Apparatus and method for projecting an array of charged particle beamlets onto a sample - Patent Application 20070122997

Abstract

An apparatus for inspecting a sample, the apparatus comprising: a sample holder for holding the sample at a sample plane; and a charged particle column for generating an array of charged particle beamlets and directing the array towards the sample holder. The charged particle column comprises an objective lens for focusing the charged particle beamlets of the array into an array of charged particle beam spots at or near the sample plane. The objective lens comprises a magnetic lens common to all of the charged particle beamlets. The apparatus further comprises a position sensor for providing a signal dependent on a position of the sample along an optical axis of the charged particle column, and a control unit for controlling the position of the sample holder based on the signal from the position sensor to keep the pitch and / or orientation of the spots on the sample constant.

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for projecting an array of charged particle beamlets onto a sample. [Background technology]

[0002] Such an apparatus or method is disclosed, for example, in US 2015 / 027095 A1. This patent application describes an apparatus for inspecting the surface of a sample. The apparatus comprises a generator for generating an array of primary charged particle beamlets and a charged particle optics system having an optical axis. The charged particle optics system comprises a lens system for focusing the primary charged particle beamlets into an array of spots on the sample surface. The lens system comprises at least one electromagnetic lens common to all charged particle beams. Preferably, an objective lens for focusing the array of primary charged particle beamlets comprises the electromagnetic lens, and the electromagnetic lens is arranged to rotate the array of primary charged particle beamlets around the optical axis of the charged particle optics system. Summary of the Invention

[0003] When the charged particle optics comprises a magnetic lens common to all primary charged particle beamlets of the array of primary charged particle beamlets, the array will rotate and / or change magnification when the excitation and / or strength of the magnetic lens is changed.

[0004] It is an object of the present invention to provide an apparatus and method in which the pitch and / or rotation of an array of primary charged particle beamlets on a sample is kept at least substantially constant.

[0005] According to a first aspect, the present invention provides an apparatus for testing a sample, the apparatus comprising: a sample holder for holding a sample at the sample plane; a charged particle column for generating an array of charged particle beamlets and directing the array of charged particle beamlets toward a sample holder, wherein the charged particle column comprises an objective lens for focusing the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; a position sensor configured to provide a signal dependent on the position of the sample in a direction along the optical axis of the charged particle column; a control unit configured to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on a signal from the position sensor; Equipped with.

[0006] Due to the combined position sensor and control unit in the multi-charged particle beam inspection apparatus of the present invention, the sample holder can be controlled to hold a portion of the sample to be inspected by the array of multiple charged particle beams positioned at the sample plane and / or focal plane of the multi-charged particle beam inspection apparatus. When the setting of the charged particle column is not changed and a portion of the sample to be inspected is held at the sample plane and / or focal plane of the multi-charged particle beam inspection apparatus, the pitch and rotation of the array of multiple charged particle beamlets on the sample at the sample plane and / or focal plane of the multi-charged particle beam inspection apparatus can be kept constant. It should be noted that in order to image the sample, the sample plane is preferably located at the focal plane of the multi-charged particle beam inspection apparatus.

[0007] It should be noted that when the sample holder is arranged to position the sample at least partially in the magnetic field of the magnetic objective lens, the array of primary charged particle beamlets will rotate in the magnetic field on its way to the sample. The rotation depends, inter alia, on the magnetic field strength of the magnetic lens. Even when the sample holder is configured to position the sample surface in a magnetic field-free area, a change in the position of the sample in a direction along the optical axis of the charged particle column usually requires adjustment of the focus, and hence the magnetic field strength, in order to keep the sample in focus. Therefore, the orientation of the array of primary charged particle beamlets at the surface of the sample depends on the position of the sample relative to the magnetic lens, in particular its position along the optical axis of the charged particle optics.

