ELECTRON OPTICAL COLUMN AND METHOD FOR DIRECTING A BEAM OF PRIMARY ELECTRONS ONTO A SAMPLE

Apparatus and methods for directing a beam of primary electrons along a primary beam path onto a sample are disclosed. In one arrangement, a beam separator diverts away from the primary beam path a beam of secondary electrons emitted from the sample along the primary beam path. A dispersion device is upbeam from the beam separator. The dispersion device compensates for dispersion induced in the primary beam by the beam separator. One or more common power supplies drive both the beam separator and the dispersion device.

TECHNICAL FIELD

The present disclosure generally relates to the field of charged particle beam apparatus, and more particularly to an electron optical column and methods in which dispersion of a beam separator in a single-beam or multi-beam apparatus is compensated.

BACKGROUND

In manufacturing processes of integrated circuits (ICs), unfinished or finished circuit components are inspected to ensure that they are manufactured according to design and are free of defects. An inspection system utilizing an optical microscope typically has resolution down to a few hundred nanometers; and the resolution is limited by the wavelength of light. As the physical sizes of IC components continue to reduce down to a sub-100 or even sub-10 nanometers, inspection systems capable of higher resolution than those utilizing optical microscopes are needed.

A charged particle (e.g., electron) beam microscope, such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM), capable of resolution down to less than a nanometer, serves as a practicable tool for inspecting IC components having a feature size that is sub-100 nanometers. With an SEM, electrons of a single primary electron beam, or electrons of a plurality of primary electron beams, can be focused at probe spots of a wafer under inspection. The interactions of the primary electrons with the wafer can result in one or more secondary electron beams. The secondary electron beams may comprise backscattered electrons, secondary electrons, or Auger electrons, resulting from the interactions of the primary electrons with the wafer. The intensity of the one or more secondary electron beams can vary based on the properties of the internal and/or external structures of the wafer.

The intensity of the secondary electron beams can be determined using a detection device or detector. The secondary electron beams can form one or more beam spots at pre-determined locations on a surface of the detector. The detector can generate electrical signals (e.g., a current, a voltage, etc.) that represent an intensity of the detected secondary electron beams. The electrical signals can be measured with measurement circuitries (e.g., analog-to-digital converters) to obtain a distribution of the detected electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of the one or more primary electron beams incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used to reveal various features of the internal and/or external structures of the wafer, and can be used to reveal any defects that may exist in the wafer.

In an inspection system comprising a single primary beam and a single secondary beam (single-beam apparatus), the detector can be placed along an optical axis of the apparatus if it has a hole allowing the primary beam to pass through. However, the presence of the hole can reduce detection efficiency of the secondary beam and in some cases result in a black spot on the center of the reconstructed images. A beam separator can be used to separate the secondary beam from the primary beam and direct the secondary beam towards a detector placed off-axis. In an inspection system comprising multiple primary beams and multiple secondary beams (multi-beam apparatus), a beam separator can be used to separate the multiple secondary beams from the multiple primary beams and direct the multiple secondary beams towards a detector placed off-axis.

The beam separator comprises at least one magnetic deflector and therefore generates dispersion on the one or more primary beams and the one or more secondary beams. The dispersion can deform the round probe spot of a primary beam into an oblong shape. The dispersion can also deform the detected beam spots thereby causing deterioration in resolution of the reconstructed image.

SUMMARY

According to an aspect, there is provided an electron optical column configured to direct a beam of primary electrons along a primary beam path onto a sample, comprising: a beam separator configured to divert away from the primary beam path a beam of secondary electrons emitted from the sample along the primary beam path; a dispersion device upbeam from the beam separator, the dispersion device being configured to compensate for dispersion induced in the primary beam by the beam separator; and one or more common power supplies each configured to drive both the beam separator and the dispersion device.

According to an aspect, there is provided a method of directing a beam of primary electrons along a primary beam path onto a sample, comprising: using a beam separator to divert away from the primary beam path a beam of secondary electrons emitted from the sample along the primary beam path; and using a dispersion device upbeam from the beam separator to compensate for dispersion induced in the primary beam by the beam separator, wherein one or more common power supplies are used to drive both the beam separator and the dispersion device.

Additional objects and advantages of the disclosed embodiments will be set forth in part in the following description, and in part will be apparent from the description, or may be learned by practice of the embodiments. The objects and advantages of the disclosed embodiments may be realized and attained by the elements and combinations set forth in the claims.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure relates to systems and methods for compensating dispersion of a beam separator in a single-beam or multi-beam apparatus. A beam separator generates dispersion on the one or more primary beams and the one or more secondary beams. Embodiments of the present disclosure provide a dispersion device comprising an electrostatic deflector and a magnetic deflector configured to induce a beam dispersion set to cancel the dispersion generated by the beam separator. The combination of the electrostatic deflector and the magnetic deflector can be used to keep a deflection angle (due to the dispersion device) unchanged when the induced beam dispersion is changed to compensate for a change in the dispersion generated by the beam separator. In some embodiments, the deflection angle can be controlled to be zero and there is no change in primary beam axis due to the dispersion device. In some embodiments, the dispersion device can comprise a multi-pole lens (e.g., quadrupole lens) configured to generate a quadrupole field to cancel at least one of the impacts of astigmatism aberrations caused by the beam separator and the dispersion device on the probe spot formed by the primary beam.

Reference is now made toFIG.1, which illustrates an exemplary electron beam inspection (EBI) system100consistent with embodiments of the present disclosure. As shown inFIG.1, EBI system100includes a main chamber101, a load/lock chamber102, an electron beam tool104, and an equipment front end module (EFEM)106. Electron beam tool104is located within main chamber101.

