Patent Publication Number: US-2022223366-A1

Title: Apparatus using multiple beams of charged particles

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. application 62/492,043 which was filed on Apr. 28, 2017 and which is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an apparatus for inspecting or observing samples such as wafers and masks used in a device manufacturing process such as the manufacture of integrated circuits (ICs). 
     BACKGROUND 
     A device manufacturing process may include applying a desired pattern onto a substrate. A patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate the desired pattern. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. A single substrate may contain a network of adjacent target portions that are successively patterned. A lithographic apparatus may be used for this transfer. One type of lithographic apparatus is called a stepper, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time. Another type of lithography apparatus is called a scanner, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. 
     In order to monitor one or more steps of the device manufacturing process (e.g., exposure, resist-processing, etching, development, baking, etc.), a substrate patterned by the device manufacturing process or a patterning device used therein may be inspected, in which one or more parameters of the substrate or the patterning device may be measured. The one or more parameters may include, for example, edge place errors (EPEs), which are distances between the edges of the patterns on the substrate or the patterning device and the corresponding edges of the intended design of the patterns. Inspection may also find pattern defects (e.g., failed connection or failed separation) and uninvited particles. 
     Inspection of substrates and patterning devices used in a device manufacturing process can help to improve the yield. The information obtained from the inspection can be used to identify defects, or to adjust the device manufacturing process. 
     SUMMARY 
     Disclosed herein is an apparatus comprising: a first electrically conductive layer; a second electrically conductive layer; a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; and an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer. 
     According to an embodiment, the third electrically conductive layer comprises a plurality of holes, wherein the holes house the optics elements. 
     According to an embodiment, the optics elements are electrically insulated from the third electrically conductive layer. 
     According to an embodiment, the third electrically conductive layer is electrically connected to the first electrically conductive layer, the second electrically conductive layer, or both. 
     According to an embodiment, the first electrically conductive layer and the second electrically conductive layer comprise openings, wherein the openings and the optics elements collectively form paths of the plurality of beams of charged particles. 
     According to an embodiment, the openings have an inverted funnel or counterbore shape. 
     According to an embodiment, the third electrically conductive layer is positioned between at least two of the optics elements. 
     According to an embodiment, the first electrically conductive layer, the second electrically conductive layer, and the third electrically conductive layer collectively form cavities that accommodate the optics elements, wherein the cavities are configured to electrically shield the optics elements from one another. 
     According to an embodiment, the electrically insulating layer comprises discrete portions, each of which is physically connected to one of the optics elements. 
     According to an embodiment, the electrically insulating layer is physically connected to the first electrically conductive layer, the second electrically conductive layer, or the third electrically conductive layer. 
     According to an embodiment, the electrically insulating layer is configured to provide mechanical support to the optics elements. 
     According to an embodiment, the electrically insulating layer is positioned upstream with respect to the optics elements. 
     According to an embodiment, the electrically insulating layer is positioned downstream with respect to the optics elements. 
     According to an embodiment, the electrically insulating layer extends between the third electrically conductive layer and the first electrically conductive layer. 
     According to an embodiment, the electrically insulating layer insulates the first electrically conductive layer from the third electrically conductive layer, the second electrically conductive layer, and the optics elements. 
     According to an embodiment, the electrically insulating layer comprises an electrically conductive via through the electrically insulating layer, wherein the electrically conductive via electrically connects the first electrically conductive layer to the third electrically conductive layer. 
     According to an embodiment, the electrically conductive via encircles a cavity formed by the first electrically conductive layer, the second electrically conductive layer, and the third electrically conductive layer and housing one of the optics elements. 
     According to an embodiment, the electrically insulating layer extends between the third electrically conductive layer and the second electrically conductive layer. 
     According to an embodiment, the electrically insulating layer insulates the second electrically conductive layer from the third electrically conductive layer, the first electrically conductive layer, and the optics elements. 
     According to an embodiment, the electrically insulating layer comprises an electrically conductive via through the electrically insulating layer, wherein the electrically conductive via electrically connects the second electrically conductive layer to the third electrically conductive layer. 
     According to an embodiment, the first electrically conductive layer, the second electrically conductive layer and the third electrically conductive layer comprise a semiconductor or a metal. 
     According to an embodiment, the optics elements are selected from a group consisting of a lens, a stigmator, a deflector, and a combination thereof. 
     According to an embodiment, the optics elements are configured to generate an electric field selected from a group consisting of a round-lens electrostatic field, an electrostatic dipole field and an electrostatic quadrupole field. 
     According to an embodiment, at least one of the optics elements comprises multiple poles. 
     According to an embodiment, the first electrically conductive layer, the second electrically conductive layer and the third electrically conductive layer are configured to reduce crosstalk or field distribution deformation of the optics elements. 
     According to an embodiment, the apparatus further comprises a detector configured to capture a signal produced from an interaction of the beams and a sample. 
     According to an embodiment, the signal comprises secondary electrons or backscattered electrons, Auger electrons, X-ray, or cathodoluminescence. 
     According to an embodiment, the charged particles comprise electrons. 
     Disclosed herein is an apparatus comprising: a first electrically conductive layer; a second electrically conductive layer; a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; and an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer; wherein the electrically insulating layer extends between the third electrically conductive layer and the first electrically conductive layer, or between the third electrically conductive layer and the second electrically conductive layer. 
     Disclosed herein is an apparatus comprising: a first electrically conductive layer; a second electrically conductive layer; a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer; and a fourth electrically conductive layer in contact with and upstream to the first electrically conductive layer. 
     Disclosed herein is an apparatus comprising: a first electrically conductive layer; a second electrically conductive layer; a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer; and a fifth electrically conductive layer in contact with and downstream to the second electrically conductive layer. 
     Disclosed herein is a system comprising: a source configured to produce charged particles; an optics system configured to generate with the charged particles multiple probe spots on a surface of a sample and to scan the probe spots on the surface, the optics system comprising any of the above apparatuses. 
     According to an embodiment, the source is an electron gun. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1A  schematically shows an apparatus that can carry out charged particle beam inspection using multiple beams of charge particles, where the charged particles in the multiple beams are from a single source (a “multi-beam” apparatus). 
         FIG. 1B  schematically shows an alternative multi-beam apparatus. 
         FIG. 1C  schematically shows an alternative multi-beam apparatus. 
         FIG. 2A  schematically shows crosstalk between two optics elements in a source-conversion unit in a multi-beam apparatus. 
         FIG. 2B  schematically shows a cross-sectional view along section A-A of  FIG. 2A . 
         FIG. 3A ,  FIG. 3B ,  FIG. 3C ,  FIG. 3D ,  FIG. 3E  and  FIG. 3F  each schematically show a portion of the source-conversion unit in a multi-beam apparatus, according to an embodiment. 
         FIG. 3G  schematically shows a cross-sectional view along section B-B of  FIG. 3C  or  FIG. 3D . 
         FIG. 3H  and  FIG. 3I  each schematically show a portion of the source-conversion unit of a multi-beam apparatus, according to an embodiment. 
         FIG. 4A  schematically shows that two of the portions of  FIG. 3C  are stacked. 
         FIG. 4B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 4A  except that one of the two portions is the portion in  FIG. 3H . 
         FIG. 5A  schematically shows that three of the portions of  FIG. 3C  am stacked. 
         FIG. 5B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 5A  except that one of the three portions is the portion in FIG.  3 I 1 . 
         FIG. 6A  schematically shows that three of the portions of  FIG. 3I ) are stacked. 
         FIG. 6B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 6A  except that one of the three portions is the portion in  FIG. 3I . 
         FIG. 7A  schematically shows a source-conversion unit including three of the portions of  FIG. 3C  stacked on one another and a portion of  FIG. 3D  upstream. 
         FIG. 7B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 7A  except that one of the three portions is the portion in  FIG. 3H . 
     