[0008] Due to the combined position sensor and control unit in the multi-charged particle beam inspection apparatus of the present invention, the sample holder can be controlled to hold a portion of the sample inspected by the array of multiple charged particle beams positioned at a desired distance from the magnetic objective lens. When the settings of the charged particle column are not changed and the portion of the sample inspected is held at a desired distance from the magnetic objective lens, the pitch and rotation of the array of multiple charged particle beamlets on the sample in the sample plane can be kept constant.

[0009] Furthermore, it should be noted that the depth of focus of charged particle inspection devices, such as electron microscopes, is relatively large. Therefore, within at least a portion of the range of the depth of focus, the sample can be moved in a direction parallel to the optical axis of the charged particle optics to position the sample at a desired location along the optical axis, where the array of multiple charged particle beamlets has a desired orientation in a direction centered on the optical axis of the charged particle column. The desired orientation of the array of charged particle beam spots at or near the sample plane depends, for example, on the direction(s) in which the array of multiple charged particle beamlets can be scanned over the sample and / or the direction(s) in which the sample can be moved relative to the charged particle column. Due to the combined position sensor and control unit in the multi-charged particle beam inspection apparatus of the present invention, the sample holder can be controlled to hold the portion of the sample inspected by the array of multiple charged particle beams at a desired position and to hold the array of charged particle beam spots at or near the sample plane in a desired orientation.

[0010] In an embodiment, the position sensor is configured such that the signal depends on the position of the sample holder or sample plane relative to the objective lens. In particular, the signal depends on the distance between the sample holder or sample plane and the objective lens. As mentioned above, the orientation of the array of primary charged particle beamlets at the surface of the sample depends on the position of the sample relative to the magnetic lens, in particular its position in the direction along the optical axis of the charged particle optics.

[0011] In an embodiment, the apparatus further comprises a photodetector for detecting photons created by one or more charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on the layer of luminescent material after passing through the sample, and an optical optical assembly for projecting or imaging at least some of the photons along the optical beam path onto the photodetector.

[0012] In an embodiment, the optical-optical assembly is configured to image the sample plane onto the photodetector. This embodiment provides an improvement to so-called integrated inspection devices, characterized by the integration of a multi-beam charged particle inspection device, such as a multi-beam scanning electron microscope (multi-beam SEM), with an optical-optical inspection device, such as optical-optical microscope optics. According to the present invention, the pitch and rotation of the array of multiple charged particle beamlets on the sample at the sample plane can be kept constant when the settings of the charged particle column are not changed and a portion of the sample to be inspected is held at a desired distance from the magnetic objective lens. Furthermore, the present invention makes it possible to provide and maintain a desired alignment and pitch of the charged particle beamlets in the array of multiple charged particle beamlets for the optical-optical microscope, particularly for the optical detector of the optical-optical microscope.

[0013] In an embodiment, the light optics assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material.

[0014] In an embodiment, the apparatus further comprises an optical focus sensor configured to provide a signal dependent on the distance between the sample plane and the optical objective lens, and the control unit is configured to control the position of the sample holder at least in a direction parallel to the optical axis of the charged particle column based on the signal from the optical focus sensor. In addition to or as an alternative to using the position sensor, the optical focus sensor can also be used to control the sample holder to hold a portion of the sample to be inspected by the array of multiple charged particle beams positioned at the sample plane. The combination of the optical focus sensor and the control unit makes it possible to hold the sample within the focus of an optical optical assembly including a photodetector, thereby making it possible to hold a portion of the sample to be inspected by the array of multiple charged particle beams positioned at the sample plane. When the settings of the charged particle column are not changed and a portion of the sample to be inspected is held at the sample plane, the pitch and rotation of the array of multiple charged particle beamlets on the sample at the sample plane can be kept constant.

[0015] In an embodiment, the optical optical assembly, particularly its optical objective lens, comprises an optical optical axis, and the apparatus is configured such that the optical optical axis is substantially parallel to the optical axis of the charged particle column. In this embodiment, a change in the position of the sample in a direction along the optical axis of the charged particle column results in a similar change in the position of the sample in a direction along the optical optical axis. Thus, when the optical optical assembly is used to hold the sample at a substantially constant position along the optical optical axis, it will also hold the sample at a substantially constant position along the optical axis of the charged particle column.

