Specific improvements to the multi-beam generation unit and the multi-beam deflection unit

Abstract

Specific improvements are provided for multi-beam raster units, such as multi-beam generation units and multi-beam deflection units, of multi-beam charged particle microscopes. These improvements include designing, fabricating, and adjusting multi-beam raster units with apertures of specific shapes and dimensions. These improvements enable high-precision multi-beam generation and multi-beam deflection or astigmatism correction. These improvements are suitable for routine applications of multi-beam charged particle microscopes, for example, in semiconductor inspection and verification, where high reliability, high reproducibility, and low machine deviations are required.

Description

Technical Field

[0001] The present disclosure relates to a multi-beam raster unit such as a multi-beam generation unit and a multi-beam deflection unit of a multi-beam charged particle microscope.

Background Art

[0002] WO2005 / 024881A2 discloses an electron microscope system that operates using a number of electron beamlets for parallel scanning of an object to be inspected using a bundle of electron beamlets. The bundle of electron beamlets is generated by directing a primary electron beam at a first perforated plate having a number of openings. A portion of the electrons of the electron beam impinge on the perforated plate and are absorbed therein, and other portions of the beam pass through the openings of the perforated plate, whereby, in the beam path downstream of each opening, an electron beamlet whose cross-section is defined by the cross-section of the opening is formed. Further, an appropriately selected electric field provided in the beam path upstream and / or downstream of the perforated plate operates as a lens on each opening of the perforated plate on the electron beamlet passing through that opening, whereby each electron beamlet is focused on a surface disposed away from the perforated plate. The surface on which the focus of the electron beamlet is formed is imaged by a downstream optical system on the surface of an object or sample to be inspected. The primary electron beamlet triggers secondary electrons or backscattered electrons to be emitted from the object as a secondary electron beamlet, and the secondary electron beamlet is condensed and imaged on a detector. Each of the secondary beamlets impinges on a separate detection element, and the intensity of the secondary electrons detected by the detection element provides information about the sample at the position where the corresponding primary beamlet impinges on the sample. The bundle of primary beamlets is regularly scanned on the surface of the sample, and the electron microscope image of the sample is generated by the usual method for a scanning electron microscope. The resolution of a scanning electron microscope is limited by the focal diameter of the primary beamlet impinging on the object. As a result, in multi-beam electron microscope analysis, all of the beamlets form the same small focus on the object.

[0003] The systems and methods described in detail in the example of electrons in WO2005 / 024881 are found to be generally very well applicable to charged particles. Correspondingly, the present invention aims to propose a charged particle beam system that operates using a number of charged particle beams and can be used to achieve higher imaging performance, such as better resolution and a narrower resolution range for each of the plurality of small beams of the plurality of small beams. A plurality of small beams for a multi-beam charged particle microscope (MCPM) are generated in a multi-beam generation unit. A multi-beam charged particle microscope (MCPM) typically uses both micro-optical (MO) elements and macroscopic elements in a charged particle projection system.

[0004] The multi-beam generation unit comprises elements that divide, partially absorb, and act on a beam consisting of charged particles. Therefore, a set of small beams of charged particles is generated in a predefined raster configuration. The multi-beam generation unit comprises a first porous plate, an additional porous plate, micro-optical elements such as micro-optical deflection elements, and macroscopic elements such as lenses in a special element design and a special configuration.

[0005] The multi-beam generation unit can be formed, for example, in an assembly of two or more parallel planar substrates or wafers created by silicon microstructuring. During use, a plurality of electro-optical elements are formed by apertures aligned in at least two of such planar substrates or wafers. Some of the apertures may comprise one or more vertical electrodes arranged axially symmetrically around the aperture, for example forming an electrostatic lens array. It is known that the optical aberrations of such an electrostatic lens array are very susceptible to the effects of manufacturing inaccuracies of the plurality of apertures. The roughness of the contour or edge of each aperture generates spherical aberration and higher-order aberrations. The inner surface of the electrostatic lens is usually formed of silicon by vertical anisotropic etching, and the inner surface typically has a roughness between 100 nm and 500 nm.

[0006] For the generation of a predefined electro-optical element, it is important to precisely control the distance between electrodes in the transmission direction of a plurality of charged particle sub-beams, together with, for example, the geometric arrangement of the electrodes and the horizontal alignment for each sub-beam of the plurality of charged particle sub-beams. Deviations in the manufacturing process of a planar substrate, electrodes, and the assembly of the planar substrate cause aberrations in the electro-optical element, leading to aberrations such as those of individual sub-beams and deviations from the predefined raster configuration of the sub-beams. Furthermore, deviations in the manufacturing process generate scattered particles, and those scattered particles degrade the image quality of the MCPM.

[0007] According to the current manufacturing process, the diameter of the apertures of the multi-beam generation unit may vary between apertures on one wafer or may vary between wafers. For example, roughness is increased by the inhomogeneity of the steps of isotropic etching, causing aberrations and image blur in conventional porous plates. Aberrations at the upper edge of the aperture, which are the cause of the generation of multiple sub-beams from the incident charged particle beam, are increased by the manufacturing processes of the prior art, thereby exhibiting significant inhomogeneity between the beams. Furthermore, deviations in the manufacturing process generate deviations between several individual multi-beam generation units used in several different individual MCPMs. Such differences, also called mechanical deviations, are undesirable in many applications of MCPMs.

[0008] With the current adjustment process, the horizontal alignment of two or more parallel planar substrates or wafers for forming a multi-beam generation unit or a multi-beam deflection unit is not sufficiently precise. The electro-optical element formed during use between at least two planar substrates requires a very high-precision horizontal alignment of at least two planar substrates or wafers. An increase in imaging performance and a reduction in mechanical deviations require a horizontal alignment of less than 0.5 μm.

[0009] The porous plate has a thin film made, for example, from a wafer by a thinning process. Deformations of the film generated during fabrication or induced, for example, by thermal expansion create different distances between some of the porous plates, thus causing differences in the plurality of electrostatic elements formed during use between at least two of the porous plates. Changes in the deformation of the film can further cause deviations in the image plane curvature of the plurality of foci of the small beam or deviations in the telecentricity characteristics of the plurality of small beams.

[0010] In the prior art, means for improving the theoretical properties of the porous array have been considered. For example, U.S. Patent Application Publication No. 2003 / 0209673 (A1) discloses means for reducing crosstalk between a plurality of primary charged particle small beams. U.S. Patent Application Publication No. 2003 / 0209673 (A1) discloses an electrostatic Einzel lens array for a plurality of electron small beams that reduces crosstalk. This electrostatic Einzel lens array is disposed in the electron beam path downstream of the aperture array and includes an upper electrode, an intermediate electrode, and a lower electrode of the Einzel lens, with each pair of electrodes being spaced apart by a distance as large as 100 μm. Crosstalk is reduced by shielding electrodes provided between the upper electrode and the intermediate electrode and between the intermediate electrode and the lower electrode. In other examples, means for reducing design aberrations are considered. DE 102014008083 filed on May 30, 2014 or corresponding U.S. Patent No. 9,552,957 shows an example of a porous plate with an array of lenses that reduces spherical aberration. The reduction of design aberrations is achieved by a lens aperture that is large compared to the beam diameter. DE 102014008083 A1 proposes a distance between porous plates in the range of 0.1 to 10 times the aperture diameter to avoid the charging effect on the electrodes, but this large range alone is ultimately not sufficient to prevent the undesirable charging effect of the electrodes from scattered charged particles. SUMMARY OF THE INVENTION

[0011] Therefore, it is an object to minimize aberrations in a multi-beam raster unit such as a multi-beam generation unit or a multi-beam deflection unit. In a pre-defined raster configuration, it is an object to provide a multi-beam generation unit that can form a sharp small beam with a small focal diameter and minimal residual aberrations, thereby suppressing the generation of scattered particles during use as much as possible. It is an object to provide a multi-beam deflection unit that can deflect a small beam with high precision without generating or increasing the aberrations of the small beam, thereby suppressing the generation of scattered particles during use as much as possible.

[0012] Therefore, it is an object to provide a design of a multi-beam raster unit such as a multi-beam generation unit or a multi-beam deflection unit that is less affected by deviations, does not significantly generate or increase aberrations, generates few scattered particles, and enables the production of a multi-beam generation unit or a multi-beam deflection unit with higher reproducibility.

[0013] Therefore, it is an object to provide a multi-beam raster unit having at least two porous plates, including providing a manufacturing process for the porous plates, which is less affected by deviations, has a small generation of aberrations, has a small generation of scattered particles, and enables the production of a multi-beam generation unit or a multi-beam deflection unit with high stability and reproducibility.

[0014] The object of the present invention is solved by the independent claims. The dependent claims relate to advantageous embodiments.

[0015] Contrary to the prior art US Patent Application Publication No. 2003 / 0209673 (A1) and DE102014008083A1 which are referred to, the present invention discloses structural means for alleviating manufacturing errors in order to improve the beam quality of a plurality of small beams, such as reducing the roughness of the inner sidewall for reducing the scattering of transmitted electrons, or preferably selecting the diameter and distance of apertures for reducing the influence of manufacturing errors of electrodes of electric field elements.

[0016] In one embodiment of the present invention, the multi-beam raster unit is configured to minimize the influence of the deviation of the apertures from a pre-defined shape. The multi-beam raster unit according to this embodiment is provided with a smooth and distinct conductive surface on the bottom side and includes a first porous plate that serves as a counter electrode for the formation of an electric field element by a subsequent second porous plate. A multi-beam raster unit, such as a multi-aperture unit or a multi-beam deflector or a multi-beam stigmator, is configured to form a plurality of electric field elements that affect a plurality of transmitting beamlets of charged particles during use. The multi-beam raster unit is configured such that the edges of the apertures and the inner sidewalls are arranged at a large distance from the transmitting beamlets of charged particles, and thus the influence of the deviation between the edges of the electrodes and the inner sidewalls of the apertures on the beam characteristics is minimized to one-tenth or less. This problem is realized by a porous plate having a special geometric arrangement and apertures of a special shape.

[0017] In one embodiment, a method for designing and fabricating a porous plate that minimizes the deviation of the apertures from a pre-defined shape is provided such that the apertures have a distinct, clear, and smooth shape (surface and edges), a low roughness value, and no local errors or deviations.

[0018] A multi-beam raster unit according to one embodiment includes a first porous plate having an inner zone that forms a film of a first thickness L1 and having a plurality of first apertures, a beam incident side, and a beam exit side. The film of the first porous plate has at least a first partial thickness L1.1 and includes a first segment having a plurality of cylindrical apertures having a first diameter D1 on the beam incident side and a second segment having a plurality of apertures having a second diameter D2 on the beam exit side. The multi-beam raster unit further includes a second porous plate having an inner zone that forms a film having a plurality of second apertures and a beam incident side, the plurality of apertures having a third diameter D3 on the beam incident side, and the first and second porous plates forming a gap of thickness L2 between the films of the first and second porous plates. The multi-beam raster unit further includes at least a first electrode configured proximate to the first apertures of the first porous plate on the beam exit side of the first porous plate and at least a plurality of second electrodes configured proximate to the plurality of second apertures of the second porous plate on the beam incident side of the second porous plate to form a plurality of electric field elements between the plurality of first and second apertures of the first and second porous plates during use. The multi-beam raster unit is configured to have a second diameter D2 that is larger than the first diameter D1, and the second diameter D2 is in the range between the second thickness L2 and twice the second thickness L2, thus L2 < D2 < 2*L2. Also, the thickness L1.1 of the first segment is less than 10 μm, preferably less than 5 μm. In one example, in the direction of a plurality of transmitted small beams of charged particles, the first porous plate is disposed in the beam path upstream of the second porous plate and forms the upper or incident porous plate of the multi-beam raster unit. In another example, in the direction of a plurality of transmitted small beams of charged particles, at least a third porous plate is disposed in the beam path upstream of the first and second porous plates.

[0019] In one embodiment, at least a portion of the inner wall of the aperture in the second portion has a surface shape that slopes in a direction away from the transmission microbeam of charged particles. In one embodiment, the surface shape curves and slopes in a direction away from the transmission beam of charged particles. In one embodiment, the surface shape in the direction of the transmission microbeam of charged particles is spherical. In one embodiment, the inner sidewall surface of at least one of the plurality of apertures in the second portion has a surface shape such that the diameter of the aperture opening continuously increases as the z coordinate increases. In one embodiment, the spherical shape is formed by isotropic etching.

[0020] In one embodiment, the first porous plate includes an absorption layer on the beam incident side, and the absorption layer is connected to the ground level during use. During use, most of the incident electrons are absorbed in the absorption layer, and a current corresponding to the number of absorbed electrons is generated. For example, when D1 = 30 μm and the pitch between adjacent apertures is 150 μm, approximately 97% of the incident electrons from the incident electron beam are absorbed, generating a large current of electrons. Therefore, the absorption layer exhibits a fluctuating voltage difference corresponding to the induced current during use and is thus not suitable for forming an electrode for an electric field element.

[0021] In one embodiment, the first porous plate includes a conductive layer on the beam exit side, and the conductive layer forms the first electrode. Preferably, the conductive layer basically completely covers the beam exit surface of the first porous plate, and the aperture holes themselves are not covered. However, it is also possible that only a part of the beam exit side is covered by the conductive layer. According to one embodiment, the conductive layer forms the first electrode as an inert or counter electrode with respect to the driven second electrode.

[0022] In one embodiment, the beam emission side of the first porous plate includes a ring-shaped conductive layer or electrode around or in proximity to an aperture of diameter D2 that is connected to a constant potential during use, and the ring-shaped conductive layer or electrode forms the first electrode. Preferably, the ring-shaped conductive layer or electrode forms the first electrode as an inert or counter electrode with respect to the driven second electrode.

[0023] In one embodiment, the beam emission side of the first porous plate does not include a shielding electrode. According to the present invention, there is no need to apply such a shielding electrode to reduce crosstalk.

[0024] In one embodiment, the second porous plate includes a ring-shaped electrode disposed around the second aperture, and during use, a driving voltage is applied to the ring-shaped electrode, and the ring-shaped electrode forms the second electrode.

