Patent Description:
The applicant has realized charged-particle multi-beam apparatuses, which incorporate one or more electromagnetic lenses of the mentioned type, and developed the corresponding charged-particle optical components, pattern definition devices, and writing methods for multiple beams, and commercialized a <NUM> keV electron multi-beam writer called eMET (electron Mask Exposure Tool) or MBMW (multi-beam mask writer), used to realize arbitrary photomasks for <NUM> immersion lithography, as well as masks for EUV lithography and templates for nanoimprint lithography. The applicants system has also been called PML2 (Projection Mask-Less Lithography) for electron beam direct writer (EBDW) applications on substrates.

For increasing throughput in high-volume industrial manufacturing, with particular regard for mask-less lithography and direct-writing on substrates (e.g. wafers), there is the need to increase the electrical current carried by the charged-particle beam passing through the charged-particle nano-pattering apparatus; this is usually at the cost of limiting the resolution due to Coulombic interactions between the charged particles and will require a corresponding compensation by reduction of the magnitude of the optical aberrations introduced by the apparatus through other mechanisms. To this end, the applicant has developed a charged-particle multi-beam apparatus consisting of multiple parallel optical columns combined in a multi-column fashion, each column having a reduced ("slim") cross-section diameter. Such a multi-beam apparatus, of which one embodiment is discussed below referring to <FIG>, enables significantly larger charged-particle beam electrical currents, while overcoming the limitations due to the trade-off between electrical current and optical aberrations found in single-column systems. This is due to the fact, that the total current delivered to the target is split into multiple optical axes, while the resolution limitation is dominated by the amount of current per optical axis. Single columns of this type are well known in prior art, such as <CIT>, <CIT> (= <CIT>) and <CIT> (= <CIT>) of the applicant.

A typical multi-column system includes multiple optical sub-columns, each of which comprises an illuminating system that delivers a broad telecentric charged-particle beam to a pattern definition system followed by a charged-particle projection optics, which includes, e.g., a number of electrostatic and/or electromagnetic lenses. For using such a system as a high-throughput wafer-direct-writer it will be necessary to place a substantial number of sub-columns above one semiconductor wafer, e.g. in the order of <NUM> sub-columns. This, however, requires that each sub-column has a diameter which is a fraction of the width of the wafer, for instance in the case of a <NUM> (<NUM>") wafer, a diameter of <NUM> or below. Slim-diameter magnetic lenses, on the other hand, cannot be realized by coil-based magnetic lenses for the generation of the desired magnetic field, because reduction of the column diameter would correspond to extremely large Joule heating due to the large electrical currents needed to operate the coils for generating sufficiently strong magnetic fields. Due to the tight space requirements, there is insufficient space for an adequate temperature-control system including high-precision sensors and isotropic and homogeneous cooling, which would be required for conventional coil-based-magnetic lenses. Additionally, the tight space requirements, which result from the target diameters of the slim columns and their arrangement in a suitable multi-column system, would hinder fabrication of a suitable coil-based magnetic lens. The mentioned limitations driven by heat and geometrical requirements are severe, but can be overcome by employing magnetic lenses based on permanent magnets within a high-permeability housing body for generating the magnetic field, such as the possible embodiments of the present invention. However, such permanent-magnet based systems cannot be recalibrated after completion of manufacturing and assembly processes, and this represents a serious disadvantage with respect to coil-based magnetic lenses, whose magnetic field can be controlled by re-adjusting the electrical current passing through the coils, especially given inherent limitations on the precision of the targeted magnetic field due to manufacturing and assembly accuracies. Current precision limitations correspond to a deviation of approximately ±<NUM> % to the targeted magnetic field, which cannot be compensated for without including some additional components that allow for compensating the lack of required magnetic field precision.

Electromagnetic lenses based on permanent magnets and high-permeability housing bodies combined with electrostatic elements for fine-adjustment are known in prior art, such as <CIT>, which however does not incorporate high-precision adjustment means such as multipoles, nor charged-particle-collecting calibration apertures, hence is unable to provide the means of altering the beam shape, calibrating its positions and generating a desired pattern. <CIT> discloses a charged-particle beam lithogarphiy apparatus including a doublet magnetic lens, composed of an upper magnetic lens and a lower magnetic lens which are mutually separate and combined with an electrostatic lens. <CIT> discloses a permanent-magnet-based electromagnetic lens combined with a coil-based magnetic lens for fine adjustment, which has the above-mentioned heating and geometrical problems. Furthermore, the magnetic field of the above state-of-the art magnetic lenses is insufficiently confined to the space of the lens itself, causing severe cross-effects in the case of a number of lenses arranged side by side.

In view of the above, it is an object of the present invention to provide an electromagnetic lens which includes permanent magnets but allows for adjusting the optical properties (e.g. focal length) with high precision. At the same time, the effect of stray magnetic field shall be limited.