EFEM106includes a first loading port106aand a second loading port106b. EFEM106may include additional loading port(s). First loading port106aand second loading port106bcan receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples are collectively referred to as “wafers” hereafter). One or more robot arms (not shown) in EFEM106can transport the wafers to load/lock chamber102.

Load/lock chamber102is connected to a load/lock vacuum pump system (not shown), which removes gas molecules in load/lock chamber102to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) can transport the wafer from load/lock chamber102to main chamber101. Main chamber101is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in main chamber101to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool104.

Reference is now made toFIG.2A, which illustrates exemplary components of electron beam tool104consistent with embodiments of the present disclosure.FIG.2Aillustrates an electron beam tool104A (also referred to herein as apparatus104A) comprising an electron source206, a gun aperture212, a condenser lens214, a primary electron beam210emitted from electron source206, a beam-limit aperture216, a beam separator222, a deflection scanning unit226, an objective lens228, a sample stage (not shown inFIG.2A), a secondary electron beam220, and an electron detector218. Electron source206, gun aperture212, condenser lens214, beam-limit aperture216, beam separator222, deflection scanning unit226, and objective lens228can be aligned with optical axis202of apparatus104A.

Electron source206can comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam210with high energy (e.g., 8-20 keV), high angular intensity (e.g., 0.1-1 mA/sr) and a crossover (virtual or real)208. Primary electron beam210can be visualized as being emitted from crossover208. Gun aperture212can block off peripheral electrons of primary electron beam210to reduce Coulomb effect. The Coulomb effect can cause an increase in size of a probe spot236.

Condenser lens214can focus primary electron beam210and beam-limit aperture216can limit the size of primary electron beam210. The electric current of primary electron beam210downstream of beam-limit aperture216can be varied by adjusting the focusing power of condenser lens214or by changing the radial size of beam-limit aperture216. Objective lens228can focus primary electron beam210onto a sample238for inspection. Primary electron beam210can form probe spot236on surface of sample238.

In response to incidence of primary electron beam210at probe spot236, secondary electron beam220can be emitted from sample238. Secondary electron beam220can comprise electrons with a distribution of energies including secondary electrons (energies ≤50 eV) and backscattered electrons (energies between 50 eV and landing energies of primary electron beam210).

Beam separator222can be a beam separator of Wien filter type comprising an electrostatic deflector generating an electrostatic dipole field E1and a magnetic dipole field B1. For a beam separator of Wien filter type, the force exerted by electrostatic dipole field E1on an electron of primary electron beam210is equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field B1. Primary electron beam210can therefore pass straight through beam separator222with zero deflection angle. However, the total dispersion of primary electron beam210generated by beam separator222is non-zero. For a dispersion plane224of beam separator222,FIG.2Ashows dispersion of primary electron beam210with nominal energy V0and an energy spread ΔV into beam portion230corresponding to energy V0−ΔV/2, beam portion232corresponding to energy V0, and beam portion234corresponding to energy V0+ΔV/2. The total force exerted by beam separator222on an electron of secondary electron beam220is non-zero. Beam separator222can therefore separate secondary electron beam220from primary electron beam210and direct secondary electron beam220towards electron detector218. Electron detector218can detect secondary electron beam220and generate a corresponding signal.

Deflection scanning unit226can deflect primary electron beam210to scan probe spot236over a surface area of sample238. Electron detector218can detect corresponding secondary electron beam220and generate corresponding signals used to reconstruct an image of surface area of sample238.

An object plane204of objective lens228can shift with changes in focusing power of condenser lens214. For primary electron beam210, if dispersion plane224of beam separator222and object plane204of objective lens228do not coincide, beam portions230,232, and234stay separated and probe spot236is extended in the dispersion direction. This can cause deterioration in resolution of reconstructed image of sample238.

Reference is now made toFIG.2B, which illustrates an electron beam tool104B (also referred to herein as apparatus104B) comprising an electron source206, a gun aperture212, a condenser lens214, a primary electron beam210emitted from electron source206, a source conversion unit252, a plurality of beamlets254,256, and258of primary electron beam210, a primary projection optical system260, a sample stage (not shown inFIG.2B), multiple secondary electron beams276,278, and280, a secondary optical system282, and an electron detection device284. Primary projection optical system260can comprise an objective lens228. Electron detection device284can comprise detection elements286,288, and290. Beam separator222and deflection scanning unit226can be placed inside primary projection optical system260.

Electron source206, gun aperture212, condenser lens214, source conversion unit252, beam separator222, deflection scanning unit226, and objective lens228can be aligned with a primary optical axis250of apparatus104B. Secondary optical system282and electron detection device284can be aligned with a secondary optical axis292of apparatus104B.

Electron source206can comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam210with a crossover (virtual or real)208. Primary electron beam210can be visualized as being emitted from crossover208. Gun aperture212can block off peripheral electrons of primary electron beam210to reduce Coulomb effect. The gun aperture212may be referred to as a Coulomb aperture; the plate in which the aperture is defined may be referred to as a Coulomb aperture plate. The Coulomb effect can cause an increase in size of probe spots270,272, and274.

Source conversion unit252can comprise an array of image-forming elements (not shown inFIG.2B) and an array of beam-limit apertures (not shown inFIG.2B). The array of image-forming elements can comprise an array of micro-deflectors or micro-lenses. The array of image-forming elements can form a plurality of parallel images (virtual or real) of crossover208with a plurality of beamlets254,256, and258of primary electron beam210. The array of beam-limit apertures can limit the plurality of beamlets254,256, and258.

