Aberration-corrected and energy-filtered low energy electron microscope with monochromatic dual beam illumination

One embodiment relates to an apparatus for correcting aberrations introduced when an electron lens forms an image of a specimen and simultaneously forming an electron image using electrons with a narrow range of electron energies from an electron beam with a wide range of energies. A first electron beam source is configured to generate a lower energy electron beam, and a second electron beam source is configured to generate a higher energy electron beam. The higher energy beam is passed through a monochromator comprising an energy-dispersive beam separator, an electron mirror and a knife-edge plate that removes both the high and low energy tail from the propagating beam. Both the lower and higher energy electron beams are deflected by an energy-dispersive beam separator towards the specimen and form overlapping illuminating electron beams. An objective lens accelerates the electrons emitted or scattered by the sample. The electron beam leaving the specimen is deflected towards a first electron mirror by an energy-dispersive beam separator, which introduces an angular dispersion that disperses the electron beam according to its energy. A knife-edge plate, located between the beam separator and first electron mirror, is inserted that removes all of the beam with energy larger and smaller than a selected energy and filters the beam according to energy. One or more electron lenses focus the electron beam at the reflection surface of the first electron mirror so that after the reflection and another deflection by the same energy-dispersive beam separator the electron beam dispersion is removed. The dispersion-free and energy-filtered electron beam is then reflected in a second electron mirror which corrects one or more aberrations of the objective lens. After the second reflection, electrons are deflected by the magnetic beam separator towards the projection optics which forms a magnified, aberration-corrected, energy-filtered image on a viewing screen.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electron beam apparatus and electron microscopy methods.

2. Description of the Background Art

Optical microscopes, the simplest and most common instruments used to image objects too small for the naked eye to see, uses photons with visible wavelengths for imaging. A specimen is illuminated with a broad light beam, and a magnified image of the specimen can be observed using an eyepiece or camera. The maximum magnification of a light microscope can be more than 1000× with a diffraction-limited resolution limit of a few hundred nanometers. Improved spatial resolution in an optical microscope can be achieved when shorter wavelengths of light, such as ultraviolet wavelengths, are utilized for imaging.

An electron microscope is a different type of microscope. It uses electrons to illuminate the specimen and create a magnified image. The microscope has a greater resolving power than a light microscope, because it uses electrons that have wavelengths few orders of magnitude shorter than visible light. Electron microscopes can achieve magnifications exceeding 1,000,000×.

Scanning electron beam microscopes (SEMs), the most widely used electron microscopes, image the specimen surface by scanning it with a tightly focused high-energy beam of electrons in a raster scan pattern, pixel by pixel. In a typical SEM, an electron beam is emitted in a vacuum chamber from an electron gun equipped with a thermionic (tungsten, lanthanum hexaboride), thermally assisted (Schottky, zirconium oxide) or cold field emission cathode. The electron beam, which typically has an energy ranging from a few hundred eV to few tens keV, is collimated by one or more condenser lenses and then focused by the final objective lens to a spot about 1 nm to 100 nm in diameter. The beam is deflected by pairs of magnetic scanning coils or electrostatic deflector plates, sweeping in a raster fashion over a rectangular area of the specimen surface. Primary electrons can generate various signals from elastically scattered electrons, secondary electrons (due to inelastic scattering), characteristic Auger electrons and the emission of electromagnetic radiation. Each of the generated signals can be detected by specialized detectors, amplified and displayed on a CRT display or captured digitally, pixel by pixel on a computer.

Low energy emission microscopes (LEEM) and photoemission electron microscopes (PEEM) are projection (as opposed to scanning) electron microscopes, and thus resemble a conventional light microscope. In a LEEM, the electron gun forms a broad electron beam that is accelerated to typically 10 to 30 keV and passed through a beam separator, an energy-dispersive magnetic prism that separates the illumination and projection optics and bends the beam into the axis of a cathode objective lens containing the specimen. The objective lens is called a cathode lens as the specimen forms the negative electrode in this lens. A parallel flood beam then uniformly illuminates the specimen that is electrically biased at approximately the same potential as the cathode of the electron gun, so that illuminating electrons are decelerated in the objective lens, striking the specimen at energies typically between 0 to about 1000 eV. In the opposite direction, i.e. upward from the specimen, the objective lens simultaneously accelerates the scattered electrons and forms a magnified image of the specimen. As the electrons reenter the beam separator, they get deflected into the projection optics. The projection zoom optics forms an electron image on the scintillating screen that is then viewed by a CCD camera and further processed on a computer. The extremely low energy of the illuminating electrons makes LEEM an exquisitely sensitive surface imaging technique, capable of imaging single atomic layers with high contrast. The low landing energy of electrons is also critical for avoiding radiation damage, as high energy electrons in all keV kinetic energy ranges can cause unavoidable damage to many types of specimens.

Photoemission electron microscopes (PEEM) are projection electron microscopes, where the specimen is illuminated with UV photons or X-rays rather than electrons. Similar to a LEEM, the objective lens is a cathode lens with the specimen at a high negative bias. The photon flood beam uniformly illuminates the specimen, and the photoemitted electrons are accelerated by the objective lens and form a magnified image of the specimen.

One of the main drawbacks of conventional LEEM/PEEM is lateral resolution. In spite of the short deBroglie wavelength in the Angstrom range, the lateral resolution of conventional LEEM instruments is limited to a few nm and sub-nm resolution has not been achieved yet; and PEEM resolution ranges from 10 to 20 nm. The electron lenses used for imaging in a LEEM/PEEM, in particular the cathode objective lens, introduce spherical and chromatic aberrations that deteriorate the spatial resolution of a LEEM/PEEM. Effective means for improving the spatial resolution are therefore desirable if LEEM/PEEM instruments are to be used for imaging at higher spatial resolution.

Another drawback of a conventional LEEM/PEEM is its lack of energy-filtered imaging. The primary electrons scattered by the specimen produce electrons over a wide range of energies, from secondary electrons in the range of a few eV, to hundreds to thousands of eV for characteristic Auger electrons, and near the landing energy for elastically scattered electrons. X-ray photons result in the generation of photoemission electrons with a wide spectrum of energies, containing element-specific peaks that can be used to characterize the specimen. Electrons with different energies produce different image contrast and can provide comprehensive information about the specimen, including specimen topography, composition, crystalline structure as well as electrical and magnetic properties. In order to obtain detailed information about the chemical composition, interatomic bonding and local electronic states of non-periodic objects such as nanoparticles, interfaces, defects and macromolecules, an energy resolution of 0.1 eV or less is necessary to discern their characteristic electronic states. Effective means for selecting electrons emitted from the sample with a narrow range of energies for imaging as well as utilizing monochromatic illumination with an energy spread smaller than the desired energy resolution are therefore desirable for detailed characterization of specimens.

One approach to improve the spatial resolution and provide energy-filtering capability in a LEEM/PEEM is to use an aberration corrector based on an electron mirror, such as the one disclosed in U.S. Pat. No. 5,319,207, which is entitled “Imaging system for charged particles” and which issued Jun. 7, 1994 to inventors Harald Rose, Ralf Degenhardt and Dirk Preikszas. As shown inFIG. 1a, this approach employs a dispersion-free magnetic beam separator and an electron mirror for aberration correction. The absence of energy dispersion after each deflection facilitates minimum combined aberrations between the energy dispersion and the chromatic and spherical aberrations of the electron mirror. However, this prior technique is disadvantageous in some aspects. The practical implementation of this approach is rather difficult, due to complexity of the magnetic beam separator. The dispersion-free beam separator is a rather complicated electron-optical element, consisting of a large number of coils with complex shapes that are difficult to construct and align. The machining, tight tolerances and assembly are challenging which makes tuning and alignment of the whole microscope difficult. In addition, the dispersion-free magnetic prism separator cannot be used for energy filtering, and an additional energy filter must be included in the projection optics which further complicates the microscope design, assembly and alignment.