[0016] It should be noted that the depth of focus of the optical-optical assembly at the sample is typically much smaller than the depth of focus of the charged particle inspection device. Therefore, by positioning and / or holding the sample at the focal point of the optical-optical assembly, the portion of the sample inspected by the array of multiple charged particle beams is also positioned and / or held at a substantially fixed distance from the magnetic objective lens. When the settings of the charged particle column are not changed and the portion of the sample inspected is held at a desired distance from the magnetic objective lens, the pitch and rotation of the array of multiple charged particle beamlets above the sample at the sample plane can be held constant.

[0017] According to a second aspect, the present invention provides an apparatus for testing a sample, the apparatus comprising: a sample holder for holding a sample at the sample plane; a charged particle column for generating an array of charged particle beamlets and directing the array of charged particle beamlets toward a sample holder, wherein the charged particle column comprises an objective lens for focusing the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; a photodetector for detecting photons created by one or more of the charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on the layer of luminescent material after passing through the sample; an optical optical assembly for projecting or imaging at least a portion of the photons along an optical beam path onto a photodetector, wherein the optical optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material; an optical focus sensor configured to provide a signal dependent on the distance between the sample holder or sample plane and the optical objective; a control unit configured to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on a signal from the optical focus sensor; Equipped with.

[0018] In a combination of a multi-beam charged particle device and an optical inspection device, an optical inspection device can be used.

[0019] The combination of the optical focus sensor and the control unit allows the sample to be kept in the focus of the light optical assembly comprising the photodetector and / or allows the portion of the sample to be inspected by the array of multiple charged particle beams to be kept positioned at the sample plane. When the settings of the charged particle column are not changed and the portion of the sample to be inspected is held at the sample plane, the pitch and rotation of the array of multiple charged particle beamlets over the sample at the sample plane can be kept constant.

[0020] In an embodiment, the optical objective lens has an optical axis and the apparatus is configured such that the optical axis is substantially parallel to the optical axis of the charged particle column.

[0021] In an embodiment, the optical-optical assembly is configured to image the sample plane onto the optical detector. This embodiment provides an improvement to so-called integrated inspection apparatus, characterized by the integration of a multi-beam charged particle inspection apparatus, such as a multi-beam scanning electron microscope (SEM), with an optical-optical inspection apparatus, such as optical-optical microscope optics, where the optical-optical microscope is used to set and maintain the alignment of an array of charged particles on the sample at least substantially constant during inspection of the sample.

[0022] According to a third aspect, the present invention provides a method for testing a sample, the method comprising: placing a sample in a sample holder; generating an array of a plurality of charged particle beamlets using a charged particle column and directing the same towards the sample, wherein the charged particle column comprises an objective lens that focuses the charged particle beamlets of the array of a plurality of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of a plurality of charged particle beamlets; using a position sensor to provide a signal dependent on the position of the sample in a direction along the optical axis of the charged particle column; using a control unit to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on a signal from the position sensor; Equipped with.

[0023] In an embodiment, the position sensor is configured such that the signal depends on the distance between the sample holder or sample plane and the objective lens.

[0024] In an embodiment, the method comprises: The method includes adjusting the position of the sample holder and / or the charged particle column relative to each other at least in a direction parallel to the optical axis of the charged particle column, and / or focusing and setting a desired pitch and orientation of an array of multiple charged particle beamlets at the sample plane.

[0025] According to a fourth aspect, the present invention provides a method for testing a sample, the method comprising: placing a sample in a sample holder; generating an array of a plurality of charged particle beamlets using a charged particle column and directing the same towards the sample, wherein the charged particle column comprises an objective lens that focuses the charged particle beamlets of the array of a plurality of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of a plurality of charged particle beamlets; using a photodetector to detect photons created by one or more of the charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on the layer of luminescent material after passing through the sample; using an optical optical assembly to project or image at least a portion of the photons along an optical beam path onto a photodetector, wherein the optical optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material; using an optical focus sensor to provide a signal that is dependent on the distance between the sample holder or sample plane and the optical objective; using a control unit to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on a signal from the optical focus sensor; Equipped with.