Condenser lens214can focus primary electron beam210. The electric currents of beamlets254,256, and258downstream of source conversion unit252can be varied by adjusting the focusing power of condenser lens214or by changing the radial sizes of the corresponding beam-limit apertures within the array of beam-limit apertures. Objective lens228can focus beamlets254,256, and258onto a sample238for inspection and can form a plurality of probe spots270,272, and274on surface of sample238.

Beam separator222can be a beam separator of Wien filter type comprising an electrostatic deflector generating an electrostatic dipole field E1and a magnetic dipole field B1(both of which are not shown inFIG.2B). If they are applied, the force exerted by electrostatic dipole field E1on an electron of beamlets254,256, and258is equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field B1. Beamlets254,256, and258can therefore pass straight through beam separator222with zero deflection angle. However, the total dispersion of beamlets254,256, and258generated by beam separator222is non-zero. For a dispersion plane224of beam separator222,FIG.2Bshows dispersion of beamlet254with nominal energy V0and an energy spread ΔV into beamlet portions262corresponding to energy V0, beamlet portion264corresponding to energy V0+ΔV/2, and beamlet portion266corresponding to energy V0−ΔV/2. The total force exerted by beam separator222on an electron of secondary electron beams276,278, and280is non-zero. Beam separator222can therefore separate secondary electron beams276,278, and280from beamlets252,254, and256and direct secondary electron beams276,278, and280towards secondary optical system282.

Deflection scanning unit226can deflect beamlets254,256, and258to scan probe spots270,272, and274over a surface area of sample238. In response to incidence of beamlets254,256, and258at probe spots270,272, and274, secondary electron beams276,278, and280can be emitted from sample238. Secondary electron beams276,278, and280can comprise electrons with a distribution of energies including secondary electrons (energies ≤50 eV) and backscattered electrons (energies between 50 eV and landing energies of beamlets254,256, and258). Secondary optical system282can focus secondary electron beams276,278, and280onto detection elements286,288, and290of electron detection device284. Detection elements286,288, and290can detect corresponding secondary electron beams276,278, and280and generate corresponding signals used to reconstruct an image of surface area of sample238.

Reference is now made toFIG.3A, which is a schematic diagram illustrating exemplary dispersion devices, consistent with embodiments of the present disclosure.FIG.3Aillustrates a dispersion device310comprising an electrostatic deflector and a magnetic deflector. The electrostatic deflector can generate an electrostatic dipole field E2and the magnetic deflector can generate a magnetic dipole field B2, wherein E2and B2are superposed substantially perpendicular to each other and to an optical axis330. The electrostatic dipole field E2exerts a force Feand the magnetic dipole field B2exerts a force Fmon an electron of an electron beam210propagating along optical axis330. The forces Feand Fmact in substantially opposite directions. The total force exerted by the electrostatic dipole field E2and the magnetic dipole field B2on an electron with nominal energy V0and nominal velocity v0can be calculated using the following equation:

For an electron with energy V0+dV and velocity v0+dv, the total force exerted by the electrostatic dipole field E2and the magnetic dipole field B2can be calculated using the following equation:

Reference is now made toFIG.3B, which illustrates a dispersion device311consistent with embodiments of the present disclosure. Dispersion device311, similar to dispersion device310, comprises an electrostatic deflector and a magnetic deflector capable of generating a corresponding electrostatic dipole field E2and magnetic dipole field B2. The electrostatic deflector and magnetic deflector can be arranged wherein E2and B2are superposed substantially perpendicular to each other and to an optical axis331. In dispersion device311, electrostatic dipole field E2and magnetic dipole field B2can be controlled wherein the total force (Fe+Fm) can be substantially zero when changing E2and B2. Accordingly, the nominal deflection angle is zero as illustrated inFIG.3B. The deflection dispersion induced by dispersion device311at a dispersion plane341can be controlled by varying E2and B2while maintaining the deflection angle at zero.

Reference is now made toFIG.3C, which illustrates a dispersion device312consistent with embodiments of the present disclosure. Dispersion device312, similar to dispersion devices310and311, comprises an electrostatic deflector and a magnetic deflector capable of generating a corresponding electrostatic dipole field E2and magnetic dipole field B2. The electrostatic deflector and magnetic deflector can be arranged wherein E2and B2are superposed substantially perpendicular to each other and to an optical axis332. In dispersion device312, electrostatic dipole field E2and magnetic dipole field B2can be controlled wherein the total force (Fe+Fm) can be a constant non-zero value when changing E2and B2. Accordingly, the nominal deflection angle α is non-zero as illustrated inFIG.3C. The deflection dispersion induced by dispersion device312at a dispersion plane342can be controlled by varying E2and B2while maintaining the deflection angle at α.

In the following discussion the terms deflection dispersion and dispersion may be used interchangeably to refer to any spreading of an electron beam caused by an energy dependence of deflection angle. The dispersion induced by dispersion device311or312can be controlled while maintaining a fixed deflection angle (equal to zero or a in the example ofFIGS.3B and3C). When the dispersion device311or312is positioned upbeam of a beam separator222, the dispersion of the dispersion device311or312can be used to compensate for dispersion induced in the beam separator222. The dispersion of the dispersion device311or312can be arranged to be equal and opposite to the dispersion induced in the beam separator222for example. Examples of embodiments having a beam separator222and a corresponding dispersion device311or312upbeam of the beam separator222are described below with reference toFIG.4A,4B,5-13.