[0025] In one embodiment, the ring-shaped electrode extends substantially through the second porous plate. Preferably, the ring-shaped electrode extends entirely through the second porous plate from the beam incidence side to the beam emission side.

[0026] In one embodiment of the present invention, the porous plate of the multi-beam raster unit is configured to have etching stop rings around a plurality of apertures, such as the apertures on the beam emission side of the first porous plate or the apertures on the beam incidence side of the second porous plate. The etching stop rings enable the realization of a high-precision shape and roughness of the aperture of the aperture of the incident or emission surface of the porous plate. A manufacturing process for a porous plate having etching stop rings is provided.

[0027] In one embodiment, the surface roughness of the inner sidewall surface of at least one of the plurality of apertures in a portion is less than 50 nm rms, preferably less than 25 nm rms, more preferably less than 10 nm rms. A method for producing an aperture in which the roughness of the inner sidewall is less than 50 nm rms, preferably less than 25 nm rms, more preferably less than 10 nm rms is provided.

[0028] The diameter D1 of the upper edge has the function of stopping and shaping the charged particle beam in the multi-beam generation unit. In one embodiment, the diameter D1 is in the range of 10 μm ≤ D1 ≤ 30 μm, while for the optimal performance of the electric field element, the average aperture diameter and the diameter D2 at the lower or beam exit edge are larger than D1. In one example, the second diameter D2 is in the range between the first thickness L1 and twice the first thickness L1, thus L1 < D2 < 2*L1. In one example, the second diameter D2 is in the range between the thickness L1.1 of the first portion and twice the thickness of the first portion, thus L1.1 < D2 < 2*L1.1. In a further example, the second diameter D2 is in the range between the second thickness L2 and twice the second thickness L2, thus L2 < D2 < 2*L2. In one example, the third diameter D3 is larger than the first diameter D1, for example the third diameter D3 is in the range between the first diameter D1 and the second diameter D2, thus D1 < D3 < D2.

[0029] In one embodiment, a method for producing a porous plate is provided in which the edge of the electrode and the inner sidewall of the aperture are arranged at a greater distance from the transmitted small beam. A microstructuring process for a porous plate of a multi-beam raster unit having apertures of a special shape, for example a curved shape, is provided. The porous plate is produced by double-sided processing on a homogeneous planar substrate or a single composite substrate (e.g., SOI, i.e., silicon on insulator) wafer. In one embodiment, a method for double-sided processing for the precise production of the indispensable aperture stop at the exit side together with the incident side of the porous plate is provided.

[0030] In one embodiment, the multi-beam raster unit includes a first porous plate and a second porous plate having at least two portions. In one example, the second porous plate includes first and second portions separated by an insulating and etch stop layer. The first portion includes a plurality of electrodes formed around a plurality of apertures to form a plurality of electric field elements during use. The second portion has a large thickness L7 between 100 μm and 400 μm, and the aperture diameter D7 is equal to or greater than the aperture diameter D3 of the first portion. The second portion provides a shield around the electric field and improves the performance of the plurality of electric field elements.

[0031] In one embodiment, the multi-beam raster unit includes a second porous plate having first and second portions and a third portion, each separated by an insulating and etch stop layer. The first portion includes a plurality of electrodes formed around a plurality of apertures to form a plurality of electric field elements during use. The second portion has a large thickness L7 between 100 μm and 400 μm, and the aperture diameter D7 is equal to or greater than the aperture diameter D3 of the first portion. The second portion provides a shield around the electric field and improves the performance of the plurality of electric field elements. The third portion is disposed between the first portion and the second portion and has apertures formed in conjunction with the apertures of the first portion to minimize the influence of alignment errors due to double-sided processing of the second porous plate. The thickness L8 of the third portion is equal to or less than the thickness L3 of the first portion.

[0032] In one embodiment, the beam incident side of the second porous plate is covered by a shielding layer having at least one plunging extension to at least one of the apertures of the second porous plate.

[0033] In one embodiment, the influence of deviations on the optical performance is reduced by a functional separation between the upper edge of the porous plate that acts as a beam stop and forms the beam, and the lower edge of the porous plate that forms an electrostatic lens together with a subsequent second micro-optical raster element. In one example, the first porous plate includes two or more separate planar portions or substrates that are each processed separately by one-sided or two-sided structuring and assembled together to form a composite first porous plate. Thereby, a higher resolution size, position, and low roughness of both edges are achieved. The first and second portions are separate portions attached to each other. The porous plate is realized by alignment and mutual fixation of its parts, for example by bonding, or enables fine passive or active alignment and is fixed to a special holder for mutual alignment and adjustment to maintain the porous plate in an aligned state. In an example of active alignment, the substrate is adjustable using actuators formed between the separate substrates. The diameter D1 of the upper edge, which has the function of stopping and shaping the charged particle beam, is given in the range of 10 μm ≦ D1 ≦ 30 μm, and the diameter D2 of the lower or beam exit edge is larger for optimal performance of the electrostatic element.

[0034] In one embodiment, a method of manufacturing for a porous plate of a multi-beam raster unit including two or more separate portions is provided. The separate portions are manufactured by two-sided processing on a homogeneous planar substrate or on one composite substrate (for example, an SOI, i.e., silicon-on-insulator) wafer.

[0035] In one embodiment, the multi-beam raster unit includes a holder for the mutual alignment and adjustment of at least first and second porous plates. In one embodiment, a porous plate is provided having a membrane region with first, second, and third portions. The third portion is made of a different material or material composition, is between the first portion and the second portion, and forms an etching stop and an insulating layer. In one embodiment, a method of fabricating a porous plate having a membrane region with three portions is provided. In one example, the inner sidewall of at least one of the plurality of apertures of the second portion is curved.

[0036] In one embodiment, the multi-beam raster unit includes a porous plate having an internal region for forming a membrane from an SOI wafer having at least three layers in the order of silicon, silicon dioxide, and silicon. In one example, the porous plate having an internal region for forming a membrane is formed from an SOI wafer having five layers in the order of silicon, silicon dioxide, silicon, silicon dioxide, and silicon.

[0037] In one embodiment, a structure and method for adjusting the deformation of a porous plate are provided. At least the membrane of one porous plate includes a layer of a predetermined thickness, and the layer induces stress-induced deformation of the membrane.

[0038] In one embodiment, the multi-beam raster unit includes at least first and second porous plates that form an internal region or a membrane region and a support region for the mutual attachment of the porous plates. In one example, the first porous plate includes an internal region with a diameter of O4 and a support region. The support region of the second porous plate is attached to the internal region or the membrane region of the first porous plate, and forms a z-distance of less than 50 μm, preferably less than 30 μm, more preferably less than 20 μm between the membrane regions of the first and second porous plates in the direction of a small beam of a plurality of charged particles passing through. In a further example, the multi-beam raster unit includes a porous plate having a thick membrane with a thickness between 100 μm and 500 μm.

[0039] In one embodiment, a structure for 3D alignment of a multi-beam raster unit comprising at least two porous plates is provided. Each porous plate further comprises a support region having a thickness of >100 μm or >50 μm and a maximum deviation in thickness of less than 10%. The porous plates further comprise through-apertures having different aperture diameters A1 and A2 for mutual alignment of the first and second porous plates. In one example, the multi-beam raster unit has an additive thickness ZU as a result of stacking a plurality of at least three porous plates in the direction of a plurality of small beams of charged particles passing through, and a z-distance ZA of the through-apertures, whereby ZA is smaller than ZU, enabling mutual alignment of the porous plates. In one embodiment, at least one porous plate has a reduction or depression in the first through-aperture to form a z-distance to a corresponding through-aperture of an adjacent porous plate, and the z-distance is less than 30 μm to enable mutual alignment of the porous plates. In one example, the multi-beam raster unit comprises at least two alignment axes for mutual alignment of at least two pairs of porous plates, has at least two reductions or depressions in the first and second through-apertures, and forms a z-distance to a corresponding through-aperture of an adjacent porous plate for each, and the z-distance is less than 30 μm.

[0040] In one embodiment, a plurality of transmitted small beams propagate through a plurality of apertures of a plurality of porous plates in a first direction, a high-voltage supply wiring connection is provided to a first electrode in at least one of the porous plates from a second direction perpendicular to the first direction, and a low-voltage supply wiring connection is provided to a second electrode in at least one of the porous plates from a third direction perpendicular to the first and second directions.

[0041] In one embodiment, a multi-beam charged particle microscope comprises a multi-beam raster unit according to one of the above embodiments.

[0042] Embodiments of the present disclosure will be described in more detail with reference to the following drawings.

Brief Description of the Drawings

[0043]

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Mode for Carrying Out the Invention

[0044] A multi-beam charged particle microscope (MCPM) according to one embodiment is illustrated in FIG. 1. The charged particle system 1 includes an illumination system 3. The illumination system 3 is configured to focus a set of small beams of charged particles 5 at the image plane 7. At the image plane 7, the sample 9 can be arranged on a sample stage having an actuator (not shown) so that the sample 9 can be accurately arranged at the image plane 7 with five or more degrees of freedom. The set of small beams 5 includes a raster configuration, for example, several small beams of charged particles in a hexagonal raster, or small beams dispersed on a circle. The number of small beams can be 10 to 10,000 or more, and a typical example of a hexagonal raster configuration includes, for example, more than 90 small beams. Each small beam of the set of small beams 5 is focused on the sample 9 at one spot. The spot distance at the image plane 7 (on the object 9) is usually 5 μm to 15 μm, but other distances such as 1 μm to 200 μm are also possible. For high resolution, the illumination system 3 is configured to generate a small spot diameter, for example, less than 5 nm, less than 3 nm, and even less than 1 nm. The set of small beams 5 of charged particles 5 can include an electron small beam, but other charged particles are also possible. This charged particle interacts with the sample 9, and secondary charged particles including secondary electrons and backscattered charged particles are generated. The secondary charged particles from each spot together form a set of small beams of secondary charged particles 15. The imaging system 11 is configured to collect at least a part of the secondary charged particles and condense the set of small beams of secondary charged particles 15 on the detection plane 19. The imaging system 35 of the illumination system 3 and the projection system 11 in the beam path of the secondary charged particles leading from the image plane 7 to the detection plane are configured such that the image plane 7 of the imaging system 35 coincides with the object plane 17 of the imaging system 11. The imaging system 35 of the illumination system 3 includes a global deflection unit 67. The imaging systems 35 and 11 are coupled by a beam splitting and combining unit 65. The objective system 37 exists in a common imaging path for both the imaging systems 11 and 35. The imaging system 11 further includes an additional imaging element 69. At the detection plane 19, a spatially resolved charged particle detector 13 is arranged. For each of the small beams of secondary charged particles 15, the spatially resolved charged particle detector 13 detects the secondary charged particles of the small beam using at least one detection element.

[0045] The illumination system 3 comprises a light source unit 21 that generates a set of small beams of charged particles 5. The multi-beam generation unit is described, for example, in U.S. Patent Application No. 16 / 277,572, U.S. Patent Application Publication No. 2019 / 0259575, both of which are incorporated herein by reference, and U.S. Patent Application No. 16 / 266,842 filed on February 4, 2019.

[0046] The light source unit 21 includes a charged particle emitter 22 that generates a divergent beam 23 of charged particles. The condenser system 25 collimates the beam 23 with respect to the porous mechanism 27. The porous mechanism 27 includes a first porous plate having a plurality of apertures and a micro-optical raster element. The porous mechanism 27 converts the beam 23 into a set of small beams of charged particles 5. The set of small beams of charged particles 5 is focused on an intermediate focal surface 29, and for each of the small beams of charged particles 5, a focal point 31 is formed on the intermediate focal surface 29. This focusing is achieved by the micro-optical raster element of the porous mechanism 27 in combination with the collimating power of the condenser system 25 and any additional imaging elements 33. Thus, the intermediate focal surface 29 can be curved to compensate for the field curvature of the imaging system 35. The imaging system 35 images the intermediate focal surface 29 onto an image plane 7. The imaging system 35 includes an objective system 37 and an additional imaging element 39. The illumination system 3 is configured to focus the set of small beams 5 onto the sample 9 such that the set of small beams 5 forms a telecentric bundle of small beams, where each small beam is inclined perpendicular to the image plane 7 on which the sample 9 is disposed. To achieve a telecentric set of small beams 5, the multi-beam deflection unit 41 is disposed on the intermediate image plane 29. The deflection unit 41 includes a micro-optical raster element configured as an array of deflectors for individually adjusting the propagation angle for each small beam of the set of small beams 5. A multi-beam raster unit, such as the multi-beam generation unit 27 or the multi-beam deflection unit 41, includes a plurality of porous plates or micro-optical raster elements formed on a planar substrate or wafer. A representative multi-beam raster unit 71 including three porous plates 73.1, 73.2, 73.3 is illustrated in FIG. 2. FIG. 2 shows a part of the internal region or film region of the porous plates 73.1, 73.2, 73.3. For example, the first porous plate 73.1 is configured to have a set of openings or apertures 75.1, 75.2, 75.3. The set of apertures can be arranged in, for example, a hexagonal raster or a different raster configuration such as, for example, a circular arrangement. In one example, the porous plate is configured to generate a set of a plurality of electron small beams 77.1, 77.2, 77.3 from a single incident electron beam 70.In this configuration, a single incident electron beam 70 passes through the aperture, thereby forming a plurality of electron sub-beams 77.1, 77.2, 77.3 of a raster configuration. In other examples, for example, in a multi-beam deflection unit, a first porous plate 73.1 is configured to transmit a pre-formed set of electron sub-beams 77.1, 77.2, 77.3 and includes apertures 75.1, 75.2, 75.3 of a raster configuration of the pre-formed set of electron sub-beams. In each example, at least one of the plurality of electron sub-beams 77.1, 77.2, 77.3 passes through or transmits through the corresponding apertures 75.1, 75.2, 75.3 of the micro-optical raster element 73.1.