Another approach to improve the spatial resolution and provide energy-filtering capability in a LEEM/PEEM is to use an aberration corrector based on an electron mirror, such as the one disclosed in U.S. Pat. No. 7,348,566, which is entitled “Aberration-correcting cathode lens microscopy instrument” and which issued Mar. 25, 2008 to inventor Rudolf Tromp. Unlike the prior technique disclosed in U.S. Pat. No. 5,319,207 that uses a complex dispersion-free beam separator, the apparatus and method disclosed in the U.S. Pat. No. 7,348,566 do not require the separator to be free of dispersion in order to achieve aberration correction. Instead, as shown inFIG. 1b, it uses two dispersive magnetic beam separators of a practical design with simple square shaped coils that are much easier to machine, assemble and align. In addition, the technique disclosed in the U.S. Pat. No. 7,348,566 does not require an additional energy filter to carry out energy-filtered imaging. However, this technique using two beam separators is disadvantageous in some aspects. The additional deflection that transports the beam into the projection optics introduces energy dispersion that generates additional combination aberrations, including image tilt and off-axis astigmatism which can affect the image quality when sub-nm resolution is needed. In addition, the two beam separators have to be identical to ensure that the dispersion and all combination aberrations of the first prism separator are cancelled by the second prism separator, which may be difficult to achieve practically.

Another approach to improve the spatial resolution and provide energy-filtering capability in a LEEM/PEEM is to use an aberration corrector based on an electron mirror, such as the one disclosed in U.S. patent application Ser. No. 13/251,266, which is entitled “Compact arrangement for aberration correction of electron lenses” and which was filed Oct. 2, 2011 by inventor Marian Mankos.

Unlike the prior technique using a dispersion-free magnetic beam separator discussed above in relation toFIG. 1a, the apparatus and method disclosed in U.S. patent application Ser. No. 13/251,266 does not necessarily require a complex dispersion-free beam separator in order to achieve aberration correction. Instead, it uses a single energy-dispersive magnetic beam separator with practical design and simple square shaped coils that are much easier to machine, assemble and align.

Unlike the prior technique disclosed by Tromp in U.S. Pat. No. 7,348,566 using two energy-dispersive magnetic beam separators and discussed above in relation toFIG. 1b, the apparatus and method disclosed in U.S. patent application Ser. No. 13/251,266 does not require the use of an additional magnetic beam separator to achieve aberration correction. The prior technique disclosed by Tromp in U.S. Pat. No. 7,348,566 may result in additional combination aberrations in the projection optics including image tilt and off-axis astigmatism which can affect the final image quality. In addition, small differences in the geometry and excitation between the two energy-dispersive magnetic beam separators may result in incomplete cancellation of the dispersion combination aberrations, which may prevent the aberration corrector from fully correcting the objective lens aberrations. Instead, as shown inFIG. 1c, it uses one dispersive magnetic beam separator and two electron mirrors to achieve aberration correction. The specimen is illuminated with an electron, UV-photon or X-ray beam, and a magnetic beam separator deflects the electron beam emitted from the specimen and magnified by a cathode objective lens towards a first electron mirror. The magnetic beam separator introduces an angular dispersion that disperses the incoming electron beam according to its energy. An electron lens is configured to focus the dispersed electron beam at the reflection surface of a first electron mirror and introduce symmetry so that the reflected electron beam passes through the magnetic beam separator a second time and exits without energy dispersion. The electron beam then enters a second electron mirror that is configured to correct for one or more aberrations of the cathode objective lens and reflect the electron beam back into the magnetic beam separator. After a third deflection through the magnetic beam separator the electron beam is transported into the projection optics and magnified on a viewing screen.

Another important drawback of a conventional LEEM/PEEM is its lack of imaging capability for insulating samples. When a conventional LEEM instrument is used to image insulating specimens, the low landing energy exacerbates charging effects resulting in significantly reduced image quality. The imbalance between the arriving and leaving flux of electrons causes the surface to charge up, resulting in increased blur and distortions. In many cases, the built-up surface charge can rapidly discharge in an arc, resulting in specimen damage. On a homogeneous insulator surface, the charging can be suppressed by operating at a landing energy resulting in a net electron yield of 1. However, this approach restricts the landing energy and typically does not work when different insulating materials are present on the surface. Effective means for controlling local surface charging are therefore desirable if LEEM instruments are to be used for imaging of insulating samples. None of the above mentioned aberration correction inventions shown inFIG. 1a-chave a provision to mitigate the charging effects.

One possible approach that can be used to solve the charging problem in a LEEM/PEEM is the dual illumination beam approach. In a LEEM with dual-beam illumination, two electron beams with different landing energies are used to mitigate the charging effect. The low-energy electron beam with landing energy near 0 eV is partially mirrored and partially absorbed, charging the surface negatively. The high-energy electron beam (˜100 eV or more) emits secondary electrons with an electron yield that exceeds 1, charging the surface positively. However, when two beams with opposite charging characteristics, i.e. a low-energy mirror electron beam and a high-energy electron beam are superimposed on the specimen, charging effects can be neutralized. The challenge is to devise an electron optical system that can deliver overlapping illumination of the low-energy mirror and high-energy electron beams at preferably normal incidence on the specimen, i.e. a system that combines two parallel electron beams with different energies and beam currents at the specimen surface.

One approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 6,803,572, which is entitled “Apparatus and methods for secondary electron emission with a dual beam” and which issued Oct. 12, 2004 to inventors Lee H. Veneklasen and David L. Adler. As shown inFIG. 1d, this approach employs two co-planar guns with different beam energies and inclined beam axes that generate two illumination beams. The guns are configured such that the angle of inclination is equal to the difference in bending angles caused by the magnetic prism separator. However, this prior technique is disadvantageous in some aspects. The practical implementation of this approach is rather difficult, due to the small difference in deflection angles. For example, for a 30 keV electron beam energy and beam energy differential of 300 eV, the difference in deflection angles amounts to only about 5 mrad, i.e. about ⅓ of a degree. This means that the guns must be impractically far from the prism in order not to overlap. In principle, one can increase the angular separation by biasing a drift tube in the beam separator at high negative potential and thus lowering the beam energy while electrons pass through the beam separator. However this is not desirable due to increased Coulomb interactions and geometric aberrations that deteriorate the spatial resolution. In addition, it complicates the design and increases the likelihood of high-voltage arcing.

Another approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 6,803,571, which is entitled “Method and Apparatus for Dual-Energy E-Beam Inspector” and which issued Oct. 12, 2004 to inventors Marian Mankos and David L. Adler, is shown inFIG. 1e. Unlike the prior technique using two inclined beams discussed above, the apparatus and method disclosed in U.S. Pat. No. 6,803,571 do not necessarily require biasing of the separator at high voltage in order to achieve sufficient angular separation of the low and high energy beams. In addition, the presently disclosed technique does not require two electron guns to be in close proximity to each other. The apparatus includes a dual-beam electron gun that is configured to generate both a high-energy electron beam component and a low-energy electron beam component. In one implementation, the dual-beam electron gun is composed of two concentric cathodes, an inner disc and an outer annulus. The inner disc may be biased at a high negative voltage with respect to the specimen, while the outer annulus may be biased by an additional negative voltage with respect to the inner disc. However, this prior technique using a dual-beam electron gun is disadvantageous in some aspects. The proximity of the two cathodes in the gun at different temperatures and potentials results in complex crosstalk effects, beam current drift and long settling times, which makes it difficult for practical use and may reduce stability and reliability of the electron beam apparatus. These issues can be resolved when an electron mirror and prism are used to recombine two spatially separate electron beams.