    
    
     DETAILED DESCRIPTION 
     There are various techniques for inspecting the sample (e.g., a substrate and a patterning device). One kind of inspection techniques is optical inspection, where a light beam is directed to the substrate or patterning device and a signal representing the interaction (e.g., scattering, reflection, diffraction) of the light beam and the sample is recorded. Another kind of inspection techniques is charged particle beam inspection, where a beam of charged particles (e.g., electrons) is directed to the sample and a signal representing the interaction (e.g., secondary emission and back-scattered emission) of the charged particles and the sample is recorded. 
     Charged particle beam inspection may have higher resolution than optical inspection due to the shorter wavelengths of the charged particles used in the former than the light used in the latter. As the dimensions of the patterns on the substrate and the patterning device become smaller and smaller as the device manufacturing process evolves, charged particle beam inspection becomes more widely used. The throughput of charged particle beam inspection is relatively low due to interactions (e.g., the Coulomb effect) among the charged particles used therein. More than one beam of charged particles may be used to increase the throughput. 
     In an example, multiple beams of charged particles can simultaneously scan multiple regions on a sample. The scanning of the multiple beams may be synchronized or independent. The multiple regions may have overlaps among them or may be isolated from one another. Signals generated from the interactions of the beams and the sample may be collected by multiple detectors. The number of detectors may be less than, equal to, or greater than the number of the beams. The multiple beams may be individually controlled or collectively controlled. 
     Multiple beans of charged particles may form multiple probe spots on a surface of a sample. The probe spots can respectively or simultaneously scan multiple regions on the surface. The charged particles of each beam may generate signals from the locations of the probe spots. One example of the signals is secondary electrons. Secondary electrons usually have energies less than 50 eV. Another example of the signals is backscattered electrons when the charged particles of the beams are electrons. Backscattered electrons usually have energies close to lading energies of the electrons of the beams. The signals from the locations of the probe spots may be respectively or simultaneously collected by multiple detectors. 
     The multiple beams may be from multiple sources respectively, or from a single source. If the beams are from multiple sources, multiple columns may scan and focus the beams onto the surface, and the signals generated by the beams may be detected by detectors in the columns, respectively. An apparatus using beams from multiple sources may be called as a multi-column apparatus. The columns can be either independent or share a multi-axis magnetic or electromagnetic-compound objective lens (see U.S. Pat. No. 8,294,095, whose disclosure is hereby incorporated by reference in its entirety). The probe spots generated by a multi-column apparatus may be spaced apart by a distance as large as 30-50 mm. 
     If the beams are from a single source, a source-conversion unit may be used to form multiple virtual or real images of the single source. Each of the images and the single source may be viewed as an emitter of a beam (also called a “beamlet” as all of the beamlets are from the same source). The source-conversion unit may have an electrically conductive layer with multiple openings that can divide the charged particles from the single source into multiple beams (also called “beamlets”). The source-conversion unit may have optics elements that can influence the beamlets to form multiple virtual or real images of the single source. Each of the images can be viewed as a source that emits one of the beamlets. The beamlets may be spaced apart by a distance of micrometers. A single column, which may have a projection system and a deflection scanning unit, may be used to scan and focus the beamlets on multiple regions of a sample. The signals generated by the beamlets may be respectively detected by multiple detection elements of a detector inside the single column. An apparatus using beams from a single source may be called as a multi-beam apparatus. 
     There are at least two methods to form the images of the single source. In the first method, each optics element has an electrostatic micro-lens that focuses one beamlet and thereby forms one real image. (see, e.g., U.S. Pat. No. 7,244,949, whose disclosure is hereby incorporated by reference in its entirety). In the second method, each optics element has an electrostatic micro-deflector which deflects one beamlet thereby forms one virtual image (see, e.g., U.S. Pat. No. 6,943,349 and U.S. patent application Ser. No. 15/065,342, whose disclosures are hereby incorporated by reference in their entirety). Interactions (e.g., the Coulomb effect) among the charged particles in the second method may be weaker than that in the first method because a real image has a higher current density. 