[0026] According to a fifth aspect, the present invention provides a computer-readable medium having computer-executable instructions adapted to cause an apparatus as described above, or an embodiment thereof, to perform a method as described above, or an embodiment thereof.

[0027] The various aspects and features described and illustrated in this specification may, wherever possible, be applied individually. These individual aspects, in particular those described in the accompanying dependent claims, may be the subject of divisional patent applications.

[0028] The invention will now be described on the basis of exemplary embodiments shown in the accompanying drawings. [Brief explanation of the drawings]

[0029] [Figure 1] 1 shows a schematic diagram of a first exemplary embodiment of an apparatus typically improved by the present invention; [Figure 2] 10 schematically illustrates a second exemplary embodiment of an apparatus typically improved by the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0030] 1 shows a schematic representation of a first exemplary embodiment of an apparatus typically improved by the present invention. The apparatus according to this first example comprises a multi-beam scanning electron microscope (MBSEM). The MBSEM 1 comprises a multi-beam charged particle generator 2 for generating an array of primary charged particle beamlets, in this case, an array of primary electron beamlets 3. The multi-beam electron generator 2 comprises at least one electron source 4 for generating a divergent electron beam 5. The divergent electron beam 5 is split into an array of focused primary electron beamlets 3 by an aperture lens array 6. The primary electron beamlets 3 are then directed towards a sample 15 in a sample holder 17, as indicated diagrammatically by arrows P.

[0031] Multiple images of the source 4 are positioned in the object principal plane of an accelerator lens 7. The accelerator lens 7 directs the primary electron beamlets 3 towards an optical axis 8, creating a first common crossover 9 for all primary electron beamlets 3. The first common crossover 9 is imaged by a magnetic condenser lens 10 onto a variable aperture 16, which functions as a current-limiting aperture. At the variable aperture 16, a second common crossover for all primary electron beamlets 3 is created.

[0032] The MBSEM comprises a lens system 13, 14 for directing the primary charged particle beamlets from a common crossover at a variable aperture 16 towards a sample surface 15 and focusing all the primary charged particle beamlets 3 into an array of individual spots on the sample surface 15. The lens system comprises an intermediate magnetic lens 13 for imaging the variable aperture 16 onto the coma-free plane of the objective lens 14, which creates an array of focused primary electron beamlets on the sample surface 15.

[0033] In addition, the MBSEM is provided with a scanning coil 18 for scanning the array of focused primary electron beamlets over the sample surface 15 .

[0034] The MBSEM therefore comprises a charged particle column for generating an array of charged particle beamlets 3 and directing the array of charged particle beamlets 3 towards a sample holder 17, the charged particle column comprising an objective lens 14 for focusing the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near the sample surface 15, the objective lens 14 comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets 3.

[0035] 1 further comprises a position sensor 19 configured to provide a signal dependent on the position of the sample 15 in a direction along the optical axis 8 of the charged particle column, and a control unit 20 connected to the position sensor 19 to receive the signal and connected to the sample holder 17 to control an actuator in the sample holder 17 to move the sample 15. The control unit 20 is configured to control the position of the sample holder 17 in at least a direction parallel to the optical axis 8 of the charged particle column based on the signal from the position sensor 19. The position sensor 19, the control unit 20, and the sample holder 17 are configured to provide a control loop for positioning the sample 15 in a direction parallel to the optical axis 8. In this way, the sample 15 can be placed at a desired position along the optical axis 8, at which position the array of charged particle beamlets 3 has a desired orientation or alignment, and the control loop can be used to hold the sample 15 at said desired position. During inspection of the sample 15, the settings of the MBSEM preferably remain unchanged, so that the pitch and orientation of the array of multiple charged particle beamlets 3 remains constant.