It has been found that dispersion compensation is highly sensitive to unwanted fluctuations in the voltage or current used to power the dispersion device311,312or beam separator222.FIG.8depicts an example arrangement in which a dispersion device311is provided upbeam from a beam separator222. In this example, the beam separator222and dispersion device311are Wien filters. A Wien filter is an electron optical component that applies crossed electric and magnetic fields. The electric and magnetic fields may be powered by respective voltage and current sources, although for ease of illustration only the voltage source is depicted. The crossed electric and magnetic fields can be arranged to provide a deflector having a tunable pass velocity (or pass energy) for electrons passing through the filter. For electrons having a selected nominal velocity, the electric and magnetic forces cancel and the electron is not deflected by the filter. Electrons having velocities below or above the nominal velocity will be deflected. A single Wien filter can be used to separate a primary electron beam from a secondary electron beam because the primary electrons and secondary electrons pass through the Wien filter in different directions. The primary electrons and secondary electrons are thus affected differently by the magnetic field applied by the Wien filter. For example, in a case where a Wien filter is configured to direct primary electrons along an optical axis onto a sample, the Wien filter will deflect the secondary electrons away from the optical axis. A single Wien filter causes dispersion in an electron beam due to the inherent spread in the velocities of electrons in the beam. The dispersion leads to an angular spread of the electron beam at the sample238. The angular spread results in a blurred focus of the electron beam, thereby reducing effective resolution. Providing a second Wien filter as a dispersion device311allows compensation of the dispersion effect. In the example shown inFIG.8, the second Wien filter is configured to apply crossed electric and magnetic fields that are in opposite directions to the electric and magnetic fields applied by the Wien filter of the beam separator222. The combined action of such a Wien filter doublet (formed by the beam separator222and dispersion device311) is depicted inFIG.8. A primary beam passes through both the dispersion device311and the beam separator222and impinges on a sample238. A portion1040of the primary beam has a nominal beam energy corresponding to a pass energy of both the Wien filters. The portion1040of the primary beam thus passes undeflected through the Wien filters. A portion1041of the primary beam has a different beam energy. The portion1041of the primary beam is deflected first in one direction (to the right at the dispersion device311) and then back in the opposite direction (to the left at the beam separator222). Both portions1040and1041are subsequently focused to the same position on the sample238. Nominal voltages VAand VBapplied to electrodes in each Wien filter (and/or nominal currents applied to coils in each Wien filter) are selected so that the effects of dispersion cancel at the sample238to provide a sharp focused spot. In practice, significant fluctuations in the voltage and/or current used to power the Wien filters occur. A representative fluctuation dV in the voltage VAis depicted inFIG.9. The fluctuation dV causes the portion1040of the primary beam to be deflected at the beam separator222(in contrast to the situation inFIG.8). This deflection shifts the focus away from an intended focus position. The portion1041of the primary beam is also deflected so as to be focused at the shifted focus position. The fluctuation dV thus disturbs the focus at the sample238. Fluctuations may also occur in the voltage VBand in the current applied to either or both of the Wien filters. These fluctuations may further disturb focusing at the sample238.

In some embodiments of the disclosure, the focus disturbance described above with reference toFIGS.8and9is reduced or removed by providing one or more common power supplies1002,1004that each drive both the beam separator222and the dispersion device311,312. The common power supplies may comprise either or both of a common current source1002and a common voltage source1004. An illustrative example is depicted inFIG.10(with only a common voltage source1004shown). More detailed example configurations are described with reference toFIG.11-13. Since the electric and/or magnetic fields applied by the beam separator222and the dispersion device311,312are oriented oppositely to each other, any fluctuations in these fields caused by fluctuations in the common power supplies will at least partially compensate for each other. Adjustable electronics (e.g. voltage and/or current dividers) can be used to optimize the levels of voltage and/or current to remove unwanted beam displacement at the sample238. In the illustrative example ofFIG.10, a common voltage source1004is configured to provide a nominal voltage V and is subject to a fluctuation dV. In contrast to the arrangement shown inFIG.9, the fluctuation dV is applied to both the beam separator222and the dispersion device311. The fluctuation dV thus causes additional deflections of portions1040and1041of the primary beam at the dispersion device311. The additional deflections compensate for respective deflections of portions1040and1041at the beam separator222caused by the fluctuation dV acting at the beam separator222, thereby reducing or removing displacement of the focus position at the sample238caused by the fluctuation dV. In a typical configuration of the type depicted inFIG.9, it is expected that, without compensation, power supply fluctuations could cause unwanted beam displacement comparable to SEM image resolution or even larger (e.g. of the order of 10 nm). Using the compensation scheme ofFIG.10, unwanted beam displacements can typically be reduced to well below the SEM image resolution (e.g. to 0.5 nm or lower).

FIGS.4A,4B and5-7describe example contexts in which beam separators222and corresponding dispersion devices311,312may be driven by common power supplies. In each example, both a common current source1002and a common voltage source1004are depicted. It is also possible for each example to be implemented using only the common current source1002or only the common voltage source1004. Connections between the common power supplies and the beam separators222and corresponding dispersion devices311,312are not shown for clarity purposes inFIGS.4A,4B and5-7. Example arrangements for these connections are shown inFIG.11-13.

Reference is now made toFIG.4A, which illustrates an exemplary single-beam apparatus400, consistent with embodiments of the present disclosure. Single-beam apparatus400can be electron beam tool104A ofFIG.2Afurther comprising dispersion device311ofFIG.3B.FIG.4Aillustrates operation of dispersion device311for a case where object plane204of objective lens228is above objective lens228.FIG.4Billustrates operation of dispersion device311for a case where object plane204of objective lens228is below objective lens228. As described below, disclosed embodiments can compensate beam dispersion without limiting the operation mode of objective lens228.