The above object is met by an electromagnetic lens configured to modify (e.g. shape, focus/defocus or otherwise manipulate) a charged-particle beam of a charged-particle optical apparatus, the lens being provided with a passage opening extending along a longitudinal axis and allowing passage of the charged-particle beam, the lens comprising a magnetic circuit assembly comprising at least one ring magnet and a yoke body and a sleeve insert, the sleeve insert surrounding the passage opening and extending between a first end and a second end thereof along the longitudinal axis, wherein the sleeve insert comprises one or more electrically conductive electrode elements, preferably at least two, which are configured to be applied respective electric potentials (with respect to the electric potential of the housing, which is identified with a ground potential) so as to generate an electrostatic field within the passage opening, and the yoke body comprising an outer yoke shell and an inner yoke shell, arranged circumferential around the longitudinal axis and comprising a magnetic permeable material, wherein the inner yoke shell is arranged surrounding at least a central portion of the sleeve insert, and the outer yoke shell surrounds the inner yoke shell and the sleeve insert, with the ring magnet(s) arranged circumferentially around the inner yoke shell and arranged between the inner and outer yoke shells, the ring magnet(s) comprising a permanent magnetic material being magnetically oriented with its two magnetic poles towards the inner yoke shell and the outer yoke shell, respectively, wherein in the magnetic circuit assembly, the inner yoke shell, the at least one ring magnet, and the outer yoke shell form a closed magnetic circuit but having at least two gaps, located at an axial end of the inner yoke shell towards a respectively corresponding (inner surface) portion of the outer yoke shell, configured to generate a defined magnetic field reaching inwards into the passage opening and spatially overlapping with the electrostatic field generated by the electrode elements of the sleeve insert.

In many embodiments, the sleeve insert comprises at least two electrically conductive electrode elements, where the electric potential applied to these element may be defined with regard to the housing or another external component, and/or with respect to each other.

Thus, the electromagnetic lens according to the invention will include: (<NUM>) A magnetic circuit assembly for the generation of a static magnetic field which can exert a lensing effect on the charged-particle beam propagating through the electromagnetic lens. The magnetic circuit assembly includes one or more cylindrical permanent magnets, which preferably are stacked along and concentric to the optical axis of the system, and accommodated within a housing body made of high-permeability magnetic material, configured to direct magnetic field lines into targeted sections of the optical axis, i.e. defined sections of the passage opening, in particular sections corresponding to the two or more gaps provided in the housing body. (<NUM>) An electrostatic inlay, provided for the generation of an electric field that can be used to fine-adjust the lensing effect of the electromagnetic lens on the traversing charged-particle beam, and furthermore, where desired, for altering the shape and deflection with respect to the optical axis of the particle beam and possibly also for modifying optical aberrations introduced by the electromagnetic lens itself and/or the charged-particle beam optical apparatus into which the electromagnetic lens is to be incorporated. Furthermore, for operating the electromagnetic lens, a cabling system may be used for interconnecting the elements of the electrostatic inlay, such as the liners and the rods of the multipoles, to external power supply units for individual voltage adjustment.

The inlay according to the invention enables the generation of an electrostatic field superposing to the aforementioned magnetic field, which allows to compensate for the discussed deviation of the magnetic field from design target, by allowing an in-situ fine-adjustment of the focal-length of the lens with high precision, for instance in the order of <NUM> ppm or below, as well as control of specific properties of the charged-particle beam such as shape and optical aberrations. Accordingly, the invention significantly facilitates layout, construction, fine-adjustment and controlling of writer tools, and in particular of a multi-column multi-beam mask-writer tool.

In contrast to <CIT>, the electromagnetic lens of the invention has a magnetic loop which is completely closed except for a number of air gaps in the housing body, which allow to focus the magnetic field at desired regions of the optical axis, thus strongly reducing the degrading influence of stray fields on the performance of the electromagnetic lens as employed in charged-particle multi-beam nano-patterning apparatuses. This fact can be readily understood in view of Ampere's circuital law. Ampere's law states that the line-integral of the magnetic field around a closed curve is proportional to the total electrical current flowing across a surface enclosed by such a closed curve, which equals zero in a current-free system, as is the case here by virtue of using permanent magnets. Therefore, by employing (at least) two gaps towards the optical axis and by layout generating two (or, multiple) spatially sharply confined magnetic fields of opposing direction in axial direction, acting as magnetic lenses in those regions, the layout enables to fulfill Ampere's law without incurring additional undesired stray-fields. In contrast, for a single gap layout, which generates only one spatially sharply confined magnetic axial field in one direction along the optical axis, Ampere's law predicts the existence of additional (but with respect to the application, undesired) axial magnetic stray-fields along the optical axis in opposing direction. Those fields are charged-particle-optically problematic as they are not spatially sharply-confined, thus leading to unwanted interactions with the beam, and hence aberrations in the system. Therefore, a layout where the generated axial magnetic fields along the longitudinal direction intentionally cancel other each other out (on the whole), minimizes the presence of stray-fields that would otherwise impact not only each sub-column, but also potentially the neighboring sub-columns, and thus, would lead to non-rotationally symmetric distortions in the overall system, which would thus make the system unsuitable for the industries' high-tech node-precision write-quality requirements. This approach to minimize stray fields is not employed in the above mentioned state-of-the-art systems.

For at least the above reasons, the present invention and its application in writer tools such as multi-column multi-beam charged-particle nano-patterning systems for direct writing of substrates, offer a unique combination of magnetic, electrical and calibration components, which is expected to significantly impact the development of high-throughput industrial processes for integrated circuits.

In many embodiments the magnetic circuit has two gaps located at either axial end of the inner yoke shell towards a respectively corresponding portion of the outer yoke shell, wherein each gap generates a defined magnetic field reaching inwards into the passage opening and spatially overlapping with the electrostatic field generated by the electrode elements of the sleeve insert. However, it will be clear that the number of gaps may be higher, such as three or four or more, depending on the individual application of the lens.