[0036] It should be noted that the position sensor 19 preferably measures the position of the sample 15 on the sample holder 17, which is coincident with the optical axis 8. Because the position sensor 19 is positioned perpendicular to and spaced from the optical axis 8, the distance between the sensor 19 and the sample holder 17 is sensitive to tilt errors. Such tilt errors can be substantially prevented by using two position sensors 19, 19', as shown schematically in FIG. 1.

[0037] Additionally or alternatively, the two position sensors 19 , 19 ′ may be configured to use triangulation to determine the position of the sample 15 along the optical axis 8 .

[0038] It is further noted that an MBSEM such as that presented in Figure 1 is typically located in a vacuum chamber. In addition, the MBSEM is typically provided with sensors for detecting secondary charged particles and / or electromagnetic radiation that are generated from the sample 15 when the primary charged particle beamlets 3 impinge on the sample 15.

[0039] 2 shows a schematic diagram of a second exemplary embodiment of an apparatus typically improved by the present invention. The apparatus according to this second example features the integration of a multi-beam charged particle inspection apparatus, such as a multi-beam scanning electron microscope 30 (MBSEM), with an optical-optical inspection apparatus, such as an optical-optical microscope 50. The multi-beam scanning electron microscope 30 (MBSEM) and the optical-optical inspection apparatus, such as the optical-optical microscope 50, are mechanically coupled.

[0040] The MBSEM 30 can be essentially the same device as that presented in the first example above. Therefore, the MBSEM 30 includes a multi-beam charged particle generator 32 for generating an array of primary charged particle beamlets, in this case, primary electron beamlets 33. The multi-beam electron generator 32 includes at least one electron source 34 for generating a divergent electron beam 35. The divergent electron beam 35 is split into an array of focused primary electron beamlets 33 by an aperture lens array 36. The primary electron beamlets 33 are then directed toward a sample 45 in a sample holder 47. Multiple images of the light source 34 are positioned in the object principal plane of an accelerator lens 37. The accelerator lens 37 directs the primary electron beamlets 33 toward an optical axis 38, creating a first common crossover 39 for all primary electron beamlets 33. The first common crossover 39 is imaged by a magnetic condenser lens 40 onto a variable aperture 46, which functions as a current-limiting aperture. A second common crossover of all primary electron beamlets 33 is created at a variable aperture 46. The MBSEM includes lens systems 43, 44 for directing the primary charged particle beamlets from the common crossover at the variable aperture 46 towards a sample surface 45 and focusing all primary charged particle beamlets 33 into an array of individual spots on the sample surface 45. The lens system includes an intermediate magnetic lens 43 for imaging the variable aperture 46 onto a coma-free plane of the objective lens 44, which creates an array of focused primary electron beamlets on the sample surface 45. In addition, the MBSEM is provided with a scan coil 48 for scanning the array of focused primary electron beamlets 33 over the surface of the sample 45.

[0041] Preferably, the MBSEM is also provided with a charged particle sensor 49, which is configured to detect secondary charged particles and / or electromagnetic radiation emerging from the sample when the primary charged particle beamlets 33 impinge on the sample 45. For clarity, the trajectories of the secondary charged particles are not shown in the figures and the charged particle sensor 49 is also presented very diagrammatically.

[0042] As shown in FIG. 2, the MBSEM is placed inside a vacuum chamber 31, which includes an output port 70 for connecting the vacuum chamber 31 to a vacuum pump (not shown).

[0043] A light-optical microscope 50 is positioned below the sample holder 47. The light-optical microscope 50 includes a microscope objective 51, which is positioned inside the vacuum chamber 31. The other main parts of the light-optical microscope system are positioned outside the vacuum chamber 31. Light from the sample 45 is collected by the microscope objective 51 and imaged via a mirror 52, a window 53, and a semi-transparent mirror or dichroic 56 onto a photodetector 54, such as a CCD sensor.