Another approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 7,217,924, which is entitled “Holey mirror arrangement for dual-energy e-beam inspector” and which issued May 15, 2007 to inventors Marian Mankos and Eric Munro. As shown inFIG. 1f, the apparatus includes a illumination configuration with two perpendicular branches which are connected by a magnetic prism beam combiner. The first branch includes a first electron gun at a first (lower) energy, and the second branch includes a second electron gun or source at a second potential energy. The second branch also includes a semitransparent electron mirror that reflects the lower energy beam and transmits the higher energy beam. This prior technique allows the use of two conventional single beam guns, which simplifies the gun design and makes the operation more reliable. However, this prior technique is disadvantageous in some aspects. The column requires an additional bending element, i.e. a magnetic prism array, and complex transfer optics to assure cancellation of the dispersion.

Another approach to combine two illuminating electron beams with different charging characteristics to mitigate the deleterious charging effects is disclosed in U.S. Pat. No. 8,258,474, which is entitled “Compact arrangement for dual-beam low energy electron microscope” and which issued Sep. 4, 2012 to inventor Marian Mankos. As shown inFIG. 1g, a first electron beam source is configured to generate a low-energy electron beam, and an energy-dispersive device deflects the low-energy electron beam towards an Einzel lens that acts as an semitransparent electron mirror. The Einzel lens is biased to reflect the low-energy electron beam. A second electron beam source is configured to generate a high-energy electron beam that passes through an opening in the Einzel lens. Both the low- and high-energy electron beams enter the same energy-dispersive device that deflects both beams towards the specimen. A deflection system positioned between the high-energy electron source and Einzel lens is configured to deflect the high-energy electron beam by an angle that compensates for the difference in bending angles between the lower- and higher energy electron beams introduced by the energy-dispersive device, therein allowing both the lower- and high-energy beams to strike the specimen at normal incidence.

However, none of the above-mentioned prior dual-beam beam approaches allow for aberration correction, monochromatic illumination and energy filtered imaging. In addition, none of the above-mentioned aberration-correction and energy-filtering approaches allow for dual beam illumination. An improved LEEM/PEEM apparatus and methods for providing simultaneous aberration correction, monochromatic illumination, energy filtering and dual beam illumination are desirable.

SUMMARY

One embodiment pertains to an apparatus for generating an aberration-corrected, energy-filtered image of a specimen illuminated by two spatially overlapping electron beams. A first electron beam source is configured to generate a high-energy electron beam, and passed through a monochromator utilizing an electron mirror and a knife-edge plate that removes the high and low energy tail from the propagating beam. A second electron beam source is configured to generate a low-energy electron beam, and a first energy-dispersive beam separator deflects the low-energy electron beam towards an electrostatic lens. The electrostatic lens is biased negatively to reflect the low-energy electron beam. The monochromatic high-energy beam passes through the negatively biased electrostatic lens. Both the low- and high-energy electron beams enter the first energy-dispersive beam separator that deflects both beams towards a transfer electron lens. The transfer electron lens is configured to focus the dispersed electron beams at the achromatic plane of a second energy-dispersive beam separator and introduce symmetry so that after the deflection by second energy-dispersive beam separator towards the cathode objective lens, both electron beams become coaxial. The cathode objective lens is configured to illuminate the specimen, and electrons scattered and emitted by the specimen form an electron beam with a range of energies. The scattered and emitted electrons return to the second energy-dispersive beam separator and are deflected towards a first electron mirror. The second energy-dispersive beam separator introduces an angular dispersion that disperses the incoming electron beam according to its energy. A knife-edge plate removes the electrons with energies higher and lower than the selected energy for imaging. An electron lens is configured to focus the electron beam into an electron mirror so that the electrons are reflected symmetrically. This assures that the energy dispersion is removed after the energy-filtered electron beam is deflected by the second energy-dispersive beam separator toward a second electron mirror. The second electron mirror is configured to correct for one or more aberrations of the cathode objective lens and reflects the electron beam back into the second energy-dispersive beam separator. After a third deflection through the second energy-dispersive beam separator the electron beam is transported with dispersion through the electron transfer lens towards the first energy-dispersive beam separator. The first energy-dispersive beam separator then deflects the energy-filtered electron beam into the projection optics. This deflection removes the energy dispersion and the energy-filtered electron beam forms an aberration-corrected, energy-filtered image that is magnified on a viewing screen.

Another embodiment pertains to a method for aberration correction and energy filtering when a cathode objective lens is used to image a specimen illuminated by electron beams with different energies. A high-energy electron beam, passed through a monochromator, and a low-energy electron beam illuminate a specimen and a cathode objective lens forms an image with electrons scattered and emitted by the specimen. The beam of electrons scattered and emitted by the specimen is deflected with energy dispersion towards a first electron mirror. A knife-edge plate removes both the high or low energy tail from the electron beam. The dispersed electron beam is focused by an electron lens at the reflection surface of the first electron mirror and reflected back. After a second deflection the energy dispersion is cancelled and the beam enters a second electron mirror. One or more aberrations of the image formed by the dispersion-free electron beam are corrected by the second electron mirror. The electron beam is deflected into the projection optics and magnified on a viewing screen.

Another embodiment pertains to an apparatus for generating an aberration-corrected, energy-filtered image of a specimen illuminated by spatially overlapping electron and UV or X-ray photon beams. A first electron beam source is configured to generate an electron beam. When a monochromatic electron beam is needed, a monochromator utilizing an electron mirror and a knife-edge plate is used to remove the high or low energy tail from the propagating electron beam. Both the electron and UV or X-ray photon beams are configured to illuminate the specimen, and electrons scattered and photoemitted by the specimen form an electron beam with a range of energies. The scattered and emitted electrons are deflected by a first energy-dispersive beam separator towards a first electron mirror. The energy-dispersive beam separator introduces an angular dispersion that disperses the incoming electron beam according to its energy. A knife-edge plate removes the electrons with energies higher and lower than the selected energy for imaging. An electron lens is configured to focus the electron beam into an electron mirror so that the electrons are reflected symmetrically. This assures that the energy dispersion is removed after the energy-filtered electron beam is deflected by the second energy-dispersive beam separator toward a second electron mirror. The second electron mirror is configured to correct for one or more aberrations of the cathode objective lens and reflects the electron beam back into the second energy-dispersive beam separator. After a third deflection through the second energy-dispersive beam separator the electron beam is transported with dispersion through the electron transfer lens towards the first energy-dispersive beam separator. The first energy-dispersive beam separator then deflects the energy-filtered electron beam into the projection optics. This deflection removes the energy dispersion and the energy-filtered electron beam forms an aberration-corrected, energy-filtered image that is magnified on a viewing screen.