Advantageously, the electromagnetic lens will often have an overall rotationally symmetric shape along said longitudinal axis, wherein the inner yoke shell and the outer yoke shell are coaxial to each other.

According to a suitable geometric layout the inner yoke shell may extend between two axial ends thereof along a passage space receiving the sleeve insert, and the outer yoke shell may surround the inner yoke shell radially outward of the inner yoke shell while extending to either sides corresponding to the axial ends of the inner yoke shell.

In many embodiments the at least one ring magnet may have a magnetization oriented substantially radially, and/or may be realized as radially magnetized ring magnet. Furthermore, the at least one ring magnet may be composed of three or more (e.g. four, six, eight) magnet parts arranged uniformly around the longitudinal axis along a circumferential direction. Generally, while the ring magnet will usually have a shape of a hollow cylinder or polygonal hollow prisma, the at least one ring magnet can have a general ring form where ring portions are distributed around the longitudinal axis along the circumferential direction; this may also include gaps of some angular extension between the ring portions. For instance, the magnet parts may be substantially wedge-shaped elements forming sectors with respect to the longitudinal axis. Alternatively to this or in combination with this, the ring magnet (or some or all of the magnet parts) may be composed of two or more layers (segments) stacked along the longitudinal axis.

As one highly advantageous aspect of the invention, the electrode elements may be configured to form a particle-optical lens in conjunction with the magnetic field within the passage opening at one of the gaps, at each one of several of the gaps, or preferably at each gap, and in the case of multiple gaps at each of these gaps a particle-optical lens may be formed. The focal length of such particle-optical lens(es) may be adjustable through modifying the electric potentials applied to the electrode elements. For instance, the electrode elements may often be configured (mechanically and electrically) to form at least one Einzel lens.

Furthermore, in many embodiments of the invention, at least one of the electrode elements may include an electrostatic multipole electrode comprising a number of sub-electrodes arranged uniformly around the longitudinal axis along a circumferential direction.

In many embodiments of the lens of the invention, in particular in those cases where the lens is intended to be used in connection with a PD system, among the electrode elements may be a beam aperture element forming a delimiting opening with a defined radius around the longitudinal axis, the delimiting opening limiting the lateral width of a charged-particle beam propagating along the longitudinal axis. This delimiting opening may be used as a particle-collecting calibration aperture used to collect particles, including those intentionally deflected in a PD system, so as to prevent them from reaching the target of the charged-particle beam. Furthermore, the beam aperture element may be connected to a current measurement device, which may be used e.g. to measure the amount of the charged-particle beam absorbed at the beam aperture element. In front of such a beam aperture element, as seen in the direction of propagation of the beam along the longitudinal axis, it is often advantageous to provide an electrostatic multipole electrode, which comprises a number of sub-electrodes arranged uniformly around the longitudinal axis along a circumferential direction, preferably configured to determine a transversal position of the beam with respect to the longitudinal axis, by applying different suitable electrostatic potentials to the sub-electrodes.

In advantageous embodiments, the sleeve insert may comprise a ceramic body on which the electrode elements are realized as conductive coatings of respectively limited shape and area.

A further aspect of the invention is directed at a charged-particle optical apparatus including an electromagnetic lens according to the invention and configured for influencing a charged-particle beam of said apparatus propagating through the lens along the longitudinal axis thereof, wherein said lens is part of a projection optic system of said apparatus. In particular, the apparatus may preferably realize a multi-column system comprising a plurality of particle-optical columns, each column using a respective particle beam and comprising a respective projection optic system, of which at least one, preferable several and most preferably all, includes a respective electromagnetic lens.

In the following, in order to further demonstrate the present invention, illustrative and non-restrictive embodiments are discussed, as shown in the drawings, which show schematically:.

The detailed discussion of exemplary embodiments of the invention given below discloses the basic ideas, implementation, and further advantageous developments of the invention. It will be evident to the person skilled in the art to freely combine several or all of the embodiments discussed here as deemed suitable for a specific application of the invention. Throughout this disclosure, terms like "advantageous", "exemplary", "typical", or "preferred" indicate elements or dimensions which are particularly suitable (but not essential) to the invention or an embodiment thereof, and may be modified wherever deemed suitable by the skilled person, except where expressly required. It will be appreciated that the invention is not restricted to the exemplary embodiments discussed in the following, which are given for illustrative purpose and merely present suitable implementations of the invention. Within this disclosure, terms relating to a vertical direction, such as "upper" or "down", are to be understood with regard to the direction of the particle-beam traversing the electromagnetic lens, which is thought to run downwards ("vertically") along a central axis (or longitudinal axis). This longitudinal axis is generally identified with the Z direction, to which the X and Y directions are transversal.

<FIG> depicts an electromagnetic lens <NUM> according to a first embodiment of the invention, in a longitudinal sectional view, i.e. along a section plane passing through its central axis c1. For the sake of better clarity, the components are not shown to size. The lens <NUM> may be used to implement the final lens <NUM> of the writer tool <NUM> of <FIG> (see below), where it is used as objective lens, but it will be appreciated that it is suitable for use in many other particle-optical devices which may implement a multi-column or single-column architecture, as disclosed in <CIT> and <CIT> of the applicant, and the disclosure of those documents are herewith included by reference into the present disclosure.