[0047] The multi-beam raster unit includes second micro-optical raster elements 73.2 and 73.3 either upstream or downstream in proximity to the porous plate 73.1 in the direction of the incident electron beam 70. Each of the second micro-optical raster elements 73.2 and 73.3 includes a similar or identical set of apertures in a raster configuration as the porous plate 73.1, and a set of electric field elements configured to act individually and independently on each small beam of the set of electron sub-beams. For example, the electric field lens element 80 is generated as an electric field (not shown) by applying a drive voltage to a ring-shaped electrode 79 disposed around the aperture of the micro-optical raster element 73.2 and the lower or bottom side 107 of the first porous plate 73.1 operating as a counter electrode. In use, the electric field lens element 80 focuses the small beam 77.1 onto the focal surface 29 (see FIG. 1). Another example of an electric field element is, for example, a multipole element having four or eight electrodes 81.1 and 81.2 (only two are shown) disposed around the aperture of the third micro-optical raster element 73.3 for individually and independently correcting the shape of the small beam 77.3, or for operating as a deflector, an astigmatism corrector, and for fine adjustment of the focal length. The example of FIG. 2 illustrates the focusing power of the lens element 80 in the example of the small beam 77.1 and the deflection of the small beam 77.3 using the electrodes 81.1 and 81.2. The small beam 77.3 is deflected, for example, in the multi-beam generation unit 27 to minimize the deviation from a pre-defined position in the raster configuration, or in the multi-beam deflection unit 41 to achieve a telecentric imaging state.

[0048] In one example, the multi-beam generation unit 71 includes a planar element 84, such as an electric field element downstream of the micro-optical raster element 73.3 in the direction of the electron beam 70. The planar element 84 is configured with at least one electrode 82 and is used for the global focusing or deflection of the entire set of small beams. The electric field element including the set of electrodes 82 arranged around the aperture is configured to act on the plurality of charged particle small beams as a whole. In one example, a single electrode 82 is provided to form a ring electrode. During use, a homogeneous electric field is formed between the global ring electrode 82 and the proximal porous plate 73.3, and a small electric field is formed inside the apertures of the porous plate 73.3 that equally focuses each of the plurality of small beams.

[0049] The multi-beam charged particle microscope (MCPM) of FIG. 1 further includes a supply hardware and a control unit that supply voltage and current to the optical components of the charged particle microscope, and an operation unit 43 that controls the operation of the MCPM according to user instructions and control software. The operation unit 43 is connected to an image processing unit 49 that processes the image data from the detector 13. The operation unit 43 is further connected to a stage control unit 51 that controls the position or movement of the stage using an actuator, such as a 5-axis stage, that holds the sample 9. The operation unit 43 is further connected to a user interface and other devices such as a storage or a network (not shown). The operation unit 43 is connected to a multi-beam raster control unit 45 that controls at least one of the multi-beam raster units, such as the multi-beam generation unit 27 or the multi-beam deflection unit 41. Each of the multi-beam raster units further includes local control units 47.1 and 47.2. During use, the multi-beam raster control unit 45 and the local control units 47.1 and 47.2 supply voltage and current to the plurality of electrodes of the multi-beam raster unit for the individual control of the plurality of electric field elements, such as a plurality of electric field lenses, deflectors, or aberration correction devices.

[0050] Each electric field element is formed during use between electrodes on at least two different planar substrates of the micro-optical element. For example, a plurality of electric field lenses 80 are formed between a first porous plate 73.1 and a second porous plate 73.2, a plurality of deflectors or correction devices are formed during use between porous plates 73.2 and 73.3, and a global electric field lens is formed between porous plate 73.3 and planar element 84. Each of the porous plates 73.1 to 73.3, or the planar element 84, is realized as a planar substrate or wafer. Therefore, for each electric field element, precise adjustment of two planar substrates or wafers is required. Spacers 83.1, 83.2, and 83.3 are arranged between the elements of the multi-beam generation unit 71 to maintain a constant pre-defined distance between the elements and to insulate the elements from each other. The high-precision horizontal alignment according to an embodiment of the present invention will be described in detail below.

[0051] Throughout the present disclosure, in an illumination beam path having a multi-beam generation unit or multi-beam deflection unit 71, a beam of charged particles 70 and a plurality of small beams 77 propagate in the positive z-direction when the z-direction points downward. It is understood that an arrangement of planar substrates or wafers such as porous plates 73.1, 73.2, 73.3, etc. is arranged continuously in the direction of the charged particle beam or small beam passing through in the positive z-direction. It is understood that the beam incident side or upper side 74 is the first surface or side of the element in the direction of the charged particle beam or small beam passing through, and the lower side or beam exit side 107 is the last surface or side of the element in the direction of the charged particle beam or small beam passing through. However, the present disclosure is not limited to only charged particle beams in the positive z-direction when the z-direction points downward.

[0052] Each electric field element is constituted by a set of one or more electrodes symmetrically arranged around a plurality of apertures of, for example, two porous plates 73.1 and 73.2, and the set of electrodes is configured to form an electric field element such as an electric field lens, a beam deflector, or a beam aberration corrector for each of the plurality of electron microbeams 77. During use, different voltages are applied to each of the electrodes, and an electric field is generated within the apertures and between some of the porous plates. In one example, the electric field element 80 is arranged close to at least one aperture 85 of the porous plate 73.1 and is configured to construct, for example, an electric field lens when the electron microscope is in use. When operating, a specific voltage is applied to the various electrodes 79, 81.1, 81.2 of the electric field element, and a predefined electric field is generated. An example is illustrated in detail in FIG. 3. The porous plate 73.1 is formed by a conductor element having an aperture such as a through hole or aperture 85. For simplicity, FIG. 3 shows only one of the plurality of apertures. The function of the first porous plate 73.1 is to divide the incident beam 70 into a plurality of microbeams including the microbeam 77.1, and the plurality of divided microbeams further propagate in the electron-optical projection system. During use, the electrons of the incident beam 70 either pass through an aperture such as the aperture 85 of the first porous plate 73.1 or are absorbed by the conductor element of the first porous plate 73.1. Thus, the first porous plate 73.1 is configured to have a stopping power for high-energy particles on the incident side of the incident electron beam 70. To avoid charging and generation of repulsive potential, the first porous plate 73.1 is coated with an absorption and conductive layer 99, and the absorption and conductive layer 99 is connected to a large capacitance, for example, to empty or discard the absorbed charge or to ground it. At the aperture 85, the electron beam 70 passes through the aperture 85 of the porous plate 73.1, and the electron microbeam 77.1 is generated. The microbeam 77.1 passes through a predefined electric field lens element 80 and is focused on the focal surface 29 (see FIG. 1).

[0053] In this example, the first porous plate 73.1 operates as a counter electrode together with at least the second planar substrate 73.2, thereby forming an electrostatic lens 80 between the first porous plate 73.1 and the second planar substrate 73.2. This is achieved, for example, by configuring the bottom side or the beam emission side 107 of the first porous plate 73.1 using a layer of the conductive material 108. The electrostatic lens element 80 is generated by applying a driving voltage difference to the electrodes 79 and the conductive layer 108, and the electrostatic field 80 expands in the free space between the electrode 79 and an additional conductive or insulating element close to the electrodes such as the conductive layer 108 having the apertures 85. The equipotential curves of the electrostatic field of the electrostatic lens 80 are illustrated in FIG. 3. Specifically, the electrostatic field further penetrates through the apertures 85 of the porous plate 73.1.

[0054] The optical aberration of a micro-optical element such as the electrostatic lens element 80 is known to be highly susceptible to the influence of manufacturing inaccuracies of the apertures. The deviation of the aperture cross-section from a pre-defined shape is illustrated in the lower half of FIG. 3 and shows the upper or top surface 74 of the porous plate 73.1. The upper or beam incident surface 74 is generally the first surface of the porous plate in the direction of the charged particle beam. The outer surface shape 89 of the aperture 85 deviates from the ideal circular shape indicated by the dashed circle 91. The shape of the electrodes, the shape of additional conductive or insulating elements, in particular the deviation of the apertures such as the aperture 85 of the porous plate 73.1, generates a deviation 93 of the electric field 80 from a pre-defined electric field during use, thereby causing aberrations such that an electron micro-beam such as the small beam 77.1 deteriorates from its pre-defined characteristics. For example, a slowly varying deviation of the pre-defined or ideal outer surface shape 89 of the aperture 85 causes, as shown by the electron beam trajectory 97, for example, tilt or astigmatism and deviates from the ideal trajectory of the small beam 77.1 as shown by the dashed line. High-frequency deviations such as roughness, defined as the standard deviation of the minimum distance between the outer shape and the ideal shape, cause higher-order aberrations or cause the generation of scattered particles. Typical deviations due to the etching process are on the order of up to 0.5 μm rms. Generally, when the power or voltage of the electric field element is applied to the electrodes, the aberration increases. Scattered particles are generated, for example, at the upper edge or the aperture 85 on the incident side of the electron beam 70. In the prior art, the shape of the upper edge and the inner wall deteriorates from the designed shape due to statistical process variations of strong etching and residual inhomogeneities, thereby exhibiting significant roughness, inhomogeneity, and deviation from the designed shape. The diameter of the lower outer shape forming the exit surface diameter of the aperture of the first porous plate can vary between apertures and between wafers. The roughness is increased by the inhomogeneity of the isotropic etching step. These deviations cause a deviation of the electric field in the vicinity of the aperture and thus cause aberrations and image blur in the small beam. Furthermore, for example, the inclination angle of the inner wall 87 can cause an increase in the scattering of electrons passing through a cylindrical aperture such as the aperture 85, as shown by the scattered electron trajectory 95.

[0055] The first porous plate 73.1 of the multi-beam generating unit 27 is a major cause of scattered particles. However, other micro-optical raster elements may also be susceptible to deviations from a predefined shape due to inaccuracies during fabrication that reduce the quality of the image, such as the resolution or contrast of the image generated by the multi-beam electron microscope 1. It is an object of the present invention to minimize aberrations and scattered charged particles in order to achieve high resolution, high contrast, and high throughput.

[0056] In a first embodiment of the present invention, the multi-beam raster unit is configured such that the edge of the electrode and the inner sidewall of the aperture are arranged at a greater distance from the transmitted charged particle micro-beams. Thus, the influence of the deviation between the edge of the electrode and the inner sidewall of the aperture on the beam characteristics is minimized.

[0057] One embodiment of the present invention is illustrated in FIG. 4. In this embodiment, at least a portion of the inner wall of the aperture stop 85 is configured such that the charged particle beam passing through curves and tilts in a direction away from the aperture, with the exit diameter forming the electrode being larger than the entrance diameter of the aperture. By "tilting in a direction away," it is generally understood that a portion of the inner wall is configured such that the tangent at a point on the inner wall in the direction of the z-axis points away from the charged particle beam passing through. As a result, as the z-coordinate increases, the diameter of a portion of the aperture stop continuously increases. In this example, the lower or second portion increases in diameter in the direction of the transmitted electron beam, forming, for example, a concave circular portion at the intersection plane (x-z plane of FIG. 4). In FIG. 4, an example of one aperture 85.1 of the porous plate 73.1 is shown. Generally, this porous plate includes a plurality of apertures 85 as described above in a raster configuration, and only one of them is shown in FIG. 4. The inner wall of the aperture 85.1 is configured to be rotationally symmetric about the symmetry axis 105 of the aperture 85.1, and the symmetry axis 105 is parallel to the incident electron beam 70. The porous plate includes an absorption layer 99 on the incident side 74. The absorption layer is connected to the ground potential.

[0058] During use, most of the incident electrons from the electron beam 70 are absorbed in the absorption layer 99, and a current corresponding to the number of absorbed electrons is generated. For example, when D1 = 30 μm and the pitch is 150 μm, about 97% of the incident electrons from the incident electron beam 70 are absorbed, and a large current of electrons is generated. Therefore, the absorption layer 99 exhibits a fluctuating voltage difference corresponding to the induced current during use, and is thus not suitable for forming an electrode for the electric field element 80.

[0059] The porous plate 73.1 having the aperture 85 includes a first portion 101.1 and a second portion 101.2 having z-extensions L1.1 and L1.2. The incident electron beam 70 strikes parallel to the z-direction and is either absorbed in the absorption layer 99 and diverted or passes through the aperture 85.1. At the incident surface 74, the aperture 85.1 has a diameter D1. The second portion 101.2 is configured to have an inner sidewall forming a circular portion recessed in the x-z plane, and the continuously increasing diameter and the tangent vector 103 in the x-z plane point away from the main direction of the passing electron beam 77. Thereby, the inclination of the inner wall of the second portion 101.2 is directed away from the passing electron beam 77 and terminates with a maximum aperture diameter D2 at the exit surface or bottom surface 107 of the porous plate 73.1. The maximum aperture diameter D2 at the exit surface 107 is larger than the average aperture diameter D2A within the second portion 101.2 which is larger than the aperture diameter D1 of the first portion 101.1. The beam exit surface 107 is coated by a conductive layer 108 connected to a potential, for example, the ground potential. The conductive layer having the boundary or edge of the diameter D2 forms a counter electrode for a subsequent second porous plate 73.2 in the z-direction adjacent to the first porous plate 73.1. In use, to form an electric field element, the second porous plate 73.2 is configured to have a ring electrode 79 around the aperture 85.2 of diameter D3 to form an electric field lens 80 (see also FIG. 3) between the conductive layer 108 and the ring electrode 79 during use. At least in portion 102.1, the inner wall of the aperture 85.2 of the second porous plate 73.2 can be similarly configured to curve or incline away from the passing electron beam 77. In other examples, the inner wall 102.1 of the electrode can be configured parallel to the z-axis. In the example of FIG. 4, the embodiment of the second porous plate 73.2 includes the portion 102.1 having the electrode 79, and in other examples, the second porous plate 73.2 includes a further portion 102.2 (see below). The second porous plate with the electrode 79 has a length L3 of about 30 μm to 300 μm. Examples of the design and manufacturing method of the second porous plate 73.2 are described in U.S. Patent Application Publication No. 2019 / 0259575, which is incorporated herein by reference.