Another embodiment pertains to a method for aberration correction and energy filtering when a cathode objective lens is used to image a specimen illuminated with an overlapping electron and UV or X-ray photon beam. The cathode objective lens forms an image with scattered or photoemitted electrons, and the beam of electrons scattered and emitted by the specimen is deflected with energy dispersion towards a first electron mirror. A knife-edge plate removes the electrons with energies higher and lower than the selected energy for imaging. The dispersed electron beam is focused by an electron lens at the reflection plane of the first electron mirror and reflected back so that the electron beam after a second deflection exits without energy dispersion. One or more aberrations of the image formed by the dispersion-free electron beam are corrected by a second electron mirror. The electron beam is deflected into the projection optics and magnified on a viewing screen.

Other embodiments are also disclosed.

DETAILED DESCRIPTION

While some of the above discussed prior apparatus and methods provide aberration correction and energy filtering, they do not allow for dual beam illumination. Other discussed prior apparatus and methods provide for dual beam illumination, however they do not allow for aberration correction, monochromatic illumination and energy filtered imaging. Improved apparatus and methods for providing aberration correction, energy filtering and dual beam illumination in a single LEEM/PEEM instrument are desirable. One distinct technique for providing aberration-corrected imaging, energy filtering and dual electron beam illumination with a monochromator in a single apparatus is disclosed herein and discussed below.

Unlike the prior technique of Rose et al. (FIG. 1a) employing a dispersion-free magnetic beam separator and an electron mirror for aberration correction, the presently disclosed apparatus and method does not require the separator to be free of dispersion in order to achieve aberration correction. In addition, the presently disclosed technique provides dual electron beam illumination which mitigates charging effects on insulating specimens, as well as monochromatic illumination, which further improves the spatial resolution as well as spectroscopic resolution for energy-filtered imaging.

Unlike the prior technique of Tromp (FIG. 1b) using two dispersive magnetic beam separators for aberration correction, the presently disclosed apparatus and method does not generate additional combination aberrations including image tilt and off-axis astigmatism in the projection optics which can affect the image quality when sub-nm resolution is needed. In addition, the presently disclosed apparatus and method does not require two identical beam separators to ensure that the dispersion and all combination aberrations of the first beam separator are cancelled by the second beam separator. Further, it provides dual electron beam illumination which mitigates charging effects on insulating specimens, as well as monochromatic illumination, which improves the spatial resolution as well as the spectroscopic resolution for energy-filtered imaging.

Unlike the prior technique of Mankos (2011, U.S. patent application Ser. No. 13/251,266,FIG. 1c) using one dispersive magnetic beam separator and two electron mirrors for aberration correction, the presently disclosed apparatus and method provides dual electron beam illumination which mitigates charging effects on insulating specimens, as well as monochromatic illumination, which improves the spatial resolution as well as the spectroscopic resolution for energy-filtered imaging.

Unlike the prior dual beam illumination techniques of Veneklasen and Adler (FIG. 1d), Mankos and Adler (FIG. 1e), Mankos and Munro (FIG. 1f), and Mankos (2012, U.S. Pat. No. 8,258,474,FIG. 1g), the presently disclosed apparatus and method provides aberration correction, which significantly improves obtainable spatial resolution, and energy filtering with monochromatic illumination, which further improves the spatial resolution as well as spectroscopic resolution for energy-filtered imaging.

A schematic layout of a LEEM/PEEM apparatus200, combining two independent illumination beams with an aberration corrector, energy filter and monochromator, is shown inFIG. 2. Such an apparatus200may be used to improve the spatial and spectroscopic resolution of LEEM/PEEM microscopes and to allow imaging insulating or composite (metal, semiconductor, insulator) specimens that otherwise charge up during electron beam illumination.

The illumination configuration ofFIG. 2has two branches, one for an imaging beam202(solid lines) and one for a charge balance beam204(dash lines), which are recombined by the main beam separator210, an energy dispersive element composed of an array of uniform magnetic fields of different length and strength so as to provide a mechanism for deflection and stigmatic focusing. The beam separators used here have a practical and proven design with rectangularly shaped coils. It is easy to manufacture and simplifies overall microscope alignment and tuning. Beam separator210simultaneously stigmatically images the incoming electron beam and deflects this beam by 90 degrees, i.e. the beam separator images as a conventional round lens while deflecting the incoming beam by 90 degrees, which greatly simplifies set-up, alignment and operation of the apparatus.

The illumination optics of the imaging beam202includes a second beam separator220that in combination with electron mirror222operates as a monochromator such as the one disclosed by M. Mankos in the U.S. Pat. No. 8,183,526, which is entitled “Mirror monochromator for charged particle beam apparatus” and issued on the 22ndof May, 2012. The monochromator is designed to reduce the electron energy spread of the imaging beam202to less than 0.1 eV.

The energy spread of electron sources used commonly in LEEM instruments, e.g. thermionic (W, LaB6) and thermally assisted (Schottky, ZrO2) field emission cathodes, is in the range of 0.5 to 2 eV. The electron source205, biased at high negative voltage, thus emits electrons with an energy spread ΔE of 0.5-2 eV. After the illumination optics206, the beam passes through the beam separator220, which deflects this beam into electron mirror222. The electrons202with nominal beam energy E0(solid lines) are deflected by 90 degrees, while electrons208with slightly lower energy (long dash lines) are deflected by a slightly larger angle and electrons209with slightly higher energy (dotted lines) are deflected by a slightly smaller angle, due to the energy dispersion of prism separator220. The axial bundle of rays with energies in the range (E0−ΔE, E0+ΔE) exiting beam separator220appears to emanate from a point near the center plane of the beam separator220, also known as its achromatic point221(plane). As the electrons proceed towards electron mirror222, a knife edge-shaped aperture224stops one portion of the energy distribution, e.g. the higher energy electrons209with energies E0+ΔE. The transfer optics226focuses the achromatic point221at the reflection surface223of electron mirror222, which is biased to a slightly more negative potential than the electron source205by a few hundred V to a few kV, and thus reflects the electrons back into beam separator220. As the remaining electrons proceed back to the beam separator, the lower energy electrons208with energies E0−ΔE are stopped by the same knife edge-shaped aperture. This arrangement allows the use of a simple knife edge224as the energy-selecting device, which is a much simpler and more reliable design when compared to the narrow, often sub-micrometer slits needed in typical monochromator applications. The remaining nearly monochromatic electrons202then enter the beam separator220which deflects this beam by 90 degrees back into the axis of the electron source and towards the main beam separator210. After the double pass through the monochromator formed by prism separator220and electron mirror222the dispersion of the monochromator vanishes due to the imposed mirror symmetry, which is desirable for high resolution imaging.

After exiting the beam separator220, the nearly monochromatic imaging beam202passes through transfer optics227and Einzel lens228, a three-electrode electrostatic lens with both outer electrodes at ground potential and the central electrode biased at a high negative potential. The nearly monochromatic imaging beam202then passes through transfer optics229and enters the main beam separator210, which deflects this beam toward a third beam separator230. After the 90 degree deflection by beam separator230, the dispersion of imaging beam202introduced by the 90 degree deflection in beam separator210vanishes due to the imposed symmetry which is desirable for high resolution imaging.