As described above with reference toFIG.3B, the nominal dispersion angle associated with dispersion device311is zero and primary electron beam210can pass straight through dispersion device311. Dispersion device311can induce a beam dispersion based on the values of E2and B2. Primary electron beam210can also pass straight through beam separator222of Wien filter type. Beam separator222can also induce a beam dispersion based on the values of E1and B1. The beam dispersion induced by beam separator222can be referred to as main dispersion (MDS) and the beam dispersion induced by dispersion device311can be referred to as compensation dispersion (CDS). Dispersion device311can be configured and controlled to generate CDS opposite in direction to the MDS. For example, with reference toFIG.4A, an electron with energy>nominal energy V0can be deflected towards −x direction by beam separator222and towards +x direction by dispersion device311(corresponding to beam path430). An electron with energy<nominal energy V0can be deflected towards +x direction by beam separator222and towards −x direction by dispersion device311(corresponding to beam path434). The magnitude of CDS generated by dispersion device311can be controlled to make electrons with energies different from nominal energy V0(for example, electrons corresponding to beam paths430and434) to virtually focus on object plane204. Accordingly, objective lens228focuses primary electron beam210onto sample238to form probe spot236.

Reference is now made toFIG.5which illustrates an exemplary single-beam apparatus600, consistent with embodiments of the present disclosure. Single-beam apparatus600can comprise electron source206, gun aperture212, condenser lens214, primary electron beam210emitted from electron source206, beam-limit aperture216, dispersion device312, beam separator510, deflection scanning unit226, objective lens228, secondary electron beam220, and electron detector218. Beam separator510comprises a magnetic deflector and therefore associated deflection angle642has a non-zero value. Electron source206, gun aperture212, condenser lens214, beam-limit aperture216, dispersion device312, beam separator510, deflection scanning unit226, and objective lens228can be aligned with respect to optical axis602of single-beam apparatus600.

As described above with reference toFIG.3C, the nominal dispersion angle associated with dispersion device312is non-zero and primary electron beam210can pass through dispersion device312with a nominal deflection angle641and with an associated beam dispersion CDS. For single-beam apparatus600, an electron of primary electron beam210traveling along optical axis602with nominal energy V0can be deflected by angle641at deflection plane342(of dispersion device312) and can be incident at deflection plane520(of beam separator510) at an incident angle641. An electron traveling along optical axis602with energy>V0can be incident at beam separator510with an incident angle<angle641. An electron traveling along optical axis602with energy<V0can be incident at beam separator510with an incident angle>angle641.

Beam separator510can deflect primary electron beam210with a nominal deflection angle642and an associated beam dispersion MDS. An electron with nominal energy V0can be deflected at deflection plane520by an angle642. An electron with energy>V0can be deflected at an angle less than angle642. An electron with energy<V0can be deflected at an angle greater than angle642.

The CDS generated by dispersion device312can be controlled wherein the incident angle variation generated by CDS for electrons with different energies can compensate the deflection angle variation generated by MDS. Accordingly, the electrons with different energies can be controlled to virtually focus on object plane204. Further, objective lens228can focus the electrons with different energies (corresponding to beam paths630,632, and634) onto sample238to form probe spot236. Dispersion device312comprises an electrostatic deflector and a magnetic deflector and the CDS can therefore be varied while maintaining deflection angle641constant. Therefore the CDS can be changed to match the position variation of object plane204and no restrictions are placed on operation modes of objective lens228. Further dispersion device312can be controlled to maintain angles641and642equal. So optical axis602can be maintained parallel to optical axis of beam separator510. This can simplify the arrangement and alignment of various components of single-beam apparatus600.

Reference is now made toFIG.6, which illustrates an exemplary multi-beam apparatus700, consistent with embodiments of the present disclosure. Multi-beam apparatus700can be electron beam tool104A ofFIG.2Bfurther comprising dispersion device311ofFIG.3B.

As described above with reference toFIG.3B, the nominal dispersion angle associated with dispersion device311is zero and beamlets254,256, and258can pass straight through dispersion device311. Dispersion device311can induce a CDS for beamlets254,256, and258. Dispersion device311can be placed above primary projection optical system260.

Beamlets254,256, and258can also pass straight through beam separator222of Wien filter type. Beam separator222can induce a MDS for the beamlets. As described above with reference toFIG.4AandFIG.4B, dispersion device311can be configured and controlled to generate CDS opposite in direction to the MDS. The magnitude of CDS generated by dispersion device311can be controlled to make dispersed electrons of each beamlet (for example, electrons corresponding to beam paths720and724) virtually focus on object plane of objective lens228. Accordingly, objective lens228focuses the dispersed electrons of beamlets254,256, and258onto sample238to form corresponding probe spots270,272, and274.

Reference is now made toFIG.7, which illustrates an exemplary multi-beam apparatus900, consistent with embodiments of the present disclosure. Multi-beam apparatus900can comprise electron source206, gun aperture212, condenser lens214, primary electron beam210emitted from electron source206, source conversion unit252, plurality of beamlets254,256, and258of primary electron beam210, primary projection optical system260, multiple secondary electron beams930,932, and934, secondary optical system282, and electron detection device284. Primary projection optical system260can comprise objective lens228. Electron detection device284can comprise detection elements286,288, and290. Dispersion device312, beam separator510and deflection scanning unit226can be placed inside primary projection optical system260.

Electron source206, gun aperture212, condenser lens214, source conversion unit252, dispersion device312, beam separator510, deflection scanning unit226, and objective lens228can be aligned with a primary optical axis902of apparatus900. Secondary optical system282and electron detection device284can be aligned with a secondary optical axis292of apparatus900.