[0060] According to the example of FIG. 4, only one pair of apertures 85.1, 85.2 is shown, but a plurality of electric field elements of the raster configuration between pairs of apertures such as apertures 85.1 and 85.2 are constituted by a plurality of electrodes such as the ring electrode 79 on the incident side of the second porous plate 73.2 and a plurality of electrodes provided on the exit side of the first porous plate 73.1 such as the conductive layer 108. The plurality of electric field elements having the ring electrode 79 form a plurality of electrostatic lenses of the raster configuration during use. In other embodiments, the plurality of electric field elements are configured to have a plurality of electrodes 81 of the raster configuration in order to form a plurality of deflectors or correction devices of the raster configuration, for example, together with the conductive layer 108, during use. At least the second or lower portion of the first porous plate 73.1 that forms the counter electrode for the electrode of the second porous plate 73.2 is curved or inclined in a direction away from the passing electron beam 77 and includes an inner wall configured to form an aperture diameter D2 on the beam exit side that is larger than D1. The fabrication tolerance of the aperture opening at the exit surface 107 generates optical aberration of the electric field element. By increasing the second diameter D2 by more than 10% with respect to the first diameter D1, it is possible to reduce the optical aberration by one-fifth to one-tenth. The first beam incident portion 101.1 of the first porous plate 73.1 is configured as a conductive film having apertures (openings), and the film region between the apertures serves as an aperture stop for the incident electron beam 70 and defines the range of the set of small beams 77.

[0061] In the beam incident portion 101.1, the aperture is cylindrical with a diameter of D1 in the horizontal direction and has a small extension portion L1.1 in the z direction, where 2 μm ≤ L1.1 ≤ 5 μm. Therefore, the thickness of the first portion having parallel sidewalls is significantly reduced compared to the thickness L1 of the first porous plate 73.1. The thickness of the beam incident portion is determined as an adjustment between the function of blocking incident charged particles between the apertures and the control of scattering along the sidewalls described above. By reducing the thickness of the parallel sidewalls of the porous plate by half, the scattering is reduced by 50%. In this example, the second portion 101.2 having curved sidewalls reduces the thickness of the cylindrical aperture to L1.1 = L1 / 2, thereby reducing the scattering of the first porous plate 73.1 by up to 50%. By further reducing L1.1 to L / 3, the scattering of the first porous plate 73.1 is reduced to approximately 30%.

[0062] The distance P1 between the aperture centers is usually in the range of 30 μm ≤ P1 ≤ 250 μm. The area ratio of the surface of the aperture opening to the entire surface of the aperture plate determines the range of the transmittance of the aperture plate. The transmittance tr is usually 5% ≤ tr ≤ 20%, preferably 10% ≤ tr ≤ 15%.

[0063] In one example, the following parameters are selected. D1 = 30 μm, D2 = 40 μm, D3 = 55 μm, D4 = 55 - 75 μm, L1.1 = 5 μm, L1.2 = 5 μm, L1 = 10 μm, L2 = 20 μm, and L3 > 400 μm. In other examples, the following parameters are selected. D1 = 18 m, D2 = 25 μm, D3 = 20 μm, L1.1 = 5 μm, L1.2 = 10 μm, L1 = 15 μm, L2 = 20 μm, and L3 > 40 μm. Preferably, L1.2 ≤ 10 μm.

[0064] In other examples, in the range between the first thicknesses, the relationship of L1.1 ≤ D2 ≤ 4 * L1.1, preferably L1.1 ≤ D2 ≤ 3 * L1.1, and even more preferably L1.1 ≤ D2 ≤ 2 * L1.1 is maintained.

[0065] Generally, the incident-side aperture diameter D1 of the absorption layer 99 is in the range of 10 μm ≤ D1 ≤ 50 μm, the horizontal-direction exit surface diameter D2 of the aperture of the electrode layer 108 is larger than D1, that is, D2 > D1, and D2 can be configured in the range of 15 μm ≤ D2 ≤ 60 μm. The thickness of the first porous plate 73.1 is preferably 3 μm ≤ L1 ≤ 15 μm. It is generally beneficial to have a thin first porous plate 73.1 where L1 < D2, or L1 < D2 / 2, or even better L1 < D1, or even better L1 < D1 / 2. In one example, L1 is at least D2 / 4. It is even more beneficial to have D2 in the range of L1 to 5 times L1, or L1 < D2 < 5*L1, preferably L1 < D2 < 4*L1, more preferably L1 < D2 < 2*L1. In each configuration, the thickness L1 is thin enough to avoid unnecessary scattering of transmitted charged particles, and at the same time thick enough to achieve sufficient absorption of the incident beam of charged particles between the apertures. The incident diameter D3 and the exit diameter D4 of the second porous plate 73.2 are preferably between 15 μm and 75 μm. The distance L2 between the first porous plate 73.1 and the second porous plate 73.2 is preferably very small, for example, 10 μm ≤ L2 ≤ 30 μm. The electric field crosstalk between the plurality of electric field elements is avoided by the axial distance L2 (in the z direction) that is smaller than the aperture diameter D2 or D3. Preferably, D2 is larger than L2 but smaller than 3 times L2, preferably smaller than 2 times L2. Preferably, the first portion 102.1 of the second aperture plate 73.2 has a thickness L3 similar to the diameter D3 or D4. In one example, the incident diameter D3 of the aperture of the second porous plate 73.2 having the electrode 79 is between D1 and D2, so D1 < D3 < D2, but in other examples, D3 can also be larger compared to D2, so D3 > D2. Generally, D1 has the minimum diameter such that D1 < D2, D1 < D3, D1 < D4. By the above-described design and fabrication where D2 is larger than L1 together with D1, preferably 2*L2 > D2 > L2, more preferably D3 > D2, the influence of any deviation in the edge profile of the electrodes of the elements 73.1 and 73.2 that affect the performance and characteristics of the electric field elements is reduced.

[0066] As described in detail below, the first porous plate may include a third portion 101.3. By configuring the inner wall of at least the second portion 101.2 or the third portion 101.3 of the aperture so as to be curved and inclined in a direction away from the passing charged particle beam 77, and further by forming the second or third or fourth diameter D2-D4 of the aperture of the porous plate to be larger than the diameter D1 of the first thin portion of the aperture, the scattering of the passing charged particles 77 is reduced. By configuring the inner wall of the lower portion 101.2 of the porous plate 73.1 so as to be inclined in a direction away from the passing charged particle beam 77 such that the second portion terminates with a larger aperture diameter D2 at the exit surface 107 or the porous plate 73.1, the deviation of the aperture plate of the electric field of the electric field lens 80 and the adverse effects of the fabrication tolerances during fabrication are reduced, and as shown in FIG. 3, the aberration of the passing charged particle beam 77 is reduced. High shape accuracy and position accuracy of the electrodes are achievable, such as the counter electrode at the lower boundary or edge of the aperture 85 having an increased diameter D2>D1 at the exit surface 107 of the porous plate 73.1 covered by the electrode 79 and the conductive layer 108 in the porous plate 73.2. In one example, the conductive layer 108 also covers the side walls of the apertures of at least the second portion 101.2 of the porous plate 73.1. Thereby, secondary electrons or scattered electrons that collide with the side walls are guided in a direction away and do not interfere with the electric field elements formed below. The high-precision distance L2 is realized, for example, by a spacer, and during use, a plurality of electric field elements of the multi-beam generation unit or the multi-beam deflection unit are formed with high precision. Each of the first beam incident portion 101.1 and the second beam exit portion 101.2 is formed on one substrate or slice by the double-sided processing described in detail below.

[0067] The first portion 101.1 of FIG. 4 has parallel inner side walls, similar to a conventional aperture. In other embodiments, it is also possible to configure the first portion 101.1 to have an inner wall that is inclined in a direction away from the passing charged particle beam 77. In such a case, the transmission of the electron beam forms a transition region where the transmission rate gradually decreases as the distance to the aperture center 105 increases.

[0068] FIG. 4 shows only a part of the internal region or the membrane region of the porous plates 73.1 and 73.2. As will be described in detail below, the porous plate further includes a support region that supports the thin film region to achieve mechanical stability.

[0069] In the second embodiment of the present invention, the multi-beam raster unit 71 is composed of at least two separated parts. The influence of the deviation on the optical performance is reduced by the functional separation between the upper edge of the porous plate that operates as a beam stop to form a beam and the lower edge of the porous plate that forms an electrostatic lens by a subsequent second micro-optical raster element. In one example, the first porous plate includes two or more separated planar separation parts or substrates, each of which is processed by one-sided or double-sided structuring. These parts are assembled to form a composite porous plate. Therefore, higher-precision sizes and positions of both edges and lower roughness are realized by separating and processing at least two parts. In addition, the edge of the electrode and the inner side wall of the aperture are arranged at a greater distance from the small beam of charged particle transmission, and thus the influence of the deviation between the edge of the electrode and the inner side wall of the aperture on the beam characteristics is further minimized.

[0070] A porous plate separated into at least two parts providing different functions is shown in FIG. 5. The two parts 101.1 and 101.2 are formed as separated parts and adjusted and joined together using known techniques. Thereby, a functional separation is achieved between a first beam incident part 101.1 forming a charged particle beam 77 passing through and a lower second beam exit part 101.2 forming an electrostatic lens together with an adjacent porous plate 73.2 having an electrode 79. The first part 101.1 is coated with a conductive metal film 99 having the function of absorbing and conducting most of the incident charged particles 70. The second part is formed as a silicon or SOI film having a conductive metal layer 108 forming a counter electrode for a subsequent electrode along the z direction. Both films are aligned and fixed together by fixing points 111 such as adhesion points, for example by adhesion or by bonding or welding. It is preferable to keep the distance L1.3 between the two separated parts 101.1 and 101.2 as small as possible, for example less than the thickness L1.2 of the second part 101.2 or less than the thickness L1.1 of the first part 101.2. Similar to the example described in relation to FIG. 4, it is beneficial to constitute a D2 that is larger than the total thickness L1 formed by the sum of L1.1, L1.2, and L1.3 but not more than 5 times L1, preferably not more than 3 times L1, preferably not more than 2 times L1. In addition, the diameter D2 of the second part 101.2 is larger than D1. Generally, for the dimensions, limitations similar to those given above are preferable. In other embodiments, the inner wall of the second part 101.2 can also be shaped to be inclined in a direction away from the passing charged particle small beam 77. The thin second part 101.2 is precisely constructed and aligned with the adjacent aperture plate 73.2 having the electrode 79 and thus defines a high-precision and aberration-free electrostatic lens during use. In the case of the first porous plate 73.1 comprising at least two separated parts 101.1 and 101.2, each part can be formed by single-sided processing.Each of portions 101.1 and 101.2 can be structured by planar processing techniques known from planar substrates such as metal films, silicon or SOI wafers, and can be accurately aligned with the second porous plate 73.2 along a plurality of symmetry axes 105 (one shown in FIG. 5) of the plurality of apertures 85 (one shown in FIG. 5).

[0071] In other examples, the two portions 101.1 and 101.2 are attached to each other by a special holder that allows passive or active alignment. For active alignment, the holder comprises a guiding element and an actuator such as a piezoelectric actuator.

[0072] The upper aperture diameter D1 at the incident surface 74 can be in the range of 10 μm ≤ D1 ≤ 50 μm, and the exit aperture diameter D2 is larger, for example, it can be in the range of 15 μm ≤ D2 ≤ 60 μm. A smaller distance L2 is preferred in the range of 10 μm ≤ L2 ≤ 30 μm and can be controlled with very high precision that requires sub-μm accuracy. Due to the configuration of the two separated parts 101.1 and 101.2, the first part 101.1 is constructed from the top or incident side 74 to achieve high precision of the incident aperture diameter D1, and the second part 101.2 is constructed from the bottom side 107, thereby achieving high precision of the aperture diameter D2 at the bottom side 107 of the second part 101.2. By configuring the porous plate 73.1 with at least the first separated part 101.1 and the second separated part 101.2 such that the second part 101.2 terminates with an aperture diameter D2 larger than D1 at the exit surface 107 or the porous plate 73.1, the deviation of the aperture plate of the electric field of the electric field lens 80 and the adverse effects of the fabrication tolerances during fabrication are reduced, and as shown in FIG. 3, the aberration of the passing charged particle beam 77 is reduced. Therefore, the shape of the passing charged particle beam 77 is very well controlled, and the electric field element formed between the lower or bottom side 107 having the conductive film 108 and the electrode 79 is well defined. With such design and fabrication, high shape accuracy and position accuracy of the electrode 79, the lower boundary or edge below the aperture 85 due to the increase in diameter D2 > D1 at the exit surface 107 of the second part 101.2 of the porous plate 73.1, and the distance L2 are achieved, and during use, a plurality of electric field elements of the multi-beam generation unit or the multi-beam deflection unit are formed with high precision. The position accuracy of the electrode 79 and the lower boundary or edge below the aperture 85 at the exit surface 107 are well defined with an accuracy better than 0.5 μm.

[0073] In the third embodiment, structural members that reduce aberration and scattering, thereby improving resolution and image contrast, are combined. As a result, the aberration is further reduced. An example is illustrated in FIG. 6. The first porous plate 73.1 of the present embodiment includes three parts and further includes two separate components. The absorption and conductive surface metal layer 99 forms a part of the first beam incident portion 101.1 of the first porous plate 73.1. The second part 101.2 is formed as shown in the example of FIG. 4, and the inner wall curves and slopes in a direction away from the beam through which the charged particles 77 pass. The first part 101.1 and the second part 101.2 have similar characteristics and dimensions as in the example of FIG. 4 and form, for example, one component fabricated by double-sided processing. The first porous plate 73.1 is formed as a separated part made of, for example, silicon or SOI, having through holes or apertures, and further includes a third thick beam emission portion 101.3 attached to the bottom side of the second part 101.2 by, for example, an attachment point 111. Thereby, a small gap with a width L1.3 between 5 and 15 μm is maintained. In other examples, the part 101.3 can be directly attached to the part 101.2 and no gap is formed. The length of the third part 101.3 is about 50 to 300 μm, and the bottom surface of the third part 101.3 forms the bottom surface or the emission surface 107 of the porous plate 73.1. The apertures on the emission side 107 have a maximum diameter D2 > D1, and the emission diameter D2 at the emission surface 107 can be in the range of 30 μm ≦ D2 ≦ 70 μm. The emission surface 107 of the third part 101.3 is connected to a constant potential, for example, the base level, and includes a ring-shaped conductive layer or electrode 114 around or in the vicinity of the aperture with a diameter D2 that forms a counter electrode for the subsequent second porous plate 73.2 in the z direction. As an alternative, the emission surface 107 of the second part 101.3 and the inner sidewalls of the plurality of apertures of the third part 101.3 can also be coated with a conductive layer such as the layer 108.