       FIG. 1A  schematically shows an apparatus  400  that can carry out charged particle beam inspection using multiple beams of charge particles, where the charged particles in the multiple beams are from a single source (a multi-beam apparatus). The apparatus  400  has a source  401  that can produce charged particles in free space. In an example, the charged particles are electrons and the source  401  is an electron gun. The apparatus  400  has an optics system  419  that can generate with the charged particles multiple probe spots on a surface of a sample  407  amid scan the probe spots on the surface of the sample  407 . The optics system  419  may have a condenser lens  404  and a main aperture  405  upstream or downstream with respect to the condenser lens  404 . The expression “Component A is upstream with respect to Component B” as used herein means that a beam of charged particles would reach Component A before reaching Component B in normal operation of the apparatus. The expression “Component B is downstream with respect to Component A” as used herein means that a beam of charged particles would reach Component B after reaching Component A in normal operation of the apparatus. The optics system  419  has a source-conversion unit  410  configured to form multiple virtual images (e.g., virtual images  402  and  403 ) of the source  401 . The virtual images and the source  401  each can be viewed as an emitter of a beamlet (e.g., beamlets  431 ,  432  and  433 ). The source-conversion unit  410  may have an electrically conductive layer  412  with multiple openings that can divide the charged particles from the source  401  into multiple beamlets, and optics elements  411  that can influence the beamlets to form the virtual images of the source  401 . The optics elements  411  may be micro-deflectors configured to deflect the beamlets. The electric current of the beamlets may be affected by the sizes of the openings in the electrically conductive layer  412  or the focusing power of the condenser lens  404 . The optics system  419  includes an objective lens  406  configured to focus the multiple beamlets and thereby form multiple probe spots onto the surface of the sample  407 . The source-conversion unit  410  may also have micro-compensators configured to reduce or eliminate aberrations (e.g., field curvature and astigmatism) of the probe spots. 
       FIG. 1B  schematically shows an alternative multi-beam apparatus. The condenser lens  404  collimates the charged particles from the source  401 . The optics elements  411  of the source-conversion unit  410  may comprise micro-compensators  413 . The micro-compensators  413  may be separate from micro-deflectors or may be integrated with micro-deflectors. If separated, the micro-compensators  413  may be positioned upstream to the micro-deflectors. The micro-compensators  413  are configured to compensate for off-axis aberrations (e.g., field curvature, astigmatism and distortion) of the condenser lens  404  or the objective lens  406 . The off-axis aberrations may negatively impact the sizes or positions of the probe spots formed by off-axis (i.e., being not along the primary optical axis of the apparatus) beamlets. The off-axis aberrations of the objective lens  406  may not be completely eliminated by deflection of the beamlets. The micro-compensators  413  may compensate for the residue off-axis aberrations (i.e., the portion of the off-axis aberrations that cannot be eliminated by deflection of the beamlets) of the objective lens  406 , or non-uniformity of the sizes of the probe spots. Each of the micro-compensators  413  is aligned with one of the openings in the electrically conductive layer  412 . The micro-compensators  413  may each have four or more poles. The electric currents of the beamlets may be affected by the sizes of the openings in the electrically conductive layer  412  and/or the position of the condenser lens  404 . 
       FIG. 1C  schematically shows an alternative multi-beam apparatus. The optics elements  411  of the source-conversion unit  410  may comprise pre-bending micro-deflectors  414 . The pre-bending micro-deflectors  414  are micro-deflectors configured to bend the beamlets before they go through the openings in the electrically conductive layer  412 . 
     Additional descriptions of apparatuses using multiple beams of charge particles from a single source may be found in U.S. Patent Application Publications 2016/0268096, 2016/0284505 and 2017/0025243, U.S. Pat. No. 9,607,805, U.S. patent application Ser. Nos. 15/365,145, 15/213,781, 15/216,258 and 62/440,493, and PCT Application PCT/US17/15223, the disclosures of which are hereby incorporated by reference in their entirety. 
     When two optics elements (e.g., the micro-deflectors or micro-compensators in  FIG. 1A ,  FIG. 1B  or  FIG. 1C ) in a source-conversion unit (e.g., the source-conversion unit  410  in  FIG. 1A ,  FIG. 1B  or  FIG. 1C ) are close to one another, there may be crosstalk between them. Namely, the electric field caused by one optics element may extend into the beam path of another optics element and affect the beam transmitting through the other optics element. For example, the micro-deflectors in the source-conversion unit  410  may bend the beamlets by electric fields and the electric field generated by one micro-deflector may extend into the beam path of a neighboring micro-deflector, thereby causing the beamlet through the neighboring micro-deflector to bend by an amount in addition to the bending caused by the electric field generated by the neighboring micro-deflector. Similarly, for example, the electric field (e.g., a quadrupole field or a round-lens field) generated by one of the micro-compensators in the source-conversion unit  410  may extend into the beam path of a neighboring micro-compensator, and thereby influencing the beamlet through the neighboring micro-compensator, by an amount in addition to the influence caused by the electric field generated by the neighboring micro-compensator. 