The electromagnetic lens <NUM> includes a magnetic circuit assembly <NUM> and a sleeve insert <NUM>, which is also referred to as beam-control inlay or simply "inlay". The magnetic circuit assembly <NUM> comprises one or more ring magnets <NUM>, <NUM> made of a permanent magnetic material, which have <NUM> to <NUM> outer radius, <NUM> to <NUM> (radial) thickness, and are about <NUM> long, and typically will have a remanence of about <NUM> T; as well as a housing body <NUM> which also serves as a yoke body for the ring magnets. Such a housing body consisting of two concentric cylinders with inner radius r1 of around <NUM> and outer radius r3 of about <NUM>, where each of the cyclinders has a thickness of <NUM> to <NUM>, and a length between <NUM> and <NUM>. The sleeve insert <NUM>, which is also referred to as beam-control inlay or simply "inlay", comprises several electric active and passive components acting as electrostatic electrodes, apertures, or field-termination caps, which have outer diameters below <NUM> and lengths between <NUM> and <NUM>, as discussed in detail below. The lens <NUM> is usually arranged in a particle-beam exposure system (such as the writer tool <NUM> of <FIG>) such that its central axis c1 coincides with the optical axis of the exposure system; but the skilled person will appreciate that also other relative arrangements may be chosen depending on the application of the electromagnetic lenses according to the invention. The dotted lines <NUM> symbolize an envelope of the particle beam as it propagates through the electromagnetic lens <NUM> within the particle-beam exposure system.

The magnetic circuit assembly <NUM> is discussed in the following. The housing body <NUM> comprises an internal part and an external part, referred to as inner yoke shell <NUM> and outer yoke shell <NUM>, respectively, with the latter encasing the ring magnets <NUM>, <NUM>, the inner yoke shell <NUM>, as well as the inlay <NUM> when the latter is mounted into the lens <NUM>. Both the magnets <NUM>, <NUM> and the housing body <NUM> are concentric with respect to the central axis c1. The ring magnets <NUM>, <NUM> preferably have the shape of rotationally symmetric rings or ring sectors. The sizes of the ring magnets are chosen as suitable for the respective application and charged-particle apparatus; in the shown embodiments the geometric dimensions are typically in the order of several millimeters (e.g., <NUM> outer radius, <NUM> thickness, and about <NUM> in length). Multiple magnets may be used, which are preferably arranged in a sequential stacking along the longitudinal axis of the system (<FIG>).

In a preferred embodiment of the invention, the permanent magnets forming the ring magnets <NUM>, <NUM> exhibit radial magnetization (see discussion regarding <FIG> below). The ring magnets act as a source of the magnetic flux in a magnetic circuit realized in the magnetic circuit assembly <NUM>. The inner yoke shell <NUM> is, e.g., realized as a hollow cylinder of sufficient length so as to protrude from the stack of ring magnets at both ends. The outer yoke shell <NUM> is, e.g., realized having a cylindrically symmetric shape having double C-like longitudinal section; in other words, it comprises a central body portion <NUM> shaped as a hollow cylinder, which is concentric with the hollow cylinder of the inner yoke shell <NUM> but with a larger radius, and two end parts <NUM>, <NUM> of disk-like shape with a central bore, attached at both longitudinal ends of the central body portion <NUM>. The central bore of each end part <NUM>, <NUM> preferably has an inner radius which is the same as the inner radius of the hollow cylinder body of the inner yoke shell <NUM>. Thus, the central bores and the hollow space of the inner yoke shell surround a passage space <NUM> (of radius r1) traversing the magnetic circuit assembly <NUM> along the axis c1. The geometric dimensions of the hollow-cylinder portions <NUM>, <NUM> are chosen suitably such that the cylinders envelope the magnets <NUM>, <NUM> inwardly and outwardly; they advantageously contact the respective faces of the magnets minimizing or preferably avoiding a gap between the magnets and the respective surface areas of the cylinder bodies. In contrast gaps 14a, 14b are provided between the end faces 103a, 103b of the inner yoke shell and the corresponding inner faces 141a, 142b of the end parts of the outer yoke shell, which represent pole-pieces of the magnetic circuit <NUM>. The radial thickness of the hollow cylinders is typically, and without loss of generality, in the order of a few millimeters. The inner and outer yoke shells <NUM>, <NUM> of the housing body <NUM> are made of a magnetically permeable material, preferably of high magnetic permeability (such as ferrimagnetic or ferromagnetic material); and by virtue of its shape it can enhance and concentrate the magnetic flux generated by the magnets. Around the regions of the gaps 14a, 14b, the magnetic flux of the circuit will also generate a defined magnetic field of magnetostatic type reaching inwards into the passage space <NUM> at specific portions of the central axis c1. Thus, in accordance with the invention, the housing body act as yokes of a magnetic lens, where the distribution of the magnetic flux is formed by the pole-pieces 103a, 103b, 141a, 142b. In particular, with respect to the passage space <NUM>, the magnetic field generated by the magnetic circuit <NUM> is restricted to two regions: namely, an upper region within the gap 14a, formed by the pole piece faces 103a, 141a; and an lower region within the gap 14b, formed by the pole piece faces 103b, 142b. Thus, two magnetic lenses are formed at the portions of the longitudinal axis corresponding to the gaps 14a, 14b respectively. The magnetic field comprises an axial component and a radial component; while the radial component is of little importance, the resulting axial component of the magnetic field BZ is exploited for the lens effect of the electromagnetic lens. The strength <NUM> of the axial component of the magnetic field BZ at the location of the central axis c1 as a function of the longitudinal coordinate is depicted in <FIG> (solid line); a typical value of the peak value of the magnetic field BZ is in the order of <NUM> T in applications where the charged particles are electrons. Thus, the magnetic circuit will generate two regions of (comparatively) high magnetic field intensity, which will act as two consecutive magnetic lenses with well-defined focal lengths and optical aberrations. Magnetic coupling of the two lenses via a common yoke <NUM> strongly reduces the effect of magnetic stray fields in regions other than the regions of the gaps 14a, 14b, which would otherwise be inevitably associated with permanent magnets in particle lenses of conventional layouts.