[0074] The second porous plate 73.2 having the electrode 79 is configured to have a distance L2 between 15 μm and 30 μm with respect to the first porous plate 73.1, and forms an electric field lens together with the electrode of the third portion 101.3 during use. The third portion 101.3 is constructed from the bottom side 107, thereby realizing high precision of the aperture diameter D2 of the third portion 101.3. Accordingly, the shape of the passing charged particle beam 77 is very well controlled, and the electric field element formed between the electrode 114 and the electrode 79 is well defined. By such design and fabrication, high shape accuracy and positional accuracy of the electrode 79, the lower boundary or edge of the diameter D2 at the emission surface 107 of the third portion 101.3 covered by the conductive layer 114, and the distance L2 are realized, and during use, a plurality of electric field elements of the multi-beam generation unit or the multi-beam deflection unit are formed in a raster configuration with high precision. Over the entire thickness L1.3 of the third portion 101.3, the minimum inner diameter D5 of each aperture passing through the third portion 101.3 is larger than D1, and thus, even when the thickness L1.3 is large, scattering of charged particles can be avoided.

[0075] In the fourth embodiment of the present invention, the porous plate is configured to have an etching stop ring around a plurality of apertures. The etching stop ring realizes high-precision shape and roughness of the aperture opening of the incident or emission surface of the porous plate. In the example of FIG. 7, the etching stop ring 109 having a length L1.22 is formed in the bottom or emission surface 107 of the porous plate 73.1 covered by the conductive layer 108. The etching stop ring 109 is formed with high precision by lithography and direct etching, and the inner diameter D2 is larger than D1. Each etching stop ring is etched, for example, into a silicon or SOI wafer by direct etching to form a circular trench, for example SiO 2Or another suitable etch stop material is replenished. By an etch stop ring 109 precisely formed around the periphery of the aperture opening 85 of the exit surface 107, the aperture opening can be etched, for example, by isotropic etching, which is limited by the etch stop ring 109. The etch stop ring 109 enables high-precision control of the diameter D2 to achieve better accuracy of 0.5 μm, or 0.25 μm, or 0.1 μm or less, and the roughness can be controlled to be less than 0.1 μm or less than 0.05 μm. As a result, together with the porous plate 73.2 provided with the electrode 79, an electric field lens is formed with high precision and very small aberration during use.

[0076] The present invention relates to an example of a second porous plate 73.2 provided with a ring electrode 79 for forming an electric field lens element 80 between a first porous plate and a second porous plate, as illustrated in FIGS. 4, 5, 6, and 7. In other examples, the second porous element 73.2 may include a plurality of electrodes 81 for forming an electric field astigmatism correction element, or a correction element, or a deflection element, with the plurality of electrodes 81 arranged around each aperture. Although these embodiments have been described with respect to examples of circular apertures, the present invention is not limited to circular apertures and is applicable, for example, to elliptical apertures or apertures of other shapes, including corresponding-shaped etch stop rings and corresponding-shaped electrodes.

[0077] FIG. 8 illustrates an embodiment of the porous plates 73.1, 73.2 according to the example of the embodiment according to FIG. 7. The porous plate 73.1 includes an internal or central region or membrane 123 and a support region 121 disposed at the outer peripheral edge of the porous plate 73.1. In each of FIGS. 2 to 7, only a part of the internal or central region or membrane 123 having a thickness L1 of, for example, about 10 μm is illustrated for each porous plate. At least the membrane 123 is coated with an absorption and conductive metal layer 99 and includes a plurality of apertures 85 (only four are illustrated) having a distance P1 for the transmitted small beams 77.1, 77.2, 77.3 and 77.4 of charged particles from the incident charged particle beam 70. The internal or central region has a thickness L1 < 15 μm. The apertures in the exit surface 107 are provided with an etching stop ring 109 (only one is illustrated by reference numeral 109). The support region 121 has an additional support structure for improving the mechanical rigidity of the porous plate 73.2, such as a first layer 117 having a thickness L0 and a second layer 119 having a thickness LS. The first layer 117 and the second layer 119 may include an insulating or conductive or semiconductor material such as silicon dioxide (SIO2), metal, or silicon. Adjusting elements 115.1, 115.2 for the attachment and adjustment of subsequent porous plates are attached to the bottom surface 107. By machining the top surface 74 and the bottom surface 107 in a planar machining operation, high-precision aperture openings at the top surface 74 and the bottom surface 107 are maintained.

[0078] According to one embodiment, each porous plate is formed in an assembly of one, two or more parallel planar substrates created, for example, by silicon macrostructuring. Other manufacturing techniques are, for example, additive manufacturing. In silicon microstructuring, the planar substrates are fabricated on a silicon or SOI wafer. The central part of the wafer is constructed as a membrane with a thickness of 3 - 20 μm having openings or through-holes. The wafer with openings or through-holes includes two regions. The openings are realized by an etching process including an isotropic etching process such as SF6 plasma etching, an anisotropic vertical etching such as DRIE (gas chopping or cryogenic), or wet etching. The local etching is controlled by a mask formed by a standard lithography process using a high-precision lithography mask. The wafer is optionally coated with a conductive layer such as gold to enhance both the conductivity and the stopping power for high-speed incident (30 keV) electrons.

[0079] The manufacturing process of the first porous plate 73.1 uses double-sided processing with the first processing on the incident side 74 and the second processing from the rear or bottom side 107. The inner sidewalls forming the circular part are fabricated, for example, by isotropic plasma etching of silicon or SOI using SF6 plasma. This processing can also start from the processing of the second part 101.2 from the bottom or lower side 107 of the porous plate 73.1, and the beam bundle defining the openings of the first part 101.1 is fabricated in the second step. The second porous plate 73.2 is attached to the first porous plate 73.1, for example, by spacers at the periphery of the opening plate, and an insulating vacuum gap is formed between the two opening plates 73.1 and 73.2.

[0080] FIG. 9 details a method of fabricating a porous plate according to an embodiment. Although porous plate 73.1 is referred to, the steps of the fabrication method can be similarly applied to other porous plates 73.2 or 73.3. In step S1, the SOI wafer is provided with three layers: a first thick top layer 129.1, a second intermediate layer 129.2, and a third or device layer 129.3. The intermediate layer 129.2 is formed as a buried oxide layer. The third layer 129.3 is polished to a thickness equal to approximately 10 μm. Optionally, the bottom surface 107 is further configured, for example, by wet etching, to have alignment or adjustment spacers around the wafer. After step S1, in step S2, a circular etching stop ring 109 is formed in the device layer 129.3 from the bottom side 107. A first circular trench is formed by directional RIE etching through a first etching mask (not shown) formed by lithography on the bottom side 107 having a depth of approximately 30 - 75% of the SOI thickness, for example, 3 μm to 7.5 μm. The first etching mask (not shown) is removed, and the trench is filled by deposition of SiO2 to form the etching stop material of the etching stop ring 109. After step S2, in step S3, a second portion 101.2 is formed by isotropic silicon plasma etching. For that plasma etching, a second etching stop mask 131 is formed by photoresist deposition and lithographic patterning. The second etching stop mask 131 includes a plurality of small circular openings 133 concentric with the plurality of circular etching stop rings 109. After isotropic silicon plasma etching, a structure 125 having the shape of a portion of a sphere is formed in the device layer 129.3. The etched shape is controlled by the openings 133, the etching time, and the etching stop ring 109. After step S3, in step S4, a through hole 127 is formed in the device layer 129.3 by directional plasma-assisted etching, for example, DRIE (deep reactive ion etching) with silicon. The etching depth is limited by the intermediate layer 129.2 which serves as an etching stop layer.Thereby, a first portion 101.1 having a cylindrical sidewall and a second portion 101.2 having a curved sidewall of the porous plate 73.1 are formed. The bottom side 107 can be further protected by a protective coating (not shown). In step S5, the results of further etching of the first layer 129.1 and the second layer 129.2 are shown. The first layer 129.1 and the second intermediate layer 129.2 are subjected to through-etching by a known process to form the support structures 117 and 119 of the support region 121. The second etching stop mask 131 and any protective coating are removed. After step S5, in step S6, an electron stopping and conductive layer 99 is formed by deposition of, for example, gold or other thick metal at least at the incident portion on the upper side of the porous plate 73.1. The conductive layer 114 is formed on the bottom side or the lower side 107 to form a counter electrode for a subsequent porous element. Optionally, the porous plate 73.1 is completely covered by the conductive layer 99 or 114 from all sides.

[0081] Figure 10 illustrates a method for fabricating apertures in the porous plate 73.1 by a double-sided directional etching process. In the steps of those processes, a plurality of apertures in the porous plate 73.1 or 73.2 or 73.3 are formed in the SOI substrate by a double-sided etching process, and each of the steps of the etching process is limited by a buried oxide layer consisting of an SOI wafer. Although referring to the porous plate 73.1, the steps of the fabrication method can be similarly applied to the other porous plates 73.2 or 73.3. In the internal region or the film region 123 (see Figure 8) of the porous plates 73.1, 73.2 or 73.3, a wafer having layers 135.1, 135.2 and 135.3 is configured after step S7. In step 8, a first etching mask 137 is formed on the incident side 74, and a plurality of circular apertures 141 (only one is shown) having a diameter D1 are formed. By DRIE etching (RIE1) from the incident side, the aperture 141 is etched through the first layer 135.1. The second layer 135.2 serves as an etching stop layer. The first etching mask 137 is removed, and in step S9, a second etching mask 139 is formed on the back side 107 opposite to the incident side 74. The second etching mask 139 includes a plurality of circular apertures 143 (only one is shown) having a diameter D2 larger than D1. Each of the circular apertures 143 in the etching stop layer 135.3 is formed with lithography accuracy concentric with one of the circular apertures 141 in the first layer 135.1. Thereafter, the aperture of the third layer 135.3 is etched by DRIE from the back or the opposite side 107. In other embodiments, smaller apertures are formed in the etching mask 139, and isotropic etching is applied as described in step S3. After isotropic silicon plasma etching, a curved structure having the form of a part of a sphere is formed in the device layer 135.3. The etched shape is controlled by the aperture (not shown), the etching time, and the etching stop layer 135.2. This method uses a lithography process and an etching process from both sides of the wafer, and thus a very high-precision aperture is realized on the emission side together with the incident side of the porous plate.Thereafter, in step 10, the intermediate layer 135.2 is removed at least at the opening 141, for example, by wet etching from the back side 107. Together with the electrode layer 108, the absorption and conductive layer 99 is applied.

[0082] The porous plate fabricated according to any of steps S7 to S10 includes a first layer 135.1 and a second layer 135.3 together with a third layer 135.2 made of a different material, such as SiO2, in the internal region. Thus, the film region of the corresponding porous plate 73.1 includes a layer made of a different material between the first portion 101.1 and the second portion 101.2. In one example, a part of the inner sidewall of at least one of the plurality of apertures has a curved shape.

[0083] Figures 9 and 10 illustrate the fabrication of a porous plate according to an embodiment in an example of the first porous plate. Steps S1 to S10 should not be understood as necessarily consecutive steps in the order from 1 to 10. Those skilled in the art will understand that other sequences including selected steps in a specific order are possible, as well as variations in combinations with individual steps or other steps not described herein. For example, the electrode layer 114 is deposited on the back or the exit surface 107 of the porous plate in step S1, and through-etching is performed using the same lithography mask to form an etching stop ring in step S2. In another example, the absorption layer 99 is formed on the incident side 74 in step S7 before the RIE etching of the aperture 141. By such a process, some layers of the porous plate are etched together, reducing the alignment error between some lithography processes and etching processes, and further enhancing the performance of the porous plate. For example, SiO 2The batch etching of layers of different materials, including a layer composed of SiNx, silicon, and metal, is based on wet or gaseous chemical etching, together with plasma fluoride etching (RIE or DRIE). Further, alignment marks or holes, described later, can be constructed and fabricated together with the apertures using the same lithography mask and etching process as those described above. The fabrication method including steps S1 and S10, as illustrated in the example of the first porous plate 73.1, is generally applicable as well to the porous plates used in the multi-beam raster unit according to the embodiments of the present invention.

[0084] The second porous plate according to the present embodiment includes an electrode that forms an electric field element during use so as to act on a plurality of charged particle microbeams passing therethrough. In one embodiment, similar to the first porous plate described above, the second porous plate is configured as a porous plate including at least a first portion 102.1 and a second portion 102.2. The second porous plate including at least the first portion 102.1 and the second portion 102.2 is fabricated by a fabrication process including the process steps as described above. An example of the second porous plate 73.2 including at least the first and second portions is shown in FIG. 11. The second porous plate 73.2 includes a first portion 102.1 and a second portion 102.2 both of which contain doped silicon as a bulk material, and an insulating etching stop layer 305 made of, for example, SiO2 therebetween. The first portion 102.1 includes an electrode 79.3 such as a ring electrode around the apertures 85.31, 85.32, which is connected to a voltage source 47.1 or 47.2 (see FIG. 1) via an electrical wiring 175 (only one is shown). In another example, the second porous plate includes a plurality of electrodes for an astigmatism correction device or a deflector as described above. The electrode 79.3 is insulated from the bulk material 183 by an insulating gap 309 filled with an insulating material such as SiO2. There is an insulating layer 311 in which, for example, the wiring 175 is embedded and insulated, on top of the bulk material 183 such as silicon and the electrode 79.3 made of, for example, metal or silicon. A shielding layer 177 exists on top of the porous plate 73.2. The thickness L3 of the first portion 102.1 is in the range of 30 μm ≤ L3 ≤ 50 μm.