As described above with reference toFIG.3C, the nominal dispersion angle associated with dispersion device312is non-zero and primary electron beam210can pass through dispersion device312with a nominal deflection angle908and with an associated beam dispersion CDS. An electron of beamlets254,256, and258traveling along optical axis902with nominal energy V0can be incident at beam separator510with an incident angle908. An electron traveling along optical axis902with energy>V0can be incident at beam separator510with an incident angle<angle908. An electron traveling along optical axis902with energy<V0can be incident at beam separator510with an incident angle>angle908. Dispersion device312can be placed above primary projection optical system260.

Beam separator510can deflect beamlets254,256, and258with a nominal deflection angle equal to angle910and an associated beam dispersion MDS. An electron with nominal energy V0can be deflected at an angle equal to angle910. An electron with energy>V0can be deflected at an angle less than angle910. An electron with energy<V0can be deflected at an angle greater than angle910.

The CDS generated by dispersion device312can be controlled wherein the incident angle variation generated by CDS for electrons with different energies can compensate the deflection angle variation generated by MDS. Accordingly, the electrons with different energies can be controlled to virtually focus on the object plane of objective lens228. Further, objective lens228can focus the electrons with different energies (corresponding to beam paths920,922, and924) onto sample238to form corresponding probe spots270,272, and274. Dispersion device312comprises an electrostatic deflector and a magnetic deflector and the CDS can therefore be varied while maintaining deflection angle908constant. Therefore the CDS can be changed to match the position variation of object plane204and no restrictions are placed on operation modes of objective lens228. Further dispersion device312can be controlled to maintain that angles908and910are equal. So optical axis902can be maintained parallel to optical axis906of beam separator510. This can simplify the arrangement and alignment of various components of single-beam apparatus900.

FIG.11-13depict portions of further examples of an electron optical column having one or more common power supplies1002,1004for driving a beam separator222and a dispersion device311,312. The optical column is configured to direct a beam of primary electrons along a primary beam path1045onto a sample238. The optical column comprises a beam separator222. The beam separator may take any of the forms described above with reference toFIG.2-10. The beam separator222diverts away from the primary beam path1045a beam1060of secondary electrons emitted from the sample238along the primary beam path1045. In the example ofFIG.11, the diversion away from the primary beam path1045happens at point1070. A dispersion device311,312is provided upbeam from the beam separator222. The dispersion device311,312may take any of the forms described above with reference toFIG.3-10. The dispersion device311,312compensates for dispersion induced in the primary beam by the beam separator222. One or more common power supplies1002,1004are provided for driving both the beam separator222and the dispersion device311,312.

In some arrangements, as exemplified inFIG.11, the beam separator222diverts the beam1060of secondary electrons using a magnetic field generated by a beam separator coil1111. The magnetic field may be perpendicular to an optical axis of the column. In the example shown, the magnetic field is perpendicular to the page. The dispersion device312at least partially compensates for the dispersion induced by the beam separator222(e.g. by the beam separator coil1111) using a magnetic field generated by a dispersion device coil1110. In arrangements of this type, the one or more common power supplies may comprise a common current source1002that drives both the beam separator coil1111and the dispersion device coil1110. When an electric current is driven through the coils the coils generate a corresponding magnetic field. The beam separator coil1111and the dispersion device coil1110may be formed on respective magnetic cores. The magnetic field generated by the beam separator coil1111is oppositely oriented to the magnetic field generated by the dispersion device coil1110. In an embodiment, the beam separator coil1111and the dispersion device coil1110are connected in series with each other and to the common current source1002, as depicted schematically by connections1005. A current divider arrangement may be provided to adjust the relative sizes of currents in the beam separator coil1111and the dispersion device coil1110. Alternatively or additionally, a number of turns of either or both coils may be changed to adjust the relative sizes of magnetic excitations in the beam separator coil1111and the dispersion device coil1110.

In some arrangements, as exemplified inFIG.12-13, the beam separator222comprises beam separator electrodes1121. The beam separator electrodes1121apply an electric field to primary electrons in the beam separator222. The electric field is such as to apply a force to the primary electrons that is opposite in direction to a force applied to the primary electrons by the magnetic field generated by the beam separator coil1111. In some arrangements, as discussed above with reference toFIG.10for example and exemplified inFIG.13, the force applied by the electric field to a portion of the primary beam in the beam separator222having a selected nominal energy is substantially equal in magnitude to the force applied to the same portion of the primary beam by the magnetic field. In arrangements of this type, the beam separator222may be referred to as a Wien filter. The portion of the primary beam having the nominal energy will pass through the beam separator222undeflected. In other arrangements, as exemplified inFIG.12, the force applied by the electric field is different in magnitude to the force applied by the magnetic field for all portions of the beam. The force applied by the magnetic field may, for example, be twice as strong as the force applied by the electric field to minimize dispersion. This condition arises because the angle of deflection of electrons with mass m, charge q, and speed v in crossed electric and magnetic fields applied over a distance l is given by θ=ql(vB−E)/(mv2), and dθ/dv=0 when vB=2E. In this situation, a deflection angle θBthat would be imparted by the magnetic field on its own would be twice as large as a deflection angle θEthat would be imparted by the electric field on its own and in the opposite direction, such that θB=−2θE. The beam1060of secondary electrons emitted from the sample238is travelling in the opposite direction to the primary beam and is deflected in the beam separator222by both the magnetic field and the electric field in the same direction. The cumulative effect of the two forces results in a larger deflection of −3θEor more (because the secondary electrons may have lower energy than the primary electrons). As discussed above with reference toFIG.3C, arranging for the electric and magnetic fields not to cancel allows a fixed non-zero deflection to be applied to the primary beam.

