or wherein the unit comprises an output aperture arranged at the output and comprising an aperture that is substantially centered on the center line , preferably wherein the output aperture is connectable to a voltage source .13 . The unit according to any one of the claims 1 - 12 , wherein the unit comprises a stigmator which is arranged between the input and the output , preferablywherein the stigmator is arranged at or near a position where, in use , the deflected beam of charged particle beam is bending back in the direction of the center line , preferably wherein the stigmator is arranged between the deflector plates of the second deflector .14 . The unit according to any one of the claims 1 - 13 , wherein the unit comprises a stack of silicon wafer parts , wherein each silicon wafer part comprises one electrostatic deflector of the series of electrostatic deflectors or one ring-shaped electrode of the one or more ring-shaped electrodes , preferably wherein the input aperture comprises a silicon wafer part with a membrane deposited on top of the wafer part , wherein the membrane comprises an aperture opening, preferably wherein the output aperture comprises a silicon wafer part with a membrane deposited on top of the wafer part , or comprises a membrane without a silicon wafer part , wherein the membrane comprises an aperture opening .

15. The unit according to claim 14 , wherein the unit comprises a spacer plate which is arranged in between two adj acent silicon wafer parts of said stack of silicon wafer parts , wherein the spacer plate is made from material that is substantially electrically isolating, preferably wherein the unit comprises a spacer plate in between each two ad acent silicon wafer parts of said stack of silicon wafer parts , preferably wherein the spacer plate is a glass plate .

16. The unit according to claim 14 or 15, wherein the unit is manufactured using MEMS production techniques17 . A charged particle beam device comprising aunit according to any one of the claims 1 - 16, wherein the charged particle beam device comprises a charged particle source configured for emitting a beam of charged particles along a beam path in the charged particle beam device, wherein the unit is arranged in said beam path .18 . The charged particle beam device according to claim 17 , wherein the unit is arranged in a substantially linear part of the beam path, preferably wherein the centerline of the unit substantially coincides with said substantially linear part of the beam path .

19. A method for filtering and / or correcting a charged particle beam in a charged particle beam device , wherein the charged particle device comprises a charged particle source that emits a charged particle beam along a beam path in the charged particle beam device, wherein the method comprises the steps of : providing a unit according to any one of the claims 1 - 16 , arranged in said beam path such that the centerline substantially coincides with said beam path, providing a first voltage dif ference over deflector plates of the first deflector to deflect the charged particle beam away from the center line, providing a second voltage dif ference over deflector plates of the second deflector to bend the deflected charged particle beam back in the direction of the center line, and providing a third voltage di fference over deflector plates of the third deflector to bend at least part of the charged particle beam back along the center line, providing voltages to one or more of the ringshaped electrodes to provide an electrostatic lens , and wherein the voltages provide a dispersion of the charged particle beam in the unit to arranged charged particles with dif ferent energy next to each other along a line substantially parallel to a direction of deflection in the deflectors .20 . The method according to claim 19, wherein the voltages are configured to provide a negative chromatic aberration at the output of the unit .21 . The method according to claim 19, wherein the unit is used as an energy filter, wherein the method further comprises the step of using the spatial filter to block the passage of charged particles according to their energy .22 . The method according to claim 19 , 20 or 21 , wherein the one or more ring-shaped electrodes comprises an input ring-shaped electrode between the input and the first deflector, a first ring-shaped electrode between the first deflector and the second deflector, a second ringshaped electrode between the second deflector and the third deflector, and an output ring-shaped electrode between the third deflector and the output , wherein the method comprises the step of : providing voltages to the input , first , second and output ring-shaped electrodes to provide an Einzel lens , preferably for imaging a plane at the input on a plane at the output .-o- o-o-o-o- o-o-o-BP / HZ

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