[0044] It should be noted that it is also possible to use a light-optical microscope 50 equipped with a light source 55, e.g., an LED. Light emitted from the light source 55 is directed onto a semi-transparent mirror or dichroic 56 and directed through a window 53 into the vacuum chamber 31. This light is coupled into the microscope objective 51 via a mirror 52 to illuminate the sample 45 from the bottom side.

[0045] In use, the photodetector 54 is configured to detect light reflected from the sample 45. Additionally, the photodetector 54 is configured to detect photons created by one or more of the charged particle beamlets of the array of charged particle beamlets 33 when they impinge on the sample 45 or when they impinge on the layer of luminescent material 42 after transmitting through the sample 45.

[0046] The photodetector 54 can be used as an optical focus sensor, for example, by providing a signal that depends on whether the sample 45 is in focus of the photodetector 54. Additionally or alternatively, the light optical microscope can be provided with a separate optical focus sensor configured to provide a signal that depends on the distance between the sample holder 47 or sample surface 45 and the optical objective lens 51, using well-known techniques for providing autofocus, such as, for example, triangulation, phase detection, or contrast detection.

[0047] The signal from the photodetector 54 or a separate optical focus sensor is provided to a control unit 60 configured to control the position of the sample holder 47 at least in a direction parallel to the optical axis 38 of the MBSEM 30 based on the signal from the photodetector 54 or a separate optical focus sensor.

[0048] It should be noted that the optical focus sensor preferably uses a wavelength that is at least substantially different from the wavelength of the emission light from the sample, so that the optical focus sensor does not interfere with the collection of the emission light from the sample.

[0049] As presented above, the final MBSEM lens 44 comprises a magnetic lens common to all charged particle beamlets of the array of the plurality of charged particle beamlets 33. The pattern of the array of the plurality of charged particle beamlets 33 projected by the magnetic lens 44 onto the sample 45 is generally non-telecentric. Even if the sample holder 47 is configured to position the sample 45 in a magnetic field-free area, a change in the position of the sample 45 in the direction along the optical axis 38 usually requires adjusting the focus of the magnetic lens 44, and hence the magnetic field strength, to keep the sample 45 in focus. Therefore, when the excitation / strength of the magnetic lens 44 is changed (e.g., for focusing), the pattern of the array of the plurality of charged particle beamlets 33 will rotate and / or change magnification. To solve this problem, the present invention provides a method comprising the following steps:

[0050] a. The position of the sample 45 along the optical axis of the MBSEM 30 is adjusted so that the sample 45 is in focus of the photodetector 54. b. An array of multiple charged particle beamlets 33 is focused onto the sample 45 and set to the desired pitch (magnification). The sample 45, the photo-optical focus, and the MBSEM focus are now in the same plane. c. A photodetector 54 and / or a separate optical focus sensor configured to provide a signal that is a measurement of the distance between the light optical focus and the sample surface 45 is provided. d. This signal is used in the control unit to control the sample holder 47 to keep the sample surface 45 in the focus of the photodetector 54.

[0051] In this way, the sample 45 is kept in the focus of the photodetector 54, and hence in the focus of the MBSEM. During inspection of the sample 45, the settings of the MBSEM remain substantially unchanged, so that the pitch and orientation (particularly rotation about the optical axis 8) of the array of charged particle beam spots on the sample surface 45 remains substantially constant.

[0052] Note that steps a and b above can be repeated to obtain the desired alignment.

[0053] It should be understood that the above description is included to illustrate the operation of the preferred embodiments and is not intended to limit the scope of the invention. From the above discussion, many variations will become apparent to those skilled in the art that would still be encompassed by the scope of the invention.