[0085] The second part 102.2 separated by the insulating layer 305 has a thickness L7 between 100 μm and 400 μm. The apertures of the second part of the second porous plate 73.2 have an incident diameter D3 between the electrodes 79.3 and optionally a larger diameter D7 by the second part. The insulating etch stop layer 305 enables double-sided machining of the apertures 85.31 and 85.32 from both sides, as described above in conjunction with fabrication steps S7 - S10. Thereby, the position and diameter of the apertures are controlled with very high precision, and the parts of the apertures 85.31 and 85.32 are aligned within an accuracy of less than 1 μm. The thick second part 102.2 is connected to a given potential, such as a ground potential, and operates as a confinement for the electric fields generated during use, improving the performance of a plurality of electric field elements while reducing crosstalk between adjacent electric field elements. The bottom side or beam emission side 107 has an increasing distance with respect to the electric field elements, whereby, during use, the electric fields are shielded from external adverse effects at the apertures of diameter D7 of the second part 305.

[0086] In the double-sided processing by steps S7 to S10, the residual alignment error between the etching masks 137 and 139 for the structuring of both sides of the porous plate (see FIG. 10) can induce residual aberration in the electric field element during use. The residual aberration caused by the alignment error can be minimized by a third portion in the second porous plate. FIG. 12 shows an example of a second porous plate including at least the first and second portions. The second porous plate 73.2 in FIG. 12 shows a first portion 102.1, a second portion 202.2, and a third portion 102.3 disposed between the first portion and the second portion. These three portions 102.1, 102.2, and 102.3 are separated by insulating and etching stop layers 305 and 313. The fabrication process steps S7 to S10 are used for fabrication. The third portion is etched from above to form the inner sidewall of the third portion 102.3 together with the inner sidewall of the first portion 102.1 having a diameter D3. The second portion 102.2 has an aperture portion with a larger diameter D7 to minimize electron scattering. The thickness L3 of the first portion is about 30 μm to 50 μm, the thickness L8 of the third portion is about 20 μm to 50 μm, and the thickness L7 of the third portion is about 50 μm to 300 μm. The bottom side or the beam emission side 107 has an increasing distance with respect to the electric field element, so that during use, the electric field is shielded from external adverse effects at the aperture with a diameter D7 of the second portion 102.2. The configuration of the third portion 102.3 where the first portion 102.1 abuts against the inner sidewall of the aperture, together with the between the first portion 102.1 and the third portion 102.3, improves the accuracy of the electric field element during use, thereby reducing the adverse effect of an alignment defect dx of about 1 μm in aperture formation in the second portion 102.2 formed by backside deep processing. During use, the electric field expands accurately in conjunction with the aperture portion of the first portion 102.1 and is shielded from external influences at the aperture portion of the third portion 102.3. The aperture diameter D7 in the second portion 102.2 is larger than D3, for example, larger by more than 10 μm, so that the alignment defect dx of about 1 μm has no effect and electron scattering is reduced.In the above example, apertures 85.31 and 85.32, 85.41 and 85.42 are etched through at least some of the layers and portions formed in the second porous plate 73.2 in one pass using a single lithography mask and dry etching step to form holes in the upper insulating layer 311, the first doped silicon layer 183, the etch stop layer 305, and the third insulating layer 102.3. In one example, the diameter D7 of the apertures in the second portion 102.2 is the same as the aperture diameter D3 in the first portion 102.1, and apertures 85.31 and 85.32, 85.41 and 85.42 are etched through the second porous plate 73.2 in one pass using a single lithography mask and dry etching step.

[0087] By fabricating a porous plate with two separated parts, which enables the fabrication of the indispensable aperture openings on the incident and exit sides of the porous plate with high lithography accuracy, the multi-beam raster unit is fabricated with high precision and is less affected by fabrication inaccuracies on the optical performance of the electric field elements. By fabricating apertures having curved sidewalls inclined in a direction away from the small beam of transmitted charged particles, the multi-beam raster unit is fabricated with high precision and is less affected by fabrication errors on the optical performance of the electric field elements. Further improvement can be achieved by improving the method of fabricating the apertures. For example, in step S4, the through-hole 127 having a cylindrical sidewall is etched in silicon by the DRIE method, and the aperture edge and sidewall still have a roughness up to about 100 - 500 nm rms. Scattering is reduced by the thin thickness L1.1 of less than 10 μm, preferably less than 5 μm, of the first part 101.1. In one embodiment, a method is provided to significantly reduce the roughness to about 100 nm rms by controlled smoothing of the sidewalls by thermal oxidation of the silicon surface and removal of the formed silicon oxide layer. By oxidation, the protrusions on the sidewalls with larger surfaces are removed. This roughness is reduced by more than half by this method of smoothing by oxidation. Thereby, after application of the smoothing process by oxidation, the surface roughness of about 100 nm is reduced to less than 50 nm rms, for example less than 25 nm rms. The first aperture 127 is formed by DRIE etching with a reduced aperture diameter, reduced by, for example, 0.5 μm to 1 μm. The silicon oxide layer is formed by heat treatment for a predetermined time, and the surface layer of the silicon substrate is oxidized. The thickness of the silicon oxide layer is controlled by the oxidation time, whereby, for example, an oxide layer of 1 μm to 2 μm can be formed. The silicon oxide layer is removed by etching with HF acid, whereby the aperture diameter in silicon is increased by about 0.5 - 1 μm, thereby reducing the roughness of less than 50 nm rms and achieving an aperture sidewall with the desired aperture diameter. In one example, the smoothing by oxidation can be repeated, and a surface roughness of the inner sidewall of less than 25 nm rms, or less than 10 nm rms, is achieved.Furthermore, the doping and doping concentration in the sidewalls of the apertures and in the portions adjacent to the sidewall surfaces are changed by oxidation. Accordingly, scattering in the sidewalls of the first portion 101.1 of the porous plate with a thin thickness L1.1 is further reduced. The smoothing by oxidation can be further applied to other apertures in at least a part of the porous plate. In one example, the roughness of the aperture sidewalls inside the apertures having electrodes is reduced by smoothing by oxidation, and in this case, the aberration of the electric field elements in use is thereby reduced.

[0088] The advanced MCPM requires a complex multi-beam generation unit or multi-beam deflection unit. In an embodiment of the present invention, a multi-beam raster unit 71 such as the multi-beam generation unit 27 or the deflection unit 41 is formed by stacking at least two porous plates and spacers. However, an advanced multi-beam generation unit or multi-beam deflection unit includes a large number of porous plates with, for example, four or more or six or more wafers or plates stacked on top of each other and has a large expansion in the z direction. For the individual and high-precision shaping and correction of a plurality of transmitted charged particle beams, three or more porous elements are required. An example of a multi-beam generation unit includes a first porous plate for performing correction for telecentricity, a second porous lens array for focusing, a third porous aberration correction device for performing aberration correction, a fourth porous lens array for performing image field curvature correction, and a fifth porous deflector for performing deflection. A number of different arrangements of the porous elements are possible, and the numbering does not limit the arrangement of the porous plates. In an example of a multi-beam generation unit, a first porous plate 73.1 for multi-beam generation is arranged in sequence upstream of two third porous aberration correction devices for performing aberration correction, such as a porous plate 73.3 that operates in common as an aberration correction device and a deflector, and a fourth porous lens array for correcting image field curvature, such as a porous plate 73.2. Finally, a porous lens array for focusing and a global lens such as an element 84 follow. High-precision alignment between at least the first wafer and the second wafer is realized, for example, by mechanical alignment or attachment structures provided on the first and second wafers that engage with each other. The alignment or attachment structure is fabricated with lithographic precision and has an accuracy of less than 1 μm or less than 0.5 μm. The alignment or attachment structure is formed to maintain a predefined gap between wafers, such as to maintain a predefined distance L2 between the lower surface or bottom surface of the first porous plate 73.1 and the second porous plate 73.2. In one embodiment, the multi-beam generation unit or multi-beam deflection unit includes a spacer for defining the distance L2 with high precision, for example.For example, the multi-beam generation unit includes four spacers with a thickness of 10 μm. Thereby, the separation S between the porous plate and the subsequent porous plate is maintained with high accuracy of less than 1 μm or less than 0.5 μm within a given range S (10 μm ≤ S ≤ 30 μm).

[0089] In one embodiment, horizontal high-precision alignment with a horizontal accuracy of less than 1 μm is achieved by active 3D alignment of a plurality of porous plates. The z position and z distance of the wafers in the stack can be precisely controlled by planar fabrication of the wafers and spacers, and the horizontal alignment across the charged particle beam is obtained by horizontal active alignment and fixation in the aligned state. The active alignment can be realized, for example, by horizontal displacement of the wafers forming the porous plates with the assistance of 3D alignment marks extending over a large z distance. FIG. 13 shows an example of that embodiment. A multi-beam raster unit 71, such as a multi-beam generation unit or a multi-beam deflection unit, includes four porous plates 151.1 to 151.4 fabricated according to, for example, the description according to one of the above embodiments, and a support unit 153. At least some of the porous plates 151.1 to 151.4 have an internal region 123 forming a film with a thickness of about 10 μm to 50 μm having a plurality of apertures 157 (only 5 are shown), and a support region 121 around each internal region 123. Each support region 121 of each of the porous plates 151.1 to 151.4 has z extensions Z1 to Z4 in the direction of the charged particle beam, each of which is on the order of 100 μm or larger. The porous plates 151.1 to 151.4 are attached to each other with mechanical precision in a specific order and are on the order of several μm in the horizontal position. To achieve higher-precision alignment, at least each subsequent pair of the porous plates 151.1 to 151.4 has at least two alignment holes A1 to A4 or alignment marks fabricated by a high-precision lithography process along at least two alignment axes 155.1 and 155.2. The diameters A1 to A4 are selected between 50 μm and 200 μm. The thickness of the film in the internal region and the thickness of the spacer for each porous plate are between 10 μm and about 100 μm.By forming a porous plate with a thin film in inner regions 157 and support regions 121 of different diameters, the porous plates 151.1 to 151.4 can be stacked at a small distance of the film, and the z-expansion part ZA between the first alignment hole A1 and the last alignment hole A4 is between 75 μm and several hundred μm. In this example, the diameter O4 of the fourth aperture plate 151.4 is configured to be small so that the fourth aperture plate 151.4 can be attached to the film of the third porous plate 151.3. To achieve higher mechanical stability of the film of the third porous plate 151.3, the third porous plate 151.3 is first attached to the second porous plate 151.2. Then, the fourth porous plate 151.4 is attached to the film of the third porous plate 151.3 having improved stability. Thereby, the alignment holes A1 to A4 are formed so as to enable high-precision deep optical alignment. High-precision alignment on the order of 1 μm requires an optical numerical aperture (NA) of 0.35 for a conventional wavelength of 0.5 to 0.65 μm for alignment, which corresponds to a large numerical aperture with a very low depth of focus of less than several micrometers. Therefore, conventional high-precision alignment techniques cannot be used for porous plates having a large z-distance. Therefore, using an optical microscope with a depth of focus greater than 30 μm and a low NA of about 0.1 or less, for example, an NA less than 0.07 and a depth of focus of 75 μm, the diameters A1 to A4 and positions of the alignment apertures are selected to enable deep optical alignment of the lowermost porous plate 151.4 through the upper porous plates 151.1 to 151.3 with better accuracy than 1 μm, preferably 0.5 μm. Therefore, the aperture diameters A1 to A4 increase in the reverse order from A4 to A1, enabling a semi-aperture angle B limited, for example, to about 6° to 8°. When looking in the z-direction through the alignment axis using an optical microscope, the apertures A1 to A4 are visible, and high-precision alignment can be achieved even if the alignment apertures A1 to A4 extend over a large distance ZA, and the resolution of the microscope alone is on the order of several μm, for example, 3 μm.The horizontal accuracy of 1 μm and even 0.5 μm is obtained by edge detection and image processing for an image of circular apertures having diameters A1 to A4, formed with lithography accuracy having better accuracy than 100 nm. Therefore, the centers of the circular apertures A1 to A4 are extracted with high accuracy. The horizontal alignment is performed by horizontal movement by a highly accurate actuator optically monitored until concentric rings formed by the holes A1 to A4 accurately aligned are realized. This is schematically illustrated in the lower part of FIG. 13.