     The crosstalk tends to be more serious if the optics elements are closer to one another (e.g., integrated into one wafer or a stack of wafers). As an example,  FIG. 2A  schematically shows crosstalk between two optics elements  611  and  612  integrated in a stack of wafers.  FIG. 2B  schematically shows a cross-sectional view along section A-A of  FIG. 2A . The optics elements  611  and  612  are attached to an insulation layer  616 . There is an electrically conductive plate  617  with openings (e.g., circular in shape) upstream to the optics elements  611  and  612 , or an electrically conductive plate  618  with openings (e.g., circular in shape) downstream to the optics elements  611  and  612 . The optics elements  611  and  612  may have circular electrodes or multiple poles. When the optics element  611  is set at a potential different from the potentials of the conductive plates  617  and  618 , an electric field is generated to influence (e.g., compensate for field curvature aberration of) a beamlet transmitting along a beam path  613  through the optics element  611 . The equipotential lines  615  in  FIG. 2A  and  FIG. 2B  represent an electric field when the conductive plates  617  and  618  and the optics element  612  are at the same potential that is different from the potential of the optics element  611 , the openings in the electrically conductive plates  617  and  618  are circular, and the optics elements  611  and  612  have circular electrodes. As shown by the extensive size of the equipotential lines  615 , the electric field extends into a beam path  614  through the optics element  612 . A beamlet passing through the optics element  612  thus may be affected not only by the electric field the optics element  612  generates but also by the electric field the optics element  611  generates, i.e., optics elements  611  and  612  have crosstalk. 
     As shown in  FIG. 2B , the equipotential lines  615  are not axisymmetric because the structure surrounding the optics element  611  is not axisymmetric. Hence the electric field the optics element  611  generates comprises not only an axisymmetric component (also called a round-lens field), which compensates for the field curvature aberration, but also high order rotation-symmetric components such as a deflection field and a stigmator field. The high order components add aberrations to the beamlet passing through the optics element  611 . 
       FIG. 3A  and  FIG. 3B  each schematically show a portion  700  of a source-conversion unit of a multi-beam apparatus, according to an embodiment. The portion  700  has multiple optics elements  710  (e.g., micro-deflectors, micro-lenses, micro-stigmators, micro-compensators) sandwiched between a first electrically conductive layer  720  and a second electrically conductive layer  740 . The first electrically conductive layer  720  and the second electrically conductive layer  740  have openings that, collectively with the optics elements  710 , form paths (e.g., paths  701 ,  702  and  703 ) of the beamlets. The portion  700  also has a third electrically conductive layer  730  with multiple holes that house the optics elements  710 . The third electrically conductive layer  730  extends between the first electrically conductive layer  720  and the second electrically conductive layer  740 . The third electrically conductive layer  730  is electrically connected to the first electrically conductive layer  720 , or the second electrically conductive layer  740 , or both. The third electrically conductive layer  730  is positioned between at least two of the optics elements  710 , i.e., a plane the beam paths of the two optics elements  710  are on crosses the third electrically conductive layer  730 . The walls among the holes of the third electrically conductive layer  730  can provide electrostatic shielding among the optics elements  710 . The portion  700  has an electrically insulating laver  750 . The electrically insulating layer  750  is physically connected to the optics elements  710 . For example, the electrically insulating layer  750  may have discrete portions, each of which is physically connected to one of the optics elements  710 . The electrically insulating layer  750  is physically connected to the first electrically conductive layer  720 , the second electrically conductive layer  740 , or the third electrically conductive layer  730 . The electrically insulating layer  750  can provide mechanical support to the optics elements  710  and electrically insulate the optics elements  710  from the first electrically conductive layer  720 , the second electrically conductive layer  740 , and the third electrically conductive layer  730 . The first electrically conductive layer  720 , the second electrically conductive layer  740 , and the third electrically conductive layer  730  collectively form cavities  760  electrically shielded from one another, where each of the cavities  760  accommodates one of the optics elements  710 . The cavities  760  may have circular cross-sectional shape. The first electrically conductive layer  720 , the second electrically conductive layer  740 , and the third electrically conductive layer  730  may also be magnetically conductive to magnetically shield the beam paths from outside stray magnetic fields. In the examples shown in  FIG. 3A  and  FIG. 3B , the electrically insulating layer  750  is positioned upstream and downstream with respect to the optics elements  710 , respectively. In the examples shown in  FIG. 3A  and  FIG. 3B , the third electrically conductive layer  730  is electrically connected to both the first electrically conductive layer  720  and the second electrically conductive layer  740 . 