[0054] In summary, the present invention provides an apparatus for inspecting a sample, the apparatus comprising: a sample holder for holding the sample at a sample plane; and a charged particle column for generating an array of charged particle beamlets and directing the array towards the sample holder. The charged particle column comprises an objective lens for focusing the charged particle beamlets of the array into an array of charged particle beam spots at or near the sample plane. The objective lens comprises a magnetic lens common to all of the charged particle beamlets. The apparatus further comprises a position sensor for providing a signal dependent on the position of the sample along an optical axis of the charged particle column, and a control unit for controlling the position of the sample holder based on the signal from the position sensor to keep the pitch and / or orientation of the spots on the sample constant. The following is a summary of the claims as originally filed: [1] An apparatus for testing a sample, said apparatus comprising: a sample holder for holding the sample at a sample plane; a charged particle column for generating an array of charged particle beamlets and directing the array of charged particle beamlets towards the sample holder, wherein the charged particle column comprises an objective lens for focusing the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; a position sensor configured to provide a signal dependent on the position of the sample in a direction along the optical axis of the charged particle column; a control unit configured to control a position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on the signal from the position sensor; and An apparatus comprising: [2] The apparatus of [1], wherein the position sensor is configured such that the signal depends on the position of the sample holder or sample plane relative to the objective lens. [3] The device comprises: a photodetector for detecting photons created by one or more charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on a layer of luminescent material after passing through the sample; an optical optical assembly for projecting or imaging at least a portion of the photons along an optical beam path onto the photodetector; The device according to [1] or [2], further comprising: [4] The apparatus of [3], wherein the optical optical assembly is configured to image the sample plane onto the photodetector. [5] The apparatus of [3] or [4], wherein the light-optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material. [6] The apparatus further comprises an optical focus sensor configured to provide a signal dependent on a distance between the sample plane and the optical objective; [5] The apparatus described in [5], wherein the control unit is configured to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on the signal from the optical focus sensor. [7] The apparatus described in [5] or [6], wherein the optical objective lens has an optical axis, and the apparatus is configured such that the optical axis is substantially parallel to the optical axis of the charged particle column. [8] An apparatus for testing a sample, the apparatus comprising: a sample holder for holding the sample at a sample plane; a charged particle column for generating an array of charged particle beamlets and directing the array of charged particle beamlets towards the sample holder, wherein the charged particle column comprises an objective lens for focusing the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; a photodetector for detecting photons created by one or more of the charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on a layer of luminescent material after passing through the sample; an optical optical assembly for projecting or imaging at least a portion of the photons along an optical beam path onto the photodetector, wherein the optical optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material; an optical focus sensor configured to provide a signal dependent on the distance between the sample holder or sample plane and the optical objective; a control unit configured to control a position of the sample holder in a direction at least parallel to an optical axis of the charged particle column based on the signal from the optical focus sensor; and An apparatus comprising: [9] The apparatus of [8], wherein the light-optical assembly is configured to image the sample plane onto the light detector.

[10] A method for testing a sample, the method comprising: placing the sample in a sample holder; generating an array of charged particle beamlets and directing them towards the sample using a charged particle column, wherein the charged particle column comprises an objective lens that focuses the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near a sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; using a position sensor to provide a signal dependent on the position of the sample in a direction along the optical axis of the charged particle column; using a control unit to control the position of the sample holder in a direction at least parallel to an optical axis of the charged particle column based on the signal from the position sensor; A method comprising:

[11] The method according to

[10] , wherein the position sensor is configured such that the signal depends on the distance between the sample holder or sample plane and the objective lens.

[12] A method for testing a sample, the method comprising: placing the sample in a sample holder; generating an array of charged particle beamlets and directing them towards the sample using a charged particle column, wherein the charged particle column comprises an objective lens that focuses the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near a sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; using a photodetector to detect photons created by one or more of the charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on a layer of luminescent material after passing through the sample; using an optical optical assembly to project or image at least a portion of the photons along an optical beam path onto the photodetector, wherein the optical optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material; using an optical focus sensor to provide a signal dependent on the distance between the sample holder or sample plane and the optical objective; using a control unit to control the position of the sample holder in a direction at least parallel to an optical axis of the charged particle column based on the signal from the optical focus sensor; A method comprising:

[13] A computer-readable medium having computer-executable instructions adapted to cause an apparatus described in any one of [1] to [7] to perform the method described in

[10] or

[11] , or to cause an apparatus described in any one of [7], [8], or [9] to perform the method described in

[12] .