[0090] FIG. 14 illustrates another example of the present embodiment. A multi-beam raster unit 71, such as a multi-beam generation unit or a multi-beam deflection unit, includes five porous plates 151.1 to 151.5 fabricated according to, for example, one of the above-described embodiments, and a support unit 153. At least some of the porous plates 151.1 to 151.5 have an internal region 123 forming a film with a thickness of about 10 μm to 50 μm having a plurality of apertures 157 (only five are shown), and a support region 121 around each internal region 123. Each support region of each of the porous plates 151.1 to 151.5 has z-expansion portions Z1 to Z5 in the direction of the transmitted charged particle beam, each of which is on the order of 100 μm or greater. The multi-beam generation unit or the multi-beam deflection unit 71 includes at least one porous plate having a thickness greater than 100 μm, such as 400 μm, similar to, for example, the second porous plate 73.2, such as the thick film of the porous plate 151.3 in FIG. 14. The thickness of the other films of the internal region and the thickness of the spacers for each porous plate are between 15 μm and about 100 μm. To achieve more accurate alignment, each of the porous plates 151.1 to 151.5 includes at least two alignment holes A1 to A5 fabricated by a high-precision lithography process along at least two alignment axes 155.1 and 155.2. The alignment holes A1 to A5 are shaped to enable high-precision deep optical alignment. Accordingly, the diameters A1 to A5 and positions of the alignment apertures are selected to enable high optical precision deep alignment. Horizontal alignment is performed by horizontal movement by a high-precision actuator optically monitored until concentric rings formed by the accurately aligned holes A1 to A5 are realized. This is schematically illustrated in the lower part of FIG. 14. To achieve individual high-precision alignment between selected porous plates, together with alignment in two horizontal coordinates, some of the alignment holes may be arranged in parallel.For example, two or more alignment holes or marks include, for example, a counter alignment structure, and are provided with at least one round hole as described with reference to FIGS. 13 and 14. At least one of these round holes is such that at least two of the porous plates are through-etched by lithography accuracy, and the diameters of the holes in the at least two porous plates increase in the direction of the alignment axis. Since high-precision alignment using image processing requires that the aperture diameter conforms to the mirror force field of the microscope, the conical aperture for observation needs to enable an aperture angle corresponding to the numerical aperture of the microscope. For example, in the case of a semi-aperture angle greater than 6°, the diameter of the viewing hole in the upper porous plate may be too large to align the upper porous plate with sufficient accuracy of less than 1 μm. When the diameters A2 and A3 of the porous plates 151.2 and 151.3 for deep alignment of the lower porous plates 151.4 and 151.5 are too large for high-precision horizontal alignment, a second alignment structure with smaller diameters A1 and A2 is provided for alignment of the upper porous plates 151.2 and 151.3 parallel to the first holes for alignment of the deeper porous plates 151.4 and 151.5. An example is illustrated in FIG. 15. The two lower porous plates 151.2 and 151.3 are aligned with each other by the alignment axis 155.3 from above (in the positive z direction) by a large hole having a large hole diameter A1.1 passing through the porous plate 151.1 and a reduction or lowering portion 160 having a large hole diameter A1.1 in the thick second porous plate 151.2 to reduce the z distance ZA3 of the alignment marks with diameters A2.3 and A3.2 to less than 30 μm, enabling mutual optical alignment of the deep alignment edges of the porous plates 151.2 and 151.3 having a z distance with a semi-aperture angle B1 of about 8° and a horizontal accuracy of less than 1 μm.For example, the first two upper porous plates 151.1 and 151.2, which require a very high-precision horizontal alignment with a horizontal accuracy of about 0.5 μm, are provided with additional alignment marks of diameters A1.2 and A2.1 on the second alignment axis 155.4 for the alignment of only the first and second porous plates. The first porous plate further has a first reduction or lowering portion 160 to reduce the z-distance ZA2 of the alignment marks of diameters A1.2 and A2.1 to less than 30 μm. A larger opening angle B2 of about 10° is made possible by the reduction or lowering portion 160 having a larger diameter A0.2. The third mutual alignment of the lower porous plates 151.3 and 151.4 can be achieved from the back side using alignment marks of diameters A3.4 and A4.3 by optical alignment in the negative z-direction of the coordinate system shown in FIG. 15. For alignment from the back side, the alignment mark diameter A3.4 of the porous plate 151.3 is selected to be smaller than the alignment mark diameter A4.3 of the porous plate 151.4, and the alignment mark diameter A4.3 of the porous plate 151.4 is selected to be smaller than the diameter A5.4 of the through-hole in the lower porous plate 151.5, enabling a semi-opening angle B3 of about 8° for 3D mutual optical alignment with an accuracy of about 1 μm for the porous plates 151.3 and 151.4. For example, when the exit side of the porous plate is coated with a metal film having a high reflectivity compared to silicon or SOI, alignment from the back side or the bottom side is beneficial. In one example, the surface of the second porous plate to which the first porous plate having the alignment hole of diameter A1 is aligned includes alignment marks such as a round hole or a reflective (metal) structure having a diameter A2 smaller than diameter A1. A reflective structure such as a circular structure ensures high contrast and high brightness and enables alignment in the deep alignment hole.

[0091] After realizing a horizontal 3D alignment with an accuracy of less than 1 μm, at least a pair of the porous plates or the entire stack of porous plates is fixed by a fixing technique such as clamping, adhesion, or adhesive bonding. In the case of the direct etching process, each hole is axisymmetric (along the axis of the hole) and parallel to the alignment axis in the coordinate system shown in FIGS. 13 and 14, which is parallel to the z-axis. The 3D alignment structure is provided at at least two locations on the porous plate to enable rotation along with the alignment in the horizontal direction.

[0092] After design, fabrication, and assembly according to one of the above-described embodiments, the multi-beam raster unit is fabricated with high precision and with little influence from fabrication inaccuracies and deviations in the optical performance of the electric field elements. However, the optical performance of the electric field elements decays during use, for example, due to bending or deformation of the film during use, which leads to, for example, curvature of the focal plane of a plurality of charged particle micro-beams or deviation from the telecentricity characteristics of the plurality of micro-beams. The telecentricity characteristics generally refer to the beams that are each of the plurality of micro-beams. A perfect bundle of micro-beams is either concentric or telecentric, meaning that the plurality of micro-beams either converge to a single coincidence point or diverge from that coincidence point. Strictly speaking, telecentricity means that the convergence point is at infinity and all the micro-beams propagate in parallel, for example, parallel in the z-direction. Deviations from the telecentricity characteristics are the deviations of the individual micro-beams from a pre-defined coincidence point. The multi-beam raster unit is designed for a specific shape of the field plane 29, for example, a planar shape or a spherical shape, or for specific telecentricity characteristics, for example, a plurality of perfectly parallel micro-beams or a plurality of micro-beams having a pre-defined coincidence point. In one embodiment, the multi-beam raster unit includes a deformable membrane, and a pre-defined shape of the field plane 29 or specific telecentricity characteristics are realized by passive or active deformation of the membrane. The membrane deformation induced, for example, by a thermal effect or the active membrane deformation as described later can be monitored by a capacitance sensor.

[0093] Passive deformation is realized, for example, by the application of a coating of a specific thickness. The coating applies either compressive stress or tensile stress to the coated element, and the coated element is deformed either convexly or concavely. According to one embodiment of the present invention, the deformation of the porous plate is measured after fabrication, and the film thickness of the stress compensation film is determined to generate a specific amount of deformation for the porous plate. The film is deposited on the porous plate, thereby compensating for unwanted deformation or realizing a predetermined deformation in the porous plate. The stress compensation layer can be, for example, a layer made of SiNx having a thickness between 100 nm and 1 μm. By the method of stress engineering using this additional stress compensation layer, passive deformation between 20 μm and 30 μm with an accuracy of less than several μm, for example less than 1 μm, in the z direction is realized.

[0094] In one embodiment, active deformation is realized by applying a voltage difference to electrodes on at least two porous plates or to electrodes for deforming at least one of the films in the internal region of an additional porous plate using an existing electrode. In other embodiments, active deformation of the film of the porous plate is realized by heating or cooling a film having layers of different thermal expansion, such as a metal layer. In another example, active deformation of a film provided with apertures or electrodes for forming an electric field element is realized by an actuator in the support region of the film that generates a bending force on the film in the internal region of the porous plate during use. Thereby, active deformation between 20 μm and 30 μm with an accuracy of less than 1 μm in the z direction is obtained during use.

[0095] FIG. 16 illustrates an example of the deformable membrane 161 by deformation control. In the multi-beam raster unit 71, the membrane 161 of the second porous plate 73.2 is deformed in the internal region 123 according to a predefined deformation in order to achieve image plane curvature. The designed deformation is realized by a SiNx coating 163 having a predetermined thickness by stress engineering. The capacitance sensor 165 monitors the deformation of the membrane 161 in the internal region 161 of the second porous plate 73.2, thereby controlling the distance to the membrane 171 of the first porous plate 73.1. During use, this deformation is changed by an actuator 169, such as a piezoelectric actuator, which generates a bending force with respect to the outer periphery of the membrane 161. Thereby, the designed image plane curvature is obtained by passive and active deformation of the membrane 161 and maintained during use.

[0096] During use, the incident beam of charged particles either is absorbed by the first porous plate or passes through a plurality of apertures, forming a plurality of small beams. The plurality of small beams pass through the plurality of apertures of the plurality of subsequent porous plates, and each small beam is focused, aberration-corrected, or deflected by an electric field element in a predetermined manner as described above. Even with the improvements in design and fabrication as described above, some charged particles are still scattered and absorbed, for example, on the upper surface of the second or third porous plate, the beam incident surface, or the inner sidewalls of the apertures. The scattered charged particles can generate x-rays. During use, the absorbed charged particles or x-rays can have an adverse effect on the electric field element and thus cause aberrations, such as generating surface charges in an isolator. In a further improvement of embodiments of the present invention, a design and fabrication method for reducing the absorption of charged particles is provided. According to that improvement, in addition to the conductive and absorbing layer 99 on the beam incident side of the first porous plate, the conductive layer is provided on the beam incident sides of the second porous plate of the multi-beam raster unit and further subsequent porous plates. In one example, the electrodes arranged near the edges of the plurality of apertures, the wiring connections, and the insulator of the electric field element that serves to insulate the substrate are shielded by a conductive shielding layer. An example in an example of the first portion 102.1 of the second porous plate 73.2 is shown in FIG. 17. The second porous plate 73.2 includes an internal region or film having a plurality of apertures 85.21 and 85.22 (only two are shown) and ring electrodes 79 and 79.2 arranged around the apertures 85.21 and 85.22. These apertures are aligned with the plurality of apertures of the first porous plate to allow a plurality of charged particle small beams 77.4 (only one is shown) to pass through. The ring electrodes are insulated from the bulk silicon or SOI substrate 183 via an insulating interval 185, for example, by silicon oxide of an insulating material. Each ring-shaped electrode 79, 79.5 is electrically connected to a voltage source via electrical wirings 175, 175.2 to a voltage maintaining unit (not shown) and is insulated from the substrate 183 by an insulating material 179 such as silicon oxide. The wiring connections 175, 175.2 can be from the upper or lower side of the second porous plate or can alternate from both sides between the electrodes.The insulating material 179 extends over the wiring 175 such that the wiring 175, the electrode 70, and the bulk material 183 are completely covered on the upper side. The inner sidewall of the electrode 79 is not covered by the insulating material 179. On top of the insulating material, a conductive shielding layer 177 is formed to form the beam incident surface or the upper surface 173 of the second porous plate. The conductive layer has a protruding extension 189 and extends into the aperture, and a small insulating gap 181 is formed between the conductive shielding layer 177 and the electrode 79.1, thereby insulating the conductive layer 177 from the electrode 79.1. The conductive layer 177 is connected to a large capacitance, such as grounded (U = 0V) for example. Thereby, during use, the scattered charged particles 187 are absorbed by the conductive shielding layer 177, guided in a divergent direction, and the interfering surface charges are avoided. When the conductive layer 177 is connected to a large capacitance, a stable electric field element is generated during use. The surface charges in the small insulating gap 181 disappear due to the small distance of the insulating gap 181. Thereby, the surface charges during use are reduced to less than 10% compared to the conventional second porous plate. After passing through the first porous plate, the scattered particles and secondary particles formed by the transmitted charged particle small beam 77 have low kinetic energy, and the conductive shielding layer 177 made of a thin or low-density metal, such as aluminum for example, compared to the conductive layer 99 of the first porous plate 73.1, absorbs the scattered charged particles and deflects them. In one example, the multi-layer electrodes 79 and the shielding layer 177 of the electric field element are realized by an "integrated etching" approach using a series of dry etching processes using one and the same etching mask with apertures corresponding to the apertures 85.2 (two apertures 85.21 and 85.22 are shown in FIG. 17). Thereby, high-precision fabrication of the shielding layer 177 having the protruding extension 189 aligned with the inner wall of the electrode 79 is provided, and during use, a plurality of electric field elements are provided with high reproducibility and no additional astigmatism or deviation of the charged particle beam caused by misalignment of the protruding extension 189 with respect to the electrode 79. In an alternative configuration, an aperture through the electrode 79 is first formed, the roughness of the inner sidewall is reduced by the smoothing process by oxidation described above, and the etching of the shielding layer 177 is performed from the back side of the second porous plate 73.2.An extension protruding into the apertures of the second porous plate is provided with high precision in the conductive shielding layer 177, and by forming a small gap with respect to the electrode, during use, an electric field element is generated with higher precision and stability, and aberration is reduced.

[0097] In use, to enhance the performance of a multi-beam charged particle microscope, for example, by individual focus correction using a plurality of individually controlled ring electrodes 79, or a plurality of individually controlled electrodes of an astigmatism correction device or a deflector, each of the plurality of charged particle micro-beams is individually controlled. The individual control of the plurality of electrodes is realized by wiring, and additional wiring is provided for the capacitance sensors and actuators or other sensors described above, and for the shielding and absorption layers described above. One multi-beam raster unit for a plurality of micro-beams such as 100, for example, comprises about 1000 or more electrodes together with about 1000 or more individual wiring connections. The electrodes and the shielding or absorption layers require drive voltages of different orders of magnitude, for example, between 10 V and 1 kV. For example, compound focus correction requires 100 high-voltage wirings of about 200 V, compound astigmatism correction requires, for example, 800 low-voltage wirings for a few volts with very low noise, and the absorption layer generates a large current. Wirings having such voltage differences are easily influenced by each other, thereby degrading the performance of the multi-beam raster unit. In one embodiment, the multi-beam raster unit comprises design features and structures that minimize the influence of voltage differences. The multi-beam raster unit comprises a mixed-signal architecture for different voltages and currents. The high voltage is provided by an external controller. The low voltage is provided by an ASIC arranged in vacuum having a digital interface to the external controller. Signal routing and voltage supply are obtained via a UHV flange. The separation of wirings having different voltages is realized by supplying voltages from different directions. In the case of the first direction (z direction) of the transmitted charged particle micro-beam, for example, the low voltage is supplied from the second direction (x direction), and the high voltage is supplied from the third direction. The large-current connection to the absorption layer can be provided, for example, from a fourth direction such as the z direction, or in parallel from the third direction. All wirings can be individually shielded, or the low-voltage supply wirings can be shielded in the low-voltage wiring group. The fewer the high-voltage wirings, the greater the distance that can be provided. In one embodiment, the wiring connections to the ring electrodes for the electrostatic lens are provided alternately from above and below between the electrodes to keep the distance between the wirings as long as possible.FIG. 18 shows an example of the embodiment. A multi-beam raster unit 71 including five porous plates 73.1 to 73.5 each having a film and a support structure in a support region 197 arranged in parallel in a film region 199 is mounted on a support plate 153. Through the support structure and the support plate, a high-voltage wiring connection 201 is provided to a ring electrode of at least one electrostatic lens of the porous plates having a plurality of apertures 195 (only 4×5 are shown, 91, 100, 300, or more are also possible) in the positive and negative y directions. The high-voltage wiring connection is shielded by a grounded ground wire 203. In the peripheral region, the high-voltage wiring is shielded by coaxial shielding and insulation 205 (four high-voltage wiring connections and coaxial shielding are shown, only one is indicated by reference numerals 201 and 205). Low-voltage wiring connections 207 and 209 for the spherical aberration correction device and the deflector are provided from both the x directions (only the positive direction is shown) from ASICs 211 and 215 mounted on the support plate 153. The low-voltage wiring is further shielded from each other by a ground wiring (not shown) between the low-voltage wirings. The ASIC obtains power supply by low-voltage supply lines 219.1 and 219.2 together with digital signals by digital signal lines 217.1 and 217.2. Thereby, the high-voltage signal and the low-voltage signal are separated as much as possible, the adverse effect of mutual induction of leakage is reduced, and more reliable optical performance of the multi-beam raster unit is realized.