The lens <NUM> further comprises a sleeve insert or inlay <NUM>, which is inserted into the passage space <NUM> of the housing body <NUM> along the optical axis c1. Correspondingly, the physical dimensions of the inlay <NUM> are appropriately chosen with respect to the housing body <NUM> discussed above. The inlay <NUM> comprises a number of beam control elements, including one or several electrically active elements that are employed to generate an adjustable electrostatic field <NUM> (dashed line in <FIG>) superposing the magnetostatic field <NUM> (solid line in <FIG>) in the passage space. The beam control elements are generally ring-shaped components serving as electrically active elements, and they are stacked along the central axis c1 and oriented with their geometric axes concentric and parallel to the central axis c1, having a common inner diameter r2; thus they define a passage opening <NUM> which transverses the lens <NUM> along the central axis c1 and which serves as channel for the beam-path during operation of the magnetic lens. In the embodiment shown in <FIG>, the beam control elements comprise, going downwards in <FIG>, a first liner <NUM>, two multipole electrodes <NUM>, <NUM>, and a second liner <NUM> - all made of electrically conductive material. Each of the multipole electrodes <NUM>, <NUM> is, e.g., realized as a composite metallic ring, composed of multiple sections of equal arc length as further discussed below with reference to <FIG>; their (radial) thickness is, e.g., below <NUM>, and their lengths between <NUM> and <NUM>. In addition, preferably an electrically passive ring <NUM> is interposed between the two multipoles <NUM>, <NUM>; the component is referred to as calibration aperture, which is further discussed below. While the multipoles <NUM> and <NUM> are composed of a plurality of sector components arranged around the central axis c1 along the circumferential direction (for instance, and without loss of generality, <NUM>, <NUM> or <NUM> sector components each), all other inlay elements are preferably rotationally symmetric with respect to the central axis c1. Electrically active elements <NUM>, <NUM>, <NUM>, <NUM> may preferably be connected to respective power supplies <NUM>, <NUM>, <NUM>, <NUM> of electrostatic voltages so that their electrostatic potentials can be individually adjusted (<FIG>); in a variant, the power supplies may be combined in a common power supply device which provides the individual supply voltages. Finally, the electrically active elements <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are electrically separated from each other and terminated at both ends by elements referred to as field termination caps <NUM>, <NUM>, <NUM>, <NUM>, whose electric potential is same as the housing body <NUM>. The field termination caps <NUM>, <NUM>, <NUM>, <NUM> serve to confine the electric field to the inner space <NUM> of the inlay; they thus provide a well-defined "field boundary" of the inlay towards surrounding components (such as other particle-optical columns <NUM>, see <FIG>). The spaces between the field termination caps <NUM>, <NUM>, <NUM>, <NUM> and respective neighboring electrically active elements <NUM>,<NUM>,<NUM>,<NUM>,<NUM> are electrically insulating, e.g. realized as vacuum or filler material using a non-conductive, preferably voltage-resistant, material such as ceramics. The various elements <NUM>-<NUM> of the inlay are supported and held together by a supporting ring body of hollow-cylindrical shape (between radii r1 and r2) and made of electrically insulating material such as e.g. ceramic or plastic, symbolized in <FIG> by two dashed rectangles. The electrode elements may be realized, for instance, as discrete ring-shaped elements joined and held together within the ring body, or as conductive coatings formed at the inner surface of the ring body, so as to have respectively limited shape and area. The strength <NUM> of the electric field EZ (axial component, i.e. along Z-direction) as function of the coordinate along the axis c1 is depicted in <FIG> (dashed line); a typical value of the peak value of the electric field EZ is in the order of <NUM><NUM> V/m.

In a typical embodiment, the first and second liners <NUM>, <NUM> are between <NUM> and <NUM> long and have a small radial thickness e.g. below <NUM>. Each liner <NUM>, <NUM> is placed corresponding to one of the magnetic flux regions of the gaps 14a, 14b, respectively, that act as magnetostatic lenses. In particular, the liners <NUM>, <NUM> can be used as Einzel lenses in order to generate an electrostatic field <NUM> of, e.g., the order of <NUM><NUM> V/m superposing the magnetic field <NUM>. This allows fine adjusting of the focal length of the corresponding magnetic lenses, and the system is provided with a certain adjustment range for the focal length. Without the liners <NUM>, <NUM>, the accuracy of the focal length would suffer from limited precision in mechanical manufacturing, assembly accuracy limitations, and magnetization accuracy of the permanent magnets <NUM>, <NUM>, which is typically in the order of <NUM>% to <NUM>%. However, with respect to the purpose of a magnetic lens as intended by the applicant, a precision of even below <NUM>% would be desired. In addition, permanent magnets are known to have aging effects, i.e. often the magnetic field becomes weaker over time, and change strength depending on temperature. Thus, using permanent magnets in an charged-patricle-optical application, requires suitable compensation means for these various effects. The invention, also by including integrated corrections means, allows to overcome all the above-mentioned limitations relating to manufacturing, assembly, magnetization strength and aging effects, as electric fields can be adjusted and controlled with a precision in the ppm (parts-per-million) regime. In addition, the voltages of the liners can be adjusted in combination with other optically and electrically active elements of the system, in order to change the property of the particle-beam exposure apparatus with respect to optical aberrations and/or alter the height of the image-plane produced by the lens with respect to the target.