       FIG. 3C  and  FIG. 3D  each schematically show a portion  700  of a source-conversion unit of a multi-beam apparatus, according to an embodiment. The example shown in  FIG. 3C  is the same as the example in  FIG. 3A , except that the electrically insulating layer  750  extends between the third electrically conductive layer  730  and the first electrically conductive layer  720 . The electrically insulating layer  750  electrically insulates the first electrically conductive layer  720  from the optics elements  710 . The electrically insulating layer  750  electrically may also insulate the first electrically conductive layer  720  from the third electrically conductive layer  730 , and the second electrically conductive layer  740 . The electrically insulating layer  750  may have one or more electrically conductive vias  723  through it and the conductive vias  723  electrically connect the first electrically conductive layer  720  to the third electrically conductive layer  730 . The electrically insulating layer  750  can be very thin, such as less than 100 microns, less than 50 microns or less than 10 microns. The electrically insulating layer  750  can be an oxide, nitride or any other suitable material. 
     The example shown in  FIG. 3D  is the same as the example in  FIG. 3I , except that the electrically insulating layer  750  extends between the third electrically conductive layer  730  and the second electrically conductive layer  740 . The electrically insulating layer  750  electrically insulates the second electrically conductive layer  740  from the optics elements  710 . The electrically insulating layer  750  may also insulate the second electrically conductive layer  740  from the third electrically conductive layer  730  and the first electrically conductive layer  720 . The electrically insulating layer  750  may have one or more electrically conductive vias  723  through it and thereby electrically connect the second electrically conductive layer  740  to the third electrically conductive layer  730  by the one or more electrically conductive vias  723  and the conductive vias  723  electrically connect the second electrically conductive layer  740  to the third electrically conductive layer  730 . The electrically insulating layer  750  can be very thin, such as less than 100 microns, less than 50 microns or less than 10 microns. The electrically insulating layer  750  can be an oxide, nitride or any other suitable material. 
       FIG. 3E  and  FIG. 3F  each schematically show a portion  700  of a source-conversion unit of a multi-beam apparatus, according to an embodiment. The examples shown in  FIG. 3E  and  FIG. 3F  are the same as the examples in  FIG. 3C  and  FIG. 3D , respectively, except that the electrically insulating layer  750  has an electrically conductive via  723  encircling each of the cavities  760  and the electrically conductive via  723  electrically connects the third electrically conductive layer  730  to the first electrically conductive layer  720  or the second electrically conductive layer  740 .  FIG. 3G  schematically shows a cross-sectional view along section B-B of  FIG. 3C  or  FIG. 3D . 
     FIG.  3 I 1  schematically shows a portion  700  of a source-conversion unit of a multi-beam apparatus, according to an embodiment. The example shown in  FIG. 3H  is the same as the example in  FIG. 3C , except that the portion  700  additional has a fourth electrically conductive layer  721  upstream to the first electrically conductive layer  720 . The fourth electrically conductive layer  721  may be directly bonded to the first electrically conductive layer  720 . The fourth electrically conductive layer  721  and the first electrically conductive layer  720  may have the same material. The fourth electrically conductive layer  721  has openings whose sidewall is thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel or counterbore shape as shown in  FIG. 3H . 
       FIG. 3I  schematically shows a portion  700  of a source-conversion unit of a multi-beam apparatus, according to an embodiment. The example shown in  FIG. 3I  is the same as the example in  FIG. 3D , except that the portion  700  additional has a fifth electrically conductive layer  741  downstream to the second electrically conductive layer  740 . The fifth electrically conductive layer  741  may be directly bonded to the second electrically conductive layer  740 . The fifth electrically conductive layer  741  and the second electrically conductive layer  740  may have the same material. The fifth electrically conductive layer  741  has openings whose sidewall is thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel or counterbore shape as shown in  FIG. 3I . 
     The materials suitable for the first electrically conductive layer  720 , the third electrically conductive layer  730  and the second electrically conductive layer  740  may include semiconductors (e.g., silicon), and metals (e.g., gold). The first electrically conductive layer  720 , the third electrically conductive layer  730  and the second electrically conductive layer  740  do not necessarily have the same materials. The first electrically conductive layer  720 , the second electrically conductive layer  740 , the third electrically conductive layer  730  can respectively be made from a semiconductor wafer, a semiconductor wafer with metal (e.g., gold) coating, or a dielectric plate with metal (e.g., gold) coating. The optics elements  710  can be made from a semiconductor wafer. The optics elements  710  and the third electrically conductive layer  730  can be made from a same semiconductor wafer, a same semiconductor wafer with metal (e.g., gold) coating, or a same dielectric plate with metal (e.g., gold) coating. The source-conversion unit shown in  FIG. 3A - FIG. 3I  can help reduce or eliminate crosstalk and field distribution deformation of the optics elements  710 . 