Claims

1. 1. An apparatus for testing a sample, the apparatus comprising: a sample holder for holding the sample at a sample plane; a charged particle column for generating an array of charged particle beamlets and directing the array of charged particle beamlets towards the sample holder, wherein the charged particle column comprises an objective lens for focusing the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near the sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; and a light optical microscope, the light optical microscope comprising: a photodetector for detecting photons created by one or more of the charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on a layer of luminescent material after passing through the sample; an optical optical assembly for projecting or imaging at least a portion of the photons along an optical beam path onto the photodetector, wherein the optical optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material; an optical focus sensor configured to provide a signal dependent on the distance between the sample holder or sample plane and the optical objective; The apparatus further comprises a control unit configured to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on the signal from the optical focus sensor to hold a portion of the sample to be inspected by the array of charged particle beamlets at a desired distance from the magnetic lens and to hold the array of charged particle beam spots at a desired orientation at or near the sample plane.

2. The apparatus of claim 1 , wherein the light-optical assembly is configured to image the sample plane onto the light detector.

3. 3. The apparatus of claim 1, wherein the optical objective lens has an optical axis, and the apparatus is configured such that the optical axis is substantially parallel to the optical axis of the charged particle column.

4. The apparatus of claim 1 , wherein the apparatus is configured to provide a fixed distance between the magnetic lens and a focal point of the light-optical assembly.

5. The apparatus of claim 1 , wherein the light-optical assembly is configured to image the sample plane onto the light detector.

6. the apparatus comprising a position sensor configured to provide a signal dependent on a position of the sample in a direction along the optical axis of the charged particle column; The apparatus of claim 1 , wherein the control unit is configured to control a position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on the signal from the position sensor.

7. The apparatus of claim 6 , wherein the position sensor is configured such that the signal depends on the position of the sample holder or sample plane relative to the objective lens.

8. 1. A method for testing a sample, the method comprising: placing the sample in a sample holder; generating an array of charged particle beamlets and directing them towards the sample using a charged particle column, wherein the charged particle column comprises an objective lens that focuses the charged particle beamlets of the array of charged particle beamlets into an array of charged particle beam spots at or near a sample plane, the objective lens comprising a magnetic lens common to all charged particle beamlets of the array of charged particle beamlets; using a photodetector to detect photons created by one or more of the charged particle beamlets of the array of charged particle beamlets when they impinge on the sample or when they impinge on a layer of luminescent material after passing through the sample; using an optical optical assembly to project or image at least a portion of the photons along an optical beam path onto the photodetector, wherein the optical optical assembly comprises an optical objective lens for collecting photons from the sample and / or the layer of luminescent material; using an optical focus sensor to provide a signal dependent on the distance between the sample holder or sample plane and the optical objective; using a control unit to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on the signal from the optical focus sensor to hold a portion of the sample to be inspected by the array of charged particle beamlets at a desired distance from the magnetic lens and to hold the array of charged particle beam spots at a desired orientation at or near the sample plane; A method comprising:

9. The method of claim 8 , wherein the focal point of the light optical assembly is located at a fixed distance relative to the magnetic lens.

10. The method comprises: using a position sensor to provide a signal dependent on the position of the sample in a direction along the optical axis of the charged particle column, wherein the position sensor is configured such that the signal depends on the distance between the sample holder or the sample plane and the objective lens; using a control unit to control the position of the sample holder in a direction at least parallel to the optical axis of the charged particle column based on the signal from the position sensor to hold a portion of the sample to be inspected by the array of charged particle beamlets at a desired distance from the magnetic lens and to hold the array of charged particle beam spots at a desired orientation at or near the sample plane; The method of claim 8 , comprising:

11. A computer-readable medium having computer-executable instructions adapted to cause the apparatus of claim 1 to perform the method of claim 8.

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