In some embodiments, the dispersion device comprises dispersion device electrodes1120. The dispersion device electrodes1120apply an electric field to primary electrons in the dispersion device311,312. The electric field is such as to apply a force to the primary electrons that is opposite in direction to a force applied to the primary electrons by the magnetic field generated by the dispersion device coil1110. In some arrangements, as discussed above with reference toFIG.10for example and exemplified inFIG.13, the force applied by the electric field to a portion of the primary beam in the dispersion device311having a selected nominal energy is substantially equal in magnitude to the force applied to the same portion of the primary beam by the magnetic field. In arrangements of this type, the dispersion device311may be referred to as a Wien filter. The portion of the primary beam having the nominal energy will pass through the dispersion device311undeflected. In other arrangements, as exemplified inFIG.12, the force applied by the electric field is different in magnitude to the force applied by the magnetic field for all portions of the beam. The force applied by the magnetic field may, for example, be twice as strong as the force applied by the electric field. As discussed above with reference toFIG.3C, arranging for the electric and magnetic fields not to cancel in this manner allows a fixed non-zero deflection to be applied to the primary beam.

In some embodiments, as exemplified inFIG.12-13, the one or more common power supplies comprises a common voltage source1004configured to drive both the beam separator electrodes1121and the dispersion device electrodes1120. The common voltage source1004may thus apply a voltage (potential difference) across both the beam separator electrodes1121and the dispersion device electrodes1120. The beam separator electrodes1121and the dispersion device electrodes1120may be connected in parallel relative to each other, as depicted schematically inFIGS.12and13. The beam separator electrodes1121and the dispersion device electrodes1120may be connected so that the voltage across the beam separator electrodes1121is opposite in polarity to the voltage across the dispersion device electrodes1120. A voltage divider arrangement may be provided to adjust the relative sizes of the electric fields in the beam separator electrodes1121and the dispersion device electrodes1120. Alternatively or additionally, positions and/or geometries of either or both of the beam separator electrodes1121and the dispersion device electrodes1120may be changed to adjust the relative influences of the beam separator electrodes1121and the dispersion device electrodes1120on electrons passing through the beam separator222and dispersion device311,312. For example, the electrodes may be made longer or shorter along the beam path to change the period during which the electric field acts on the electrons or a separation between the electrodes may be changed to vary the electric field for a given applied voltage.

In some embodiments, as exemplified inFIG.13, either or both of the beam separator222and the dispersion device311comprises adjustment electrodes1130,1131. The adjustment electrodes1130,1131apply an electric field to primary electrons that is perpendicular or oblique to the electric field applied by the beam separator electrodes1121or the dispersion device electrodes1120. In the example ofFIG.13, adjustment electrodes1130,1131are provided in both the beam separator222and the dispersion device311. The adjustment electrodes1130,1131are parallel to the plane of the page and therefore generate an electric field perpendicular to the plane of the page. The force applied to the primary electrons by the adjustment electrodes1130,1131is thus perpendicular to the forces applied by the beam separator coil1111, the dispersion device coil1110, the beam separator electrodes1121and the dispersion device electrodes1120. The adjustment electrodes1130,1131may be used for fine tuning of the beam of primary electrons by deflection in the direction perpendicular to the forces applied by the beam separator coil1111, the dispersion device coil1110, the beam separator electrodes1121and the dispersion device electrodes1120. The electric fields needed to perform the fine tuning are likely to be significant smaller than the electric fields applied in the beam separator electrodes1121and the dispersion device electrodes1120. Errors due to fluctuations in the power supply used to power the adjustment electrodes may thus have only a limited negative effect on performance. In embodiments of this type, independent power supplies1005and1006may be provided to respectively drive the adjustment electrodes1130,1131. The independent power supplies1005and1006are independent of the one or more common power supplies1002,1004.

In some arrangements, one or more adjustment coils1132may be provided at the beam separator222and/or at the dispersion device311,312. In the example ofFIG.13, the beam separator222comprises a pair of adjustment coils1132. In other arrangements, the dispersion device311,312may comprise one or more adjustment coils or the beam separator222and the dispersion device311,312may each comprise one or more adjustment coils. The adjustment coils1132apply a magnetic field to primary electrons that is perpendicular or oblique to the magnetic field applied by the beam separator coil1111or dispersion device coil1110. The adjustment coils1132may be used for fine tuning the beam of primary electrons by deflection in the direction perpendicular to the forces applied by the beam separator coil1111, the dispersion device coil1110, the beam separator electrodes1121and the dispersion device electrodes1120. The magnetic fields needed to perform the fine tuning are likely to be significant smaller than the magnetic fields applied in the beam separator coil1111and the dispersion device coil1110. Errors due to fluctuations in the power supply used to power the adjustment coils may thus have only a limited negative effect on performance. In embodiments of this type, an independent power supply1007may be provided to drive the adjustment coil or adjustment coils1132at the beam separator222or dispersion device312. The independent power supply1007is independent of the one or more common power supplies1002,1004and, where present, of power supplies1005and1006for driving adjustment electrodes1130,1131. Connections between the independent power supplies1005,1006and1007and the adjustment electrodes1130,1131and adjustment coils1132are omitted inFIG.13for clarity.