Referring to <FIG> and <FIG>, the inlay <NUM> may preferably include a passive element referred to as calibration aperture <NUM>, which serves as a stopping component for deviating or deflected parts of the particle beam. <FIG> shows an enlarged detail of a longitudinal section of the calibration aperture. The calibration aperture <NUM> is provided with an inner ring structure projecting towards the axis c1, thus forming an aperture of small diameter d6. The aperture serves to limit the size of the beam <NUM> traversing the inlay <NUM> by absorbing the parts <NUM> of the beam which travel outside of the diameter d6. In a preferred embodiment of the invention, one of the preceding inlay elements, for instance a multipole electrode <NUM>, is configured to steer the beam <NUM> along a direction transversal to the axis c1 by applying a dipole field (typically using voltage in the range of ±<NUM> V for instance). The multipole electrode <NUM> enables defining the transversal location of the beam <NUM>, <NUM> with respect to the longitudinal axis c1, by varying the dipole voltage applied to selected electrodes of the multiple electrode in at least two linearly independent directions, which can also be used for beam alignment. In the particle-beam columns of the writer tool <NUM> described above, the particle beam is split into a bundle of beamlets that can selectively pass through the pattern-definition system <NUM> with or without an additional deflection, introduced by said pattern-definition system. Such deflection is introduced in order to prevent the beamlets from reaching target and hence define a writing pattern. The deflected beamlets will reach an area beside the calibration aperture <NUM>, rather than traveling through it, and will thus be absorbed therein, without causing unwanted charge-up of other parts of the system, which could otherwise generate undesired stray electric fields. For this purpose, the beam calibration aperture is shaped as a cylinder that is, e.g., about <NUM> long and has a hook-like cross-section at the bottom (<NUM>) with a minimum diameter of a few hundreds of micrometer. The absorption of the beam parts <NUM> will cause generation of an electric charge in the element forming the calibration aperture <NUM>, which may be eliminated, i.e. drained off, through the electric connection of the beam aperture, for instance towards a measuring device <NUM> as discussed below with reference to <FIG>. The inlay <NUM> is usually inserted into the passage space <NUM> of the housing body <NUM> such that its longitudinal axis c3 coincides with the central axis c1 of the housing body <NUM>.

In a variant of the inlay, one or more of the electrode elements of the inlay may be geometrically shaped in a way such that the aberrations of the system are intentionally changed or kept constant within a defined range of supply voltages. To achieve this purpose, the respective electrode(s) may have a modified shape, such as having a cross-section with a tipped shape towards the longitudinal axis, thus realizing a constricted inner diameter, and are applied a suitable electric potential with respect to ground potential.

<FIG> illustrates one exemplary embodiment of an inlay <NUM>' comprising a liner <NUM> having a constriction of inner diameter. <FIG> is a detail of the "bottom" end of the inlay <NUM>'. By virtue of the constriction to a reduced inner width, i.e. diameter d3, the liner <NUM> allows to alter aberrations. In particular, if the diameter of the beam in the area of liner <NUM> is approximately <NUM>% of the diameter d3 or larger, significant spherical aberrations are induced, which could then be used e.g. to alter radial spatial or angular distortions, or the image-field curvature. In other respects, the inlay <NUM>' corresponds to the inlay <NUM> discussed above, in particular the field termination caps <NUM>, <NUM> having an inner diameter d2 = <NUM>·r2 at either sides of the liner <NUM> may be realized identical to the field termination caps <NUM>, <NUM>.

<FIG> shows a schematic sectional view of a multi-column writer tool <NUM> incorporating an electromagnetic lens according to one embodiment of the invention. The writer tool employs a charged-particle beam formed from charged particles which may be electrons or ions (for instance ions of positive electric charge).

The writer tool <NUM> comprises a vacuum housing <NUM> for the multi-column charged-particle optics <NUM>, a target chamber <NUM> onto which the multi-column charged-particle optics is mounted by means of by means of a column base plate <NUM>. Within the target chamber <NUM> is an X-Y stage <NUM>, e.g. a laser-interferometer controlled air-bearing vacuum stage onto which a substrate chuck <NUM>, preferably an electrostatic chuck, is mounted using a suitable handling system. The chuck <NUM> holds the substrate <NUM> serving as target, such as a silicon wafer with an electron or ion beam sensitive resist layer.