[0098] FIG. 19 schematically illustrates a plurality of low voltage wiring connections 107 of a porous plate 73.3, which comprises a plurality of anastigmatic correction devices or deflectors formed during use by eight electrodes 81.1 ··· 81.8 for each of a plurality of apertures 85 having a pitch P1 (only two are shown by reference numerals 85.1 and 85.2). Electrodes in the directions of 0°, 90°, 180° and 270° are connected to the peripheral ASICs 211, 215 (see FIG. 18) shown in FIG. 19, in the x-direction, on the beam incident side of the porous plate 73.1. Correction electrodes in the directions of 45°, 135°, 225° and 315° are connected to the peripheral ASICs 211, 215 (see FIG. 18) in the backside x-direction or in different layers of the porous plate 73.3 and are thus invisible in FIG. 19. During use, a plurality of low voltages in the range of 5 V to 100 V are applied to the plurality of electrodes in the x-direction. The low voltage is separated from, for example, a high voltage applied to the first porous plate, where the high voltage is supplied in the y-direction.

[0099] The multi-beam raster unit of the above embodiment is described in an illumination beam path having charged particle beams propagating in the positive z-direction when the z-direction points downward. However, the multi-beam raster unit is also applicable in an imaging beam path, in which case the charged particle micro-beams propagate in the negative z-direction in the coordinate system of FIG. 1. Still, a series of porous plates are arranged in order in the propagation direction of the transmitted charged particle beam or micro-beam. It is understood that the beam incident side or the upper side is the first surface or side of the element in the direction of the transmitted charged particle beam or micro-beam, and the lower side or the beam exit side is the last surface or side of the element in the direction of the transmitted charged particle beam or micro-beam.

[0100] Although the multi-beam raster unit according to the present invention has been described in some of several embodiments as a multi-beam generation unit, the features of those embodiments are equally applicable to other multi-beam raster units according to the present invention, such as a multi-beam deflector or a multi-beam anastigmatic correction unit.

[0101] The features of the embodiments improve the performance of a multi-beam charged particle microscope in order to achieve a resolution of less than 5 nm, preferably less than 3 nm, more preferably less than 2 nm, and even more preferably less than 1 nm. Such improvements are particularly suitable for further development of multi-beam charged particle microscopes using a large number of small beams, such as more than 100 small beams, more than 300 small beams, more than 1000 small beams, and even more than 10000 small beams. Such multi-beam charged particle microscopes require, for example, porous plates with a larger diameter, more apertures and electrodes, including more wiring connections. These improvements are particularly suitable for the daily application of multi-beam charged particle microscopes in semiconductor inspection and verification, where high reliability and reproducibility, and low mechanical deviation are required.

[0102] The above embodiments provide a charged particle beam system that operates using a large number of charged particle beams and can be used to achieve higher imaging performance. In particular, a narrower range of resolution for each small beam of a plurality of small beams is achieved. By the features and methods described in the embodiments and combinations thereof, each small beam of the plurality of small beams achieves a small beam diameter in the range of, for example, 2 nm to 2.1 nm, with an average resolution of 2.05 nm, and the range of resolution achieved by the features and methods of the embodiments is less than 0.15% of the average resolution, preferably less than 0.1%, and more preferably less than 0.05%.

[0103] The present invention is not limited to the embodiments described above. The embodiments can be combined with each other, either wholly or partially. As can be seen from the above description, many variations and modifications are possible, and it is clear that the scope of the present application is not limited by specific embodiments.

[0104] Although the improvements have been described in the context of an example of a multi-beam charged particle microscope, they are not limited to multi-beam charged particle microscopes and are also applicable to other multi-beam charged particle systems such as multi-beam lithography systems.

[0105] Throughout the above embodiments, electrons should generally be understood as charged particles. Although some embodiments have been described in the context of electrons, they are not limited to electrons and are fully applicable to all types of charged particles, such as helium or neon ions, for example.

[0106] A list of reference numerals is provided below. 1 Charged Particle System 3 Illumination System 5 Set of Small Charged Particle Beams 7 Image Plane 9 Sample 11 Imaging System 13 Spatial Resolution Charged Particle Detector 15 Set of Small Secondary Charged Particle Beams 17 Object Plane 19 Detection Plane 21 Light Source Unit 22 Charged Particle Emitter 23 Divergent Beam of Charged Particles 25 Condensing System 27 Multi-Beam Generation Unit 29 Intermediate Focus Surface 31 Multiple Foci 33 Imaging Element 35 Imaging System 37 Objective System 39 Imaging Element 41 Multi-Beam Deflection Unit 43 Operation Unit 45 Multi-Beam Raster Control Unit 47 Local Control Unit 49 Image Processing Unit 51 Control Unit 65 Beam Splitting and Combining Unit 67 Global Deflection Unit 69 Imaging Element 70 Incident Electron Beam 71 Multi-Beam Raster Unit 73.1, 73.2, 73.3, 73.5 Porous Plate 74 Beam incident side or upper side 75.1, 75.2, 75.3 Set of openings or apertures 77, 77.1, 77.2 Multiple electron sub - beams 79 Ring - shaped electrode 80 Electric field lens element 81.1, 81.2 Electrodes 82 Electrode 83.1, 83.2, 83.3 Spacers 84 Planar element 85 Aperture 87 Inner wall 89 Outer surface shape of the aperture 91 Ideal circular shape 93 Electric field deviation 95 Scattered electron trajectories 97 Electron beam trajectory 99 Absorbing and conductive layer 101.1, 101.2, 101.3 Parts of the first porous plate 102.1, 102.2, 102.3 Parts of the second porous plate 103 Contact vector in the x - z plane 105 Symmetry axis of the aperture 107 Bottom side or beam exit side 108 Layer of conductive material 109 Etching stop ring 111 Fixed point or attachment point 114 Electrode 115.1, 115.2 Adjusting elements 117 First layer of the support structure 119 Second layer of the support structure 121 Support area 123 Inner area or membrane area 125 Structure having the shape of a part of a sphere 127 Through - hole 129.1, 129.2, 129.3 Layers 131 Etching stop mask 133 Circular opening 135.1, 135.2, 135.3 Layers 137 First etching stop layer 139 Second etching stop layer 141 Circular opening 143 Circular opening 151.1, 151.2, ···, 151.5 Porous plates 153 Support unit 155.1, 155.2, 155.3, 155.4 Alignment axes 157 Multiple apertures 160 Reduction or lowering part 161 Deformed membrane of the second porous plate 163 SiN coating 165 Capacitive sensor 169 Actuator 171 Membrane of the first porous plate 173 Beam incident surface or upper surface of the second porous plate 175 Electrical wiring 177 Conductive shielding layer 179 Insulating material 181 Insulating gap 183 Bulk material 185 Insulating gap 187 Scattering particles 189 Expansion part 195 Multiple apertures 197 Support area 199 Membrane area 201 High voltage wiring connection 203 Ground wire 205 Coaxial shielding and insulation 207 Low voltage wiring connection 209 Low voltage wiring connection 211 ASIC 215 ASIC 217.1, 217.2 Digital signal lines 219.1, 219.2 Low voltage supply lines 305 Etching stop or insulating layer 309 Insulating gap 311 Insulating layer 313 Etching stop or insulating layer

Claims

1. A multi-beam raster unit such as a porous unit, a multi-beam deflector, or a multi-beam aberration corrector, configured to form a plurality of electrostatic elements that affect a plurality of transmitted sub-beams of charged particles during use, - a first porous plate having an internal region forming a film of a first thickness L1, having a plurality of first apertures, a beam incident side, and a beam exit side, - a film of the first porous plate comprising at least a first portion having a first partial thickness L1.1 and having a plurality of cylindrical apertures having a first diameter D1 on the beam incident side, and a second portion having a plurality of apertures having a second diameter D2 on the beam exit side, - a second porous plate having an internal region forming a film having a plurality of second apertures and a beam incident side, the plurality of apertures having a third diameter D3 on the beam incident side, the second porous plate, - a gap of thickness L2 between the films of the first and second porous plates, - at least a first electrode configured proximate to at least the first apertures of the first porous plate on the beam exit side of the first porous plate, - at least a plurality of second electrodes configured proximate to the plurality of second apertures of the second porous plate on the beam incident side of the second porous plate to form the plurality of electrostatic elements between the plurality of first and second apertures of the first and second porous plates during use, comprising, - the second diameter D2 is greater than the first diameter D1, and the second diameter D2 is in the range between the second thickness L2 and twice the second thickness L2, thus L2 < D2 < 2*L2, - the first partial thickness L1.1 is less than 10 μm, - the first electrode and each of the plurality of second electrodes are configured to form the electrostatic elements acting on the plurality of transmitted sub-beams of charged particles during use, a multi-beam raster unit.

2. The multi-beam raster unit according to claim 1, wherein an inner sidewall surface of at least one of the plurality of apertures in the second portion has a surface shape that slopes away from the transmitted sub-beam of charged particles.

3. The multi-beam raster unit according to claim 2, wherein the surface shape is a surface shape curved in the direction of the transmitted sub-beam of charged particles.

4. The multi-beam laser unit according to any one of claims 1 to 3, wherein an inner side wall surface of at least one of the plurality of apertures in the second portion has a surface shape such that the diameter of the aperture continuously increases as the z coordinate increases.

5. The multi-beam laser unit according to claim 3 or 4, wherein the surface shape in the direction of the transmitted small beam of charged particles is spherical formed by isotropic etching.

6. The multi-beam laser unit according to any one of claims 1 to 5, further comprising an etching stop ring in the aperture on the beam emission side of the first porous plate or on the beam incident side of the second porous plate.

7. The multi-beam laser unit according to any one of claims 1 to 6, wherein in the direction of the plurality of transmitted small beams of charged particles, the first porous plate is disposed in the beam path upstream of the second porous plate.

8. The multi-beam laser unit according to any one of claims 1 to 7, wherein in the direction of the plurality of transmitted small beams of charged particles, at least a third porous plate is disposed in the beam path upstream of the first and second porous plates.

9. The multi-beam laser unit according to any one of claims 1 to 8, wherein a surface roughness of an inner side wall surface of at least one of the plurality of apertures in the first portion is less than 50 nm rms.

10. The multi-beam laser unit according to any one of claims 1 to 9, wherein the second diameter D2 is in a range between the first partial thickness L1.1 and four times the first partial thickness L1.1, and thus L1.1 ≤ D2 ≤ 4 * L1.

1.

11. The multi-beam laser unit according to any one of claims 1 to 10, wherein the third diameter D3 is larger than the first diameter D1.

12. The multi-beam laser unit according to any one of claims 1 to 11, wherein the third diameter D3 is in a range between the first diameter D1 and the second diameter D2.

13. The multi-beam laser unit according to any one of claims 1 to 12, wherein the first porous plate (73.1) includes an absorption layer (99) on the beam incident side (74), and during use, the absorption layer (99) is connected to a ground potential.

14. The first porous plate (73.1) has a conductive layer (108) on the beam emission side (107), and the conductive layer (108) forms the first electrode. The multi-beam raster unit according to any one of claims 1 to 13.

15. The beam emission side (107) of the first porous plate (73.1) has a ring-shaped conductive layer or electrode (114) around or in the vicinity of the opening with diameter D2 that is connected to a constant potential during use, and the ring-shaped conductive layer or electrode (114) forms the first electrode. The multi-beam raster unit according to any one of claims 1 to 14.

16. The beam emission side (107) of the first porous plate (73.1) is provided without a shielding electrode. The multi-beam raster unit according to any one of claims 1 to 15.

17. The second porous plate (73.2) has a ring-shaped electrode (79) arranged around the second opening, and during use, a driving voltage is applied to the ring-shaped electrode (79), and the ring-shaped electrode (79) forms the second electrode. The multi-beam raster unit according to any one of claims 1 to 16.

18. The ring-shaped electrode basically extends through the second porous plate (73.2). The multi-beam raster unit according to claim 17.

19. The beam incident side of the second porous plate is covered by a shielding layer having at least one protruding extension to at least one of the openings of the second porous plate. The multi-beam raster unit according to claim 17.

20. The first and second parts are separate parts attached to each other. The multi-beam raster unit according to any one of claims 1 to 19.

21. It includes a holder for mutual alignment and adjustment of the first and second parts of the first porous plate. The multi-beam raster unit according to claim 20.

22. It includes a holder for mutual alignment and adjustment of at least the first and second porous plates. The multi-beam raster unit according to any one of claims 1 to 21.

23. At least said membrane of one porous plate comprises a layer of a predetermined thickness, said layer inducing stress-induced deformation of said membrane, the multi-beam raster unit according to any one of claims 1 to 22.

24. Each of said porous plates further comprises a support region having a thickness greater than 50 μm, the maximum deviation of said thickness being less than 10%, the multi-beam raster unit according to any one of claims 1 to 23.

25. Said porous plate further comprises through holes having different aperture diameters A1 and A2 for the mutual alignment of said first and second porous plates, the multi-beam raster unit according to claim 24.

26. At least one porous plate comprises a reduction or lowering portion in a first through hole in order to form a z-distance to corresponding through holes of adjacent porous plates, the z-distance being less than 30 μm in order to enable the mutual alignment of said porous plates, the multi-beam raster unit according to claim 25.

27. Said plurality of transmitted small beams propagate through a plurality of apertures of a plurality of porous plates in a first direction, the high voltage supply wiring connection being provided to a first electrode in at least one of said porous plates from a second direction perpendicular to said first direction, and the low voltage supply wiring connection being provided to a second electrode in at least one of said porous plates from a third direction perpendicular to said first and second directions, the multi-beam raster unit according to any one of claims 1 to 26.

28. A multi-beam charged particle microscope comprising the multi-beam raster unit according to any one of claims 1 to 27.

Images