The charge balance beam204generated by electron source215, biased to a potential less negative by a few hundred volts than electron source205, is used to mitigate charging effects. The illumination optics216collimate the charge balance beam204into the main beam separator210, which deflects the beam by 90 degrees towards transfer optics229and Einzel lens228. The fraction of charge balance beam electrons204with nominal beam energy E0are deflected by 90 degrees, while electrons with slightly lower energy are deflected by a slightly larger angle and electrons with slightly larger energy are deflected by a slightly smaller angle, due to the energy dispersion of prism separator210. The axial bundle of electron rays with energies in the range (E0−ΔE, E0+ΔE) appears to emanate from a point near the center plane of the beam separator210, the achromatic plane211. Transfer optics229is configured to focus the achromatic plane at the reflection surface233of the Einzel lens228. The reflection is caused by the central electrode of Einzel lens228which is biased slightly more negative than the electron source215, and thus behaves like an electron mirror and reflects the charge balance beam electrons204back towards the beam separator210. The Einzel lens228is configured to reflect the incoming electrons symmetrically, i.e. electrons entering at an angle with respect to the normal to the reflection surface233exit the Einzel lens228at the same angle and symmetrically with respect to the normal to the reflection surface233. As the charge balance beam204proceeds back to the prism separator210, the axial bundle of electron rays with energies in the range (E0−ΔE, E0+ΔE) is focused by transfer lens229back at the achromatic plane211of beam separator210and deflected towards third beam separator230. The charge balance beam204has lower kinetic energy while passing through magnetic beam separator210when compared to imaging beam202and is thus deflected by a larger angle. Transfer lens235focuses charge balance beam204emanating from achromatic plane211of beam separator210at the achromatic plane231of beam separator230. This ensures that after the next 90 degree deflection by beam separator230, both imaging beam202and charge control beam204are again coaxial.

After the deflection by beam separator210and focusing by transfer lens235, both beams are deflected by a third beam separator230that passes both beams through cathode objective lens240. After the 90 degree deflection by beam separator230the dispersion of imaging beam202introduced by main beam separator210vanishes due to the imposed symmetry which is desirable for high resolution imaging. In objective lens240the electrons are decelerated from the transport beam energy of a few tens of keV and focused to form parallel flood beams illuminating a specimen250. The electrons scattered and emitted by specimen250are accelerated and focused by objective lens240to form a magnified, two-dimensional image that is blurred by the aberrations of objective lens240. The scattered and emitted electron beam then enter beam separator230and get deflected towards symmetry electron mirror260. The strength of beam separator230is adjusted so that scattered imaging beam electrons202with nominal energy E0are deflected by 90 degrees and enter along the axis of electron mirror260, while scattered electrons202a(long dashed lines) with slightly lower energy E0−δE1are deflected by a slightly larger angle and electrons202b(dotted lines) with slightly larger energy E0+δE2are deflected by a slightly smaller angle, due to the energy dispersion of beam separator230. The axial bundle of electron rays with energies in the range (E0−δE1, E0+δE2) appears to emanate from a point near the center plane of beam separator230, the achromatic plane231. Transfer lens262is configured to focus the achromatic plane231at the reflection surface261of the electron mirror260, which is biased more negative than specimen250by few hundred V to a few kV, and thus reflects the electron rays202back towards beam separator230. The charge balance beam204has much lower kinetic energy while passing through magnetic beam separator230and is thus deflected by a larger angle and strikes knife edge aperture264. The charge balance electrons204as well as any secondary electrons generated by the specimen can be stopped by a knife edge aperture264so as to not blur the image contrast formed by the scattered imaging beam202. Knife edge aperture264in combination with beam separator230and electron mirror260selects electrons with nominal energy E0and thus operates as an imaging energy filter, such as the one described by M. Mankos in U.S. Pat. No. 8,334,508, which is entitled “Mirror energy filter for electron beam apparatus” and issued on the 18th of December, 2012. The nominal energy E0selected for imaging can be adjusted in the range from near 0 eV (charge balance electrons) to the electron landing energy (elastically backscattered electrons) by tuning the strength of beam separator230. The width of the energy range (and thus the energy resolution of the final filtered image) can be selected by adjusting the position of knife edge aperture264. This arrangement utilizes a simple knife-edge plate264as the energy selecting device, which is much simpler when compared to the narrow, often sub-micrometer slits needed in typical imaging energy filter applications. The mechanical design and manufacture of a knife-edge plate is much less complex when compared with a narrow slit aperture with straight and parallel edges and thus allows selection of a much narrower energy distribution. In this design, an energy width of 0.1 eV or less is achievable. In addition, the reliability of operation of a knife-edge plate under heavy electron bombardment is much improved when compared to slit apertures, as sub-micrometer slits tend to clog with electron-beam induced contamination.

The scattered electrons202include electrons with a range of energies.FIG. 3depicts the energy distribution300of the scattered electron beam202leaving the specimen250. The scattered electron beam202includes electrons emitted over a range of solid angles with energies ranging from near zero (secondary electrons302) to the landing energy (elastically transmitted or backscattered electrons308). Electrons with intermediate energies include Auger electrons304and also electrons that suffered plasmon losses306. Each group of electrons can be used advantageously to form images of specimen250with different contrast by utilizing an energy filter which selects a narrow portion of the electron energy spectrum for imaging.

Coming back toFIG. 2, electron mirror260is configured to reflect the incoming electrons symmetrically, i.e. electrons entering at an angle with respect to the normal to the electron mirror reflection surface261exit the mirror at the same angle and symmetrically with respect to the normal to the electron mirror reflection surface261of electron mirror260. Electron rays contained in scattered imaging beam202are refocused by transfer lens262at the achromatic plane231of magnetic beam separator230and deflected towards transfer lens272which focuses the scattered imaging beam202into second electron mirror270that is configured as an aberration corrector. After the second deflection by beam separator230the dispersion vanishes due to the imposed symmetry, which is a prerequisite for aberration correction. It is critical to remove the energy dispersion of the beam prior to entering electron mirror270, as otherwise combination aberrations due to the dispersion of beam separator230and the chromatic and spherical aberrations of the electron mirror270are introduced that can be larger than the original aberrations and thus preclude the desired aberration correction.

Electron mirror270is configured to correct for one or more aberrations of cathode objective lens240, and reflects the electron beam back towards beam separator230. The aberration-corrected imaging beam202is refocused by transfer lens272at the achromatic plane231of beam separator230and deflected towards main beam separator210. After a third 90 degree deflection through magnetic beam separator230, aberration-corrected imaging beam202becomes again energy-dispersed. Transfer lens235focuses aberration-corrected imaging beam202emanating from achromatic plane231of beam separator230at the achromatic plane211of beam separator210. This ensures that after the next 90 degree deflection by beam separator210the dispersion of aberration-corrected imaging beam202vanishes due to the imposed symmetry which is desirable for high resolution imaging in the projection optics280.

After the final deflection by beam separator210the aberration-corrected imaging beam202exiting beam separator210is transported into the projection optics280and magnified on a viewing screen290. The detection may be performed by a CCD camera detector or other detection system.

FIG. 4is a flow chart depicting a method400for imaging of a specimen using aberration correction, energy filtering, and dual beam illumination with a monochromator in accordance with an embodiment of the invention. This method400may use the structure200described above in relation toFIG. 2.

An imaging electron beam with nominal energy E0and energy width ΔE is generated402by a first electron gun. The energy spread of the imaging beam is reduced by passing it through a monochromator404. After exiting the monochromator, the nearly monochromatic imaging beam passes through a negatively biased electrostatic lens406, due to the fact that it has kinetic energy that is higher than the potential energy of the negative electrostatic lens.

A second, charge balance electron beam with a nominal energy a few hundred to a few thousand electron Volts lower than the imaging electron beam is generated410by a second electron gun. The charge balance electron beam is deflected412towards negatively biased electrostatic lens, and reflected 414 off negatively biased electrostatic lens due to the fact that it has kinetic energy that is smaller than the potential energy of the electrostatic lens.