The multi-column optics <NUM> comprises a plurality of sub-columns <NUM> (the number of columns shown is reduced in the depiction for better clarity, and represent a much larger number of columns that are present in the multi-column apparatus in a realistic implementation). Preferably, the sub-columns <NUM> have identical setup and are installed side-by-side with mutually parallel axes. Each sub-column has an illuminating system <NUM> including an electron or ion source 411a, an extraction system 411b, and an electrostatic multi-electrode condenser optics 411c, delivering a broad telecentric charged-particle beam to a pattern definition (PD) system <NUM> being adapted to let pass the beam only through a plurality of apertures defining the shape of sub-beams ("beamlets") permeating said apertures (beam shaping device), and a demagnifying charged-particle projection optics <NUM>, composed of a number of consecutive electro-magneto-optical projector stages, which preferably include electrostatic and/or magnetic lenses, and possibly other particle-optical devices. In the embodiment shown in <FIG>, the projection optics <NUM> comprises e.g. a first lens <NUM> which is an accelerating electrostatic multi-electrode lens, whereas a second lens <NUM>, located downstream of the first lens, is realized using an electromagnetic lens according to the invention, such as lens <NUM> of the first embodiment (<FIG>).

In each sub-column <NUM>, the first lens <NUM> of the projection optics forms a first cross-over of the particle beam, whereas the second lens <NUM> forms a second cross-over. In the second lens a beam aperture <NUM> (corresponding to the beam aperture <NUM> of <FIG>) is configured to filter out beam parts which deviate from the respective optical axis since they have been deflected in the PD system. Each second lens <NUM> of the sub-columns may be preferably mounted on a reference plate <NUM> which is mounted by suitable fastening means <NUM> onto the column base plate <NUM>. Mounted onto the reference plate <NUM> are parts <NUM> of an off-axis optical alignment system.

The reference plate is fabricated from a suitable base material having low thermal expansion, such as a ceramic material based on silicon oxide or aluminum oxide, which has the advantage of little weight, high elasticity module and high thermal conductivity, and may suitably be covered with an electrically conductive coating, at least at its relevant parts, in order to avoid charging (by allowing electrostatic charges being drained off).

The PD system <NUM> serves to form the particle beam into a plurality of so-called beamlets which contain the information of the pattern to be transferred to the target. The structure, operation and data-handling of the PD system are disclosed in <CIT> and <CIT> of the applicant, and the disclosure of those documents are herewith included by reference into the present disclosure.

As mentioned, the inlay comprises multipole electrodes <NUM> and <NUM>. Each of the multipole electrodes <NUM>, <NUM> is composed of three or more metallic ring sectorial components serving as electrodes of the multipole electrode (sub-electrodes), which hereinafter are also called rods. Preferably, the rods are of identical geometry. A depiction of a multipole electrode <NUM> with eight rods <NUM> (sectorial electrodes) in a cross-sectional view is shown in <FIG> shows an enlarged detail of a gap range between two of the rods. <FIG> shows a variant of a multipole electrode <NUM> with four sectorial electrodes <NUM>. Preferred numbers of rods within a multipole electrode are four, six, eight, twelve or sixteen, depending on the desired effects to be achieved.

Referring to <FIG>, the rods <NUM> are located within a surrounding sleeve serving as support <NUM>. The rods <NUM> can be individually applied electric potentials by means of their respective external power supply units <NUM>, <NUM> (<FIG>). Additionally, a global offset voltage may be applied to have them behave as additional electrostatic lenses. By virtue of the different voltages applied to the individual rods <NUM> various field configurations of dipole, quadrupole or higher order electrostatic fields can be realized, with the purpose of shaping the particle beam crossing their corresponding transversal section of the optical axis <NUM>. With respect to a typical application in the context of the embodiment of <FIG>, the voltages applied to the rods <NUM> are typically in the order of up to a few tens of volts. Such beam shaping can be used to compensate for errors due to imperfections of the optical system, such as magnetic inhomogeneities, mechanical manufacturing and/or assembly accuracies. In this respect, the multipoles can correct the beam position with respect to the optical axis <NUM> when used as dipoles, whose directions in the plane defined by the X and Y axes can be arbitrary if at least four different voltages +V1 (hatched rods to the right hand side), -V1 (hatched rods to the left hand side), +V2 (cross-hatched rods to the top), -V2 (cross-hatched rods to the bottom) are applied to the rods. Additionally, it is possible to compensate for astigmatism or other higher-order distortions by the multipoles when the latter are used as quadrupole or higher-order multipoles, by applying suitable voltages at the individual rods, in a manner similar to the dipole case. As for the liners <NUM>, <NUM>, also the common offset potential of multipole electrodes <NUM>, <NUM> may be adjusted in order to tune optical aberrations of the magnetic lenses formed by the lens <NUM>. The gaps <NUM> between the sectorial electrodes forming the rods <NUM> are advantageously angled (or zig-zag shaped, "labyrinth"), so as to avoid that particles <NUM> diverting from the beam which propagates within the central space of the multiple electrode can travel to the outside of the multiple electrode, possibly affecting the support <NUM> or other outer components, but will impinge on a surface of a rod. At the outer sides of the rods <NUM> the gaps <NUM> end in pouches which serve to collect the particles and drain their electric charge to a drain electrode (not shown). This serves to avoid the build-up of electric charges and associated stray electric fields which otherwise might affect the charged-particle beam propagating through the multipole electrode <NUM>.

Furthermore, by appropriate choice of the electric multipole and the location and shapes of the rods and applying a suitable common supply voltage to all rods of a multipole electrode, it is possible to achieve a change in the focal length of one or both of the magnetic lenses corresponding to the two gaps 14a, 14b formed by the yokes.