     A source-conversion unit may have portions  700  in  FIG. 3A - FIG. 3I  stacked on one another. For example,  FIG. 4A  schematically shows a source-conversion unit including two of the portions  810  and  820  of  FIG. 3C  stacked on each other. The second electrically conductive layer  740  of the portion  810  upstream in the stack and the first electrically conductive layer  720  of the portion  820  downstream in the stack can be merged as a single layer. The optics elements  710  in the portion  810  may be micro-deflectors; the optics elements  710  in the portion  820  may be micro-compensators, which may comprise micro-lenses and micro-stigmators. In alternative, the optics elements  710  along the same path may collectively serve as a micro-compensator. The openings in the second electrically conductive layer  740  of the portion  820  or the openings in the first electrically conductive layer  720  of the portion  810  may serve as the openings in the electrically conductive layer  412  in  FIG. 1A ,  FIG. 1B  or  FIG. 1C . The sidewall of the openings may be thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel or counterbore shape as shown in  FIG. 4A . 
       FIG. 4B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 4A  except that the portion  810  is the portion in FIG.  3 I 1 . 
       FIG. 5A  schematically shows a source-conversion unit including three of the portions  910 ,  920  and  930  of  FIG. 3C  stacked on one another. The second electrically conductive layer  740  of the portion  910  upstream in the stack and the first electrically conductive layer  720  of the portion  920  in the middle of the stack can be merged as a single layer; the second electrically conductive layer  740  of the portion  920  in the middle of the stack and the first electrically conductive layer  720  of the portion  930  downstream in the stack can be merged as a single layer. The optics elements  710  in the portions  910  and  920  may be micro-compensators, micro-lenses and micro-stigmators; the optics elements  710  in the portion  930  can be micro-deflectors. The openings in the first electrically conductive layer  720  of the portion  910  may serve as the openings in the electrically conductive layer  412  in  FIG. 11  or  FIG. 1C . The sidewall of the openings may be thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel or counterbore shape as shown in  FIG. 5A . 
       FIG. 5B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 5A  except that the portion  910  is the portion in  FIG. 3H . 
       FIG. 6A  schematically shows a source-conversion unit including three of the portions  910 ,  920  and  930  of  FIG. 3D  stacked on one another. The second electrically conductive layer  740  of the portion  910  upstream in the stack and the first electrically conductive layer  720  of the portion  920  in the middle of the stack can be merged as a single layer; the second electrically conductive layer  740  of the portion  920  in the middle of the stack and the first electrically conductive layer  720  of the portion  930  downstream in the stack can be merged as a single layer. The optics elements  710  in the portions  910  and  920  may be micro-compensators, microlenses and micro-stigmators; the optics elements  710  in the portion  930  can be micro-deflectors. The openings in the first electrically conductive layer  720  of the portion  910  may serve as the openings in the electrically conductive layer  412  in  FIG. 1B  or  FIG. 1C . The sidewall of the openings may be thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel or counterbore shape as shown in  FIG. 6A . 
       FIG. 6B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 6A  except that the portion  930  is the portion in  FIG. 3I . 
       FIG. 7A  schematically shows a source-conversion unit including three of the portions  910 ,  920  and  930  of  FIG. 3C  stacked on one another (i.e., the structure of  FIG. 5A ), and a portion  940  of  FIG. 3D  upstream to the stack of the portions  910 ,  920  and  930 . The optics elements  710  in the portions  910  and  920  may be micro-compensators, micro-lenses and micro-stigmators; the optics elements  710  in the portion  930  can be micro-deflectors. The openings in the first electrically conductive layer  720  of the portion  910  may serve as the openings in the electrically conductive layer  412  in  FIG. 1C . The sidewall of the openings may be thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel shape as shown in  FIG. 6A . The optics elements  710  in the portion  940  may be pre-bending micro-deflectors. The sidewall of the openings in the first electrically conductive layer  720  of the portion  940  may be thinned to reduce scattering of charged particles thereon. To thin the sidewall, the openings can have an inverted funnel or counterbore shape. 
       FIG. 7B  shows another example of the source-conversion unit, which is similar to the source-conversion unit in  FIG. 7A  except that the portion  910  is the portion in  FIG. 3H . 
     Although the disclosure above is made with respect to multi-beam apparatuses (i.e., apparatuses that can carry out charged particle beam inspection using multiple beams of charge particles, where the charged particles in the multiple beams are from a single source), the embodiments may be applicable in multi-column apparatuses (i.e., apparatuses that can carry out charged particle beam inspection using multiple beams of charge particles, where the multiple beams of charge particles are produced from multiple sources). Additional descriptions of multi-column apparatuses may be found in U.S. Pat. No. 8,294,095, the disclosure of which is hereby incorporated by reference in its entirety. 
     The embodiments may further be described using the following clauses: 
     1. An apparatus comprising: 
     a first electrically conductive layer; 
     a second electrically conductive layer; 
     a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; 
     a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; and 
     an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer. 
     2. The apparatus of clause 1, wherein the third electrically conductive layer comprises a plurality of holes, wherein the holes house the optics elements.
 