The low- and high-energy components are joined to form coaxial electron beams and deflected420by a magnetic beam separator towards an electron transfer lens. During this deflection420, energy dispersion is introduced in the two beam components. In other words, the two components are bent by different bending angles, due to the difference in electron energy. The transfer lens focuses422both electron beams emanating from the achromatic plane of first beam separator210into the achromatic plane of the next beam separator230. This ensures that after the next 90 degree deflection by beam separator230, both imaging beam202and charge balance beam204are again coaxial, i.e. the energy dispersion is cancelled.

The imaging and charge balance beam components are deflected424by a magnetic beam separator towards the objective lens and form coaxial electron beams. Using the objective lens, the imaging and charge balance beam components are decelerated, collimated and focused426to illuminate an area of the specimen. Impingement of the two electron beam components onto the specimen area generates scattered electron beams.

The scattered electron beams are bent428by a beam separator away from the illumination system and instead towards first electron mirror that in conjunction with the beam separator operates as an energy filter. The electron beam with energy Esselected for imaging is deflected by 90 degrees to introduce dispersion according to the electron energy. The beam separator deflects the high-energy component of the beam at less of an angle in comparison to its deflection of the low-energy component of the beam, such that the higher and lower energy electron-beam components exit the beam separator at different angles of trajectory. By adjusting the strength of one or more sectors in the beam separator, the selected electron energy Escan be tuned over a range of values covering the full range of the energy distribution of the scattered electron beam leaving the specimen.

One or more lenses are used to focus the achromatic plane of the beam separator, located near its center, at the reflection surface430of the electron mirror. The electron beams are reflected in this electron mirror symmetrically, i.e. electrons entering at an angle with respect to the normal to the electron mirror reflection surface exit the mirror at the same angle and symmetrically with respect to the normal to the electron mirror reflection surface. Due to the symmetry introduced by this electron mirror, the energy dispersion introduced by the previous 90 degree deflection is canceled by the dispersion introduced by the next 90 degree deflection.

When energy filtering is desired440, a knife-edge plate is inserted445between the beam separator and electron mirror into the beam path which removes one portion of the beam (either the beam with energy larger than selected energy Esor with energy lower than selected energy Es) during the path towards the electron mirror. The portion of the beam except the beam with selected energy Esis removed by the same knife-edge plate during the beam path from the electron mirror towards the beam separator. When energy filtering is not needed, the knife edge aperture is removed from the beam path.

The electron beam reflected by the electron mirror is then deflected450by 90 degrees into a mirror aberration corrector. Due to the symmetry introduced by the electron mirror, the energy dispersion introduced by second deflection450cancels the dispersion introduced by first deflection428. The dispersion-free electron beam is then reflected in the second electron mirror which is configured to correct one or more aberrations of the objective lens.

After the reflection, electrons are directed towards the magnetic beam separator and deflected452by 90 degrees towards the electron transfer lens. During this deflection420, energy dispersion is introduced in the electron beam. In other words, the beam components with different kinetic energies are bent by different bending angles, due to the difference in electron energy. The transfer lens is already focused422so that electron beams emanating from the achromatic plane of first beam separator210are focused into the achromatic plane of the next beam separator230. This means that the transfer lens also focuses the electron beam emanating from the achromatic plane of beam separator230into the achromatic plane of the beam separator210. Beam separator210deflects the electron beam by 90 degrees454towards the projection optics and introduces energy dispersion which cancels the energy dispersion due to the previous 90 degree beam deflection by beam separator230. The projection optics forms456a magnified, aberration-corrected and (if elected) energy-filtered image on a viewing screen.

The above disclosed apparatus200combining two independent illumination beams with an aberration corrector, energy filter and monochromator, as shown inFIG. 2, can be advantageously operated in PEEM mode. The aberration corrector removes the spherical and chromatic aberrations that deteriorate the spatial resolution of conventional PEEM instruments, the energy filter allows for energy-selective imaging, and the charge balance beam allows investigation of insulating specimens, which otherwise charge up under UV or X-ray photon illumination.

FIG. 5is a diagram depicting one implementation of the photoemission electron microscopy mode500utilizing apparatus200comprising further an X-ray or UV source505. In this microscope500, the flood beam of photons510generated by X-ray or UV source505illuminates specimen250and generates a beam of photoemission electrons520(solid line) with a range of kinetic energies. Photoemission electrons520are accelerated by cathode objective lens240and form a magnified image blurred by the aberrations of the cathode objective lens240.

In this photoemission electron microscopy mode500, the charge balance beam504is generated by electron source215, biased to a potential within a few volts of the specimen250. The charge balance beam504is used to mitigate charging effects caused by the X-ray or UV photon illumination510. The illumination optics216collimate the charge balance beam504into the main beam separator210, which deflects the beam by 90 degrees towards transfer optics229and Einzel lens228. Transfer optics229is configured to focus the achromatic plane at the reflection surface233of the Einzel lens228. The reflection is caused by the central electrode of Einzel lens228which is biased by a few hundred volts to kilovolts more negative than the electron source215, and thus behaves like an electron mirror and reflects the charge balance beam electrons504back towards the prism separator210. The Einzel lens228is configured to reflect the incoming electrons symmetrically, i.e. electrons entering at an angle with respect to the normal to the electron mirror reflection surface233exit the Einzel lens228at the same angle and symmetrically with respect to the normal to the electron mirror reflection surface233. As the charge balance beam504proceeds back to the prism separator210, the axial bundle of electron rays with energies in the range (E0−ΔE, E0+ΔE) is focused by transfer lens229back at the achromatic plane211of beam separator210and deflected towards toward third beam separator230. The charge balance beam504has lower kinetic energy while passing through magnetic beam separator210when compared to photoemission electron beam520passing through magnetic beam separator230and is thus deflected by a larger angle. Transfer lens235focuses charge balance beam504emanating from achromatic plane211of beam separator210at the achromatic plane231of beam separator230. This ensures that after the next 90 degree deflection by beam separator230, the charge control beam504enters the objective lens240along its symmetry axis.

After the deflection by beam separator210and focusing by transfer lens235, the charge balance beam504is deflected by beam separator230towards objective lens240. Objective lens240decelerates the electrons from the transport beam energy of a few tens of keV and focuses to form a parallel flood beam illuminating a specimen250. A fraction of the charge balance beam is absorbed and balances the charge build-up formed by the emission of photoemission electrons520.

The photoemission electron beam520as well as the reflected portion of charge balance beam504then enter beam separator230and are deflected towards symmetry electron mirror260. The strength of beam separator230is adjusted so that photoelectrons520with nominal energy Esselected for imaging are deflected by 90 degrees and enter along the axis of electron mirror260, while photoemission electrons520a(long dashed lines) with lower energy Es−δE1are deflected by a larger angle and photoemission electrons520b(dotted lines) with larger energy Es+δE2are deflected by a slightly smaller angle, due to the energy dispersion of beam separator230. Charge balance electrons have the lowest kinetic energy Eminand are deflected by the largest angle. The axial bundle of electron rays with energies in the range (Emin, Es+δE2) appears to emanate from a point near the center plane of beam separator230, also known as the achromatic plane231. Transfer lens262is configured to focus the achromatic plane231at the reflection surface261of the electron mirror260, which is biased more negative than specimen250by few hundred V to a few kV, and thus reflects the photoemission electron rays520back towards beam separator230. The mirror beam504has much lower kinetic energy while passing through magnetic beam separator230and is thus deflected by a larger angle and strikes knife edge aperture264. The charge balance beam electrons504as well as any electrons except a small range of selected energies near selected energy Esgenerated by the specimen can be stopped by a knife edge aperture264so as to not blur the image contrast formed by the electrons with selected energy Es. Knife edge aperture264in combination with beam separator230and electron mirror260selects electrons with nominal energy Esfor imaging and thus operates as an imaging energy filter. The nominal energy Esselected for imaging can be adjusted in the range from near 0 eV (charge balance electrons) to the maximum electron energy (corresponding to the maximum photon energy) by tuning the strength of beam separator230. The width of the energy range (and thus the energy resolution of the final filtered image) can be selected by adjusting the position of knife edge aperture264.