Referring again to <FIG>, advantageously the two magnetic lenses may be used for realization of a so-called cross-over cv of the particle beam. In <FIG> the dotted envelope lines <NUM> indicate the evolution of the beam size (cross section diameter) along the optical axis (c1). As already mentioned, a first magnetic lens is formed at the level of the first gap 14a, and this first lens can be used to focus the beam, so it converges into a crossover cv, i.e. a position or region where the lateral width of the beam attains a minimum. The magnetic lens is advantageously configured so as to create the crossover cv at a position located at, or in the vicinity (e.g. <NUM> or less) of the longitudinal location of the calibration aperture <NUM> into a crossover cv. Thereafter, the beam diverges again and traverses a second magnetic lens formed at the level of the second gap 14b, which preferably has the effect of making the beam telecentric again (with respect to the source of the particle beam). Thus, the beam forms the image of the apertures that shape the beam in the PD system <NUM> so that the images are produced at the target <NUM>. Therefore, it is highly desired that an accurate matching of the focal lengths of the two lenses be ensured for a successful image formation at the target. In this context, the liners <NUM>, <NUM> are crucial elements for the correct functioning of the system. Focusing the beam at the location of the calibration aperture <NUM> makes the latter particularly useful for beam-alignment procedures, where the beam <NUM> can be scanned across the aperture <NUM>, while corresponding current measurements <NUM> (or measurements of charge accumulation) serve to monitor the beam location in order to center the beam to the optical axis c1.

<FIG> shows a schematic overview of the electric voltage supplies <NUM> for the inlay <NUM> and the monitoring device <NUM> carrying out the current measurement <NUM>. It should be remarked that the multipole electrodes <NUM>, <NUM>, could be used as (quasi-)static or as dynamic elements, i.e. having time-varying voltages, depending on the application. Indeed, the upper multipole electrode <NUM> may be used in a time-varying dipole configuration to deflect the beam during the scanning across the calibration aperture <NUM>. Moreover, with the lower multipole electrode <NUM> it is possible to keep the beam aligned with respect to the target <NUM> on a moving stage <NUM>. The skilled person will appreciate that the mentioned uses of beam control elements are mentioned as exemplary applications and not as restrictions on the functionalities that can be accomplished with the present invention.

<FIG> illustrates one embodiment of a ring magnet <NUM> suitable for being used as a component of a magnetic circuit of a lens according to the invention (e.g. as one of the ring magnets <NUM>, <NUM> of <FIG>). <FIG> is a schematic perspective view, <FIG> a schematic sectional view along the longitudinal axis c8, and <FIG> a corresponding schematic cross-sectional view; the dimensions are not to scale. The ring magnet <NUM> is composed of a number of ring magnet elements <NUM> stacked in a co-axial arrangement along the common longitudinal axis c8. Each of the ring magnet elements <NUM> has a radially oriented magnetization, as indicated by dashed arrows in <FIG>, so as to have e.g. a "north" pole N formed towards the inner space of the ring magnet <NUM>, whereas the outer sides represent the "south" pole type S of the magnetization. Ring magnet elements having radial magnetization as shown are commercially available, made of a ferromagnetic material such as sintered NdFeB, SmCo<NUM> or ferrite. The ring magnet elements are joined by gluing or clamping or any other suitable means. The number of the ring magnet elements <NUM> in a ring magnet <NUM> may be any number, such as one, two, three or more, depending on the dimension (in particular height) of the elements and the desired dimensions of the ring magnet <NUM>.

Claim 1:
An electromagnetic lens (<NUM>) configured to modify a charged-particle beam of a charged-particle optical apparatus (<NUM>), the lens being provided with a passage opening (<NUM>) extending along a longitudinal axis (c1) and allowing passage of the charged-particle beam, the lens comprising:
- a magnetic circuit assembly (<NUM>) comprising at least one ring magnet (<NUM>, <NUM>) and a yoke body (<NUM>); and
- a sleeve insert (<NUM>),
said sleeve insert (<NUM>) surrounding the passage opening (<NUM>) and extending between a first end and a second end thereof along the longitudinal axis (c1), wherein the sleeve insert (<NUM>) comprises at least one electrically conductive electrode element (<NUM>, <NUM>, <NUM>, <NUM>), each electrode element being configured to be applied a respective electric potential via the power supplies (<NUM>, <NUM>, <NUM>, <NUM>) so as to generate an electrostatic field within the passage opening,
said yoke body (<NUM>) comprising an outer yoke shell (<NUM>) and an inner yoke shell (<NUM>), arranged circumferential around the longitudinal axis and comprising a magnetic permeable material, wherein the inner yoke shell is arranged surrounding at least a central portion of the sleeve insert, and the outer yoke shell surrounds the inner yoke shell and the sleeve insert,
said at least one ring magnet (<NUM>, <NUM>) arranged circumferentially around the inner yoke shell and arranged between the inner and outer yoke shells, said at least one ring magnet comprising a permanent magnetic material being magnetically oriented with its two magnetic poles towards the inner yoke shell and the outer yoke shell, respectively,
wherein in the magnetic circuit assembly (<NUM>), the inner yoke shell, the at least one ring magnet, and the outer yoke shell form a closed magnetic circuit but having at least two gaps (14a, 14b) located at an axial end (103a, 103b) of the inner yoke shell towards a respectively corresponding portion (141a, 142b) of the outer yoke shell, configured to generate a magnetic field (<NUM>) reaching inwards into the passage opening (<NUM>) and spatially overlapping with the electrostatic field (<NUM>) generated by the electrode elements of the sleeve insert.