3. The apparatus of any one of clauses 1-2, wherein the optics elements are electrically insulated from the third electrically conductive layer.
 
4. The apparatus of any one of clauses 1-3, wherein the third electrically conductive layer is electrically connected to the first electrically conductive layer, the second electrically conductive layer, or both.
 
5. The apparatus of any one of clauses 1-4, wherein the first electrically conductive layer and the second electrically conductive layer comprise openings, wherein the openings and the optics elements collectively form paths of the plurality of beams of charged particles.
 
6. The apparatus of clause 5, wherein the openings have an inverted funnel or counterbore shape.
 
7. The apparatus of any one of clauses 1-6, wherein the third electrically conductive layer is positioned between at least two of the optics elements.
 
8. The apparatus of any one of clauses 1-7, wherein the first electrically conductive layer, the second electrically conductive layer, and the third electrically conductive layer collectively form cavities that accommodate the optics elements, wherein the cavities are configured to electrically shield the optics elements from one another.
 
9. The apparatus of any one of clauses 1-8, wherein the electrically insulating layer comprises discrete portions, each of which is physically connected to one of the optics elements.
 
10. The apparatus of any one of clauses 1-9, wherein the electrically insulating layer is physically connected to the first electrically conductive layer, the second electrically conductive layer, or the third electrically conductive layer.
 
11. The apparatus of any one of clauses 1-10, wherein the electrically insulating layer is configured to provide mechanical support to the optics elements.
 