The photoemission electrons520include electrons with a range of energies. The energy spectrum is composed of a set of peaks which correspond to the elements present in the specimen250, and the shape of the individual peaks is determined by the electronic and bonding states of the elements. When a single peak is used for imaging, a map of the element location on the specimen can be generated.

Electron mirror260is configured to reflect the incoming photoemission electrons520symmetrically, i.e. electrons entering at an angle with respect to the normal to the electron mirror reflection surface261exit the mirror at the same angle and symmetrically with respect to the normal to the electron mirror reflection surface261of electron mirror260. Electron rays contained in photoemission electron beam520are refocused by transfer lens262at the achromatic plane231of magnetic beam separator230and deflected towards transfer lens272which focuses photoemission electron beam520into second electron mirror270that is configured as an aberration corrector. After the second deflection by beam separator230the energy dispersion vanishes due to the imposed symmetry, which is a prerequisite for aberration correction. It is critical to remove the energy dispersion of the beam prior to entering electron mirror270, as otherwise combination aberrations due to the dispersion of beam separator230and the chromatic and spherical aberrations of the electron mirror270are introduced that can be larger than the original aberrations and thus preclude the desired aberration correction.

Electron mirror270is configured to correct for one or more aberrations of cathode objective lens240, and reflects the photoemission electron beam520back towards beam separator230. The aberration-corrected photoemission electron beam520is refocused by transfer lens272at the achromatic plane231of beam separator230and deflected towards main beam separator210. After a third 90 degree deflection through magnetic beam separator230, aberration-corrected photoemission electron beam520becomes again energy-dispersed. Transfer lens235focuses aberration-corrected photoemission electron beam520emanating from achromatic plane231of beam separator230at the achromatic plane211of beam separator210. This ensures that after the next 90 degree deflection by beam separator210the dispersion of aberration-corrected photoemission electron beam520vanishes due to the imposed symmetry which is desirable for high resolution imaging in the projection optics280.

After the final deflection by beam separator210the aberration-corrected photoemission electron beam520exiting beam separator210is transported into the projection optics280and magnified on a viewing screen290. The detection may be performed by a CCD camera detector or other detection system.

FIG. 6is a diagram depicting another implementation of a photoemission electron microscopy mode600utilizing apparatus200comprising further an X-ray or UV source505. In this microscopy mode600, the flood beam of photons510generated by X-ray or UV source505illuminates specimen250and generates a beam of photoemission electrons620(solid line) with a range of kinetic energies. Photoemission electrons620are accelerated by cathode objective lens240and form a magnified image blurred by the aberrations of the cathode objective lens240.

In this photoemission electron microscopy mode600, the charge balance beam602is generated by electron source205, biased to a potential within a few volts of the specimen250. The charge balance beam602is used to mitigate charging effects caused by the X-ray or UV photon illumination510. After the illumination optics206, the charge balance beam602passes through the beam separator220, which deflects this beam into electron mirror222. The electrons602with nominal beam energy E0(solid lines) are deflected by 90 degrees, while electrons608with slightly lower energy (long dash lines) are deflected by a slightly larger angle and electrons609with slightly higher energy (dotted lines) are deflected by a slightly smaller angle, due to the energy dispersion of beam separator220. The axial bundle of rays with energies in the range (E0−ΔE, E0+ΔE) exiting beam separator220appears to emanate from a point near the center plane of the beam separator220, also known as its achromatic point221(plane). As the electrons proceed towards electron mirror222, a knife edge-shaped aperture224stops one portion of the energy distribution, e.g. the higher energy electrons609with energies E0+ΔE. The transfer optics226focuses the achromatic point221at the reflection plane223of electron mirror222, which is biased to a slightly more negative potential than the electron source205by a few hundred V to a few kV, and thus reflects the electrons back into the beam separator220. As the remaining electrons proceed back to the beam separator, the lower energy electrons608with energies E0−ΔE are stopped by the same knife edge-shaped aperture. The remaining nearly monochromatic electrons602then reenter the beam separator220which deflects this beam by 90 degrees back into the axis of the electron source and towards the main beam separator210. The energy spread of charge balance beam602is thus significantly reduced, which is advantageous for improving the charge balance equilibrium. After the double pass through the monochromator formed by beam separator220and electron mirror222, the energy dispersion of this monochromator vanishes due to the imposed mirror symmetry, which is desirable for high resolution imaging.

After exiting the beam separator220, the nearly monochromatic charge balance beam602passes through transfer optics227and electrostatic Einzel lens228, which in this case is turned off, i.e. all three electrodes of electrostatic Einzel lens228are at ground potential. The nearly monochromatic charge balance beam602then passes through transfer optics229and enters the main beam separator210, which deflects this beam toward a third beam separator230. After the 90 degree deflection by beam separator230, the dispersion of charge balance beam602introduced by the 90 degree deflection in beam separator230vanishes due to the imposed symmetry which is desirable for high resolution imaging.

The charge balance beam602has lower kinetic energy while passing through magnetic beam separator210and230when compared to photoemission electron beam620passing through magnetic beam separator230and is thus deflected by a larger angle. Transfer lens235focuses charge balance beam602emanating from achromatic plane211of beam separator210at the achromatic plane231of beam separator230. This ensures that after the next 90 degree deflection by beam separator230, charge balance beam602enters the objective lens240along its symmetry axis.

After the deflection by beam separator210and focusing by transfer lens235, the charge balance beam602is deflected by beam separator230towards objective lens240. Objective lens240decelerates the electrons from the transport beam energy of a few tens of keV and focuses to form a parallel flood beam illuminating a specimen250. A fraction of the charge balance beam is absorbed and balances the charge build-up formed by the emission of photoemission electrons620.

The photoemission electron beam620as well as the reflected portion of charge balance beam602then enter beam separator230and are deflected towards symmetry electron mirror260. The strength of beam separator230is adjusted so that photoemission electrons620with nominal energy Esselected for imaging are deflected by 90 degrees and enter along the axis of electron mirror260, while photoemission electrons620a(long dashed lines) with lower energy Es−δE1are deflected by a larger angle and photoemission electrons620b(dotted lines) with larger energy Es+δE2are deflected by a smaller angle, due to the energy dispersion of beam separator230. Charge balance electrons have the lowest kinetic energy Eminand are deflected by the largest angle. The axial bundle of electron rays with energies in the range (Emin, Es+δE2) appears to emanate from a point near the center plane of beam separator230, also known as the achromatic plane231. Transfer lens262is configured to focus the achromatic plane231at the reflection plane261of the electron mirror260, which is biased more negative than specimen250by few hundred V to a few kV, and thus reflects the photoemission electron beam620back towards beam separator230. The charge balance beam602has much lower kinetic energy while passing through magnetic beam separator230and is thus deflected by a larger angle and strikes knife edge aperture264. The charge balance beam electrons602as well as any electrons except a small range of selected energies near selected energy Esgenerated by the specimen can be stopped by a knife edge aperture264so as to not blur the image contrast formed by the electrons with selected energy Es. Knife edge aperture264in combination with beam separator230and electron mirror260selects photoemission electrons with nominal energy Esfor imaging and thus operates as an imaging energy filter. The nominal energy Esselected for imaging can be adjusted in the range from near 0 eV (charge balance electrons) to the maximum electron energy (corresponding to the photon energy) by tuning the strength of beam separator230. The width of the energy range (and thus the energy resolution of the final filtered image) can be selected by adjusting the position of knife edge aperture264.