Various arrangements are described above in which a beam separator222and dispersion device311,312are provided in series electron-optically. One or more beams of primary electrons pass first through the dispersion device311,312and then through the beam separator222before reaching the sample238. The dispersion device311,312compensates in advance for at least a portion of dispersion induced in the primary beam by the beam separator222. The dispersion device311,312is a feed-forward corrector or compensator. The compensation is introduced to the beam prior to the beam separator222operating on the beam311,312. For efficient compensation, it is desirable for the beam separator222and dispersion device311,312to be directly consecutive along the primary beam path. In such arrangements, no other element having a significant influence on electrons is therefore present between the beam separator222and dispersion device311,312. Any region between the beam separator222and dispersion device311,312is free of any element that could significantly alter a trajectory or energy of electrons, such as another electron-optical element, an obstacle or a filter. Preferably, trajectories of electrons in the primary beam are parallel to each other between the beam separator222and dispersion device311,312. The trajectories are such that no intermediate focus is formed between the beam separator222and dispersion device311,312. Such an electron-optical design may be described as asymmetric with respect to the beam separator222and dispersion device. The beam path is continuous between the beam separator222and dispersion device311,312and any focus such as intermediate focus is upbeam or downbeam along the primary beam path of both beam separator222and the dispersion device311,312.

The beam separator222and dispersion device311,312may be substantially symmetrical with respect to each other. For example, an electric field applied by the beam separator222may be substantially equal in size and opposite in direction to an electric field applied by the dispersion device311,312. A magnetic field applied by the beam separator222may be substantially equal in size and opposite in direction to a magnetic field applied by the dispersion device311,312. A length of a portion of the primary beam path along which electrons are influenced by the beam separator222may be substantially the same as a length of a portion of the primary beam path along which electrons are influenced by the dispersion device311,312. The beam separator222and the dispersion device311,312may thus have substantially the same size. Making the influences of the electric and/or magnetic fields symmetric in this manner may facilitate optimum compensation of dispersion. However, the inventors have found that effective levels of compensation may be achieved even where some asymmetry is present. An effective level of compensation may include correction but not entire elimination of the dispersion generated by the beam separator. This makes it possible to achieve a desirable balance between space saving and dispersion compensation. For example, in some arrangements the dispersion device311,312is made deliberately smaller than the beam separator222to allow space to be saved in the region where the dispersion device311,312is to be located. Thus, the dispersion device311,312may be configured to influence electrons along a smaller portion of the primary beam path than the beam separator dispersion device311and can thereby be made smaller.

It will be appreciated that the present invention is not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof. It is intended that the scope of the invention should only be limited by the appended claims and clauses. There is provided the following clauses:

Clause 1: An electron optical column configured to direct a beam of primary electrons along a primary beam path onto a sample, comprising: a beam separator configured to divert away from the primary beam path a beam of secondary electrons emitted from the sample along the primary beam path; a dispersion device upbeam from the beam separator, the dispersion device being configured to compensate for dispersion induced in the primary beam by the beam separator; and one or more common power supplies each configured to drive both the beam separator and the dispersion device.

Clause 2: The column of clause 1, wherein: the beam separator is configured to divert the beam of secondary electrons using a magnetic field generated by a beam separator coil; and the dispersion device is configured to compensate for the dispersion using a magnetic field generated by a dispersion device coil.

Clause 3: The column of clause 2, wherein the one or more common power supplies comprises a common current source configured to drive both the beam separator coil and the dispersion device coil.

Clause 4: The column of clause 2 or 3, wherein the magnetic field generated by the beam separator coil is oppositely oriented to the magnetic field generated by the dispersion device coil.

Clause 5: The column of any of clauses 2-4, wherein: the beam separator comprises beam separator electrodes configured to apply an electric field to primary electrons in the beam separator; and the electric field is such as to apply a force to the primary electrons that is opposite in direction to a force applied to the primary electrons by the magnetic field generated by the beam separator coil.

Clause 6: The column of clause 5, wherein the force applied by the electric field to a portion of the primary beam in the beam separator having a selected nominal energy is substantially equal in magnitude to the force applied to the same portion of the primary beam by the magnetic field.

Clause 7: The column of clause 5 or 6, wherein: the dispersion device comprises dispersion device electrodes configured to apply an electric field to primary electrons in the dispersion device; and the electric field is such as to apply a force to the primary electrons that is opposite in direction to a force applied to the primary electrons by the magnetic field generated by the dispersion device coil.

Clause 8: The column of clause 7, wherein the force applied by the electric field to a portion of the primary beam in the dispersion device having a selected nominal energy is substantially equal in magnitude to the force applied to the same portion of the primary beam by the magnetic field.

Clause 9: The column of clause 7 or 8, wherein the one or more common power supplies comprises a common voltage source configured to drive both the beam separator electrodes and the dispersion device electrodes.

Clause 10: The column of any of clauses 5-9, wherein either or both of the beam separator and the dispersion device comprises adjustment electrodes configured to apply an electric field to primary electrons that is perpendicular or oblique to the electric field applied by the beam separator electrodes or the dispersion device electrodes.

Clause 11: The column of clause 10, further comprising at least one independent power supply configured to drive the adjustment electrodes, each independent power supply being independent of the one or more common power supplies.

Clause 10: The column of any preceding claim, wherein the dispersion device is configured to influence electrons along a smaller portion of the primary beam path than the beam separator.

Clause 13: The column of any preceding claim, wherein the beam separator and dispersion device are directly consecutive along the primary beam path.

Clause 14: A charged particle assessment tool comprising the column of any of clauses 1-13.

Clause 15: A method of directing a beam of primary electrons along a primary beam path onto a sample, comprising: using a beam separator to divert away from the primary beam path a beam of secondary electrons emitted from the sample along the primary beam path; and using a dispersion device upbeam from the beam separator to compensate for dispersion induced in the primary beam by the beam separator, wherein one or more common power supplies are used to drive both the beam separator and the dispersion device.