12. The apparatus of any one of clauses 1-11, wherein the electrically insulating layer is positioned upstream with respect to the optics elements.
 
13. The apparatus of any one of clauses 1-12, wherein the electrically insulating layer is positioned downstream with respect to the optics elements.
 
14. The apparatus of any one of clauses 1-13, wherein the electrically insulating layer extends between the third electrically conductive layer and the first electrically conductive layer.
 
15. The apparatus of clause 14, wherein the electrically insulating layer insulates the first electrically conductive layer from the third electrically conductive layer, the second electrically conductive layer, and the optics elements.
 
16. The apparatus of clause 14, wherein the electrically insulating layer comprises an electrically conductive via through the electrically insulating layer, wherein the electrically conductive via electrically connects the first electrically conductive layer to the third electrically conductive layer.
 
17. The apparatus of clause 16, wherein the electrically conductive via encircles a cavity formed by the first electrically conductive layer, the second electrically conductive layer, and the third electrically conductive layer and housing one of the optics elements.
 
18. The apparatus of any one of clauses 1-13, wherein the electrically insulating layer extends between the third electrically conductive layer and the second electrically conductive layer.
 
19. The apparatus of clause 18, wherein the electrically insulating layer insulates the second electrically conductive layer from the third electrically conductive layer, the first electrically conductive layer, and the optics elements.
 
20. The apparatus of clause 18, wherein the electrically insulating layer comprises an electrically conductive via through the electrically insulating layer, wherein the electrically conductive via electrically connects the second electrically conductive layer to the third electrically conductive layer.
 
21. The apparatus of clause 20, wherein the electrically conductive via encircles a cavity formed by the first electrically conductive layer, the second electrically conductive layer, and the third electrically conductive layer and housing one of the optics elements.
 
22. The apparatus of any one of clauses 1-21, wherein the first electrically conductive layer, the second electrically conductive layer and the third electrically conductive layer comprise a semiconductor or a metal.
 
23. The apparatus of any one of clauses 1-22, wherein the optics elements are selected from a group consisting of a lens, a stigmator, a deflector, and a combination thereof.
 
24. The apparatus of any one of clauses 1-23, wherein the optics elements are configured to generate an electric field selected from a group consisting of a round-lens electrostatic field, an electrostatic dipole field and an electrostatic quadrupole field.
 
25. The apparatus of any one of clauses 1-24, wherein at least one of the optics elements comprises multiple poles.
 
26. The apparatus of any one of clauses 1-25, wherein the first electrically conductive layer, the second electrically conductive layer and the third electrically conductive layer are configured to reduce crosstalk or field distribution deformation of the optics elements.
 
27. The apparatus of any one of clauses 1-26, further comprising a detector configured to capture a signal produced from an interaction of the beams and a sample.
 
28. The apparatus of clause 27, wherein the signal comprises secondary electrons or backscattered electrons, Auger electrons, X-ray, or cathodoluminescence.
 
29. The apparatus of any one of clauses 1-28, wherein the charged particles comprise electrons.
 
30. An apparatus comprising:
 
     a first electrically conductive layer; 
     a second electrically conductive layer; 
     a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; 
     a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; and 
     an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer; 
     wherein the electrically insulating layer extends between the third electrically conductive layer and the first electrically conductive layer, or between the third electrically conductive layer and the second electrically conductive layer. 
     31. An apparatus comprising: 
     a first electrically conductive layer; 
     a second electrically conductive layer; 
     a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles; 
     a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; 
     an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer; and 
     a fourth electrically conductive layer in contact with and upstream to the first electrically conductive layer. 
     32. An apparatus comprising: 
     a first electrically conductive layer; 
     a second electrically conductive layer; 
     a plurality of optics elements between the first electrically conductive layer and the second electrically conductive layer, wherein the plurality of optics elements are configured to influence a plurality of beams of charged particles: 
     a third electrically conductive layer between the first electrically conductive layer and the second electrically conductive layer; 
     an electrically insulating layer physically connected to the optics elements, wherein the electrically insulating layer is configured to electrically insulate the optics elements from the first electrically conductive layer, and the second electrically conductive layer; and 
     a fifth electrically conductive layer in contact with and downstream to the second electrically conductive layer. 
     33. A system comprising: 
     a source configured to produce charged particles; 
     an optics system configured to generate with the charged particles multiple probe spots on a surface of a sample and to scan the probe spots on the surface, the optics system comprising the apparatus of any one of clauses 1-32. 
     34. The system of clause 33, wherein the source is an electron gun. 
     While the concepts disclosed herein may be used for inspection on a sample such as a silicon wafer or a patterning device such as chrome on glass, it shall be understood that the disclosed concepts may be used with any type of samples, e.g., inspection of substrates other than silicon wafers. 
     The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.