The photoemission electrons620include electrons with a range of energies. The energy spectrum is composed of a set of peaks which correspond to the elements present in the specimen250, and the shape of the individual peaks is determined by the electronic and bonding states of the elements. When a single peak is used for imaging, a map of the element distribution on the specimen can be generated.

Electron mirror260is configured to reflect the incoming electrons symmetrically, i.e. electrons entering at an angle with respect to the normal to the electron mirror reflection surface261exit the mirror at the same angle and symmetrically with respect to the normal to the electron mirror reflection surface261of electron mirror260. Electron rays contained in photoemission electron beam620are refocused by transfer lens262at the achromatic plane231of magnetic beam separator230and deflected towards transfer lens272which focuses photoemission electron beam620into second electron mirror270that is configured as an aberration corrector. After the second deflection by beam separator230the energy dispersion vanishes due to the imposed symmetry, which is a prerequisite for aberration correction. It is critical to remove the energy dispersion of the beam prior to entering electron mirror270, as otherwise combination aberrations due to the dispersion of beam separator230and the chromatic and spherical aberrations of the electron mirror270are introduced that can be larger than the original aberrations and thus preclude the desired aberration correction.

Electron mirror270is configured to correct for one or more aberrations of cathode objective lens240, and reflects the photoemission electron beam620back towards beam separator230. The aberration-corrected photoemission electron beam620is refocused by transfer lens272at the achromatic plane231of beam separator230and deflected towards main beam separator210. After a third 90 degree deflection through magnetic beam separator230, aberration-corrected photoemission electron beam620becomes again energy-dispersed. Transfer lens235focuses aberration-corrected photoemission electron beam620emanating from achromatic plane231of beam separator230at the achromatic plane211of beam separator210. This ensures that after the next 90 degree deflection by beam separator210the dispersion of aberration-corrected photoemission electron beam620vanishes due to the imposed symmetry which is desirable for high resolution imaging in the projection optics280.

After the final deflection by beam separator210the aberration-corrected photoemission electron beam620exiting beam separator210is transported into the projection optics280and magnified on a viewing screen290. The detection may be performed by a CCD camera detector or other detection system.

FIG. 7is a flow chart depicting a method700for imaging of a specimen with photoemission electrons using an aberration corrector, energy filter and a charge balance electron beam in accordance with an embodiment of the invention. This method700may use the photoemission modes500and600utilizing structure200described above in relation toFIGS. 5 and 6.

A photon beam is generated702by a UV or X-ray source. A charge balance electron beam is generated704by an electron gun. When low energy spread of the charge balance beam is desired710, the beam is passed through a monochromator which removes714portions of the beam with larger and smaller energy than the nominal beam energy. The charge balance electron beam is deflected716towards the objective lens.

Both the photon and charge balance beams illuminate720the specimen, and the objective lens forms an image with the photoemission electrons emitted by the specimen. The photoemission electron beam and the reflected portion of the charge balance beam are deflected722by a beam separator away from the illumination system and instead towards first electron mirror that in conjunction with the beam separator operates as an energy filter. The electron beam with energy Esselected for imaging is deflected by 90 degrees to introduce dispersion according to the electron energy. The beam separator deflects the high-energy component of the beam at less of an angle in comparison to its deflection of the low-energy component of the beam, such that the higher and lower energy electron-beam components exit the beam separator at different angles of trajectory. By adjusting the strength of one or more sectors in the beam separator, the selected electron energy Escan be tuned over a range of values covering the full range of the energy distribution of the photoemission electron beam emitted by the specimen.

One or more lenses are used to focus the achromatic plane of the beam separator, located near its center, at the reflection surface730of the electron mirror. The photoemission and charge balance electron beam are reflected in this electron mirror symmetrically, i.e. electrons entering at an angle with respect to the normal to the electron mirror reflection surface exit the mirror at the same angle and symmetrically with respect to the normal to the electron mirror reflection surface. Due to the symmetry introduced by this electron mirror, the energy dispersion introduced by the previous 90 degree deflection is canceled by the dispersion introduced by the next 90 degree deflection.

When energy filtering is desired740, a knife-edge plate is inserted745between the beam separator and electron mirror into the beam path which removes one portion of the beam (either the beam with energy larger than selected energy Esor with energy lower than selected energy Es) during the path towards the electron mirror. The portion of the beam except the beam with selected energy Esis removed by the same knife-edge plate during the beam path from the electron mirror towards the beam separator. When energy filtering is not needed, the knife edge aperture is removed from the beam path.

The photoemission electron beam reflected by the electron mirror is then deflected750by 90 degrees into a mirror aberration corrector. Due to the symmetry introduced by the electron mirror, the energy dispersion introduced by second deflection750cancels the dispersion introduced by first deflection722. The dispersion-free electron beam is then reflected in the second electron mirror which is configured to correct one or more aberrations of the objective lens.

After the reflection, electrons are directed towards the magnetic beam separator and deflected752by 90 degrees towards the electron transfer lens. During this deflection752, energy dispersion is introduced in the electron beam. The transfer lens is already focused752so that electron beams emanating from the achromatic plane of beam separator210are focused into the achromatic plane of the next beam separator230. This means that transfer lens also focuses electron beam emanating from the achromatic plane of beam separator230into the achromatic plane of the beam separator210. Beam separator210deflects the electron beam by 90 degrees754towards the projection optics and introduces energy dispersion which cancels the energy dispersion due to the previous 90 degree beam deflection752by beam separator230. The projection optics forms756a magnified, aberration-corrected and energy-filtered photoemission image on a viewing screen.

CONCLUSION

A combined aberration corrector for electron lenses, electron energy filter, and monochromator with dual beam illumination comprising energy-dispersive beam separators and electron mirrors is disclosed herein. Advantageously, the above-disclosed technique allows the microscope user to simultaneously obtain aberration-corrected and energy-filtered images of specimens illuminated with two electron beams.

In accordance with certain embodiments of the invention, the aberration corrector provides the opportunity to improve the spatial resolution of images acquired by an electron microscope. Hence, more detailed information about the local structure and morphology is obtainable in electron microscopes used to characterize specimens.

In accordance with certain embodiments of the invention, the energy filter provides the opportunity to use scattered electrons with a very narrow range of energies for the formation of an image of the specimen. Hence, detailed information about the chemical composition, interatomic bonding and local electronic states is obtainable in electron microscopes used to characterize specimens.

In accordance with certain embodiments of the invention, the use of two illuminating electron beams provides the opportunity to image insulating specimens which charge up under single beam illumination. Hence, higher spatial resolution imaging is obtainable in electron microscopes used to characterize insulating specimens.

In accordance with certain embodiments of the invention, the monochromator provides the opportunity to illuminate specimens with an electron beam with a narrow energy spread, resulting in improved spatial and spectroscopic resolution. Hence, higher spatial resolution imaging is obtainable in electron microscopes used to characterize specimens.